No thanks! I would like to know more about CHROMacademy

 Over 3000 E-Learning topics / 5000 Articles & Applications
 

The CHROMacademy Essential Guide to
GC Troubleshooting - Column & Detector Issues

The Essential Guide from LCGC’s CHROMacademy presents the second in our series of webcasts on Practical GC Troubleshooting. In this session, Dr. John Hinshaw (Senior Scientist , BPL Global Ltd) and Tony Taylor (Technical Director, Crawford Scientific), present practical troubleshooting and maintenance information associated with capillary GC columns and common detectors.

The session includes a wealth of practical information on avoiding issues when preparing, installing and conditioning capillary GC columns. We also consider causes of capillary column aging and how to ensure your column remains fit for purpose for as long as possible! Also included are a quick revision on common GC Detectors (FID / TCD / ECD / NPD) as well as practical tips for avoiding problems and optimizing detector response.

We conclude by considering the symptoms of detector problems and building a list of troubleshooting and maintenance items which will be useful to the GC practitioner. Throughout, we consider both instrument and chromatographic symptoms. A must see for everyone working with GC instruments.

 

Topics includeautosampler

  • Proper column installation and conditioning
  • Capillary column performance degradation
  • Protecting the column and prolonging column lifetime
  • Detector principles (FID / NPD / ECD / TCD)
  • Detector performance optimization
  • Troubleshooting detector response / sensitivity issues
  • Maintenance Schedule

Who Should Attend:

  • Anyone who uses GC equipment and who wants to improve their troubleshooting and instrument maintenance skills

Key Learning Objectives:

  • Learn how to properly prepare a GC column for installation
  • Strategies for proper column installation and conditioning to ensure rapid column readiness
  • Methods for monitoring column performance and protecting the column from degradation
  • Ways to restore column performance and tell tale chromatographic symptoms of performance issues
  • Overview of the working principles of common GC detectors
  • Optimizing GC detector response
  • Common detector problems and how to avoid them
  • Recognizing baseline and chromatographic symptoms of detector issues
  • Build a practical maintenance schedule
Tony Taylor
Technical Director
Crawford Scientific

John V Hinshaw
Senior Scientist
BPL Global Ltd.

 

The Camtasia Studio video content presented here requires JavaScript to be enabled and the latest version of the Adobe Flash Player. If you are using a browser with JavaScript disabled please enable it now. Otherwise, please update your version of the free Adobe Flash Player by downloading here.

The CHROMacademy Essential Guide Tutorial
GC Troubleshooting - Column & Detector Issues

The CHROMacademy Essential Guide to
GC Troubleshooting - Column & Detector Issues

In this essential guide, we provide practical troubleshooting and maintenance advice on capillary columns and selected detectors used for capillary GC.

The guide is intended to provide useful practical information on identifying problems when they occur and how to best address them.  We will use both chromatographic as well as hardware symptoms to do this and will provide information on good practice and preventative maintenance to ensure instrument downtime is minimised.

 

 

 

  Waters Acquity

Share this tutorial

In gas chromatography, as with all forms of chromatography, the column is the heart of the separative system.[1]  The proper preparation, installation and conditioning of the GC column is critical in optimising performance and lifetime.  Protection from harmful impurities in the carrier gas, as well as less volatile matrix components, is also vital.  These topics are considered in the sections below.autosampler

 
 

 


Like CHROMacademy on Facebook and keep up-to-date with the latest webcast, tutorial and eLearning release schedules.

Facebook

     

We considered the importance of carrier gas cleanliness in a previous Essential Guide (February 2012), however a re-cap of the important facts are given below.[2]

Clean carrier gas is essential to prolong the lifetime of GC columns, and is required to achieve less noisy baselines and good peak shape. Figure 1 shows a typical oxygen catalysed stationary phase degradation mechanism, which iwll be discussed in more detail in a subsequent section. Detector gases are also susceptible to impurities which lead to increased background signal, baseline noise and reduced sensitivity.

The three main gas contaminants of concern are Oxygen, Moisture and Hydrocarbons. The first two can enter the gas stream through permeation of tubing and fittings, the latter can arise from grease and lubricating oils within tubing, gas compressors used to generate air or plastic tubing used in gas generators.

Gas suppliers all have proprietary names for high quality gas – however in general they all carry products that can be categorized as ‘Certified’ and ‘High Purity’. Wherever possible the gas supplied to the GC should contain less than 1ppm of the relevant impurity and be designated as (‘five nines’) 99.999% pure or better

Table 1 shows typical gas purity when using commercial gas filter systems.  This can be achieved with an input gas of ‘four nines’ (99.99%) purity or above:

 
 
Trap Capacity (g) Purity (ppb)
Oxygen (high capacity) 3.5 < 10
Oxygen indicating 0.05 < 0.1
Moisture 12 < 10
Hydrocarbons 8 < 1

Table 1.  Typical trap capacities.

 
 

Siloxane bleed

 

Figure 1. Siloxane bleed – six membered ‘back-biting’ O2 catalysed degradation of polysiloxane based capillary GC stationary phases

 
 

Moisture traps usually contain molecular sieve that is heat treated and packed under vacuum. Removing moisture from the carrier and detector fuel gases prolongs column lifetime and ensures low noise baselines with good detector sensitivity. The moisture trap will also protect the oxygen filter further down the gas line.

The indicator for modern moisture traps have been altered from the traditional cobalt blue dye according to European Safety directives. Newer traps use a silica based indictor which changes from orange/brown to off-white when exhausted.

 

Moisture trap« Figure 2. Moisture trap.  Courtesy of SGE

 

 

 

Modern moisture traps have capacities of around 7-10 grams (H2O) and will typically reduce the carrier hydrocarbon content to less than 1ppm.

Oxygen contamination in the carrier gas can produce excessive column bleed at elevated column temperatures caused by oxidative degradation of the stationary phase.

Oxygen traps usually contain aluminium oxide which has a high capacity for getting and binding (fixing) moisture.

Modern oxygen traps have capacities of around 1000 mL (O2) and will typically reduce the carrier hydrocarbon content to less than 50ppb.

 

 

Moisture trap« Figure 3. Oxygen indicating trap.  Courtesy of SGE.

 

 

 

 

column bleed

 

 

 

« Figure 4. Oxygen contaminated carrier gas can cause severe
column bleed at elevated temperature.  This can be both increased baseline response as well as discrete peaks within the chromatogram

 

 

 
 

Hydrocarbons cause ghost peaks, increase baseline noise and reduce detector sensitivity.  It is not essential to have a hydrocarbon trap connected to the carrier inlet stream – however most manufacturers would recommend this for good practice.

Modern Hydrocarbon traps are self-indicating and available in cartridge format.  These traps plug into a base plate and can be conveniently changed without interrupting the gas flow.  Indicating traps are typically constructed from glass or impermeable plastics which do not allow the passage of oxygen and moisture.

 

Stainless steel oxygen trap« Figure 5. Hydrocarbon trap.  Courtesy of SGE.

 

 
 
 
Impure carrier can cause noisy baselines

« Figure 6. Impure carrier can cause noisy baselines, high background and poor sensitivity.

 

Modern hydrocarbon traps have capacities of around 6-12 grams (measured as n-butane) and will typically reduce the carrier hydrocarbon content to less than 100ppb.

Removal of hydrocarbons from the carrier and detector gases will ensure a stable baseline with low noise levels and detection limits.

 

 


Like CHROMacademy on Facebook and keep up-to-date with the latest webcast, tutorial and eLearning release schedules.

Facebook

     

To prepare any capillary GC column for installation we you will need to have the correct nut and ferrule placed onto the column which should be properly cut at both inlet and detector ends and the exterior surface of the column needs to be contaminant free.

 

Tools and Accessories

There are a few tools and accessories that you will need to prepare and  install your column

  • Column cutting tool (ceramic wafer, fused silica column cutter, etc)
  • Magnifying glasses (to inspect your column cut)
  • Spanners
  • Nuts, ferrules, septum
  • Supply of non-retained compound (such as methane or butane for FID; argon or air for MS)
  • Standard compound to test system is in order (optional)
  • Electronic flow meter (or a detecting solution)
  • Electric leak detector
  • Safety glasses (you MUST wear them as you are going to cut the GC column)
  • Powder free latex gloves
  • Ruler
 

 

Column Nuts and Ferrules

Despite their importance, ferrules are sometimes underestimated; however, without ferrules the airtight seal that is required at the inlet and detector of a GC system would be impossible to achieve. The ideal ferrule will provide a leak free seal, must not stick to the column and must tolerate temperature changes during programming.[3]

Leaks cause a number of problems, including poor retention time and peak area reproducibility, loss of accuracy, drifting detector baselines, high detector noise, shortened column life, and increased gas consumption.

A leak is not just a wasteful one-way path from the high pressures inside the tubing to the atmosphere; oxygen can diffuse from the atmosphere back into areas where the partial pressure is lower. High oxygen levels in the carrier gas can cause excessive stationary phase bleed and greatly reduced column life, as well as increased detector noise and background signal levels.

 

Figure 7. » Ferrule Selector Tool


Key ferrule facts:

1. When selecting a ferrule consider:

  • Injector temperature
  • Type and sensitivity of the detector
  • Material that provides the optimum seal to avoid leaks under the given operating conditions (see Figure 7 - Ferrule Selector Tool)

2. How to avoid problems:

  • Change regularly or as described below
  • Avoid finger grease and other contaminants on the column external surface
  • Ensure the correct column nut is used and fit the ferrule in the correct direction (all ferrules are ‘directional’ as defined by the fitting into which they are tightened – follow manufacturers advice)
  • Do not over tighten the nut & ferrules

3. When to change the ferrule:

  • When the ferrule is visibly deformed
  • When cutting the column at the inlet end to restore chromatographic performance
  • When unable to achieve or maintain the desired column head pressure
  • When encountering baseline drift (potentially caused by oxygen ingress at the column connection which is subsequently causing oxidative degradation)
  • If no other cause of peak area irreproducibility can be identified
  • If no other cause of peak retention time irreproducibility can be identified
  • If encountering an increase in the detector background signal

 

 

Selecting the correct ferrule internal diameter to match the column internal (actually outside) diameter is important.  Whilst some variation will be seen from manufacturer to manufacturer, in general the following guide holds true:

 

Ferrule Material and Use Ferrule ID (mm) Use with columns of this ID (mm)
Graphite Short Ferrules (General Purpose)

0.5

1.0

0.1, 0.2, 0.25, 0.32
0.53
85% Vespel, 15% Graphite Short Ferrules (General Purpose)

0.4

0.5

0.8

0.1, 0.2, 0.25
0.32
0.45, 0.53
100% Vespel Short Ferrules (for isothermal analysis only)

0.4

0.5

0.8

0.1, 0.2, 0.25
0.32
0.45, 0.53

Table 2.  Matching ferrule i.d. to capillary column i.d

 

Failure to use the correct ferrule may result in a non-gas tight / leaky connection and therefore reduced sensitivity.[7, 9]

chromatogram

Figure 8.  Failure to use the correct ferrule may adversely affect sensitivity, peak shape and basline position/drift

 

 

figure 9Cutting and Cleaning the Column

 

The column nut and ferrule should be placed onto the column prior to cutting at the inlet or detector end.  Failure to do so may risk shards of ferrule material entering the column and giving rise to poor peak shape.  Some operators prefer to use a septum to hold the column nut and ferrule in place (correct distance from the column end) during installation.  This should also be done prior to column cutting.[4-8]

 

» Figure 9.  Insert septum, column nut and ferrule onto column prior to cutting

Using a scribe (ceramic wafer, diamond tipped pen), score the polyamide coating of the column and, holding firmly just below the score, flick the column above the score away from you.  The wafer has two edges, the rough edge should be used to score the polyimide coating.

 

 

 

 

Score and remove about 2 cm of the column » Figure 10. Cutting the column.  Usually 2.0 cm of severed column are enough to guarantee high quality stationary phase column although many workers prefer to take >20cm.

 

 

 

 

 

 

 

 

cut edgeInspect the cut edge with a 10-20X magnifier.  The cut end should be at a 90o angle relative to the tubing wall.  There should be no burrs or large, jagged areas.  If necessary, re-cut the column until a proper cut is obtained.  If a good column cut is not made, then significant activity (resulting in poor quantitative precision) and split or tailing peaks may be observed in the chromatogram.

 

» Figure 11.  Examples of good and bad column cuts.  Courtesy of Agilent Technologies.

 

Poorly cut columns expose excess silanol groups and subject the analytes entering the column to turbulent eddy currents. Secondary retention mechanisms and turbulent eddies cause peak tailing and splitting problems.

 
 

peak tailing and splitting problems

Figure 12. Tailing peaks caused by a jagged column cut

Figure 13.  Column cutting and installation

 
 

Some workers prefer to use fused silica column cutters of the type shown in figure 14

  1. Insert capillary column (A) in the column cutter as indicated
  2. Fix the capillary column by turning the securing disk (B)
  3. Cut the column by turning the cutting disk (C), one full rotation is usually sufficient
  4. Use the magnifier (D) to verify the cut

Wipe the column with acetone to remove finger grease and other residues from the exterior surface of the column.

» Figure 14.  Fused silica column cutter.  Courtesy of SGT

 

 

Fused silica column cutter
 


Like CHROMacademy on Facebook and keep up-to-date with the latest webcast, tutorial and eLearning release schedules.

Facebook

     

System Checks
Before installing the capillary column, ensure old septa and injection port liners have been replaced with new ones. Use gloves and/or clean tweezers to handle items to ensure cleanliness and inactivity of septa and liners. Inspect your gas purifiers for colour change. Change the gas purifiers if required.

This is a good time to check our gas supply and pressure control essential guide:[2]

 

The CHROMacademy Essential Guide to GC Troubleshooting - Gas Supply and Inlet Issues.

 
 
 

Place the Column in the GC Oven
Make sure the column tubing does not touch any of the sides of the oven.  If the column touches the hot oven wall the polyimide will ‘bake out’ and become brittle.  You risk column breakage when the column cooling fan starts.

Unwind enough column at both sides to perform the installation.  In practical terms, 30 cm (12 inches) of column at each end is usually sufficient to achieve proper column installation.

 
 

Column installation into the GC inlet.Making the Connection

Start by installing the column into the GC injection port. 

Push the column through a septum, keeping the septum at 90o to the column – this will be used to support the nut and ferrule at the correct point on the column whilst securing in the injector and detector. Feed the column through the nut and ferrule prior to cutting the column as described in the sections above.[7]

 

 

 

» Figure 15. 
Column installation into the GC inlet.

 

Please bear in mind that the use of a septum is not mandatory, but it helps with the installation process.

 
 

Column Insertion Depth –Injection Port

The distances between the ferrule tip and the column inlet are crucial and differ between manufacturers.column insertion depth

» Figure 16.   Setting the column insertion depth (L).

Ensure that guidelines for column insertion distances are followed closely. Use the septum to keep the nut and ferrule at the correct position on the column whilst inserting the column.  Note that you MUST consult your GC system manufacturer to find out what the correct column connection length (either L1 or L2) must be used.

 

If the column is incorrectly positioned broad peaks or incorrect peak area ratios will be seen due to the increase in dead volume or incorrect sampling of analytes into the column or analyte degradation.

 

 

Incorrect GC column positioning

 

Figure 17.  Incorrect GC column positioning is the source for a multitude of GC peak shape and quantitation problems

 
 

Coupling the column into the GC Injection Port

Once the column length has been properly adjusted, insert the column into the inlet and finger tighten.  If necessary, slide the septum to properly reposition the column.  Tighten the column nut an additional ½ turn. 

 
 

GC injection port

Figure 18.  Installing the column into the GC injection port

 
 
Tighten the column nut

Ensure that the column is securely held in the injection port and that no leaks are present. Care should be taken not to over tighten the column nut as this can break the ferrule (leaks and reduced sensitivity) and damage the end of the column (split or tailing peaks).

 

« Figure 19.  Tighten the column nut an additional half turn past the point at which the ferrule ‘just grips’ the column in position.  Do not over tighten the nut, as ferrule and /or column damage might result.

 
 

Flow Rate Initial Considerations

Set the carrier gas pressure to the value stipulated in the analytical method you are about to use.

Place the detector end of the GC capillary column into a container filled with an appropriate solvent (water or methanol or acceptable solvents), and check that there is a continuous flow of bubbles to indicate that carrier is able to flow through the column and partially verify the installation to this point.  Remove from the liquid prior to turning off the carrier flow.  Wipe the end of the column and then clean with acetone to remove any residues.[7, 8]

 

Preliminary flow rate checking

Figure 20.  Preliminary flow rate checking.

 
 

Connecting the GC Column to the Detector Port

Repeat the same procedure already described for the coupling of the GC column with the injection port in the case of the detector.

 

Preliminary flow rate checking

Figure 21.  Couple the column to the detector port.

 

Different column manufacturers have different recommendations for insertion distances into the detector port.  These must be followed to avoid peak shape and quantitation issues.

 

Failure to install the column properly in the detector may lead to the introduction of large dead volumes in the sample path.  This may manifest itself as tailing or, where the dead volumes are large, peak broadening.

 

dead volume

Figure 22.  Detector dead volumes lead to tailing effects. 

These are best investigated by highly diffusive analytes such as methane or other permanent gas probes.

 
 

Check for leaks at both column fittingsPreliminary Tests –Leaks

Once the column has been properly installed in both inlet and detector ports, check for leaks and correct as appropriate, which may involve re-making the connection using a fresh ferrule.  Use an electronic leak detector if possible as these are by far the most reliable means of ensuring leak free operation.  Remember that leaks can be small, and whilst you will be able to obtain and maintain the required GC inlet pressure, the fitting may still be leaking – risking ingress of oxygen and causing severe column damage.  Where MS detectors are used, small leaks at the detector fitting may seriously compromise the vacuum level within the detector and cause high background signals.[2, 7]

 

» Figure 23.  Check for leaks at both column fittings.

 

 

NOTE:  prior to column conditioning, check for leaks using the flow rate stipulated in your intended analytical method but do not heat the column!

 


Like CHROMacademy on Facebook and keep up-to-date with the latest webcast, tutorial and eLearning release schedules.

Facebook

     

Some column and detector combinations may require the column to be purged or conditioned before the detector connection is made.  This usually involves MS detectors and columns with thick films where a high amount of column bleed can contaminate the detector, but please refer to manufacturers instructions prior to column conditioning

GC columns should be conditioned according to the column dimensions, phase type, film thickness and intended analysis temperature.  The purpose of conditioning is to remove degraded stationary phase and contaminants which would otherwise give rise to ghost peaks or increased background signal during sample analysis.

NOTE: at this point the GC inlet should be at ambient temperature

The first part of the GC column conditioning is to purge the column with carrier gas.  This removes dissolved oxygen from the stationary phase and will prevent wasteful stationary phase oxidative degradation which might otherwise occur if the column is immediately heated without first purging the dissolved gases from within the bonded phase.

Use the information in Table 3 to choose the correct purge time.
 
 
Column ID (mm) Minimum Flow Rate (mL/min.) Minimum Purge Time(min.)
0.53 5.0 10
0.32 1.5 20
0.25 1.0 25
0.18 0.8 30
0.10 0.5 40

Table 3.  Matching column purge time to column i.d

 
 

Before embarking upon column conditioning please bear in mind that conditioning is intended to produce an acceptable background signal, free from system peaks, with the required signal to noise ratio for the intended application.  Any further conditioning will unnecessarily shorten the working lifetime of your column.  The practice of overnight column conditioning at elevated temperature should be avoided.  This is usually only necessary when using very thick film columns with highly sensitive detectors or when the column is intended to be operated at its upper temperature limit for extended periods of time.

  1. Program the column oven from ambient to 20oC above your intended maximum operating temperature or to the columns isothermal temperature maximum limit (whichever is lower) at 10oC per minute
  2. Program the inlet temperature from ambient to the intended operating temperature as the oven begins to heat
  3. Hold the column at the upper temperature according to table 4 below
  4. Cool the oven and, with the carrier still flowing, connect to the detector if required
 
Column Length (meters) Column Film Thickness (µm) Time for conditioning (mins.)
< 15 < 0.25
0.5 - 1.0
1.0 - 1.5
1.5 - 3.0
20
40
60
80
15 - 30 < 0.25
0.5 - 1.0
1.0 - 1.5
³ 1.5
40
60
80
100
³ 60 < 0.25
0.5 - 1.0
1.0 - 1.5
³ 1.5

80
120  
160  

200

Table 4.  Suggested column conditioning times according to column geometry

 
 

column conditioningPlease note that these times are suggestions and you should condition the column sufficiently so as to meet the background response and signal to noise criteria acceptable for your particular analysis.

 

« Figure 24.    After proper column conditioning, the baseline drift at increased column temperature should be reduced, with fewer system peaks and acceptable signal to noise ratio

 
 

Settings

solvent peak An injection of solvent is often more convenient than using a gaseous non-retained compound.  Furthermore – because the solvent typically has a reasonably large expansion volume (1μL of methylene chloride expands to 356μL at 250oC / 13 psi), the sample band will more readily highlight issues with column, installation or instrument settings.  To perform the test, inject 1μL of a solvent (methylene chloride is a good solvent to use) in the split mode (10:1) at 40°C isothermal and examine the peak shape.

A tailing solvent peak is a good indicator of

  • a poorly deactivated or contaminated inlet line
  • problems with inadequate make-up gas flow
  • improper column insertion into the detector
  • grossly contaminated column front end


» Figure 25.  Use of solvent to identify problems with GC column installation

 

 


Like CHROMacademy on Facebook and keep up-to-date with the latest webcast, tutorial and eLearning release schedules.

Facebook

     

Column bleed is the elution of degradation products of the stationary phase causing a background signal in the GC detector.  All columns produce bleed products and the most common degradation reaction is the 6 member so-called ‘back biting’ reaction shown in Figure 1 above.  It should be noted that a rising baseline during a GC temperature program is often attributed solely to ‘column bleed’ whereas this may also be caused by septum bleed products, sample matrix bleed, changes in the carrier flow rate when using mass flow sensitive detectors and operating in ‘constant pressure’ mode etc.[10, 11]

Bleed is best characterised using a ‘bleed profile’ – a temperature program that ramps to the column top operating temperature and holds for 10-15mins. A typical profile is shown in Figure 26 –any major deviations from this type of profile are not due to column bleed.  Bleed is a continuous process - any peaks in a blank run are not from column bleed, but most likely originate from contaminants in the GC system.

Column bleed increases with stationary phase polarity, film thickness and column age.  Exposing the column to oxygen (air) or consistently using the column at its upper temperature limit accelerates the onset of higher column bleed.  A sudden or rapid increase in column bleed is usually an indicator of column damage or a problem in the GC system.  Prolonged heating of a column above its upper temperature limit, constant exposure of the column to oxygen (usually via a leak), or repeated injection of damaging compounds (water, acetonitrile etc.) are the most common causes of problems.

 
 
 

Important Column Bleed Characteristics

  1. The baseline is relatively flat during the lower temperature region of the blank run.
  2. A sharp rise in the baseline begins at 30-40oC below the upper limit of the column and continues until the upper temperature limit is reached.
  3. Upon holding at the upper temperature limit, a fairly flat baseline is obtained.  Several minutes may elapse before the baseline becomes completely flat.  Major deviations from this profile are not due to column bleed.
  4. Column bleed is an abnormal elevated baseline at high temperature –it is not an elevated baseline at low temperature, a wandering or drifting baseline or (most typically) discrete peaks.
 

Column Bleed Characteristics

Figure 26.  Important characteristics of column bleed

 
 

Excessive column bleed appears as a larger rise in the baseline at the higher temperature regions.  There is no absolute measurement to indicate when column bleed is excessive.

Column bleed is best measured as the difference or change in the background signal at two temperatures –Relative Bleed.

Usually the column’s upper temperature limit and a lower value around 100oC are used.  The absolute background signal is a composite of the background generated by the entire GC system.  It is not possible to determine the contribution of column bleed to this total background signal.  By measuring the relative amount of column bleed, the other contributors to the background signal are subtracted out.

Most columns are tested using an FID detector.  The output signal for an FID is in picoamps (pA).  Bleed levels are usually reported as the difference (DpA) in the FID signal at two temperatures.

 

relative column bleed

Figure 27.  Measuring relative column bleed

 
Column Temperature Limits          
    1 2   3
Consider for the 30m x 0.25mm x 0.25mm,   -60/ 325   350)oC

Columns have lower and upper temperature limits (1-3 above).[12]

  1. If a column is used below its lower temperature limit, broad peaks are obtained (i.e., loss of efficiency). No column damage will occur; however, the column does not function properly. Using the column at or above its lower limit maintains good peak shapes.

    Upper temperature limits are often stated as two numbers

  2. The lower one is the isothermal temperature limit. The column can be used indefinitely at this temperature and reasonable column bleed and lifetime are realized.

  3. The upper number is the temperature program limit. A column can be maintained at this temperature for 10-15 minutes without severely shortening column lifetime or experiencing excessively high column bleed. Exposing the column to higher temperatures or for longer time periods results in higher column bleed and shorter column lifetimes. Exceeding the upper temperature limits may damage the stationary phase and the inertness of the fused silica tubing.

 

 

 


Like CHROMacademy on Facebook and keep up-to-date with the latest webcast, tutorial and eLearning release schedules.

Facebook

     

Over time, the inlet end of a capillary GC column can become contaminated from the accumulation of non-volatile sample matrix components. The phase in the front portion of the column can also be damaged from the continuous condensation and vaporization of solvent and analytes. Inevitably, active analytes will adsorb to this contaminated/damaged section causing peak tailing (through secondary interaction), loss of resolution, loss of effciency and reduced analyte response. When the chromatographic system degrades to an unacceptable level, performance can often be restored by ‘trimming’ the contaminated/damaged section from the inlet end of the column – typically at least 20cm is removed from the column. A very slight decrease in retention times and resolution occurs each time the column is trimmed, as theoretical plates are lost. Eventually, the column will need to be replaced.

The use of a guard column is an inexpensive technique to extend the lifetime of capillary columns.   Typically a 1-10 meter length of deactivated fused silica tubing attached to the inlet end of the column is used. Deactivated fused silica tubing does not contain any stationary phase; however, the surface is deactivated to minimize solute interactions. A suitable union is used to attach the tubing to the column. In most cases, the diameter of the retention gap or guard column should be the same as the column. If the tubing sizes are different, it is better to have a larger diameter guard column or retention gap than a smaller one.  The onset of peak shape problems is the usual indicator that the guard column needs trimming or changing.[10, 11, 13]

 
 
guard column to protect the GC column

» Figure 28. Use of a guard column to protect the GC column

 

Retention gaps are used to improve peak shapes for some types of samples, columns, and GC conditions. Usually a minimum of 3-5 meters of tubing is required to obtain the benefits of a retention gap. The situations that benefit the most from retention gaps are large volume injections (>2μL) and solvent-stationary phase polarity mismatches for splitless, Megabore direct and on-column injections. Peak shapes are sometimes distorted when using combinations of these conditions. Polarity mismatches occur when the sample solvent and column stationary phase are very different in polarity. The greatest improvement is seen for the peaks eluting closest to the solvent front or solutes very similar to the solvent in polarity. The benefits of a retention gap are often unintentionally obtained when using a guard column.

 

 

 


Like CHROMacademy on Facebook and keep up-to-date with the latest webcast, tutorial and eLearning release schedules.

Facebook

     

Signs of an Aging Column

Like all columns in chromatography, the GC column will deteriorate and begin to show signs of old age!  Perhaps the most typical symptom of column aging is peak broadening, caused by loss of column efficiency (theoretical plates)due to loss of bonded phase from the column – inherently reducing the available stationary phase surface on which to adsorb (into which to absorb) but also exposing silica (silanol groups) which will also lead, ultimately, to peak tailing.

As columns age they may also exhibit unacceptably high levels of column bleed.  One should always measure the bleed profile of a new GC column against which to compare at regular intervals to assess column aging / degradation.

 

 

» Figure 29. Signs of an aging GC column include loss of efficiency (top) and excessive column bleed (bottom)

 

 

 

Excessive peak tailing Other chromatographic symptoms such as gross peak tailing or split / shouldered peaks may be fixed by trimming the column, ensuring proper column positioning, fixing inlet septa issues, reducing dead volumes and ensuring the analyte is not labile at the inlet temperatures used.

 

» Figure 30. Excessive peak tailing and/or split peaks can sometimes be overcome by trimming the inlet portion of the capillary GC column.

 

For more information on Inlet issues please see our previous Essential Guide

//www.chromacademy.com/gc_troubleshooting_gas_supply_and_inlet_issues_essential_guide.asp

 


Like CHROMacademy on Facebook and keep up-to-date with the latest webcast, tutorial and eLearning release schedules.

Facebook

     

GC DetectorThere are a multitude of GC detectors and a commensurate number of GC detector problems which one might encounter.  This section aims to outline the operating principle of one or two of the most common detectors and to pinpoint common issues, troubleshooting techniques and maintenance which apply to each of them.  We will deal with Mass Spectrometric detector troubleshooting in a future Essential Guide.

 

 

 

 


Like CHROMacademy on Facebook and keep up-to-date with the latest webcast, tutorial and eLearning release schedules.

Facebook

     

 

The column effluent is mixed with hydrogen and a make-up gas (for capillary systems) before exiting via a small orifice (jet-tip) which is surrounded by a high flow of air.  The Hydrogen is combustible in air and can be lit via a remote glow plug.[14, 15, 16]

As the column effluent is burned in the resulting flame, ions are created which form a small current when a potential difference is applied.  When no analyte (carbon containing compounds) are being burned, a small background current (10-20 picoamperes) arises from impurities in the carrier and detector gases.  The exact mechanism of ion production is not well characterized, however the formation of carbon ions via pyrolysis and of small organic fragment ions produced via high energy combustion products are popular theories.  The flame ionisation detector produces a proportional response to the number of carbon atoms in a molecule.

 

» Figure 31. Operating principle of a typical FID detector

 

Three gases are required for FID detection with capillary columns :

  • A FUEL gas, typically Hydrogen
  • An OXIDISER gas, typically Air
  • A MAKEUP gas, typically Nitrogen or Helium
 
 

loss of efficiency The Fuel gas and Oxidiser combine to form the flame in which the analyte will combust and ionise. The makeup gas assists the analyte transition into the flame and minimizes dead volume effects.[17, 18, 19]

 

» Figure 32. Typical chromatograms showing loss of efficiency with no change in analyte retention time

 
 

Incorrect Gas Stoichiometry

The ratio’s of detector gas flow rates are crucial for optimum sensitivity and good peak shape.  Most FID detectors operate at maximum sensitivity with a volumetric flow rate of between 20 and 40 mL/min.  This is a combination of carrier and makeup gas flows.  As most capillary columns operate between 0.1 and 10 mL/min, well below the optimum required for maximum sensitivity, we can set the makeup gas flow rate accordingly.[16, 17, 18, 19]

The makeup gas is usually chosen to be different from the carrier –nitrogen is the most popular as its viscosity ensures good mixing with the effluent stream.

 
Typical FID gas default flow rates:
Carrier Gas: 1mL/min.
Makeup Gas: 30mL/min.
Hydrogen: 30mL/min.
Air: 300mL/min

Table 5.  Typical FID gas default flow rates

 

If the makeup gas flow is too high, then peak shape will be good but sensitivity may be reduced as the analyte is swept through too quickly for the detector to respond fully.

The ratio of fuel to makeup + carrier is vital in determining the sensitivity of the instrument –this ratio can be optimised for each analysis if required.  The air flow (ratio) is not so important (as can be seen opposite) so long as a minimum flow is established.

The makeup (nitrogen in this case) and hydrogen ratio have an effect on the sensitivity of the detector – this is demonstrated on the plot.  Each makeup flow (actually carrier + makeup) is plotted against a range of fuel (hydrogen) flow rates.  As can be seen below there is a maximum value for each combination representing a possible increase of around 10% response by optimising the gas flow rates.

 

FID detector sensitivity

« Figure 33.  Optimising the FID detector sensitivity by optimising the stiochiometric ratio of fuel and make-up gases

 

Once past a critical minimum value the actual flow of the oxidiser seems to make little difference to the detector sensitivity.  Typically this value will be between 300 and 400mL/min.  The critical ratio for sustaining a flame is 8-12% hydrogen in air.

detector sensitivity

Figure 34.  Air (oxidiser) and Hydrogen flow rate effects on detector sensitivity.

 

The sensitivity of the detector will typically deviate with hydrogen flow as shown in Figure 34.

If hydrogen flow rate is too high, this tends to reduce the dynamic linear range of the detector and one should use the manufacturers recommended hydrogen flow rate wherever possible as a starting point.

 
 

Jet Contamination

Detectors become contaminated with combustion products that form within the small orifice of the jet.  Eventually these deposits will begin to occlude the orifice and flow rates of the carrier + make-up stream will alter – thus affecting sensitivity.

Follow manufacturer’s instructions to clean the jet – taking extra care not to deform the orifice otherwise on-going problems with noisy baselines and peak area irreproducibility will result.

 
 

Poorly Positioned Column

If the column is positioned such that the column end is protruding into or through the flame, the analyte will not be efficiently burned within the flame and detector sensitivity will reduce.

 

» Figure 35.  Typical symptoms of peak broadening (loss of efficiency) with an FID detector

 

loss of efficiency

 

The position of the column within the detector housing relative to the flame is vitally important.  If the column is positioned too low (away from the flame) then the dead volume created between the column end and the flame will cause sample diffusion and hence loss of efficiency in the analyte bands as they elute from the column.

Follow manufacturers recommendations when installing the column into the detector.


» Figure 36.  Typical off-scale response

 

.  Typical off-scale response

 

 

 
 

Shorted Detector

A high off-scale response usually indicate the breakdown of electrical isolation between the detector anode (typically the jet) and cathode (typically the walls of the detector or a collector suspended above the flame).

Over time, the deposition of combustion products within the detector may coat any insulating elements within the device causing a ‘short’ between the anode and cathode – leading to the symptoms described in Figure 35.

Follow manufacturers advice to clean the detector

Most FID detectors contain an ‘Electrometer’ device which collects the very small signals (current) generated by the FID and detects and amplifies the signal.  If this device malfunctions or the connection between the detector and electrometer is compromised then a short may also occur.  Follow manufacturers troubleshooting advice to locate and correct the problem.[16-19]

 

Baseline drop and loss of signal

 

Figure 37.  Baseline drop and loss of signal following solvent peak

 
 

Solvent Causing a ‘Flame-Out’

The passage of a large solvent band through the detector, especially where the solvent is water, can extinguish the flame momentarily.

Most modern FID detector systems have measures which prevent this occurring or which can re-ignite the flame.

One may try to divert the solvent using a switching device or simply reduce the injection volume to avoid the problem.

Check the detector gas stoichiometry to ensure the gas ratios are correct as poorly adjusted gases can contribute to the flame being more easily extinguished.

 

Noisy baselines or baselines spikes

Figure 38.  Noisy baselines or baselines spikes

 

Noisy baselines or spiking baselines in an FID are typically created when the GC column end is poorly positioned in the flame.  If the column tip is within the flame region of the detector, the polyimide column coating will bake and small particles will be emitted into the flame – causing baseline noise or spikes.

 

Upward baseline drift

 

Figure 39  Upward baseline drift encountered with an FID detector

This type of baseline drift can be seen with a poorly conditioned column in temperature program mode or with a mass flow sensitive detector if operating in temperature programming mode using ‘constant pressure’ rather than ‘constant flow’ carrier gas modes.  One should eliminate these potential causes of the symptom prior to troubleshooting the detector.  More information can be sought from our previous GC troubleshooting Essential Guide:[2]

 

 

 

//www.chromacademy.com/gc_troubleshooting_gas_supply_and_inlet_issues_essential_guide.asp

Typically, FID detectors may give rise to baseline drift when the gas stoichiomtery is incorrect.  One should pay particular attention in this case to the hydrogen flow rate as a non-optimal setting can give rise to pronounced baseline drift.

If the drift is cyclical in nature, one should check the pressure stability of the in-coming gases – especially when using gas generator devices.

 
 

FID –Maintenance

Typically one would periodically disassemble the detector and clean the internal components, paying particular attention to the jet tip orifice and the surfaces of electrical isolators, anode and cathode.

Always follow manufacturers recommendations when carrying out maintenance on your GC detector.

 


Like CHROMacademy on Facebook and keep up-to-date with the latest webcast, tutorial and eLearning release schedules.

Facebook

     

 

Although the nitrogen phosphorous detector (NPD) is based on a similar design to the FID, and whilst belonging to the family of ‘ionising detectors’ it works on a different principle to the FID.[14, 17, 18, 19]

Whilst the main structure of the detector looks essentially similar to the FID, the main difference is the addition of a resistively heated bead / thermionic source just above or in the vicinity of the jet.  The bead contains or is coated with an alkali metal salt –usually caesium or rubidium silicate.  When heated this bead emits thermionic electrons which migrate to the collector electrode and form the background current.

The hydrogen flow into the detector is appreciably less than used in the FID and is too low to sustain a flame at the jet tip. Rather a ‘plasma’ of fuel (hydrogen), oxidiser (air), carrier effluent and makeup passes over the bead –at which point partial combustion occurs due to the heating filament.  When a suitable analyte is eluted into the plasma, the partially combusted nitrogen or phosphorous materials are adsorbed onto the surface of the bead.  This reduces the work function of the bead surface – effectively making it easier to emit electrons at the applied voltage / temperature.  Thus the emitted electron density increases, which causes an increase in the current in the det ector which is amplified and becomes the chromatographic peak.

 

» Figure 40. Working principle of the Nitrogen Phosphorous Detector.

 

 

 

  » Figure 41.  Loss of sensitivity seen with an NPD detector

Loss of sensitivity seen with an NPD detector

 
 

Bead Voltage Non-optimised
The voltage applied to the bead (and hence the resulting temperature) does have a marked effect on the detector response.  If unsure – start at 2V and make 10mV adjustments until the optimum response is obtained.

 
 

Hydrogen Flow / Make-up gas Non-optimised
It is essential to ensure that the hydrogen flow rate is low enough NOT to sustain a flame at the jet tip – otherwise measurement of nitrogen containing compounds will not be possible.  The detector is sensitive to variations in the hydrogen flow rate and a constant flow of hydrogen is recommended to ensure steady baselines.

Nitrogen is preferred over helium as make-up gas due to its lower thermal conductivity. Using nitrogen, the source requires a lower heating current.

 
 

Bead Surface Depleted / Contaminated
The main disadvantage of this detector is that its performance deteriorates with time –usually seen via the need to increase the bead voltage in order to generate a signal.  The water formed from combustion of hydrogen hydrolyses the alkali silicate to the metal hydroxide and silica.  As the alkali metal hydroxide is volatile under typical operating conditions, the rubidium or caesium is constantly lost from the bead –ultimately leaving an inert bead of silica.

To conserve bead life it is possible to turn off the detector (conventionally by interrupting the hydrogen flow) whilst solvent peaks elute or between injections when the GC oven is cooling.

To increase bead lifetime:

  • Use the lowest practical bead voltage
  • Run clean samples
  • Turn the bead off when not in use
  • Keep the detector temperature high (320 to 335°C)
  • Turn the hydrogen flow off during solvent peaks and between runs
  • If your NPD is off for a long time in a high-humidity environment, water may accumulate in your detector.  To evaporate this water:
  • Set the detector temperature at 100°C and maintain it for 30 minutes
  • Set the detector temperature to 150°C and maintain it for another 30 minutes
 
NPD –Noisy Baseline / Baseline Drift  

 

« Figure 42. Noisy / drifting baselines are a typical symptom of fluctuating / non-optimised hydrogen flow

 
 

Irregular Hydrogen Flow Rate
The NPD is particularly sensitive to changes in hydrogen flow rate.  It is essential that hydrogen flow is optimised and that the supply pressure / flow is constant.  This is particularly importat when using gas generators.

 
 

Constant Flow vs. Constant Pressure
The detector is mass-flow sensitive and as such it is better to operate in constant flow as opposed to constant pressure mode when performing temperature programmed analysis.

 
 

Bead Contamination
Noisy baselines are caused by contamination of the bead.  SNOOP and other surfactant based leak detection solutions should be avoided when using NPD detectors.

 

» Figure 43.  Baseline drop and loss of signal following solvent peak

 

 

Figure 43.  Baseline drop and loss of signal following solvent peak

 
 

The "Solvent quenching effect" is observed when there is a large negative baseline upset at solvent elution with no recovery to the original baseline.  This is usually due to the heating current / bead temperature being too low.

This can be remedied by slightly increasing the heating current / bead temperature.

There are several causes for an inherently high background signal for an NPD detector.  These issues (and solutions) include:

  • Heater current too high (reduce the heater temperature)
  • Hydrogen flow not optimised (optimise hydrogen flow)
  • Air / Make-up gas flow rate too low (increase as necessary)
  • Excessive column bleed (condition or replace the column)
 

NPD –High Off-scale Response

Figure 44.  Typical off-scale response

 


Like CHROMacademy on Facebook and keep up-to-date with the latest webcast, tutorial and eLearning release schedules.

Facebook

     

It is a selective detector that responds to compounds capable of ‘capturing electrons’ –more especially, halogenated compounds.  The radionuclide is 63Nickel which emits beta particles (low energy electrons).[14, 18, 19]


These negatively charged particles collide with carrier gas molecules and produce further, higher energy, electrons (remember that beta particles are themselves low energy electrons.

The electrons formed by this process establish a high standing current between the anode

 

(usually the inlet tube or detector body) and the cathode (usually a cylindrical electrode in the centre or top of the detector).  The potential difference applied is usually 20-100V dc).  When an electronegative analyte elutes, the analyte molecules capture some of the ‘background’ electrons and this results in a reduction of the standing (background) current.

The negative analyte ions formed are slow moving and are not collected by the anode.  The extent of electron absorption, and hence the reduction in standing current, is proportional to the concentration of the analyte.

 

Figure 45.  Operating principle of an Electron Capture Detector (ECD)

 

Figure 46.  Pulsed ECD Operation

 
 

Improved performance and linearity can be obtained by operating the detector in a ‘pulsed’ mode.  A square wave pulse (amplitude 50V, width 1m s at intervals of 20-50 μs) is applied at a frequency that maintains a constant current in the detector cell –in order to maintain the current in the presence of an analyte the pulse frequency has to be increased.  The signal is generated in proportion to the frequency of the applied pulse.

 

 

» Figure 47.  Loss of sensitivity seen with an ECD detector

 

 

Loss of sensitivity seen with an ECD detector

 

The cleanliness of the detector needs to be maintained at all times, which often means care with sample preparation.  Coating of either the emitter or collector can lead to a drastic reduction in detector response.  It is possible that a dirty or depleted detector actually produces a large absolute response – however the signal to noise ratio will reduce markedly.

Typically, when the detector performance in deteriorating a marked deviation away from the expected linear range is seen –especially at higher analyte concentrations as shown.

The ECD behaviour can be affected by a change of the total flow of the gases through the cell. The resulting response is also related to the nature and purity of the gases.

When using helium or hydrogen as a carrier gas with capillary or wide bore columns the detector should be fed with nitrogen or argon/methane through the make-up gas line, the flow rate of which should be optimised.

ECD –High Background Signal The sensitivity of the electron capture detector is variable, depending upon the electron affinity of the analyte.  For compounds of high electron affinity, such as halogenated compounds, minimum detectability of picograms on column are not untypical –with the added benefit that chromatograms are much simplified due to the very high selectivity.

 

» Figure 48.  Typical off-scale response

 

 

When using a pulsed detector – the pulse rate should be selected to match the make-up gas being used

Most modern ECD detectors use the change in pulse frequency required to maintain a constant current between the emitter and collector in order to measure analyte response.  As the analyte ‘captures’ the electrons (beta particles) emitted by the foil, the pulse frequency increases in order to maintain a constant current.  Some issues will cause a very high background pulse frequency – this ultimately reduces the sensitivity of the detector.  These issues (and suggested fixes) include:

  • Make-up gas flow rate too low (increase the make-up gas flow rate)
  • Excess column bleed (condition or replace the column)
  • Dirty collector electrode (clean if recommended by manufacturer or have the detector re-conditioned)
  • Contaminated / depleted radioactive source (manufacture detector re-condition)
  • Set correct pulse width for make-up gas (nitrogen or argon/methane)

 

 

Figure 49.  Noisy / Drifting baseline

 

Increased baseline noise or baseline drift is often seen when the detector becomes contaminated with highly responding species such as chlorinated solvents.

The carrier gases used for ECD operation should be pure and dry. Oxygen and water are both electronegative and as such contribute to a noisy baseline if they are present in the carrier or makeup gases even in trace amounts. 

A peculiar phenomenon of the ECD detector is that when either the foil or collector become dirty, the chromatogram will show distinctive negative dips after each of the peaks.

 

 

 

 

Figure 50. Negative dips after each peak

 


Like CHROMacademy on Facebook and keep up-to-date with the latest webcast, tutorial and eLearning release schedules.

Facebook

     

The detector is a heated metal (usually stainless steel) block drilled with two passageways or cavities.  Each cavity contains a filament made from a high temperature coefficient of resistance metal such as Rhenium or Tungsten.  These two filaments are heated and connected to the arms of a Wheatstone Bridge as shown in Figure 50.  Pure carrier (or reference gas) is allowed to flow over one filament (the reference cell) whilst the column effluent flows over the other (the analysis cell).[14, 18, 19]

» Figure 51.  Schematic representation of a Typical TCD Detector

 

 

  Thermal Conductivity Detector
 
 
When an analyte elutes with the carrier, the thermal conductivity of the gas in the analysis cell changes, the rates of heat loss of the two filaments will not be balanced and a current will have to be applied to balance the Wheatstone bridge. It is this current which is recorded as the signal.  The size of the applied current will be related to the concentration of analyte in the effluent gas.  The size of the cell is critical in determining the sensitivity of the detector –with 140 μL being typical in a detector used for packed column GC.  These devices cannot be used with narrow bore capillary columns and the cell volume must be drastically reduced.  Several manufacturers offer ‘single cell’ devices of very low volume (~5μL) for use with capillary columns in which the reference and analysis streams are switched onto the single filament in the order of 5-10Hz.  Ultra-small detector devices etched onto silica wafers are also available but are not used routinely.
 

TCD detector

Figure 52.  Single (switched) cell (low internal volume) TCD detector suitable for use with capillary GC columns

 
» Figure 53.  Loss of sensitivity seen with an TCD detector   Loss of sensitivity seen with an TCD detector
 
 

It is important that the filaments are held at a temperature ABOVE that of the body of the detector so that the effects of filament temperature change are optimised.

The larger the heating current applied to the filament, the greater the temperature differential and the greater the detector sensitivity.  However, high filament temperatures also mean shorter filament life and the potential that the filament will burn out if no gas is flowing in the cell.

The detector is susceptible to changes in the detector body, the filament temperature (and hence the current supplied), and the nature and flow of the carrier gas.  Therefore, all of these parameters should be kept as stable as possible.  This usually means covering the detector with a thermal cladding and ensuring that constant flow mode is used during temperature programming analysis. 

The sensitivity of the detector very much depends upon the difference in thermal conductivity between the eluting analyte and the carrier / reference gas. Interestingly this precludes the use of Area Normalisation as a method of quantitation!  Hydrogen and helium have a much larger thermal conductivity value than most common analytes and as such they are preferred as the carrier.  The use of nitrogen or argon as the carrier severely impairs the sensitivity of the detector.

 

Table 6.  Thermal Conductivities and TCD Response Values for Selected Compounds

  Compound Thermal Conductivity Relative Molar Response

Carrier

gases

Argon 12.5  
Carbon Dioxide 12.7  
Helium 100  
Hydrogen 128  
Nitrogen 18  
Samples
Ethane 17.5 51
n-Butane 13.5 85
n-Nonane 10.8 177
i-Butane 14.0 82
Cyclohexane 10.1 114
Benzene 9.9 100
Acetone 9.6 86
Ethanol 12.7 72
Chloroform 6.0 108
Methyl iodide 4.6 96
Ethyl Acetate 9.9 111

Thermal Conductivity values relative to Helium
Relative Molar Response in Helium:  Standard
Benzene = 100

 

 
 

Filaments become coated with sample residues over a period of time and need to be periodically cleaned by applying a very high temperature for 30 mins whilst the voltage is turned off.

Sometimes a gradual loss of sensitivity is observed over a period of weeks or months. This loss usually is caused by slow degradation of the filaments or thermistor beads due to corrosion or to contaminant deposition. The only practical solution is to replace the damaged elements. Note that replacement elements are sold as matched pairs. Do not try to replace only one of them — you probably would not be able to balance the detector properly. If you have a low-volume detector with feedthrough filaments, the whole block will need to be sent to the manufacturer to be rebuilt.

 

Negative peaks / extra peaks

Figure 54  Negative peaks / extra peaks seen with carrier gas contamination in TCD detection

 

As described above, the signal generated in a TCD detector is primarily due to differences in thermal conductivity between the reference (carrier) and sample (carrier plus sample) streams.

If the carrier contains impurities, or if the sample matrix changes markedly, then the chromatogram may contain extra peaks.  Extra peaks also occur if the injection routine has inadvertently introduced air or water into the system.

If the carrier is grossly contaminated, or if the carrier and sample thermal conductivity do not differ enough, then all peaks may be negative with a high baseline offset.  Peaks that represent solutes with thermal conductivities greater than the carrier gas, such as helium in nitrogen carrier or hydrogen in argon carrier are expected to have the opposite polarity of solutes with lower thermal conductivities than the carrier gas, such as hydrocarbons in helium, nitrogen, or argon carrier. If all peaks are inverted, then the detector polarity is backwards. When two columns are in use, one connected to either side of the thermal conductivity detector, the TCD polarity should be reversed when injecting on the second column.

 
 

The detector responds to changes in column or reference flow rates. If one of the flows is changed, it might be necessary to rebalance the detector or make a corresponding change to the other flow. During a temperature-programmed run, the flow through the column changes because of changes in the carrier-gas viscosity. Even using a mass-flow controller, significant transient fluctuations can occur in the column flow. These temperature-related effects, which are seen in addition to drift from column bleed, cause the baseline to drift up or down during a run. A matching reference column or restrictor often is installed in the oven so that both sides of the detector experience the same flow changes with changing oven temperature. Even in this case, the two flows might not perfectly cancel each other out; electronic baseline profile compensation is very useful for ironing out these residual effects.  One should operate in constant flow rather than constant pressure mode wherever possible.

 

TCD –Baseline Drift

Figure 55.  Noisy / Drifting baseline

 


Like CHROMacademy on Facebook and keep up-to-date with the latest webcast, tutorial and eLearning release schedules.

Facebook

     

1. “Theory and Instrumentation of GC”.  “Introduction” from CHROMacademy
2. “GC Troubleshooting –Gas Supply and Inlet Issues”  Essential Guide from CHROMacademy
3. “Fundamental GC-MS”.  “GC Considerations” from CHROMacademy
4. Dean Rood.  “The Troubleshooting and Maintenance Guide for Gas Chromatographers”  Chapter 9. WILEY-VCH Verlag GmbH & Co. KGaA. 2007
5. “Agilent J&W GC Column Selection Guide”  Pp 117-132.  Agilent Technologies. Printed in the USA February 15, 2007 5989-6159EN
6. “GC Accessories.  Instrument Supplies”  www.restek.com
7. “Practical GC Troubleshooting and Maintenance”  Chapter 5.  Crawford Scientific 2007.
8. “Make Clean GC Column Cuts…” Hewlett-Packard Company. CC Brochure B2 5/11/98
9. “Capillary GC Troubleshooting Guide: How to Locate Problems and Solve Them”  Supelco.  Bulletin 853.  Sigma-Aldrich Co. 1999.
10. “GC Method Development”  Chapter 9.  Crawford Scientific 2010.
11. “Fundamental Gas Chromatography”  Chapter 4.  Crawford Scientific 2009.
12. “GC Columns” from CHROMacademy’s “Theory and Instrumentation of GC”
13. “Maintaining your Agilent GC and GC/MS Systems”  5989-1925EN.  Agilent Technologies. 2005
14. Instrumentation of HPLC.  “Detectors” from CHROMacademy
15. Robert L. Grob and Eugene F. Barry. “Modern Practice of Gas Chromatography” Chapter 6.  John Wiley & Sons 2004
16. Dean Rood.  “The Troubleshooting and Maintenance Guide for Gas Chromatographers”  Chapter 8. WILEY-VCH Verlag GmbH & Co. KGaA. 2007
17. “Fundamental Gas Chromatography”  Chapter 8.  Crawford Scientific 2009.
18. “Practical GC Troubleshooting and Maintenance”  Chapter 13.  Crawford Scientific 2007.
19. “GC Method Development”  Chapter 15.  Crawford Scientific 2010.

 

 


Like CHROMacademy on Facebook and keep up-to-date with the latest webcast, tutorial and eLearning release schedules.

Facebook

     

 

Learn chromatography from the experts

Whether you work in a lab, or manage a lab, you will benefit from being a member of CHROMacademy.

As a member of CHROMacademy, you will get access to our vast e-Learning archive full of great interactive content and animations.
All our Essential Guide Webcasts and tutorials and LCGCs archive of magazine articles and webcasts from your favourite authors - John Dolan, John Hinshaw, Mike Balough, and Ron Majors. Plus vendor application notes, electronic laboratory tools and calculators and with our 'Ask the Expert' function - help is always at hand.

Analytical Chemists

  • I feel empowered to fix things
  • I know where to get applications
  • I’m more employable
  • My career is progressing
  • I understand my analyses
  • I’m up to date
  • I can troubleshoot effectively
  • I know where to go for help
 

Laboratory Managers

  • Flexible workforce with a common standard
  • Improved equipment utilization
  • Faster method development/problem solving
  • Better quality data
  • Get up to speed quicker
  • Lower T&E
  • Less reliant on me
  • I spend less time on training
 

Subscribe for $399 per/year and access:

  • The entire e-Learning archive
  • All Essential Guide Webcasts and Tutorials
  • LCGCs archive of articles and webcasts
  • Expert troubleshooting advice when needed
subscribe now
 
 
     
 

The following subjects are covered in CHROMacademy.com

The Theory Of HPLC
Introduction (1.5hrs)
Chromatographic Parameters (3hrs)
Band Broadening (3hrs)
Column chemistry (4hrs)
Reverse phase (partition) chromatography (6hrs)
Ion-Pair Chromatography (3hrs)
Normal phase (absorption) chromatography (3hrs)
Gradient HPLC (3hrs)
Quantitative and Qualitative HPLC (3hrs)
FAST HPLC (4.5hrs)
HILIC (3hrs)
SFC (3hrs)
Ion Chromatography(3hrs)

Theory and Instrumentation of GC
Introduction (1.5hrs)
Chromatographic Parameters (3hrs)
Band Broadening (3hrs)
Gas Supply and Pressure Control (2hrs)
Sampling Techniques (4.5hrs)
Sample Introduction (5hrs)
GC Columns (5.5hrs)
GC Temperature Programming (3hrs)
GC Detectors (2.5hrs)
SFC (3hrs)

Instrumentation of HPLC
Mobile Phase Considerations (3.5hrs)
Solvent Pumping Systems (4hrs)
Autosamplers (4.5hrs)
Detectors (4.5hrs)

Solid Phase Extraction
Molecular Properties (4hrs)
SPE Overview (3.5hrs)
SPE Mechanisms (4.5hrs)
Method Development (5.0hrs)
Primary Sample Preparation Techniques (2hrs)
Liquid / Liquid Extraction Techniques (1.5hrs) Approaches to Automation for SPE (1.5hrs)

Fundamental GC-MS
Introduction (1.5hrs)
GC Considerations (4.5hrs)
GC -MS Interfaces (2.5hrs)

Fundamental LC-MS
Introduction (1.5hrs)
Electrospray Ionisation Theory (6hrs)
Electrospray Ionisation Instrumentation (4hrs)
Mass Analyzers (9.5hrs)
Atmospheric Pressure Chemical Ionisation (3.5hrs)
Atmospheric Pressure Photoionisation (3hrs)
Solvents, Buffers and Additives (3.5hrs)
Vacuum Systems (3hrs)
Flow Rates and Flow Splitting (3hrs)
Orbitrap Mass Analyzers (3hrs)

MS Interpretation
General Interpretation Strategies (11hrs)
Intro to MS Proteomics Research (3.5hrs)

loading data
loading data
loading data
loading data
loading data
Home | About UsContact Us | SubscribeTerms and Conditions | Advertise | Privacy Policy |

loading data

loading data

loading data

 

loading data


loading data