Absorption, detection method

Absorbance peaks, proteins and peptides, 20-21 Absorption, detection method in CE, 16r Acetonitrile... 

Multiplying the transmittance by 100 gives the percent transmittance (%T), which varies between 100% (no absorption) and 0% (complete absorption). All methods of detection, whether the human eye or a modern photoelectric transducer, measure the transmittance of electromagnetic radiation. 

The efficient recovery of volatile nitrosamines from frankfurters, followed by gc with chemiluminescence detection, has been described (133). Recoveries ranged from 84.3 to 104.8% for samples spiked at the 20 ppb level. Methods for herbicide residues and other contaminants that may also relate to food have been discussed. Inorganic elements in food can be deterrnined by atomic absorption (AA) methods. These methods have been extensively reviewed. Table 8 Hsts methods for the analysis of elements in foods (134). 

Physical detection methods are based on inclusion of substance-specific properties. The most commonly employed are the absorption or emission of electromagnetic radiation, which is detected by suitable detectors (the eye, photomultiplier). The / -radiation of radioactively labelled substances can also be detected directly. These nondestructive detection methods allow subsequent micropreparative manipulation of the substances concerned. They can also be followed by microchemical and/or biological-physiological detection methods. 

The several experimental methods allow a wide range of relaxation times to be studied. T-Jump is capable of measurements over the time range 1 to 10 s P-jump, 10 to 5 X 10" s electric field jump, 10 to 10 s and ultrasonic absorption, 10 to 10 " s. The detection method in the jump techniques depends upon the systems being studied, with spectrophotometry, fluorimetry, and conductimetry being widely used. 

During an XAS experiment, core electrons are excited. This produces empty states called core holes. These can relax by having electrons from outer shells drop into the core holes. This produces fluorescent X-rays that have a somewhat lower energy than the incident X-rays. The fluorescent signal is proportional to the absorption. Detection of this signal is a useful method for measuring absorption by dilute systems such as under potential deposited (UPD) monolayers. 

W. Pennincks, P. Vankeerberghen, D.L. Massart and J. Smeyers-Verbeke, A knowledge-based computer system for the detection of matrix interferences. Atomic Absorption Spectrometric Methods, J. Anal. Atom. Spectrom. inch Atomic Absorption Spectrom. Updates, 10 (3) (1995) 207-214. 

A molecular absorption spectrophotometry method, using a sharp-line irradiation source, has been developed for the determination of sulfide (as hydrogen sulfide) in water and sludge samples. The method was tested with measurements of real waste-water samples. The limit of detection was 0.25 g (1-10 mL sample volume). 

The most commonly used method for the identification of carotenoids is high-pressure liquid chromatography (HPFC) combined with the UV-Vis absorption detection. The introduction of diode array detection enabled parallel collection of pigment spectra, which greatly aids the quantification and localization of unknown compounds. Coupling HPFC with the mass-spectrometer significantly... 

A battery of sensitive techniques is being developed to probe the photofragments resulting from photolysis of metal complexes in collision free conditions. The aim is to characterize the energy content, structure and chemistry of the photoproducts. These methods rely on ultraviolet (UV) laser photolysis followed by detection methods based on UV absorption (20), chemical trapping (21,22), IR absorption (23,24.25) and ionization (5,6,26,27). 

The principal analytical methods for complex samples are those that separate the mixture by differential migration and then detect the separated components. The separation methods are chromatography, electrophoresis, and field flow fractionation the detection methods—which need not be selective but must be sensitive—include absorption, laser-induced fluorescence, electrochemistry, and mass... 

Sturgeon et al. [59] have described a hydride generation atomic absorption spectrometry method for the determination of antimony in seawater. The method uses formation of stibene using sodium borohydride. Stibine gas was trapped on the surface of a pyrolytic graphite coated tube at 250 °C and antimony determined by atomic absorption spectrometry. An absolute detection limit of 0.2 ng was obtained and a concentration detection limit of 0.04 pg/1 obtained for 5 ml sample volumes. 

A hanging mercury drop electrodeposition technique has been used [297] for a carbon filament flameless atomic absorption spectrometric method for the determination of copper in seawater. In this method, copper is transferred to the mercury drop in a simple three-electrode cell (including a counterelectrode) by electrolysis for 30 min at -0.35 V versus the SCE. After electrolysis, the drop is rinsed and transferred directly to a prepositioned water-cooled carbon-filament atomiser, and the mercury is volatilised by heating the filament to 425 °C. Copper is then atomised and determined by atomic absorption. The detection limit is 0.2 pg copper per litre simulated seawater. 

Tony et al. [951] have discussed an online preconcentration flame atomic absorption spectrometry method for determining iron, cobalt, nickel, magnesium, and zinc in seawater. A sampling rate of 30 samples per hour was achieved and detection limits were 4.0,1.0,1.0,0.5, and 0.5 xg/l, for iron, cobalt, nickel, magnesium, and zinc, respectively. 

It has been reported that the differential determination of arsenic [36-41] and also antimony [42,43] is possible by hydride generation-atomic absorption spectrophotometry. The HGA-AS is a simple and sensitive method for the determination of elements which form gaseous hydrides [35,44-47] and mg/1 levels of these elements can be determined with high precision by this method. This technique has also been applied to analyses of various samples, utilising automated methods [48-50] and combining various kinds of detection methods, such as gas chromatography [51], atomic fluorescence spectrometry [52,53], and inductively coupled plasma emission spectrometry [47]. 

Valkirs et al. [105] have conducted an interlaboratory comparison or determinations of di- and tributyltin species in marine and estuarine waters using two methods, namely hydride generation with atomic absorption detection and gas chromatography with flame photometric detection. Good agreement was obtained between the results of the two methods. Studies on the effect of storing frozen samples prior to analysis showed that samples could be stored in polycarbonate containers at - 20 °C for 2 - 3 months without significant loss of tributyltin. 

This preliminary work demonstrated well the use of CL as a highly sensitive and selective detection method in CE by its application to the separation of luminol and ABEI using the same experimental conditions cited previously for the CL reaction of luminol in HPLC. Detection limits (S/N = 3) of 100 amol and 400 amol were obtained for the compounds mentioned, respectively, achieving an improvement in sensitivity of 2-3 orders of magnitude with respect to the ones obtained using UV absorption for detection. 

The sensing methods summarized thus far are intended for absorption detection of molecules in the ambient, but molecules (or indeed thin films) on the microresonator surface can also be detected. In particular, if the surface is covered to such an extent that the optical energy absorbed heats the microresonator, the resulting thermal bistability in the frequency-scan response can be used to determine the absorption and/or thickness of the thin-film coating. This and surface characterization by measurement of the thermal accommodation coefficient were described in Sect. 5.5. These methods offer quite precise measurement, provided that certain reasonable and easily implemented assumptions are satisfied. 

In soil analysis, HPLC is used much like GC in that soil is extracted and the extract, after suitable cleanup and concentration, is analyzed. One major difference between them is that HPLC does not require the components to be in the gaseous phase. They must, however, be soluble in an eluent that is compatible with the column and detector being used. A second difference is that both a syringe and an injector are used to move the sample into the eluent and onto the column. Detection is commonly by UV absorption, although RI, conductivity, and mass spectrometry are also commonly used. Conductivity or other electrical detection methods are used when analysis of ionic species in soil is carried out [3,78],... 

Several kinds of detection systems have been applied to CE [1,2,43]. Based on their specificity, they can be divided into bulk property and specific property detectors [43]. Bulk-property detectors measure the difference in a physical property of a solute relative to the background. Examples of such detectors are conductivity, refractive index, indirect methods, etc. The specific-property detectors measure a physico-chemical property, which is inherent to the solutes, e.g. UV absorption, fluorescence emission, mass spectrum, electrochemical, etc. These detectors usually minimize background signals, have wider linear ranges and are more sensitive. In Table 17.3, a general overview is given of the detection methods that are employed in CE with their detection limits (absolute and relative). 

The most often used detection method for the optical sensors are based on absorption, luminescence, reflectance, and Raman scattering measurements. The basic theory and instrumentation of most of these... 

Pulse radiolysis, using as time-resolved detection methods optical absorption, luminescence, electrical conductivity or electron spin resonance can be expected to give information on the formation of transient or permanent radiation products and on their movement. 

Recent developments in laser technology and fast detection methods now allow the kinetic behaviour of the excited state species arising from absorption of radiation by polymers to be studied on time-scales down to the picosecond region ( ). An example of a time-resolved fluorescence spectrometer which can be used to study such ultrafast phenomena is illustrated in Figure 5 Q). 

In any chromatographic analysis the method of detection is determined by the nature of the analyte and the mobile phase used must not interfere with this system. The use of ultraviolet absorption detection systems is very common but the solvents used must not absorb significantly at the wavelength used. For instance, absorption at 280 nm is frequently used to detect protein but some solvents, e.g. acetone, absorb at this wavelength. Similarly the use of concentration gradients in the mobile phase may present problems with refractive index and electrochemical detection systems. 

Fluorescence detection is usually more sensitive than absorption detection, but the number of naturally fluorescent compounds is limited. Pre- or postcolumn derivatization can also be applied for this type of detection. Chemiluminescence detection is the most sensitive method for some fluorescent compounds. 

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