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Anal. Chem. 1983, 55,2011-2013
-
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Figure 4. Standard mixture of ethylene and propylene glycols at
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100 wg/mL per component in water. Peak identification: (A) ethylene propylene glycol, (C)diethylene glycol, (D) dipropyiene glycols, glycol, (6) (E) triethylene glycol, and (F) tripropylene glycols. All components and condklons are the same as those given in Figure 2 except temperature program: 60 OC (hold 2 min) to 200 OC at 8 ‘C/min.
average RSDs for all peak areas of 4.1% and 5.6% for ths heptane and hexane hydrocarbon test solutions, respectively. Evaluation of column efficiency using n-dodecane at a K’of -5.5 with the 1.0 mm and 2.0 mm i.d. vaporization tubes
indicated 2000 effective theoretical plates per meter which agreed favorably with the column manufacturer’s test data. The chromatograms shown in Figures 3 and 4 illustrate two applications of the fixed low splitting ratio injector. Although the described fixed low splitting injector is imperfect with respect to solute discrimination relative to boiling point differences, being somewhat positive for high volatility components and somewhat negative for those of lower volatility, we have found that injector reproducibility is not sacrificed. This characteristic, as well as its simplicity, ruggedness. ease of installation. and amenabilitv to Dacked column gas chromatographs should make the fixed low splitting ratio injector a useful device. LITERATURE C I T E D (1) Grob, K., Jr.; Neukom, H. P. J . High Resolut. Chromatogr. Commun. 1979, 2, 563. (2) Grob, K., Jr.; Neukom, H. P. J . Chromatogr. 1982, 236, 297-306. (3) Schomburg, G.; Behlau, H.; Deilmann, R.; Weeke, F.; Husman, H. J . Chromatogr. 1877, 142, 87-102. (4) Jennings, W. G. J . Chromatogr. Sci. 1975, 13, 185-187. (5) Novotny, M.; Harlow, R. J . Chromatogr. 1075, 103. 1-6.
RECEIVED for review April 22, 1983. Accepted May 26, 1983.
Determination of Monovalent Cations by Ion Chromatography with Ion-Selective Electrode Detection Koji Suzuki,* Hiroshi Aruga, a n d Tsuneo Shirai Department of Applied Chemistry, Keio University, 3-14-1 Hiyoshi-cho, Kohoku-ku, Yokohama 223, J a p a n There are several special advantages to the use of ion-selective electrodes (ISEs) in flowing solutions (1). The most attractive advantage is the excellent reproducibility of response potential which is mainly attributable to continuous “washing” of the membrane surface of ISE with fresh flowing solution. Unfortunately, all ISEs respond to more than one ion species to some extent, and this is the most serious limitation to use of ISE. The response potential is therefore affected by interference from ions in a sample, even though its reproducibility is quite satisfactory when using the flow-through method. However, the problem can be solved, if the sample contains only one sensed ion species. These matters led us to the idea of using ISEs together with an ion separation apparatus such as an ion-exchange column for ion chromatography ( 2 ) . Although similar efforts have been performed for determination of halide ions (3)and anioiis ( 4 , 5 ) ,we report here the determination of monovalent cations (alkali metal ions and ammonium ion) by the ion chromatography equipped with an ion-selective electrode detector using several electrodes based on neutral carrier ligands (valinomycin, benzo-15-crown-5, nonactin, and tetranactin). EXPERIMENTAL S E C T I O N Ion-Selective Electrode Detector. A cross section of the detector is shown in Figure 1which was constructed with an ISE, a reference electrode, and two miniature three-way Teflon joints (commerciallyavailable from Sanyo Rikagaku Kikai Seisakusho, Tokyo, Japan) used as a detector block. The ISEs used were homemade poly(viny1 chloride) (PVC) matrix membrane electrodes which were basically similar to the coated-wire electrodes by Cattrall and Freiser (6). A bended platinum wire 0.4 mm in diameter was covered with a PVC tube 0.8 mm in inner diameter and one of the wire edges was coated with the PVC-tetrahydrofuran (THF) solution containing active ligand and bis(2ethylhexyl) sebacate (BEHS)as a solvent mediator. The electrode 0003-2700/83/0355-2011$01 .BO/O
membrane was composed of 3 wt % active ligand, 70 wt % BEHS, and 27 wt % PVC after evaporation of THF. The ligands used were valinomycin (obtained from Sigma Chemical Co., Saint Louis, MO), benzo-15-crown-5 (Merck, Dermstadt, West Germany), nonactin (Sigma Chemical Co.), and tetranactin (Chugai Pharmaceutical Co. Ltd., Tokyo, Japan). Only the tetranactin was of 90% quality containing 5% of both di- and trinactin. These ligands were used without further purification. The reference electrode was a fiber junction silver-silver chloride type electrode (TOA Denpa Kogyo Co. Ltd., Tokyo, Japan, type HS-907). The glass body 5 mm in diameter was filled with the saturated potassium chloride bridge solution. The cell volume between electrodes was about 50 wL. The EMF (mV) produced between the two electrodes as detector response was measured with a pH/mV meter (TOA Denpa Kogyo, type HM-2OE) and the variation in ion chromatogram was drawn with a recorder (Yokogawa Electric Works Ltd., Tokyo, Japan, type 3066). Ion Chromatography System. The system was composed of a pump (Japan Spectroscopic Co. Ltd., Tokyo, Japan, Model Trirotar-11)for HPLC, sample injector (Rheodyne, Inc., Cotati, CA, Model 7125), cation separation column (Wescan Instruments Inc., Santa Clara, CA, 269-004 Cation Column), and ion-selective electrode detector. The eluent used was acidic solution at pH 2.3-2.7 prepared with nitric acid (Wako Pure Chemical Industries, La., Osaka, Japan, No. 140-04016) and highly purified deionized water (>2 X lo7 Q-cm). Although the temperature was not specially controlled in the eluent and the system apparatus, the reference electrode potential was corrected automatically with a thermistor type sensor (TOA Denpa Kogyo, type HR-105) placed in a eluent drain bottle to maintain the potential constant. R E S U L T S AND DISCUSSION Figure 2 shows the typical ion chromatograms obtained by using ion-selective electrode detectors with different electrodes. The injected solution is the standard mixture of monovalent cations (Li+, Na+, NH4+, K+, Rb+, and Cs+) of the same concentration. Since the acidic solution around pH 2.5 is 0 1983 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 12, OCTOBER 1983
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Flgure 1. Schematic view of the ion-selective electrode detector: (1) reference electrode body; (2) O-ring (silicone rubber); (3) miniature three-way joint (Teflon); (4) Teflon packing; (5) silicone rubber tube; (6) Teflon tube; (7) separation column outlet; (8) ion-selective electrode; (9) PVC (bonding agent); (10) platinum wire; (11) PVC tube; (12) PVC matrix membrane.
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Figure 2. Ion chromatograms of monovalent cations obtained by using ion selective electrode detectors with different electrodes: column, Wescan, Cation, 269-004; eluent, HNO,, pH 2.5, 1.5 mL/min; sample, 1 X M each cation (Li', Na', NH,' K', Rb', Cs'); injection volume, 100 pL.
required for cation separation with the ion-exchange column, the ISEs based on some active ligands (valinomycin and nonactin) were chosen for the reason that they were sufficiently highly selective for alkali metal and ammonium ions compared with hydrogen ion (7). Thus the detector response peaks basically depend on the cation selectivity sequences of the ISE, although the sensitivity (peak heights in chromatogram) was determined by the difference in selectivity between analyte cations and hydrogen ion in the eluent. The typical calibration curves of the detector using a nonactin electrode are shown in Figure 3. The fact that the response time is not always constant at a definite ion concentration is a classical problem for ISE. However, the rewhich is the most sensitive ion tQ the detector, sponse to is linear when plotted against its logarithmic activity and
",+,
200 8
7
6
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4
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-I o g a Flgure 3. Calibration curves of the ion-selective electrode detector using a nonactin electrode: (1) calibration curve for NH,+ (injection volume, 2 mL); (2) calibration curve for NH4+(carrier flow, deionized water, 1.5 mL/min; noncolumn; injection volume, 2 mL); (3) base line potential of eluent; (4) base line potential of deionized water (1.5 mL/min). Column: Wescan, Cation, 269-004; eluent, HN03, pH 2.5, 1.5 mL/min, 20 'C; sample injection volume, 100 ILL (except curve 1 and 2).
near-Nernstian response. The calibration curves of the other ions did not exhibit linear relation to their logarithmic activity. However they were still quite useful because of their good reproducibility. In Figure 3 the calibration curve of ammonium ion was also compared with that obtained by the same detector when deionized water was employed as carrier flow and noncolumn system. Because of the change of the base line potential due to hydrogen ion in the eluent, the detector response to ammonium ion is markedly affected in low concentration range. Consequently, the range of the linear response to NH4+became narrow and the lower limit of the range was l X M NH4+ for a 100-WLinjection. The detection limit defined as a two times higher response peak than the noise is below 1 X M for NH4+ (100-pL injection). However, since the limit also depended on the reproducibility of response peaks, the effective determination limit was 2 X lo* M NH4+ (100 pL), where the coefficient of variations for 10 times the sequential standard sample injection was k3.470.In the low concentration range from 2 X lo4 M to 1 X lo-&M NH4+,its calibration curve is almost linearly related to the activity of NH4+on a nonlogarithmic scale. One of the important factors for determining the response peak was injection volume. In Figure 3 the calibration curves of NH4+for a 100-pL injection was also compared with that for 2 mL (2000 pL). It was noticeable that the difference in response potential of both curves was almost proportional to the ratio of the injection volumes. Although the maximum injection volume was limited by the separation ability of an ion-exchange column, larger injection volumes were generally quite useful for increasing the response peak and decreasing the detection limit. The electrode potential defined as a base line in the chromatogram changed rapidly for the first 15 to 60 min and
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Anal. Chem. 1983, 55,2013-2016
reached a constant potential. The detector used for several weeks tended to require longer time to reach a constant base line potential than a new one. The variation of the potential was 0-10 mV for 10 h except for the first 1h of use. The noise (short time drift) on the base line was less than 0.1 mV. Since the system uses a low conductivity eluent such as weak acidic solution, the variation of dynamic flow resistance causes a large drift of the base line potential. Therefore, the following two points are important to reduce the noise; one is to use a pulse-free delivery pump and the other is to ground all the apparatus of the ion chromatograph including the eluent. A remarkable "tailing peak" was observed in the chromatogram when a sample was injected a t a very high concentration, but in most cases the response dropped to the base line potential previously observed. The sensitivity of most constructed detectors was kept almost constant for a t least 3 weeks. Further measurement of the lifetime is in progress. The detector can be easily reconstructed with exchangeable detector assemblies including ISEs of a coated-wire type, which is also prepared with ease when it is not usable any longer. This provides an additional advantage to the detectlor.
ACKNOWLEDGMENT The authers gratefully thank Koji Suzuki of Research Laboratories, Chugai Pharmaceutical Co., Ltd., for providing nactin chemicals. Registry No. Li, 7439-93-2; Na, 7440-23-5;NH4+,14798-03-9; K, 7440-09-7;Rb, 7440-17-7;Cs, 7440-46-2;valinomycin, 2001-95-8; nonactin, 6833-84-7;benzo-15-crown-5,14098-44-3;tetranactin, 33956-61-5. LITERATURE C I T E D (1) TBth, K.; Nagy, G.; Pungor, E. "Ion-Selective Electrode Methodology"; Covington, A. K., Ed.; CRC Press: Boca Raton, FL. 1979; Vol. 11, Chapter 4. (2) Small, H.; Stevens, T. S.;Bauman, W. C. Anal. Chem. 1975, 4 7 , 1801. (3) Franks, M. C.; Pullen, D. L. Anafyst (London) 1974, 9 9 , 503. (4) Schultz, F. A,; Mathis, D. E. Anal. Chem. 1974, 4 6 , 2253. (5) Suzukl, K.; Ishlwada, H.; Inoue, H.;Shirai, T. Presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, NJ, March 9, 1982; Abstract No. 332. (6) Cattrall, R. W.; Freiser, H. Afal. Chem. 1971, 4 3 , 1905. (7) Morf, W. E.; Simon, W. Ion-Selective Electrodes in Analytical Chemistry"; Freiser, H., Ed.; Plenum Press: New York, 1978; Voi. 1, Chapter 3.
RECEIVED for review March 2, 1983. Accepted June 1, 1983.
Determination of Aliphatic Aldehydes in Air by Liquid Chromatography K a z u h i r o Kuwata,* Michiko Uebori, Hiroyasu Yamasaki, a n d Yoshio Kuge
Environmental Pollution Control Center, 62-3, 1 Chome, Nakamichi, Higashinari-ku, Osaka City 537, Japan Yoshiyuki Kiso
Applied Physics and Chemistry, Faculty of Engineering, Hiroshima University, Shitami, Saijo, Higashihiroshima City 724, Japan Aldehydes have received much attention as hazadrous and odorous substances in studies of air pollution. They often enter the environment from industrial plants, incinerators, and automobiles, and they are even photochemically produced under sunlight in the atmosphere. The method widely proposed or discussed to determine such aldehydes is to form their 2,4-dinitrophenylhydrazonesand then to determine the derivatives by high-performance liquid chromatography (HPLC)
(1-7). In conventional methods, however, the analysis undergoes rather tedious procedure especially in sampling aldehyde vapors with impingers or bubblers. The troublesome background peaks observed in trace analysis of aldehyde samples are difficult to eliminate because the reagents and the analytical tools used are more or less exposed to the ambient air and because the reagents and the solvents naturally contain traces of aldehydes as impurities. Besides, the unhandy sampling apparatuses result in difficulty in conducting a field investigation where a number of samples should be simultaneously collected. A convenient silica column in which 2,4dinitrophenylhydrazine (DNPH) and hydrochloric acid are coated is used to trap low parts-per-million (ppm) (v/v) levels of formaldehyde in air sample ( 4 ) . On the other hand, glass cartridges packed with glass beads impregnated with DNPIl in phosphoric acid-saturated poly(ethy1ene glycol) are used to trap hundreds of parts-per-billion (ppb) (v/v) or low ppb levels of formaldehyde, acetaldehyde, benzaldehyde, and other carbonyl compounds, anid the DNPH derivatives of carbonyl compounds are determined by HPLC with a 3-pm particle ODS column ( 5 , 6 ) . However, the preparation of these Sampling tubes is considerably tedious and substantial background
peaks still seem to be observed in determining low ppb levels of the aldehydes in air samples. The recently developed Sep-PAK CLB(SP)cartridge is often used to enrich and cleanup trace components from food (8-11), environmental (12-15), and biological samples (16-20) with savings of the number of steps and total time. T o date, the SP cartridge, however, has not been applied to analysis of air samples. In this study, a rapid and simple HPLC method using a S P cartridge impregnated with pure DNPH and phosphoric acid is presented to sample and determine CI-C4 aliphatic aldehydes in the low ppb or low ppm range in the atmosphere, industrial emissions, and incinerator emissions. The use of the cartridge resulted in minimized background effects. EXPERIMENTAL SECTION Reagents and Materials. The acetonitrile used was of chromatographic grade and distilled for recrystallization of the 2,4-dinitrophenylhydi-azine (DNPH).DNPH, which was of special grade from Wako (Osaka, Japan), was recrystallized with 25% acetonitrile in water. The C1-C4 aldehyde-DNPH derivatives as the standards were synthesized and recrystallized with ethanol as described elsewhere (3). The standard solutions for calibration were prepared by dissolving the derivatives in acetonitrile and by appropriately diluting the solution with acetonitrile. The Sep-PAK CIB(SP) cartridge was from Waters Associates (Milford, MA). Apparatus. A Waters Associates ALC/GPC 244 liquid chromatograph with a U6K injector and an ultraviolet absorbance detector adjusted to 365 nm was employed. The analytical column used was a 15 cm X 4.0 mm i.d. tube packed with Develosile ODs-3 (3 wm) (Nomura Kagaku, Aichi, Japan) (21). The mobile phase
0003-2700/83/0355-2013$01.50/00 1983 American Chemical Society
