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SEPARATIONS AND PURIFICATIONS
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other metals of toxicology concerns like mercury and cadmium. In addition, colorimetric assays are subjective (ie, analyst dependent) and the outcome can be infl uenced by the presence of colored ions such as copper or nickel resulting in false positive fi ndings. Instrumental methods such as atomic absorption and atomic emission spectroscopy based on fl ame or electrothermal atomization are superior to USP <231> colorimetric methods. However, these methods have given way to inductively coupled plasma (ICP) spectroscopy, which is reproducible, highly sensitive, and can perform multi-elemental analyses. Also, these instruments have become more accessible and aff ordable. In inductively coupled plasma optical emission spectrometry (ICP-OES), the sample solution is aspirated into plasma at elevated temperatures (~10 000 K). This results in rapid vaporization of sample aerosols, liberating free atoms in the gas phase. Collision of atoms within the plasma results in the conversion of atoms to ions and the subsequent promotion of ions to an excited state. Relaxation of excited atoms and ions results in the emission of characteristic wavelengths unique for each element resulting in their identifi cation. The number of photons emitted is directly proportional to the concentration of the element present in the sample enabling their quantitation. ICP-OES as a means of metal analysis has been used extensively in diff erent industries. It has been used for metal analysis in the environmental, food and agriculture, mining, and semiconductor industries, and is appropriate for residual elemental analysis in the pharmaceutical industry.3
To determine the residual metals levels present in API samples, a generic ICP-OES method was successfully developed, optimized, and validated for 9 elements commonly used in API synthesis: nickel, copper, zinc, iron, magnesium, tin, palladium, platinum, and ruthenium. Regulatory agencies require the validation of non-compendia methods used for the release testing of APIs used in clinical trials and commercial products to ensure the accuracy, precision, and reproducibility of the results. Method validation involves assessing specifi city, repeatability, linearity, limits of detection, and quantitation using an approved protocol with preset acceptance criteria to demonstrate the suitability of the method. With the generic method validation approach, attributes other than the sample specifi city and accuracy are validated once, and for subsequent compounds, the accuracy and specifi city are verifi ed. The use of a generic method as a platform technology requires minimal method validation for new compounds in the pipeline, eliminating the need for a complete revalidation of the method. This allows for considerable savings in eff ort and resources, especially for compounds in the early stages of development.4
While geared towards the pharmaceutical industry, the generic method described here is applicable to any industry requiring metal analysis using a pre-validated method.
Experimental
Chemical and Reagents All reagents used were ultra-pure and of a grade traceable per the National Institute of Standards and Technology unless otherwise stated.
38 | | January/February 2015
Ultra-pure hydrochloric acid was used in all sample preparations. The Milli-Q purifi ed water was used as diluent.
Instrumentation
A PerkinElmer Optima 3300 DV ICP-OES inductively coupled plasma spectrometer was used for this study. Table 1 shows the instrument detail condition and operating parameters.
Standard Preparation
Intermediate stock solutions of 100 μg/mL of nickel, copper, zinc, iron, magnesium, tin, palladium, platinum, and ruthenium were prepared from individual standard solutions, 1000 μg/mL. Working standard solutions ranging from 0.1 μg/mL to 10 μg/mL were prepared by serial dilution of 100 μg/mL intermediate stock solution. All standards were prepared in 2% hydrochloric acid. A check standard solution was prepared at 1 μg/mL to assess the method.
The ICP-OES was set up according to the instrument parameters shown in Table 1. The primary and secondary wavelengths characteristic of each element are shown in Table 2.
Operating Parameter Source power View height Pump rate Rinse time
Nitrogen purge
Plasma argon fl ow Auxiliary fl ow Nebulizer fl ow
Replicate readings Mode
Table 1. ICP-OES Instrument Operating Parameters Condition
1400 watts 15 mm
1.5 mL/min 60 seconds 4 L/min 15 L/min 0.5 L/min 0.8 L/min 3
Axial
Table 2. Analytes of Interest with Primary and Secondary Analytical Wavelengths for ICP-OES Analysis
Element Primary Wavelength (nm) Secondary Wavelength (nm) Ni
Cu Zn Fe
Mg Sn Pd Pt
Ru
231.604 324.752 206.200 238.204 285.213 235.485 340.458 265.945 240.272
232.003 327.393 213.857 239.562 279.077 283.998 363.470 214.423 349.894
Results and Discussion
The ICP-OES method was validated for 9 elements commonly used in the pharmaceutical industry (nickel, copper, zinc, iron, magnesium, tin, palladium, platinum, and ruthenium) following ICH
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