Comparison with ICP-MS results.
Analytical results obtained with the UV–vis method have been validated by comparison with those acquired with a well-established
ICP-MS method from a second laboratory. Table II provides method validation results for three different samples. Comparing
the lead results obtained from the ICP-MS and from our UV–vis method, we can conclude that the UV–vis method is reliable and
Table II: Comparison of analytical results of lead from UV–vis and ICP-MS methods (μg/g).
Lead analysis results.
Using the UV–vis method described above, lead in several samples and at various levels has been successfully determined.
Each sample was analyzed in triplicate and the average was reported. Table III lists analysis results of lead in some agricultural,
food, and raw dietary supplement materials.
Table III: Lead analytical results of some common samples (n = 3).
In all the samples listed in Table III, 2.0 μg of standard lead were always spiked for quality assurance. The accepted recovery
result at our R&D department is set at 1.8–3.0 μg (90– 150% of the spiked lead). For all the samples, the percentage recoveries
always fall in the range of 90–120%, and none are out of the accepted range.
Based on the lead analytical and recovery test results, the procedure described previously is effective for all materials
included in the study. For most of the samples, the digestion time is <1 h. For some complicated dietary supplements, which
might either contain certain compounds with high molecular weights or a mixture of organic and minerals ingredients, the digestion
time may take as long as 2 h. In comparison, to digest this type of sample could take as long as 8 h, or even a couple days,
if using the USP method (24).
The last two samples in Table III, hydrox 6% FD and olive extract, are samples that would be extremely difficult to digest
if using the USP method. For such samples, repeated treatment with H2SO4–HNO3 or H2SO4–H2O2 would dramatically shorten the digestion time, usually by 2 h. For salt samples of alkaline earth metals, such as dicalcium
phosphate, the USP digestion method does not work (24). Dicalcium phosphate contains ~29% calcium and 70% phosphate. If concentrated
H2SO4 is used for the digestion, then a heavy precipitation is formed at pH adjustment (step 7). Mere HNO3 can also dissolve the sample, but if not enough masking reagent is used, then the calcium phosphate also will precipitate
out when ammonium hydroxide is added to adjust the pH. In both cases, the precipitation makes it impossible to separate the
lead from the sample solution. After trying different approaches and various masking reagents, we found the best way to avoid
this problem was to:
- Use smaller amount of samples (e.g., 2 g instead of 5–10 g) (step 1)
- Add 30 mL of ammonium citrate as masking reagent instead of 5 mL (step 6)
- Stir the sample solution while slowly adding ammonium hydroxide to adjust the pH (step 7).
Following all other procedures as stated previously, trace amounts of lead in the salts of alkaline earth metals was successfully
determined (see Table I).