Advantages and limits of adsorption sampling for physico-chemical measurements of odorous compounds

Abstract

A lot of activities such as agricultural and industrial plants can be responsible of malodorous emissions that induce annoyance for human beings [1]. Odours, which are due to the presence in the air of organic and inorganic complex mixture at very low concentrations, are still not easy to analyse. Gas chromatography coupled with mass spectrometry (GC/MS) is generally used and seems to be the most powerful analytical technique available today to analyse organic odorous compounds. According to the very low perception levels, an adsorption step on a suitable solid sorbent is often required as sample preconcentration. Sample recovery is generally carried out by thermal desorption (TD) followed by analytes separation and detection using gas chromatography and mass spectrometry (GC/MS). This last technique is convenient for evaluating present organic molecules and their concentration [2]. For many applications, it has become the standard method for the determination of target compounds in atmosphere [3][4][5][6].Difficulties in the overall analysis of malodorous atmospheres arise from the different properties of their components, such as volatility, polarity and reactivity. None of commercial sorbents can be considered universal, i.e. available to retain quantitatively and later to release all volatile organic pollutants. So, the choice of a sorbent is very important to preserve sample integrity. In order to trap odorous compounds with very different properties, multi-bed sorbents seem to be useful [7] and therefore two sorbents: a carbon molecular sieve, Carboxen 564, and a porous polymer, Tenax TA were used alone or combined in this study. In a two-bed sorbent, Tenax TA is placed first to trap heavy molecules and Carboxen 564 is put secondly to trap smaller compounds.The influence of several factors such as temperature, concentration, flow rate and humidity should be considered on adsorption process [8]. In some cases, field sampling depends on weather conditions. For example, 90 % relative humidity (r.h.) at 25°C refers to a water vapour concentration of 21000 mg.m . Thus, when air is collected during rain or high atmospheric humidity, water may be the only major compound retained on sorbent material because of organic compound levels which are typically lower [9]. Presence of water in the collected sample is also unacceptable because it presents a major problem in the subsequent GC/MS analysis. There is a gradual desactivation of the chromatographic column after repeated analysis and injection of water can cause failure when using mass spectrometry. In humid conditions, Tenax TA and Carboxen 564 were studied. Several methods have also been developed to remove water vapour from the sample stream before reaching the sorbent tube, involving absorption with hygroscopic salts [10][11] or Nafion dryers [12], but water removal leads to losses of polar compounds. Another approach is to trap the water of the effluent by condensation at low temperatures [13]. However, their suitability for the whole set of compounds is stil...

Os biocombustíveis no Brasil: panorama atual, emissões gasosas e os métodos analíticos de monitoramento da qualidade do ar referente a gases de natureza orgânica / Biofuels in Brazil: the current scene, gaseous emissions and analytical methods on air quality monitoring concerned to organic gases

“…O método envolve, basicamente, o uso de tubos amostradores ou caixas (também conhecidas com a utilização de adsorventes hidrofóbicos, o vapor d'água pode interferir no sentido de danificar equipamentos e alterar os resultados das análises. Seu controle pode ser feito por meio de sais higroscópicos (como o carbonato de sódio, sulfato de magnésio, cloreto de cálcio) (GUILLOT et al, 2000); banho como badge). Em geral, os tubos amostradores possuem um longo caminho axial de difusão e pequena área transversal, o que resulta em taxas de amostragem relativamente baixas.…”

“…Assim, a recuperação da amostra é normalmente feita em equipamentos de dessorção térmica automática (ATD, do inglês "automatic thermal desorption"), ou por meio do uso de solventes (extração). Só então a amostra segue para a separação e detecção no sistema (GUILLOT et al, 2000;MIRANDA, 2007). O uso do ATD facilita significativamente a extração de gases orgânicos pré-concentrados em tubos adsorventes, levando a uma melhor concentração e transferência dos analitos dessorvidos para a coluna cromatográfica (WOOLFENDEN, 2010b).…”

Rapid Preconcentration and Enrichment Techniques for the Analysis of Food Volatile. A Review

“…The weaker sorbent (Tenax TA) is placed first to trap the heavier molecules and the lighter compounds are retained on the stronger sorbent located in second position. Desorption always takes place in the reverse direction to the adsorption step (Guillot et al, 2000).…”

“…Therefore efforts have been made to develop a sorbent with low water affinity. But even in the case of the hydrophobic Tenax TA or Carboxen 564, the trapping of water can cause trouble when the relative humidity of the sample is above 90% (Guillot et al, 2000). Too much water entering the system can also damage the MS-detector (Hinshaw, 1990) and induce a modification of the spectrum, rendering identification difficult (Westendorf, 1992).…”

“…It was shown that of the three hygroscopic salts, sodium carbonate, magnesium sulphate and calcium chloride, only sodium carbonate seemed not to absorb significant amounts of the volatile components (Guillot et al, 2000). Phosphorus pentoxide is also very effective for selective absorption of water (Valero et al, 1997).…”