IR Sampling

A recent innovation in IR sample preparation is the use of disposable sample cards made from thin sheets of either polyethylene (PE) or polytetrafluoroethylene (PTFE). 

In this IR sampling technique, a thermogravimetric (TG) analyzer is interfaced to an IR spectrophotometer so that the evolved gas from the sample/TG furnace is directed to an IR gas cell. This IR sampling technique lends itself to the identification and quantitation of residual solvent content for a pharmaceutical solid [17], and also to the investigation of pharmaceutical pseudopolymorphs. 

The pharmaceutical industry comprises the largest segment, roughly 15 to 20%, of the infrared (IR) market. Modern mid-infrared instrumentation consists almost exclusively of Fourier transform (FT) instruments. Because of its ability to identify molecular species, FT-IR is routinely used as an identification assay for raw materials, intermediates, drug substances, and excipients. However, the traditional IR sample preparation techniques such as alkali halide disks, mulls, and thin films, are time-consuming and not always adequate for quantitative analysis. 

Rather than measure the light transmitted through a sample, as in all of the IR sampling methods discussed thus far, some methods measure the light reflected from a sample. There are various methods employed to measure this reflectance, depending on the properties of the sample. 

It is essential that oxygen was contained in subsurface Pt layers, dissolving there with increasing temperature. This oxygen, whose removal is extremely difficult, can affect the constants of surface reactions. For example, the initial sticking coefficient of 02 on the oxidized sample is S02 = 0.05, whereas for the Ir sample that was not exposed to oxygen we have Sq2 - 0.26 [78,106,142]. Since the literature lacks detailed information, our model does not account for this fact. 

One challenge in the analysis of the CWA is the analysis of their precursors and degradation products, which are often nonvolatile. The CWA degrade, for example, hydrolyze or oxidize easily. Traditional IR sampling techniques, like KBr pellets and liquid cells are well suited for analysis of neat or concentrated nonvolatile chemicals. Environmental samples containing these kind of chemicals, however, normally require derivatization before GC/FTIR analysis. 

ATR is a technique based on total internal reflections at the crystal surface. The infrared spectrum is measured from a very thin volume surrounding the infrared transparent ATR crystal. This technique is one of the best IR sampling techniques suitable for analyses of chemicals in water. Detection limit is less than 1 mg/ml for nerve agents (10). 

The liquid from an LC is compatible with normal IR sampling and is less of a problem. However, LC mobile phases may not be transparent in the IR, and water is a particularly difficult solvent to handle. For volatile organic solvents, evaporation is possible, and the remaining nonvolatile analytes can be deposited in KBr and pressed into pellets. Further discussion can be found in reference 20. 

The sample chamber is supplemented by a second IR window, which is separated from the basic IR window by a thin spacer of several micrometers. Fig. 6.6-9 shows an outline of the IR sample chamber. 

UV-VIS spectra (Perkin-Elmer Lambda 9) were registered in reflectance using a evacuable optical cell. Powder of zeolites (around 500 mg) were placed into the cell, pretreated as the IR samples. The following measurements were performed, monitoring the generation of surface species at increasing (i) hydrocarbon loading and (ii) temperature. 

The activity of catalysts was tested in the parallel HDN/HDS of pyridine (PY) and thiophene (TH) in an flow reactor at 320°C and 20 bar. In case of reduced Ir samples, single HDN of pyridine was performed. The feed contained 220 ppm of PY and 240 ppm of TH (or only PY) in H2 at a flow rate of 0.4 mol/h. The HDS of thiophene was described by pseudo-first-order rate constant Rth- The HDN was described for simplicity by two rate constants for pyridine HY, kpv, and piperidine HDN, kcs- Further details concerning the evaluation of the catalyst activity can be found elsewhere [10,11]. 

A Nicolett 7199 FT-IR spectrometer was used to monitor the cure of silicone gel. The decreasing absorption of the Si-H absorption at "2129cm-1 was monitored during the cure of the silicone gel (see Figure 3). Silicone gel (Part A and Part B) was freshly mixed at a prescribed ratio prior to the cure study. A NaCl salt plate was used as the IR sample cell. Results of the FT-IR study are shown in Figure 4. 

This chapter will focus on the appHcation of FTIR spectroscopy in the quantitative analysis of foods. Following a brief discussion of the fundamental principles of IR spectroscopy, we wiU describe the instrumentation, data handling techniques, and quantitative analysis methods employed in FTIR spectroscopy. We will then consider the IR sampling techniques that are most useful in FTIR analysis of foods. Finally, a survey of FTIR applications to the quantitative analysis of food will be presented. Although important, the so-called hyphenated techniques, such as GC-FTIR, will not be covered in this chapter. Similarly, near-IR (NIR) spectroscopy, which has found extensive use in food analysis, is beyond the scope of this chapter. 

Unless otherwise noted, the air temperature was held at 60 1 C and the dew point was 25 +2 C. No condensing humidity cycle was employed. Samples doped with HALS 2 and the IR samples were mounted approximately 6 cm away from two FS20 UV-B sunlamps. At various times, the samples were removed from the chamber and analyzed. IR spectra were obtained with a Nicolet Fourier transform IR. Samples were mounted such that the same spot on the plate was always measured. In this way it was possible to obtain information on changes in film thickness as well and chemical composition on degradation. The ESR samples were placed in a special sample holder and the nitroxide concentration determined using an IBM-Bruker ESR spectrometer equipped with an Aspect 2000 data system (42). 

The classic IR sampling technique is the alkali halide pellet preparation (18). This technique involves mixing the solid-state sample of interest with an alkali halide (typically KBr or KC1) at a 1-2% w/w sample/alkali halide ratio. The mixture is pulverized into a finely ground homogeneous mixture, placed into a die (typically stainless steel), and subjected to approximately 10,000 psi of pressure for a period of time to produce a glass pellet. The pellet (with the sample finely dispersed throughout the glass) may then be placed into the IR spectrophotometer for spectral data acquisition. 

With the ever-increasing need to improve quality and productivity in the analytical pharmaceutical laboratory, automation has become a key component. Automation for vibrational spectroscopy has been fairly limited. Although most software packages for vibrational spectrometers allow for the construction of macro routines for the grouping of repetitive software tasks, there is only a small number of automation routines in which sample introduction and subsequent spectral acquisition/data interpretation are available. For the routine analysis of alkali halide pellets, a number of commercially available sample wheels are used in which the wheel contains a selected number of pellets in specific locations. The wheel is then indexed to a sample disk, the IR spectrum obtained and archived, and then the wheel indexed to the next sample. This system requires that the pellets be manually pressed and placed into the wheel before automated spectral acquisition. A similar system is also available for automated liquid analysis in which samples in individual vials are pumped onto an ATR crystal and subsequently analyzed. Between samples, a cleaning solution is passed over the ATR crystal to reduce cross-contamination. Automated diffuse reflectance has also been introduced in which a tray of DR sample cups is indexed into the IR sample beam and subsequently scanned. In each of these cases, manual preparation of the sample is necessary (23). In the field of Raman spectroscopy, automation is being developed in conjunction with fiber-optic probes and accompanying... 

Storing IR sample cells and KBr powder-cells are always stored in desiccators to prevent fogging by absorption of moisture. KBr powder must be dried in the oven, cooled and kept in a desiccator. 

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