- Email: [email protected]
Biogenic volatile organic compounds
Trends in Analytical Chemistry, Vol. 30, No. 7, 2011
Editorial
Editorial Biogenic volatile organic compounds Jo Dewulf, Herman Van Langenhove Biogenic volatile organic compounds (BVOCs) are organics originating from various biological organisms and play an important role in biological, environmental, food and health sciences. From a quantitative point of view, there is no doubt that plants are the dominant source, with about 1035 Tg C/year emitted as BVOCs into the global atmosphere. These plant metabolic products are hydrocarbons (e.g., isoprene, monoterpenes and sesquiterpenes); others are oxygenated organics {e.g., alcohols [2-methyl-3-butene-2-ol (MBO)], terpene alcohols (C10H18O) or esters (hexenyl acetate), the so-called green-leaf compounds}. Ambient air concentrations of BVOCs are in the ppbv– pptv range. Their atmospheric life times vary from minutes to hours, illustrating the gas-phase reactivity of those compounds. In this way, BVOCs interfere with the atmospheric hydroxyl-radical pool. Furthermore in the presence of nitrogen oxides, BVOC oxidation can result in both increased ozone (greenhouse gas) concentrations and secondary organic aerosol (SOA) formation, influencing the radiation balance. As a result, BVOC emissions affect climate change through direct and indirect effects. Algorithms have been elaborated relating temperature and photosynthetic active radiation (PAR) to BVOC emissions for a number of important plant species under controlled conditions. These algorithms predict increased BVOC emissions with increasing global temperature. Jo Dewulf* Herman Van Langenhove Research Group ENVOC, Ghent University, Coupure Links 653, B-9000 Ghent, Belgium
*
Corresponding author. Tel.: +32 9 2645949; Fax: +32 9 2646243; E-mail: [email protected]
However, because of the non-linearity of interactions between biogeochemical cycles and climate change, the overall effect is quite uncertain as a result of possible feedbacks (e.g., drought, land-use change, and changing nutrient inputs). Furthermore, it becomes more and more apparent that, within the context of BVOC emissions, plants have to be looked upon not as isolated species but as ecosystems (e.g.. since infestation by herbivores induces considerable shift in the BVOC-emission profiles). Next to biological and environmental relevance, BVOC emissions are also associated with microbial metabolism in food processing/conservation or in infectious diseases. Adequate analytical approaches may lead to using these BVOCs as tracers or even as early warning signals. Although total amounts emitted are much smaller than emissions from vegetation, their impact as indicators can be very relevant. Chemical analysis and preferably realtime monitoring are essential and a challenge in all research related to the above-mentioned topics. Challenges related to BVOC analysis go along with those for anthropogenic VOC analysis in general, e.g., ever driving towards higher sensitivity, higher resolution and shorter analysis time. Nevertheless, we can highlight some specific challenges for BVOC analysis, as follows. (1) High temporal resolution of the analysis – for example, to collect data that can be combined with results from micrometeorological measurements (eddy covariance) in order to calculate canopy emissions or allowing fast detection of changing microbial processes at the initial stage of food deterioration. Today, the possibilities of methods based on proton transfer reaction mass spectrometry (PTR-MS) or selected ion flow tube mass spectrometry (SIFT-MS) are fully involved/ explored for this purpose. Advantages are the sensitivity and real-time monitoring possibilities but less certainty about the exact meaning of the signals measured.
0165-9936/$ - see front matter ª 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.trac.2011.06.002
935
Editorial
(2) The need for a high level of speciation that, from a technical point of view, is often incompatible with the aim above. In this case, more classical sorption-sampling, thermal-desorption strategies with gas chromatography mass spectrometry (GC-MS) are still prevalent. BVOCs belong to a rather limited number of groups of chemical compounds, but large numbers of isomers (including enantiomers) occur within those groups. Compound identification is the strong point here, but, because of the need for a sampling stage, real-time monitoring is impossible. (3) The need for proper standards and reference materials allowing quantitative results, especially for compounds such as sesquiterpenes. Standards are not always commercially available, and, when they are, the purity of the isomer is not always warranted. Where no standards are available, two strategies can be followed: chemical synthesis; or, the search for a natural source from which the compound can be extracted and purified. Both ways are time consuming and labor intensive, and are therefore rather the exception than standard procedure at present. (4) The need for quality control with respect to artifact formation by either reactivity of the compounds (light/temperature), which results in
936
http://www.elsevier.com/locate/trac
Trends in Analytical Chemistry, Vol. 30, No. 7, 2011
detection of a compound that is not the one originally emitted, or sorptive losses. The latter is of concern, especially for the low vapor-pressure compounds and where samples are transferred over considerable distances (e.g., canopies) through sampling lines. When all necessary quality control measures have been implemented, the choice of the methodology (monitoring/discrete sampling) will often depend on the final goal of the analysis. The real added value of the analytical methods is using them in order to get complementary information that mutually backs up and reinforces the information derived from either method. This Special Issue on BVOCs covers different aspects of BVOC analysis. It presents an overview of different methodologies and approaches next to examples of stateof-the-art applications in the field of environmental science (including ecochemistry), food science and medical applications. Finally, we would like to thank all contributors to this special issue. We hope that the interested readers are convinced that BVOC research is a challenging field and that they will at least get a glimpse of possible pitfalls but, above all, will see the possibilities of these state-ofthe-art methodologies.
Editorial
Editorial Biogenic volatile organic compounds Jo Dewulf, Herman Van Langenhove Biogenic volatile organic compounds (BVOCs) are organics originating from various biological organisms and play an important role in biological, environmental, food and health sciences. From a quantitative point of view, there is no doubt that plants are the dominant source, with about 1035 Tg C/year emitted as BVOCs into the global atmosphere. These plant metabolic products are hydrocarbons (e.g., isoprene, monoterpenes and sesquiterpenes); others are oxygenated organics {e.g., alcohols [2-methyl-3-butene-2-ol (MBO)], terpene alcohols (C10H18O) or esters (hexenyl acetate), the so-called green-leaf compounds}. Ambient air concentrations of BVOCs are in the ppbv– pptv range. Their atmospheric life times vary from minutes to hours, illustrating the gas-phase reactivity of those compounds. In this way, BVOCs interfere with the atmospheric hydroxyl-radical pool. Furthermore in the presence of nitrogen oxides, BVOC oxidation can result in both increased ozone (greenhouse gas) concentrations and secondary organic aerosol (SOA) formation, influencing the radiation balance. As a result, BVOC emissions affect climate change through direct and indirect effects. Algorithms have been elaborated relating temperature and photosynthetic active radiation (PAR) to BVOC emissions for a number of important plant species under controlled conditions. These algorithms predict increased BVOC emissions with increasing global temperature. Jo Dewulf* Herman Van Langenhove Research Group ENVOC, Ghent University, Coupure Links 653, B-9000 Ghent, Belgium
*
Corresponding author. Tel.: +32 9 2645949; Fax: +32 9 2646243; E-mail: [email protected]
However, because of the non-linearity of interactions between biogeochemical cycles and climate change, the overall effect is quite uncertain as a result of possible feedbacks (e.g., drought, land-use change, and changing nutrient inputs). Furthermore, it becomes more and more apparent that, within the context of BVOC emissions, plants have to be looked upon not as isolated species but as ecosystems (e.g.. since infestation by herbivores induces considerable shift in the BVOC-emission profiles). Next to biological and environmental relevance, BVOC emissions are also associated with microbial metabolism in food processing/conservation or in infectious diseases. Adequate analytical approaches may lead to using these BVOCs as tracers or even as early warning signals. Although total amounts emitted are much smaller than emissions from vegetation, their impact as indicators can be very relevant. Chemical analysis and preferably realtime monitoring are essential and a challenge in all research related to the above-mentioned topics. Challenges related to BVOC analysis go along with those for anthropogenic VOC analysis in general, e.g., ever driving towards higher sensitivity, higher resolution and shorter analysis time. Nevertheless, we can highlight some specific challenges for BVOC analysis, as follows. (1) High temporal resolution of the analysis – for example, to collect data that can be combined with results from micrometeorological measurements (eddy covariance) in order to calculate canopy emissions or allowing fast detection of changing microbial processes at the initial stage of food deterioration. Today, the possibilities of methods based on proton transfer reaction mass spectrometry (PTR-MS) or selected ion flow tube mass spectrometry (SIFT-MS) are fully involved/ explored for this purpose. Advantages are the sensitivity and real-time monitoring possibilities but less certainty about the exact meaning of the signals measured.
0165-9936/$ - see front matter ª 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.trac.2011.06.002
935
Editorial
(2) The need for a high level of speciation that, from a technical point of view, is often incompatible with the aim above. In this case, more classical sorption-sampling, thermal-desorption strategies with gas chromatography mass spectrometry (GC-MS) are still prevalent. BVOCs belong to a rather limited number of groups of chemical compounds, but large numbers of isomers (including enantiomers) occur within those groups. Compound identification is the strong point here, but, because of the need for a sampling stage, real-time monitoring is impossible. (3) The need for proper standards and reference materials allowing quantitative results, especially for compounds such as sesquiterpenes. Standards are not always commercially available, and, when they are, the purity of the isomer is not always warranted. Where no standards are available, two strategies can be followed: chemical synthesis; or, the search for a natural source from which the compound can be extracted and purified. Both ways are time consuming and labor intensive, and are therefore rather the exception than standard procedure at present. (4) The need for quality control with respect to artifact formation by either reactivity of the compounds (light/temperature), which results in
936
http://www.elsevier.com/locate/trac
Trends in Analytical Chemistry, Vol. 30, No. 7, 2011
detection of a compound that is not the one originally emitted, or sorptive losses. The latter is of concern, especially for the low vapor-pressure compounds and where samples are transferred over considerable distances (e.g., canopies) through sampling lines. When all necessary quality control measures have been implemented, the choice of the methodology (monitoring/discrete sampling) will often depend on the final goal of the analysis. The real added value of the analytical methods is using them in order to get complementary information that mutually backs up and reinforces the information derived from either method. This Special Issue on BVOCs covers different aspects of BVOC analysis. It presents an overview of different methodologies and approaches next to examples of stateof-the-art applications in the field of environmental science (including ecochemistry), food science and medical applications. Finally, we would like to thank all contributors to this special issue. We hope that the interested readers are convinced that BVOC research is a challenging field and that they will at least get a glimpse of possible pitfalls but, above all, will see the possibilities of these state-ofthe-art methodologies.