When prosecutors present the results of a piece of evidence analyzed in the lab, a question often comes up: how good is the measurement? It’s an important one to ask, because the error bars tell you how good the measurement is. If a Breathalyzer measurement was collected by police, and the result was near the limit for driving while intoxicated, it makes a huge difference if the error bar is 50% or 0.1%. At 50% the correct value could be half of the measurement. When the difference is spending time in jail or not, that 50% matters. This is what police in Massachusetts discovered when it was determined that their Breathalyzer machines weren’t calibrated .
In the article “Caution in interpreting FTIR/ATR spectral intensity values,” we discuss a similar issue with measuring liquids or particulate solids . The Fourier Transform InfraRed Attenuated Transmission Resonance (FTIR/ATR) instrument enables rapid measurement of chemicals that are in a solution or powder, but as we discuss, it does NOT do well at measuring HOW MUCH.
The FTIR/ATR stimulates vibrations/movements within a molecule which enables identifying the molecules present. The intensity of the measured signal at a specific wavelength does not linearly correlate to a greater amount of a chemical in a sample. While a greater amount of material will give a stronger signal, the technique is not reproducible across instruments, nor consistent over time.
We discuss the problem using three different FTIR/ATR instruments as examples, pointing out the difficulty in extrapolating quantitative data from instrument to instrument. In the field of forensics, a spectrum of an unknown substance is obtained and then compared to a library of spectra provided by the FBI (from their own machine, which would have drifted in its response with time) to determine identification and possibly the relative amounts of the components in the material. Due to the inherent error of the FTIR/ATR method, a secondary method to determine the relative amounts of chemicals within the material is necessary at the present time.
With this information, any criminal case dependent on FTIR/ATR results alone is vulnerable to doubt as to its accuracy. A thorough calibration procedure must be followed and documented. The original article can be found in the October 2017 publication of Analyst.
 State withheld evidence breathalyzer machine was faulty, lawyers charge By Maria Cramer GLOBE STAFF AUGUST 23, 2017
 Caution in interpreting FTIR/ATR spectral intensity values by Elizabeth A. Jensen, Doug Smith, Patricia Chapela and Susan M. Lederer ANALYST Issue 20 October 2017