A Valve Switching Ion ChromatographySystem With Integrated AutomationManagement

by Richard Jack, Rong Lin, Brian De Borba, Kannan Srinivasan,Yoko Sekiguchi, Yuichi Nakanishi, and Kazue Yoshimura

Valve switching applications are becoming popular in ionchromatography (IC). Example applications includematrix diversion or matrix elimination prior to analysis oftrace components. A conventional ion chromatographysystem is usually equipped with a single injection valve,and installing additional valves for various applicationssuch as two-dimensional (2-D) separations and samplepreparation applications is cumbersome. The instrumentdetailed here has an integrated automation managementmodule supporting various valve configurations and a reac-tion coil heater for postcolumn reaction applications.Here, the authors will discuss the utility of the system forvarious valve switching applications.

First, the instrumentation details of the system and theautomation module will be discussed. Next examples ofapplications will be shown, including 2-D analyses of per-chlorate in high-ionic-strength matrices and a matrix neu-tralization approach showing analysis of trace anions in con-centrated base. With the 2-D analysis method inconjunction with suppressed conductivity detection, it ispossible to detect low levels of perchlorate without the use ofan expensive mass spectrometer.

System descriptionThe ICS-3000 Reagent-Free™ IC (RFIC™) system(Dionex Corp., Sunnyvale, CA) combines traditionalinstrument improvements for signal enhancement withvalving and control for easy implementation of complexapplications. The system consists of a pump (single or dual,SP or DP, respectively), eluent generator (EG),detector/chromatography compartment (DC), automationmanager (AM), autosampler (AS), and tablet PC. The sys-tem can be configured as a single or dual system and incorpo-rates the latest advancements in RFIC system technology(Figure 1). When configured as a single system, the addi-tional pump from the DP can be used for miscellaneousapplications such as pumping a deionized (DI) water streamfor sample preparation applications.

From ordinary DI water, RFIC systems generate high-purity eluents that perform superior separations, andthen neutralize the eluent back to DI water in the sup-pression step to facilitate sensitive detection using a con-ductivity detector. Using the EG with the continuouslyregenerated-trap column (CR-TC) minimizes baselineshift during the run, when pursuing eluent concentrationstep changes or gradients.

The DC compartment consists of three sections for separa-tion, detection, and automation. The separator columns arehoused in the DC in a controlled thermal environment.Capable of single or dual analysis, the DC houses conduc-tivity and electrochemical detectors. Two detector cells areavailable for “plug-and-play” conductivity and electro-

Figure 1 The automation manager (AM) in the ICS-3000 detec-tor/chromatography compartment (DC) is highlighted above. Mountedas a sliding tray for easy plumbing, the AM is versatile enough to handledifficult applications by supporting a variety of valve configurations.

Reprinted from American Laboratory September 2006

The schematic for matrix removal and signal enhancementis shown in Figure 2. There are several advantages of the 2-D matrix diversion approach. Initial sample loading ontothe 4-mm column allows a large sample injection volume(large amount of sample) due to the high capacity of theanalytical column and higher selectivity for perchloraterelative to the matrix ions. Second, it is possible to focusthe perchlorate peak that is partially resolved in the firstdimension onto a concentrator column in the seconddimension. The suppressed effluent with hydroxide eluentis water, which provides the ideal environment for ion-exchange retention and focusing. Third, the seconddimension is operated at a lower flow rate relative to thefirst dimension, thereby enhancing the detection sensitiv-ity. Finally, this approach also allows the potential to com-bine two different chemistries in two dimensions, therebyenabling a selectivity not possible using only a singlechemistry dimension.

Figure 3a shows the analysis of a sample in the first dimen-sion consisting of 5 ppb perchlorate in the presence of 1000ppm matrix ions. As can be seen in this figure, the matrixinterferes with the detection of the perchlorate peak,which is broadened and difficult to quantify. This samesample was analyzed using the 2-D approach by using the 2-mm column in a second dimension. As shown in Figure 3b,perchlorate is well resolved and free from any matrixeffects. Chromatography with matrix diversion and con-centration followed by analysis on a 2-mm column and aconcentration-sensitive detector yields sensitivity propor-tional to the flow rate ratio of the first dimension versusthe second dimension: a fourfold gain. Due to the physical

Figure 2 Diagram highlighting components and configuration for 2-D IC in the ICS-3000.

chemical operation. The DC also contains two valves in thelower compartment for injection and sample preparationapplications. All valves are on slide-out trays for easy instal-lation and troubleshooting. An important feature withinthe DC is the AM, a slide-out tray that is configurable withup to two high-pressure valves, two low-pressure valves, anda postcolumn reaction heater. The AM facilitates valveswitching applications.

The AM is mounted inside the DC compartment so thatsample preparation steps such as preconcentration andmatrix removal are in the near vicinity of the separatorcolumns. In conventional IC systems, mounting the samplepreparation valves close to the separator column is difficultand often leads to excessive band dispersion. Due to theclose proximity of the AM to the separator columns, banddispersion issues are minimized.

Two-dimensional perchlorate analysisAlthough there are no federal drinking water regulationsfor perchlorate, various states have adopted their ownadvisory levels that range from 1 to 18 ppb. Trace-levelanalysis in IC is typically done using concentratorcolumns or large-volume injections. When high levels ofmatrix ions are present, large-volume injections are pre-ferred over concentration methods since the matrix ionscan elute off the analyte of interest from the concentratorcolumn. It should be noted that large-volume injectionsnot only enhance the sensitivity of trace components, butalso enhance the sensitivity of matrix components. Insome cases, the matrix components interfere with theanalysis by coeluting or eluting the trace component intoa broad peak, leading to poor detection. This is the casewith perchlorate analysis.

Perchlorate in drinking water is typically analyzed in the ppbrange, whereas the matrix concentrations are at the hundreds-of-ppm level. U.S. EPA Method 314.0 was a direct injectionmethod but required an off-line matrix elimination step withsolid-phase extraction cartridges for samples containing highlevels of matrix ions. U.S. EPA Method 314.1 was publishedas an update and allowed improved detection of perchloratein high-ionic-strength matrices. Alternatively, an automated2-D heart-cutting method can be used as an in-line approach.

The ICS-3000 system was configured as a two-channel dualsystem for this application. The injection valve in the firstdimension was fitted with a large-volume sample loop (4mL). The injection valve in the second dimension was fittedwith a concentrator column that focused the heart-cut ana-lyte peak from the first dimension for further analysis in thesecond dimension.

proximity of the two dimensions in the system and minimaldelay volume, it was possible to achieve excellent peakshape and recovery.

AutoNeutralizationAnother example of valve switchinginvolves a technique known asAutoNeutralization™ (Dionex Corp.).This technique is used when it is neces-sary to quantify anionic contaminants inconcentrated bases. The strategy mostoften employed is to dilute the sample.This dilution reduces the concentration

of the interfering matrix ion to a level that does not affectseparation. However, dilution also reduces the concentrationof trace anions, compromising their detection.

AutoNeutralization solves the analytical problem of achiev-ing good detection limits of trace anions in concentratedbases by neutralizing the base using a membrane-based neu-tralizer device. The sample anions are in a water back-ground after neutralization and can be focused back onto aconcentrator column that is located on a third valve in thelower compartment.

In this setup, the system was used in conjunction with anAM module.

Two six-port valves on the AM module were used for neutral-ization. The first valve was used for loading the sample ontoan ASRN™ neutralizer device (Dionex Corp.). The secondvalve was used for holding the sample in the collection loopand rerouting the sample back through the neutralizer anddiverting the neutralized stream onto a concentrator column.

This plumbing configuration is shown schematically inFigure 4. In the AutoNeutralization process, the concen-trated base sample is loaded into the 25-µL sample loop ofthe sample valve. The sample loop is switched in-line andflushed with a stream of deionized water (also known as thecarrier solution). The carrier solution transfers the concen-trated base from the sample loop to the ASRN, where thesample is partially neutralized, then transferred to the 5000-µL loop. The recycle valve is then actuated to pass the sam-ple through the ASRN again so that it is completely neutral-ized. The completely neutralized sample is finally deliveredto a concentrator column; since the anions are now in water,they are concentrated as a tight band. Finally, the anions areeluted from the trap column to the analytical column, sepa-rated, and detected.

Figure 5 shows the results of anion analysis of a concentratedbase sample with (a) and without (b) neutralization. With-out neutralization, the anions are broadened and show poorsensitivity in a 1% sodium hydroxide sample matrix. In the50% sample matrix the presence of the matrix causes a high

Figure 3 Chromatograms of a 1-D (a) and 2-D (b) analysis of per-chlorate in a high-salt matrix. Note that perchlorate is not detected due tointerference from the matrix ions in the 1-D separation. In the 2-D sepa-ration, the interfering matrix has been removed and the perchlorate peakis enhanced and also quantifiable.

a

b

Figure 4 Plumbing configuration required for AutoNeutralization applications.

background that makes detection of additional anions virtu-ally impossible. Figure 5a shows the results after neutraliza-tion. Note that the large background has been removed anddetection of anions is now possible with good recovery.

The AM allows easy implementation of the autoneutral-ization application without any additional externalvalves or controls. The system configuration for thisapplication was simplified with the AM installed. Allvalves, plumbing, trap columns, and ASRN remain

inside the DC compartment, minimizing tubing lengthsand allowing all manipulation of sample to be containedin a precisely controlled environment. In addition, allvalves are recognized through Chromeleon® control soft-ware (Dionex Corp.), eliminating the use of externalTTL controls or triggers.

ConclusionThe ICS-3000 system provides an effective platform for pur-suing standard and multiple valve switching applications.Applications have been shown that demonstrate the utilityof the system and the AM module for valve switching appli-cations. Due to the close proximity of the valves to thecolumns and detector cells, this design provides a low-dispersion platform for implementing multidimensionalsample preparation and analysis schemes. The inherent flex-ibility and precise control of the system make such nonrou-tine applications easier to implement.

Dr. Jack, Dr. Lin, Mr. De Borba, and Dr. Srinivasan are with DionexCorp., 1228 Titan Way, Sunnyvale, CA 94086-4015, U.S.A.; tel.: 408-

737-0700; fax: 408-739-4398; e-mail: Richard.jack@dionex.com. Dr.

Sekiguchi, Mr. Nakanishi, and Ms. Yoshimura are with Nippon Dionex,K.K., Osaka, Japan.

Figure 5 Chromatographic results of anion analysis with (a) andwithout (b) AutoNeutralization, from caustic sample matrix.

a

b