Trace gas analysis in real-time using PTR-MS

On-line gas-phase detection and process monitoring

Volatile organic compounds (VOCs) are ubiquitous in the environment. As the term ‘volatile’ suggests, VOCs have a high vapour pressure and as such are typically present in the gas-phase at ambient temperatures. VOCs in the environment have manifold origins, but these can essentially be categorised as being either from biogenic emissions (plants, trees, organic matter, etc.) or from anthropogenic sources (manmade materials, fossil fuel burning, industrial processes, etc.). Despite a high degree of commonality of VOCs in the emissions of various sources, specific processes often result in the release of unique VOCs or VOC profiles. The detection, identification and monitoring of specific VOCs can therefore deliver key information about products, processes and people.

The Department of Sensory Analytics has extensive expertise and diverse state-of-the-art equipment for use in the on-line detection of gas-phase VOCs in all manner of applications. Specifically, we employ proton-transfer-reaction mass spectrometry (PTR-MS) and ion-mobility spectrometry (IMS) to detect and quantify VOCs in real-time. The former includes high sensitivity PTR-quadrupole-MS and high sensitivity, high resolution PTR-time-of-flight-MS (respectively, PTR-QMS and PTR-TOFMS) instruments, which are capable of monitoring discrete changes in the concentration of VOCs in real-time down to parts-per-trillion (pptv) levels.

On-line detection of food-related volatiles

Foods and beverages are rich in VOCs, most commonly in the form of odour-active aroma compounds that contribute to a food’s flavour. Such aroma compounds are continually released from the solid or liquid food matrix into the gas-phase. On the one hand this proceeds freely depending on diverse factors such as the physicochemical properties of the aroma compounds, the food matrix composition, temperature, etc., which imparts a food with its characteristic aroma that is enjoyed by consumers immediately prior to eating. On the other hand, food undergoes additional mechanical and enzymatic processes during mastication that act to release aroma compounds from the food matrix into the oral cavity, which subsequently imparts the flavour experience to a consumer via retronasal aroma perception in combination with the taste receptors on the tongue.

On-line tools for monitoring VOC generation and release processes in foods can be of great benefit in the study of various food applications. Nosepace analysis, for example, might offer clues as to the aroma release processes occurring during mastication (cf. Fig. 1). In nosespace analysis the aroma compounds that are exhaled via the nose following their release into the oral cavity and subsequent retronasal perception can be used to investigate correlations between food matrix formulations and compounds-specific release processes. This presents just one example of how our on-line VOC detection tools can be used in the analysis of foods. Other applications include:

  • non-destructive headspace analyses of VOCs to determine the release of specific compounds from the unperturbed food [1]
  • monitoring food spoilage processes (oxidation, thermal degradation, microbial spoilage) on diverse foods (milk, meat, etc.) in view of shelf-life depending on packaging (vacuum-packaging, modified atmosphere) and storage (temperature, lighting) conditions [2, 3]
  • characterising VOC profiles for determining product origins or screening for suspected adulteration [4]
  • optimising food matrix formulations to promote the release of desirable aromas and prevent the release of undesirable off-flavours [5-7]
  • comparing VOC profiles of foods undergoing different treatments
  • assessing the influence of packaging materials on the aroma profiles of packed foods

 

Fig. 1: The aroma release from food during mastication ca be monitored in vivo via PTR-MS nosespace analysis

Real-time process monitoring

Following the changes in concentration, or the generation or disappearance of specific VOCs, can be of direct utility in process-monitoring applications. This might be used, for instance, to study, control or optimise continuous processes, including:

  • fermentation (in food as well as pharmaceutical applications) [8]
  • industrial or agricultural emissions (e.g., in view of abatement strategies for offensive odours)
  • packing or sterilisation steps (e.g., rapid screening of contamination in reusable bottles)
  • materials science (assessment of dynamic processes, e.g. by monitoring volatiles produced during manufacturing)


Physiology and metabolism monitoring

The human body emits a large number of individual VOCs – numbering more than 800 – that are either of endogenous or exogenous origin. Environmental factors, as well as health, influence this so-called volatilome, with VOCs principally excreted via breath, sweat and urine. Volatile food constituents entering the body after ingestion can pass through unperturbed or undergo metabolism before excretion, that latter equally contributing to the volatilome. Health status can also shift the VOCs emission profile of the body in one or the other direction. On-line detection of VOCs exhaled in the breath, passed in the urine, or emanated via sweat, can lead to clues in various scientific areas of applications, including:

  • medical diagnostic assessments (e.g., through the analysis of volatile disease-specific biomarkers for non-invasive diagnostics) [9, 10]
  • metabolism of food constituents in view of bioavailability and nutrition [11]
  • pharmacokinetics assessment of exogenous substances in exhaled breath or urine (e.g., drugs, medication, or food constituents) [12]
  • estimation of adverse outcome pathways (AOPs) after exposure to xenobiotics [13, 14]

Fig. 2: above: A buffered end-tidal (BET) sampler for breath gas analysis; below: The PTR-TOFMS mass spectrum contains a wealth of information about the constituent VOCs of a sample gas.

Peripheral equipment in our on-line laboratories expands our analytical capabilities. The available calibration systems cater for the generation of both gas and liquid standards (advanced gas and liquid calibration units; GCU-a and LCU-a, respectively) at defined concentration in gas matrices of pre-defined humidity for improved accuracy of quantitation of our on-line systems. The resolution power of both the PTR-TOFMS and IMS instruments is enhanced by the use of the (optional) gas chromatographic (GC) column at the sampling inlet, allowing for separation of many isomeric compounds. Both the PTR-TOFMS and IMS systems are additionally fitted with an autosampler for high sample throughput, for use as necessary. A PerMaSCal system fitted to the PTR-TOFMS ensures optimum results in terms of mass scale positioning. Breath analysis is aided by use of a buffered end-tidal (BET) sampler for on-line sampling and breath collection unit (BCU) for offline analysis.

For structural elucidation of the respective VOCs, on-line diagnostics are cross-validated with the use of our advanced GC and MS arsenal of instruments (link Methods GC-MS). This allows for a comprehensive characterisation of VOCs with a high resolution down to trace levels.


References

[1]  Beauchamp J and Herbig J 2015 The Chemical Sensory Informatics of Food: Measurement, Analysis, Integration, ed B Guthrie, et al.: American Chemical Society) pp 235-51

[2]  Beauchamp J, Zardin E, Silcock P and Bremer P J 2014 Monitoring photooxidation-induced dynamic changes in the volatile composition of extended shelf life bovine milk by PTR-MS J. Mass Spectrom. 49 952-8

[3]  Silcock P, Alothman M, Zardin E, Heenan S, Siefarth C, Bremer P J and Beauchamp J 2014 Microbially induced changes in the volatile constituents of fresh chilled pasteurised milk during storage Food Packaging and Shelf Life 2 81-90

[4]  Araghipour N et al 2008 Geographical origin classification of olive oils by PTR-MS Food Chem. 108 374-83

[5]  Siefarth C, Tyapkova O, Beauchamp J, Schweiggert U, Buettner A and Bader S 2011 Influence of polyols and bulking agents on flavour release from low-viscosity solutions Food Chem. 129 1462-8

[6]  Tyapkova O, Bader-Mittermaier S, Schweiggert-Weisz U, Wurzinger S, Beauchamp J and Buettner A 2014 Characterisation of flavour–texture interactions in sugar-free and sugar-containing pectin gels Food Res. Int. 55 336-46

[7]  Tyapkova O, Siefarth C, Schweiggert-Weisz U, Beauchamp J, Buettner A and Bader-Mittermaier S 2016 Flavor release from sugar-containing and sugar-free confectionary egg albumen foams LWT-Food Sci. Technol. 69 538-45

[8]  Keupp C, Zardin E, Schneiderbanger H and Beauchamp J 2014 Monitoring dynamic changes in the release of aroma compounds during fermentation of wheat beer wort 11th International Trends in Brewing (Gent, Belgium)

[9]  Kohl I et al 2013 First observation of a potential non-invasive breath gas biomarker for kidney function J. Breath Res. 7 017110

[10]  Beauchamp J D and Pleil J D 2015 Biomarker Validation. Technological, Clinical and Commercial Aspects, ed H Seitz and S Schumacher (Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA)

[11]  Kirsch F, Beauchamp J and Buettner A 2012 Time-dependent aroma changes in breast milk after oral intake of a pharmacological preparation containing 1,8-cineole Clinical Nutrition 31 682-92

[12]  Beauchamp J, Kirsch F and Buettner A 2010 Real-time breath gas analysis for pharmacokinetics: monitoring exhaled breath by on-line proton-transfer-reaction mass spectrometry after ingestion of eucalyptol-containing capsules J. Breath Res. 4 026006

[13]  Pleil J D, Miekisch W, Stiegel M A and Beauchamp J 2014 Extending breath analysis to the cellular level: current thoughts on the human microbiome and the expression of organic compounds in the human exposome J. Breath Res. 8 029001

[14]  Pleil J D, Beauchamp J D, Miekisch W and Funk W E 2015 Adapting biomarker technologies to adverse outcome pathways (AOPs) research: current thoughts on using in vivo discovery for developing in vitro target methods J. Breath Res. 9 039001