Anal. Chem. 2005, 77, 3045-3052
Determination of Air-To-Water Partition Coefficients Using Automated Multiple Headspace Extractions A. Brachet and A. Chaintreau*
Corporate R&D Division, Firmenich SA, 1 Route de Jeunes, 1211 Geneva 8, Switzerland
Air-to-water partition coefficients are experimentally determined using a multiple headspace extraction procedure and an automated headspace cell. The approach is first validated with 2-butanone and then applied to a homologous series of methyl ketones. As adsorptions of the most hydrophobic compounds occurred in the sampling cell, technical improvements have been tested. This study represents the first attempt to overcome analyte adsorptions by studying and minimizing the effect of the cell’s adsorption of hydrophobic analytes on the determination of their partition coefficients. The present method allows the measurement of several analytes at the same time, in the ppm range, without calibration, and with a limited manpower. During the past decade, the use of partition coefficients has gained great importance in various areas such as environmental studies1 and the food and flavor industry.2,3 Therefore, there is an increasing need to determine accurate partition coefficients for a large number of organic compounds. Various methods have been proposed that differ in their reliability, rapidity, and ease of use. Their possible sources of inaccuracy, leading to discrepancies between literature values, have already been discussed in previous works.4,5 More recently, two automated methods were proposed by Chai and Zhu.6,7 The former plays on phase ratio differences, and the latter involves multiple headspace extractions (MHE). In both cases, the sample is collected from the static headspace (S-HS) of pressurized vials. This may influence the partition and does not allow an enrichment of volatiles present in the gas phase for moderately volatile compounds. Moreover, only methanol, ethanol, and 2-propanol were tested. Miller and Stuart have proposed * To whom correspondence should be addressed. E-mail: alain.chaintreau@ firmenich.com. (1) Sander, R. Compilation of Henry’s law constants for inorganic and organic species of potential importance in environmental chemistry (version 3). http:// www.mpch-mainz.mpg.de/∼sander/res/henry.html. 1999. (2) Graf, E.; de Roos, K. B. In Thermally generated flavors; Parliment, T. H., Morello, M. J. McGorrin, R. J., Eds.; American Chemical Society: Washington, DC, 1994; Chapter 37. (3) Nahon, D. F.; Harrison, M.; Roozen, J. P. J. Agric. Food Chem. 2000, 48, 1278-84. (4) Chaintreau, A.; Grade, A.; Mun ˜oz-Box, R. Anal. Chem. 1995, 67, 3300-4. (5) Staudinger, J.; Roberts, P. V. Crit. Rev. Environ. Sci. Technol. 1996, 26, 205-97. (6) Chai, X. S.; Zhu, J. Y. Anal. Chem. 1998, 70, 3481-7. (7) Chai, X. S.; Zhu, J. Y. J. Chromatogr., A 1998, 799, 207-14. 10.1021/ac0401220 CCC: $30.25 Published on Web 04/12/2005
© 2005 American Chemical Society
another MHE approach, which appeared to be time-consuming. In addition, a lack of agreement was observed for low volatile compounds in comparison with published Henry’s constants.8 As a solid-phase microextraction (SPME) autosampler now exists, this technique could be automated to measure the partition coefficients as proposed by Dewulf et al.9 However, according to their assumptions and accuracy evaluation, significant deviations must be expected for compounds with nonpolar retention indices higher than 900 and partition coefficients less than unity. Other authors also used SPME according to an unclear model involving the stationary-phase volume and an external calibration without determination of the GC response factors, both of which are inaccurate.10 In a previous work, a new method based on a static and trapped headspace sampling (S&T-HS) was proposed. It allows measurements at atmospheric pressure, with an enrichment of the volatile compounds of the gas phase to improve the sensitivity.4 However, this technique requires ∼2 days and significant manpower to fill and clean the HS cell and perform the subsequent GC analyses. These disadvantages are a limitation for the analysis of large numbers of compounds. Therefore, a more rapid technique was needed, while maintaining the highest possible accuracy. In addition, to fulfill the need of a quick and reliable method, the present work attempts the measurement of partition coefficients at low concentrations to fit the infinite dilution criterion of the thermodynamics and the real use conditions of aromas and perfumes. This paper first investigates the applicability of the MHE procedure to measure the air-to-water partition coefficient for a model compound, 2-butanone. Then, this methodology is applied to the simultaneous determination of methyl ketone coefficients (from C4 to C9) in comparison with the literature values. As adsorption phenomena occur for the most hydrophobic compounds, technical improvements of the headspace cell are considered in order to overcome their consequences. EXPERIMENTAL SECTION Chemical Products. Methyl ketones from C4 to C9, were purchased from Fluka (Buchs, Switzerland) and Acros (Geel, Belgium). Sylon CT (5% dimethylchlorosilane in toluene) was (8) Miller, M. E.; Stuart, J. D. Anal. Chem. 2000, 72, 622-5. (9) Dewulf, J.; Vanlangenhove, H.; Everaert, P. J. Chromatogr., A 1999, 830, 353-63. (10) Liu, Z.; Wene, M. J. J. Chromatogr. Sci. 2000, 38, 377-82.
Analytical Chemistry, Vol. 77, No. 10, May 15, 2005 3045
Figure 1. Scheme of the automated headspace cell: (a) sample cell, (b) headspace chamber, (c) Tenax cartridge, (d) glass or stainless steel tube, (e) bottom end piece, (f) end of the piston, (g) removable glass receptor, (h) glass stir bar, (i) stainless steel tube, (j) double jacket, (k) thermostated fluid, (l) motor, and (m) captor.
supplied by Supelco (Bellefonte, PA). All solvents were of analytical grade. Methanol, ethanol, and toluene were obtained from Carlo Erba Reagenti (Rodano, Italy) and acetone was from Fluka. Deionized water was provided by Seradest (Basel, Switzerland). Cell Automation. The sampling cell for measuring the partition coefficient has been previously described.4 Several modifications introduced to the previously reported instrumentation have been made as shown in Figure 1. The main differences between previous and current equipment are the automation of the sampling and the cell material constituents. The piston was driven by a PC-controlled motor, and the cell was double jacketed to circulate a thermostated fluid. A magnetic stirring was added in the sample cavity to shorten the equilibration 3046
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of the analyte between both phases. The sample was placed in a removable glass receptor with the glass stir bar. Stirring of the solution was maintained during equilibration (1 h) at the investigated temperature and was stopped when the piston was pressed down. MHE Procedure. The glass receptor was filled with 1 mL of a methyl ketone solution at 8 ppm. The sampling cell was then closed with the piston in the bottom position. The piston was raised to fill the cell with ambient air (Vg ) 250 mL), and the sample solution was equilibrated for exactly 1 h at 25 °C. The whole gas phase was then evacuated by pressing down the piston at a constant rate to maintain a flow of 15 mL/min through the Tenax cartridge (100 mg of Tenax/cartridge). Traps were desorbed and analyzed as described below. Preliminary experiments showed that stirring for 30 min was sufficient for 2-butanone equilibrium to be established. To ensure a reasonable reproducibility, the chosen equilibrium time was 1 h for all other methyl ketones. All the sampling cell parts directly in contact with the liquid or the vapor phases were washed with water and dried in a vacuum oven at 40 °C for 1 h. Tested Materials. Tenax (35-60 Mesh) was supplied by P. H. Stehelin & Cie AG (Basel, Switzerland).The sampling cell was a 5 cm × 15 cm Pyrex tube (Omnifit, Cambridge, U.K.). Removable glass receptors and glass stir bars were made in-house. All glass pieces were silanized before use with Sylon CT to reduce adsorptions. To test the adsorptive properties of the glass material, this latter was ground (60-270 mesh) after being cleaned with acetone. One part of the powder was silanized before being tested for adsorptions. Teflon and Viton O-rings were purchased from Angst & Pfister (Geneva, Switzerland). After cleaning O-rings with ethanol, they were roughly ground under liquid nitrogen (10-60 mesh for both materials). The stainless steel material was cleaned with acetone and ground down by filing (35-270 mesh). All cartridges were conditioned under a nitrogen stream at 250 °C for 12 h before use. Instrument Conditions. A Flache 5000 thermal desorber (Brechbu¨hler, Plan-les-Ouates, Switzerland) coupled on line with a HRGC 5300 Mega Series gas chromatograph (Carlo Erba, Rodano, Italy) was used. The GC was equipped with a flame ionization detector (FID) and a nonpolar wide-bore CP-SIL 5 CB fused capillary column (50 m × 0.53 mm, 1.0-µm film thickness; Chrompack, Middleburg, The Netherlands). Helium was used as the carrier gas at 80 kPa. The traps were desorbed at 240 °C for 5 min. The volatiles were focused in a cold trap at -150 °C and then desorbed at 240 °C for 3 min. The column temperature was programmed from 60 to 190 °C at 5 °C/min. The detector was maintained at 250 °C. Curve Fitting. The equation of the MHE curve was determined by linear regression of the data points using the Excel software (Microsoft, Redmond, WA). The confidence interval was calculated as (t(SD))/N 1/2, where t is the Student variable at 95% confidence level, SD, the experimental standard deviation, and N the number of experiments.
RESULTS AND DISCUSSION In contrast to the previously proposed method,4 the MHE procedure only requires a single sample of the investigated solution. Moreover, from a series of headspace extractions, the depletion of the analyte in the gas phase only depends on the partition coefficient and phase volume ratio. The latter is “a key parameter” to apply the mass balance rule in a closed vessel.6,7 For a satisfactory sensitivity, the gas volume must be large enough to allow a sufficient amount of volatiles to be transferred in the headspace of a dilute solution and subsequently concentrated in the Tenax cartridge. This may be achieved with the previously developed S&T-HS technique.4 The headspace and sample volumes are accurately determined, and the sampling can be performed under normal temperature and pressure conditions, avoiding any modification of the partition. To save some manpower and improve the sampling reproducibility, the use of the HS cell was automated for the present work (see the Experimental Section). Principle and Theoretical Model. The principle of the MHE procedure is based on a series of successive samplings of volatiles present in the headspace from a single liquid sample in a closed cell after having reached the phase equilibrium before each recovery step. First Extraction. Since the initial liquid mass ml,0 is distributed between both phases, the mass balance can be written as
ml,0 ) mg,1 + ml,1
(
)
(2)
with Vg and Vl the headspace and liquid volumes, respectively. The remaining concentration in the liquid phase is then
ml,1 ) µml,0
(3)
with
(
µ ) 1 + kgl
)
Vg Vl
-1
Second Extraction. After the second extraction, the remaining concentration in the liquid phase is reequilibrated with the same headspace volume Vg
ml,1 ) mg,2 + ml,2
(4)
Replacing ml,1 using eq 3 and writing mg,2 as a function of kgl, the mass of the solute in the liquid phase can be expressed as
ml,2 ) µ2ml,0
mg,2 ) µ(1 - µ)ml,0
(6)
Extraction n and n + 1: From eqs 5 and 6, the mass of the solute in the gas and liquid phases after the extraction n should be
ml,n ) µnml,0
(7)
mg,n ) µn-1(1 - µ)ml,0
(8)
and
From the mass balance ml,n ) mg,n+1 + ml,n+1, replacing ml,n using eq 7 and the expression mg,n+1 as a function of kgl gives in the headspace
mg,n+1 ) µn(1 - µ)ml,0
(9)
and in the liquid phase
ml,n+1 ) µn+1ml,0
(10)
(1)
with mg,1 and ml,1 the masses of the solute in the headspace and the liquid, respectively, after the first equilibrium. If we define air-to-water partition coefficient as kgl ) Cg/Cl, with Cg and Cl the concentrations of the solute in the headspace and the liquid, respectively, eq 1 becomes
Vg ml,0 ) 1 + kgl ml,1 Vl
Replacing in eq 4, ml,1 using eq 3 and ml,2 with eq 5, the mass of the solute in the gas phase is
(5)
Both eqs 9 and 10 for the nth extraction are valid for the following extraction n + 1 and are applicable to any step of the MHE. The natural logarithm of eq 8,
ln(mg,n) ) ln[(1 - µ)ml,0] + (n - 1) ln(µ)
(11)
represents a straight line giving ln(mg,n) as a function of the number of extractions n - 1. From its slope, the partition coefficient kgl can be calculated. It is noteworthy that this method does not depend on the initial amount of the analyte and therefore does not require any calibration. Validation of the MHE Methodology Using 2-Butanone. 2-Butanone was selected to test the MHE approach as many methods have been used to determine its air-to-water partition coefficient. It has been directly measured under either static or dynamic conditions (Table 1) or calculated from other thermodynamic constants. Moreover, its sampling was carried out in different ways: either manually by using a gastight syringe or automatically and under various pressure conditions. The determination of the partition coefficient of 2-butanone was carried out by a six-step MHE procedure using a silanized Pyrex headspace cell. The equilibration time was 1 h under stirring at 25 °C. The experiment was repeated four times with solutions freshly prepared at a fixed concentration (8 ppm) and with different phase volume ratios (Vg/Vl from 33 to 255). All correlation coefficients of the regression curves of ln(area) versus n - 1 lay between 0.9982 and 1.0000. The resulting kgl mean value (kgl ) 0.0024) was in agreement with the literature value measured at atmospheric pressure (Table 1). These four replica measurements gave a relative standard deviation of less than 2%, representing the total variation of the procedure (phase Analytical Chemistry, Vol. 77, No. 10, May 15, 2005
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Table 1. Experimental Values of Air-to-water Partition Coefficients of Methyl Ketones (from C4 to C9) from the Literature compounds
kgl
temp, °C
method
ref
2- butanone
0.0019 0.003 0.0023 0.0021 0.0021 0.0021 0.0053 0.0030 0.0029 0.0039
25 30 25 30 25 25 25 25 25 30
11 12 13 4 14 15 16 17 18 19
2-pentanone
0.0026 0.0044 0.0041 0.0034 0.0032 0.0053 0.0059 0.0062 0.0069 0.0077 0.0073 0.015 0.01 0.016
25 28 30 25 25 30 25 30 25 25 30 25 30 30
0.0097 0.0148
25 25
gas loop/atm press glass syringe/atm press gas loop/atm press sampling cell/atm press glass syringe/atm press gas stripping method glass syringe/atm press pressurized viala pressurized viala glass syringe pressurizable/ atm press gas loop/atm press glass syringe/atm press glass syringe/atm press gas stripping method gas stripping method glass syringe/atm press gas loop/atm press glass syringe/atm press gas stripping method gas loop/atm press glass syringe/atm press gas loop/atm press glass syringe/atm press mean value unpublished exptl conditions (static HS) semiexperimentalb semiexperimentalb
2-hexanone 2-heptanone 2-octanone 2-nonanone
b
11 20 12 21 22 12 11 12 21 11 12 11 12 23
Figure 2. Decrease of the extracted methyl ketone amount in the headspace as a function of headspace extraction steps (n ) 6).
24 25
a Extrapolated values from the relationship between log k and 1/T. gl Partition coefficients obtained from experimental and theoretical data.
equilibrium, gas aliquot transfer, trapping, desorption, and GCFID analysis). These results suggest a satisfactory accuracy and repeatability of the MHE method. This latter was significantly faster than the previous method4 as a determination was achieved within 1 day with limited manpower. To ascertain the satisfactory precision and accuracy of the method, measurements were also performed with other methyl ketones. Partition Coefficient of a Model Mixture (Methyl Ketones from C4 to C9). The air-to-water partition coefficient of the homologous series of methyl ketones (from C4 to C9) was determined in a single MHE experiment using exactly the previously described procedure. For each compound, Figure 2 plots natural logarithms of chromatographic peak areas versus the number of extraction steps minus one. Linear regression coefficients of MHE curves for methyl ketones from C4 to C7 were higher than 0.999 and equal to 0.9941 and 0.9794 for 2-octanone and 2-nonanone, respectively. The curvature observed for the two latter compounds did not allow an accurate determination of their partition coefficient. Among the MHE examples reported in the literature, a curvature was similarly observed by Wenzl but for the first MHE points26 and was attributed to adsorption phenomena. No curvature (11) Buttery, R.; Ling, L. C.; Guadagni, D. G. J. Agric. Food Chem. 1969, 17, 385-9. (12) Nawar, W. W. J. Agric. Food Chem. 1971, 6, 1057-9. (13) Vitenberg, A. G.; Ioffe, B. V.; Dimitrova, Z. St.; Butaeva, I. L. J. Chromatogr. 1975, 112, 319-27.
3048 Analytical Chemistry, Vol. 77, No. 10, May 15, 2005
Figure 3. Comparison of MHE profiles for 2-nonanone with different phase volume ratios. Experiments were conducted from the same stock solution.
was mentioned for the later points in the literature, mainly because the MHE procedure was conducted in only four steps17,26,27 or with compounds of high volatility or low molecular weight such as common organic solvents.6,28 However, a slight curvature or low regression coefficients may be observed in figures of some papers, (14) Rohrschneider, L. Anal. Chem. 1973, 45, 1241-7. (15) Zhou, X.; Mopper, K. Environ. Sci. Technol. 1990, 24, 1864-9. (16) Ashworth, R. A.; Howe, G. B.; Mullins, M. E.; Rogers, T. N. J. Hazard. Mater. 1988, 18, 25-36. (17) Kolb, B.; Welter, C.; Bichler, C. Chromatographia 1992, 34, 235-40. (18) Ettre, L. S.; Welter, C.; Kolb, B. Chromatographia 1993, 35, 73-84. (19) Friant, S. L.; Suffet, I. H. Anal. Chem. 1979, 51, 2167-72. (20) Nelson, P. E.; Hoff, J. E. J. Food Sci. 1968, 33, 479-82. (21) Shiu, W. Y.; Mackay, D. J. Chem. Eng. Data 1997, 42, 27-30. (22) Piringer, O.; Sko ¨ries, H. Analysis of volatiles; Walter de Gruyter & Co: Berlin, 1984. (23) Voilley, A. Interactions of food matrix with small ligands influencing flavour and texture; Food Science and Technology, COST Action 96; 1996; pp 339. (24) Li, J. J.; Carr, P. W. Anal. Chem. 1993, 65, 1443-50. (25) Abraham, M. H.; Whiting, G. S. J. Chem. Soc., Perkin Trans 2 1990, 291300. (26) Wenzl, T.; Lankmayr, E. P. J. Chromatogr., A 2000, 897, 269-77. (27) Kolb, B. Chromatographia 1982, 15, 587-94. (28) Maggio, A.; Milana, M. R.; Denaro, M.; Feliciani, R.; Gramiccioni, L. J. High Resolut. Chromatogr. 1991, 14, 618-20.
Figure 4. Sorption of methyl ketones by ground cell materials (in parentheses: amount of material in the first headspace cartridge). The percentage of the methyl ketones adsorbed by the investigated material was calculated as the ratio between the GC area of methyl ketones adsorbed by the investigated material and the total area of methyl ketones adsorbed by the investigated material and the Tenax.
Figure 5. Comparison of MHE profiles for 2-nonanone using a Pyrex cell with or without the Viton O-ring and a stainless steel cell without the Viton O-ring. Experiments were conducted using different stock solutions.
but it was not commented on in the corresponding discussions.8,29,30 The FID response was checked to be linear over the investigated concentration range and was not responsible for the curvature. When reducing the analyte concentration by a factor of 10, the curvature increased considerably (data not shown), suggesting that adsorption phenomena had occurred. To ascertain the occurrence of adsorption on cell walls or other surfaces such as O-ring seals, two experiments were performed. Change of the Gas-Phase Volume. Change of the gas-phase volume should not affect partition coefficient values. However, it changes the exposed glass area and thus modifies the level of (29) Milana, M. R.; Maggio, A.; Denaro, M.; Feliciani, R.; Gramiccioni, L. J. Chromatogr. 1991, 552, 205-11. (30) Uhler, A. D.; Miller, L. J. J. Agric. Food Chem. 1994, 36, 772-5.
adsorption. Silanizing the Pyrex surfaces should limit these drawbacks, but an incomplete deactivation might be sufficient to cause adsorption. Figure 3 shows that decreasing the gas volume from 251 to 33 mL with a constant liquid volume (Vl ) 1 mL) improves the linearity of MHE curves, because the available active glass area is reduced, while using the same total amount of analyte. Conversely, if half the gas cell volume is used (i.e., Vg ) 127 mL), while maintaining the phase volume ratio constant (Vl ) 0.5 mL, Vg/Vl ) 255 mL), an increased curvature was observed for this reduced quantity of 2-nonanone in the cell (Figure 3). Change of cell Materials. Adsorption on the cell walls has already been highlighted by Buttery11 and recently confirmed when Teflon and nonsilanized glass surfaces were used for very Analytical Chemistry, Vol. 77, No. 10, May 15, 2005
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Table 2. Values of Partition Coefficients at 25 °C Computed (1) from the First Four Points of the Six-Step MHE Procedures while Stirring the Solution and (2) from Three MHE Steps without Stirring the Solution 1 h,a stirring (n ) 4)
C4 C5 C6 C7 C8 C9 Figure 6. Comparison of MHE profiles for 2-nonanone obtained with or without stirring using the stainless steel cell (identical equilibrium time).
nonpolar compounds.31 These latter surfaces, using aqueous solutions, gave significant adsorption31-33 and subsequent changes in the partition coefficient. To investigate which materials were responsible for adsorption, their sorptive properties were tested. Two cartridges in series were connected to the sampling cell. The first one, directly connected to the sampling cell, was filled with the ground tested material (between 100 and 300 mg). The second one was filled with Tenax (100 mg). All materials used in the cell construction were investigated, such as Pyrex, stainless steel, and seals (Viton and Teflon O-rings). None of the materials was quite free of adsorption as illustrated in Figure 4. Among them, Viton exhibited the highest affinity for hydrophobic compounds with an adsorption of roughly 90% of the initial 2-nonanone concentration. Nonsilanized Pyrex and Teflon were the second highest adsorbing materials. Although silanizing Pyrex considerably reduced 2-nonanone adsorption, it did not prevent it completely. Only stainless steel gave rise to a negligible adsorption and was considered as the most appropriate material. Cell Improvement. To lower adsorption phenomena, the Viton O-ring was replaced by a Teflon O-ring. A six-step MHE procedure was then conducted under exactly the same conditions as those of validation. The curvature of the 2-nonanone curve was drastically decreased in the absence of Viton (Figure 5). As the adsorption of silanized Pyrex was found to be greater than stainless steel (Figure 4), a new cell was built with this latter material. According to the lower curvature of the curve, MHE determinations achieved with this stainless steel cell decreased adsorptions (Figure 5). Because a slight curvature still exists, a MHE procedure was performed after having introduced a Teflon O-ring within the cell to increase the Teflon area in contact with the headspace. The same MHE curve was obtained (data not shown), thus suggesting that the additional Teflon area did not significantly influence the partition coefficient measurements. The influence of the removable glass receptor and the glass stir bar on partition coefficients was also investigated with the (31) Ackerman, A. H.; Hurtubise, R. J. Talanta 2000, 52, 853-61. (32) Baltussen, E.; Sandra, P.; David, F.; Janssen, H.-G.; Cramers, C. Anal. Chem. 1999, 71, 5213-6. (33) Vaes, W. H. J.; Mayer, P.; Oomen, A. G.; Hermens, J. L. M.; Tolls, J. Anal. Chem. 2000, 72, 639-41.
3050 Analytical Chemistry, Vol. 77, No. 10, May 15, 2005
12 h,a no stirring (n ) 3)
kglb
SDc
kgl
0.00237 0.00341 0.0043 0.0057 0.0075 0.0092
0.00003 0.00004 0.00012 0.00011 0.00021 0.00034
0.0025 0.0033 0.0041 0.0056 0.0070 0.0089
a Equilibration time in 1 h (in parentheses: n, number of MHE extraction steps used for the calculation). b Mean value of five experimental values of partition coefficient. c SD, experimental standard deviation only refers to experiments under stirring.
Figure 7. Air-to-water partition coefficients for six homologous series of methyl ketones determined at 25 °C, as a function of the carbon chain length (experimental values from this work and the literature).
stainless steel cell. The same experiment was first conducted without the glass receptor (Figure 6), leading to a slightly lower curvature. However, the influence of the receptor on the adsorption was not significant. The MHE was also performed without the glass stir bar and the glass receptor (i.e., without any stirring). Figure 6 shows an excellent linear relationship between the logarithm of the peak area and (n - 1) MHE steps, with a good correlation coefficient (0.9996). However the slope was not representative of the partition coefficient because the equilibration time (1 h) was insufficient in the absence of stirring. Therefore, the remaining slight curvature observed when the solution was stirred could be attributed to the presence of the glass stir bar. Despite the silanization of its glass coating, its continuous friction on the stainless steel surface may be sufficient to mechanically alter its surface, resulting in a reactivation of the sites. Accordingly, a Teflon-coated stir bar was tested. The resulting MHE curve was similar to that obtained when stirring was effected with a glass stir bar (Figure 6), confirming once again that Teflon has very little influence on partition coefficient measurements. Final Proposed Procedure. In conclusion, adsorption phenomena could be dramatically lowered when a sampling cell made
Figure 8. Natural logarithm of air-to-water partition coefficient kgl versus the reciprocal of absolute temperature (1/T) for methyl ketones.
Figure 9. Temperature dependence for 2-butanone, according to experimental values from this work and the literature.
of stainless steel and Teflon O-rings, without any glass parts and stir bar was used. Without any stirring, adsorption phenomena could presumably be completely overcome if a longer equilibration time, i.e., a longer experimental procedure, was used. Based on these observations, kgl measurements were performed in the absence of any glass surface (i.e., with the stainless steel cell, without stirring and glass receptor) and with an increased equilibrium time (12 h) in order to determine the most accurate kgl values. Results are reported in Table 2. From the previous six-step MHE experiments (using the stainless steel cell, the glass stir bar with 1-h equilibration time), kgl values were recalculated using only the first four MHE points that gave a linear relationship. The five measurements (Table 2) gave a relative standard deviation less than 4.0% with the highest relative standard deviation for the most hydrophobic compounds. Recalculated kgl values did not significantly differ from those obtained with no stirring. This suggests that adsorption phenomena were negligible with the first four-step extractions. This
observation will be further discussed and supported elsewhere by modeling the kinetics of equilibration and adsorption during the MHE procedure.34 Thus, a four-step MHE procedure under stirring (1-h equilibration time) using cell materials with the lowest sorptive properties was assumed to be sufficient to rapidly and accurately determine partition coefficients, even for the most hydrophobic compounds. The final air-to-water partition coefficient values are reported in Figure 7 versus the length of the carbon chain and compared with the experimental values taken from the literature (Table 1). The partition coefficient of the homologous series increases with the increase in the number of carbons as already highlighted by Buttery.35 Moreover, a relatively good agreement between partition coefficient values from this work and the literature was observed (34) Brachet, A.; de Saint Laumer, J.-Y.; Chaintreau, A. Anal. Chem. 2005, 77, 3053-3059 (ac040123s). (35) Buttery, R.; Bomben, J. L.; Guadagni, D. G.; Ling, L. C. J. Agric. Food Chem. 1971, 19, 1045-8.
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3051
except for 2-nonanone. For this latter compound, its significant adsorption to the walls or seals probably contributes to the observed discrepancy of literature values. Temperature Dependence. According to Ioffe,36 the temperature dependence of air-to-water partition coefficients was found to be ln kgl ) a(1/T) + b with a and b regression coefficients and T the temperature (in kelvin). Figure 8 shows MHE procedures performed at three different temperatures (20, 25, and 30 °C). Air-to-water partition coefficients were determined using cell materials with the lowest sorptive properties and by taking into account the first four points. Linear regression coefficients between ln kgl and 1/T lay between 0.9958 and 1.0000 for all methyl ketones. Such regression coefficients (calculated from three points) seem to a posteriori confirm the adequacy of the proposed kgl determination. Comparison with literature values at different temperatures was only possible for 2-butanone. Figure 9 represents experimental and extrapolated kgl values from this work and published results. The data of Kolb and Ettre were extremely close to one another, as they were obtained with the same analytical instrument using pressurized vials.17,18 Zhou’s relationship based on gas stripping agreed quite well with ours.15 Ashworth’s values16 were not (36) Ioffe, B. V.; Vitenberg, A. G. Head-space analysis and related methods in gas chromatography; John Wiley & Sons: New York, 1984.
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reported in Figure 9, as relative standard deviations were reported to lie between 6 and 82% when using a static headspace method (EPICS, equilibrium partitioning in closed systems). CONCLUSION The present work validates the measurement of air-to-liquid partition coefficients using the MHE procedure, as it associates good precision and accuracy with a reasonable experimental time and manpower, without requiring any calibration. For the first time, such an approach takes into account all possible occurrences of adsorptions into the cell. Owing to an appropriate selection of cell materials, a four-point MHE determination can overcome the remaining adsorptions, which allows an accurate calculation of partition coefficients. Such observations will be further investigated and supported by a theoretical model of equilibration and adsorption kinetics in a forthcoming paper.34 ACKNOWLEDGMENT The authors are grateful to Mr. A. Hugon and J.-P. Probst for the modifications of the sampling cell, Mr. C. Debonneville for his collaboration, and Dr. R. L. Snowden and Dr. J. -Y. de Saint Laumer for reviewing the manuscript. Received for review July 6, 2004. Accepted February 9, 2005. AC0401220
