Presence of Diphenhydramine in Ultrapure Water and Influence on LC-MS/MS Analysis

Liquid chromatography-tandem mass spectrometry (LC-MS/MS) is a powerful analytical technique which enables highly specific determination of drugs even at the ng/L level, especially for hair samples from victims of drug-facilitated sexual assault. However, there are issues that coexist with such level of sensitivity. We experienced a false-positive result wherein a diphenhydramine (DPH, an OTC sleep aid) peak constantly appeared at trace levels even in the blank tests. In this study, the cause of this phenomenon was investigated to ensure the reliability of analysis. As a results, it has been revealed that the ultrapure water, prepared from tap water for making the mobile phases, contained DPH as an impurity at pg/L level. DPH in the mobile phase was probably concentrated at the top of the reverse-phase LC column by the gradient program, and then was eluted with increased percentage of organic solvent in the mobile phase. DPH in the ultrapure water is probably attributed to contamination of aquatic environment by pharmaceuticals and personal care products (PPCPs).Thus, in the high-sensitivity analysis of drugs, great attention should be paid to the fact that ultrapure water, used for sample preparation or mobile phases, can contain very small amounts of PPCPs.