198 Chapter 8
as shown in Figure 1. The quality of spectra is dependent on intimate contact
between the diamond crystal and the sample, so a pressure device is used to
achieve this. Diamond has a higher refractive index than most plastics (2.4
and approximately 1.5, respectively), so when the beam comes into contact
with the sample, it penetrates only about 2m into the material before being
totally internally reflected by it into the detector. A detector examines the fre-
quencies absorbed compared with those present in the incident radiation and
produces a spectrum or trace which is definitive for all infrared-sensitive
chemical bonds present in a particular material or mixture. A spectrum can be
obtained 30 s after placing the sample in the instrument.
4000,0 3000 2000 1500 1000 600,0
cm-^1%Tnylonlow density polyethylenepolyester (Melinex)Figure 1Attenuated Total Reflection accessory for FTIR spectrometer with diamond crystal
through which infrared beam is reflected (upper). Plastic samples must have good
contact with the surface of the diamond to obtain well-resolved spectra (lower)