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This single reflection, horizontal internal reflection (ATR or HATR) attachment is a powerful analytical tool for repeatable qualitative and quantitative FT-IR spectroscopy studies of a wide range of samples. These include aqueous and organic liquids, pastes, and organic and inorganic powders.
The simple efficient optical design of this ATR accessory combines high optical throughput with the convenience of horizontal sampling. Installation and alignment are simple. Sample preparation is virtually eliminated. The high energy throughput allows excellent infrared ATR spectra to be obtained even with weakly absorbing samples. Conventional transmission cells can cause a degradation of spectral information obtained on highly absorbing samples due to severe energy losses, making accurate analytical work impossible. For such samples, the attenuation of this single reflection horizontal ATR accessory is essential.
Figure 1. Optical Drawing of the FastIR™.
An optical drawing of this ATR accessory is shown in Figure 1. Light from the infrared spectrometer source strikes mirror M1. The reflected light then strikes the 45° single reflection crystal at normal incidence and passes through the element until it is internally reflected at the sampling surface. The light interacts with the sample and exits the other 45° face at normal incidence. The radiation then reflects from mirror M2 to the detector of the spectrometer. This design provides a high degree of insensitivity to precise alignment. It also allows the mirror configuration of the attachment to be used not only for internal reflection (ATR) but also for external or specular reflection at a fixed 45° angle of incidence.
The standard ATR crystal is made from ZnSe. This has a wide useful spectral range (20,000 cm-1 to 500 cm-1) and durable mechanical properties. Alternative materials are also available. The ATR crystal is held into its mounting block using a Kalrez gasket. This gasket does not interact with the sampling radiation. Thus spectra are free from any spurious peaks or energy degradation due to the interaction of the light with the adhesive.
Liquids and pastes may be analyzed by simply placing the sample on top of the crystal. For volatile samples, a glass cover slide and a low volume, o-ring sealed liquid cell with luer fittings are provided. The o-ring of the sealed liquid cell does not come in contact with the sampling light, so spectra are free from any interference or degradation associated with the o-ring. Additional accessories for the FastIR™ include flow-through liquid reservoirs and trough-style liquid cells. Heatable solid and liquid cells are available on special order.
Solid materials, such as fine powders, flexible polymers, and soft films, may also be analyzed using this ATR accessory. In order to compress the sample against the ATR crystal, a pressure plate and clamp are provided. Repeatable spectroscopy measurements can be obtained using the optional Torque Screwdriver or Slip-Clutch.
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As one example of the versatility of this accessory, quantitative FT-IR measurements of silicone oil were performed. The ability to analyze these samples directly eliminated the need for a tedious chromatographic separation. The samples exhibit high absorbance in the infrared and high viscosity. Use of short pathlength transmission liquid cells would prove difficult due to the viscosity of the samples. The short effective pathlength and ease of sample introduction for this accessory, however, make analysis of samples of this nature extremely straightforward. Figures 2a-c show the almost identical absorbance spectra of the silicone oil samples: pure component 1, pure component 2, and a mixture of the two. Using the absorbance at 1257 cm-1, the concentration of components 1 and 2 in the mixture were quickly and accurately determined.
Figures 2a and 2b. ATR Spectra of Silicone Oil: Components (a) 1 and (b) 2.
Figure 2c. ATR Spectrum of Silicon Oil Mixture.
Figure 3. ATR Spectrum of Talcum Powder.
A second example is given in Figure 3. Here, the infrared spectrum of talcum powder is shown. This spectrum was taken using the attachment with the pressure plate and clamp.
These two examples demonstrate the utility and flexibility of this single reflection FastIR™ accessory.
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