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Área de trabajo: Análisis de suelo, Medio Ambiente.

Título: Tecnología de monitoreo de VOCs en aire y su aplicación en suelos contaminados.

Título original:  VOC air monitoring technology and its application to contaminated land.

Autor: Markes International Ltd, UK.

Introduction

Background to sorbent tube air sampling

Sorbent tube sampling together with thermal desorption (TD)–GC(MS) analytical technology has been optimised and validated over many years for monitoring vapour phase volatile organic chemicals (VOCs) in air. Both diffusive1–3 and pumped4–6 sampling mechanisms have proved their worth for a wide range of common volatile organic pollutants at concentrations ranging from hundreds of parts per million in polluted industrial air to low/sub-part per billion levels in the ambient environment.

While not so well-suited to the most volatile species such as C2 hydrocarbons, sorbent tubes are perhaps the most versatile of all the available air sampling technologies. Data are reported for C3 to n-C26 7,8 hydrocarbons, halogenated hydrocarbons 9,10, ketones 9,10, esters 9,10, aldehydes 9,10, glycol esters 9,10, alcohols 9,10, organic nitriles 9,10, aromatic amines 9,10, polyaromatic hydrocarbons (such as anthracene 11) or some of the more volatile polychlorinated biphenyls, toluene diisocyanate 12 and organic sulfur compounds (see Application Note TDTS 32).

The cost and performance of sorbent tube monitoring methods have also been enhanced by the now widespread acceptance of TD–GC(MS) analytical technology. Thermal desorption offers a significant improvement in sensitivity and numerous practical advantages over conventional solvent extraction methods13. It eliminates manual sample preparation and allows sorbent tubes to be reused up to 100 times without regeneration or repacking. These factors combine to make sorbent tubes among the most affordable of air sampling techniques.

 

Monitoring contaminated land

The quality of such air monitoring methods contrasts sharply with many conventional soil monitoring procedures. Soil is a complex and non-homogeneous matrix, typically of unknown organic/inorganic composition and with its own ecosystem of micro- and macro-organisms. It is difficult, if not impossible, to make absolute analytical measurements given the number of variables involved. Conventional soil analysis involves a sampling step followed by some form of sample preparation (addition of biocides, buffers, etc.), sample screening and then quantitative purge-and-trap–GC(MS) analysis. It is the sampling of the soil itself that presents the most difficult challenge.

Even relatively large, kilogram-level samples cannot be considered representative of a wider area, and the very act of soil removal disturbs and modifies contaminant levels. The presence of micro-organisms also means that organic concentrations vary with time. Additional considerations include the expense of manual sample collection and preparation, the limitations of short sample turnaround times, and the cost of both screening and quantitative laboratory work. These all make it difficult to get enough samples to overcome soil non-homogeneity issues and adequately map out the profile of underground pollution. The method is even open to abuse by ploughing/turning-over the land to reduce pollutant levels prior to sample collection.

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