The Chemistry of Random Breath Testing

Under new legislation in Western Australia your licence is now revoked immediately if you return a blood alcohol reading of 0.08% or higher.

Last week in his weekly discussion with Millsy on 6PR Tom Percy QC expressed his disagreement with this law on the grounds that it smacks of a “guilty until proven innocent” mindset, and would be unfair if a person who had lost their licence was later exonerated.

No doubt this is based on the protocols involving drug testing of athletes when A and B samples are taken, and if a positive result is returned for the A sample, the B sample is analysed to either confirm or quash the initial result.

The problem is that with athletes, all uncertainty in the process involves the chain of custody.  That is, if there is a discrepancy between the A and B samples, no one ever says that the lab made a mistake the first time – the issue always is that one of the samples was tampered with between the sampling point and the lab.

With blood testing of alcohol, however, this is not an issue.  If you are tested in a booze bus, a blood sample is taken and analysed on the spot.  Therefore, if it returns a result of over 0.08% then your blood alcohol is indeed over 0.08% – there is no explanation other than this.  In other words, you ARE guilty.

That’s why there is a booze bus.  It’s essentially a lab on wheels.  Inside the lab is the instrument that is used to analyse your blood – a gas chromatograph.  And this is sufficiently mature technology now that it is feasible to make them portable in the back of a bus.

With performance enhancing drugs however this is not the case – the instruments required for this process are highly sophisticated instruments that are far too delicate and complex to be put in the back of a bus.  Therefore the sampling and analysis are done in different locations.

Want about the little things you blow in?  These are small fuel cells – exactly the type of units that were used in the Apollo 13 spacecraft.  A gas passes across one of the electrodes, reacts with an electrolyte and generates a voltage.

When the Apollo spacecraft lost its oxygen supply, it therefore lost its power.  In the case of the roadside units, the alcohol is the reactant.  The more alcohol there is in your breath, the higher the voltage.  The reason you cannot be convicted of drink driving purely on the basis of this is twofold: firstly, they are not accurate enough, but are only a screening device.

But secondly, and more importantly, to be convicted of a blood alcohol reading, the blood must be tested, and not your breath.  There is however a strong correlation between the amount of alcohol in your breath and the amount in your blood.

So if Tom Percy QC has a problem with this law, he’ll need to find another argument.

6790cookie-checkThe Chemistry of Random Breath Testing