Repeated lyophilization: A neo-method for urine-based reference materials preparation

In urine drug testing, a cut-off value is often imposed to determine whether the sample is negative or positive. A matrix containing a reference substance helps counteract the adverse effects of the urine matrix across different laboratories to improve the consistency of final results. However, as a biological matrix, urine is prone to corruption and other problems that make it difficult to use as a reference sample. In this study, morphine, nitrazepam, lorazepam, buprenorphine, zolpidem, midazolam, diazepam, and clozapine commonly used in clinical practice were selected as target analytes, and the preparation process was further optimized to repeated lyophilization, in order to obtain more effective, stable, and accurate urine matrix reference materials (mRMs). The appropriate urine density (1.010–1.017 kg/m³) for preparing lyophilized samples was investigated through density determination. Conducting repeated lyophilizations resulted in a denser powder with reduced susceptibility to collapse and improved the quality of lyophilized urine samples. Lyophilized urine mRMs could be stored at room temperature for one month or under refrigeration conditions (4 ℃) for six months.