Resonance lines

A result of tlie relaxation processes is a shortened lifetime of the spin states giving rise to a broadening of the EPR line, which for most magnetic resonance lines dominated by homogeneous linewidth can be written as... 

Feher G 1956 Observation of nuclear magnetic resonances via the electron spin resonance line Rhys. Rev. 103 834-7... 

Use of glow-discharge and the related, but geometrically distinct, hoUow-cathode sources involves plasma-induced sputtering and excitation (93). Such sources are commonly employed as sources of resonance-line emission in atomic absorption spectroscopy. The analyte is vaporized in a flame at 2000—3400 K. Absorption of the plasma source light in the flame indicates the presence and amount of specific elements (86). 

Garnets have played an important role in the development of highly sophisticated microwave devices since the development of yttrium—iron garnet, yttrium iron oxide [12063-56-8]. The iron is strongly constrained to be trivalent in order to maintain electrical neutraUty in the crystal, which is essential for low microwave losses. Garnets have lower values of saturation magneti2ation than spinels, but provide superior performance in microwave devices because they have a narrower resonance line width. 

In contrast to the low-pressure lamps (1—130 Pa) which primarily emit at the resonance line at A = 254nm, high-pressure lamps (lO —10 Pa) also produce numerous bands in the UV and VIS regions (Fig. 16). Table 3 lists the emission lines and the relative spectral energies of the most important mercury lamps (see also [44]). The addition of cadmium to a mercury vapor lamp increases the numbei of emission lines particularly in the visible region of the spectrum [45] so that it i. also possible to work at A = 326, 468, 480, 509 and 644 nm [46]. 

This reaction has been studied in 0-enriched water, the line broadening of the carbonyl and diol O resonance lines being analyzed to give the exchange rate The exchange is catalyzed by acid, the exchange rate being related to the second-order rate constant by t = This rate constant was found to be 470... 

JesJ73 Jesson, J. P., Meakin, P. Determination of mechanistic information for nuclear magnetic resonance line shape for intramolecular exchange. Accounts of Chem. Res. 6 (1973) 269-275. 

At its simplest, 13C NMR makes it possible to count the number of different carbon atoms in a molecule. Look at the l3C NMR spectra of methyl acetate and 1-pentanol shown previously in Figures 13.3b and 13.6b. In each case, a single sharp resonance line is observed for each different carbon atom. 

Predict the number of carbon resonance lines you would expect in the 13C NMR spectra of the following compounds ... 

Most 13C spectra are run on Fourier-transform NMR (FT-NMR) spectrometers using broadband decoupling of proton spins so that each chemically distinct carbon shows a single unsplit resonance line. As with NMR, the chemical shift of each 13C signal provides information about a carbon s chemical environment in the sample. In addition, the number of protons attached to each carbon can be determined using the DEPT-NMR technique. 

AH C-C bonds are equivalent one resonance line in both and l3C NMR spectra. 

Calculation shows that only a small fraction of the atoms are excited, even under the most favourable conditions, i.e. when the temperature is high and the excitation energy low. This is illustrated by the data in Table 21.1 for some typical resonance lines. 

It would appear that measurement of the integrated absorption coefficient should furnish an ideal method of quantitative analysis. In practice, however, the absolute measurement of the absorption coefficients of atomic spectral lines is extremely difficult. The natural line width of an atomic spectral line is about 10 5 nm, but owing to the influence of Doppler and pressure effects, the line is broadened to about 0.002 nm at flame temperatures of2000-3000 K. To measure the absorption coefficient of a line thus broadened would require a spectrometer with a resolving power of 500000. This difficulty was overcome by Walsh,41 who used a source of sharp emission lines with a much smaller half width than the absorption line, and the radiation frequency of which is centred on the absorption frequency. In this way, the absorption coefficient at the centre of the line, Kmax, may be measured. If the profile of the absorption line is assumed to be due only to Doppler broadening, then there is a relationship between Kmax and N0. Thus the only requirement of the spectrometer is that it shall be capable of isolating the required resonance line from all other lines emitted by the source. 

A schematic diagram showing the disposition of these essential components for the different techniques is given in Fig. 21.3. The components included within the frame drawn in broken lines represent the apparatus required for flame emission spectroscopy. For atomic absorption spectroscopy and for atomic fluorescence spectroscopy there is the additional requirement of a resonance line source, In atomic absorption spectroscopy this source is placed in line with the detector, but in atomic fluorescence spectroscopy it is placed in a position at right angles to the detector as shown in the diagram. The essential components of the apparatus required for flame spectrophotometric techniques will be considered in detail in the following sections. 

With regard to position within the flame, it can be shown that in certain cases the concentration of atoms may vary widely if the flame is moved either vertically or laterally relative to the light path from the resonance line source. Rann and Hambly42 have shown that with certain metals (e.g. calcium and... 

The main advantages of electrothermal atomisers are that (a) very small samples (as low as 0.5 pL) can be analysed (b) often very little or no sample preparation is needed, in fact certain solid samples can be analysed without prior dissolution (c) there is enhanced sensitivity, particularly with elements with a short-wavelength resonance line in practice there is an improvement of between 102- and 103-fold in the detection limits for furnace AAS compared with flame AAS. 

As indicated in Fig. 21.3, for both atomic absorption spectroscopy and atomic fluorescence spectroscopy a resonance line source is required, and the most important of these is the hollow cathode lamp which is shown diagrammatically in Fig. 21.8. For any given determination the hollow cathode lamp used has an emitting cathode of the same element as that being studied in the flame. The cathode is in the form of a cylinder, and the electrodes are enclosed in a borosilicate or quartz envelope which contains an inert gas (neon or argon) at a pressure of approximately 5 torr. The application of a high potential across the electrodes causes a discharge which creates ions of the noble gas. These ions are accelerated to the cathode and, on collision, excite the cathode element to emission. Multi-element lamps are available in which the cathodes are made from alloys, but in these lamps the resonance line intensities of individual elements are somewhat reduced. 

Spectral interferences in AAS arise mainly from overlap between the frequencies of a selected resonance line with lines emitted by some other element this arises because in practice a chosen line has in fact a finite bandwidth . Since in fact the line width of an absorption line is about 0.005 nm, only a few cases of spectral overlap between the emitted lines of a hollow cathode lamp and the absorption lines of metal atoms in flames have been reported. Table 21.3 includes some typical examples of spectral interferences which have been observed.47-50 However, most of these data relate to relatively minor resonance lines and the only interferences which occur with preferred resonance lines are with copper where europium at a concentration of about 150mgL 1 would interfere, and mercury where concentrations of cobalt higher than 200 mg L 1 would cause interference. 

Selection of an alternative resonance line will overcome spectral interferences from other atoms or molecules and from molecular fragments. 

A more detailed table of the resonance lines is given in Appendix 9. 

Element Wavelength of main resonance line k (nm) Flame Working range (fig mL"1)... 

Procedure (ii). Make certain that the instrument is fitted with the correct burner for an acetylene-nitrous oxide flame, then set the instrument up with the calcium hollow cathode lamp, select the resonance line of wavelength 422.7 nm, and adjust the gas controls as specified in the instrument manual to give a fuel-rich flame. Take measurements with the blank, and the standard solutions, and with the test solution, all of which contain the ionisation buffer the need, mentioned under procedure (i), for adequate treatment with de-ionised water after each measurement applies with equal force in this case. Plot the calibration graph and ascertain the concentration of the unknown solution. 

A double-beam atomic absorption spectrophotometer should be used. Set up a vanadium hollow cathode lamp selecting the resonance line of wavelength 318.5 nm, and adjust the gas controls to give a fuel-rich acetylene-nitrous oxide flame in accordance with the instruction manual. Aspirate successively into the flame the solvent blank, the standard solutions, and finally the test solution, in each case recording the absorbance reading. Plot the calibration curve and ascertain the vanadium content of the oil. 

Notes. (1) If the lead concentration is too high to be measured directly using the 283.3 nm resonance line then further dilution of the sample solution is necessary. 

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