The science of odour - part 2

23rd of August 2023
The science of odour - part 2

Odour plays an important role in the cleaning products industry. Fragrances can be added to products to elicit specific associations and emotions, or to mask a malodour. Specific technologies have been designed to battle malodours such as tobacco and pet odours. Lieke van Genderen at Olfasense tells us more in the second of a two-part report written for ECJ. Read part one on page 35 of the April/May edition, or by clicking here.

There are many parameters on which the panellists can evaluate an odour. The most common are the odour intensity, hedonic tone, odour character and odour concentration.

These first three parameters can all be determined by so-called ‘direct sniffing’. Panellists evaluate the odour samples undiluted and blindly, directly from Nalophan bags or other specific recipients. The odour intensity can be assessed for the overall odour of a sample, but also for specific odours. For example, when a cleaning product with a fragrance is used it is often interesting to ask the panellists for the fragrance and malodour intensities separately.

Panellists should be trained in the usage of odour intensities scales with reference odours. Reference odours are also used to train panellists in the odour character description. For example, while the odour of cigarette smoke might be more easily described by most people, the subtle notes of a fragrance require more training. The hedonic tone shows the pleasantness of an odour. As this is a subjective impression, panellists cannot be trained in this parameter.

The parameter of odour concentration is measured with the use of an olfactometer.  The aim of this method, known as dynamic olfactometry, is to determine how often an odour can be diluted until the panellists are just able to detect the odour, ie,  the detection threshold. This is expressed in an amount of  European Odour Units, the OUE. A higher OUE means that an odour has a lower detection threshold and is thus stronger. By measuring before and after cleaning conditions, the odour reduction efficacy can be expressed as a percentage in OUE reduction.

Important to note, is that the odour concentration looks at the overall odour. This means that when a fragrance is added to a cleaning product, the amount of OUE directly after cleaning might be higher than before cleaning, resulting in a negative odour removal efficacy while the odour might have become more pleasant. In cases with perfumed products, is it therefore important not to consider this method (dynamic olfactometry) but measure the malodour and fragrance intensities as well as the hedonic tone.

Instrumental odour analyses

Instrumental analyses are used when the chemical composition of a sample is of importance. This can be the case when there are concerns about health and safety. Chemical analyses are also used during new product development. After all, in order to create new techniques to remove specific odours one needs to know which chemical components are the relevant ones. Often used methods are GC-MS and GC-IMS.

The GC-MS method consists of gas chromatography (GC) and mass spectrometry (MS) analysis. Usually, the odour samples are collected in thermal desorption tubes for further analysis. Desorption of the volatile organic compounds present in the odour sample is done with high temperature (eg, 280-300°C), followed by a low temperature adsorbent trap (eg, 0-5°C) and another heating round (eg,, 300-350°C) to release the adsorbed volatile compounds into the gas chromatograph (GC) column. In the GC column, the volatile organic compounds are separated based on their interaction with the GC column and physical-chemical properties.

After going through the column, the volatile compounds arrive at the MS, whose measurements are transformed into a chromatogram. In the chromatogram, the volatile compounds are represented as peaks showing the intensity and time of detection of the signal. Based on the information of these peaks, the different volatile compounds can be chemically identified and quantified. A GC-MS analysis results in a list of identified chemical compounds and their chemical concentrations.

GC-IMS  is often seen as an alternative method to GC-MS, it consists of gas chromatography and ion-mobility spectrometry. The GC-IMS provides less detailed information but is much faster and thereby allows for continuous sampling. Thanks to the relatively small size of the GC-IMS, it can be brought to a sampling location for direct measurements.

The volatile compounds at the GC-IMS inlet are separated by a GC column, after which they are ionised, drift into an electrical field and are separated based on their charge, mass and shape. After going through the drift region, the volatile compounds reach the detector which transforms the data in a so called ‘fingerprint’. The fingerprints of different samples can be compared, for example to monitor a room’s air over time.

Selection of the analysis method

As there are many different analysis options, it can be difficult to select the best analysis for specific research questions.

For the cleaning product against cat urine odour, a study to make an odour claim on the packaging makes sense. In order to claim odour reduction, a sensory odour analysis is required. One option would be to work with odour concentration analyses for the untreated condition, the new product cleaned condition and the reference product cleaned condition. By comparing the odour concentrations of these different scenarios, it is possible to calculate the odour reduction percentage which can be achieved by using the anti odour product.

If one of the products is perfumed, using only odour concentration would not be sufficient. The odour concentration is about the strength of an odour, and adding a perfume can increase the amount of odour. The odour reduction percentage might then become negative, while the odour did become more pleasant. In such cases, it is important to either include the hedonic tone or to switch to odour intensity of the malodour and perfume odour separately.

To determine the efficacy of odour cleaning services, both sensory and instrumental analyses could be used. The sensory analyses, such as odour intensity for malodours, hedonic tones and odour characters, can be used to show if there is a difference in the odour before and after cleaning. In cases with chemical spills, it can be very important to also assess the safety of the resulting odour.

After cleaning, the indoor air should not exceed safety levels for specific chemicals. GC-MS can be used to detect and quantify a large range of chemicals. For some chemicals, such as formaldehyde, specific cartridges can be used to quickly quantify the amount.


Typical odour studies in the cleaning product industry focus on quantification of the odour reduction efficacy of products and services. Sampling methods are highly individual and are based on the specific product or service. The evaluation methods also depend on the research questions. In order to create a new anti-odour technology, insight in the chemical composition of specific malodour can be needed.

This can be done with the use of instrumental analyses, such as GC-MS and GC-IMS. In order to make a claim about the efficacy of a finished  product or service in terms of the perceived odour, sensory odour analyses are needed.


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