2048
Anal. Chem. 1988, 58,2048-2052
(12) Theng, 6 . K. G. Formetion and prspertles of Cky-Polvmer Complex8s; Elsevler: New York, 1979 p 362. (13) Stumm, W.; Morgan, J. J. Aquatic Chemistty; Wley-Interscience: New York, 1970; p 583.
RECEIVED for review January 21,1986. Accepted April 7,1986.
This is contribution no. ERLN-NO19 of the Pacific Division (Newport, OR) Of the Research Laboratory a t Narragansett, RI. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.
Evaluation of Graphitized Carbon Black Cartridges for Rapid Organic Trace Enrichment from Water: Application to Priority Pollutant Phenols Claudio Borra,' Antonio Di Corcia,* Marcello Marchetti, and Roberto Samperi Dipartimento di Chimica, Universitd "La Sapienza" di Roma, Piazza Aldo Moro, 00158 Roma, Italy
Trace enrlchment of the 11 prlorky pollutant phenM on a graphltked carbon black (We) cartrldge has been studled In order to evaluate the feadbUlty of tMs swbent In retalnlng very polar compotMdr from aqueous sokltlon. Optlmlzatlon studies of the eluant have been performed In order to recover quantltatlvely all phenols conskkd from the GCB surface. Recovery of phenols was unaffected by the sampllng rate even at a flow rate of 32 mL/mln. The extraction efficiency of the sodwnt wasassemed bysamplhg up to 4 L of dWiUed water. At such water volume sampled, only phenol and In part o-chkrophenol were lost. The mstrbc effect was studled by extractslg phenols rplked in water samples from dlfferent sources. The effklency of the Carbopack cartrdge In trapping phenols was compared wlth that of a C,@bomled phase column. When coupled wlth a HPLC method, samplng of water by the Carbopack cartrklge allows determlnation of phenoleto beperfomed wlth a Itm# ofdetectknof 4-40 parts per trllllon (pptr), except for phenol, wMch can be detected at the level of 250 pptr In surface water.
Up until some years ago, sample pretreatment for isolation and/or enrichment of organic compounds from aqueous solution was invariably done by liquid-liquid extraction. The recent introduction of adsorbents having elevated chemical inertness and reproducible chemicophysical characteristics has prompted searchers to adopt solid-phase extraction of organics from an aqueous matrix instead of liquid-liquid partitioning, especially when organic trace enrichment is necessary. In this case, the solvent extraction method requires large volumes of expensive, toxic, and flammable organic solvents. In addition, the solvent evaporation time is timeconsuming and bias may be introduced in the analysis due to solvent impurities or evaporative losses of volatile sample constituents. Among these adsorbing media of relatively recent introduction, chemically bonded phases for both off-line and on-line extraction (1-4) have received particular attention. Although promising results have been obtained, this class of sorbents has two drawbacks. One is that very polar compounds have small breakthrough volumes. The second limiting factor is 'Present address: Department of Chemistry, Indiana University, Bloomington, IN 47405.
that a relatively low sampling rate has to be used in order to avoid losses of analytes. Vulcan is a well-known example of graphitized carbon black (GCB)having a nonporous surface and being essentially free of chemical heterogeneities. This adsorbent has already been employed for enriching some organic compounds from water (5, 6) as well as for isolating analytes from biological fluids ( 7-9). The object of this work has been that of evaluating the ability of an experimental kit consisting of fine particles of Vulcan packed in plastic tubes in rapid and reliable enrichment from water of very polar compounds. For this purpose, we selected the 11priority pollutant phenols, as they comprise a large range of polarity and they are important environmental pollutants.
EXPERIMENTAL SECTION Apparatus. A 6 cm x 1cm i.d. cylindrical polypropylene tube was one-sixth full with 250 mg of Vulcan. The surface area of this adsorbing material was reported to be about 100 m2/g (10). The particle size range of the sorbent was between 100 and 60 pm. Polyethylene frits were located above and below the GCB bed to hold minute particles in place and keep the chromatographic column intact. All the materials cited above were kindly supplied by Supelco, Bellefonte, PA. The sample reservoir had a narrow opening at the bottom that fitted into the cartridge down to 3 cm of the top of the GCB bed. The extraction cartridge fitted directly into the vacuum manifold below. Vacuum was done by a water pump. No care was taken to ensure reproducible and constant vacuum applied to each water sample. Also, no unwelcome effect was observed if the Vulcan bed was casually allowed to go dry during an experiment. Standards. Analytical standards were from various commercial sources. Organic solvents were of analytical grade (Carlo Erba, Milano, Italy) and were used as supplied. The standard solution used to spike water samples was prepared by dissolving the 11 phenols in methanol at the individual concentration of 0.1 mg/mL. Tetramethylammoniumhydroxide (TMAOH)was purchased from Fluka (Bucks, Switzerland). Procedure. The GCB cartridge was cleaned by passing sequentially 3 mL of methylene chloride/methanol (70/30, by volume), 2 mL of methanol, and 5 mL of water acidified with HC1 (pH 2). This acidic pretreatment was necessary to eliminate some chemical heterogeneities on the GCB surface capable to give a pH value of water equal to 10.5 (IO).In this situation, low recovery of some phenols was observed. The water samples spiked with phenols were acidified with HCl (pH 3) and filtered when necessary. After the samples had passed through, 400 pL of methanol was percolated through the extraction column to eliminate water.
0003-2700/86/0358-2048$01.50/00 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 9, AUGUST 1986
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Table I. Recovery from Vulcan by Using Two Different Desorbing Mixtures % recovery
A CHzClZ/CHaOH (30:70, v/v) 60 m M
B: CHZClz/CH30H (7030, v/v) 60 mM
HCl
TMAOH
100
101 98 85 98 97 13 60 80 98 96 99
phenol p-nitrophenol o-chlorophenol 2,4-dinitrophenol o-nitrophenol 2,4-dimethylphenol 4-chloro-m-cresol 2,4-dichlorophenol 4,6-dinitro-o-cresol 2,4,64richlorophenol DentachloroDhenol
98 99 4 98 97 97
101 8
102 6
Thereafter, 3 mL of methylene chloride/methanol (30/70, by volume) acidified with HC1, 60 mmol/L, followed by 4 mL of methylene chloride/methanol(70/30, by volume) basified with TMAOH, 60 mmol/L, were passed to recover phenols. Solvent was removed under a nitrogen stream at 45 OC until a volume of about 100-200 pL was reached. The degree of alkalization is rather critical and must be cantrolled carefully in order to avoid loss of phenols during solvent evaporation. In fact, when the ambient was made more alkaline than that resulting above, that is about 9 mmol/L of TMAOH, some loss of 2,4-dinitrophenol and 4,6-dinitrceo-creaol d, likely due to decoplposition. Vice versa, when the solvent mixture was insufficiently alkalized, a drastical loss of phenol, o-chlorophenol, and o-nitrophenol was observed, likely due to evaporation. To the solution resulting above, 15 pL of methanol containing HC1,4 mol/L, and estrone, 0.5 mg/mL, used as internal standard, were added. Twenty microliters of this solution was injected into the HPLC apparatus. Instrumentation. A Series 3B liquid chromatograph (Perkin-Elmer Corp., Norwalk, CT)equipped with a Rheodyne Model 7125 injector having a 20-pL loop and with a Model 2050 UV detector (Varian Associates, Walnut Creek, CA) was used. A 4.6 mm X 25 cm column filled with 5 pm (average particle size) C18 reversed-phase packing and a guard column containing “Pelliguard”,both from Supelco,were used. The chromatographic elution of phenols was performed in a way similar to that reported elsewhere (11). Briefly, solvent A was acetonitrile and solvent B was water, both containing 1%(v/v) acetic acid. The flow rate was 1.5 mL/min. Just after injection, gradient elution was run from 30 to 85% acetonitrile at 3%/min. Except for pentachlorophenol,which was deteded at 254 nm, detection of phenols was done by setting the UV detector at 280 nm. In our case, however, reproducible retention times of phenols were achieved even by omitting a time of reequilibrium of the mobile phase between two chromatographicruns. Recovery data were d&ted by measuring the peak height of each phenol relative to that of estrone and comparing them with those of a standard solution prepared by dissolving a given amount of the starting standard solution into the solvent mixture used to desorb phenols from GCB and submitting this resulting solution to the evaporation step.
RESULTS AND DISCUSSION Nowadays, there is an increasing interest in detecting organics in water even at the parts-per-trillion level in order to develop theoretical models of long-distance transport of organic compounds by water or to develop accurate maps of pollution around a hazardous waste site. Therefore, there is the need for adsorbing media able to quantitatively and rapidly extract trace organics from many, large volume water samples. We evaluated the ability of various organic solvents or mixtures of them in quantitatively desorbing phenolic compounds extracted from water by Vulcan. In Table I, recovery data for two selected eluting phases are reported. The ineffectiveness of the acidic solvent mixture in removing the moet acidic phenols from the carbon surface may be explained
A followed by 101 97 96 98 98 99 98 97 97
100
101
by the presence on the GCB surface of relatively few chemical heterogeneities having a particular affinity for ionogenic compounds. In two previous papers (IO, 12),it was shown that the GCB surface is contaminated by a particular carbon-oxygen complex that, on oxidation in the presence of acidified water, is rearranged to form a postively charged molecular structure similar to those of benzpyrilium salts. In this situation, a GCB surface is able to act simultaneously as an anion exchanger and a nonspecific adsorbent. Phenolic compounds having sufficiently high pK, values, such as 2,4-dinitrophenol, 2,4-dinitro-o-cresol, and pentachlorophenol, are partially dissociated in water a t pH 3 and, therefore, they can be captured by the positively charged impurities immobilized on the carbon surface. This specific bond is so strong that the compounds considered are not extracted from Vulcan even by acidifying a suitable solvent mixture. This expedient generally suffices in displacing anions from common anion exchangers. Moreover, only partial recovery of the three phenols cited above was achieved by adding KOH to a methylene chloride/methanol mixture. We reached the aim of complete elution of the most acidic phenols from the carbon cartridge only by dissolving TMAOH in the solvent mixture. This is likely due to the 2-fold action displayed by TMAOH, that is, displacement from the anion exchanger sites of phenoxide ions by the OH ions and formation of ion pairs wellsoluble in the organic solvent mixture by the tetraalkylammonium ions. When we tried to use only the alkalized mobile phase for recovering all 11 phenols, or we inverted the sequence of percolation of the two mobile phases through the Vulcan column, a drastic loss of 2,4-dimethylphenol occurred. Evidently, in an anhydrous, basic ambient, this compound is irreversibly adsorbed by the same surface chemical heterogeneities cited above or by other, unknown ones. Although this mechanism of chemisorption is unclear to us, it may be supposed that 2,4-dimethylphenol, which has a very low pK, value under the experimental conditions considered, is converted to phenoxide and is able to form a covalent bond with surface chemical impurities via a C-alkylation-like reaction (13). In order to assess the extent a t which high sampling rates can affect the extraction efficiency of the GCB cartridge, we sampled distilled water spiked with the 11 phenols at an individual concentration of 10 pg/L at two different flow rates, that is 6 mL/min, which is the sampling rate usually employed with bonded-phase extraction columns, and 32 mL/min the maximum flow rate possible with the apparatus used. Recovery data reported in Table I1 make evident that the trace enrichment capability of the sorbent under discussion is unaffected by the sampling rate for phenol itself, in spite of the fact that its breakthrough volume measured by us is lower than the sampled water volume, that is, 0.18 L. This positive feature shown by Vulcan is not surprising if consideration is
2050
ANALYTICAL CHEMISTRY, VOL. 58, NO. 9, AUGUST 1986
Table II. Recovery by Varying the Sampling Rate. Sample: 0.2 L Water at 10 pg/L of Each Compound
Table IV. Recovery at Increaaing Water Volumes Sampled Water Spiked with 3 pg/L of Each Compound at a Flow Rate of 32 mL/min
70 recovery
at 6 mL/min phenol p-nitrophenol o-chlorophenol 2,4-dinitrophenol o-nitrophenol 2,4-dimethylphenol 4-chloro-m-cresol 2,4-dichlorophenol 4,6-dinitro-o-cresol 2,4,6-trichlorophenol pentachlorophenol __-_
9: 93 94 96 95 97 96 96 99 102 101
98 95 94 95 95 96 99 94 99 101 100
0.5 L
% recovery
50 pgJL
100 FgJL
200 PgJL
100 95.6 94.8 97.3 96.4 97.8 98.2 97.9 96.6 99.1 99.2
92.1 96.6 96.7 97.8 98.4 97.5 98.4 97.9 96.0 99.4 99.4
56.0 99.1 97.4 98.4 98.8 97.0 97.6 97.5 96.4 99.3 99.7
made that, as already established (14, 15), the time of equilibrium for adsorption on the flat surface of GCB is extremely short, so that the broadening of the chromatographic band is scarcely influenced by varying the flow rate of the mobile phase. The loading capacity of the Vulcan cartridge was estimated by percolating through it water samples supplemented with increasing amounts of the phenols of interest. Recovery data are shown in Table 111. As one can read, a sharp decrease of the extraction efficiency of phenol occurred at the maximum concentration investigated. Likely, under such conditions, the carbon surface is to some extent saturated and "displacement chromatography" of phenol by the other stronger retained compounds takes place. The extent of such effect was further investigated by measuring breakthrough volumes of o-chlorophenol and o-nitrophenol when they were both separately and together dissolved in water. In any case, they were added to water a t an individual concentration of 100 pg/L. When alone, we measured breakthrough volumes of o-chlorophenol and o-nitrophenol of 0.66 and 1.2 L, respectively. On the other hand, when these compounds were added together with water, the breakthrough volume of ochlorophenol drastically decreased to 320 mL while that of o-nitrophenol was virtually unaffected by the presence of o-chlorophenol. This effect of "displacement chromatography" occurring at the expense of the less strongly adsorbed compounds should be carefully taken into consideration in order to perform reliable measurements of traces of given organic compounds in actual water samples. When the total content of organics in water is unknown, an easily practicable expedient could be that of sampling water two times by varying the volume sampled and checking whether the amounts of the analytes of interest trapped increase proportionally as the water volume is increased. The extraction efficiency of a carbon cartridge at increasing water volumes sampled was evaluated. Distilled water was
2L
4L
phenol 46.2 f 1.6" 9.8 f 1.4 1.5 f 0.6 p-nitrophenol 97.8 f 2.2 98.0 f 2.3 98.3 f 2.0 98.4 f 2.0 o-chlorophenol 95.0 f 2.4 19.7 f 1.5 2,4-dinitrophenol 98.3 f 2.4 101 f 2.7 95.1 f 2.9 o-nitrophenol 98.2 f 4.4 99.1 f 4.2 76.4 f 2.6 2,4-dimethylphenol 101.3 f 1.3 98.7 f 1.9 79.4 f 3.3 4-chloro-m-cresol 99.9 f 1.6 99.4 f 2.1 98.0 f 2.4 2,4-dichlorophenol 101 f 2.1 96.7 f 1.9 97.8 f 2.9 4,6-dinitro-o-cresol 96.2 f 4.3 101 f 4.0 96.7 f 4.0 2,4,6-trichlorophenol 100 f 1.5 103 f 1.7 101 f 2.3 99.7 f 1.9 pentachlorophenol 99.6 f 2.0 93.7 f 2.6 a
Table 111. Recovery at Different, Individual Concentrations of Phenols in 200 mL of Water
phenol p-nitrophenol o-chlorophenol 2,4-dinitrophenol o-nitrophenol 2,4-dimethylphenol 4-chloro-m-cresol 2,4-dichlorophenol 4,6-dinitro-o-cresol 2,4,6-trichlorophenol pentachlorophenol
% recovery
at 32 mL/min
Standard deviation calculated from five determinations.
spiked with the 11 phenols a t an individual concentration of 3 pg/L. We chose to report recovery data at increasing water volumes processed instead of breakthrough volume measurements, as the f i t method, with respect to the second one, gives more useful, practical information about the maximum water volume which can be sampled and still obtain a satisfying extraction efficiency, say 85-90%. As an example, we observed that, at the level of 50 pg/L in water, the breakthrough volume of o-chlorophenol was 0.71 L. Nevertheless, due to the slow rise of the breakthrough curve reaching the steady-state condition, another 0.4 L of water could be sampled with a recovery not less than 90%. Recovery data are shown in Table IV. Except for phenol, whose breakthrough volume (reported above) is low if compared with the other phenolic compounds, the data reported show that the GCB cartridge is extremely well-suited for the trace enrichment of even soluble phenols in water, such as nitrophenols. The astonishing ability of the Vulcan cartridge in trapping phenolic compounds from very diluted aqueous solutions may be due to adsorption of phenols on particular high-energy adsorption sites of the GCB surface. Let us consider that aromatic adsorbate molecules lie flat on the Vulcan surface (16) and assume that adsorption can take place up to the formation of one monolayer. Under these hypotheses, assigning a surface area of 100 m2/g (10)to Vulcan and a value a loading of 60 A2to the molecular area of o-chlorophenol capacity of the Carbopack cartridge for the model compound of about 9 mg can be calculated. Frontal chromatography of o-chlorophenol at three different concentrations in water, viz., 25, 50, and 100 pg/L, gave breakthrough volumes of 1.3, 0.71, and 0.66 L, respectively. These values correspond to very small percentage surface coverages of, respectively, 0.36,0.40, and 0.73%. It can be concluded therefore that adsorption of a phenolic compound initiates preferentially on high-energy adsorption sites that are progressively overloaded as the concentration in water of the phenolic compound increases to the point that these sites are totally occupied and adsorption proceeds on the remaining part of the GCB surface. The influence that other organic compounds present in a real matrix have on the extraction efficiency of the carbon cartridge for phenols was estimated by adding them a t the individual concentration of 2 pg/L to 1 L of water samples of different origin. Results are shown in Table V. As expected, under the experimental conditions chosen, almost no recovery of phenol was obtained. We observed that quantitative recovery, that is %!%, of this compound could be achieved by sampling not more than 0.15 L of a surface water specimen. Under this condition, the limit of detection (signal to noise ratio 4) for phenol was 0.25 pg/L. Figure 1 shows
(In,
ANALYTICAL CHEMISTRY, VOL. 58, NO. 9, AUGUST 1986
Table V. Matrix Effect on the Extraction Efficiency of the GCB Cartridge with a Sample Volume of 1 L
2051
I
% recovery
tap water phenol p-nitrophenol o-chlorophenol 2,4-dinitrophenol o-nitrophenol 2,4-dimethylphenol 4-chloro-rn-cresol 2,4-dichlorophenol 4,6-dinitro-o-cresol 2,4,6-trichlorophenol pentachlorophenol
ground water
surface water 5 92 39 87 90 82
12
10
98
99
99 94 102
98 92 101 100 96 97 99 102 101
100 96 97
100 103 102
91
89 87 91 103
1 . 0
4
8
12
TIME
I
L 0
(min.)
Flgure 2. Isocratic elution (solvent A 70%) after sampling 1 L of Tevere water spiked wRh 0.2 pg/L of pentachlorophenol.
Table VI. Trace Phenol Enrichment as Performed by Vulcan and C,*-BondedPhase 4
8
12
16
% recovery
20
Vulcan
TIME (mln)
Flgure 1. Chromatogram obtained on sampling 0.15 L of river water (levere), October 1985, spiked with 2 pg/L of each phenol: (1) phenol; (2) p-nitrophenol; (3) o-chlorophenol; (4) 2,4dinitrophenoi; (5) onitrophenol; (6) 2,4dimethylphenol; (7) Cchloro-m-cresol; (8) 2,edichlorophenol; (9) 4,6dinitro-o-cresol; (10) 2,4,6-trichlorophenol; (1 1) pentachlorophenol.
a typical chromatogram obtained for a surface water sample. By use of a 1-L sample of water, the recovery of o-chlorophenol was dependent upon the origin of the water specimen. The maximum volume of surface water that could be sampled without loss of o-chlorophenol was 0.5 L, with a limit of detection of 0.04 pg/L. For the other nine phenols considered, a generally slight decrease of the extraction efficiency of Vulcan can be observed by increasing the level of organic contamination of the sample. For these phenols, the limit of detection varied from 0.04 pg/L for 2,4,6-trichlorophenol to 0.004 pg/L for 4,6-dinitro-o-cresol. Among the phenols considered, pentachlorophenol is likely the most important water contaminant because it is widely used as a wood preservative. When determination of this compound has to be performed in a highly contaminated water sample, ita peculiar behavior on the carbon surface previously discussed can be advantageously exploited in that the acidic eluant phase is discarded before desorbing pentachlorophenol with the basic one. In such way, potentially interfering compounds with the HPLC analysis of pentachlorophenol may be eliminated. Moreover, this expedient drastically reduces
phenol p-nitrophenol o-chlorophenol 2,4-dinitrophenol o-nitrophenol 2,4-dimethylphenol 4-chloro-rn-cresol 2,4-dichlorophenol 4,6-dinitro-o-cresol 2,4,6-trichlorophenoI pentachlorophenol
82 95 94 96
101 93 94 103
98 99 100
Cl8
3
5 40 3
25 101 85 101 24 77
101
the presence of a lot of compounds that, having low K’values on the HPLC column, produce a large front with a severe drift of the base line. Figure 2 depicts a chromatogram obtained by modifying the sampling procedure as suggested above. The efficiency of GCB in trace enrichment of phenols was compared to that of a widely used chemically bonded sorbent, that is, a C18-bonded phase. A 314-mg portion of this sorbent was obtained by voiding a Sep-PAK cartridge (Waters) and packing it in a tube equal to those used by us. For this comparison, we used 0.2-L portions of a surface water sample fortified with 5 pg/L of each phenol and acidified to pH 3. After water was passed through the bonded phase bed, 4 mL of methanol was used to recover phenols, which were suitably alkalized with TMAOH 9 mmol/L to avoid loss of phenols during the solvent elimination step at 45 O C . As can be observed from the data reported in Table VI, Vulcan is un-
Anal. Chem. 1986. 58,2052-2056
2052
doubtly more suitable for trapping very polar compounds from water than a chemically modified silica. The reusability of the GCB cartridge was evaluated by doing repeated extractions of phenols from water on the same cartridge. After each extraction the column was restored with 3 mL of methanol and 5 mL of water. After five such concentrations the recovery of the 11 phenols considered was unchanged within the precision of the method. The sole effect observed was that by sampling more than 1 L of water each time, the flow rate was progressively decreased from 32 to 14 mL/min. Registry No. HzO, 7732-18-5;p-nitrophenol, 100-02-7;2,4dinitrophenol,51-285; o-nitrophenol,8875-5; 2,4-dimethylphenol, 105-67-9;4-chloro-m-cresol,59-50-7;2,4-dichlorophenol,120-83-2; 4,6-dinitro-o-cresol, 534-52-1; 2,4,6-trichlorophenol, 88-06-2; pentachlorophenol, 87-86-5; phenol, 108-95-2;o-chlorophenol, 95-57-8.
LITERATURE CITED (1) Saner, W. A.; Adamec, J. R.; Sager, R. W. Anal. Chem. 1979, 57, 2180-2 188. (2) Shoup. R. E.; Mayer, G. S. Anal. Chem. 1982, 5 4 . 1164-1169. (3) WerkhovenOoewle, C. E.; Brinkman. U. A. Th.; Frei, R. W. Anal. Chem. 1981, 53, 2072-2080.
(4) Rostad, C. E.; Pereira, W. E.; Ratcliff. S. M. Anal. Chem. 1984, 56, 2856-2860 -- - - - - - - .
(5) Bacaloni, A.; Ooretti. G.; Lagan& A.; Petronio, B.; Rotatori, M. Ana/. Chem. 1980, 5 2 , 2033-2037. ( 6 ) Mangani, F.; Crescentini, G.; Bruner, F. Anal. Chem. 1981, 53, 1627-1631. (7) Andreolini, F.; Di Corcia. A.; Lagan& A.; Samperi, R. Clin. Chem. (Winston-Salem, N.C.)1983, 29, 2076-2078. (8) Andreollni, F.; Borra, C.; Di Corcia, A.; Samperi. R. J . Chromatogr. 1984, 370,208-212. (9) Andreolini, F.; Bora, C.; Caccamo, F.; Di Corcia, A,; Samperi, R. Clin. Chem. (Winston-Salem, N.C.)1985, 3 7 , 1698-1702. (IO) Di Corcia, A.; Samperi, R.: Sebastiani, E.; Severini, G. Anal. Chem. 1980, 52, 1345-1350. (11) Reallni, P. A. J. Chromatogr. Sci. 1981, 19, 124-129. (12) Campanella, L.; Di Corcia, A.; Samperi, R.; Gambacorta, A. Mater. Chem. 1982, 7 , 429-438. (13) Kornblum, N.; Berrigan, P. J.; Le Noble, W. J. J. Am. Chem. SOC. 1963, 8 5 , 1141-1147. (14) Kiselev, A. V.; Yashin, Y. I. Gas-Adsorption Chromatography; Plenum Press: New York, 1969. (15) Ciccioli, P.; TaDDa, . . R.; Di Corcia, A.; Liberti. A. J. Chromatogr. 1981, 206, 35-42. (16) Ross, S.; Olivier, J. P. On Physical Adsorption; Wiiey: New York, 1964. (17) Snyder, L. R. Principles of Adsorption Chromatography;Marcel Dekker: New York, 1988.
RECEIVED for review December 9, 1985. Accepted April 7, 1986.
Cyclical-Field Field-Flow Fractionation: A New Method Based on Transport Rates J. Calvin Giddings Department of Chemistry, University of Utah, Salt Lake City, Utah 84112
A new form of field-flow fractionation (FFF) Is described In whkh the flew strength Is cycled up and down many times during a run. I t is shown that thk method, termed cyclical FFF (CFFF), leads to rnlgmtion rates depmwht on transport coe(fickrds. AHu devekpment of the general theory, resdts are obtained for coherent (no dmuslon) squarewave operation. For the latter, migration rates are shown to depend on the generalized mobility of the partldes and upon system parameters such as cycle time and fleM strength. Other varlants of CFFF are noted and several advantages are Indicated. A gravttathal CFFF system operating according to sedhrentatkm coemclents Is described and certain programming options are discussed.
Field-flow fractionation (FFF) is mainly an analytical separation tool, providing convenient operation and high selectivity for the separation of macromolecules, colloidal particles, and larger particles up to 100 hm diameter (1-6). A major advantage of FFF is that migration rates through the thin FFF flow channel can be rather exactly related to the various physicochemical parameters of the particles such as particle mass, charge, diffusion coefficient, etc. (7);thus, these latter parameters can be conveniently and accurately determined by observing the time at which the particles are eluted from the system. For polydisperse particle samples, one can measure the corresponding parameter distribution curves, e.g., the mass or molecular weight distribution curve (8).
With the exception of the subtechnique of flow FFF, most variants of FFF have migration rates that depend upon equilibrium or particle-specific parameters such as mass, density, thermal diffusion factor, charge, etc. With flow FFF, migration velocity depends on the fundamental transport parameter termed the friction coefficient, which can be expressed as a diffusion coefficient through the Plank-Einstein equation (2, 9). However, flow FFF is difficult to apply in a calculable way to particles much over 1Fm diameter because of steric effects (10,II). Often, it is useful to characterize the transport of larger particles. For example, sedimentation processes, involving mainly larger particles, have deposited much of our earth's surface and even now are responsible for the deposition of many toxic materials adsorbed on particles. To fully characterize such sedimentation processes, one needs access to the distribution of sedimentation coefficients within a particulate sample. The method detailed below should provide a convenient means for measuring distributions of such coefficients and, in principle, other transport coefficients as well. Normal FFF is based on the formation of a steady-state cloud (see Figure 1)whose effective thickness 1 is determined by the balance between the field-derived forces pushing the particles toward the accumulation wall of the channel at velocity U (absolute velocity IUl), and diffusion, measured by coefficient D, acting to disperse the particle cloud. It is found that 1 is the ratio of these two coefficients (12) 1 = D/lUl (1) Ordinarily, both D and IUl are inversely proportional to the
0003-2700/88/0358-2052$01.50/00 1986 American Chemical Society
