Sensitive and stable pre-calibrated solid-phase extraction columns for environmental and forensic quantification using isotope dilution mass spectrometry

Abstract

A method was developed to pre-load solid-phase extraction columns (SPE) with isotopic calibrant for use with isotope dilution mass spectrometry (IDMS). The pre-calibration method was developed, optimized, and validated for the quantification of the pesticide glyphosate in drinking water using anion-exchange SPE, electrospray-ionization, and time-of-flight mass spectrometry (ESI-TOF-MS). The instrumental method obtained a mass-accuracy of 3 ppm and a limit of quantification (LOQ) of 0.3 ng mL⁻¹. Quantification of glyphosate by IDMS significantly improved quantitative error and LOQ compared with the calibration curve. The pre-loading methodology was optimized for stability over time and validated in drinking water, exhibiting an accuracy of 1.25% ± 0.87% error with no significant difference from certified concentrations or traditional SPE. Method LOQ was 0.4 ng mL⁻¹. Quantifying glyphosate in spiked drinking water sample produced high accuracy up to two-weeks after pre-loading columns, with an accuracy of 6.41% ± 7.10% error. A potential forensic application was investigated by adapting the pre-loading method to the quantification of seven drugs in synthetic urine using a mixed-mode SPE column. The ESI-TOF-MS method using traditional SPE produced accurate quantification of all seven drugs in synthetic urine with a mean error of 4.16% ± 3.07% and LOQ of 0.780 ng mL⁻¹. The pre-loading method produced accurate quantification with 5.36% ± 4.73% error, with no significant difference from traditional SPE or certified standards. Five of the seven drugs were quantified at high accuracy one week after pre-loading, with 5.40% ± 4.57% error from certified values. This method may be applicable to analysts seeking to develop methods to improve the transfer of high-accuracy and precision methods between laboratories.