GC Troubleshooting in Petrochemical Analysis

A guide to simple troubleshooting steps in gas chromatography (GC) with an emphasis on petrochemical analysis.

Gas chromatography (GC) is a core analytical technique in the petrochemical sector, primarily used to analyze the main process stream components in fuel production but also to detect trace impurities that can impact the production process and final product. Against this background, GC technology has advanced towards higher sensitivity (or lower detection limits), and detection of a greater number of chemical components within a sample. Multiple detectors can be combined for the analysis of complex mixtures, resulting in instruments with multifaceted and highly involved configurations that can analyze 30 or more components from a single sample injection. Another key trend is the miniaturization of GC instruments, allowing on-site analysis and a reduction in refinery running costs because they require very low flow rates of carrier gas.

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Troubleshooting GC Analysis

As in other industries, chromatographers in the petrochemical sector face the same issues as all GC users. However, knowing where to start troubleshooting can be difficult because there are so many potential impurities, a broad range of analytes, and wide concentration ranges encountered during analysis. This is further complicated where instrumentation is contained within one "black box". Merging and packaging different technologies within one GC unit can simplify analysis, but this creates a greater number of issues because it is impossible to visualize each step of the analysis.

Calibration: A common cause of GC problems is a lack of precision in the calibration of the instrument and detector:

• The certificate supplied with the calibration gas mixture must be read and clearly understood to ensure that the component concentrations are similar to the concentrations that will be measured.

• The accuracy of the reported values in the calibration mixture should be appropriate for the measurement being undertaken and all required components must be present in the calibration gas mixture.

• The certificate should be checked to ensure that the gas mixture is within its shelf life or expiry date.

Beyond these fundamentals, the 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.

Troubleshooting Unexpected Peak Shapes in Gas Chromatograms

A fundamental error in process or malfunction of the equipment can be diagnosed if a chromatogram displays results far removed from expectations. 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. Below are key hints and tips on how to approach troubleshooting unexpected chromatograms.

"Fuzzy" Chromatograms: A problem occasionally encountered is that peaks may become smeared to the point where there is no apparent difference between peaks, referred to as a "fuzzy" chromatogram. This can be caused by using a damaged GC column, or using a GC column that is not capable of reaching the level of separation required. The column should be replaced or exchanged for a different column that will achieve a better separation.

Another possible cause is that the carrier gas is not appropriate for the application. Hydrogen has a low viscosity and high separation velocity and will often achieve the fastest results, but helium will generally achieve a better peak resolution despite a slightly slower response time. A change in carrier gas can sometimes address the issue.

Unexpected Peaks: The appearance of unexpected peaks can sometimes be the result of impurities in the carrier or detector gas. Check the correct grade of gas has been connected; for example, a purity gas of 99.8% has been connected to a GC system that requires a purity of 99.999% or higher.

The next step is to check the system for leaks that can let gases out of the system, but also allow contaminant gases in. Leaks are particularly problematic because they lower method sensitivity and can result in a loss of carrier gas, with associated costs and potential safety issues. If leaks are found, connections should be tightened, and the system allowed to settle with gas flowing through to purge before resuming analysis. It is also possible that there may be damage to the GC column from the moisture in incoming air.

If no leaks are found, the carrier, detector, and gas cylinders should be replaced. It is important when changing from an old cylinder to a new cylinder to use appropriate techniques such as purging and leak testing 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 if any change in the results occurs. If so, it could indicate that the generator produces gas with non-favourable impurities for the specific analysis.

Masking Effect: Peaks in a chromatogram can sometimes appear to be overlapping creating a "masking effect". Solutions on how to address this are:

• Impurities in the carrier gas: Check for gas purity and system leakages.

• The sample volume is too high: Typical GC sample volumes are millilitres or micro litres, so if too much volume is introduced to the system, the detector or separator could become overloaded and this leads to masked peaks.

• Carrier gas: 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.  

Peak Shifting: If the carrier gas flow rate is too high or too low, peaks will show up in places where they are not expected, effectively shifting the whole chromatogram to the left or the right. The first step is to check the carrier gas flow rate. "Pressure creep" is characteristic of single-stage gas pressure regulators; as the cylinder empties, carrier gas flow rate can increase. Using two-stage pressure regulation will maintain a stable gas inlet pressure to the GC.

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, the flame will burn with an unstable characteristic and 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.

Off-the-Scale Peaks: Peaks on the chromatogram scale can disappear off the paper for the following reasons:

• High detector sensitivity: If it is possible, the simplest troubleshooting solution would be to reduce the detector sensitivity level.

• Sample volume: Reduce the sample volume or dilute the sample prior to, or during, injection to the GC.

To be sure the GC works well and is fit for purpose, good practice would be to run a method specific system suitability test. In addition, to track any system drift over time, known samples could be analyzed regularly during the analytical run.

Sample Considerations

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. The wrong sample may have been introduced to the instrument, or samples may have become contaminated or decayed. If this is suspected, the sampling technique, sample preparation, and storage should be reviewed.

Sample Decay: Sample decay, or changes in sample composition, can take place during the chromatography process. For example, if hydrogen is used as a carrier gas, any unsaturated hydrocarbons or aromatic hydrocarbons present in a sample are likely to react. This will be vastly accelerated in the GC column oven. In this case, the best troubleshooting advice would be to change the carrier gas.

Sample Collection: Samples can also be inadvertently transformed prior to injection into the column. 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. Collecting samples in evacuated sample containers or using temperature control during sample transportation can be effective troubleshooting remedies.

The above risks can be significantly mitigated by taking and analyzing multiple samples that will significantly increase the chance that a sample handling error will be detected.

Petrochemical Analysis Focus

Gas Chromatography–Flame Ionization Detection (GC–FID):

Overview: Perhaps the most common gas chromatography technique used in refining and petrochemical applications is gas chromatography with a flame ionization detector (GC–FID). The FID detects analytes by measuring an electrical current generated by electrons from burning carbon particles in the sample. FID harnesses a combination of hydrogen and oxygen. The oxygen for the flame combustion is normally supplied by the use of high purity synthetic air to minimize the amount of impurities coming into the detector.

Key Considerations: It is important when changing over from one cylinder of synthetic air to another to ensure that the composition of the air in the new cylinder is consistent with that of the previous cylinders, in terms of blend tolerance. For example, the target oxygen concentration might be 20%, but that mixture might have a blend tolerance of plus or minus 1% absolute (5% relative) meaning that the actual oxygen concentration can be between 19% and 21%. While a small change in the consistency of the contents of the new cylinder might be acceptable, more pronounced differences will influence how the FID flame burns and could lead to a very different analytical result, even though the sample has not changed.

The same principle applies to ordering calibration gas mixtures. Using a calibration gas mixture with an analytical accuracy of plus or minus 10% could create an apparent shift in process parameters when a process analyzer is recalibrated and the instrument then begins to respond differently.

Troubleshooting Steps: The first step is to check that the fuel gas to the detector has been switched on, that the flame is functioning, and has been successfully ignited. Troubleshooting relies on checking gas flow rates and re-ignition of the flame prior to re-running the sample. It should also 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.

Gas Chromatography–Sulphur Chemiluminescence Detection (GC–SCD):

Overview: The sulphur chemiluminescence detector (SCD) has emerged as a powerful tool in refinery GC, and is primarily used for the quantitative determination of various sulphur organic species (such as hydrogen sulphide, mercaptans, thiophenes, benzothiophenes, and sulphides in hydrocarbon samples). It is a highly sensitive and useful technique for the characterization of crude oils of different origin, because sulphur speciation is essential during oil catalytic processing in a refinery.

Key Considerations: A key consideration when performing GC–SCD analyses are the physical properties of the sample delivery lines. It is essential that these delivery lines are constructed from an inert material, because using the wrong material could result in sample components reacting with the walls of the line. The most common material used in general industry is 316 stainless steel, but it is not appropriate for refinery analysis. This is because certain sulphur compounds in the sample line can adhere to the walls and therefore not reach the analyzer at the same time as the bulk of the sample. The best alternatives are highly chemically resistant non-metallic materials such as Teflon and Kel-F or Hastelloy C-22, a nickel-chromium-molybdenum-tungsten alloy with high corrosion resistance.

Troubleshooting Steps: This problem can go undetected because analysts do not know that these compounds are present until the line becomes saturated, resulting in a sudden concentration of the substance being released and detected as an anomaly. If sulphur compound peaks appear in an analysis result, but several hours after they might have been expected, the reason could be that the sulphur concentration is actually several hours old. To validate or rule out this issue, test injections of known concentration calibration gas mixtures into the sample delivery pipework upstream of the analyzer could validate or rule out this problem.

Summary

The analysis of chemical components for petrochemical plant process control has been elevated to unprecedented levels of accuracy. As legislation becomes ever more stringent, the importance of quantifying and qualifying emission pollutants in an accurate and transparent manner through GC has become a priority. Emissions measurement has serious financial implications and compliance to measurement is critical.

Stephen Harrison is a British Chartered Engineer (MIChemE) with a career in industrial gases spanning 26 years, over 12 of which have been focused in the area of specialty gases. He has worked in an international capacity for both Linde Gases and previously BOC and now leads Linde's global Specialty Gases & Specialty Equipment business from Munich, Germany. Stephen has a Masters degree in chemical engineering from Imperial College, London, UK.

E-mail: hiq@linde-gas.com

Website: http://hiq.linde-gas.com/

This article is from LCGC's digital magazine, The Column. Click here to view the issue.