Spectrophotometers slit width

The experimental technique is simple. The cell containing the solution to be titrated is placed in the light path of a spectrophotometer, a wavelength appropriate to the particular titration is selected, and the absorption is adjusted to some convenient value by means of the sensitivity and slit-width controls. A measured volume of the titrant is added to the stirred solution, and the absorbance is read again. This is repeated at several points before the end point and several more points after the end point. The latter is found graphically. 

Procedure. Charge the titration cell (Fig. 17.24) with 10.00 mL of the copper ion solution, 20 mL of the acetate buffer (pH = 2.2), and about 120mL of water. Position the cell in the spectrophotometer and set the wavelength scale at 745 nm. Adjust the slit width so that the reading on the absorbance scale is zero. Stir the solution and titrate with the standard EDTA record the absorbance every 0.50 mL until the value is about 0.20 and subsequently every 0.20 mL. Continue the titration until about 1.0 mL after the end point the latter occurs when the absorbance readings become fairly constant. Plot absorbance against mL of titrant added the intersection of the two straight lines (see Fig. 17.23 C) is the end point. 

The final optically clear solution is evaluated as to intensity of color at 555 m/x, and with slit width at 0.02 to 0.04 mm., on a Beckman spectrophotometer adjusted to 100% transmittance with an untreated sample freshly processed in parallel with the unknown sample. From a standard calibration curve (see Figure 3), the concentration of parathion may readily be ascertained, and an appropriate factor converts this value to micrograms of parathion present in the original field sample. 

Insufficient resolution leads to a decrease in the extinction coefficient across the wavelength axis, and therefore inaccurate quantitation results. The sensitivity of the measurement is also compromised. From a qualitative point of view, the fine features in the spectrum may be lost. The resolution of a UV-Vis spectrophotometer is related to its spectral bandwidth (SBW). The smaller the spectral bandwidth, the finer the resolution. The SBW depends on the slit width and the dispersive power of the monochrometer. Typically, only spectrophotometers designed for high-resolution work have a variable slit width. Spectrophotometers for routine analysis usually have a fixed slit width. For diode array instruments, the resolution also depends on the number of diodes in the array. 

Spectra were obtained with a Perkin-Elmer Model 13 spectrophotometer (double beam) modified to scan and record linearly in frequency [9]. A calibrated LiF prism was used with estimated frequency accuracy rh 4 cm"1. The spectral slit- width was about 9 cm "1 at 3600 cm"1 and 6 cm"1 at 3000 cm 1v Transmission accuracy is estimated at 0 5% in the region 30-50% T, where most measurements were made. The zero and 100% transmission values were measured for each spectrum, and a correction was applied for false energy. 

For the study of the first overtones of the O—H, N—H, and C—H bonds, situated in the range from 8000 to 5500 cm-1 we used a double beam recording autocollimating spectrophotometer with glass prisms, similar to that described before [19, 22] and constructed in the laboratory. The recording was photographic. The spectral slit width was 15 cm-1 at 7000 cm-1. 

Spectra were run on a Perkin-Elmer model 125 grating spectrophotometer at room temperature, ignoring the heating effect of the IR beam. Reference beam attenuation was used. The spectral slit width was less than 3 cm-1 in the region 1600-1400 cm-1. Optical density measurements were carried out as previously described (19). 

Structural Determinations. Wafers containing 1.5 mg of zeolites in 1 gram of KBr were used. Spectra were scanned on a Perkin-Elmer model 225 grating spectrophotometer with a spectral slit width of ca. 3 cm-1. 

Measure enhancement at 450 nm in a fluorescence spectrophotometer with an excitation of 360 nm (slit width 20 nm). 

Apparatus Use a suitable spectrophotometer (Perkin-El-mer Model 6000, or equivalent), a graphite furnace containing a L vov platform (Perkin-Elmer Model HGA-500, or equivalent), and an autosampler (Perkin-Elmer Model AS-40, or equivalent). Use a lead hollow-cathode lamp (lamp current of 10 mA), a slit width of 0.7 mm (set low), the wavelength set at 283.3 nm, and a deuterium arc lamp for background correction. 

Procedure Concomitantly determine the absorbances of the Sample Blank, the Diluted Standard Lead Solutions, and the Sample Preparation at the lead emission line of 283.3 nm, using a slit-width of 0.7 nm. Use a suitable atomic absorption spectrophotometer equipped with a lead electrodeless discharge lamp (EDL), an air-acetylene flame, and a 4-in. burner head. Use water as the blank. 

Slit. The intensity of light emitted through any filter or monochrometer may be too intense or too weak for a light-sensing device to record. It is therefore necessary to be able to adjust the intensity of the incident light If) by placing a pair of baffles in the light path to form a slit. Simple colorimeters often have a fixed slit, but more sophisticated spectrophotometers usually have a variable-slit-width adjustment mechanism. 

The concentration of benzenediazoninm ion can be determined by the absorbance at wavelengths between 295 and 325 nm. Below 295 nm, prodncts of the reaction prodnce interfering absorption and above 325 nm, the molar absorption coefficient is too small to permit effective measnrement of changes in the benzenediazoninm ion concentration." The absorbance will be measnred nsing a snitable spectrophotometer such as a Beckman model DU-20 UV detailed instrnctions for operating the instrnment will be provided in the laboratory. A wavelength of 305 nm shonld be nsed, with a slit width of 0.3 mm. Make two absorbance readings on each sample. 

When using an ultraviolet/visible spectrophotometer, instrument parameters will include wavelength and slit width. If the wavelength is changed then the measured absorbance of the sample solution is likely to change. 

Figure 2-12. Effect of slit width on the resolution of a recording spectrophotometer. (From R. L. Manning, Ed., Introduction to Spectroscopy, Pye Unicam Ltd., Cambridge, England, 1969.)...

Spectrophotometric Measurements. Spectrophotometric measurements were made with a Cary Model 14 recording spectrophotometer. Absorption spectra of solutions were obtained in silica cells. Absorption spectra of the crystalline salts were obtained using mixtures of the materials with petrolatum between glass or silica plates using the Cary Model 1417200 source. Blanks for the solid spectra were CaCOa mulls in petrolatum plus aqueous starch solution if necessary to produce a flat base line. The reference was adjusted so the base line was flat in the 520 to 600 m/x region where the U(VI) acetate complexes do not absorb. Slit widths for spectra of solids were typically <0.1 mm. 

Second, the quantity x does not necessarily cause the regression of y, for another factor z may vary in a regular way with x and so be the actual cause. For example, the rate of an air-oxidation reaction could vary with pH and be the actual cause of a regression of titration volume with pH. A n, the slope of a least-squares plot of absorbance against concentration often is interpreted directly as a molar absorptivity, whereas the slope may in fact be affected by a third variable, such as the slit width of a spectrophotometer. Sometimes the calculation of simple correlation coefficients can elucidate such problems. 

Infrared Spectral Measurements. Sample preparation techniques and apparatus have been described previously (3, 4). The spectra were recorded on disks of powdered material with a Cary-White Model 90 infrared spectrophotometer. Spectra were obtained from 4000 to 1200 cm" at a spectral slit width of 4 cm" and a scan speed of 3 cm Vsec. The infrared cell was so constructed as to permit calcinations to be done... 

A Beckman DK-2A spectrophotometer was calibrated with a standard benzene vapor spectrum. The spectrophotometer was adjusted for the highest resolution with a tolerable signal-to-noise ratio. Since high resolution demands a narrow slit width, instrument sensitivity was set as high as possible. As a maximum signal response was needed, a low time constant was set, and a relatively slow scanning speed was used. 

Another method for the determination of salicylic acid in aspirin powder by second derivative ultra-violet spectrometry was done by Kitamura et al. (28). A differentiator with electronic derivative circuit and incorporating three circuits with differencial time constants of 27, 82 and 22 was connected to a double beam spectrophotometer derivative spectra were recorded with a slit width of 1 mm and scanning speed f 120 nm min 1. The second derivative spectrum of salicylic acid showed a trough at 292 nm and a satellite peak at 316 nm, in the presence of large amounts of aspirin, the trough disappeared, but the peak was unaffected. The relationship between the height of this peak and the concentration of salicylic acid was rectilinear for 1 to 10 //g ml 1. 

Q8.16 Why should instrumental parameters (excitation wavelength, slit width, etc.) on the fluorescence spectrophotometer not be changed when measuring luminescence intensities for a given series of solutions ... 

Measurement. The optical density of the HiCN solution is measured after at least 30 minutes using a Beckman DU or similar spectrophotometer with water as a blank (X = 540 nm slit width = 0.02 mm I = 1.000 cm). The optical density of the diluting solution at 540 nm should be zero when measured against water. 

A Beckman DU spectrophotometer was used for all measurements. The slit width was chosen so that a half-intensity band width of 1.6 nm was obtained in the region X = 650-590 nm, 0.6 nm in the region X = 590-A60 nm, and 0.15 nm in the region of the Soret bands. The wavelength of the instrument had been checked with the aid of the mercury emission spectrum. Measurements at X > 590 nm were made in a layer thickness of 0.0993 cm, at X < 590 nm in a layer thickness of 0.0128 cm. The latter was obtained by inserting a plane parallel glass plate into the 0.0993-cm cuvette (Fig. 13). The layer thickness of the cuvette had been determined spectrophotometrically using HiCN solutions and a 1.000-cm cuvette as a reference. 

A Spex Fluorolog 212 spectrophotometer was used for recording the emission and excitation spectra of the polyimide films and the model compounds. The slit width used for the films was 2 mm and for the model compounds was 1 mm. Excitation and emission spectra were subsequently normalized with respect to the lamp intensity fluctuations by dividing each spectrum by that obtained with a Rhodamine-B standard solution. Absorption spectra were obtained with... 

Discuss the effect of the slit width on the resolution of a spectrophotometer and the adherence to Beer s law. Compare it with the spectral slit width. 

In TLC, reflectance of the UV light from the plate is usually measured by a spectrophotometer. The separated compound spots are detected by scanning over the whole plate with a given slit width which controls the resolution in space. 

In using a diode-array transducer, the slit width of the spectrometer is usually adjusted so that the image of the entrance slit just fills the surface area of one of the diodes that make up Ihe array. Thus, the information obtained is equivalent lo that recorded during scanning with a traditional spectrophotometer. With the array, however, information about the entire spectrum is accumulated essentially simultaneously and in discrete Clements rather than in a continuous way. 

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