Analytical chemistry introduction
I said in my about me that I’m an analytical chemist and use equipment with initials for names, like GC-MS and LC-MS, and in case you’re wondering what any of that means, I thought I’d write an introduction to analytical chemistry and the terms we use.
First of all, what is analytical chemistry? Well, in their textbook Instant Notes in Analytical Chemistry, Kealey and Haines define it as “the application of a range of techniques and methodologies to obtain and assess qualitative, quantitative and structural information on the nature of matter”1. That might not help a lot, as it uses a lot of technical jargon! My summary of that statement would be that analytical chemistry is about looking at what’s present in something and how much of a given substance there is, but let’s have a quick look at some of the terms used in that quote.
Qualitative: a qualitative analysis is looking at what is in the sample being tested. We often refer to this type of analysis as “untargeted” because we’re not looking for anything specific; just an idea of what’s there.
Quantitative: here we are looking at how much there is of a particular substance in our sample - we “quantitate” or “quantify” (both terms are used and mean the same thing) it. We call this a “targeted” analysis as we know what we are looking for. Quantitative analysis can be used after qualitative analysis – once you’ve worked out what’s there, you can start to work out how much of it there is.
The difference between these is illustrated in the diagram below. In (a) the qualitative analysis tells us that the sample (the purple circle) is made of red and blue elements, but that’s all we know. When we use qualitative analysis (b), we find out that the red components account for 25% of the sample and the other 75% are blue.
Now we know a bit more about what analytical chemistry is, what about those equipment names? Well, GC-MS stands for Gas Chromatography – Mass Spectrometry and LC-MS is Liquid Chromatography – Mass Spectrometry. Sounds impressive, but what do they do?
The term chromatography comes from the Greek for “colour writing” and was coined in 1900 by Russian-Italian botanist Mikhail Tsvet, who used the technique to separate plant pigments.
Chromatography is a method of separating different components of a sample using what’s known as a stationary phase (something that remains in one place) and a mobile phase that carries the sample along or through the stationary phase. An example of chromatography that you might have seen in school science lessons is the separation of inks on filter paper, where the filter paper acts as the stationary phase and the water added to the ink sample is the mobile phase.
In GC, the mobile phase is a gas (most commonly helium) and in LC a mixture of water and other solvents is used as the mobile phase. The method of separation of the components of a sample varies depending on the type of chromatography, the compounds we’re interested in and the sample type; methods include boiling point, electrical charge, size and other chemical characteristics. You can see an example of what a chromatogram (as we call the resulting trace we get from an experiment) below.
Mass spectrometry is an analytical technique that separates compounds and bits of compounds, known as fragments, by their mass (technically, their mass-to-charge ratio, but for many small molecules the charge is 1, so it’s the same as their mass). If we couple a chromatography instrument to a mass spectrometer, we can use the masses of compounds and their fragments to give us more confident identifications and also differentiate between compounds that may not be separated by the chromatography system. The images below show an example of a mass spectrum and a GC-MS instrument.
I hope this has made some of the terms in my about me a bit clearer. There are lots of great resources on the web if you want to find out more about these topics, and I will almost certainly revisit some of the concepts in more detail in later blog posts.
1. Kealey, D.; Haines, P. J., BIOS Instant Notes in Analytical Chemistry (1st ed.). 1st ed.; Taylor & Francis: London, 2002.