Pathlengths

W L Bragg [7] observed that if a crystal was composed of copies of identical unit cells, it could then be divided in many ways into slabs with parallel, plane faces whose distributions of scattering matter were identical and that if the pathlengths travelled by waves reflected from successive, parallel planes differed by integral multiples of the... 

Kinetic measurements were performed employii UV-vis spectroscopy (Perkin Elmer "K2, X5 or 12 spectrophotometer) using quartz cuvettes of 1 cm pathlength at 25 0.1 C. Second-order rate constants of the reaction of methyl vinyl ketone (4.8) with cyclopentadiene (4.6) were determined from the pseudo-first-order rate constants obtained by followirg the absorption of 4.6 at 253-260 nm in the presence of an excess of 4.8. Typical concentrations were [4.8] = 18 mM and [4.6] = 0.1 mM. In order to ensure rapid dissolution of 4.6, this compound was added from a stock solution of 5.0 )j1 in 2.00 g of 1-propanol. In order to prevent evaporation of the extremely volatile 4.6, the cuvettes were filled almost completely and sealed carefully. The water used for the experiments with MeReOj was degassed by purging with argon for 0.5 hours prior to the measurements. All rate constants were reproducible to within 3%. 

A 5.00 X 10 M solution of an analyte is placed in a sample cell that has a pathlength of 1.00 cm. When measured at a wavelength of 490 nm, the absorbance of the solution is found to be 0.338. What is the analyte s molar absorptivity at this wavelength ... 

Infrared spectroscopy is routinely used for the analysis of samples in the gas, liquid, and solid states. Sample cells are made from materials, such as NaCl and KBr, that are transparent to infrared radiation. Gases are analyzed using a cell with a pathlength of approximately 10 cm. Longer pathlengths are obtained by using mirrors to pass the beam of radiation through the sample several times. 

Sensitivity The sensitivity of a molecular absorption analysis is equivalent to the slope of a Beer s-law calibration curve and is determined by the product of the analyte s absorptivity and the pathlength of the sample cell. Sensitivity is improved by selecting a wavelength when absorbance is at a maximum or by increasing the pathlength. 

The transmittance of a solution is found to be 85.0% when measured in a cell whose pathlength is 1.00 cm. What is the percent transmittance if the pathlength is increased to 10.00 cm ... 

A second instrumental limitation to Beer s law is stray radiation. The following data were obtained using a cell with a pathlength of 1.00 cm, when stray light is insignificant... 

A thorough treatment of the multicomponent quantitative analysis of samples based on Beer s law, and the analysis of samples for which the pathlength is indeterminate is found in the following review article. 

This is similar to Equation (2.16) in which Iq and / are the intensities of the incident and transmitted radiation, respectively, and f is the pathlength of the absorbing sample. 

The amount of rotation observed in a polarimetry experiment depends on the number of optically active molecules encountered by the light beam. This number, in turn, depends on sample concentration and sample pathlength. If the concentration of sample is doubled, the observed rotation doubles. If the concentration is kept constant but the length of the sample tube is doubled, the observed rotation is doubled. It also happens that the amount of rotation depends on the wavelength of the light used. 

To express optical rotations in a meaningful way so that comparisons can be made, we have to choose standard conditions. The specific rotation, [a ]D, of a compound is defined as the observed rotation when light of 589.6 nanometer (nm 1 nm = 10-9 m) wavelength is used with a sample pathlength / of 1 decimeter (dm 1 dm = 10 cm) and a sample concentration C of 1 g/mL. (Light of 589.6 nm, the so-called sodium d line, is the yellow light emitted from common sodium lamps.)... 

Problem 14,14 A knowledge of molar absorpiivities is particularly important in biochemistry, where UV spectroscopy can provide an extremely sensitive method of analysis. For example, imagine that you wanted to determine the concentration of vitamin A in a sample. If pure vitamin A has Amax = 325 (e = 50,100), what is the vitamin A i concentration in a sample whose absorbance at 325 nm is A = 0.735 in a cell with 1 a pathlength of 1.00 cm ... 

Long-path absorption measurements for [HO ] have been primarily developed by researchers in Germany (122,123,124,125). This approach measures the uv absorption by HO over a pathlength up to several kilometers. The result is an HO concentration spatially averaged over the... 

Besides the photometric method, a HPLC method could also be used. (Cf. the first constraint under the earlier head PROBLEM. The HPLC has a higher relative standard deviation for two reasons The transient nature of the signal results in a short integration time, and the short pathlength makes... 

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