Enhancing sensitivity and precision on NIR reflectance determination of an API at low concentration: Application to...

The use of a mixed calibration sample set (intact production tablets and powdered doped samples used to enlarge calibration range) is a usual procedure for the NIR reflectance determination of the API content of a pharmaceutical solid preparation. However, the high difference in scattering properties and the intrinsic low sensitivity of NIR make difficult the achievement of a good precision when API is at a low mass proportion (≈1%, w/w). The compression of the calibration powdered samples has been studied as a very simple procedure to enhance the sensitivity of NIR reflectance measurements and, consequently, to improve precision. Different pretreatments (SNV, 1D, 2D and their combinations) have been applied to reduce the spectral difference between powdered and compressed samples. Although none eliminates completely this difference, the combined pretreatment SNV+2D has proved to be the one with a better performance. Results obtained by using both calibration sample sets (powdered and compacted) in the quantification of estradiol valerate (VE, 2mg/tablet, ≈1.6%, w/w) and medroxyprogesterone (MPA, 10mg, ≈8%, w/w) in intact tablets of the hormonal preparation show that a slight but significant improvement in precision is obtained when using compacted samples for calibration. A HPLC procedure was developed to be used as reference method.