Spectroscopic sample preparation

This chapter deals mainly with (multi)hyphenated techniques comprising wet sample preparation steps (e.g. SFE, SPE) and/or separation techniques (GC, SFC, HPLC, SEC, TLC, CE). Other hyphenated techniques involve thermal-spectroscopic and gas or heat extraction methods (TG, TD, HS, Py, LD, etc.). Also, spectroscopic couplings (e.g. LIBS-LIF) are of interest. Hyphenation of UV spectroscopy and mass spectrometry forms the family of laser mass-spectrometric (LAMS) methods, such as REMPI-ToFMS and MALDI-ToFMS. In REMPI-ToFMS the connecting element between UV spectroscopy and mass spectrometry is laser-induced REMPI ionisation. An intermediate state of the molecule of interest is selectively excited by absorption of a laser photon (the wavelength of a tuneable laser is set in resonance with the transition). The excited molecules are subsequently ionised by absorption of an additional laser photon. Therefore the ionisation selectivity is introduced by the resonance absorption of the first photon, i.e. by UV spectroscopy. However, conventional UV spectra of polyatomic molecules exhibit relatively broad and continuous spectral features, allowing only a medium selectivity. Supersonic jet cooling of the sample molecules (to 5-50 K) reduces the line width of their... 

Principles and Characteristics Extraction or dissolution methods are usually followed by a separation technique prior to subsequent analysis or detection. While coupling of a sample preparation and a chromatographic separation technique is well established (Section 7.1), hyphenation to spectroscopic analysis is more novel and limited. By elimination of the chromatographic column from the sequence precol-umn-column-postcolumn, essentially a chemical sensor remains which ensures short total analysis times (1-2 min). Examples are headspace analysis via a sampling valve or direct injection of vapours into a mass spectrometer (TD-MS see also Section 6.4). In... 

Near-infrared spectroscopy is quickly becoming a preferred technique for the quantitative identification of an active component within a formulated tablet. In addition, the same spectroscopic measurement can be used to determine water content since the combination band of water displays a fairly large absorption band in the near-IR. In one such study [41] the concentration of ceftazidime pentahydrate and water content in physical mixtures has been determined. Due to the ease of sample preparation, near-IR spectra were collected on 20 samples, and subsequent calibration curves were constructed for active ingredient and water content. An interesting aspect of this study was the determination that the calibration samples must be representative of the production process. When calibration curves were constructed from laboratory samples only, significant prediction errors were noted. When, however, calibration curves were constructed from laboratory and production samples, realistic prediction values were determined ( 5%). 

Modern spectroscopy plays an important role in pharmaceutical analysis. Historically, spectroscopic techniques such as infrared (IR), nuclear magnetic resonance (NMR), and mass spectrometry (MS) were used primarily for characterization of drug substances and structure elucidation of synthetic impurities and degradation products. Because of the limitation in specificity (spectral and chemical interference) and sensitivity, spectroscopy alone has assumed a much less important role than chromatographic techniques in quantitative analytical applications. However, spectroscopy offers the significant advantages of simple sample preparation and expeditious operation. 

The same can be said for the sections concerning the instrumental techniques of GC, IR, NMR, and HPLC. The chromatographic techniques of GC and HPLC are presented as they relate to thin-layer and column chromatography. The spectroscopic techniques depend less on laboratory manipulation and so are presented in terms of similarities to the electronic instrumentation of GC and HPLC techniques (dual detectors, UV detection in HPLC, etc.). For all techniques, the emphasis is on correct sample preparation and correct instrument operation. 

Molecular spectroscopic techniques have been widely used in pharmaceutical analysis for both qualitative (identification of chemical species) and quantitative purposes (determination of concentration of species in pharmaceutical preparations). In many cases, they constitute effective alternatives to chromatographic techniques as they provide results of comparable quality in a more simple and expeditious manner. The differential sensitivity and selectivity of spectroscopic techniques have so far dictated their specihc uses. While UV-vis spectroscopy has typically been used for quantitative analysis by virtue of its high sensitivity, infrared (IR) spectrometry has been employed mainly for the identihcation of chemical compounds on account of its high selectivity. The development and consolidation of spectroscopic techniques have been strongly influenced by additional factors such as the ease of sample preparation and the reproducibility of measurements, which have often dictated their use in quality control analyses of both raw materials and finished products. 

The procedures used to record NIR spectra for samples are much less labor-intensive than those involved in other spectroscopic analytical techniques. NIR spectral information can be obtained from transmission, reflectance and transflectance measurements (Figure 14.1) this allows the measurement process to be adapted to the physical characteristics of the sample and expedites analyses by avoiding the need for sample preparation. 

SEC became the most widely used method for molar mass and molar mass distribution determination due to its broad applicability, easy sample preparation, and the large amount of information resulting from the full distribution curve. The commercially available SEC systems work automatically with small sample amounts and even at elevated temperatures. In addition, chromatographic systems coupled with spectroscopic methods giving chemical information on the separated fractions gain more and more importance for analysis of complex polymer systems and mixtures. 

The resulting styrene/maleic acid copolymer is soluble in hot water, in contrast to the starting material the aqueous solution of the product gives a distinctly acid reaction. The disappearance of the anhydride moiety can be verified by IR or C-NMR spectroscopic methods.The IR spectra of polymers should be recorded from a film of the sample prepared on a KBr pellet (freshly made from KBr powder). For this, a drop of a solution of the polymer in a low-boiling solvent (e.g.,THF, methylene chloride) is placed on the pellet.The residual solvent can often be removed directly in the IR beam.The resulting spectra are characterized by their sharp bands. 

Sample preparation methods are similar to those used for FAAS and ICP-AES. However, nitric acid is favoured for sample digestion since the other mineral acids contain elements which cause spectroscopic inteferences. 

Two types of coal ash samples have been prepared routinely for analysis at the Illinois Geological Survey. Low-temperature ash samples (12), in which the bulk of the mineral matter remains unchanged, are prepared by reaction of the coal with activated oxygen in a radiofrequency field. The effective temperature produced by this device is approximately 150 °C. Such samples were unsatisfactory for emission spectroscopic analysis. It is postulated that the presence of largely unaltered mineral matter, such as carbonates, sulfides, and hemihydrated sulfates (12), caused the observed nonreproducibility of results. High-temperature ash samples, prepared in a muffle furnace, consisted mainly... 

Transient absorption spectra from valerophenone in Na-ZSM-5 and Cs-ZSM-5 could be assigned to triplet-triplet transitions no spectra which can be attributed to the hydroxy-1,4-biradicals were detected. Furthermore, the decay of the transient signals could not be fit to either a single or double exponential expression, and samples prepared under apparently identical conditions exhibited half-lives that varied by a factor of 2 [292], All of these spectroscopic observations suggest that the valerophenone molecules reside in a distribution of sites within a zeolite and migration among them under the experimental conditions is slow. Thus, the Norrish II photoproduct ratios must be interpreted in terms of not only the relative populations of alkanophenones at each site type, but also the quantum efficiencies of each and the conformational preferences of the intermediate BR in each environment. 

The combination of preconcentration by electrodeposition with stripping by voltammetry is probably the most sensitive electroanalytical method in common use today. Consequently, this popular technique is discussed in more detail in Chapter 24. Preconcentration of material by an electrode reaction has been used in sample preparation for atomic absorption, neutron activation, x-ray fluorescence, microprobe, and several other spectroscopic techniques. 

IR data. Spectroscopic techniques were then employed to determine more about the composition of the fracture surfaces. ATR of the adhesive side of the fracture surfaces showed only slight differences in the composition of the organic phase near the interface as a result of applying primer to the adherend surface before applying the adhesive. The spectrum shown in Fig. 7C (obtained with the 45° KRS-5 reflection element) is the difference between a sample prepared from an unprimed adherend (Fig. 7B) and one prepared from a primed... 

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