Soil and Environmental Analysis: Modern Instrumental Techniques - Chapter 5 pptx

Depending on the analytical accuracy and precision required, ion chromatographs can be divided into two major groups: those that operate on the principle of eluent suppression dual-colum

of high-performance liquid chromatography (HPLC), has gained popularity for accurate and precise determination of anions and cations in soils, plants, water, and other environmental materials, as well as samples from clinical, metal plating, power generation, semiconductor fabrication, and other industrial sources Several books have been published on IC, including the development and use of its components, and the potential of the technique as an analytical tool (Sawicki et al., 1978; Mulik and Sawicki, 1979a; Fritz et al., 1982; Smith and Chang, 1983; Weiss, 1986; Tarter, 1987; Small, 1989).

This is a revised version of the corresponding chapter in the previous edition (Tabatabai and Basta, 1991) It covers the basic principles of IC, the instruments and methods that have been developed, and the application

of these methods to the analysis of soil, plant, water, and environmental samples New approaches for application of IC to chemical speciation are described Application of IC to soil and environmental analysis has been previously reviewed by Frankenberger et al (1990), Tabatabai and Basta (1991), Tabatabai and Frankenberger (1996), and Karmarkar (1998) Several IC methods are available for the determination of ions other than those discussed in this chapter, but these methods have not been evaluated for soil analysis (Sawicki et al., 1978; Mulik and Sawicki, 1979a, b; Johnson, 1987).

II BASIC PRINCIPLES

Ion chromatography has its roots in pioneering work in the area of ion exchange, including the development of synthetic ion-exchange resins This technique falls under the broad category of liquid chromatography A review of the work published on these topics is beyond the scope of this chapter, but information on the basic principles involved in the operation

of ion chromatographs is presented Typical components of an IC system (Fig 1) include an optional autosampler, a high-pressure pump, and an injection valve with a sample loop of suitable size (typically 10–250 mL), a guard column (also called a precolumn), an analytical column, a postcolumn reaction system, a flow-through detector, and a data station ranging in complexity from a chart recorder to a computerized data system A suitable

conductivity detector; PAD, pulse amperometric detector.

and NH 4 ; (2) ion exclusion, which is used for the separation of low-molecular-weight organic acids (e.g., adipic, acetic, formic, malic, malonic, oxalic, succinic, and tartaric acids); and (3) ion pair separation, including separation of heavy metals and transition metal ions (e.g., Cd 2þ ,

Co 2þ , Cu 2þ , Fe 2þ , Fe 3þ , Pb 2þ , Mn 2þ , Ni 2þ , and Zn 2þ ) Details of each of these separation modes are described by Haddad and Jackson (1990).

The first IC system developed in the early 1970s used conductimetric detection, but recent IC equipment features colorimetric (UV-VIS), pulse amperometric or spectroscopic detection systems, including inductively coupled plasma (ICP) spectrometry or hydride generation and atomic absorption spectrometry The development of new detection modes has increased the capability of IC to measure a great number of analytes with improved detection limits.

Depending on the analytical accuracy and precision required, ion chromatographs can be divided into two major groups: those that operate

on the principle of eluent suppression (dual-column system) and those with

no suppressor column (single-column system) Detailed comparisons between eluent-suppressed and nonsuppressed ion chromatography have been presented by Pohl and Johnson (1980) and Tarter et al (1987) Both types employ conductimetric detection systems, based on the variation in electrical conductivity of a solution with the concentration

of ions present These detectors are used for the determination of all ionic species (inorganic anions and cations, and organic acids) in solution Calibration graphs of specific conductance vs ion concentration used in IC are usually linear at low concentrations of each ion (< 100 mg L
1 ).

A Systems with Conductimetric Detectors

1 Eluent-Suppressed Ion Chromatography

Until the late 1980s, the suppressed-type IC was only marketed by Dionex Corporation (Sunnyvale, CA) Figure 2 shows its basic components For simplicity, the reservoirs of the eluent and water used for regeneration of the suppressor column and the valving system involved in the IC are not shown The instrument employs the following components:

1 An eluent pump and reservoir

2 A sample injection valve (the sample loop can be adjusted from about 50 mL to several hundred mL)

3 An ion-exchange separation column

4 A suppressor column coupled to a conductivity detector, meter, and output device

5 A regenerating pump with electronic timer and controls

Several types of column are commercially available for the exchange separation of the common inorganic and organic anions via eluent-suppressed IC The resin material used and the available columns were described by Weiss (1995).

ion-In the eluent-suppressed IC, the ion species are resolved by ventional elution chromatography followed by passage through an eluent stripper, or ‘‘suppressor,’’ column, wherein the eluent coming from the separating column is stripped or neutralized Thus only the ion species of interest leave the bottom of the suppressor column; anions emerge in a background of H 2 CO 3 , which exhibits a low conductivity, while cations emerge in water These ions are monitored subsequently in the conductivity

each working day This device had two disadvantages: (1) a relatively large volume of the suppressor column resulted in band broadening, which resulted in loss in chromatographic efficiency; and (2) the detector response to the ions of strong acids or bases decreased, whereas the response

to ions of weak acids or bases increased as the active sites of the suppressor column were steadily depleted The lack of a steady state resulted in poor precision Despite these disadvantages, the packed-bed suppressor provided the foundation on which the suppressed IC was developed The background suppression (eluent, NaHCO 3þNa 2 CO 3 ) was achieved according to

1 A hollow-fiber membrane suppressor (Steven et al., 1981) and membrane suppressor (Franklin, 1985; Stillian, 1985) that is generated

micro-electrochemically (Henshall et al., 1992); commercially available with IC systems from Dionex.

2 The QuikChem small suppressor that is regenerated after every sample using chemicals (Karmarkar, 1996); commercially available with an IC system from Zellweger Analytics, Inc., Lachat Instruments Div (Milwaukee, WI).

3 A set of two parallel small suppressor columns: one is regenerated electrochemically while the other is being used (Saari-Nordhaus and Anderson, 1996); commercially available with an IC system from Alltech Associates, Inc (Deerfield, IL).

4 A device with postcolumn addition of a colloidal suspension of a high-capacity ion exchange material, also called solid phase reagent (Gjerde and Benson, 1992); available commercially from Sarasep, Inc (San Jose, CA).

5 A self-regenerating suppressor (SRS), which utilizes autosuppression

to enhance analyte conductivity while decreasing eluent conductivity, thus resulting in a significant improvement in analyte detection limits,

is marketed by Dionex The ions required for eluent suppression are generated in the SRS by the electrolysis of water The SRS combines the best features of micromembrane suppressor—high suppres- sion capacity, minimal peak dispersion, solvent compatibility, and continuous use—with the added advantage of effortless operation and

no maintenance.

With the use of a hollow-fiber membrane suppressor or brane suppressor, the problems associated with the original packed-bed suppressor technique, such as band broadening, ion exclusion, and oxidation of NO

micromem-2 , are eliminated The disposable solid-phase chemical suppressor (SPCS) simplifies the instrumentation required to perform suppressed-based IC by eliminating the regeneration system and the complex postcolumn reaction system needed with other suppression techniques (Saari-Nordhaus et al., 1994) The lifetime of the SPCS cartridge

is dependent on the ionic strength and flow rates of the eluent, varying from

7 to 12 h.

In a variant of the suppressor column system, the resin in the suppressor column is replaced by an ion-exchange membrane in tubular form to condition the eluent continuously (Stevens et al., 1981) This membrane (sulfonated polyethylene hollow fiber) acts exactly like the suppressor resin in that ions are exchanged from the membrane for ions in the eluent system The innovation is that for the analysis of anions the membrane is regenerated continuously by a gravity-fed (or low-pressure) flow of low-concentration H 2 SO 4 that continuously replaces the ions that

The reactions involved in the separator column and suppressor column (or one of the devices listed above) in the determination of anions, alkali metals, and alkaline earth metals are shown in Table 1 In the determination of anions, the IC is equipped with a separator column packed with a low-capacity anion-exchange agglomerated resin in the HCO

3 form, and the suppressor column contains a strong acid high-capacity cation- exchange resin in the Hþ

form or one of the other suppression devices listed above The eluent used normally is a mixture of dilute NaHCO 3 and

Na 2 CO 3 , although other dilute mixtures (e.g., Na 2 CO 3þNaOH) are also used (Johnson, 1987) The anions are separated and converted to their strong acids in a background of H 2 CO 0 3 , which has no charge and low conductivity The presence of strong acids in H 2 CO 3 is measured by a conductivity cell and reported as peaks on a stripchart recorder or integrator The peak height is directly proportional to the concentration

of ions in solution From calibration graphs prepared for peak height versus concentration of ions in standard solutions containing the ions of interest, the concentrations of the ionic species in the sample are calculated Because

of the excellent signal-to-noise ratios, when equipped with a suppressed conductivity detector the IC system can achieve detection limits two orders

of magnitude lower than those obtained in a nonsuppressed IC system The mixture of the standards can be prepared from reagent-grade chemicals Figure 3 shows a typical chromatogram of a standard solution containing

2 mg L
1 each of F
, Cl
, PO 3
4 -P, NO
3 -N, SO 2
4 -S The separation of

PO 3
4 from several other oxyanions is shown in Fig 4.

Recent developments by Dionex involve the use of an autosuppression with the anion self-regenerating suppressor, which uses water as a regenerant In this system, water undergoes electrolysis to form oxygen

Table 1 Reactions in Separator and Suppressor Columns in Determination of Anions and Alkali/Alkaline Earth Metal Cations

by Ion Chromatography

Component

Reaction

Separator column

2 þ2C1

b M ¼ alkali metal, A ¼ associated anion.

c M ¼ alkaline earth metal, A ¼ associated anion.

dm-PDA.2HCI ¼ m-phenylenediamine dihydrochloride.

gas and hydronium ions in the anode chamber, and hydrogen gas and hydroxide ions in the cathode chamber The hydroxide ions generated

at the cathode are excluded from the eluent chamber by Donnan exclusion Cation exchange membranes allow hydronium ions to move from the anode chamber into the eluent chamber to neutralize hydroxide eluent, while

is suppressed to a less conductive medium, water, (2) analyte conductivity

system.

increases because the analyte anions associate with the more conductive hydronium ions, and (3) sample counter ion peaks typical of nonsuppressed

m-phenylenediamine (PDA) dihydrochloride is used as the eluent (Table 1) Suppression of the 5 mM HCl eluent used for measuring monovalent cations, or of PDA dihydrochloride eluent used for determining divalent cations, is achieved according to

RþOH
þHCl ! RþCl
þH 2 O

2RþOH
þPDAð2HClÞ ! 2RþCl
þPDA þ 2H 2 O:

system (A) 0.05 mM; (B) 0.5 mM with respect to each of the oxyanions (Karmarkar and Tabatabai, 1992.)

Typical chromatograms of standard solutions containing mixtures of

Liþ, Naþ, Kþ, Rbþ, and Csþ, and Mg 2þ , Ca 2þ , Sr 2þ , and Ba 2þ , respectively, are shown in Fig 5.

Another recent development by Dionex involves the use of an autosuppression with the cation self-generating suppressor (CRSR) This system also uses hydrolysis of water as described above for ARSR Anion exchange membranes allow the hydroxide ions to move from the cathode chamber into the eluent chamber to neutralize hydronium ions in the eluent, while eluent counterions (e.g., methanesulfonic acid, MSA) moves across the membrane into the anode chamber, maintaining the charge balance Response is maximized by association of the analyte cations with the more conductive hydroxide ions As with the ARSR, the result is a significant improvement in signal-to-noise ratio.

2 Single-Column Ion Chromatography

Two alternative methods to that described above are now available for ion separation and determination In both methods, no suppressor column is needed (single-column systems) Instead, moderately conducting eluents are

(Tabatabai and Basta, 1991.)

used to elute a variety of ions, which then flow directly into a conductivity detector The typical eluents used in nonsuppressed IC are phthalic acid and

p-hydroxybenzoic acid for determination of anions and methanesulfonic

acid for determination of cations The equivalent conductance values of Cl

,

SO 2
4 , and other common anions are appreciably greater than the conductance of the eluent anion, so a positive peak is detected as the ions are carried through the detector Conversely, the equivalent conductance values of Naþ

, Kþ

, and other common cations are appreciably smaller than the conductance of the eluent cation, so a negative peak is detected as the cations are carried through the detector.

One technique is a variation of conventional HPLC, in which based column packings provide ion separations In a second similar approach, specially synthesized macroporous polystyrene-divinylbenzene resins with low capacities are coupled with moderate-conductivity mono- or polyvalent eluting ions (Smith and Chang, 1983) In the early 1980s, dedicated systems for single-column IC were introduced by Wescan (Santa Clara, CA), Hewlett-Packard (now Agilent) (Palo Alto, CA), and Brinkman (Westbury, NY) The instrument distributed by Brinkman was manufac- tured by Metrohm in Switzerland.

silica-Compared with suppressed IC, nonsuppressed IC is easy to operate It

is also a useful technique for determining ions of weak acids, such as cyanide and sulfide, that do not conduct in chemically suppressed systems However, for several reasons, nonsuppressed IC has not gained as much acceptance

as suppressed IC, especially in environmental analysis One reason is that regulatory methods, such as the USEPA method 300.0, are based on suppressed IC The other is that the signal-to-noise ratio is much greater with suppressed IC than that with nonsuppressed IC Lastly, the suppression devices developed since 1981 eliminate the drawbacks of the original packed-bed suppressor.

The basic components of a nonsuppressed-type (single-column system) ion chromatograph (SCIC) are shown in Fig 6 The technique employs the following main components:

1 An eluent pump and eluent reservoir

2 A sample injection valve (a sample loop of 500 mL is normally used

3 An ion-exchange separator column

4 A conductivity detector coupled to an output device

In this system, a low-capacity exchange column and low-conductivity eluent are used without the need for a suppressor column (Gjerde and Fritz,

1979, 1981; Gjerde et al., 1979, 1980) Eliminating the suppressor column reduces the postcolumn dead volume, resulting in faster analyses, but the SCIC system is about two orders of magnitude less sensitive than the

eluent-suppressed system Appropriate low-capacity exchange columns used

in the SCIC systems include a macroporous polystyrene divinylbenzene resin (Gjerde and Fritz, 1979, 1981) or a surface-quaternized silica (Girard and Glatz, 1981) Organic acids (phthalate, benzoate, or citrate) are often used in the mobile phase of SCIC (Gjerde and Fritz, 1981; Jupille, 1987), with phthalic acid being the most common because of its wide range of retention control (via pH adjustment) and equivalent conductance (Jupille et al., 1983) Anions of interest elute in the hydrogen form (e.g., HCl, HNO 3 ,

H 2 SO 4 ) against a background of ionized phthalate ions A number of equilibria affect SCIC Buffer ions (usually weak acid ions) equilibrate with the free acid in solution Both of these species, in turn, equilibrate with their bound forms at the surface of the stationary phase (Jupille, 1987) Details of the reactions involved and factors affecting ion-exchange separations in the SCIC system, information on other types of separations, and column technology were presented by Jupille (1987) Most of these systems, however, have not been used for soil analysis.

For the determination of NHþ

4 and alkali metals, the mobile phase used in the SCIC system must have a strong affinity for the ion-exchange resin in order to displace separated ions from the analytical column Maximum sensitivity is achieved when the equivalent conductance of the ionic species gives a detection signal well above the eluent background (Gjerde et al., 1979) Dilute HNO 3 is used for the determination of

Basta, 1991.)

NHþ4 and the alkali metals (Fritz et al., 1980) Use of 10 mM HNO 3

(pH 2.1) has been shown to give excellent resolution of monovalent cations, with elution complete in < 6 min when a Vydac 401 TP cation- exchange column (Separation Group, Hesperia, Calif.) is used (Nieto and Frankenberger, 1985) Ethylenediammonium dinitrate (5 mM, pH 6.1) competes more strongly with divalent cations in solution than does HNO 3 , thus providing better resolution and peak symmetry for the divalent cations Fritz et al (1980) recommended a solution pH of 6.1 so that all carbonic acid species would elute as bicarbonate and cause no interference with the analysis.

Background conductance and minimum detection limits of both alkali and alkaline earth metals increase with increasing concentration of the electrolyte mobile phase (Iskandaranl and Pletrzyk, 1982), whereas retention times decrease with increasing eluent concentration and decreasing resin capacity (k0) (Gjerde et al., 1980) The commercially available columns (e.g., Vydac 401 TP cation-exchange column) have relatively low k0

[0.10 mol(-) kg
1 ], although resins of even lower k0 have been synthesized for chromatographic separation of ions (Boyd et al., 1954; Fritz and Story, 1974a,b; Gjerde and Fritz, 1979; Gjerde et al., 1979).

B Systems with Spectroscopic Detectors

Many inorganic ions display strong absorbance in the lower range of UV.

At first, these wavelengths were not readily accessible to IC photometers, but when UV detectors that could reach down to  200 nm became available, inorganic anions such as NO

3 , NO

2 , Br
, I
, BrO

3 , IO

3 , and S 2 O 2
3 could be determined (Small, 1983) Nitrate, NO
2 , and Br
have been determined in such diverse environments as river and waste treatment waters, rain, eutectic salt mixtures, and saliva, although little information is available on the use of UV detectors for the determination of these or other inorganic ions in soils Also, ions such as SCN

S 2 O 2
3 and several polythionate species have been measured successfully by using low- capacity resins and NaClO 4 as an eluent Cortes (1982) used silica-based columns with amino functional groups for the effective separation of both organic and inorganic anions that are UV-absorbing Another approach involves ‘‘postcolumn derivatization’’, whereby the separated ions are converted into complexes that absorb ultraviolet and visible (UV-VIS) light (Fig 7) This is accomplished by merging separator column effluent with a stream of complexing agent to form absorbing complexes prior to a UV-VIS detector (Figs 8, 9) Postcolumn derivatization detection has extended the use of IC to measure trace levels of transition and other heavy

metals (Fig 10) in soil and sewage sludge digests (Basta and Tabatabai,

1990, 1991).

C Systems with Pulse Amperometric Detectors

Pulse amperometric detection (PAD) is useful for IC analysis of anion or weak acids with pK a>7 These anions cannot be measured by IC based on suppressed conductivity because they form poorly conducting weak acids after chemical suppression Anions in this category include S 2
and CN

Ion chromatography based on amperometric detection has been reviewed by Weiss (1986) Applications of HPIC-PAD to the determination of such organic species as saccharides, aminosaccharides, and aminoacids are described in Sec III.A.3 below.

D Design and Operational Features

The two crucial milestones in the IC system developed by scientists at Dow Chemicals in the early 70s (Small et al., 1975) were the development of

with membrane reactor and UV-VIS detector (Basta and Tabatabai, 1990.)

low-capacity ion-exchange resins for efficient chromatographic separation and chemical suppression for enhanced S/N ratio As mentioned before, chemical suppression lowers the background conductance and enhances the signal by converting the ions to their highly conducting forms The chemical

(Basta and Tabatabai, 1991.)

suppression devices available now essentially fall into three broad categories In the first type, the suppression reactions occur in a packed bed of high-exchange-capacity resin material (Karmarkar, 1996; Saari- Nordhaus and Anderson, 1996) In the second type, the suppression reactions occur as the eluent stream mixes with the flowing stream of high- capacity resin material (Gjerde and Benson, 1992) In the third type, the reactions occur across an ion exchange membrane (Stevens et al., 1981; Stillian, 1985; Henshall et al., 1992).

There are at least three types of commercially available packed bed suppressors; in each case the geometry of the suppressor is significantly smaller than that of the pioneering invention of Small et al (1975) In the first type, a 4.6  20 mm cartridge is regenerated after every sample

by pushing through it about 1 mL of 0.25 M H 2 SO 4 followed by about 2 mL

of deionized water (Karmarkar, 1996) In the second type, there are two small cartridges, one being used while the second one is regenerated electrochemically The suppression system toggles between these two

membrane (Basta and Tabatabai, 1991.)

cartridges (Saari-Nordhaus and Anderson, 1996) In the third type, commercially available on IC systems from Metrohm, three suppressor cartridges are used While one of the three is being used for IC analysis, the second one is being regenerated with H 2 SO 4 , and the third one receives a deionized water wash The IC systems employing these cartridges have been

chromatography with (A) 2,6-pyridinedicarboxylic acid and (B) oxalic acid as eluent (Basta and Tabatabai, 1990.)

found useful for the analysis of waters and soil and plant extracts The second type of suppression, with flowing stream resin material, has not found many commercial applications (Gjerde and Benson, 1992).

The third type of suppression, using membranes for transport of eluent and suppressing ions, has been an evolutionary process driven by Dionex Corp At the beginning of the 1980s the fiber-based suppressor was developed (Stevens et al., 1981), followed by the micromembrane-based suppressor (Stillian, 1985) The reactions involved in the micromembrane device are shown in Fig 11 The IC systems employing these two types of suppressors were evaluated for the analysis of soil and plant extracts (Karmarkar and Tabatabai, 1991, 1992) Further improvements to the micromembrane suppressor were then made in which, instead of regenerat- ing the ion exchange membranes with a chemical, the regenerating ions were formed in situ by electrolysis of deionized water (Henshall et al., 1992) The reactions involved in the autosuppression with the anion and cation self-generating suppressor are shown in Figs 12 and 13, respectively The IC system using this improved micromembrane suppressor has not yet been evaluated for the analysis of soil and plant extracts.

The columns used in eluent-suppressed-type IC were initially made

of glass, but present versions are made of plastic, with a performance equivalent to or better than that of glass columns and no breakage Typical diameters are from 4 to 9 mm; the lengths vary from 50 to 250 mm Dionex Corp is the sole distributor.

The columns in single-column-type (nonsuppressed) IC can be glass, plastic, or (most commonly nowadays) stainless steel The phthalate and

benzoate eluents used in SCIC have pH values ranging from 3 to 7, so little corrosion is expected The injection of samples with pH values higher than 7

is not advisable because the silica packing will degrade severely Samples with pH values > 7 are normally treated with eluent until the proper pH balance is achieved.

self-regenerating suppressor.

self-regenerating suppressor.

signal changes to the amplifier The signal caused by the presence of conductive ions in the cell, after temperature compensation, results in meter and recorder-pen deflection.

The SCIC instruments have many of the components of suppressed–type instruments The main difference between these two types

eluent-of instruments lies in the column packing, and the lack eluent-of a regeneration pump and timer in the SCIC systems.

E Commercial IC Systems

Current ion chromatographic systems are available from Dionex (Sunnyvale, CA), Lachat (Milwaukee, WI), Brinkman (Westbury, NY), Wescan/Alltech (Deerfield, IL), Waters (Milford, MA), and Agilent (Hewlett-Packard)(Palo Alto, CA), or their associate/subsidiary companies

in other countries All IC systems feature ion separation and detection modes Other instruments are available from most manufacturers that involve postcolumn reaction systems for the determination of polyphosphates and the transition metals in aqueous solutions No information is available, however, on the use of these instruments for soil or plant analysis.

Several advanced eluent-suppressed IC models manufactured by various companies have not been evaluated for soil analysis, but it should not be difficult to adapt most of the methods available for this purpose to make them compatible with those IC systems In using any IC instrument, the operator must be familiar with the principle of operation and the reactions involved Knowledge of the sample composition is also very useful Cleanup procedures for most IC instruments are provided by the manufacturers Although most of these procedures are not difficult to

perform, experience with the IC system and familiarity with the functions of its components are helpful.

F Sample Preparation and IC Conditions

One of the most important requirements of the IC technique is that the sample injected for analysis should be free of particulates Loss of resolu- tion can result from a contaminated precolumn or analytical column Reproducibility may be affected by a contaminated column, an insufficiently conditioned column, or microbial growth in the eluents when stored at room temperature for several days, especially those used with single-column systems Therefore, the eluent used with the single-column system should be prepared freshly on a daily basis The precolumn, analytical column, and suppressor column can be used for many months Degradation of the columns’ resins can be detected easily from inconsistent peak heights and lack of peak resolution The relative retention time for the eluent-suppressed

IC system is affected by the eluent composition, and for the single-column system is affected by the pH and ionic strength of the mobile phase An increase in the ionic strength and pH of the eluent causes the solute retention time to decrease In general, retention of ionic species is directly proportional to column length and inversely related to eluent flow rate Increasing the column length generally results in greater resolution of the solute; however, the time required for analysis is increased (Dick and Tabatabai, 1979; Tabatabai and Dick, 1983) Analysis time is decreased at high flow rates, but this can lead to poor resolution with overlapping peaks Analyte retention time is also indirectly proportional to the concentration of the sample injected, but this effect is minor at low analyte concentrations (Small et al., 1975; Iskandaranl and Pietrzyk, 1982) Another factor that significantly affects peak height, peak resolution, and reproducibility of results is temperature (approximately 2%C
1 for peak height) Tempera- ture variations may also cause changes in retention time and produce baseline drift Waterbaths, jackets, or column heaters may be used to eliminate fluctuations in laboratory temperature, but these are costly and difficult to operate at room temperature with small columns In most situations, the fluctuation in laboratory temperature can be overcome easily

by running standard samples more often during the workday or placing the instrument in an air-conditioned room to eliminate severe fluctuations

in daily temperature The sensitivity of the IC system can be adjusted

by changing the range of the conductivity detector and/or sample size (sample loop) Sample pretreatment is often required before analysis by IC systems Some of the techniques used in sample preparation are summarized

in Table 2.

H - or OH -saturated resin for basic or acidic samples, respectively (1994) Ion collection and

dissolution for

airborne samples.

Collection of airborne samples with an impinger followed by liquid extraction or sparging the gas through a collection solvent

Frankenberger et al.

(1990) NIOSH (1994) OSHA (1991)

In-line techniques

Elimination of matrix Heart-cut column switching to eliminate matrix.

Examples: (1) elimination of organic matrix in an analgesic formulation for sulfite determination, and (2) elimination of phosphate matrix in determination of sulfate in sodium phosphate

Kilgore and Villasenor (1996)