Step 6.
Add 10 mL of DI water to the sample beaker and stir to dissolve any water-soluble salt. Add 5 mL of 30% ammonium citrate
solution, 2 mL of 20% hydroxylamine hydrochloride solution, and mix well. (Note: For alkaline earth-salt samples, do not add
the 10 mL of DI water, instead add 20 mL of 30% ammonium citrate and 2 mL of 20% hydroxylamine hydrochloride solution, and
mix well.)
Step 7.
Add two drops of 0.1% phenol red in ethanol. Carefully adjust the pH with ammonium hydroxide until the solution changes colors
from pink to yellow then back to pink again. Cool the solution to room temperature. Completely transfer the sample to a 125-mL
separatory funnel for liquid–liquid extraction of the lead.
Step 8.
Into the separatory funnel containing the sample solution, add 2 mL of 5% potassium cyanide solution and 5 mL of 0.003% dithizone
extraction solution. Shake the separatory funnel for ~30 s, carefully releasing pressure from time to time. Drain the bottom
dithizone phase to a 60-mL separatory funnel containing 20 mL of 1% HCl, which is used for back extraction of lead.
Repeat the extraction three more times. Collect all the dithizone phases in the 60-mL separatory funnel. The original aqueous
sample solution should be collected in a specific container for waste treatment.
Step 9.
Shake the 60-mL separator funnel for 45 s to back-extract lead to the acidic aqueous phase. Drain the dithizone phase into
a beaker containing 20 mL of DI water to recycle the dithizone solution.
Step 10.
In the 60-mL separatory funnel, add 4 mL of 2% ammonium cyanide and 5 mL of 0.001% standard dithizone solution. Shake the
separatory funnel for 45 s. Drain the dithizone solution into a 10-mL of centrifuge tube for the spectrophotometric determination
of lead.
Step 11.
To construct the standard calibration curve, extract lead standard solutions containing 0.0, 1.0, 5.0, 10.0, and 25 μg of
lead in the same way as described in steps 6–10. Using chloroform as a reference, measure the absorbances of the solutions
at 520 nm. Create a calibration curve from the measured results. The R
2
value should be .0.990.
Step 12.
Measure the absorbance of the samples at 520 nm. Calculate the lead concentration based on the standard calibration curve
obtained at Step 11.
For quality assurance, we performed triplicate analyses for all samples.
Results and discussion
Method validation.
The method was validated using two means: standard addition or method recovery and comparison with an accepted ICP-MS method.
Table I: Lead recovery test results (n 5 3).
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Method recovery results.
Two samples with nondetectable lead were specifically chosen for lead recovery testing. These samples were spiked with 0.5
–3.0 μg of lead, and triplicate analyses were performed in the same manner as for the regular samples. Table I lists the lead
recovery test results. In these tests, 10 g of dried hotdog bun powder and 2 g of green tea extract were used.
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