Sample Preparation for Quantitative Metal Impurity Testing - Pharmaceutical Technology

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Sample Preparation for Quantitative Metal Impurity Testing
The author examines sample-preparations methods used in inductively coupled plasma–optimal emission spectroscopy for four test metals.


Pharmaceutical Technology
Volume 35, pp. s31-s33

Results


Table I: Sample preparation methods using a test article of sodium chloride for testing for arsenic, cadmium, lead, and mercury with an inductively coupled plasma–optical emission spectrometer.
The results are summarized in Tables I and II. Evaluation of the sodium chloride results showed that the indirect solution preparation using closed-vessel microwave digestion gave the best recovery values of the four methods. All four elements met the USP proposed accuracy criterion. The worst of the sample-preparation methods was refluxing. Although sodium chloride is freely soluble in water, the direct aqueous solution method was not the most appropriate for the metals that were analyzed. The cadmium recovery was below 70%.


Table II: Sample preparation methods using a test article of polysorbate 80 for testing for arsenic, cadmium, lead, and mercury with an inductively coupled plasma–optical emission spectrometer.
The polysorbate 80 results showed that no one method gave acceptable recovery for all four metals. The direct aqueous solution was the best method, but had mercury recovery above 150%. The indirect solution preparation using closed-vessel microwave digestion was the second best method, but had mercury recovery slightly below 70%. Mercury recovery may be improved using a stabilizer. The worst of the sample preparation methods was refluxing.

Conclusion

These examples illustrate that solubility of the test article may not be the primary factor when choosing a sample-preparation method. Just because a test article is water soluble, does not mean that the direct aqueous solution method is the best choice. Validation studies need to be performed as described in the USP proposed General Chapter <233> to ensure the method is accurate, specific, precise, and rugged when used as a quantitative method (1).

Gayla Velez is director of analytical laboratory services, SGS Life Science Services, 616 Heathrow Drive, Lincolnshire, IL 60069, tel. 847.821.8900,
.

References

1. USP Proposed General Chapter <233> "Elemental Impurities— Procedures," Pharmacopeial Forum 37 (3), Rev. May 26, 2011, http://www.usppf.com/pf/pub/data/v373/CHA_IPR_373_c233.html#CHA_IPR_373_c23, accessed Aug. 22, 2011.

2. EPA, Method 3050B, "Acid Digestion of Sediments, Sludges, and Soils" (Washington, DC), http://www.epa.gov/osw/hazard/testmethods/sw846/pdfs/3050b.pdf, accessed Aug. 1, 2011.

3. USP 34–NF 29 "USP Purified Water Monograph Water for Pharmaceutical Purposes" (USP, Rockville, MD, 2010), p. 4598.

4. USP, Proposed General Chapter <232>"Elemental Impurities— Limits," Pharmacopeial Forum 37 (3), Rev. May 26, 2011, http://www.usppf.com/pf/pub/data/v373/CHA_IPR_373_c232.html, accessed Aug. 22, 2011.

5. USP 34–NF 29 "Reference Tables: Description and Solubility" (USP, Rockville, MD, 2010), pp. 1035 and 1040.


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