9.2 Dynamic Range in Sensitivity

The GC/MS system is the workhorse instrument of this laboratory. It is used for general purpose screening (after preliminary or presumptive testing) as well as high sensitivity analysis of trace samples. For reasons of utility, therefore, we do not always operate the instrument at maximum sensitivity. There is a broad dynamic range of sample concentrations that can be tested with the instrument, depending upon how injections are made, how the instrument is tuned and the acquisition mode. Changes from some regions of the complete dynamic range are quite easy and quick to make.

• The least sensitive mode is full scan (complete mass spectrum) acquisition, standard EI spectrum tuning, 60:1 split injection of 1 $\mu$L. These are methods that end in ``1,'' such as GEN1, VOL1, LSD1, etc. When dealing with solutions of unknown concentration or composition, these should be the first methods used in a set.
• If a ``1'' method is found to be just barely too insensitive (for example, maybe TIC peaks are present, but some small signals lack isotope peaks), one may simply inject a slightly larger sample. The ``2'' methods, such as GEN2, are the same as the ``1'' methods except that 3 $\mu$L of sample are injected; this triples the solute placed on column.
• If large signals lack isotope peaks or TIC integrated areas are too small to be detected by automated data analysis runs, the injected amount can be increased by approximately 50 times by changing to a splitless method, such as GEN3. The ``3'' methods are the same as the ``1'' methods except for using splitless injection. Note that the splitless parameters are generic and are not tuned for any particular analysis in any of the general purpose ``3'' methods. Therefore, the chromatograph peak shapes and quantitative precision may be poor.
• Each of the method classes ``1'', ``2'' and ``3'' involve only changes to the GC parameters. If a ``3'' method remains too insensitive, the mass spectrometer can be tuned for increased sensitivity. There is an autotune program to produce increased sensitivity, but if maximum full scan sensitivity is required, the operator may wish to manually tune after such an autotune. To achieve higher sensitivity in the mass spectrometer tuning, the EM voltage is increased ('normal' tuning uses a relatively 'cool' multiplier to increase multiplier life), as well as possibly tuning for a specific mass range. Note that such tuning may generate 'non-standard' EI spectra that will not be comparable to standard spectra; this is partly why we interpret spectra to identify compounds, not simply compare to other spectra.
• To achieve maximum sensitivity and for true trace work, one may have to abandon the collection of the full mass spectrum at each scan. Remember that the quadrupole is not a multiplexing analyzer like TOF, meaning that ions are destroyed during mass filtering. This can be overcome somewhat by 'parking' the mass analyzer on a mass of interest, such as the base peak in the spectrum of a target analyte. This technique is called Selected Ion Monitoring (SIM) and increases the overall sensitivity of the GC/MS system by about a factor of 100 (so that in principle, 10 femtograms of hexachlorobenzene should yield an S/N of 10:1 or greater). Methods designed for use with SIM have an ``s'' appended to the method name, such as GEN1s.