Quality Assurance and Method Validation - Forensic Applications of Gas Chromatography - page 55

5.6.3.6 Specificity
This is the ability of a method to assess unambiguously the analyte in the
presence of components that may be expected to be present. These may
include impurities, products of degradation and the matrix.

5.6.3.7 Limit of Detection (LOD)
This is the lowest amount of analyte concentration in a sample that can be
detected but not necessarily quantitated as an exact value. Typically, this
parameter is established when carrying out linearity and, when appropri-
ate, limit of quantitation (LOQ). The concentration is reduced and the sig-
nal from the GC analysed. The limit of detection will be established when
the signal-to-noise (S/N) ratio is ≥3:1. Figure 5.2 shows a chromatogram
obtained when establishing LOD; it can be seen that the peak at 7 min is
the largest peak. If this is the peak associated with the analyte of interest in
the validation, the area under this peak must be at least three times greater
than the area under the peak of the next largest peak in the chromatogram.
For example, the peaks at 4, 12 and 14 min appear to be the next largest
peaks (in relation to the analyte peak). The peak at 4 min has a peak area of
20; at 7 min, the peak area is 60; at 12 min, peak area is 18; and the peak at
14 min has a peak area of 8. The peak of interest has a peak area of 60 with
the next largest peak having a peak area of 20. The peak of the analyte is
exactly three times greater than that of the next largest peak; therefore, this
is acceptable.

70

60

50

40

30

20

10

0

0 2 4 6 8

Retention Time (mins)

10 12 14 16 18

-10

Figure 5.2 Chromatogram obtained when establishing LOD.