Anal. Chem. 2001, 73, 4694-4703
Two-Dimensional Detection in Ion Chromatography: Sequential Conductometry after Suppression and Passive Hydroxide Introduction Rida Al-Horr and Purnendu K. Dasgupta*
Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061 Rebecca L. Adams
DOW Chemical U.S.A., A-915, Freeport, Texas 77541
An improved method that uses sequential suppressed and nonsuppressed IC for the sensitive detection of both common anions and very weak acid anions is described. After suppressed conductometric detection of an electrolytically generated hydroxide eluent and an electrolytic suppressor, the eluent is passed into a membrane device where KOH is passively introduced into the eluent stream using Donnan forbidden leakage. A second conductivity detector then measures the conductivity of the stream. The background conductance of the second detector is typically maintained at a relatively low level of 20-30 µS/ cm. The weak acids are converted to potassium salts that are fully ionized and are detected against a low KOH background as negative peaks. The applicability of different commercially available cation exchange membranes was studied. Device configurations investigated include planar, tubular, and a filament-filled annular helical (FFAH) device. The FFAH device provided more effective mixing of the penetrated hydroxide with the eluent stream, resulting in a noise level of e7 nS/cm and a band dispersion value of less than 82 µL. Optimal design and performance data are presented. Ion chromatography (IC) continues to play a leading role in many areas of analytical chemistry, with applications that range from trace analysis in semiconductor fabrication to environmental analysis. Small et al.1 pioneered the technique of suppressed conductometry in 1975; it is still considered the key feature that distinguishes IC from the liquid chromatographic analysis of ions. The mainstay of IC is in the analysis of anionic analytes, and we will therefore confine our attention to this area with the note that identical considerations apply to cation analysis systems. From a standpoint of detectability, suppression is greatly beneficial in the determination of strong acid anions, and even for anions derived from weak acids, at least up to a pKa value of 5. It is integral to the practice of modern IC; detection limits that result from removing the conductive eluent ions and converting the analyte to a highly conducting acid are unsurpassed by other techniques.2-4 (1) Small, H.; Stevens, T. S.; Bauman, W. S. Anal. Chem. 1975, 47, 18011809.
4694 Analytical Chemistry, Vol. 73, No. 19, October 1, 2001
However, very weak acid anions are not easily detectable by suppressed IC. Anions derived from acids with pKa >7 are virtually undetectable. Hence, the concept of converting such weakly dissociated acids to more dissociated compounds was developed. Berglund and Dasgupta published a series of papers in which the weak acid HX was converted by two sequential steps (HXf NaX f NaOH) to NaOH5 or in a simultaneous cation/anion exchange step to LiF.6 The best results were, however, achieved by combining both suppressed and single-column IC. Following a conventional suppressed IC, a controlled amount of NaOH was electrically introduced into the detector effluent by a microelectrodialytic NaOH generator (MENG). With a ∼0.1 mM NaOH background, the noise level was 20 nS/cm; the exact band dispersion was not measured.7 In a subsequent more detailed paper,8 the dispersion was measured to be 94 µL for a device of 15-mm active length. Further developments led to planar MENG devices that exhibited noise levels as good as 8 nS/cm with band dispersions in the range of 78-90 µL.9 Caliamanis et al. have developed an altogether different approach. A commercial suppressor unit bearing cation exchange membranes and an NaOH-EDTA external bathing solution is used to convert HX to NaX.10-13 Yuan et al.14 suggested operating a suppressor in a mode such that the eluent is just short of completely neutralized. However, it is very difficult to maintain such operation with a constant background, low-noise environment. (2) Dasgupta, P. K. Anal. Chem. 1992, 64, 775A-83A. (3) Strong, D. L.; Joung, C.-U.; Dasgupta, P. K. J. Chromatogr. 1991, 546, 159173. (4) Strong, D. L.; Dasgupta, P. K. Anal. Chem. 1989, 61, 939-945. (5) Berglund, I.; Dasgupta, P. K. Anal. Chem. 1991, 63, 2175-2183. (6) Berglund, I.; Dasgupta, P. K. Anal. Chem. 1992, 64, 3007-3012. (7) Berglund, I.; Dasgupta, P. K.; Lopez, J. L.; Nara, O. Anal. Chem. 1993, 65, 1192-1198. (8) Sjogren, A.; Dasgupta, P. K. Anal. Chem. 1995, 67, 2110-2118. (9) Sjogren, A.; Dasgupta, P. K. Anal. Chim. Acta 1999, 384, 135-141. (10) Caliamanis, A.; McCormick, M. J.; Carpenter, P. D. Anal. Chem. 1997, 69, 3272-3276. (11) Caliamanis, A.; McCormick, M. J.; Carpenter, P. D. Anal. Chem. 1999, 71, 741-746. (12) Caliamanis, A.; McCormick, M. J.; Carpenter, P. D. J. Chromatogr., A 1999, 850, 85-90. (13) Caliamanis, A.; McCormick, M. J.; Carpenter, P. D. J. Chromatogr., A 2000, 884, 75-80. (14) Huang, Y.; Mou, S.; Liu, K. J. Chromatogr., A 1999, 832, 141-148. 10.1021/ac0105336 CCC: $20.00
© 2001 American Chemical Society Published on Web 08/25/2001
Figure 1. Theoretical response plots (note a-c have log-log axes and show first detector response): (a) pure water assumed to be background, (b) 10 µM CO2 assumed to be the background, (c) 1 µM H2SO4 assumed to be the background, and (d) response of first and second detectors in linear axes.
The present work elaborates on previous studies that utilized base introduction after a conventional suppressed IC. It is the added and different dimensionality brought about by the additional detector that makes the overall approach attractive.7-9 It differs from previous work in that passive rather than electrodialytic base introduction is used, requiring no electronic control. Further, we have studied several different available membranes in different physical designs and in different thicknesses with different bases to determine the optimum conditions so that results as good as the best of the previous electrodialytic base introduction efforts can be realized in a simpler manner. The recent commercial availability of electrodialytic eluent generators15 capable of producing highly pure hydroxide eluents, which lead to nearly invariant backgrounds even with gradient elution, makes two-dimensional ion chromatography (2DIC) more attractive than ever before. We present a simple way to practice the same. (15) Liu, Y.; Avdalovic, N.; Pohl, C.; Matt, R.; Dhillon, H.; Kiser, R. Am. Lab. 1998, 30 (22), 48C. Liu, Y.; Kaiser, E.; Avdalovic, N. Microchem. J. 1999, 62, 164-173.
PRINCIPLES Analytes elute from a suppressor as an acid HX (when we are concerned with weak acids, even if a given analyte may be multiprotic, consideration of ionization beyond the first proton is typically unnecessary).8 The suppressed conductometric signal is related to 0.5(λH+ + λX-)((Ka2 + 4CKa)0.5 - Ka)), where C and Ka are the eluite concentration and the dissociation constant of HX, respectively, under conditions where autoionization of water can be neglected. For most practical purposes, the presence of trace acids in the background, whether from regenerant leakage in a chemically regenerated suppressor or from omnipresent CO2, is a more urgent concern than the autoprotolysis of water. Figure 1 depicts the nature of the problem. All of these computations were carried out with the following assumptions: temperature 25 °C, monoprotic acid analytes HX (with λX- equal to 50 and pKa ranging from strong acid 0 to10) and the analyte concentrations represented in the abscissas are those at the point of measurement in the detector (injected concentrations, accounting for chromatographic dispersion, would typically be an order of magnitude Analytical Chemistry, Vol. 73, No. 19, October 1, 2001
4695
higher). Numerical computations were carried out by solving the complete charge balance equation for a given system using the nonlinear curve fitting capabilities of Microsoft Excel Solver16 with a numerical accuracy of seven significant digits in the computed H+ concentration. Specific analyte concentrations solved were 0.1, 0.3, 1, 3, 10, 30, and 100 µM and the lines shown are spline fits through these points. Panel a shows the situation for a hypothetical pure water background. For clarity, the first three panels are in log-log scales. The minimum ordinate value is 1 nS/cm, slightly below the current state of the art of the noise levels encountered in suppressed hydroxide eluent anion chromatography. Realistically, 10 nS/cm is the level at which a peak could be detected by a current state-of-the-art system. In general, at low analyte concentrations, there is little difference from a strong acid down to a pKa of ∼5. Past a pKa of 7, the response begins to decrease ∼1 log unit with each log unit decrease in Ka. The possibility that acids with pKa >7 can be detected at low concentrations is obviously remote. In reality, when auxiliary acids such as CO2 (in panel b we assumed 10 µM ΣCO2, 120 ppb total inorganic C, and background 0.76 nS/cm; pure water saturated with atmospheric CO2 contains 13-17 µM ∑CO2) or H2SO4 (in panel c, we assumed 1 µM H2SO4, typical minimum leakage from a chemically regenerated suppressor, resulting in a background of 0.86 nS/ cm) are present, the detectability of weaker acids deteriorates considerably. In panels b and c, the pKa 10 case disappears from our viewing region and in fact it is clear that there is little hope of detecting acids weaker than pKa of 7 even at relatively high concentrations. In addition, the detectability of a weak acid analyte in a real matrix that may contain other, more ionized constituents at higher concentrations is likely to be far worse if there is any possibility of coelution. Even when a weak acid analyte elutes on the tail of a stronger acid peak, it may never be seen, both due to the suppression of ionization of the weak acid and due to the intrinsically lower response. The introduction of a low but constant concentration of a strong base to the effluent from the above conventional suppressed conductometric IC system prior to detection by a second conductivity detector has been proposed previously.7-9 An analysis of the relative response behavior is noteworthy. In Figure 1d, we show (in a linear scale) the response behavior of analytes from a strong acid to a pKa of 10 for the 10 µM ∑CO2 background, as well as the responses resulting from the second detector upon introduction of 125 µM NaOH (no volumetric dilution or dispersion is assumed, the background is ∼25 µS/cm, and such signals have no significant dependence on whether some weak or strong acids, such as CO2/H2SO4, are present in the background). These signals appear as negative peak responses (which they are). For a strong acid HX with λX- of 50, the response is 37% in magnitude for the base introduction system relative to that of the conventional suppressed system (increases to 48% for λX- of 20). For the strong acid case, this represents a 2-3-fold loss of sensitivity and is not attractive. However, the base introduction system shows the same response (within (3.8%) from a strong acid to an analyte with a pKa of 8, a response comparable in magnitude to the response of an analyte with a pKa of 5 in a suppressed IC system but with better linearity. With analytes of pKa >5, the base introduction response is favored by 1 order of magnitude with each order of magnitude decrease in Ka. With analytes of acidity weaker than a 4696 Analytical Chemistry, Vol. 73, No. 19, October 1, 2001
Figure 2. Cassidy plot of response sensitivity in linear axes. An ideally linear response produces a flat curve of zero slope. The top trace assumes a 1 µM H2SO4 background; all others assume a 10 µM CO2 background.
Figure 3. Experimental system. Key: P, chromatographic pump (1 mL/min); EEG, electrodialytic eluent generator; V, injection valve (25 µL); GC, AG11HC (4 mm) guard; SC, AS11HC separator; EDS, electrodialytic suppressor; D1, first detector; BID, base introduction device; D2, second detector; R, exit restrictor. KOH flow into BID is 0.5 mL/min by nitrogen pressure.
pKa of 8, the pH afforded by the introduction of 125 µM NaOH is insufficient to maintain full ionization. By the time a pKa of 10 is reached, the sensitivity has decreased to 40% to that for the corresponding case of a strong acid, but it is still 4 orders of magnitude more sensitive than the corresponding suppressed detection response. Indeed, the response in the second detector to an analyte of pKa 10 is significantly better than that of an analyte of pKa 6 in the first detector, with much better response linearity. The linearity of response is best examined with a Cassidy plot,17 as shown in Figure 2. It is interesting to note that, in the absence of a strong acid in the background, theory predicts that there will be considerable nonlinearity in the response at very low analyte concentrations in the conventional suppressed conductometric detection mode. This behavior is due to the pliant nature of the baseline, which in the limit is constituted of water, a weakly ionized acid. Appearance of an analyte peak on the baseline causes (16) Walsh, S.; Diamond, D. Talanta 1995, 42, 561-572. (17) Cassidy, R. M.; Chen, L. C. LC‚GC Mag. 1992, 10, 692-696.
Figure 4. Base introduction device designs: (a) planar sheet membrane design that can be operated electrodialytically or by Donnan leakage, (b) straight tube in shell design, and (c) filament-filled annular helical design.
decreased dissociation of the background constituents, similar to subsidence of soil upon erecting a structure. This was quantitatively probed for carbonate eluents by Doury-Berthod et al.,18 where a large amount of carbonic acid is present as the background but at the detection limits possible today, this behavior will be expected at low analyte concentrations even with pure water as background. The fact that sufficient strong acid may be present in a real eluent background (even one electrodialytically generated) can constitute a blessing in disguise insofar as response linearity at low concentrations is concerned. All responses shown in Figure 2 assume a 10 µM CO2 background, which may be the least contaminated background that can be attained in practice. In the conventional detection mode, the response per unit concentration is initially low due to the CO2 background and also decreases at the high concentration end for all but a strong acid analyte. As a result, analytes of intermediate pKa values, most notably at 4 and 5, show a peak in sensitivity as a function of concentration. The general nonlinearity of response and the drastic decrease in response at analyte pKa values of g6 is apparent in this depiction, in marked contrast to the essentially uniform response for the base introduction detection mode, at least up to a pKa value of 8. The latter also shows usable response up to a pKa value of 10.
In the present system, negatively charged hydroxide ions are introduced through a negatively charged cation exchange membrane, Donnan-forbidden ion penetration19 is the mechanism of base introduction. The relevant parameters are thus (i) the concentration gradient across the membrane, (ii) the characteristics of the membrane, and (iii) nature of the counterion accompanying OH-. The penetration rate of the forbidden ion decreases with increasing size and charge,19 and introduction of OH- is thus easier than most other anions. The penetration rate is also inversely related to the membrane thickness and directly to the available surface area. These parameters are optimized in this work.
(18) Doury-Berthod, M.; Giampoli, P.; Pitsch, H.; Sella, C.; Poitrenaud, C. Anal. Chem. 1985, 57, 2257-2263.
(19) Dasgupta, P. K.; Bligh, R. Q.; Lee, J.; D’Agostino, V. Anal. Chem. 1985, 57, 253-257.
EXPERIMENTAL SECTION Figure 3 represents the system used in this work. The base introduction device was placed between two conductivity detectors. The system temperature was controlled at all times by placing columns, detector cells, the base introduction device, and all connecting tubing in a chromatographic oven. Base Introduction Device. Three different devices designs were investigated (see Figure 4). Device A is made up of two Plexiglas blocks, each containing an inscribed channel (0.6 × 0.6 × 40 mm) with 10-32 threaded ports that connect them to the
Analytical Chemistry, Vol. 73, No. 19, October 1, 2001
4697
outside. Platinum wires (0.3 × 15 mm) partially fill the channels and exit through additional independent 10-32 threaded ports, as shown. These wires are used as electrodes connected to a constant-current source for electrodialytic introduction of base. The cation exchange membrane is placed between the blocks and separates the two flow channels; bolts hold the blocks together. Several different cation exchange membranes were investigated. Donor hydroxide solution flows through one channel while the suppressed effluent from the first conductivity detector D1 flows through the other side to detector cell D2. The other two designs are based on perfluorosulfonate Nafion membrane tubing. Terminal bores of 1.5-mm-o.d., 0.25-mm-bore PTFE tubes were enlarged by drilling. Nafion tubes, the terminal ends of which are strengthened by PTFE or PEEK tubular inserts, can be put into the end-enlarged PTFE tubes and sealed by standard compression fittings. Each end terminates in a tee such that the donor base solution can be made to flow in a jacket that connects the two tees and surrounds the Nafion tube. Device B uses a 90-mm-long Nafion tube in linear configuration. Two membranes were tested, with respective dry dimensions of 0.35 × 0.525 and 0.30 × 0.40 mm (i.d. × o.d.). Device C represents the third design in which a 0.25-mm nylon monofilament-filled Nafion tube (250 × 0.30 i.d. × 0.40 mm o.d.) was coiled into a helical structure before incorporation into an external jacket, following the design of a filament-filled annular helical (FFAH) suppressor.20 All experiments were carried out with a DX-500 ion chromatography system, consisting of a GP-40 gradient pump equipped with a degasser, an LC-30 chromatography oven, an EG-40 eluent generator, and CD-20 and ED-40 conductivity detectors. All connections utilized 0.25-mm PEEK tubing. For chromatography, Dionex AG11 and AS11 guard and separator columns were used. Data collection and analysis utilized PeakNet 5.1, all from Dionex Corp. (Sunnyvale, CA). All experiments were carried out at 30 °C with a chromatographic flow rate of 1 mL/min. All conductance values are corrected to 25 °C by assuming a temperature coefficient of 1.7%/°C. Except as stated, the hydroxide flow rate was 0.5 mL/min (observed values were affected at flow rates less than 0.4 mL/min) and 100 mM KOH was used as feed. Band Dispersion Measurements. Band dispersion was calculated as the square root of the difference between the squares of the band half-widths of the first and second detector response.19 Band dispersion calculated in this way decreases with increasing band volumes. Dispersion affects sharp narrow peaks more than it affects broad peaks. Therefore, band dispersion was computed on sharp early-eluting peaks of 0.25 mM acetate (injection volume 25 µL, 5 mM KOH eluent).
RESULTS AND DISCUSSION Electrodialytic Base Introduction through Different Membranes. Most ion exchange membranes are available in sheet form. Base introduction capabilities were therefore tested with device design A (Figure 4a), which allowed both electrodialytic and Donnan-forbidden passive penetration to be tested. Baseline noise was taken to be the standard deviation of the baseline over a 15-min period. In Figure 5, we show the background conductivi(20) Dasgupta, P. K. Anal. Chem. 1984, 56, 103-105.
4698 Analytical Chemistry, Vol. 73, No. 19, October 1, 2001
Figure 5. Current efficiencies observed with electrodialytic devices with different membranes.
ties generated with different membranes as a function of the current. Exact Faradaic behavior and a membrane with no zero current leakage will result in a background conductance of 27.1 µS/cm (100 µM KOH) for a drive current of 160 µA. This ideal behavior is shown as the thick solid line. The behavior of most of the membranes falls into one group, and a collective best-fit line is drawn through them. This exhibits a small background bleed (∼1.1 µS/cm, ∼4 µM KOH) and a mean slope that is 78% of theoretical. One membrane, a radiation grafted PTFE cation exchange membrane, falls in a class by itself and exhibits very significant zero current penetration of 16.8 µS/cm (over 60 µM KOH) and a relatively low current dependence of KOH generation (47% of Faradaic). The background noise levels observed with the different membranes are obviously of interest since they control the detection limits that could ultimately be attained. Figure 6a shows the noise levels observed as a function of background conductance. It is clear that the strong cationic Teflon membrane again falls in a class by itself by providing the lowest background noise. However, since this membrane also exhibits a very high zero current background conductance it is instructive to look at the noise as a function of the electrodialytic drive current; this is shown in Figure 6b. In this depiction, the noise appears to be largely independent of the membrane. Rather, it is linearly proportional to the electrodialytic drive current. If microbubbles of electrolytic gas, the amount of which is expected to be proportional to the drive current, is the dominant contributor to the observed noise, then this behavior is understandable. Whether or not bubbles are specifically involved, the data strongly suggest that the observed noise in the background conductance is directly related to the drive current, more than any other factor. Passive Introduction of Base through Different Membranes. The foregoing experiments suggested that the simpler expedient of passive, Donnan-forbidden introduction of base to the desired extent (∼100 µM) may not only be possible but may be desirable from a standpoint of background noise. It has been suggested in previous studies18 that when maintaining a sufficient flow rate prevents buildup on the receiver side, the Donnan
Figure 6. Background noise in electrodialytic devices with different membranes as a function of (a) the observed conductance (0.1 mM KOH ) 27.2 µS/cm) and (b) the electrodialytic drive current. Internal flow, 1 mL/min in this and subsequent figures.
Figure 7. Passive Donnan leakage of KOH through various sheet membranes as a function of feed KOH concentration.
penetration rate (A) of the forbidden ion is a quadratic function of the feed concentration (m) as follows:
m2 ) RA2 + βA + γ
(1)
where R and β are positive constants and γ is a constant of either sign. Figure 7 shows the observed concentration of KOH in the receiver (as determined from the conductance) as a function of the feed concentration for several different membranes. The line
through the points is the best fit for each case to eq 1 above. The Dow perflurosulfonate ionomer (PFSI) membrane and the thin grafted Teflon membrane both have very high penetration rates, and the desired degree of Donnan leakage can be achieved with relatively low feed concentrations. The Dow PFSI was an experimental material available in very limited quantity, and further work was done with the thin Teflon membrane only. Dependence of Penetration Rate on the Nature of the Cation. Hydroxides of the alkali metals, LiOH, NaOH, KOH, and CsOH, were used individually as feed solutions. and the penetration rates were measured for the thin Teflon membrane. The penetration rates, shown in Figure 8, are in the order LiOH . NaOH > KOH > CsOH and directly reflect the order of the ion exchange affinities of these ions for cation exchange sites, Li+ being the most easily replaced. This is logical since we would expect ion exchange sites on the feed side of the membrane to be saturated with the metal ion (because of both its high concentration and its high alkalinity) such that the overall rate is likely to be controlled by the rate that the metal ion leaves the membrane on the receiver side. Note that this behavior is opposite to that expected for diffusive transfer through a passive, e.g., a dialysis membrane, because the diffusivity is much lower for the large solvated Li+ ion than the Cs+ ion. Regrettably, these series of experiments were performed after most other experiments described in this paper. It is obvious that, for base introduction purposes, it should be preferable to use LiOH, even though KOH was used for most of the experiments in this study. For detection after base introduction, one is interested in maintaining some constant concentration of base introduced. Because LiOH has the lowest equivalent conductance among the alkali hydroxides, it also provides the least background Analytical Chemistry, Vol. 73, No. 19, October 1, 2001
4699
Figure 8. Dependence of Donnan leakage on tubular membrane dimensions. Nafion membrane tubes are used.
conductance at the same concentration (the conductance due to 100 µM MOH is 23.7, 24.9, 27.2, and 27.6 µS/cm for M ) Li, Na, K, and Cs, respectively) and should therefore provide the least conductance noise at the same background base concentration. Effects of Temperature on Penetration Rate. The effect of temperature was examined for KOH penetration through the thin Teflon membrane from 25 to 40 °C. The penetration increased from 62.5 to 68.4 µM, essentially linearly at 0.39%/°C. Effects of Membrane Thickness on Penetration Rate. It is intuitive that penetration rate should increase with decreasing membrane thickness, and the data in Figure 7 already provide some support toward this. However, the membrane types differ in that experiment and no clear conclusions can be drawn. The two tubular membranes used for the construction of device B were identical in length but varied in radial dimensions (525 × 350 vs 400 × 300 µm in o.d. × i.d., respectively). Compared to the first, the second tube provides a 42% lower external surface area but the wall thickness is also 43% lower. The data presented in Figure 9 make it clear that the wall thickness is by far the dominant factor. A complete understanding of the exact dependence will require experiments that were beyond our ability to perform since availability of the same membrane in different thickness was not within our control. In the above experiment, the decrease in inner diameter increases the flow velocity by 36% at the same volumetric flow rate, this may also have a small effect on increasing the penetration rate by decreasing the stagnant boundary layer thickness. Device Performance. Noise and Dispersion. As previously noted, experiments with device A showed passive penetration was superior in terms of noise performance than electrolytic introduction of base. The conductance noise level measured directly at the exit of device A fabricated with the thin Teflon cation exchange membrane with KOH feed concentration adjusted to produce ∼100 µM KOH in the effluent was 28 ( 2 nS/cm. It was also observed that incorporation of lengths of connecting tubing between the base introduction device and the detector reduces the noise. This 4700 Analytical Chemistry, Vol. 73, No. 19, October 1, 2001
Figure 9. Detection of 0.6 µM borate in a sample mixture on the second detector. This presentation used a moving average routine to reduce baseline noise. The S/N ) 3 LOD will be 0.6 µM based on the baseline noise observed in the raw detector signal.
Figure 10. Donnan leakage of different alkali hydroxides through the RAI PTFE membrane.
suggested that mixing within the device is incomplete. Incorporation of a 0.75-mm-i.d., 750-mm-long mixing coil woven in the Serpentine II design21 reduced the noise level to 7 ( 2 nS/cm. However, the band dispersion induced by the device, already at a significant value of 96 ( 8 µL, increased by a further 55 µL with the addition of the mixing coil. Both versions of device B exhibited noise levels similar to that of device A (without mixer). However, dispersion in straight open tubes is the highest of all,21 and even with the narrower membrane tube, the band dispersion was measured to be 110 ( 4 µL (148 ( 6 µL for larger tube). Incorporation of a mixer to reduce noise will clearly make this even worse. (21) Waiz, S.; Cedillo, B. M.; Jambunathan, S.; Hohnholt, S. G.; Dasgupta, P. K.; Wolcott, D. K. Anal. Chim. Acta 2001, 428, 163-171.
Figure 11. Second detector response to various analytes using a commercial membrane suppressor (containing an ion exchange screen) as the base introduction device.
A logical solution seemed to be the incorporation of base introduction and mixing functions within the same device. The helical geometry is known to induce good mixing while minimizing band dispersion due to the development of secondary flow that is perpendicular to the axial flow. This secondary flow flattens the parabolic profile of the axial flow velocity observed in a linear tube and leads to both reduced axial dispersion and increased radial mixing inside the tube.21,22 FFAH devices, albeit of somewhat larger dimensions, have previously been used as suppressors.19,21,23 Built along this design, device C indeed exhibited the best performance. Even though the tube itself was nearly 3 times as long as device B, the band dispersion was measured to be 78( 4 µL. Under isocratic elution conditions, the noise level was measured to be 5 ( 2 and 10 ( 2 nS/cm under a demanding steeply changing gradient elution condition. Because of its larger surface area relative to device B, a lower concentration of feed KOH is needed to reach a ∼100 µM concentration in the receiver. At 30 °C, a 50 mM KOH feed leads to a background conductance of 28 µS/cm with an eluent flow rate of 1 mL/min. Under a given feed condition, the penetration of KOH remains constant. In one experiment, the flow rate of 35 mM electrodialytically generated KOH used as eluent was varied between 0.5 and 1.75 mL/min in 0.25 mL/min increments. The electrodialytically suppressed conductance always remained below 0.8 µS/cm. The suppressor effluent (essentially water) was passed through a FFAH device with 65 mM carbonate-free KOH (electrodialytically generated by a second electrodialytic generator) acting as feed. The observed background conductance was linearly related to the reciprocal of the eluent flow rate with a linear r2 value of 0.9999. The device showed excellent reproducibility. Taking borate, a classic weak acid analyte, the reproducibility at the 50 µM (22) Dasgupta, P. K. Anal. Chem. 1984, 56, 96-103. (23) Dasgupta, P. K. U.S. Patent 4,500,430, 1985.
injected level was 2.0% in RSD, the S/N ) 3 limit of detection was 0.6 µM (6.5 ppb B, 25-µL injection, 15 pmol) with a linear r2 value of 0.9997 for response in the 5-100 µM range (7 mM KOH isocratic elution, tR ∼6.3 min). This performance is notable because boric acid has a pKa of 9.23 and under the above conditions elutes as a relatively broad peak (w1/2 ∼40 s). Response from 0.6 µM borate (and several other ions at trace levels) is shown in Figure 10. Base Introduction versus Ion Exchange. Effect of Device Design. Different membrane devices are commercially available as suppressors. The purpose of such devices in anion chromatography is to exchange large concentrations of eluent cations and, as such, requires significant ion exchange capacities. As a result, such suppressor devices are often designed with ion exchange screens between ion exchange membranes;24 these screens are particularly valuable in gradient elution because of their ability to provide reserve ion exchange capacity. While these devices can undoubtedly be used for base introduction, it is to be noted that they are capable of ion exchange on the screens, without immediate and concomitant base introduction. This process can occur in addition to the base introduction process. Note that when the sole process is introduction of the base MOH through the membrane, the reaction that occurs for any analyte HX (within the limits that HX does not exist as an un-ionized acid at a pH of ∼10 (∼100 µM MOH)) is
MOH + HX f MX + H2O
(2)
In this case, all signals are uniformly negative and the signal intensity is controlled by the analyte concentration and the difference in equivalent conductance between the analyte ion and OH-. If the analyte HX is significantly ionized, the resulting H+ (24) Stillian, J. R. LC‚GC Mag. 1985, 3, 802-812.
Analytical Chemistry, Vol. 73, No. 19, October 1, 2001
4701
Figure 12. 2D ion chromatogram under standard conditions using gradient elution, 25-µL injection volume.
can be ion exchanged for M+ at the interior membrane surface:
M+membrane + H+aq f H+membrane + M+aq
(3)
Processes 2 and 3 cannot be distinguished in practice because the M+ that is being exchanged at the membrane surface would have otherwise been introduced as MOH. There is the apparent difference in principle that process 2 results in a production of an additional water molecule. In practice, with trace level analysis, the difference in the hydration of ions in the membrane versus free solution and the high water permeability of all ion exchange membranes will make it impossible to differentiate processes 2 and 3. If, however, the same process as that in 3 occurs on the ion exchange screens, the outcome will be different:
M+screen + H+aq f H+ screen + M+aq
(4)
The screen ion exchange sites are regenerated on a much slower scale, and process 4 will therefore lead to the production of MX in addition to the introduction of MOH. For poorly ionized analytes, only process 2 can occur. But for ionized analytes, processes 2/3 and 4 can occur in competition. If the latter dominates, the result will be a positive MX peak atop a MOH background. (The screen sites will be regenerated more slowly, resulting in an eventual change in baseline.) The use of a suppressor for base introduction purposes results in the chromatograms shown in Figure 11. This behavior obviously results in an interesting and immediate differentiation between strong and weak acid analytes and may be useful in some situations. The possibility of coeluting peaks in opposite directions may, however, complicate interpretation of the data in real samples. Illustrative Applications. Figure 12 shows a 2-D chromatogram with the two detector signals being shown for several strong and weak acid anions. Weak acid analytes such as arsenite, silicate, borate, and cyanide are invisible in the first detector and produce easily measurable responses in the second detector. 4702 Analytical Chemistry, Vol. 73, No. 19, October 1, 2001
Figure 13. 2D ion chromatogram of an air filter sample extract (Houston, TX, July 2000). The inset shows the 18-21-min region magnified.
Previous work has elaborated on how such 2-D data can be exploited for the diagnosis of coelution, estimation of analyte pKa values, calculation of analyte equivalent conductance (and thereby providing a means of identification) values, and performance of universal calibration.7-9 The advent of commercial electrodialytic eluent generators has made possible nearly pure water backgrounds that, in conjunction with passive base introduction devices, make the practice of 2-D IC detection simpler, more sensitive, and more attractive than ever. User-friendly software that can fully utilize the 2-D data is needed for the complete exploitation of the technique. Recent advances in the understanding of ion exchange devices in ion chromatography may even make possible 3-D detection schemes (HX, MX, MOH).25 However, even the present state of development provides a very useful tool to the interested user as detailed below. We are interested in the composition of airborne particulate matter and in that context have collected numerous samples of airborne particulate matter and analyzed them by ion chromatography, for example, during the EPA supersite campaigns in Atlanta and in Houston26 (for a detailed description of alternately washed/ sampled filter collection, see ref 27). While major components such as sulfate, nitrate, and chloride are readily identifiable and quantifiable, there are numerous other analytes also present in these samples that are often hidden by the major analyte peaks. Even with IC-MS, coelution makes identifying the occurrence (25) Srinivasan, K.; Saini, S.; Avdalovic, N. Recent Advances in Continuously Regenerated Suppressor Devices. 2001 Pittsburgh Conference, New Orleans, LA, March, 2001, Abstract 136. (26) http://www-wlc.eas.gatech.edu/supersite/; http://www.utexas.edu/research/ ceer/texaqs/index.html (27) Samanta, G.; Boring, C. B.; Dasgupta, P. K. Anal. Chem. 2001, 73, 203440.
and identification of trace constituents a very challenging task. (Contrary to popular belief, in our hands, IC-MS actually provides considerably poorer detection limits than either of the detectors in 2D IC, especially when a total ion scan must be conducted for a totally unknown analyte.) Figure 13 shows a 2D chromatogram of an air filter sample extract collected in Houston during the summer of 2000. Note that the data immediately reveal that the asterisked peak is clearly an acid weaker than a common aliphatic carboxylic acid (see response to acetate in Figure 12). This information would have been impossible to discern by any other means. Of the numerous other nuances that are present in this chromatogram but are too difficult to see without further magnification, we focus only on the 18-21-min region. The peak at ∼19 min is completely invisible in the suppressed chromatogram and must be due to a very weak acid. The peak at ∼20 min is seen as a perfectly clean Gaussian response in the suppressed chromato-
gram while the second dimension immediately reveals that it is actually a mixture of two partially coeluting analytes, probably in a ratio of ∼1:3. In summary, 2DIC in its presently developed form is simple to implement and practice, and aside from improving the detectability and response linearity characteristics of weak to very weak acids, it provides a wealth of information that is otherwise difficult or impossible to obtain. ACKNOWLEDGMENT This research was supported in part by Dionex Corp. and by the Dow Chemical Co.
Received for review May 8, 2001. Accepted July 25, 2001. AC0105336
Analytical Chemistry, Vol. 73, No. 19, October 1, 2001
4703
