Measurable absorption

A promising technique is cavity ringdown laser absorption spectroscopy (307), in which the rate of decay of laser pulses injected into an optical cavity containing the sample is measured. Absorption sensitivities of 5 x 10 have been measured on a ]ls time scale. AppHcations from the uv to the ir... 

Gas/Liquid Interfacial Area This has been evaluated by measuring absorption rates like those of CO9 in NaOH. A correlation by Charpentier (Chem. Eng. Journal, 11, 161 [1976]) is... 

Spectrometer (Section 13.1) Device designed to measure absorption of electromagnetic radiation by a sample. 

Equation (4-30) shows that when w is very small, alf- = A -I- B absorption occurs by both the chemical and classical modes. In terms of Fig. 4-2, there is no phase-lag between the pressure and concentration curves. When to is very large, a// = B the chemical system is unable to absorb energy from the high-frequency forcing function. When w is comparable with 1/t, a.lf - is a function of frequency, and alf- passes through an inflection point when w = t thus t is found by measuring absorption as a function of frequency. 

Before dealing with the experimental details of A AS or FES determinations it is necessary to consider the mode of treatment of the experimental data obtained. To convert the measured absorption values into the concentration of the substance being determined it is necessary either to make use of a calibration curve, or to carry out the standard addition procedure. 

Legend No number of measurement. Cone concentration in fig, CN"/100 ml Absorb absorbance [AU] slope slope of regression line t CV intercept see slope res. std. dev. residual standard deviation Srts -n number of points in regression LOD limit of detection LOQ limit of quantitation measurements using a 2-fold higher sample amount and 5-cm cuvettes—i.e., measured absorption 0. .. 0.501 was divided by 10. 

A qualitatively different approach to probing multiple pathways is to interrogate the reaction intermediates directly, while they are following different pathways on the PES, using femtosecond time-resolved pump-probe spectroscopy [19]. In this case, the pump laser initiates the reaction, while the probe laser measures absorption, excites fluorescence, induces ionization, or creates some other observable that selectively probes each reaction pathway. For example, the ion states produced upon photoionization of a neutral species depend on the Franck-Condon overlap between the nuclear configuration of the neutral and the various ion states available. Photoelectron spectroscopy is a sensitive probe of the structural differences between neutrals and cations. If the structure and energetics of the ion states are well determined and sufficiently diverse in... 

H E A / be (A is measured absorption b is the optical path-length c is the molar concentration)... 

The total anthocyanin content can often be determined in crude extracts containing other phenolic materials by measuring absorptivity of the solution at a single wavelength (Table 6.3.1). This is possible because anthocyanins have typical absorption bands in the 490 to 550 nm region of the visible spectra — far from the absorption bands of other phenolics with spectral maxima in the UV range. ... 

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. 

A series of reactions with gases have been selected for the rapid quantification of many of the major products from the oxidation of polyolefins. Infrared spectroscopy is used to measure absorptions after gas treatments. The gases used and the groups quantified include phosgene to convert alcohols and hydroperoxides to chloroformates, diazomethane to convert acids and peracids to their respective methyl esters, sulfur tetrafluoride to convert acids to acid fluorides and nitric oxide to convert alcohols and hydroperoxides to nitrites and nitrates respectively. 

Figure 6.11 shows the measured absorption spectra of miconazole (reference and sample). As precipitation takes place to varying degrees at different pH values, the spectra of the sample solutions change in optical densities, according to Beer s law. This can be clearly seen in Fig. 6.12 for the sample spectra, where the sample spectra have the lowest OD values at pH 9.0 and systematically show higher OD values... 

Water and sludge Acidify sample measure absorption at 196.0 nm using the selenium atomic line. AAS 0.25 NR Parvinen and Lajunen 1994... 

In the following section, the calculation of the VolSurf parameters from GRID interaction energies will be explained and the physico-chemical relevance of these novel descriptors demonstrated by correlation with measured absorption/ distribution/metabolism/elimination (ADME) properties. The applications will be shown by correlating 3D molecular structures with Caco-2 cell permeabilities, thermodynamic solubilities and metabolic stabilities. Special emphasis will be placed on interpretation of the models by multivariate statistics, because a rational design to improve molecular properties is critically dependent on an understanding of how molecular features influence physico-chemical and ADME properties. 

HPLC has been used, with an ultraviolet absorption detector set for 254 nm, for the determination of aromatic hydrocarbons and with a flow calorimeter for the detection of all hydrocarbons. Increased sensitivity and decreased interference can be achieved with the ultraviolet absorption detector by measuring absorption at two wavelengths and using the ratios of the absorption at those wavelengths [28]. 

Fig. 5.4 From top down the measured absorption profiles of methane, methyl chloride, and ethylene obtained using a WGM locked to the laser. In each case, the top trace shows the amplified variation in dip depth and the bottom trace is the transmission profile of the gas in a 16 cm absorption cell. The frequency axis shows the tuning range. Reprinted from Ref. 4 with permission. 2008 Optical Society of America...

Physicochemical profiling at the early discovery stage is important in the pharmaceutical industry because poor bioavailability is a leading factor in compound attrition. The ability to rapidly measure absorption properties such as solubility, log P, and log D allows promising compounds to quickly pass into exploratory development. 

There are two ways to approach this issue, and both should be investigated in any questionable measurement. First, it is often assumed that any absorption or other measurement that is three or four times larger than the noise is real. This is a good start. However, there are other more scientific approaches to this problem. If repeated measurements on different subsamples or aliquots produce the same absorption or measurement, with minor variations, then it is probably a real result and not noise. On the other hand, if, during repeated measurements, absorption features occur in exactly the same location and have exactly the same characteristics such as shape and area under the peak, then they are probably not due to the sample because some variation in measurement always occurs. This type of problem is usually a result of instrument malfunction and this must be investigated. 

A continuously monitoring detector of high sensitivity is required and those that measure absorption in the ultraviolet are probably the most popular. These may operate at fixed wavelengths selected by interference filters but the variable wavelength instruments with monochromators are more useful. Wavelengths in the range of 190-350 nm are frequently used and this obviously means that a mobile phase must not absorb at those wavelengths. 

Figure 5-19. Representation of the measured absorption profiles in the U space.

---