Gas chromatography mass spectrometry (GC-MS) sample preparation is performed on smaller and more volatile samples including environmental pollutants, industrial byproducts, food contaminants, pesticides, and metabolites of illicit and designer drugs. These molecules are more challenging to ionize and separate (i.e., resolve) using liquid chromatography mass spectrometry (LC-MS), and so are subjected to GC-MS instead.
GC-MS sample preparation can sometimes occur using LC-MS, as the following figure shows. Furthermore, advances in GC-MS technology, such as the introduction of the Orbitrap mass analyzer, have enabled analysis of larger and more complex compounds, propelling GC-MS into traditional LC-MS fields such as metabolomics.
Gas chromatography (GC) is first employed to volatize the sample, after which is it injected into the mobile phase, which is typically composed of a gas such as helium or argon. This mobile phase “carries” the sample until the stationary phase inside the GC column is reached. This stationary phase often consists of a chemical that is formulated to preferentially interact with select sample compounds.
Once at the stationary phase, the GC sample separates. This occurs because the GC column is ramped, or gradually heated, and compounds with lower boiling points are eluted first. Likewise, the pressure of the mobile phase can be varied to fine-tune separation. Finally, there are the chemical interactions between the sample compounds and the stationary phase to consider; weaker interactions will dissociate faster and elute earlier than stronger ones. A compound can be measured from the time of its injection until the time of its elution from the GC column; this is called the compound’s retention time.
Lower molecular weight compounds will elute from a GC column sooner than those with a higher molecular weight because of boiling point differences.
After this process of elution is complete, compounds undergo electron ionization (EI) or chemical ionization (CI) and become charged. They then undergo mass analysis within a mass spectrometer, and their unique mass (m) and charge (z) information is reported as numerical m/z ratios.
Once these values are reported, they are typically displayed as ion peaks. The values are also compared against previously compiled libraries of known mass spectra using analytical software programs. Matching spectra are identified and characterized. Mass spectrometry analysis enables the determination of compound molecular weight and formula, as well as functional groups.
GC-MS samples often contain dirty, labile, and volatile compounds that sometimes need further processing before they are introduced into the gas chromatograph. Different manual and automated sample extraction processes are often used prior to gas chromatography, and will differ based on the degree of selectivity required during sample preparation as well as the initial cleanliness of the samples.
Headspace sampling: In this technique, the liquid or solid sample is added to a glass vial until it establishes equilibrium. Some of the analytes vaporize from the liquid or solid and enter the headspace above the sample. If this gas phase is then directly injected into the gas chromatograph for separation, the method is referred to as static headspace sampling. If an inert gas is passed into the sample and the vaporized analytes accumulate on an absorbant surface or cryogenic trap, the method is termed dynamic headspace sampling or purge and trap. This method of analysis is often used with blood, cosmetics, plastics, solid, and materials with high water content.
Pyrolysis: This step is performed prior to GC-MS and involves heating samples to 120 C in either an inert environment or in air, resulting in their breakdown to smaller fragments. Samples that typically undergo pyrolysis include plastics, paints, dyes, resins, cellulose, wood, oils, and rubber, as well as larger samples that might be heavily crosslinked or insoluble. Because pyrolysis negates the need to use solvents, it is useful for identification of additives, solvents, and additives.
Solid phase extraction (SPE): This manual extraction technique typically involves the use of a solid packing material, often contained within a cartridge, to separate sample components, remove interferences, or fractionate the sample matrix. The sample itself will typically (although not always) be in liquid form. Samples frequently extracted via SPE include biological samples such as urine, saliva, and plasma, environmental samples such as water, and food products such as beverages.
Solvent extraction: Many analytes, such as pesticides, are highly polar and therefore amenable to extraction with solvents such as acetonitrile. Upon addition of salts such as magnesium sulfate, phase separation of acetonitrile from water occurs. This is the guiding principle behind the QuEChERS technology.
Solid phase micro extraction (SPME): Used before both gas chromatography or HPLC, SPME is a sample extraction technique that does not use solvent. Instead, a fused silica fiber, which is coated with a stationary (polymer) phase, is added to the sample. This fiber can be exposed to the vapor phase (headspace) above the liquid or solid or immersed into the liquid sample directly. The analyte(s) of interest diffuse onto the stationary phase; afterwards, the fiber is placed into the injection port of the GC-MS system and the analytes are transferred to the analytical column after thermal desorption. SPME is a fast and simple process that has obvious advantages for field application work. It is frequently used on high background samples such as food.
Stir bar sorptive extraction (SBSE): This method consists of exposing a magnetic stirring rod that is coated with a sorptive layer to the sample. Following exposure, the bar is removed, rinsed with deionized water, and dried. It is then placed inside the desorber tube and undergoes thermal desorption. During this process, the analytes are transferred onto the chromatographic system. SBSE has the advantage of higher sensitivity due to the larger amount of sorptive phase used; the trade-off is a longer extraction time and mostly manual procedure.
The following sample preparation protocols enable faster throughput and lowered costs as a result of automation:
Automated SPE: Automated extraction systems can process many SPE samples in just a few hours, enabling quick analysis of compounds of interest including pesticides, flame retardants, semivolatiles, nitrosamines, and steroids.
Accelerated solvent extraction (ASE): Automated batch sample processing of solids and semi-solid samples including pesticides, oils, nutritional supplements, and biofuels is frequently accomplished using automated extraction systems. In this way, compounds are quickly extracted from the sample using a minimum amount of time, solvent, and overall cost.
