GAS CHROMATOGRAPH QUANTITATIVE ANALYSIS BY GAS CHROMATOGRAPHY
As previously noted, the size of the peak on a gas chromatogram is proportional to the amount of material that reaches the detector. Strictly speaking, it is the area under the peak (geometrically speaking), which is the important quantity. Suppose, in the case with the heroin and cocaine, one wanted to determine the percent cocaine in the drug exhibit. The following steps would be taken:
1. A sample of the exhibit would be weighed out and dissolved in a suitable solvent that also contained an internal standard. An internal standard is a compound that elutes near the analyte and is used to standardize the procedure.
2. A sample of known cocaine would be weighed out and dissolved in the same solvent with the same amount of internal standard.
3. Both would then be chromatographed and the areas under the cocaine and internal standard peaks for both the known cocaine and case exhibit are calculated. Through simple proportions, the weight of cocaine in the exhibit can be calculated and then the percent can be determined by knowing the weight of the powder that was dissolved. In most jurisdictions, the percent of a drug is not a legal issue in the sense that the penalty for possession or distribution does not depend on the percent of cocaine. Why then would a drug chemist do this quantitative analysis? In some cases, the investigative agency asks for these data because it can help in determining how far up the distribution chain the seized drugs are. For example, if normal “street” cocaine is 40% pure and this particular exhibit is 80%, it means that the person arrested for possession of this exhibit is probably not an ultimate user, but is more likely a distributor. Also, sometimes judges want to know the purity of a drug exhibit so that they can impose appropriate sentences when someone is found guilty of possession or distribution.
