Spectrophotometer, measuring

Determination of organolead metabolites of tetraalkyllead in urine can be carried out after solid-phase enrichment and end analysis using reversed-phase HPLC with chemical reaction detector and by LC-MS (thermospray127). The chemical derivation consists of conversion to the dialky Head form, as shown in reaction 1, followed by complex formation with 4-(2-pyridylazo)resorcinol (11) and spectrophotometic measurement at 515 nm128. 

In contrast, a spectrophotometer measures an absorbance, a quantity that is expressed on an absolute scale. 

There are some potential problems that should be taken into account when interpreting such spectra. A spectrophotometer measures transmission (and maybe also reflection) but not absorption. What is measured as absorption is a transmission measurement that is mathematically manipulated to convert it to absorption. Absorption is usually measured as absorbance. A, which by definition is given by... 

The spectrophotometer measures the transmission and, if an absorption measurement is carried out, converts the transmission into absorbance using these equations. This conversion works fine for samples where there is no reflection, either specular or diffuse, as is the case for nonturbid solutions. However, for films there is invariably some reflection, which is often quite large, particularly for films of high dielectric constant (or refractive index) materials, such as PbS and PbSe. Additionally, if the films are not completely transparent, then scattering introduces an extra element of reflection. Therefore, to measure the real absorption of a film, a reflection measurement must also be carried out and correction for this reflection made. The correction will be approximate and depends on the nature of the film itself. However, that most commonly used is... 

Many studies present optical absorption or transmission spectra of the resulting films. (A reminder that a spectrophotometer measures transmission, not ab-... 

A calibration curve shows the response of an analytical method to known quantities of analyte.8 Table 4-7 gives real data from a protein analysis that produces a colored product. A spectrophotometer measures the absorbance of light, which is proportional to the quantity of protein analyzed. Solutions containing known concentrations of analyte are called standard solutions. Solutions containing all the reagents and solvents used in the analysis, but no deliberately added analyte, are called blank solutions. Blanks measure the response of the analytical procedure to impurities or interfering species in the reagents. 

The spectrophotometer measures and displays the increase in absorbance at 410 nm as a function of time (AA/At). Whether the output from the instrument is in the form of a strip chart or is collected by a computer, the reaction velocities are usually expressed in terms of change in concentration per unit time, or converted to specified units of enzyme activity. The International Unit (U) for enzyme activity is defined as the amount of enzyme that transforms 1 pmol substrate to product in 1 min under specified assay conditions. The SI unit for activity is the katal, which is defined as the amount of enzyme that transforms 1 mol substrate per second under specified conditions. Thus 1 U = 16.7 nkatal. To convert slopes AA/At values) to velocities (v), the following equation is used ... 

Figure F5.1.7 A color spectrophotometer measures the reflectance of a sample to compute color. Figure courtesy of GretagMacbeth. This black and white facsimile of the figure is intended only as a placeholder for full-color version of figure go to http //www.currentprotocols.com/colorfigures...

The model was initially compared with LOWTRAN 7 (5) results and with Brewer spectrophotometer measurements carried out at Lisbon in 1990. LOWTRAN-MESTRad results show a reasonable good agreement over 290-400 nm range with a mean difference of about 10%. Figure 4 shows de Model/LOWTRAN ratios for direct, diffuse and global spectral irradiances computed for the same conditions of ozone (332 Dobson Units), solar zenith angle (45.2°) and aerosol optical depth (0.0). 

Total chromium and Cr in samples of stainless steel welding dusts were determined after extraction of the samples with a pH 4 buffer solution (Girard and Hubert, 1996). The analytical method involved the use of an FI system equipped with two detectors. The first detector (spectrophotometer) measured Crvl as the diphenylcarbazide complex. The second detector (AA spectrometer) measured total chromium. The recovery of chromium was 96%, and the lower detection limit was 0.005 mg ml-1. 

Procedure Set out three quartz cuvettes suitable for use with a UV spectrophotometer. For each of the following solutions, shake the contents of each cuvette well and, using the spectrophotometer, measure the absorbance at 235 nm at 0 and 10 min. 

Readily Carbonizable Substances Transfer 1.00 + 0.01 g of finely powdered Citric Acid to a 150-mm x 18-mm (od) tube previously rinsed with 10 mL of 98% sulfuric acid at 90° or used exclusively for this test. Add 10 + 0.1 mL of 98% sulfuric acid, carefully agitate the tube until solution is complete, and immerse the tube in a water bath at 90° + 1° for 1 h. Occasionally remove the tube from the water bath and carefully agitate it to ensure that the Citric Acid is dissolved and gaseous decomposition products are allowed to escape to the atmosphere. Cool the tube to ambient temperature, carefully shake the tube to ensure that all gases are removed, and using an adequate spectrophotometer, measure the absorbance and transmission of the solution at 470 nm in a 1-cm cell. The absorbance does not exceed 0.52, and the transmission is equal to or exceeds 30%. 

Both reflectance and transmittance measurements can be applied to the same database. For plastics and translucent liquids and for printing or coating on nonopaque substrates, this is a very important feature. In these applications it is not sufficient to match a standard in reflectance only a transmission match is equally important. Contemporary color-matching systems can combine both types of matches in one calculation using only one database. To use this capability, it is necessary that the spectrophotometer measure both the reflectance and total transmittance of the sample. 

Activity can also be followed conveniently by measuring O2 utilization with a recording O2 electrode. This procedure is as sensitive as the spectrophotometric method, as simple to perform, and can be used with solutions that would be optically unsatisfactory in the spectrophotometer. Measurement of O2 consumption by Warburg manometry should be avoided. Not only is it cumbersome and less precise, but lipoxygenase, especially when somewhat purified, is highly sensitive to surface dena-turation on shaking. 

The concentration of HCl at various tim was determined from atomic absorption spectrophotometer measurements of the calcium and magnesium ions. 

Q A spectrophotometer measures the absorption of specific wavelengths of light by a reactant or product as a reaction progresses to determine the specific rate constant for the reaction. 

A spectrophotometer measures the sample transmittance, T = I Ir, which is the ratio of the light transmitted through the sample and that transmitted through a reference. 

The UV spectrophotometer measures the light absorbed by a sample at a specific wavelength. For NOM the UV/VIS spectra are somewhat featureless with no distinct peaks in absorbance. This is due to the fact that NOM is a mixture of many compounds. 

Glass pH electrodes are convenient in the lab but are not well suited to longterm environmental measurements because electrode potentials drift We compensate for drift in the lab by frequent calibration in standard buffers. To monitor pH in natural waters, a spectrophotometer measures the ratio of absorbance of light at wavelengths corresponding to the two colored forms of an indicator. This ratio provides stable measurements without calibration. The precision (repeatability) of shipboard spectrophotometric pH measurements is 0.000 4 pH units. 

An ordinary spectrophotometer measures the relative attenuation of the beam I X L)IIq X), not the absolute intensities of incident and transmitted light. However, in photochemical applications absolute intensity is important since the fundamental quantity of interest is the quantum yield cpx,... 

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