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Anal. Chem. 1991, 63,2175-2183
Two-Dimensional Conductometric Detection in Ion Chromatography, Postsuppressor Conversion of Eluite Acids to a Base Ingemar Berglund a n d P u r n e n d u K. Dasgupta* Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061
A converter, conslstlng of a serlally connected catlon-exchanger and an anlon-exchanger membrane tube, woven In a low dispersion configuration and bathed externally with NaOH, can convert an elulte acld HX (wtth varying degrees of efflclency, dependlng on the pK of HX) to NaOH. Such a converter Is installed after the conductivity detector of a conventlonal NaOH eluent suppressed Ion chromatography system and followed by a second conductlvlty detector. The two dimensional response data readily reveal peaks due to weak and very weak acids hidden In the suppressed basellne or overlapped wlth strong acld elulte peaks. The sensitivity for strong acld analytes generally decrease by a factor of 2 for the converted slgnal vs the suppressed Ion’s signal whereas for a weak acld analyte, dependlng on the pK, concentratlon, and the number of protons involved, the converted slgnal can be more than 1 order of magnitude greater. The method allows an estimation of the pK of the elutte peak and permlts approxlmate quantltatlon wlthout standards. Numerlcal slmulatlons of a theoretlcal model of the process are presented and compared with experimental results.
-
Increasing the dimensionality of the information output of any analytical method for multicomponent samples can be attempted either at the separation or the detection stage. In both cases it is necessary that the selectivity of individual separation/detection methods be different. The power of a truly multidimensional separation technique is impressive; this is well illustrated by the recent marriage of high-performance liquid chromatography to capillary zone electrophoresis (I). Multiwavelength optical absorbance measurements in an HPLC system, carried out typically with photodiode array detectors, represent on the other hand a commonly practiced approach with a respectable amount of multidimensional information output that is obtained a t the detector end. In favorable cases, the information for only two carefully chosen wavelengths can indicate the purity of an eluting peak (as determined from the constancy of the absorbance ratio) and can assist in confirming the identity of the eluite (from the value of the absorbance ratio) (2). We are primarily concerned with the separation of ionic and ionizable analytes. Pseudomultidimeneional chromatographic schemes (sequential class-selective elution) have been described for such systems (3),and the power of the technique can conceivably be enhanced with multifunctional stationary phases (4). While these schemes have their own merits, they are generally incompatible with conductometric detection, the mainstay of ion chromatography (IC). Further, a large number of ions of interest have no useful or characteristic optical absorption (indeed, suppressed conductometric IC ( 5 ) was developed to solve this problem) and multiwavelength absorbance detection is of little value. 0003-2700/91/0363-2175$02.50/0
Some unique approaches are, however, possible in conductometric IC. Wilson et al. (6) utilized separate sequential single-column IC determination of the same sample with two different eluents, enabling them to assess the ionic mobility of the individual eluites and permitting solute-independent calibration. This technique was explored subsequently in more detail by Renn and Synovec (7) who also simplified the approach by using simultaneous injection into two independent chromatographic systems. An implicit requirement of these techniques is that the eluite anion be ionized to the same extent in the two eluent systems. The general approach is confined by the need to devise two practical eluents with very different eluent ion mobilities and is compounded by the limited sensitivities attainable in single-column IC analysis under such conditions. More recently, Synovec et al. (8)have shown the ability to determine the existence of the overlap of eluite peaks and the ability to perform accurate quantitation despite significant overlap by using sequential chromatography of a sample as such and after spiking with an analyte. This approach can be applicable to any chromatographic technique; however, like the foregoing, the necessity of carrying out chromatography twice remains. We propose here an altogether different approach to increase the dimensionality of information output in a conductometric suppressed IC system. Following the emergence and detection of the eluite acids at a first conductivity detector in the conventional manner, the scheme calls for the translation of the eluite acid into some other compound and detection a t a second conductivity detector. The orthogonality of the information content is increased if very weak acids can be detected sensitively after conversion, since they are not sensitively detected by the first detector. Poor detectability of very weak acid analytes is not confined to supressed IC. Although the sensitivity for such analytes is better with a strongly alkaline eluent like NaOH (91,most typical singlecolumn anion separations utilize phthalate, borategluconate, or p-hydroxybenzoate eluents (10). PRINCIPLES Both from a theoretical and practical standpoint it is simplest to consider a background of nearly pure water, as may be obtained with a NaOH eluent in a suppressed IC system. Recently developed electrodialytic on-line ultrapure eluent generators and suppressors ( I I , I 2 ) can indeed attain eeeentially pure water as the detector background. An eluite acid H X may be converted to a corresponding alkali metal salt, e,g., NaX, by continuous cation exchange of the eluite with Nata Such an approach has previously been used (13) in an ion-exclusion chromatography (lEC) system with a strong acid eluent to convert both the eluent and the eluite acids to the corresponding Na salts followed by detection of the weak acid salts by the virtue of their alkaline nature. However, the orthogonality of the attainable information output in the present case can presumably be increased if a second ion@ 1991 American Chemical Society
2176
ANALYTICAL CHEMISTRY, VOL. 63, NO. 19, OCTOBER 1, 1991
The ratio of eqs 6 and 8 yields R* = s,*/s,* = (AH+ + Ax-)/(AN~+
+ AoH-)
(9)
Equation 9 predicts a flat ratiogram for a pure peak (strong acid eluite) similar to absorbance ratiograms. Putting in values of AH+, AOH-, and ANa+ (350, 200, and 50, respectively, in appropriate units) and rearranging eq 9, one may evaluate AXto aid identification: Ax- = 250R* - 350 (10)
Figure 1. System schematic.
exchange step is carried out, to convert the NaX as generated above into NaOH, by using continuous anion exchange with OH-. Such a two-step ion-exchange process has also been previously demonstrated for an IEC application. Tanaka and Fritz (14) converted eluite C 0 2 (in a water eluent) system first into KHC03 and then to KOH to improve its conductometric detectability by sequential passage through K+-form and OH--form cation- and anion-exchange resin packed columns, respectively. The system we wish to consider is depicted in Figure 1. A conventional NaOH eluent suppressed IC system is followed by a sequential two-stage converter containing Na+and OH--exchanger stages and a second conductometric detector, The suppressed and "converted" conductance signals Saand S,, respectively, from detectors D, and D, are acquired by a personal computer via a data acquisition interface. The signal Sa (in pS/cm) may be given by
Since both Sa* and S,* are linearly proportional to the instantaneous concentration, Ax- may be evaluated, more conveniently, by replacing R* in eq 10 by the ratio of the corresponding peak areas, A,* and A,*. The use of area values obviate corrections for dispersion in the converter. The concentration C can be evaluated readily from either eq 11 and 12, permitting solute-independent calibration.
C = S,*/1000(350 + Ax-)
(11)
C = Sc*/250000
(12)
In practice this is likely to have limited utility; in few situations is an analyst blessed with samples that contain only strong acid eluites. However, this limitation is comparable to the need for the eluite to be ionized to the same extent in previously described single-column systems (6, 7) for soluteindependent calibration except that chromatography need not be carried out twice. The expression comparable to eq 9 that is generally applicable may be obtained by substituting eq 4 into eq 1; whence R = S,/S, =
0.5(350 + Ax-)(-K
where the concentrations are expressed in molar units and AH+ and Ax- represent the mobilities of H+ and X-, respectively. The relevant electroneutrality expression is
[H+] = K,/[H+]+ [X-]
(2)
+ [H+])
(3)
where = CK/(K [X-]
K being the dissociation constant of H X and C being the instantaneous molar concentration of eluite H X responsible for the signal. Unless K and C are both very low
[H+] = [X-] = (-K
+ (P+ 4KC)'/2)/2
(4)
which reduces to the familiar form
[H+] = [X-1= C
(5)
for a strong acid a t not too dilute concentrations. In such a case, eq 1 becomes
sa*= 1oooc(AH++ AX-)
(6)
where the asterisk denotes a strong acid. Discounting dispersion in the converter and the transit time between the two detectors, the signal S, is expected to depend on the efficiency E with which the eluite H X can be converted to NaOH. sc
= 10008C(ANa++ XOH-)
(7)
where AN: and AOH- represent the mobilities of Na+ and OH-, respectively. The value of E is expected to depend on C and K . Attainment of complete conversion ( E = 1) cannot be expected when K is low and lack of complete dissociation inhibits the H X NaX conversion. Otherwise, for a strong acid eq 7 may be written as
-
sc* = 1oooc(kNa+ +
XOH-)
(8)
+ (P+ 4KC)'/2)/250EC (13)
In order to evaluate the nature of eq 13 a better perspective of how E may be controlled by K and C is necessary. A reasonable model is that it is the first step of the exchange process HX NaX that controls the overall conversion. All of the unconverted HX in the effluent from the first stage will be readily converted to water in the second stage (resin-OH + H X resin-X + H20). We assume presently that in a multiplate OH- exchanger, NaX will be efficiently converted to NaOH. (If X- has relatively poor affiiity for the exchanger, the number of plates available for accomplishing this may become a significant determining factor.) Although modeling of ion-exchange processes in multiplate systems are generally carried out by assuming that equilibrium is attained at each plate, we believe that in the present case, a more appropriate assumption is that the HX NaX conversion is kinetically limited. This is especially true for very weak acid eluites, of special importance in this work, where [H+]in the eluite band is very low. If an equilibrium model is applied, for example, to pure water as influent, substantial effluent NaOH concentrations are predicted; this is not experimentally observed. Indeed, because of this limitation, the two-step HX NaX NaOH is preferred over the one-step HX HC1 exchange. In the latter case, as soon as conversion is initiated and some HC1 is formed, further conversion is inhibited by the suppression of H X dissociation by the H+ formed. Admittedly, [H+] available for exchange also decreases as NaX is formed and a HX/NaX buffer system is generated; however, experiments show that greater conversions are obtained in such a system. Consider C molar H X (dissociation constant K ) influent into an N-plate converter system. At each plate, a fraction f of the H+is converted into Na+ such that at any plate i (i I 1)
-
-
-
- -. -
[Na+Ii = [Na+Ji-l + f[H+]i-l
(14)
ANALYTICAL CHEMISTRY, VOL. 63, NO. 19, OCTOBER 1, 1991
400 1
2177
I
-0 a)
.-S
0.1
Y
+
6
=\
0.01
t
. e, _
.-0
i i
w
0.001
a)
G?
5
a
0.0001
2
0 X
u
0 00001
0
50
100
150
200
Number of Plates
-0 -
aJ
.-c 0
.-0 't w
0.01
Time ( a r b i t r a r y u n i t s l Flgure 3. (a) Suppressed and (b) converted signal profiles for a Gaussian eiulte band of 1 mM peak concentration.
L
0
5
0.001
X
0.000001
0,00001
0.0001
0.001
Eluite Concentration, M
0.01
v) Y
10000 1
C
3
Figure 2. Computed exchange efficiencies (a) for an 1 mM eluite as a function of plates available and (b) for 200 plates as a function of concentration.
1000: 0
2 a
1
And a t each plate the new composition is governed by the electroneutrality requirement: [H+l + [Na+l = K,/[H+]
+ CK/(K + [H+])
(15)
In the following, the results of an iterative numerical simulation are presented with assumed values o f f = 0.05 and N = 200 (200-plate converter, 5% conversion at each stage). Figure 2a shows a semilogarithmic plot of the overall exchange efficiency as a 1 mM eluite proceeds through the converter for various values of the pK of the eluite acid. Figure 2b shows the overall exchange efficiency attained by the converter effluent as a function of eluite concentration and pK. The consequence of this behavior on a real chromatographic eluite band is shown in Figure 3a, which plots the suppressed signal output for a Gaussian chromatographic band (peak concentration 1mM) as a function of the pK of the eluite acid (in this and subsequent figures ionic mobility of the eluite anion is assumed to be 60). The shape of the concentration profile closely resembles that shown for the response function of pK = 2. Superposition of the exchange efficiency information presented in Figure 2b on the concentration profile allows us to generate the corresponding converted signal profiles shown in Figure 3b.
0.1
1
10
100
1000
10000
Suppressed Signal Area Counts Figure 4. Converted vs suppressed area counts as a function of pK; Gaussian peaks of 1 1 M to 1 mM apex concentrations are assumed.
At the present time, software for taking the ratio of two chromatographic signals with a temporal difference is not readily available; when it is necessary to correct for dispersion, this is not a trivial task. A variety of software, however, readily provide chromatographic peak areas, and it is of interest to examine a plot of the suppressed vs converted peak areas as a function of pK according to the present model. Hypothetical Gaussian chromatographic peaks with peak maximum concentrations ranging from 1 x lo4 to 1 X M were numerically integrated. The results are shown in Figure 4. Only the qualitative pattern is emphasized.
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 19, OCTOBER 1, 1991
Table I. Computed Band Dispersion and Change in Band Asymmetry Factor for Column vs Membrane-Based Converters
measurement technique water injected as sample into unsuppressed NaOH carrier, no column present, ‘peaks” are negative actual chromatography using 20 mM NaOH eluent chloride (0.25 mM)’ sulfate (0.25 mM) oxalate (0.5 mM)
aPproxb approxa band half-height asymm bandwidth at at D., D,, r L ‘40.1 80
2.15
computed band dispersion, pLc column membrane 71
55
factor of bandd asymm change column membrane 0.91
1.05
160 66 2.38 1.17 90 2.00 355 3.46 350 170 1.02 0.17 630 4.18 695 negative 1.22 0.17 a Termed approximate because column and membrane experiments were carried out at different times and some differences were observed in the two experimental sets. *Peak width at 10% peak height, trailing half/leading half. Computed as the square root of the difference of the square of the D, and D, peak half-widths. dFactor by which asymmetry changed from D, to D,. A value less than 1 indicates that the asymmetry actually decreased. e Injected concentration.
EXPERIMENTAL S E C T I O N Equipment. In the experimental arrangement shown in Figure 1,the chromatographic pump was a Beckman llOA followed by
a 4.6 x 250 mm column packed with 10-pm unfunctionalized poly(styrenediviny1benzene)particles (Hamilton Co., Reno, NV) functioning as a pulse dampener; a flow rate of 1 mL/min was used throughout. An electropneumatically driven dual stack slider valve (Dionex Corp., Sunnyvale, CA) equipped with 25- or 63.7-pL calibrated loops was used for sample injection. The suppressor was a filament-filled helical tubular Nafion membrane device externally resin-packed, and a 20 mM dodecylbenzenesulfonic acid solution (Bio-Soft S-100, Stepan Chemical Co., Northfield, IL) was used as regenerant (15). The detectors used were Model 213 conductivity detectors (Wescan Instruments, Santa Clara, CA); data acquisition from both the detectors was accomplished by a 80386-based personal computer via a AI-450 advanced computer interface and accompanying chromatography software (both from Dionex Corp.) All chromatographic experiments were conducted on a Dionex AS 5A-5-rm column. No attempts were made to optimize individual separations. Some of the response data for pure solutes were generated without a column, in a flow-injection mode. Converter. The packed column converter was a 50 X 2 mm dry-packed with Na+-form sulfonic acid type cation-exchange resin (No. 044-87-3, Dionex) for the upstream half and OH--form strong base type anion-exchange resign (No. 212-65, Dionex) for the downstream half. The membrane-based converter similarly consisted of a 75 cm long, 500 pm i.d. Nafion tube (a gift from Perma-Pure Products, Toms River, NJ) filled with a 400 pm diameter fishing line, and the second half consisted of an anion-exchange membrane tube. We experimented with three type of anion-exchange membrane tubes-a perfluorinated membrane that uses Nafion 020 as the starting material (see ref 16, Tosoh, Tokyo, Japan), a radiation-grafted, aminated, quaternized thinwalled PTFE tube (RAI Research, Hauppauge, NY; see ref 17), and a 250 pm i.d. radiation-grafted poly(ethylviny1acetate) tube (previously available as a cation suppressor refill fiber, Dionex Corp.). The first type of fiber rapidly lost functionality and the second could not be woven into a low-dispersion form without shutting off flow. Resulta are therefore reported only for the third type of fiber. Most of the data reported here were generated with a 50 cm length anion-exchange tube except that the dispersion data and some of the chromatograms shown utilized a 1m long anion-exchange tube. The anion-exchange tube was not filament-filled; this avoided excessive back-pressure. To provide a low-dispersion converter device, the membrane tubes were three-dimensionally woven on a 1000 pm grid spacing polypropylene screen (Small Parts Inc., Miami, FL) following the Serpentine-I1 design of Curtis and Shahwan (18). The cationand anion-exchange membrane tube segments were connected by a fine-bore syringe needle tube segment (ca. 5 mm long), and either a Dionex micro membrane suppressor shell or a 30 mL
-
capacity syringe barrel was used for housing the converter, with a pneumatically aided flow of 10 mM NaOH at -1 mL/min external to the membrane. Reagents. Sodium hydroxide solutions (prepared from 50% stock, Fisher Scientific) of various concentrations were used throughout as eluent and protected during use from the intrusion of C02with a soda-lime trap. Analyte solutions were prepared either from the corresponding alkali-metal salts or, occasionally, from the corresponding acids. Typically, these analytes were reagent grade chemicals and injections were carried out at five different concentrations in the 0.05-10 mM range using replicate ( n = 5) samples. Computations. The simulations presented were carried out in iterative programs written in BASIC, and nonlinear least-squares fitting was done by a Marquardt-Levenberg algorithm (MINSQ, Micromath Scientific Software, Salt Lake City, UT). Chromatographic data files obtained with the Dionex AI-450 system were exported to and postprocessed via BASIC and GRAPHER (Golden Software, Golden, CO).
RESULTS AND DISCUSSION’ Converter Design. Packed Column or Membrane? Few meaningful arguments for a packed-column suppressor can further be made (19). However, a converter is designed only to exchange the analyte; small capacities are adequate, making possible low-dispersion devices. On-line regeneration with eluent NaOH is also possible. Both packed column and membrane devices initially appeared useful and much data were obtained with both types. Some asymmetry and dispersion data appear in Table I. A converter is neither a passive inert conduit nor a stoichiometric exchanger. The extent of dispersion induced by such an intrinsically nonlinear device is difficult to quantify. The first row of data in Table I represents an arrangement that comes closest to eliciting passive behavior. The other measurements represent actual chromatographic conditions. The computed dispersion data are uniformly lower for the membrane device than the column. However, computed dispersion normally decreases with increasing input bandwidth. Ready evaluation is possible only for chloride where the input band is relatively narrow with little tailing and can be expected to be fully ionized both at D, and D,. The column converter appears to display some chromatographic retention. This contributes to the dispersion of the eluite and leads to increased tailing. With the membrane device and tailing input bands, asymmetry actually improves possibly because at very low concentrations on the trailing edges, conversion stops altogether, resulting in apparently negative dispersion. If cessation of conversion at some finite value of H+is incor-
ANALYTICAL CHEMISTRY, VOL. 63, NO. 19, OCTOBER 1, 1991 n
E
2.00
,
2 50E-005
I\
2170
Table 11. Response Behavior of Strong Acids
calibration slopeo converted analyte suppressed area area
suppressed/ converted signal area ratio exptl theor
1.94 f 0.22 1.71 4.51 2.60 chloride 2.02 f 0.28 1.72 bromide 4.20 2.24 1.96 f 0.24 1.69 nitrate 4.21 2.24 3.83 2.43 1.88 f 0.25 (1.73)* sulfate OArea counts in arbitrary units per microequivalent of injected sample. * Hypothetical value assuming complete ionization in the sumressed detector.
0
0.00
0
Flgure 5. Concentration and signal profiles, eluite acid pK = 6 , 2 5 pM apex concentration, threshold for H+ exchange assumed to be 0.2 pM. 1.2€+008
Propionate KCGOConverted Si pal M O M Suppressed 8 g n a l
1 .OE+OO8
/
m -+ c
,,
0
/ /
/ /
8.OE+007
/ /
0
/
d 6.OE+007
/
/
6
/ /
2.OE+007
O.OE+OOO
0.00
2.00
4.00
6.00
8.00
10.00
Injected Concentration (mM) Figure 6 . Suppressed and converted signal calibration for propionic
acM.
porated in the model presented earlier, negative dispersion is indeed predicted. In Figure 5, we assume the concentration profile of a highly asymmetric and badly tailing peak of a weak acid at a low concentration (pK = 6,25 pM at peak maximum). Even a t such a low concentration, due to the depression of ionization, the suppressed signal shows greater band asymmetry and tailing than the concentration profile. This is perceptibly reduced in the converted signal. A second consequence of this is that the apparent dispersion measured is concentration dependent and decreases with increasing eluite concentration. The membrane devices are not necessarily superior to the packed-column devices in all respects. Many more exchange plates are possible in the column compared to its membrane counterpart, resulting in significantly greater converted signals in many cases. Even a pure 5050 mixed-bed resin in Na+-OHform provides significant conversion. However, much of the eluite is then wasted in the undesired reaction resin-OH + H X resin-X H20. Some eluite peaks like sulfide, however, disappear from the converted signal with the column device, possibly due to impurity heavy metals present in the resin. The baseline noise for D, was generally found to be higher with the higher back-pressure associated with a column converter. On the basis of a combination of these factors,
-
+
further experiments were conducted solely with membrane devices and only such data are presented in the following. Behavior of S t r o n g Acid Analytes. The relevant data are shown in Table 11. Both detector channels produce reasonably linear responses (slopes of log-log plots for these analytes are 1.01 and 1.06 for the suppressed and converted calibrations, respectively) in the 0.05-10.0 mN injected concentration range with the linear slopes indicated. The observed ratio varies as a function of the injected concentration, and the respective means ( h d ) are indicated. The observed ratio passes through a minimum in all cases as a function of concentration; e.g., for nitrate, we observe ratio values of 2.30, 2.14, 1.96, 1.76, and 1.85 at 0.05,0.10, 1.00,5.00, and 10.00mN injected concentrations, respectively. At very low concentrations, exchange efficiency drops because of the existence of an H+-exchange threshold concentration. At high concentrations, the exchanger does not apparently have the requisite capacity to effect complex exchange. At any concentration, the comparison of the experimental vs theoretical ratios indicates that less than stoichiometric conversion is being attained. The degree of variation in the observed ratio as a function of concentration and the specific analyte precludes an exact determination of Ax- and identification of Xon that basis. The suppressed signal is so dominated by the large value of AH+, an accurate estimation of Ax- would be difficult even if the variation in the observed ratio was far smaller. Approximate quantitation of the analytes from the converted channel is possible, however, by assuming a solute-independent calibration slope. The accuracy obtained by such an approach is comparable to that for quantitation directly from the suppressed signal, assuming an analyte-independent average calibration factor. It is only for sulfate (bisulfate is likely incompletely ionized at the high end of the injected concentration range) that we obtain significantly more accurate solute-independent quantitation from the converted channel data. Based on the above, it may seem at first sight that practicing the proposed approach is of limited value. It will be shortly seen, however, that the observed ratio values immediately identify the analyte in regard to its pK value, a fact not known a priori in a real chromatogram if retention-based identification is inconclusive. Weak Acid Analytes. The advantages of the present system becomes readily apparent with weak acid analytes. Calibration behavior of propionic acid, a typical carboxylic acid, is shown in Figure 6. Not only does it yield a much higher response, the linearity of the response is substantially better (especially at concentrations
