1264
Anal. Chem. 1984, 56, 1264-1268
Gradient Elution Ion Chromatographic Determination of Inorganic Anions Using a Continuous Gradient James G. Tarter Department of Chemistry, North Texas State University, Denton, Texas 76203
A method is described for the production of a gradient elution solvent delivery system for ion chromatography. The system uses elther a two pump or a three pump configuration wlth a pressure of less than 400 psi. A 100-mm separator column and a fiber suppressor column are used along with both etectrochemical and conductometrlc detectlon. The Initial eluent is a 0.003 M NaHCO, to which a strong eluent (elther 0.004 M Na,CO, or 0.002 M Na,CO, 0.002 M NaOH) is added. With the component arrangement described, F-, Ci-, and NO2- elute under essentlaily isocratic conditions. I-, SCN-, Cr2072-,and S20:ail elute within 22 min. Retention times of less than 22 min are given for 18 different anions. The preparatlon and chemical composltlon of the gradients are discussed and gradlent proflles are presented.
+
Ion chromatography as developed by Small et al. (1) has significantly altered the determination of inorganic and organic ions (2). One of the limitations to ion chromatography that restricts its use is that the relative retention times for species of interest can vary widely reducing the number of species amenable to analysis in any one given sample injection. A prominent example of this variation is in the analysis of the halide ions. The conditions necessary for the analysis of Fand Cl- ions result in an unacceptably long retention time and peak shape for the I- ion. Conversely, the conditions necessary for the efficient analysis of I- results in little or no separation between the F- and C1- peaks (3). Three common approaches, when confronted with samples containing species with widely varying retention times, are to use multiple injections under different analytical conditions (4,5),stronger eluents (6),or traditional eluents with a shorter separator column (7). These methods result in either extended analysis time or significant changes in the early portion of the chromatogram including loss of peaks. Silica-coated polyamide crown resins have been employed (8)but the resolution of common ions suffers to some extent. A method developed by Wang et al. (3) used a sequential electrochemical and conductometric detection scheme to analyze for F-, C1-, Br-, and I- in one sample injection. The analysis time reported by Wang et al. was approximately 30 min, but the method did require that the operator switch a valve a t a critical time in the analysis to redirect the fluid flow and thus minimize the analysis time. The concept of gradient elution chromatography has been widely and successfully employed in high-performance liquid chromatography for the analysis of species with widely varying retention times (9). Gradient elution has been applied to the analysis of SOS2-,Sod2-, and Sz02- containing solutions by Sunden et al. (10) but no attention was given to the early eluting species such as F- or C1-. A potential limitation of the method of Sunden et al. is the necessity of a frequency generator to assist in the gradient production. The gradient elution techniques reported here provide a method for the more efficient analysis of complex solutions by allowing a larger number of species to be analyzed per 0003-2700/84/0356-1264$0 1.50/0
Table I. I!istrumental Conditions instruments
primary eluent (weak eluent) secondary eluents (strong eluents) conductance scale electrochemical detector chart recorder speed separator column suppressor column injection volume
Dionex Model 10 ion chromatograph, Dionex electrochemical detector, Perkin-Elmer Series 10 liquid chromatograph pump, Houston Instruments Omni-Scribe chart recorder 0.003 M NaHCO, 0.004 M Na,CO,, 0.002 M Na,CO, t 0.002 M NaOH 30 OSfull scale 0.4 V applied potential, 100 nA/V
full scale 0.5 cm/min 100-mm Dionex fast run anion column or 150 mm for Figure 7 Dionex fiber suppressor for anion analysis 0.10 mL
sample injection. The analysis time is slightly longer than normal but is still quite reasonable given the number of species which can be detected and measured. The techniques do not require the use of a frequency generator as in other methods nor do they require the intervention of the operator at a critical moment in the analysis in order to minimize time (3,10). The techniques also use materials and equipment already available for instruments similar to the one used in this work.
EXPERIMENTAL SECTION Apparatus. A modified Dionex Model 10 ion chromatograph was used for this work. Modifications of the ion chromatograph plumbing which facilitate this work have been reported previowly (11). This instrument comes with two LDC Mini-Pumps as standard equipment. In addition, a Perkin-Elmer Series 10 liquid chromatograph pump was used at various times during the research. The general instrument conditions are given in Table I. The arrangement of the components in the gradient elution ion chromatograph is shown in Figure 1. The mixing chamber consists of a 16 cm tallglass cylinder with a 3 cm internal diameter. The chamber has two outlets at the bottom: one for the connection to the pump, and the other for connection to a waste line to use for rinsing the chamber between chromatograms. The 100-mm separator column was prepared from a 250-mm fast run anion column (Dionex)by cutting the long column to the desired length and reattaching the fittings. No adverse affects from this cutting have been observed (3). An alternate method would be to use two 50-mm precolumns in series to obtain the desired length. The electrochemical detector was placed after the suppressor column instead of the customary position between the separator and the suppressor columns in order to minimize the base line upset due to the eluent gradient. Reagents. Stock solutions of 1000 ppm anion concentration were prepared from the sodium or potassium salts. The only exception was the sulfide solution which was prepared from a relatively old bottle of sodium sulfide which was partially liquified. The concentration of the sulfide ion was not determined for the sample injected. Aliquots of these stock solutions were diluted to the appropriate concentration necessary for the instrumental conditions selected. (The concentrations ranged from 2 ppm F0 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 8, JULY 1984 1265 1.2 r 1.1
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0.0
5
15
10
20 25 TIME IN MINUTES
40
Flgure 2. Two-pump gradient profiles illustrating changes in relative strength of eluents. Continuous line is for method 4 and broken line is for method 5. Flgure 1. Schematic diagram of gradient elution ion chromatograph: P I , primary pump; P2 and P3, secondary pumps; MC, mixing chamber; PG, pressure gage; IV, injection valve; SE, separator column; SU,
suppressor column; ED, electrochemical detector; CD, conductance detector; CR, chart recorder. to 50 ppm I-.) All solutions, including the ion chromatographic eluent, were prepared from distilled deionized water which was not purged of carbon dioxide absorbed from the atmosphere. Procedure. Four different gradients were investigated during the course of this work. The gradients involved the use of two different strong eluents introduced into the mixing chamber using two different pump configurations. The two strong eluents produced similar results for the same pump configurations but the two pump configurations produced different results. This difference indicates that the method of gradient production is of more importance than minor changes in the eluent concentration. Table I1 lists the different methods which were used to produce the gradients along with the single eluent conditions with which the gradients are compared. Two-PumpGradient System. The two-pump gradient system was produced in two different ways. In one method the Perkin-Elmer Series 10 liquid chromatograph pump was used as P1 and one of the LDC minipumps waa used aa P2 as shown in Figure 1. In later experiments, the two LDC minipumps were used to produce the gradients. No difference in the quality of the gradients was observed with the change in pumps. It was noticed that the flow rate of the minipumps was not as easy to control as was the Perkin-Elmer pump and had to be carefully calibrated by hand to ensure that the desired conditions were actually being obtained. The mixing chamber was filled with 0.003 M NaHC03 and P1 was started at a flow rate of 3 mL/min. After a stable base line was obtained, the chamber was adjusted to a volume of 60 mL. The sample was injected and P2 was turned on at a flow rate of 1.0 mL/min of the stronger eluent. The solution was stirred at all times with a magnetic stirrer to ensure rapid and efficient mixing of the two solutions. After the analysis was complete, P2 was turned off and the mixing chamber was thoroughly rinsed with several aliquots of the weak eluent. The column was then reequilibrated to the weak eluent prior to the injection of the next sample. A stable base line indicative of reequilibration was achieved in 5 min. The entire system volume of the gradient ion chromatograph used in this work was approximately 6 mL from the mixing chamber to the detector. (This volume can be altered easily by changing the length of tubing between the mixing chamber and Pl.) The 6-mL system volume implies a 2-min system volume. The volume of 6 mL was chosen because that corresponded to the lengths of prepared tubing available initially. It soon became apparent that 6 mL was a fortuitous choice since the fluoride and chloride peaks would then elute under essentially isocratic conditions and not be affected by the gradient with a concomitant loss of resolution. Three-Pump Gradient System. The three-pump gradient system provides for a more rapid gradient to be produced since the mixing chamber does not have to be filled with as large an
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0.6 0.5 0.4 I
1
0
5
10
15
20
25
30
35
40
45
TIME IN MINUTES
Flgure 3. Gradient profiles comparlng relative strengths of eluents with two-pump and three-pump gradients. Continuous line is for method 4 and broken line is for method 6.
initial volume of eluent as in the two-pump system. The initial mixing chamber volume was 25 mL and the primary pump was maintained at a flow rate of 3.0 mL/min. The two secondary pumps were used to produce the gradient. P2 pumped weak eluent into the mixing chamber in order to provide a continuous flow of eluent for the primary pump. P 3 pumped the strong eluent. The system was equilibrated with the weak eluent, the sample injected, and the system flushed at the end of the analysis as in the two-pump gradient system. In the three-pump system, the Ferkin-Elmer pump served as P1 and the two minipumps served as P2 and P3. One advantage of the three-pump gradient system is that the analysis is not time limited due to the initial volume of the eluent reservoir. In the three-pump system the flow into the mixing chamber is equal to the flow out of the mixing chamber. In the two-pump system, the flow out is 2.0 mL/min more than the flow in, resulting in a practical limit of 30 min on the length of analysis under these conditions. Gradient Profiles. Gradient profiles obtained in the following manner were determined in order to understand how the production of the gradient would affect the resulting separation of the anions. The eluent from the exit end of the separator column was directed to a flow-through pH electrode assembly. The suppressor column had to be bypassed for the pH detector since the suppressor would compensate for any pH changes due to the introduction of the strong eluent. The electrode assembly consisted of a pH combination electrode placed inside the barrel of a 6-mL disposable syringe. The connection was made with a female luer adapter. The solution was pumped through the assembly from bottom to top to prevent air bubbles from disrupting the measurements. Experiments indicated that the pH detector responded quickly to changes in the eluent. Figures 2 and 3 show comparative gradient profiles. The concentration ratios were calculated from the measured pH of the effluentfrom the column in order to compensate for the carbon dioxide absorbed from the air. Figure 2 shows the profile for the two strong eluents used in the two-pump gradient system, The
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 8, JULY 1984
Table 11. Solutions and Flow Rates Used for Production of Gradients pump la s o ht io n
method
4 5
0.003 M NaHCO, 0.004 M Na,CO, 0.002 M Na,CO, + 0.002 M NaOH 0.003 M NaHCO, 0.003 M NaHCO,
3.0 3.0
6 7
0.003 M NaHCO, 0.003 M NaHCO,
3.0 3.0
1 2 3
a
See Figure 1.
pump 2a solution
flow rateb
flow rateb
pump 3 a solution
flow rateb
3.0 3.0 3.0 0.004 M Na,CO, 0.002 M Na,CO, + 0.002 M NaOH 0.003 M NaHCO, 0.003 M NaHCO,
1.0 1.0
2.0 2.0
0.004 M Na,CO, 0.002 M Na,CO, t 0.002 M NaOH
1.0 1.0
mL/min.
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D 2 4 6 8 10 12 14 16 16 20 22 24 26 20 30 32 34 36 38 TIME IN MINUTES
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Figure 4. Chromatograms of halide containing solution with weak and strong eluents Only: A, method 1; B, method 2. Peak identification: 1, 4 ppm F-; 2, 8 ppm CI-; 3, 10 ppm Br-; 4, 50 ppm I-.
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Flgure 5. Chromatograms of thiosulfate containing solution with weak and strong eluents only: A, method 1; B, method 2. Peak identification: 1, 2 ppm F-; 2, 3 ppm CI-; 3, 20 ppm Sot-; 4,40 ppm S2032-.S2032was retained on the column in method 1.
0.004 M Na2C03gradient produces a faster change in the eluent strength as measured by the [CO,2-]/[HCO
