Metabolite profiling using gas chromatography coupled to mass spectrometry (GC–MS) is one of the most frequently applied and standardized methods in research projects using metabolomics to analyze complex samples. However, more than 20 years after the introduction of non-targeted approaches using GC–MS, there are still unsolved challenges to accurate quantification in such investigations. One particularly difficult aspect in this respect is the occurrence of sample-dependent matrix effects. In this project, we used model compound mixtures of different compositions to simplify the study of the complex interactions between common constituents of biological samples in more detail and subjected those to a frequently applied derivatization protocol for GC–MS analysis, namely trimethylsilylation. We found matrix effects as signal suppression and enhancement of carbohydrates and organic acids not to exceed a factor of ~2, while amino acids can be more affected. Our results suggest that the main reason for our observations may be an incomplete transfer of carbohydrate and organic acid derivatives during the injection process and compound interaction at the start of the separation process. The observed effects were reduced at higher target compound concentrations and by using a more suitable injection-liner geometry.

On-site analysis of volatile organic compounds (VOCs) with miniaturized gas chromatography–mass spectrometry (GC–MS) systems is a very rapidly developing field of application. While, on the one hand, major technological advances are improving the availability of these systems on the market, on the other hand, systematic studies to assess the performance of such instruments are still lacking. To fill this gap, we compared three portable GC–MS devices to a state-of-the-art benchtop (stationary) system for analysis of a standard mixture of 18 VOCs. We systematically compared analytical parameters such as the sensitivity and similarity of the signal response pattern and the quality of the obtained mass spectra. We found that the investigated mobile instruments (i) showed different response profiles with a generally lower number of identified analytes. Also, (ii) mass spectral reproducibility (% relative standard deviation (RSD) of the relative abundance of selective fragments) was generally worse in the mobile devices (mean RSD for all targeted fragments ~9.7% vs. ~3.5% in the stationary system). Furthermore, mobile devices (iii) showed a poorer mass spectral similarity to commercial reference library spectra (>20% deviation of fragment ion relative intensity vs. ~10% in the stationary GC–MS), suggesting a less reliable identification of analytes by library search. Indeed, (iv) the performance was better with higher-mass and/or more abundant fragments, which should be considered to improve the results of library searches for substance identification. Finally, (v) the estimation of the signal-to-noise ratio (S/N) in mobile instruments as a measure of sensitivity revealed a significantly lower performance compared to the benchtop lab equipment (with a ratio among medians of ~8 times lower). Overall, our study reveals not only a poor signal-to-noise ratio and poor reproducibility of the data obtained from mobile instruments, but also unfavorable results with respect to a reliable identification of substances when they are applied for complex mixtures of volatiles. Graphical Abstract

Continuous long-term in-situ measurements of reactive trace gases in observational and environmental chamber platforms is essential to identify their impact on future air quality under climate change and the switch to renewable energy. Despite new and more advanced technologies for online and/or offline monitoring using pre-concentration and thermal desorption (TD), followed by gas chromatography (GC) coupled to mass spectrometry (MS) and/or flame ionization detection (FID), the complexity of such measurements is still challenging. Within the framework of the ACTRIS Centre for Reactive Trace Gases in Situ Measurements CiGAS, this work presents the characterization and orthogonal comparison of two modern systems for online and offline sampling, respectively, coupled to GC dual-columns (Deans Switch heart cut approach) and FID/MS detection. We evaluate the advantages and limitations of these systems regarding the versatility of the sample inlets during collection (e.g., direct line, canisters, and multi-bed TD tubes) as well as automated calibration strategies (e.g., variable sample volume through MFC and loop mode), providing differences in compound coverage, signal responses and sorbent background. We evaluate their performance in terms of sensitivity, reproducibility, linearity, and relative recovery by performing, (1) laboratory calibrations using high purity certified mixtures of non-methane hydrocarbons and monoterpenes and, (2) ambient measurements using the characterized and calibrated analytical methods with particular emphasis on urban emissions from cooking, cleaning, and traffic. In addition, we compare results from the TD-GC-FID/MS to a high time resolution chemical ionization mass spectrometer, called VOCUS, and present the resulting volatile profiles, their variability, and interferences among the different measurements methods.