Determining the analytical figures of merit from LC-MS/MS data
Johnson, Renee Michelle
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Synthetic drugs such as piperazines are among the most commonly abused drugs and are typically consumed by younger populations. Because of their popularity, developing optimized analytical strategies designed to improve detection and interpretation of synthetic piperazines is of interest to the forensic community. To improve the likelihood that a substance of interest is detected, careful evaluation into the mass spectrometry signal is required. However, with all analytical pursuits, there is a limit at which the substance cannot be detected with certainty; thus a threshold is commonly referred to as the limit of detection (LOD). Formally, the LOD is the minimum amount of analyte (concentration, mass, number of molecules, etc.) that can be detected at a known confidence level. The purpose of this research was to use common analytical methods to calculate the LOD and verify the results with previous work at the Boston University forensic toxicology laboratory. Data from the Liquid Chromatography-tandem Mass Spectrometer (LC-MS/MS) was previously generated and consisted of signal intensity information in the form of peak height and peak area, from titrations of eight synthetic piperazines that included: Benzylpiperazine (BZP), 1-(3-chlorophenyl)-piperazine (mCPP), 3-trifluoromethylphenylpiperazine monohydrochloride (TFMPP), methylbenzylpiperazine (MBZP), 1-(4-fluorobenzyl)-piperazine (FBZP), 2,3-dichlorophenylpiperazine (DCPP), para-fluorophenylpiperazine (pFPP) and para-methoxyphenylpiperazine (MeOPP). Generally, the LOD is determined by first evaluating the signal in the absence of analyte and determining the probability that signal, , crosses the signal threshold, . The signal threshold is based upon the false detection rate the laboratory can withstand for a given interpretation scheme. In instances where very small levels of false detections can be tolerated, a large is chosen. In other circumstances, where noise detection can adequately be interpreted, a low is chosen. In chromatography and radiography the typical one sided =0.003. The number of molecules for each analyte at each concentration (20 ng/mL, 50 ng/mL, 200 ng/mL, 500 ng/mL, 1000 ng/mL and 2000 ng/mL) was determined and used throughout this work. Peak area signals and ratios versus number of molecules for each analyte were used to, first, visually inspect the linearity of signal to analyte level. It was determined that using internal standards improved linearity, as expected; however, the data suggested that absolute signal intensity was sufficient to compute the LOD for these compounds. Generally accepted methods of calculating LOD were not used for this research as the signal from the blank was not detected most likely due to the sensitivity of the instrument used. This study used an extrapolation of the data and propagation of errors method to calculate the LOD as the signal from the blank was not needed. For all eight analytes, the LOD calculated was similar to the lowest concentration (20 ng/mL) used when validating this method. This research needs to be expanded on to include more concentration points and see the plateau effect at higher concentrations. This will provide information to analytical chemists when a blank signal is not available about how the LOD can be calculated with high confidence.