ATR method

Figure 5. SPR sensor based on ATR method and angular modulations (upper) and corresponding reflectivity calculated for two different refractive indices of sample (lower). Sensor configuration SF14 glass prism, 50 nm thick gold layer, sample, wavelength - 682 nm.

Table 1+ Ratio of Absorption of IR-Spectrum Using ATR Method...

The ATR technique is now routinely used for IR spectroscopy as it allows measurement of spectra for a variety of sample types with minimal preparation. The crystals employed are generally prismatic in shape, allowing contact of a flat surface with the sample. The ATR method was first adapted for HP IR spectroscopy by Moser [29-33], who realised that a conventional autoclave could easily be adapted for in situ IR spectroscopy by fitting an ATR crystal of cylindrical cross section. The technique developed by Moser is thus known as cylindrical internal reflectance (CIR) spectroscopy and high pressure cells based upon the CIR method have been commercialised by Spectra-Tech. A typical CIR cell is illustrated in Figure 3.8. 

For example, a high yield of an intermediate in a consecutive reaction depends sensitively on the instant in time when the reaction is quenched. For such applications, the fast response of the ATR method more than compensates for deficiencies related to sensitivity when the ATR technique is compared with other methods. The design of the equipment depends on the specific requirements of the application. 

When fluid is pumped through a cell such as that shown in Fig. 12, transport of dissolved molecules from the cell inlet to the IRE by convection and diffusion is an important issue. The ATR method probes only the volume just above the IRE, which is well within the stagnant boundary layer where diffusion prevails. Figure 13 shows this situation schematically for a diffusion model and a convection-diffusion model (65). The former model assumes that a stagnant boundary layer exists above the IRE, within which mass transport occurs solely by diffusion and that there are no concentration gradients in the convection flow. A more realistic model of the flow-through cell accounts for both convection and diffusion. As a consequence of the relatively narrow gap between the cell walls, the convection leads to a laminar flow profile and consequently to concentration gradients between the cell walls. 

The example furthermore shows that diffusion from the bulk fluid phase toward the volume near the IRE, which is probed by the evanescent field, has to be accounted for because it may be the limiting step when fast processes are investigated. The importance of diffusion is more pronounced when a catalyst layer is present on the IRE, because of the diffusion in the porous film is much slower than that in the stagnant liquid film. Indeed, the ATR method, because of the measurement geometry, is ideally suited to characterization of diffusion within films (50,66-68). Figure 16 shows the time dependence of absorption signals associated with cyclohexene (top) and i-butyl hydroperoxide (TBHP, bottom). Solutions (with concentrations of 3mmol/L) of the two molecules in cyclohexane and neat cyclohexane were alternately admitted once to... 

Attenuated Total Reflectance (Micro-ATR and ATR8). The ATR method discussed above is designed to record spectra from the surface of a solid... 

The attenuated total reflection (ATR) method measures the reflection coefficients of vertically and horizontally polarized light reflected from a polymer layer adsorbed on a transparent surface63. These coefficients allow the thickness of the adsorbed layer and the polymer concentration in it to be determined. 

In principle, the ATR method would provide information about the segment distribution in the adsorbed layer if light could penetrate in different depths into the layer, but this possibility still remains untested. 

Peyser and Stromberg63 used the ATR method to measure the thickness of a polystyrene layer adsorbed on a quartz surface from cyclohexane solution at 35 °C and compared it with the thickness obtained by ellipsometry. Good agreement was observed although the ellipsometric measurements were made for the chrome plate. 

Figure 8. ATR method for exciting surface electromagnetic waves by using a single prism as developed by A. Otto (11). The angle of incidence of the light on...

Another label-free optical detection method—FTIR-ATR—has been applied for detection of thrombin by means of DNA aptamers [73], The antithrombin DNA aptamer previously developed by Tasset et al. [17] was immobilized covalently onto Si surface using UV irradiation method. As a quantitative measure, the area of N-H and CH2 bands was used. This method allowed to detect thrombin with a sensitivity around 10 nmol/L. The specificity of binding of protein to aptamer was also investigated using DNA with no binding site for thrombin. It has been noted that for effective binding study by FTIR-ATR method, the concentration of protein should be kept lower than 100 nmol/L. 

The most commonly used technique for obtaining a spectrum is the attenuated total reflectance (ATR) method in the multiple internal reflectance (MIR) mode. In this method, the infrared beam is passed into a special crystal of a selenide (KRS-5). The angle of incidence is such that the beam will bounce along the crystal. A sample of the polyurethane is placed hard up against the crystal. The infrared just penetrates into the material before it continues down the crystal. A number of internal bounces are obtained along the crystal. Up to 25 reflectances are obtained from a 2 mm crystal. Figure 9.8 illustrates the infrared path in an ATR cell. 

The CLD-1 chromophore in a PMMA polymer host was used to obtain the TACAN results. The VK results are consistent with the electro-optic coefficients determined by ATR methods and discussed earlier in this review Pacific Wave Industries (Los Angeles, CA) have constructed single modulation arm Mach-Zehnder interferometers using CLD-1 in either PMMA or APC polymer matri-... 

There are restrictions in the selection of the sample preparation or introduction method when the laboratory is taken on site. Methods requiring supplies difficult to provide in field conditions, typically liquid nitrogen, cannot be applied on site. Thus, GC-based and ATR methods are not normally applicable on site. A suggestion for a basic on-site sample preparation kit is shown in Table 1. 

S. Banerjee, R.P. Semwal and S. Agarwal, Measurement of diffusivities of sulfur mustard (SM) and its analog oxygen mustard (OM) in cured butyl, nitrile, and natural-rubber sheets by weight-gain and FTIR-ATR methods, J. Appl. Polym. Sci., 57, 1483-1490 (1995). 

In the case of infrared spectroscopy, various types of infrared measurement methods (transmission method, attenuated total reflection (ATR) method, reflection-absorption (RA) method, surface-enhanced method, etc.) may be applied. 

It is common to use the peak absoiption coefficient instead of the integral absorption coefficient. In most cases only small errors are introduced by this simplification. It is evident that the detection limit decreases with the membrane method as I//At/.s - For gases extracted by the membrane the usual rotational vibrational bands vanish if the compound is dissolved in a polymer, since the molecule is no longer able to rotate freely. As a result, one relatively sharp absorption band is observed which has the same integral absorption coefficient as the rotational vibrational absorption band. So, for gases the detection limit is decreased by an additional factor h,> (see Table 6.5-3). For the ATR-method the thickness of the sample is the effective thickness multiplied by the number of reflections N. So we get as detection limit for the membrane method... 

IR radiation is emitted from the electrically modulated light source. The analytically relevant spectral range is transmitted through an interference filter, the sample chamber, and the membrane. This radiation is focused on a thermal detector (Dl), pyroelectrical or thermopile. The reflected radiation from the filter is used as a reference (D2). A comparison of the ATR-, the fiber-, and the transmission-method. Secs. 6.5.2.1, 6.5.4.2, and 6.5.4.4, shows that the ATR method is most versatile for all applications and that the transmission method allows the lowest limit of detection for gases (Hadziladzaru, 1994). The properties of the ATR method by employing wavelength selection with tunable interference filters has been studied by Lebioda (1994). 

A number of experimental alternatives to traditional IR transmission spectroscopy are suitable for overcoming some of these complicating experimental factors. In the technique of diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) (Hartauer et al. 1992 Neville et al. 1992) the sample is dispersed in a matrix of powdered alkali halide, a procedure which is less likely to lead to polymorphic transformations or loss of solvent than the more aggressive grinding necessary for mull preparation or pressure required to make a pellet (Roston et al. 1993). For these reasons, Threlfall (1995) suggests that DRIFTS should be the method of choice for the initial IR examination of polymorphs. He has also discussed the possible use of attenuated total reflection (ATR) methods in the examination of polymorphs and provided a comparison and discussion of the results obtained on sulphathiazole polymorphs from spectra run on KBr disks, Nujol mulls and ATR. 

It can be stressed, therefore, that not only is the ATR method surface-sensitive, but it is also sensitive to species located inside d. . The penetration of p-polarized light is slightly greater than that of s-polarized light and both depend on the incidence angle. Using the ATR method, surface substances as well as those in solution can be detected. 

An alternative to ATR methods is SPME with a hollow waveguide sampler, which may be of advantage because of the higher sensitivity, the shorter analysis times, the absence of practical limitations to sample volume and the possibility of performing mass analyses. The analytes need not penetrate deeply into the plastic layer and it is sufficient that they... 

The IR spectra of polyester oligomers are almost identical to those of polyester fibres. The oligomers can be prepared as KBr pellets, as a molten film between NaCl or KBr platelets or else they can be investigated directly by the diffuse reflection or the diamond ATR method. 

---