ALCOHOL MEASUREMENT TECHNOLOGY
In this paper we identify six modes of alcohol measurement:
Fuel Cells
A fuel cell sensor is an electrochemical device in which the substance of interest, e.g. alcohol, undergoes a chemical oxidation reaction at a catalytic electrode surface to generate a quantitative electrical response.
Fuel cells are characterised by the following positive analytical features towards breath alcohol testing:
high analytical specificity to the chemical of interest
high analytical accuracy, especially at 'low' alcohol concentrations
linear response to alcohol vapour over a wide concentration range
no external power requirement for normal operation, so low battery usage
long working life
stable sensitivity with time i.e. low frequency of instrument recalibration
high reliability under various field conditions
Disadvantages with fuel cell sensors for breath alcohol analysis are:
a relatively high unit cost
a small, temporary 'fatigue' effect in response output when subjected to repeated doses of alcohol vapour at high concentrations; and
the need for periodic calibration
It must also be remembered that a fuel cell requires a separate sampling system, to inject or draw into it a small but fixed volume of the sample vapour to be analysed. Fuel cells, unlike infrared systems, are not continuous flow analysers. This means they cannot monitor the expired alcohol concentration curve, either to determine the passage of deep lung air, or to detect the presence of mouth alcohol.
Semiconductors
This sensor consists of a small bead of a transition metal oxide, heated to a temperature of around 300 °C, across which a voltage is applied to produce a small standing current. The magnitude of this current is determined by the conductivity of the surface of the bead, which may be affected by the presence [and concentration] of any substances adsorbed on to it.
So when alcohol [or one of many other substances] comes into contact with this bead, it is adsorbed on to the surface, changes the surface resistivity, and hence the standing current. This change in current is taken as a measure of the concentration of alcohol in the sample. The effect is a purely physical one: it is not specific to the alcohol molecule.
Semiconductors are non-specific to alcohol, non-linear in response to alcohol vapour concentration and unstable in sensitivity with time: and their effective working life is rarely longer than one year, but this actually depends to a large extent on how often they are used.
Further, since the surface effect by which they operate is dependent on the atmospheric partial pressure of oxygen, semiconductors have been found to vary in sensitivity to alcohol with changes in climate, and even more so at changing altitudes of operation.
Furthermore, caution should be taken when reviewing the claims made by some suppliers and distributors about the type of alcohol sensor employed in their instruments. The term 'fuel cell' has been used to describe sensing devices which are clearly semiconductors, as opposed to true fuel cells operating on elctrocatalytic principles and being dependent on the chemical [as opposed to purely physical] nature of the process.
Infrared Absorption
These analysers operate on the principle that organic substances absorb infrared light at various wavelengths depending on their atomic make up and molecular structure. The quantity of radiation absorbed depends on the concentration of absorbing substance present in the sample, and is thus a measure of it.
In such circumstances, therefore, the difference between the amount of infrared light entering one end of the sample chamber from that received at the other end is measured and taken as being proportional to the concentration of absorbing chemical vapour [breath alcohol] present in that chamber.
One advantage that infrared systems have over and above fuel cell and semiconductor sensors is that they are continuous flow analysers. This means that they are able to track the shape of the alcohol concentration curve during the course of an expiration, which allows the presence of both deep lung breath and mouth alcohol to be detected.
There is an argument in the scientific world as to whether infrared light in the 3 or 9 microns region of the spectrum should be used for breath alcohol testing. Each has its benefits.
Gas Chromatography
This technique involves injecting a small sample of the substance of interest [in this case, breath] into a heated separating column, through which it is then forced by a carrier gas. . The volatile components are separated from each other as they pass through the column, and enter a detector in discrete bands. The time taken for the appearance of each substance may be used to help identify it, whereas the size of the detector response is used as a measurement of its concentration.
Colorimetry
The oxidation of alcohol by an acidified solution of potassium dichromate, resulting in a quantitative yellow to green colour change, has been used in various early instruments.
The analytical principle is also still employed in disposable alcohol detector tubes (the bag and tube) used essentially for screening purposes.
Dual Sensing
Some companies are now combining two technologies - e.g. infrared and fuel cell - into one instrument.
In this paper we have discussed four major descriptors. It can be seen from descriptor (1) (Applications) that all instruments can be used for screening but that not all instruments can be used evidentially: some devices have multi applications. Thus in the instrument classifications proposed only (2), (3) and (4) are deemed appropriate to be descriptors.....
