Rapid Determination of Different Ripening Stages of Occidental Pears (Pyrus communis L.) by Volatile Organic Compounds Using Proton-Transfer-Reaction Mass Spectrometry (PTR-MS)

Determination of Occidental pear (Pyrus communis) ripening is difficult because the appearance of Occidental pears does not change significantly during the ripening process. Occidental pears at different ripening stages release different volatile organic compounds (VOCs), which can be used to determine fruit ripeness non-destructively and rapidly. In this study, VOCs were detected using proton-transfer-reaction mass spectrometry (PTR-MS). Notably, data were acquired within 1 min. Occidental pears harvested at five separate times were divided into three ripening stages: unripe, ripe, and overripe. The results showed that the composition of VOCs differed depending on the ripening stage. In particular, the concentrations of esters and terpenes significantly increased during the overripe stage. Three ripening stages were clearly discriminated by heatmap clustering and principal component analysis (PCA). This study provided a rapid and non-destructive method to evaluate the ripening stages of Occidental pears. The result can help fruit farmers to decide the optimum harvest time and hence reduce their economic losses.

Attempts to Detect Lipid Metabolites from a Single Cell Using Proton-Transfer-Reaction Mass Spectrometry Coupled with Micro-Scale Supercritical Fluid Extraction: A Preliminary Study

Proton-transfer-reaction (PTR) mass spectrometry (MS), a widely used method for detecting trace-levels of volatile organic compounds in gaseous samples, can also be used for the analysis of small non-volatile molecules by using supercritical fluid as a transporter for the molecules. Supercritical fluid extraction (SFE) is a method that permits lipophilic compounds to be rapidly and selectively extracted from complex matrices. The combination of the high sensitivity of PTR MS with the SFE is a potentially novel method for analyzing small molecules in a single cell, particularly for the analysis of lipophilic compounds. We preliminarily evaluated this method for analyzing the components of a single HeLa cell that is fixed on a stainless steel frit and is then directly introduces the SFE extracts into the PTR MS. A total of 200/91 ions were observed in positive/negative ion mode time-of-flight mass spectra, and the masses of 11/10 ions could be matched to chemical formulae obtained from the LipidMaps lipids structure database. Using various authentic lipophilic samples, the method could be used to detect free fatty acids in the sub-femtomole to femtomole order in the negative ion mode, the femtomole to sub-picomole order for fat-soluble vitamins, and the picomole order for poly aromatic hydrocarbons in both the positive and negative ion mode.

Modern mass spectrometry in atmospheric sciences: Measurement of volatile organic compounds in the troposphere using proton-transfer-reaction mass spectrometry

Volatile organic compounds (VOCs) in the troposphere are emitted from a wide variety of natural and man-made sources and resulting in environmental issues such as air pollution and climate change. Proton-transfer-reaction mass spectrometry (PTR-MS), a chemical ionization mass spectrometry technique using H₃ O⁺ reagent ions, allows real-time measurements of various VOCs in air with high sensitivity (parts-per-trillion to parts-per-billion level) and fast time response (<1 s). Modern PTR-MS equipped with time-of-flight mass analyzers can measure hundreds of compounds simultaneously. The applications of PTR-MS have greatly promoted understanding VOC sources and their roles in environmental issues, and therefore, PTR-MS has become a well-established analytical technique for in situ VOC measurements in atmospheric sciences. Here, the principles and implementation of PTR-MS are described. We additionally highlight modern PTR-MS techniques for accurate identification and comprehensive quantification of various VOCs. Finally, recent applications for atmospheric science are presented, using examples from research in biomass burning emissions and chemistry.

Effect of medium compositions on microbially mediated volatile organic compounds release profile

AIMS

To monitor temporal changes in the volatile organic compounds' (VOCs) profile generated by the metabolic activities of Pseudomonads in real time.

METHODS AND RESULTS

Three Pseudomonas strains were cultivated in Vogel's broth, supplemented with glucose (0·5 or 1%) and/or protein (egg white powder at 0 or 2%) at 25°C. Glucose or egg white protein contents influenced the VOCs' release profile for alcohols, carbonyls and sulphur derivatives. Increasing glucose content resulted in higher alcohol and ketone contents. Glucose showed a lower effect on the VOCs' release profile, mainly impacting on individual compounds, such as m/z 89 (3-methyl-1-butanol). In contrast, egg white protein enhanced production of VOCs such as m/z 75 (2-methyl-1-propanol) and m/z 63 (dimethyl sulphide) regardless of glucose level present in the medium. At the end of bacteria growth phase (54, 60 and 72 h), the fingerprint of VOCs was different from the early growth phase. Cells near to the end of their growth phase (54, 60 and 72 h) produced a distinctly different array of compounds compared to those produced in early growth phase, for example, cyclic compounds were detected in early growth phase, whereas sulphur derivatives were more common in late growth phase.

CONCLUSIONS

Pseudomonads-mediated VOCs' fingerprint as a response to varying growth conditions can be identified as latent biomarkers.

SIGNIFICANCE AND IMPACT OF THE STUDY

Understanding how microbially mediated VOCs' release profile responds to varying growth conditions can potentially be used as a rapid method for detecting microbial activities in controlled conditions such as food quality systems.