The concentration of the control material falls between the 1 and 2 ppm standards, while the concentration of the patient sample falls between that of the 2 and 3 ppm standards. Therefore, you decide to bracket the sample and control between the appropriate standards. You make a set of measurements for the sample, using the 2.000 µg/mL and 3.000 µg/mL Ca standards and a different set of bracketing measurements for the control (SRM 909b) using the 1.000 µg/mL and 2.000 µg/mL Ca standards. You also measure the reagent and sample blanks as shown below, in the order starting with set 1, completing all samples, and then continuing on through set 5.

Sample ID
Set 1
Set 2
Set 3
Set 4
Set 5
Reagent blank:
5
7
11
13
17
Sample blank:
0
1
4
7
11
1.000 µg/mL Ca standard:
241
239
238
251
247
SRM 909b:
439
451
448
445
439
2.000 µg/mL Ca standard:
441
439
443
421
450
Patient samples:
530
529
524
520
524
3.000 µg/mL Ca standard:
620
621
614
619
610

Now use the method of bracketing to determine the concentration of SRM 909b and the patient sample. What are your results? Explain why you should use the method of bracketing rather than a calibration curve based on the average of all sample sets. Do the results of your calculations agree with the certified value for SRM 909b? Can you do a statistical test to verify your conclusion? Does the method meet the accuracy and precision requirements of 2% designated at the beginning of this question? Does the patient need further testing?

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