In the Laboratory
Topics in Chemical Instrumentation
edited by
David Treichel Nebraska Wesleyan University Lincoln, NE 68504
Ion Chromatography: Analysis of Ions in Pond Waters
W
Kumar Sinniah* and Kenneth Piers** Department of Chemistry and Biochemistry, Calvin College, Grand Rapids, MI 49546; *
[email protected], **pier @calvin.edu
Chemistry departments continue to be challenged to offer relevant courses that help students grasp the central role of chemistry in the understanding of our world. Environmental science is an area in which chemistry does indeed play that role. The health of the nation’s surface and ground water resources is an issue of interest to all citizens, and analysis of natural water samples is an excellent venue for introducing students to a wide variety of scientific skills and techniques. Since analytical water chemistry has obvious environmental applications, it frequently piques the interest of undergraduate students; however, using the lengthy wet chemical laboratory procedures associated with classical analytical methods often dampens their enthusiasm. With ion chromatography (IC), sample preparation requirements are usually minimal, and the instrument is easy to operate and provides rapid analysis with high sensitivity for a variety of anions and cations. Our purpose in this article is to provide a brief overview of IC instrumentation and to show that it can easily be introduced into the undergraduate curriculum already starting with firstyear students. Background Water analysis is not a new topic to this Journal (1–7). However, many of the standard wet chemical methods used in water analysis are not readily adaptable to introductory labs because they involve lengthy procedures that cannot be completed within a typical laboratory period. Using classical methods, determination of the concentration of individual ions such as fluoride, chloride, sulfate, nitrate, and phosphate generally requires that ions be determined one at a time, separately from all other ions present. Sample preparation for each anion often requires a long time and sometimes requires the use of hazardous analytical reagents. Ion chromatography bypasses these obstacles. Within the past decade, reliable IC instruments have become available that virtually eliminate sample preparation time and allow simultaneous determination of many anions or cations of interest during a single pass. There are a number of commercial vendors of these instruments (Dionex, Alltech, Brinkman, Metrohm, Zellweger Lachat) and they offer systems at a variety of price levels. A basic isocratic (fixed eluent composition) system equipped with an autosampler can be obtained for less than $30,000. The system we purchased [a Dionex DX500, which can operate in either isocratic or gradient (variable eluent composition) mode and is equipped with an autosampler] was obtained for about $40,000. Ion chromatography, of course, is not the only instrumental method available for cation and anion analysis. Capillary electrophoresis (8–10) and flow-injection analysis
358
(11, 12) are two other relatively new instrumental methods available for ion analysis. Although capillary electrophoresis techniques offer higher separation power than IC, thus enabling a larger number of ions to be separated, IC affords lower detection limits and better reproducibility (13, 14). Flow injection analysis has shorter analysis times than IC, but is limited to analysis of a single ion per run, whereas IC is capable of detecting multiple ions in a single sample run. In recent years IC has become a standard method for the determination of inorganic anions in water in environmental studies and in the food and beverage, pharmaceutical, nuclear, and semiconductor industries (13). Current-generation IC instruments provide high sensitivity and selectivity, reproducibility, and ease of operation, with minimal sample preparation. The Environmental Assessment Program at Calvin College Several years ago our college initiated an interdisciplinary environmental assessment program, the Calvin Environmental Assessment Program (CEAP) (15). In this program students in a variety of courses, from the natural sciences to the social sciences to philosophy, do investigative projects in which they study how human activities on the campus and its surroundings impact the campus environment. As part of this program we have the students in our firstyear general chemistry and quantitative analysis courses gather water samples from ponds located on the college grounds. They carry out measurements on these samples, including IC analysis for several anions and cations, which provide information about water quality in the campus ponds. Students in the first-year course perform this analysis in the fall semester as part of a laboratory unit on acid–base titration. They prepare and standardize solutions of a strong acid and base and then, using these standard solutions, carry out titrimetric analysis on a variety of test solutions. Among these are an acid neutralizing capacity (ANC) measurement and a pH measurement on a sample of natural water that the students collected from one of the campus ponds. An aliquot of the same water sample is submitted for ion chromatography. The IC analysis generates concentration data for five anions: fluoride, chloride, nitrate, phosphate, and sulfate. Working in pairs, students in the different lab sections generate about 150 water samples collected from 6 sites on the campus— about 25 samples per site. The ANC, pH, and ion concentration data for each site are entered on a spreadsheet. There are a sufficient number of data points to allow some simple data analysis. The students test the data points for validity using Huber’s method (16 ). For outlier data points, they identify potential factors that could contribute to the creation of an
Journal of Chemical Education • Vol. 78 No. 3 March 2001 • JChemEd.chem.wisc.edu
In the Laboratory
invalid data point. After eliminating outliers, they obtain the mean value for each measurement, the standard deviation, and the 95% confidence interval for each measured value. We spend a short time explaining the significance of each of these statistical measures. After obtaining all the data for the different collection sites we spend some time (approximately one class period) discussing the results. We talk about the environmental significance of the ANC (is the pond likely to become acidic if exposed to acid precipitation?). We discuss why a natural water sample can have a pH near 7 but still have a large ANC. For the IC results we note which ponds have relatively high concentrations of one anion or another and ask students to identify possible sources of these ions (e.g., a couple of ponds have relatively high chloride concentrations compared to the others; these ponds lie immediately adjacent to heavily traveled city streets that receive a lot of salt in the winter), and we briefly discuss the environmental significance of each anion. These data are generated and logged each year to provide a longitudinal data set in which we can see how the campus ponds change over time with respect to the parameters measured. A number of review articles and books are available on the theory and applications of ion chromatography (17–21). Reports on the use of ion chromatography for detection of phosphate in beverages and to study the common ion effect have been published in this Journal (22, 23). IC Instrument Components An IC system comprises several components integrated within a unit similar to a high-performance liquid chromatography (HPLC) system. These include the eluent supply jar, the pump system, the injection system (manual or auto-injection), the ion exchange column, a suppressor, the detection system, and the computer-controller, as shown in Figure 1.
Eluent Solutions Depending on whether one is analyzing for anions or cations, different eluent solutions are used. The most common eluent for anion analysis is a dilute buffer solution containing sodium bicarbonate and sodium carbonate. Alternatively, dilute aqueous sodium or potassium hydroxide may be used as the eluent. Since these eluents are conductive, the background signal is generally large with conductivity detectors. Although dilute eluents may be used directly with conductivity detectors,
Figure 1. Schematic of an ion chromatography system.
incorporation of a membrane suppressor or suppressor column to lower the background conductivity and thereby enhance the analyte signal is common with today’s IC instruments. Typically we use the bicarbonate–carbonate eluent system for anion analysis. We purchase stock solutions of each of these substances and dilute them with Type I (18 MΩ-cm) water to achieve the desired eluent properties. A less costly option would be to make your own stock eluent solutions using high-purity reagents and Type I water. In cation analysis, the eluent is normally a dilute acid solution—usually a mineral acid, but other acids may also be used depending on the analytical task. Again, we use a commercial metal-free-grade acid solution and dilute it to appropriate levels with Type I water.
Pump Systems Several instrument manufacturers offer modular ion chromatography systems that operate either in isocratic-only mode or with both isocratic and gradient capabilities. While isocratic IC is the standard choice for routine ion analysis, the gradient technique enables the separation and analysis of a considerably wider range of anions and is essential for successful analysis of organic anions (carboxylates, for example). The gradient technique, however, has limited use for cation analysis. We use the IC instrument mainly to support our teaching program and the isocratic mode of operation has been satisfactory for most of our purposes. Recently we have begun studies involving analysis of organic anions, in which we are making use of the gradient capabilities of the instrument. However, in view of the limited use we have made of the gradient technique within the past year and a half, our opinion is that the purchase of an isocratic system is sufficient for most teaching purposes. If the instrument is to have significant use in research projects, a combined isocratic– gradient pump system is desirable. Injection Systems While instruments equipped only for manual injection of samples are less costly, our IC system is equipped with an autosampler that can accommodate 60 samples in one run. We have found this feature to be invaluable. It allows us to run all the samples from a lab section in a single overnight run, and we feel that the convenience of an autosampler well justifies its cost. We note that the autosampler accessory also requires the use of specialized sample containers (disposable plastic tubes with caps), which introduces some additional operational cost. Ion Exchange Columns For anion analysis the ion exchange column is usually a resin with attached alkyl quaternary ammonium chains. When the bicarbonate–carbonate eluent flows through these columns the eluent anions form the counter ions to the fixed ammonium ions on the resin. When solutions containing other inorganic anions are eluted into the column, they compete with the eluent anions for the fixed positive centers on the resin. Depending on how well or how poorly these anions compete with the eluent anions for positive charge centers on the resin, they are carried more slowly or more quickly through the column.
JChemEd.chem.wisc.edu • Vol. 78 No. 3 March 2001 • Journal of Chemical Education
359
In the Laboratory
The same principles apply to cation analysis. In this case the ion exchange column is composed of a resin carrying fixed negative charge centers—sulfonate, carboxylate, etc. The acidic eluent solution provides protons as the counter ion to these negative charge centers. Then, when other cations are eluted through the column, they compete with the protons for the fixed negative charge centers on the resin. Once again separation is achieved on the basis of differing affinities for the negative charge centers among various cations. A typical ion exchange column has a significantly long lifetime provided that it is not subjected to unusually harsh or dirty solutions. We have used the same columns for nearly two years without diminished performance. Typically the column is protected by a short guard column, which is replaced from time to time. Considering the rather high initial cost of the analytical column (a typical column-plus-guard combination costs around $850) it is fortunate that the lifetime is long. Several IC instrument manufacturers now provide the convenience of switching columns from an anion to a cation column at the touch of a button. Although our Dionex DX-500 system does not have the automated column switching capability, the manual switchover from an anion column/ suppressor/eluent to a cation column/suppressor/eluent can be accomplished in less than an hour.
Therefore, practically, its use is limited to alkali metal, alkaline earth metal, and ammonium cations. During ion suppression involving cations, water undergoes electrolysis. The hydroxide ions formed at the cathode migrate through a membrane into the eluent solution—replacing the bisulfate ions of the sulfuric acid eluent, which migrate through a membrane toward the anode compartment of the electrolysis cell. The hydroxide ions that enter the eluent solution combine with the protons in the eluent solution to form water and essentially eliminate the conductivity due to the eluent acid. But at the same time the metal analytes in the solution may be converted to insoluble hydroxides, if these have very low water solubility— which is characteristic of many transition metal and lanthanide metal cations. Therefore detection of most transition metal ions requires the use of direct conductivity and not suppressed conductivity, and the separation of lanthanides usually is achieved by post-column derivatization in combination with an absorbance detector. Membrane suppressors for IC instruments are expensive, approximately $800 for each suppressor membrane system. However they are self-regenerating and have a long useful life. In the time we have had our instrument (nearly two years) we have had no membrane failures, nor have we noted any diminished performance.
Ion Suppression Systems Because the detection system most commonly employed in IC is electrolytic conductance using a conductivity cell, it is generally necessary to remove the eluent ions from the analyte solution before the solution enters the detector. Otherwise the sensitivity of the technique would be low, because one would be measuring a small incremental increase in the conductivity due to the presence of the analyte ions against the rather large background conductivity of the eluent solution itself. So, especially in anion analysis, eluent ion suppression is the norm. Early IC instruments often featured an ion suppression column through which the analyte solution was passed. These columns typically had short lifetimes and required regeneration from time to time. More recent instruments have improved ion suppression technology and use membrane suppression. Details of the operation of membrane suppressor technology are available in the literature (17 ). The basic ion suppression chemistry for a common anion exchange eluent such as carbonate/bicarbonate and an analyte (NaCl) is shown below.
Detection Systems The standard detection system in most IC instruments consists of an electrochemical detector using a conductivity cell. In our instrument, this detector can be temperature controlled between 30 and 45 °C. As the name implies, such a detector generates a signal by measuring the conductivity of the water sample passing through it. The electrochemical detector supplied by Dionex also is capable of driving a pulsed Table 1. Detection Limits of Two Dionex IC Systems Ion
DX120
Fluoride
3.5
4
Chloride
2.9
16
Nitrite
6.5
6
Bromide
7.8
14
Nitrate o-Phosphate Sulfate
NaCl + resin–SO3H → resin–SO3Na + HCl
360
DX500 Anions a
NaHCO3 + resin–SO3H → resin–Na + [H2CO3] → resin–SO3Na + [H2O + CO2] In this manner the eluent anions are transformed into nonconducting water and carbon dioxide, whereas the analytes are converted to the strongly conducting acidic form. By removing large quantities of the eluent anions (bicarbonate and carbonate) the sensitivity of the measurement is greatly increased. Indeed, using suppression, it is possible to detect several common ions whose concentrations are as low as 10 parts per billion in 25 mL of the injected sample. Table 1 shows typical detection limits for number of anions and cations using IC. Although suppression is widely used in anion IC, its use in cation analysis is more limited, mainly because it is restricted to cations that have fairly water-soluble hydroxides.
Detection Limit/µg L᎑ 1
7.7
17
20.2
59
8.2
40
Cations b Lithium
1
3
Sodium
4
31
Ammonium
5
37
Potassium
4
28
Magnesium
5
22
Calcium
8
28
aUsing
an IonPac AS14 column (24–26). a CS12 column (private communication with P. Jackson, Dionex Corporation 22 Feb 2000). The MDL’s for cations for DX-500 and DX-120 are somewhat comparable. However, they were obtained using two different methods, 3× signal-to-noise and EPA Method 300.0, respectively. bUsing
Journal of Chemical Education • Vol. 78 No. 3 March 2001 • JChemEd.chem.wisc.edu
In the Laboratory
amperometric cell. A third detection option in IC is an absorbance detector. These latter two detection systems are usually used with post-column derivatization to enhance selectivity and sensitivity. Acquisition of two or more detection systems can extend the range of useful experiments that can be run on an IC instrument. These additional detectors will provide a wider range of selectivity and improve the sensitivity for certain ions. However, we have found a single conductivity detector sufficient for our purposes. Sample Preparation Normally, sample preparation is easy and the volume of sample needed is minimal (about 5 mL in our system). Typically a pond water sample is vacuum-filtered and placed into a 5-mL autosampler vial. The vial is then fitted with a 0.45-µm filter assembly and placed in the autosampler to be run at a suitable time. Anion Analysis
Figure 2. (a) Anion analysis of a 5-anion standard. (b) Anion analysis of pond waters. Column: IonPac AS14; eluent: 1.0 mM NaHCO3 + 3.5 mM Na2CO3; flow rate: 1.2 mL/min; detection suppressed conductivity. Solute concentrations in pond: 0.26 ppm fluoride, 43.6 ppm chloride, 5.5 ppm nitrate, and 12.6 ppm sulfate.
The versatility of the IC system is its ability to analyze multiple components simultaneously. Figure 2a shows the ion chromatogram of several standard anions used in water analysis. Figure 2b shows anions present in the waters from a campus pond adjacent to a residential neighborhood. Although we have limited IC water analysis in our firstand second-year classes to samples from ponds and streams located on campus and the college’s neighboring ecosystem preserve, IC could be easily applied in an undergraduate laboratory to test snow, rain water, municipal water, groundwater, and wastewater. IC can also be used to investigate foods and beverages. We have used the IC instrument for individual student projects such as investigating the efficiency of commercial household water filters in removing several inorganic anions and cations ions, and determining a number of organic acids in fruit juices. In the quantitative analysis course, students use IC, ion-selective electrodes, and spectrometric methods as comparative techniques for anion analysis of pond water samples. In addition to the analysis of inorganic ions, IC columns are available for analyzing a variety of organic anions with conductivity detection. IC is routinely used for carbohydrate analysis employing pulsed amperometric detection, but we have not yet made use of that capability. Cation Analysis
Figure 3. (a) Cation analysis of a 6-cation standard. (b) Cation analysis of pond waters. Column: IonPac CS12A; eluent: 11 mM H2SO4; flow rate: 1.0 mL/min; detection: suppressed conductivity. Solute concentrations in pond: 2.8 ppm sodium, 0.2 ppm ammonium, 3.5 ppm potassium, 7.3 ppm magnesium, and 24.0 ppm calcium.
Although several other rapid techniques are available for the analysis of cations, IC has the advantage of simultaneously analyzing alkali metals, alkaline-earth metals, and simple amines using suppressed conductivity. An analysis of a standard cation mixture by suppressed conductivity detection is shown in Figure 3a, and analysis of water from a pond in the ecosystem preserve is shown in Figure 3b. We have analyzed a number of ponds and streams in the our ecosystem preserve to obtain a baseline of the standard cations present. These data are part of the ongoing environmental assessment program and will be useful in coming years to assess the impact of humans on such ecosys-
JChemEd.chem.wisc.edu • Vol. 78 No. 3 March 2001 • Journal of Chemical Education
361
In the Laboratory
tems. In the quantitative analysis laboratory, students have further compared cation analysis of pond waters obtained by the IC instrument with flame and graphite furnace atomic absorption spectrometry. Such instrumental comparisons give students insight into the comparative ease of use of various analytical technologies and also give them a basis for comparing the analytical capabilities of these technologies. Conclusion Ion chromatography continues to generate tremendous interest for anion and cation analysis in industry. It is able to quantitate, with great sensitivity, several anions or cations simultaneously with run times on the order of minutes. The convenience of using IC to analyze standard ions in an undergraduate setting is remarkable. With the technological improvements that have already taken place and the lower cost of instrumentation, this technique will soon replace the wetchemical methods used for ion analysis in undergraduate laboratories. We have successfully introduced this instrument to firstyear students in introductory laboratories and second-year students in quantitative analysis, as well as to individual students participating in independent projects. Operation of the instrument can be easily handled by first-year students having no detailed understanding of chromatography theory. However, method development and verifying proper data analysis require knowledge of analytical chemistry and basic understanding of chromatography (27). To demystify IC, we give all firstyear students a handout explaining the basics of ion chromatography. This handout is available on the college’s Web site (http://www.calvin.edu/academic/chemistry/ionchrom/ ). Acknowledgments Partial support to purchase the ion-chromatography instrument was provided by the Nation Science Foundation’s Division of Undergraduate Education through grant DUE # 9850906) and Calvin College. P. Jackson (Dionex Corp.) is thanked for providing the MDL’s for the Dionex DX500 system. KS also wishes to acknowledge Calvin College for additional support provided through a reduced teaching load. W
Supplemental Material
Instructions for first-year and quantitative analysis students for the IC work described in this paper are available in this issue of JCE Online.
362
Literature Cited 1. Myers, R. L. J. Chem. Educ. 1998, 75, 1585. 2. Kelter, P. B.; Grundman, J.; Hage, D. S.; Carr, J. D.; CastroAcuna, C. M. J. Chem. Educ. 1997, 74, 1413. 3. Juhl, L.; Yearsley, K.; Silva, A. J. J. Chem. Educ. 1997, 74, 1431. 4. Herrera-Melian, J. A.; Dona-Rodriguez, J. M.; HernandezBrito, J.; Perez-Pena, J. J. Chem. Educ. 1997, 74, 1444. 5. Butala, S. J.; Zarrabi, K.; Emerson, D. W. J. Chem. Educ. 1995, 72, 441. 6. Hermann, M. S. J. Chem. Educ. 1994, 71, 323. 7. Lisensky, G.; Reynolds, K. J. Chem. Educ. 1991, 68, 334. 8. Demay, S.; Martin-Girardeau, A.; Gonnord, M. J. Chem. Educ. 1999, 76, 812. 9. Boyce, M. J. Chem. Educ. 1999, 76, 815. 10. Harrold, M. P.; Wajtusik, M. J.; Riviello, J.; Henson, P. J. Chromatogr. 1993, 640, 463. 11. Ruzicka, J.; Henson, E. H. Flow Injection Analysis, 2nd ed.; Wiley: New York, 1988. 12. Valcarcel, M.; Luque de Castro, M. D. Flow Injection Analysis: Principles & Applications; Ellis Horwood: Chichester, England, 1987. 13. Erickson, B. E. Anal. Chem. 1999, 71, 465A. 14. Lucy, C. A. J. Chromatogr., A 1996, 739, 3. 15. Calvin College. CEAP; http://www.calvin.edu/academic/geology/ ceap/ (accessed Nov 2000). 16. Meier, M. C.; Zund, R. E. Statistical Methods in Analytical Chemistry; Wiley-Interscience: New York, 1993. 17. Weiss, J. Ion Chromatography, 2nd ed.; VCH: New York, 1995. 18. Small, H. Ion Chromatography; Plenum: New York, 1989. 19. Dasgupta, P. K. Anal. Chem. 1992, 64, 775A. 20. Fritz, J. S. Anal Chem. 1987, 59, 335A. 21. Singh, R. P.; Abbas, N. M.; Smesko, S. A. J. Chromatogr., A 1996, 733, 73. 22. Bello, M. A.; Gustavo Gonzalez, A. J. Chem. Educ. 1996, 73, 1174. 23. Koubek, E.; Elert, M. J. Chem. Educ. 1992, 69, A146. 24. Dionex Corporation. Determination of Inorganic Anions in Drinking Water by Ion Chromatography; Application Note 133; Dionex: Sunnyvale, CA; reported MDL’s are for n = 7 at 99% confidence level. 25. Rabin, S.; Stillian, J.; Barreto, V. J. Chromatogr. 1993, 640, 97. 26. Dhillon, H. S.; Heckenberg, A.; Heckenberg, A.; Jekot, J. Am. Lab. 1997, 28 (Feb), 33N; reported MDL’s are for n = 7 at 99% confidence level). 27. Statler, J. In Handbook of Instrumental Techniques for Analytical Chemistry; Settle, F., Ed.; Prentice Hall: Englewood Cliffs, NJ, 1997; pp 199–220.
Journal of Chemical Education • Vol. 78 No. 3 March 2001 • JChemEd.chem.wisc.edu
