Question

what are the advantages of GC/Ms compared to GC with other detectors?

Answer 1

Gas chromatography ("GC") and mass spectrometry ("MS") make an effective combination for chemical analysis. This article serves to demonstrate tools for an effective attack or defense of GC/MS evidence. To effectively use GC/MS evidence one must understand the process. First, the GC process will be considered, then the MS instrument will be presented. After a background in GC and MS is obtained, the reader will learn how to analyze the evidence produced by these instruments. The focus of this article lies in presenting the limitations to GC/MS analysis.

Before analyzing a sample, the technician should tune and calibrate the instrument.   Tuning can be accomplished using specific concentrations of Decafluorotriphenylphosphine and p-Bromofluorobenzene. A technician can process a spiked sample (containing a known concentration of a substance) to check calibration and tuning.   If the GC/MS instrument does not detect the substance or shows a greater or lesser concentration than the known concentration, the technician must recalibrate the instrument.   Also, the technician can use a blank sample (containing no detectable compounds) to test the GC/MS instrument's data reporting accuracy.   If the device indicates the presence of a substance in the blank sample, the device may contain residue from prior analysis.   If this occurs, the technician must retune and recalibrate the GC/MS instrument.

    Proper scientific practice requires that the GC technician compare the spectral output with a known standard sample of the suspected substance.   The standard sample must be analyzed with the same instrument, under the same conditions, immediately before and immediately after analyzing the unknown specimen.   If the resulting three spectral outputs do not agree, the technician can not make a reliable identification of the specimen based on the GC analysis.

Carrier Gas

    If the GC instrument uses hydrogen for the carrier gas, the technician must consider whether the hydrogen may react with any of the compounds present in the specimen. If the hydrogen does react, a broad peak will result. When using a thermal conductivity detector, care should be taken as a false peak may occur if the carrier gas's thermal conductivity is in the range of the thermal conductivity of any compound in the specimen. An unstable carrier gas flow rate may produce a drifting baseline and false broad peaks. A carrier gas should be pure. Regular changing of the gas filter should prevent significant impurities.