Question: Do you use glass in your work? If so, how?

Andrew McKinley answered on 10 Nov 2014:

I have a drinking glass on my desk here… but I assume that’s not what you meant!

Spectrometers use a vast array of optics, however very little is actually ‘glass’. The problem is that ‘glass’ is spectroscopically very active – it has lots of silicon-oxygen bonds that can vibrate and absorb a lot of infra-red radiation, while impurities in standard ‘soda glass’ will quite happily absorb lots of ultraviolet radiation. It’s fine for plain visible light, but sometimes you want to look beyond the visible.

For our ‘windows’ in spectroscopy then we need to do something a bit special… In infra-red spectroscopy, we typically use windows made of Potassium Bromide – KBr. If you look for similar elements, you’ll find you can swap the potassium for Sodium, and the bromide for Chloride – we’re basically using salt for windows! This does not absorb infrared radiation at all, which makes it great for infrared spectroscopy.

In electronic spectroscopy (ultraviolet/visible) we need to use something different to hold our sample – typically our samples are liquid, and we don’t want to dissolve the salt! This time we use fused quartz crystals. This is chemically identical to glass (silicon dioxide) but it is extremely pure, so does not have the ultraviolet absorption of cheaper ‘soda glass’.