Gas Chromatography/Mass Spectrometry
GC/MS can handle mixtures. The GC effluent is directed into the mass spectrometer, where a spectrum of each component is obtained as it elutes from the column. The Mass Spectrometer detector gives a response based on number of ions formed as sample elutes from the column, in the form of a total ion chromatogram (TIC), which is similar to other GC detectors. The difference is that each point in the chromatogram is actually a mass spectrum. Thus a mass spectrum can be retrieved for any peak in the chromatogram. Conversely, a selected ion chromatogram can be retrieved for any mass. For example, aromatic compounds tend to give the tropylium ion at mass 91. A selected ion chromatogram of mass 91 will show the chromatographic peaks attributable to aromatic compounds.
We can use only narrow bore (0.25-0.35 mm ID) fused silica capillary columns for GC/MS. We only stock DB5 columns. If you wish to use a different column, it will need to be purchased and provided or purchased by the lab and billed.
LREI or LRCI: You will receive a total ion chromatogram and mass spectra of the chromatographic peaks you have specified on the sample submission form.
Special Instructions
Capillary columns: Only wall-coated open tubular fused silica columns with an ID of 0.25mm are used in the laboratory. Because of the small ID of these columns, the sample capacity of the 0.25 mm columns is limited to about 50 to 100 nanograms per component of a mixture. Larger bore (0.5 mm ID) are available but not usable here. Generally, one must split the sample injection onto the column with an inlet injector splitter system. Split ratios of 10:1 to 100:1 are used for 0.5 to 1.0 microliter injections.
Sample size: For capillary column operation the maximum amount injected onto the column should be no more than 100 ng per component whether you are performing a split or splitless injection. The lower limit for normal GC/MS is about 1 ng injected onto the column per component.
What to have when you are ready to submit a sample: You may have your GC conditions optimized for good chromatographic resolution and peak shape. A linear carrier gas velocity of 20 to 30 cm/sec is good for 0.25 mm ID columns. You should provide literature containing information about GC column type and the oven temperature program if you don’t have GC in your lab to optimize the GC conditions.
