Cation chromatography principles

Mobile phases useful for suppressed conductivity detection of anions include sodium hydroxide, potassium hydroxide, and the sodium and potassium salts of weak acids such as boric acid. In nonsuppressed conductivity detection, the ionic components of the mobile phase are chosen so that their conductivities are as different from the conductivity of the analyte as possible. Large ions with poor mobility are often chosen, and borate-gluconate is popular. For cations, dilute solutions of a strong acid are often used for nonsuppressed conductivity detection. For more information on the application of electrochemical detection to inorganic analysis, see Ion Chromatography Principles and Applications by Haddad and Jackson,17 which provides a comprehensive listing of the sample types, analytes, sample pretreatments, columns, and mobile phases that have been used with electrochemical detection. 

Modem suppressors for cation chromatography are both efficient and self-regenerating. The principles are similar to the suppressors for anion chromatography, described in Chapter 6. The mechanism of suppression for a cation self-regenerating suppressor is illustrated in Fig. 7.1 and described in some detail by Rabin et al. [3]. Suppressors for cation chromatography are limited to those cations that do not form precipitates with the hydroxide ions from the suppressor. 

An acidic eluent such as methanesulfonic acid (MSA), which is used in cation chromatography, works on essentially the same principle. The of the eluent is... 

Figure 12.22 SFC-GC analysis of aromatic fraction of a gasoline fuel, (a) SFC trace (b) GC ttace of the aromatic cut. SFC conditions four columns (4.6 mm i.d.) in series (silica, silver-loaded silica, cation-exchange silica, amino-silica) 50 °C 2850 psi CO2 mobile phase at 2.5 niL/min FID detection. GC conditions methyl silicone column (50 m X 0.2 mm i.d.) injector split ratio, 80 1 injector temperature, 250 °C earner gas helium temperature programmed, — 50 °C (8 min) to 320 °C at a rate of 5 °C/min FID detection. Reprinted from Journal of Liquid Chromatography, 5, P. A. Peaden and M. L. Lee, Supercritical fluid chromatography methods and principles , pp. 179-221, 1987, by courtesy of Marcel Dekker Inc.

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). 

Based on preliminary results from Helfferich130, further developments by Davankov and co-workers5 131 133 turned the principle of chelation into a powerful chiral chromatographic method by the introduction of chiral-complex-forming synlhetie resins. The technique is based on the reversible chelate complex formation of the chiral selector and the selectand (analyte) molecules with transient metal cations. The technical term is chiral ligand exchange chromatography (CLEC) reliable and complete LC separation of enantiomers of free a-amino acids and other classes of chiral compounds was made as early as 1968 131. 

Ion chromatography (1C) is a separation technique related to HPLC. However, because it has so many aspects such as the principle of separation and detection methods, it requires special attention. The mobile phase is usually composed of an aqueous ionic medium and the stationary phase is a solid used to conduct ion exchange. Besides the detection modes based on absorbance and fluorescence, which are identical to those used in HPLC, ion chromatography also uses electrochemical methods based on the presence of ions in a solution. The applications of ion chromatography extend beyond the measurement of cations and anions that initially contributed to the success of the technique. One can measure organic or inorganic species as long as they are polar. 

Figure 26-9 Principle of ion-pair chromatography. The surfactant sodium octanesulfonate added to the mobile phase binds to the nonpolar stationary phase. Negative sulfonate groups protruding from the stationary phase then act as ion-exchange sites for analyte cations such as protonated organic bases, BH+.

The book commences with a chapter in which the principles and theory of various chromatographic techniques are discussed. Ion chromatography (Chapter 2) is a relatively recently introduced technique that has found extensive applications in the analysis of mixtures of anions and to a lesser extent of organic compounds and cations. Codetermination of anions and cations is possible. A variant of ion chromatography, namely electrostatic ion chromatography has to date found a very limited application to the determination of anions and is discussed in Chapter 3. 

Temperature control of the short resin column was of particular importance. At 22-24°C, results corresponded closely to values found on macrocolumns, For every 1 C rise in temperature, there is an increase of Hb Aj concentration by a 0.5% (D16) or 1% (RIO) point value. Most commercial methods supply temperature correction charts (RIO) or factors (H6). While some have found a linear relation for assay temperatures between 16 and 30°C and the increase of Hb Aj value (D16, RIO), others have reported it as only linear between 16°C and 22°C (H6). Alternatives are to use a temperature-controlled room or a Perspex water bath to take 20 to 40 minicolumns, with water at 23°C circulated from a constant-temperature water bath. The smaller the elution volume, the more critical is the influence of temperature and the necessity for strict control (S42). The results for all short-column methods should be corrected to a 23°C value and so expressed. Hammons et al. (H4) have presented a valuable evaluation of three commercial minicolumn kits. Recently, an automatic low-pressure liquid chromatographic system has been described (B18), and Diamandis et al. (Dll) have reported favorably on an automated Hb Aj. analyzer, whose separation principle is a combination of reversed-phase partition and cation-exchange chromatography. 

The cations are replaced by protons. We discuss the principles of ion exchange chromatography in Chapter 21. 

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