Volatile Organic Compounds

Volatile organic compounds (VOCs) are chemical substances that have high enough vapour pressures under normal conditions to significantly vaporize and enter the atmosphere (Grossmannova et al. 2007). The volatility of a chemical depends on the size (molecular weight), polarity, and structure of the molecule and can be expressed as the

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vapour pressure. Thus molecules with a high molecular weight and a high polarity have a low vapour pressure. As an example, the highly volatile ethyl acetate has a vapour pressure of 76 Torr at room temperature, while on the other end of the vapour pressure scale a low volatile compound is nonacosane (nC29) with a vapour pressure of 5.0 x 10^-10 Torr at room temperature (Schulz 2001). However, international agencies define VOCs as organic chemicals containing carbon atoms and having a vapour pressure larger than 10 or 13.3 Pa at 25°C, equal to 7.5 x 10^-2 or 9.9 x 10^-2 Torr, according to the EU Solvents Directive (1999/13/EC) and the American Society for Testing and Materials (method D3960), respectively. A wide range of carbon-based molecules, such as aldehydes, acids, alcohols, ketones, esters, hydrocarbons and terpenoids are VOCs. Moreover, various oxygen-, nitrogen-, sulfur-, and halogen-containing molecules are also VOCs (Hunter et al. 2000).

They are released from several sources as shown in some examples in Table 1-1.

VOCs can contribute to pleasant or nasty odours, e.g. odours from flowers and foods are favoured, while smells of paints or moulds are not favoured. Moreover, VOCs could cause sickness. Recently it was revealed that microorganism-infested buildings released compounds affecting human health, generally known as “sick building syndrome”

(Jaakkola et al. 2007). On the other hand, VOCs can be used in promoting human health, as the use of natural volatile compounds in aromatherapy.

An increasing interest of studying VOCs is coming from chemoeclogical sciences, since several VOCs have been found to play important roles in nature as chemical signals among different organisms and ecosystems. As an example, the so-called “cry for help”

phenomenon is perhaps the most remarkable one. In this case plants released specific volatile compounds (e.g. methyl salicylate) as an external signal for the recruitment of beneficial insects (Forouhar et al. 2005). Therefore, VOCs are an issue of major concern for many scientists worldwide, being active in different disciplines such as wood technology, food, flavour and fragrances, medical, pharmaceutical, forensic sciences, and particularly environmental sciences (Demeestere et al. 2007).

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-Table 1-1 Examples of VOCs from different sources

Source Compound Odour Chemical structure Reference

mushroom 1-octen-3-ol mushroom odour

OH

Tressl et al.

1982

green leaves cis-3-hexen-1-ol green-leaf odour

O

vanilla orchid vanillin vanilla

OH CHO

OMe

Pomerantz et al. 1957

banana isopentyl acetate banana-like odour

O

tree α-pinene pine-like arome Koukos et al.

2000

To increase the knowledge on the occurrence of VOCs in all fields of interest, precise and accurate analytical techniques are necessary. There are two main steps for volatile characterisation: volatile sampling and volatile analysis.

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1.3.1. Volatile Sampling

Appropriate volatile sampling and pre-concentration techniques are required particularly in environmental conditions where VOCs concentrations are often very low, varying mostly from levels of pg/l to µg/l in air. Sample preparation and sampling methods are often the bottleneck and most time consuming task in VOCs analytical scheme. They can be roughly categorised in: passive and active samplings as shown in Figure 1-1.

Passive sampling is performed without air circulation, i.e., static headspace sampling.

The VOCs are sampled in static condition where they diffuse to specific absorbent materials. For example, solid phase microextraction (SPME) is a passive sampling technique where volatiles are adsorbed on polymer matrixes coated on silicon fibres (Zhang et al. 1994). In the last years, SPME has become an attractive and widely used sampling technique, despite its relative recent character (Belardi and Pawliszyn 1989;

Arthur and Pawliszyn 1990). Another passive sampling technique is the direct headspace sampling, where the headspace volatiles are taken using a syringe and are accumulated in a cold trap, before being analysed. Passive sampling techniques are often adopted for indoor air measurements and when air samples are taken from a confined area. They are often chosen being less elaborated than active sampling techniques.

Active sampling is carried out by promoting an air circulation. The air is forced to pass through adsorbents where VOCs are trapped. Widely used absorbents are activated charcoal and polymer matrixex (i.e. TENAX^®, Gerstel, Mülheim an der Ruhr, Germany).

The entrapped volatiles are later eluted with solvent for further analysis in case of chemical desorption (activated charcoal) or are eluted by hot gas and directly analysed in case of thermodesorption (TENAX). Active sampling requires power supply and may necessitate expensive equipments and skilled staff. Moreover, in this case there is a higher risk of contamination since pumps, loops and bags are often necessary. The advantages of active sampling are linked with a general higher sensitivity of the techniques and the possibility to quantify the volatile concentration and releasing rate with higher accuracy.

Moreover, in case of solvent elution methods there is a possibility to store for long time the VOCs samples allowing further analyses.

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-Figure 1-1 Schematic drawing of three volatile sampling methods. (A) passive sampling of static air with headspace-SPME device and (B) active sampling of circulating air with TENAX-TA tube (left) and activated charcoal tube (right).

1.3.2. Volatile Analysis

The analysis of volatile compounds is traditionally performed using gas chromatography (GC). There are many detectors which can be used in gas chromatography, each one giving different types of selectivity. Gas chromatography- flame ionisation detection (GC-FID) uses ionised combustion products and is the most common detector. Gas chromatography-mass spectrometry (GC-MS) uses chemical masses of ionised fragments for interpretation. Gas chromatography- electroantennographic detection (GC-EAD) uses insect antennae as detectors as depicted in Figure 1-2. Volatiles dissolved in solvents are

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injected into the GC injection port and run through the GC column for separation.

Volatiles absorbed in a polymer matrix have to be first desorbed, trapped and later heated up and run thought the GC column.

After the volatile samples are detected by the GC-MS, their chemical identification is done by interpreting and matching their mass spectra and retention times to the ones of authentic compounds. The Mass Spectral Search Library of the National Institute of Standards and Technology (NIST) and the Wiley GC-MS database are two mass spectra libraries widely adopted for this comparison.

Figure 1-2 Schematic drawing of a gas chromatograph-mass spectrometer/electroantennographic detector setup (GC-MS/EAD). MS: mass spectrometer, GC: gas chromatograph, ODP:

olfactory detection port and EAD: electroantennogram (modified from Weissbecker et al.

2004).

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