Working principle of breath analyser

4 minutes read

Drinking and driving is an offense in many countries and its necessary to be an offense. However, consumption of alcohol is not an offense. There is a limit to which a person can drink and drive.

BAC- Blood Alcohol Concentration/content determines the quantity of alcohol consumed by a person [1]. In India, the legal limit of BAC is 0.03%. That means presence of 30mg of alcohol in 100 ml of blood [2].

Concentration of alcohol present in ones body can be easily detect by carrying out blood tests or urine tests. But sampling of blood and urine on the roads and carrying out test there seems impractical. Breath test/ breath analysis is easiest and efficient way to estimate the concentration of consumed alcohol.

Ever wondered about the device that policemen use on roads to check whether a person is driving after consuming alcohol or not. If yes, what is amount of alcohol a person has consumed?

What is that device called? How change of color takes place? How it tells about the amount of alcohol consumed?

Answer to all these questions is breath analyser or breathalyser. It’s a device that gives estimation of blood alcohol concentration (BAC) by testing exhaled air of suspected  person.

It works upon basic phenomenon of chemistry which is oxidation. Alcohol oxidizes readily in presence of oxidizing agent.  Most of breath analysers work on the principle of oxidation of alcohol by a suitable oxidizing agent under certain conditions that produces a change of color due to occurrence of a chemical reaction.

Chemical equation for the reaction:

Here, ethanol react with potassium dichromate (oxidizing agent) under acidic conditions and in presence of silver nitrate as a catalyst. Ethanol gets oxidized to acetic acid while red orange colored potassium dichromate, oxidizing agent  gets reduced to chromium sulphate which is green in color.

Higher is the concentration of ethanol in breath, more of the potassium dichromate will be used and higher will be the intensity of green color. Similarly, lower the concentration of ethanol in breath, less of potassium dichromate will  get used and intensity of green color will be less. In very low concentrations of ethanol, red color persists.

Thus, amount of change in color tells the concentration of ethanol.

The redox reaction shown above is an irreversible reaction. But, a single breath analyser contain enough oxidizing agent so that, it can perform 250-300 tests. It can also be left unused for approximately 12 months and still get accurate results [3].

But, not all breath analysers work on oxidative reaction of alcohol and showing color changes.

Two other majorly used breath analysers are:  

• Infrared Detection [ Intoxilyser] 

This breath analyser detects alcohol  on the basis of infrared spectroscopy. Infrared spectroscopy involves interaction of infrared radiation with matter. Every functional group absorbs and emit IR radiations at different wavelengths that is unique to a particular functional group. Alcohol absorbs IR radiations at wavelengths of approximately 3.4-9.3 µm. Thus, if alcohol is present in breath it will show absorption spectra nearly around wavelength 3.4-9.3 µm.

To calculate concentration of alcohol we use Beer Lambert’s law, which states that 

 

Where I = final intensity of IR,     = initial intensity of IR,      a = absorption coefficient, b = path length,  c = concentration of alcohol

The breath sample of suspected is radiated inside a sample chamber. If alcohol is present in the breath, it will absorb the IR radiations, thus decreasing the intensity of IR rays reaching detector. Higher the concentration of ethanol in the breath, lower will be the intensity of radiations falling on detector. Electronically one can measure the decrease in intensity.                                                                      

Fuel Cell    [Alcosensor III and IV]

 Its also called as electrochemical detector. Fuel cell breath analyser works on the principle that ethanol after being oxidized, can produce direct current. Pure ethanol doesn’t conduct electricity. But when ethanol gets oxidized to carbon dioxide in presence of water it yields  electrons  which  produces direct current.

 Equation shown above is basic reaction that takes place in fuel cell breathalyzer.  As we can see, number of electrons produced depends on number of ethanol molecules present. Thus, intensity of current produced is directly proportional to the concentration of ethanol present.

We can find out concentration of alcohol by measuring the intensity of current.

Now the question that arises here is, how ethanol concentration of exhaled air tells us about Blood Alcohol Concentration?

When a person consumes ethanol, blood quickly absorbs it via mouth, throat, stomach and intestine. But its digestion is not as instant as its absorption is.

So, when blood passes through lungs, alcohol present in blood diffuses into alveolar air (air present in alveoli of lungs) as alcohol is a volatile substance. Person exhales this air and thus, breath of drunken person will have ethanol in it.

Concentration of alcohol present in breath is in fixed ratio with BAC. This fact is based upon Henry’s law which states that concentration of gas dissolved in a liquid is directly proportional to the partial pressure of gas above the solution.

Its assigned that blood:breath ration for ethanol is 2100:1. Which means 2100 ml of breath contains equal concentration of ethanol as present in 1 ml of blood [4].

Everything has its pros and cons. So, is true in case of breath testing as well.

1.) Mouth alcohol affect [MAE]

One can find Alcohol vapours in the oral cavity of a person who had recently consumed alcohol. These vapours interfere with the exhaled air from lungs and can show false positive results for a higher BAC [5].

Affect of MAE lasts only for about 10 minutes after the consumption of alcohol. So, before going for breath analysis, it must be checked that person has not taken alcohol in span of less than 10 minutes before testing.

2.) Non cooperation of suspect

Poor breath samples could be obtained if the suspected person is not exhaling air properly as he is asked to do. This results in fall of results to about 20-40%.

But, inspite of these two drawbacks. The probability of showing  inaccurate results by breath analyser device is very low practically.

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