fers to the 4 cm-l resolution setting measured using one scan, the lower trace refers to two scans at 2 cm-' resolution (ie., four times the measurement time). It can be seen that the peak-to-peak noise is the same in the upper and lower trace. VARIABLE THROUGHPUT CASE
In this case, if the resolution is to be increased by a factor of two (numerically halved), the value of ymanmust be reOn reduction of -ymax by duced by a factor of the area of the focus, and consequently the energy impinging on the detector, is reduced by a factor of two. Thus, besides the fourfold increase required by the arguments in the previous section, another fourfold increase in measurement time is needed to recover the S/N lost because of the reduction in energy at the dytector, and the total increase in measurement time amounts to a factor of sixteen or Z4. This case is illustrated in Figure 2, which demonstrates the case when the source energy in the FTS-14 is stopped down to make ymax small enough fhat 0.5 cm-l resolution can be achieved in the mid IR, wheri vmax = 3800 cm-l. The trading rules derived above are exactly the same as those demonstrated for dispersion spectrometers (2), which shows that the advantage of a given interferometer over a given dispersion spectrometer is always a constant and does not increase with resolution. On the other hand, in the case when the optical throughput of an interferometer remains constant, there is a greater benefit in going to higher resolution for the measurement of bands whose half-width is less than the highest resolution.
di.
45,
EFFECT OF APODIZATION
The interrelationship between S/N, Av, and T is true irrespective of the apodization function, provided that this function is not changed. However, if spectra measured using a certain retardation and number of scans are com-
puted using boxcar and triangular apodization, respectively, a comparison shows that the resolution in the former case is almost exactly twice as good, but the spectrum is noisier (see Figure 3). by a factor of From this it may be deduced that spectra of the same Av and S/N may be measured with the same number of scans using either a certain retardation and triangular apodization or half that retardation and boxcar apodization. In the latter case, measurement time is halved, at the expense of line shape, since sharp bands will show side lobes. If a spectrum has only broad absorption bands so that the side lobes are not seen, it is obviously preferable to use the shorter retardation so that the measurement time is kept as short as possible,
4
NOISE FREQUENCY
One possible disadvantage of Fourier transform spectroscopy is found in the fact that weak bands, which are as narrow or narrower than the resolution setting used for their measurement, can be difficult to distinguish from the noise, since noise spikes are never narrower than the resolution. Thus, whereas it is occasionally possible to distinguish a band where the S/N is unity in certain spectra measured using an analog dispersive spectrometer, the corresponding S/N for spectra measured by Fourier transform spectroscopy should be at least two. PETERR. GRIFFITHS Sadtler Research Laboratories Inc. 3316 Spring Graden St. Philadelphia, Pa. 19104 RECEIVED for review March 6, 1972. Accepted May 30, 1972.
Rapid Atomic Absorption Determination of Silver and Copper by Sequential Atomization from a Graphite Rod SIR: Several manufacturers currently produce multielement hollow-cathode lamps for atomic absorption which contain both silver and copper, in addition to other elements. When these sources are being used in atomic absorption with flame atomization to determine silver in the presence of significant concentrations of copper, it is necessary that a narrow monochromator spectral bandwidth be used (preferably narrow enough to give a spectral bandwidth of less than 0.3 nm), on account of the proximity of the second-most-sensitive copper line at 327.40 nm to the most seiisitive silver line at 328.07 nm. This avoids the inclusion of a contribution from the copper line in the measurement of the silver absorption. When a graphite rod atomizer is used, however, it is possible in some cases to turn this potential source of spectral interference to good use. Previous work with such an atomizer ( I ) used atomizing temperatures of 1400 "C for silver and 1640 "C for copper. However, experiments with silver in several different physical and chemical states (aqueous solution, organo(1) C . J. Molnar, R. D. Reeves, J. D. Winefordner, M. T. Glenn, J. R. Ahlstrom, and J. Savory, Appl. Spectrosc., in press.
metallic compound in oil solution, wear metal in engine oils) have shown that satisfactory results can be obtained with atomization at temperatures as low as 1000 "C, where atomization of copper is very slow. This indicates that, with a suitable temperature program, sequential atomization of these two elements from a graphite rod might be achieved. Furthermore, by setting the monochromator at a wavelength between 327.4 and 328.1 nm, with a slit-width to give a spectral bandwidth of about 0.7 nm, it should be possible to obtain separate atomic absorption peaks for silver and copper, using only one light source and photomultiplier, and a fixed wavelength setting. In general, simultaneous determinations of two or more elements in flame atomic absorption have required the use of multiple slits and photomultipliers, as in the apparatus of Butler and Strasheim ( 2 ) , or of rapid-scanning monochromator systems such as that devised by Dawson, Ellis, and Milner (3). In terms of instrumental simplicity, the (2) L. R. P. Butler and A. Strasheim, Spectrochirn. Acta, 21, 1207 ( 1965).
(3) J. B. Dawson, D. J. Ellis, and R. Milner, ibid., 23, 695 (1968).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
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.IO
Table I.
Comparison of Silver and Copper Determinations Ag concn/(pg tnl-1) Cu concn/(pg ml-1) (i) (ii) (iii) (i) (ii) (iii) Sample 69-6A 5.5 4.8 4 . 9 i 0 . 4 “ 6.8 6.8 6.8 i 0 . 6 69-12A 0.97 0.91 l . O & 0.1 4.2 4.8 4 . 4 i 0.3 71-3A 0.70 0.88 0.7 i 0 . 2 1.7 2.0 1.8 i 0.3 71-10A 0.45 0.52 0.5 i 0.15 9.0 8.8 8 . 1 zk 0.7 (i) Graphite atomizer, sequential atomization. (ii) Graphite atomizer, separate determinations at Ag and Cu peak wavelengths. (iii) US.Air Force S.O.A.P. laboratories, flame atomic absorption, Standard deviation of results from S.O.A.P. laboratories.
.01
.06 absarha
.M
.02
0
2
4 6 cancmirrtioi/(,.t~
d)
a
10
Figure 1. Analytical curves for silver and copper using sequential atomization. Monochromator set at 327.7 nm, bandwidth 0.7 nm
+
15 % Absorption
10
5
I
L1 be
I
1
1
18
W6A
II-IOA
Figure 2. Recorder traces for atomization of silver and copper in oils Molecular absorption and scattering by oil combustion products (6) Silver atomic absorption (c) Copper atomic absorption Standard solutions containing 1, 3, 10 pg/ml of both Ag and Cu; jet engine oils, S.O.A.P. samples 69-6A, 71-10A (a)
sequential atomization technique is a n attractive alterliative where rapid determination of both silver and copper is required-e.g., in the analysis of used lubricating oils, gold, lead, ores, and other mining samples. In the present work, the technique has been demonstrated with used jet-engine oils provided by the U.S. Air Force Spectrochemical Oil Analysis Program (S.O.A.P.). 1914
A Perkin-Elmer hollow-cathode lamp, containing Ag, Cu, Cr, and Ni was used in conjunction with the graphite atomizer and auxiliary apparatus, which have been described in detail elsewhere ( I ) . Standard solutions containing 0-10 pg/ml of silver 2-ethylhexanoate and bis-(l-phenyl-l,3-butanedieno)copper (11) in a synthetic oil (Phillips Condor 105) were used for the construction of the analytical curves shown in Figure 1. In this case, the monochromator was centered at 327.7 nm, with a spectral band-width of 0.7 nm. The analytical curves show only slight curvature in the absorbance range involved here, although considerable bending is to be expected a t higher absorbances. (If the measured light intensity, I,,, can be regarded as the sum of intensities Io’ from the copper line and Io” from the silver line, and other causes of curve-bending are neglected, then the analytical curves will be asymptotic to Zo’/I0’’) and log,,(l Zo”/Zo’) for absorbances of log,,(l copper and silver, respectively.) The simultaneous measurement of the light intensity from the two line sources also leads to a loss of analytical sensitivity for each element, but this did not appear t o be critical in the present work. For both the standards and the used jet-engine oils, 0.5-pl samples (A Hamilton 1.0-p1 syringe-7101 N-CH with Chaney adaptor and a tungsten plunger extending to the tip of the needle was used; this syringe minimized cleaning problems and eliminated air- or vacuum-bubble problems in withdrawing oil samples; also, reproducibility of measurements on 0.5 pl used oils-one element at a time-was as good as that for homogeneous standards.) were placed in a 1-p1 cavity in the top of a Poco FX91 graphite rod (Poco Graphite, Jnc., Decatur, Texas 76234). After the sample had been ashed a t 440 O C for 18 seconds, the silver was atomized by suddenly increasing the current to give a cavity temperature of about 1000 “C after 4 seconds. A further sudden current increase for 4 seconds, giving a maximum cavity temperature of about 1800 “C, caused rapid atomization of the copper. Recorder traces for a series of standards and used oils are shown in Figure 2. In Table I, the copper and silver values (i) obtained by the sequential atomization technique are compared with (ii) values found from separate determinations using the graphite atomizer, with the monochromator centered on the appropriate peak wavelength for each element and with a reduced spectral bandwidth ( 4 ) , and (iii) mean values from the U.S. Air Force S.O.A.P. laboratories using conventional flame atomic absorption techniques. The standard deviations of the results from the S.O.A.P. laboratories are also shown. There is generally good agreement among the three values, with the
+
(4) R. D. Reeves, C . J. Molnar, M. T. Glenn, and J. D. Winefordner, ANAL.CHEM., submitted.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
results from the graphite rod atomizer measurements usually falling within the S.O.A.P. standard deviations. Although the precision obtained in sequential atomization (relative standard deviations of 4-7 %) is not as satisfactory as can be achieved by separate determinations, there may be occasions when the time-saving outweighs the loss of precision. With more sophisticated apparatus-e.g., a rapid-scanning monochromator-the principle of sequential atomization from the graphite rod may be useful in determining other pairs of elements in microliter samples. Elements such as arsenic, lead, and silver all appear to be completely atomized at temperatures where elements such as copper, nickel, iron, and chromium remain on the graphite.
R. D. REEVES’ C. J. MOLNAR J. D. WINEFORDNER~ Department of Chemistry University of Florida Gainesville, Fla., 32601 RECEIVED for review March 27, 1972. Accepted May 26, 1972. This work was supported by AF-AFOSR-70-1880H. 1 On leave, Department of Chemistry and Biochemistry, Massey University, Palmerston, North, New Zealand. 2 Author to whom reprint requests should be sent.
Molecular Design-Tetrachloroterephthaloyl Liquid Phases for Gas Chromatography SIR: Stationary phase design in gas-liquid chromatography is of special interest because the principles involved are potentially important in the design of liquid materials for extraction and in understanding operative forces in solution generally. Here, we describe the design and choice of several tetrachloroterephthaloyl oligomers as liquid phases from molecular structural considerations, and principles and concepts described earlier (I-3). These materials are noteworthy because of their selectivity over a wide termperature range, their thermal stability, and their well defined chemical structure. Such properties make them useful in the range of 100 “C to more than 200 “C. Many selective liquid phases available heretofore are volatile in this temperature range ( 4 ) . Other materials with only moderate selectivity are polymers of poorly defined composition. The tetrachloroterephthaloyl nucleus was chosen as the basic structural unit because of its potential participation in P type charge transfer interaction (3,5,6) with aromatics and olefins, the tendency for formation of linear, and therefore controlled, structures with para isomers (7, s), and the relative thermal and chemical stability ( 9 - / I ) of the tetrahalogenated aromatic nucleus. The non-selective intermolecular void (1) S . H. Langer, ANAL.CHEM., 39,524 (1967). (2) S . H. Langer and R. J. Sheehan, “Progress in Gas Chromatography,” J. H. Purnell, Ed., Interscience-Wiley, New York, N.Y., 1968, pp 289-323. (3) S . H. Langer, B. M. Johnson, and J. R. Conder, J. Phys. Chem., 72, 4020 (1968). (4) M. J. S . Dewar and J. P. Schroeder, J. Amer. Chem. Soc., 86,
Oligomers as
volume of simple dialkyl tetrachloroterephthalates (3) was reduced by employing oligomers, thus increasing the concentration of the selective tetrahalogenated nuclei per unit volume ( I , 2). This conforms with our principle of “increasing the concentration of the selectively interacting groups in the solvent” to improve selectivity, provided group interaction does not interfere ( I , 2). Oligomer formation allows a decrease in ratio of methylene t o tetrahaloterephthaloyl groups “to minimize solvating (attractive) interactions which may act counter to the desired separation” or do not contribute to it ( I , 2). Shortening the esterified alkyl groups with simple tetrachlorinated diesters to accomplish this gives materials with increased volatility and higher melting points (3, 12). Both properties are undesirable for gas chromatographic applications. Furthermore, oligomer melting points and liquid phase temperature ranges should be controllable by molecular design and blending. We prepared the following: ROXO(CHz),OXOR I. R = Butyl (a) n = 2, mp 183-184 OC (b) n = 3, m p 108.3-109.7 O C (c) n = 4, m p 184-185.5 “C 11. R = Propyl n = 3, mp 127-128 “C 111. ROXO(CHz)~OXO(CHz)~OXOR R = Propyl, mp 148-149.5 O C
5235 (1964). ( 5 ) S. H. Langer, C. Zahn, and G. Pantazopolos, Chem. Ind.
(London), 1958, 1145. ( 6 ) S. H. Langer, C. Zahn, and G. Pantazopolos, J. Chromatogr., 3, 154 (1960). (7) V. V. Korshak and S . V. Vinogradova, “Polyesters,” translated by B. J. Hazzard, Pergamon Press, London, 1965 pp 19-23. (8) R. W. Lenz, “Organic Chemistry of High Polymers,” John Wiley, New York, N.Y., 1967, p p 67-72 (9) A. H. Frazer, “High Temperature Resistant Polymers,” Interscience-Wiley, New York, N.Y., 1968, pp 18, 24, 118, 124
(l0,J. H. Golden, SCI (SOC.Chem. Znd. London) Monogr., 13, 231 (1961). (11) J. M. Cox, B. A. Wright, and W. W. Wright, J. Appl. Polym. Sci., 9, 513 (1965).
As expected, the melting point is a function of end group, R , and decreases with increase in group length (I3-/6). The lower melting points of compounds containing a n odd number of carbons in the connecting chain between tereph(12) S . H. Langer, P. Mollinger, B. M. Johnson, and J. Rubin, J. Chem. Eng. Data, in press, 1972. (13) L. Mandelkern, “Crystallization of Polymers,” McGraw-Hill, New York, N.Y., 1964, pp 122-125. (14) R. Hill and E. E. Walker, J. Polym. Sci., 3, 609 (1948). (15) Reference 7, pp 311-336. (16) Reference 8, pp 91-95 and references therein.
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