Suppressed-Conductivity Detection

With suppressed conductivity detection, an acidic cationic eluent is used to separate the sample cations. The column effluent with zones of separated cations passes 

For example, if a dilute nitric acid eluent is used and sodium and potassium sample ions are to be separated, the following reactions take place in the suppressor unit  

The background conductivity is very low after the eluent passes through the suppressor unit theoretically it is that of pure water, fhe equivalent conductance of sample ions is high it is the sum of conductances of the alkali metal cation and the hydroxide counter ion. 

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. 

Excellent separations of all the alkali metal cations plus ammonium in 10 min or less with a strong acid (sulfonic acid) cation exchanger and a dilute solution of a strong acid as the eluent (incomplete sentence). However, divalent metal cations are more strongly retained by this column and require either an eluent containing a divalent cation or a more concentrated solution of the H eluent. 

With suppressed-conductivity detection, an acidic cationic eluent is used to separate the sample cations. The column effluent with zones of separated cations passes directly into the suppressor unit containing an anion-exchange membrane in the hydroxide form. The eluent cation is neutralized and the counteranions associated with the sample metal ions are exchanged for the more highly conducting hydroxide ion. 

By using an ion exchanger with carboxyl groups or with both carboxyl and phosphonate groups, it is possible to separate both monovalent alkali metal cations and certain divalent metal cations in a single run. A dilute solution of a strong acid such as methanesulfonic acid is generally used as the eluent. Particular emphasis has been placed on the separation of Li, Na, NH, K, Mg and 

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Figure 3 Gradient separation of anions using suppressed conductivity detection. Column 0.4 x 15 cm AS5A, 5 p latex-coated resin (Dionex). Eluent 750 pM NaOH, 0-5 min., then to 85 mM NaOH in 30 min. Flow 1 ml/min. 1 fluoride, 2 a-hydrox-ybutyrate, 3 acetate, 4 glycolate, 5 butyrate, 6 gluconate, 7 a-hydroxyvalerate, 8 formate, 9 valerate, 10 pyruvate, 11 monochloroacetate, 12 bromate, 13 chloride, 14 galacturonate, 15 nitrite, 16 glucuronate, 17 dichloroacetate, 18 trifluoroacetate, 19 phosphite, 20 selenite, 21 bromide, 22 nitrate, 23 sulfate, 24 oxalate, 25 selenate, 26 a-ketoglutarate, 27 fumarate, 28 phthalate, 29 oxalacetate, 30 phosphate, 31 arsenate, 32 chromate, 33 citrate, 34 isocitrate, 35 ds-aconitate, 36 trans-aconitate. (Reproduced with permission of Elsevier Science from Rocklin, R. D., Pohl, C. A., and Schibler, J. A., /. Chromatogr., 411, 107, 1987.)...

Figure 10 Separation of monochloroacetate, dichloroacetate, and trichloroacetate on a sulfonated poly(styrene-divinyl benzene) column with suppressed conductivity detection. Column 2% cross-linked sulfonated poly(styrene-divinyl benzene) capacity 0.02 meq/g. Flow rate 64 ml/hr. Eluant 15 mM sodium phenate. Suppressor 0.28 x 25 cm Dowex 50W X8 column (200-400 mesh). Detector Chromatronix conductivity cell connected to a Dow conductivity meter. (Reprinted with permission from Small, H., Stevens, T. S., and Bauman, W. C., Anal. Chem., 47,1801,1975. 1975 Analytical Chemistry.)...

Nowak, M. and Seubert, A., Application of experimental design for the characterization of a novel elution system for high capacity anion chromatography with suppressed conductivity detection, /. Chromatogr. A, 855, 91,1999. 

Many IC techniques are now available using single column or dual-column systems with various detection modes. Detection methods in IC are subdivided as follows [838] (i) electrochemical (conductometry, amper-ometry or potentiometry) (ii) spectroscopic (tJV/VIS, RI, AAS, AES, ICP) (iii) mass spectrometric and (iv) postcolumn reaction detection (AFS, CL). The mainstay of routine IC is still the nonspecific conductometric detector. A significant disadvantage of suppressed conductivity detection is the fact that weak to very weak acid anions (e.g. silicate, cyanide) yield poor sensitivity. IC combined with potentiometric detection techniques using ISEs allows quantification of selected analytes even in complex matrices. The main drawback... 

High-sensitivity detection of non-chromophoric organic ions can be achieved by combining the power of suppressed conductivity detection with these columns. Suppressed conductivity is usually a superior approach to using refractive index or low UV wavelength detection. 

Although AS and AES can be detected at a low UV wavelength, sensitivity is lacking and a more suitable detection was achieved using indirect photometric detection, post-column colour formation reactions, or a pre-column derivatisation, suppressed conductivity detection and refractive index detection [1,42,43]. A comparison of detection limits for the determination of these anionic surfactants shows that photometric and conductivity detectors are better (picomole or nanogram range) than refractive index or fluorometry detectors by about a factor of 1000 [40],... 

Ion chromatography has become an essential tool of the pharmaceutical analytical chemist. The high sensitivity of the technique, coupled with the wide dynamic operating range made possible with modern high-capacity stationary phases makes it ideal for the analysis of ions in pharmaceutical applications. The combination of gradients and suppressed conductivity detection provides a powerful screening... 

For ions that are UV transparent, detection is possible through the use of indirect detection. A wide variety of different eluent systems have been described in the literature. Eluents commonly used for indirect UV detection are similar to those used in non-suppressed conductivity detection phthalate and p-hydroxybenzoic acid along with other... 

Bromate has also been measured using 1C with conductivity detection. For example, EPA Method 302.0 uses two-dimensional 1C with suppressed conductivity detection to measure bromate at 0.12 pg/L detection limits [166]. Bromate, chlorite, and chlorate can also be measured by an earlier EPA Method (Method 300.1), which uses 1C with conductivity detection [167]. Method detection limits ranging from 0.45 to 1.28 pg/L can be achieved. 

Wagner P, Pepich BV, Pohl C, Srinivasan K, De Borba B, Lin R, Munch DJ (2009) EPA Method 302.0. Determination of bromate in drinking water using two-dimensional ion chromatography with suppressed conductivity detection. U.S. EPA, Cincinnati, OH, Available at http //water.epa.gOv/scitech/drinkingwaterAabcert/upload/met302 0.pdf... 

Non-suppressed conductivity detection furnishes a signal that is the sum of the conductance of the analyte ion, its co-ion, and the decrease in the eluent counterion that remains on the column... 

Suppressed conductivity detection is the most common mode of detection and differs from the previous approach for the use of an additional device, called suppressor, whose function is to reduce the background conductivity of the eluent prior to the conductivity cell and to increase the signal of the analyte. 

Figure 10.1 Ion-exchange chromatographic separation of main anions found in water using a Dionex HPLC and AS-11 HC column. Conditions column Dionex AS-11 HC 250 X 4 mm solvent Milli-Q water and 3 mM NaOH for 6 mins then to 30 mM NaOH over 15 min flow rate 1.5 ml/min suppressed conductivity detection.

Eluants other than carbonate/bicarbonate have also found wide application in many environmental and nonenvironmental analyses. Some common eluants are listed in Table 1.11.2. Sodium hydroxide solution has now become an eluant of choice for many ion chromatography analyses using suppressed conductivity detection. The schematic representation of the method is outlined in Figure 1.11.2. 

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. 

Ultraviolet absorbance detection is the most prevalent type of detection in CE, and UV detectors operate in both the direct and indirect modes. Laser-induced fluorescence detection is often used for high-sensitivity work. Conductivity detection, suppressed conductivity detection, and mass spec-... 

Souza e Silva, R. et al. Separation and determination of metaUocyanide complexes of Fe(II), Ni(II) and Co(III) by ion-interaction chromatography with membrane suppressed conductivity detection applied to analysis of oil refinery streams (sour water). J. Chromatogr. A. 2006, 1127, 200-206. 

Ion chromatography is used for the separation of ionic solutes such as inorganic anions and cations, low molecular-mass water-soluble organic acids and bases as well as ionic chelates and organometallic compoimds. The separation can be based on ion-exchange, ion-pair and/or ion-exclusion effects. Special detection techniques like ion-suppressed conductivity detection or indirect UV detection have to be used because most analytes are transparent to conventional UV detection... 

The kind of counter ion of an ion-pair reagent is vitally important for selecting the appropriate detection method. If suppressed conductivity detection is applied, the ion-pair reagent is used in its hydroxide form. With direct conductivity detection, salicylate is perferred as the counter ion for tetraalkylammonium cations [19,20], since these salts exhibit a lower background conductance in aqueous solution. According to Wheals [21], eluents such as cetyltrimethylammonium bromide in combination with citric acid at pH 5.5 have proved suitable for UV-, RI-, and amperometric detection, as well as for direct conductivity detection. This example is an impressive illustration of the versatility of ion-pair chromatography. 

The three above described solutions were subsequently shipped to a group of ca. 30 laboratories for an interlaboratory study which allowed to explain variations in standard deviations for ion chromatography due to e.g. the application of dilTerent columns, different eluents, the use of chemical or electronic suppression (conductivity detection) etc. This intercomparison actually allowed to constitute a group of experts and to prepare them for the certification campaign. In addition, this study enabled to confirm the suitability of the procedure used for the preparation of candidate... 

Eluent strength. Anion eluents vary considerably in their ability to elute sample anions. Several anions commonly used for IC with suppressed conductivity detection are listed in Table 6.5. An anion of higher charge, such as carbonate, has greater eluting strength than an eluent containing a monovalent anion. 

The major devices for suppressed conductivity detection in ion chromatography have been reviewed [2], These are described in chronological order in the following sections. 

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