USP Chapters <232>1 and <233>,2 dealing with the assay of metal content of foods and drugs with inductively coupled plasma (ICP), were issued in December 2014 and formally adopted on January 1, 2018, replacing Chapter <231>. Chapter <232> “Elemental Impurities—Limits” focuses on preanalytics, while <233> “Elemental Impurities—Procedures” provides guidance on the analytics, including optical and mass spectrometric detection. Combined, they are a formidable list of requirements that should be considered by any stakeholder. Even if you decide to use a commercial laboratory to perform the assay, you need assurance that the assay provider can and will comply with <233>. Noncompliance may lead to nonacceptance.
However, a more interesting option is to choose to run the assays in your laboratory. Now, you need to plan and implement the plan. A recent paper describes the analytics with a special emphasis on operation and maintenance of ICP instruments.3
For preanalytics, I’m impressed with a concise guide provided by CEM Corporation— “Complete Sample Preparation Guide for Analysis of Elemental Impurities.”4 This guide leads the analyst though the many considerations involved in preparing samples for the assay of metals in a variety of matrices. The particular focus is on pharmaceuticals, including raw materials and impurity profiles. Matrices also include nutritional supplements, natural products, and possibly even cannabis.
Historically, Chapters<232> and<233> replace Chapter <231>, which used sulfide precipitation technology developed decades ago. The purpose of <231> was to demonstrate that metallic impurities were less than 10 ppm. In 2008, the USP recognized that this was inadequate for public safety and was obsolete technically.
Chapters<232> and<233> ran and survived a gauntlet of reviews. They became effective on January 1, 2018 in the U.S. These chapters are specification sheets, in contrast to method formats provided by ASTM. However, they contain valuable information that can be used to calculate sample quantities and working standards.
A laboratory considering comparing assays run in-house or outsourced to an external laboratory should carefully consider all the costs involved in running an ICP with optical or mass detection. If the decision is made to use an outside service, the prudent buyer will need assurance that the laboratory is compliant with the requirements of assays using ICP.
For example, Column 2 of Table 2 of Chapter<232> lists the maximum permitted daily exposure (PDE) for 15 listed metals in drug products in the U.S.A. It assumes a 50-kg person. These limits can be modified for large-volume parenterals (LVPs) such as intravenous solutions (Column 3). The ICH approach provided in ICH Q3D differs somewhat in the number of listed metals (24), PDE values, and design. Thus, one may need to choose between ICH and USP. The difference may seem small, but when talking with regulators, small differences can magnify quickly.
CEM’s guide focuses on the preanalytics and converts the specifications of Chapters<232> and <233> and ICH Q3D into specific topics. For example, ICPs need a clean, particle-free solution of standards and samples to be nebulized prior to entering the torch. The concentration of analytes should be less than 0.2%.
The section on sample preparation procedures lists five options: 1) neat; 2) direct injection of aqueous solution; 3) direct injection of organic solution; 4) indirect solution, usually by heating the sample in an appropriate solvent; and 5) closed-vessel digestion, which uses high temperature combined with concentrated mineral acid(s). The latter is most common and is also referred to as “microwave digestion.”
The guide compares the suitability of common acids, including HNO3, HCl, HF, etc., and also provides a general protocol suitable for many matrices, using 9 mL of HNO3 and 1 mL of HCl (aqua regia).
The suitability of the preanalytic method is a critical concern. One needs to bracket the assay to the measurement range immediately higher and lower then the PDE. This is covered in Section J of Chapter<233>. The value is called the J-Value, and is defined as the PDE concentration of the analyte of interest after sample preparation and dilution to the instrument’s working range.
The following excerpt is from page 25 of the CEM guide4:
So let’s take the determination of Pb by ICP-MS as an example. The PDE limit for Pb defined in Chapter <232> is 5 μg/day. Based on a suggested dosage of 10 g of the drug product/day, that’s equivalent to 0.5 μg/g Pb. The optimum dissolved solids content to analyze samples by ICP-MS is <0.2%. So if 1 g of sample is digested/dissolved and made up to 500 mL, that’s a 500-fold dilution, which is equivalent to 1 μg/L. So the J value for Pb in this example is equal to 1.0 μg/L. The method then suggests using a calibration made up of 2 standards: Standard 1= 1.5J, Standard 2= 0.5 J. So for Pb, that’s equivalent to 1.5 μg/L for Std 1 and 0.5 μg/L for Std 2
Similar calculations need to be made for each of the 24 elements listed in ICH Q3D or the 15 elements in USP<232>. Hopefully, some standards vendors will offer commercially prepared standards.
The specification limits calibration drift for a particular sample set to less than 20% for each element. This is calculated by repeating the first sample after the sample set has been finished.
The suitability of the assay protocol should be validated by appropriate studies showing: 1) detection behavior, including linear range; 2) detection and quantitation limits; 3) precision; 4) specificity, including interferences; 5) accuracy; and 6) ruggedness. These topics are amplified in the CEM guide. Table 5 (page 32) compares the PDE, J-Values, and J-Value/limit of quantitation. Similarly, Table 6 presents a comparison for ICP-MS.
CEM provides applications assistance for microwave digestion.5 For example, “Digestion of Difficult APIs and Gel Capsules in Accordance With USP <233>” reports on use of the CEM iPrep vessel for digestion of fish oil gelatin capsules.6 iPrep vessels facilitate microwave digestion using aqua regia. The dual-seal design reduces risk of sample loss. Sample temperature directly measures the sample temperature, not the block or tube.
It is important to note that neither the book nor the application notes describes the workflow in the degree of detail that is required for validated methods. Validated methods must provide a detailed description for the particular purpose, sample, sample prep protocol, assay instrument, data processing, and report preparation. Validation should be done in the specified laboratory by qualified staff. This is expensive, but needs to be considered in making the decision to run samples in-house or to outsource to a specialty service laboratory.
Should you decide to proceed to develop and support the assay in-house, you will probably want to work closely with relevant vendors. For more than 30 years, CEM has been helping its customers make microwave digestion work for them. For more information, visit http://www.cem.com.
References
- http://www.usp.org/sites/default/files/usp/document/our-work/chemicalmedicines/key-issues/c232-usp-39.pdf
- https://hmc.usp.org/sites/default/files/documents/HMC/GCs-Pdfs/c233.pdf
- https://www.labcompare.com/347319-Routine-Maintenance-in-ICP-MS-Critical-When-Monitoring-Elemental-Impurities-in-Pharmaceuticals/
- http://166.62.95.202/im/wpcontent/uploads/2017/08/CEM_USP_232_233_Guide_web.pdf
- http://cem.com/usp232
- http://cem.com/en/digestion-of-difficult-apis-and-gel-capsules-in-accordancewith-usp
Robert L. Stevenson, Ph.D., is Editor Emeritus, American Laboratory/Labcompare; e-mail: [email protected]