Direct Analysis of Triterpenes from High-Salt Fermented Cucumbers Using Infrared Matrix-Assisted Laser Desorption Electrospray Ionization (IR-MALDESI)

Abstract

Abstract. High-salt samples present a challenge to mass spectrometry (MS) analysis, particularly when electrospray ionization (ESI) is used, requiring extensive sample preparation steps such as desalting, extraction, and purification. In this study, infrared matrix-assisted laser desorption electrospray ionization (IR-MALDESI) coupled to a Q Exactive Plus mass spectrometer was used to directly analyze 50-μm thick slices of cucumber fermented and stored in 1 M sodium chloride brine. From the several hundred unique substances observed, three triterpenoid lipids produced by cucumbers, β-sitosterol, stigmasterol, and lupeol, were putatively identified based on exact mass and selected for structural analysis. The spatial distribution of the lipids were imaged, and the putative assignments were confirmed by tandem mass spectrometry performed directly on the same cucumber, demonstrating the capacity of the technique to deliver confident identifications from highly complex samples in molar concentrations of salt without the need for sample preparation.

Mass Spectrometry Imaging: A Review of Emerging Advancements and Future Insights

“…216 Lipids also have the potential to show tumor boundaries, specifically in breast cancer. 217 Because of their diverse structure, several common ionization sources, including MALDI, 1,20,42,97 DESI, 22,217,218 SIMS, 219–221 and IR-MALDESI, 18,19 have found success in lipid imaging. Notably, newly developed laser spray ionization (LSI), with its simple sample preparation, is becoming more commonly used for lipids as well.…”

“…3,91,236 Furthermore, the use of plants as an alternative research system is commonplace so, naturally, researchers have also transitioned to imaging plant-based samples. 234,242 In the last 2 years, food products, such as onions, 243 cucumbers, 18 and citrus peel extracts, 237 have been analyzed, but it is clear that theoretically any item can be imaged with the appropriate equipment. 244,245 For example, DESI-MSI was utilized to understand the thin-layer chromatography (TLC) separation of the citrus peel extracts directly on the TLC plates (Figure 8).…”

Development of a relative quantification method for infrared matrix‐assisted laser desorption electrospray ionization mass spectrometry imaging of Arabidopsis seedlings

“…Recently IR‐MALDESI has been applied to plant tissues, which have many different inherent sampling challenges from mammalian tissues . IR‐MALDESI has proved to be a highly effective method for the analysis of a diverse range of plant tissues . Each of these experiments answered specific fundamental biological questions.…”

“…18 IR-MALDESI has proved to be a highly effective method for the analysis of a diverse range of plant tissues. [19][20][21] Each of these experiments answered specific fundamental biological questions. Naturally, quantification is the next step towards providing plant metabolite analysis with the analytical rigor applied to other sample types.…”

Direct screening of enzyme activity using infrared matrix‐assisted laser desorption electrospray ionization

“…Since most HTS experiments are performed in a semi‐targeted manner, the total analysis time can be reduced by performing the experiments at lower resolving powers. Furthermore, the ESI post‐ionization in IR‐MALDESI allows for direct analysis of analytes from matrices containing high salt and protein concentrations, that are used to perform enzymatic reactions …”