2137
Anal. Chem. 1901, 53, 2137-2138
h
11
11 Ctj20H
C,H3
11
' 1
1
I1
6=35ms
1
1
_ _ J L 2 b L
'I
A
1
hL
Flgure 2. 99.6-MHz 'H PGSE/4-spectra from (a) neat ldecanol and (b) a 50/50 mixture of decane and ldecanol. The signals have been recorded with arbitrary signal amplltudes, and the data manipulation represented In the upper traces has been made so as to cancel out the -CHpOH signals (see text). Total measurement and data manipulation time was 15 min.
contribution to the band shape. An experiment of the present kind could be characterized as "size-resolved NMR spectrometry". Please note that these are all proton spin-echo spectra and that signal amplitudes, as compared to normal spectra, are distorted by J modulation and T2effects. For example, the alkyl -(CHZ),,- bands are strongly attenuated relative to the -CH3 signals and the -CH2-CHz0H signals from n-decanol are almost completely absent in the spectra. Carbon13 Tlmeasurements are presently utilized by several groups (see, e.g., ref 7 and 8) to characterize crude oil fractions. Tlvalues are only partly determined by molecular size and may differ vastly within a molecule. They are therefore of limited value. It would appear likely that the present method should be more useful in similar applications. It does not require any special equipment and can be implemented on any modern FT-NMR spectrometer. Proton and carbon signals could also be cross-correlated through parallel determinations of proton and carbon selfdiffusion coefficients. (The protons and carbons within a
molecule, of course, diffuse at equal rates.) The mentioned J-modulation effects in proton spectra complicate this, however. On very high field spectrometers (Bo = 5 tesla or more) J-modulation effecte are much simpler, and an approach of this kind could be feasible. A referee pointed out that NOE (nuclear Overhauser effect) suppressed carbon-13 spectra provide complementary infcrmation in that relative signal areas should be equal (or related by integer fractions) for a particular molecular type in a solution. The present method is definitely faster and more general. Peak overlap is no problem in qualitative applications. Furthermore, suppressed-NOE spectra give no inclications about molecular size. The unique feature of the present approach is ithat measured self-diffusion coefficienlts are directly related to the dimensions of the molecules in solution.
ACKNOWLEDGMENT Thanks are due to U. Obenius for technical assistance and stimulating discussions and to M. E. Moseley for stimulating discussions. LITERATURE CITED (1) Stllbs, P.; Moseley, M. E. Chem. Scr. 1980, 75, 178. (2) Stllbs, P.; Moseley, M. E. Chem. Scr. 1080, 75, 215. (3) Vold, R. L.; Waugh, J. S.; Klein, M. P.; Phelps, D. E. J . Chem. Phja. 1068, 48, 3831. (4) James, T. L.; McDonald, Q. Q. J . Ma@. Reson. 1073, 1 7 , 58. (5) Stelskal, E. 0.; lanner, J. E. J. Chem. Phys. 1085, 42, 288. (8) Hahn, E. L. Phys. f b v . 1950, 80, 580. (7) Yoshlda, T.; Maekava, Y.; Uchlro, H.; Yokoyama, S. Anal. Chem. 1080,-52, el?. (8) Sklenar, V.; Hjek, hA.; Sebor, 0.; Lang, I.; SucMnek, M.; StarEuk, 2. Anal, Chem. 1060, 52, 1794.
Peter Stilbs Institute of Physicall Chemistry Uppsala University Box 532, 5-751 21 Uppsala, Sweden RECEIVED for review March 31,1981. Accepted July 5,1981. This research was supported by the Swedish Natural Sciencles Research Council.
Zn2+-Nitric Acid Eluent for the Ion Chromatographic Separation of Alkaline Earth Metals Sir: In their original publication on ion chromatography using eluent suppression, Small et al. described a means for separating the alkalin e earth ions using Ag+ ions in the eluent (1). The Ag+ ions were removed as AgCl by passing the eluent through a strong base resin in the chloride form. Later, Dionex Corp., manufacturers of ion chromatographs using eluent suppression, recommended use of m-phenylenediamine dihydrochloride nitric acid solutions as the standard eluent for separating the alkaline earth ions. Recently, Nordmeyer et al. described a system using Ba2+or Pb2+in the eluent (2). The Ba2+tind Pb2+ions are suppressed as insoluble sulfates in a resin loaded with sulfate. In our work, we have determined that, HN03 solutions containing Zn2+ions make a good eluent for the separation of the alkaline earth ions. EXPERIMENTAL SECTION Equipment. A Dionex Model 16 ion chromatograph with a 0.1-mL sample loop was used for all experiments. Columns employed included Dionex Corp. 3 X 150 mm anion and cation 0003-2700/81/0353-2137$01.25/0
precolumns, 6 X 250 imm cation separator, 3 X 500 mm anion separator, and a 9 X 250 mm cation suppressor column. Chemicals. AH chemicals used in this work were reagent grade quality made up in deionized water.
RESULTS AND DISCUSSION When separating monovalent cations such as Na+, K+, and NH4+,one normally uses a dilute solution of HN03 or HC1 as the eluent. However, unless one uses a rather concentrated acid solution, the elution of the alkaline earth ions requires unacceptably long times. This is, of course, due to the fact that the divalent alkaline earth ions have a much stronger affinity for the separating resin than does the hydrogen ion. Furthermore, the use of relatively concentrated acid eluent8 is not practical because the suppressor resin would be depleted too rapidly. Therefore, in order to effect good separations in a reasonable time, one must add to the eluent a specie(s) which has a much sitronger affinity for the separating resin than the hydrogen 0 11981 American Chemlcal Soclety
2138
ANALYTICAL CHEMISTRY, VOL. 53, NO. 13, NOVEMBER 1981
1
20
I
I
10
0
TIME ( M I N )
P
u 0
L
I
I
20
IO
0
TIME ( M I N )
Flgure 1. (a) Separation on a Dionex 3 X 500 mm anion separator column: eluent 0.003 M HN03-0.005 M Zn(NO,),.H,O; 40% flow rate; concentrations Na, K, NH, = 5 ppm, Mg = 10 ppm, Ca = 15 ppm, Sr = 20 ppm, Ea = 25 pprn. (b) Separation on a Dionex 6 X 250 mm cation separator column: eluent 0.004 M HNO,-0.0025 M Zn(N03)pH20; 40% flow rate: concentrations Na, K, NH4, Mg = 5 ppm, Ca = 10 ppm, Sr = 15 ppm, Ea = 20 ppm.
ion. Also, if one is using ion chromatography with suppression, the added component must be amenable to substantial reduction in the suppressor column so that the background conductivity is not too high. Another factor to consider is that the suppressor resin should be easily regenerable when its capacity has been depleted. We have determined that an eluent which is 0.003-0.004 M in HN03 and about 0.0025 to 0.005 M in Zn(N03).H20meets all of the requirements listed. The Zn2+ions are removed by the strong base suppressor resin as Zn(OH)2and the HN03 is neutralized. Upon regeneration with 0.5 M NaOH, the Zn(OH)2is dissolved according to the following reaction (3): Zn(OH), 20H- +=ZnOZ2- 2H20
+
+
We have found that regeneration requires only 30 min with 0.5 M NaOH followed by 30 min with HzO. Also we have noticed no significant system pressure changes as the suppressor becomes depleted. In an earlier report, we disclosed that the Dionex anion separator column could be used for cation separations (4). Figure 1illustrates separations of the alkaline earth ions on Dionex anion and cation separator columns, respectively. It should be noted here that if one desires to separate the monovalent cations such as Na+, K+, and NH4+ions after use of the Zn2+eluent, we have determined that washing the separator column with 1 M NaOH for 1h, deionized water for 3 h, and 0.005 M HN03 for 1h will completely restore the column for these separations. One could restore the column by washing with standard cation eluent (0.005 M “OB) but it would require very long times. We wish to also report here that Al(N03)3was tested as an eluent since it exhibits a similar chemistry as Zn2+in the suppressor column. However, we found that Sr2+ is not separated from Ca2+using this eluent. We believe that the Zn2+eluent described here possesses some advantages when compared to the others mentioned earlier. First, the suppressor column is more easily regenerated when compared to Ag+, Ba2+,or Pb2+containing eluents. A disadvantage with the latter eluents is that anions in a real sample such as C1-, Br-, I-, or S2- would precipitate when contacted with the Ag+ ions and SO4“ would do the same thing with Ba2+or Pb2+ions. This could conceivably degrade the separator resin by loading it with insoluble salts. Another disadvantage to the use of Ag+, Ba2+,or Pb2+in the eluent is the fact that these ions are considered hazardous by the EPA. Therefore, one would have a pollution problem when using these ions in the eluent. The advantage of the Zn2+eluent when compared to the m-phenylenediamine eluent is that the former is stable indefinitely. Dionex recommends that the latter eluent be prepared frequently. Our experience with this eluent is that it turns brown in a few days and it deposits a dark colored material on the precolumn when used for relatively short periods of time. Attempts to dissolve the dark colored material (such as flushing with strong acids or bases) have proved unsuccessful. In conclusion, we believe that the Zn2+-HN03eluent described here is an advancement when one desires to separate the alkaline earth ions and possesses several advantages when compared to other eluents used for this purpose.
LITERATURE CITED (1) Small, H.; Stevens, T. S.; Bauman, W. C. Anal. Chem. 1075, 47, 1801-1809. (2) Nordmeyer, F. R.; Hansen, L. D.; Eatough, D. J.; Rollins, D. K.; Lamb, J. D. Anal. Chem. 1980, 52,852-856. (3) West, P. W.; Vick, M. M.; Lerosen, A. L. “Qualitative Analysis and Analytical Chemical Separations”; MacMillan: New York, 1853; pp 56-62. (4) Wimberley, J. W. Anal. Chem., in press.
Jerry W. Wimberley R&D Department Analytical Research Section Conoco Inc. Ponca City, Oklahoma 74603 RECEIVED for review June 1, 1981. Accepted July 27, 1981.
