Internal reflectance crystal

Another kind of cell, made by Graham and Curran, was based on an internal reflection crystal [80]. A gold minigrid was mounted directly on a prism (9 x 9 x 45 mm) and on top of this was a zinc selenide prism. The distance (observation) between the minigrid and the prism is typically 13-15 pm, which results in a very short response time. For a potential-step experiment, maximum absorbance is achieved within a couple of seconds. The cell is especially well-suited for potential-scan experiments because the intermediate generated at the electrode will rapidly fill out most of the observation distance even when moderately fast sweep rates (50 mV s ) are applied. Some memory effect is, however, present, because the diffusion layer will not be completely evolved on this timescale. At smaller sweep rates (2 mV s ) all of the observation layer behaves like a thin layer, where the concentrations are in equilibrium with the electrode surface concentrations. The cell has been used to study the reduction process of Fe(CO)s by CV, where it was pos-... 

Fig. 4. Arrangement of the internal reflection crystal, electrode (medium 2), and electrolyte (medium 3) in an electrochemical cell based upon the internal reflection of infrared light.

The enhancement of the electromagnetic fields at the surface of an internally reflecting crystal arise via the excitation of surface plasmon polaritons (SPP) [30] by the incident IR light. These SPPs are collective electronic excitations at metal surfaces and, theoretically, provide a sensitive probe of the optical properties of the interfacial region via resonance with the in-... 

In order to achieve SPP excitation, the light must be incident at the inner surface of the crystal near the critical angle and be p-polarised [39]. The applied potential was modulated by 50 mV either side of the point of zero charge of the evaporated/partially crystallised gold layer on a quartz internal reflectance crystal. 

A Harrick variable-angle internal reflectance attachment and quadruple diamond polarizer were used to obtain spectra of the first 2000 A of the surface of the films formed in contact with the glass plate. Germanium internal reflectance crystals (25 x 5 x 2 mm) having face cut angles of 60° were used at a 60° incident angle with perpendicular (90°) polarized radiation. Absorbance spectra of the internal reflectance crystals were obtained prior to placement of the sample films, and were stored in memory for later subtraction. Thus, only spectra of the sample films were obtained after the proper arithmetic operations. 

FIGURE 17-13 AIR apparatus. In (a) a solid sample is shown mounted on an internal reflection crystal of high refractive index. The materials used as ATR crystals include KRS-5, AgCl, Ge. Si. and the Irtran materials. Solid samples can be pressed against the crystal to obtain optical contact, tn (b), a typical attachment for ATR is shown. With many attachments, the internal reflection plate can be positioned in the holder to provide several incident angles. 

Figure 4.21 Schematic ATR sampling accessory. The internal reflection crystal permits multiple reflections. At each reflection a small amount of IR energy penetrates the sample and absorption occurs at the vibrational frequencies for the sample. (Courtesy of Pattacini, Pattacini Associates, LLC, Danbury, CT.)...

The use of cylindrical internal reflectance cells for HP-IR was pioneered by Moser and further modified by others.This method involves the use of an optically transparent internal reflectance crystal (typically ZnS, ZnSe, sapphire). Due to the inherently short path length, the method is not as sensitive as transmission-based IR, and a Fourier transform infrared (FTIR) spectrometer is therefore generally required. In addition, the type of crystal may need to be changed depending on the reaction of interest, as the optics may be corroded by some reagents or catalysts. However, as the path length is fixed regardless of conditions, it is much easier to quantify catalyst species, and unlike transmission systems the cells can also be used for the study of liquid-solid and gas-liquid-solid mixtures. ... 

Figure 4.21 Schematic ATR sampling accessory. The internal reflection crystal permits multiple reflections.

Due to the careless handling of cells, pressed discs, plates, films, internal reflection crystals, etc., spurious bands may be observed in spectra due to a person s fingerprints. These bands may be due to moisture, skin oils or even laboratory chemicals. Unfortunately, such carelessness is a common source of error. If an instrument experiences a sudden jolt, a sharp peak may be observed in the spectrum. Similarly, excessive vibration of the spectrometer may result in bands appearing in the spectrum. 

Internal reflection is an important technique but one that may be misused. When it first became available in the early 1960s, it was thought to be a panacea for every kind of sampling problem. Hours were spent preparing a sample for internal reflection when it would have taken minutes for a KBr disk or a mull. Figure 13.20 shows a schematic for an internal reflection crystal. 

Fig. 13.27 Residue left on internal reflection crystal after running spectrum of soap overwrap. 

The spectrum of the fabric was obtained by pressing the fabric against a 25-reflection KRS-5 internal reflection crystal (Figure 14.14a). This is the spectrum of a polyester. There is no evidence for the presence of a fire retardant in this spectrum. 

There are several variations of the techniqne. Commercial devices have been offered which provide for evacuation and pyrolysis of the sample heated on a platinum ribbon. The liquid pyrolyzate is condensed on an internal reflection crystal mounted directly in the pyrolysis accessory. Spectra of any vapor-phase species may also be obtained with this accessory. A similar device constructed from available laboratory equipment will be described in a later example. 

The C—H stretching bands in the internal reflection spectrum would seem to indicate only sp C—H stretching modes. This band, however, is due to an impurity on the internal reflection crystal possibly occurring as a result of handling the crystal. It is very difficult to eliminate such impurity films on internal reflection crystals, and a spectrum of the crystal should be obtained without sample before obtaining the sample spectrum. 

In another design, the internal reflectance crystal has a hemispherical shape. In this arrangement, the radiation enters into the curved surface, and is internally reflected off the flat surface, and then exits through the curved surface. In one version of this, the sample is in optical contact... 

---