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Evaluation of capacity of a TD-GCO-MS system in air sample analysis

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L. Vera1, E. Pagans, R. Domingues, A. Van Harreveld

Parc de Recerca UAB, Edificio EUREKA Espacio P2M2, 08193 Bellaterra. Tel: 935 929 048 Barcelona. 1 lvera@odournet.com (Luciano Vera)

Keywords: GC-TOFMS, odour, VOC, sensitivity, resolution

 

Abstract

olfactogram vs chromatogramKnowing the chemical composition of an air sample is a complex challenge because the abundance of substances of different nature and different concentrations. This complexity increases if we want to identify those compounds that cause odors, since many of them are present at very low concentrations, hardly detectable by more conventional instrumental systems. Gas Chromatography (GC) coupled at Mass Spectrometry (MS) GC-MS, is undoubtedly the more robust and consolidated technique in air sample analysis. The more common types of GC-MS are Mass Spectrometers with single quadrupole detector. However, the continuous technological development of analytical instrumentation and especially in Mass Spectrometry allows ever more robust analyses, more sensitive and with improved resolution. In recent years, the interest in GC-TOFMS systems (TOF: time-of-flight) has been increasing, because it involves a faster detector with higher sensitivity and resolution compared to the conventional single quadrupole GC-MS. This paper compares a GCO-TOFMS system recently acquired by Odournet to a more conventional GC-MS quadropolemethod. Both used thermal desorption (TD) for the chemical and sensory analysis of air samples collected in the output of biofilters. The presented results show a clear difference in terms of sensitivity and resolution for GC-TOFMS compared to a conventional GC-MS identifying almost50% more compounds in the same air sample. Additionally an analysis using GCO (GC-sniffing) was performed, showing good complementarity with the results obtained by GC-TOFMS for the same sample.

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Cromatografía de Gases

Wygasa
El Cromatógrafo de gases acoplado con un detector de masas (CG/MS), se considera generalmente como un dispositivo de laboratorio de ahí su limitado uso para análisis en campo. Aun así, existen algunas unidades portátiles para medidas en campo.

  El CG/MS puede ser usado tanto para identificar como para medir la concentración de un gas.

  Muestras de aire muy pequeñas se inyectan en una corriente de gas portador (nitrógeno o helio) que pasa a través de la columna de GC/MS. La columna adsorbe y desorbe los compuestos quñimicos en el aire a diferentes velocidades parasepararlos. Después de la separación, el gas portador junto con los compuestos químicos separados pasa por el detector. La señal de salida del detector identifica el compuesto químico y la concentración de este en la muestra.

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Colorimetric methods

Wygasa

 The colorimetric methods are able to identify, in many cases in a quantitative way, the presence of a gas. It is possible to use patches or colorimetric tubes or badges.

Badges.

  The badges are a one-use piece of cardboard with a small surface coated with a chemical that changes its colour when exposed to a certain gas. Both the time exposure and the intensity of the colour change are important. The badges show an integrated averaged value not very precise but useful when dealing with personal protection exposure.

 

These devices might be hanged anywhere, or be carried on the worker's cloths or they might be combined with small fans for different applications. Hydrogen sulphide badges are well used in farming, for example to limit H2S exposure in workers.

 

Colorimetric tubes.

  There are different types of colorimetric tubes to measure a wide range of gases. The Colorimetric tubes are glass tubes with both ends closed. To perform a measurement with a colorimetric gas tube, both ends of the tube must be broken. Then the tube is connected to a pump (manual or electronic). The pump makes the gas to circulate through the tube with a known volume of odorous gas. The media inside the tube reacts with the gas and change its colour according with the type of gas sampled. A scale is used to measure the amount of gas that reacts with the media. This scale indicates the concentration of the gas detected.

The colorimetric tubes usually have limited scales and the precision is around 10-20% of the full reading scale of the tube. The colorimetric tubes offer almost instant readings with detection limits down to 0,2 ppm in the case of Hydrogen sulphide (H2S). The cost of a manual pump stay between 300-500 €. The diffusion tubes that measure a mean concentration are also available for certain gases.

It is possible to find colorimetric tubes that work by means of a mechanism of diffusion, without the need of a pump. To use these passive samplers, one of the ends of the tube is opened and then the tube is placed in the site to be monitored. After a known period of time (normally 6 to 8 hours), a reading is taken by the colour change observed in the tube. This parameter and the exposition time are used to calculate average concentration for the time period sampled. This tubes have a price of around 6-8 € each.

You can check the different colorimetric tube makers in the directory of this site, or just clicking here.

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