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  • Crystal Selection, ATR
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The following are several criteria necessary to consider when selecting an ATR crystal material for a specific sample:

Refractive Index - the crystal should have a higher index of refraction than the sample. The majority of organic samples have refractive indices in the area of 1.5. Refractive indices of standard ATR crystals span from 2.4 to 4.0 - which in most cases provides sufficient sample to crystal differentiation. Inappropriate refractive index ratios may cause distortion of spectral features. These may be manifested by diminished peak symmetries, sharp baseline/peak shoulder transitions, and in extreme cases, presence of derivative-like features in the spectrum.

Spectral Range - all ATR crystals have different spectral ranges. Specifically, in mid-IR the cutoff at low wavenumbers varies from approximately 780 cm-1 for Ge to 250 cm-1 for KRS-5. To a certain extent, the cutoff values are also affected by the length (thickness) of the crystal. In light of these facts, it is important to determine whether the spectral features of the sample correspond with the spectral range of the ATR crystal selected. Useful spectral ranges of the most popular ATR materials are listed in the table below.

Chemical and Physical Properties - for obvious reasons, the ATR crystal must be chemically and physically compatible with the sample. Some crystal materials may react with samples. This will typically damage the crystal surface and may produce unpleasant side effects (e.g. acidic solutions, pH<5, may etch the ZnSe crystal, strong acids will generate toxic hydrogen selenide). Physical considerations are equally important as some crystals are more susceptible to pressure and temperature changes than others. Selected precautions are listed in the table below.

Sensitivity - effective pathlength of the infrared beam in the sample must be sufficient to produce an adequate spectrum. This parameter is affected by the number of reflections (more reflections yield higher absorbance), and the depth of penetration - which is a function the refractive indices and the angle of incident beam. Most ATR accessories are already optimized to provide the best possible sensitivity. Standard configurations provide 9 to 20 reflections at 45 degree angle of incidence. Changing these parameters, which is possible with variable angle ATRs (or special, dedicated crystal plates) can improve results in some cases. However, when selecting special configurations the following issues should be considered:

• A higher angle of incidence results in less reflections, and decreased penetration depth, lowering the overall absorbance of the spectrum. This is useful when highly absorbing or high refractive index samples are being measured.
• A lower angle of incidence results in more reflections and an increased penetration depth. This results in the best sensitivity but can result in band distortions due to the large variation in sample refractive index in the region of absorbance bands (anomalous dispersion).

Optical Design - the overall optical design of an HATR accessory - its optical path, mirrors quality and throughput has great effects on analytical results. Placing a good quality HATR accessory with a 45 degree ZnSe crystal in the sample compartment of an FTIR spectrometer should result in an energy throughput of 25 to 40 % T. To verify performance of an ATR accessory, collect the background spectrum with an empty sample compartment. Place the accessory in the spectrometer and collect the transmission spectrum. The resulting spectrum should be in the 25 to 40% T range.