# Quantification of Cocaine by GCMS

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This method relies on the response of the detector being in direct proportion to the concentration of drug in the sample being evaluated. A series of standards of known concentrations is prepared and analysed, with each analysis being separated by a blank.

SAQ 6.3

Why should each sample be separated by a blank in the analytical process?

The same solution system must be used to prepare both the standards and the samples.

SAQ 6.4

Why are internal standards necessary?

SAQ 6.5

Why is there a requirement to use the same solution for the preparation of the standards and the samples?

 Cocaine Internal standard Y-valuec X2 X x Y concentration peak area peak area (mgml-1 )b (arbitrary units) (arbitrary units) 0.188 280959 8 062 322 0.035 0.035 0.007 0.188 309454 9 364403 0.033 0.035 0.006 0.375 779 646 9 165 776 0.085 0.141 0.032 0.375 764902 9 423 551 0.081 0.141 0.030 0.750 1184 352 9411496 0.126 0.563 0.094 0.750 1390950 10 782 244 0.129 0.563 0.097 1.500 2007 799 9 231 358 0.217 2.250 0.326 1.500 2107 793 9 542 613 0.221 2.250 0.331 3.000 3 002 575 6 672401 0.450 9.000 1.350 3.000 4129815 9 572136 0.431 9.000 1.294 11.625 8825855 91228 300 1.809 23.977 3.568

cRatio of cocaine peak area to internal standard peak area.

cRatio of cocaine peak area to internal standard peak area.

Next, the peak areas of the drugs in the standards are obtained, and the relative responses calculated. Some exemplar standard data for the quantification of cocaine in a drug sample are presented in Table 6.3. Using these, a calibration graph of relative response against cocaine concentration, as the free base, is plotted (Figure 6.3), and from the simultaneous equations:

the appropriate regression equation can be calculated. For the example being considered here, this equation is as follows:

and therefore, using the HPLC data obtained from the drug sample, the concentration of cocaine can be calculated.

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