1617
Anal. Chem. 1983, 5 5 , 1617-1619
tween pump strokes. This is done eitber by changing the length of the resistance element or by altering the amount of air in the capacitor. The values for these parameters can be calculated as follows. The resistance to fluid flow in a cylindrical tube is proportional to the length and inversely proportional to the square of the radius of the tube. This is expressed in the HagenPoiseuille equation (5:)as
ar4 8P.L
U=---ap
(1)
where U is the volume flow rate of fluid, r is the radius of the tube, p is the dynamic viscosity, L is the length of the tube, and AP is the pressurle drop across the length of the tube. The capacitance of the fiiter is formulaited by using the ideal gas law. The instantaneous pressure in the syringe is equal to Pi
=
Po vo
d Vi dt
r i l l I
0
2
3 t (SI
I
I
I
I
5
Figure 4. Instantaneous volume vs. time calculated for: (A) pentane, (C) methanol, (D) water, and (E) 2-propanol. Open
(e) acetonltrlle,
v, + v, - vi
trlangles represent values determined for water in the present system.
where Vo is the amount of air initially in the syringe a t pressure Po (the capacitance), V, is the volume of the pump stroke, and Vi is the volume of fluid that has been drawn through the resistance tube. Atmospheric pressure less this quantity is the pressure drop AP of eq :l.This substitution allows us to write an expression for the amount of liquid flowing through the fillter a t any time ~
E //
106~r4Vo 1oeTr4 + 8pL( Vo + V, - Vi) 8 j L
of viscosities commonly encountered are included in the figure. Also included is the observed response of the filter with distilled water. It is clear that eq 5 accurately predicts the behavior of the device. In order to design a filter for use with any pump, the user first determines the time between pump strokes of volume V,. The value for p is that of the most viscous solvent used. Of the parameters V, and L , one is chosen and the other calculated from eq 6 or 7.
I-
I-
(3)
Rearrangement and substitution yield
(4) where A = -106ar4/8pL. Integration of eq 4 gives an expression for the time required to pass a given volume of liquid, Vi, through the filter lJ7i
v,
t = l d t = - A f A-[[In
ACKNOWLEDGMENT We thank B. D. Johnson for helpful discussions on the fluid mechanics of the system and W. D. Jamieson, S. Whiteway, E. Lewis, and E. C. V. Butler for helpful comments on the manuscript.
L Pi
P O
AP
(V, - Vi) - In V,]
(5)
r
t
U
Solving for L and Vo gives
Vi VO VP P
and
GLOSSARY length, cm instantaneous pressure, g cm-I s - ~ initial pressure, g cm-l s+ pressure drop, g cm-I s - ~ tube radius, cm time, s volume flow rate of fluid, cm3 s-l instantaneous volume of fluid, cm3 initial volume of air, cm3 volume of pump stroke, cm3 dynamic viscosity, g cm-l s-l LITERATURE CITED
(7) where B = In (V, - Vi) - In Vp. Figure 4 shows the rlesponse of the present filter ( L = 25, V, = 0.139 and Vo = 0.5) as calculated from eq 5. Solvents often used in liquid chromatography representing the range
(1) Bedard, P.; Purdy, W. C. Anal. Lett. 1983, 16, 149-158. (2) Brady, J. E.; Carr, P. W. J. Chem. Educ. 1983, 60,83. (3) Billiet, H. A. 14.; Keehnen, P. D. M.; De Galan, L. J . Chromatogr. 1979, 185, 515-528. (4) Saunders, D. L. J. Chromatogr. Sci. 1977, 75, 129-136. (5) Brodkey, R. S. “The Phenomena of Fluid Motions”, 2nd ed.; AddisonWesley: Reading, MA, 1967; Chapter 9.
RECEIVED for review February 23,1983. Accepted May 9,1983.
Ion Chromatographic Deterrnination of Fluorine, Chlorine, Bromine, and Iodine with Sequential Electirochemical and Conductometric Detection Chung-Yu Wang, Scott D. Bunday, alnd James C. Tartar* Department of Chemistry, North Texas State University, Denton, ‘Texas 76203
Ion chromatography (IC) as developed by Small et al. ( I ) has proven to bo a very useful technique for determinatiosn
of inorganic anions at the parts-per-million and sub-partper-million range. Applications and limitations of IC with
00O3-27O0/83/0355-1617$01.50/00 1983 American Chemical Society
1618 0 ANALYTICAL CHEMISTRY, VOL. 55, NO. 9, AUGUST 1983
Table I. Ion Chromatographic Parameters eluent
0.003 mol/L NaHCO, and 0.0024 mol/L Na,CO,
flow rate precolumn separator column
156 mL/h 50 mm fast run 100 mm fast run 150 mm fast run 100 mm anion suppressor column 100 pL4 Ag 0.2 v 300 nA/V or 1 pAIV
suppressor column injection volume working electrode working potential electrochemical Pull scale conductance full scale standard stock solution
Was
1 0 pmho 3 ppm Cl-, 3 ppm F-,8 ppm Br-, 20 ppm SO,*-, 10 ppm I-, 20 ppm SCN-
conductometric detection have been documented by others (2,3). In general, large polarizable anions such as I- and SCNhave strong affiiities for anion exchange resin and elute slowly with the normal instrumental conditions used in ion chromatography. The conventional way to separate and detect these large anions is to increase the eluent strength and/or use shorter columns, which often results in the loss of resolution of the faster eluting species. Other methods involving I-and/or SCN- use different anion exchange resins such as brine anion separator columns (Dionex), silica-coated polyamid low-capacity XAD-1 resin columns crown resin columns (4), (5,6),or pellicular silica based columns (7). Again resolution is frequently lost or severely reduced for the faster eluting species. Other techniques involve the measurement of the absorbance of these anions by UV detection a t 205-215 nm (8, 9). Iiowever, for samples containing both strongly and weakly retained species, these separations and detections need additional lab work and prolong the time required to perform the analysis. The electrochemical detector is a powerful instrument for the measurement of easily oxidized species. The silver working electrode can detect C1-, Br-, I-, CN-, SCN-, Sz032-and S2at different selective potentials without the need of a suppressor column. This report describes a method for combining conductometric and electrochemical detection to determine halides, sulfate, and thiocyanate quantitatively in one injection. Ion chromatography has been shown to work well for different combinations of the halides but the determination of all four halides in one sample injection has been difficult in the past. Bond et al. (10) analyzed sulfide and cyanide, Rocklin and Johnson (11) analyzed sulfide, iodide, and bromide. Both groups used electrochemical detection. Colaruotolo and Eddy (12) used conductometric detection to detect chloride and bromide. The optimum analytical conditions for all four halides have not been previously reported. The technique described in this paper provides a mechanism for such analyses to be performed routinely.
EXPERIMENTAL SECTION Apparatus. The ion chromatograph used in these experiments was a Dionex Ion Chromatograph 10 equipped with standard conductance detector and with the optional Dionex electrochemical detector. It is often the case that electrochemical detectors may be more difficult to use than conductivity detectors; however, both detectors have been in use for over 9 months following the general precautions outlined by the manufacturer and have needed no maintenance or repairs. A Dionex fast run anion analysis column kit was obtained to perform the separation of the species. The 250 mm separator column which comes with the column kit was cut into two pieces with a razor blade to form two individual columns of 100 mm and 150 mm length. The cutting of the column into two sections did not alter the response of the column. Chromatogramsof solutions made before and after
Figure 1. Schematic diagram of ion chromatograph component arrangement: P, pump; VI, separator system valve; Vp, suppressor system value; C1, 50 mm anion separator column; C2, 100 mm anion separator column; CB, 150 mm anion separator column; C,, anion suppressor column; D1, electrochemical detector; D,, conductometric
detector; R, chart recorder (injection valve not shown). cutting were identical with respect to retention times, peak heights, and peak shapes. A dual channel-dual pen chart recorder was used to record the chromatograms (Houston Instruments). Table I lists the operating conditions of the ion chromatograph during the experimental work. Reagents. A solution containing F-, C1-, Br-, Sod2-, I-, and SCN- was prepared from reagent grade chemicals. All solutions, including the ion chromatographic eluent, were prepared from distilled-deionized water. Table I lists the concentrations of the anions in the standard stock solution. Procedure. Initial chromatographic work was carried out by using conductometric detection to ascertain the retention times for the species of interest for different column lengths. The location and size of the three separator column components were optimized to provide the best possible resolution with symmetric peak shapes in a minium of time. Figure 1shows the schematic of the chromatographic system. It was determined that F-, C1-, Br-, and SO-: would have completely traversed the entire system length before the I- or SCN- had passed through 150 mm of separator column. The electrochemical detector should be located after the 50 mm separator column to minimize peak broadening and tailing. It was determined experimentally that the sulfate peak was totally eluted in approximately 12 min and that the iodide peak began to elute from the 50 mm plus 100 mm components at approximately 13 min. The analytical procedure was as follows. The 100-pL sample was injected and the injection time was noted. Immediately after the sulfate peak from the conductometric detector reached base line the conductometric detector was shut off and the valve V2 was switched to waste. The system was allowed to run in this mode for 20 min to allow time for all I- and SCN- to have been completely flushed from the system. After this 20-min period, the valve Vz was switched back on line and after 1 min the conductometric detector was turned on. Reequilibration time was less than 5 min. It should be noted the system as described in Figure 1 now contains a separator column component in what is normally a suppressor column system. The separator column should be removed before regeneration of the suppressor. Removing this separator from the suppressor system requires less than 5 min and can be done without the use of tools since all column fittings on this instrument are hand-tightened plastic fittings. Since the
ANALYTICAL CHEMISTRY, VOL. 55, NO. 9, AUGUST 1983
1619
Table 11. Relative Peak Heights of Anions anions concn, ppm peak height range, mm mean height, mm no. of injections % RSD mean retention time, mm 2
I
Br
F3 111-116 113
9 1.6 3.2
conductivity detector c1Br3 8 64.5-68 39-47 67 44 9 9 2.3 6.3 4.2 7.4
Br
50,220 150-153 152 9 0.8 9.6
Table 111. Varying Dilutions of the Stock Solution
anion
I?c1Br-
so,*-
Br' 1SCNa Off scale.
0 4 8 12 L-J---Q+-
16 36 40 44 4 8 52 RETENTION TIME (MINUTES)
Figure 2. Typical chromatogram of standard stock solution (seeTable
I) showing two sample Injections.
suppressor column is useld less than 50% of the time, suppressor use between regeneration is extended to over 20 h without problem. Care should also be taken when switching valve V2 to waste to ensure that the sulfate peak has reached base line. There is a pressure drop of about 200 psig when the valve is switched to waste and this drop could cause changes in the chromatographic peak shapes and retention times. The electrochemicaldotector responds to Cl-, Br-, I-, and SCNin our system. Because CY can be measured more accurately and sensitively with the conductometric detector, it was not included in the electrochemical d,ata. The Br- ion can be detected easily by either detector and the choice may rest orr the presence of other interfering peaks not inlcluded in this study. A very high concentration nitrate ion peak may cause slight overlapping interference with a low concentration bromide ion peak using the conductivitiy detector. :No nitrate interference is seen with the electrochemical detector since the nitrate ion does not respond to this type of detector.
RESULTS AND DISCUSSION The six-component Eitandard stock solution was analyzed for system stability and reproducibility. A representative chromatogram is shown in Figure 2. The Rolution was injected throughout the course of the day, and the results of the data analysis are shown in Table 11. These results indicate that there are no adverse instrumental effects from the off/on cycling of the suppressor column as a reeiult of the switching of value V2 The average time between injections for the nine injection series was 38 min. The experiment was repeated on a different day with no significant deviation in the results. The concentrations of individual anions in the stock solution were then increased one a t a time by varying factors. F- and
electrochemical detector 1SCN8 10 20 123-140 79-81.5 36-41 132 80 39 9 9 9 4.1 1.2 4.3 1.2 5.9 8.6
Br-
first dilution concn, S/N ppm ratio Conductivity 46 0.3 37 0.8 17 2.0 71
second dilution concn, SIN ppm ratio 0.03 0.03 0.08 0.20
12
Electrochemical 0.8 141 0.08 1.0 33 0.10 2.0 28 0.20
9 6 6
0.3
34 a
3
C1- were each increased by a factor of 20, Br- by a factor of 15, and Sod2by a factor of 10. The four-component analyte solutions were then run through the conductivity detector. The results revealed minimal overlapping. The same procedure was then followed for the electrochemically detected anions I- and SCN-. Each was increased by a factor of 10 relative to the other and then the chromatograms were run. The results revealed partial overlap, but both anions could still be quantified with correct standard preparation. This experiment showed that any one of the six anions in the stock solution could be present in a t least 10-fold excess over the others with little impairment of the results. In order to verify the measurement of lower concentrations by this technique, the standard stock solution was diluted by a factor of 10 and by a factor of 100. The 10-fold dilution was as consistent a8 the original stock solution. The only trouble that evolved from the 100-fold dilution was that of the water dip and background. In order to get a clear reading of the 100-fold dilution and decreased the noise, 1mL of l 0 X eluent solution was added per 100 mL of analyte solution thereby minimizing the water dip. The resulb are shown in Table 111.
LITERATURE CITED (1) Small, H.; Stevens, T. S.; Bauman, W. C. Anal. Chem. 1975, 4 7 , 1801-1809. (2) Smith, F. C., Jr.; Chang, R. CRC Crlt. Rev. Anal. Chem. 1980, 9 (3), 197-217. (3) Mulik, J. D.; Sawicki, E. Environ. Sci. Technol. 1979, 73,804-809. (4) Igawa, M.; Baito, K.; Tsukamoto, J.; Tanaka, M. Anal. Chem. 1981, 53, 1942-1944. (5) Gjerde, D. T.; Frltz, J. S.; Schmuckler, G. d . Chromatogr. 1079, 786, 509-5 19. (6) Qjerde, D. T.; Frltz, J. S.:Schmuckler, G. J . Chromatogr. 1980, 187, 35-45. (7) Pohl, C. A.; ,Johnson, E. L. J . Chromatogr. Sci. 1980, 78,447-452. (8) Skelly, N. E. Anal. Chem. 1982, 5 4 , 712-715. (9) Gassldy, R. M.; Elchuk, S. Anal. Chem. 1982, 5 4 , 1558-1583. (10) Bond, A. M.; Herltage, I. D.; Wallace, G. G.; McCormlck, M. J. Anal. Chem. 1982, 5 4 , 582-585. (11) Brocklln, R. D.; Johnson, E. L. Anal. Chem. 1083, 55, 4-7. (12) Colaruotolo, J. F.; Eddy, R. S. Anal. Chem. 1977, 4 9 , 884-885.
RECEIVED for review January 10, 1983. Accepted April 21, 1983. This work was funded by a grant from the North Texas State University Faculty Research Fund.
