Analyzing Samples
30 Agilent Cary 630 FTIR Spectrometer User’s Guide
ZnSe ATR Sampling Accessory
The ZnSe attenuated total reflectance (ATR) sampling accessory takes
advantage of the physical properties of light when encountering two
materials with differences in index of refraction. When coming in
contact with a sample having a different index of refraction, the
infrared light creates an evanescent wave, which provides a very small
and specific depth of penetration into the sample before reflecting
back into the detector of the spectrometer. This small and consistent
pathlength means no sample preparation is required to get good
measurement results from a variety of samples. The key to obtaining
good results with the ZnSe ATR accessory is to make good contact
between the sample and ATR crystal. The ZnSe ATR technique should
be used primarily for analyzing liquids, gels and some soft solids.
The ZnSe ATR accessory uses a Zinc Selenide crystal as the interface
between the sample and the infrared energy. Advantages of a ZnSe
crystal are its high throughput without the diamond band interference
seen with the DATR. It can accept samples with a pH range from 5 to
9. Hard or abrasive samples as well as strong acids should not be
analyzed using the ZnSe ATR.
The ZnSe ATR is a single-reflection ATR. It is typically used as an all-
purpose ATR for measuring liquids, powders, gels and soft solids.
Powder and solid samples are best measured with the single-reflection
ATR, by using the sample press device, which applies relatively low
pressure to powder and solid samples to ensure good contact with the
ZnSe crystal. The single-reflection ATR is also a good choice when the
amount of available sample is limited. The single-reflection ZnSe ATR,
similar to the diamond ATR, has a 1 mm diameter sampling surface
with 200 μm active area and provides approximately 2 μm depth of
penetration for infrared energy at 1,700 cm
-1
. The single-reflection
ATR protrudes slightly above the metal mounting plate to ensure good
contact and allow the user to clean the crystal effectively.
In the sample loading and cleaning mode, the sample press should be
in the highest position, so the sample press tip is well above the ZnSe
sampling window (see Figure 5). It is in this position that the sample
mounting area can be easily accessed for loading the sample into place
and for cleaning the sampling surfaces before the next sample is
analyzed.
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