Instant Notes: Analytical Chemistry

with stringent analytical requirements to develop, test and validate analytical

methods.

Proficiency testingschemes are designed to test the competence of individual

laboratories to perform specific analyses or a range of determinations of

analytes in various matrixes and possibly using alternative techniques.

Some typical examples of collaborative studies and proficiency testing are

● alcohol in beverages;

● metals or volatile organics in soils;

● hazardous airborne substances;

● trace metals or pesticides in domestic water supplies;

● drugs of abuse by chromatographic methods;

● adulterants in foodstuffs;

● additives in polymers and composites.

In statistical terms, collaborative trials are designed to reveal the presence of a

bias in the results arising from determinate errors that may occur in addition to

inherent indeterminate (random) errors. Statistics that are used in these schemes

include estimated standard deviationand coefficient of variation, confidence

limits(Topic B2) and tests of significanceincluding ANOVA(Topic B3). Of

particular value is a z-scoreto indicate the accuracy of results from the partici-

pating laboratories. The z-valueis a statistical factor based on a Gaussian or

normal distribution, which is included in the equation for confidence limits of

an experimental value in relation to a true or accepted value (Topic B2, equation

(5)). Rearrangement of this equation, for a single determination, xi, (N=1) gives

z=

where mis the true or accepted mass or concentration of the analyte and sis a

value for the standard deviation for the method selected by the organizers of the

study (strictly an estimated value, s). A typical chart of z-scoresfor a collabora-

tive study involving 22 laboratories is shown in Figure 2.

Results that have z-scoresof between +1.96 and -1.96 are considered to have

acceptable accuracy, as these values correspond to confidence limits at the 95%

probability level (Topic B2, Fig. 3). In the example, only laboratories 12 and 18

xi−m

s

B5 – Quality control and chemometrics 51

19 13 21 22 20 10 5 15 16 1

Laboratory number

11 8 6 14 2 17 7 4 9 3 12 18

3

2

1

0

–1

–2

–3

z-score

Fig. 2. z-score chart for results from 22 laboratories.