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Carbonic Acid Eluent Ion Chromatography Phitchan Sricharoen, Nunticha Limchoowong, Charles Phillip Shelor, and Purnendu K. Dasgupta Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b05627 • Publication Date (Web): 05 Feb 2019 Downloaded from http://pubs.acs.org on February 6, 2019
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Analytical Chemistry
Carbonic Acid Eluent Ion Chromatography Phitchan Sricharoen, Nunticha Limchoowong, Charles Phillip Shelor,* Purnendu K. Dasgupta Department of Chemistry and Biochemistry, University of Texas at Arlington, Arlington, Texas 76019-0065, United States. *Corresponding Author Email:
[email protected] ABSTRACT: Alkali metals, amines and alkanolamines are separated on a poly(butadiene)-maleic acid on silica stationary phase using a carbonic acid (H2CO3*) eluent with and without a mineral acid. The H2CO3* eluent is prepared in-situ by high pressure permeative introduction of gaseous CO2 through thin membranes supported upon a porous steel disk. The permeation flux and thus the eluent concentration is controlled by varying the applied CO2 pressure. This novel frit-supported membrane device tolerates much higher liquid and gas pressures than Teflon AF capillaries, permitting [H2CO3*] exceeding 0.53 M and attaining a pH of 3.3. Silicone was presently preferred over Teflon AF, both as planar membranes, as mechanical properties of the latter change as large amounts of CO2 dissolve in it. After separation, the CO2 can be efficiently removed via another gas permeable membrane device permitting detection of the eluting bicarbonate salt conductometrically in a background of nearly pure water. Most analytes are more sensitively detected after anion conversion to hydroxide using a standard suppressor, permitting 3-17 pmol LODs on a 2 mm bore columns. The data, particularly comparisons with an HNO3 eluent, with or without H2CO3* indicate that proton exchange alone does not account for the retention behavior, some reactive addition of HCO3- is involved.
We recently described a new method for eluent preparation; CO2 was introduced through a gas permeable membrane (GPM) to dissolve in an aqueous flow stream. At a given flow rate, the amount of CO2 permeated depends upon the membrane dimensions and the CO2 gas pressure. With water or KOH as the influent, we used the H2CO3 or KHCO3-K2CO3 formed for ion chromatography by exclusion (ICE) separations1 or anion exchange chromatography with capabilities to program pH and concentration.2 Removing CO2 postsuppression is established with HCO3--CO32- eluents;3 presently with pure H2CO3 eluents for ICE, it can also be removed postseparation. With improved CO2 removal devices,2 the detector influent is nearly pure water with only traces of H2CO3. The CO2 introduction devices operate precisely obverse to degassers, and hence called engassers. We demonstrate here carbonic acid can also be used as an eluent for cation exchange chromatography. Upon hydration, CO2 forms CO2∙H2O, part of which forms ionizable H2CO3. With its first dissociation constant (2 × 10-4) being comparable to that of formic acid (1.8 × 10-4). However, the majority of the un-ionized CO2 in solution exists as CO2∙H2O, rather than H2CO3; the two together are often denoted as H2CO3* (Stumm and Morgan notation4). At 25 C and 1 atm, the first dissociation constant K* (the second ionization step is not important for the present work) is 4.27 × 10-7 (pK* 6.37) and increases slightly with pressure.5 When CO2 is dissolved in pure water, [CO32-] is negligible, [H+] equals [HCO3-] and thus [H+] equals (K*[H2CO3*]). The total carbonate species concentration then CT thus becomes: CT = [H2CO3*](1 + K*/[H+]) …(1) With increasing CT and hence increasing [H+], CT [H2CO3*] and [H+] increases linearly with √CT. Henry’s law governs the maximum attainable equilibrium concentration (KH,CO2 = 0.037 M/atm or 0.365 M/MPa). The critical temperature for CO2 is 31.0 C; at room temperature, liquid CO2 in a cylinder provides a maximum pCO2 of ~5.9 MPa to allow
a maximum equilibrium concentration of 2.15 M H2CO3* or 9.6 × 10-4 M H+ (pH 3.02). Still, this [H+] is relatively low for use as eluent when compared to the current practice of cation chromatography. Stationary phases with high H+-affinity, e. g., polybutadienemaleic acid (PBMA) on porous silica6 (Schomburg columns7) that allow separations with modestly acidic eluents8 seemed most suitable. Nevertheless, engassers we had previously built fell well short of reaching the desired [H2CO3*]. Previous engassers relied on Teflon AFTM (TAF) capillaries up to 1.6 m long. Of different TAF varieties, TAF 2400 has the highest gas permeability, next only to poly(siloxanes), but is mechanically superior with greater burst strength. The previous 1.6 m engasser reached to 63% of the equilibrium value; attaining 90% will require a 3.7 m length.1 TAF 2400 is an expensive polymer; very long capillaries hinder utility. Overmore, we found that CO2 dissolution in TAF2400 lowers its burst strength; pressures >2.5 MPa could not be used with the capillaries anyway. To overcome these limitations, we constructed what amounts to an asymmetric membranes capable of use with high differential gas pressures. A fritted stainless steel platform was used to support thin planar membranes for use with pCO2 values that approaches the critical pressure of CO2, thus achieving higher [H2CO3*] than previously possible. This stainless steel frit- supported membrane (SSFM) based carbonic acid generator thus provided a carbonic acid concentration sufficient to carry out cation exchange chromatography on a PBMA column, the first such example with a completely volatile eluent. We show attractive separations of alkali metals, aliphatic amines, and alkanolamines and compare retention observed with a strong acid eluent of same pH and thus probe possible interactions of CO2 with the analytes and a stationary phase.
EXPERIMENTAL SECTION
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Materials. All chemicals were reagent grade or better and used as received; suppliers are listed in the Supporting Information (SI). Carbon dioxide used (www.praxair.com) was 99.5%. Deionized water (DIW, www.ariesfilterworks.com, resistivity 18.2 MΩ cm) was used throughout, stock and working standards were stored refrigerated (4 °C).
Figure 1. SSFM engasser. Two opposing membranes and frits sandwich a circular channel accessible by standard 10-32 chromatographic ports. The two large threaded male nuts, one on each side, retain the membranes/frits in place and contain a threaded port to introduce CO2. The frits act as a diffuser for the gas introduced.
Engasser. Initial experiments were with tubular TAF engassers (unspecified grade,150/0.152/0.635 mm length/i.d./o.d.) constructed as previously described.1,2The SSFM, based on two stainless steel frits (19 mm dia., 1 mm thick, 2 µm pores, www.idex.com), is shown schematically in Figure 1. The eluent chamber volume is ~350 L. A more detailed description appears in the SI. Circular membranes were cut from thin sheets of 132 ± 8 and 277 ± 7 µm thick silicone rubber (separate batches, www.plastics.saint-gobain.com) or 60 µm thick TAF2400. (www.biogeneral.com) using a craft cutter (CAMEO, www.silhouetteamerica.com).The operational setup is shown in Fig. 2.
Figure 2. Experimental arrangement
The CO2 pressure in the SSFM was controlled using an electronic pressure regulator (EPR, www.proportionair.com, GX1-ANKKZP900PSG1). All pressures hereinbelow are absolute pressures, the presently relevant metric. The barometric pressure in the lab (98.3-102.1 kPa) was read daily and added to the gauge pressure. The maximum attainable CO2 at typical lab temperature (22 °C) is ~6 MPa. While this EPR could control pressures up to 6.2 MPa, we limited pCO2 to 5.5 MPa to avoid potential condensation of liquid CO2 in the SSFM. A PEEK column blank (9 x 60 mm x L) was used as a ballast and connected between the EPR and a tee that supplied CO2 to both sides of the SSFM. A DAQ card (minilab 1008, www.mccdaq.com) allowed 10-bit analog EPR control and 12bit recording of the EPR pressure output with software written in LabView. The SSFM effluent conductance was monitored by a flow-through high pressure conductivity cell (HPCC) comprising two 1/16” stainless steel tubes (functioning as electrodes) joined by a nonconducting PEEK union connected to a conductivity detector (CDM-1, www.thermofisher.com). Chromatography Setup. Except as stated, all instrumentation was from www.thermofisher.com. A GP40 gradient pump was used to deliver water to the system when using the engasser or HNO3 for comparison studies. When HNO3 was used, the SSFM was removed from the flow path. Otherwise the eluent proceeds through the SSFM, a gradient mixer (V-100, www.agilent.com) and enters a controlled temperature column oven, going through a thermal equilibration coil, the HPCC, 25 µL loop injector, separation columns (2x5 and 2x150 mm guard and analytical columns, respectively, www.metrohm.com, Metrosep C4), exits the oven to enter a CO2 removal device (2 mm CRD-200), a cation suppressor (2 mm CSRS 500), and lastly, a conductivity detector (model CD 25). CO2-scrubbed air (bubbled through 0.1 M NaOH) was drawn through the CRD by a vacuum pump. The suppressor was used in the recycle mode. Safety Considerations. This work utilizes compressed CO2 at high pressures. Except for the SSFM, all contact materials were certified to be compatible for use with CO2 and capable of withstanding the pressures used. All contained pressurized gas volumes were minimized to reduce explosion hazards; small bore steel tubing was used at the outlet of the EPR to restrict gas leaks in the event of engasser failure. Experiments were conducted in a well-ventilated area.
PRINCIPLES CO2 Hydration. The thermal equilibration coil and mixer of this system serve additional roles. Generation of H+ from CO2 is a two-step process: hydration of CO2 to H2CO3 and dissociation. The hydration step has a half-life of ~25 s while the ionization is rapid.4 The volume between the engasser and the column (~760 µL, residence time ≳ 6 half-lives at 0.3 mL/min) allows >98% completion of the hydration process. Membrane Permeation. The permeative transport rates of a gas across a membrane is typically linearly related to the differential pressure (Δp) of the gas across the membrane. For planar membranes, the transport rate, J, (in mol/s) increases linearly with the membrane surface area (A), and inversely with the thickness (δ). The porous support may prevent the entire surface area from being utilized however, and an additional porosity coefficient, 030% apparent increase in P if the difference in end points result was used. The latter data span a higher-pressure region and this apparent increase may be due to (a) an actual change in P as more and more CO2 dissolves in the membrane, plasticizing it and altering its properties,14 or (b) under pressure, the membrane sinks into the pores of the frit, essentially having densely pitted but locally thinned regions. Compared to the degree of increase in P with pCO2 reported for silicone,14 the presently observed increase is much greater. In contrast, the literature reports a greater change in P with pCO2 through TAF,15 not observed here at all. Given also that TAF is more rigid than silicone (tensile modulus 1.5-1.6 vs. 0.001-0.05 GPa) TAF and silicone may be behaving differently because of this reason. However,
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Analytical Chemistry if plasticization of silicone is occurring under our conditions at much lower pressures than literature reports, plasticization effects cannot be ruled out. Teflon AF Affected by High pCO2. While increasing pCO2 may not markedly increase P of TAF under our conditions, there is no question that TAF undergoes observable changes, including a loss of mechanical strength. The TAF capillary could tolerate no more than 4 MPa pCO2 without irreversible damage, the visual appearance of the material also changes, becoming less transparent. While the same capillaries are routinely used in IC systems to >20 MPa, backpressures for the removal of electrolytic gases, in the presence of extant or recent exposure to pCO2 at 4 MPa, the capillaries could not tolerate back pressures 8 MPa. While the substantially thinner TAF sheet easily survived these conditions when backed by the SS frit over a short term, over days of continuous use we observe leaks to form. The membrane deforms at the O-ring-frit interface: better future designs are needed. Silicone. Permeability Exponential Dependence of P on pCO2. A steep increase in P with pCO2 at high pCO2 values has been previously reported and rationalized by plasticity changes14 but the magnitude of the present increase is much greater and is noticeable beginning at lower pressures. Our measurement conditions do differ greatly from standard permeability measurement practice, where no membrane contact with a flowing highly pressurized reactive liquid is involved. The use of the frit may also lead to the previously alluded to pitting/thinning effects that will augment the apparent increase in P. It is possible that this local thinning near the pores makes the soft SiMs effectively have the same thickness regardless of the original uncompressed thickness. No matter the reason, the overall behavior is well represented by a power law. The data in Table 1 clearly indicate that a power law fit to the TAF data show no evidence for an exponent significantly different from unity while the silicone data consistently show an exponent of 1.2-1.3 whatever the method of measurement. Figure 3 shows this behavior for CO2 introduction into water, the concentration being measured conductometrically. 0.6
Choice of Engasser Design and Membrane. The capillary based engasser is initially easy to construct, but any change in the nature or length of the capillary requires an entire reconstruction. Increased mechanical strength of a capillary comes from a reduction in the do/di value but at the expense of the permeation flux (eq 4). In addition, polymers such as TAF or polymethylpentene (PMP) that display both good mechanical strength and gas permeability, are few, and fewer are available in the capillary format (we found P for CO2 through PMP to be insufficient). The SSFM must be custom machined but once made it is easy to replace membranes, especially necessary during initial research stages. The frit support has prevented the rupture of the thinnest membranes we have tested thus far. Based on these results and given difficulties with the TAF sheet long-term, we chose the 292 µm SiM-based SSFM for most of the subsequent work. There was no permanent membrane deformation and it survived repeated assembly and disassembly of the SSFM. The supralinear behavior can actually be an advantage for eluent generation where the primary eluent parameter of interest is [H+], which increases with √[CO2,aq]. It is important to note that quantitative consideration of the data in Figure 3 would indicate that the attained concentrations are far smaller than equilibrium concentrations (at 5.5 MPa pCO2, [CO2,aq] has reached only 21% of the Henry’s law equilibrium value, the system is kinetically limited. There is considerable room for improvement. As eq 6 implies, presently the attainment of equilibrium is a first order process. Other things remaining the same, a reduction in effective thickness to 30 µm would permit attainment of 90% of the equilibrium value. But this assumes the nominal membrane thickness value as the effective value: due to pitted thinning, the actual effective thickness may be a lot lower. Greater introduction rates can possibly be attained (a) by using much thinner membranes, e.g., by solution casting the membrane directly on the frit surface or other nanoporous support similar to that originally done for CO2 removal,3 or (b) through the use of polymers that show much greater permeability to CO2 than silicone.16 Admittedly, while increasing the carbonic acid concentrations further for eluent use is still desirable, we show in the following that it is nevertheless sufficient already to demonstrate the use of CO2,aq as an attractive eluent and to probe its interactions. 160
Specific Conductance (μS/cm)
Slope
r2
1.010x 10-4 0.9972 1.002 x 10-4 0.9995
Silicone 0.2
132 µm 277 µm
y = a * xn a
1.412 x 10-5 7.481 x 10-6
0 0
2000
n
ΔtR = 3.65
ΔtF = 9.00
pCO2,ext 1st Run 2nd Run 3rd Run pCO2 (aq) x 4
3
80
2 90% tF (37.84, 55.64) 10% tR (7.52, 51.76)
40
1
r2
1.184 0.9990 1.271 0.9997 4000
120
pCO2,ext / pCO2,aq (x4) MPa
10% tF (28.84, 141.07)
60 µm Teflon AF Teflon AF capillary
0.4
4
90%tR (11.17, 141.07)
pH ~3.32
CO2 Concentration (mol/L)
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0 6000
0 0
40
60
Time (min)
Pressure (kPa)
Figure 3. Conductometric CO2 concentration, water influent at 0.3 mL/min. Silicone shows supralinear behavior.
20
Figure 4. Conductivity response (overlapped black, purple and camouflage green) to the pCO2 steps (blue) with water flowing through the SSFM @ 0.3 mL/min. The bright green trace is the CO2 pressure developed in the solution. Points corresponding to the
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Analytical Chemistry 10% and 90% of the amplitude of the conductance trace are used to calculate 10-90% rise and 90-10% fall times.
Response Time. Response times of the SSFM engasser (277 m silicone) were measured conductometrically by applying up and down pCO2 steps (0.343.45 MPa), results are in Fig. 4. The 1090% rise/fall rimes (tR/tF) of the conductivity trace are seen to be 220 and 540 s, respectively. To ascertain whether this comes primarily from the residence volume or the membrane, we measured the corresponding times for the TAF capillary based engasser (Table S2): tR was measured to be 67 s @ 0.3 mL/min and decreased to 19 s @ 2.0 mL/min, suggesting residence volume as the primary factor in this case. As this tR is much smaller that of the SSFM, it offers the ability to assess the role of the membrane vs. the residence volume of the SSFM in its observed residence time by (a) putting the capillary engasser ahead of the SSFM, (b) using polyester (PE) sheets (very low P for CO217) of approximately the same thickness as the SiM and thus eliminating any role of the SiM. Placing the PE sheet containing SSFM after the capillary engasser increased tR to 152 s @ 0.5 mL/min, suggesting that the residence volume plays the dominant role. SiM and PE containing SSFM devices placed after the engasser showed indistinguishable tR values at 2.0 mL/min, confirming the major role of the residence volume in the relatively slow response. Packing the central channel and/or a better-swept geometry should improve the response time.
nS/cm) and can thence be related to the equivalent CO2 pressure in the solution, pCO2,aq as: pCO2,aq = (𝜆
𝐻+
𝐺 + 𝜆𝐻𝐶𝑂 ―
)2/(𝐾 ∗ 𝐾𝐻) …(7)
3
where H+ and HCO3- are the equivalent conductance values of the respective ions in mS (cm.M)-1. This pCO2,aq value is also shown in Fig. 4 as the green trace 4x magnified; based on comparison with the external pCO2 trace (blue), somewhat less than 25% of the equilibrium value is reached at the plateau. It is interesting to note that in terms of H2CO3* or pCO2,aq tR and tF are far more comparable at 295 and 350 s. Chromatographic Separations. The generated carbonic acid was used for the separation of a variety of alkali metals, aliphatic amines, and ethanolamines; divalent cations such as Mg2+ or Ca2+ could only be eluted with a complexing agent added, a common practice even with a strong acid eluent.8 Representative suppressed chromatograms of 12 ions at several selected pCO2,ext values are shown in Figure 5. Unsuppressed detection was also possible using a post-detection cell restrictor to apply ~1.4 MPa backpressure and thus avoid CO2 outgassing (Fig. S2). This allowed accurate background conductance measurements at the time of elution to determine the exact [H+] to evaluate if it is only eluent [H+] that determines analyte retention by comparison with a strong acid eluent bearing a noncomplexing anion, e. g., HNO3 (0.25- 1 mM, Fig. S3). In ion exchange, the log (retention factor, k) is linearly proportional to the log (eluent ion concentration), the slope being the ratio of the charge of the analyte to the eluent ion. Such plots for the analytes in Figure 5 appear in Figure S4. The slopes of the resulting equations with their uncertainties appear in Table 2 (the r2 values for linear fits uniformly equal or exceed 0.999, see Table S3). The drift observed in Fig. 5 is likely due to reequilibration of a highly retained contaminant ion in the feed rather than fluctuations in CO2 concentration which are stable as seen Fig. S2 and further mitigated by the suppressor. Table 2. Regression Slope Values for log k – log [H+] Plots Ion
Figure 5. Cation separation. Isocratic elution @ 0.3 mL/min using 2.76-5.52 MPa pCO2 on a 60 m TAF sheet – SSFM engasser. Injection 25 µL, Column temperature 30 ⁰C, suppressed conductivity detection. Analyte identification (concentration in mg/L in parentheses): 1: Li+ (0.10); 2: Na+ (0.12); 3: NH4+ (0.3); 4: Monoethanolamine (0.8); 5: Methylamine (0.4); 6: Diethanolamine (2); 7: Rb+ (1.2); 8: Triethanolamine (8); 9: NMethyldiethanolamine (0.5); 10: Butylamine (1.5); 11: N,NDimethylethanolamine (2); 12: Trimethylamine (2).
The rise time (rather than tF) is the focus above because only the former is of importance during a gradient run of increasing pCO2. It is also interesting to note that although conductance is the measurand, the primary chemical signature is that of H2CO3* which is proportional to the square of the conductance G (in
Negative Slope (uncertainty)a
pb
H2CO3*
HNO3
Lithium
1.059 (0.006)
0.996 (0.003)
0.0002
Sodium
1.061 (0.007)
0.994 (0.003)
0.0004
Ammonium
1.061 (0.008)
1.004 (0.006)
0.0027
Monoethanolammoniu m
1.061 (0.008)
1.005 (0.008)
0.0073
Methylammonium
1.062 (0.008)
1.007 (0.010)
0.0164
Diethanolammonium
1.064 (0.008)
1.006 (0.012)
0.0234
Rubidium
1.060 (0.010)
1.008 (0.010)
0.0211
Triethanolammonium
1.061 (0.021)
1.004 (0.009)
0.1084
Nmethyldiethanolammoni um
1.076 (0.020)
0.998 (0.009)
0.0416
Butylammonium
1.102 (0.026)
0.987 (0.004)
0.0023
Trimethylammonium
1.144 (0.042)
0.978 (0.006)
0.0195
aStandard errors of the fits. bProbability that the slopes are the same.
Is H2CO3* Behaving as More than Just a Protonic Eluent? Table 2 shows that the regression slopes for H2CO3* as eluent are consistently greater than those with HNO3: the difference is
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Analytical Chemistry
-0.95
1
-1.05
345 6 7 8
2 3 45 6
-1
8
7 -1.1
12
9
9 -1.05
10 12
-1.15
-1.2
1. Lithium 2. Sodium 3. Ammonium 4. Monoethanolammonium 5. Methylammonium 6. Diethanolammonium 7. Rubidium 8. Triethanolammonium 9. N-methyldiethanolammonium 10. Butylammonium 12. Trimethylammonium
-1.25
-1.1
-1.15
HNO3 log [E] - log k Regression Slope
10
1 2
-1.2 -2.8
-2.6
-2.4
-2.2
-2
-1.8
-1.6
Intercept of log [H+] - log k Regression Line
Figure 6. Slopes of log k vs. log [H+] plots vs. the intercept for HNO3 (right ordinate) and H2CO3* as eluent (left ordinate). Note that both ordinates have the same span.
The possibilities include that alkali metal cations or the various ammonium ions (at the operating pH of 3-4 all of the tested (alkanol)amines will be in the protonated form, the pKb’s of range from 3.4 - 6.24) can form a bicarbonate adduct. Complexation constants for such alkali metals have generally been considered too small to have been studied (such data do exist for alkaline earth metals); however, no investigations have ever been made in the presence of a solid phase and with such high concentrations of CO2 being present. Amine and alcohol functionalities can react to form carbamates18 and alkylcarbonic acids /bicarbonates,19,20 respectively. Alkanolamines have been widely studied for their utility in capturing and recycling CO2.21 The available data overwhelmingly suggest no carbamate formation: only alkylcarbonic acids are formed.21,22 Regardless of whether complexation or formation of an alkylcarbonic acid is occurring, the uncharged alkali metals and ammonium bicarbonates or aminoalkylcarbonic acid, zwitterionic ammoniumalkylbicarbonate, and anionic aminoalkylbicarbonate will not be retained by cation exchange. Much as the fraction of the free metal cation is computed in a metal-ligand system,23 in all of the above systems, the fraction of the analyte present as the free cation, c, will be given as:
Lithium Sodium Ammonium Rubidium 1:1 correspondence line
50
kc Predicted (best fit eq 9)
-1
c = 1/(1 + K[HCO3-]) … (8) where K is the constant for the adduct/complex formation and kc = kc …(9) kc being the effective retention factor in the presence of CO2, and k being the retention factor in absence of any HCO3interaction, solely that predicted based on [H+]. Figure 7 shows the agreement for the inorganic ions of the observed kc values with best fit values to eq 9. Similar agreements are seen for amines (Fig. S7) and alkanolamines (Fig, S8) as well.
40
30
20
10
0 0
10
20
30
40
50
kc Observed
Figure 7. Observed retention factors vs. best fits from eq 8-9.
The best fit values for K appear to be much higher than those expected (K = 450, 340, 570, and 910 for Li+, Na+, NH4+ and Rb+, respectively) for any such equilibria in dilute aqueous solutions. In addition, the predicted KLi is curiously higher than KNa. Activity product constants (Kap) for these metal bicarbonates can be estimated from their solubilities and activity coefficients and should be directly related to the association constant K in eq 8. In Table S4 of the SI and accompanying discussion we show, however, that the Kap values for these MHCO3 salts do follow the same curious order, Rb+ >> NH4+ > Li+ >Na+. 1
Monoethanolamine Methylamine Diethanolamine Triethanolamine Methyldiethanolamine
0.95
k~1 mM HNO3/k~1 mM HNO3 + H2CO3*
significant at the 95% confidence level for all but triethanolamine. If we define the behavior with HNO3 as the benchmark (non-protonic eluent interaction unlikely) we can examine the ratio of the regression parameters of the H2CO3* vs. HNO3 as eluent. Consider that the intercept of a log k vs. log [H+] plot is a measure of the analyte retention at unity [H+]. As such, if a single mechanism applies to the interaction of (similarly charged) analytes with the eluent, the retention dependence on [H+], i.e., the slope, should either be constant or any dependence on the intercept should be monotonic, it must be a single valued function. This is observed with HNO3 as eluent (Fig. 6) but the last three analytes dramatically depart from this trend for H2CO3* as eluent. Note in addition the large increases in the uncertainties of both the slopes and uncertainties for the last three analytes with H2CO3* as eluent, suggesting that the general linear log k - log [E] model is failing, possibly because of processes other than simple cation exchange retention of the analytes as eluents. Similar pictures emerge from slope ratio plots (Fig. S5) or individual slope and intercept plots in the order of retention (Fig. S6).
H2CO3* log [E] - log k Regression Slope
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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0.9
0.85
Trimethylamine
Butylamine
0.8 Dimethylethanolamine 0.75 0
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80
H2CO3*, mM
120
160
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Analytical Chemistry Figure 8. Retention factor ratios of ions eluted by HNO3 and H2CO3* to HNO3 alone.
Adding H2CO3* to HNO3? Thus far we have compared the effects of H2CO3* vs. HNO3 as eluent. As the conductometric concentration of the former is subject to interpretation by the applicable dissociation constants and ionic mobilities, it can be argued that there are inaccuracies in the assumed values of these under the exact operating conditions and [H+] is simply being underestimated in the H2CO3* solutions; the effect attributed to HCO3- is an artifact. In an H2CO3* solution, [H+] = [HCO3-]. If we add CO2 to a ~1 mM HNO3 solution, however, the increase in [H+] will remain equal to [HCO3-] but either will be relatively small compared to the [H+] originally present. If the imputed adduct formation with HCO3- is an artifact, this should hardly be observable under these conditions and will nearly disappear. We therefore studied the effect of adding CO2 (pCO2,ext = 0, 7, 1.4, 2.8, and 5.5 MPa, corresponding to 0, 28, 42, 78, and 140 mM H2CO3*) to 0.93 mM HNO3 (all determined conductometrically, Figures S9-12). The addition of H2CO3* in the stated amounts increased the measured [H+] by 1.4, 2.1, 3.8 and 6.7% over no addition. The change in the retention factor was remarkable for all analytes tested but most notably for dimethylethanolamine, see Fig. 8. This leaves very little doubt that while we are carrying out nominally cation exchange separations with high concentrations of H2CO3*, the elution behavior cannot simply be explained by proton exchange; H2CO3* or species derived therefrom play a significant role.
Regardless, the selectivity for the different analytes can obviously be modified by introducing various amounts of H2CO3*, which afterwards is easily removed. Amine groups are obligatorily present in proteins and peptides while it has already been demonstrated that sugars, sterols and alcohols readily and reversibly react with H2CO3*.24,25 The ability to tune separations by using different and easily programmable levels of H2CO3* may be of particular value, especially as the original analyte is readily recovered by post column degassing and can be easily monitored by mass spectrometry. Even without degassing, CO2 introduction into a mass spectrometer does not present a problem. The Merit of Suppression. The system was tested with and without a suppressor following the CO2 degasser. In so far as the signal to noise ratio (SNR) and hence detection limits are concerned, suppression brings significant benefits (see Figure 9). After suppression, the signal increased on average 1.8 times while noise (measured as the drift corrected standard deviation over 1 minute) decreased by a factor of 20. The latter is likely to improve some with better degassers and a non-suppressed system may benefit the separation and conductometric detection of very weak bases. The weakest base among the present analytes was triethanolamine (pKb 6.24), and SNR was improved >5x using the suppressor despite a 73% reduction in sensitivity. Depending on the level of CO2 present prior to detection, a low level of a basic analyte may actually result in the conductance dipping below the background, resulting in dips before or (more commonly) after the peaks. The limits of detection and the calibration curves are provided in Table S5 for the suppressed arrangement. Limits of detection (S/N = 3) for all analytes tested were
