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

Materials and methods

To illustrate how important method selection is in obtaining accurate results, several examples are provided. An analysis of arsenic, cadmium, lead, and mercury was performed to support oral dosage products using the limits within the USP proposed General Chapter <232> Elemental Impurities—Limits (4). Testing was performed at 1.5 μg/g arsenic, 0.5 μg/g cadmium, 1.0 μg/g lead, and 1.5 μg/g mercury, corresponding to the 100% specification level. One unspiked test article and triplicate spiked test articles at 100% of the specification were prepared. Standards were made at 50%, 100%, and 150% of the specification.

To determine if the sample preparation method is acceptable, the USP proposed general chapter provides validation criteria for elemental impurity testing. Two sets of criteria are included: one for a limit test and the second for a quantitative procedure. The work performed in this study followed the quantitative-procedure criteria. Accuracy and specificity were performed, but repeatability and ruggedness were not included in this study. The acceptance criterion for accuracy per the USP proposed General Chapter <233> Elemental Impurities—Procedures is 70–150% recovery for the mean value at each concentration (1). This study was performed using spiked test articles prepared at the 100% concentration only.

Test articles used were reagent-grade sodium chloride (EMD Chemicals) and polysorbate 80 (Tween 80, Fisher Scientific). All concentrated acids were trace-metal grade. Sodium chloride is freely soluble in water, and polysorbate 80 is very soluble in water (5). Sample-preparation methods used included direct aqueous solution, indirect solution using closed-vessel microwave digestion, refluxing, and an acid-digestion method under EPA 3050B. Instrumental analysis was performed using a ICP–OES spectrometer (Thermo Electron IRIS Intrepid II XDL ICP–OES) with a high-solids nebulizer. A mercury stabilizer was not added to the solutions because the ICP analysis was performed immediately after standard and sample-solution preparation.

Direct aqueous solution preparation method . 0.5 g of each test article were dissolved and diluted to 10.0 mL with 1% hydrochloric acid. Spiked test articles were prepared by adding 1.0 mL of spiking standard to a mixture of 0.5 g of the test article and 9.0mL of 1% hydrochloric acid. A blank and standards were prepared in 1% hydrochloric acid.

Indirect solution-preparation method . 0.5 g of each test article were transferred to a Teflon microwave vessel. 1.0 mL of USP purified water or spiking standard and 4 mL of nitric acid were added to the vessel. The solution was left to stand for 5 min before microwaving. The microwave program consisted of ramping the temperature to 120 C in 2 min, and the temperature was maintained at 120 C for 20 min. Once the Teflon vessels had cooled to room temperature (20–25 C), the contents were quantitatively transferred to 10-mL volumetric flasks with USP purified water and diluted to volume with USP purified water. A blank and standards were prepared in 1% nitric acid.

Refluxed solution-preparation method . 0.5 g of each test article were transferred to a 50-mL round-bottom flask. 5.0 mL of 2% nitric acid or a mixture of 1.0-mL spiking standard and 4.0 mL of 2% nitric acid were added to the appropriate flask and refluxed for 30 min using water-cooled condensers. Once the flasks had cooled to room temperature, the contents were quantitatively transferred to 10-mL volumetric flasks with USP purified water and diluted to volume with USP purified water. A blank and standards were prepared in 2% nitric acid.

EPA 3050B solution-preparation method . 0.5 g of each test article were transferred to a 150-mL glass beaker, and 0.4 mL of nitric acid and 2 mL of hydrochloric acid were added. 1.0 mL of spiking solution was added for spiked samples. The beaker was covered with a ribbed watch glass and refluxed at 95 C for 15 min on a hot plate. The samples were filtered through Whatman Grade No. 41 filter paper into a 10-mL volumetric flask and washed through the filter paper with 0.4-mL of hot hydrochloric acid and 2-mL of hot USP purified water. The filter paper was placed in the original beaker used for refluxing and 2 mL of hydrochloric acid were added. The beakers were heated to 95 C on a hot plate until the filter paper dissolved. The resulting solution was quantitatively transferred through fresh Whatman Grade No. 41 filter paper into the 10-mL volumetric flask. All flasks were diluted to volume with USP purified water. A blank and standards were prepared in 2% nitric acid.

All solutions were analyzed by ICP-OES for arsenic, cadmium, lead, and mercury.


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