Suppressor devices

A small volume of the sample, typically <0.5 mL, is introduced into the injection system of an ion chromatograph. The sample is mixed with an eluent and pumped through a guard column, a separation column, a suppressor device, and a detector, normally a conductivity cell. The separation column is an ion exchange column that has the ability to separate the ions of interest. The separation column is often preceded by a shorter guard column containing the same substrate as the separation column in order to prevent the separation column from becoming overloaded and/or blocked by particles. Different types of separation columns, eluents, and suppression devices have to be used for anions and cations. Each ion is identified by its retention time within the separation column. 

A simpler and technologically superior approach is the measurement of the direct electrical conductance. The background conductivity of the mobile phase is electronically subtracted, not requiring a suppressor device. One example of direct conductivity detection is the simultaneous determination of potassium nitrate and sodium monofluorophosphate in dentrifices [76]. Alendronate, a bisphonate, can be directly detected in intravenous solutions and tablets using an anion-exchange column and conductivity detection [77]. Another example, from one of the author s (JA) laboratory is shown in Figure 5.3. Direct conductivity detection makes it possible to selectively detect choline in the presence of an equal molar amount of an antibiotic which is not detected. 

The first approach uses a suppressor device which is located between the analytical column and the detector cell. This device chemically removes the mobile-phase buffer counterions, thus reducing the background conductivity. This type of detector increases postcolumn dead volume and puts... 

Suppressor devices include packed column suppressors, hollow-fiber membrane suppressors, micromembrane suppressors, suspension postcolumn reaction suppressors, autoregenerated electrochemical suppressors, and so forth. 

Another approach of ion chromatography is the nonsuppressed single column, in which no suppressor device is used. In this case, the only method for improving the sensitivity is to maximize the difference between mobile-phase conductivity and analyte conductivity. 

Recently, membrane-based suppressor systems are increasingly used in addition to conventional suppressor columns. The characteristic properties of the various suppressor devices are discussed below. 

Ion chromatography with suppressed conductivity detection has been extremely successful in filling a large gap that previously existed in inorganic analysis. However, the necessity for a suppressor device does add to the complexity of the instrumentation. It also restricts the type of eluent that can be used and to some extent limits the separating ability of the method. 

S. Rabian, J. Stillian, V. Barreto, K. Friedman and M. Toofan, New membrane-based electrolytic suppressor device for suppressed conductivity detection in ion chromatography, J. Chromaiogr.,... 

Figure 4.1 Schematic of an ion chromatograph instrument. The classic modular building design of liquid chromatography is seen here again, yet with the difference that the separation is generally performed isocraticaUy. The configuration shows a suppressor device installed after the column and in series with a conductivity detector. The suppressor serves to eliminate the ions arising from the eluent to improve sensitivity.

The principle of functioning of a conductivity detector lies in differential measurement of mobile phase conductivity both before and during solute ion elution. The conductivity cell is placed either directiy next to an analytical column or after a suppressor device, which is required to reduce background conductivity, in order to increase the signal-to-noise ratio, and thus sensitivity. In IC, without eluate conductivity suppression the signal-to-noise ratio can be maximized if a low-conductivity mobile phase at a low concentration is used. 

The quahty of the eluent solvent is critical to achieve a low background signal. The amount of residue after evaporation must be less than 1 mg L . Solvents must be filtered through compatible sub-micron filters (0.4 or 0.2 pm). Ttlso, if possible, samples must also be filtered with special filters before injection. Obviously, the salt or buffer in the eluent is also important because this can produce a background signal. The ideal eluent solvent for this detector contains no salts or buffers. Another possibility is to remove the eluent buffer with a suppressor device before detection. Of course this may Hmit the types of chromatography... 

The characteristic properties of various suppressor devices are discussed below. 

Dasgupta et al. [22] as well as Avdalovic et al. [23] independently succeeded to miniaturize a conductivity cell and a suppressor device down to the scale required for CE. Since the sensitivity of conductivity detection does not suffer from miniaturization, detection limits achieved for totally dissociated anions and low molecular weight organics compete well with those of ion chromatography techniques. Thus, capillary elechophoresis with suppressed conductivity detection can be regarded as a complementary technique for analyzing small ions in simple and complex matrices. 

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