A paper by Yan, et. al. [Yan, X.-P.; Sperling, M.; Welz, B. Anal. Chem. 1999, 71, 4216-4222] describes a newly developed analytical method for the analysis of biological and environmental samples containing Pb2+ in the presence of a variety of interfering ions.
(a) In their procedure the authors use a stock solution containing 200 mg Pb2+/L. Express this concentration in ppm (parts per million), %w/v, and molarity (M).
(b) When the authors analyzed their solution of 200 mg Pb2+/L under conditions where there were no interferences, they obtained a mean result of 197.25 mg Pb2+/L and a standard deviation of 3.99. Assuming that these are good predictors of μ and σ, what percentage of all trials with this sample should give results between 190 and 210 mg Pb2+/L?
(c) When Fe3+ is present at a concentration of 10 g/L it interferes in the analysis, producing an error of -19% when analyzing a solution of 200 mg Pb2+/L. Using this information, determine the value for the selectivity coefficient, KPb,Zn. Is the method more selective for Pb2+ or Fe3+? Clearly explain your reasoning.
(d) To prevent interferences the authors developed a procedure for removing potential interferents prior to analyzing samples. The following results were obtained for several replicate analyses of a sample before and after removing interferences.
| μg Pb2+/L before removing interferent | μg Pb2+/L after removing interferent |
| 176 | 200 |
| 156 | 198 |
| 162 | 200 |
| 154 | 194 |
| 160 | 196 |
Using the correct significance test, show that there is a significant difference between results obtain before removing the interferent and the results obtained after removing the interferent. Use a significance level of α = 0.01 for any t-test, and a = 0.05 for any F-test.
(e) The authors used their method to analyze a variety of samples including a lobster. When analyzing a sample containing no Pb2+ a signal of 0.00096 was recorded (units are arbitrary). Analyzing a standard sample of 200.0 mg Pb2+/L gave a signal of 0.1070. A 0.1038-g sample of lobster tissue was mixed with 5.00 mL of 65% w/v HNO3 and analyzed giving a signal of 0.1165. How many mg of Pb2+ are in the tissue sample?
(f) When the authors analyzed a standard blood sample known to contain 0.394 mg Pb2+/L they obtained a mean value of 0.42 mg Pb2+/L with a standard deviation of 0.03 for five trials. Is there any evidence at α = 0.05 for a systematic error in the analysis?
