**3.5. Detectors**

Current LC detectors have a wide dynamic range normally allowing both analytical and preparative scale runs on the same instrument.

An ideal detector should have the following properties: low drift and noise level (particularly crucial in trace analysis), high sensitivity, fast response, wide linear dynamic range, low dead volume (minimal peak broadening), cell design which eliminates remixing of the separated bands, insensitivity to changes in type of the solvent, flow rate and temperature, operational simplicity and reliability. It should be non-destructive.

**Electrical conductivity detector** is commonly use. The sensor of the electrical conductivity detector is the simplest of all the detector sensors and consists of only two electrodes situated in a suitable flow cell. The sensor consists of two electrodes sealed into a glass flow cell. In the electric circuit, the two electrodes are arranged to be the impedance component in one arm of a Wheatstone bridge. When ions move into the sensor cell, the electrical impedance between the electrodes changes and the 'out of balance signal' from the bridge is fed to a suitable electronic circuit. The 'out of balance' signal is not inherently linearly related to the ion concentration in the cell. Thus, the electronic circuit modifies the response of the detector to provide an output that is linearly related to the ion concentration.

conditions. Care needed to be taken with controlling the suppressor current in order to avoid damage to the suppressor and also the generation of ionic components from oxidation of the organic solvents (especially methanol) present in the eluent. Further potential problems, arising when using suppressors as de-salting devices with organic analytes, are the possibility of analytes loss in the suppressor as a result of adsorption or precipitation effects and disper‐

Weakly acidic analytes are anionic in the presence of the high pH eluents used with anionexchange IC, but become protonated in the suppressor and are therefore prone to hydrophobic adsorption or precipitation. Similarly, weakly basic analytes are separated as cations with low pH eluents but are deprotonated in the suppressor to form neutral species. The micromembrane suppressor consists of layered ion-exchange membranes and fibrous chamber screens with the regenerant chamber screen modified to possess a high ion-exchange capacity which serves as a reservoir for regenerant ions. There is also a possibility of losses of analytes resulting from penetration of the analyte through the suppressor membrane into the regener‐ ant chamber. Theoretically, anionic analytes are not able to penetrate the cation-exchange

Introduction of a suppression device between the column and the detector can be expected to cause some degree of peak broadening due to diffusional effects. The shape of the analyte band will also be influenced by hydrophobic adsorption effects, especially when the adsorption and desorption processes are slow. Examination of peak shapes and analyte losses can therefore provide important insight into the use of suppressors with organic analytes which are weakly acidic or weakly basic. It can be expected that peak area recovery rates after suppression are governed by a combination of hydrophobic interactions with the suppressor and permeation through the membranes with the balance between these mechanisms being determined by

Current LC detectors have a wide dynamic range normally allowing both analytical and

An ideal detector should have the following properties: low drift and noise level (particularly crucial in trace analysis), high sensitivity, fast response, wide linear dynamic range, low dead volume (minimal peak broadening), cell design which eliminates remixing of the separated bands, insensitivity to changes in type of the solvent, flow rate and temperature, operational

**Electrical conductivity detector** is commonly use. The sensor of the electrical conductivity detector is the simplest of all the detector sensors and consists of only two electrodes situated in a suitable flow cell. The sensor consists of two electrodes sealed into a glass flow cell. In the electric circuit, the two electrodes are arranged to be the impedance component in one arm of a Wheatstone bridge. When ions move into the sensor cell, the electrical impedance between the electrodes changes and the 'out of balance signal' from the bridge is fed to a suitable electronic circuit. The 'out of balance' signal is not inherently linearly related to the ion

membranes of the anion suppressor due to the effects of Donnan exclusion.

eluent composition, suppression conditions and analyte properties.

preparative scale runs on the same instrument.

simplicity and reliability. It should be non-destructive.

sion of the analyte band in the suppressor.

10 Column Chromatography

**3.5. Detectors**

The amplifier output is then either digitized, and the binary number sent to a computer for storage and processing, or the output is passed directly to a potentiometric recorder. This would result in a false change in impedance due to the generation of gases at the electrode surfaces. The frequency of the AC potential that is applied across the electrodes is normally about 10 kHz. In its simplest form, it can consist of short lengths of stainless steel tube insulated from each other by PTFE connecting sleeves.

**Amperometric detection** is a very sensitive technique. In principle, voltammetric detectors can be used for all compounds which have functional groups which are easily reduced or oxidized. Apart from a few cations (Fe3+, Co2+), it is chiefly anions such as cyanide, sulfide and nitrite which can be determined in the ion analysis sector. The most important applications lie however in the analysis of sugars by anion chromatography and in clinical analysis using a form of amperometric detection know as Pulsed Amperometric Detection (PAD).

**Mass Spectrometry**: Mass to charge ratio (m/z) allows specific compound ID determination. Several types of ionization techniques: electrospray, atmospheric pressure chemical ionization, electron impact. The detector usually contains low volume cell through which the mobile phase passes carrying the sample components.

**Detector sensitivity** is one of the most important properties of the detector. The problem is to distinguish between the actual component and artifact caused by the pressure fluctuation, bubble, compositional fluctuation, etc. If the peaks are fairly large, one has no problem in distinguishing them however, the smaller the peaks, the more important that the baseline be smooth, free of noise and drift. Baseline noise is the short time variation of the baseline from a straight line. Noise is normally measured "peak-to-peak": i.e., the distance from the top of one such small peak to the bottom of the next. Noise is the factor which limits detector sensitivity. In trace analysis, the operator must be able to distinguish between noise spikes and component peaks. For qualitative purposes, signal/noise ratio is limited by 3. For quantitative purposes, signal/noise ratio should be at least 10. This ensures correct quantification of the trace amounts with less than 2% variance. The baseline should deviate as little as possible from a horizontal line. It is usually measured for a specified time, e.g., 1/2 hour or one hour and called drift. Drift usually associated to the detector heat-up in the first hour after power-on.

Sensitivity can be associated with the slope of the calibration curve. It is also dependent on the standard deviation of the measurements. The higher the slope of your calibration curve the higher the sensitivity of your detector for that particular component, but high fluctuations of your measurements will decrease the sensitivity. The more selective the detection, the lower is signal/noise and the higher the sensitivity. The detector response is linear if the difference in response for two concentrations of a given compound is proportional to the difference in concentration of the two samples.

### **3.6. Data system**

The main goal in using electronic data systems is to increase analysis accuracy and precision, while reducing operator attention. In routine analysis, where no automation (in terms of data management or process control) is needed, a pre-programmed computing integrator may be sufficient. For higher control levels, a more intelligent device is necessary, such as a data station or minicomputer.

< SO4

respectively.

*4.2.1. Bromate*

2− for the anions and Na+

 < NH4 + < K+

the above anions or cations was demonstrated within 35 min each. Detection limits calculated were 0.05–0.58 ppm for the anions and 0.05–0.38 ppm for the cations, whereas repeatability values were below 2.26, 2.76, and 2.90% for peak height, peak area and retention time,

**Figure 5.** Schematic diagram of the instruments used for simultaneous separation of anions and cations [10].

Bromate has been classified as a human carcinogen by both the IARC (International Agency for the Research on Cancer) and the USEPA (United States Environmental Protection Agency) and is known to be toxic to fish and other aquatic life [11, 12]. Bromate could be produced in aquatic systems upon the oxidation of aqueous bromide. Controlled ozonation has been considered as an effective disinfectant tool in aquatic systems [13] but when sea water is subjected to ozonation, oxy-bromide ozonation by-products (OBP) are produced and these are important both in terms of their disinfection ability and also in relation to their potential toxicity. When seawater is oxidized, aqueous bromide (Br-) is initially converted to hypobro‐ mite (OBr¯) which can then either be reduced back to bromide or oxidized further to bromate (BrO3-) which is known to be toxic to fish and other aquatic life and classified as a human carcinogen. There has been thus a considerable interest in bromate analysis so that trace

analysis of bromate in water has received considerable attention in recent years.

**4.2. Qualitative and quantitative analysis of halides**

< Mg2+ <Ca2+ for the cations. Complete separation of

Ion Exchange Chromatography - An Overview

http://dx.doi.org/10.5772/55652

13
