Diffuse Reflection Sampling Technique

An excellent description of all aspects of difiuse reflectance can be found in [4]. 

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Depth profiling of a solid sample may be performed by varying the interferometer moving-mirror velocity (modulated IR radiation). By increasing the mirror velocity, the sampling depth varies, and surface studies may be performed. Limitations do exist, but the technique has proven to be quite effective for solid samples [21]. In addition, unlike diffuse reflectance sampling techniques, particle size has a minimal effect upon the photoacoustic measurement. 

In each of the aforementioned studies, qualitative IR spectroscopy was used. It is important to realize that IR is also quantitative in nature, and several quantitative IR assays for polymorphism have appeared in the literature. Sulfamethoxazole [35] exists in at least two polymorphic forms, which have been fully characterized. Distinctly different diffuse reflectance mid-IR spectra exist, permitting quantitation of one form within the other. When working with the diffuse reflectance IR technique, two critical factors must be kept in mind when developing a quantitative assay (1) the production of homogeneous calibration and validation samples, and (2) consistent particle size for all components, including subsequent samples for analysis. During the assay development for... 

The application of NIRS does not resolve all concerns associated with accurate blend uniformity determination. Key questions arise, such as (1) is the NIR beam sampling within a unit dose and (2) are an effective number of unit doses being sampled throughout a blend These questions are not limited to NIRS, but are applicable to any diffuse reflectance optical technique. 

For kinetics studies, powdered samples are less likely to give rise to diffusion-limited regimes in the reaction rates. Even loose powdered catalysts—a further step toward real conditions—can now be studied using diffuse-reflection infrared techniques (62), and it is to be hoped that more studies of this type will be made. 

In the past, extensive investigations were made to obtain better insight into the limitations of the diffuse reflectance measurement technique. Studies demonstrated that sample properties such as particle size and packing affect, in addition to the optical constants, the diffuse reflectance spectrum. The characteristics of the diffuse and specular components were studied for different particle sizes and dilution within a non-absorbing inert matrix. It was found that specularly and diffusely reflected radiation coexist in the measured diffuse reflectance spectrum, even in KCl-diluted samples. In addition, the specular component, which is certainly sample-dependent, is not necessarily the same as from the front-surface reflection.To prove this, a top layer of pure KCl powder was... 

Infrared spectra may be obtained from surfaces using a variety of techniques. These included attenuated total internal reflectance (ATR) and specular and diffuse reflectance. These techniques involve the infrared beam passing through only the outer few micrometers of the sample. The most widely applicable is ATR. A typical two-layer plastic material used for lidding on plastic food trays consists of polyethylene and poly(ethylene terephthalate) bound together with an adhesive. ATR infrared spectra of the two surfaces will easily identify one surface as... 

Diffuse Reflectance The phenomenon that takes place when infrared radiation reflects off a rough surface. The light approaches the surface in a specific direction, but the diffusely reflected light leaves the surface in many directions. A reflectance sampling technique known as DRIFTS is based on this phenomenon. 

DRIFTS Stands for Diffuse Reflectance Infrared Fourier Transform Spectroscopy, a reflection sampling technique that makes use of the phenomenon of diffuse reflectance and is used primarily on powders and other solid samples. 

FTIR instrumentation is mature. A typical routine mid-IR spectrometer has KBr optics, best resolution of around 1cm-1, and a room temperature DTGS detector. Noise levels below 0.1 % T peak-to-peak can be achieved in a few seconds. The sample compartment will accommodate a variety of sampling accessories such as those for ATR (attenuated total reflection) and diffuse reflection. At present, IR spectra can be obtained with fast and very fast FTIR interferometers with microscopes, in reflection and microreflection, in diffusion, at very low or very high temperatures, in dilute solutions, etc. Hyphenated IR techniques such as PyFTIR, TG-FTIR, GC-FTIR, HPLC-FTIR and SEC-FTIR (Chapter 7) can simplify many problems and streamline the selection process by doing multiple analyses with one sampling. Solvent absorbance limits flow-through IR spectroscopy cells so as to make them impractical for polymer analysis. Advanced FTIR... 

Diffuse Reflectance IR Fourier Transform Spectroscopy (DRIFTS) can be employed with high surface area catalytic samples that are not sufficiently transparent to be studied in transmission. In this technique, the diffusely scattered IR radiation from a sample is collected, refocussed, and analysed. Samples can be measured in the form of loose powders. 

Although most often connected with investigations of solid dosage forms, diffuse reflectance spectroscopy can also be used to characterize alternative formulations. Through the use of a special sample cell, the technique has been used to study the stability of emulsions [37]. In this work, it was found that information could be obtained that pointed toward subtle changes in the emulsion microenvironment. 

The utilization of IR spectroscopy is very important in the characterization of pseudopolymorphic systems, especially hydrates. It has been used to study the pseudopolymorphic systems SQ-33600 [36], mefloquine hydrochloride [37], ranitidine HC1 [38], carbovir [39], and paroxetine hydrochloride [40]. In the case of SQ-33600 [36], humidity-dependent changes in the crystal properties of the disodium salt of this new HMG-CoA reductase inhibitor were characterized by a combination of physical analytical techniques. Three crystalline solid hydrates were identified, each having a definite stability over a range of humidity. Diffuse reflectance IR spectra were acquired on SQ-33600 material exposed to different relative humidity (RH) conditions. A sharp absorption band at 3640 cm-1 was indicative of the OH stretching mode associated with either strongly bound or crystalline water (Fig. 5A). The sharpness of the band is evidence of a bound species even at the lowest levels of moisture content. The bound nature of this water contained in low-moisture samples was confirmed by variable-temperature (VT) diffuse reflectance studies. As shown in Fig. 5B, the 3640 cm-1 peak progressively decreased in intensity upon thermal... 

In the diffuse reflectance mode, samples can be measured as loose powders, with the advantages that not only is the tedious preparation of wafers unnecessary but also diffusion limitations associated with tightly pressed samples are avoided. Diffuse reflectance is also the indicated technique for strongly scattering or absorbing particles. The often-used acronyms DRIFT or DRIFTS stand for diffuse reflectance infrared Fourier transform spectroscopy. The diffusely scattered radiation is collected by an ellipsoidal mirror and focussed on the detector. The infrared absorption spectrum is described the Kubelka-Munk function ... 

Infrared spectra of zeolitic samples can be measured in several different modes. These include transmission, diffuse reflectance, attenuated total internal reflection (ATR) and emission. Transmission and diffuse reflectance are by far the most widely used of these techniques. In the transmission mode, the sample is placed directly in the infrared beam of the instrument and the light passing through or transmitted is measured by the detector. This transmitted signal (T) is ratioed to the open beam (no sample) signal (To) to get the transmission spectrum of the sample. The transmission spectrum is converted to an absorbance spectrum ... 

If used for continuous monitoring applications and if the sample is a powder then the technique of choice is most likely NIR or Raman. NIR works well with diffuse reflectance, and Raman is comparable in its method of measurement. 

PR. Griffiths and l.M. Olinger, Continnnm theories in diffuse reflection, in Handbook of Vibrational Spectroscopy, l.M. Chalmers and P.R. Griffiths (Eds), Sampling Techniques Vol. 2, John Wiley Sons, Ltd, Chichester, 2002. 

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