A student analyzed a sample of tap water for the presence of C. To ensure accura

Question

A student analyzed a sample of tap water for the presence of C. To ensure accuracy and precision a reagent blank, sample duplicate and sample spike were analyzed To the sample spike 0.05 ug of Cl were added. The results are in the table below. 2. Sample Tap water Tapwater Duplicate Tapwater Spike Blank Cl (ug) 0.12 0.16 0.20 0.00 What is the purpose of the blank, duplicate and spike. What do the results of each of them tell the chemist about the method? a. b. What is the % recovery? C. What is the % deviation?

Explanation / Answer

Question 2 a- A blank sample is used to calibrate the instrument, and to trace sources of artificially introduced contamination. A duplicate sample to proves that the results can be repeated within established limits. A spike sample is a sample to which known concentrations of specific analytes have been added in such a manner as to minimize the change in the matrix of the original sample.

Results of each of them tell the chemist if the instrument is properly calibrated, and if the measurements made are fairly accurate and replicable.  

Question 2 b- Usually some analyte is lost during analysis. Percent recovery is the amount in percentage recovered without loss. It can be calcualted with help of a spike sample, with following formula-

(spike value -unspike value )*100/( spike amount)

(0.20-0.12)*100/0.05 = 160%

Question 2 c- Percent deviation is the deviation from average value of multiple sets of observations

Average value for sample and duplicate sample analysis = (0.12+0.16)/2 = 0.14

Deviation = (0.02+0,02)/2 = 0.02  

Percent deviation = 0.02/0.14x100 = 14.28

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