Mar-2011

GC-MS: an ideal tool for forensic analysis

Gas chromatography-mass spectrometry (GC-MS) presents the ideal analytical tool for forensic analysis in the petrochemical industry.

Stephen Harrison, Linde Gas

Article Summary

In fact, its simplicity, sensitivity and effectiveness in separating and identifying components has made GC-MS one of the most important tools in analytical chemistry across a spectrum of industries today.

GC-MS can be harnessed across the entire value chain in the oil and petrochemical arena, from the prospecting stage to the refining process to monitor and improve quality, to final refined and formulated fuels and lubricants, as well as to ensure compliance with environmental standards, and even as a forensic tool in criminal cases in which fuel has been used to perpetrate arson.

Where other analytical techniques fundamentally determine the quantitative issues arising from analysis of a specific sample — answering the question “how much is present?”— GC-MS is one of a very few techniques able to qualitatively identify the actual nature of chemicals in the sample. It answers the question, “What molecules are present?”

Of these two questions, in many analytical scenarios the “what?” is actually more important than the “how much?”. Conducting quantitative work alone on a sample with unknown content can be fruitless. The qualitative method is especially relevant to research applications and lays the correct foundation for the analysis. Only when it is known which chemicals are present can the quantitative analysis be performed.

The GC principle is that molecules in a sample separate in the chromatography column because of differences in their chemical properties. The MS breaks the components as they exist from the column into ionised species and separates these based on their mass-to-charge ratio. This is the great advantage of the combination of GC as the first separation step and the MS as the qualitative detector.

With the massively burgeoning importance of petroleum products and their derivatives in all aspects of people’s lives all over the world today, GC-MS comes to the fore as a very important analytical technique. It is one of few techniques to determine exactly what is in a sample. Characterised by its quick screening abilities, GC-MS has been widely heralded as the gold standard for forensic substance identification.

On the same standing with GC-MS, liquid chromatography-mass spectrometry (LC-MS) is also a qualitative analytical chemistry technique that combines the physical separation capabilities of high-performance liquid chromatography with the qualitative analysis capabilities of mass spectrometry. Both techniques involve using a mass spectrometry detector, but GC-MS is used to screen a sample using a gaseous-phase component separation process in the gas chromatography column, while LC-MS is able to detect and identify chemicals using a liquid- phase component separation process in the liquid chromatography column.

The medium in which the sample exists and is most effectively separated in the chromatography column, gaseous or liquid, determines which technique is more appropriate. While many samples can be vapourised for GC-MS, other samples are better dissolved in a suitable solvent and examined using LC-MS. However, GC-MS is preferred when quick screening is required because the column separation is generally faster in the gaseous phase.

Sample preparation
In the petrochemical sector, GC-MS can be dynamically applied across a broad range of applications. However, critical to the success of the analytical process is the correct handling and preparation of the sample. Samples requiring GC-MS analysis could include heavy liquids, such as tars, which behave like solids, or solids such as coal. In cases like this, analysts are able to use Pyrolysis mass spectrometry (MS) to convert the sample into the gaseous phase required for characterisation by GC-MS.

Pyrolysis MS is an important technique in the overall analysis armoury and has been hailed for its ability to analyse small amounts of material with minimum sample preparation to obtain, within minutes, fingerprint data that can be used for identification and typing. Pyrolysis MS samples are heated up so rapidly to very high temperatures that they convert from solid to gaseous phase instantaneously and can then be passed through the GC-MS process.

Another sample preparation technique is called thermal desorption mass spectrometry (TDMS). This technique involves collecting desorbed molecules from a surface when the surface temperature is increased and then introducing these individual components into the GC-MS process. This method is commonly used to analyse volatile organic compounds, commonly the source of odours in the ambient air, either inside a building or outside in the open air. Samples are collected using thermal desorption tubes. A third technique for preparing samples is called Headspace GC-MS and involves heating a liquid sample to boiling point, collecting the vapours emitted from the liquid as it is heated up to about 100–200°C and passing them through a GC-MS.

GC-MS applications
GC-MS has an important role to play in prospecting for crude oil or natural gas. As drilling proceeds, exploration personnel need to determine the value of the resources they come across. GC-MS will be able to tell you exactly what you have uncovered. It is the perfect tool for determining the quality of the stream of crude oil or natural gas emanating from the well. A significant part of the financial valuation of petrochemical companies is based on the reserves that companies believe they hold — much more than the value of their processing assets, ships or oil rigs. It is all about the potential value of the oil field that the company has rights to. Therefore, an accurate assessment of the quality of these reserves is absolutely critical for a fundamental evaluation of the reserves being declared. A technique such as GC-MS can play a fundamental role in quantifying the value of such oil field reserves.

The most typical use of GC-MS in the petrochemical industry it is for process troubleshooting. The composition of crude oil is never consistent, creating the opportunity for operational problems to arise as raw material feedstock changes between incoming crude oil batches. Production pressures require the source of the problem to be identified as quickly as possible and very often the best way to achieve this is to use GC-MS as a forensic tool to identify the presence and type of chemical responsible and at what point it entered the process.