Quantitative measurement of crystalline impurity in a pharmaceutical tablet by X-ray powder diffraction and method validation

A powder X-ray diffraction method was developed and validated to measure the crystalline impurity 4-(5-cyclopentyloxy-carbonylamino-1-methyl-indol-3-ylmethyl)-3-methoxy-N-o-tolylsulfonylbenzamide hydrate in a pharmaceutical tablet ranging from 0.6 to 3% (w/w). The calibration plot was found to be linear with a correlation coefficient (r2) of 0.996, and was reproducible among operators. The detection limit was determined to be 0.6% with a signal-to-noise ratio of 3:1. The quantitation limit was determined to be 1% with a signal-to-noise ratio of 5:1. Instrument precision at the quantitation limit was 5.8%. Method precision was 6.1% at the quantitation limit and 7.4% at the detection limit. Intermediate precision at the quantitation limit was 7.3% during a 6-month study. Accuracy measurements using crystalline impurity standards prepared in an excipient mixture ranged from 89.3 to 105.5%. Accuracy measurements using tablets containing spiked quantities of crystalline impurity ranged from 72.0 to 92.7%. Accuracy measurements using spiked tablets were complicated because the crystalline impurity was lost during the manufacturing process and a correction factor was used. Ruggedness was assessed by evaluating repetitive assay, repetitive packing, sample packing, and sample stability. Repetitive assays show the exposure of standards to a relative humidity in excess of 57% caused displacement error because of an increase in sample volume and a peak-position shift. Repetitive-packing studies show the analyte was extracted from the sample at a low relative humidity because of a static-charge induction. Sample-packing studies show that two subjective packing techniques were equivalent, and that under- and over-packing samples cause changes in sample density which would not affect results within ±16%. Sample-stability studies show that the quantitation-limit standard was stable as long as the sample was exposed to a relative humidity below 57%.

Genetic Regression as a Calibration Technique for Solid-Phase Extraction of Dithizone-Metal Chelates

The application of genetic regression (GR) to reflectance spectra of a solid-phase colorimetric extraction for the determination of Hg(II) is demonstrated. GR is a technique that combines wavelengths that optimize linear regression using a genetic algorithm. Solid polystyrene (PS) beads with a molecular weight distribution of 125,000–250,000 were impregnated with Hg(II)-dithizonate, filtered, allowed to dry, and packed in quartz cuvettes, and their reflectance spectra were collected. Quantitation with the use of normal calibration methods could not be achieved because of baseline fluctuations which originate in sample packing, presentation to the spectrometer, and particle size. Results indicate that GR can easily compensate for these baseline fluctuations and provide excellent calibrations.