ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979
the coupled column system is the method of choice.
ACKNOWLEDGMENT The authors are indebted to H. M. Baine, Jr. for assistance in the development and application of this method.
LITERATURE CITED (1) Williams, R . C.; Schmit, J. A,; Henry, R. A. J. Chromatogr., Sci. 1972, 10. 494-501. (2) Tompkins, D. F.; Tscherne, R . J. Anal. Chem. 1974, 46, 1602-4. (3) Osadca, M.; Araujo, M. J . ASSOC. off. Anal. Chem., 1977, 6 0 , 993-7. (4) MacDonald, J. C. Am. Lab. 1977, (8),69-76.
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(5) Nunes, M.; Robinson, J. R., Mead Johnson Co., unpublished work, 1975. (6) Barnett. S. A.. Mead Johnson Co., unDublished work, 1976. i 7 j Schmit, J. A; Henry, R. A.; Williams, R. c',; Dickman. J. F. J . Chromatogr. Sci. 1971, 9. 645. (8) Snyder. L. R.; Kirkland, J. J. "Modern Liquid Chromatography", American Chemical Society: Washington, D.C.. 1973. (9) Frick, L. W., Mead Johnson Co., unpublished work, 1977. (10) Karger, 6.L.; Snyder, L. R.; Horath, C. "An Introduction to Separation Science", John Wiley and Sons: New York, 1973.
RECEIVED
for review August 24, 1978. Accepted January 29,
1979.
Universal Detector for Monitoring Organic Carbon in Liquid Chromatography Rolf Gloor' and Hans Leidner Federal Institute for Water Resources and Water Pollution Control, Ueberlandstr. 133, CH-8600 Dubendorf-Zurich, Switzerland
A detector based on the principle of measuring organic carbon as C 0 2 after oxidation has been developed. The construction and principle of the system are described. Sensitivity is discussed and compared to other LC detectors (UV, RI). The minlmum detectable concentration of organlc carbon is 3 X lod g/mL. Applications using Sephadex gel chromatography and reversed phase HPLC are shown. The detector Is restricted to nonorganic solvents.
The majority of detectors used in liquid chromatography (LC) today (UV-vis, fluorescence, electrochemical) are sensitive only for specific types of compounds, rather than being universal in their use (1-3). Among the universally used detectors, only the refraction index (RI) has been employed extensively, while others (transport-FID, photoionization, LC-MS) are still in an experimental stage, or their sensitivity is insufficient for many applications. In addition to its lack of sensitivity, RI yields both positive and negative signals which make its use very questionable for monitoring complex mixtures whose components may coelute. Therefore, there is still a need for a universal type detector comparable to the flame ionization detector (FID) in gas chromatography (GC). As part of a general effort in the biological department of our Institute to characterize organic water constituents, we have used gel permeation chromatography to gain information on the molecular weight distribution of organic matter in water samples of various types. Since dissolved organic carbon (DOC) is one of the key parameters in water analysis and gives the best information for comparison (4,we were searching for a detection system capable of monitoring the organic matter in the column effluent according to its carbon content. DOC is generally determined by oxidation of organic matter, followed by measurement of the COz produced with infrared absorbance or by FID (after reduction to methane). Axt ( 5 ) reported earlier on an analyzer capable of measuring organic carbon continuously in water samples by using the infrared technique. Based on this work, we developed a simple and inexpensive on-line detector for monitoring organic carbon in column effluents.
EXPERIMENTAL Construction of the Detection System. An illustration of the detection system is shown in Figure 1. The oxidation oven (1)(a 50 X 2.5 cm ceramic tube, 98% Alu203
standard unit, Koppers Dusseldorf, West Germany) is vertically
inserted into a furnace (2) (Heraeus, 6450 Hanau, West Germany). The oven is filled for 30 cm with ceramic splinters approximately 5 mm in length. A 5-cm-long copper tube (3), loosely filled in the lower half with platinum wool and copper pieces, is placed on top of the ceramic splinters (4). The copper tube has an outer diameter that fits snugly inside the ceramic tube. Copper forms an oxide surface in the high temperature of the oven and thus acts as a catalyst for oxidation of organic carbon. The copper tube also shields the outer ceramic tube from temperature shocks caused by the inlet flow (which enters as drops, as discussed below). Both ends of the ceramic tube, which extend 10 cm out of the oven on each side, are sealed with Teflon stoppers. If the oven is properly insulated at both ends, the temperature of the Teflon stoppers should not exceed 100 "C. The sample and carrier gas inlet is a '/4-in. stainless-steel tube with a 1/16-in.capillary tubing inserted into the top Teflon stopper. All connections (T-piece for carrier gas inlet, T-piece 1/16-in.for splitter (5)) are standard Swagelok units. The condenser (6) is a specially constructed intensive water condenser with a built-in syphon unit (7). A 20-mm syphon efficiently closes the system from the atmosphere. The infrared cell used is a Beckman 865 infrared analyzer (8) (Beckman, Fullerton, Calif.). Mode of Operation. The column effluent drops continuously into the hottest part of the oxidation oven (1) (800-900 "C) onto the platinum/copper oxide catalyst, which immediately oxidizes organic matter in the effluent to COP. A carrier air stream (C02-free)supplies the oxygen necessary for oxidation and carries the COz vapor mixture into the condenser (6). The condensed water separates from the gas phase and leaves via the syphon (7) which prevents any access of atmosphere to the system. About 30% of the carrier gas also leaves through the syphon, while the rest passes the infrared cell (8) through a side channel (9). The concentration of COPin the carrier gas is monitored on a strip chart recorder (11). The cell gas effluent reenters the system after having passed the COz filter (10). Fresh air, replacing the 30% lost through the syphon, is added by a needle valve (12) to the carrier stream. The respective amount of fresh air added is calculated from the two flowmeter readings (13, 13') and is adjusted to make up about 30% of the total carrier flow. This arrangement of the air circulation ensures a constant flow through the IR cell while the syphon acts as a pulse damper for pressure fluctuations caused by falling drops. It is not necessary to know the exact amount of carrier gas passing through the cell since the concentration of COz in the carrier stream is measured rather than the absolute amount.
RESULTS AND DISCUSSION Sensitivity. Figure 2 illustrates the excellent sensitivity
of the detection system. The response of the detector for three different compounds (fructose, propionic acid, phenol) was compared to that of a UV absorbance detector (Varichrom,
0003-2700/79/0351-0645$01.00/0 @ 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979 column effluent
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Flgure 2. Sensitivity of DOC detection compared with UV detection.
I Injection
of standard solutions into the detectors (for details, see text); flow rate 0.5 mL/min. (A) 2 pg of fructose in 0.5 mL of water; (B) 2 pg of propionic acid in 0.5 mL of water; (C) 2 Fg of phenol in 0.5 mL of water. UV detector: Varichrom (Varian)set at 210 nm. DOC detector: as described, carrier gas 9 L/h, attenuation 3, gain 5 0
co
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Figure 1. Schematic diagram of the DOC detection system
Varian). Aliquots (0.5 mL) of a 4-ppm solution of each compound were injected into a system consisting of a pump, a loop injector, and UV and DOC detectors connected in series. Distilled water was pumped through the system at a flow rate of 1 mL/min. The MDQ (minimum detectable quantities) calculated from the different detector responses for these three compounds indicate that DOC detection is superior in the case of a weak UV-absorbing compound (fructose at 210 nm). In the case of a medium UV-absorbing compound (propionic acid a t 210 nm), the sensitivities of both detectors are comparable, while UV detection is superior for a strong UV-absorbing compound (phenol at 210 nm). Since the individual molecular carbon content and the peak volumes (2 mL) of the test compounds are known, a minimum detectable concentration (MDC) for organic carbon can be calculated from Figure 2 as 3X g/mL. Thus, DOC detection is approximately an order of magnitude greater in sensitivity than an RI detector ( I , 2). For any organic compound, the MDQ value determined from the DOC detector signal depends exclusively on its molecular carbon content. Therefore, the detector response is predictable after calibration with a standard sample. The DOC detector response was not precisely linear, owing to characteristics of the infrared analyzer used. The nonlinearity varied around 5%. In case a linearized output is an ultimate need, an optional linearizer board is available from the manufacturer (6). Applications. Ever since the introduction of Sephadex in 1959, this gel has been used increasingly for separation of
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Figure 3. Molecular-size distribution of carbohydrates on Sephadex G 25: 0.5 mL of standard containing 30 wg of each, dextran blue, raffinose, and fructose. Column: Sephadex G 25 800 X 16 mm. Eluent: lo-' M phosphate buffer (Na,HP04/KH,P0,), pH 7.0. Flow rate: 1 mL/min. DOC detector: carrier gas 9 L/h, attenuation 2, gain 8
compounds in aqueous systems on the basis of molecular size. Figures 3 and 4 illustrate the use of DOC detection for monitoring Sephadex column effluent. Figure 3 represents a calibration run of a standard mixture on Sephadex G 25, while the chromatogram in Figure 4 shows an actual run of a concentrated ground water sample on the same column. As this detection system is destructive, the column effluent was split prior to entering the oven inlet. Thirty percent of the column effluent was monitored for DOC; the remainder was
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