A cautionary note on the effects of laboratory air contaminants on ambient ionization mass spectrometry measurements

Impurity contribution to ultraviolet absorption of saturated fatty acids

Saturated fatty acids are abundant organic compounds in oceans and sea sprays. Their photochemical reactions induced by solar radiation have recently been found as an abiotic source of volatile organic compounds, which serve as precursors of secondary organic aerosols. However, photoabsorption of wavelengths longer than 250 nanometers in liquid saturated fatty acids remains unexplained, despite being first reported in 1931. Here, we demonstrate that the previously reported absorption of wavelengths longer than 250 nanometers by liquid nonanoic acid [CH3(CH2)7COOH)] originates from traces of impurities (0.1% at most) intrinsically contained in nonanoic acid reagents. Absorption cross sections of nonanoic acid newly obtained here indicate that the upper limit of its photolysis rate is three to five orders of magnitude smaller than those for atmospherically relevant carbonyl compounds.

Algorithms and Databases: Unlocking Non-Targeted Screening of Small Molecules with Ambient Ionization Mass Spectrometry

Almost all sectors of analytical chemistry are finding applications for ambient ionization mass spectrometry (AI–MS) because of its ease of use, speed of analysis, and sensitivity. Although emphasis has been placed on developing new hardware that can help analyze unique samples across various applications, there has not been much innovation in the functionality of software tools and mass spectral libraries to support applications like non-targeted searching. In this article, we discuss new algorithms and libraries that have enabled non-targeted analysis of small molecules using AI–MS, as well as some of the key considerations and outstanding questions in the field.

Characterization of a Modulated X-ray Source for Ion Mobility Spectrometry

As a highly deployed field instrument for the detection of narcotics, explosives, and chemical warfare agents, drift tube ion mobility spectrometry relies heavily upon the performance of the ionization source and mechanism of ion beam modulation. For this instrumental platform, ion chemistry plays a critical role in the performance of the instrument from a sensitivity and selectivity perspective; however, a range of instrumental components also occupy pivotal roles. Most notably, the mechanism of ion modulation or ion gating is a primary contributor to peak width in a drift tube ion mobility experiment. Unfortunately, physical ion gates rarely perform perfectly, and in addition to serving as physical impediments to ion transmission, their modulation also has undesirable field effects. Using a recently developed modulated, non-radioactive X-ray source, we detail the performance of an ion mobility spectrometry (IMS) system that is free of a gating structure and utilizes the pulsed nature of the modulated X-ray source (MXS) for both ion generation and initiation of the IMS experiment. After investigating the influence of pulse duration and spatial X-ray beam width on the analytical performance of the instrument, the possibility of using multiplexing with a shutterless system is explored. By increasing ion throughput, the observed multiplexing gain compared to a signal-averaged spectrum approaches the theoretical maximum and illustrates the capability of the MXS-IMS system to realize significant signal to noise improvements.

Combining First and Second-Tier Newborn Screening in a Single Assay Using High-Throughput Chip-Based Capillary Electrophoresis Coupled to High-Resolution Mass Spectrometry

BACKGROUND

Most first-tier newborn screening (NBS) biomarkers are evaluated by a 2-min flow injection analysis coupled to tandem mass spectrometry (FIA-MS/MS) assay. The absence of separation prior to MS/MS analysis can lead to false positives and inconclusive results due to interferences by nominal isobars and isomers. Therefore, many presumptive positive specimens require confirmation by a higher specificity second-tier assay employing separations, which require additional time and resources prior to patient follow-up.

METHODS

A 3.2-mm punch was taken from dried blood spot (DBS) specimens and extracted using a solution containing isotopically labeled internal standards for quantification. Analyses were carried out in positive mode using a commercially available microfluidic capillary electrophoresis (CE) system coupled to a high-resolution mass spectrometer (HRMS).

RESULTS

The CE-HRMS platform quantified 35 first- and second-tier biomarkers from a single injection in <2-min acquisition time, thus, successfully multiplexing first- and second-tier NBS for over 20 disorders in a single DBS punch. The CE-HRMS platform resolved problematic isobars and isomers that affect first-tier FIA-MS/MS assay specificity, while achieving similar quantitative results and assay linearity.

CONCLUSIONS

Our CE-HRMS assay is capable of multiplexing first- and second-tier NBS biomarkers into a single assay with an acquisition time of <2 min. Such an assay would reduce the volume of false positives and inconclusive specimens flagged for second-tier screening.

Plug-and-play laser ablation-mass spectrometry for molecular imaging by means of dielectric barrier discharge ionization

The plug-and-play hyphenation of UV-laser ablation (LA) and mass spectrometry is presented, using dielectric barrier discharge ionization (DBDI). The DBDI source employed here is characterized by its unique geometry, being directly mounted onto the inlet capillary of a mass spectrometer. In the literature, this particular kind of DBDI source is also referred to as active capillary plasma ionization. It has been commercialized as soft ionization by chemical reaction in transfer (SICRIT) and will be addressed as DBDI in this study. LA-DBDI-MS was used for the direct, molecule-specific and spatially resolved analysis of various solid samples, such as coffee beans and pain killer tablets without extensive sample preparation. The combination of fast washout UV-laser ablation and the principle of the DBDI source used here allowed for highly efficient soft ionization as well as high spatial resolution down to 10 μm for molecular imaging.

Critical evaluation of ambient mass spectrometry coupled with chemometrics for the early detection of adulteration scenarios in Origanum vulgare L

Nowadays, most of the screening methods in food manufacturing are based on spectroscopic techniques. Ambient Mass Spectrometry is a relatively new field of analytical chemistry which has proven to offer similar speed and ease-of-use when compared to other fingerprinting techniques, alongside the advantages of good selectivity, sensitivity and chemical information. Numerous applications have been explored in food authenticity, based either on the target detection of adulteration markers or, less frequently, on the development of multivariate classification models. The aim of the present work was to evaluate and compare the capabilities of Direct Analysis in Real Time (DART) and Atmospheric Solid Analysis Probe (ASAP) Mass Spectrometry (MS) for the high-throughput authenticity screening of commercial herbs and spices products. The gross addition of bulking material to dried Mediterranean oregano was taken as case study. First, a pilot sample set, constituted by authentic dried oregano, olive leaves (a frequently reported adulterant) and mixtures thereof at different levels (i.e. 10, 20, 30 and 50% w/w) was used. Each sample was fingerprinted by both ambient-MS techniques. After appropriate pre-processing, the whole mass spectra were used for the subsequent multivariate data analysis. Soft Independent Modelling of Class Analogy was adopted as classification algorithm and the model was challenged with both new authentic oregano and in-house prepared blends. To the best of our knowledge, this is the first report of DART-MS and ASAP-MS used in full scan mode and coupled to chemometric modelling as rapid fingerprinting approach for food authentication. Although both the techniques provided satisfactory results, ASAP-MS clearly showed greater potential, leading to reproducible, diagnostic feature-rich mass spectra. For this reason, ASAP-MS was further tested under a more convoluted scenario, where the training and validation sets were enlarged with additional authentic oregano samples and a wider range of adulterant species, respectively. Overall good results were achieved, with 93% model predictive accuracy, and screening detection capability estimated between 5-20% (w/w) addition, depending on the adulterant considered with the only exception of majorana. Investigation of Q residuals could highlight the statistically-relevant chemical markers which could be tentatively annotated by coupling the ASAP probe with a high resolution mass analyser. The results from the validation study confirmed the great potential of ASAP-MS in combination with chemometrics as fast MS-based screening solution and demonstrated its feasibility for classification model building.

Direct analysis in real time accurate mass spectrometry determination of bisphenol A in thermal printing paper

Contact with thermal printing paper is a relevant source of dermal exposure to unbonded bisphenol A (BPA). In order to limit this exposure route, the European Union has introduced a drastic reduction in the maximum allowed concentration of BPA in thermal paper produced after beginning of year 2020. This study investigates the suitability of direct analysis in real time (DART), combined with accurate mass spectrometry, as a faster alternative to chromatography-based methods for the quantitative determination of BPA, and three analogues species, in receipts and tickets usually printed on thermal paper. The ionization efficiency of these compounds is evaluated under different conditions, and the effect of instrumental parameters of the DART source in the observed responses is discussed. The yield of the DART desorption-ionization process was greatly improved when compounds are previously converted into their acetyl derivatives; thereafter, the temperature of electronically excited helium atoms was the most relevant of the evaluated instrumental parameters. Under optimized conditions, the reported method provided recoveries in the range from 90 to 110%, a limit of quantification of 0.004% (w:w), well below the maximum concentration established after 2020 for BPA (0.02%, w:w), and permitted to perform duplicate determinations of each sample extract with a response time around 1 min. The accuracy of BPA levels found in non-spiked samples was confirmed using GC-EI-MS as reference technique. BPA was systematically noticed in the processed samples with concentrations ranging from 0.005% to more than 6%.

Positive matrix factorization: A data preprocessing strategy for direct mass spectrometry-based breath analysis

Interest in exhaled breath has grown considerably in recent years, as breath biosampling has shown promise for non-invasive disease diagnosis, therapeutic drug monitoring, and environmental exposure. Real time breath analysis can be accomplished via direct online mass spectrometry (MS)-based methods, which can provide more accurate and detailed data and an enhanced understanding of the temporal evolution of exhaled VOCs in the breath; however, the complicated chemical composition and large raw datasets involved in breath analysis have hindered the discovery of sources contributing to the exhaled VOCs. The positive matrix factorization (PMF) receptor model has been widely used for source apportionment in atmospheric studies. Since the exhaled VOCs contain compounds from various sources, such as alveolar air, mouth air and respiratory dead-space air, PMF may be also helpful for source apportionment of exhaled VOCs in the breath. Thus, this study explores the application of PMF in the pretreatment of direct breath measurement data. The results indicate that (i) endogenous compounds and background contaminants sources can be readily distinguished by PMF in data obtained from replicate measurements of human exhaled breath at single time points (~30 s/measurement), which may benefit both exhalome investigations and the identification of exposure biomarkers; (ii) sources resolved from online measurement data collected over longer periods (1.5 h) can be used to isolate the evolution of exhaled VOCs and investigate processes such as the pharmacokinetics of ketamine and its major metabolites. Therefore, PMF has shown promise for both data processing and subsequent data mining for the ambient MS-based breath analysis.