202
5 Qualitative Aspects of Analytical Chemistry
S.l Introduction
Qualitative analysis is the first step in the hierarchy formed by the three primary
goals of Analytical Chemistry according to the type of information required or
delivered, viz. qualitative, quantitative and structural (see Fig. 1.20). In fact,
quantifying an analyte entails previously checking that it is present in the sample
concerned.
Samples can be classified into three broad categories depending on how deep
available knowledge about them is - and hence on how difficult their qualitative
analysis can be expected to be. White samples are samples with well-defined
properties which, with some exceptions (e.g. those of spring water subjected
to routine laboratory controls), remain virtually constant as a whole; as such,
their qualitative analysis is straightforward and unequivocal. On the other
hand, grey samples are only approximately known as regards composition; such
is the case with samples analysed for environmental pollutants at a central
laboratory receiving them from several factories. Obviously, grey samples are
more complicated to analyse qualitatively than are white samples. Finally, black
samples are samples of absolutely unknown composition (e.g. those involved in
sporadic analyses for special purposes); this type of sample is the most difficult
to analyse qualitatively of the three as it may in principle contain an indefinite
number of species of widely variable nature and be subject to interferences
among them.
__ Box 5.1
Qualitative Analysis is designated in various ways in the analytical literature and in daily
practice - with slightly different connotations, however. Two such designations are "detection" and "identification:
The word detection is usually employed to refer to a measurement process for qualitative
purposes, as opposed to N determination' ; which is usually reserved for Quantitative Analysis.
The distinction is not categorica l, even though such combinations as "quantitative detection"
and "qualitative determinations· are less frequently used.
The word identification is normally used to refer to a qualitative analytical process by
which the analyte (or its reaction product) is recognized (identified) from some physical or
physico-chemical property. Identifying entails the prior use of a standard to obtain one or
more signals that are subsequently compared with those produced by the sample.This word
is consistent with the comparison involved in metrologica l activity and hence more meaning -
ful and appropriate than "detection" as an alternative to "Qualitative Analysis' :
Figure 5.1 summarizes the essential features of qualitative (bio)chemical information. The most usual way of expressing it is a YES/NO binary response. As
shown in this chapter, such a seemingly simple response possesses some quantitative connotations. In fact, the response can be qualified with some of the
capital, basic and accessory properties described in Chap. 2 - with some
adaptations, however. In clear contrast with chemical and biological metrology,
qualitative information is of little significance to physical metrology.
5 Qualitative Aspects of Analytical Chemistry
S.l Introduction
Qualitative analysis is the first step in the hierarchy formed by the three primary
goals of Analytical Chemistry according to the type of information required or
delivered, viz. qualitative, quantitative and structural (see Fig. 1.20). In fact,
quantifying an analyte entails previously checking that it is present in the sample
concerned.
Samples can be classified into three broad categories depending on how deep
available knowledge about them is - and hence on how difficult their qualitative
analysis can be expected to be. White samples are samples with well-defined
properties which, with some exceptions (e.g. those of spring water subjected
to routine laboratory controls), remain virtually constant as a whole; as such,
their qualitative analysis is straightforward and unequivocal. On the other
hand, grey samples are only approximately known as regards composition; such
is the case with samples analysed for environmental pollutants at a central
laboratory receiving them from several factories. Obviously, grey samples are
more complicated to analyse qualitatively than are white samples. Finally, black
samples are samples of absolutely unknown composition (e.g. those involved in
sporadic analyses for special purposes); this type of sample is the most difficult
to analyse qualitatively of the three as it may in principle contain an indefinite
number of species of widely variable nature and be subject to interferences
among them.
__ Box 5.1
Qualitative Analysis is designated in various ways in the analytical literature and in daily
practice - with slightly different connotations, however. Two such designations are "detection" and "identification:
The word detection is usually employed to refer to a measurement process for qualitative
purposes, as opposed to N determination' ; which is usually reserved for Quantitative Analysis.
The distinction is not categorica l, even though such combinations as "quantitative detection"
and "qualitative determinations· are less frequently used.
The word identification is normally used to refer to a qualitative analytical process by
which the analyte (or its reaction product) is recognized (identified) from some physical or
physico-chemical property. Identifying entails the prior use of a standard to obtain one or
more signals that are subsequently compared with those produced by the sample.This word
is consistent with the comparison involved in metrologica l activity and hence more meaning -
ful and appropriate than "detection" as an alternative to "Qualitative Analysis' :
Figure 5.1 summarizes the essential features of qualitative (bio)chemical information. The most usual way of expressing it is a YES/NO binary response. As
shown in this chapter, such a seemingly simple response possesses some quantitative connotations. In fact, the response can be qualified with some of the
capital, basic and accessory properties described in Chap. 2 - with some
adaptations, however. In clear contrast with chemical and biological metrology,
qualitative information is of little significance to physical metrology.