Anal. Chem. 1989, 6 1 , 631-632
CONCLUSIONS The spin echo technique is well suited for the measurement of conventional carbon NMR spectra of HUS. Spectra with smooth base lines are readily obtained, and quantitative evaluations of the spectra become feasible. The combined use of the DEPT and QUAT pulse techniques allows a quantitation of the carbon type distribution in aquatic FA and HA. A surprisingly large fraction (78%) of quaternary carbons is found. Moreover, comparing NMR results and elementary analysis reveals that about 75% of the hydrogen of the investigated HUS is bound to oxygen. Short spin-spin relaxation times of the carboxylate carbons in HUS confirm the latter as preferred ligands for paramagnetic ions. Considerable improvement of the accuracy of carbonyl and methyl carbon determination is anticipated (a) from the implementation of composite pulses (20) into the pulse sequences used and (b) from the use of probe heads with shorter pulse widths.
ACKNOWLEDGMENT The stimulating discussions with Professor G. Tolg, Dortmund, are gratefully acknowledged.
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(3) Florence, T. M.; Batley, G. E. Chemical Speciation in Natural Waters. CRC Crit. Rev. Anal. Chem. 1980, 9 ,219-296. (4) Ryan, D. K.; Weber, J. H. Anal. Chem. 1982, 5 4 , 986-990. (5) De Haan, H. In Aquatic and Terrestrial Humic Materials; Christman, R. F., Gjessing, E. T., Eds.; Ann Arbor Science: Stoneham, MA, 1983; pp 165-182. (6) Wershaw, R. L. In Humic Substances in Soil, Sediment and Water; Aiken, G. R., McKnight, D. M., Wershaw, R. L., Maccarthy, P., Eds.; Wiley: New York, 1985; pp 561-582. (7) Wilson, M. A. NMR Techniques and Applications in Geochemistry and Soil Chemistry; Pergamon Press: 1987; pp 182-203. (8) Hanninen, K. I. Sci. Environ. 1987, 62, 193-200. (9) Preston, C. M.; Blackwell, B. A. Soil Scl. 1985, 739,88-96. (10) Steelink, C.; Petson, A. Sci. Environ. 1987, 62, 165-174. (1 1) Kolle, H. Erfahrungen bei der Aufarbeitung eines reduzierten huminstoffhaltigen Grundwassers im Wasserwerk Fuhrberg der Stadtwerke Hannover; Abschlussbericht zum Forschungsvorhaben 02 WT 606 des BMFT; Hannover, FRG, 1981. (12) Turner, C. J. Prog. Nucl. Magn. Reson. Spectrosc. 1984, 16, 311-370, especially p 314. (13) Pegg, D. T.; Doddrell, D. M. J . Chem. Phys. 1982, 7 7 , 2745-2752. (14) Bendall, M. L.; Pegg, D. T. J . Magn. Reson. 1983, 53,272-296. (15) Freeman, R.; Kempsell, S. P.; Levitt, M. H. J . Magn. Reson. 1979, 3 4 , 863-667. (16) Morris, G. A.: Freeman, R. J . Am. Chem. SOC. 1979, 707, 760-762. (17) Brown, D. W.; Nahashima, T. T.; Rabenstein, D. L. J . Magn. Reson. 1981, 4 5 , 302-314. (18) Netzel, D. A. Anal. Chem. 1987, 59, 1775-1779. (19) Pan, S.L.; Sykes, B. D.J . Chem. Phys. 1972, 56,3182-3184. (20) Levin. M. H. Prog. Nucl. Magn. Reson. Spectrosc. 1988, 78, 61-122.
LITERATURE CITED (1) Bohn, H. L. SoilSci. SOC.Am. J . 1978, 4 0 , 468-469. (2) Stevenson, F. J.. Humus Chemistry, Wiley: New York, 1982.
RECEIVED for review July 11, 1988. Accepted December 1, 1988.
CORRESPONDENCE Model for Conductometric Detection of Carbohydrates and Alcohols as Complexes with Boric Acid and Borate Ion in High-Performance Liquid Chromatography Sir: In recent articles ( I d ) , Okada has demonstrated the utility of indirect conductometric detection of electrically neutral sugars and alcohols through their complexes in boric acid solution. The use of a boric acid eluent provides a highly sensitive means of detection for monosaccharides, lactose, and sugar alcohols but not for polysaccharides (other than lactose) and simple alcohols. Addition of sorbitol, mannitol, or fructose to the boric acid eluent allows detection of the polysaccharides and simple alcohols, as well as lactose, glucose, fructose, and presumably other monosaccharides and sugar alcohols. These results were interpreted ( I ) in terms of the ability of an analyte to form either dissociated or undissociated complexes with boric acid. This interpretation was quantified with a mathematical description of the complexation equilibria and the conductivity due to ionic species. Unfortunately, the mathematical model contains some incorrect assumptions that severely limit the utility of the derived equations and may prevent optimization of this potentially important technique. We present here a more general mathematical model that does not suffer from these limitations. A basic flaw in the earlier interpretation was the assumption that the complex between boric acid and an analyte either dissociates or remains nonionic. Actually, most polyols complex to some degree either with boric acid (undissociated) or with borate ion (dissociated). Addition of any poly01 to a boric acid solution generally lowers the pH (6, 7), indicating that
the complex with borate ion is stronger than that with the acid. Mannitol, sorbitol, and fructose show very strong effects, while the drop in pH is considerably less for comparable concentrations of glucose, maltose, glycerine, or ethylene glycol. In describing the eluent containing mannitol, sorbitol, or fructose and the analyte, Okada focused on the conductivity due to ionic borate complexes. However, since no other cations are present, the change in concentration of hydrogen ion must be equivalent to the sum of the changes in concentrations of all anions. Therefore, any changes in conductivity are primarily due to changes in concentration of the highly mobile proton. For a generalized model, the eluent is assumed to contain boric acid (HB) at formal concentration CBand a complexing agent (R) a t formal concentration CR. These are a t equilibrium with protons (H'), borate ions (B-), and a series of complexes that may be dissociated (BRk-) or undissociated (HBR,). Normally, j and k are no greater than 2. The eluent may also contain an analyte (S) and its complexes which also may be dissociated (BS-) or undissociated (HBS). Because of its low concentration, only 1:l complexes of the analyte are considered. There is also the possibility or borate complexes of higher order, but consideration of these species contributes more confusion than illumination a t this point. The equilibria may be described
0003-2700/89/0361-0631$01.50/0 0 1989 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 6, MARCH 15, 1989
HB=H++BHB B-
+ j R + HBR,
+ kR + BRk-
KRdk, h = 1, 2 , ..., K
+ S + BS-
Ksu KSd
The ionization constants of the complexes also might be considered, but these equilibria are redundant with those given above. Mass balance requires CB =
[HB]
+ [B-] + [HBS] + [BS-] + C[HBR,] +
c [BR!4-1
CR = [R] + Cj[HBR,]
+ Ck[BRk-] Cs = [SI + [HBS] + [BS-]
and for electrical neutrality [H’] = [OH-]
+ [B-] + [BS-] + C[BRk-]
For solutions with pH of 5 or less, the concentration of hydroxide ion can be neglected relative to other concentrations, and CB -
[H+] = [HB]
[H+1 = [B-IIl + KSd[Sl + CKRdk[Rlk) [H+1 = [HB1{1+ KSulS] + cKRuj[Rl’)
. The equations may be solved for [B-] and [HB] for substitution into the ionization expression
[“12(1 + KSu[Sl + CKRuj[Rl’)
K, = IcB
-
in which [H+],[HB], and [B-] are determined by the formal concentration of boric acid, with [HB] >> [B-] for reasonable values of CB. T h e detectability of a n analyte is not determined b y t h e absolute magnitude of t h e complexation constants, KSd a n d K,,, but by t h e difference between these values. However, detectability is inversely related to the quantity Q, which is more sensitive to undissociated complexes ([HB]Ksu) than dissociated ([B-]KSd).If Ksu > KSd, the hydrogen ion concentration will decrease as the analyte appears at the detector, but because of the effect on Q, the amount of decrease will be less than the corresponding increase would be if the magnitudes of the equilibrium constants were reversed. The compounds that Okada was able to detect with boric acid eluent all gave increased conductivity (KSd >> KsU). The undetectable compounds apparently have a smaller difference between these values. Addition of mannitol, sorbitol, or fructose to boric acid solution causes a strong decrease in pH, indicating that C K w [ R l k>> CKRuj[R]’for these compounds. Assuming that CKW[Rlk>> 1and CKRuj[R]’
