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A Practical Guide to Ion Chromatography an INTRODUCTION and TROUBLESHOOTING MANUAL

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A Practical Guide to Ion Chromatography an INTRODUCTION and TROUBLESHOOTING MANUAL A Practical Guide to Ion Chromatography AN INTRODUCTION AND TROUBLESHOOTING MANUAL SeQuant Innovators in Chemical Analysis™ An introduction and troubleshooting manual · 1 A PRACTICAL GUIDE TO ION CHROMATOGRAPHY An introduction and troubleshooting manual SeQuant © Copyright, 1998-2007, SeQuant AB. All rights reserved. 2 · A Practical Guide to Ion Chromatography A Practical Guide to Ion Chromatography ISBN 978-91-631-8056-9 Published by SeQuant AB, Box 7956, 907 19 Umeå, Sweden. 2:nd edition. 1:st original print. Printed at Original, Umeå, Sweden, March 2007. Revisions and additional material can be found on SeQuant’s website (www.sequant.com). Questions and comments regarding the content in this booklet can be e-mailed to [email protected]. For more information, see our website at http://www.sequant.com A PRACTICAL GUIDE TO ION CHROMATOGRAPHY An introduction and troubleshooting manual Foreword The ions are thereby slowed down to an extent characteristic of each ion and arrive separated at This guide will show you some of the most com- the “suppressor”, where the eluent and analyte ions mon practical issues that occur during analysis of are transformed before they reach the detector. anions by ion chromatography. Many users find the technique difficult and hard to understand. It The detector measures the electrolytic conductivi- is our hope that we will be able to explain a few of ty of the eluate and the function of the suppressor the most fundamental prerequisites for analysis of is both to reduce the background of the eluent and water samples. We also want to give you guidance to increase the sensitivity for the sample ions. The on troubleshooting of the instruments. If your ion detector signals can be evaluated manually from chromatographic problems cannot be solved with recorder charts or automatically with an integrator this guide, SeQuant will be happy to assist you. or a computer-based data-acquisition system with suitable chromatography software. First of all, we recommend that you visit our web pages (http://www.sequant.com), where you will As with all chromatographic techniques it is very find more information on applications and our important to recognize the chromatographic terms products. efficiency, capacity and selectivity. If these factors are kept in mind, it is easier to achieve good sepa- The Ion Chromatographic System rations in a time-saving and economic way: Figure 1 shows an ion chromatographic system Efficiency with suppressor. Eluent is pumped by an eluent An efficient chromatographic system will generally pump through an injection valve, where exactly produce high and narrow chromatographic peaks, reproducible sample volumes are injected. and thus provide good sensitivity. The efficiency is controlled by the column choice and careful selec- tion of other components of the flow system. Capacity The capacity term has to do with the ability of the column to attract ions and the eluent strength re- quired to elute these through the column. To put it simply, the capacity relates to the retention time of the ions and the ability to accommodate samples with a large span in ion concentrations. Selectivity Figure 1: Anion chromatography system comprising a SAMS™ Selectivity refers to the capability of the separation suppressor with CARS™ 4WD for continuous regeneration. system in achieving separation between different analytes. The selectivity depends on the chemical The sample ions are carried by an eluent flow to a and physical qualities of the column, that result in separation column, where interaction with fixed interactions with the ions to be separated, and also ions of opposite charge (i.e., positive) take place. on the choice of eluent. © Copyright, 1998-2007, SeQuant AB. All rights reserved. 4 · A Practical Guide to Ion Chromatography The Eluent Isocratic Elution and Gradient Elution The eluent transports the sample through the sys- The most common type of elution is isocratic, i.e., tem and contributes to the selectivity of the sepa- where the eluent has a constant concentration and ration. The eluent is a solution of a salt in water, composition during the entire run. Another possi- alternatively a solution of several salts, that also bility of elution is a gradient, i.e. where the eluent act as a buffer, providing a stable pH. The table concentration is changed in a reproducible way in below describes how the qualities of the eluent are each run. The reason for using gradient elution is affected by different parameters. that ions with widely different retentions can be Table 1: Influence by different eluent parameters on the separated in the same run with good separation retention in anion chromatography. efficiency and reasonable retention times. Hydroxide eluents are commonly used for Parameter Effect on retention in anion IC gradient elution in anion chromatography. By gra- Ion strength The eluting ability increases with eluent dually increasing the concentration of hydroxide ionic strength. The selectivity among ions ions, the eluting power of the eluent increases. As of equal charge is only marginal, whereas a result, ions with high retention stay at the top of the selectivity between ions of different the column almost without being eluted until the charge (mono- or polyvalent) is far more eluent concentration is sufficiently high. They then sensitive to changes in ion strength. elute in sequence as sharp peaks. A pump that is capable of mixing the gradient must be used, and pH The retention times for anions of weak because the eluent ionic strength increases during acids increase when the pH of the eluent the run, the suppressor system must work very increases in the vicinity of the pKa of the effectively to minimize the baseline drift of the acid. This is due to the sample ion charge system. After each gradient run the column must being controlled by the eluent pH – the be equilibrated with the start concentration of the more basic the eluent, the stronger the eluent before the next sample can be injected. negative charge. In a suppressed system, the eluent pH is substantially higher than Choice of Eluent the pKa's of most analytes, with the excep- tion of, e.g., acetate or phosphate ions. Several considerations govern the choice of eluent. The first factor is the kind of sample ions that will Temperature The ion exchange rate between the statio- be separated, but the type of separation column is nary and the mobile phase increases with also important. The two most common eluents are increasing temperature. The viscosity of based on hydroxide or carbonate as eluting anion. the eluent, and thereby the column back- Their different qualities are described below. pressure, decreases and can give a better separation efficiency. The temperature Carbonate Eluents can also affect the column selectivity. The carbonate eluent has traditionally been used in anion analysis. The eluent is an aqueous Flow rate The ions are eluted faster with high eluent solution of carbonate and hydrogen carbonate flow rates, but faster elution will decrease salts, and has the advantage that the total ionic the separation efficiency. The flow rate is strength as well as the proportions of the mono- also limited by the pressure durability of – 2– valent (HCO3 ) and the divalent (CO3 ) ions can be the separation column. varied to optimize the retention time and the Buffer salt The eluting power of the eluent, and also selectivity between monovalent and multivalent to some extent its selectivity, are affected sample ions. by the anion of the eluent (through its pKa Carbonic acid (H2CO3) is formed when the carbo- value). A change of salt will normally also nate eluent passes through the suppressor. Despite result in a change of the eluent pH. the relatively high pKa of carbonic acid, it still gives For more information, see our website at http://www.sequant.com An introduction and troubleshooting manual · 5 some background conductivity after the suppres- Because of the high pH of the hydroxide eluent, sor. For typical carbonate eluents, the eluate pH is the retention order is different than with a carbo- around 4 due to the carbonic acid, which produces nate eluent. Phosphate, e.g., is eluted mainly as –1 3– a conductivity in the vicinity of 10-20 µS·cm . trivalent phosphate ion (PO4 ) and will elute after 2– the sulfate ion (SO4 ). HCO ' H+ + HCO – pK = 6.36 2 3 3 a1 Preparation of Eluent At this pH, analytes that have been converted into The eluent must be prepared from freshly tapped, – – 2– strong acids (e.g. Cl , NO3 and SO4 ) will be fully deionised laboratory water with low conductivity. dissociated, while the pH of the eluate controls the Deionised water should not be stored, neither in degree of dissociation of weak acids. Acetic acid is glass vessels, nor in plastic bottles, since the con- for example only about 20 % dissociated at this pH ductivity increases fast with an accompanying de- and because only the dissociated ions give rise to terioration in eluent quality. electrolytic conductivity, a major part is presented The sodium hydroxide that is used should be of to the detector in a form that produces no signal, the purest quality. In order to obtain as low back- with lower sensitivity as a result. ground conductivity as possible, the hydroxide The Hydroxide Eluent eluents should be prepared from carbonate-free The advantage of using a hydroxide eluent is that 50 weight-% NaOH or KOH. it is transformed into pure water in the suppressor Hydroxide eluents are very corrosive and must reaction, and consequently gives a very low back- not be spilled on the skin or in the eyes. Use ground conductivity. eye protection and other safety equipment. To obtain a really low and stable background level, the hydroxide eluent has to be protected from air, The table below shows the molecular weights of since the carbon dioxide present in the air is easily some common salts that are used for preparation absorbed by the basic eluent and converted into of eluents.
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