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How should I select an ATR crystal material?

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 materials have refractive indices in the area of 1.4. 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 ATR range cutoff at low wavenumber varies from approximately 1500 cm-1 for Si to 400 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. Spectral ranges of selected ATR materials are listed 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 while strong acids may generate toxic hydrogen selenide) or Alkalies, pH>9 can be harmful to ZnSe or AMTIR. Physical considerations are equally important since some crystals are more susceptible to pressure and temperature changes than others.

Sensitivity - the 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. For high absorbing samples, Ge is a good choice due to relatively low depth of penetration.

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 >20 % T for multi-reflection systems and > 35 % T for single reflection systems.

Cost - the material should be economical considering the value of the experiment. ZnSe is the most popular because of this. Diamond is the most expensive and is needed only when very harsh or abrasive samples are common.

The following is a basic review of common ATR crystal materials:

AMTIR - produced as a glass from selenium, arsenic and germanium. It is highly toxic during the manufacturing process. However, the brittle nature of this material and its total insolubility in water makes it safe for use as an internal reflectance element. It has a similar refractive index to zinc selenide and can be used in measurements that involve strong acids.

Diamond (Raw or Synthetic) - offers excellent chemical and physical properties when used as a sampling interface in infrared experiments. This hard, scratch-resistant material is suitable for applications involving a wide range of chemicals. It withstands highly acidic and basic samples very well. It does not react with strong oxidizers or complexing agents. Diamond ATRs can be used to analyze hard powders and other difficult to analyze solid samples. The main disadvantage of the diamond is its relatively high absorbance (when used as an ATR element) in the 2,500 cm-1 to 1,650 cm-1 region. It also is the most expensive ATR material, which is less practical for multi-reflection applications.

Germanium - has been used extensively as a higher refractive index material for samples that produce strong absorptions (e.g. rubber). The crystal is also used when analyzing samples that have a high refractive index, such as in passivation studies on silicon. Although slightly higher than ZnSe, it is fairly low cost.

KRS-5 was the most widely used material for ATR elements prior to the common availability of Zinc Selenide. Although it has a wide spectral range, KRS-5 is very soft and is easily damaged. Like the Zinc based compounds, the thallium in KRS-5 is readily complexed by ammonium compounds and amino-based chelates. The main advantage of KRS-5 is its wide spectral range. It is moderately expensive.

Silicon has a relatively high refractive index and it is useful for analyzing highly absorbing samples. Silicon is scratch resistant and is insoluble in water and organic solvents. However, it is affected by strong acids and is soluble in alkalis. Another limitation of a silicon crystal is its relatively narrow infrared spectral range. Although slightly higher than Ge, Si is still priced moderately.

ZnSe is the preferred replacement for KRS-5 for all routine applications. Its useful spectral range is less at the low frequency end than that of KRS-5, but the mechanical strength of this rigid, hard polycrystalline material is superior. Although it is a general-purpose material, it has limited use with strong acids and alkalies due to the surface becoming etched during prolonged exposure to extremes of pH. Complexing agents, such as ammonia and EDTA, will also erode the surface because of the formation of complexes with the zinc. It is the lowest cost ATR material available today.

What makes an ATR Accessory useful in spectrometry?

The heart of an ATR accessory is a trapezoidal or parallelogram crystal mounted in the infrared beam. There are several different crystal materials used for ATR crystals. Their common characteristic is a relatively high refractive index. The infrared radiation enters the crystal through a set of mirrors, and is internally reflected (typically several times) until it exits at the other end. These internal reflections create an evanescent wave, which extends beyond the surface of the crystal into a sample placed on its surface (the evanescent wave decays rapidly with distance from the surface, therefore, the sample must be in intimate contact with the crystal surface).

In a typical IR experiment, part of the evanescent radiation is absorbed by the sample producing an absorption spectrum. The resulting absorbance intensity is proportional to the number of reflections of the infrared beam in the crystal and the depth of penetration of the evanescent wave into the sample. These two factors determine an effective pathlength, which is equivalent to a cell pathlength or sample thickness in a traditional transmission experiment. A typical number of reflections provided by various ATR accessories lies between 1 to 15, which translates to an effective pathlength of 1 to 25 micrometers - depending on the accessory, crystal material and the angle of incidence of the IR 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).

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 20 to 40 % T depending on the number of reflections.

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 line should be in the 20 to 40% T range.

Why does PIKE Technologies use Indium in sealing the ATR crystal?

PIKE Technologies Horizontal ATR trough plates feature a unique crystal seal. The crystal is attached to the plate mechanically and sealed with an indium gasket. Thanks to this design, the plate is completely leak-proof and works well at high and low temperatures. No O-rings, glue or epoxy resins are used in the crystal plate assembly.

Indium is a very soft, silvery-white metal with brilliant luster. Its foil has been found to be an excellent gasket material. The foil is placed between the crystal and the supporting plate and then clamped. The low tensile strength of the foil allows it to flow into the gaps between the clamped materials, assuring intimate mechanical and thermal contact everywhere on the interface. Indium sealing characteristics are not affected by temperature changes and the foil gasket can withstand any temperature below its melting point of 156.6 degrees Celsius, down to very low cryogenic temperatures.

And why is it important that no O-rings or epoxies are used in the crystal plate assembly? The reasons are multiple - rubber and adhesives can be affected by many organic solvents, which may diminish their strength and lead to leaching and unwanted spectral features. Epoxies may not offer an adequate mechanical support for the entire crystal, since only the edges of the crystal are attached to the base and the crystal may break at extreme temperatures. Indium provides for excellent vibration damping and mechanical shock as well.

Why is it useful to dilute samples for Diffuse Reflection experiments?

The specular reflectance component in diffuse reflectance spectra causes changes in band shapes, their relative intensity, and in some cases it is responsible for complete band inversions (restrahlen bands). Dilution of the sample with a non-absorbing matrix minimizes these effects (particle sizes and loading also play an important role).

Refractive index effects result in specular reflectance contributions (spectra of highly reflecting samples will be more distorted by the specular reflectance component). This can be significantly reduced by sample dilution.

Other Factors:

Particle Size - reducing the size of the sample particles reduces the contribution of reflection from the surface. Small particles improve the quality of spectra (narrow bandwidths and better relative intensity). The recommended size of the sample/matrix particles is 50 micrometers or less (comparable to the consistency of the finely ground flour).

Homogeneity - samples prepared for diffuse reflectance measurements should be uniformly and well mixed. Non-homogenous samples will lack reproducibility and will be difficult to quantify.

Packing - the required sample depth is governed by the amount of sample scattering. The minimum necessary depth is about 1.5 mm. The sample should be loosely packed in the cup to maximize IR beam penetration.

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