Suppressor system

In most cases, the separation of alcohols, usually methanol, ethanol, and glycerol, is carried out contemporaneously with the separation of sugars and organic acids, and almost always the desire is to quantify all these analytes. It is seen, therefore, that the mobile phase is often an aqueous acid solution, even though only water may be used (5,9). Sulphuric acid is the one most frequently used, although phosphoric acid is preferred by some, since it is less corrosive on the components of the HPLC system (10). The concentration of sulphuric acid normally varies between 0.004 N and 0.01 N or more. The choice of acid may, however, be dictated by other considerations. This is the case, for example, with the use of a conductivity detector, which requires an appropriate conductivity suppressor system. If such a device is not available for a particular... 

A wide variety of different types of ion suppression columns are available for this purpose. It should be pointed out that any suppressor system introduced between the column and the detector that has a finite volume will cause some band dispersion. Consequently, the connecting tubes and suppression column must be very carefully designed to reduce this dispersion to an absolute minimum. 

The Sionic Sound Suppressor System was designed by Mitchell Werbell at Military Armament Corp. (MAC)in 1970. The system is named for Werbell s previous corporate venture, Sionics Inc., an acronym for Studies I n Operational Negation of insurgency and Countersubversion. 

The most commonly employed detector in ion chromatography is the conductivity detector, which is used with or without a suppressor system. The main function of the suppressor system as part of the detection unit is to chemically reduce the high background conductivity of the electrolytes in the eluent, and to convert the sample ions into a more conductive form. In addition to conductivity detectors, UV/Vis, ampero-metric and fluorescence detectors are used, all of which are described in detail in Chapter 6. 

Table 3-7 presents an overview of the various eluents being compatible with suppressor systems and their elution power. With the eluents listed in Table 3-7 and with the aid of organic additives described above, nearly all anions that are detected via conductivity may be analyzed using one of the many available anion exchangers. This multitude of stationary phases, with their different selectivities, enables the vast number of potential eluents to be reduced to a few versatile systems. Therefore, the statement that suppressor systems limit the choice of eluents is not valid. 

A suppressor system is used for the sensitive detection of ions via their electrical conductance. Its function is to chemically reduce the background conductivity of the electrolyte used as eluent before it enters the conductivity cell. Therefore, the suppressor system may be regarded as part of the detection system. 

The advantage of the suppressor technique is its higher sensitivity. In addition, the specificity of the method is also increased, since the chemical modification of eluent and sample in the suppressor system converts the conductivity detector from a bulk property detector into a solute specific detector [52]. Thus, exchanging eluent and sample cations with protons means that only the sample anions to be analyzed are detected by the conductivity detector and appear in the resulting chromatogram. 

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. 

While the parameters listed in Section 3.3.4.1 that determine the retention are fundamental in character, the parameters concerning the eluent discussed below depend on the detection system being used. This particularly applies to conductivity detection, which is possible directly or in combination with a suppressor system. These two modes of conductivity detection are fundamentally different and require eluents that not only differ with regard to their type but also to their concentrations and pH values, respectively. Therefore, the influence of these parameters on both modes is discussed separately for the most important detection system. 

The term system peak refers to signals that may not be attributed to solutes. System peaks are characteristic for ion chromatographic systems that have have no suppressor system when weak organic acids are used as the eluent. Despite numerous publications concerning this subject [72-76], system peaks were often the reason of misinterpretations. However, some facts about the thermodynamic and kinetic processes that occur within the separator column may be inferred [77,78] from their occurrence and help in understanding the chromatographic processes. 

Detection is usually carried out by measuring the electrical conductivity. When combined with a suppressor system, this detection method is superior to all other detection methods such as, for example, refractive index or UV detection at low wavelengths with regard to specificity and sensitivity. 

The detection of aliphatic carboxylic acids usually involves the measurement of the electrical conductivity. In ion-exclusion chromatography, therefore, suppressor systems are used to chemically reduce the background conductivity of the acid that acts as the eluent. Due to a lack of modern membrane suppressors in the past, suppressor columns... 

Fig. 4-8 shows a chromatogram of various organic acids obtained by using a modern membrane-based suppressor system. The acid concentration in the mobile phase determines the retention in the analysis of di-basic and poly-basic acids. It affects the degree of dissociation and, thus, the retention time of the carboxylic acid to be analyzed. In general, a higher resolution is observed with increasing add concentration. This effect... 

Fig. 4-8. Separation of organic acids upon application of a membrane-based suppressor system. — Separator column IonPac ICE-AS1 eluent 0.001 mol/L octanesulfonic acid flow rate 1 mL/min detection suppressed conductivity injection volume 50 pL solute concentrations 50 ppm oxalic acid (1), 50 ppm tartaric acid (2), 25 ppm fluoride (3), 50 ppm lactic acid (4), 50 ppm formic acid (5), 50 ppm acetic acid (6), and 100 ppm propionic acid (7).

HPICE Separator Column Suppressor System Conductivity Cell... 

Ion-pair chromatography is also suited for the analysis of metal complexes. For their chromatographic separation, the complexes must be thermodynamically and kinetically stable. This means that complex formation must be thermodynamically possible and furthermore an irreversible process. Metal-ETDA and metal-DTPA complexes exhibit a corresponding high stability. To separate the Gd-DTPA complex (Fig. 5-21), which is of great relevance in the pharmaceutical industry, TBAOH was used as the ion-pair reagent [36], Detection was carried out by measuring the electrical conductivity in combination with a suppressor system. 

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