Instant Notes: Analytical Chemistry

multiple of the estimated standard deviation of the method, usually 95% or

±1.96s/N1/2;

● action linesare positioned to correspond to a selected probability level

or multiple of the estimated standard deviation of the method usually 99.7% or

±3.01s/N1/2;

● the pairs of warning lines will move inwards towards the target line as the

number of results per plotted point, N, increases, so the value of Nmust be

fixed before the chart is used;

● in the absence of determinate errors (bias), 95% of values should fall within

the upper and lower warning lines. The system is then considered to be

stable or under control;

● two or more consecutive values falling outside the warning lines but within

the action lines indicate a possible loss of control;

● two or more consecutive values falling outside the action lines indicate the

occurrence of one or more determinate errors; the system is then considered

to be out of control and remedial action should be taken;

● trends in plotted values are an indication of incipient problems, as are ten or

more consecutive values on one side of the target line.

The chart shows that the system is in control during the first 6 samples

analyzed, but the upper warning level has been breached by result 7. However,

the next 8 results are within the warning levels, but results 17 to 20 indicate a

downward trend culminating with both lower limits being breached indicating

a loss of control due to one or more determinate errors. At this point, the causes

should be sought and remedial action taken.

The principal purpose of collaborative testingis to assess the accuracy of

results from a group of laboratories performing one or more specific quantita-

tive determinations. Interlaboratory studies are also used to assess other perfor-

mance characteristics of a particular method or the competence of individual

analysts in relation to a specified technique, analyte or group of analytes. They

are frequently used by trade associations, standards organizations or agencies

Collaborative
testing

Sample number

Mass or concentration

of analyte, x

3s/N1/2

2s/N1/2

x

2s/N1/2

3s/N1/2

0 5 10 15 20

Fig. 1. A Shewart averages control chart.

50 Section B – Assessment of data