Physical-chemical analysis

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   Both olfactometry and physical-chemical analysis are two methods used to evaluate the somehow relative annoyance caused by odorous substances. These techniques have several advantages and also some drawbacks although they can provide complementary information.

  With the olfactometry technique odour threshold can be quantified but in most cases these parameters cannot be chemically linked to a substance or group of substances. On the other hand, a physical-chemical analysis can identify single substances that could be related with odorous episodes, but in most of the cases these relation is not possible. Physical-chemical analysis proportionates also information about the toxicity of certain odorous gases, however, there will hardly be a link between concentration of a substance and odour annoyance using this technique. As a short example, more than 500 molecules have been identified in a sample taken from a landfill (terpenes, sulphides, aldehydes, acids, acetones, alcohols, aromatic compounds, chlorides, esters, etc.).

  An odorous gas can be sampled with different sampling devices such as packed tubes, Tedlar®/Nalophan® bags, canisters, etc. The election of the sampling method has to be evaluated as previous step in the planning process. The criteria to use one or the other method is out of the scope of this article but as a general rule, when odour sensation on site is well distinct, Tedlar®/Nalophan® bags can perfectly be used both for physical-chemical analysis and dynamic olfactometry. In any case, independently of the sampling technique, the substance sampled has to remain in stable conditions when transported to the lab for analysis.

  In certain cases, it is possible to measure directly on site. This is quiet convenient when the influence of changes in processes on a direct emission of odours is the object of the study or when there is a incident with immediate risks to the population. In this case there are several methods such as colorimetric passive tubes, electronic detectors or portable Gas Chromatography analysers coupled with several detectors (MS, NPD, FID, PID, FPD, etc). Several pros and cons can be outlined but in any case and when money is not problem, a portable GC/MS is a very good option. As an example, the regional environmental ministry of Andalusia is regularly using a GC/MS to measure high hydrocarbon emissions in oil refineries in the Gibraltar area to identify toxic substances on site.

 It is also possible to evaluate odour concentration on site by means of mobile labs in vans or similar. In this case the calibrated odour panel is on site and samples are evaluated inside the van. These vans or similar have to be odourless and perfectly isolated from the ambient air. Some companies nowadays use these mobile olfactometric laboratories, eg. EOG (France), ECOMA (Germany) or the Odour Unit Pty Ltd (Australia) which are able to relocate to meet clients' specific needs.

 Due to the fact that in most cases an odour is made of several substances at extremely low concentration, it is very difficult and in some cases costly to find out the exact composition of an odour. In any case, physical-chemical analysis is always a good add on for any evaluation of odour annoyance.

There is a link between odour threshold and chemical composition for some substances. Here, there is one small compilation for some compounds.

chemical compound

Odour threshold, ppm

 

Aldehidos

 

Acetaldehido

0.21

Propionadehido

0.0095

 

Acidos grasos volátiles

 

Ácido acético

1.0

Ácido Propionico

20.0

Ácido Butyrico

0.001

 

Compuestos nitrogenados

 

Metilamina

0.021

Dimetilamina

0.047

Trimetilamina

0.00021

Skatol

0.019

Amoniaco

46.8

 

Compuestos azufrados

 

Metanotiol

0.0021

Etanotiol

0.001

Propanotiol

0.00074

t-Butiltiol

0.00009

Dimetil sulfito

0.001

Hydrogen sulfide

0.0072

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