274
6 Quantitative Aspects of Analytical Chemistry
interferences are of the multiplicative type, i. e. if they primarily affect the
sensitivity (see Sect. 2.5.3). These errors can be minimized by using the extrapolation or multiple standard addition method, which involves the following
steps:
(a) A number n of sample aliquots are supplied with increasing concentrations
(e. g. 0 - 6 arbitrary units) of a pure standard of the analyte, all containing an
analyte concentration equal to the combined original (sought) and added
concentrations.
(b) The n aliquots of spiked real samples are subjected to the analytical process
in order to obtain one signal per aliquot as shown in Fig. 6.9.
(c) The signals thus obtained are plotted against the concentrations of standard
added (0-6). As can be seen from Fig. 6.lOB, a straight calibration line is
obtained that is parallel to that provided by the pure standards in the absence of matrix effects. Otherwise, the two lines will not be parallel. The
signal on the y-axis is that yielded by the unspiked sample (that to which a
zero concentration of standard is added).
(d) The analyte concentration in the sample is obtained by extrapolating the
calibration curve; its intercept with the x-axis gives the datum sought.
Box 6.13
Relative Interpolation and Extrapolation Methods
The practical implementation of these two types of method is illustrated here with the same
analytical problem (viz. the determination of iron in wine). The choice in each case will be
dictated by the characteristics of the instrumental technique to be used.
With atomic absorption spectroscopy, one can use the direct interpolation mode, which is
highly selective and provides signals that are not significantly affected by the sample matrix
(usua lly not very complex in any case). Six iron standard solutions of variable concentration
(O.5- 5 ppm) are prepared and delivered to the flame by nebulization, their absorbances then
being recorded (see Fig. 6.10). Next, the sample is aspirated to obtain a Signal that is interpolated into the curve to determine the iron concentration in the wine.
The CMP can also be based on UV-Visible absorption (photometry). for example. This
entails supplying the sample with a ligand L (o-phenanthroline) to form the chelate FeL~ ,
which is deeply coloured ().ma~ = 510 nm); a reductant (hydroxylamine or ascorbic acid) is
previously added to convert all iron present into Fe2+ ion. The selectivity is lower here as the
procedure can be interfered by a number of factors including the wine's base colour, the
presence of metal ions forming chelates with the ligand, the addition of an inadequate
amount of reductant, the presence of iron-masking ligands, etc It is therefore advisable to use
the extrapolation approach based on multiple standard additions. Thus, six aliquots of the
wine are supplied with increasing concentrations (0-4 ppm) of Fe 2 and subjected to the
CMf> Six signals are thus obtained such as those of Fig. 6.10 B. The amount of analyte present
in the unknown is determined by extrapolation.
6 Quantitative Aspects of Analytical Chemistry
interferences are of the multiplicative type, i. e. if they primarily affect the
sensitivity (see Sect. 2.5.3). These errors can be minimized by using the extrapolation or multiple standard addition method, which involves the following
steps:
(a) A number n of sample aliquots are supplied with increasing concentrations
(e. g. 0 - 6 arbitrary units) of a pure standard of the analyte, all containing an
analyte concentration equal to the combined original (sought) and added
concentrations.
(b) The n aliquots of spiked real samples are subjected to the analytical process
in order to obtain one signal per aliquot as shown in Fig. 6.9.
(c) The signals thus obtained are plotted against the concentrations of standard
added (0-6). As can be seen from Fig. 6.lOB, a straight calibration line is
obtained that is parallel to that provided by the pure standards in the absence of matrix effects. Otherwise, the two lines will not be parallel. The
signal on the y-axis is that yielded by the unspiked sample (that to which a
zero concentration of standard is added).
(d) The analyte concentration in the sample is obtained by extrapolating the
calibration curve; its intercept with the x-axis gives the datum sought.
Box 6.13
Relative Interpolation and Extrapolation Methods
The practical implementation of these two types of method is illustrated here with the same
analytical problem (viz. the determination of iron in wine). The choice in each case will be
dictated by the characteristics of the instrumental technique to be used.
With atomic absorption spectroscopy, one can use the direct interpolation mode, which is
highly selective and provides signals that are not significantly affected by the sample matrix
(usua lly not very complex in any case). Six iron standard solutions of variable concentration
(O.5- 5 ppm) are prepared and delivered to the flame by nebulization, their absorbances then
being recorded (see Fig. 6.10). Next, the sample is aspirated to obtain a Signal that is interpolated into the curve to determine the iron concentration in the wine.
The CMP can also be based on UV-Visible absorption (photometry). for example. This
entails supplying the sample with a ligand L (o-phenanthroline) to form the chelate FeL~ ,
which is deeply coloured ().ma~ = 510 nm); a reductant (hydroxylamine or ascorbic acid) is
previously added to convert all iron present into Fe2+ ion. The selectivity is lower here as the
procedure can be interfered by a number of factors including the wine's base colour, the
presence of metal ions forming chelates with the ligand, the addition of an inadequate
amount of reductant, the presence of iron-masking ligands, etc It is therefore advisable to use
the extrapolation approach based on multiple standard additions. Thus, six aliquots of the
wine are supplied with increasing concentrations (0-4 ppm) of Fe 2 and subjected to the
CMf> Six signals are thus obtained such as those of Fig. 6.10 B. The amount of analyte present
in the unknown is determined by extrapolation.