Gas chromatography: Trends and troubleshooting

Trends in gas chromatography are always changing and today one of the most notable trends relates to making the laboratory environment easier and more productive for the analytical teamWhen it comes to measurement for chemicals plant process control and optimisation, it becomes increasingly evident that it is not only the main process stream components which influence a process, but also trace impurities that can also have a definite influence on the final result,” comments Stephen Harrison, Global Head of Speciality Gases and Speciality Equipment at Linde. “Therefore another GC trend is the move towards higher sensitivity, or lower detection limits. This involves identifying more chemical components in a sample and detecting them even if they occur in very small quantities.”

GC technology is also making rapid headway out of the laboratory setting and into the realm of miniaturisation, with instruments small enough to be located in the field — for example, on a refinery or chemicals production plant. These miniature GC systems are placed at the location where the sample is generated, making it possible to conduct an analysis without having to move the sample to the laboratory. Micro-gas chromatographs also have a positive impact on plant running costs because they require very low flow rates of carrier gas, reducing overall gas consumption on the plant.

In today’s world of complex analysis, there is also a move towards combining multiple detectors in series and in parallel, configurations that create highly complicated instruments designed for special tasks. A specialist GC like this could analyse 30 components, or more, from a single sample injection. The high sensitivity and low detection levels presently required demand ever higher purity carrier gases, often as high as a quality of 7.0 – that is, 99.99999% purity – or not having more than 0.1 parts per million (ppm) total reported impurity level. The purity of the carrier gas is crucial for performance, maintenance and longevity of equipment. Impurities, especially hydrocarbons, can cause base line noise and reduced sensitivity and might increase detection limits. Traces of water and oxygen may also decompose the stationary phase, which leads to premature destruction of the column.

“Yet another trend is the evolution of GC itself to become more of an all-round, built-in, do-everything kind of instrument,” continues Harrison. “In the past, laboratories purchased add-on instruments, but nowadays GC is becoming more of a ‘black box’ – everything needed to conduct a specific type of analysis is built into the box. Examples of this are flow controllers, purifiers and gas pressure regulators which are all built into the GC instrument, so that it can be used on a ‘plug and play’ basis with maximum convenience and reliability.”

Harrison comments that although this specialised equipment is fundamentally not all that complex, operators sometimes encounter problems which occasionally beset all GC users. With such a broad range of analytes, potential impurities and concentration ranges encountered during this type of analysis, there are naturally some problems related to these applications that can be attributable to instrumentation gases or gas distribution equipment.

“Linde is committed to helping our customers troubleshoot these problems,” he says. “Particularly in cases where everything is contained in one box, troubleshooting can be somewhat challenging, because in order to identify a problem, you need to examine what’s going on in each stage of the process. Although the merging of a number of instruments into one unit simplifies the process when everything is running smoothly, when problems are encountered the identification of the cause can be more complicated than in simpler systems used 20 years ago due to the larger number of potential failure points contained inside the instrumentation setup.”

Gases are essential for efficient operation of a gas chromatograph. Separation takes place in the gaseous phase by introducing a sample which is transported by a carrier gas that separates the sample over the static medium in the column. Typical carrier gases for a range of applications include helium, usually supplied in cylinders, nitrogen supplied in cylinders from liquid sources or via a gas generator, argon for niche GC applications – also supplied in cylinders – or hydrogen in cylinders or via gas generators. The choice of carrier gas depends on the type of detector, column, application and safety requirements. But the choice of the carrier gas is also dependent on re-quirements in terms of separation efficiency and speed.

Hydrogen is finding increased popularity as an effective substitute for helium, which is becoming scarce. Hydrogen also has certain benefits over helium, making its use preferable on occasion. Notably it has the lowest viscosity of all gases, thereby providing the highest mobile phase velocity and the shortest analysis time. Helium, on the other hand, gives the best overall performance and peak resolutions for many applications, making it an optimum choice of carrier gas in those cases.

A GC’s detector will often harness speciality gases such as a combination of air and hydrogen which are used in a flame ionisation detector (FID), or helium which is used in a helium ionisation detector (HID). Other gases used in association with GC are calibration gas mixtures to calibrate the detector in order to ensure accurate measurement, or zero gases to set a zero reading on the detector.

The information received by laboratory technicians using a GC is basically a graph called a chromatogram (chromatograph), with peaks representing the different chemicals in the sample (analytes) and the area of the peaks indicating the amount of these analytes. Each chromatogram illustrates the fingerprint of the mixture of chemicals introduced to the GC from the sample, the specialty gases used and any contaminant gases unintentionally introduced from the surrounding atmosphere. With the large number of specialty gases being used, and the complexity of modern GC operations, the possibility exists for things to go wrong. And, when this happens, the errors are likely to show up in the chromatogram and cause problems in the interpretation of the analytical results. “Unfortunately, real life in a laboratory isn’t just plug and play,” says Harrison. “When you’re constantly conducting measurements of different types, the instrument and components within it age and can deteriorate. Looking at the chromatogram generated during the process, a problem sometimes encountered is that instead of displaying clear and discreet peaks, these peaks might be all smudged together, so that there is no apparent or clear differential in the peaks. This is often referred to as a ‘fuzzy chromatogram’.”

One of the common causes of this is possible damage to one of the GC’s separation columns. Alternatively, this result could simply be attributable to using a column that’s not suitable to achieve the level of separation required. In this case, a solution to consider is using a different column that will achieve a better separation, or in the case of a damaged column, to replace it.

Occasionally, there might appear to be unexpected additional peaks in the chromatogram or anticipated peaks may be larger than they should be. This could be attributable to impurities in the carrier or detector gas. In this event, the first troubleshooting step in this case would be to check that the correct grades of instrumentation gases have been connected to the GC setup. It is possible that an error has been made and a technical grade gas with purity of, for example, only 99.8% has been connected to a GC system that requires a purity of 99.999% or higher.

If it is determined that this is not the fault, as a second step, the system should be checked for leaks which not only let the instrumentation gases out of the system, but allow contaminant gases from the atmosphere into the GC setup. If any leaks are found, connections should be tightened to eliminate leaks, and then allowing the system to settle before resuming analysis. Leaks are particularly problematic as they lower the sensitivity of the method, result in the loss of carrier gas – with the associated costs and potential safety issues – and could damage the column by allowing ingress of moisture from the ambient air. If this leak test doesn’t solve the problem, as the next troubleshooting step, one or more of the carrier gas or detector gas cylinders could be replaced to see if there is an observable change in results. In this case, it will also be important to change the cylinder using appropriate techniques such as purging and leak testing once the new cylinder is connected to avoid the introduction of contaminants during the cylinder change over. If the carrier gas or detector gas is sourced from a gas generator, the gas could be replaced by a high purity specialty gas cylinder to see whether any change in the results occur. If so, it could indicate that the generator produces gas with non-favourable impurities for the specific analysis.

Impurities in the carrier gas can also cause the chromatogram to display peaks that overlap each other, so that there is no apparent differential. The solution in this case is the same: check for gas purity and system leakages. “This so-called masking effect could also be occurring because the sample volume is too high, so reducing the sample volume is another possible way to resolve this problem,” Harrison suggests. “Typical GC sample volumes are millilitres or microlitres, so if too much volume is introduced to the system, the detector or separator could become overloaded and this leads to masked peaks. Masking can also be caused by the carrier gas itself. The carrier gas is present in huge quantities in the GC column and detector and analytes with a similar separation coefficient will elute at a similar time and can be masked by the carrier gas. If this is suspected, the best troubleshooting idea is simply to switch to a different type of carrier gas.

Where analysts find themselves with a chromatogram that displays results far removed from expectations, this could indicate a fundamental error in process or malfunction of the equipment. If the result is not what was anticipated, or the result indicates only a small number of components in a complicated chemical mixture, it is possible the operator has chosen a set-up for the separator column and detector which are simply not suitable for the sample being measured. It is also feasible that, for example in the case of a GC-FID setup, the fuel gas to the detector has not been switched on, or the flame has malfunctioned, or has not been successfully ignited. Troubleshooting here relies on checking gas flow rates and re-ignition of the flame prior to re-running the sample. Alternatively, if the carrier gas flow rate is too high, or too low, peaks will show up in places where they are not expected. Effectively the whole chromatogram shifts to the left or the right. As an initial troubleshooting suggestion, the carrier gas flow rate should be checked. Use of suitable two-stage pressure regulation will maintain a stable gas inlet pressure to the GC from the supply cylinder over a long period of time. This will help to avoid ‘pressure creep’ as the cylinder empties, which can cause increased carrier gas flow rates.

Inappropriate gas flow rates can also cause problems in the detector. The FID flame operates best when gas flow rates produce an even flame with laminar flow and the correct stoichiometric mix of fuel and oxidant gases. If the fuel gas (normally hydrogen) or oxidant gas (normally synthetic air) flow rates are not matched, then the flame will burn with an unstable characteristic and this can cause erratic sample detection. The remedy here is simple. Gas flow rates should be checked and it should be ensured that high quality gas regulators are used to deliver the gases to the FID detector to avoid pressure fluctuations that may cause the gas flow rates to change. In some modern GC-FID setups the flame will not ignite if the fuel gas flow rates are unsuitable. While a good feature, if the sample is run through the GC-FID without the flame being ignited, the results will clearly be wrong.

“In extreme cases, a skewed result may have nothing to do with the flow rate, volume of sample, purity of the carrier gas, or any leakages,” explains Harrison. “Fundamentally, the operator could have chosen the wrong instrumentation set-up and a fundamental review of the laboratory procedure or use of an alternative test method would be required. Another explanation could be that the analyst has introduced the wrong sample to the instrument or that samples have become contaminated or decayed. If this is suspected, a review of the sampling technique, sample preparation and storage would be recommended. One simple troubleshooting recommendation related to sampling would be to take and measure multiple samples. This can significantly increase the chance that a sample handling error will be detected.”

Sample decay, or changes in sample composition due to chemical reactions, can also take place during the chromatography process. For example, if an un-saturated hydrocarbon or aromatic hydrocarbon is present in a sample and hydrogen is used as the carrier gas, it is likely that the hydrogen will react with these components. As the GC column is placed in an oven, even such a reaction might take place slowly at room temperature, it will be vastly accelerated in the GC column oven. In this case, the best troubleshooting advice would be to change the carrier gas, possibly trying helium or nitrogen as alternatives and see if the chromatogram shows different results.

Samples can also be transformed prior to injection into the column. For example volatile components can evaporate from the sample mixture or components within the sample can react with each other or with air or moisture from the ambient environment. In these cases collecting samples into properly evacuated sample containers or use of temperature control during sample transportation can be some of the most effective troubleshooting remedies. This risk can also be significantly mitigated by taking and analysing multiple samples which will significantly increase the chance that a sample handling error will be detected.” When operators encounter peaks in the chromatogram scale that disappear off the paper, the reasons could include use of the wrong detector setup or excessive sample introduction.  Most GC systems come with a control to adjust the sensitivity of the detector, so when the peak is off scale, this could be a result of detector sensitivity being set too high. If it is possible, the simplest troubleshooting solution would be to reduce the detector sensitivity level. Another possible cause of an off-scale peak could be that the sample volume introduced to the column is too big. This could be addressed by reducing the sample volume or by diluting the sample prior to, or during, injection to the GC.

To be sure the GC works well and is fit for purpose, a good practice procedure would be to run a method specific system suitability test. In addition, to track any system drift over time, known samples could be analysed regularly during the analytical run. “Sometimes analysts see a certain analyte coming through the detector, but are unable to quantify the amount of that particular analyte,” Harrison says. “For example, mass spectroscopy will identify what is present, but an additional instrument is needed to identify the quantity of the chemical, so depending on whether qualitative, or quantitative information is required will determine the most appropriate instrumentation configuration and for some methods, a combination of different types of detector at the back end of the GC column may be the right answer.”

Zero setting is also a critical process step in the instrumentation setup. The carrier gas is generally the zero gas for a GC setup, so the higher the purity, the more accurate the zero setting of the instrument. Use of high purity carrier gas also reduces noise and will therefore allow much higher sensitivity during operation, because sensitivity is simply the signal to noise ratio. To observe the impact of such a change, using a carrier gas of 6.0 purity grade (99.9999%) instead of the more commonly used 5.0 (99.999%) purity gas is suggested, of course, using 7.0 grade (99.99999%) is also possible.

Many of the troubleshooting steps up to now have focused on instrument operation and sample handling. Another common cause of problems is lack of precision in the calibration of the instrument and detector. The most fundamental troubleshooting step in this instance is checking the quality of the calibration gas mixture. Ensure that the certificate supplied with the calibration gas mixture has been read and clearly understood, and check that the component concentrations are similar to the concentrations that will be measured. Also confirm that the accuracy of the reported values in the calibration mixture is appropriate for the measurement being undertaken and check that all required components are present in the calibration gas mixture. Check that the gas mixture is still within its shelf life or expiry date. Beyond these fundamentals, use of appropriate cylinder connection techniques is vital and this may involve purging the system with an inert gas to remove atmospheric air after calibration cylinder connection, but prior to calibration sample introduction.