Absorption measurements

Example quantitative determination of alanine in dry extract of marshmallow root 

Method. This page is identified by the name of the method (in this case AS 15). All the parameters for the measurement have previously been entered in the corresponding computer file. 

Peak list (short). In this list, only the substances with the automatic peak assignments are used. (For reasons of space, this hst is not given here.) 

Results (short). Printouts are given as for Results (long) , but without listing the calibration solutions. (Again, this hst is not given here for reasons of space.) 

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O Keefe A and Deacon DAG 1988 Cavity ring-down optical spectrometer for absorption-measurements using pulsed laser sources Rev. Sol. Instrum. 59 2544-51... 

Zalicki P and Zare R N 1995 Cavity ring-down spectroscopy for quantitative absorption-measurements J. Chem. 

Methods for improving the precision of absorption measurements (a) high-absorbance method (b) low-absorbance method and (c) maximum precision method. Abbreviations Sa = sample St = standard. 

AT the path length, and P (A) the mass absorption coefficient at wavelength A. Between absorption edges, P (A) is proportional to Z A and is nearly independent of physical or chemical state. An absorption measurement on each side of an absorption edge is required for each element analyzed. X-ray absorption is especially useful in determining heavy elements in mixed materials of lower Z, such as lead in gasoline and uranium in aqueous solution. 

Although the most sensitive line for cadmium in the arc or spark spectmm is at 228.8 nm, the line at 326.1 nm is more convenient to use for spectroscopic detection. The limit of detection at this wavelength amounts to 0.001% cadmium with ordinary techniques and 0.00001% using specialized methods. Determination in concentrations up to 10% is accompHshed by solubilization of the sample followed by atomic absorption measurement. The range can be extended to still higher cadmium levels provided that a relative error of 0.5% is acceptable. Another quantitative analysis method is by titration at pH 10 with a standard solution of ethylenediarninetetraacetic acid (EDTA) and Eriochrome Black T indicator. Zinc interferes and therefore must first be removed. 

Optical absorption measurements give band-gap data for cubic sihcon carbide as 2.2 eV and for the a-form as 2.86 eV at 300 K (55). In the region of low absorption coefficients, optical transitions are indirect whereas direct transitions predominate for quantum energies above 6 eV. The electron affinity is about 4 eV. The electronic bonding in sihcon carbide is considered to be predominantiy covalent in nature, but with some ionic character (55). In a Raman scattering study of vahey-orbit transitions in 6H-sihcon carbide, three electron transitions were observed, one for each of the inequivalent nitrogen donor sites in the sihcon carbide lattice (56). The donor ionization energy for the three sites had values of 0.105, 0.140, and 0.143 eV (57). 

Calculations of Giierrieri (S.M. thesis in chemical engineering, MIT, 1932) from room-temperature absorption measurements of Cohlentz (Investigations of Infrared Spectra, Carnegie Institution, Washington, 1905) with poor allowance for temperature. 

Revised structures have been proposed for these compounds. The brownish-yellow acid obtained with maleic anhydride has been shown by ultraviolet, infrared, and nuclear magnetic resonance absorption measurements and oxidative degradation to have the tricyclic structure... 

Acids require vigorous conditions for successful reductions on a synthetic scale, but they can be reduced to the alcohol in small yield over rhodium even at ambient conditions. This fractional reduction is without utility but it is sufficient to cause errors in absorption measurements when a carboxylic acid is used as a solvent. At 150 C and 2000 psig, RhjOj becomes a useful catalyst for carboxylic acid hydrogenation 13). 

Parker [55] studied the IN properties of MEH-PPV sandwiched between various low-and high work-function materials. He proposed a model for such photodiodes, where the charge carriers are transported in a rigid band model. Electrons and holes can tunnel into or leave the polymer when the applied field tilts the polymer bands so that the tunnel barriers can be overcome. It must be noted that a rigid band model is only appropriate for very low intrinsic carrier concentrations in MEH-PPV. Capacitance-voltage measurements for these devices indicated an upper limit for the dark carrier concentration of 1014 cm"3. Further measurements of the built in fields of MEH-PPV sandwiched between metal electrodes are in agreement with the results found by Parker. Electro absorption measurements [56, 57] showed that various metals did not introduce interface states in the single-particle gap of the polymer that pins the Schottky contact. Of course this does not imply that the metal and the polymer do not interact [58, 59] but these interactions do not pin the Schottky barrier. 

Electrical measurements on devices with different layer thickness have shown that the diode current depends on the applied field rather than the drive voltage. This is similar to what has been observed with our alternating PPV copolymers [68]. It indicates that field-driven injection determines the electrical characteristics. From Figure 16-39 it is evident that U-OPV5 has the lowest onset for both current and emission. By means of Fowler-Nordhcini analysis of the /-V -charac-teristics and optical absorption measurements, wc estimated the injection barrier for holes and the HOMO-LUMO gap, respectively [119]. The results of... 

The amount of reddish-purple acid-chloranilate ion liberated is proportional to the chloride ion concentration. Methyl cellosolve (2-methoxyethanol) is added to lower the solubility of mercury(II) chloranilate and to suppress the dissociation of the mercury(II) chloride nitric acid is added (concentration 0.05M) to give the maximum absorption. Measurements are made at 530nm in the visible or 305 nm in the ultraviolet region. Bromide, iodide, iodate, thiocyanate, fluoride, and phosphate interfere, but sulphate, acetate, oxalate, and citrate have little effect at the 25 mg L 1 level. The limit of detection is 0.2 mg L 1 of chloride ion the upper limit is about 120 mg L . Most cations, but not ammonium ion, interfere and must be removed. 

Aqueous solutions may sometimes be analysed directly without any pretreatment, but it is a matter of chance that the given solution should contain the correct amount of material to give a satisfactory absorbance reading. If the existing concentration of the element to be determined is too high then the solution must be diluted quantitatively before commencing the absorption measurements. Conversely, if the concentration of the metal in the test solution is too low, then a concentration procedure must be carried out by one of the methods outlined at the end of this section. 

In order to concentrate the lead extract, remove the lead from the organic solvent by shaking this with three successive 10 mL portions of the dilute hydrochloric acid solution, collecting the aqueous extracts in a 250 mL beaker. To the combined extracts add 5 mL of 20 per cent ascorbic acid solution and adjust to pH 4 by the addition of concentrated ammonia solution. Place the beaker in a fume cupboard, add 3 mL of the 50 per cent potassium cyanide solution and immediately adjust the pH to 9-10 with concentrated ammonia solution. Transfer the solution to a 250 mL separatory funnel with the aid of a little de-ionised water, add 5 mL of the 2 per cent NaDDC reagent, allow to stand for one minute and then add 10 mL of methyl iso butyl ketone. Shake for one minute and then separate and collect the organic phase, filtering it through a fluted filter paper. This solution now contains the lead and is ready for the absorption measurement. 

Absorption measurements by Barkla revealed the existence of characteristic x-ray emission lines before x-ray wavelengths could be measured. There is no better evidence for the fundamental importance of the absorption process. 

Fig. 1-11. Fate of an x-ray beam. Two types of events occur as an x-ray beam is absorbed in matter—more precisely, as x-ray quanta disappear from the beam. These types of events are photoelectric absorption, measured by r, and scattering, measured by a. (See Eq. 1-8.)...

X-ray absorption measurements were made on several of the more... 

Comparative x-ray absorption measurements were used in the identification of various new compounds that could contain at most the following elements carbon, hydrogen, fluorine, and chlorine. The presumed composition of each compound, known in advance, was duplicated by properly blending carbon tetrachloride, benzotrifluoride, heptane, and benzene the latter also was used as solvent for the unknown. Under conditions intended to be identical, the amount of unknown... 

When the x-ray absorption measurements were made, nothing was known about the samples except that AOT-1 and B62M-1 were the only two base stocks involved. The sixteen values of equivalent thickness obtained were interpreted substantially as in Table 3-4 by those who supplied the samples. 

Kig. 5—1. Yortii American Philips (Philips Plodronics> -rav spectrometer converted Ion spectrophotometer for absorption measurements. (( ourlrsv o Hughes and Wilc/.ew ski, I roc.. Am. I droleiun hint., 30 M(III), 11.)... 

Various methods may be used for the determination of gas holdup—for example, displacement measurements and tracer experiments. Farley and Ray (F2) have described the use of gamma-radiation absorption measurement for the determination of gas holdup in a slurry reactor for the Fischer-Tropsch synthesis. 

These are chemically independent of Eqs. (11-51) and (11-52). Each follows in time after the quenching event. The back reactions do not involve luminescent quenching, and so they are generally studied by absorption measurements. In these cases the values17 are 53 = 5 x 106 Lmol-1 s-1 and 54 = 4.5 X 107 Lmol-1 s-1. 

This value agrees well with the rest found in [191] but not with point (4), which was shown to be ZR = 3.1 in ultrasonic experiment [216], No such discrepancy was found for pure nitrogen. Therefore it may be attributed to the low sensitivity of ultrasonic absorption measurements when nitrogen is present at small concentration in a gas mixture. 

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