What are the merits of GC-MS and electronic noses?
Gas Chromatography - Mass Spectrometry (GC-MS) has an excellent sensitivity (about 0.2 ppb) and separation for a gas mixture of like odorous compounds and can ideally be used for identification and quantification of odours. Gas chromatography separates individual components according to their vapour pressures and solubility inside the GC column material. Mass spectrometry identifies the eluted components by their ionized molecular fragmentation patterns. With proper selection of GC column material and GC temperature programming procedures, the method can be adapted theoretically for analyzing animal odorants. To date, more than 200 specific odorants have been reported. Major categories of odorants found within samples of odorous air are:
- Inorganic compounds: Hydrogen Sulfide, Ammonia;
- Sulfide compounds: Sulphides, Mercaptans, Thiophenol;
- Nitrogen compounds: Amines, Indoles, Skatoles;
- Volatile Fatty Acids: Acetic acid, Butyric acid and Propyl acid.
GC-MS can be used to determine chemical concentrations and compositions of an odorous sample. Much effort has been put into the development of chemical analysis technology using GC-MS. The major limitations of the technique are:
- Much identification remains ambiguous or questionable as a result of the presence of unknown components at very low concentration level (ppt). However, preparation of standard gases for calibration of GC-MS analysis has to date proved impracticable.
- Chemical concentrations corresponding to the odour detection thresholds of complex mixtures of odorous compounds can not be determined using chemical analytical techniques because of the synergistic olfactory effects of stimuli comprising complex mixtures of gases.
- No indication is obtained as to the relevance of individual compounds to the odour of the sample as a whole. Even if individual chemical concentrations and their odour threshold values are known, it is not possible to deduce the overall sample odour threshold or the odour character of the mixture of odorants.
For the reasons given above, the characteristics of complex odours cannot be derived reliably from the individual chemical characteristics and chemical concentrations of the odorous compounds present in a gas mixture. However, chemical analysis may help a process design engineer to select equipment if the type of odour is unknown, and may help researchers understand the mechanisms of odour removal.
Like GC-MS, the Electronic Nose (E-Nose) uses an array of chemical sensors (4 - 32) that respond to the presence of odorous compounds in air. E-Nose development is progressing in the food and wine industries. For environmental odours, the use of an E-Nose faces a large number of uncertainties. In addition to the reasons mentioned above, sensitivity and selectivity of E-Nose might not be satisfactory. The influence of humidity can lead to some degree of fluctuation in the response signal. Integration of data from 4 - 32 sensors requires a high degree of computation. Chemical (GC-MS) and sensory (dynamic olfactometry) calibrations are needed to confirm the repeatability and reproducibility of E-Nose techniques.
In summary, GC-MS is useful to partially quantify the chemical constitution of an odour. Results can be assessed to help understand the physical, chemical and biological processes occurring in the emission sources of interest. The practical application of E-Nose technology to environmental odours would require significant development. For odour related issues, dynamic olfactometry has become the normal tool for odour impact assessment purposes.