A reagent test is  a chemical test that looks for specific parts of a molecule.  For example, a reagent that tests for the alcohol (OH) group on an organic chemical would potentially detect many compounds that have the OH group on them, such as ethanol, methanol, propanol, etc. Some reagents are more specific, but are generally  unable to distinguish among at least closely related compounds. 

In contrast, tests that measure the physical properties of a substance tend to be rather specific for what they are measuring.  Each compound has a specific melting point, a specific  absorption spectra, etc.  Ethanol, methanol, and isopropanol, for example, could be distinguished by their boiling points.

In the case of gas chromatography/mass spectroscopy, the detection system uses two physical features to identify the compound.  First, the gas chromatography basically heats up the sample and “boils off” the compound of interest.  Because each compound has a unique boiling point, the temperature at which it boils off can be used to help identify the compound.

The second test is to take the compound that has been “boiled off” and break it down into smaller pieces.  Even for complex chemicals such as cocaine, the heated chemical breaks down into a characteristic number of smaller pieces of specific weights.  The mass spectroscope looks for those pieces.  Because each chemical has a characteristic mass spectrum, it should be possible to compare the mass spectrum of the unknown sample to that of a reference standard to identify the makeup of the unknown sample.

It may be to make a compound that contains the same number of hydrogen, carbon, oxygen, and nitrogen atoms as cocaine.  However, this chemical would have a different melting point than cocaine.  Also, when it breaks up, the pieces would look different, and therefore show up with at least some pieces of different weight than the pieces of broken down cocaine would on the mass spectrometer.

To give a simple example, propanol has the formula CH3CH2CH2OH, while isopropanol is CH3CH(OH)CH3.  Each has the same chemical makeup and same weight, but would have different boiling points.  On the mass spectra, propanol might (as an example) have pieces CH3, CH2, and CH2OH, while isopropanol would have CH3, CHOH, and CH3.    The two compounds could be distinguished because (1) they have different boiling points and (2) they break down into different pieces with different weights, even though they weigh the same and have the same elemental makeup.  This is a very simplistic and probably incorrect version of what would actually happen if you ran propanol and isopropanol on a GC/MS, but it at least gives you an idea of what is measured.

By having a recorded standard for isopropanol and propanol, the analyst would be able to say that the GC/MS spectrum of the unknown ant the standard are similar.

As with any machine, GC?MS needs to be calibrated by running against known standards.  However, once the machine is calibrated, it should be possible to identify a compound based upon its characteristic temperature it comes of the gas chromatograph, and its characteristic mass spectrum.

The machine is an electronic device that uses an electromagnetic field to separate out the various size pieces and to detect those pieces.  It is a qualitative analysis, and not a quantitative analysis.  However, they tend to be very stable once calibrated.

A regular GC machine can only tell you what is present, not how much.

Dean P. Loven

Assistant Public Defender