Gas Chromatography-Mass Spectrometry
Gas chromatography-mass spectrometry (GC/MS) combines the fine separating power of GC with the uniquely powerful detection capabilities of MS. This powerful technique is particularly suitable for the analysis of mixtures of volatile and low relative molecular mass compounds (< 800) such as hydrocarbons, fragrances, essential oils and relatively non-polar drugs. Chemical derivatisation, e.g., trimethylsilylation, can often be employed to increase the volatility of compounds containing polar functional groups (-OH, -COOH, -NH2, etc) thereby extending the range of suitable analytes to such compounds as steroids, polar drugs, prostaglandins, bile acids, organic acids, amino acids, and small peptides.
An adequate sample preparation is needed before proceeding with the GC-MS analysis. Samples such as water, soil, urine, blood plasma, etc., have to be subjected to a “clean-up” procedure prior to analysis in order to extract and concentrate the more volatile, low molecular mass components. Extraction can be performed by organic solvents or by solid phase extraction (SPE).
Where considered necessary, the extract can be derivatised with a choice of special reagents. For example, thermally labile and polar carboxylic acids groups can be methylated with BF3/methanol (or TSM-diazomethane) or trimethylsilated with a variety of commercially available reagents.
GC-MS analysis. Typically, a solution of the analyte mixture (1 uL) is injected on to the GC column via a heated injection port, which is normally held at 250-300 °C to facilitate vaporisation Following volatilisation in the heated injector, the mixture is pushed by a pressurized carrier gas (usually helium) through the GC column, which is heated in an oven. Separation relies on the fact that different compounds dissolve to different extents in this liquid and move through the column at different rates depending on their partition between the stationary liquid and the mobile carrier gas.
The GC-MS system currently available in the facility is supported with an electron ionisation (EI) source. The MS detector can operate in 2 different modes: scanning and selected ion monitoring (SIM). Quantitative work can be performed by integration of selected ion chromatographic peaks.