Anal. Chem. 1990, 62, 2471-2478
progress to explore these possibilities.
ACKNOWLEDGMENT The authors are grateful to Mark Bartelt for his competent technical assistance. LITERATURE CITED (1) Chem. Eng. News 1900, March, 19, 38. (2) Amstrong, D. W.; Han, S. M. CRC Crit. Rev. Anal. Chem. 1998, 19, 175. (3) Hinze, W. L. Sep. Pwff. Methook 1981, 10, 159. (4) Armstrong, D.W. Anal. Chem. 1987, 59. 84A. (5) Armstrong, D. W. Sep. pwff. Methods 1985, 14, 213. (6) Hinze, W. L.; Armstrong, D. W. ordered M i a In Chemical Separat h s ; American Chemical Society: Washington, DC, 1987. (7) Purdie, N. Rog. Anal. Spectfosc. 1987, 10, 345. (8) Purdle, N.; Swallow, K. A. Anal. Chem. 1989, 67, 77A. (9) Raw H. Chem. Rev. 1983, 83, 535. (10) Tran, C. D.; Xu, M. Rev. Sci. Inshum. 1989, 6 0 , 3207. (11) Leach, R. A.; Harris, J. M. J . Chromatogr. 1981, 218, 15. (12) Enscoe, R. F.: Kocka, R. J. Lasers Appl. 1984, (June), 91. (13) Tran, C. D.; Drake, A.,F. Blochem. B/ophys. Res. Commun. 1981, 101, 66. (14) Tran, C. D.; Beddard, G. S. Biochim. Biophys. Acta 1981, 678, 497.
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(15) Izumoto, S.; Miyoshi, K.; Yoneda, H. Bull. Chem. SOC. Jpn. 1987, 60, 3199. (16) Yoneda, H. J . Liq. Chromatogr. 1979, 2 , 1157. (17) Synovec, R. E.; Yeung, E. S. Anal. Chem. 1985, 5 7 , 2606. (18) Tran, C. D. AIM/. chem. 1988, 60, 182. (19) Tran, C. D.; Van Fleet, T. A. Anal. Chem. 1988, 60, 2478. (20) Phillips. C. M.; Crouch, S. R.; Lerol, G. E. Anal. Chem. 1988, 5 6 , 1710. (21) Tran, C. D.; Xu, M. Unpublished results. (22) Thomas, M. P.; Patonay, G.; Warngr, I. M. Rev. Sci. Inshum. 1988, 5 7 , 1308. (23) Shao, Y. Y.; Rice, P. D.; Bobbitt, D. R. Anal. Chim. Acta 1989, 221, 239. (24) Lloyd, D. K.; Goodail, D. M.; Scrivener, H. Anal. Chem. 1980, 67, 1238. (25) Chan, K. C.; Yeung, E. J . Chromatgr. 1989, 457, 421. (26) Synovec, R. E.; Yeung, E. S. J . Chromatogr. 1988, 368, 85. (27) Christensen, P. L.; Yeung, E. S. Anal. Chem. 1989, 67, 1344.
RECEIVED for review May 18,1990. Accepted August 13,1990. Acknowledgment is made to the National Institutes of Health (Grant BRC 1 R03 RR05305-01A1) for financial support of this research.
Concentration of Organics from Aqueous Solutions Using Uncoated CapiIlary Columns Albert Zlatkis* and Ravindra P. J. Ranatunga Department of Chemistry, University of Houston, Houston, Texas 77204-5641
Brian S . Middleditch Department of Biochemical & Biophysical Sciences, University of Houston, Houston, Texas 77204-5500
A novel technlque has been developed for the concentratlon of trace organics In aqueous solutlons. Concentratlonof organics b carded out by passage of aqueous samples through uncoated plastlc and metal caplllary tublng. The organlc compounds are removed on the waL of the column wMle the water Is allowed to elute from the capillary. Detailed investlgations were also carrled out on parameters that affect the effklency of trapplng such as the column length, column Internal dlameter, column Internal surface area, trapping temperature, solute concentratlon, lonlc strength of aqueous solutlons, and the flow rates of solutlons through trapplng cap Illarles. It was determined that plastic type materlals, and dlkone In partlcular, are capable of removlng trace organics from solutlon extremely effklently. The trapping temperature and the flow rate of solutlons through the columns are important parametersthat contribute to the removal of organics.
INTRODUCTION The presence of trace organics in water and wastewater has posed numerous problems in diverse fields of interest. Many of these trace organics, especially those present in industrial effluents, have toxic, carcinogenic, mutagenic, and teratogenic properties. Hence, their presence in water is important from an environmental and social point of view. In spite of their low concentrations, they have significant activity in terms of toxicity, thereby raising questions on how to control the occurrence of these harmful chemicals in the environment, permissable “safe” levels in which they may occur in natural matrices including aqueous systems, and how best to analyze 0003-2700/90/0362-2471$02.50/0
for these substances. As a result, the Environmental Protection Agency has adopted 15 methods for the analysis of these organic Priority Pollutants ( 1 ) . Methods 601-612 are chromatographic techniques. Methods 613,624, and 625 are combined gas chromatography/mass spectrometry techniques. Each class of compounds has its own analytical procedure, which includes sampling, storage, apparatus, purge and trap specifications (where applicable), extraction methods, and GC or GC/MS analysis specifications as well as quality control and data handling. Clearly, the analysis of Priority Pollutanta in this manner is labor intensive and costly and may be prone to error during the handling of samples for the different methods involved. A major problem associated with the analysis of trace organics is the lack of available techniques for such analyses due to the low concentration levels involved. Capillary gas chromatography is undoubtadly the best available method for such studies. However, due to limitations in sample size that can be introduced into bonded-phase capillary columns, it is necessary to carry out preconcentration of organics prior to analysis. Many different methods have been employed in order to concentrate trace organics from aqueous solutions. They include liquid-liquid extraction with or without further concentration, microextraction, headspace analysis, purge and trap analysis, adsorption of trace organics in aqueous samples on solid adsorbents such as carbon black, ion-exchange resins, macroreticular porous polymers, zeolites, polyurethanes, bonded-phase sorbents similar to those used in high-pressure liquid chromatography and graphite fluoride, among other methods, as reviewed by Poole et al. (2). Subsequent to concentrating the trace organics, they are analyzed by solvent 0 1990 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 22, NOVEMBER 15, 1990
extraction or thermal desorption or by a combination of both procedures or closed-loop stripping (3). Yet another analytical method is where trace organics are preconcentrated on coated metal columns (4, 5) and fused silica capillary columns (6-8)prior to desorption and subsequent analysis. The liquid sample is pushed through the column, and the water is allowed to elute from the exit end of the column. It is found that the trace organics present in the aqueous sample are trapped by the column and remain behind. Along the Same principles, it has now been found that trace organics could be removed from an aqueous sample by passage through uncoated metal and plastic capillaries (9). Sample sizes as much as 100 mL or more may be sent through these columns. During passage of the sample through the capillary, the organics are removed by the column, presumably by some type of an adsorption mechanism. The extent of removal of organics may be determined by chromatographing the solution before and after elution through the capillary column. We have studied the extent of removal of organics from aqueous solutions in this manner by various types of plastic and metal capillary columns (also referred to as “trapping columns” hereafter). Further, the extent of such removal has been studied as a function of column length, internal diameter, internal surface area, the temperature a t which these trapping columns are operated, and the flow rate of aqueous solutions through them. Factors such as the solubility of organics and their polarities may play an important role in the removal of organics from the bulk aqueous phase, and compounds covering a wide range of solubilities and polarities were employed in this study. We have also attempted to determine whether the concentration of an organic solute and the ionic strength of the solution affect the extent by which the organics are removed from solution. After the organics are trapped, Le., concentrated inside these trapping capillaries, they can be desorbed quantitatively in order to analyze them. In this publication we wish to report on the factors that affect the removal of organics from aqueous solutions when they flow through uncoated trapping columns. Application of this concentration procedure in the development of methods for the analysis of trace organics in solution by solvent desorption and thermal desorption subsequent to their concentration have been presented in related publications (10, 11).
EXPERIMENTAL SECTION Materials. Several different stock solutions were utilized throughout this study. They are as follows: (i) 5OOO ppm benzene, toluene, and ethylbenzene in methanol; (ii) 5000 ppm chlorobenzene, 1,2-dichlorobenzene,and 1,3-dichlorobenzenein methanol; (iii) 5000 ppm l,l,l-trichloroethane, 1,1,2-trichloroethane, and tetrachloroethylene in methanol; (iv) 5000 ppm mesitylene, decane, and 1-undecene in methanol; (v) 5000 ppm mesitylene, decane, and octane in methanol; (vi) 5000 ppm 1-octanol, acetophenone, and nitrobenzene in methanol; (vii) 5000 ppm 4methyl-2-pentanone, cyclohexanone, benzaldehyde, and 2-butanone in methanol; (viii) 1% (w/w) pyridine, morpholine, 4methylpyridine, and 4-ethylpyridine in water; (ix) saturated solution of carbon tetrachloride in water; (x) saturated solution of chloroform in water; (xi) saturated solution of dichloromethane in water; (xii) saturated solution of 1,2-dichloroethane in water; (xiii) saturated solution of l,l,l-trichloroethane in water; (xiv) saturated solution of tetrachloroethylene in water. Aqueous solutions containing organics at the trace level were prepared by serial dilution of these solutions in deionized water obtained from a Milli-Q filtration system (Millipore, Bedford, MA). These different standard solutions have been used to demonstrate the applicability of this concentration method to a variety of substances covering a wide range of polarities, solubilities, and volatilities. Also, a number of the experiments reported were carried out with a test solution containing benzene, toluene, ethylbenzene, chlorobenzene, and 1,3-dichlorobenzene at the 1 ppm level in 2% methanol.
Apparatus. Three different gas chromatographic systems were used throughout this study. They are as follows: (i) HewlettPackard Model 5890A GC unit with a microprocessor-controlled integrator, Hewlett-Packard Model 3393A. The GC unit as supplied by the manufacturer was equipped with a split/splitless injector and a flame ionization detector. (ii) Hewlett-Packard Model 588OA GC unit with a microproceeaor-controlledintegrator, Hewlett-Packard Model 588OA Level 4. The GC unit as supplied by the manufacturer was equipped with with a split injector and a flame ionization detector. (iii) Varian Model 3700 GC unit with a Shimadzu bench type automatic balancing recorder, Model R 11. The GC unit as supplied by the manufacturer was equipped with a split injector, a flame ionization detector, and an electron capture detector. An additional electron capture detector was added in house. Analytical Columns and Conditions. Several different analytical columns and conditions were utilized throughout this project. They are as follows: (i) 12 m X 0.32 mm i.d. X 0.2 pm crosslinked methyl silicone (Hewlett-Packard Co., Avondale, PA), carrier gas (helium) flow rate = 3.0 mL/min, split ratio = 1O:l; (ii) 40 m x 0.32 mm i.d. X 5.0 pm crosslinked methyl silicone (Hewlett-Packard Co., Avondale, PA), carrier gas (helium) flow rate = 3.0 mL/min, split ratio = 1O:l; (iii) 30 m X 0.20 mm i.d. X 0.33 jtm crosslinked methyl silicone (Hewlett-Packard Co., Avondale, PA), carrier gas (helium) flow rate = 1.9 mL/min, split ratio = 1O:l; (iv) 30 m X 0.52 mm i.d. X 1.5 pm DB-5 (J & W Scientific, Rancho Cordova, CAI, carrier gas (helium) flow rate = 1 2 mL/min, split ratio = 10:l. For certain types of analysis (depending on the compounds being studied), these columns were operated isothermally, and at other times they were operated with temperature programming. Individual temperature program conditions qre listed where appropriate. The split injectors were operated at a temperature of 275 “C, and the flame ionization detectors were operated at 325 “C. Sample introduction into the gas chromatographic columns was made by using a 26-guage 10-pL Hamilton Model 701 microliter syringe (Hamilton Co., Reno, NV). Procedure. (a) Investigation of the Effect of Column Material of the Trapping Capillaries on the Efficiency of Trapping. (i) Efficiency of Removal of Organics by Plastic Trapping Capillaries, Initial studies on the removal of organics from aqueous solutions were carried out by using 30 m X 0.81 mm i.d. Teflon (Cole-Parmer Instrument Co., Chicago, IL), 15 m X 1.0 mm i.d. silicone, and 15 m X 1.6 mm i.d. polyethylene tubing (Bel-Art Products, Pequannock, NJ). Each tube was washed with 25-mL aliquots of deionized water, methanol, acetone, and dichloromethane solvents and dried under nitrogen for 1/2 h before use. The efficiency of removal of these columns was studied by using a wide range of compounds, as listed above. Solutions (1ppm) of these compounds in 2% methanol were used as test solutions. Methanol was added to solubilize the compounds efficiently as well as improve the removal of organics on trapping columns. It is reported that the adsorption of weakly adsorbing substances is strengthened by the addition of solvents such as methanol, ethanol, or acetone to the aqueous solutions (12). Such observations have been made with solutions containing much higher percentages of alcohol than have been used in this study, but it was felt that results obtained for solutions with the addition of methanol to obtain such concentations would not reflect the removal of organics from a “true” aqueous sample. Aliquots (100 mL) of these solutions were used for evaluation of the efficiency of removal of organics by the trapping capillaries. The test solutions were introduced into the trapping capillaries by means of an apparatus described by Zlatkis et al. (9). The efficiency of removal of organics was calculated from the differences in area counts for each compound as reported by the GC integrator for solutions before and after passage through trapping capillaries. Some typical results obtained on the removal of organics by these columns are shown in Table I. (ii) Efficiency of Removal of Organics by Metal Trapping Capillaries. Initial studies on the removal of organics from aqueous solutions were carried out by using 30 m X 1.0 mm i.d. stainless steel, 30 m x 0.51 mm i.d. copper (SmallTube Products, Altoona, CA), and 15 m X 2.0 mm i.d. aluminum tubing. Each
ANALYTICAL CHEMISTRY, VOL. 62, NO. 22, NOVEMBER 15, 1990
on the Efficiency of Trapping on Teflon Columns. A 10-mL aliquot of a solution of benzene, toluene, and ethylbenzene in 2% methanol was sent through 15 m X 1.1mm i.d. and 30 m X 1.1 mm i.d. Teflon columns under identical conditions. Injections (1 pL) of the eluted solutions and the standard solution before introduction into the trapping capillaries were made into a 30 m X 0.52 mm i.d. X 1.5 pm DB-5 column operated isothermally at 80 "C. The results on the removal of organics are summarized in Table 111. (ii) The Effect of Column Length on the Efficiency of Trapping on Stainless Steel Columns. A 10-mL aliquot of a 1 ppm solution containing benzene, toluene, ethylbenzene, chlorobenzene, 1,2-dichlorobenzene, mesitylene, decane, and 1-undecene in 2% methanol was sent through 15 m X 1.0 mm i.d. and 30 m x 1.0 mm i.d. stainless steel columns under identical conditions. Injections (1 pL) of the eluted solutions and the standard solution before introduction into the trapping capillaries were made into the same analytical column above operated at an initial temperature of 80 OC (3 min) and temperature-programmed at 5 OC/min to a final temperature of 115 "C. (c) Investigation of the Effect of Column Internal Diameter of the Trapping Capillary on the Efficiency of Trapping. Ten-milliliter aliquots of a 1 ppm solution containing benzene, toluene, ethylbenzene, chlorobenzene, and 1,2-dichlorobenzene in 2% methanol were sent through 15 m X 0.31 mm i.d., 15 m X 0.56 mm i.d., 15 m X 0.81 mm i.d. and 15 m X 1.1mm i.d. Teflon capillaries under identical conditions. Injections (1 pL) of the eluted solutions and the standard solution were analyzed as before, and the results obtained are shown in Table IV. (d) Investigation of the Effect of Surface Area on the Efficiency of Trapping. Two strategies were adopted to study the influence of surface area on the removal of organics from aqueous solutions when they flow through trapping capillaries: (1)etching the inner walls of Teflon trapping capillaries prior to passage of standard solutions containing trace organics and then comparing these results with results obtained for unetched capillaries having similar lengths and (2) oxidizing the inner walls of copper trapping columns prior to passage of standard solutions containing trace organics and then comparing these results with results obtained from regular (unoxidized) copper capillaries having similar lengths. An etching reagent was made in the laboratory by dissolving an equimolar mixture of metallic sodium and naphthalene in tetrahydrofuran (all reagents from J. T. Baker Chemical Co., Phillipsburg, NJ) and bubbling the solution with purified nitrogen (13). The columns were activated by filling them with the reagent and leaving them for 12 h. Thereafter the columns were emptied of the etchant, dried under nitrogen for 15 min, rinsed with 100 mL of acetone, and dried again with nitrogen for 1/2 h. It is reported that the surface area increases dramatically to as high as 4000 m2/g by etching Teflon in this manner (14). In order to oxidize the inner walls of a copper capillary, a 30 m X 0.51 mm i.d. copper column was treated with 25 mL of a 40% nitric acid solution, rinsed with 100 mL of distilled water, 25 mL of acetone, and then dried under nitrogen for 1/2 h. Next, oxidation was performed by flushing dry oxygen (22 mL/min) through the column for 7 h at a temperature of 250 "C. It is reported that a capacity 4-32 times that of untreated tubes is obtained in this way (15). (i) Trapping Efficiencies on Etched and Unetched Teflon Capillaries. Ten-milliliter aliquots of standard solutions containing benzene, toluene, and ethylbenzene at the 5 ppm level in 2% methanol were sent through the etched and unetched
Table I. Efficiency of Removal of Organics at the 1 ppm Level on Plastic Trapping Capillariesa % removal on a 15 m X
?& removal on a 15 m X
1.0 mm i.d. silicone capillary
1.6 mm i.d. polyethylene capillary
100 100 100 100
100 100 100 100
100
100
100
100
35 100
99 50
19
25
benzene toluene ethylbenzene chlorobenzene 1,2-dichlorobenzene 1,3-chlorobenzene 2- butanone 4-methyl-2pentanone cyclohexanone
a Organics were present at the 1 ppm level in the aqueous sample containing 2% methanol. Numbers are the percentages of each component removed during a single passage through the capillary.
Table 11. Efficiency of Removal of Organics at the 1 ppm Level on Metal Trapping Capillaries % removal on a 30 m X
% removal on a 30 m X
0.51 mm i.d. copper
capillary
1.0 mm i.d. stainless steel capillary
52 55 61
64 64 69
51
59
61
76
40
82
74 89 81
95 86 80
benzene toluene ethylbenzene chlorobenzene 1,2-dichlorobenzene 1,3-dichlorobenzene mesitylene decane 1-undecene
Table 111. Efficiency of Trapping on Teflon Columns of Different Lengths % removal on a
% removal on a
15 m X 1.1 mm i.d. Teflon capillary
benzene toluene ethylbenzene
30 m X 1.1 mm i.d. Teflon capillary
56 76 77
65 80 85
of these tubes was washed in the same manner as previously described. Studies on the removal of organics on these tubes were also conducted in the same manner as described earlier. Some results obtained on the removal of organics by the copper and stainless steel columns are shown in Table 11. (b) Investigation of the Effect of Column Length on the Efficiency of Trapping. (i) The Effect of Column Length
Table IV. Efficiency of Trapping on Teflon Capillaries of Different Internal Diameter
benzene toluene ethylbenzene chlorobenzene 1,2-dichlorobenzene
% removal on a 15 m X 0.31 mm i.d. Teflon capillary
% removal on a 15 m X 0.56 mm i.d. Teflon capillary
70removal on a 15 m X 0.81 mm i.d. Teflon capillary
% removal on a 15 m X 1.1 mm i.d. Teflon capillary
60
66
72
81
80
86 77
89 77
69 79
80
80
67 74 67 61
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90
78 81
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 22, NOVEMBER 15. 1990
Table V. Efficiency of Trapping of Organics on a Teflon Column at Different Temperatures % removal
benzene toluene ethylbenzene chlorobenzene 1,2-dichlorobenzene
%
removal
%
removal
at 0 "C
at 25 "C
at 70 "C
59
66 78 88 78 84
88 94 97 94 96
64 70 63
62
Table VI. Efficiency of Trapping on a Copper Column at Different Temperatures ~~
removal at 0 "C
90 removal at 27 "C
%
benzene ethylbenzene chlorobenzene 1,2-dichlorobenzene mesitylene decane
65 66 70 59 54 70 44
1-undecene
65
to1u en e
50 52 57
52 49 67 43 62
Teflon trapping columns under identical conditions, and the eluted solutions and the standard solution were analyzed by GC. (ii) Trapping Efficiencies on Oxidized and Regular (Unoxidized) Copper Trapping Capillaries. Ten-milliliter aliquots of a standard solution containing 1-octanol,acetophenone, and nitrobenzene at the 5 ppm level in 2% methanol were sent through the oxidized and regular copper trapping columns under identical conditions, and the eluted solutions and the standard solution were analyzed under identical conditions. (e) Investigation of the Effect of Solute Concentration on the Efficiency of Trapping. (i) Removal of Organics Present in Different Concentrations in Aqueous Solutions on Teflon Trapping Columns. Ten-milliliter aliquots of standard solutions containing benzene, toluene, and ethylbenzene at the 1, 5, 25, and 100 ppm levels in 2% methanol were sent through a 15 m X 1.1mm i.d. Teflon capillary under identical conditions, and the trapping efficiencies were calculated as before. (ii) Removal of Organics Present in Different Concentrations in Aqueous Solutions on Copper Trapping Columns. Ten-milliliter aliquots of standard solutions containing chlorobenzene and 1,2-dichlorobenzene at the 2, 3, and 6 ppm levels in 2% methanol were sent through a 30 m X 0.51 mm i.d. copper capillary and analyzed under identical conditions. (f) Investigation of the Effect of Temperature on the Efficiency of Trapping. (i)The Effect of Temperature on the Efficiency of Removal of Organics on Teflon Trapping Columns. The trapping efficiency of Teflon trapping columns at different temperatures was studied by using benzene, tduene, ethylbenzene, chlorobenzene, and 1,2-dichlorobenzene at the 3 ppm level in a 2 % methanol solution. Ten-milliliter aliquots of this solution were sent through a 15 m X 1.1 mm i.d. Teflon trapping column operated at 0, 27, and 70 "C while all other operating parameters were kept the same. Aliquots (1pL) of the eluted solutions and the corresponding standard solution were
analyzed, and the results on the removal of organics at these temperatures are summarized in Table V. (ii) The Effect of Temperature on the Efficiency of Removal of Organics on Copper Trapping Columns. The trapping efficiency of copper columns at different temperatures was studied by using benzene, toluene, ethylbenzene, chlorobenzene, 1,2-dichlorobemne, mesitylene, decane, and 1-undewne at the 2 ppm level in a 2% methanol solution. Ten-milliliter aliquots of this solution were sent through a 30 m X 0.51 mm i.d. copper trapping column operated at 0 and 27 "C while all other operating parameters were kept the same. 4-Chlorotoluene was added to 5-mL portions of the eluted solutions and the standard solution as an external standard such that it would be at the 5 ppm level. The results obtained on the removal of organics at these temperatures are summarized in Table VI. (g) Investigation of the Effect of Ionic Strength of a n Aqueous Solution on the Efficiency of Trapping. (i) The Effect of Ionic Strength of a n Aqueous Solution on the Efficiency of Removal of Organics on Teflon Trapping Columns. A solution containing benzene, toluene, and ethylbenzene (volatileand moderately soluble organics) at the 1ppm level in water was evaluated with respect to trapping on a 30 m x 1.1mm i.d. Teflon column with and without the solution being "salted out". Salting out was done by the addition of 1.5 g of anhydrous sodium sulfate to a 10-mL portion of the solution and then rapidly shaking the volumetric flask containing the solution such that the electrolyte dissolved within 1 min of addition. Thereafter the solution WEB immediately sent through the trapping column. The results obtained for the efficiency of removal of organics were compared with similar experiments carried out without the solution being salted out. Additionally, a 30 m X 0.81 mm i.d. Teflon trapping column was evaluated in the exact same manner using a 5 ppm solution of 1-octanol,acetophenone, and nitrobenzene (nonvolatile and fairly soluble organics). (ii) The Effect of Ionic Strength of an Aqueous Solution on the Efficiency of Removal of Organics on Copper Trapping Columns. A solution of 1-octanol, acetophenone, and nitrobenzene at the 5 ppm level in water was evaluated with respect to trapping on a 15 m X 0.51 mm i.d. copper column and a 30 m X 1.0 mm i.d. stainless steel column with and without the solution being salted out. (h) Investigation of the Effect of Flow Rate of Aqueous Solutions through Trapping Columns on the Efficiency of Trapping. (i) The Effect of Flow Rate of Aqueous Solutions through Teflon Trapping Capillaries on the Efficiency of Trapping. Ten-milliliter aliquots of a solution containing benzene, toluene, ethylbenzene, chlorobenzene, 1,2-dichlorobenzene, mesitylene, decane, and 1-undecene at the 1ppm level in 2% methanol were made to flow through a 15 m X 1.1mm i.d. Teflon capillary at flow rates of 0.51, 1.1,2.6, and 4.9 mL/min, and the results obtained on the removal of organics at these different flow rates are summarized in Table VII. (ii) The Effect of Flow Rate of Aqueous Solutions through Copper Trapping Capillaries on the Efficiency of Trapping. Ten-milliliter aliquots of the same solution used above were made to flow through a 30 m X 0.51 mm i.d. copper trapping column at flow rates of 0.41 and 0.99 mL/min, and the trapping efficiencies at the different flow rates were calculated as before. (i) Removal of Organics in Various Fractions of the Aqueous Solution. (i) Trapping Profile on Plastic Trapping
Table VII. Efficiency of Removal of Organics from Aqueous Solutions Flowing at Different Flow Rates through a 15 m X 1.1 m m i.d. Teflon Capillary % removal at a flow rate of 0.51
mL/min
benzene toluene ethylbenzene chlorobenzene 1,2-dichlorobenzene mesitylene
decane 1-undecene
73 85 94 86 90 97 92 93
%
removal at a
%
removal at a
%
removal at a
flow rate of 1.1
flow rate of 2.6
flow rate of 4.9
65 76 86 74 78 92 80 82
61 69 77 70 70 84 72 71
44 60 69 58
mL/min
mL/min
mL/min
60 77 70
67
ANALYTICAL CHEMISTRY, VOL. 62, NO. 22, NOVEMBER 15, 1990 1
2
1
1 3
2
4 3
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5
M
1’”
ri
1
1
2 2
4 3
4 3
5
5
F Chromatograms obtalned for a 1 ppm solution of benzene ( l ) , toluene (21, ethylbenzene (3), chlorobenzene (4), and 1,2dlchlorobenzene (5) In 2% methanol (a, top) before and (b, bottom) after elution of 100 mL of the solutbn through a 30 m X 0.81 mm i.d. Teflon Figure 1.
capillary. Capillaries. A 100-mL aliquot of a solution containing benzene, toluene, ethylbenzene, chlorobenzene, and 1,2-dichlorobenzene at the 1 ppm level in 2% methanol was sent through a 15 m X 1.1mm i.d. Teflon trapping capillary, and the eluted solution was collected in fractions of 10 mL. The results obtained on the removal of organics in each of the fractions were plotted in graphical form. A 100-mL aliquot of a solution containing benzene, toluene, and ethylbenzene at the 1% (w/w) level in methanol was sent through a 15 m X 1.0 mm i.d. silicone trapping column, and the eluted solution was collected in fractions of 10 mL. Aliquots (1 pL) of the standard solution and the fractions so collected were analyzed as before, and the results were plotted in graphical form. (ii) Trapping Profile on Copper Trapping Columns. A 100-mL aliquot of a solution containing benzene, toluene, ethylbenzene, chlorobenzene, and 1,2-dichlorobenzene at the 1 ppm level in 2% methanol was sent through a 30 m X 0.51 mm i.d. copper trapping column, and the eluted solution was collected in fractions of 10 mL and analyzed.
RESULTS AND DISCUSSION (a) The Effect of Column Material on the Efficiency of Trapping. Figure la is a chromatogram for a standard solution containing benzene, toluene, ethylbenzene, chlorobenzene, and 1,2-dichlorobenzene in 2% methanol, and Figure l b is a chromatogram obtained after passage of 100 mL of this solution through a 30 m X 0.81 mm i.d. Teflon trapping column. Parts a and b of Figure 2 are representative chromatograms obtained before and after elution of the above standard solution through a 30 m X 0.51 mm i.d. copper
Figure 2. Chromatograms obtained for a 1 ppm solutlon of benzene (I), toluene (2), ethylbenzene (3), chlorobenzene (4), and 1,2dlchlorobenzene(5) in 2% methanol (a, top) before and (b, bottom) after elution of 100 mL of the solutlon through a 30 m X 0.51 mm i.d.
copper capillary.
I
L
z
P
$ P
5
Flgure 3. Chromatogam obtained for water (blank) (a, top) before and (b, bottom) after elution of 10 mL through a 15 m X 1.1 mm 1.d. Teflon
capillary.
trapping capillary. Parts a and b of Figure 3 are chromatograms obtained for a blank of water obtained from the MiUi-Q filtration system before and after elution through the same Teflon capillary above. From Table I it can be noted that polyethylene and silicone columns are capable of removing many trace organics to a great degree; under the right conditions, it is possible to quantitatively remove almost all organics utilizing silicone trapping columns, including some which are infinitely soluble in water. In one particular study carried out with the passage of 10 mL of a 1 ppm solution of pyridine, morpholine, 4-methylpyridine, and 4-ethylpyridine in water through the silicone trapping capillary at a flow rate of 0.40 mL/min, it was found that all four solutes were removed completely from solution. However, due to difficulties associated with desorption of the trapped organics from silicone tubing as mentioned in related publications (IO,II),
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much of the data presented here for plastic type materials are for those from which it is possible to obtain quantitative desorption (e.g., Teflon). The reproducibility of the trapping efficiency was checked by repeated pasage of 10-mL portions of a 1ppm solution of benzene, toluene, and ethylbenzene in 2% methanol through a 30 m X 1.1 mm i.d. Teflon capillary under identical conditions (e.g., flow rate, trapping temperature), and under the conditions used, the mean percent removal was found to be 77%,82%, and 98%, respectively, for the three compounds with a relative standard deviation of *6.4%, f4.270, and f2.870. When trapping efficiencies were calculated, the results reported by the GC integrator for each test compound before and after introduction of the test solutions into trapping capillaries were statistically treated by applying the Q test (16) at the 90% confidence level. (b) The Effect of Column Length on the Efficiency of Trapping. The results obtained point out that the trapping processes on Teflon and stainless steel are different from each other. In general, the longer the trapping column, the greater is the removal of organics from aqueous solutions. Conceivably, if a trapping column of sufficient length is used coupled with other conditions such as low flow rates of solution through them (as discussed later), it would be possible to trap organics completely on the walls of a trapping column. However, this would involve long trapping times, making it impractical toward developing a routine analytical scheme. Another general trend that can be seen is that as the solubility of an organic species decreases, it is removed to a greater extent. There are, however, exceptions to this trend, and thus, there are factors besides the solubility of compounds in water which determine their removal. (c) The Effect of Column Internal Diameter on the Efficiency of Trapping. As the results shown in Table IV indicate, trapping efficiencies on Teflon columns of different diameters are not substantially different from each other although the extent of removal of organics on tubing of 0.31 mm i.d. is lower than on any of the other tubes. This phenomenon cannot be due to differences in internal surface meas since if it were the case, it should be possible to observe similar differences in terms of trapping efficiencies among other trapping capillaries as well. Therefore, (1) the volume of solution/surface area may be an important factor in terms of trapping, and (2) there may be a certain “capillary effect“-the manner in which a solution flows through a capillary-that influences the removal of organics contained in the solution. To verify the latter, a simple experiment was performed. A 30 m x 0.81 mm i.d. Teflon capillary has an internal surface area of approximately 0.075 m2. Fluoropak 80, a fluorocarbon support used in chromatography, and a material much like Teflon, has a specific surface area of 1.3 m2/g (1 7). Thus, approximately 60 mg of Fluorpak 80 has a surface area of 0.075 m2/g. A small glass tube was packed with 60 mg of Fluoropak 80 having a mesh size greater than 60. Next, 10 mL of an aqueous solution containing benzene, toluene, and ethylbenzene at the 1ppm level was sent through this adsorbent tube. No external pressure was required for the solution to flow through. It was found that 39% of benzene, 43% of toluene, and 47% of ethylbenzene was removed in this way. In comparison, when 10 mL of the same solution was sent through a 30 m x 0.81 mm i.d. Teflon trapping column a t the same flow rate and with the other trapping conditions being identical in the two cases, the removal was 61%, 70%,and 80%, respectively, clearly indicating that there indeed exists such a “capillary effect”, which contributes to the mechanism by which the organics are retained by the trapping capillaries.
Another point of interest which may be noted is that the removal of these solutes keeps increasing in the order ethylbenzene > toluene > benzene when the solution is sent through the trapping capillary and the powdered adsorbent material. These results can be explained by considering the solubilities of the test compounds and the velocity profile of the solution within the trapping column. (d) The Effect of Column Internal Surface Area on the Efficiency of Trapping. The results indicate that etching of Teflon capillaries has had no significant effect on the removal of aromatics from the aqueous solution. However, based on the observations made, the role that oxidation of copper plays on the extent of removal of organics from solution is not immediately clear. (e) The Effect of Solute Concentration on the Efficiency of Trapping. On the basis of the results obtained, it can be concluded that the extent by which benzene, toluene, and ethylenebenzene are removed by Teflon trapping columns is more or less constant within the concentration range investigated. A similar observation is made for the copper trapping capillary as well. From these results, it would seem that an equilibrium of the solute molecules in the aqueous phase and on the walls of the trapping column exists. In this regard, one possible explanation for the independent behavior of the trapping process on solute concentration over the range investigated may be the absence of interactions between those molecules that are already adsorbed on the surface of a trapping capillary. If there were such (lateral) interactions, then the amounts trapped should gradually decrease with increasing solute concentration due to repulsive forces between adsorbed molecules. (f) The Effect of Trapping Temperature on the Efficiency of Trapping. The trapping temperature plays a significant role in the removal of organics by Teflon capillaries. Trapping efficiencies are exclusively higher at higher trapping temperatures. This phenomenon may be explained by the fact that plastic materials soften at higher temperatures. In such instances, the surfaces of plastics may become amenable to hold organics better. In copper capillaries, in contrast, the amounts removed at higher trapping temperatures are lower than at lower trapping temperatures. This observation may be due to a combination of two factors: the changes occurring on the copper surface are insignificant within the temperature range used, and increased solubility of organics at higher trapping temperatures contribute to a greater retention of organics in the aqueous phase. (g) The Effect of Ionic Strength of the Aqueous Solution on the Efficiency of Trapping. The results obtained suggest that salting out the aqueous solution has indeed assisted the Teflon trapping column in the removal of both volatile and nonvolatile organics that have low to moderate solubilities in water. On the other hand, it is found that those organics that are extremely soluble in water (e.g., pyridine and substituted pyridines) are not removed by the capillaries over a wide concentration range (5 ppm-0.1% v/v) irrespective of whether the solutions are salted out or not. With regard to the two metal capillaries investigated, it seems that salting out has no significant effect on the removal of organics. Additionally, the effect of temperature on the removal of organics from salted out solutions by copper trapping columns was checked by the use of a 1 ppm solution of mesitylene, decane, and 1-undecene in 2% methhol. Aliquots (10 mL) of this solution were salted out as described before and sent through a 30 m X 0.51 mm i.d. copper capillary operated a t 25 and 70 “C.The trapping efficiencies calculated indicate that the removal of organics from salted out solutions does
ANALYTICAL CHEMISTRY, VOL. 62, NO. 22, NOVEMBER 15, 1990
not depend on the trapping temperature. The differences in trapping observed with and without the solution being salted out would seem to indicate that the surface of the copper capillary is modified by the ionic character of the solution, which results in changes in selectivity of the surface for some organics over others. (h) The Effect of Flow Rate on the Efficiency of Trapping. The results presented clearly indicate that the efficiency of trapping is greatly dependent on the flow rate of aqueous solutions through trapping capillaries, be they plastic or metal. The lower the flow rate, i.e., the greater the residence time of the solution inside trapping capillaries, the better the removal of trace organics-probably due to the fact that the increased residence time enhances the possibility of organics coming into contact with the trapping surface. Furthermore, slower flow rates would produce only minor differences in flow rates of individual channels inside the trapping capillary, and therefore it may be possible for even comparatively lesser soluble substances such as ethylbenzene to distribute fairly evenly within the bulk of the solution than to be confined to the inner channels of liquid closer to the center of trapping capillaries. There is also the possibility that less turbulant flows are created so that the displacement of molecules already trapped is decreased. For the application of this method to an analytical scheme for the analysis of organics in aqueous solutions, it is important that the trapping time be minimized so that the overall analysis time is decreased. Therefore, for rapid concentration of organics from aqueous solutions, fairly high flow rates should be used. In one particular study conducted with the passage of 100 mL of a solution containing trichloroethylene and tetrachloroethylene at the 10 ppm level in 0.2% methanol at a flow rate of 21 mL/min through a 15 m X 1.6 mm i.d. Tygon trapping capillary, it was found that 91% of the trichloroethylene and 95% of the tetrachloroethylene was removed by the trapping column. Upon passage of 1 L of the same solution at a flow rate of 20 mL/min through the same trapping column, it was observed that 84% of the trichloroethylene and 95% of the tetrachloroethylene was removed from the solution, indicating that it is indeed possible to use high flow rates for trapping purposes without significantly compromising the trapping efficiencies. (i) Removal of Organics in the Various Fractions. The results obtained on the removal of organics in various fractions on both plastic and metal capillaries are indeed quite intriguing and surprising. It could be expected that the removal of organics would be the highest in the first fraction, because when the solution first comes in to contact with the trapping surface, none of the adsorption sites are occupied and therefore the receptiveness of the surface to adsorbent molecules will be high. With increasing coverage of these sites, the removal of organics from later eluting fractions would gradually decrease if monolayer formation is the predominant form of removal of organics. Furthermore, the removal of organics depends greatly on the flow rate of the aqueous solution through the trapping column as pointed out before. When a solution flows through a capillary, the first few (i.e., 1-3) 10-mL fractions take different times to elute, meaning that it takes some time before the flow stabilizes. The next few 10-mL fractions (4-7) take more or less the same time to elute, while the final few 10-mL fractions (8-10) have different flow rates from each other. Thus, it may be expected that the removal of a particular organic species may show some variation in the first few and last few 10-mL fractions but remain fairly constant within the middle fractions. Another possibility to be considered is the formation of multilayers of adsorbed material. The actual patterns of removal of organics may be due to a combination of some or all these possibilities.
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An exact mechanism controlling the retention process is difficult to be formulated due to many reasons. Adsorption of organics may take place by physisorption, chemisorption, and/or by electrostatic interaction processes. On materials such as polyethylene, we probably have physical adsorption processes taking place, as demonstrated by the ability to desorb the trapped organics by using solvents (11). On Teflon columns, the process of adsorption may be either physical or chemical in nature, since thermal desorption of trapped organics is possible at fairly low desorbing temperatures (e.g., 80 “ C ) (IO). On silicone capillaries, the processes occurring are both adsorption and absorption, since (a) very low desorbing efficiencies are obtained with higher desorption temperatures (e.g., 150 “C) and (b) it is possible to actually feel the silicone capillary swell and to see the desorbing solvent disappear inside the column during solvent desorption. On metal capillaries, on the other hand, we have irreversible adsorption or some other form of loss of the organics, since they cannot be quantitatively desorbed by either solvent or thermal desorption. We also have evidence of a certain “capillary effect”, which contributes to the retention of organics in our system, as described earlier. It is difficult to present an absolute scale for the retention efficiency for test materials since it is a function of a great many variables such as the column material, the length of the column, column internal diameter, the flow rate of solutions through the columns, the trapping temperature, the ionic strength of the solution, etc. However, on a particular trapping column, keeping all other conditions the same, the retention efficiency generally increases with decreasing polarity and solubility and increasing molecular weight of compounds, i.e, with increasing hydrophobicity. Many theoretical approaches have been used to describe the adsorption phenomena observed on solid adsorbents. Prominent among them are the Polyani Adsorption Potential model, Solvophobic Interaction model, Langmuir Adsorption model, and the Vacancy Solution theory, as reviewed by Derylo-Marczewska, et al. (18). A large number of variations to each of these models have been proposed by a number of authors such that a dozen or more extensions of these models exist at present. Many of these models are based on adsorption data obtained from static systems as opposed to a dynamic (flow) system used in the present study and by using single-solutesolutions. As such, it is quite difficult to correlate adsorption phenomena observed by other authors with the present study. This novel concentration method offers some significant advantages over other methods currently available for the analysis of trace organics in solution. One is that there is no restriction of sample size that can be used in the concentration system. Sample volumes as large as 4 L have been used, and there is no reason why even larger volumes cannot be used. Consequently, the detection limits for compounds can be extended to enable the analysis of organics at the parts per trillion level or below using common gas chromatographic detectors. A second advantage is that the analysis time is significantly reduced. Allowing, for example, 20 min for the concentration step and 20 min for the desorption step (IO, I I ) , the total analysis time would be 40 min + GC analysis time, which is a substantial decrease in terms of time compared to, for instance, batchwise liquid-liquid extraction methods. Thirdly, since the sample is not handled, uQlike in other methods currently in use, errors caused due to sample handling are minimized. Further, since steps such as solvent reduction are avoided ( I I ) , sample losses are minimized as well.
CONCLUSIONS Removal of trace organics from aqueous solutions by their passage through various kinds of capillary columns was
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evaluated. It was determined that plastic capillaries, and silicone tubing in particular,were the best in terms of trapping (i.e. concentration) efficiencies, although metal capillaries are capable of concentrating trace Organics in substantial amounts as well. Many factors that affect the removal of organics from aqueous solutions were studied in detail. In general, the lower the solubility and lesser the polarity of a molecule, the better it is removed on a trapping capillary. One of the most important factors that affects the removal of organics was found to be the flow rate of a solution through a trapping column. The lower the flow rate, the greater is the removal of organics from solution. In addition, the temperature of trapping is quite significant for plastic capillaries, but not as much for metal capillaries. At higher temperatures, organics are removed more efficiently by plastic type capillaries.
Mecky, D. A. M.; Hussein, M. M. J . clwametq~.t979, 178, 291. Mackey. D. A. M.; liwetn. M. M. J . .IN2,243,43. ZkAkls, A.; Wang, F.-S.; Shentkld, H. Anal. 0".I-, 54, 2406. Zlatkls,A.; Wang. F.-S.; ShanRekl, H. Anal. Chem. 1W, 55, 1840. Rwaade, J.; Bbmberg, S. I n PIoCeedhgs of Ihe Mth Intemnfiiml Symposim on CapMety Chromatography; Sandra, P., Ed.; HutMg: Heidelberg, 1988; p 614.
Zlatkls, A.; WeiSner, S.; Ghaoul. L. Chromto@aptfi~ 1986, 21, 19. Zlatkls, A.; Ranatunga, R. P. J.; M w c h , E. S. chromerogrephhr 1990, 29, 523. Zlatkis, A.; Ranatunga, R. P. J.; Middleditch. B. S. chrOmet~8phi8 1990, 30. 149. Radeke, K. H.; Schmldt, R. East German Patent 252,128. 1987. 1901, 14, 19. Bomblck, D.; Dinunzb, J. C h Jansta, J.; Doussk..d. P.; Riha. d - J . Aopl,polymer Sci. 1975, 19. 3201. Tentzsch, 0.;Hovermann, W. J . Chromatogr. 1063, 1 1 , 440. Dsan, R. 8.; Dlxm, W. J. Anel. Ghem. -ISH, 23, 636. Pode, C. F.; Schuette, S. A. Contempomry Practice of C h m t o g m phy; Elsevier: Amsterdam, 1985 p 59. Derylo-Marczewska, A.; Jaronice, M. I n Swfece and ColkM Sdence; Matijevlc, E., Ed.; 1987; Vol. 14, p 301.
LITERATURE CITED (1) Fed. Regkit. 1979, 44, 69464. (2) Poob, C. F.; Schwne, S. A. J . H@h Resolift. Chromatcgr. chrometogr. Cantnun. 1983, 8, 526. (3) &ob, K. J . Chrome?togr.1973, 84, 255.
RE"
for review April 11,1990. Accepted August 28,1990.
This work was supported in part by the Gulf Coast Hazardom Substance Research Center.
Electrophoretic Separations of Proteins in Capillaries with Hydrolytically Stable Surface Structures Kelly A. Cobb, Vladislav Dolnik,' a n d Milos Novotny*
Department of Chemistry, Indiana University, Bloomington, Indiana 47405
A procedure for owdnlng rJohly stable coated capRlark for useInorpWorydectrsphomk(CE)k-. Rerrctkn
creasing number of applications, the separation of proteins has often been emphasized (1-7). According to the theory of o f m ~ t r l m l d l c s ~ w l t h t h e Q l o M l d CE (81, the low diffusion coefficients of biological macromolecules, such as proteins, should lead to separation effireagent, "rgAerLmbrom#.,Mowdby r.roskn ofthe ciencies on the order of 106theoretical plates. This is deairable vlnyl groq with acrylrmld., remtts In an lnwmblkod layer for both the resolution of complex protein mixtures and their ofPab--tlroughhrclrdlrtlcdly-= isolation a t nanoscale levels. bonds. Thk m.thod is an ext.ndon of the capMary coating Practitioners of CE have realized that proteins present procedure dercrtbed previously by Hjertm, dlffdng In the unique challenges to the separation method, due to their means by wMch the polyacrylamkle layer Is bonded to the inherent tendency to adsorb to the inner walls of fused silica capMary walk. CapUhks treated In the manner described capillaries. Such adsorption leads to considerable peak here can be used over a pH range of 2-10.5, wHhout nobroadening and asymmetry, making it difficult to attain the tlceabk ckompmRion of the coathg. I n comparlron lo unimpressive efficiencies predicted by theory. Several efforts coated ,cseparatkr#, of protdnr udng wch coated have thus been made to prevent adsorption and improve caplllarles are Improved due to a reductlon In proteln adprotein separation capabilities. The strategies undertaken sorption to the capillary walls, although lnterac#on Is sUll can be grouped into two main categories: (1) chemically present tomnedagreeas4MMencd by an inrrblllyto owdn bonding a neutral material to the inner walls of fused silica plate counts as Mgh as those predkted by theory. Eleccapillaries to eliminate surface charge and adsorption sites; t r o e m d c f i o w l e emhated ~ htha coaed cpplarkr, and (2) manipulation of buffer pH and ionic strength to caw rewlUng In Improved reproduclbHltles of proteln migration the proteins and capillary walls to experience Coulombic repulsion. In the former case, various materials have been tlmes In comporkon to uncoated caplllarles. AdcNtlonaUy, peak skew le evaluated for m0d.l proteh and lmprovef#wnb reported as fairly successful in reducing protein adsorption and enhancing separation efficiencies. Jorgenson and Lukacs are noted lor the coated c . R.rultrarepresented (1)bonded glycol-containing materials to fused silica; Hjerten for separatknc of modd prot.kr mkturos, comparing the (9) reported the use of methylcellulose and non-cross-linked pwb"of th. vinyl-bownlpdyacryIamW coatod cappolyacrylamide bonded through an organosilane reagent; Illarks and unceatd captllarles at both hlgh and bw pH exMcCormick (3) has employed a poly(vinylpyrro1idinone) tremes. coating, as well as organosilane derivatization; Bruin et al. (7) also coated capillaries with poly(ethy1ene glycol). In the category of buffer manipulation, low pH phosphate buffers INTRODUCTION have been used by McCormick (3) for the purpose of reducing During the past several years, capillary electrophoresis (CE) the negative charge of fused silica as well as inducing some has experienced a significant growth. Among its rapidly inprotective screening of the silica surface by phosphate groups. Lauer and McManigill (2), followed by Walbroehl and Jor] Current address: Institute of Analytical Chemistry, Czechoalovak Academy of Sciences, Brno, Czechoslovakia. genson (IO), have employed high pH buffers with added ionic 0003-2700/90/0362-2478$02.50/0
@ 1990 American Chemlcal Society
