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The CHROMacademy Essential Guide to Troubleshooting GC Separations | Retention Time Efficiency and Peak Shape Issues
Thursday 14th June 2012, 11:00am EDT, 4.00pm BST
.

Presenters:          
Tony Taylor
Technical Director
Crawford Scientific
  John V Hinshaw
Senior Scientist
BPL Global Ltd.
 

The Essential Guide from LCGC’s CHROMacademy presents the third 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 tips for GC separations, with special emphasis on retention time variability, loss of efficiency and poor peak shape. The session includes a wealth of practical information on GC separation issues and all our real world problems are related to both separation chemistry and hardware issues. We consider the common problems and highlight causes and remedial actions in a very practically relevant way. A must see for everyone working with GC instruments.

Topics include

  • Investigate causes of retention time variability in GC including carrier and injection issues
  • Investigate tailing, fronting and shouldering peak behaviors – identify the major causes and learn practical remedies for the problem
  • Learn about loss of efficiency in capillary GC and identify the major causes, including injection and column related issues
  • Learn how to quickly identify problems and investigate the root cause
  • Build up a portfolio of preventative and corrective maintenance operations
 

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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 quickly identify separation problems in GC
  • Identify strategies to confirm the problem and investigate the causes
  • Investigate the major causes of retention time variability and how to fix / prevent these issues including problems with carrier gas flow rate reproducibility and injection acquisition parameter issues
  • Highlight major causes of peak shape deformation, including the problems associated with peak tailing caused by activity within the GC system and how to take preventative measures to minimize tailing and maximize sensitivity and area reproducibility
  • Investigate the reasons for efficiency loss in GC and develop strategies to prolong column lifetime and maximize the efficiency of the system you are using
  • Develop curative and preventative maintenance operations to optimize the quality of your GC data

The CHROMacademy Essential Guide to Troubleshooting GC Separations -
Retention Time Efficiency and Peak Shape Issues

This essential guide examines the common causes of retention time drift, loss of efficiency and peak shape issues (such as splitting, fronting, tailing and shouldering) in modern capillary gas chromatography. The guide provides useful practical information on identifying problems when they occur and how to best address them as well as giving general maintenance tips.

 

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The CHROMacademy Essential Guide to Troubleshooting GC Separations -
Retention Time Efficiency and Peak Shape Issues

This essential guide examines the common causes of retention time drift, loss of efficiency and peak shape issues (such as splitting, fronting, tailing and shouldering) in modern capillary gas chromatography. The guide provides useful practical information on identifying problems when they occur and how to best address them as well as giving general maintenance tips.

 

Overview

The retention time, defined as the interval between the instant of injection and the apex of the chromatographic peak (detector response) for that component, is known as the retention time,[1] and is the primary means for analyte identification in GC. Retention time per se or retention indices (relative retention compared to a time based ‘marker’ or ‘markers’) can be used. For confirmation of analyte identity additional information, such as MS data, is often used in tandem. The retention time is influenced by numerous parameters such as carrier gas flow rate, temperature, stationary phase chemistry, etc.

From a practical perspective, only two parameters are typically varied during most GC analyses: column temperature (‘temperature programming’) and carrier gas flow rate (‘constant flow’ as opposed to ‘constant pressure’ GC). Broadly speaking, temperature has the greatest effect on the separation, but in most modern instruments carrier gas flow rate flow rate can be maintained at an optimum value (pressure programming using pneumatic electronic controllers) during temperature programming to avoid broad , late eluting peaks which are sometimes encountered with constant pressure analysis.

 
 
 

Retention Time Shifts

Retention time shifts are not uncommon in GC, and the magnitude of the shift can be both highly significant and problematic. Small retention time shifts will not necessarily mean there is a problem, but are representative of a system in which some natural variability is inevitable - we are, after all, using resistively heated components to heat a box of air in which we hang a very long silica column through which a gas is passing! If one suspects a retention time problem, then determine whether or not the retention factor has been affected (see below). As a rule of thumb, you should confirm the existence of the problem at least twice – that is, perform the same analysis at least twice to confirm the retention variability. Whilst this is a minimum requirement, in most cases it would be better to perform a statistically significant number of injections (6 is typically used) to obtain a relative standard deviation of the retention time variability.

It is difficult to suggest an estimate of acceptable retention time variability, applicable to all situations, as the retention time precision required for various applications will differ and of course different regulatory or industry standards / guidelines may apply. Roughly speaking, retention time deviations within a window of ± 10% of the peak width at base are within most regulatory limits. However, from a purely pragmatic practical point of view, if all other system suitability criteria for efficiency and resolution are met, the retention time criteria could be much less stringent.

The primary root causes of retention time shifts are:

  • Change in column temperature
  • Changes in carrier gas linear velocity
  • Changes in column (stationary phase, dimensions, phase ratio, etc.)
  • Changes in sample solvent (retention factor changes are mainly detected for peaks eluting near the solvent front)

These causes will be further investigated in the following pages.

 
 

Temperature

Temperature plays an important role in GC; as both the kinetics and thermodynamics of the chromatographic process are temperature dependent.[2-4]

In GC separations, an increase in temperature will reduce analyte retention; likewise, when retention time is increased, a decrease in column temperature should be suspected.

The oven temperature control system in a modern gas chromatograph will consist of a box containing resistive heating elements through which air is forced to rapidly heat the enclosed column to a pre-set temperature.  This arrangement allows the column temperature to be accurately maintained and quickly changed to meet the desired temperature profile for the analysis.  To ensure the accuracy of the oven temperature during temperature programming, it is possible to use accurate thermocouples attached to the column to confirm instrument readout data; note that such thermocouples require periodic calibration.  In most laboratories, a check of the validity of and repeatability of the oven temperature profile would be part of the Operational Qualification / Performance Verification (OQ / PV) testing.

 
 

Isothermal Operation

In this case the column is held at a constant temperature during the analysis.  In simple terms, the lower the oven temperature, the longer the analytes remain on the column – it can be said that the solubility of the analyte in mobile phase is lower at lower temperature. A general observation from isothermal analysis is that peaks broaden as retention time increases.  These effects are shown in Figure 1.

 
Figure 1.  Effect of column temperature on retention time.
 

As a rule of thumb, retention factor times decrease by a factor of two for every 20oC increase in column temperature. 

In order to perform peak identification, it is a good practice to establish what retention time variability is acceptable.  As suggested above, retention factor deviations within a window of ± 10% of the peak width at base is more than sufficient to perform peak identification by absolute retention time.  For capillary columns this is usually achieved by implementing temperature control of at least ± 0.05oC.  Certain columns, however, can deliver adequate retention time repeatability with less stringent temperature control.  Fast GC separations might require even better temperature control.

 
 

Temperature Programmed Operation

In this case the column is subjected to an increasing temperature throughout the analysis.[5]

Temperature is one of the two most important retention variables in GC, along with the chemical nature of the stationary phase. The retention factor (k’) decreases as temperature increases – they are inversely proportional - this is a fundamental relationship in Gas Chromatography.

 
Figure 2.  Determining retention factor (k).
 

A decrease in vapor pressure (due to decreasing temperature (T)) results in a decrease in the relative amount of analyte in the mobile phase and an increase in retention factor (and retention time). This is described by the Clausius-Clapeyron and Van’t Hoff equations.

 
Table 1. The Clausius-Clapeyron and Van’t Hoff equations.
 
Clausius-Clapeyron Equation
 
Van’t Hoff Equation

Where:
Po = analyte vapor pressure at a given absolute temperature T (K).
ΔH = enthalpy of vaporization at absolute temperature T (K).
R = the gas constant (R = 8.314472 J mol-1 K-1)

  • At low carrier gas (oven) temperature, the retention factor is high –the analytes are interacting strongly with the stationary phase and their vapor pressures are low
  • Increasing the oven temperature by 20-30oC brings about a 2 fold reduction in retention factor (based on the Van’t Hoff equation)
  • When retention factors are very high or very low, the quality of the separation is reduced.  Retention factors below 1 indicate that analytes may well be poorly resolved and may elute with the solvent peak and other non-retained components.  High k’ values indicate the analyte will be excessively retained and peak shape will be poor and analysis times long

In a plot of the natural logarithm of retention volume (analogous to tr) versus the inverse of temperature (Figure 3), the relationship between retention factor and temperature is clearly obvious from the linear nature of the plots for all analytes.

 
Figure 3.  Net retention volume versus 1/T.
 
  • The equations dealing with the temperature / retention relationships in GC are complex as can be seen.  In the practical context it is enough to remember that retention is inversely proportional to temperature and that a plot of the natural logarithm of retention factor against the inverse of the temperature should give an approximately straight line relationship such as that shown
  • The Clausisus-Clapeyron equation shows us that as temperature decreases, the analyte vapor pressure also decreases (-1/T).  As the analyte vapor pressure decreases it partition more readily into the stationary phase and is retarded in the column longer –and hence it’s retention factor and retention time increase
  • The Van’t Hoff Equation indicates that as temperature is increased the natural logarithm of the retention factor increases –this is a direct proportionality between retention and the inverse of temperature
 
Note: Selectivity increases with decreasing temperature but so does analysis time and band broadening.
 

In terms of retention time variability in temperature programmed GC, the same criteria used with isothermal operation apply, so ovens must be able to reproduce the time-temperature curve with the same precision as required for isothermal operation (around ± 0.05oC).  A high degree of run to run repeatability is crucial to retention time acceptability.

Note that ovens and columns require an equilibration period after cooling down before a new analysis can commence.  The column temperature could be higher than the temperature sensor due to its inherent thermal mass.  An equilibration time is required for the column, and most importantly the carrier gas travelling through it,  to reach thermal equilibrium with the oven  - 1 to 5 minutes is usually adequate for most situations.[6]

From a practical point of view, no oven can guarantee instantaneous isothermal conditions along the whole column.  However, differences up to 1oC across the column will not affect retention time drastically, as long as the gradient program is stable and reproducible.  Given that no electrical problems are occurring, heating problems will be minimized by making sure the oven fan, heater and temperature sensor are all in good working order.

Gradients that fluctuate from run to run will affect retention time; if this fluctuation is fast, peak shape could be affected. 

It should be noted that each GC will have a maximum temperature (reproducible) programming rate, effectively dictated by the instrument and power source design.  One should take care not to exceed this limit or irreproducible temperature programs may be encountered

Use the slider to investigate the effect of temperature on retention time (and retention factor).[7]

 
Figure 4.  Resolution as a function of retention factor (k’) and time.
 

Note how, under GC temperature program operation, chromatographic peaks tend to have approximately the same widths as they elute from the column.

 
 

The Carrier Gas Flow Rate

One of the primary parameters to investigate when retention time issues occur is the carrier gas flow rate.  The practical implications of setting an incorrect flow rate are tremendous, of course, and not only retention time but significant changes in resolution and peak distortion can occur.  Clean carrier gas is essential to prolong the lifetime of GC columns, and is required to achieve low levels of noise and good peak shape.[8, 9, 10]  The carrier gas flow rate has to be accurately controlled using the electronic pneumatic controllers of the GC system - typically micro fluidic devices working with digital solenoids.  This is especially true when operating in Constant Flow mode where an algorithm within the instrument firmware will alter the pressure applied to the carrier gas in order to maintain constant carrier flow as the column temperature increases.  Older GC instruments use manual pressure control system with regulators and are typically not able to operate in Constant Flow mode.

The carrier gas flow rate is temperature dependent.  At moderate conditions of temperature and pressure, the so called “ideal gas” law holds:

There are many situations where the ideal gas law cannot be used and expressions that account for the real gas behavior should be used instead.  This treatment is out of the scope of the present guide; however, the message is the same, the PVT properties (pressure, volume and temperature) of a gas are closely related, and you cannot act on one of them without altering the state of the remaining variables.

 
 

Constant Flow and Pressure

The viscosity of gases tend to increase with temperature.  A flow controller is used to keep carrier gas flow constant by altering pressure to compensate for changes in temperature.  Pressure controllers, on the other way, keep pressure constant as the temperature is changed.  In general terms:

  • Constant pressure mode: when temperature increases, pressure remains constant and carrier flow rate decreases
  • Constant flow mode: when temperature increases, pressure increases to maintain constant carrier gas flow

Using electronic pressure control allows the instrument to compensate for changes in gas pressure as the column oven temperature is increased. This will result in a flatter baseline, better peak shape and shorter elution times for more highly retained analytes.[8]

The chromatogram in figure 5, was obtained under constant pressure mode conditions; the initial analysis flow rate of 0.92 mL/min (at 50oC) falls to 0.38 mL/min. at the end of the analysis– the carrier gas pressure is 5 psi.

 
Figure 5.  Constant pressure GC separation of acrylic acid impurities using FID detection.
 

The chromatogram in figure 6, was obtained under constant flow mode conditions; unlike in the previous example, the carrier gas flow rate was kept constant to a value of 0.92 mL/min. throughout the whole analysis.  The pressure being ramped from 5psi to a final value of 11.5 psi at 300oC.

 

Figure 6. Constant flow GC separation of acrylic acid impurities using FID detection.

 

The previous figures reveal that the chromatography has been improved in several respects when using constant carrier flow in Temperature Programmed GC.  There has been an overall gain in sensitivity (the peaks areas have increased), mainly due to a decrease in peak width.  The rising baseline has been eliminated due to the constant flow into the mass flow sensitive detector – allowing more reproducible integration and quantitation of peaks 4 and 5.  The retention time has decreased by a factor of three.

 
 

Figure 7. Electronic Pressure (Pneumatic) control unit (EPC).  Courtesy of Agilent Technologies (Santa Clara, Ca, USA).

 
  • Electronic pressure regulating solenoids and read back can fail
  • Leads to an inability to achieve the set point flow (pressure) OR variable carrier / split / septum purge flow (pressure)

There are many reasons why GC instrument do not reach the required flow rate (pressure) or a constant carrier gas flow rate is not achievable.  These include:

  • Leaks at the column fitting or inlet septum fitting
  • Insufficient gas supply
  • Wrong system configuration (typically low flows with large internal diameter capillary columns)
  • Problems with the proportioning valves of the electronic pressure control system

Most modern electronic pressure controllers are well suited to perform carrier gas control at pressures greater than 5 psig (best performance at pressures of at least 20 psig).  Pressure fluctuations are a less significant contributor to retention time variability than temperature variations.  Pressure controllers might work not properly when operated below 3 psig; in such cases, a flow-controlled system would be desirable.

 
 

Measuring Gas Flow

As temperature and ambient air pressure may vary from place to place, standard reference conditions for testing, reporting and calibration should be used.  Selected standard reference conditions for gases are listed in the table below:

 
Table 2. Selected gas reference conditions (approximated).
 
Reference Pressure (kPa)* Temperature (oC)
STP 100.000 0
NTP 100.325 20
SATP 101.000 25
ICAO 100.325 15
* Approximately 1 atmosphere (1atm ≈ 101325 Pa ≈ 760 torr ≈ 14.7 psi)
 

Many GC systems express their readings on a standard 25oC and 1 atmosphere basis.  Therefore, a flow measured with an external device (such as a soap bubble meter) may not agree with your instrument reading,[11]  as flow was measured at local rather than standard conditions.  Make sure you know and use the same reference conditions than your GC system uses.  In case of doubt, consult your GC manufacturer’s handbook.

 

Example:  When using an electronic flow-meter you find a flow rate of 5 mL/min (measured at 740 torr and 18oC), calculate the equivalent flow rate (also in mL/min) that would appear in the dashboard of your GC system (which uses STP conditions).

 

Considerations:

  • Use absolute temperatures
  • Make sure that units are consistently used (for example, do not use P1 in bar and P2 in psi)
  • At moderate conditions of temperature and pressure, the so called “ideal gas” law holds
 

Lab Conditions

V1 = 5 mL/min
T1 = 18oC = (18 + 273.15 ) K = 291.15 K

STP Conditions

VSTP = ?? mL/min
TSTP = 0oC = (0 + 273.15 ) K = 273.15 K
PSTP = 100.0 kPa

Therefore:

 
 

Linear Velocity

The carrier gas flow rate (mL/min) can be expressed as an average linear velocity (cm/s) at which a non retained compound travels through the column.  This velocity can be found by injecting a highly volatile analyte that is not retained by the column, and using the following expression:

 

Where:

is the average carrier gas linear velocity (cm/s)
L is the total column length (cm)
tm is the retention time of the non-retained compound (seconds, minutes)

 

In a similar way, the flow rate (F) is given by:

Where rc is the column’s internal radius (usually expressed in mm) and j is can be calculated as follows:

 
 

Where pi is the inlet pressure and po the outlet pressure

Every time the column is shortened, which is the case when performing column installation or maintenance, the retention time is decreased.  Note that other parameters such as separation efficiency and resolution are changed when the column length is altered.  Selected non-retained analytes currently used to measure carrier gas flow rate are listed in the table below.

 
Table 3. Selected compounds used to measure carrier gas linear velocity.
 
GC Detector Compound
FID Methane, butane
TCD Methane, butane
ECD Methylene chloride*, halogenated methanes*
PID Ethylene, acetylene
NPD Acetonitrile*
MS Butane, air, halogenated methanes*

* Liquid at room temperature

The effective column length is key to understanding retention time decrease when shortening the column.  Count the number of full turns of the column on the hanger and multiply by the average length of a single turn, add the remaining length of column connected to the inlet and detector.  The correct linear velocity for your shorter column can be estimated by:

 
Where and L denote velocity and column length respectively.
 

The links below provide user friendly GC flow calculators.

//www.chem.agilent.com/en-US/Support/Downloads/Utilities/pressureflowcalc/Pages/GcPressureFlow.aspx

//www.chem.agilent.com/en-US/products/columns-supplies/gc-gc-mscolumns/Pages/GCapp.aspx

 
 

Other Parameters that Affect Retention Time

Although the main reasons that account for retention time variability were already explained, there are a few more scenarios that are worth considering. 

It may seem obvious, however one should ensure that the correct column is used (stationary phase, dimensions, phase ratio, particle size distribution, etc.).

For more information in column selection, please visit the links below:[12, 13]

GC Columns

Column Choice for Capillary GC

 
 

Column Bleed and Degradation

Column bleed, the natural loss of components from the stationary phase, is another factor that compromises retention time reproducibility (as well as peak shape).  This process can be accelerated by misusing the column, for example:[8, 9, 14]

  • Using the column at temperatures outside its recommended working range
  • Heating the column in the presence of oxygen or moisture
  • Using poor quality carrier gas
  • Lack of gas filters or traps

Remember to carefully read and follow the instructions of your column manufacturer.

For more information on column bleed please follow the links below:

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

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

 
 

Column Contamination

Contamination will compromise the performance of any GC column.  In terms of retention time variability, the amount of contamination has to be severe, so compounds are prevented from interacting with the stationary phase for a significant portion of the column.  In such situations, the retention factor is usually decreased.  In some exceptional circumstances, analytes are strongly retained by contaminants, with an increase in the retention factor.  Remember to use guard columns to minimize the incidence of contamination thus prolonging the life time of your analytical column.

For more in column contamination please visit the link below:

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

 
 

Sample Solvent

Small changes in retention time (and retention factor) for peaks eluting near the solvent front might occur if there is a change in the sample solvent.  In general the larger the difference in polarity between the sample solvent and analytes, the larger the extent of this problem.

Changes in solvent polarity will lead to retention factor variability.  Basically, the retention factor, a measurement of the tendency of an analyte to remain in the stationary phase over the mobile phase, will be more affected for early eluting peaks (or while the injection solvent is part of the mobile phase).

 
 

Amount of Sample Entering the Column

A large change in the amount of a sample entering the column (usually hundred times or even more) will compromise retention time.  Good sample preparation and injection technique are crucial to avoid this problem.

During split injection, the amount of sample entering the column will be dependent upon the split ratio, so a carefully controlled split ratio must be set.

Peak fronting, a common form of peak distortion, usually reveals column degradation or contamination.  The root causes of peak fronting include any condition or event causing a small portion of the analyte molecules to:

  • travel down the column at a faster rate than the rest of the analyte molecules
  • enter the column earlier than the rest of the analyte molecules

So it is not possible to assign a unique retention time representative of the eluting analyte band.  As an illustration, consider splitless injection with cold trapping (low temperature).  If your sample was injected in a solvent whose polarity is very different to that of the stationary phase; then condensation at different positions would occur and sample will be unevenly infused into the column.  Please refer to figure 8.

 
Figure 8. Large difference in polarity between injection solvent and stationary phase will lead to inefficient focusing and sample being unevenly infused into the column.
 
 

Leaks

Leaks are a major root cause of retention time variability.  In this case, the flow rate is decreased unless electronic pressure controllers are used.

GC detectors are prone to developing leaks, especially when installing the GC column.  When installing your GC column, please bear in mind the following pointers:

  • Column cutters are needed for a clean square cut at column end
  • Look for crushed column remnants
  • Ensure that guidelines for column insertion distances are followed closely

If the GC system lacks pressure controllers, then a leak in the injector will cause an increase in retention time. 

The situation with pressure controllers is a slightly more complicated, as flow will be changed (usually increased) to compensate for the leak.  Here, a fraction of the flow will leak while the rest would go to the column.  If the amount of carrier gas to the column is large enough, then a reduction in retention time is possible, otherwise retention time will increase as the limits of the pressure control system are exceeded.

If suspicious, one might check for leaks using the following general procedure:

Step 1.  In general terms, the maximum input pressure that most GC systems can handle safely is around 80 psi (check manufacturers information for exact values).  As a rule of thumb, the carrier gas pressure regulator, at the gas cylinder, should be set 20 psi higher than the highest required carrier gas head pressure

Step 2.  Look for leaks in the lines and connections between the gas cylinders and flow controllers.  In order to perform this step, the GC system should turned off; close each of the gas cylinders one at a time and closely monitor the pressure indicator on the cylinder regulator (second stage which measures line pressurerath er thgan the first stage whgich measures bottle pressure) to see if pressure remains constant within a reasonable time frame (a 5 minutes pressure plateau is a good indication this portion of the GC system is leak free).  If pressure drops significantly during the testing period, then use a leak detector to verify the integrity of the gas supply lines.  Usually 1:1 mixtures of water and alcohol (methanol, ethanol, propanol or isopropyl alcohol) make useful bubble mixtures

 

Figure 9. Leak testing using a water / alcohol solution.

 

Warning: do not use soap and water as a leak checking solution.  Soapy solutions may get inside the tubing or fitting and contaminate the GC system with residues which give rise to baseline issues.

 

Figure 10. Electronic leak detector.

 

Alternatively, you can use an electronic  leak detector.  These are very practical and can be used even when testing hot spots, although obviously are a little more expensive.

 

Figure 11. Modern electronic leak detector.

 

Step 3.  Look for leaks between flow controllers and injection port.  You should start by disconnecting the column and inserting a Swagelok nut with a blanked ferrule (or a paperclip in place of the column will work as a blanking device) and blank the split line outlet with a blanking nut.  With the inlet set to split mode - apply a column flow / pressure via the instrument and note the instrument head pressure.  Close the line regulator into the instrument.  The instrument head pressure reading should not drop appreciably (>5%) over a two minute period if there are no leaks within the inlet or inlet connections

Step 4.  Reinstall the GC column and check for leaks in the same manner described in step 2.

Last but not least, verify your septum is in good working order and replace it as required.

 

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Overview

The efficiency of a chromatographic peak is a measure of the dispersion of the analyte band as it travelled through the GC injection system, column and detector. In an ideal world, chromatographic peaks would be pencil thin lines –however, due to dispersion effects the peaks take on their familiar ‘Gaussian’ shape.

A reduction in peak efficiency may not adversely affect the resultant data; determine if the analytical results are truly compromised before taking any corrective actions.  However, if gradual peak broadening has been observed over a long period of time, then it is indicative of a general deterioration in the column itself (and replacement actions should be taken).  From a practical point of view, it makes sense to evaluate if resolution is compromised.

The plate number (N) is primarily a measure of the peak dispersion in the GC column, reflecting the column performance. N is derived from an analogy of Martyn and Synge who likened column efficiency to fractional distillation, where the column is divided into ‘Theoretical Plates’.

 

Each plate is the distance over which the sample components achieve one equilibration between the stationary and mobile phase in the column.  Therefore, the more (‘theoretical’) plates available within a column, the more equilibrations possible and the better quality the separation.  The more traps or ‘plates’ there are, the narrower the carbon number range collected from that trap.[7]  Therefore, the higher the number of plates (N) the narrower the ‘carbon distribution’ obtained from that trap.  This concept can be directly related to the peak ‘efficiency’ in GC where a column with a high number of plates gives rise to narrower peaks.

Similarly, for a fractioning tower of a given length (L), the higher the number of plates, the lower will be the distance between each plate, shown as plate height in the diagram.  Therefore, for high efficiency separations, the plate number (N) will be high and the plate height low.  Note that plate height is often called Height Equivalent to a Theoretical Plate (HETP).

These two terms are related through the expression:  H = L / N

 

Figure 12.  Factional distillation model of efficiency theory.

 

The ‘number of theoretical plates’ is often used to establish the efficiency of a column for a given method.  The method developer may decide that a given method is no longer valid when the plate number falls below a predetermined value –at which time the column would be replaced.

Some typical column efficiencies for capillary GC are shown in table 4.  Figures are plates (N) / meter (m) and plates (N) / column for a standard 30m column.  The phase used was polydimethylsiloxane (PDMS) –a popular non-polar stationary phase.

 
Table 4. Capillary column efficiencies.
 
Column I.D. (mm) N/m (N/column)
0.10 10,000 (300,000)
0.20 4,500 (135,000)
0.32 3,200 (96,000)
0.53 1,500 (45,000)
 

GC separations are primarily driven by the efficiency of the columns used –therefore it is important to maintain high efficiencies where possible.

There are many factors that contribute to the broadening of the peak – or actually the injected ‘band’ of analyte vapor as it travels through the chromatographic system.

The biggest contributor to band broadening (and hence lower efficiency) is usually the column itself.  Column efficiency is affected by many factors: the column length, stationary phase type, film thickness, column internal diameter, quality of column packing or coating.  There are other important factors affecting efficiency which are not column related and include:

  • GC operating mode (isothermal vs. gradient)
  • Incorrect column installation and accessories
  • Injection amount and sample volume
  • Flow rate
  • Type of carrier gas used
  • Column contamination and degradation
 
 

GC Operation mode

As was explained, with GC temperature programming, as long as peaks elute during a given ramp, then peak widths remain approximately constant.  Under isothermal conditions, peak width increases in proportion to the retention time.  If under isothermal GC conditions, late eluting peaks are too broad, GC temperature programming should be considered.

As a matter of fact, extra-column effects are largely responsible for efficiency reduction and as such care must be exercised when installing the GC column.

 

Figure 13.  Efficiency calculation.

 

The dependency between efficiency (N) and the retention time to peak width ratio (tr/wb or tr/w1/2) will indicate that any event or conditions that changes efficiency will affect retention time and/or peak width.

There are many parameters that affect chromatographic efficiency (carrier gas flow rate, temperature, extra-column dead volume, etc.).  Large extra-column dead volume have been long recognised the overriding factor behind retention time and efficiency reduction.

 
 

Column Installation

Column installation is a continuous source of problems for many chromatographers.  Incorrect column installation practices (use of incorrect tools, deficient leak testing, use of large dead volume fittings and accessories, etc.) will be reflected in a multitude problems (peak shape distortion, reduced efficiency, etc.).  Important parameters in column cutting and installation are as follows:

  • Column is cut at 90o to the column wall with no jagged edges or burrs
  • The column is positioned correctly within the inlet and detector to avoid dead volume and ensure optimum sampling efficiency
  • There are no leaks at the inlet or detector fittings during the whole temperature program cycle
 

Figure 14.  Incorrect GC column positioning is the source for a multitude of GC peak shape and quantitation problems.

 

Poor column installation into the inlet and especially the detector may introduce large dead volumes into the system.  In gas phase separations  this is particularly serious as the sample will quickly diffuse to fill the void space and peak efficiency will be significantly reduced.

In order to maximize peak efficiency, system dead volume should be reduced by following your manufacturers recommended column installation instructions for both the inlet and detector connections.  For more information on column installation, please visit the link below:

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

 
 

Injection Volume and Backflash Considerations

In terms of injection volume, care needs to be exercised in order to obtain good balance between analytical sensitivity, peak shape and efficiency.  If too much sample is loaded, peak shape will deteriorate.  This problem can be prevented by injecting less sample.  In split injection this is avoided by increasing the split ratio.

Overloading the column will lead to problems including peak distortion and inconsistent retention times.  There are two ways of overloading your column:

  • Concentration overloading: sample concentration is increased while the injected sample volume remains constant (injecting a more concentrated sample)
  • Volume overloading: sample concentration is kept constant while the injected sample volume is increased (injecting a larger sample volume)
 

Figure 15.  Typical peak fronting and loss of efficiency associated with column overloading.

 

Column capacity is defined as the maximum amount of sample that can be loaded onto a column without separation efficiency and peak integrity being compromised – usually determined using peak asymmetry measurements.  As a rule of thumb, column capacity will increase with the amount of stationary phase (i.e. will increase with column diameter and film thickness, see Table 5).  Packed columns tolerate larger amounts of analyte than their capillary counterparts – typically up to a few hundreds of micrograms (of analyte) per injection; capillary column loadability will not usually exceed 30μg per component.

Table 5 gives approximate sample capacity for selected GC columns.  For more accurate data, please consult your column manufacturer.

Table 5. Approximate GC sample capacity as a function of column diameter and film thickness (note that amounts are PER ANALYTE not per sample).
 
Column I.D. (mm) Film Thickness (μm)
0.1 0.25 0.5 1.0
0.10 10ng 30-40ng 50-70ng 100-200ng
0.18 20-30ng 60-80ng 100-150ng 250-350ng
0.25 30-40ng 125-175ng 175-250ng 400-500ng
0.32 50-60ng 200-250ng 250-350ng 600-800ng
0.45 70-80ng 300-400ng 400-500ng 800-1000ng
0.53 100-120ng 400-500ng 500-700ng 1000=1500ng

* Where 1 ng = 10-9 g

 

One might use Table 5 to calculate the sample mass injected on a per analyte basis.  If a sample contains an analyte of interest at approximately 1% and a 0.32mm x 0.5mm i.d. column is being used then the maximum sample load would be 250 – 350mg

For more information, please follow the link below.

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

 
 

The effect of carrier gas type and flow rate velocity are well described using the Van Deemter equation.  For any particular carrier gas, there is a region of carrier gas linear velocity where efficiency is maximized -  corresponding to minimum plate height (H).  Methods should aim to use carrier gas linear velocities in this range.

 

Figure 16.  Effect of carrier gas type and velocity on the GC separation.

 

Anything which takes the method out of this range (a change in column geometry, carrier flow or gas type), will compromise the efficiency of the separation and so may require some remedial action or a return to the original conditions.

The number of theoretical plates is often used to establish the efficiency of a column for a given method.  The user may decide, perhaps under the guidance of regulatory limits, that a given method is no longer valid when the plate number falls below a predetermined value.  At that time, the column would be replaced with a new one.

 
 

Column Degradation and Contamination

Column contamination, thermal oxidation and thermal decomposition (column operating outside its recommended working range) are the root causes for GC column performance degradation leading to reduction in peak efficiency as well as other issues such as retention time changes and peak distortions.

Over time any GC column will deteriorate and begin to show signs ageing, and the earliest signs will include peak broadening, caused by loss of column efficiency (reduced theoretical plates) due to loss of bonded phase from the column – inherently reducing the available stationary phase surface on which to adsorb 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 17.  Comparison of two chromatograms with the same selectivity and different efficiency (and resolution).

 

Column bleed, the natural process of column degradation by thermal means, cannot be entirely suppressed, but it can be slowed by preventing oxidising reagents such as oxygen and moisture, from entering the column, so they will not react with the stationary phase at elevated temperature.  This is typically achieved using the correct gas filters.

The incidence of oxidising reagents and gas contamination can be reduced by using high purity gases are used in conjunction with gas purifiers (‘traps’). The traps are fitted as close as possible to the GC instrument to reduce contamination between the trap and the instrument.  For more information in GC traps, please follow the link below:

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

 

Column contamination should be avoided by implementing best sample preparation practices (using high quality reactants, implementing sample filtration or solid phase extraction where intractable matrices are used, etc.) and using column protection (guard columns), where problems occur.

Peaks due to column contamination, tend to be broader than the other peaks in the chromatogram.

 
 

Analyte Focussing

The lack of solvent effect or cold trapping, when performing splitless or on-column injection can result in peak shape abnormalities, such as peak tailing and broadening.

The analyte is slowly introduced from the inlet during the whole of the splitless time (the inlet volume may be exchanged as few as two times during this whole splitless period).

This slow sample vapour transfer can result in the analyte band entering the column over a period of 30 – 60 seconds or so, depending upon the exact analytical conditions. Without proper focussing these effects can entirely negate any efficiency gained through the use of capillary columns and the resulting chromatographic peaks would be unacceptably broad.

Figure 18 illustrates the principles of focussing in splitless injection.  The general rules of thumb are:

  • Initial column oven temperature at least 10oC below the sample solvent boiling point
  • Match the polarity of the sample solvent with that of the stationary phase chemistry
 

Figure 18. Sample focusing.

 
 

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Overview

Ideal GC peaks are sharp and symmetrical.  Peak shape deformations can aid in the identification of many problems associated with the GC system components.[2-5, 15-21]

Peak shape problems may not adversely affect the resultant data; determine if the analytical results are truly compromised before taking corrective actions.  The following section describes the cause of certain peak deformations including:

  • Peak tailing
  • Peak fronting
  • Peak split (or doubling)
  • Negative peaks
  • Solvent peak shapes
 

From a practical point of view, compounds are not detected at the very moment they elute from the column.  The main reason being the distance (note there’s an inherent dead volume) between the end of the column and the detection point.  This dead volume can lead to peak distortion and reduced peak intensity.

The interaction between analytes and the tubing after the column could be strong enough to generate peak distortion (mainly broadening and tailing) or even to permanently absorb analyte molecules, thus reducing peak area and intensity permanently.  Only in the first case the use of a makeup gas will solve the problem.

 
 

Peak Tailing

Perhaps regarded as one of the most common peak shape problems, peak tailing usually reveals some secondary retention effect, either physical or chemical, which causes a portion of the analytes in the sample band to be retained more than the bulk.  The root causes of peak tailing include any condition or event causing a small portion of the analyte molecules to:

  • travel down the column at a slower rate than the rest of the analyte molecules
  • enter the column later than the rest of the analyte molecules
 
Figure 19. Tailing peaks.
 
 

Column Considerations

Silanol groups are responsible for classical mixed mode effects with polar analytes due to dipole-dipole and hydrogen bonding interactions.  Oxidizing agents (such as oxygen and water) will degrade the column through a process that is irreversible.  As a consequence, the number of secondary Si-OH interactions will increase and with it the degree of peak distortion.  This effect is most noticeable with polar compounds.

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.  Ensure the column is properly cut and installed to avoid these 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.  Note that extra-column dead volumes will increase peak tailing and broadening.

 
Figure 20.  Detector dead volumes lead to tailing effects.
 

Sample decomposition in the column may also lead to peak tailing.  Make sure your sample is not thermally labile.  Under certain circumstances, the thermal decomposition of the analyte in the inlet may lead to a ‘chair’ shaped peak as shown in figure 21.

 

Figure 21. Chair shaped peaks which result from sample decomposition in the inlet.

 

Note that peak coelution can under certain circumstances be mistaken with peak tailing.  A closer inspection may reveal the true nature of the problem, and changing analysis conditions or MS detection can help to confirm this situation.

 
 

Liners

Inlet liners (sleeves) can be a source of peak distortion problems.  The quartz glass surfaces of the liner and quartz wool packing material both need to be chemically deactivated to reduce interactions between the silanol groups on their surface and polar analyte functional groups.  Over time, especially in the presence of water,  the deactivating reagent will re-hydrolyse, exposing the silanol groups and leading to analyte tailing.

The glass wool used within the liner, could also account for peak distortion problems, especially if it is too tightly packed or incorrectly placed in the liner.

Contamination is also a potential cause of peak distortion as strongly adsorbed sample components chemically interact with analytes as they pass into the GC column.[3, 4, 14]

There are different GC inlet cleaning protocols, in general terms, they are similar but differences can arise depending upon manufacturers and models. A generic cleaning protocol can be found at the following link:

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

 

Figure 22. Chromatograms obtained with inlets in a) poor and b) good working conditions.

 
 

Injection Solvent

When the polarity of the injection solvent and stationary phase are very different, peak distortion (tailing but also fronting, shouldering or broadening) may occur.  Likewise, large differences in the pH of the sample and stationary phase may lead to the same type of problems.[3, 4, 14]

Sample Condensation in the Injector

Cold spots within the inlet body and poorly heated regions around the column or detector connections are prone to partial sample condensation.  Peak tailing may occur in these regions and is usually more severe for the less volatile (later eluting) compounds.  Ensure that column connections into the inlet and detector are well lagged to prevent cold spot formation.

Inlet Considerations

During splitless injection, it is of vital importance that the inlet is purged of residual vapours once the analyte has been transferred to the capillary column. If this is not done, the solvent peak will show a high degree of tailing and the GC baseline signal may be noisy and rise markedly as the analysis progresses. This is due to the slow bleed of excess solvent and sample (not analyte) components from the inlet into the capillary column. An example showing a splitless injection in which the inlet is not properly purged is shown below.

 

Figure 23. Split/splitless injection (inlet is not purged).

 
 

The inlet purge is achieved by actuating the split (purge) valve that allows a high split flow through the liner, which quickly purges the residual solvent vapours from the inlet. The split flow is high as the aim is to quickly purge the inlet – split flows of 100-200 mL/min. are typical. The time from the beginning of the injection to the time at which the split line is turned on is known as the splitless or purge time.

Broadly speaking, high boiling point analytes will require longer purge activation times than analytes with lower boiling points.  Typical purge activation times are in the order of 40 to 90 seconds.  Sample focusing will also help to overcome this problem.

Excessively long purge activation times will increase solvent front size without any gain in analyte peak intensity.  This situation can occur when dealing with high boiling point sample solvents or when setting the inlet temperature too low.

Note that excessively short purge activation times will render peaks of small intensity.  A similar problem can develop when setting a low split ratio.  The goal is, of course, to maximise analyte peak size and minimise solvent front size.[14]

 

Figure 24. Broad injection front.

 

For splitless injectors, the use of a retention gap will normally improve peak shape for analytes that rely on the solvent effect for focusing.

A poor manual injection technique could also be responsible for large tailing solvent fronts, especially when performing a very slow depression of the syringe plunger.

Dips after solvent front are not uncommon and they usually indicate contamination (primarily with ECD) or detector overload (ECD, NPD and FPD).

 
 

Peak Fronting

Fronting peaks have a pronounced leading edge and peak asymmetry values less than 1.  The root causes of peak fronting include any condition or event causing a small portion of the analyte molecules to:

  • travel down the column at a faster rate than the rest of the analyte molecules
  • enter the column earlier than the rest of the analyte molecules
 
Figure 25. Fronting peaks appear to be similar to mirror images of tailing peaks.
 
 

Column Considerations

Peak fronting is the result of any condition or event causing a small portion of the analyte molecules to enter or move through the column faster than the bulk.  Peak co-elution could be mistaken with peak fronting.  The use of a different (shallower) temperature gradient or oven starting temperature, the use of MS detection or an orthogonal stationary phase chemistry could be used to further investigate the problem.

One of the most common conditions that account for peak fronting is column overloading.  Column dimensions, film thickness and stationary phase all play a part in determining the column loadability.  If in doubt, consult with your column manufacturer.

Column capacity will increase as film thickness increases (please refer to table 5), so for a given column with fixed dimensions and stationary phase, larger amounts of sample can be introduced whenever the film thickness is increased.

 

Figure 26. Column capacity as a function of film thickness for a 0.32 mm ID column.

 

For more information, please follow the link below.

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

 
 

Injection Solvent

When the polarity of the injection solvent and stationary phase are very different, peak distortion (tailing, fronting, shouldering, broadening) would occur.  Likewise, large differences in the pH of the sample and stationary phase will lead to the same type of problems.[3, 4, 15-21]

Figure 25 shows a typical separation process in GC. Each sample component ‘partitions’ between the gaseous mobile phase and liquid stationary phase (often coated onto the inner wall of a long thin capillary tube). The rate and degree of partitioning depends upon the chemical affinity of the analyte for the stationary phase and the analyte vapour pressure – which is governed by the column temperature.

The distribution coefficient (Kc) measures the tendency of an analyte to be attracted to the stationary phase.  Large Kc values lead to longer retention times.

Where:

Cs is the concentration of analyte in the stationary phase
Cm is the concentration of analyte in the mobile phase

The value of Kc is controlled by several parameters including the chemical nature of the stationary phase, the stationary phase film thickness, column temperature and the properties of the mobile phase.  The presence of solvents (and so their properties) as well as the carrier gas used will influence the distribution coefficient (and the retention time).
 

Figure 27. The gas chromatographic process and the distribution coefficient.

 
 
Figure 28. Injection of a solution of n-alkanes in methanol on a dimethylsiloxane (non-polar) stationary phase.
 
 

Negative Peaks

Negative peaks are often detector or compound related.  There are instances when all peaks are negative, in such cases, check for a reversal of the recorder connections or settings.[10, 20, 21]

 

Figure 29. ‘Negative’ chromatogram caused by reversed detector polarity or high response in the carrier / background.

 

Certain detectors can be “blinded” by a high concentration of sample or a compound present in the carrier which causes a high detector response; in those cases, the response rapidly drops and then returns to normal.  During the process, negative peaks can be created.  This condition known as “detector overload” is commonly found with specific detectors such as ECD, NPD and FPD.

 
 

Nitrogen Phosphorous Detector (NPD) Considerations

Due to the fact that surface chemistry at the NPD bed is not well understood, people tend to underestimate its effects.  The electronic work-function of the surface bed will determine signal response; this parameter depend upon several factors including:

  • Chemical composition of the surface bed
  • Magnitude of the hydrogen flow
  • Magnitude of the heating current and temperature
  • Flow rate and composition of the gases flowing through the detector

If the NPD bead is contaminated, negative peaks are possible.  The negative signal usually appears as a ‘dip’ after a positive peak.

 
 

Electron Capture Detector (ECD) Considerations

Negative peaks are not unusual with ECD.  A small negative peak after the solvent front is not uncommon.  As long as this small peak does not interfere with any other signals of interest, they are not of major concern.  From a practical perspective, negative signals are an indication of a contaminated detector or exhausted source in some cases.

Analytes capable of generating negative peaks in ECD include certain electropositive compounds that are capable of donating electrons to the ECD cell, thus increasing the current cell and generating a negative signal.  A good example of such compounds are hydrocarbons, which are well know to quench ECD signal intensity and even appear as negative peaks.

 

Figure 30. Dirty ECD symptoms.

 

As the ECD is a non-destructive detector, contamination builds up on the inner surfaces of the detector. 

 

Figure 31.  Negative dips after each peak.

 

When either the foil or collector become dirty, the chromatogram will show distinctive negative dips after each of the peaks.  Typically, when the detector performance is deteriorating a marked deviation away from the expected linear range is seen –especially at higher analyte concentrations.

 
 

Thermal Conductivity Detector (TCD) Considerations

Negative peaks are not unusual with TCD.  If all peaks are negative, then check the polarity of the detector and recorder.

Some compounds are capable of decreasing the thermal conductivity of the gas flowing through the TCD cell.  In this case, the filament will heat up and a negative signal would result.

 

Figure 32.  Negative peaks / extra peaks seen with carrier gas contamination in TCD detection.

 

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.

 
 

Split Peaks

Split peaks are mainly caused by sample injection problems, so sample is loaded / introduced into the column in several (two or more) portions.

 

Figure 33. Peak splitting.

 

A poor injection technique (such as erratic syringe plunger depression) will often produce split peaks.  Remember that a portion of volatile sample in the needle could volatilise and leave the syringe before the plunger is pressed.

Incorrect column installation errors can give rise to peak splitting of the sort described in figure 31, especially when the column is placed too high in the inlet. 

Poorly cut columns can give rise to such pronounced Eddie turbulences that a significant portion of the analytes are help up prior to entering the column, which give rise to shouldered or split peaks.  Always ensure the column cut is at 90o to the column wall and is free from burrs.

Sample degradation at the inlet could appear as peak splitting if the analyte elutes close to the decomposition compound peak.  Typically this type of split has a special appearance often referred to as ‘chair shaped’ and it is described in figure 34.

 

Figure 34. Chair shaped peaks which are indicative of analyte decomposition within the inlet or column.

 

Problems with analyte decomposition can be investigated by lowering the inlet temperature and observing the effect on peak shape and peak area reproducibility.  All but the most of labile analytes should have an inlet temperature high enough to volatilise the sample without causing drastic analyte decomposition.

Peak coelution due to the bleed of contamination products or phase bleed in MS detection could appear as splitting peaks. 

 
 

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  1. IUPAC Gold Book. Retention Time in Chromatography. International Union of Pure and Applied Chemistry. //goldbook.iupac.org
  2. Practical GC Troubleshooting and Maintenance.  Crawford Scientific 2007
  3. Fundamental Gas Chromatography.  Crawford Scientific 2009
  4. GC Method Development.  Crawford Scientific 2010
  5. “GC Temperature Programming” from “Theory and Instrumentation of GC”. CHROMacademy
  6. John V. Hinshaw. “GC Ovens — A Hot Topic” GC Connections. LC-GC Europe - July 2001
  7. “Chromatographic Parameters” from “Theory and Instrumentation of GC”. CHROMacademy
  8. “GC Troubleshooting Gas Supply and Inlet Issues. –The Essential Guide” CHROMacademy
  9. “Gas Supply and Pressure Control” from “Theory and Instrumentation of GC”. CHROMacademy
  10. Robert L. Grob and Eugene F. Barry. “Modern Practice of Gas Chromatography” Fourth Edition. Chapters 2, 3 and 6. John Wiley & Sons 2004
  11. Agilent 6890 Series Gas Chromatograph. Agilent Technologies, Inc.
  12. “GC Columns” from “Theory and Instrumentation of GC”. CHROMacademy
  13. CHROMacademy Resolver Issue 3: Column Choice for Capillary GC
  14. “Sample Introduction” from “Theory and Instrumentation of GC”. CHROMacademy
  15. Dean Rod. “The Troubleshooting and Maintenance Guide for Gas Chromatography”. ISBN 978-3-527-31373-0. Wiley-VCH. Germany 2007
  16. GC Troubleshooting. Thermo
  17. “Troubleshooting and Reference Guide” Agilent Technologies Inc. 5988-6191EN. 2009
  18. “Gas Chromatography Troubleshooting and Reference Guide”. MSP Kofel. 2005
  19. “GC Inlet Resource Guide” Agilent Technologies Inc. 5988-3466EN. = 2001
  20. “GC Detectors” from “Theory and Instrumentation of GC”. CHROMacademy
  21. Raymond P. W. Scott. “Chromatographic Detectors” Chapters 4-7. Marcel Dekker, Inc. USA 1996
 
Further reading and resources: *** CHROMacademy Registered users only ***
 
Related articles

When Peaks Collide Part I

When Peaks Collide Part II

When Peaks Collide Part III

GC Spring Cleaning

Extreme Leaks!

Pinning down tailing peaks Part I

Pinning down tailing peaks Part II

Using Computerized Pneumatics Part I

Using Computerized Pneumatics Part II

 
CHROMacademy GC E-Learning Modules

GC Temperature Programming

Sample Introduction

GC Detectors

 
CHROMacademy Webcasts / Essential Guides

GC Troubleshooting - Gas Supply and Inlet Issues

GC Troubleshooting – Column and Detector Issues

Column Selection for Capillary GC

 
 

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The following subjects are covered in CHROMacademy.com

The Theory Of HPLC
Introduction (1.5hrs)
Chromatographic Parameters (3hrs)
Band Broadening (3hrs)
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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
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SPE Overview (3.5hrs)
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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)

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