2232
Anal. Chem. 1987,59, 2232-2237
Quantitative Determination of Volatile Carboxylic Acids in Formation Waters by Isotachophoresis Tanja Barth
Department of Chemistry, University of Bergen, Allegt. 41, N-5000 Bergen, Norway The composition and concentration of the hlghly water soluble volatile organic aclds found In waters assoclated wlth ollbearlng formations are lnterestlng in several geochemical contexts. Caplilary lsotachophoresls has been found to be wltable for the quantltatlve determlnatlon of these compounds In formatlon water samples. The separation capacity for a comprehensive set of mono- and pdyfunctlonal standards has been Investigated. Calibration curves are stralght Ilnes wlth correlation coefflclents > O S % Concentration levels of 0.02 to 20 mM of each compound can be determlned. No sample preparation Is required. The procedure Is applled to the analysls of formatlon waters from the Norweglan contlnental shelf, showing levels of acetic acid up to 16 mM.
Analysis of a large number of formation waters from oil reservoirs has shown that carboxylic acids are the major dissolved organic compounds in the water phases. The acids have been identified as mainly acetic acid (up to 83 mmol/L), with decreasing amounts of higher aliphatic acid homologues (1). Experimental investigations show that these acids are produced by maturation of the deposited organic material in a process roughly parallel to oil generation (2). The geochemical significance of such acid production is now being investigated by several groups from different points of view: as possible petroleum percursors (3),as possible gas percursors (4,as buffers determining the pH level of the subsurface waters (5),as complexing agents in the dissolution of minerals in source or reservoir rocks ( 6 ) ,as indicators of microbial activity in the oil or in oil production (7), and as maturity and proximity indicators for oil (1,8). Furthermore, the corrosive effects of high levels of organic acid anions in produced waters and their influence on some possible EOR (enhanced oil recovery) techniques must be considered in technical contexts. All of these investigations require knowledge of the composition and amounts of the organic compounds dissolved in waters that also contain moderate to high levels of inorganic salts. Volatile acids in water phases are not easily determined by the standard analytical procedures of extraction, derivatization, and gas chromatography, and the high levels of inorganic salts pose additional problems. Methods based on extraction or total evaporation of the water phase are most frequently used and are reported to give acceptable recoveries of aliphatic monofunctional acids (1, 2, 9), but they are time-consuming and not suitable for analysis of bifunctional acids or other complex acidic compounds that can be important in several of the geochemical applications mentioned above. This laboratory has adopted a completely different approach. As an immediate screening procedure, the water samples are analyzed for organic acids by capillary isotachophoresis (ITP),an analytical electrophoretic technique. The analysis registers the ionic organic compounds in the sample. It is rapid and quantitative and requires no sample preparation. A quantitative analysis for C,-C8 mono-, di-, and hydroxy acids, both aromatic and aliphatic, can be performed in less than 30 min. High concentrations of inorganic salts are removed in an on-line precolumn separation. The ap-
plicability of ITP for analysis of these acids is well documented in food chemistry and biochemical analysis (10) but the technique has not yet been adapted to geochemical applications.
EXPERIMENTAL SECTION Apparatus. The analysis is performed on a LKB 2127 Tachophor with a 0.32 mm i.d. analytical quartz capillary. The Tachophor has been rebuilt with a precolumn. Figure 1 shows a schematic diagram of the apparatus. The precolumn is a wide-bore (2 mm) channel with a tell-tale conductivity detector at the junction with the capillary analytical column. The analytical column has two detectors, a conductivity detector with linear and differential signal output and a UV detector at 254 nm. The linear conductivity signal and the UV absorbance are plotted on a Cole-Parmer dual-channel plotter, while the differential signal is registered on a HP 3390 A intergrator for quantification from calibration curves. The electrolyte systems used are specified in Table I. Reagents. Electrolyte Solutions. Deionized water was used for all solutions and freshly distilled methanol for the pH 4.3 leading electrolyte. All produced solutions were boiled to drive off dissolved gases. For quantitative measurements 0.100 N HC1 solutions were prepared from M & B Volucon standard volumetric concentrates, and this stock solution was diluted 10-fold to give 0.010 M chloride ion solutions for the leading electrolytes in the analytical separation. Organic buffers were freshly recrystallized. The pH of the electrolytes was adjusted by careful addition of the buffer compound (@-alanineor histidine) until the selected pH value was reached. For the preseparation leading electrolyte, a 0.5 70 HPMC (hydroxymethylpropylcellulose)solution was cleaned by passing through an anion exchanger in the OH- form. One milliliter of 1 M HC1 was added to 100 mL of the HPMC solution, which was buffered as above and boiled in a water bath. The terminating electrolyte was prepared by weighing the appropriate amount of MES (2-N-(morpholino)ethanesulfonicacid), adding water and adjusting the pH value with 1 M NaOH. The electrolytes in the separation system must be renewed for each run. After five runs the electrolyte solutions in the reservoirs were changed to ensure buffer capacity, and the separation capillary washed with a solution of 0.3 % HPMC and 0.01 % HTAB (hexadecyltrimethylaonium bromide) to supress electroosmosis and electrode reactions at the conductivity detector electrodes. Standards. The standard substances listed in Table I1 were reagent grade and used as received, except for the n-aliphatic acids which were distilled before use. Sodium salts of reagent grade were used to make the quantitative acetic and propanoic acid standards. Standard solutions were 1 mM in each acid, except where other concentrations are specified. Dilute NaOH solutions were used when necessary to produce sufficient solubility of the standards. Run Parameters. The preseparation was performed with a driving current of 465 pA, giving a run time of 5-10 min depending on salt concentrations. When the tell-tale detector signal registered the end of the leading electrolyte zone, the preseparation arm containing inorganic ions was closed off and the current switched to the analytical separation. The driving current was set at 115 pA for 7 min and then reduced to the final level of 15 pA for zone registration. Separation and Step Heights of Standards. Standard solutions (1 mM) were coinjected with acetic acid to give the relative step heights. All compounds with similar step heights were coinjected to check for overlapping zones. Calibration Curves. The curves are based on the measurement of zone lengths in minutes with a 15-pA driving current.
0003-2700/87/0359-2232$01.50/0 0 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 18, SEPTEMBER 15, 1987
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Table I. Electrolyte Systems for Determination of Carboxylic Acids 1: pH 4.3 precolumn leading electrolyte
anion concn counterion PH additive
analytical leading electrolyte
2: pH 5.5 precolumn leading electrolyte
analytical 1eading electrolyte
c10.01 M histidine 5.5 0.5% HPMC"
0.010 M histidine 5.5
c1-
c1-
0.01 M &alanine 4.3 0.5% HPMC"
0.010 M &alanine 4.3 10% MeOH
c1-
terminating electrolyte MESb 0.01 M
OH7.0
" Hydroxypropylmethylcellulose. 2-N-(Morpholino)ethanesulfonicacid. Table 11. Step Heights of Acids Standards Relative to Acetic Acid no.
compound
UV abs
1: pH 4.3 step height
interference
2: pH 5.5 step height
interference
Normal Acids 1 2
3 4 5
6 7
acetic acid formic acid propanoic acid butanoic acid pentanoic acid hexanoic acid octanoic acid
-
1
-
0.23 1.28 1.31 1.40 1.47 1.60
20,b 22," 23b
1
13, 19 8, 14, 20 9, 10
11, 18
0.43 1.25 1.43 1.55 1.65 1.80
4 5, 10 5, 9 7, 18
1.43 1.55 1.55 1.65
4, 14, 20 5, 10 5, 9 6
0.80 1.25 1.40
17 3, 19 4, 8, 20
0.28 0.51 0.79
2 12
8 ' 9, 10
11
Branched Aliphatic Acids 8
9 10 11
12
13 14
2-methylpropanoic acid 2-methylbutanoic acid 3-methylbutanoic acid 2,2-dimethylbutanoic acid
-
2-hydroxyacetic acid 2-hydroxypropanoic acid 4-hydroxybutanoic acid
-
1.31 1.40 1.40 1.60
-
-
Aliphatic Hydroxy Acids 0.49 24, 29," 28 0.61 1.26 26" Aliphatic Diacids
oxalic acid malonic acid succinic acid
-
0.19 0.38 0.77
18
cyclohexanoic acid
-
Cyclohexanoic Acids 1.60
19 20
benzoic acid phenylacetic acid 2-methylbenzoic acid 3-methylbenzoic acid 4-methylbenzoic acid
++ + +(+I ++ +++
15 16 17
(+)
11, 7
1.71
21"
1.25 1.41 1.50 1.50 1.50
3, 13 4, 8, 14 22, 23, 25" 21, 23, 25" 21, 22, 25n
21," 22," 23"
14"
1.10 1.49 1.57
12, 24 12,' 24
0.91 0.72 0.72
29 28
Benzoic Acids
21
22 23
0.87 1.03 0.84 1.00 1.07
l b
19" 1," 20b I*
Hydroxybenzoic Acids 24 25 26
2-hydroxybenzoic acid 3-hydroxybenzoic acid 4-hydroxybenzoic acid
+ ++ +++
0.58 0.95 1.26
12, 28, 29
Benzodiacids 27 28 29
phthalic acid 1,3-benzenediacid 1,4-benzenediacid
++ +(+I +++
0.56 0.48 0.47
" Separate zones observed on the UV trace. Zone with largest step height moves by enforced migration in front of lower zone. Quantitative standards were injected at levels of 1to 20 nmol in 1-10 WL solution, using 1 and 10 mM standards. Samples. Samples of water produced from a North Sea installation were received from Statoil, and samples from well testa on the Norwegian continental shelf were provided by Statoil and Norsk Hydro. Procedure. ITP is an analytical electrophoretic separation performed in an electrolyte solution in a quartz or Teflon capillary (11). The driving force of the separation is the electric current delivered from a constant current, high-voltage unit. The capillary
is filled with the leading electrolyte, where an ion more mobile than the sample components is the only available carrier for the current. The sample is injected at the interface between the leading and terminating electrolyte. The terminating electrolyte is made with an ion that has a lower mobility in the system than all the target compounds. When the current is applied, the ions are "pulled" through the system by the potential difference. The different components separate into zones of molecules with equal mobility, ideally consisting of just one compound per zone. The zones are separated in order of decreasing mobility. Only elec-
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 18, SEPTEMBER 15, 1987
I
1
I
I
U
n
uv
I+
3
t---
6 7
Figure 1. The tachophor: 1, electrolyte reservoirs; 2, preseparation channel; 3, analytical capillary; 4, injection port; 5, draining valve; 6, conductivity detectors; 7, UV detector; 8, high-voltage switch; 9, three-way valve; 10, high-voltage, constant current unit.
trically charged molecules are mobile, and the mobility depends, among other things, on the degree of dissociation of weak acids and so on the pH values of the electrolytes. The constant current in the capillary requires that the concentration of charged particles carrying the current must be equal in all sections of the capillary, and thus the concentration of the leading ion determines all subsequent zone ion concentrations. Timing the length of each zone as it passes by the conductivity detector gives a measure of the concentration of ions in each of the zones in the sample relative to the leading ion. The step height, which gives the zone conductivity relative to the leading ion, depends on the effective mobility of each ion and is specific for each compound at a given pH value and buffer system. Compounds with equal mobilities in the system have equal step heights and give overlapping zones. For each application, a suitable set of electrolytesmust be found where the target compound mobilities are sufficiently different to give separated zones. Tables of calculated separation indexes are available and are useful for this selection (12). For complex mixtures several different electrolyte systems should be used to minimize the risk of undetected overlap of sample components. Identification of the sample components is best done by internal standard addition or coinjection of standard solution, as small changes in the electrolyte solutions and different combinations of sample components can influence the step heights. For quantitative work either calibration curves should be established for each compound or standard addition methods used. Simple, one-step ITP is easily overloaded by high levels of background compounds and is not suitable for brine analysis. Brine samples therefore require a preseparation ITP column (13), consisting of a wide-bore channel with a high sample capacity. This is connected to a separate leading electrolyte chamber and high-voltage supply and coupled to the analytical capillary with a three-way valve. A tell-tale detector at the junction indicates when the zones of interest should be switched into the analytical separation. The separation principle is identical with the analytical step, but high sample load capacity enables large amounts of interfering compounds to be removed before the analytical run or dilute samples to be concentrated in the preseparation chamber.
RESULTS AND DISCUSSION Qualitative Analysis. The step heights of a selection of standard acids were determined at different pH values for the leading electrolyte, spanning from pH 3.5 to 6.5. Two values were chosen for further investigation, pH 4.3 and pH 5.5. Under these conditions a good separation is obtained for the expected major components while the background interference is kept at a minimum. The electrolyte compositions are given in Table I. The list of standards, registered UV absorbance, registered step heights relative to acetic acid, and a specification of overlapping compounds is given in Table 11. Figure 2 shows the tachograms of n-aliphatic acids of carbon number 1to 6 and 8 in the two electrolyte systems. In the conductivity detector trace the Y axis gives the step heights identifying each compound, while the X axis gives zone lengths as a measure of concentration. At pH 4.3 the acids are only partly dissociated (25-30%), and the separation depends primarily on the p K values (“separation according to pK values” (ref 11, p 83)). The
A R
B
uv
1
A
f
R
-
P
I
A,MIN
e
Figure 2. Tachograms of standard solutions of naliphatlc acids with carbon numbers 1-6 and 8 at (A) pH 4.3 and (B) 5.5: upper trace, UV absorbance; lower trace, zone resistance (from the Conductivity detector). Abclssa: P = preseparation (driving current 465 PA), A = analytical separation (driving current 15 MA), zone length in minutes.
influence of additional chain or rihg substituents on the acid constants, and hence on the relative mobility of the compounds, can be roughly be predicted from their Hammet/Taft u/u* values (14), though the group of aromatic acids is somewhat displaced relative to the aliphatic acids. At pH 5.5 the acids are nearly fully dissociated (>80%) and the absolute mobility controls the separation (“separation according t o mobility” (ref 11, p 83)). This is not, at the moment, theoretically predictable, but the values are tabulated for a number of ions (12),or they can be measured isotachophoretically (15). Absolute mobilities are less influenced by the substituent
2235
ANALYTICAL CHEMISTRY, VOL. 59, NO. 18, SEPTEMBER 15, 1987 A
c
B
uv
1 1
.I
i
Flgure 3. Spiking of formation water sample with acetic and propanoic acids: (A) formation water no. 2, (B) acetic and propanoic acid standard, (C) formation water spiked with the standard; eiectroiyte system 1, 15-pA driving current; traces as in Figure 2.
pattern than the pK values, and isomeric forms show a greater tendency to coelute. This simplifies the interpretation of the tachograms. Both electrolyte systems show a good separation of naliphatic acids C1-C8. Methyl-substituted aliphatic acids are not separated from n-acids of equal carbon numbers. Diacids, both aliphatic C2-C, and benzoic diacids, are very mobile and easily identified. The UV-responses indicate aromatic compound zones and the dimension of the extinction coefficient but do not give sufficiently specific information for precise interpretation. For the longer-chain acids, the separation is best at pH 5.5, while the polyfunctional acids that have step heights between formic and acetic acid are best separated at pH 4.3. Quantitative Analysis. Calibration curves for acetic, propanoic, and benzoic acids have been established by registering zone lengths for standard substances at levels of 1to 20 nmol injected in the course of 12 runs (eight for benzoic acid); see Table 111. The resulting curves are linear over the concentration range, and the correlation of the measured values to the straight line calculated by linear regression is better than 0.99 for all curves. The value of the intercept b, which reflects the contamination level in the electrolytes, changes with time and must be checked every 3-5 days. The value of the slope a is stable for a given stock solution. New calibration curves are required when stock solutions are renewed. In this system configuration the practical minimum level for quantitative determination is 1nmol of a compound injected, or 50 1L of an 0.02 mM solution. The limiting factor is the precision of the differential conductivity detector output response and registration, which is not satisfactory at shorter zone lengths. Overloading the system, i.e., injecting more than 30-40 nmol of each standard, gives mixed zones and loss of linearity. Overloading with one component does not disturb the analysis of other, minor components if the mobilities are sufficiently different. Repeatability and Spiking of Samples. The repeatability of the quantification is good as long as the electrolyte buffer capacity is sufficient. As an example, samples 2 and 3 have been quantitated repeatedly (5 to 11parallels). The mean concentrations and standard deviations for each electrolyte system are given in Table IV. The relative standard deviation increases rapidly when the amount of the injected compound is less than 1nmol.
Table 111. Calibration Curves for Acetic, Propanoic, and Benzoic Acids" compound
a
acetic acid propanoic acid benzoic acid
0.172 0.183 0.185
b
n
corr coeff
0.15 0.14
12 12
0.05
8
0.997 0.995 0.999
pH 4.3
pH 5.5 acetic acid propanoic acid benzoic acid
0.18
0.158 0.164 0.168
0.10
12 12
0.06
8
0.994 0.995 0.998
" Expressed as straight lines y = ax + b where y = zone length in minutes and x = nanomoles standard injected. n = number of points. Concentration of leading electrolyte = 0.010 M C1-, driving current 15 MA. Table IV. Repetability of Quantification of Samples 2 and 3
sample number: electrolyte system:
2 1
mean acetic acid concn std dev no. of runs mean propanoic acid concn std dev no. of runs mean butanoic acid concn std dev no. of runs
6.811 0.243 5 0.524 0.111 5
tr
2 6.694 0.325 11 0.544 0.066 11 0.14 0.10 11
3 1
2
1.159 0.054 5 0.078 0.040 5
1.073 0.139 5 0.149 0.019 5
The same samples have been quantified by standard addition. The zone lengths are additive, but the calculated concentration values are somewhat more uncertain than the determinations based on calibration curves. The correspondence is within 10% of the calibration curve values for major components. Since the calibration curve does not pass through zero but always has a positive intercept, blank values must be deducted in standard addition calculations. An example of the corresponding tachograms of sample 2, an acetic and propanoic acid standard, and the sample spiked with the standard is shown in Figure 3. Applications. Samples of water produced in well tests on different North Sea oil fields have been analyzed, giving values
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 18, SEPTEMBER 15, 1987
us
a
i
2 m -
P
8
A , MIN.
e
c
0
Figure 4. Tachogram of produced water from the separator system, Statfjord A, Norwegian Continental shelf: electrolyte system 2, 15-hA driving current: traces as in Figure 2.
of acids representative of the in situ formation water values. Three sets of produced water, taken at different points in the separator system at the Statfjord A oil platform (North Sea) have been analyzed for organic acids as part of a project registering microbial activity. The results are given in Table V. Figure 3A shows a typical formation water tachogram (well test water, sample no. 2) and Figure 4 shows the composition of the produced water in the coalescer of the separator system (sample no. 8). The analysis shows that acetic acid is the dominant compound in all the samples except no. 4. The concentration level of the higher homologues decreases rapidly, the concentration of propanoic acid is a factor of 10 lower than that of acetic acid. The composition of the well test waters, samples 1-5, is simpler than the separator system samples, possibly indicating bacterial activity in the separation chambers. The listed acids contribute 70-100% of the registered total acid concentration; for most samples the contribution is higher than 90% * Sample no. 4 is clearly different from the rest, containing mainly polyfunctional acids and showing a large UV absorbance. Only one electrolyte system has been used, as the sample deactivated the conductivity detector electrodes. This indicates that the sample is contaminated by drilling fluid, and the acids that were registered were probably not originally in the formation water. The compounds are possibly products of aerobic biodegradation of drilling mud components. Comparison of the concentrations determined in the twoelectrolyte systems shows a reasonable correspondence. For some samples the divergence in acetic acid concentrations is somewhat high, but this can be due to a time lapse between the determinations. The acid levels tend to sink with storage time. Undetected zone overlap of minor components which changes between the electrolyte systems may explain smaller variations. The results show that the analytical method is very suitable for the rapid determination of the concentrations of volatile acids in formation water samples. The main components in
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Anal. Chem. 1987, 5 9 , 2237-2241
the sample lie within the optimum range of I T P analysis. Extraction and derivatization losses are avoided. As described, the method has two weaknesses. Firstly it is not preparative, and identification of sample components must be done by standard addition. This necessitates the availability of quite a wide range of standard compounds and still leaves some uncertainty in identification. Secondly, the reduction of selectivity and separation capacity a t higher molecular weights limits the applicability. For compounds with carbon number >6 the increasing number of isomers and the decreasing differences in p K and mobility values make analysis of complex mixtures problematical. However, when the method is used as a quantitative screen for formation waters, these drawbacks do not seriously disturb the analysis. The scope of the I T P analysis is sufficient to give a clear picture of the levels of all the target compounds considered most important in the geochemical applications; the C1 to C6aliphatic acids, benzoic and methylbenzoic acids, and bifunctional compounds of the same carbon numbers. I t is also faster, more easily performed and as accurate as the alternative methods while covering a broader spectrum of compounds.
ACKNOWLEDGMENT
I am grateful to Statoil and Norsk Hydro for supplying samples of formation waters for analysis. Registry No. Formic acid, 64-18-6;acetic acid, 64-19-7;propanoic acid, 79-09-4; butanoic acid, 107-92-6;pentanoic acid, 109-52-4;hexanoic acid, 142-62-1;heptanoic acid, 111-14-8;octanoic acid, 124-07-2;benzoic acid, 65-85-0;oxalic acid, 144-62-7; malonic acid, 141-82-2;lactic acid, 50-21-5; 2-methylpropanoic acid, 79-31-2; 2-methylbutanoic acid, 116-53-0; 3-methylbutanoic acid, 503-74-2;2,2-dimethylbutanoic acid, 595-37-9;2-hydroxyacetic acid, 79-14-1; 2-hydroxypropanoic acid, 50-21-5; 4hydroxybutanoic acid, 591-81-1; succinic acid, 110-15-6; phenylacetic acid, 103-82-2; 2-methylbenzoic acid, 118-90-1; 3-
2237
methylbenzoic acid, 99-04-7; 4-methylbenzoic acid, 99-94-5; 2hydroxybenzoic acid, 69-12-7; 3-hydroxybenzoic acid, 99-06-9; 4-hydroxybenzoic acid, 99-96-7; phthalic acid, 88-99-3; 1,3benzenediacid, 121-91-5; 1,4-benzenediacid, 100-21-0; water, 7732-18-5; cyclohexanoic acid, 98-89-5.
LITERATURE CITED Carothers, William W.; Kharaka, Yousif K. AAPG Bull. 1978, 62, 2441-2453. Kawamura, Kimitaka; Tannenbaum, Eli; Huizinga, Bradley J.; Kapian, Isaac R. Geochem. J . 1985, 20,51-59. Kawamura, Kimitaka; Tannenbaum. Eli; Huizinga, Bradley, J.; Kapian, Isaac R. Org. Geochem. 1988, 70, 1059-1065. Kharaka, Yousif K.; Carothers, Wiiiiam W.; Rosenbauer, Robert J. Geochim. Cosmochim. Acta 1983, 47,397-402. Kharaka, Yousif; Law, Leroy M.; Carothers, William W.; Goerlitz, Donaid F. I n Roles of Organic Matter in Sediment Diagenesis: SEPM Special Publication 38, SEPM: Tulsa, OK, 1988; pp 111-122. Surdam, Ronald C.; Boese, Stephen W.; Crossey, Laura J. In Clastic Diagenesis; McDonald, D. A.; Surdam, R. D., Eds.; AAPG Memoir 37, AAPB: Tulsa, OK, 1984; pp 127-151. Norenkova, I . K.; Arkhangei’skaya, R. A,; Tarasova, T. G. Geokhimliya 1978, 3,408-414. Chem. Abstr. 197& 88, 194026~. Barth, Tanja, Chemolab, in press. Pempowiak, Janusz J . Chromatogr. 1983, 258,93-102. Bocek, Petr; Gebauer, Petr; Dolnik, Viadislav; Foret, Frantisek, J . Chromatogr. 1985, 334, 157-195. Everaerts, Frans M.; Beckers, Jo L.; Verheggen, Theo P. E. M. Isotachophoresis ; Journal of Chromatography Library 6, Eisevier: Amsterdam, 1976. Hirokawa, Takeshi; Nishino, Makoto; Aoki, Nobuyaki; Kiso, YoshiYuko: Sawamoto, Yasnyo; Yagi, Takao; Akiyama, J.-I. J . Chromatogr. 1983, 277,D1-106. Everaertes, Frans M.; Verheggen, Theo P. E. M.; Mikkers, Frans E. P. J . Chromatogr. 1979, 769, 21-38. Perrin, D. D.; Dempsey, Boyd; Serjant, E. P. pK, Prediction for Organic Acids and Bases; Chapman and Hail: London, 1981. Pospichal, Jan; Deml, Mirko; Zemiova, Zdenka; Bocek, Petr J . Chrornatogr. 1985, 320, 139-146.
RECEIWQfor review January 27,1987. Accepted May 29,1987. This work was supported by the Norwegian Research Council and Statoil, who have kindly permitted the publication of this material.
Enrichment of Enantiomers and Other Isomers with Aqueous Liquid Membranes Containing Cyclodextrin Carriers Daniel W. Armstrong* and Heng L. Jin
Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-4260
Water-based liquid membranes utilizing CY-, p-, and y-cyclodextrin carriers are able to selectively transport certaln isomers. Enantlomerlc enrlchments were demonstrated from slx racemic mixtures, two of whlch were drugs. Isomeric enrichments from mixtures of several geometrical and structural Isomers were demonstrated as well. Two separation schemes are ldentlfled and discussed. The factors that affect and control isomeric selectivity are examined. This study Is an important first step in demonstrating the feaslbliity of membranebased Isomer separatlons. These and analogous membrane systems have tremendous potential as a tool for large, continuous, preparatlve-scale isomer separatlons.
The isolation and purification of isomers is one of the more important and interesting areas of separation in science. Recently, considerable attention has been focused on the resolution of enantiomers, particularly by liquid chromatography (LC) methods ( I d ) . It is apparent that much progress
has been made in separating a variety of enantiomers and other isomers on the analytical scale. Much less has been written about preparative-scale separation of isomers although some commercially available chiral stationay phases (CSPs) are available in preparative and semipreparative columns. It is known that scaling sensitive analytical separations generally results in a loss of resolution. However, isomeric separations continue to be done via preparative LC because of a lack of viable alternatives. Many of the problems of routine, preparative-scale LC have been addressed (6). From an industrial point of view, the selectivity and efficiency of preparative LC are attractive while the low-capacity, high-cost, and batchseparation approach are detriments to the technique. A low-cost continuous separation process that has the isomeric selectivity of CSP-LC would be particularly attractive. Unfortunately, there are few such alternatives a t the present time. Membrane mediated separations are attractive from a preparative standpoint, because they can be used in continuous processes and the cost and energy requirements are often reasonable (7).However, they generally lack the selectivity
0003-2700/87/0359-2237$01.50/00 1987 American Chemical Society
