166
4 The Measurement Process in Chemistry
Fig.4.15. Principal goals of analytical separation techniques (ST). (1) Preconcentrating an
analyte (A). (2) Removal (clean-up) of interferents to isolate the analyte (A) from the other
sample components (2a) or separate the different components (considered to be the analytes)
from one another (2b)
Box 4.9
A flame atomic absorption spectrometer (FAAS) is available to determine lead in water from
wells near a mining area. The limit of detection achieved by direct insertion of aqueous
standards of Pb 2 + into the instrument is 1.0 mg/L (i. e. in the parts-per-million range). However, when real water samples are inserted, no signal is obtained; this suggests that their lead
contents are below the limit of the detection .The sensitivity of an FAAS is thus inadequate for
the purpose as the legal threshold for lead in water for animal or human consumption is
0.5 ~lg/L (i. e. in the parts-per -billion range). One must therefore increase the sensitivity of the
available technique by preconcentrating the lead.
To this end, a volume of 4 litres of water is passed through a chelating column (Chelex100)' which selectively retains Pb 2 +. Subsequently, the column is eluted with 2 mL of 0.1 N
HN0 3 and the eluate is inserted into the instrument. In this way, a preconcentration factor of
4000: 2 = 2000 is achieved. The new lim it of detection is thus 2000 times lower, i.e. it has
been reduced to 0.5 ~lg/L, which, coincidentally, is the same as the legal limit. Based on the
underlying criterion of the analytical problem, if a preconcentrated real sample now provides
a signal, then the water can be assumed to be contaminated with lead.
One could also have used a more sensitive technique [e.g. anodic stripping voltammetry
(ASV), electrothermal-atomization atomic absorption spectrometry (ETAAS)] to avoid the
need for such a high concentration factor and the time-consuming manipulations involved
- in fact, passing 4 litres of water through the column takes a long time and using a high
flow-rate to expedite the process may decrease the analyte retention efficiency below the
minimum 90 - 95 % required to ensure quantitative results.
4 The Measurement Process in Chemistry
Fig.4.15. Principal goals of analytical separation techniques (ST). (1) Preconcentrating an
analyte (A). (2) Removal (clean-up) of interferents to isolate the analyte (A) from the other
sample components (2a) or separate the different components (considered to be the analytes)
from one another (2b)
Box 4.9
A flame atomic absorption spectrometer (FAAS) is available to determine lead in water from
wells near a mining area. The limit of detection achieved by direct insertion of aqueous
standards of Pb 2 + into the instrument is 1.0 mg/L (i. e. in the parts-per-million range). However, when real water samples are inserted, no signal is obtained; this suggests that their lead
contents are below the limit of the detection .The sensitivity of an FAAS is thus inadequate for
the purpose as the legal threshold for lead in water for animal or human consumption is
0.5 ~lg/L (i. e. in the parts-per -billion range). One must therefore increase the sensitivity of the
available technique by preconcentrating the lead.
To this end, a volume of 4 litres of water is passed through a chelating column (Chelex100)' which selectively retains Pb 2 +. Subsequently, the column is eluted with 2 mL of 0.1 N
HN0 3 and the eluate is inserted into the instrument. In this way, a preconcentration factor of
4000: 2 = 2000 is achieved. The new lim it of detection is thus 2000 times lower, i.e. it has
been reduced to 0.5 ~lg/L, which, coincidentally, is the same as the legal limit. Based on the
underlying criterion of the analytical problem, if a preconcentrated real sample now provides
a signal, then the water can be assumed to be contaminated with lead.
One could also have used a more sensitive technique [e.g. anodic stripping voltammetry
(ASV), electrothermal-atomization atomic absorption spectrometry (ETAAS)] to avoid the
need for such a high concentration factor and the time-consuming manipulations involved
- in fact, passing 4 litres of water through the column takes a long time and using a high
flow-rate to expedite the process may decrease the analyte retention efficiency below the
minimum 90 - 95 % required to ensure quantitative results.