Anal. Chem. 1981, 53, 1195-1199 Table IV. Determination of Se in Food Composite Samples by Two Methods amt of S e , pg/g --hydride fluorescence sample methoda methoda diet 3 diet 60 diet 57 a
0.026 0.025 0.024
0.020 0.032 0.023
1195
(3) Fiorino, J. A.; Jones, J. W.; Capar, S. G. Anal. Chem. 1978, 48, 120-1 25. (4) Ihnat, M. J. Assoc. Off. Anal. Chem. 1977, 60, 813-825. (5) Clinton, 0. E. Analyst(London)1977, 102, 187-192. (6) Knechtel, J. R.; Fraser, J. L. Analyst(London)1978, 103, 104-105. (7) Flanjak, J. J. Assoc. Off. Anal, Chem. 1978, 61, 1299-1303. (8) Egaas, E.; Julshamm, K. At. Abs. News/. 1978, 17, 135-138. (9) Siemer, D. D.; Hagemann, L. Anal. Left. 1975, 8, 323-337. (10) Tam, G. K. H.; LaCroix, G. J. Environ. Scl. Health, Part 8 1979, 814, 515-516. (11) Reamer, D. C.; Veillon, C.; Tokousbalides, P. T. Anal. Chem. 1981, 53, 245-248. (12) Veilion, C.; Vallee, B. L. Meth. Enzymol. 1978, 54, 446-484.
Wet weight.
fluorescence method. The data (Table IV) show that agreement was good consideiring these low concentrations of selenium.
LITERATURE CITED (1) Anawical Methods Cornmittee Analyst(London)1979, 104, 778-787. (2) Ihnat, M. J. Assoc. Off. Anal. Chem. 1974, 57, 368-372.
RECEIVED for review January 30, 1981. Accepted March 25, 1981. D.C.R. is a Research Associate, Children’s Hospital, Boston, MA, and is supported in part by General Cooperative Agreement No. 58-32U4-0-127 with the U.S. Department of Agriculture. Specific manufacturer’s products are mentioned herein solely to reflect the personal experiences of the authors and do not constitute their endorsement nor that of the Department of Agriculture.
Humic Acid Fractionation Using a Nearly Linear pH Gradient Michael A. Curtls, Alllan F. Witt,’ Steven B. Schram,2 and L. B. Rogers’
Department of Chemistry, University of Georgia, Athens, Georgia 30602
Model compounds and humlc acids from commerlcal sources have been eluted from Amberllte XAD-8 resin using a nearly llnear pH gradient generated by mixing a universal buffer with a solutlon of sodlum hydroxlde. Model compounds eluted In order of decreaslng acld strength. Two humlc acid samples could readlly be dlstinguished from one another by their elution proflles. Due to thie retention of buffer components by the column, the shape of the pH gradient was found to depend upon both the flow rate and buffer concentratlon.
Humic acids have been the subject of intense research because of their significance in agricultural and environmental processes (1-5). Due to their complexity, they must first be fractionated into a series of less complex mixtures. Gjessing (6-9) has used gel permeation chromatography to fractionate humic acids while others have adsorbed them onto hydrophobic resins and eluted fractions using buffers at various pH values. As the pH was increased, components having progressively higher pKa values were ionized and, thus, desorbed. Mantoura and Riley (10) used XAD-2 resin and four buffer solutions of progressively higher pH to obtain four fractions. Malcolm et al. (11)demonstrated the superior properties of XAD-8, a methyl methacrylate resin. MacCarthy et al. (12) and Thurman and Malcolm (13) generated a continuously changing pH gradient by titrating phosphoric acid with sodium hydroxide and separated humic acids into two fractions. Those fractions eluted from XAD-8 resin a t the inflection points in the titration curve of phosphoric acid. In the above approaches, the pH gradient had sharp inflections at one or more points. It seemed desirable to generate a linear pH gradient so as to better separate any mixtures of ‘Present address: Monsanto Chemical Co., St. Louis, MO 63116. 2Present address: Sid E. Williams Research Center, Life Chiropractic College, Atlanta, GA 30060.
acids having many different pK, values. Our approach was to use a universal buffer (14) which, when titrated with strong base, gave a fairly linear titration curve without sharp inflections. The universal buffer chosen was that of Prideaux (15), an equimolar mixture of phosphoric, acetic, and boric acids. This buffer does not absorb significantly at 254 nm. Hence, a mixture of aromatic test acids covering a wide range of pKa values could be used to test the feasibility of the technique. Two commercial humic acid samples and a synthetic humic acid, prepared from hydroquinone, were chosen, not with high purity in mind, but to see if differences, due to the acids themselves or to the impurities, could be found.
EXPERIMENTAL SECTION Apparatus. Two Altex Model 110-A solvent metering pumps (Altex Scientific Inc., Berkeley, CA) were used to generate solvent gradients. They were controlled by an LSI-11 microcomputer (Digital Equipment Corp., Maynard, MA) having 32K words of memory and a 12-bit, 4-channel digital-to-analog converter. Samples were introduced by means of an air-actuated 6-port Valco valve, Model AC V-6UHPA (Valco Instrument Co., Houston, TX), having a 25-pL sample loop. Two detectors were used in the study: a Laboratory Data Control UV Monitor I11 254-nm fixed-wavelength detector (Division of Milton Roy Co., Riviera Beach, FL) and a Hitachi Model 100-10single-beam UV-visible spectrophotometer (Hitachi, LM. Tokyo, Japan) equipped with an Altex flow cell. A Markson flow-through pH electrode, Model 737 (Markson Science Inc., Delmar, CA), and a Corning Model 130 pH meter (Corning ScientificInstruments, Medfield, MA) were used to measure the pH of the column effluent. A Corning pH electrode (No. 476022) and a calomel reference electrode (No. 476002) were used for test titrations. Optical absorbance and pH data were recorded on a Linear Model 585 dual-channel recorder (Linear Instruments Corp., Irvine, CA) and also recorded by computer for storage on floppy disk. Reagents. Water, used to dissolve samples and to prepare the eluents, was purified by a two-stage deionization followed by
0003-2700/81/0353-1195$01.25/00 1981 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 8,JULY 1981
passage through an activated charcoal column to remove trace organics before being distilled. Phosphoric acid, glacial acetic acid, and boric acid (all reagent grade, J. T. Baker Chemical Co., Phillipsburg, NJ) were used to prepare buffer solutions. Sodium hydroxide solutions were prepared by diluting 50% sodium hydroxide solution (J. T. Baker) with distilled water, Amberlite XAD-8 resin was purchased from Supelco, Inc., Bellefonte, PA). Ethanol, acetonitrile, ethyl ether, and toluene (All J. T. Baker, reagent grade) were used as received to clean the resin. Humic acid samples were purchased from Aldrich Chemical Co., Metuchen, NJ, and Tridom Chemical Co., Hauppage, NY. Hydroquinone (J.T. Baker) was used to prepare synthetic humic acid. The model compounds, p-toluenesulfonic acid (Fisher Scientific Co., Fairlawn, NJ), phenol, benzoic acid (both J. T. Baker), and vanillin (Aldrich), were used as received. Certified buffer solutions (Fisher Scientific Co.) were used to calibrate the pH electrodes. Column Preparation. Amberlite XAD-8 resin, as received, contains impurities, primarily unreacted monomers, which must be removed if the background absorbance is to be minimized (16, 17). The resin beads were sucessively Soxhlet extracted for 24 h with each of the following solvents: ethanol, acetonitrile, ethyl ether, toluene, and again with ethanol. The beads were then dried and pulverized in a commercial food grinder. The 37-75-pm range was isolated by dry sieving with US. standard screens to remove fines that would result in an excessive pressure drop in the column (18). The sieved particles were placed in methanol, stirred, and allowed to settle. Any material which did not settle within 15 min was discarded. This process was repeated until the methanol appeared to be clear at the end of the settling period. Columns, 25 cm X 4.6 mm i.d., were constructed from 316 stainlevs steel (Alltech Associates, Deerfield, IL) and 6.35 mm to 1.59 mm Swagelok stainless steel reducing unions (Georgia Valve and Fitting Co., Atlanta, GA) which had been drilled out so as to minimize dead volume. The column hardware was washed with 6 M nitric acid, followed by distilled water, methanol, tetrahydrofuran, and, finally, hexane before being dried under a stream of dry nitrogen. The 37-75-pm resin particles were drypacked by using the conventional tap-and-fill method. Twomicrometer pore size removable frits (Alltech Associates) kept the packing in the column. A newly packed column was conditioned by passing 100 mL of 0.2 M sodium hydroxide through at a rate of 1.0 mL/min and ramping down the solvent composition to pure 0.1 M Prideaux buffer over a 15-min period. This procedure was repeated until the base line shift had minimized (ca. 0.01 AU). Sample Preparation. The test compounds p-toluenesulfonic acid, benzoic acid, vanillin, and phenol were dissolved in distilled water at concentrations of 5 mg/mL, 1mg/mL, 0.25 mg/mL, and 0.75 mg/mL, respectively. Aldrich humic acid, sodium salt, was dissolved in distilled water and the pH adjusted to 4 with dilute acetic acid. The solution was then diluted to 5 mg/mL. The Tridom humic acid was dissolved in 0.01 M NaOH, taken to pH 4 with acetic acid, and then diluted t o 5 mg/mL. Synthetic humic acid was made by dissoving 10 g of hydroquinone in 300 mL of distilled water. Concentrated sodium hydroxide solution was added with stirring until the pH reached 10.3. The mixture became dark red upon addition of the base. Distilled water was then added to bring the total volume to 400 mL. The solution was placed in a stoppered 500-mL Erlenmeyer flask and stored in the dark at room temperature for 24 h. This produced a dark brown liquid which was then adjusted to pH 1 with 6 M HCl. Although the resulting mixture appeared cloudy, no precipitation occurred upon standing overnight. More distilled water was added to bring the total volume to 800 mL. This mixture had such a high UV absorbance that it had to be diluted by a factor of 10 to bring the absorbance value onto the scale. Both the natural and synthetic humic acid samples were stored under refrigeration in the dark. Fresh samples were prepared every week. Chromatography. At the beginning of a run, the mobile phase consisted of 0.1 M Prideaux buffer (0.1 M in phosphoric, acetic, and boric acids). Upon starting the run, the sample valve was set to the inject position by a signal from the computer. After 1min, the valve was automatically reset to the load position. Upon
.
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sample injection, the computer began taking both absorbance and pH data at 2-9 intervals. As the percentage of sodium hydroxide in the eluent increased, elution of sample components began. A study was undertaken to assess the effects of flow rate and buffer concentration on the pH profile. Three concentrations of buffer were used: 0.025 M, 0.05 M, and 0.10 M. In each case the concentration of the sodium hydroxide solution in the second pump was twice that of each component in the acid buffer. For each concentration, pH gradients were run at five different flow rates: 0.5,0.7, 1.0,1.2, and 1.5 mL/min. The same pump control parameters were used for all of these experiments. At the end of a run, the column was regenerated by ramping the solvent gradient from sodium hydroxide solution to pure buffer over a 15-minperiod. Buffer was then passed through the column until the pH had stabilized before the next sample was injected. The flow-through pH electrode was calibrated each day before use. The software allowed simultaneous control of up to four pumps. Solvent gradient programs of up to 20 different steps could be used. Convex and concave curvature as well as linear changes between concentration values were attainable. Pump control parameters as well as data files were stored on floppy disk.
RESULTS AND DISCUSSION Preliminary Experiments. Figure 1shows the titration curve of 0.1 M Prideaux buffer with 0.2 M sodium hydroxide. The four slight inflections correspond to the four acid groups in the buffer, not including the third hydrogen of phosphoric acid. The mixture showed appreciable buffer capacity over a range of 10.5 pH units. The pH profile was influenced by the column. The first and third inflections in the pH profile were more pronounced when there was a column in place than when an empty piece of 0.75 mm i.d. tubing was substituted for the column. (See Figure 2.) The results for the flow rate and buffer concentration study are shown in Figure 3. The data for 0.5 mL/min have been omitted since at this flow rate the pH did not rise above 6 for any buffer concentration. The results show that an increase in either buffer concentration or eluent flow rate resulted in a more nearly linear pH profile. However, flow rate had a much greater influence than did buffer concentration; i.e., doubling the flow rate at a given buffer concentration resulted in a better pH profile than that obtained by using solutions of twice the original concentration at the lower flow rate. Since chromatographic efficiencies decreased at high flow rates, 1.0 mL/min represented the best compromise. Separations of Mixtures. A separation of the four model compounds is shown in Figure 4. The compounds eluted in the order of decreasing acid strength. Presumably as the pH approached the pK, of a given acid, that acid ionized and was desorbed from the resin.
ANALYTICAL CHEMISTRY, VOL. 53, NO. 8, JULY 1981 I
1197
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Figure 5. Fractionations of natural and synthetic humic acids using the same gradient and flow rate as in Figure 2: (A) pH profile; (B) Aldrich humic acid, 5 mg/mL, 25-pL sample; (C) Tridom humic acid, 5 mg/mL, 25-pL sample; (D)synthetic humic acid, 1.25 mg/mL, 25-pL sample.
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Figure 3. Effect of buffer concentration and flow rate on the pH profiles from an XAD-8 column using the same gradient as in Figure 2: (left to right) 0.7 mL/min, 1.0 mL/min, 1.2 mL/min, 1.5 mL/min; (top to bottom) 0.025 M buffer vs. 0.05 M NaOH, 0.05 M buffer vs. 0.1 M NaOH, 0.1 M buffer vs. 0.2 M NaOH.
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Figure 4. Separation of model compounds by pH gradient elution on XAD-8: p-toluenesuifonic acid, benzoic acid, vanillin, and phenol using the same gradient and flow rate as in Figure 2.
Figure 5 shows the separations of the natural and synthetic humic acids under identical conditions. The separations of the two natural humic acids have several features in common. Except for the small amount of unretained material in the Aldrich sample, no elution occurred until the pH rose to about 3, following which some material eluted at all pH values between 3 and 12. For both samples, the absorbance signal decreased toward base line as the pH rose above 12. The chromatograms exhibited three unresolved peaks, the first and third being taller than the second. Each of those peaks corresponded to one of the inflections in the pH profile. If the gradient was changed to make one of the inflections sharper, the peak corresponding to that inflection became more prominent. This implies that these peaks are merely
artifacts caused by the inflections in the pH gradient. The temporary increase in the rate of pH change caused more material to be desorbed in a given time period. The separations of Aldrich and Tridom humic acids could be readily distinguished from one another on the basis of three criteria. The Aldrich humic acid contained some material which was unretained at low pH while no unretained fraction was present in the Tridom sample. For the Aldrich humic acid the peaks corresponding to the second and fourth inflections were of equal height while in the Tridom sample, the peak eluting at the fourth inflection was higher than that which eluted during the second inflection. Also, the valleys separating the first and third major peaks from the middle peak were of equal height in the case of the Aldrich humic acid whereas in the Tridom sample, the second valley was higher. The synthetic humic acid gave results which were significantly different from either of the two natural samples. Virtually all of the material eluted above pH 10. The reaction products of hydroquinone in basic aqueous solution are believed to consist of phenolic polymers (19). This would agree with their observed elution at high pH values. Fractions were collected, acidified, and readsorbed onto the resin in order to check the integrity of the separation procedure. They were then rerun under the same gradient. The results are shown in Figure 6. Note that most of the material collected in a given pH range eluted in the same pH range when reinjected. Spectroscopic methods are frequently used to further characterize separated humic acid fractions. One drawback with this buffer with respect to its use in the fractionation of humic acids was that it contained acetic acid. That compound would interfere with determinations of the dissolved organic carbon in the separated fractions. Also, the acetic acid would interfere with infrared spectroscopy and with UV measurements below 250 nm. For that reason the following procedure was devised for removing the acetic acid. A 25-pL portion of 0.1 M acetic acid containing 1mg/mL of Tridom humic acid was injected into a column which had been equilibrated with 0.1 M phosphoric acid. More phos-
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 8. JULY I981
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flections in the titration of the universal buffer were much smaller than those in a single acid-base reaction, they were still present. This is seen both in the titration cruve of the buffer itself (Figure 1) and in the pH profiles of the c h r e matograms. In the latter, the region between pH 10 and 11 had the highest slope. This region lies midway between the pK, of boric acid and pK8 of phosphoric acid (12.32) where the pKa values were more than two units apart, so the aolution had its lowest buffer capacity there. As mentioned previously, two of the pH inflections in the gradient were more prominent when the eluent was passed through a packed column than when an empty piece of tubing was placed between the injection valve and the pH electrode. Any attempt to explain this phenomenon must account for three observations. First, the second pH inflection was strongly affected by the presence of the column while the fourth inflection was less strongly influenced. The first and third inflections were unaffected by the column. Second, increasing the flow rate of the mobile phase dramatically decreased the effect of the column. Finally, increasing the concentrations of the buffer and base solutions by the same amount slightly reduced the effect of the column. The influence of the column on the pH profile may be explained as follows. When the percentage of strong base in the eluent was increased from zero a t the beginning of a run, one hydrogen ion was removed from phosphoric acid. Next the base neutralized some of the acetic acid, hut some was retained by the resin. After the acetic acid had heen neutralized in the eluent entering the column, additional base removed a second proton from the phosphoric acid. The monohydrogen phosphate anions entered the column and readed with the adsorbed acetic acid. Thus the column itself acted as a reservoir of acid to slow down the rate of pH increase measured a t the column outlet. After the last of the adsorbed acetic acid had been consumed, the pH abruptly rose to the value it would have had if the column had not heen present. This breakthrough phenomenon was observed as a sharpening of the second pH inflection. A similar proms can explain the fourth inilection if a small quantity of boric acid was retained by the column. When the pH at the column entrance was so high that no free boric acid was present in the incoming eluent, the retained boric acid reacted with incoming phosphate anion until the boric acid was consumed. A decrease in the rate of pH change followed by a sharp increase upon complete reaction of the absorbed species OcClvTed in a manner analogous to the acetic acid case. Since acetic acid is organic, its affinity for the hydrophobic resin should be greater than that of boric acid. Hence, a greater adsorbed amount of acetic acid compared to boric acid would explain why the effect of the column was considerably greater on the second inflection than on the fourth. This mechanism can also explain why the third inflection was unaffected. After the second endpoint of phosphoric acid had heen reached and HlB08- was present in the incoming eluent, there was no uncharged acid species to be retained by the column and later reprotonate the incoming HzBOa-. Phosphoric acid apparently does not adsorb onto the column since the f i t inilection point was not affected by the presence of the column. The retention of acetic and boric acids also accounts for the effects of flow rate and concentration. During the run, the column equilibrated with the buffer and a fixed amount of the neutral acid was retained. When the flow rate was higher, the mas flow rate of the basic species into the column was faster so that the adsorbed acid was consumed faster and the inflection was smaller. Using more concentrated solutions of acids and bases under the same gradient conditions did not produce the same effect. Although the mass flow rate of the
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Flgum S. (A) Fractionation of Aidrich humic acid showing regions where fractions ware Qlt, fa later reinjection. The same ga&t and Row rate were used in Fbue 2. (e) First fraction. (C) second fraction. (D) Third fraction. (E) Distilled water blank. 100
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phoric acid was pumped through the column a t 1 mL/min for 8 min. Then, 0.1 M sodium hydroxide was pawed through the column a t 1 mL/min while the effluent was monitored a t 230 nm using the variable-wavelength detector. It was found that the acetic acid eluted at low pH and was completel~ resolved from the humic material. (See Figure 7.) Our fractionations of humic acids showed continuous elution of material, without drastic variations in the rate, over a wide pH range. With the present method, the user can arbitrarily select the pH range over which to collect a fraction. In contrast, it is clear that in methods which employ stepwise pH gradients, the type of buffer used determines the pH range over which a chromatographic peak will appear. Hence, species covering a wide range of pK. values will elute a t the same time. The reasoning behind usinga mixture of acids in the buffer is that, by having several overlapping buffer regions, pH can be controlled over a wide range. Since the practical buffer range of a weak acid or weak base is 2 pH units (pK 1). the pK, values of the acid groups should OCCUT at intervals of two. For the five acid groups in the Prideaux buffer, the pK. values are 2.23, 4.76, 7.21, 9.23, and 12.32. Although the pH in-
*
Anal. Chem. 1981,
1199
53, 1199-1202
basic species was higher than with more dilute solutions, it was compensated for by adsorption of a greater amount of neutral acid from the more concentrated buffer. The fact that an increase in the concentrations of both solutions slightly reduced the effect of the column implies that the capacity of the resin to adsorb acetic and boric acids did not increase linearly with concentration. To ensure that those effects were actually due to the column packing, we ran the same gradient without the column a t different flow rates and concentrations. No effect on the size of the inflections was found. Increasing the flow rate of the mobile phase appears to be a convenient method for minimizing the influence of the column on the pH profile. The decrease in column efficiency associated with high flow rates could be compensated by using a higher efficiency column packed with smaller resin particles. Dieterle et al. (20) have shown that microparticle size ranges of XAD resins can be prepared and slurry-packed to produce high-efficiency columns. This would allow the use of higher flow rates and, thus, more nearly linear pH profiles while maintaining high chromatographic efficiency. This pH gradient method shows considerable promise both as a fingerprinting technique for humic acids and as a means for fractionating these materials prior to further characterization. The fractionations of replicate samples were quite reproducible making preparative scale analysis amenable to automation. After fractionation, the acetic acid from the buffer can be removed easily prior to characterizing the humic acid species, by readsorbing the fraction on a short column of large particle XAD resin, rinsing with dilute mineral acid to remove the buffer components, and flushing with sodium hydroxide. This procedure would have the advantage of
concentrating the humic acid fraction into a small volume of base.
LITERATURE CITED (1) Kononova, M. M. “Soil Organic Matter”, 2nd English ed.; Pergamon Press: Oxford, 1966; pp 183-228. (2) Schnitzer, M.; Khan, S. U. “Humic Substances in the Environment”; Marcel Dekker: New York, 1966; pp 281-302. (3) Gjessing, E. T. “Physical and Chemical Characteristics of Aquatic Humus”; Ann Arbor Science: Ann Arbor, MI, 1976; pp 56-73. (4) Felback, G. T. in “Soil Biochernlstry”; McLaren, A. D., Skujins, J., Eds.; Marcel Dekker: New York, 1971; Vol. 2, pp 36-59. (5) Szalay, A. Ark. Mineral. Geol. 1969, 5 , 23-36. (6) Gjessing, E. T. Chem. Environ. Apuat. Habitat, Proc. IBP-Symp. 1967, Paper No. 18, 191-201. (7) Gjessing, E. T. TMSskr. Kjemi, Bergves. Metall. 1967, 27, 8-15; Chem. Abstr. 1967, 67, 5593p. (8) Gjessing, E. T.; Lee, G. F. Environ. Sci. Techno/. 1967, 1 , 631-638. (9) Gjessing, E. T. Vatten 1970, 26, 135-141. ( I O ) Mantoura, R. F. C.; Riley, J. P. Anal. Chim. Acta 1975, 76, 97-106. (1 1) Malcolm, R. L.; Thurman,E. M.; Aiken, G. R. Proc. An. Conf. Tr. Subs. Environ. Health, 11th 1978, Paper No. 37, 307-315. (12) MacCarthy, P.; Peterson, M. J.; Malcolm, R. L.; Thurman E. M. Anal. Chem. 1979, 51, 2041-2043. (13) Thurman, E. M.; Malcolm, R. L. US. Geol. Survey, Water Resources Division, Denver, CO, 1979, Water Supply Paper No. 1817. (14) Bates, R. G. “Determination of pH: Theory and Practice“, 2nd ed.; Wiley-Interscience: New York, 1973; Chapter 5. (15) Prideaux, E. B. R. Proc. R . SOC.London, Ser. A 1916, 92, 463-468. (16) Pietrzyk, D. J.; Chu, C. H. Anal. Chem. 1977, 49, 757-763. (17) Thurman, E. M.; Malcolm, R. L.; Aiken, G. R. Anal. Chem. 1978, 50, 775-779. (18) Pietrzyk, D. J.; Chu, C. H. Anal. Chem. 1977, 49, 660-8613. (19) Flaig, W.; Beutelspacher, H.; Rietz, E. “Soil Components; Volume One: Organic Components”; Springer-Verlag: Berlin, 1975; Chapter 1. (20) Dieterle, W.; Faigle, J. W.; Mory, H. J . Chromatogr. 1979, 168, 27-34.
RECEIVED for review January 5, 1981. Accepted March 30, 1981. Supported in part by National Science Foundation Grant No. CHE 78-13269.
Multichannel, Positive Displacement Teflon and Glass Sampler for Trace Organics in Water David C. Tigweli,‘ David J. Schaeffer,” and Luther Landon Illinois Environmental Protection Agency, 2200 Churchill Road, Springfiehl, Illinois 62706
A multlchannel positlve displacement apparatus for collecting composite samples of water-borne trace organic compounds is described. A Teflon line connects each source to a separate three-way Teflon valve. Each valve is connected to a separate 50-mL syrlnge held in a machined mount which prevents lateral shear. This apparatus delivers precise volumes of 10.0-40.0 mL per channel per cycle. The all Teflon-glass construction minimizes adsorptive losses in the sample train. Multiple sources and/or multiple collection devices can be sampled simultaneously.
Numerous recent articles describe procedures for, and results obtained from, the analysis of aqueous environmental samples for organic compounds present a t trace levels (1-3). While sample storage, compound isolation, and identification procedures are documented, relatively little information is Current address: D. C. Tigwell and Associates, 2100 Tanglewilde
#71, Houston, TX 77063.
available on equipment and methods of sampling for trace levels of organic compounds. With few exceptions, most aqueous samples have been collected as “grab” samples, although some reports describe the collection of composite samples (4-6). Garrison et al. ( 5 ) described a new sampler which uses a pump with Teflon bellows, glass-ball check valves, and Teflon intake lines. Stephan et al. (6)developed a sampler employing peristaltic pumps to force sample through columns containing XAD-2 and XAD-7 resin. Several commercial manufacturers are currently advertising modified versions of their standard composite samplers (7-9) which use Teflon delivery lines and glass vacuum chambers. Three years ago the Illinois Environmental Protection Agency initiated a program to evaluate the chemical, biologic, and toxicologic significance of organic compounds being discharged to, and found in, the State’s waters (1). From the outset, we recognized that the most critical part of this project was the requirement of collecting a representative sample (10-13). In order to accomplish this, we developed an extensive list of sampling requirements (6-9,14). The sampler
0003-2700/81/0353-1199$01.25/0 0 1981 American Chemical Society
