ANALYTICAL CHEMISTRY, VOL. 50, NO. 4 , APRIL 1978
of alkylamine-silanized metal oxide electrode. We can draw from this t h e intimation t h a t a redox reagent bound to the alkylamine-silane may find itself in a spectrum of slightly different chemical environments. The redox reagent may as a consequence exhibit a spectrum of formal electrochemical potentials, which in turn has the effect of broadening the electrochemical surface wave observed fur the reagent. To what extent broadening we have observed in surface waves on alkylamine-silanized electrodes is due to this chemical heterogeneity as opposed to interactive effects within the chemically bound layer (62) or to both, remains tu be resulved.
ACKNOWLEDGMENT Assistance from D. N. Smith in ESCA bandfitting is gratefully acknowledged
LITERATURE CITED (1) P. R. Moses, L. Wier, and R. W. Murray, Anal. Chem., 47, 1882 (1975). (2) P. R. Moses and R. W. Murray, J . Electroanal. Chem., 77, 393 (1977). (3) J. R. Lenhard and R. W. Murray, J . Electroanal. Chem., 78, 195 (1977) A. Diaz, J . Am. Chem. Soc., 99, 5838 (1977);G. J. Leigh and C. J. Pickett, J . Chem. Soc., Dalton frans., 1797 (1977). (4) C. M. Elliott and R. W. Murray, Anal. Chem., 48, 1247 (1976). (5) P. R. Moses, J. C. Lennox, J. Lenhard, and R. W. Murray, 173rd National Meeting, American Chemical Society, New Orleans. La., March 1977. (6) D. F. Untereker, J. C. Lennox, L. M. Wier, P. R. Moses, and R. W. Murray, J . Electroanal. Chem., 81, 309 (1977). (7) M. Fujihira, T. Matsue, and T. Osa, Chem. Lett.. 875 (1976). (8) D. F. Untereker, P. R. Moses, L. M. Wier. C. M. Elliott, and R. W. Murray, 149th Electrochemical Society Meeting, Washington, D.C., May 1976. (9) E. Grushka, Ed., "Bonded Stationary Phases in Chromatography", Ann Arbor Science Publications, Ann Arbor, Mich., 1974. (10) C. H. Lochmuiler and C. W. Amoss, J . Chromafogr., 108, 85 (1975). ( 1 1 ) E. Grushka and E. J. Kikta, Jr., Anal. Chem., 46, 1370 (1974). (12) H. H. Weetall, Sep. Purif. Methods, 2, 199 (1973). (13) D. M. Hercules, L. E. Cox, S. Onisick, G. D. Nichols, and J. C Carver, Anal. Chem.. 45, 1973 (1973). (14) D. E. Leyden and G. H. Luttrell, Anal. Chem., 47, 1612 (1975). (15) R. L. Burwell, Chem. Techno/., 370 (1974). (16) E. P. Ruddemann, Adhes. Age, 18,36(1975);Chem. Abstr., 83, 148735
(1975). (17) K. Hardee and A. J. Bard, J . Electrochem. Soc., 122, 739 (1975). (18) C. N. Reilley and W. S. Woodward, to be submitted for publication. (19) H. P. Boehm. "Chemical Identification of Surface Groups," Adv. Cafal., 16, 179 (1966). (20)R. K. Gilpin and M. F. Burke. Anal. Chem.. 45, 1383 (1973). (21) E. W. Thornton and P. G. Harrison, J . Chem. Soc. Faraday Trans. 7 , 71, 461 (1975). (22) D. J. C. Yates, J . Phys. Chem., 65, 746 (1961). (23) H. Kuhn, "Spectroscopy of Monobyer Assemblies", in "Physical Methods of Chemistry", A. Weissberger and 8. Rossiter, Ed., Wiley, New York, N.Y.. 1972,Part IIIB, p 577. (24) P. R. Moses and R. W. Murray, J . A m . Chem. Soc., 98, 1435 (1976). (25) M. Gleria and R. Memming, Z . Phys. Chem. (Frankfurt am Main), 98,
303 (1975).
585
A. A. Oswald, L. L. Murrei. and L. J. Boucher, Am. Chem. Soc., Div. Pet. Chem., Abstr., 168th National Meeting, American Chemical Society, Los Angeles, Calif., 1974. P. G. Harrison and E. W. Thornton. J . Chem. Soc., Faraday Trans. 7 ,
1310 (1976) A. Ciaz, J . Am. Charn. Soc., 99, 6/80 (1977). V. S. Srinivasan and W. J. Lamb, Anal. Chem., 49, 1639 (1977). J. H. Scofield, Lawrence Livermore Laboratory Report No. UCRL-51326, January 1973. D. M. Hercules, Anal. Chern., 42 ( l ) , 20A (1570). S. Pignataro and G.Distelano, J . Electron Spectrosc. Relat. Phenom., 2, 171 (1973). H. F. Weetall and L. S . Hersh, Biochim. Siophys. Acta. 206. 54 (1970). L. Lee, J ColloidInrerface Sci., 27. 751 (1968). E. P. Plueddeman, J . Adhes , 2, 184 (1970). M. L. Hair, "Infrared Spectroscopy in Surface Chemistry", M. Dekker, New York, N.Y., 1967,p 102. F. J. Kahn, Appl. Phys. Lett., 22, 386 (1973). H. R. Anderson, F. M. Fowkes, and F. H. Hielscher, J . Polm. Sci., PoWm. Phys. Ed., 14, 879 (1976). J C Lennox and R W. Muiray, J , Elecrroanai. Clmrn., 78, 195 (1977). J. N. Butler, "Ionic Equilibria", Addison Wesley, Reading, Mass., 1964, D
466.
R. F. Reilman. A. Msezane. and S. T Manson, J . Electron Spectrosc. Relat. Phenom., 8. 389 (1976). C. D. Wagner, A n d . Chem., 49, 1282 (1977). D. R. Penrl, J . Electron Spectrosc. Relat. Phenom., 9, 29 (1976). J. C. Helmer and N. H. Weichert, Appl. Phys. L e t , 13, 266 (1968). J. S. Johannessen, W. E. SDicer. and Y. E. Strausser, Thin SolidFilms, 32, 311 (1976). P. H. Holioway, and H. J. Stein, J . Electrochem. Soc., 123. 723 (1976). A. W. C. Lin, N. R. Armstrong. and T. Kuwana, private communication. March 1977. K. S. Kim and N. Winograd, J . Catal., 35, 66 (1974). N. S . McIntyre and M. G. Cook, Anal. Chem., 47, 2208 (1975). W. Dianis and J. E. Lester, Surf. Scl., 43, 602 (1974). T. Robert, M. Bartel, and G. Offergeld, Surf. Sci., 33, 123 (1972). K. S. Kim, A. F. Gossmann, and N. Winograd, Anal. Chem., 46, 197 (1974). G.C. Allen, M. T. Curtis, A. J. Hooper, and P. M. Tucker, J . Chem. Soc., Dalton Trans., 1525 (1974). K. Kishi and S . Ikeda, Bull. Chem. Soc Jpn., 46, 341 (19r3). J. P . Hynd and A. K. Rastogi, Surf. Sci., 48, 22 (1975). S. R. Nagel, J. Tauc. and B. G. Bagley. Solid State Commun., 20, 245
(1976). N. R. Armstrong, A. W. C. Lin, M. Fujihira, and T. Kuwana, Anal. Chem., 48, 741 (1976). C. D. Wagner, Anal. Chem., 44, 1050 (1972). K S. Kim, W. E. Baitinger, J. W. Amy, and N. Winograd, J , flectron Spectrosc. Relat. Phenom.. 5, 351 (1974). L. Wier, unpublished results, Univerisity of North Carolina, 1976. R. C. Baetzoldanel and G. A. Somorjai. J . Catal.. 45, 94 (1976). F. C. Anson. U.S.- Japan Seminar, San Francisco, Calif, May 1977.
RECEIL EU for review September 14, 1977. Accepted Uecember 19, 1977. This paper is Part IX of a series on "Chemically Modified Electrodes". This research was assisted by the National Science Foundation under grants MPS75-07863, CHE76-24564, DMR72-03024 and by the Office of Naval Research.
Analysis of Commercial Sodium Tripolyphosphates by Phosphorus-3 1 Fourier Transform Nuclear Magnetic Resonance Spectrometry Stanley A. Sojka" and Roger A. Wolfe Hooker Chemicals and Plastics Corporation, Research Center, Grand Island Complex, M.P.O. Box 8, Niagara Falls, N e w York 14302
Commercial sodium tripolyphosphate samples were analyzed by phosphorus-31 Fourier transform nuclear magnetic resonance spectrometry. The accuracy and precision of the method were assessed and found to be satisfactory. Identification of phosphorus-containing species was possible. I n some cases, the presence of high polyphosphatesinterfered with the analysis.
T h e Fourier transform (FT) approach ( I ) has added new 0003-2700/78/0350-0585$01 OO/O
dimensions to 31PNMR spectrometry. The time savings and sensitivity enhancement inherent in the FT approach have been realized in practice, and analytical applications are becoming increasingly apparent (2). Detection of organophosphorus compounds a t the level of the parts-per-million range has been reported (3). This paper describes the use of 31PFT NMR spectrometry to analyze commercial samples of sodium tripolyphosphate (4,5),including the determination of accuracy and precision. The advantages of this approach over other analytical procedures, such as gel chromatography C 1978 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 4, APRIL 1978
Table I. "P F T NMR Parameters for Phosphates* Phosphate NaH,PO, Na,HPO, Na,PO, Na;P,O, Na3P30, NaSP301
0
Na,P,O,
2
ppmb 0.8
3 1 ~ ,
3.3 5.7 - 5.3 - 21.0 4.7c -19.ld - 23.4
TI, s
-
14.1
-
3.3 23.1 7.6 6.0
-
a Chemical shifts and T, values are subject to sample In conditions (e.a., pH. concentration). See Ref. 15. . ppm relative t o 85% H,PO,. Doublet, Jpp = 19 Hz. Triplet, J,,, = 1 9 Hz.
( 6 ) ,paper chromatography (7),ion exchange chromatography (a),and colorimetry (9),are that it (1) enables identification of the phosphorus-containing constituents, (2) requires a short time for each analysis, and (3) requires simple sample preparation.
EXPERIMENTAL Apparatus. A Varian XL-100-12 NMR spectrometer was operated at 40.49 MHz with an internal deuterium field-frequency lock. The spectrometer was used in conjunction with a Nicolet TT-100 Fourier transform unit. A 30" pulse took 3.83 ws and the time between pulses was 60 s. The sweep width was 1600 Hz and 8K computer points were used for time domain data accumulation. Proton decoupling was not used. Sixteen free induction decays were accumulated before Fourier transformation. Quadrature phase detection was used to detect signals. Chemical shifts are reported relative to 85% H3P04 which was contained in a concentric capillary. Negative chemical shifts are associated with signals to higher field strength than 85% H3P04(to the right of 85% H,PO,). The inversion-recovery pulse sequence was used to measure spin-lattice relaxation times (TJ. A three-parameter exponential fit was used to obtain Ti from experimental data (10). The probe temperature was 26 O C . Sample Preparation. Samples were made as concentrated as possible using D20as solvent. A standard mixture sample was made with known weights of four phosphates to test accuracy and precision. The standard phosphates were analytical grade and commercially available. Their purity was checked by 31PNMR spectrometry. RESULTS AND DISCUSSION There are several problems to overcome in order to perform quantitative work by 31P NMR spectrometry. Proton decoupling can produce a nuclear Overhauser effect (NOE) (11, 12) which enhances signal intensities such that peak areas may no longer be proportional to the number of nuclei. Although there are various means of avoiding this possibility (13, 14), the measurements here did not use proton decoupling and, thus, the NOE did not exist. Secondly, signal intensity decreases with increasing separation from the carrier frequency. This is because the irradiation power decreases with distance from the carrier frequency. This problem was surmounted by using a small sweep width and quadrature phase detection. Thus, all signals are within 600 Hz of t h e carrier frequency. With a 11.5 p s 90° pulse, there is no meaningful dropoff of rf power, even several kiloHertz away from the carrier frequency. Finally, the time between pulses must be chosen so t h a t t h e magnetization is given sufficient time t o return t o equilibrium before the next pulse. This time is governed by the spin-lattice relaxation time (TI). This obstacle t o obtaining quantitative results may be overcome by reducing all spin-lattice relaxation with the addition of a paramagnetic relaxagent (2, 3, 5 ) or by using a small pulse flip angle and long pulse delay. Since these samples are highly concentrated, we chose to d o the latter.
- 25 6 P-31
Figure 1. 31P FT NMR spectrum of a commercial sodium tripoly-
phosphate sample
s P-31 Figure 2. 31P FT NMR spectrum of a commercial sodium tripoly-
phosphate sample containing high polyphosphates Table 11. Statistical Data for "P FT NMR Assay of Known Phosphate Sample Phosphate Na,HPO, Na,P,O, Na,P,O, Na,P,O,,
Avrel wt, 7%
Known Re1 % Std wt, % accuracy dev, S 1.55 13.03
0 1.69
8.86
8.88
76.75
76.54
0.23 0.27
1.55 12.81
0.04 0.63 0.24 0.73
I t has been shown that the Ti's, as well as the chemical shifts and coupling constants for these compounds may be sensitive to pH, concentration, temperature, and the nature of the counterion (15,16). In the p H range of these samples (pH = 9.0 f 0.5), the Ti's should be relatively short ( 1 5 ) . In order to assist in determining adequate pulse conditions, the spin-lattice relaxation times of the phosphates were measured under typical sample conditions. Table I shows the 31P NMR chemical shifts and the T1 values for standard samples. Using a 30% pulse flip angle and a time between pulses of 60 s will enable sufficient magnetization to return to equilibrium so that quantitative peak area measurements can be made. Figure 1 shows the 31PFT NMR spectrum of a commercial tripoly sample. T h e signal-to-noise ratio was excellent while all signals are well separated and easily integrated. The accuracy and precision of this quantitative method were assessed by performing six analyses on a known standard
ANALYTICAL CHEMISTRY, VOL. 50, NO. 4, APRIL 1978
587
The presence of higher polyphosphates will interfere with this analysis. Figure 2 shows the 31PFT NMR spectrum of another commercial tripoly sample. T h e signal3 from Na2HP04,Na4P2O7,and Na5P3OIoare clearly seen. However, upon scale expansion of the Na5P3010triplet region, as in Figure 3, numerous signals from higher polyphosphates can be detected (16). The analysis can still be performed, although the accuracy will be reduced.
LITERATURE CITED
2 -
~~
-,--
~.-~---~l
-18
-19
-.
.+
. 3
P-31
--
. : : 8
--.
..----x----x^-l
- 20
____
T
-21
Scale expansion of spectrum in Figure 2 clearly showing numerous signals from high polyphosphates
Figure 3.
mixture of Na2HP04,Xa4P207,Na3P309,and NajP3OIo. Table I1 shows the average relative weight percent, the known weight percent, and the relative percentage accuracy. For an NMR method of analysis, both t h e accuracy and precision are outstanding. T h e average relative weight percent may be taken as the average absolute weight percent since all the constituents of commercial tripoly samples so far analyzed contained phosphate, thus eliminating the necessity of adding an internal standard.
(1) T. C. Farrar and E. D. Becker, "Pulse and Fourier Transform NMR". Academic Press, New York, N.Y., 1971. (2) I . K. O'Neill and N. A. Pringuer, Anal. Chem., 49, 558 (1977). (3) T. W. Gurley and W. M. Ritchey, Anal. Chem., 48, 1137 (1976). (4) J. G. Coison and D. H. Marr, Anal. Chem., 45, 370 (1973). (5) T. W. Gurley and W. M. Ritchey, Anal. Chem., 47, 1444 (1975). (6) N. Yoza, K. Kouchiyama, T. Miyajima, and S. Ohashi, Anal. Lett., 8, 641 (1975). (7) R. H. Koiioff, Anal. Chem., 33, 373 (1961). (8) D. P. Lundgren and N. P. Loeb, Anal. Chem., 33, 366 (1961). (9) W. B. Chess and D. N. Bernhart. Anal. Chem., 30, 111 (1958). (10) J. Kowaiewski, G. C. Levy, L. F. Johnson, and L. Palmer, J . Magn. Reson., 26, 533 (1977). ( 1 1) J . H. Noggie and R. E. Shrmer, "The Nuclear Gveti?auser Effect', Academic Press, New York, N.Y., 1971. (12) P. L. Yeagie, W. C. Hutton, and R. B. Martin, J . Am. Chem. SOC.,97, 7175 (1975). (13) 0. A. Gansow and W. Shittenhelm, J . Am. Chem. SOC.,93, 4294 (1971). (14) T. Gionek, J . Am. Chem. Soc.. 98, 7090 (1976). (15) T. Gionek, P. J . Wang, and J. R. VanWazer. J . Am. Chem. SOC., 98, 7968 (1976). (16) T. Glonek, A. J. R. Costello, T. C. Myers, and J. R. VanWazer, J . Phys. Chem., 79, 1214 (1975).
RECEIVED for review August 10, 1977. Accepted January 3, 1978.
Microcomputer Assisted, Single Beam, Photoacoustic Spectrometer System for the Study of Solids Harry E. Eaton" and James D. Stuart Chemistry Department, U-60, University of Connecticut, Storrs, Connecticut 06268
A photoacoustic spectrometer system, for the study and analysis of solid materials, is described in detail. The system has been designed to make the photoacoustic technique available to most laboratories. Expensive, commercial, data acquisition units have been replaced by commonly available, inexpensive, and easily constructed components. The problems associated with the single beam mode of operation (i.e., source output correction, background compensation, and temporal variations) have been accounted for with the use of a microcomputer for data acquisition and reduction. Evaluation of the system is presented along with the spectra of Ho,O,, Er,O,, and UF4. Resolution is approximately 15 nm. Standard sample size is generally 5 mg while the lower limit of material necessary for an analysis is at the submilllgram level-being both analyte and matrix dependent.
Photoacoustic spectrometry (PAS) has received attention lately as a tool for the investigation of the ultraviolet, visible, and infrared transmission properties of solid and semisolid materials which heretofore were inaccessible because of the physical nature. The technique has been used in the analysis 0003-2700/78/0350-0587$01 .OO/O
and study of. among others, thin-layer chromatography plates ( I ) ; semiconductor materials ( 2 ) ;low magnitude optical absorption coefficients (3-5); thermal diffusivity constants (6); fluorescence in solids (7-9); radiationless transitions ( 1 0 , I I ) ; qualitative identification of uranium tetrafluoride (12);biological systems (13-16); and excited states of a solid (17). Articles in the literature have advanced theoretical expressions to explain signal generation ( 4 , 5 , 18,19). The basis of the technique is the radiationless conversion of resonant, periodic, incident radiation into heat within the sample surface. The result is a periodic change in the kinetic energy of gas molecules in contact with the sample to produce pressure changes detectable as sound. For an incident wavelength, the magnitude of the sound is dependent on the optical absorption coefficient of the sample for that particular wavelength, the thermal diffusivity, and the sample thickness. By monitoring the magnitude of the sound vs. the incident radiation wavelength, a photoacoustic spectrum is produced which is analogous to a conventional absorption or transmission spectrum. While literary descriptions of photoacoustic spectrometer systems have appeared for both single (20) and double beam instrumentation (21), experimentation connected with t h e G 1978 American Chemical Society
