Spectroscope/spectrometer types

Quality control, pharmaceutical product identity checks, and quantification are important fields in the broad application of the different spectroscopic methods. There are many spectroscopic aspects, e.g., concerning sample preparation, influences from different accessories, and possibly spectrometer effects, which certainly influences quantitative measurements. The latter problems could be solved using calibration transfer between different spectrometer types, for example, a scanning and an FT-near-IR spectrometer. 

Matrix isolation is a powerful tool for studying highly reactive species. The matrix provides an inert environment, which increases the lifetime of the reactive species to the extent that it may be thought of as a stable species. This allows for its spectroscopical identification and/or characterization using normal-type spectrometers. It is also possible to learn something about the intrinsic reactivity of the species, because, under such conditions, intermolecular reactions can be excluded. 

Cells of the second type were initially developed by Tinker and Morris at Monsanto [4] and subsequently by Penninger [5]. In these systems, the reaction solution is circulated from the autoclave through an external IR cell of relatively small volume. This arrangement means that the cell can be isolated from the main reaction vessel relatively easily (for example in the event of window failure) thus protecting the spectrometer. Cells of this sort can, in principle, be fitted to plants or pilot plants to monitor liquid streams. However, the circulation of solution from the main reaction vessel through an external cell introduces some potential problems. A pressure drop in the circulation system can lead to release of dissolved gas, which may accumulate between the cell windows and interfere with the spectroscopic measurement. A change in pressure may also influence the catalyst specia-tion, such that the observed spectra may not be truly representative of the bulk reaction solution. 

The type of arrangement of the monomeric units in polymeric dienes can be determined qualitatively and quantitatively by IR spectroscopy. For this purpose thin films are prepared by dropping an approximately 2% solution in carbon disulfide (spectroscopically pure) on to suitable rock salt plates and allowing the solvent to evaporate at room temperature.The plates are placed in the spectrometer beam and the IR spectrum is recorded.The different types of chemical linkages are associated with characteristic IR bands as summarized in Table 3.9. 

In principle, all these capabilities will enhance the performance of any type of atomic spectrometry, independently of the nature of the spectroscopic technique used (e.g. a procedure that separates trace elements from a large volume of a highly saline medium and releases them into a smaller volume of dilute nitric acid can be used in conjunction with any type of spectrometer). 

One important practical aspect of PLS is that it takes into account errors both in the concentration estimates and spectra. A method such as PCR will assume that the concentration estimates are error free. Much traditional statistics rest on this assumption, that all errors are in the dependent variables (spectra). If in medicine it is decided to determine the concentration of a compound in the urine of patients as a function of age, it is assumed that age can be estimated exactly, the statistical variation being in the concentration of a compound and the nature of the urine sample. Yet in chemistry there are often significant errors in sample preparation, for example, accuracy of weighings and dilutions and so the independent variable (c) in itself also contains errors. With modem spectrometers, these are sometimes larger than spectroscopic errors. One way of overcoming this difficulty is to try to minimise the covariance between both types of variables, namely the x (spectroscopic) and c (concentration) variables. 

Durst et al. (29) made microscale synthesis of several sarin analogues in autosampler vials and analyzed them directly with a gas chromato-graphy/infrared spectroscope/mass spectrometer (GC/IR/MS) instrument. Ninety different alkyl and cycloalkyl alcohols were used. They presented a couple of light-pipe IR spectra and a table of typical gas-phase frequencies (vp F, vp=0, pP methyl, 8S P methyi, vp 0 c) for 12 sarin-type chemicals. 

Principal component analysis is most easily explained by showing its application on a familiar type of data. In this chapter we show the application of PCA to chromatographic-spectroscopic data. These data sets are the kind produced by so-called hyphenated methods such as gas chromatography (GC) or high-performance liquid chromatography (HPLC) coupled to a multivariate detector such as a mass spectrometer (MS), Fourier transform infrared spectrometer (FTIR), or UV/visible spectrometer. Examples of some common hyphenated methods include GC-MS, GC-FTIR, HPLC-UV/Vis, and HLPC-MS. In all these types of data sets, a response in one dimension (e.g., chromatographic separation) modulates the response of a detector (e.g., a spectrum) in a second dimension. 

The details of spectroscopic instrumentation belong in a book dedicated to the subject, but it is still important that you have a feel for the nature of the experiment. The basic elements of many types of instruments, including IR spectrometers, are illustrated in Figure 7-7. First, you need a source of radiation in the appropriate frequency range and then you need to get the light focused on... 

---