Mixture Analysis: The Importance of Reference Spectra

Mixture Analysis: The Importance of Reference Spectra

I’ve been struggling the last few weeks analyzing complex mixture samples sent to me by clients (performing FTIR sample analyses is part of my business…call for details). My challenges with these samples have me musing yet again on mixture analysis. As I teach in my Fundamentals of FTIR course, perhaps the biggest practical disadvantage of FTIR is mixtures. The problem is that the more molecules there are in a sample the more difficult it becomes to figure out what peaks are due to what molecules. FTIR is frequently sold as a great technique for identifying molecules in samples. It is-if the sample is pure or relatively simple in composition. However, my recent struggles with complex mixture spectra have shown me yet again the limitations of FTIR as a mixture analysis technique. My infrared spectral interpretation skills by themselves have not been enough to make any firm conclusions about the composition of these samples. I have found reference spectra to an invaluable aid in handling this type of problem.

By comparing the spectrum of your unknown mixture to spectra of known molecules you think are present, it is sometimes possible to identify the presence of molecules in a mixture. However, one needs to be careful. When comparing sample and reference spectra you can only conclude with 100% certainty if a molecule is present in a sample if all its peaks are present in the reference and the sample. Unfortunately, overlapping peaks and signal to noise problems can prevent you from clearly seeing all the peaks for a given molecule in a mixture spectrum. In these cases you have to find as many matching peaks as you can and use your judgment as to what to conclude. Frequently, phrases such as “suggest the presence of”, “are consistent with”, or “infer” should be used in these circumstances.

Serious consideration needs to be given to the reference spectrum to use. As I teach in my Infrared Spectral Interpretation course, when comparing sample and reference spectra it is a requirement that they be measured at the same resolution and important they be measured on the same instrument using the same sample preparation technique. You also need to consider the chemical matrix of the sample and reference. If, for example, the unknown is a solution of things dissolved in water it would be best to compare it to a water solution of the molecules suspected of being in the unknown. An appropriate reference spectrum gives you additional information along with your own interpretation of the spectrum when tackling mixture spectra.