Debunking FTIR Myths I: "FTIR Can't Identify Things in Mixtures"

I am not normally one to trash other chemical analysis techniques. But when people mouth untruths about FTIR it raises my dander and I feel compelled to respond. I have talked to several people lately who have said, “FTIR can’t be used to identify things in mixtures. It’s easier to shoot the sample into the GC-MS instead.” I am fully aware, and I always teach in my courses, that FTIR has difficulty with mixtures. However, as I also always teach, there are strategies for getting around the mixture analysis problem, some of which I have talked about in previous blog posts. These techniques include purifying the sample, mixture analysis software, subtraction, library searching, and the “process of elimination”. These techniques make analyzing mixtures doable, and frequently some of the components in a mixture can be identified from the infrared spectrum of a sample as a result. I have identified components in mixtures hundreds of times during my career using these tools. FTIR can be used to identify things in mixtures if the analysis if performed properly.

GC-MS stands for Gas Chromatography-Mass Spectrometry. In this technique the GC purifies mixtures into their components, and then the mass spectrometer identifies each component. Thus, it appears GC-MS does not suffer from a mixture problem like FTIR. Additionally, GC-MS is more sensitive than FTIR and does a better job of handling aqueous solutions. I have in fairness sung the praises of GC-MS.

Here is why the phrase “It’s easier to shoot the sample through the GC-MS” raises my ire. A GC-MS is always more expensive, more time consuming, and more difficult to use than an FTIR. This is why so many more labs have an FTIR than a GC-MS. Also, GC-MS does suffer from a mixture problem. The retention time of a peak in a gas chromatograph is not unique to that molecule. Molecules with different chemical structures can, by coincidence, elute at the same time. Thus, there is always the possibility that what we think is a pure component coming off a GC column may in fact contain two or more components. The other dirty little secret about a mass spectrum is that it does not provide a completely unique fingerprint of a molecule. Mass spectrometry does not distinguish between structural isomers. These are molecules that have the same chemical formula but different chemical structures. For example, a mass spectrometer normally can’t distinguish ortho-, meta-, and para- xylene from each other. These three molecules are easy to distinguish by FTIR. Infrared spectroscopy can distinguish between structural isomers, and provides a unique fingerprint for a given molecule. A mixture of structural isomers would be difficult to distinguish by MS. It could thus be argued that you can’t identify things in mixtures by GC-MS either.

There is really is no victor in the FTIR/GC-MS smackdown except hopefully the truth. Both techniques have their strengths and weaknesses, and you need to have an understanding of both of these to pick the right technique for the right sample. Since FTIR is faster, simpler, and cheaper than GC-MS I believe it should the first choice for analyzing unknown mixtures.