Cation suppressed-conductivity detection

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. 

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. 

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

Gradient elution is feasible with modem suppressed conductivity detection. Several aliphatic amines were separated as the protonated amine cations in Fig. 7.3 using a sulfuric acid gradient [5]. The increasing acidity served to reduce the effective exchange capacity of the ion exchanger and thereby speed up elution of the larger monoamines and the diamines. 

Riviello et al. made a careful comparison of conductivity changes in cation chromatography between direct- and suppressed conductivity detection [7]. The calculation example is outlined in Fig. 7.5. The change in conductivity, AG, is actually slightly greater with non-suppressed conductivity. Flowever, the noise is much higher in the non-suppressed detection mode. Noise may be defined as the random signal that... 

The equivalent conductances of the aromatic bases listed in Table 7.2 are low enough for non-suppressed conductivity detection. However, the detection limits are generally lower with indirect UV detection and the resolution is often better also. In Fig. 7.8 chromatograms of the alkali metal cations are compared with direct conductivity and indirect UV absorption detection. With the aromatic base eluents Li elutes... 

Figure 5.20. Schematic representation of the operation of a membrane suppressor for suppressed conductivity detection of anions and cations. (From ref. [168] Elsevier).

Audalovic, N., Pohl, C. A., Rocklin, R. D., and Stilhan, J. R. Determination of cations and anions by capillary electrophoresis combined with suppressed conductivity detection. Anal. Chem., 65,1470-1475,1993. 

Ion chromatography (IC) is a low- to moderate-pressure liquid chromatographic (LC) technique and should be clearly distinguished from that of high-pressure LC (HPLC). IC as a determinative technique has been developed to separate both cations and anions. The instrument available to the student utilizes anion-exchange IC with suppressed conductivity detection. This technique can separate the common inorganic anions, fluoride,... 

Ion-exchange is the primary separation mode used with modern ion chromatography, although other approaches used for separation of inorganic anions and cations include ion interaction, ion exclusion, and chelation chromatography. Ion chromatography, with suppressed conductivity detection, is the most widely used and generally offers the best performance. 

This acetonitrile, acetone, and water eluent is ideal for this detector because it is completely volatile. Ammonium acetate may also be a useful eluent because of its high volatility. This eluent can provide both anions and cations as eluent ions. Sodium (or potassium or Hthium) carbonate bicarbonate eluents can be suppressed and then sent through an ELSD. This eluent operates at a high pH so can be used to separate on an anion exchanger weak acid anions such as borate or several organic acids. While these anions cannot be detected by suppressed conductivity detection (because of low conductance) it may be possible to adjust detector conditions to detect many of these weak acid anions by ELSD. 

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. 

A major difference between the two detection modes is that detection limits for alkali metal ions were 12-21 times lower with suppressed-conductivity detection [8]. Nevertheless, cations may be separated and quantified with direct detection down to fairly low concentrations. Another advantage is that the eluent remains acidic, and metal does not precipitate as easily as it might in the high pH environment of the suppressor. Figure 7.10 shows a separation of several metal cations including cesium and strontium at concentrations averaging only 1.0 ppm. 

A separation of eleven cations with indirect non-suppressed-conductivity detection is illustrated in Figure 7.18. The mobile phase contained both a strong acid (MSA) and oxalic acid as a complexing reagent [16]. The peaks actually denote decreasing conductivity. 

SC-CSRS 300 is a postcolumn electrolytic eluent suppressor package consisting of an eluent suppressor component (ES) and an analyte converter (AC) component that enables the suppressed conductivity detection of low-level ammonium and other amines. Figure 4.48 illustrates the ion movement through a SC-CSRS 300 in detail. The eluent and the ammonium analyte (an example of the cation analytes) exit the analytical column as methanesulfonic acid (H" MSA ) and ammonium methanesulfonate (NH4 MSA ). In the eluent suppressor component, the majority of the eluent is suppressed to water and the analyte is converted to the base form, NH4" OH . However, a small amount of MSA reenters the eluent chamber near the exit, which converts the analyte back to the salt form, so that the analyte exits the eluent suppressor component... 

The sequential analysis of alkaline-earth metals, which elute in the order Mg " < Ca " < Sr < Ba on strong acid cation exchangers, can also be performed using both conductivity detection modes. An eluent mixture of hydrochloric acid and 2,3-diaminopropionic acid is used for suppressed conductivity detection ethylenediammonium ions are suitable for nonsuppressed conductivity detection. Figure 4.56 shows a separation of alkahne-earth metals on Shimpack IC-Cl obtained with this eluent Because of the high elution power of the mobile phase, all monovalent cations present in the sample are eluted as one peak within the void volume of the column. As an alternative to strong eluents, shorter separator columns may be employed to reduce the retention of cations that have high affinities toward the stationary phase. 

Dionex Corporation (2004) Dionex Application Note No. 157 Comparison of Suppressed to Non-Suppressed Conductivity Detection for the Determination of Common Inorganic Cations. Dionex Corporation (now part of Thermo Fisher Scientific), Sunnyvale, CA, USA. 

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