Detector ion chromatography

Conductivity detectors provide the advantage of universal detection, as all ions are electrochemically conducting. Thus, the majority of ion chromatography detectors rely upon conductivity measurements. 

This is the classical ion chromatography detector and measures the eluate conductivity, which is proportional to ionic sample concentration (provided that the cell is suitably constructed). Its sensitivity decreases as the specific conductivity of the mobile phase increases. The active cell volume of 2 gl is very small. Good conductivity detectors have automatic temperature compensation (conductivity is highly temperature-dependent) and electronic background conductivity suppression. The linear range is not large. 

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). 

A practical method for low level perchlorate analysis employs ion chromatography. The unsuppressed method using a conductivity detector has a lower detectable limit of about 10 ppm. A suppression technique, which suppresses the conductivity of the electrolyte but not the separated ions, can further improve sensitivity (110,111). Additionally, ion chromatography can be coupled with indirect photometric detection and appHed to the analysis of perchlorates (112). 

Conductivity detectors, commonly employed in ion chromatography, can be used to determine ionic materials at levels of parts per million (ppm) or parts per bUHon (ppb) in aqueous mobile phases. The infrared (ir) detector is one that may be used in either nonselective or selective detection. Its most common use has been as a detector in size-exclusion chromatography, although it is not limited to sec. The detector is limited to use in systems in which the mobile phase is transparent to the ir wavelength being monitored. It is possible to obtain complete spectra, much as in some gc-ir experiments, if the flow is not very high or can be stopped momentarily. 

Gas Chromatography. Gas chromatography is a well recognised method for the analysis of H—D—T mixtures. The substrate is alumina, AI2O2, coated with ferric oxide, Fe202. Neon is used as the carrier gas. Detectors are usually both thermal conductivity (caratherometer) and ion chamber detectors when tritium is involved (see Chromatography). 

One example of normal-phase liquid chromatography coupled to gas chromatography is the determination of alkylated, oxygenated and nitrated polycyclic aromatic compounds (PACs) in urban air particulate extracts (97). Since such extracts are very complex, LC-GC is the best possible separation technique. A quartz microfibre filter retains the particulate material and supercritical fluid extraction (SPE) with CO2 and a toluene modifier extracts the organic components from the dust particles. The final extract is then dissolved in -hexane and analysed by NPLC. The transfer at 100 p.1 min of different fractions to the GC system by an on-column interface enabled many PACs to be detected by an ion-trap detector. A flame ionization detector (PID) and a 350 p.1 loop interface was used to quantify the identified compounds. The experimental conditions employed are shown in Table 13.2. 

Ion chromatography (IC) is a relatively new technique pioneered by Small et al.25 and which employs in a novel manner some well-established principles of ion exchange and allows electrical conductance to be used for detection and quantitative determination of ions in solution after their separation. Since electrical conductance is a property common to all ionic species in solution, a conductivity detector clearly has the potential of being a universal monitor for all ionic species. 

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. 

Detectors. Although electrical conductance has been widely used for detecting ions in ion chromatography, the scope of the technique has been considerably extended by the use of other types of detector. It is convenient broadly to classify detectors into two series. 

The conductance detector is a universal detector for ionic species and is widely used in ion chromatography (see Section 7.4). 

Eluent selection in ion chromatography is made more complex by the need to consider detector operating characteristics. 

The chromatographic pumps and flow path used in IEC must be resistant to corrosion. For this reason, polymers such as poly(etheretherketone) (PEEK , ICI Americas Wilmington, DE) have entered into widespread usage in ion chromatography. Electrochemical detectors may also be subject to corrosion by certain ions. This chapter will review the chromatographic materials, detectors, and applications of ion exchange chromatography. For some classes of compounds, where reversed phase or normal phase alternatives may have been developed, alternative separation techniques will be presented. 

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

Bayon, M. M., Rodriguez Garcia, A., Garcia Alonso, J. I., and Sanz-Medel, A., Indirect determination of trace amounts of fluoride in natural waters by ion chromatography a comparison of on-line post-column fluorimetry and ICP-MS detectors, Analyst, 124, 27, 1999. 

Simek, P., Jegorov, A., and Dusbabek, F., Determination of purine bases and nucleosides by conventional and microbore high performance chromatography and gas chromatography with an ion trap detector, J. Chromatogr., 679,1951,1994. 

Liquid cathode lithium cells, 3 464-466 Liquid chromatography, 4 618—620 6 375, 440-468 14 233. See also High performance liquid chromatography Ion chromatography applications, 4 625-626 6 457-465 brief overview, 6 384-388 column switching, 6 446-447 columns, 4 623 derivatization, 6 447—448 detectors, 4 622-623 6 386-387, 448 52... 

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. 

The power of the system to overcome the problems associated with coeluting compounds is demonstrated in conjunction with the use of deuterated (or13C-labelled compounds) as internal standards. Such techniques could not be used in conventional gas chromatography as the deuterated compounds often co-elute, making quantification difficult if not impossible. With the ion-trap detector, however, it is easily possible to differentiate between the ions arising from the different compounds and the intensities of these ions could then be used for quantification of the compounds involved. The application of such techniques can be shown by... 

Campbell, C. The ion trap detector for gas Chromatography technology and application. Finnigan MAT IDT 15. 

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