Column suppressor

In the early 1980s, conventional suppressor columns became technologically outdated and were replaced by membrane-based suppressor systems. Nevertheless, miniaturized forms of the original packed-bed suppressors are still available. 

The characteristic properties of various suppressor devices are discussed below. 

The particle size of the resin and the void volume of the suppressor column affect the quality of the separation, because both parameters determine the peak broadening. The total volume of the suppressor column should be as small as possible to prevent mixing of the already separated signals. For the resulting suppression capacity, however, the total volume of the suppressor column should be as large as possible. These two requirements are incompatible, so the dimensioning of packed-bed suppressors is always a compromise between the suppression capacity and the peak broadening caused by the void volume. Therefore, it is advisable to use suppressor columns only in combination with separator columns packed with resins that have a particle diameter of more than 15 pm. Under these conditions, the contribution to the total peak variance is small. 

The regeneration of conventional packed-bed suppressors is usually carried out for about 15 min using 0.5 mol/L sulfuric acid with a flow rate of 4mL/min. Subsequently, the system is flushed with deionized water for several minutes and then it is conditioned with eluent again. 

8 mmol/L NaHCOs + 2.2 mmol/L Na2C03 -I- 0.22 mmol/L p-cyanophenol, to 5 mmol/L 

This setup represents a certain improvement over conventional packed bed suppressors, because the void volume of the cartridges is very small. However, as a consequence, the suppression capacity of the MSM is also very small, so that this kind of packed bed suppressor can only be used for low ionic strength eluants (see Fig. 3-18 in Section 3.4.1). High-capacity anion exchangers as well as gradient elution techniques cannot be utilized with this suppressor module. 

The basic problem of packed bed suppressors, i. e. periodic regeneration of cartridges, is not addressed by the Metrohm Suppressor Module. On the other hand, the MSM does deliver low noise, as do all chemically regenerated packed bed suppressor systems. 

In 2001, the ERIS suppressor was replaced by the DS-Plus suppressor, the first packed bed suppressor that is continuously regenerated. As shown in Fig. 3-67, the eluant enters the resin-based suppressor and is split into three paths. Two paths are directed to the electrodes where electrolysis takes place, thereby providing the regenerant ions. The third path carries the analyte ions through the suppression bed. Hydronium ions generated at the anode replace 

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To minimize the mobile phase s contribution to conductivity, an ion-suppressor column is placed between the analytical column and the detector. This column selectively removes mobile-phase electrolyte ions without removing solute ions, for example, in cation ion-exchange chromatography using a dilute solution of HCl as... 

Ion-exchange chromatography in which conditions are adjusted so that an ion-suppressor column is not needed. 

In IC this problem of electrolyte background is overcome by means of eluant suppression. Thus in the above example of sodium and potassium analysis, if the effluent from the separating column is passed through a strong base anion exchange resin in the hydroxide form (suppressor column) the following two processes occur ... 

A flow scheme for the basic form of ion chromatography is shown in Fig. 7.3, which illustrates the requirements for simple anion analysis. The instrumentation used in IC does not differ significantly from that used in HPLC and the reader is referred to Chapter 8 for details of the types of pump and sample injection system employed. A brief account is given here, however, of the nature of the separator and suppressor columns and of the detectors used in ion chromatography. 

Suppressor column. Where electrical conductance is used for detection of sample ions in the effluent from the columns, an eluant background of low conductivity is required. The function of the suppressor column is to convert eluant ions... 

The low-concentration eluants used to separate the sample ions on the separator column allow a substantial number of samples (typically about 50) to be analysed before the suppressor column is completely exhausted. Clearly an important practical consideration is the need to minimise the frequency of regeneration of the suppressor column and, for this reason, the specific capacity of the column is made as large as possible by using resins of moderate to high cross-linking. Some instruments contain two suppressor columns in parallel,... 

It is appropriate to refer here to the development of non-suppressed ion chromatography. A simple chromatographic system for anions which uses a conductivity detector but requires no suppressor column has been described by Fritz and co-workers.28 The anions are separated on a column of macroporous anion exchange resin which has a very low capacity, so that only a very dilute solution (ca 10 4M) of an aromatic organic acid salt (e.g. sodium phthalate) is required as the eluant. The low conductance of the eluant eliminates the need for a suppressor column and the separated anions can be detected by electrical conductance. In general, however, non-suppressed ion chromatography is an order of magnitude less sensitive than the suppressed mode. 

The experiment described illustrates the application of ion chromatography (Section 7.4) to the separation and determination of the following anions Br", Cl , NO3 and N02 It may be readily extended to include other anions, such as F , H2PC>4, and SO -. The experiment is based on the Waters ILC Series Ion/Liquid Chromatograph which does not require the use of a suppressor column. 

The action of the suppressor column is thus to reduce the conductance of the mobile phase to a low level, by removing the HC1,... 

We would need an anion-exchange resin in the analytical column, a cation-exchanger in the suppressor column, and a mobile phase containing NaOH. In the suppressor column, Na+ would be exchanged for H +. ... 

FI There are two fairly obvious disadvantages associated with the use of suppressor columns can you see what they are ... 

The disadvantages are that the suppressor column will increase the dispersion of the system, and it will also have to be regenerated from time to time. 

An alternative means of detection involves UV spectrophotometry, the mobile phase containing the strongly absorbing phthalate ion which gives a constant high absorbance baseline signal that displays negative peaks as the sample components elute from the separator column. No suppressor column is therefore needed and the sensitivity is comparable to conductometric detection. 

Common eluents in suppressor ion chromatography are dilute solutions of mineral acids or phenylenediamine salts for cation separations and sodium bicarbonate/sodium carbonate buffers for anion separations. These eluents are too highly conducting to be used without a suppressor column or conductivity detection. Fritz et al. [54-56] have... 

However, it has to be borne in mind that salts added to the mobile phase, their type and concentration, and the pH can influence the retention order. This technique can be modified by the application of a so-called suppressor column. The suppressor column exchanges the counter-ion of the analyte after the separation column, enhancing in this manner the sensitivity of conductometric detection of analyte ions. 

A few DBFs, such as bromate, chlorate, iodate, and chlorite, are present as anions in drinking water. As a result, they are not volatile and cannot be analyzed by GC/MS. They are also difficult to separate by LC, but will separate nicely using ion chromatography (IC). At neutral pH, HAAs are also anions and can be separated using 1C. A number of methods have been created for these DBFs using both IC/ inductively coupled plasma (ICF)-MS and IC/ESl-MS. Fretreatment to remove interfering ions (e.g., sulfate and chloride), along with the use of a suppressor column prior to introduction into the MS interface, is beneficial for trace-level measurement. 

A detailed description of IC is given in reference 1 however, the basic principles of the method can best be described by an example. Figure 1 schematically represents both an anion and a cation IC analysis. In both cases, the instrumentation involves a pumping system, an eluent, an injection valve, an ion-exchange separator column, an ion-exchange suppressor column and a conductivity cell. The sample is first injected into the flow system then the well known reaction equilibrium shown in Figure 1 results in the separation of sample anions or cations on the separator column (2). 

The second column is called a suppressor column. Its function is to convert the eluent to a less conductive species while converting sample ions to a common form. This system enables conductimetric detection of the sample ions in a low conductivity background. The ion-exchange suppressor reactions are also shown in Figure 1. In the case of anion analysis, sodium carbonate and/or bicarbonate eluent is converted to a weakly conductive dilute carbonic acid while the sample ions are converted to strong-... 

H+X and Y+OH must be stable species in solution. Amphoteric species such as amino acids are retained on the suppressor column and, therefore. 

H+ and Y+OH must be species with less than about 20 carbon atoms. Larger organic species tend to absorb in the suppressor column resulting in excessive peak tailing or total absorption. 

An Ion Chromatograph has been successfully automated by interfacing it to an automatic sampler (7). Continuous unattended analysis was possible, the actual number of samples analyzed being limited by the ionic capacity of the suppressor column. The automated Ion Chromatograph was used to analyze soluble sulfates, ammonia and alkyl amines in stack and automobile exhqust samples. Excellent agreement between IC and automated barium chloroanilate titration for sulfate was obtained with a relative standard deviation less than 5%. 

Eluent 0.003 M NaHCOj/O.0024 M Na2C03 Suppressor Column 6 x 250 mm Flow Rate 138 ml/hr Anion Suppressor... 

Analyses were done on a Dionex Model 14 Ion Chromatograph (IC), equipped with a Waters WISP 7 autosampler, Linear recorder, and interfaced with a Hewlett-Packard 3354 Laboratory Automated System. The principal components of the IC, shown in Figure 2, are (A) eluent reservoir, (B) pump, (C) injection valve, (D) separator column, (E) suppressor column, (F) conductivity cell, and (G) conductance meter with a recorder (integrator). 

Samples and standards are injected into the IC in 100-yL aliquots. The ions are separated by their varying affinites for the ion exchange resin in the anion separator column, the oppositely charged ions are stripped away by the suppressor column leaving only the separated anions and water to be detected by the conductivity cell. Samples are quantitated by comparison with a calibration curve. 

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