Sample-handling techniques diffuse reflection

With the advent of the commercial FT-IR instruments, and computer techniques, it is now possible to record the infrared spectrum of almost any material regardless of its shape or form. A number of different sampling accessories are available for recording the infrared spectra. Some of these accessories such as AIR and specular reflectance have been used successfully with dispersive instruments, but the FT-IR instruments allow these accessories to be used more rapidly and with greater sensitivity. Most of the sample handling techniques have been reviewed in detail in the series of volumes on "Fourier Transform Infrared Spectroscopy" edited by J.R. Ferraro and J.R. Basile (1). In this paper, some of these techniques will be reviewed with particular emphasis on reflectance techniques (ATR and diffuse) and photoacoustic spectroscopy. Further applications such as far-IR, diamond cell, the absorption subtraction methodology can be found in the article by Krishnan and Ferraro (2). 

A.J. Eilert and D.J. Wetzel, Optics and sample handling for near-infrared diffuse reflection, in Handbook of Vibrational Spectroscopy, l.M. Chalmers and P.R. Griffiths (Eds), Sampling Techniques Vol. 2, John WUey Sons, Ltd, Chichester, 2002. 

Eilert, A.J. 8t Wetzel, D.J., Optics and Sample Handling for Near-Infrared Diffuse Reflection. In Chalmers, J.M. 8c Griffiths, P.R. (eds) Handbook of Vibrational Spectroscopy, Sampling Techniques, Volume 2 John Wiley 8c Sons Chichester, 2002 pp. 1162-1174. 

The DR technique lends itself to polymorph studies since the technique is noninvasive, the polymorph character remains intact due to limited sample handling, and the technique is quantitative (4). One disadvantage to diffuse reflectance IR is that it is a particle size-dependent technique (22). Development of quantitative polymorph assays require that the particle size of each component be limited to a specific range, including both components of a mixture, and the nonabsorbing matrix if the mixture is not sampled neat. It must also be kept in mind that for a quantitative assay, all calibration, validation, and subsequent samples to be assayed must fall within the particle size range otherwise, significant prediction errors may arise. 

One of the major sample-handling problems in FTIR analysis of carbonaceous materials is that many of them are effective blackbody absorbers and thus are too opaque for direct transmission analysis in the midinfrared spectral region. Addition of KBr intensifies the signal to obtain transmission infrared spectra. It is time consuming, and grinding conditions and moisture are known to affect the spectrum of the sample [238]. Alternative techniques such as specular reflectance, diffuse reflectance (DRIFT), photoacustic spectroscopy (FTIR-PAS), and total... 

Coarse or hard powders are not well served by either the compressed pellet or mull technique, mainly because of difficulties associated with grinding. In such situations, the best approaches require the use of an accessory, such as a diffuse reflectance or photoacoustic detector. Both diffuse reflectance and photoacoustic methods [99,100] may be applied to most forms of powdered solids. As a rule, photoacoustic measurements, which are the only form of true absorption measurement, are not significantly influenced by sample morphology. An alternative procedure for powders is ATR, especially a horizontal accessory, preferably equipped with a pressure applicator. Note that the use of pressure is recommended to ensure intimate contact between the sample and the IRE (internal reflectance element) surface. Normally, the sample must conform to the surface of the IRE, and because the strength of the IRE is typically limited, the procedure is recommended only for soft powders. However, with the introduction of diamond-based ATR accessories [101-103], it is possible to handle most types of powdered material. 

Very hard surfaces can be studied by the abrasion of the surface with a sheet of abrasive material, such as silicon carbide or carborundum paper. At this point a number of different methods may be used to analyze the abraded material. In essence, any soUd sampling technique that is capable of handling fine powders—KBr pellet, diffuse reflectance, ATR, photoacoustic, etc.—may be used to study the material. An interesting variant is to use diffuse reflectance to study the abrasive (see the reference to the silicon carbide method in Section 4) for the residual material. 

Fourier transform infrared and FT-Raman methods for the quantitation of polymorphs of cortisone acetate were compared by Deeley et al. [32]. The Raman analysis provided similar standard errors of prediction to the diffuse reflectance FTIR method of around 3.0-3.5%. Better precision and accuracy was reported in the same article for a Raman quantitative analysis of a novel research drug with a standard error of prediction of around 2.5%. The authors also outlined some of the advantages of the FT-Raman technique for quantitative analysis, primarily the minimal sample preparation that may alter polymorphic forms and that handling of the samples is unnecessary—spectra can be obtained through glass vials. Limitations of the technique were also described, notably intensity changes... 

Here k is the molar extinction coefficient and 5 is a scattering coefficient which varies with particle size and packing. While the Christiansen effect seen in transmission spectra of powders appears to be absent, spectral distortion can occur in diffuse reflection spectra if the particle size is not uniformly fine. Strongly scattering, or black samples such as coal can be handled by this technique. 

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