Analysis of Volatile Compounds in Garlic, Onion, and Ramsons using SUPER SESI-HRMS

Introduction:

In this study, SUPER SESI-Orbitrap MS was used to analyze the time-resolved gas-phase volatile organic compounds (VOCs) of garlic, ramsons, and onion.

The aim of this study was to address the difficulties of characterizing heat-labile sulfur molecules from various Allium spp. In this, we will delve into the analytical method SUPER SESI-HRMS used in this study and its applications in the analysis of volatile compounds in garlic, onion, and ramsons.

The Principle Behind SUPER SESI-HRMS:

SUPER SESI-HRMS is an analytical technique that combines Secondary Electrospray Ionization (SESI) with a High-Resolution Mass Spectrometer (HRMS). The process begins with the generation of an ultrafine aerosol charged through a nano-electrospray, directed towards the analyzed sample. When molecules in the sample come into contact with the charged aerosol, they acquire a charge and volatilize. These volatilized molecules are then ionized and transferred to the high-resolution mass spectrometer, where they are precisely separated and detected. This system does not rely on analyte separation, so the time between molecules entering the SUPER SESI and reaching the high vacuum in the Orbitrap is only about 2 sec, reducing degradation by fragmentation and bimolecular processes. Also, the ionisation is very soft and can be achieved at atmospheric pressure and low temperature. SUPER SESI-Orbitrap MS allows real-time analysis because the need for chromatographic separation is bypassed, and full-MS scans can be recorded every 0.3 s. Because no sample treatment is required, any possible sampling bias is avoided. 

Seeing the smell of garlic:

Allicin is the major active component in crushed garlic, its antimicrobial effects against multiple pathogens occur either by direct contact in solution, where it has an efficiency comparable to many commercially available antibiotics. But its reactivity and heat instability cause challenges in the analysis of gas phase samples. In most GC measurements, allicin degrades in the heated injection port or during the chromatography and is therefore not detectable. This problem can be circumvented by using a SUPER SESI-Orbitrap MS system. We showed that varying the temperature at the intake line, ionizer core and intake capillary had a huge impact on the composition and intensity of measured volatiles. Allicin has a molecular formula of C6H10OS2 with an [M + H]+ of m/z 163.0246. The SUPER SESI-Orbitrap MS apparatus used has a resolution of 70,000 and usually works within an error range of 3 ppm.

A feature with the same exact mass as protonated allicin was clearly visible in all tests, but not in the background, therefore this feature was assumed to be non-degraded allicin. 

The intensity of detected allicin decreased with increasing temperature of the SUPER SESI unit. The highest intensity was observed at 30 ◦C with 6.8×108 molecules per scan: 

SUPER SESI-Orbitrap MS allows kinetics to be followed by the ability to measure in a time resolved manner with the scan rate of the mass spectrometer, in this case, at 0.4 Hz. In the study of volatile sulfur compound chemistry (VOSC) in damaged garlic, it is observed that 2-propenesulfenic acid appears before allicin, but decreases rapidly due to rapid condensation to allicin. In onions, which produce mainly isoalliin rather than alliin, 1-propenesulfenic acid is rapidly converted to propanetial S-oxide, being indistinguishable in this system. In ramsons, which produce both alliin and isoalliin, 1-propenesulfenic acid and zwiebelanes, indistinguishable from 2-propenesulfenic acid and allicin respectively, are formed. In addition, ramsons produce large amounts of methiin, which, when treated by alliinase, yields several compounds, including allicin and isomeric allyl methyl thiosulfinate.

(a), (c), and (e) kinetics and (b), (d), and (f) show the absolute intensities of selected VOSCs at 0.5 min for garlic (a), (b), onion (c), (d), and ramsons (e), (f). All species were found as [M +H]+. The kinetics show each molecular species normalised to its own maximum and the first appearance of a VOSCs was set to zero.

Conclusion:

The use of the analytical technique SUPER SESI-HRMS has proven to be an invaluable tool in the analysis of volatile compounds in garlic, onion, and ramsons. This technique provides a detailed insight into the chemical composition of these foods, which can open up new opportunities in the food industry and in the study of natural products with medicinal properties. With its real-time analysis capability and high resolution, SUPER SESI-HRMS represents a significant advancement in the study of volatile compounds in foods and provides a solid platform for future research in this field.