Absorbance range

Both fixed and variable wavelength uv/visible detectors are available. The variable types use a deuterium and/or a tungsten filament lamp as the radiation source and can operate between about 190-700 nm. They will have a number of absorbance ranges (ranges are given... 

Radiation source wavelength range, nm absorbance ranges, aufs noise, au... 

Fig. 2.4f shows theuv spectra of azobenzene (Az, concentration 3,73 x 1Q 3 g dm- 3) and phenan-threne (P. 3.23 x 1G-3 g dm-3 both recorded in /.soocune. The wavelength dme on the instrument was 1U 11m cm" 1 and the absorbance range was 2 uufs. Measurements were made against wo-octane using 10 mm cells. 

The minimum uncertainty (ca. 3%) of photometric error ranges from approximately 20 to 60% transmittance or an absorbance range of 0.2-0.7, a 5% relative error in concentration has a photometric range of 0.1-1.0. 

What percent transmittance range and what absorbance range are considered to be the optimum working ranges for spectrochemical measurements ... 

The optimum working range for percent transmittance (to avoid instrumental deviations from Beer s law) is between 15 and 80%, which corresponds to an absorbance range of 0.10 to 0.82. 

The use of a single standard in this way assumes that the Beer-Lambert relationship is valid over the absorbance range measured and again it is necessary to confirm the relationship before using the method. 

Materials and Methods. The isomeric compositions of the four polybutadienes used are listed in Table I. Samples were prepared for infrared measurement from solutions of the polymer without further purification. Most films were cast from carbon disulfide solutions on mercury or on glass plates, but a few films were cast from hexane solutions to determine whether or not the solvent affected the radiation-induced behavior. No difference was observed for films cast from the different solvents. The films were cured by exposure to x-rays in vacuum. (Doses were below the level producing detectable radiation effects.) They were then mounted on aluminum frames for infrared measurements. The thicknesses of the films were controlled for desirable absorbance ranges and varied from 0.61 X 10 s to 2 X 10 3 cm. After measuring the infrared spectrum with a Perkin-Elmer 221 infrared spectrophotometer, the mounted films were evacuated to 3 microns and sealed in glass or quartz tubes (quartz tubes only were used for reactor irradiations). 

Thiamine shows a pH-dependent UV absorbance range of 230-270 nm. However, its UV absorbance is prone to interference by other endogenous UV absorbers in foods, such as nucleic acids (67,68). In a recent interlaboratory comparison of thiamine methods (42), the results obtained from an HPLC method using UV absorbance detection were rejected due to the presence of peaks that interfered with thiamine. In the interests of increased sensitivity and selectivity, the thiamine vitamers are generally converted to their thiochrome derivatives by alkaline oxidation and determined fluorimetrically (42,70). The thiochrome derivatives of thiamine and its phosphate esters all fluoresce at nearly identical excitation (365-375 nm) and emission (425-435 nm) maxima at pH over 8. The thiochrome derivatives are all relatively stable in alkaline solution at pH greater than 9 and room temperature. 

The Model 835 multiwavelength filter photometer (Fig.3.44) provides energy at 254 nm with a low-pressure mercury lamp and at 280,313,334 and 365 nm with a medium-pressure mercury source. Selected wavelengths between 380 and 650 nm are also available with a quartz-iodine light source. Absorbance ranges of 0.01-2.56 AUFS are provided. Short-term noise levels are 5 X 10-s AU with the low-pressure mercury source and 1 X 10 4 AU with the other lamps. The design and dimensions of the cell are the same as for Model 840. A 24-jtzl cell is standard with the medium-pressure mercury lamp and the quartz—iodine lamp. 

Two UV detectors are also available from Laboratory Data Control, the UV Monitor and the Duo Monitor. The UV Monitor (Fig.3.45) consists of an optical unit anda control unit. The optical unit contains the UV source (low-pressure mercury lamp), sample, reference cells and photodetector. The control unit is connected by cable to the optical unit and may be located at a distance of up to 25 ft. The dual quartz flow cells (path-length, 10 mm diameter, 1 mm) each have a capacity of 8 (i 1. Double-beam linear-absorbance measurements may be made at either 254 nm or 280 nm. The absorbance ranges vary from 0.01 to 0.64 optical density units full scale (ODFS). The minimum detectable absorbance (equivalent to the noise) is 0.001 optical density units (OD). The drift of the photometer is usually less than 0.002 OD/h. With this system, it is possible to monitor continuously and quantitatively the absorbance at 254 or 280 nm of one liquid stream or the differential absorbance between two streams. The absorbance readout is linear and is directly related to the concentration in accordance with Beer s law. In the 280 nm mode, the 254-nm light is converted by a phosphor into a band with a maximum at 280 nm. This light is then passed to a photodetector which is sensitized for a response at 280 nm. The Duo Monitor (Fig.3.46) is a dual-wavelength continuous-flow detector with which effluents can be monitored simultaneously at 254 nm and 280 nm. The system consists of two modules, and the principle of operation is based on a modification of the 280-nm conversion kit for the UV Monitor. Light of 254-nm wavelength from a low-pressure mercury lamp is partially converted by the phosphor into a band at 280 nm. 

Dilute all of the solutions nearly to volume with water incubate for 5 to 10 min, but no longer, in a water bath cooled with tap water and dilute to volume. Record the spectrum for each solution between 500 nm and 700 nm using an absorbance range of 0 to 1 and a 1-cm pathlength cell record all spectra on the same spectrogram. 

Figure 9 shows the substantial improvement in the performance of this revised prototype over the performance of the absorber shown in Figure 3. As the solution flow rate was increased, the heat duty transferred by this absorber ranged from 4.51 to 15.1 kW, with 545 WW-K < UA < 940 W/m -K, and 638 < hsoMim < 1648 WW-K. With only 30% (0.162 X 0.157 X 0.150 m surface area = 0.456 m ) of the surface area of the previous absorber, this revised prototype transferred similar heat duties at lower solution and coolant flow rates. Meacham and Garimella [42] attributed this increase in performance primarily to the increase in solution-side heat transfer coefficients due to improved solution distribution and wetting.

In absorption photometry the pathlength of the cuvette is usually fixed. In conventional clinical chemical methods a dilution of the sample is necessary both to run the assay under optimized conditions and to make sure that the developed color of the reaction product is within the measurable absorbance range of a spectrophotometer. The thickness of the reagent carrier in reflec-tometry which is calculated by means of the Kubelka-Munk theory, is assumed to be infinite and hence of negligible significance. Hence, the linear range in reflection spectroscopy may be expected to exceed that of absorption spectroscopy with a consequential reduction in the frequency of sample dilution prior to measurement. 

Given a chemistry which is linear, it is expected that the photometric output would exhibit basic conformance to the Beer-Bouguer Law over the absorbance range 0-2. 

---