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Chemistry

PTR-ToF-MS Coupled with an Automated Sampling System and Tailored Data Analysis for Food Studies: Bioprocess Monitoring, Screening and Nose-space Analysis

Published: May 11th, 2017

DOI:

10.3791/54075

1Department of Food Quality and Nutrition, Research and Innovation Centre, Fondazione Edmund Mach (FEM), 2Faculty of Science and Technology, Free University of Bolzano, 3Department of Agriculture, Food and Environmental Sciences, University of Foggia, 4Institute of Analytical Chemistry & Radiochemistry, Leopold-Franzens Universität Innsbruck, 5Institut für Ionenphysik und Angewandte Physik, Leopold-Franzens Universität Innsbruck

Proton Transfer Reaction Time of Flight Mass Spectrometry allows high-sensitivity, rapid and non-invasive analysis of volatile organic compounds. To demonstrate its potential, we give three examples: lactic acid fermentation of yogurt (on-line bioprocess monitoring), different apple genotypes (large-scale screening), and retronasal space after drinking coffee (nosespace analysis).

Proton Transfer Reaction (PTR), combined with a Time-of-Flight (ToF) Mass Spectrometer (MS) is an analytical approach based on chemical ionization that belongs to the Direct-Injection Mass Spectrometric (DIMS) technologies. These techniques allow the rapid determination of volatile organic compounds (VOCs), assuring high sensitivity and accuracy. In general, PTR-MS requires neither sample preparation nor sample destruction, allowing real time and non-invasive analysis of samples. PTR-MS are exploited in many fields, from environmental and atmospheric chemistry to medical and biological sciences. More recently, we developed a methodology based on coupling PTR-ToF-MS with an automated sampler and tailored data analysis tools, to increase the degree of automation and, consequently, to enhance the potential of the technique. This approach allowed us to monitor bioprocesses (e.g. enzymatic oxidation, alcoholic fermentation), to screen large sample sets (e.g. different origins, entire germoplasms) and to analyze several experimental modes (e.g. different concentrations of a given ingredient, different intensities of a specific technological parameter) in terms of VOC content. Here, we report the experimental protocols exemplifying different possible applications of our methodology: i.e. the detection of VOCs released during lactic acid fermentation of yogurt (on-line bioprocess monitoring), the monitoring of VOCs associated with different apple cultivars (large-scale screening), and the in vivo study of retronasal VOC release during coffee drinking (nosespace analysis).

Direct-Injection Mass Spectrometric (DIMS) technologies represent a class of analytical instrumental approaches that offer considerable mass and time resolution with high sensitivity and robustness, allowing the quick detection and quantification of volatile organic compounds (VOCs)1. These instrumental approaches include, among others, MS-e-noses, Atmospheric-Pressure Chemical Ionization Mass Spectrometry (APCI-MS), Proton-Transfer-Reaction Mass Spectrometry (PTR-MS), and Selected Ion-Flow-Tube Mass Spectrometry (SIFT-MS)1. The pros and the cons of each approach depend on: the kind of sample injection, the source and co....

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The protocol follows the guidelines of our institutional committee on human research ethics.

1. Sample Preparation and Autosampler Conditions

  1. On-line bioprocess monitoring: detecting VOCs released during lactic acid fermentation of yogurt
    NOTE: This section of the protocol represents part of the procedure reported by Benozzi et al.32
    1. Add 5 mL of pasteurized milk to each vial (20 mL glass vials equipped with PTFE/silicone septa). Note .......

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The volatile profile of samples resulted in a complete mass spectrum for the desired mass range acquired each second. In Figure 2, an example of the acquired average spectra during the yogurt on-line bioprocess is given32. In every spectrum, more than 300 mass peaks in the m/z range up to 250 Th can be identified32.

Figure 2

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Proton transfer reaction-mass spectrometry (PTR-MS) coupled to time of flight (ToF) mass analyzers represent a valid compromise between the need for identification and quantification of volatile organic compounds and the necessity for rapid analytical profiling. The high mass resolution that characterizes the ToF mass analyzer gives/provides relevant sensitivity and mass spectra with considerable informational content. Furthermore, the application of PTR-ToF-MS coupled with an auto-sampler and tailored data analysis tool.......

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This work is supported by the European Commission's 7th Framework Programme under Grant Agreement Number 287382. SY is a beneficiary of a European Commission's 7th Framework Programme Grant Agreement Number 287382. IK is a beneficiary of a FIRST doctoral school grant from the Fondazione Edmund Mach. For his work at University of Foggia, VC is supported by the Apulian Region in the framework of 'Future In Research' program (practice code 9OJ4W81).

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Name Company Catalog Number Comments
PTR-TOF 8000 High-Resolution PTR-TOF-MS Ionicon Analytik Ges.m.b.H. PTR-TOF 8000 An detector for volatile organic compounds (VOCs) that allows for continuous VOC quantification with a very high mass resolution
GERSTEL MPS 2XL Gerstel A multifunctional autosampler 
Gas Calibration Unit Ionicon Analytik Ges.m.b.H. GCU-s / GCU-a A dynamic gas dilution system that provides variable but known quantities of different standard compounds in a carrier gas stream
TofDaq Tofwerk AG free available at http://soft.tofwerk.com/    A data acquisition software (for spectra  acquisition)
MATLAB  MathWorks http://it.mathworks.com/products/matlab/ A technical computing language and interactive environment for algorithm development, data visualization, and data analysis
R The R Foundation free available at https://cran.r-project.org/mirrors.html   A language and environment for statistical computing and graphics

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