Step-By-Step Procedure to Identify Previously Unknown Compounds by LC-QTOF-MS Exemplified by an Intoxication With the...

In September 2019, a 22-year-old man with a history of drug abuse presented to the hospital with altered mental status. Due to a suspected drug overdose, a blood sample taken on admission and a urine sample collected 30 h thereafter were submitted to our laboratory to test for illegal drugs, pharmaceutical substances, and designer drugs. During the routine toxicological analysis of the serum sample, morphine and phenobarbital were identified by liquid chromatography-quadrupole-time-of-flight mass spectrometry (LC-QTOF-MS). Additionally, two compounds showing identical accurate masses and isotope ratios as the designer benzodiazepine diclazepam and the benzodiazepine lormetazepam were found. However, retention times differed significantly from the expected values, and the acquired MS/MS spectra did not match the library entries of the two compounds, indicating the presence of two previously unknown substances. After further investigation, SL-164 (5-chloro-3-(4-chloro-2-methylphenyl)-2-methyl-4(3H)-quinazolinone), a methaqualone analog, which has recently emerged on the research chemical market, and its hydroxy metabolite were tentatively identified by accurate mass, isotope matching, and plausible fragmentation. However, for unequivocal confirmation and quantification, a reference standard is required. As no reference material was available by the end of 2019, SL-164 was obtained from an online shop, and its identity and purity (97.8%) were confirmed by nuclear magnetic resonance spectroscopy. The subsequent quantitative analysis revealed a concentration of 390 ng/mL SL-164 in serum. In the urine sample, the parent compound was not detected, but three suspected monohydroxylated metabolites were found. This example shows that LC-QTOF-MS is a powerful approach for the (tentative) identification of unknown compounds in biological matrices.