Automated Programmable Preparation of Carbonate-Bicarbonate

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Automated Programmable Preparation of Carbonate-Bicarbonate Eluents for Ion Chromatography with Pressurized Carbon Dioxide Charles Phillip Shelor, Kenji Yoshikawa, and Purnendu K. Dasgupta Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b02808 • Publication Date (Web): 25 Aug 2017 Downloaded from http://pubs.acs.org on August 25, 2017

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Analytical Chemistry

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Automated Programmable Preparation of CarbonateBicarbonate Eluents for Ion Chromatography with Pressurized Carbon Dioxide

C. Phillip Shelor*, Kenji Yoshikawa, and Purnendu K. Dasgupta* Department of Chemistry and Biochemistry University of Texas at Arlington Arlington, TX 76019-0065, USA

*

Email: [email protected]; [email protected] Fax: (817) 272-3808 1 ACS Paragon Plus Environment

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ABSTRACT We introduce a novel carbonate-bicarbonate eluent generation system in which CO2 is introduced using programmable CO2 pressures across a membrane into a flowing solution of electrodialytically generated high purity KOH. Many different gradient types are possible, including situations where gradients are run both on the [KOH] and the CO2 pressure. The system is more versatile than current electrodialytic carbonate eluent generators and can easily generate significantly higher eluent concentrations (at least to 40 mM carbonate), paving the way for future higher capacity columns. Demonstrably purer carbonate-bicarbonate eluent systems are possible compared to manually prepared carbonate-bicarbonate eluents and with considerable savings in time. Performance in different modes is examined. The dissolved CO2 is removed by a carbon dioxide removal device prior to detection. Best case noise levels are within a factor of 2-3 of best case suppressed hydroxide eluent operation. The eluent system allows particular latitude in controlling elution order/time of polyprotic acid analytes. Although CO2 introduction is possible prior to hydroxide eluent generation, this configuration causes complications because of electroreduction of CO2 to formate.

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INTRODUCTION Although the first exposition of anion chromatography began with eluents containing NaOH and/or Na-phenate,1 the first practical eluents were carbonate-bicarbonate based2 and were the first commercially recommended eluents.3,4 Of manually prepared eluents, the carbonate eluent has continued to prevail over four decades. Compared to the electrodialytic generation of hydroxide eluents,5 similar generation of carbonatebicarbonate eluents is more complex. Two steps are involved: K2CO3 is first generated and part of the K+ is then exchanged for H+ in a controlled fashion to provide a KHCO3-K2CO3 eluent.6 Additionally, the maximum concentration of carbonate/bicarbonate that can be electrogenerated is significantly less than the corresponding hydroxide concentrations. Suppression of carbonate-bicarbonate eluents results in weakly conducting carbonic acid. Increased noise from a carbonatebased eluent, compared to a hydroxide eluent (suppression product is water) has recently been ameliorated by a two-stage suppressor design.7,8 Even before this development, it was possible to remove the volatile carbonic acid by membrane-based devices; 9-16 A bicarbonate-carbonate gradient was traditionally held as not practical because whereas monovalent HCO3- occupies one anion exchange site, a CO32- ion occupies two and introduction of the latter into the column thus results in the expulsion of an equimolar amount of bicarbonate resulting in a gradient “hump”.17 A CO2 removal device would largely remove such a gradient hump. Such a device was introduced commercially as a “Carbonate Removal Device” (CRD,18 this nomenclature by the manufacturer is obviously a misnomer). But this came long after electrogenerated hydroxide gradients, which, for a variety of reasons are perceived to be superior to


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carbonate-bicarbonate gradients. As a result, bicarbonate-carbonate gradients have not been explored in detail, taking advantage of the CRD. It is interesting to note that CO2 intrusion into water used in hydroxide eluent generators (HEGs) can also lead to “carbonate humps”. In this case, rather than removing the CO2 post-suppression with a CRD, the preferred approach has justifiably been to remove the carbonate from the eluent with an electrodialytic anion trap column. We are interested in a large range bicarbonate-carbonate gradient capability for the following reason. Given a certain suppression capacity, it will permit a much greater eluent strength span than hydroxide and will thus enable the use of very high capacity columns. The latter will facilitate the analysis of trace components in high ionic strength samples, always the Holy Grail. As noted above, membrane devices for the removal of carbonic acid as CO2 gas, were introduced more than three decades ago and efforts to improve it continues.19 Such CRDs, permit post-separation degassing of suppressed carbonate eluents to lower the conductance background (and the noise). The CRD also greatly improves the linearity of the otherwise nonlinear response observed with carbonate eluents.20,21 In the present paper we propose a new paradigm in automated programmable generation of carbonate-bicarbonate eluents. We suggest precisely the opposite of a CRD: introducing variable amounts of CO2 through a membrane-based engasser into an electrodialytically generated hydroxide or manually prepared carbonate solution to prepare carbonate-bicarbonate eluents for IC. Eluent gradients can be generated both with programmed changes in the hydroxide/carbonate concentration and/or the CO2



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pressure. The strategy can also generate carbonic acid gradients, recently demonstrated to be useful for ion exclusion chromatography.22 EXPERIMENTAL SECTION Engasser. The engasser has been described previously22 and is shown in detail in Figure S1 of the electronic supporting information (SI). Briefly, the engasser is a length of gas permeable tube (Teflon AF tube, 0.18 mm i.d., 0.74 mm o.d., taken from degasser units integral to electrodialytic eluent generators available from www.thermofisher.com) enclosed within an impermeable jacket tube that terminates at tees at each end, permitting access both to the annular space as well as the lumen of the Teflon AF tube. The engasser used throughout had an active length of 5.5 cm. Laser Pure Grade Carbon Dioxide (www.airgas.com P/N CD LZ200) was used throughout.

Chromatographic System. Figure 1 schematically shows the chromatographic system. Other than the engasser, all other components are commercial off-the-shelf components (www.thermofisher.com except as stated). A GP40 gradient pump pumped water through a 10 port stainless steel injector valve (www.vici.com, plumbed as shown), an EG40 hydroxide eluent generator (HEG) equipped with KOH EGC-III cartridge and degasser, the engasser (via which desired amounts of CO2 are introduced) followed by microfabricated metallic gradient mixer (V100, www.agilent.com), a high pressure conductivity cell, injector (25 µL loop), analytical column (IonPac AS9-HC, 4x250 mm), suppressor (ESRS 500, 4 mm), modified 2 mm


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CRD 200 carbon dioxide removal device, and CD25 conductivity detector (cell maintained at 35 °C). The high-pressure conductivity cell used to monitor the eluent consisted of 2 stainless steel HPLC tubes electrically isolated but fluidically joined by a PEEK union. The tubes functioned as electrodes for a Dionex CDM-1 conductivity detector. The high-pressure conductivity detector, injector, and column were all housed in a column oven (40 °C) that was part of a Surveyor AS-AP autosampler. The 10-port valve allowed facile reversal of the flow path through the HEG and engasser; the default condition follows the path described above. Note that the AS9-HC column is not intended for long term use with hydroxide eluents. We are using it here because of its high capacity and therefore utility with carbonate gradients. Although some experiments with pure hydroxide eluents have been described, this is purely for comparative purposes, it is not being suggested that the column be so used. Our experiments with hydroxide were limited enough that there was no significant change in capacity or selectivity due to such use.

Engasser Pressure Control. The pressure in the engasser jacket was controlled using an electronic pressure regulator (EPR, Proportion-Air GX1ANKKZP900PSG1). The EPR can control gauge pressures up to 6.2 MPa; maximum attainable CO2 pressure at ambient laboratory temperature (22 °C) is ~6 MPa. A PEEK column blank (9 mm i.d. 6 cm long) was used as a ballast and connected between the EPR and one tee of the engasser; the second tee was plugged during operation after purging the system with CO2. The permeation rate of CO2 is dependent upon the partial pressure of CO2; even at zero gauge pressure the engasser annular space has ~1 atm of CO2 available to



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permeate. Pressures henceforth are reported as absolute values. Conversion of gauge pressure readings was performed by adding the ambient laboratory barometric pressure (typically 98.5 kPa during these experiments) to the EPR reported gauge pressure. A minilab 1008 data acquisition card (www.mccdaq.com) allowed 10 bit analog EPR control and 12 bit recording of the high pressure conductivity data and pressure registered by the EPR with software written in LabViewTM.

Engasser Permeation Rate Determination. The permeation rate of the engasser was determined at 25 °C by conductometric titration with 49.80.1 mM NaOH solution (as determined by primary standard potassium hydrogen phthalate) prepared from 50% w/w NaOH (www.fishersci.com). The solution was pumped through the engasser at a constant flow rate while varying the external CO2 pressure (pCO2,ext) from 110-3450 kPa over 4 h; the experiment was repeated twice. Subsequent shorter-duration (1 h) titrations were performed periodically using 40 mM HEG KOH (a concentration relevant to the generated eluents) with pCO2 centered at 690 kPa (near the equivalence point for the eluent flow rate used) using both ascending and descending pCO2 ramps. In all titrations, 10 min isobaric zones were included before and after the pressure ramp to facilitate pressure and conductivity data alignment (See Figure S2). The KOH titrations were performed at chromatographic temperatures of 40 °C with the column in place to provide enough back pressure to prevent H2 bubble formation at the HEG.


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RESULTS AND DISCUSSION Determination of CO2 Permeation Rate of the Engasser. CO2 introduction in the present utilization of the engasser is kinetically rather than thermodynamically governed. Above a solution pH of ~8.3, pCO2 is negligible; the pK1 for carbonic acid (this term hereinafter includes hydrated CO2) being ~6.3. For any permeative transfer system, the permeation rate is proportional to the differential pressure of the gas across the barrier. If the receptor solution has a pH throughout high enough to have a negligible internal pCO2, the permeation rate R (in nmol/min) is constant across the length of the tube and is linearly proportional to pCO2,ext. From analogous heat transfer equations for a cylindrical pipe, we derive that R = (kLpCO2,ext)/ln(d0/di) ...(1) Where R is the permeation rate in nmol/min, L is the length of the tube of o.d. and i.d. do and di respectively (all in cm), and k is the permeability constant (nmol/(min.cm.kPa)), independent of pCO2,ext (kPa). Figure 2 shows the results of a single conductometric titration experiment to determine the permeation rate. The presently used EPR was designed for a greater level of gas consumption. Set point maintenance involved rather frequent automatic pressurization/depressurization (Figure S2), a moving average filter spanning 20 s was applied for the pressure data in Figure S2. The Teflon AF membrane, however, behaves as a large capacitor, the modest gas pressure oscillations were not observed in the conductivity signal, which displayed a noise level equal to the DAQ bit noise. During the titration, NaOH is converted stepwise to Na2CO3 and then NaHCO3. Further addition of CO2 does not result in perceptible ionization of the added CO2 and



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solution conductance shows no further increase. The two equispaced end-points, indicated by arrows in Figure 2, occur respectively at 1396 and 2807 kPa. Stoichiometry dictates at the first equivalence point: FC = 2R ...(2) Where C mM NaOH is flowing at F mL/min. The combination of eq 1 and 2 lead to k = [FC*ln(d0/di)]/[2*LpCO2,ext,equiv,1] ...(3) where pCO2,ext,equiv,1 is the external CO2 pressure at the first equivalence point. Likewise k may be computed using the second equivalence point (pCO2,ext,equiv,2). The perceptive reader will appreciate that the effect of any carbonate contamination of the NaOH used can be eliminated by a difference equation: k = [FC*ln(d0/di)]/[2*L(pCO2,ext,equiv,2 - pCO2,ext,equiv,1)] ...(4) For F = 1 mL/min, k was computed from eq 3 and 4 for three titrations to be 4.50 ± 0.12 and 4.53 ± 0.08 nmol/(min.cm.kPa), respectively, statistically indistinguishable (combined: 4.515 ± 0.88 nmol/(min.cm.kPa)), also indicating negligible carbonate contamination of the NaOH. For HEG KOH, we relied on the nominal concentration that the manufacturer states it is generating and only the first equivalence point was determined. Based on three titrations spanning as many weeks, k consistently ranged from 3.98 to 4.08 nmol/(min.cm.kPa), ca. ~10% lower than that obtained with the secondary standard NaOH titration. Further investigation revealed that this particular pump-HEG combination is producing an eluent concentration slightly greater than the nominally programmed value, not exhibited by other HEG setups in our laboratory. In any case, we refer to this nominal manufacturer-stated HEG KOH value throughout as the


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concentration. Also, unlike the pumped NaOH case, results of titrations using descending vs. ascending pressure ramps consistently differed by 3% indicative of some minor hysteresis in the system. Values reported are the average of the ascending and descending pressure ramp titrations. The permeation rate can also be estimated from the slope of the titration plots. The accuracy of this estimation depends on the calibration of the cell constant as well as the listed values of the equivalent conductance values of NaOH, Na2CO3, NaHCO3, etc. at the concentration level the titration is carried out. In Figure 2, the NaOH to Na2CO3 (step 1) and Na2CO3 to NaHCO3 (step 2) conversion steps display respective descending slopes of 4.25 and 0.515 µS/(cm.kPa), r2 = 0.9995 and 0.9970. The equivalent conductance () of 50 meq/L NaOH, Na2CO3, and NaHCO3 were respectively reported to be 227, 93.2, and 80.6 µS/cm per meq/L,23 the conversion factors for the difference thus being 133.8 and 12.6 µS/cm per meq/L. Converting the above slopes to concentration units one thus gets 31.8 and 40.9 µeq/(L.kPa) for step 1 and step 2, respectively. Multiplying by the flow rate and the natural log of the outer to inner diameter and dividing by the length of the engasser and stoichiometric correction, k for all 3 trials is computed to be 4.00 ± 0.11 and 5.26 ± 0.01 nmol/(min.cm.kPa) for step 1 and 2 respectively. The former was determined to be 11.4% lower on average than the value computed from the equivalence points while the latter was 16.4% higher. This apparent discrepancy likely originates from errors in the reported  values. Our data on 50 mM NaOH (based on the ordinate intercept of the step 1 fits) indicates a  of 230.9 µS/cm per meq/L, close to the reported value of 227.23 However, at ~50 meq/L,



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we determined  values of Na2CO3 and NaHCO3 to be 112 and 97.5 µS/cm per meq/L, significantly higher than that in the Dow report.23

Chromatographic Application The following basic modes of anion exchange chromatography are possible with eluents made with a CO2 engasser: (i) Use of an electrogenerated hydroxide eluent which is converted variously to carbonate and bicarbonate by the CO2 engasser. Gradients are possible: (a) hydroxide gradient at constant pCO2,ext, (b) varying pCO2,ext at constant hydroxide concentration, and (c) both are varied. (ii) Use of a pumped Na2CO3 eluent (with a gradient pump, the concentration may be varied if desired), which is converted to bicarbonate by the engasser to the desired extent. (iii) Use of a pure carbonic acid eluent with low capacity anion or cation exchangers. Because the pH remains acidic, this mode is also compatible with silica-based ion exchange columns. This application is different from the central focus of the present paper; this will be presented elsewhere. The use of pure carbonic acid as eluent in ion exclusion chromatography has already been reported.22

Modes of Deployment. Several modes of deployment are apparent: (a) The HEG precedes the engasser, the engasser runs a pCO2,ext gradient; (b) The engasser precedes the HEG; the HEG runs a gradient; (c) The engasser precedes the HEG; a pCO2,ext gradient is run on the engasser; and


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(d) The HEG precedes the engasser; a gradient is run on the HEG. Each of these results in different consequences, not to speak of the fact that within each deployment domain, a monotonic gradient in the indicated parameter may not result in a monotonic change in eluent strength. In addition, in all above deployments, gradients can be run both on the engasser and the HEG.

Species Distribution in Different Gradient Modes, Chromatographic Consequences. The consequences of operational mode a above are shown in Figure 3a. An increasing amount of CO2 (0-50 mM equivalent) is introduced into a constant concentration of 40 mM KOH. Up to the point that 20 mM CO2 has been introduced, essentially hydroxide is converted into carbonate and while the extent may vary among analytes, there will be a general increase in eluent strength. This gradient region and direction is indicated as G1. Extant instrumentation will permit the generation of mixed (heretofore unexplored) hydroxide-carbonate eluents by pumping a carbonate solution through a hydroxide generator. The carbonate concentration could even be varied if a gradient pump was used. Note that there is relatively little change in pH in this eluent system. Past the point that the maximum eluent strength is reached, further addition of CO2 will cause carbonate to be converted to bicarbonate and the eluent strength will decrease. So it is not meaningful to go across the entire width of the plot (G1+G2). To run an effective gradient in the right half, one would start at some point in the right quartile of the plot and move leftward. This gradient G2 is the same as that previously mentioned for configuration ii where a carbonate eluent is pumped through the CO2 engasser that starts with a high pCO2,ext, which decreases during the run. This increase



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in carbonate/bicarbonate ratio during the gradient results in a much greater pH change (and thus may substantially affect retention of polyprotic acid anions) than the G1 gradient. Note that towards the right end of the G2 zone, especially past the bicarbonate end point, a linear gradient in pCO2,ext does not in practice result in a linear increase of total carbonate species (CT) because the internal pCO2 level can no longer be neglected; and hence the introduction rate is no longer linearly related to pCO2,ext. This is particularly true when long engasser lengths or low flow rates are to be used.22 The present experiments used very short (5.5 cm) engassers, however; the CO2 introduction rate decreased only by ~2% when raised above the bicarbonate end point. The consequences of deployment mode b are shown in Figure 3b. The first part of the gradient (G3) results essentially in a linear increase in the bicarbonate concentration. Bicarbonate is a rather weak eluent and G3 thus represents a very weak gradient system, possibly useful only for very weakly retained ions. Continuing to the next gradient zone G4 results in the conversion of bicarbonate to carbonate with an increasing carbonate to bicarbonate ratio and rapid rise in eluent strength (and pH). When essentially all of the carbonate species are converted to CO32-, although additional hydroxide is added, most analytes are unlikely to experience a further increase in eluent strength with the additional increase in hydroxide concentration. The only difference in deployment mode a and c is the order of the devices: either hydroxide solution flows into the engasser or an aqueous CO2 solution flows into the HEG. For the same flow rate and the same engasser and maximum pCO2,ext the principal difference is the maximum CT level attainable. Especially for short engassers, with water flowing through the engasser, the effluent CO2 concentration is far from the


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equilibrium value (equilibrium CT equals HCO2*pCO2,ext, HCO2 being 0.365 mM/kPa). At pCO2,ext = 500 kPa, for example, the equilibrium CT is 183 mM. Previous experiments with Teflon AF tubes approximately half as permeable as the ones presently used indicated that at F = 0.5 mL/min, 5.5 and 160 cm long engassers reached only 4.3 and 63.1% of equilibrium.22 The present 5.5 cm engasser in deployment mode c (water influent @ 0.5 mL/min) had an effluent CT of 7.85 mM at pCO2,ext of 500 kPa. In contrast, in mode a, CT can go up to the permeation rate (2.34 nmol/(cm.min.kPa)) controlled value (as long as the effluent pH remains alkaline), which for that 5.5 cm engasser at F = 0.5 mL/min and pCO2,ext = 500 kPa translates to CT = 9.67 mM. Mode d shares with mode b constant pCO2,ext operation. This can be considered the simplest hardware option: the CO2 engasser can be operated simply connected to a CO2 cylinder and operated at a fixed pressure. As before for mode a vs. c, mode d allows greater CT than mode b, the difference being significant for long engassers. Also when the HEG is placed after the engasser, the degasser following the HEG (used to remove electrolytic gases), removes excess CO2 at eluent pH values

Automated Programmable Preparation of Carbonate-Bicarbonate

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