á233ñ ELEMENTAL IMPURITIES—PROCEDURES
INTRODUCTION
This chapter describes two analytical procedures (Procedures 1 and 2) for the evaluation of the levels of the elemental impuri-ties. The chapter also describes criteria for acceptable alternative procedures. By means of validation studies, analysts will confirm that the analytical procedures described herein 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 re-quirements herein may be used in accordance with General Notices and Requirements 6.30, Alternative and Harmonized Meth-ods and Procedures . Information on the Requirements for Alternate Procedure Validation  is provided later in this chapter.Speciation
The determination of the oxidation state, organic complex, or combination is termed speciation . Analytic
al procedures for spe-ciation are not included in this chapter, but examples may be found elsewhere in USP–NF  and in the literature.
PROCEDURES
• C OMPENDIAL  P ROCEDURES  1 AND  2
System standardization and suitability evaluation using applicable reference materials should be performed on the day of analysis.
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 ele-mental impurities generally amenable to detection by ICP–MS. Before initial use, the analyst should verify that the proce-
dure is appropriate for the instrument and sample used (procedural verification) by meeting the alternative procedure vali-dation requirements below.
Sample preparation:Forms of sample preparation include Neat , Direct aqueous solution , Direct organic solution , and Indi-
rect solution . The selection of the appropriate sample preparation depends on the material under test and is the responsibil-ity of the analyst. When a sample preparation is not indicated in the monograph, an analyst may use any of the following
appropriately validated 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 . [N OTE —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 . Digest th
e sample using the Closed vessel diges-tion  procedure provided below or one similar to it. 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 Concen-trated 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 appropri-ate, but each introduces inherent safety risks. Therefore, appropriate safety precautions should be used at all times.
[N OTE —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 . [N OTE —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.
Reagents:All reagents used for the preparation of sample and standard solutions should be free of elemental impurities,in accordance with Plasma Spectrochemistry á730ñ.
• P ROCEDURE  1: ICP–OES
Standardization solution 1:  1.5J  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 mer-cury 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 1.5J .
Blank:
Matched matrix
298 á233ñ Elemental Impurities—Procedures / Chemical Tests USP 40
Elemental spectrometric system
(See Plasma Spectrochemistry á730ñ.)
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. [N OTE—If samples are high in mineral content, rinse system well 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. [N OTE—Appropriate measures must be taken to correct for matrix-induced inter-ferences (e.g., wavelength overlaps).]
• P ROCEDURE2: ICP–MS
Standardization solution 1:  1.5J of the Target element(s) in a Matched matrix
validation verificationStandardization 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 1.5J.
Blank:Matched matrix
Elemental spectrometric system
(See Plasma Spectrochemistry á730ñ.)
Mode:ICP. [N OTE—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. [N OTE—If samples are high in mineral content, rinse sys-tem well 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. [N OTE—Appropriate measures must be taken to correct for matrix-induced interferences (e.g., argon chloride interference with arsenic determinations).]
REQUIREMENTS FOR ALTERNATE PROCEDURE VALIDATION
If the specified compendial procedures do not meet the needs of a specific application, an alternative procedure may be devel-oped (see General Notices and Requirements 6.30, Alternative and Harmonized Methods and Procedures). Alternative proce-dures must be validated and shown to be acceptable, in accordance with the validation requirements for alternative proce-dures as described below. 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. The requirements for the validation of an elemental impurities procedure for each 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.
LIMIT PROCEDURES
The following section defines the validation parameters for the acceptability of alternative limit procedures. Meeting these re-quirements must be demonstrated experimentally using an appropriate system suitability procedure and reference material. The suitability of the method must be determined by conducting studies with the material or mixture under test supplemen-ted with known concentration
s of each Target element of interest at the appropriate acceptance limit concentration. The ma-terial or mixture under test must be spiked before any sample preparation steps are performed.
• D ETECTABILITY
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
USP 40Chemical Tests / á233ñ Elemental Impurities—Procedures 299
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 solu-tion 1. [N OTE —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 . [N OTE —Correct the values obtained for each of the spiked solutions using the Unspiked sample solution .]
• P RECISION  FOR  I NSTRUMENTAL  M ETHODS  (R EPEATABILITY )
[N OTE —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
• S PECIFICITY
The procedure must be able to unequivocally assess (see Validation of Compendial Procedures á1225ñ) each Target element  in the presence of components that may be expected to be present, including other Target elements , and matrix compo-nents.
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 .
• A CCURACY
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
• P RECISION
Repeatability
Test samples:Six independent samples of material under test (taken from the same lot) spiked with appropriate refer-ence materials for the Target element(s) at the indicated level
Acceptance criteria
Relative standard deviation:NMT 20% (N  = 6) for each Target element
Intermediate precision (ruggedness)
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 anal-yses is 12.
Acceptance criteria
Relative standard deviation:NMT 25% (N = 12) for each Target element
• S PECIFICITY
The procedure must be able to unequivocally assess (see á1225ñ) each Target element  in the presence of components that may be expected to be present, including other Target elements , and matrix components.
• L IMIT  OF  Q UANTITATION , R ANGE , AND  L INEARITY
Demonstrated by meeting the Accuracy  requirement.
GLOSSARY
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, respective-ly.
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 prepara-tions.
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.
Target limit or Target concentration:The acceptance value for the elemental impurity being evaluated. Exceeding the Tar-get limit  indicates that a material under test exceeds the acceptable value. The determination of compliance is addressed in other chapters. [N OTE —When applying this chapter to Elemental Impurities—Limits á232ñ and
Elemental Contaminants in Diet-
300 á233ñ Elemental Impurities—Procedures / Chemical Tests USP 40
ary Supplements á2232ñ, Target limits can be approximated by dividing the Daily Dose PDEs by the maximum daily dose for the Drug Product Analysis Option in á232ñ or the Daily Serving PDE divided by the maximum daily serving size in á2232ñ.]
J:The concentration (w/w) of the element(s) of interest at the Target limit, appropriately diluted to the w
orking 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 10g/day using inductively coupled plasma–mass spectrometry (ICP–MS), the target limit for these elements would be 0.5 m g/g and 1.5 m g/g (see Table 2 in á232ñ). However, in this case, the linear dynamic range of the ICP–MS is known to extend from 0.01 ng/mL to 0.1 m g/mL for these elements. Therefore, a dilution factor of at least 1:100 is required to ensure that the analysis occurs in the linear dynamic range of the instrument. J would thus equal 5 ng and 15 ng/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 ma-terials (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.
á241ñ IRON
This limit test is provided to demonstrate that the content of iron, in either the ferric or the ferrous form, does not exceed the limit for iron specified in the individual monograph. The determination is made by concomitant visual comparison with a control prepared from a standard iron solution.
SPECIAL REAGENTS
Standard Iron Solution
Dissolve 863.4 mg of ferric ammonium sulfate [FeNH
4
(SO
4
)
2
·12H
2
O] in water, add 10 mL of 2N sulfuric acid, and dilute with water to 100.0 mL. Pipet 10 mL of this solutio
n into a 1000-mL volumetric flask, add 10 mL of 2N sulfuric acid, dilute with water to volume, and mix. This solution contains the equivalent of 0.01 mg (10 m g) of iron per mL.
Ammonium Thiocyanate Solution
Dissolve 30g of ammonium thiocyanate in water to make 100 mL.
STANDARD PREPARATION
Into a 50-mL color-comparison tube pipet 1 mL of Standard Iron Solution (10 m g of Fe), dilute with water to 45 mL, add 2 mL of hydrochloric acid, and mix.
TEST PREPARATION
Into a 50-mL color comparison tube place the solution prepared for the test as directed in the individual monograph and if necessary dilute with water to 45 mL; or, dissolve in water, and dilute with water to 45 mL the quantity, in g, of the substance to be tested, as calculated by the formula:
1.0/(1000L)
in which L is the Iron limit in percentage. Add 2 mL of hydrochloric acid, and mix.
PROCEDURE
To each of the tubes containing the Standard Preparation and the Test Preparation add 50 mg of ammonium peroxydisulfate crystals and 3 mL of Ammonium Thiocyanate Solution, and mix: the color of the solution from the Test Preparation is not darker than that of the solution from the
Standard Preparation.
USP 40Chemical Tests / á241ñ Iron 301

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