Increasing the linear dynamic range in LC-MS: is it valid to use a less abundant isotopologue?

Importance of Utilizing Natural Isotopologue Transitions in Expanding the Linear Dynamic Range of LC-MS/MS Assay for Small-Molecule Pharmacokinetic Sample Analysis - A mini-review

Liquid chromatography-tandem mass spectrometry (LC-MS/MS) is a widely used quantitative method in small-molecule pharmacokinetic sample analysis. The linear dynamic range of mass analyzers, typically spanning 3 orders of magnitude, is usually insufficient for this purpose. Utilization of multiple isotopologues has been proposed as a compelling approach to expand the linear dynamic range of LC-MS/MS assays, particularly when the detector is saturated. Isotopologues are a statistical mixture of molecules of the same compound but of different exact masses due to the presence of natural chemical isotopes. While the concept of isotopologues is widely recognized in large-molecule bioanalysis and small-molecule metabolite profiling, it has not been commonly implemented in small-molecule targeted quantification. To increase the awareness of the value of isotopologues in small-molecule LC-MS/MS analysis, this minireview provides the basis of isotopologue distribution in MS/MS and summarizes published studies as well as our own experience in utilizing multiple isotopologues to expand the linear dynamic range of small-molecule LC-MS/MS assays. Considering that utilizing natural isotopologue transitions in the LC-MS/MS assays represents an easy, straightforward, and robust way to expand the linear dynamic range, we believe this method deserves wide application in small-molecule pharmacokinetic sample analysis and can particularly benefit people working in pharmacokinetic labs as well as the GLP bioanalytical labs in pharmaceutical industry.

A Comprehensive Multi-Analyte Method for Hair Analysis: Substance-Specific Quantification Ranges and Tool for Task-Oriented Data Evaluation

The aim of the present study was to quantify a large number of analytes including opioids, stimulants, benzodiazepines, z-drugs, antidepressants and neuroleptics within a single sample workup followed by a single analytical measurement. Expected drug concentrations in hair are strongly substance dependent. Therefore, three different calibration ranges were implemented: 0.5 to 600 pg/mg (group 1), 10 to 12,000 pg/mg (group 2) and 50 to 60,000 pg/mg (group 3). In order to avoid saturation effects, different strategies were applied for selected transitions including the use of parent mass ions containing one or two 13C-isotopes and detuning of the declustering potential and/or collision energy. Drugs were extracted from pulverized hair by a two-step extraction protocol and measured by liquid chromatrography--tandem mass spectrometry (LC--MS-MS) using Scheduled MRM™ Algorithm Pro. In total, 275 MRM transitions including 43 deuterated standards were measured. The method has been fully validated according to international guidelines. A MultiQuant™ software based tool for task-oriented data evaluation was established, which allows extracting selected information from the measured data sets. The matrix effects and recoveries were within the allowed ranges for the majority of the analytes. The lower limits of quantification (LLOQs) were for ∼72% of the analytes in the low-pg/mg range (0.5-5 pg/mg) and for ∼24% of the analytes between 10 and 50 pg/mg. These LLOQs considered cut-offs by the Society of Hair Testing (SoHT), if recommended. The herein established multi-analyte approach meets the specific requirements of forensic hair testing and can be used for the rapid and robust measurement of a wide range of psychoactive substances. The analyte-specific wide concentration ranges open up a wide field of applications.

A quantitative UHPLC-MS/MS method for the growth hormone-releasing peptide-6 determination in complex biological matrices and transdermal formulations

Growth hormone-releasing peptide-6 (GHRP-6) is part of a group of small synthetic peptides with potent GH-releasing activity that have gained attention in the last two decades by virtue of their cyto- and cardioprotective effects. Despite numerous preclinical studies highlighting the potential cardiovascular benefits of GHRP-6, confirmation of clinical efficacy is still awaited. Recent advances in transdermal drug delivery systems have been made to address challenges related to the poor skin permeation rate of peptides by using pain-free microneedle (MN) devices. Accordingly, highly sensitive and validated analytical methods are required for the potential clinical translation of MN-based peptides. The ultra-performance liquid chromatography tandem mass spectrometry (UHPLC-MS/MS) methods developed in this study aimed to quantify GHRP-6 in biological matrices (plasma, skin) and dissolving polymeric MNs. UHPLC/MS-MS method detection limits of 0.1, 1.1, 0.9 and 1.5 ng/mL were achieved in neat solution, plasma, MN polymer solution, and skin matrices, respectively. Method validation also involved assessment of precision, accuracy, limits of quantification, linearity of matched calibration curves (R² > 0.990), extraction recovery, matrix effect, stability studies, selectivity, and carry-over effect. Additionally, quality control samples were analyzed at three concentration levels to determine recovery (85-109%) and accuracy/bias (3.2-14.7%). Intra- and inter-day precision were within the range of acceptance (RSDs of 3.0-13.9% and 0.4-14.5%, respectively). The validity and applicability of such methods were successfully demonstrated for transdermal GHRP-6 delivery using GHRP-6-loaded MN patches applied to pig skin.

Representing the Metabolome with High Fidelity: Range and Response as Quality Control Factors in LC-MS-Based Global Profiling

Liquid chromatography-mass spectrometry (LC-MS) is a powerful and widely used technique for measuring the abundance of chemical species in living systems. Its sensitivity, analytical specificity, and direct applicability to biofluids and tissue extracts impart great promise for the discovery and mechanistic characterization of biomarker panels for disease detection, health monitoring, patient stratification, and treatment personalization. Global metabolic profiling applications yield complex data sets consisting of multiple feature measurements for each chemical species observed. While this multiplicity can be useful in deriving enhanced analytical specificity and chemical identities from LC-MS data, data set inflation and quantitative imprecision among related features is problematic for statistical analyses and interpretation. This Perspective provides a critical evaluation of global profiling data fidelity with respect to measurement linearity and the quantitative response variation observed among components of the spectra. These elements of data quality are widely overlooked in untargeted metabolomics yet essential for the generation of data that accurately reflect the metabolome. Advanced feature filtering informed by linear range estimation and analyte response factor assessment is advocated as an attainable means of controlling LC-MS data quality in global profiling studies and exemplified herein at both the feature and data set level.

The challenge that chlorine presented during the development of a benzodiazepine assay

During the routine validation of a benzodiazepine method (performed on a Liquid Chromatography - Tandem Mass Spectrometer), it was noted that lorazepam, triazolam, and α-hydroxytriazolam showed a quadratic shift/bias in the calibration curve, particularly at high concentrations. The ultimate cause of this bias was determined to be due to the natural presence of chlorine (Cl) isotopes (³⁵Cl and ³⁷Cl) in these benzodiazepines. The presence of the heavy (³⁷Cl) isoforms of Cl resulted in the analyte's mass being the same as the internal standard which, in turn, caused the internal standard to appear "falsely increased", thus skewing the calibration curve. One solution to this potential issue was to take advantage of this natural phenomenon and use the Cl heavy isoforms of the respectively labeled internal standards.

Chromatographic separation strategies for precision mass spectrometry to study protein-protein interactions and protein phosphorylation

Investigating protein-protein interactions and protein phosphorylation can be of great significance when studying biological processes and human diseases at the molecular level. However, sample complexity, presence of low abundance proteins, and dynamic nature of the proteins often impede in achieving sufficient analytical depth in proteomics research. In this regard, chromatographic separation methodologies have played a vital role in the identification and quantification of proteins in complex sample mixtures. The combination of peptide and protein fractionation techniques with advanced high-performance mass spectrometry has allowed the researchers to successfully study the protein-protein interactions and protein phosphorylation. Several new fractionation strategies for large scale analysis of proteins and peptides have been developed to study protein-protein interactions and protein phosphorylation. These emerging chromatography methodologies have enabled the identification of several hundred protein complexes and even thousands of phosphorylation sites in a single study. In this review, we focus on current workflow strategies and chromatographic tools, highlighting their advantages and disadvantages, and examining their associated challenges and future potential.

A Validated Method for the Screening of 320 Forensically Significant Compounds in Blood by LC/QTOF, with Simultaneous Quantification of Selected Compounds

A broad drug screening method for toxicologically significant drugs and metabolites in whole blood using liquid chromatography time-of-flight mass spectrometry (LC/QTOF) was developed and comprehensively validated. The method qualitatively screens for 320 compounds while simultaneously quantifying 39. Compounds were extracted from the blood using alkaline liquid/liquid extraction and chromatographic separation was achieved in 12 min. The QTOF was operated using positive mode electrospray ionization using data dependent acquisition. Qualitative validation was performed for all 320 compounds, and included selectivity, recovery, limit of detection, matrix effects, carryover and extract stability. The limits of detection were in the low to sub ng/mL range for the majority of compounds. Full quantitative validation was performed for 39 compounds and accuracy and precision were within 15 and 18%, respectively. The qualitative data processing method uses an in-house retention time, accurate mass and MSMS spectral database, which can be easily updated with new compounds of interest as they emerge onto the market, without affecting method performance. The use of a non-targeted data acquisition method coupled with targeted data processing has proven to be a highly versatile, efficient and robust approach to screening, well suited to meet the needs of the modern toxicology laboratory involved in systematic toxicological analysis.

Development of a quantitative approach in blood plasma for low-dosed hallucinogens and opioids using LC-high resolution mass spectrometry

The WHO annually reports an increasing abuse of new psychoactive substances (NPS), which are a heterogeneous group of synthetic drugs and are consumed as substitute for controlled drugs of abuse. In this work, we focused on highly potent derivatives such those of phenethylamine (2C), N-2-methoxybenzyl phenethylamine (NBOMes), lysergic acid diethylamide (LSD), and fentanyl. Severe to fatal intoxications were described due to their high potency. Therefore, they have to be taken at very low doses resulting in low blood concentration in the low ng/mL range, which is a challenge for reliable analytical detection and quantification. The aim of this work was therefore to design a simple, robust, and fast method for simultaneous detection and quantification of multiple substances of the different classes in human blood plasma using liquid chromatography (LC) high resolution (HR) mass spectrometry (MS) with alternating HR full-scan (HRFS) MS and "All-ions fragmentation" (AIF) MS. The paper contains results of the method validation according to the EMA guideline, including intra-/interday accuracy and precision, matrix effects, storage and benchtop analyte stability as well as selectivity and carryover. All validation criteria were fulfilled for most tested compounds except for the NBOMe derivatives, one out of ten 2C-derivatives and butyryl fentanyl, which failed at accuracy and/or precision or at the acceptance criteria for matrix effect. Reasons for this are discussed and solutions presented. Despite some limitations, the HRFS + AIFMS analysis allowed detection of most of the analytes down to 0.1ng/mL, seamless integration of new or unexpected analytes, identification and quantification with no limitations on the number of monitored compounds, and reevaluation of the acquired data also concerning metabolism studies using group-indicating fragment ions.