Sampling accessory

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... 

Snively, C.M. and Lauterbach, J. (2002) Sampling accessories for the high-throughput analysis of combinatorial libraries using spectral imaging. Spectroscopy, 17, 26. 

Instrument configuration Types of solvents that are available for running HPLC, sampling accessories on hand for IR, detectors available for GC, and the sensitivity toward particular analytes of each detector and instrument... 

Fig. 4 Schematic representation of the diffuse reflectance sampling accessory. Key A, blocker device to eliminate specular reflectance B, path of IR beam.

The first linkage between a microscope and an IR spectrophotometer was reported in 1949 [15]. Today, every manufacturer of IR spectrophotometers offers an optical/IR microscope sampling accessory. The use of optical and IR microscopy is a natural course of action for any solid state investigation. Optical microscopy provides significant information about a sample, such as its crystalline or amorphous nature, particle morphology, and size. Interfacing the microscope to an IR spectrophotometer ultimately provides unequivocal identification of one particular crystallite. Hence, we have the tremendous benefit of IR microscopy for the identification of particulate contamination in bulk or formulated drug products. 

Clearly, the potential applications for vibrational spectroscopy techniques in the pharmaceutical sciences are broad, particularly with the advent of Fourier transform instrumentation at competitive prices. Numerous sampling accessories are currently available for IR and Raman analysis of virtually any type of sample. In addition, new sampling devices are rapidly being developed for at-line and on-line applications. In conjunction with the numerous other physical analytical techniques presented within this volume, the physical characterization of a pharmaceutical solid is not complete without vibrational analysis. 

Of the analytical techniques available for process analytical measmements, IR is one of the most versatile, where all physical forms of a sample may be considered - gases, liquids, solids and even mixed phase materials. A wide range of sample interfaces (sampling accessories) have been developed for infrared spectroscopy over the past 20 to 30 years and many of these can be adapted for either near-lme/at-lme production control or on-line process monitoring applications. For continuous on-line measurements applications may be limited to liquids and gases. However, for applications that have human interaction, such as near-line measurements, then all material types can be considered. For continuous measurements sample condition, as it exists within the process, may be an issue and factors such as temperature, pressure, chemical interfer-ants (such as solvents), and particulate matter may need to be addressed. In off-line applications this may be addressed by the way that the sample is handled, but for continuous on-line process applications this has to be accommodated by a sampling system. 

However, the improved sensitivity of FT-IR allows one to obtain better sensitivity using the conventional sampling accessories and expand the range of sampling techniques. Emission, diffuse reflectance and photoacoustic spectroscopy represent new areas where FT-IR reduces the difficulty of the techniques considerably. Greatly improved results are also achievable from reflection spectroscopy. Special effects such as vibrational circular dichroism can be observed using FT-IR instrumentation. 

Figure 6.2 General layout of the FTIR imaging spectrometer. Optical elements F and G illuminate the sampling accessory with light from the spectrometer optical elements B and E focus the light from the sampling accessory onto the FPA detector aperture C is used to control the overall light level reaching the detector, and filter B is used to block light from outside the desired spectral region.

An example of a dedicated sampling accessory is the parallel gas phase array (GPA) that was utilized in the gas phase effluent studies mentioned below. For... 

In the field of FTIR, the availability of the instruments has a dualistic nature. Basic instruments, spectrometers, and basic sampling accessories, are available from multiple sources. The problem lies in selecting the right model from all available possibilities. In GC/FTIR instruments, the case is different. While the major FTIR manufacturers produce lightpipe interfaces, only few deposition-type instruments have been available. Erickson reviewed the situation of the GC/FTIR market in 1998 (12). The article discusses general developments and future prospects of GC/FTIR. 

There are two types of Mastersizer instruments the Mastersizer Micro and the Mastersizer E, which are low cost instruments for repetitive analyses and the modular series of Mastersizer S and Mastersizer X, the ultimate in resolution and dynamic size range, which are required when samples in the form of aerosols, suspensions and dry powders need to be measured. Mastersizer X provides a selection of small size ranges using a variety of interchangeable lenses whereas the Mastersizer S provides a wider dynamic size range covered in a single measurement. For powders which are to be suspended in a solvent, emulsions, suspensions and particles in liquids there are small volume cells which require as little as 15 ml of dispersant. Where a material is either valuable or toxic the Malvern Small Volume Flow Cell, with a sample volume of 50-80 ml and full sample recovery, can be used. The X-Y sampler is a 40-sample accessory... 

Infrared spectra may be obtained for gases, liquids, orsolids. For transmittance infrared spectroscopy, the sampling techniques may involve a solution, a film, amull, or a pellet, depending on the type of sample. Reflectance spectroscopy differs from transmittance spectroscopy in that infrared radiation reflected from the surface of a material is studied. With a proper sampling accessory (obtainable from commercial sources), the materials analyzed by reflectance techniques normally require little or no sample preparation. The method is non-destructive, non-invasive, and very useful for analyzing materials that are too thick or have too much absorbance to be analyzed by transmittance spectroscopy. 

Most reflectance spectroscopy is carried out utilizing an accessory that can be easily inserted into and removed from the sampling compartment of a conventional spectrometer. These accessories are designed for each application and usually consist of mirrors or prisms for reflecting or focusing the radiation. Most of the sampling accessories for the FTIR spectrometers and the FTIR microscopes are available commercially. 

These somewhat disparate technical developments progressed more or less in parallel but converged during the evolution of Raman instrumentation starting approximately in 1986. A major result was the emergence of integrated Raman spectrometers that incorporated laser, spectrometer, sampling accessories, and software into a complete system. These newer instruments were not only much more capable than previous systems, but they were also more reliable... 

Figure 1.26 The Large Sampling Accessory by Varian, provides for FPA chemical imaging at a 1 1 magnification for the analysis of macro samples. Illustration courtesy of Varian Corporation.

Infrared spectroscopy is seldom used to study lignin in solution, though the need for such a study occasionally arises for example, for the analysis of spent pulping liquors or soluble lignin fractions. Faix [119] provides a review of sampling accessories and methods as well as information on the type of lignin samples analyzed. 

The spectrum was recorded with the use of an ATR sampling accessory and has been ratioed against a single-beam spectrum of water. 

The advent of new infrared sampling accessories has allowed in situ study of organometallic systems under conditions that were previously not readily accessible. 

Most of these instruments have sampling accessories —generally coupled on-line— to reduce human intervention in the analytical process. They often also have an independent microprocessor linked to the main one (Fig. 8.11). The aforesaid accessories can have a variety of purposes, namely ... 

If a microscope facility is not available, there are other special sampling accessories available which allow examination of microgram or microlitre amounts. This is accomplished by using a beam condenser so that as much as possible of the beam passes through the sample. Microcells are available with volumes of 4 jjul and pathlengths up to 1 mm. 

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