The existing standard EN 13725, first published in 2003, requires revision. The 9^th meeting of the group that deals with the revision of the EN 13725 just took place in Antwerp, Belgium. The members of the Task Group that is dealing with the calculation of the uncertainty agreed that n-butanol is probably not representative for all environmental odours. In addition, n-butanol does not necessarily describe the sense of smell for a panelist very well. The new Standard EN 13725 will define a new procedure to design and test any new reference odorant. Is this the end of n-butanol as a reference material for olfactometry?

   The odour concentration of an odorous gas sample is determined by presenting a panel of selected and screened human subjects with that sample, varying the concentration by diluting with neutral gas, in order to determine the dilution factor at the 50 % detection threshold. At that dilution factor the odour concentration is 1 ou_E/m³ by definition.

   The ou_E is that amount of odorant that, when evaporated into 1 m³ of neutral gas at standard conditions, elicits a physiological response from a panel equivalent to that elicited by one European Reference Odour Mass (EROM), evaporated in 1 m³ of neutral gas at standard conditions. For n-butanol, for example, one EROM is 123 μg evaporated in 1 m³ of neutral gas, at standard conditions, that is a volume fraction of 40 ppb.

However n-butanol

• Is probably not representative for all environmental odours.
• Does not necessarily describe the sense of smell for a panelist very well
• The inter laboratory comparisons with n-butanol probably does not describe the inter laboratory uncertainty component very well

  One major limitation of n-butanol is that only a few receptors can perceive the odour. Malfunction of one of these receptors can have a big influence on the panelists perception of the odour of n-butanol. The panelist might be good for environmental odours or other mixtures of odorants.

  Malfunction of one of many receptors has minor impact on the total sense of smelling. Consequently a mixture could be better also for selection purposes as a mixture can be perceived by many olfactive receptors.

   One EROM has, by definition, a concentration of 1 ou_E/m³. To produce an EROM any odorant must be evaporated into 1 m³ of neutral gas at standard conditions, in a way that will elicit the D50 physiological response, assessed by an odour panel.

  There is one relationship between the ou_E for the reference odorant and that for any odorant. This relationship is defined only at the D50 physiological response level, where:

 1 EROM = 123 μg n-butanol = 1 ou_E for any odorant.

 This linkage is the basis of traceability of odour units for any odorant or defined mixture of odorants to that of the reference odorant n-butanol. It effectively expresses odour concentrations in terms of ‘n-butanol mass equivalents’.

To establish a robust estimated value of the molar mass for a specific odorant compound or a defined mixture of odorant compounds that corresponds to the EROM, the new standard EN 13725 will define a new procedure to design and test any new reference odorant. The idea behind is to select better reference materials to improve the uncertainty due to the selection of the panellists.

  Will this be the end of n-butanol as a reference material for olfactometry? well not yet as any new European Reference Odour Material will need to have a response equivalent to that of the n-butanol...

Photo: 9th meeting of the Working Group 2 of the Technical Committee 264 of the European Committee for Standardization in Antwerp

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• How a European Norm about odours was born in 1991
• The 8th Meeting of the group dealing with the updating of the Norm EN 13725 took place in Amsterdam
• 7th Meeting to review the European Norm EN 13725
• A new meeting of the committee reviewing the EN 13725
• A New Norm to Measure Odours in Europe, the new EN 16841