Question

1. Samples of drinking water were analyzed for lead contamination. Analysis of the drinking water showed a lead concentration of 12.54 ppb. Analysis of a sample of the same drinking water spiked with 20.00 ppb lead gave a concentration of 29.63 ppb. Calculate the spike recovery. Is the method sufficiently accurate?

2. A chemical technician wants to verify the precision of an analytical procedure used to determine citric acid concentration in orange juice. She obtains 10.00 mL of a particular orange juice and dilutes it to 100.00 mL with water. She then analyzes five aliquots of the solution with the following results:

Aliquot 1                12.33 mg/mL

Aliquot 2                12.59 mg/mL

Aliquot 3                11.89 mg/mL

Aliquot 4                12.11 mg/mL

Aliquot 5                11.71 mg/mL

What can you say about the precision based on these results? Should the method be certified?

3. Your company is interested in using a new, faster method to determine copper concentration in aqueous solutions. Before implementing the new method you must check it out to see if it is sufficiently accurate and precise. You also want to know how well the new method compares to the old method. To test the new method you obtain a lake water SRM that is certified to contain 0.0283 ± 0.0002 ppm copper and analyze it by both methods with the following results:

Copper concentration (ppm)
Old Method	New Method
0.0235	0.0233
0.0240	0.0231
0.0239	0.0233
0.0236	0.0232
0.0240	0.0231

What can you say about the new method? Is it as good as the old? Is it better? Do the two methods yield essentially the same results? Is it sufficiently accurate to warrant changing to the new method? Back up your responses with statistical calculations.

Answer 1

1) Unspiked lead concentration = 12.54 ppb

Spiked lead concentration = 20.00 ppb

The concentration of lead added to the spiked portion = 29.63 ppb

Sample of the same drinking water spiked with 20.00 ppb will give a concentration = Unspiked lead concentration + Spiked lead concentration

Sample of the same drinking water spiked with 20.00 ppb will give a conc = 12.54 ppb + 20.00 ppb

Sample of the same drinking water spiked with 20.00 ppb will give a conc = 32.54 ppb

Percent recovery = (spike result / expected result) x 100

29.63 ppb/( 32.54 ppb) = 91.05 %

The % recovery should be within 90% to 110%. So % recovery is within the range. The method is not sufficiently accurate, because volume of a sample of drinking water is not included.