Ion Chromatography of Environmentally Significant Anions

The quantitative determination of levels of environ' mentally important ions, such as those discussed in the preceding chapters, can he accomplished using chromatographic methods described in this box.

The need to determine the prevalence of common anions like phosphate (P043"), nitrate (N03~), or fluoride (F~) isn't immediately clear. The biospheric significance of these ubiquitous ions is not as obvious as is, for example, the presence of PCBs, pesticides, or toxic metals like lead, mercury, or cadmium. These ionic components are important because they give an indication of the relative reduction-oxidation potential in an aqueous sample taken from an environment such as a stagnant lake (P04 ), or of the contamination of groundwater from fertilizer runoff (N03~), or of whether municipal water supplies need to be supplemented with fluoride (F~) for the health of children's teeth. Although these charged ions can be detected by widely available ultraviolet detectors common in most high-performance liquid chromatographic systems (Janos and Aczel, 1996), a more sensitive means of detection involves ionic conductivity. This chromatographic method is called ion chromatography with ionic conductivity detection. Although cations can also be separated by ion chromatography (IC), only anionic separations will be discussed here.

The heart of the separation process in an ion chromatograph is a short column ( 10—15 cm) packed with small-diameter particles called ion exchange resins. These are often made of a styrene/divinylbenzene polymer or microparticles of silica coated with compounds containing an anionic functional group such as a quaternary amine, —N(CH3)3+OH~, or a primary amine, —NH3+OH~, when they are to be used for anion separation.

The actual process of chromatopaphic separation occurs after a sample containing analyte anions (and their associated cations) is injected onto the chromatographic column. With gas chromatography (see Environmental Instrumental Analysis Box II), the mobile phase is an inert gas that does not chemically interact with the chromatographic surface. The mobile phase in ion chromatography, on the other hand, is a solution of cations and anions with a carefully controlled pH; often buffers are used. This complex mixture of mobile-phase ions—carefully chosen for each group of analyte ions to be separated—interacts with the analyte ions and the functional groups of the column's chromatographic surface. That interaction involves competition of the mobile-phase anions and the analyte anions for chromatographic sites on the packing material (the charged functional groups such as —N(CH3)3+ or —S03^). This competition yields different overall travel times for each of the analytes as they pass down the column; some are retained longer than others. (The overall down-column movement is provided by a pumping of the mobile phase by an external pump.) Different analyte travel times—as in gas chromatography— translate into different exit (or retention) times for each anion in the original mixture. The result is chromatographic separation of anions.

(continued on p, 658)

Environmental Instrumental Analysis V

Ion Chromatography of Environmentally Significant Anions (continued)

The process for anionic ion chromatographic retention by ion exchange resins can be represented by the equation

RN(CH3)3+HC03~(s) + anion-(aq)->

RN(CH3)3+ anion-(s) + HC03"(aq)

In this equation, the term anion" represents any of the analyte anions mentioned above. When a sample is injected onto the column, this anion is quickly retained by complexation with the stationary phase near the head of the column. The next step in the chromatographic process takes place as a mobile phase, with a carefully controlled amount of anionic ion such as bicarbonate, HC03~ , is pumped through the column. The presence of the bicarbonate anion in the mobile phase forces the equilibrium in the above equation to the left; the retained analyte anion is freed and moves down the column in the flowing mobile phase. As the analyte moves along, it repeatedly undergoes this same process of retention and movement (or exchange between the stationary and mobile phases). Most importantly, different analyte anions (e.g., fluoride or phosphate or chloride) undergo this exchange process to differing degrees and therefore travel at different overall rates during their time in the IC column. The result is that different analytes exit the chromatographic column at different times, i.e., separation has taken place.

The task of detecting analyte anions in the presence of the anions always present in the mobile phase is by no means a trivial one. Since both kinds of anions—the analytes' and the mobile phase's—conduct electricity, using an ordinary conductivity cell as a detector at the end of the IC column is normally not practical. The problem is especially difficult because, in order to obtain adequate separation of some important anions, the mobile phase often has to have high ionic content to displace the analyte anions from the chromatographic surface—something that is obviously required for separation. Therefore, most of the ionic conductivity passing through the detector is attributable to the mobilephase ions and not the analyte—an unworkable situation when trying to detect the analyte anions by their conductivity.

An ingenious solution to this problem is called conductivity suppression or eluent (mobilephase) suppression. This technology converts the mobile-phase anions from an easily dissociated ionic form to a (soluble) molecular form that does not strongly influence the signal produced by the conductivity detector. The suppression module is placed after the chromatographic column but before the conductivity detector. In an anion exchange system, the suppression module might carry out the following reaction:

Na (aq) + HC03 (aq) + resin"H+(s) ■

resin Na+(s) + H2C03(aq)

Here resin~H+ represents a cation exchange resin that will exchange cations—instead of anions as in the chromatographic column described. This process basically prevents (or suppresses) the mobile phase's anions from contributing to the conductivity by converting current-conducting bicarbonate anion into relatively undissociated H2C03. Therefore, the conductivity detector's signal is based almost

Injector

Mobile-

phase

pump

Ion exchange column

Analytes' retention times based on interaction of analyte, ion exchange column, and mobile-phase ions.

Bluent

suppression module

Conductivity detector

Mobile-phase ions are converted to molecular forms that do not produce significant signals in detector

Analyte ions are detected against a quiet, stable background by their electrical conductivity.

Mobile-phase ions are converted to molecular forms that do not produce significant signals in detector

Analyte ions are detected against a quiet, stable background by their electrical conductivity.

completely on the passage of analyte anions through the detector cell. (Those anions are not affected hy the cation.ion exchange resin.) This results in lower (i.e., better) detection limits for the analytes of interest and a more stable baseline (less noise and drift) than a similar system without eluent suppression.

The figure above is a schematic of an ion chromatographic system. Detailed are an injector, chromatographic column, eluent suppression module, and conductivity detector, as well as the processes that occur at each step.

The figure at right is an example of the kind of chromatogram that a system of this type would generate. The anions detected are fluoride, chloride, phosphate, and nitrate. As with all chromatograms, detector signal intensity is plotted versus time.

Researchers have used this method recently to determine nitrate and nitrite anions in dew, rain, and snow collected in Massachusetts (Zuo et al., 2006). Instead of conductivity detection, these authors used a UV absorption detector and chose an analytical wavelength at which neither the mobile phase nor other anions would absorb (205 nm). Surprisingly, dew had the highest concentrations of these

Dentist Equipment
(continued on p. 660)

Environmental Instrumental Analysis V

Ion Chromatography of Environmentally Significant Anions (continued)

Sample

Date

Nitrite (ppb)

Nitrate (ppm)

Dew 1

27.09.2005

640

4.87

2

27.09.2005

620

4.79

3

26.09.2005

830

5.99

Rain 1

02.10.2005

< DL*

2.63

2

02.09.2005

< DL*

2.62

3

28.05.2005

140

1.20

Snow 1

29.01.2005

21

0.320

2

18.01.2005

32

0.376

3

12.01.2005

32

0.60

i

12.01.2005

, 26

1 . 0.56 1_

* Detection limit for nitrite 10 ppb.

* Detection limit for nitrite 10 ppb.

anions compared to rain and snow collected at the same site (see table above). The authors proposed that these dew nitrate concentrations, ranging from 4-79 to 5.99 juig/mL, suggest that dew is acting as a nighttime sink for these anionic species; they also note that this may be important for vegetation because these anions are held in contact with the leaf surface for long time periods as dew forms, and the concentration may spike as dew evaporates in the morning.

Since photolysis of both these anions in shallow aqueous solution can lead to the formation of hydroxyl radical and hydrogen peroxide, this may be a source of oxidative stress for plants on which the dew forms (Kobayashi etal.,2002):

N02" + H 20 + light-> OH + NO + OH"

N 03" + HzO + light-> OH + N02 + OH"

Studies of this process on red pine needles on trees on Mount Uokurakuji in western Japan have concluded that 40% of hydroxyl radial production in dew on those trees originates from nitrite and nitrate (Nakatani et al., 2001).

References: P. Janos and P. Aczel, "Ion Chromatographic Separation of Selenate and Selenite Using a Polyanionic Eluent," Journal of Chromatography A 749 (1996): 115-122.

T. Kobayashi, N. Naianani, T. Hirakawa, M. Suzuki, T. Miyake, M. Chiwa, T. Yuhara, N. Hashimoto, K. Inoue, K. Yamamura, N. Agus, J. R. Sinogaya, K. Nakane, A. Kume, T. Arakaki, and H. Sakugawa, Environmental Pollution 118 (2002): 383-391.

N. Nakatani, T. Miyake, M. Chiwa, M. Hashimoto, T. Arakaki, and H. Sakugawa, "Photochemical Formation of OH Radicals in Dew Formed on the Pine Needles at Mt. Gokurakuji," Water, Air and Soil Pollution U0 (2001) 397-402.

Y. Zuo, C. Wang, and T. Van, "Simultaneous Determination of Nitrite and Nitrate in Dew, Rain, Snow and Lake Water Sample by Ion-Pair High-Performance Liquid Chromatography," Talanca (2006): 281-285.

PART V

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