Anion chromatography detectors

Amperometric detection is a very sensitive technique. In principle, voltammetric detectors can be used for all compounds which have functional groups which are easily reduced or oxidized. Apart from a few cations (Fe , Co ), it is chiefly anions such as cyanide, sulfide and nitrite which can be determined in the ion analysis sector. The most important applications lie however in the analysis of sugars by anion chromatography and in clinical analysis using a form of amperometric detection know as Pulsed Amperometric Detection (PAD). 

In suppressed anion chromatography, the effluent from the ion exchange column comes into contact with a cation-exchange device (Catex-H" ) just before the liquid stream passes into the detector. This causes the following reactions to occur. 

Gjerde, Fritz and Schmuckler [5] devised a form on anion chromatography in 1979 that used a conductivity detector but did not require the use of a second stripper, or suppressor column. The anion-exchange resin used had a capacity of only 0.007-... 

Gjerde and Benson discovered that post-column addition of a suspension of sulfo-nated polystyrene particles may be used to reduce the background conductance of basic eluents used in anion chromatography [7]. The eluent cation (typically Na" ) is also replaced in the analyte ion bands by the more highly conducting as the counterion to a sample anion. Since the added reagent is a solid, it is invisible to detectors that respond only to the liquid phase, for example, conductivity and potentio-metric detectors. 

The term sensitivity will be used here in its technically correct context as the change in detector signal per unit concentration. It should not be confused with detection limits that are dependent on baseline noise. (Baseline noise is dependent on the signal magnitude, temperature variations, the electronics, etc. and varies from instrument to instrument.) In this section we shall examine the factors that determine the sensitivity that can be attained in anion chromatography. 

Anion chromatography (Hamilton PRP-XlOO column) of arsenic compounds (each at lo ppb) at pH 5.8 in 15 mM KjHPO /KHjPO,. with atomic fluorescence detection of As. Structures of the parent acids are shown. Arsenic compounds were converted to AsHj, which decomposes to As atoms in the detector. [Courtesy Y. Cai and L. Yehiayan, Florida International University.)... 

The ELSD may be used in conjunction with an eluent containing nonvolatile ions provided that an appropriate suppressor is inserted between the column exit and the detector. This technique is illustrated by the determination of carbohydrates in cola drinks [30]. The separation is depicted in Figure 6.24. Carbohydrates such as glucose and sucrose are anionic at high pH values and can be separated by anion chromatography with a dilute KOH eluent. As the mobile phase leaves the column and enters the suppressor, nonvolatile KOH is converted to water hence a very low background signal is obtained. Only a mist of carbohydrate particles remain to be detected upon evaporation in the ELSD. 

In this experiment phosphate is determined by singlecolumn, or nonsuppressed, ion chromatography using an anionic column and a conductivity detector. The mobile phase is a mixture of n-butanol, acetonitrile, and water (containing sodium gluconate, boric acid, and sodium tetraborate). 

Ohta and Tanaka reported a method for the simultaneous analysis of several inorganic anions and the cations Mg + and Ca + in water by ion-exchange chromatography. The mobile phase includes 1,2,4-benzenetricarboxylate, which absorbs strongly at 270 nm. Indirect detection of the analytes is possible because their presence in the detector leads to a decrease in absorbance. Unfortunately, Ca + and Mg +, which are present at high concentrations in many environmental waters, form stable complexes with 1,2,4-benzenetricarboxylate that interfere with the analysis. 

A flow scheme for the basic form of ion chromatography is shown in Fig. 7.3, which illustrates the requirements for simple anion analysis. The instrumentation used in IC does not differ significantly from that used in HPLC and the reader is referred to Chapter 8 for details of the types of pump and sample injection system employed. A brief account is given here, however, of the nature of the separator and suppressor columns and of the detectors used in ion chromatography. 

It is appropriate to refer here to the development of non-suppressed ion chromatography. A simple chromatographic system for anions which uses a conductivity detector but requires no suppressor column has been described by Fritz and co-workers.28 The anions are separated on a column of macroporous anion exchange resin which has a very low capacity, so that only a very dilute solution (ca 10 4M) of an aromatic organic acid salt (e.g. sodium phthalate) is required as the eluant. The low conductance of the eluant eliminates the need for a suppressor column and the separated anions can be detected by electrical conductance. In general, however, non-suppressed ion chromatography is an order of magnitude less sensitive than the suppressed mode. 

Thin-layer chromatography (TLC) is used both for characterization of alcohol sulfates and alcohol ether sulfates and for their analysis in mixtures. This technique, combined with the use of scanning densitometers, is a quantitative analytical method. TLC is preferred to HPLC in this case as anionic surfactants do not contain strong chromophores and the refractive index detector is of low sensitivity and not suitable for gradient elution. A recent development in HPLC detector technology, the evaporative light-scattering detector, will probably overcome these sensitivity problems. 

Analysis of the halohydrocarbons, halocarbons, and sulfur hexafluoride is usually achieved by gas chromatography that is equipped with an electron capture detector. Complex metal anions, such as cobalt hexacyanide, are used as nonradioactive tracers in reservoir studies. The cobalt in the tracer compound must be in the complex anion portion of the molecule, because cationic cobalt tends to react with materials in the reservoir, leading to inaccurate analytic information [1226]. 

Kordorouba, V. and Pelletier, M., Ion chromatography using an electrochemical detector response to non-electroactive anions, /. Liq. Chromatogr., 11, 2271, 1988. 

A conductivity detector measures the electrical conductivity of the HPLC eluent stream and is amenable to low-level determination (ppm and ppb levels) of ionic components such as anions, metals, organic acids, and surfactants. It is the primary detection mode for ion chromatography. Manufacturers include Dionex, Alltech, Shimadzu, and Waters. 

Separation and quantitation of carbohydrate mixtures may be achieved using HPLC, a method that does not necessitate the formation of a volatile derivative as in GLC. Both partition and ion-exchange techniques have been used with either ultraviolet or refractive index detectors. Partition chromatography is usually performed in the reverse phase mode using a chemically bonded stationary phase and acetonitrile (80 20) in 0.1 mol U1 acetic acid as the mobile phase. Anion- and cation-exchange resins have both been used. Carbohydrates... 

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