Analyte estimation using the generalized H‐point standard additions method and a new methodology for locating linear spectral intervals for unknown interferents

An improvement in the way to locate linear spectral intervals for unknown interferents which overlap the analyte signal in UV–visible spectrophotometry is offered as an alternative to the method previously described by Campíns‐Falcó et al. ( Anal. Chim. Acta, 302 , 323 (1995)). In that report, quotients of the second‐derivative spectra of the sample and analyte were used. The new method improves on the old one by using first‐ instead of second‐derivative data, thus eliminating the possible divisions by zero (inflection points for the analyte) obtained in the previous procedure. The linear intervals are now obtained by plotting for each wavelength the differences in first‐derivative data between the sample and an analyte reference solution against the corresponding values for the analyte reference solution. Second‐derivative data can be used to confirm the linearity supposition. If the absorbance of the unknown interferent can be considered constant in the selected wavelength interval, absorbance and first‐derivative data are used. Once linear and/or constant spectral intervals for the unknown interferent have been located, the generalized H‐point standard additions method (GHPSAM) is used to validate the chosen linear and/or constant intervals and to estimate the analyte concentration with absorbance data of the sample. The method has been used to determine phenol in a pharmaceutical product to check its validity and the results were compared with those obtained using HPLC determination. The relative error for the proposed method compared with the chromatographically obtained value is less than 7%, without the experimental work involved in HPLC. © 1998 John Wiley & Sons, Ltd.