Diffuse reflection, sample handling

In practice, very few applications of FEWS sensors can be found outside laboratory applications and demonstration systems. In the near-IR, suitable fibres are readily available but usually there is no real necessity to use them. Possible transmission pathlengths are sufficiently large to allow using standard transmission probes, while turbid samples can be measured using transflection or diffuse reflection probes. In the mid-IR, high intrinsic losses, difficulties in fibres handling and limited chemical and mechanical stability limit the applicability of optical fibres as sensor elements. 

D.L. Wetzel and J.A. Eilert, Optics and sample handling for near-infrared diffuse reflection in Handbook of Vibrational Spectroscopy, J.M. Chalmers and P.R. Griffiths (eds), vol 1, John Wiley Sons, New York, 2002. 

Diffuse-reflectance MIRS has found a number of applications for dealing with hard-to-handle solid samples, such as polymer films, fibers, or solid dosage forms. Reflectance MIR spectra are not identical to the corresponding absorption spectra, but sufficiently close in general appearance to provide the same level of information. Reflectance spectra can be used for both qualitative and quantitative analysis. Basically, reflection of radiation may be of four types specular, diffuse, internal, and attenuated total. 

Wetzel, D.L. and Eilert, A.J., Optics and Sample Handling for Near-Infrared Diffuse Reflection. In Chalmers, J.M. and Griffiths, P.R. (eds), Handbook of Vibrational Spectroscopy, vol 2 John Wiley 8c Sons New York, 2002, pp. 1163-1174. 

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. 

This class of solids is an extension of the sample types already discussed, and many of the procedures already highlighted may be used here. If the material dissolves in a suitable solvent, then a cast film may be prepared on an IR transmitting window or on the surface of an ATR element. Moldable materials, such as polymer pellets, may be prepared as hot-pressed films, with care taken to ensure that material does not thermally degrade. Grind-able materials can be handled as previously discussed for powders using the compressed halide pellet, mineral oil mull, or diffuse reflectance methods to acquire the spectrum. 

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... 

Raman provides easy sampling, whereas IR spectroscopy frequently needs some form of sample preparation. Materials which are difficult to handle in IR (highly viscous liquids, solids requiring pellets, mulls, or diffuse reflectance) are often easily measured by Raman. Unlike IR reflectance spectra, Raman spectra of solid samples are not affected by sample properties such as particle size. A significant difference with infrared absorption spectroscopy is that the Raman signal is emitted from the sample. Consequently, matrix effects are seldom as severe in RS as they are with mid-IR and NIR. Water may be used as a solvent with no loss in signal or resolution. Glass, even tinted, does not interfere with the Raman spectra. 

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. 

Samples such as suspensions and opaque films, which strongly scatter NIR radiation but transmit only a small portion, are handled in the following ways the NIR radiation which has been transmitted by the sample is introduced into an integrating sphere as shown in Figure 18.4c, or the absorption is measured by the transmission-reflection method shown in Figure 18.4d. In the transmission-reflection method, a standard reflector such as a ceramic plate is placed at the back of the sample, and the ratio of intensities of the diffuse reflection measured with and without a sample is assumed to be the absorbance of the sample. 

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). 

---