233 ELEMENTAL IMPURITIES—PROCEDURES
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INTRODUCTION
This chapter describes two analytical procedures (Procedures 1 and 2) for the evaluation of the levels of the elemental impurities. The chapter also describes criteria for acceptable alternative procedures. 2S (USP38) By means of validation2S (USP38) studies, analysts will confirm that the analytical procedures described herein 2S (USP38) are suitable for use on specified material.
Use of Alternative Procedures
The chapter also describes criteria for acceptable alternative procedures. Alternative procedures that meet the validation requirements herein may be used in accordance with General Notices and Requirements 6.30, Alternative and Harmonized Methods and Procedures. Information on the Requirements for Alternate Procedure Validation is provided later in this chapter.2S (USP38)
Speciation
The determination of the oxidation state, organic complex, or combination is termed speciation. Analytical procedures for speciation are not included in this chapter, but examples may be found elsewhere in USP–NF and in the literature.
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COMPENDIAL PROCEDURES 1 AND 2
System standardization and suitability evaluation using applicable reference materials should be performed on the day of analysis.2S (USP38)
Procedure and Detection Technique
Procedure 1 can be used for elemental impurities generally amenable to detection by inductively coupled plasma–atomic (optical) emission spectroscopy (ICP–AES or ICP–OES). Procedure 2 can be used for elemental impurities generally amenable to detection by ICP–MS. Before initial use, the analyst should verify that the procedure is appropriate for the instrument and sample used (procedural verification) by meeting the alternative procedure validation requirements below.
Sample Preparation
Forms of sample preparation include Neat, Direct aqueous solution, Direct organic solution, and Indirect solution. The selection of the appropriate sample preparation depends on the material under test and is the responsibility of the analyst. When a sample preparation is not indicated in the monograph, an analyst may use any of the following appropriately validated2S (USP38) preparation procedures. In cases where spiking of a material under test is necessary to provide an acceptable signal intensity, the blank should be spiked with the same Target elements, and where possible, using the same spiking solution. Standard solutions may contain multiple Target elements. [Note—All liquid samples should be weighed. ]
Neat: Used for liquids or alternative procedures that allow the examination of unsolvated samples.
Direct aqueous solution: Used when the sample is soluble in an aqueous solvent.
Direct organic solution: Used where the sample is soluble in an organic solvent.
Indirect solution: Used when a material is not directly soluble in aqueous or organic solvents. Total metal extraction is the preferred sample preparation approach to obtain an indirect solution.2S (USP38) Digest the sample using the2S (USP38) Closed vessel digestion procedure provided below or one2S (USP38) similar to it.2S (USP38) The sample preparation scheme should yield sufficient sample to allow quantification of each element at the limit specified in the corresponding monograph or chapter.
Closed vessel digestion: This sample preparation procedure is designed for samples that must be digested in a Concentrated acid using a closed vessel digestion apparatus. Closed vessel digestion minimizes the loss of volatile impurities. The choice of a Concentrated acid depends on the sample matrix. The use of any of the Concentrated acids may be appropriate, but each introduces inherent safety risks. Therefore, appropriate safety precautions should be used at all times. [Note—Weights and volumes provided may be adjusted to meet the requirements of the digestion apparatus used. ]
An example procedure that has been shown to have broad applicability is the following. Dehydrate and predigest 0.5 g of primary sample in 5 mL of freshly prepared Concentrated acid. Allow to sit loosely covered for 30 min in a fume hood. Add an additional 10 mL of Concentrated acid, and digest, using a closed vessel technique, until digestion or extraction is complete. Repeat, if necessary, by adding an additional 5 mL of Concentrated acid. [Note—Where closed vessel digestion is necessary, follow the manufacturer’s recommended procedures to ensure safe use. ]
Alternatively, leachate extraction may be appropriate with justification following scientifically validated metal disposition studies, which may include animal studies, speciation, or other means of studying disposition of the specific metal in the drug product.2S (USP38)
Reagents: All reagents used for the preparation of sample and standard solutions should be free of elemental impurities, in accordance with Plasma Spectrochemistry 730.
Procedure 1: ICP– OES2S (USP38)
Standardization solution 1: 1.52S (USP38) J of the Target element(s) in a Matched matrix
Standardization solution 2: 0.5J of the Target element(s) in a Matched matrix
Sample stock solution: Proceed as directed in Sample Preparation above. Allow the sample to cool, if necessary. For mercury determination, add an appropriate stabilizer.
Sample solution: Dilute the Sample stock solution with an appropriate solvent to obtain a final concentration of the Target elements at NMT 1.52S (USP38) J.
Blank: Matched matrix
Elemental spectrometric system
(See Plasma Spectrochemistry 730.)
Mode: ICP
Detector: Optical detection system
Rinse: Diluent used
Standardization: Standardization solution 1, Standardization solution 2, and Blank
System suitability
Sample: Standardization solution 1
Suitability requirements
Drift: Compare results obtained from Standardization solution 1 before and after the analysis of the Sample solution.
Suitability criteria: NMT 20% for each Target element. [Note—If samples are high in mineral content, rinse system well 2S (USP38) before introducing the Sample in order to minimize carryover. ]
Analysis: Analyze according to the manufacturer's suggestions for program and wavelength. Calculate and report results on the basis of the original sample size. [Note—Appropriate measures must be taken to correct for matrix-induced interferences (e.g., wavelength overlaps). ]
Procedure 2: ICP–MS
Standardization solution 1: 1.52S (USP38) J of the Target element(s) in a Matched matrix
Standardization solution 2: 0.5J of the Target element(s) in a Matched matrix
Sample stock solution: Proceed as directed for Sample Preparation above. Allow the sample to cool, if necessary. For mercury determination, add an appropriate stabilizer.
Sample solution: Dilute the Sample stock solution with an appropriate solvent to obtain a final concentration of the Target elements at NMT 1.52S (USP38) J.
Blank: Matched matrix
Elemental spectrometric system
(See Plasma Spectrochemistry 730.)
Mode: ICP. [Note—An instrument with a cooled spray chamber is recommended. (A collision cell or reaction cell may also be beneficial.) ]
Detector: Mass spectrometer
Rinse: Diluent used
Standardization: Standardization solution 1, Standardization solution 2, and Blank
System suitability
Sample: Standardization solution 1
Suitability requirements
Drift: Compare results obtained from Standardization solution 1 before and after the analysis of the Sample solution.
Suitability criteria: Drift NMT 20% for each Target element. [Note—If samples are high in mineral content, rinse system well 2S (USP38) before introducing the Sample in order to minimize carryover. ]
Analysis: Analyze according to the manufacturer's suggestions for program and m/z. Calculate and report results based on the original sample size. [Note—Appropriate measures must be taken to correct for matrix-induced interferences (e.g., argon chloride interference with arsenic determinations). ]
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REQUIREMENTS FOR2S (USP38) ALTERNATE PROCEDURE VALIDATION
If the specified compendial procedures do not meet the needs of a specific application, an alternative procedure may be developed (see General Notices and Requirements 6.30, Alternative and Harmonized Methods and Procedures). Alternative procedures must be validated and shown to be acceptable, in accordance with the validation requirements for alternative procedures as described below.2S (USP38) The level of validation necessary to ensure that an alternative procedure is acceptable depends on whether a limit test or a quantitative determination is specified in the monograph.2S (USP38) The requirements for the validation of an elemental impurities procedure for each2S (USP38) type of determination are described below. Any alternative procedure that has been validated and meets the acceptance criteria that follow is considered to be suitable for use.2S (USP38)
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LIMIT PROCEDURES
The following section defines the validation parameters for the acceptability of alternative limit procedures. Meeting these requirements must be demonstrated experimentally using an appropriate system suitability procedure and reference material. 2S (USP38)
The suitability of the method must be determined by conducting studies with the material or mixture under test supplemented with known concentrations of each Target element of interest at the appropriate acceptance limit concentration. The material or mixture under test must be spiked before any sample preparation steps are performed.
Detectability
Standard solution: A preparation of reference materials for the Target element(s) at the Target concentration
Spiked sample solution 1: Prepare a solution of sample under test, spiked with appropriate reference materials for the Target elements at the Target concentration, solubilized or digested as described in Sample Preparation.
Spiked sample solution 2: Prepare a solution of the sample under test, spiked with appropriate reference materials at 80% of the Target concentration for the Target elements, solubilized or digested as described in Sample Preparation.
Unspiked sample solution: A sample of material under test, solubilized or digested in the same manner as the Sample solutions
Acceptance criteria
Non-instrumental procedures: Spiked sample solution 1 provides a signal or intensity equivalent to or greater than that of the Standard solution. Spiked sample solution 2 must provide a signal or intensity less than that of Spiked sample solution 1. [Note—The signal from each Spiked sample solution is NLT the Unspiked sample solution determination. ]
Instrumental procedures: The average value of the three replicate measurements of Spiked sample solution 1 is within ±15% of the average value obtained for the replicate measurements of the Standard solution. The average value of the replicate measurements of Spiked sample solution 2 must provide a signal intensity or value less than that of the Standard solution. [Note—Correct the values obtained for each of the spiked solutions using the Unspiked sample solution. ]
Precision for Instrumental Methods (Repeatability)
[Note—Non-instrumental precision is demonstrated by meeting the Detectability requirement above. ]
Sample solutions: Six independent samples of the material under test, spiked with appropriate reference materials for the Target elements at the Target concentration
Acceptance criteria
Relative standard deviation: NMT 20% for each Target element
Specificity
The procedure must be able to unequivocally assess (see Validation of Compendial Procedures 1225) each Target element in the presence of components that may be expected to be present, including other Target elements, and matrix components.
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QUANTITATIVE PROCEDURES
The following section defines the validation parameters for the acceptability of alternative quantitative procedures. Meeting these requirements must be demonstrated experimentally, using an appropriate system suitability procedure and reference materials. Meeting these requirements demonstrates that the procedure is equivalent to the compendial procedure for the purpose of quantifying the Target elements.
Accuracy
Standard solutions: Prepare solutions containing the Target elements at concentrations ranging from 50% to 150% of J, using appropriate reference materials.
Test samples: Prepare samples of the material under test spiked with appropriate reference materials before any sample preparation steps (digestion or solubilization) at concentrations ranging from 50% to 150% of J for each Target element.
Acceptance criteria
Spike recovery: 70%–150% for the mean of three replicate preparations at each concentration
Precision
repeatability
Test samples: Six independent samples of material under test (taken from the same lot) spiked with appropriate reference materials for the Target element(s) at the indicated level
Acceptance criteria
Relative standard deviation: NMT 20% (N = 6)2S (USP38) for each Target element
intermediate precision (2S (USP38) ruggedness )2S (USP38)
Perform the Repeatability analysis again either on a different day, with a different instrumentation, with a different analyst, or a combination thereof. Combine the results of this analysis with the Repeatability analysis so the total number of analyses is 12.2S (USP38)
Acceptance criteria
Relative standard deviation: NMT 25% (N = 12)2S (USP38) for each Target element
Specificity
The procedure must be able to unequivocally assess (see 1225) each Target element in the presence of components that may be expected to be present, including other Target elements, and matrix components.
Limit of Quantitation, Range, and Linearity
Demonstrated by meeting the Accuracy requirement.
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APPENDIX
Concentrated acid: Concentrated ultra-pure nitric, sulfuric, hydrochloric, or hydrofluoric acids or Aqua regia
Aqua regia: Aqua regia is a mixture of concentrated hydrochloric and nitric acids, typically at ratios of 3:1 or 4:1, respectively.
Matched matrix: Solutions having the same solvent composition as the Sample solution. In the case of an aqueous solution, Matched matrix would indicate that the same acids, acid concentrations, and mercury stabilizer are used in both preparations.
Target elements: Elements with the potential of being present in the material under test. Include arsenic (As), cadmium (Cd), lead (Pb), and mercury (Hg) in the target element evaluation when testing is done to demonstrate compliance. Target elements should also include any elements that may be added through material processing or storage. 2S (USP38)
Target limit or Target concentration: The acceptance value for the elemental impurity being evaluated. Exceeding the Target limit indicates that a material under test exceeds the acceptable value. The determination of compliance is addressed in other chapters. [Note—When applying this chapter to Elemental Impurities—Limits 232 and Elemental Contaminants in Dietary Supplements 2232, Target limits can be approximated by dividing the Daily Dose PDEs by the maximum daily dose for the Drug Product Analysis Option in 232 or the Daily Serving PDE divided by the maximum daily serving size in 2232. ]
J: The concentration (w/w) of the element(s) of interest at the Target limit, appropriately diluted to the working range of the instrument. For example, if the target elements are lead and arsenic for an analysis of an oral solid drug product with a daily dose of 10 g/day using inductively coupled plasma–mass spectrometry (ICP–MS), the target limit for these elements would be 0.5 µg/g and 1.52S (USP38) µg/g (see Table 2 in 232). However, in this case, the linear dynamic range of the ICP–MS is known to extend from 0.01 ng/mL to 0.1 µg/mL for these elements. Therefore, a dilution factor of at least 1:1002S (USP38) is required to ensure that the analysis occurs in the linear dynamic range of the instrument. J would thus equal 5 ng2S (USP38) and 15 ng2S (USP38) /mL for lead and arsenic, respectively, when the dilution factor is added.
Appropriate reference materials: Where Appropriate reference materials are specified in the chapter, certified reference materials (CRM) from a national metrology institute (NMI), or reference materials that are traceable to the CRM of an NMI should be used. An example of an NMI in the United States is the National Institute of Standards and Technology.
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