But First, How Does FTIR Spectroscopy Work? (Briefly)
FTIR spectroscopy produces an IR spectrum from a sample, whether solid, liquid, or gas. The sample is exposed to infrared wavelengths (longer wavelengths and lower frequency than visible light). The sample can absorb light and produce a pattern of measurements or a “molecular fingerprint.” The technique is advantageous, because it can be used to collect high-resolution data over a variety of wavelengths in seconds to minute’s time (much like a UV/vis spectrophotometer). These molecular vibrations correspond to specific chemical bonds, which are unique to each molecule and, therefore, each compound absorbing the variety of wavelengths.The Apparatus
Liquid samples are much easier to prepare and involve using a liquid cell apparatus. The liquid sample is held together in place between two windows made of specific, suitable materials that do not interfere with the IR absorbance of the specific application. However, in between the two windows lies a spacer. This Teflon spacer provides a specific path length for the light to shine through and allows a quantitative measurements according to Beer’s Law:(Absorbance = concentration * path length* extinction coefficient)
The windows are sandwiched between a pair of metal plates held together by 4 screws, which are mounted in the IR chamber, similar to the way the UV/vis chamber holds a cuvette. In FTIR spectroscopy, keeping the windows clean is just as important as the sample preparation. Below is the protocol I use to keep calcium fluoride windows essentially spotless.Acid Washing/Cleaning Calcium Fluoride Windows
You will need your personal protective equipment (PPE) such as:- Gloves
- Goggles
- Lab coat
- Large glass beaker halfway filled with water
- 50 mL corning tubes halfway filled with water (2 per window)
- Small 100 mL beaker (put no more than 20 ml Sulfuric acid)
- Forceps
Procedure
- Dissolve a few crystals of potassium permanganate (KMnO4) (generally no more than 5 semi big crystals) into a beaker of sulfuric acid (no more than 20 mL). Be careful, because this is a very strong oxidant that will cause fire (and a possible explosion) if you are not mindful of what you are doing. The color should be green but not dark green.
- Use forceps to gently put the calcium fluoride window, one at a time, in the permanganic acid. Wash for no more than 10-15 seconds.
- Remove the window being very careful not to scratch it or drip acid all over. Put the window in a corning tube halfway filled with water. The water should turn slightly pink-purple.
- Repeat for the second window. Remove both windows and place them in another corning tube with water to prepare for rinsing.
- Dilute the acid: pour all of the contents of the first corning tubes and the permanganic acid into the larger beaker.
- Use soda ash to carefully neutralize the contents of the larger beaker.
- Rinse the windows carefully using deionized water and dry them using pressurized air. Use the windows immediately or store them in a safe place to prevent them from getting dirty or scratched.