Quantification of phosphatidylethanol combining dried blood spots sampling with Orbitrap-based LC-HRMS platform: Method validation and proficiency testing

Phosphatidylethanols (PEth) are phospholipids specifically synthetised by an enzymatic reaction in presence of ethanol. PEth is a mid-term direct biomarker for alcohol consumption or abstinence, leading to a growing interest for forensic toxicology purposes. Blood deposition onto a filter paper card as dried blood spots (DBS) allows PEth stabilisation which is otherwise easily degraded at room temperature. The goal of this study was to develop and validate a procedure for the quantification of PEth 16:0/18:1, the most abundant analogue, in DBS samples benefiting from the selectivity brought by Orbitrap-based high-resolution mass spectrometry (HRMS). Calibration samples and quality controls (QCs) are volumetrically spotted (10 μL) onto filter paper card (Whatman 903 protein saver) either using HemaXis DB10™ device, or using laboratory micropipette. The whole DBS is manually punched out and the PEth is then directly extracted using an in vial approach with 100 μL of PEth 16:0/18:1-D5 in methanol. Analyses are then performed by LC-HRMS in PRM mode using a QExactive Orbitrap platform. Validation was carried out on with respect to the Food and Drug Administration (FDA) criteria, using three different QC concentrations (39, 87 and 350 ng/mL) over three non-consecutive days. To assess the robustness of our method, an interlaboratory proficiency testing was carried out using 3 different commercial standards at 4 levels (50, 100, 200, and 300 ng/mL) of quality controls (QCs) provided by ACQ Science (Germany). The method was fully validated according to guidelines set forth by the FDA. Calibration curve was built using a weighting factor of 1/x. Trueness was measured between 100.1 and 103.6% for the tested concentrations. Repeatability and intermediate precision were found to be lower than 5%. Linearity was evaluated between the lower limit of quantification of 20 ng/mL and the upper limit of quantification of 2000 ng/mL with an R2 greater than 0.998. Recovery (RE) and matrix effect (ME) evaluated at 2 concentrations ranged from 76 to 99% and 8 to 10%, respectively. During the proficiency test, precision was evaluated below 8.0%, while the accuracy ranged from 93.8% to 105.9% and from 101 to 104.5% when compared with the expected concentration and the mean measured value of all the labs participating respectively. The validation results demonstrate that the use of Orbitrap-based PRM quantification present an interesting alternative over triple quadrupole platforms considered as the gold standard for LC-MS/MS quantitative approaches. Indeed, multiple reaction monitoring (MRM) and PRM provide comparable performance in terms of sensitivity, linearity, dynamic range, precision and repeatability. Moreover, the use of PRM provides access to the full fragmentation spectra without any preanalytical considerations. The proficiency testing results confirm the suitability of PRM mode showing great quantification performances. In addition, those results and especially the repeatability exhibit the precision of the volume control provided by the use of HemaXis™ device. This tool enables the standardisation of the DBS collection at the fingertip in a non-hospital environment either by minimally-trained medical staff or by the patient himself. A rapid, sensitive, and robust method was developed and fully validated for the detection and quantification of PEth 16:0/18:1 in DBS samples. The results of the validation process and the proficiency testing confirm that the use of DBS associated with HRMS PRM-based analyses does not hinder the analytical performances, while providing full information over the MS-MS spectra. The development of analytical tools enabling the use of DBS microsampling that can be performed in almost every environment opens new opportunities toward clinical and forensic toxicological applications.