Detection of volatile organic compounds in the headspace above mold fungi by GC‐soft X‐radiation–based APCI‐MS

Abstract

Mold fungi on malting barley grains cause major economic loss in malting and brewery facilities. Possible proxies for their detection are volatile and semivolatile metabolites. Among those substances, characteristic marker compounds have to be identified for a confident detection of mold fungi in varying surroundings. The analytical determination is usually performed through passive sampling with solid phase microextraction, gas chromatographic separation, and detection by electron ionization mass spectrometry (EI-MS), which often does not allow a confident determination due to the absence of molecular ions. An alternative is GC-APCI-MS, generally, allowing the determination of protonated molecular ions. Commercial atmospheric pressure chemical ionization (APCI) sources are based on corona discharges, which are often unspecific due to the occurrence of several side reactions and produce complex product ion spectra. To overcome this issue, an APCI source based on soft X-radiation is used here. This source facilitates a more specific ionization by proton transfer reactions only. In the first part, the APCI source is characterized with representative volatile fungus metabolites. Depending on the proton affinity of the metabolites, the limits of detection are up to 2 orders of magnitude below those of EI-MS. In the second part, the volatile metabolites of the mold fungus species Aspergillus, Alternaria, Fusarium, and Penicillium are investigated. In total, 86 compounds were found with GC-EI/APCI-MS. The metabolites identified belong to the substance classes of alcohols, aldehydes, ketones, carboxylic acids, esters, substituted aromatic compounds, terpenes, and sesquiterpenes. In addition to substances unspecific for the individual fungus species, characteristic patterns of metabolites, allowing their confident discrimination, were found for each of the 4 fungus species. Sixty-seven of the 86 metabolites are detected by X-ray-based APCI-MS alone. The discrimination of the fungus species based on these metabolites alone was possible. Therefore, APCI-MS in combination with collision induced dissociation alone could be used as a supervision method for the detection of mold fungi.

Characterization of volatile metabolites formed by molds on barley by mass and ion mobility spectrometry

“…The formation of mVOC as volatile metabolites of fungi can occur over a wide and mostly unknown concentration range. Another challenge is the strong variation of the response values of the compounds in APCI . Therefore, a quantitative description of the mVOC space would require an extensive calibration of each substance.…”

“…Our group already demonstrated the application of an alternative APCI source based on soft X‐radiation in MS. In two publications detailing the detection of explosives in the negative mode and the detection of volatile metabolites of fungi in the positive mode, we could show that the underlying ionization mechanism is more selective. These miniature X‐ray sources, which are not subject to any legal regulations in Germany (photon energy E X‐ray < 5 keV), have already been introduced as alternatives for radioactive 63 Ni sources in ion mobility (IM) spectrometers, eg, by the company Bruker Daltonics (Leipzig, Germany).…”

“…HS investigations of fungi were carried out with GC‐EI‐MS (7890A GC, 5975C MSD, Agilent Technologies) and GC‐APCI‐MS (5890 Series II GC, Hewlett Packard, LTQ XL, Thermo Fisher Scientific), which were already described in detail previously . For both MS methods, the same type of GC columns, but different inner diameters (15 m × 0.25/0.32 mm × 1.0 μm), containing poly(5% diphenyl‐95% dimethylsiloxan) as the active phase was used for the preseparation of the substances.…”

“…The handheld IM spectrometer (Roadrunner, Bruker Daltonics, Leipzig) has an APCI source based on soft X‐radiation (miniature X‐ray tube, 40 mm length, rhodium target on a beryllium window), with an energy of 2.7/2.8 keV ( L α / L β ‐transitions of Rh) and is therefore exempt from registration in Germany. The same type of source was used in the APCI‐MS system (LTQ XL, Thermo Fisher Scientific), where it was integrated into a home‐built ionization chamber . The original application scenario of the Roadrunner instrument is the detection of explosives and drugs on surfaces.…”

“…In this work, our previous HS‐GC‐EI/APCI‐MS investigations of different fungi on agar were extended to fungus‐contaminated barley grains. The aim was the identification of volatile fungus metabolites.…”