Qualitative Spectral Information

An EDX spectrum typical of thin-film analysis in TEM/(S)TEM is shown in Eig. 4.26. It was obtained from a polycrystalline TiC/Zr02 ceramic by use of an Si(Li) detector at 100 keV primary electron energy. Eor spectrum recording the electron probe of approximately 1 nm in diameter was focused on the triple junction between the grains in the STEM mode (Eig. 4.26a). Besides the elements expected for the material under investigation, viz. Ti and Zr, Si, Ee, and Co were also detected, hinting at the presence of a (Ee, Co) silicide as an impurity. Eor ceramic materials it is known that 

The procedure commonly used to quantify EDX spectra was originally outlined by Castaing [4.109], although for the general situation of investigating bulk materials. To a good approximation it can be assumed that the concentration Csp of an element present in an unknown sample is related to the concentration Cst of the same element in a standard specimen by 

Z - correction for the different inelastic scattering properties introduced by differences between the mean atomic numbers of the specimen of interest and the standard  

A - correction for differences between X-ray absorption and F - correction for corresponding X-ray fluorescence differences. 

For an electron-transparent specimen the absorption and fluorescence correction parts can often be neglected, this is the so-called thin-film criterion introduced by Cliff and Lorimer [4.118]. Thus, for a thin specimen containing two elements A and B yielding the net X-ray intensities I a and 1b, the concentration ratio reduces to  

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Gas chromatography also can be used for qualitative purposes. When using an FT-IR or a mass spectrometer as the detector, the available spectral information often can be used to identify individual solutes. 

Infrared spectroscopy is one of the techniques most frequently coupled to SFE — it accounts for more than 30% of reported SFE hyphenated methods — which is unsurprising as it is one of the most powerful tools available for the elucidation of molecular structures. The specificity of IR spectral information is highly useful for the real-time monitoring of SFE processes with qualitative and quantitative purposes [125]. However, the partial transparency of supercritical CO, in the IR region — it exhibits strong absorption bands at 3800-3500,2500-2150 and below 900 cm — calls for careful background correction. 

If the same sample is characterized by SIM, a clear improvement in selectivity and sensitivity is observed (see Fig. 13d). All spectral information on the peak is sacrificed as the instrument is configured to monitor only a single m/g window. Additional selectivity is obtained by monitoring an MRM transition, in this case on a triple quadrupole mass spectrometer (see Fig. 13e). The precursor ion of gabapentin, (M + FI)1+ 172 m/ 7y, is selected in Ql and fragmented in the collision cell, and the product ion, (M + FI)1+ 154 m/g, is monitored by Q3. This technique provides the highest combination of selectivity and signal to noise (S/N) with the sacrifice of qualitative information. 

Contemporary spectrometers are able to produce huge amounts of data within a very short time. This development continues due to the introduction of array detectors for spectral imaging. The utilization of as much as possible of the enclosed spectral information can only be achieved by chemometric procedures for data analysis. The most commonly used procedures for evaluation of spectra are systematically arranged in Fig. 22.2 with the main emphasis on application, i.e. the variety of procedures was divided into methods for qualitative and quantitative analysis. Another distinctive feature refers to the mathematical algorithms on which the procedures are based. The dominance of multivariate over univariate methods is clearly discernible from Fig. 22.2. 

The photodiode-array detector is a powerfiil analytical instrument that has provided enhanced detection capabilities with the addition of detailed spectral information via its multisignal detection technology. Its applications are HPLC based and can be found in basic research, automated analysis, pharmaceutical product development, and the clinical laboratory environment. Through spectral acquisition and analysis, a wealth of information can be obtained about the identity and purity of a compound. Combined with high selectivity and sensitivity, this mode of detection is essential for qualitative and quantitative HPLC analysis. 

Recent advances in hyphenated analytical techniques, where a separation device is coupled online with detectors generating spectral information, have remarkably widened the analysis field of complex biological matrices. During the last few years covered by this chapter, a number of papers describing the application of TLC, GC-MS, HPLC-UV, HPLC-UV/MS, CE-MS, and NMR to the qualitative and quantitative analysis of tropane alkaloids in toxicological, physiological, forensic, phytochemical, and chemotaxonomical studies have been published. 

In photoluminescence one measures physical and chemical properties of materials by using photons to induce excited electronic states in the material system and analyzing the optical emission as these states relax. Typically, light is directed onto the sample for excitation, and the emitted luminescence is collected by a lens and passed through an optical spectrometer onto a photodetector. The spectral distribution and time dependence of the emission are related to electronic transition probabilities within the sample, and can be used to provide qualitative and, sometimes, quantitative information about chemical composition, structure (bonding, disorder, interfaces, quantum wells), impurities, kinetic processes, and energy transfer. 

Haaland, D.M., Thomas, E.V., "Partial Least-Squares Methods for Spectral Analysis 1. Relation to Other Quantitative Calibration Methods and the Extraction of Qualitative Information" Anal. Chem. 1988 (60) 1193-1202. 

The amount of information, which can be extracted from a spectrum, depends essentially on the attainable spectral or time resolution and on the detection sensitivity that can be achieved. Derivative spectra can be used to enhance differences among spectra, to resolve overlapping bands in qualitative analysis and, most importantly, to reduce the effects of interference from scattering, matrix, or other absorbing compounds in quantitative analysis. Chemometric techniques make powerful tools for processing the vast amounts of information produced by spectroscopic techniques, as a result of which the performance is significantly... 

The set of energy levels associated with a particular substance is a unique characteristic of that substance and determines the frequencies at which electromagnetic radiation can be absorbed or emitted. Qualitative information regarding the composition and structure of a sample is obtained through a study of the positions and relative intensities of spectral lines or bands. Quantitative analysis is possible because of the direct proportionality between the intensity of a particular line or band and the number of atoms or molecules undergoing the transition. The various spectrometric techniques commonly used for analytical purposes and the type of information they provide are given in Table 7.1. 

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