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. Just as an FID gives a response based on the number of ions formed as sample elutes from the column, so does GC/MS, in the form of a total ion chromatogram (TIC). 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.EI is usually used for ionization, but CI and FI can also be used. In EI or CI, high-resolution measurements are possible. You may run the vast majority of samples yourself on the Chemistry Department's Hewlett-Packard Mass Selective Detector (MSD). This is an extremely small mass spectrometer mounted on the side of a gas chromatograph. You will need our GC/MS capabilities
- if your sample weighs more than 800 Da
- if you need other than EI ionization
- or if you require more sensitivity than the MSD provides
We can use only narrow bore (0.25-0.35 mm ID) fused silica capillary columns for GC/MS on the 70-VSE. Megabore columns have a much higher sample capacity than a narrow bore column and a much higher carrier gas flow rate. However, we can reproduce your GC run on the GC/MS by reducing the sample size. This is why we require submitting a standard sample GC trace along with your regular sample. It is of the utmost importance for you to prepare and run a sample of known concentration similar to that of one of the larger peaks in your real sample. This gives us a true idea of the concentration of the peaks in your GC run so that if we do have to dilute or concentrate your sample we do not waste several hours doing so!
GC/MS Instrument: 70-VSE Sample size: see below
LREI, LRCI, and LRFI: You will receive a total ion chromatogram and mass spectra of the chromatographic peaks you have specified on the sample submission form.
HREI and HRCI: This is performed at high (5,000-6,000) resolution. Low resolution data must have previously been obtained and all output submitted. This method will give elemental composition listings of all major peaks in the mass spectrum.
- 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 should 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 must also include a separate run of a suitable standard by injecting a known amount of compound onto your column. This gives us a rough idea of the size of your component peaks before we begin a run. Many people use dodecane or methyl stearate. You do not have to submit the solution of the compound, just the GC trace and conditions. Your GC traces must indicate chart speed or retention times and attenuator range. The sample submission form has sections for GC conditions.
- GC/MS Form: Much of the form is identical to the general form. Near the top of the form are six columns. In the first column you can list the peaks for which you want to see the mass spectrum. The corresponding expected formula goes in column 2, and the concentration range in column 4. Columns 3, 5, and 6 need only one entry each. Column 3 is for the upper mass limit of the mass scan. This should be as low as possible, since a wider range takes a longer time and thus degrades the resolution of the total ion chromatogram. Near the bottom of the form are spaces to record your chromatographic conditions.