Spectrophotometer, absorption

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. 

Several parameters are observed total amount of ozone (Dobson spectrophotometer), absorption in the 9.6-micron band, and emission from the ozone at several different zenith distances. The value of each of these observations may be expressed as the sum of contributions from the different layers of the ozone. Total amount of ozone is, of course, merely the sum of the amounts in each layer the infrared absorption, however, is pressure-dependent and a pressure-dependence term must be introduced. The sums are somewhat more complicated for the emission, but still are easily calculated. The problem is to find a distribution of ozone which will give rise to the... 

Infrared measurements were recorded on a Perkin-Elmer Model 580B Spectrophotometer. Absorption data were obtained on NaCl crystals. Standard instrument conditions were used and a computer driven data station was used to assist in analyzing the spectra. 

The spectrum of the solution in the visible region was measured with a Hitachi spectrophotometer. Absorptions at 389 nm were measured by a continuous variation method for the solutions of various molar ratios of DEC and EK or polymer matrices to confirm the formation of a charge-transfer complex. 

The absorbance (A) or transmittance (T) is recorded directly by all modem double beam spectrophotometers. Absorption spectra are plotted with absorbance (A) (or e, or logic e) (Fii- 10.56a) or transmittance (T(%)) (Fig. 10.56b) on the ordinate and with wavelength (A in nanometers or angstroms) on the abscissa. 

The infrared probe uses ATR to obtain the absorption information. The probe crystal is made from a chemically inert material, such as germanium or synthetic sapphire. The reflecting surfaces are masked so that the same area is covered by a sample each time an analysis is made. A single-beam optic system is employed, chopped at 45 Hz. Since the air path is less than 5 cm, as opposed to well over 1 m in conventional infrared spectrophotometers, absorption due to atmospheric water vapor and carbon dioxide is insignificant,... 

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%. 

Atomic absorption spectrophotometers (Figure 10.37) are designed using either the single-beam or double-beam optics described earlier for molecular absorption spectrophotometers (see Figures 10.25 and 10.26). There are, however, several important differences that are considered in this section. 

Atomization The most important difference between a spectrophotometer for atomic absorption and one for molecular absorption is the need to convert the analyte into a free atom. The process of converting an analyte in solid, liquid, or solution form to a free gaseous atom is called atomization. In most cases the sample containing the analyte undergoes some form of sample preparation that leaves the analyte in an organic or aqueous solution. For this reason, only the introduction of solution samples is considered in this text. Two general methods of atomization are used flame atomization and electrothermal atomization. A few elements are atomized using other methods. 

When the identity of the matrix interference is unknown, or when it is impossible to adjust the flame to eliminate the interference, then other means must be used to compensate for the background interference. Several methods have been developed to compensate for matrix interferences, and most atomic absorption spectrophotometers include one or more of these methods. 

Other methods of background correction have been developed, including Zee-man effect background correction and Smith-Iiieffje background correction, both of which are included in some commercially available atomic absorption spectrophotometers. Further details about these methods can be found in several of the suggested readings listed at the end of the chapter. 

When using a spectrophotometer for which the precision of absorbance measurements is limited by the uncertainty of reading %T, the analysis of highly absorbing solutions can lead to an unacceptable level of indeterminate errors. Consider the analysis of a sample for which the molar absorptivity is... 

Trace contaminants in the phosphoms may be deterrnined by oxidation of the phosphoms by various techniques. The metals are then deterrnined by an inductively coupled plasma spectrophotometer or by atomic absorption. The most important trace metal is arsenic, which must be reduced in concentration for food-grade products. Numerous other trace metals have become important in recent years owing to the specifications for electronic-grade phosphoric acid requited by the semiconductor industry (see Electronic materials Semiconductors). Some trace elements must be reduced to the low ppb range in phosphoric acid to comply. 

Quartz also has modest but important uses in optical appHcations, primarily as prisms. Its dispersion makes it useful in monochromators for spectrophotometers in the region of 0.16—3.5 m. Specially prepared optical-quality synthetic quartz is requited because ordinary synthetic quartz is usually not of good enough quality for such uses, mainly owing to scattering and absorption at 2.6 p.m associated with hydroxide in the lattice. 

For quantitative analysis, the resolution of the spectral analyzer must be significantly narrower than the absorption lines, which are - 0.002 nm at 400 nm for Af = 50 amu at 2500°C (eq. 4). This is unachievable with most spectrophotometers. Instead, narrow-line sources specific for each element are employed. These are usually hoUow-cathode lamps, in which a cylindrical cathode composed of (or lined with) the element of interest is bombarded with inert gas cations produced in a discharge. Atoms sputtered from the cathode are excited by coUisions in the lamp atmosphere and then decay, emitting very narrow characteristic lines. More recendy semiconductor diode arrays have been used for AAS (168) (see Semiconductors). 

Figure 8-8. Atomic absorption spectrophotometer with 16 mm cine projector in piace.

This is primarily engaged in analysis of boiler water treatment matters and involves on-site studies of various problems and the chemical examination of corrosion products, boiler scales, etc. It can also carry out certain types of metallurgical, fuel and inorganic analysis. Normal wet methods of analysis coupled with a visible ultraviolet and atomic absorption spectrophotometer are used for a wide range of analytical applications. Equipment in use by the engineering insurers providing these services can include an ion chromatograph, spectrometer equipment, atomic... 

The methods dependent upon measurement of an electrical property, and those based upon determination of the extent to which radiation is absorbed or upon assessment of the intensity of emitted radiation, all require the use of a suitable instrument, e.g. polarograph, spectrophotometer, etc., and in consequence such methods are referred to as instrumental methods . Instrumental methods are usually much faster than purely chemical procedures, they are normally applicable at concentrations far too small to be amenable to determination by classical methods, and they find wide application in industry. In most cases a microcomputer can be interfaced to the instrument so that absorption curves, polarograms, titration curves, etc., can be plotted automatically, and in fact, by the incorporation of appropriate servo-mechanisms, the whole analytical process may, in suitable cases, be completely automated. 

Procedure. Weigh out 0.0226 g of hydrated ammonium iron(III) sulphate and dissolve it in 1 L of water in a graduated flask 50 mL of this solution contain 100 g of iron. Place 50.0 mL of the solution in a 100 mL separatory funnel, add 10 mL of a 1 per cent oxine (analytical grade) solution in chloroform and shake for 1 minute. Separate the chloroform layer. Transfer a portion of the latter to a 1.0 cm absorption cell. Determine the absorbance at 470 nm in a spectrophotometer, using the solvent as a blank or reference. Repeat the extraction with a further 10 mL of 1 per cent oxine solution in chloroform, and measure the absorbance to confirm that all the iron was extracted. 

Procedure. Allow the whole of the sample solution (1 L) to flow through the resin column at a rate not exceeding 5 mL min . Wash the column with 250 mL of de-ionised water and reject the washings. Elute the copper(II) ions with 30 mL of 2M nitric acid, place the eluate in a small conical flask (lOOmL, preferably silica) and evaporate carefully to dryness on a hotplate (use a low temperature setting). Dissolve the residue in 1 mL of 0.1 M nitric acid introduced by pipette and then add 9 mL of acetone. Determine copper in the resulting solution using an atomic absorption spectrophotometer which has been calibrated using the standard copper(II) solutions. 

The basic principle of most colorimetric measurements consists in comparing under well-defined conditions the colour produced by the substance in unknown amount with the same colour produced by a known amount of the material being determined. The quantitative comparison of these two solutions may, in general, be carried out by one or more of six methods. It is not essential to prepare a series of standards with the spectrophotometer the molar absorption coefficient can be calculated from one measurement of the absorbance or... 

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