The internal standard diquat-d4 causes errors in diquat analysis by LC–MS/MS

Abstract

We report a more reliable method for determination of the herbicide diquat (DQ) in human biological samples using stable isotope dilution liquid chromatography–tandem mass spectrometry (LC–MS/MS). DQ-d8, a newly synthesized stable isotope was used as the internal standard (IS) for quantification of DQ instead of DQ-d4 typically used so far. The use of DQ-d8 could completely prevent errors like the misdetection of DQ and inaccurate quantification probably caused by using DQ-d4. Additionally, we developed a method for simultaneous determination of DQ and the similar herbicide paraquat (PQ) in urine, serum and whole blood by stable isotope dilution LC–MS/MS after solid-phase extraction. The stable isotopes DQ-d8 and PQ-d6 were synthesized and used as ISs. The DQ and PQ in the samples were extracted by solid-phase extraction using an EVOLUTE® WCX. The extracted DQ and PQ were separated and detected by LC–MSMS using a ZIC®-pHILIC column. The recovery rates ranged from 82.0 to 99.7%. Calibration curves showed good linearity in the 0.01–2 μg/mL. The intraday accuracy and intraday precision [% relative standard deviation (RSD)] ranged from 92.1 to 106% and from 0.4 to 4.7%, respectively. The interday precision (%RSD) ranged between 0.8 and 6.7%. The stable isotopes DQ-d8 and PQ-d6 allowed for accurate and precise quantification. Especially for analysis of DQ, the use of DQ-d8 could completely prevent errors caused by using DQ-d4. Therefore, the proposed method would be of great use in criminal investigations and emergency medicine.