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Standard and reference samples of the International Humic Substances Society have been characterized by solution state carbon-13 and hydrogen-1 nuclear magnetic resonance (NMR) spectrometry. Samples included the Suwannee River, soil, and peat standard fulvic and humic acids, the Leonardite standard humic acid, the Nordic aquatic reference fulvic and humic acids, and the Summit Hill soil reference humic acid. Aqueous-solution carbon-13 NMR analyses included the measurement of spin-lattice relaxation times, measurement of nuclear Overhauser enhancement factors, measurement of quantitative carbon distributions, recording of attached proton test spectra, and recording of spectra under nonquantitative conditions. Distortionless enhancement by polarization transfer carbon-13 NMR spectra also were recorded on the Suwannee River fulvic acid in deuterated dimethyl sulfoxide. Hydrogen-1 NMR spectra were recorded on sodium salts of the samples in deuterium oxide. The carbon aromaticities of the samples ranged from 0.24 for the Suwannee River fulvic acid to 0.58 for the Leonardite humic acid. INTRODUCTION This report presents solution state carbon-13 ( 13C) and hydrogen-1 nuclear magnetic resonance (NMR) spectra that have been recorded on standard and reference samples of the International Humic Substances Society (IHSS). The objective is not to interpret the spectra in great detail, but to expeditiously make the data available to the IHSS in a convenient format. For detailed presentations of NMR theory, the reader is referred to several recent textbooks on NMR spectroscopy: Breitmaier and Voelter (1987), Farrar (1987), Sanders and Hunter (1987), Derome (1987), Wehrli, Marchand, and Wehrli (1988), and Brey (1988). The application of NMR spectroscopy to humic substances has been reviewed by Wershaw and Mikita (1987) and Wilson (1987). These latter two books also discuss the limitations in the quantitative reliability of solid state cross polarization/magic angle spinning (CP/MAS) 13C NMR when applied to humic substances. Further information on the IHSS Suwannee River samples is available in the review by Averett and others (1989). To obtain solution state 13C NMR spectra that are quantitative, that is, in which peak areas or intensities are in direct proportion to the number of 13C nuclei resonating, two phenomena must be taken into consideration. First, of Chemistry and Geochemistry, Colorado School of Mines, Golden, Colo. differential saturation effects must be eliminated by recording the spectra using a pulse delay that is five times the longest spin-lattice relaxation time (Tj) present in the sample. T! is a time constant that is inversely proportional to the rate at which energy is transferred from the spins to the lattice, allowing the magnetization to return to its equilibrium value after a radio frequency pulse (Wehrli and Wirthlin, 1980). If too short a pulse delay is used, then 13C nuclei that have shorter TVs are exaggerated in intensity when compared to 13C nuclei that have longer Tj's. Second, differential nuclear Overhauser enhancement (NOE) effects must be eliminated. This elimination is accomplished by employing inverse-gated decoupling, in which the decoupler is on during the acquisition time but off during the pulse delay. The NOE effect occurs because proton decoupling causes changes in the population distribution of the 13C spins, resulting in an enhancement of the 13C signals (Wehrli and Wirthlin, 1980). The signal increase resulting from NOE is equal to 1+n, where n is the NOE factor. When 13C spin-lattice relaxation occurs solely by 1H13C dipole-dipole interactions, and within the extreme narrowing limit of fast molecular motion, r| reaches its maximum value of 1.988. At its maximum, therefore, the NOE effect can result in a threefold increase in signal intensity (1+1.988*3). Even if spectra are acquired under conditions that eliminate differential TI and differential NOE effects, problems may still arise with 13C nuclei that are in close proximity to paramagnetic metal ions, stable organic free radicals, or quadrupolar nuclei. Such 13C nuclei can have short spin-spin relaxation times (T2*s) and, therefore, linewidths too broad to be observed. No manipulation of the spectrometer can overcome this latter potential problem. In principle, however, spin-counting experiments (Vassallo, 1987) can be performed to determine whether the problem exists or not. This possibility may be an area of future investigation. The quantitative distributions of 13C nuclei in the fulvic and humic acids were determined by recording 13C NMR spectra using inverse gated decoupling and pulse delays of sufficient duration to ensure quantitative results. The Tj's of the samples were measured using the progressive saturation method (Freeman and Hill, 1971) to determine the necessary pulse delays. The NOE factors can provide some insight into the relaxation mechanisms of the 13C nuclei in the fulvic and humic acid molecules, and so it is convenient to measure these concurrently with the quantitative carbon distributions. This measurement is done by simply switching the decoupler between the continuous decoupling and the inverse gated decoupling modes when recording the quantitative spectra. The NOE factors are calculated by measuring the differences in peak areas between the two sets of spectra. Sometimes qualitative information is suppressed in 13C NMR spectra recorded under quantitative conditions. For this reason it also is useful to record spectra under nonquantitative conditions that maximize signal-to-noise. Under conditions of continuous decoupling and short pulse delays, the peak intensities of 13C nuclei that have short Ti's and significant NOE factors are enhanced in comparison to spectra obtained under quantitative conditions. Additionally, because greater signal to noise is obtained in a nonquantitative 13C NMR spectrum, a smaller line broadening can be applied to the free induction decay (FID) resulting in the resolution enhancement of discrete, narrow line resonances. A number of spectral editing pulse sequences were introduced in the late 1970*s and early 1980's. These pulse sequences are usually classified into spin-echo techniques, such as APT (Attached proton test; Patt and Shoolery (1982)), GASPE (Gated spin echo; Cookson and Smith (1983)), and PSCE (Part coupled spin echo; Snape (1983)), or polarization transfer techniques, such as INEPT (Insensitive nuclei enhancement by polarization transfer; Morris and Freeman (1979)) and DEPT (Distortionless enhancement by polarization transfer; Doddrell and others (1982)). APT spectra were recorded on the samples examined in this study. The APT experiment differentiates among methyl, methylene, methine, and quaternary carbons, and therefore allows more specific assignments of the 13C NMR spectra. DEPTGL (GL version of DEPT sequence; Sorensen and others, 1983) spectra also were recorded on the Suwannee River fulvic acid. EXPERIMENTAL CONDITIONS NMR Spectrometry The 13C and XH spectra were recorded on a Varian* XL-300 NMR spectrometer at 75.4 and 299.9 megahertz (MHz), respectively. All 13C NMR spectra were recorded using a Varian XL Series Generation III 10-millimeter (mm) broadband probe, which was modified to be free of any background carbon signal. For 13C spectra, approximately 200 milligrams (mg) of sample was added to 1.5 milliliter (ml) H20 and 0.5 ml D20, and adjusted to pH 7 with NaOD, in a 10 mm NMR tube. The *H spectra were recorded using a 5 mm broadband probe. Approximately 35 mg of sample was tirated to pH 8-9 in aqueous solution, and then dried in a rotary-evaporator. The sodium salts were redissolved in 100 percent D20, and then dried once again in a rotary-evaporator to exchange deuterium for hydrogen. The deuterium exchange step was repeated a second time, and the sodium salts were again dissolved in 100% DgO for NMR analysis. In most cases, the residual HOD peaks in the 1H-NMR spectra were narrow enough so homonuclear decoupling of the HOD peaks was unnecessary. Acquisition Parameters Progressive saturation experiments were recorded using a 30,000 hertz (Hz) spectral window (SW), 90° pulse width (PW), 0.2 second acquisition time (AT), and continuous decoupling in the WALTZ Modulation mode (CWD). Pulse delays (PD) were arrayed at 0.2, 1.0, 2.0, 4.0, 6.0, 8.0, 10.0, and 12.0 seconds. The samples were not degassed to remove dissolved oxygen, which may contribute to 13C relaxation through the electron dipolar relaxation mechanism (Martin and others, 1980). NOE measurements were recorded using a 50,000 Hz SW, 90° PW, 0.2 second AT, and 10.0-12.0 second PD. The decoupler was switched between the continuous and inverse-gated decoupling modes for each block size of 100 transients. Spectra recorded using inverse gated decoupling represented the quantitative spectra. *Use of trade names in this report is for identification purposes only and does not constitute endorsement by the U.S. Geological Survey. Nonquantitative spectra were recorded using a 30,000 Hz SW, 45° PW, 0.2 second AT, 1.0 second PD, and CWD decoupling. Dioxane, assumed to be 67.4 parts per million (ppm), was used as an internal reference standard. Attached Proton Test spectra were recorded using the pulse sequence described by Patt and Shoolery (1982): (PD PW -T(decoupler off) 180° (t+A) -180° A -acquire) The acquisition parameters included a 30,000 Hz SW, and 0.2 second AT. PD was 0.0 seconds, PW was 45°, t was 7.0 milliseconds, and A was 1.0 millisecond. The value of 7.0 milliseconds for T corresponds to T=1/J, where J equals 142.5 Hz, the average of the aromatic C-H ( 1J_ =160 Hz) and aliphatic C-H Ln ( 1Jx,H=125 Hz) one bond coupling constants. When T is set equal to 1/J, methylene and quaternary carbons are positive, and methyl and methine carbons negative. Proton spectra were recorded using an 8,000 Hz SW, 25° PW, 1.9 second AT, and 5.0 second pulse delay. Dioxane, assumed to be 3.6 ppm, was u

water-resources investigations report

Characterization of the International Humic Substances Society standard and reference fulvic and humic acids by solution state carbon-13 (13C) and hydrogen-1 (1H) nuclear magnetic resonance spectrometry