Heavy metal exposure of a population has been a long-standing health concern. When this exposure occurs through prescribed medicine or dietary supplements, it is even more worrisome. Metals have been monitored for years; while some are toxic, others are necessary to biological functions. Even though some metals are used as catalysts to produce the medications, all nonessential metals should be eliminated from the final product; their presence may represent product quality issues. Whatever the source, the responsibility for testing all products lies with the manufacturer.
While many regulatory bodies exist, few have put forth methods on how the testing should be done. Most recently, one of the most prominent promulgators of methods, the USP (United States Pharmacopeia), has recognized the need for a new metals testing method that provides more accuracy and better specificity in product testing. “Although still widely accepted and used in the pharmaceutical industry, these methods based on the intensity of the color of sulfide precipitation are non-specific, insensitive, time-consuming, labor intensive, and more often than hoped, yield low recoveries or no recoveries at all.”1
Table 1 Elements included in new USP 232 guidelines
USP-proposed general chapter 232 is the intended replacement for the old USP 231. The range of active pharmaceutical ingredients for foods and dietary supplements will necessitate the inclusion of many elements, shown in Table 1.
Inductively coupled plasma-mass spectrometry (ICP-MS) would be an ideal choice for meeting these needs, with its ability to analyze most of the elements in the periodic table and a wide dynamic range, spanning parts-per-trillion to parts-per-million levels. This range means that trace contaminants as well as nutritionally significant elements can be measured during the same analysis.
This article describes a sample preparation procedure that minimizes contamination and loss of volatile elements. The digested samples were measured on a NexION® 300 ICP-MS (PerkinElmer, Shelton, CT), a new generation of ICP-MS with features that make it well suited for the analysis of pharmaceutical and dietary products.
Samples were prepared using microwave-assisted digestion in closed vessels. This allows a contamination-free environment, as well as a single preparation method for all elements of interest. The microwave system used was the Multiwave™ 3000 digestion system (Anton Paar, Ashland, VA). The high-pressure 100-mL (HF100) rotor and vessels are recommended.
Table 2 - Multiwave digestion program*
The sample types used in this study consisted of vitamin pills (purchased in a pharmacy and supplied by customers), NIST® food materials, and various raw materials used in the formulation of dietary supplements. A half gram of material was weighed into an HF100 Multiwave vessel. Five mL of conc. HNO3 was added, and the solution was swirled to ensure wetting and mixing. Finally, 2 mL of H2O2 was added slowly and allowed to react. The vessels were then sealed. The Multiwave program was run using the internal temperature probe, which allows precise control of the reaction and temperature. The digestion program is a modified vitamin program from the Multiwave library, as shown in Table 2.
Upon completion of the digestion, samples were brought to 50 mL final volume with deionized water. Samples were then diluted 1:10 with deionized water prior to analysis, and the following internal standards were added: Sc (40 ppb) and Ge, In, and Bi (20 ppb each). Quantitation was done against aqueous, external calibration curves.
Table 3 - NexION 300X instrumental conditions
Table 4 - Universal cell conditions
All analyses were performed on a NexION 300X ICP-MS, using both standard and kinetic energy discrimination (KED) modes. Extended dynamic range (EDR) was also utilized as part of the Universal Cell Technology to enable the simultaneous analysis of trace and major nutritional elements, such as calcium, potassium, and phosphorus, which are present at hundreds of ppm. There are only four metals required by the State of California Proposition 65: As, Hg, Cd, and Pb. These are analyzed at very low ppb levels in all materials.
The analysis of vitamins and nutritional supplements does not require ultralow detection limits; thus KED mode was chosen instead of dynamic reaction cell (DRC) mode as a simple universal correction technique. Table 3 shows the instrumental conditions used, and Table 4 displays the cell parameters. The default elemental corrections in the NexION software were used. Elements measured were those that are commonly analyzed by customers looking to conform to the new expanded USP requirements, although this list could increase in the future.
Table 5 - NIST 1548a Total Diet (units = μg/g)*
Table 6 - Raw materials for food supplement production (units = μg/g)
Table 7 - Digestion duplicates for vitamins (units = μg/g)
Results and discussion
Internal standards were used to compensate for differences in matrix composition between the variety of samples analyzed. Multiple isotopes were used for selected elements for verification during method development. Extended dynamic range was used for the high-level elements by adjusting the RPa values specific to that isotope, as shown in Table 4. EDR allows measurements up to hundreds of ppm if necessary.
The accuracy of the method was established by analyzing a certified reference material (NIST 1548a, Total Diet), as well as spike recoveries. Table 5 shows the results, which indicate that both the certified values and spike recoveries are adequate for all elements of interest. The high spike recoveries for the Hg isotopes were due to the fact that Au was not included in the wash solutions, which meant that Hg washout was not complete.
Once the method was validated, a batch analysis containing different pharmaceutical and nutritional supplement samples was performed; the results are shown in Table 6 (raw materials for food supplements) and Table 7 (vitamins, digested in duplicate).
Instrument stability was established by comparing sensitivity before and after a two-hour batch analysis, which included the SRM, different vitamins, and a variety of raw materials used for supplement production. The sensitivity did not change, indicating that electrical parameters within the instrument did not change, i.e., there was no sample deposition within the instrument. The NexION 300 ICP-MS was designed specifically to handle high-matrix samples.
This article demonstrates the ability of the NexION 300 ICP-MS to effectively measure a wide variety of trace elements in vitamin samples, thus conforming to the proposed USP criteria. By incorporating a triple cone interface, quadrupole ion deflector, extended dynamic range, and Universal Cell Technology, the NexION 300 ICP-MS is an effective tool to meet the needs of the new USP guidelines and extend the metal analysis capabilities of the modern laboratory to handle even more challenging analyses.
- Wang, T.; Wu, J.; Hartman, R.; Jia, X.; Egan, R.S. J. Pharm. Biomed. Anal. 2000, 23, 867–90.
The authors are with PerkinElmer, Inc., 710 Bridgeport Ave., Shelton, CT 06484, U.S.A.; tel.: 800-762-4000; fax: 203-944-4904; e-mail: firstname.lastname@example.org.