Anal. Chem. 1980, 52. 1546-1548
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Figure 8. Linearity plots of aldrin in t h e same ECD as used for Figure 7, but after careful cleaning nA
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Figure 6. Calibration plots of aurin under various constant-current levels
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distinguish this state of the detector from the one of scrupulous cleanliness shown in Figure 8. The state of purity of the two detectors is reflected by the two voltage settings necessary for maximum response, 25 V vs. 12 V. As is well known, a clean detector has better sensitivity and this, in essence, leads to a n increase in linear range. In addition, the constant-current mode adds approximately another decade of linear range. In our best case, shown in Figure 8, the linear range amounts to 5000 to 1. It is interesting to note in this regard the very impressive, recent development of a nonradioactive ECD by J. J. Sullivan of Hewlett-Packard (5). Nominally, this device works in the dc mode and has a dynamic range of 20 million. Interestingly enough, its linear range (within & l o % ) and its minimum detectable amounts are very close to the data shown for constant-current operation in Figure 8. The Hewlett-Packard device works in a constant-voltage mode; by analogy, it would be interesting to see whether it, too, could improve its linear range by constant-current operation.
ACKNOWLEDGMENT
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We appreciate the competent help, and a loan of the Keithley 610 C electrometer, by E. Kenny.
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LITERATURE CITED
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Figure 7. Linearity plots of aldrin in a "dirty" ECD
T o illustrate dc ECD behavior in constant-current vs. constant-voltage modes, aldrin was again chosen as the analyte for the next two figures. Figure 7 shows its linear response, within & l o % , in a "dirty" ECD. "Dirty" indicates a detector of conventional purity; the adjective is simply used here to
( 1 ) Pellizzari, E. D. J . Cbromatogr. 1974, 98, 323. (2) Maaas. R . J.; Joynes. P. L.; Davies, A. J.; Lovelock, J. E. Anal. Chem. l s f f , 4 3 , 1966. (3) Pack, J. L.; Phelps, A. V . Phys. Rev. 1981, 721, 798. 14) A u e . W. A,: Kaoila. S. J . Chromatour 1980. 188. 1. (5) Sullivan, J J iejewlett-Packard Tec6 Pap 82, 1979
RECEIVED for review January 16, 1980. Accepted April 29, 1980. This research was supported by NSERC grant A-9604 and a Killam Scholarship for one of us (K.W.M.S.). Material taken from Ph.D. thesis work of the second author.
X-ray Photoelectron Spectra of Dithizone and Related Compounds A. Katrib" and A. Y. Kassim Chemistry Department, Kuwait University, Kuwait
Dithizone (diphenylthiocarbazone) is assumed to exist in solution as a mixture of two tautomers, the thioketone and thioenol forms.
ph-N=N-c-N-N-m
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T h e evidence presented for this assumption is based on the presence of two widely separated bands in the visible absorption spectrum. For example, in chloroform it absorbs a t 605 nm (el = 41.4 x lo3) and 440 nm ( t 2 = 15.9 x lo3)with c l / t Z = 2.59. The variation in the relative intensity ( t l / t z ) of the two bands is usually explained as being due to variation 'C 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
1547
Table I. Binding Energies of Dithizone and Other Related Compounds in Electron Volts S
com po un d diphenylthiocarbazone di(pmethylpheny1)thiocarbazone di(o-chloropheny1)thiocarbazone di(o-fluoropheny1)thiocarbazone di(pfluoropheny1)thiocarbazone di( 2-chloropheny1)thiocarbazide a
c Is 284.3, 285.7 284.4 284.5, 285.3 285.3
285.3 285.2, 286.2
N Is 399.6, 400.3, 402.3 (s)‘ 399.8, 402.2 (s) 399.7, 400.3, 402.7 (s) 400.5, 403.7 (6) 399.8, 400.5, 403.5 (s) 400.8
2P312
2P, I2
161.4
162.8 162.3
161.6
162.9 163.8 163.2 163.5
161.4
162.7 162.1
162.4
The assigned shake-up line.
of the ratio of the two tautomers. However, in the solid phase the data obtained from the reflectance spectrum tend to indicate that dithizone is predominantly present in the thioketone form as i t absorbs a t 450 nm ( 1 ) . X-ray photoelectron spectroscopy (XPS) is often very useful for determining the nature of bonding of sulfur and nitrogen atoms in organic compounds. In the case of dithizone and its related compounds there are certain spectral features that would be expected: (1) If the compounds are tautomeric mixtures, involving S in both the thiol and thione forms, the S(2p) peak will be a broad band ( 2 ) . T h e relative intensities of peaks in the band will be proportional to the tautomer concentrations. If there is only one tautomer, the peak will be t h e usual 3/2-1/2 doublet consisting of two sharp lines separated by 1.1 eV; (2) 1s nitrogen peaks for the two tautomeric forms will also have different binding energies. That nitrogen doubly bound to a carbon atom, for the case of the thiol form, will have a binding energy 1.5eV lower than the same nitrogen having a hydrogen attached in the thione form (2);(3) the 1s spectra of nitrogen atoms in azone groups might be expected to exhibit shake-up peaks. Shake-up peaks have been observed in carbon spectra by several workers ( 3 , 4 ) . In all cases the molecules contained carbon with unsaturated bonds, e.g. ethylene, propylene, butene ( 3 ) . T h e shake-up transition is believed to invole a A H* molecular orbital excitation. Since two of the nitrogens in dithizone are bound together by H orbitals, a shake-up transition is also possible here.
ev-
165
eV
160
-
285
RESULTS AND DISCUSSION T h e XPS spectrum of dithizone shows two well resolved lines a t 161.4 and 162.8 eV (Figure l a ) which are assigned to t h e S(2P/3/2,1/2) spin-orbit components, respectively. This binding energy is of the same order as that observed previously for t h e sulfur 2p orbital in the thione form. Also, the band shape indicates that there is only one type of bonding of the sulfur atom. As a result, it is concluded that dithizone in the solid phase is completely in the thioketone form. It is very difficult to differentiate between the many carbon atoms present in t h e phenyl groups which have a similar electronic environment and show an intense line a t 284.3 eV (Figure l b ) . T h e less intense line a t 285.7 eV is assigned to the Is orbital of the three carbon atoms bonded to nitrogens. This chemical shift is expected on the basis of the electronegativity difference. Figure ICshows an intense band with a maximum a t 399.6 eV and a shoulder a t 400.3 eV. This small difference in binding energy is expected for nitrogens in the -NH-NH-
400
395
A
-
(1253.6 eV) was used for measuring X-ray photoelectron spectra; a cryogenic pump in the sample chamber maintained a pressure of less than lo4 Torr. Samples were mounted on aluminum mesh; their temperatures were approximately ambient. Spectral data were computer processed to reduce instrumental broadening of the peaks. In this way, we were able to resolve peaks due to chemical shifts and shake-up effects much better.
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Figure 1. The XPS spectrum of diphenylthiocarbazone: (a)the S 2p spin-orbit components, (b) the C 1s energy region, and (c) the N 1s energy region
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EXPERIMENTAL A McPherson ESCA-36 spectrometer with a Mg K a source
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