Anal. Chem. 1981, 53, 1535-1536
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CORRESPONDENCE Anion Peak Migration in Ion Chromatography Sir: The use of ion chromatography, first described in 1975 (1) for the characterization of a variety of solutions with respect to their anionic and cationic composition is well documented (2-6). Mixed eluents containing various ratios of the carbonate-bicarbonate species are routinely used for the elution and separation of common anions in the standard methodology. As with all chromatographic techniques, the most effective use of the ion chromatographic process requires accurate characterizatioln of the retention times of individual species and identification of analysis parameters which effect the retention characteristics of these species. In this correspondence, the behavior of the species of primary interest in ion chromatography (chloride, sulfate, and nitrate) is summarized and an explanation for the observed phenomenon proposed.
EXPERIMENTAL SECTION Instrumentation. All chromatograms were obtained with a commercial Dionex Model 14 ion chromatograph equipped with a standard 3 X 500 mm anion separator column and a 6 X 250 mm suppressor column. Sample loop volume was 0.1 mL, conductivity detector response was maintained at 10 @/cm full scale deflection and laboratory temperature was maintained at 22 f 2 "C.
Reagents. Eluents used throughout the course of this study were prepared by dissolution of the appropriate amount of reagent grade Na2C03and NaHC03 in type I1 deionized water. Eluents are allowed to stabilize for a period of not less than 3 h before utilization; the pH of these eluents is recorded just prior to utilization. All samples analyzed are prepared by dilution of standard solutions; the concentrations of Na2C03and NaHC03 in the sample and the eluent used are rigidly matched. Standard solutions for chloride and nitrate are prepared by dissolution of their sodium salt while the sulfate standard is a 0.02 N H2S04 solution manufactured by the Hach Chemical Co.; the normality of this solution was confirmed by titration with a standard base. R E S U L T S A N D DISCUSSION Effect of Eluent Flow Rate. The effect of eluent flow rate on anion retention time is summarized in Table I. Increasing the flow rate causes a proportional decrease in retention time; this decrease in retention time is not dependent on the identity of the anion of interest as is illustrated by the constant ratio of retention time noted. These data illustrate that the ion chromato,graphic mechanism is significantly different than that predicted from the linear, nonideal rate theory of chromatography; the data agree poorly with respect to the von Deemter relationship which expresses the theoretical effect of flow rate of the mobile phase on the retention time of the soluble species for this treatment. Effect of Solute Concentration. Table I1 summarizes the effect of changing concentrations of sulfate on the retention of itself and nitrate in the chromatographic system; a 300-fold increase in sulfate concentration has no effect on the elution characteristics of these species. As is illustrated in Table 111,however, the concentration of nitrate in a sample clearly effects its elution characteristics; changes of retention times of up to 1.5 min a13 the concentration changes from 10 to 70 ppm are noted. Above this threshold concentration, the nitrate migration is less profound. The data indicate that the nitrate experiences an interaction with the exchange resin 0003-2700/81/0353-1535$01.25/0
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Table I. Effect of Flow Rate on Anion Retention Time retention time, flow rate min retention % mL/min NO,SO:ratio 10 15 20 30 40 50 60
0.76 1.26 1.74 2.36 2.59 3.54 5.33
15.9 9.6 7.1 4.6 3.4 2.8 2.2
1.347 1.333 1.340 1.353 1.308 1.308 1.341
11.8 7.2 5.3 3.4 2.6 2.1 1.6
Table 11. Effect of Sulfate Concentration on Anion Retention Time retention time, min sulfate concn, PPm NO,so:0 50 100 200 300
9.8 9.7 9.8 9.8 9.7
7.2 7.2 7.2 7.2
Table 111. Effect of Nitrate Concentration on Its Own Elution retention time, min concn, ppm 5 10 20 25 30 40 50 60 70 100
a
13.8 13.5 13.3 12.9 12.6 12.3 12.0 11.7
b
17.4 17.0 16.9 16.8 16.3 16.2 16.1 15.9 15.8 15.7
In 0.003 M NaHC0,/0.002 M Na,CO, eluent, 25% flow rate. In 0.003 M NaHCO,/O.006 M Na,CO, eluent, 10% flow rate. governed by nonlinear isotherms at concentrations less than 70 ppm and further suggest that the nonlinear behavior involves the desorption of the nitrate from the resin by the eluent ions. Effect of Eluent Composition. The effect of eluent composition on the relative retention characteristics of the sulfate, nitrate, and chloride peaks is quite profound. Table IV illustrates the effect of eluent pH, ionic strength, and absolute bicarbonate and carbonate concentration on the elution characteristics. Trends noted include (1) that the behavior of the sulfate species is more profoundly effected by eluent composition, (2) that changing the concentration of carbonate has a larger net effect than changing the bicarbonate concentration, and (3) that in cases where the ionic strength was maintained, pH had no effect on the elution characteristics of the three species. The trends noted in the @ 1981 American Chemical Society
Anal. Chem. 1981, 53, 1536-1538
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Table IV. Effect of Eluent Composition on Anion Retention
compositiona O.OOl/O.OOl
0.003/0.001 0.003/0.002 0.003/0.005 0.006/0.005 0.009/0.005 0.003/0.007 0.012/0.005 0.009/0.007 0.003/0.009 O.Ol2/0.009 0.23/0.019
eluent properties ionic strength,b pH M C110.10 9.87 10.11 10.38 10.22 10.03 10.56 9.89 10.17 10.74 10.17 10.25
0.0050 0.0063 0.0099 0.0171 0.0198 0.0025 0.0237 0.0252 0.0292 0.0315 0.0396 0.0792
NO3-
SO-;
6.4 6.0 5.2 4.5 4.4 4.3 4.4 4.3 4.2 4.1 3.9 3.5
26.8 14.7 13.8 12.7 10.8 12.2 10.0 8.8 7.8 5.2
a (mol/L of NaHCO,)/(mol/L of Na,CO,). assuming no significant ion pair formation.
22.0 20.4 16.0 12.4 11.8 11.1
10.8 10.7 10.2 9.9 8.8
7.4
Calculated
Table V. Ion Size Parameters for Selected Anions ion size, anion
a
c1NO,'
1.81a
1.8gb
Pauling crystal radii (7). ical radii (8). a
ion size,
a
anion
so:
-
co,2HCO 3-
2.30b 1.85; 1.63
Yatsimirskii thermochem-
first two instances above are tentatively explained as illustrating ionic sizelcharge effects. The sequence of ionic size (S042> NO, > C1-) from Table V accurately mirrors the relative effect that increased carbonate and bicarbonate
concentration in the eluent has on the species retention characteristics. This ionic size phenomenon has been attributed to the buildup of steric stress in the resin structure (9). The observation that increased carbonate ion concentration effects elution more than bicarbonate concentration reflects the electrostatic difference between the two species. Observation 3 allows us to conclude that the p H per se has no significance in determining the retention time of the species studied. It is proposed that pH effects the elution characteristics only by effecting the overall ionic strength of the eluent; in the systems studied in this paper, the effect of the OH- ion on the total ionic strength was minimal and therefore no pH dependent behavior was observed.
LITERATURE CITED Small, H.; Stevens, T.; Bauman, W. Anal. Chem. 1975, 47, 1801. Wetzel, R. Envlron. Sci. Techno/. 1979, 73, 1214. Wetzel, R.; Anderson, C.; Schlelcher, H.; Crook, D. Anal. Chem. 1979, 57. 1532. Hansen, L.: Richter, B.: Roiiins, D.:Lamb, J.: Eutouah, D. Anal. Chem. 1979, 51, 633. Lathouse, J.; Coutant, R. I n "Ion Chromatographic Analysis of Envlronmental Pollutants"; Ann Arbor Science Publishers: Ann Arbor, MI, 1978. Gjerde, D.; Schmuckier G.; Fritz, J. J. Chromatogr. 1980, 187, 35. Pauiing, L. "The Nature of the Chemical Bond"; Corneii University Press: Ithaca, NY, 1960; p 514. Waddington, T. Adv. Inorg. Chem. 1959, 7 , 160. Peters, D.; Hayes J.; Heftje, G. "Chemical Separations and Measurements"; W. B. Saunders: Philadelphia, PA, 1974; p 583.
Dennis Jenke Montana Energy and MHD Research and Development Institute P.O. Box 3809 Butte, Montana 59702
RECEIVED for review March 23, 1981. Accepted May 7,1981.
Comparative Precision of Silver Inquartation Techniques Sir: Local assayers have recently noted a decrease in the precision of results obtained by using manufactured solid inquarts and have switched to solution inquartation, claiming vastly superior precision, The manufacturer contended that the solid inquarts gave results as good as can be obtained by fire assaying ( I ) . It had been our observation that the precision of inquartation was limited by the fire assay process and that precision equal to the precision of pipetting could not be obtained, as some assayers were claiming. The precision is of interest, since the average weight of inquarts assayed as blanks is subtracted from the bead weight to obtain the weight of silver in the assayed sample of ore. The precision thus determines the detection limit for gravimetric determination of silver. We undertook comparative inquartation studies to resolve the conflicting statements. EXPERIMENTAL SECTION To study the precision of solution inquartation, we added 5-mL aliquots of a freshly prepared solution of 400 ppm silver (as silver nitrate) in 1% (v/v) nitric acid to a series of crucibles containing weighed portions (85 g) of a sample-flux-reducing agent mixture prepared in large batches (3000 g of silver-free litharge, 150 g of borax glass, 900 g of sodium carbonate, 900 g of silica, and 150 g of flour). The portion of the mixture taken corresponded t o
a sample of 15 g of silica, using a flux very close to that recommended by Bugbee for a bisilicate slag after production of the recommended size button (2). The solution inquarted crucibles were dried in a 95 "C oven for several hours and at 105 "C overnight. To study the precision of the solid inquarts, we obtained new vials of inquarts from the manufacturer, older vials from local assayers, and very old boxes (in some cases more than 10 years past manufacture) obtained from forgotten storeroom supplies. Studies were performed on unbiased splits of 10 inquarts obtained by reduction of sample size by repeated employment of a Soiltest 10-in. splitter starting with the original box or vial of 1000 pieces. These inquarts were weighed and subjected to either of two studies, fusion and cupellation or dissolution and atomic absorption. The fusions and cupellations were performed at 1800 OF by using a large Denver Fireclay Co. electric assay furnace. Beads were weighed on a Cahn Model 26 electronic microbalance. The atomic absorption used was a Perkin-Elmer Model 303 at lox scale expansion, equipped with recorder readout, an active 0.5-Hz filter, using large zero offset, and air-Mapp gas (3). The solutions for atomic absorption were prepared in 125 mL of 20% (v/v) nitric acid, The acid was dispensed with a measured precision of 10.035% relative standard deviation, equivalent to 10.0007 mg silver. A final phase of the study used 1000-pL spikes of silver nitrate solutions of appropriate concentration in addition to either solid
0003-2700/81/0353-1536$01.25/00 1981 American Chemical Society
