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?

Table 6.3 Calibration and calculated data, obtained by GC-MS analysis, used for the quantification of cocaine in a drug sample

Cocaine®

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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