"Analysis of Inorganic Anions in Potomac Water, Sediment and Floc by Ion Chromatography" FINAL REPORT. by James E. Girard, Ph.D.

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2 DC WRRC Report No. 37 "Analysis of Inorganic Anions in Potomac Water, Sediment and Floc by Ion Chromatography" FINAL REPORT by James E. Girard, Ph.D. The work upon which this publication is based was supported by, the D.C. Water Resources Research Center with the funds provided in part by the Office of Water Research and Technology, U.S. Department of the Interior, Washington, D.C. as authorized by the Water Research and Development Act of 1978 (PL ).

3 TABLE OF CONTENTS Abstract Introduction Experimental Results and Discussion Conclusion Acknowledgment References Tables List of Figures Figures

4 ABSTRACT A new single column ion chromatography (SCIC) system for the analysis of inorganic anions has been described. The factors controlling resolution and detectability limits have been investigated. This SCIC system is much simpler than the system it replaces. Although the limits of detectability are not as low as the dual column system, they are adequate (-1PPM) for work with natural samples. This technique has been successfully applied to the analysis of municipal water samples from Howard, Baltimore and Montgomery Counties in Maryland, as well as the District of Columbia. Samples of Potomac River water taken at the Chesapeake Bay/Potomac River interface are usually very difficult to analyze, since there is a small amount (10-15 PPM) of Bromide Ion in the presence of a large amount (^-1200 PPM) of chloride ion. These Potomac samples were successfully analyzed. A no cost extension has been granted until April, We intend to demonstrate the usefulness of this technique for the analysis of Potomac River/Chesapeake Bay samples. One publication has already resulted from this work, and it is attached. Contents of this publication does not reflect the views and policies of the United States Department of the Interior, Office of Water Research and Technology, nor does mention of trade names or commercial products constitute their endorsement or recommendation for use by the United States Government.

5 INTRODUCTION Since its introduction in 1975, (1) suppressed ion chromatography has given the analytical chemist a rapid and reliable method for the determination and quantitization of mixtures of inorganic anions. Ion chromatography as first developed was a combination of ion exchange, eluent suppression, and conductimetric detection. The state of the art has recently been reviewed by Pohl and Johnson (2). The Dionex Ion Chromatograph, a commercially available unit employing the suppressed ion chromatography concepts, has been widely accepted. The interested reader will find many applications already developed (3) especially in the environmental area. The Dionex Ion Chromatograph is essentially a low pressure, high performance liquid chromatograph. The anions are separated on a low capacity ion exchange column using a proprietary anion exchange resin. Solutions of NaHCO 3, Na 2 CO 3 and NaOH in the' M range are the eluents most commonly used in the Dionex system. During separation, the eluent anions and the sample anions compete for the active sites on the ion exchange resin. After separation, the anions flow through a hydrogen-form cation exchange column (suppressor column) which removes most of the background conductance. This conversion of the sample anions to their acid analogs, and eluent anions into species with low conductance, either H 2 O or H 2 C0 3, enhances their detectability by a conductivity detector. Electrical conductance is, in general, a simple function of ionic concentration, especially in the low concentration easily handled by ion chromatography (l0ug/l to lmg/l). There are two major disadvantages to the Dionex system. First, the

6 suppressor column must be regenerated after five to fifteen hours of operation to, remove the ions accumulated from the eluent stream. Second, the eluent must be a base so that after neutralization by the suppressor column it will have a low conductance. Recently, two relatively simple, non-suppressed ion chromatography systems have been developed by Fritz and co-workers (4,5) and Harrison and Burge (6). Rather than the Dionex system which uses a suppressor column to lower the background conductance, these two systems utilize a specially designed conductivity detector (7). This detector was designed to minimize the effects of temperature variation while also allowing partial conductivity of the eluent without undue background noise or drift. The circuit and cell design permit up to 100 times full-scale suppression without significant drift due to temperature variation, and detection of conductivity changes of one part in 5,000 per minute. Each system has its own ion exchange column. Harrison's (Vydac 302 I.C.) column uses a low capacity (0.1 mequiv./g) ion exchange material. This material is produced by chemically bonding to a silica substrate the proper functionalities. This results in a thin coating of ion exchange material surrounding the silica particles. The silica particles are a spheroidal shape with high mechanical strength, an average surface area of 86 m 2 /g, and an average pore diameter of 330. The Vydac 302 I.C. is commercially available. Fritz and co-workers prepare their column material using macroporous polystyrenedivinylbenzene copolymers (Rohm and Haas, XAD-1,2,4) as their starting material. The starting resin is chloromethylated

7 following a procedure by Goldstein and Schmuckler (8). Next, the resin is aminated by adding trimethylamine. The capacity of the resin can be controlled by varying the time of chloromethylation. Capacities ranging from 0.04 to 1.46 mequiv./g were obtained. Presently, this resin is not commercially available. The eluent anion used, in both these systems, contains a benzene ring in its structure, phthalate being commonly used. In order to effectively separate the sample anions, the eluent anion must be sufficiently attracted to the ion exchange material so that a very low concentration of the eluent anion will move the anions to be separated through the column. However, the eluent's conductance must also be very low so that the detector's signal due to the sample anions is significantly above the background. The concentration of the eluent also plays an important role in the detectability and resolution of the sample anions. This concentration will depend upon the capacity of the ion exchange resin; a low capacity resin will require a lower concentration of eluent anions to effectively compete with and move the sample anions down the column. The ph of the eluent will also determine its ability to separate the sample anions. Since we are dealing with a weak organic acid, for instance, o-phthalic acid, the ph of the solution will determine its ionic strength and eluting ability. During this project we investigated non-suppressed ion chromatography with the Vydac 302 I.C. column using the three phthalic acid isomers as eluents. EXPERIMENTAL Apparatus The chromatography system used is shown in Figure 1. The injection valve was

8 equipped with a 100u1 calibrated injection loop. The column used for all separations was a Vydac 302 I.C. 4.6, maintained at a temperature of 30 0 C. The conductivity cell was positioned next to the column block heater and it was insulated with styrofoam insulation. Reagents The eluents used for the separations were prepared from analytical reagent grade phthalic acids dissolved in conductivity grade water. Eluent concentrations were varied from 5 x 10-3 M to 1 x 10-4 M. The ph was varied from ph 4.0 to ph 5.4 using 0.1 M KOH solution. The fraction of divalent, monovalent, and unionized acid as a function of ph are shown in Table 1. When changing eluent species, at least one liter of solution was allowed to flow through the column to ensure complete exchange equilibrium. Standard solutions of 5, 50 and 500 ppm of the various anions were prepared from analytical grade reagents and conductivity water. Solutions were then diluted as necessary. Mixed standard solutions were also prepared from the individual solutions.. Potassium salts were used in all cases except for CO 3, which was prepared from the sodium salt. Real municipal water samples and the Potomac River pore water sample were filtered using a 0.45p Millipore filter prior to injection. RESULTS AND DISCUSSION Table 2 lists the retention times, detectability range, and co efficient of correlation (R) of the least squares analysis for a plot of peak height vs. concentration for the various anions. In all cases, the operating conditions were kept constant and are listed in Table 3. Due to limits of solubility for isophthalic and terephthalic acid, the ranges of concentration do not fully overlap.

9 Some general trends are evident. As the concentration of the eluent decreases, at a constant ph, the detection limits tend to improve, due mainly to the decrease in background conductivity and increase in detector response. As the ph increases, the detection limits appear to reach a minimum around ph 5.0. This can be attributed to the relationship between the increase in background conductivity and the decrease in retention time resulting in much sharper peak shapes. Using peak height measurements, the linearity of the calibration curves is very good over the ranges analyzed, supporting the theory that electrical conductance is a simple function of ionic concentration. o-phthalic acid proved to be a very good eluent for non-suppressed ion chromatography. It is the most soluble isomer of the benzenedicarboxylic acids. Dissolution of the solid acid is easily performed, and allows a wider range of concentrations than the other isomers. As phthalate concentration decreases at constant ph, the retention times increase. As the amount of phthalate ion decreases, competition between the eluting anion and the sample anion for the active sites on the ion exchange material decreases, as can be seen in Figure 2. This increase in retention time tends to degrade peak resolution and increase the detection limit for strongly retained anions such as S0 4. For anions that easily elute, such as Cl - and N0 3, decreasing phthalic concentration also increases retention time but does not degrade resolution to any great degree. Generally, detector response increases, asphthalate concentration decreases, due to the decrease in background conductivity. For the monovalent anions, retention times decrease as the ph increases, at constant phthalate concentration, but not to the extent observed-for the. divalent SO 4.

10 Referring to Table 1, the fraction of o-phthalic acid existing as a divalent anion increases as the ph increases. It is this divalent anion that appears to compete with the sample anions and causes their elution from the ion exchange material. Figure 3 shows the relationship between SO 4 retention time and o-phthalic species' fraction, 1 x 10-3 M. The fraction of divalent o-phthalic acid (), increases as the retention times decrease. Figure 4 shows the relationship between retention time and ph for S04, using 1 x 10-3 M o-phthalic acid as the eluent. Above ph 5.0, the retention time for sulfate decreases rapidly. The other polyvalent anions analyzed do not show any correlation between ph other polyvalent anions analyzed do not show any correlation between ph and retention time. This suggests that o-phthalic acid is not an effective eluent for these ions that are weakly conducting and strongly retained by the ion exchange resin. At ph 5.4, the retention times for Cl and NO - 3 decrease to such an extent that they co-elute with either the pseudo peak or the H 2 O valley. Since retention time appears to exponentially decrease as ph increases, the ph must be carefully chosen and controlled to ensure reproducible retention times and also to not obscure the easily eluted anions during the chromatographic process. The pseudo peak is a common feature of non-suppressed ion chromatography. It is the first peak and is related to the total salt concentration in the volume injected. When a sample is injected onto the column, anions present in the sample replace the eluent anions which are occupying active sites at the top of the column. The displaced eluent ions, together with sample cations, move with the solvent front to the conductivity detector.

11 If the concentration of these cations and anions is such that the conductance is greater than that of the eluent background, there will be a positive peak. If the conductance is less than the eluent background, a negative peak will result. Even though detector response increases with decreasing concentration and ph, the optimum o-phthalic acid eluent appears to be 1 x 10-3 M at ph 5.0. Isophthalic acid is much less soluble than o-phthalic acid, 0.013g/ l00g H 2 O versus 0.548/l00g H 2 0. For this reason, solution preparation is much more difficult. Solutions have to be heated to approximately 80 0 C to effect dissolution. Referring to Table 1, isophthalic acid is more divalently ionized at a lower ph than o-phthalic acid and, therefore, should be a more effective eluent. This proved to be true. At a concentration of 1 x 10-3 M and ph 4.6, SO 4 elutes in 9.06 minutes. When using o-phthalic acid at the same concentration and ph, SO 4 elutes in minutes. The background conductivity of isophthalic acid is slightly higher at a comparable concentration causing the detection limits to increase but are still at a very acceptable level. As the concentration of isophthalic acid decreases, the retention times increase but at a slower rate than for o-phthalic acid. Even at a concentration of 5 x 10-4 M, ph 5.4, SO 4 has a retention time of 10.0 minutes. As the ph increases, at constant isophthalic concentration, the retention times decrease. Isophthalic acid is so strong an eluent that Cl - and NO 3 - are easily lost in the pseudo peak or H 2 O valley, at any ph value above 4.6, at a concentration above 5 x 10-4 M. A concentration of 1 x 10-4 M produces an extremely effective eluent for Cl - and N The detection limits at ph 4.6 are 0.25 PPM for C1 - and 0.5 PPM for N

12 However, S0 4 is so strongly retained that over 70 minutes is required for it to elute from the column and then it produces a very._ broad and shallow peak. It appears that the optimum concentration for isophthalic acid is between 5 x 10-4 M and 1 x 10-4 M. Figure 5 shows the relationship between 80 4 retention time and isophthalic species' fractions al, and Q2 at l x 10-3 M and 5 x 10-4 M. The same relationship is also seen for C1 - and N0-3. Terephthalic acid is the least soluble of the three isomers, g/100g H 2 0. Since it is so insoluble, a concentration of 1 x 10-4 M is about the maximum useful concentration. Again, heat is necessary to effect dissolution of the acid. At this concentration, SO 4 is too strongly retained (>70 minutes) to produce an acceptable peak. Increasing ph decreases the detector's response and also increases the detection limits. Similar elution behavior for C1 - and NO - 3 was found using terephthalic acid, as can be seen in Figure 6. However, due to its insolubility, terephthalic acid does not appear to be an effective eluent for non-suppressed ion chromatography. Several separations were performed on real samples and prepared mixtures. The mixtures were used to demonstrate certain characteristics associated with changes in concentration and ph among the various eluents used. A comparison of Figures 7, 8 and 9 will show the effect on retention times and sensitivity for respective changes in eluent concentration and ph.

13 Figure 10 shows a separation for seven anions at various concentrations using isophthalic acid at 2.5 x 10-4 M, ph 4.6. Even though we do not have full baseline - separation, each peak-is clearly discernable: Total analysis time takes less than 30 minutes. Detector response is very good considering the recorder setting of 5 mv full scale. This chromatogram also shows the difference in selectivity for isophthalic vs. o- phthalic acid. Using o-phthalic acid as an eluent, CN always elutes late in the chromatogram after S0 4, regardless of ph. However, here using isophthalic acid, 2.5 x 10-4 M, ph 4.6, CN elutes very early, even before Cl. It appears that isophthalic acid discriminates more fully between the easily eluted monovalent anions creating better separation. Several real samples were analyzed by non-suppressed ion chromatography to demonstrate its practical importance for environmental studies. In general, the baseline noise was much higher than for the synthetic mixtures. Figure 11 shows a separation for a Potomac River pore water sample using o-phthalic acid, 2 x 10-3 M, ph 4.0. A recorder setting of 10 mv was necessary to keep the Cl peak on scale. Even so, 14.3 PPM Br is easily seen in the presence of PPM Cl -. The low ph was necessary to separate Cl, Br and NO 3 from each other yet it also provides extremely good detector response. This increase in detector response is responsible for the SO 4 peak being as symmetrical as it is, with a retention time of 20 minutes. Six municipal water supplies were analyzed using o-phthalic acid, 1 x 10-3 M, ph 5.0. Figure 12 shows the separation of one of them with a low level of N03. Table 4 lists the concentrations of the various anions in the samples.

14 The calibration curves for this series of separations were very linear. Coefficients of correlation obtained were Cl -, ; N03 -, ; and S0 4, CONCLUSION Non-suppressed ion chromatography offers a new inexpensive method to practitioners of ion chromatography. Since a suppressor column is not necessary for this method, the time required to regenerate the suppressor column is saved. Sensitivity for non-suppressed ion chromatography is comparable, but not as low as the suppressed mode. For non-suppressed ion chromatography, the sensitivity for Cl - is 0.25 PPM, for NO 3 - is 0.50 PPM and for SO 4 is 1.25 PPM. ACKNOWLEDGEMENT We wish to thank the Electrical Power Research Institute and the Department of the Interior, Office of Water Resource Technology for their support.

15 REFERENCES 1. H. Small, T. S. Stevens and W. C. Bauman, Anal. Chem., 47, 1801 (1975). 2. C. A. Pohl and E. L. Johnson, J. Chromatogr. Sci., 18, 442 (1980). 3. Ion Chromatographic Analysis of Environmental Pollutants, J. D. Mulik and E. Sawicki, eds., Ann Arbor Science Publishers, Ann Arbor, Michigan, Volume I, 1978, Volume II, D. T. Gjerde, J. S. Fritz and G. Schmuckler, J. Chromatography, 186, 509 (1979). 5. D. T. Gjerde and J. S. Fritz, J. Chromatography, 176, 199 (1979). 6. K. Harrison and D. Burge, Paper presented at Pittsburg Conference on Applied Spectroscopy, 4301, Cleveland, Ohio, March Wescan Model 213 Conductivity Detector, Wescan Instrument Company, Santa Clara, California. 8. S. Goldstein and G. Schmuckler, Ion Exch. Member., 1, 135 (1973).

16 TABLE 1 Species' Fractions Of The Phthalic Acid Isomers ph o-phthalic Isophthalic Terephthalic Species Acid Acid Acid K l =1.1x10 3 K 1 =3.3x10-4 K 1 =3.1x10-4 K 2 =5.5x10 6 K 2 =3.2x10-5 K 2 =1.5x = unionized acid 2 = monovalent acid 3 = divalent acid

17 TABLE 2 Retention Times, Detection Range and Linearity For The Anions Analyzed Eluent o-phthalic Acid 5 x 10-3 M ph 4.6 Anion Retention Time Minutes Detection Range,PPM C R NO CN C0 3 Cr0 4-3 P ph 5.0 C NO CN S0 4 Cr0 4 C ph 5.4 P CN S0 4 Cr0 4 CO 3-3 P

18 TABLE 2 - continued Eluent Retention Time Detection Anion Minutes Range, PPM R o-phthalic Acid 1 x 10-3 M C ph 4.6 NO 3 - S ph 5.0 C NO CN CO ph 5.4 o-phthalic Acid 5x10 4 M ph 5.0 S Cl NO Potassium Hydrogen Phthalate 1x10-3 M S ph 5.0 Cl NO 3 - S

19 TABLE 2 - continued Eluent Anion Retention Time Minutes Detection Range, PPM R Isophthalic Acid 1 x 10 3 M ph 4.6 C N ph 5.0 S0 4 ph 5.4 S Isophthalic Acid 1 x 10-4 M ph 4.6 C NO 3 S0 4 ph 5.0 N0 3 - S ph 5.4 S Isophthalic Acid 1 x 10-4 M ph 4.6 Cl NO ph 5.0 Cl NO ph 5.4 Cl NO

20 TABLE 2 - continued Eluent Terephthalic Acid 1x10-4 M Anion Retention Time Minutes Detection Range, PPM R ph 4.6 C NO ph 5.0 C NO ph 5.4 C NO

21 TABLE 3 Standard Operating Conditions Column: Vydac 302 I.C. 4.6 Column Temperature: 30ºC Flow Rate: 2.0 cc/min. Injection Volume: 100uL Recorder: 2mV full scale, 0.10 inches/min.

22 TABLE 4 Analysis Of Municipal Water Supplies Municipality Concentration, PPM Cl - NO 3 - SO 4 District of Columbia Prince George's County, MD Baltimore County, MD Howard County, MD Fairfax County, VA

23 LIST OF FIGURES Figure Description 1. Chromatographic Equipment 2. Eluent Concentration vs. SO 4 Retention Time, o-phthalic Acid, ph S0 4 Retention Time vs. o-phthalic Acid Species Fractions, 1 x 10-3 M 4. S04 Retention Time vs. ph, o-phthalic Acid, 1x 10-3 M 5. SO4 Retention Time vs. Isophthalic Acid Species; Fractions, 1 x 10-3 M 6. C1 - and NO 3 Retention Times vs. Terephthalic Acid Species Fractions, 1 x 10-4 M 7. Separation of Cl -, N0 3 -, S04 and CN, o-phthalic Acid, 5 x 10-3 M, ph 4.6, 2.0 cc/min., Recorder at 2 mv Full Scale 8. Separation of Cl -, NO 3, and SO 4, o-phthalic Acid, 1 x 10-3 M, ph 4.6, 2.0 cc/min., Recorder at 2 mv Full Scale

24 Description 9. Separation of CI -, N0 3 -, and S04, o-phthalic Acid, 1 x 10-3 M, ph 5.0, 2.0 cc/min., Recorder at 2 mv Full Scale 10. Separation of 7 Anions, Isophthalic Acid, 2.5 x M, ph 4.6, 2.0 cc/min., Recorder at 5 mv Full Scale 11. Separation of Potomac River Pore Water, o-phthalic Acid, 2 x 10-3 M, ph 4.0, 2.0 cc/min, Recorder at 10 mv Full Scale 12. Separation of Prince George's County, Maryland municipal water, o-phthalic Acid, 1 x 10-3 M, ph 5.0, 2.0 cc/min., Recorder at 2 mv Full Scale 21

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26 x10-3 1x10-3 5x10-4 Figure 2. SO 4 retention time vs. eluent concentration, o-phthalic acid, ph

27 42 oo Minutes Species' Fraction Figure 3. SO 4 retention time vs. o-phthalic acid-species' fraction oo l, monovalent, and a 2, divalent; 1 x 10-3 M 24

28 ph 6 Figure 4. SO 4 retention time vs. ph, o-phthalic acid, 1 x 10-3 M 25

29 Species' Fraction Figure 5. SO 4 retention time vs. isophthalic species' fractional, monovalent, and a 2, divalent; 1 x 10-3 M and 5 x 10-4 M 26

30 Species' Fraction 5 Figure 6. Cl - and NO 3 retention time vs. terephthalic acid species' fraction oo1, monovalent, and oo2, divalent; 1 x 10-4 M 27

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