Page 118 - Modern Analytical Chemistry

1400-CH04 9/8/99 3:55 PM Page 101

Chapter 4 Evaluating Analytical Data 101

Determine whether there is a significant difference in the 29. The following data were collected during a study of the
effectiveness of the two preservatives at a= 0.10. concentration of Zn in samples drawn from several locations
26. Using X-ray diffraction, Karstang and Kvalhein in Lake Erie (all concentrations in parts per million).
reported a new method for determining the weight 2+ 2+
27
percent of kalonite in complex clay minerals. To test the [Zn ] at the [Zn ] at the
air–water sediment–water
method, nine samples containing known amounts of
Location interface interface
kalonite were prepared and analyzed. The results (as
%w/w kalonite) are shown. 1 0.430 0.415
Actual: 5.0 10.0 20.0 40.0 50.0 60.0 80.0 90.0 95.0 2 0.266 0.238
Found: 6.8 11.7 19.8 40.5 53.6 61.7 78.9 91.7 94.7 3 0.567 0.390
4 0.531 0.410
Evaluate the accuracy of the method at a= 0.05.
5 0.707 0.605
27. Mizutani and colleagues reported the development of a new 6 0.716 0.609
28
method for the analysis of l-malate. As part of their study
they analyzed a series of beverages using both their method Determine whether there is a significant difference between
2+
and a standard spectrophotometric procedure based on a the concentration of Zn at the air–water interface and the
clinical kit purchased from Boerhinger Scientific. A summary sediment–water interface at a= 0.05.
follows of their results (in parts per million). 30. Ten laboratories were asked to determine the concentration
of an analyte A in three standard test samples. Following are
Sample Electrode Spectrophotometric
the results, in parts per million. 30
Apple juice 1 34.0 33.4
Laboratory Sample 1 Sample 2 Sample 3
Apple juice 2 22.6 28.4
Apple juice 3 29.7 29.5 1 22.6 13.6 16.0
Apple juice 4 24.9 24.8 2 23.0 14.2 15.9
Grape juice 1 17.8 18.3 3 21.5 13.9 16.3
Grape juice 2 14.8 15.4 4 21.9 13.9 16.9
Mixed fruit juice 1 8.6 8.5 5 21.3 13.5 16.7
Mixed fruit juice 2 31.4 31.9 6 22.1 13.5 17.4
White wine 1 10.8 11.5 7 23.1 13.9 17.5
White wine 2 17.3 17.6 8 21.7 13.5 16.8
White wine 3 15.7 15.4 9 22.2 12.9 17.2
White wine 4 18.4 18.3 10 21.7 13.8 16.7
Determine whether there is a significant difference between Determine if there are any potential outliers in Sample 1,
the methods at a= 0.05. Sample 2, or Sample 3 at a significance level of a= 0.05.
28. Alexiev and associates describe an improved photometric 31. When copper metal and powdered sulfur are placed in a
3+
method for the determination of Fe based on its crucible and ignited, the product is a sulfide with an empirical
catalytic effect on the oxidation of sulphanilic acid by formula of Cu x S. The value of x can be determined by
29
KIO 4 . As part of their study the concentration of weighing the Cu and S before ignition, and finding the mass
3+
Fe in human serum samples was determined by the of Cu x S when the reaction is complete. Following are the Cu/S
proposed method and the standard method. ratios from 62 such experiments.
Following are the results, with concentrations in 1.764 1.838 1.865 1.866 1.872 1.877 1.890 1.891 1.891
micromoles/L.
1.897 1.899 1.900 1.906 1.908 1.910 1.911 1.916 1.919
Sample Proposed Method Standard Method 1.920 1.922 1.927 1.931 1.935 1.936 1.936 1.937 1.939
1.939 1.940 1.941 1.941 1.942 1.943 1.948 1.953 1.955
1 8.25 8.06
1.957 1.957 1.957 1.959 1.962 1.963 1.963 1.963 1.966
2 9.75 8.84
1.968 1.969 1.973 1.975 1.976 1.977 1.981 1.981 1.988
3 9.75 8.36
1.993 1.993 1.995 1.995 1.995 2.017 2.029 2.042
4 9.75 8.73
5 10.75 13.13 (a) Calculate the mean and standard deviation for these data.
6 11.25 13.65 (b) Construct a histogram for this data set. From a visual
inspection of your histogram, do the data appear to be
7 13.88 13.85
normally distributed? (c) In a normally distributed
8 14.25 13.43
population, 68.26% of all members lie within the range
Determine whether there is a significant difference between m±1s. What percentage of the data lies within the range
–
the two methods at a= 0.05. X ±1s? Does this support your answer to the previous