Page 68 - Modern Analytical Chemistry

1400-CH03 9/8/99 3:51 PM Page 51

Chapter 3 The Language of Analytical Chemistry 51

3J PROBLEMS

1. When working with a solid sample, it often is necessary to 4. A sample was analyzed to determine the concentration of an
bring the analyte into solution by dissolving the sample in a analyte. Under the conditions of the analysis, the sensitivity is
–1
suitable solvent. Any solid impurities that remain are 17.2 ppm . What is the analyte’s concentration if S meas is 35.2
removed by filtration before continuing with the analysis. and S reag is 0.6?
In a typical total analysis method, the procedure might 2+
5. A method for the analysis of Ca in water suffers from an
read interference in the presence of Zn . When the concentration
2+
2+
2+
After dissolving the sample in a beaker, remove any of Ca is 50 times greater than that of Zn , an analysis for
2+
solid impurities by passing the solution containing Ca gives a relative error of –2.0%. What is the value of the
the analyte through filter paper, collecting the selectivity coefficient for this method?
solution in a clean Erlenmeyer flask. Rinse the beaker 6. The quantitative analysis for reduced glutathione in blood is
with several small portions of solvent, passing these complicated by the presence of many potential interferents.
rinsings through the filter paper, and collecting them In one study, when analyzing a solution of 10-ppb
in the same Erlenmeyer flask. Finally, rinse the filter glutathione and 1.5-ppb ascorbic acid, the signal was 5.43
paper with several portions of solvent, collecting the times greater than that obtained for the analysis of 10-ppb
rinsings in the same Erlenmeyer flask. glutathione. 12 What is the selectivity coefficient for this
analysis? The same study found that when analyzing a
For a typical concentration method, however, the procedure
solution of 350-ppb methionine and 10-ppb glutathione the
might state
signal was 0.906 times less than that obtained for the analysis
After dissolving the sample in a beaker, remove of 10 ppb-glutathione. What is the selectivity coefficient for
any solid impurities by filtering a portion of the this analysis? In what way do these interferents behave
solution containing the analyte. Collect and discard differently?
the first several milliliters of solution before collecting
7. Oungpipat and Alexander described a new method for
a sample of approximately 5 mL for further analysis.
determining the concentration of glycolic acid (GA)
Explain why these two procedures are different. in a variety of samples, including physiological fluids
13
2. A certain concentration method works best when the analyte’s such as urine. In the presence of only GA, the signal
concentration is approximately 10 ppb. is given as
a. If the sampling volume for the method is 0.5 mL, about
S samp,1 = k GA C GA
what mass of analyte is being measured?
and in the presence of both glycolic acid and ascorbic acid
b. If the analyte is present at 10% w/v, how would you
(AA), the signal is
prepare the sample for analysis?
c. Repeat for the case in which the analyte is present at S samp,2 = k GA C GA + k AA C AA
10% w/w.
–4
When the concentration of glycolic acid is 1.0 ´10 M and
d. Based on your results, comment on the suitability of this
–5
the concentration of ascorbic acid is 1.0 ´10 M, the ratio of
method for the analysis of a major analyte.
the two signals was found to be
3. An analyst needs to evaluate the potential effect of an
interferent, I, on the quantitative analysis for an analyte, A. She S samp,2
=.
144
begins by measuring the signal for a sample in which the
S samp,1
interferent is absent and the analyte is present with a
a. Using the ratio of the two signals, determine the value of
concentration of 15 ppm, obtaining an average signal of 23.3
the selectivity ratio
(arbitrary units). When analyzing a sample in which the
analyte is absent and the interferent is present with a k AA
concentration of 25 ppm, she obtains an average signal of 13.7. K GA AA =
,
k GA
a. What is the analyte’s sensitivity?
b. What is the interferent’s sensitivity? b. Is the method more selective toward glycolic acid or
c. What is the value of the selectivity coefficient? ascorbic acid?
–5
d. Is the method more selective for the analyte or the c. If the concentration of ascorbic acid is 1.0 ´10 M,
interferent? what is the smallest concentration of glycolic acid that
e. What is the maximum concentration of interferent relative can be determined such that the error introduced by
to that of the analyte (i.e., [interferent]/[analyte]), if the failing to account for the signal from ascorbic acid is less
error in the analysis is to be less than 1%? than 1%?