10 GC Tips to help you make your instrument sing and dance like a seasoned pro!
1. How to check gas supplies and pressure
Remember on most regulators the gauge nearest the bottle valve measures bottle pressure (between 2000 and 3000 psi on most K size cylinders) and the gauge furthest away from the bottle measure output (line) pressure which is typically between 80 and 120psi for most users. Carrier gas should have moisture and oxygen self-indicating traps fitted near the instrument and you should know the colour change which indicates that they need to be changed. If you switch off the GC(s) and shut the bottle valve, the pressure drop over time indicates how 'leaky' your tubing is and remember that leaks can cause ingress of air through the 'Venturi' effect. More than 5psi line pressure in 30 mins. means you have a pretty serious issue.
Webcast and Full tutorial on Managing Gas Supply issues
The CHROMacademy Essential Guide to GC Troubleshooting - Gas Supply and Inlet Issues »
2. Regular inlet maintenance
Regular inlet maintenance is critical and the septum and liner need to be regularly checked.
Septa don't last forever and will eventually leak during the injection phase of each analysis. Do you know the signs to spot a leaking septum?
Liners are critical to the peak shape and quantitive reproducibility of your method. Do you know how to spot when the liner needs to be cleaned or changed? Do you have a routine for liner changing?
These webcasts will help you to recognise when trouble is brewing and how to draw up effective maintenance routines;
Webcast and tutorial on
GC Troubleshooting - Gas Supply and Inlet Issues »
Troubleshooting GC Separations - Retention Time Efficiency and Peak Shape Issues »
3. The sample solvent and solvent volume injected is critical and you should know how to check if your choices are going to lead to problems
Pretty obviously the sample solvent chosen for your application needs to fully solubilise the analyte, and as such the polarity of the solvent would be similar to that of the analyte. However the sample solvent affects so many parameters within the GC method, including the injection volume and the initial oven temperature, especially when using splitless methods.
- The solvent expansion co-efficient dictates the volume of liquid that can be injected without risking carry-over due to 'backflash' processes
- The initial oven temperature in split injection is set to at least 30°C above the solvent boiling point and at least 20°C below the solvent boiling point in splitless injection
- The solvent polarity should match the column polarity to avoid peak splitting of early eluting peaks in splitless injection
4. How to recognise a reasonable purge time
Every splitless injection needs to have the split (purge) valve opened after a certain length of time, to purge the lingering solvent vapours from the inlet. This prevents large, tailing solvent peaks and rising baselines during the analysis.
The purge time is usually set by monitoring analyte peak area at increasingly longer splitless times and monitoring when a consistent peak area is obtained.
or a full explanation of this procedure - Split Splitless Injection for Capillary GC »
5. The relationship between column geometry and the application you are running
Choosing the correct column dimensions for a separation can often be confusing and even experienced method developers struggle with the choices – so it's good to have an understanding of the basic principles, just to sense check what you see in the chromatogram versus the column being used. Broadly speaking – long columns are required when there are many components to be separated. Narrow internal diameter columns are used when the chemical separating power of the stationary phase is required to separate compounds whose chemical or physico-chemical properties are similar. Thick stationary phase films are typically used when volatile species are being analysed.
The following resources explain these statements in much more detail and present all of the information required to understand column choice;
The CHROMacademy Essential Guide Webcast - Principles of GC Column Selection »
Chromatography Technical Tips - Real Life GC Column Selection »
6. The 6 most common capillary column chemistries
Just as the selection of GC column dimensions can be daunting, so can the choice of column chemistry. Whilst the general perception is that there are a myriad of phases available – in fact we only need be familiar with 5 basic monomer chemistries and which can be reasonably simply rationalised in terms of three basic inter-molecular interactions: dispersive interactions, dipole-dipole interactions and hydrogen bonding. The resources below can be used to quickly get up to speed with GC column chemistry;
The CHROMacademy Essential Guide Webcast: Principles of GC Column Selection »
7. How to properly install and condition a capillary GC column
Cutting, preparing and inspecting the column, cleaning the column end, choosing the correct ferrules and installing to the correct distances within the inlet and detector system are all important. You should know something about each of these aspects of column installation in order to avoid problems.
Similarly – there are many ways in which a column can be conditioned improperly, all of which lead to reduced column lifetime. All GC columns bleed constantly and your job is to ensure that the level of bleed is low and consistent – which means removing any phase which has degraded during storage. Most folks know that heating the column will drive off these bleed products – but very often critical measures such as purging of the phase at low temperatures are not well understood.
The following resources will help you with column installation and conditioning;
Column Installation and Conditioning Video »
Column Conditioning Tech Tip »
8. The importance of correctly setting GC detector variables
Whichever GC detector you are using, there will be combinations of gas flows and applied voltages which ensure that the detector performs optimally for your application. It's really important that you get familiar with each of the variables in order to 'critically evaluate' a method and assess if the recommended settings make sense. Of course different detectors use different gases and voltages – however each will have variables which make a big impact on sensitivity and reproducibility. Examples include the make-up gas to hydrogen ratio in FID detectors, the voltage applied to the bead in NPD detection and the auxiliary gas type and flow rate in ECD detection, however we could list every type of GC detector here!
To learn which are the important variables in each of the GC detector types you can visit the GC Detectors channel in CHROMacademy;
GC Detectors channel in CHROMacademy »
9. When you have a baseline you can trust –and how to spot trouble brewing
Rising, falling, noisy, undulating, spikey, unstable, cycling – the full list of baseline problems is a long one! However, an experienced GC operator knows that each particular baseline type tells a 'story' in terms of the health of the instrument, and that being able to decode the baseline pattern can help to avoid wasted time and, more importantly, wasted samples!
To understand more about the problems associated with each of the various baseline types;
The CHROMacademy Essential Guide to Troubleshooting GC Separation Part II – Selectivity, Resolution and Baseline Issues
10. What a 'good chromatogram' looks like
Not just in general – but what a good chromatogram associated with your current method looks like. We know this seems 'obvious', however instruments and methods tend to drift over time. Catastrophic failures are usually pretty easy to spot – however, method 'jitter' which describes the situation where instruments and methods develop many small problems over time, is often much more difficult to diagnose and prescribe. Have a chromatogram which was generated when the method was 'new' or known to be working properly can help enormously in diagnosing problems. Further, many operators will also retain a whole set of analytical results to illustrate the typical quantitative aspects of the analysis including reproducibility of QC samples, statistical performance etc.