Anal. Chem. 1997, 69, 3339-3345
Behavior of Model Ionic Compounds under Thermospray Liquid/Liquid Extraction Conditions E.S. Farrell and G.E. Pacey*
Miami University, Oxford, Ohio 45056
Our previously reported procedure for the extraction of semivolatile and nonvolatile organic compounds from aqueous samples by thermospray liquid/liquid extraction was extended to the extraction of charged organic compounds from water. By thermally disrupting or extricating the solvation shell of hydrated ions, the thermospray process facilitates the extraction of these analytes from the aqueous matrix. The effects of probe temperature, probe flow rate, and sample pH on the extraction efficiency of model compounds such as phenylalanine, benzoate ion, p-toluenesulfonate ion, and naphthalenetrisulfonic acid trisodium salt were investigated. Under optimized conditions, the percent recoveries for these compounds were 95, 97, 91, and 13%, respectively. A new extraction technique referred to as thermospray liquid/ liquid extraction (TSLLE) for rapid recovery of semivolatile and nonvolatile organic compounds from water has recently been described.1,2 Application of the technique to the extraction of charged species from aqueous media has not yet been reported. Electrostatic attractions between cations and anions or electronrich ligands are important factors in TSLLE. If charged, the analyte can be made to interact with counterions or with the solvent through such processes as Coulombic attraction or hydrogen bonding to form stable uncharged species which can then be extracted into the organic phase. The degree of attraction between these species is dependent on both the charge concentration of the analytes and the temperature at which they are assayed. While the charge concentration of an ion can be predicted from its ionic potential (defined as the ratio of the charge to the ionic radius), the interactions of these species under TSLLE conditions are still not well understood. Liquid/liquid extraction of charged species is generally unsatisfactory unless an appropriate counterion is used or the correct pH adjustment is made. If the latter is the preferred sample pretreatment procedure, separate pH adjustments and lengthy reextractions would be necessary for mixtures containing both acidic and basic species. Often, the pH-sensitive components in the mixture may preclude the necessary pH adjustments or impose time constraints on the system to prevent long exposure of the analytes to a harsh acidic/basic environment. In TSLLE, preadjustment of the sample pH or undue exposure to acids or bases is unnecessary since these adjustments can be done online via the solvent probe at the moment of mass transfer into the organic phase. (1) Farrell, E. S.; Pacey, G. E. Anal. Chem. 1996, 68, 93-99. (2) Farrell, E. S.; Pacey, G. E. 22nd Annual Conference of the Federation of Analytical Chemistry and Spectroscopy Societies, October 1995. S0003-2700(96)01182-1 CCC: $14.00
© 1997 American Chemical Society
A particularly challenging situation involves the extraction of fully ionized dipolar or zwitterionic species, such as amino acids, from water. One approach is to utilize ion-exchange membranes to preconcentrate the zwitterionic species into a receiver solution under controlled pH conditions.3 Although zwitterions are electrically neutral, the opposite electric charges at their two poles result in properties that are different from nonionic organic molecules of similar size.4 Given that analyte/solvent interactions of charged species can be affected by temperature, a study was undertaken to characterize such effect under TSLLE conditions. We have already reported the TSLLE results for a number of weakly acid phenols, where it was shown that efficient extraction was obtained without a need for ionic strength or pH adjustments.1 For the study reported in this paper, benzoic acid (BA), phenylalanine (PA), naphthalenetrisulfonic acid trisodium salt (NTA), and p-toluenesulfonic acid (TSA) were the selected test compounds. While BA is a targeted compound on the U.S. EPA’s list of semivolatile organic compounds, its sodium salt is used extensively in the food industry to prevent molding and fermentation in certain foods. At sufficiently high levels, BA, like many other additives, may produce toxic or harmful health effects. As a result, routine extraction and analysis of BA is often done to ensure regulatory compliance. Since the carboxylate group of BA is strongly solvated in water, its extraction will certainly be more challenging than for the neutral form. Complexes of the benzoate ion with metal ions of Fe, Al, Be, Ga, In, and Se have been efficiently extracted from aqueous solutions with ethyl acetate and butyl or amyl alcohol.5 This paper assesses the performance of the newly developed TSLLE for the extraction of these species and provides information about their distribution as a function of extraction parameters. EXPERIMENTAL SECTION Reagents. HPLC grade methylene chloride, chloroform, carbon tetrachloride, hexane, methanol, ammonium hydroxide, and hydrochloric acid and reagent grade glacial acetic acid, potassium chloride, and sodium hydroxide were obtained from either Fisher Scientific (Fair Lawn, NJ) or Baxter Health Care Corp. (McGraw Parl, IL) and used without further purification. Reagent grade benzoic acid was purchased from Sigma Chemical Co. (St. Louis, MO). PA, TSA, NTA, tetramethylammonium chloride, tetraethylammonium chloride, and tetrabutylammonium hydrogen sulfate were obtained from Aldrich Chemical Co., Inc. (Milwaukee, WI). Triply distilled deionized water was used for all investigations. (3) Cox, J. A.; Bien, R. Chem. Anal. (Warsaw) 1995, 40, 1-10. (4) Lehninger, A. L. Principles of Biochemistry; Worth Publishers, Inc.: New York, 1982; pp 103, 104. (5) Marrison, G. H.; Freiser, H. Solvent Extraction in Analytical Chemistry; John Wiley & Sons Inc.: New York, 1966; p 145.
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Figure 1. Transverse view of TSLLE.
Apparatus and Instrumentation. The TSLLE apparatus, which is described elsewhere,1 consists primarily of a water-cooled Pyrex glass chamber (300 mL), several thermospray vaporizer inserts (Vestec VT1460-4 vaporizer), a dual-stage condenser, and a vapor bypass assembly. Supporting instruments include a 120-V variable autotransformer, a Fluke 52 K/J thermocouple reader, an SSI Model 300 LC pump equipped with an SSI LP-21 pulse dampener (Scientific System Inc.), a Perkin-Elmer Model 250 binary LC pump, and a Neslab Coolflow 33 refrigeration cooler. In general, analysis of carboxylic acids by gas chromatography is preceded by an esterification step. But the large number of extractions involved in each TSLLE characterization made this approach impractical. Instead, a UV detection scheme was used to facilitate rapid analysis. UV/visible measurements were made on a Hewlett-Packard 8452A photodiode array spectrophotometer controlled by an IBM PC. Sample Preparation Procedure. A sample solution containing 1.03 × 10-3 M BA was formed by dissolving 0.125 g of benzoic acid in 5 mL of ∼0.1 M NaOH solution and then diluting to a final volume of 1 L with triply distilled deionized water. pH adjustments for BA and other analytes were made using 6 M HCl or 6 M NaOH. Separate solvent mixtures were prepared in a series of 100-mL volumetric flasks by diluting 5 mL of glacial acetic acid (GA) to volume with chloroform, methylene chloride, carbon tetrachloride, or hexane. A 1.73 × 10-3 M PA solution was 3340
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prepared by dissolving 0.143 g of PA in 500 mL of distilled water. Separate counterion solutions were prepared by dissolving 0.0588 g of tetrabutylammonium hydrogen sulfate, 0.0189 g of tetramethylammonium chloride, and 0.0287 g of tetraethylammonium chloride to a final volume of 100 mL with the 1.73 × 10-3 M PA solution. The solvent for ion pair extraction of PA was prepared by adding 20 mL of methanol and 2 mL of concentrated NH4OH to a 200-mL volumetric and then diluting to volume with chloroform. The 4.21 × 10-3 M TSA solution was prepared by dissolving 0.400 g of TSA in 500 mL of water. Prior to extraction, tetrabutylammonium hydrogen sulfate was added to the TSA solution at a concentration of 1.43 g/100 mL of TSA. A 9.05 × 10-4 M NTA stock solution was prepared by dissolving 0.196 g of NTA in 500 mL of water. The 4.50 × 10-5 M NTA working standard was prepared by diluting 10 mL of the NTA stock solution to a final volume of 200 mL with water. General Extraction Procedure. Both the sample and the solvent were admitted through their respective probes at optimized flow rates and probe temperatures. At the beginning of each set of extractions, triply distilled water was extracted to obtain a system blank and ascertain probe temperature equilibrium. Following this, the sample pump was briefly turned off to allow replacement of the blank with an actual sample. At the end of each extraction, an additional 5 mL of water was circulated through the sample probe to facilitate removal of residual analytes
Figure 2. Proposed scheme for the destructuring of water from the solvation shell of ionic solutes during TSLLE.
from the transfer lines. Following extraction, the temperature of the extraction chamber was elevated as necessary to breakup emulsion. Upon separation of both liquids, the organic phase was collected, diluted to 25 or 50 mL, and analyzed by UV/visible at a wavelength of 276 nm for BA, 262 nm for TSA, 282 nm for NTA, and 274 nm for PA. Batch analyses were performed by extracting the sample three times in a 250-mL separatory funnel with 10-mL portions of the extracting solvent. The combined organic fractions were diluted and analyzed as described above. With the solvent and sample temperatures set respectively at 162.3 and 190.2 °C, 25-mL portions of the TSA solution were admitted into the extractor at a flow rate of 5 mL/min and spray extracted with CH2Cl2 at a rate of 3.5 mL/min. This procedure was then repeated, but with the corresponding sample and solvent temperatures changed to 228.5 and 148.5 °C. The TSA procedure was adopted for the extraction of NTA by changing the sample and solvent temperatures to 191.3 and 163.2 °C, respectively. The organic and aqueous phases were collected and diluted to a final volume of 25 or 50 mL prior to analysis. RESULT AND DISCUSSION Destructuring Mechanism and Exploratory Work. A transverse view of the extractor used for all thermospray liquid/ liquid extraction experiments in this section is shown in Figure 1. A major fraction of the extractable ions in an aqueous medium can be extracted into organic solvents as ion pairs. Since hydrated ion pairs are significantly less soluble in organic solvents than their unhydrated counterparts, a major objective of ion pair extraction by TSLLE would be to enhance solubility in the organic phase by depletion of the hydration shell prior to extraction. This is shown schematically in Figure 2, where ion pair formation is depicted to involve the conversion of unpaired solvated ions with primary and secondary solvation shells to contact ion pairs with one common primary solvation shell.6 Thus, before the ions can come into contact, their solvation shell must be at least partially disrupted. The energy needed for this process is supplied by the heated thermospray probes. This factor together with the intrinsic properties of the ions may determine how efficiently charged species can be extracted. The ability of TSLLE to strip away or diminish the solvation shell of an ion was the focus of the exploratory work using the 4.50 × 10-5 M NTA. For comparison, NTA was exhaustively extracted in a separatory funnel using all the organic solvents listed in this paper. Unlike those for TSLLE, the samples for batch processing were spiked to a final concentration of 6 M in NaCl and/or a 10-fold excess of tetrabutylammonium hydrogen sulfate, tetramethylammonium chloride, or tetraethylammonium chloride. But extraction of NTA by the batch procedure was still unsuc(6) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry; VCH Publishers: New York, 1988; p 43.
Figure 3. Comparison batch extraction and TSLLE using NTA.
cessful. While NTA was not efficiently extracted by TSLLE, Figure 3 shows that destructuring its hydration shell may have contributed to the 13% extraction at low pH (see extraction highlights in Table 1). These extractions were performed with the sample and solvent probes set respectively at 191.3 and 163.2 °C and 5.0 and 3.5 mL/min. The low recovery of NTA is a reflection of the strong interaction of its nine oxygen atoms with water. pH Considerations. The objective of the present procedure was to determine the useful pH range over which some desired measure of extraction can be attained for both batch and thermospray liquid/liquid extraction of BA and to explore alternative extraction procedures in regions of unsatisfactory analyte recovery. To commence this process, a 50-mL aliquot of a 1.03 × 10-3 M BA solution was extracted at a flow rate of 5 mL/min and a temperature of ∼190 °C. Chloroform was admitted in the solvent probe at a rate of 3 mL/min and a temperature of ∼178 °C. As is evident from Figure 4, there is good agreement between TSLLE and batch extraction processes. Below pH 3.0, where the benzoate ion is mostly uncharged, some loss in extraction efficiency of TSLLE relative to the batch process is observed. An explanation for this deviation will be given when overall temperature effects are considered. Despite changes in probe parameters, the recovery of BA at higher pH is comparatively less efficient. This is predicated on the fact that, while TSLLE is effective at stripping water molecules from the solutes, only neutral species can be extracted into organic solvents. Thus, the low recovery at basic pH is more a reflection of the inability of charged solutes to migrate into the organic phase than it is the inability of the probes to create small droplets. Interesting information about the TSLLE extraction process can be obtained by adopting a model that requires minimum exposure to harsh pH environments or precludes direct pH adjustments of the sample prior to analysis. For this purpose, traditional LLE, in which pH adjustment is made well in advance of a time-consuming extraction step, offers limited possibility. In TSLLE, pH adjustments can be made via a separate probe or, whenever possible, by admitting the buffer as an integral part of Analytical Chemistry, Vol. 69, No. 16, August 15, 1997
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Table 1. Highlights of the Extraction Data for Model Compounds % recovery compound benzoic acid p-toluenesulfonic acid p-toluenesulfonic acid phenylalanine phenylalanine naphthalenetrisulfonic acid a
condition
TSLLE
batch or single-cycle extrn
5% GA/CH3Cl ion pair extraction, 228.5 °C ion pair extraction, 190.2 °C ion pair extraction saturated with KCl low pH
97a
75 93 94 nab nab 0
91 85 95 51 13
Solvent temperature, 140 °C. b Not applicable.
Table 2. Extraction Conditions for Selected Samples sample
flow rate (mL/min)
entry
figure
vol (mL)
concn (×103/M)
1 2 3 4 5 6 7
8 8 9 9 16 16 na
25 25 25 25 25 25 25
1.03 1.03 1.03 1.03 1.03 1.03 1.73
Figure 4. Extraction of BA as a function of pH.
the solvent. Unless otherwise stated, the latter approach was adopted for the work reported in this paper. Effect of Flow Rate on Extraction Efficiency of BA. The effect of sample probe flow rate on extraction efficiency was examined using the BA solution and a 5% GA/hexane described in Table 2, entries 1 and 2. While there is a decrease in recovery with increasing sample flow rate for the two cases shown in Figure 5, higher solvent velocity and lower solvent temperature seemed to favor increased extraction efficiency. In a separate experiment in which the percent recovery was observed as a function of the hexane flow rate at 110 °C and then again at 178 °C (see Figure 6 and Table 2, entries 3 and 4), higher solvent flow rates and lower temperatures do indeed favor increased analyte recovery. Another factor that could have contributed to the gain in extraction 3342 Analytical Chemistry, Vol. 69, No. 16, August 15, 1997
probe temp (°C)
sample
solvent
sample
solvent
varying varying 5 5 3 5 5
3 5 varying varying 3 5 3
160 160 160 160 varying varying 190
154 136 178 110 136 151 157
Figure 5. Effect of sample probe flow rate on a single-cycle TSLLE of BA in hexane.
efficiency is the elevated probe pressure which, at higher flow rates, resulted in greater sample/solvent impact and mass transfer. Effect of Probe Temperature on the Recovery of Charged Species in Different Solvents. In thermospray liquid/liquid extraction of neutral compounds, probe parameters are chosen to allow removal of water from the solvent shell of the solutes and, by so doing, bring about intimate and more effective contact with the extracting solvent. For these species, it is generally assumed that the selected parameters (especially temperature) will not adversely affect their distribution. However, when the extraction entails ionic analytes, these effects must be accounted for. Consider again the extraction of benzoate ion. The control-
Figure 6. Effect of solvent probe flow rate on a single-cycle TSLLE of BA in hexane.
Figure 7. Effect of bath temperature on the equilibrium distribution of BA.
ling equilibrium for this process can be represented by the expression
HA(org) H HA(aq) H H+(aq) + A-(aq) where HA is the protonated form of the benzoate ion. Since the unionized compound is extracted into the solvent, the H+ or counterion concentration must be adjusted to ensure the formation or HA(org), or suitable ion pairs. While it is true that the carboxylate ion can be stabilized by hydrogen bonding, pH adjustments, or ion pair formation,7 the temperature effect on these interactions cannot be overlooked. Heat effects will be very pronounced in cases where ion-pair formation or hydrogen bonding-interactions are the modes of mass transfer. An appreciation for the magnitude of the heat effect was obtained when 25mL portions of a 1.03 × 10-3 M BA solution (pH 11.06) were heated on a water bath for ∼10 min prior to extraction with 15mL portions of a 5% GA/hexane solvent. As Figure 7 shows, the percent recovery decreased by several percent in going from a bath temperature of 25 to 87 °C. To find out whether such effect was a problem in TSSLE, the sample solution was spray extracted at varying solvent probe temperatures. For this set of experiments, the BA solution was adjusted to pH 8.10, and the sample probe temperature was set at 165 °C to ensure nebulization at a flow rate of 5 mL/min. Each of the three solvents used contained 5% GA (v/v) and was admitted at a flow rate of 3 mL/min. Since the TSSLE is a closed system, it is not surprising that Figure 8 shows an increase in the percent recovery with increasing temperature up to a critical optimum (Topt), beyond which the recovery decreased with additional temperature increments. A similar decrease was observed by varying the sample probe temperature from 167 to 266 °C while the pH 8.10 sample was spray extracted with pure chloroform (without GA) at 143 °C and (7) Shirai, M.; Smid, J. J. Am. Chem. Soc. 1980, 102, 2863. (8) Cox, B. G.; Hedwig, G. R.; Parker, A. J. Aust. J. Chem. 1974, 27, 477. (9) Blumberg, R. Liquid-Liquid Extraction; Academic Press: London, 1988; p 120.
Figure 8. Effect of solvent probe temperature on the recovery of BA in different solvents.
a sample flow rate of 5 mL/min (see Figure 9). Without GA, analyte recovery was low because only a small fraction of the analyte is protonated at this pH. But the temperature effect is still the same. A number of interesting features in Figure 8 are worth further considerations. The first is a concern that the decrease in extraction efficiency beyond Topt was due to evaporative loss. While this possibility was ruled out by leak checks and material balance analysis, the graphical profile beyond Topt is in itself instructive. Notice that, between 180 and 230 °C, the temperature profile is relatively flat. Rather than the flat response shown, a trend toward evaporative loss should reflect continued drop in recovery with increasing temperature. In the 140-180 °C temperature range, where the problem is more pronounced, one would expect a correlation between vapor loss and slopes of the solvents, with the most volatile solvent suffering the greatest loss. Since Topt appears at relatively the same temperature range for each solvent, it would seem logical to infer that the same phenomenon, in this Analytical Chemistry, Vol. 69, No. 16, August 15, 1997
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Figure 9. Effect of sample probe temperature on the recovery of BA at basic pH.
case redistribution of the analyte due to the type of dissociation described for Figure 7, is occurring within each solvent. In like manner, for Figure 4, the loss in TSLLE extraction efficiency below pH 3 is essentially due to redistribution of the analyte on account of the excessively high probe temperatures employed. Once proper pH adjustments are made, the process is essentially an extraction of neutral entities for which the desolvation temperature would be comparatively lower. The superior performance of chloroform over methylene chloride and hexane as depicted in Figure 8 is in part due to its unusual properties. Although chloroform is not a true hydrogen bond donor (HBD) solvent, it is certainly capable of some degree of bonding interaction with hydrogen bond acceptor (HBA) solutes. For example, chloroform possesses the ability to hinder the formation of biologically important hydrogen bonds via the reaction scheme10
CHCl3 + R2NH‚‚‚OdCR2 h Cl3CH‚‚‚OdCR2 + R2NH
Its unusual property was again observed in the stabilization of trans-(ethoxycarbonyl)methylenetriphenylphosphorane where it was established that increasing solvent polarity led to increasing stability, except for chloroform ( ) 4.8), which was a better stabilizing solvent than the more polar acetonitrile ( ) 37.5) and nitromethane ( ) 35.9). This discrepancy was explained as the result of hydrogen-bonding interaction between the negative oxygen atom of the solute and chloroform.11 The observation by Eliel and Hofer that decreasing solvent polarity decreases the stability of cis-2-isopropyl-5-methoxy-1,3-dioxane, except for unexplained deviation in chloroform and methylene chloride, is another example of the unusual solvating ability of chloroform.12 Smith et al. pointed out that the ability of protic solvents to form hydrogen bonds is often not reflected in their dielectric constants (10) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry; VCH Publishers: New York, 1988; p 17. (11) Snyder, J. P. Tetrahedron Lett. 1971, 215. (12) Eliel, E. L.; Hofer, O. J. Am. Chem. Soc. 1973, 95, 8041.
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Figure 10. Effect of sample probe temperature on the recovery of BA in hexane.
or in the dipole moments.13 These solvents behave as more polar solvents than their dielectric constant would lead one to predict.14 It is therefore not surprising that, from the point Topt and beyond in Figure 8, the slopes of the halogenated solvents show greater sensitivity to temperature effects than that of hexane for which this type of HBD ability is nonexistent. In Figure 9 where no acid is added to either the solvent or the sample, the extraction of benzoate ion may in part be due to the type of interaction described above. Assuming that heat transfer occurs upon sample/solvent impact, an interaction that is truly temperature dependent would be observed irrespective of which probe supplies the necessary heat. Not surprisingly, excessive temperature increases in either the sample probe or the solvent probe resulted in some loss in analyte recovery (see Figures 8 and 9). Since temperature decay was minimal for applications involving hexane, a similar response should be observed with the sample probe for the reason just discussed. For verification, BA was again extracted with a 5% GA/hexane mixture in keeping with the conditions shown in Table 2, entries 5 and 6. As seen in Figure 10, at lower sample flow rates, the temperature is sufficiently high to guarantee complete nebulization of the sample over the range tested. But, as expected, loss of efficiency with increasing temperature was not observed in either case. General Consideration for TSLLE of Ion Pairs and Zwitterions. Many of the parameters that are relevant to thermospray liquid/liquid ion pair extraction (TSLLIPE) are described in the expression15
K ) 4πNe2Q(b)/1000kT
where K is the ion pair formation constant, N is Avogadro’s number, e is the charge, T is the absolute temperature, k is Boltzmann constant, Q(b) is a calculable function, b equals e2/ aekT, a is the distance between paired ions, and e is the dielectric (13) Smith, S. G.; Fainberg, A. H.; Winstein, S. J. Am. Chem. Soc. 1961, 83, 618. (14) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry; VCH Publishers: New York, 1988; p 115. (15) Marrison, G. H.; Freiser, H. Solvent Extraction in Analytical Chemistry; John Wiley & Sons Inc.: New York, 1966; p 31.
constant. This equation implies that ionic components with higher charge density, smaller interionic distances, and low dielectric constant should lead to greater electrostatic interaction and the formation of more stable ion pairs. But the effect of e on ion pair extraction must be considered in conjunction with its response to temperature variations. For solvents with high dielectric constants, increasing the temperature leads to marked reduction of e and, consequently, the eT value. In such solvents, ion pairing increases with increasing temperature. In solvents with low dielectric constant, where e does not change much with increasing temperature, eT increases with temperature, and ion association falls off. The foregoing observation about the temperature dependence of eT is significant to TSLLE, for it would mean that ion pairing reaction in water (high e) should increase with temperature elevation, while that in the organic phase (low e) should decrease with temperature elevation. This was seen in the extraction of a 4.21 × 10-3 M TSA solution with tetrabutylammonium hydrogen sulfate as the counterion. With the sample and solvent temperatures set respectively at 190.2 and 162.3 °C, the extraction efficiency for TSA was only 85%. When the sample temperature was increased to 228.5 °C and that of the solvent probe reduced to 148.5 °C, the extraction efficiency of TSA increased to 91%. The preceding observations are in reasonable agreement with the temperature profiles depicted in Figure 8, where the recovery of BA in a variety of organic solvents appears to indicate that, beyond ∼145 °C, extraction efficiency does indeed decrease with increasing solvent temperature. The increases observed up to this point on the graph is perhaps due to surface area enhancement. Phenylalanine was used as a test compound for the extraction of zwitterionic species by TSLLE. While it is a zwitterion in both the crystalline and aqueous states, PA exists as a neutral molecule (H2NCR2CO2H) in the gas phase.16 In theory, the extraction (16) Locke, M. J.; McIver, R. T. J. Am. Chem. Soc. 1983, 105, 4226. (17) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry; VCH Publishers: New York, 1988; p 121. (18) Hughes, D. L.; Bergan, J. J.; Grabowski, E. J. Org. Chem. 1986, 51, 2579.
efficiency of these species should improve as gas phase conditions are approached. One reason is that gas phase ionization of molecules into free ions is a very endothermic process which requires other means of energy other than solvation with solvents.17 However, under typical gas phase conditions of low flow rate, low pressure, and high temperature, the increased risk of thermal decomposition and salt deposition within the probes must be weighed against any gains that might be achieved. For this very reason, gas phase experiments were not performed. An alternative approach is to neutralize the amino group by making the sample solution basic and use a counterion to ion pair with the carboxylate end of the molecule. Under the conditions described in Table 2, entry 7 for a pH 11.8 solution, the extraction efficiency for the 1.73 × 10-3 M PA using the tetrabutylammonium counterion was 95%. Extraction of PA at acidic pH or with other counterions was less efficient. This is probably because of the greater solvation of the carboxylic group in water compared to the ammonium group.18 CONCLUSION We have used BA, PA, TSA, and NTA as model compounds to study the behavior of charged species under TSLLE conditions. Using the data from the systems studied, we conclude that probe temperature is the single most important factor affecting the extraction of these particular charge compounds into the organic solvents studied or their redistribution into the aqueous phase. Under optimized conditions, the extraction efficiencies for BA, PA, TSA, and NTA were 97, 95, 91, and 13%, respectively. This study also demonstrates that on-line adjustment of sample pH to prevent preexposure of the analyte to harsh pH environments is possible when the TSLLE technique is employed. Received for review November 21, 1996. Accepted April 17, 1997.X AC961182B X
Abstract published in Advance ACS Abstracts, July 15, 1997.
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