296
7 The Analytical Problem
liminary operations such as interference removal and preconcentration in order to boost the
selectivity and sensitivity, respectively (see Sect. 4.3.4).
Example 2
Problem: The presence of a preservative banned by European legislation in a soft drink.
If a liquid chromatograph (LC) is available (see Box 5.16), then the CMP will be very
sim ple: it will suffice to degas the sample and insert an aliquot into the chromatograph
(identification can be done as described in Box 5.18). If only a rudimentary photometer is
available, however, interferences will preclude direct measurements of the preservative
absorbance, so some prelimina ry operation (e. g. a liquid-liquid or solid-phase extraction) wi ll
be required to partly overcome this drawback; alternatively, a derivatizing reaction can be
used to accomplish the effect depicted in Fig.5.12.
7.5.4 Monitoring the Results
The fourth step of the analytical problem-solving process involves two distinct
actions both of which are related to validation (see Sect. 4.6) and to the first two
steps of the process. First, one must evaluate internal quality, which entails
validating the results with respect to the levels sought in the analytical properties
and their compromises; this is the responsibility of the analytical chemist, in the
second step of the process. Second, one must validate the results in relation to the
information requested by the client and to data supplied by other professionals;
the purpose is to check for external quality, which, again, requires establishing
the interface involved in the first step. If both evaluations (comparisons with
references) are favourable, the analytical problem will have been solved (see
Fig. 7.7).
Box 7.8
Fourth Step of the Analytical Problem-solving Process
The evaluation of the results produced by the CMPs designed in the third step of the process
should rely on comparisons with two types of reference, namely: the characteristics of the
analytical information specified in the second step and the information needs confirmed in
the first. This evaluation will reveal whether the problem is solved or a fifth step is required.
Example 1. Presence of cadmium in yellow toys
After the CMP is conducted, the final concentration in the treated sample is found to fall
beyond or near the limit of quantitation for the linear range of the calibration curve.
Obviously, the methodology cannot be validated for quantitation purposes as the uncertainty of the result in this concentration region will be very high. However, if the information
requested is of the purely qualitative type and such a concentration exceeds the limit of
detection, the binary response provided will be quite valid.
Example 2. Contamination of fruit with pesticides
After the CMP is carried out, real samples are found to provide no signals (peaks) in the
chromatogram. The easiest interpretation is that the fruit is "ciean':However, as in the previous
7 The Analytical Problem
liminary operations such as interference removal and preconcentration in order to boost the
selectivity and sensitivity, respectively (see Sect. 4.3.4).
Example 2
Problem: The presence of a preservative banned by European legislation in a soft drink.
If a liquid chromatograph (LC) is available (see Box 5.16), then the CMP will be very
sim ple: it will suffice to degas the sample and insert an aliquot into the chromatograph
(identification can be done as described in Box 5.18). If only a rudimentary photometer is
available, however, interferences will preclude direct measurements of the preservative
absorbance, so some prelimina ry operation (e. g. a liquid-liquid or solid-phase extraction) wi ll
be required to partly overcome this drawback; alternatively, a derivatizing reaction can be
used to accomplish the effect depicted in Fig.5.12.
7.5.4 Monitoring the Results
The fourth step of the analytical problem-solving process involves two distinct
actions both of which are related to validation (see Sect. 4.6) and to the first two
steps of the process. First, one must evaluate internal quality, which entails
validating the results with respect to the levels sought in the analytical properties
and their compromises; this is the responsibility of the analytical chemist, in the
second step of the process. Second, one must validate the results in relation to the
information requested by the client and to data supplied by other professionals;
the purpose is to check for external quality, which, again, requires establishing
the interface involved in the first step. If both evaluations (comparisons with
references) are favourable, the analytical problem will have been solved (see
Fig. 7.7).
Box 7.8
Fourth Step of the Analytical Problem-solving Process
The evaluation of the results produced by the CMPs designed in the third step of the process
should rely on comparisons with two types of reference, namely: the characteristics of the
analytical information specified in the second step and the information needs confirmed in
the first. This evaluation will reveal whether the problem is solved or a fifth step is required.
Example 1. Presence of cadmium in yellow toys
After the CMP is conducted, the final concentration in the treated sample is found to fall
beyond or near the limit of quantitation for the linear range of the calibration curve.
Obviously, the methodology cannot be validated for quantitation purposes as the uncertainty of the result in this concentration region will be very high. However, if the information
requested is of the purely qualitative type and such a concentration exceeds the limit of
detection, the binary response provided will be quite valid.
Example 2. Contamination of fruit with pesticides
After the CMP is carried out, real samples are found to provide no signals (peaks) in the
chromatogram. The easiest interpretation is that the fruit is "ciean':However, as in the previous