1500
Anal. Chern. 1980, 52, 1500-1503
mulate during the clean-up procedure is probably the primary factor in limiting the sensitivity potential at this time.
(1 1) Norstrom, A.; Chaudhary. S. K.: Aibro, P. W.: McKinney. J D. ChemOsphere 8 0 . 1979, 331-343. (12) Erlanger, B. F.; Borek, F.; Beiser, S. M.; Liberman, S. J . B o / . Chem. 1957, 228, 713-727. (13) Albro, P. W.; Luster, M. I.; Chae, K.; Chaudhary S.K.;Clark, G.; Lawson. L. D.: Corbett, J. T.; McKinney, J. D. Toxicol. Appl. Pharmacol. 1979, 5 0 , 137-146. (14) Luster, M. I.; Albro, P. W.; Clark, G.; Chae, K.; Chaudhary, S.K.; Lawson, L. D.; Corbett, J. T.; McKinney, J. D. Toxicol. Appl. Pharmacol. 1979, 50, 147-155. (15) Aibro. P. W.; Corbett, J. T. Chemosphere 1977, 7 , 381-385. (16) Luster, M. I.; Albro, P. W.; Chae, K.; Chaudhary, S. K.;McKinney, J. D. Chapter in "The Chemistry of Environmental Agents as Potential Human Hazards", McKinney, J. D., Ed.; Ann Arbor: Ann Arbor. Mich., in press. (17) Hunter, W. B. "Handbook of Experimental Immunology", 2nd ed.; Blackwood Books: Oxford, 1973; Chapter 17. (18) Karush. F. Adv. Immunol. 1962, 2 , 1-23. (19) Albro, P. W.; Parker, C. E. J . Chromatogr. 1979, 169, 161-166. (20) Moore, J. A.; McConneli, E. E.; Daigard, D. W.; Harris, M. W. Ann. N .Y . Acad. Aci. 1979, 320, 159-163.
LITERATURE CITED (1) Bowes, G. W.; Mulvihill, M. J.; Simonet, B. R. T.; Burlingame, A. L.; Risebrough, R. W. Nature (London) 1975, 256, 305-307. (2) Goldstein, J. A.; Friesen, M.; Linder, R. E.; Hickman. P.; Hass, J. R.; Bergman, H. Biochem. Pharmacol. 1977, 26, 1549-1557. (3) Roach, J. A. G.: Pomerantz. I. H. Bull. Environ. Contam. Toxicol. 1974, 12, 338-342. (4) Huckins, J. N.; Stalling, D. L.; Smith, W. A. J . Assoc. Off. Anal. Chem. 1970, 6 1 , 32-38. (5) Ahling, B.; Lindskog, A,; Jansson, B.; Sundstrom, G. Chemosphere 1977, 6 , 481-466. (6) Naaayama. J.; Masuda. Y.; Karatsune. J. Food Cosmet. Toxicol. 1977. 15: i95-203. (7) Rappe, C.; Buser, H. R.; Kuroki, H.; Masuda, Y. Chemosphere 1979, 8 , 259-266. (8) Moore, J. A.; Gupta, B. N.;Vos, J. G. "Proceedings of the National Conference on Polychlorinated Biphenyls"; Environmental Protection Agency: Washington, D.C., 1976; pp 77-80. (9) Buser, H. R. Anal. Chem. 1977, 49, 918-922. (IO) Hutzinger. P.; Safe, S.; Zitko, V . Int. J . Environ. Anal. Chem. 1972, 2 , 95-102.
RECEIVED for review February 15, 1980. Accepted May 19, 1980.
Carbon Monoxide Detection in Nitrogen Gas by Atmospheric Pressure Ionization Mass Spectrometry Hideki Kambara,
Yukiko Ogawa, Yasuhiro Mitsui, and Ichiro Kanomata
Central Research Laboratory, Hitachi, Ltd., Kokubunji, Tokyo 185, Japan
The carbon monoxide content in a purified nitrogen gas is detected by a highly sensitive atmospheric pressure ionization (API) method. Although CO has the same molecular weight as N,, its ionization potential is quite different. Most ion species produced from nitrogen disappear when 100 ppm of Kr is added to the sample gas, and only ions relating to CO ((CO),' and Kr"C0) can be successfully and selectively detected. Relation curves between CO concentration and (CO),', as well as Kr'CO ion intensities are obtained. The concentration of CO included in a purified nitrogen gas is estimated to be 110 ppb. The detection limit of Kr'CO ions in this system is determined by the interference from Kr'N, ions and is 15 ppb or less.
Detection of carbon monoxide included in nitrogen gas has generated much interest in the field of air pollution measurement (I). Several approaches for detecting trace CO have been reported, in which the detection limit was around 0.1 ppm (2,3). However, the detection limit required for impurity analysis of purified standard gases for air pollution measurement is less than 0.02 ppm of CO. No adequate analytical method with this capacity has been reported yet. Generally, mass spectrometry has a high sensitivity for a large number of compounds. However, it has been impossible to detect trace CO included in nitrogen gas by mass spectrometry. This is because the mass difference of CO and Nz is extremely small (0.01123 amu). These two ions can only be distinguished by high resolution mass spectrometry. However, even high resolution mass spectrometry has not been capable of detecting CO trace in nitrogen gases. Atmospheric pressure ionization (API) (4-7) has been shown to be a very sensitive method for detecting various air pollution components. This paper reports the ability of the API method t(J reliably detect trace CO in nitrogen gas. The technique involved has been successfully applied to estimating CO 0003-2700/80/0352-1500$01 .OO/O
concentration in a highly purified nitrogen gas. Ion Molecule Reactions. Both N, and CO produce ions of mass 56 (N4+and (CO),+) in the APl ion source. T h e ionization potentials of N2 and CO are 15.6 and 14 eV, respectively (8). The bond dissociation energies of Nz+-N2and CO+-CO have been reported to be 1.0 and 1.1 eV (9, IO), respectively. Therefore the ionization potentials of N4 and (CO)2 are estimated to be 14.6 and 12.9 eV, respectively. If any stable reactant gas, the ionization potential of which is between 12.9 and 14.6 eV, is added to the mixture gas of CO and N2, N4+ ions will be quenched and not observed while (CO),' ions will be observed. Krypton is adequate for this purpose. The ionization potential of Kr is 14 eV. Ions of Kr2+ are produced in the API ion source, the bond dissociation energy of which is about 1 eV (11). Therefore, the ionization potential of Kr2 is between those of N4 and (CO)2. The collision number of a n ion with neutral molecules in an API ion source is about lo5,consequently a highly selective ionization occurs. As a result, from 10 to 100 ppm of Kr in the nitrogen gas is sufficient t o quench N4+ions through ion molecule reaction sequences. T h e following ion molecule reactions initiated by a corona discharge occur in the ion source:
+ - + + + + + + - + + - + + + + +
corona discharge N+,N2+
2N2
N+,N2+
Ns+,N4+
N2
N4+ K r K r + N 2 N2 Krz+ N 2 K r + N 2 Kr N4+
O2
0,'
Kr2+
O2
02+ 2Kr Kr'CO Kr
Kr2+
Kr+CO
CO
CO
2N2
(CO),+
Kr
(1)
12) (3)
(4) (5)
(6) (7)
(8)
When the quantity of Kr is not sufficient to produce clusters C 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
m-
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Flgure 1. Schematic view of the API system. (1) Highly purified nitrogen gas, (2) purified nitrogen 9 s including 500 ppm of Kr, (3)purified nitrogen gas including 5 . 2 ppm of CO (standard gas for calibration),(4) liquid nitrogen, ( 5 , 6) molecular sieve traps, (7) needle electrode,(8) ionzation region, (9) collision chamber of Krz+,ions of Kr+N2are strongly observed together with N4+ ions. These ions disappear when Kr concentration is increased (see the next section). When oxygen or carbon monoxide are in the nitrogen gas, ions of 02+ or Kr'CO as well as (CO),' are observed.
EXPERIMENTAL A schematic view of the system is shown in Figure 1. It consists of a gas supply system, an ion source operated at atmospheric pressure, a collision chamber at 1 Torr, and an analyzing region at 2 X lo-' Torr. Primary ions were produced by a corona discharge with a needle electrode. Discharge current was 0.5 PA. Ion source temperature was fixed a t room temperature ( 2 5 "C) throughout the experiment. The collision chamber was separated from the ion source and the analyzing region with two aperture electrodes and evacuated with a rotary pump. The length of the chamber was 6 mm. The diameter of the first and the second apertures was 0.1 and 0.2 mm, respectively. A 10-V drift voltage was supplied between these two aperture electrodes. This improved the transmission efficiency of ions drifting into the analyzing region. The quadrupole mass spectrometer used here could detect ions of masses between 1and 170. Details of the apparatus were reported in previous papers (7, 22). A highly purified nitrogen gas including 100 ppm of Kr was used as a carrier. Purified nitrogen gas and nitrogen gas including 500 ppm of Kr were mixed to produce the mixture gas. The sample nitrogen gas including CO was mixed with the carrier and introduced into the ion source at a flow rate of 1.1L/min. A small amount of water was included in the gas. This prevented CO detection because of the production of water clusters H+(H,O), (n = 1, 2 , . .). Molecular sieve traps were inserted just before the ion source to remove the water from the gases. An equilibrium between CO adsorption on and desorption from the molecular sieves was established within 20 min after the start of the gas flow. It was confirmed by observation that Kr+CO or (CO),+ ion intensity did not change after 20 min had elapsed at a constant gas flow speed. A relation curve between ion intensity and CO concentration was obtained using a standard gas, Le., nitrogen gas including 5.2 ppm CO. Glass tubing and metal fittings used in the gas supply system were thermally purified until the major ions in the API spectrum for nitrogen gas became N4+. Generally, even highly purified nitrogen gas includes a small amount of CO. The trap put in a carrier gas stream was cooled with liquid nitrogen to remove any CO trace in the carrier and to investigate the detection limit of the system.
RESULTS AND DISCUSSION Background Spectrum. T h e API mass spectrum of nitrogen gas is shown in Figure 2. T h e major ion species were N4+ a n d N2+ which dissociated from N,+ in the collision chamber. T h e details of collisional dissociation phenomena in a n API system were reported previously (12, 13). Ions of HzO+, 02+,N3+, and H 2 0 + N zwere also observed in t h e spectrum. T h e relative ion intensity of 02+ was 0.2%. Oxygen concentration in the nitrogen gas was estimated as being about
ti26 0-
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Figure 2. Background spectrum for purified nitrogen gas
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Figure 3. Kr concentration dependence of API spectra. (a) 100 ppm of Kr, (b) 60 ppm of Kr, (c) 30 ppm of Kr, (d) 10 ppm of K r 40 ppb from a quantitative API study of oxygen analysis which will be published elsewhere. T h e API spectra changed drastically, as shown in Figure 3, by adding a small amount of K r (from 10 t o 100 ppm) t o the gas stream. As 10 ppm of Kr was added in the gas stream, ion intensities of Nz+and N4+went down with Kr+ and Kr+N2 appearing strongly in the spectrum instead. There are several isotopes of K r . T h e isotope a b u n d a n c e is 80Kr:82Kr:83Kr:&LKr:s6KI = 23:11.6:11.6:57:17. Although the ion intensities of N2+,N4+,Kr+, and Kr+N2decreased as the K r concentration was increased from 10 t o 100 ppm, those and N3+ stayed almost constant for various K r of H20+,02+, concentrations. These three species have ionization potentials lower t h a n those of K r or (Kr),. Clusters of Kr2+ were observed, although, they are not shown in the figure. T h e spectrum was not changed in a Kr concentration range higher
1502
ANALYTICAL CHEMISTRY, VOL. 5 2 , NO 9, AUGUST 1980
,
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Figure 5. Relation curves of CO concentration and (CO),+as well as Kr'CO ion intensities
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56
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Figure 4. CO concentration depenaence of API spectra (Kr concentration is 100 ppm). (a) No addition of CO, (b) 0 . 4 9 ppm of CO, (c) 0.98 ppm of CO, (d) 2.1 ppm of CO, (e) 4 . 2 ppm of CO
t h a n 100 ppm. I t was concluded that 100 ppm of Kr was enough to quench nitrogen ions of N4+or Kr+N2and to detect ions produced from CO. R e l a t i o n C u r v e s of I o n I n t e n s i t i e s a n d C O Concentration. A nitrogen gas including 5.2 ppm of CO from Nippon Sanso Co. was used as the standard gas for obtaining the relation curves of ion intensities and CO concentration. The standard gas was put into the carrier gas, i.e., highly purified nitrogen gas including Kr, a t various flow ratios giving a CO concentration from 0.4 to 4 ppm. T h e Kr concentration in the total gas remained constant (100 ppm) throughout the experiment. Changes in the spectra are shown in Figure 4. Ions of masses around 112 increased with CO concentration. These were identified as Kr+CO. Ion intensity of mass 56 was proportional to the square of CO concentration, indicating that this species was (CO),+ and not CO+Nz. T h e relation curves of both ion intensities of (CO),' and Kr+CO and CO concentration are shown in Figure 5. Electron multiplier gain was about lo4. Ion transmission efficiency was very poor in a high mass range and the Krz+ion intensity (Kr2+ being the major component in the spectra) was not measured appropriately. Consequently, relative ion intensities of the above components, which would be more adequate for obtaining these relation curves, were not obtained. These relation curves did not depend on the concentration of coexisting Kr in a concentration range from 100 ppm to at least 500 ppm. A of T h e result shows t h a t 1 ppm of CO gives 0.35 X Kr+CO ion current under this experimental condition. Q u a n t i t a t i v e A n a l y s i s of C O i n a H i g h l y P u r i f i e d N i t r o g e n Gas. T h e CO concentration in a highly purified nitrogen gas, Le., 99.999% pure, was estimated by observing Kr+CO ion intensity. To estimate Kr+N2interference on the ion intensity of mass 112 (@Kr 28) and t h a t of CO not included in the gas, the background spectrum was observed by cooling the molecular sieve trap with liquid nitrogen. The A. After the obion current of mass 112 was 0.017 X servation of background spectrum, the molecular sieve trap was warmed up to room temperature. After an hour, the ion intensity of (@Kr 28)' became constant. This suggested t h a t the adsorption and desorption equilibrium between the
+
+
molecular sieve or glass tubing surfaces and the flowing gas was established. T h e ion intensity of mass 112 was 0.055 i~ A, indicating that the actual Kr+CO ion current 0.010 X A. T h e due to CO in the gas flow was 0.038 f 0.010 X CO concentration was estimated to be 110 f 30 ppb from the relation curves in Figure 5. T h e background ion current of (@Kr+ 28) (0.017 X lo* A) corresponds to 50 ppb of CO. This determined the practical detection limit of CO by the API method. There were three possible sources of the species (Kr + 28)': (a) CO desorption from the glass tubing between the molecular sieve trap and the ion source, (b) a small amount of CO flowing out through the molecular sieve trap cooled with liquid nitrogen, and (c) production of Kr+Nz. T h e glass tubing was thermally re-purified over flowing carrier gas through a molecular sieve trap cooled with liquid nitrogen to investigate the possibility of case a. However, any significant change in the ion intensity of (Kr + 28)' was not observed, suggesting that case a was a minor source. T h e ion intensity of (Kr + 28)' increased t o a quantity corresponding to several ppm of CO a t fist and then decreased to 0.06 X lo* A by removing the liquid nitrogen from the trap. This indicated that CO was condensed in the molecular sieve trap when it was cooled with liquid nitrogen. Consequently, case b must not be a major source of the residual (Kr + 28)' ions. There were two probable cluster production processes in case c. One could be that Kr+N2 was produced in the ion source, where the Kr+Nz ion intensity should be dependent on Kr concentration. However, the intensity was independent of the Kr concentration in a concentration range from 100 to 300 ppm. The result was that these ions were not produced in the ion source. Another Kr+Nz production process could be a switching reaction of Krz+ with Nz through collisional excitation of drifting Kr2+ ions in the collision chamber. Similar switching reactions occuring in the collision chamber have been reported previously (13,14). This switching reaction seems to be the major Kr+N2 creation process because the Kr+Nz ion intensity decreased to one third when the drift voltage was lowered from 10 to 6 V. Generally, collisional excitation, therefore the switching reaction, can be lessened by reducing the drift voltage. Consequently, it seemed that most of the residual (Kr 28)' ions were produced through this collisional excitation process in the collision chamber and that the background signal could be diminished to a quantity corresponding to less than 15 ppb of CO by reducing the drift voltage.
+
Anal. Chem. 1980, 52, 1503-1505
LITERATURE CITED (1) Fox, D. L.; Jeffries,H. E . Anal. Chem. 1979, 57, 22R. (2) Chane, L. W.; McClenny, W. A. Environ. Sci. Technoi. 1977. 1 1 , 1186. (3) Polasek, J. C.; Buliin, J. A. Environ. Sci. Technol. 1978, 12, 708. (4) Horning, E . C.; Horning, M. G.; Carroll, D. I.; Dzidic, I.; Stillwell, R . N . Anal. Chern. 1973, 45, 936. (5) Siegel, M. W.; Fite, W . L. J . Phys. Chem. 1978, 80,2871. (6) Reid, N. M.; French, J. B.; Buckky, J. A,; Lane, D. A,; Lovett, A. M. Sciex Inc. Application Note No. 677, 1977. (7) Kambara, H.; Kanomata. I. Anal. Chem. 1977, 49, 270. (8) Rosenstock, H. M.; Draxl, K.; Steiner, B. W.; Herron, J . T. J . Phys. Chem. Ref. Data 1977, 6 , 267.
1503
(9) Payzant, J. D.; Kebarle, P. J . Chem. Phys. 1970, 53, 4723. (10) Meot-Ner, M.; Field, F. H . J . Chern. Phys. 1974, 6 1 , 3742. (11) Ng, C. Y.; Trevor, D. J.; Mahan, B. H.; Lee, Y . T . J . Chem. Phys. 1977. 66, 446. (12) Kambara, H.; Kanomata, I. Int. J . Mass Spectrom. Ion phys. 1977, 25, 129. (13) Kambara, H.; Mitsui, Y . ; Kanomata, I. Anal. Chern. 1979, 5 1 . 1447. (14) Kambara, H.; Misui, Y.; Kanomata. I. Int. J . Mass Spectrorn. Ion phys.,
in press.
RECEIVED for review January 28, 1980. Accepted April 14, 1980.
Tubular Flow Donnan Dialysis James A. Cox" and Zbigniew Twardowski Department
of Chemistry and
Biochemistry, Southern Illinois University at Carbondale, Carbondale, Illinois 6290 1
Pumping the receiver electrolyte through cation-exchange tubing is demonstrated to yield more efficient Donnan dialysis enrichments than the use of static systems. Twofold enrichments are achieved in a 1.4min dngie pass experiment, and 50-fold enrichments are attained in 20 min by circulation of an aliquot of electrolyte through a receiver loop. The dialysis rate is directly proportional to sample concentration and independent of sample matrix factors over a wide range of conditions; hence, quantitation of real samples can be performed with a linear calibration curve. In this method the metals in the dialysate are determined by differential pulse polarography or anodic stripping voitammetry.
The determination of trace quantities of ions in real samples often requires pretreatment steps. The original matrix may not be compatible with the analytical method, matrix variation may not be corrected by a n internal standard or standard addition approach, or the sensitivity of the analytical method may be too low t o perform the determinations without preconcentration. Donnan dialysis has been demonstrated to be a useful technique for matrix normalization and ion enrichments (1-3). Typically it has been performed by contacting a 50-200 mL stirred sample to a few milliliters of static receiver electrolyte through a n ion-exchange membrane. The uses of stirred receiver electrolytes ( 4 ) and circulated samples and receivers (5) have also been reported. The latter had a receiver chamber which was a 0.16 cm deep rectangular solid indentation with t h e ion-exchange membrane covering one face; baffles were used to increase turbulence. The static receiver approach has the disadvantage of yielding rather modest enrichment factors; values of 5-10 were achieved with 1-h dialyses. With the stirred receivers, greater enrichments were attained; values of 100 were possible by allowing the systems to reach Donnan equilibrium. For monovalent ions, 2 h was required whereas about 20 h was needed to reach Donnan equilibrium for divalent species ( 4 ) . T h e above limitations are a result of a n unfavorable ratio of receiver volume to ion-exchange membrane surface area in cases where milliliter-size aliquots are needed for the quantitation step. For example, 10-fold enrichments can be attained in a few minutes if microliter volumes are used for t h e receiver ( 3 ) . 0003-2700/80/0352-1503$01 .OO/O
The availability of tubular cation-exchange membranes introduces a convenient means of decreasing the receiver volume-to-surface ratio while using receiver volumes in the milliliter range. Further, by pumping the receiver through the tubing rather than using a static system, the effect of concentration polarization can be decreased. Only recently (6) has concentration polarization in the receiver been suggested to be important in determining the overall transport rate. In the present paper, it is demonstrated that such a flow system results in significantly greater enrichment factors while maintaining the matrix normalization function of Donnan dialysis. EXPERIMENTAL The cation-exchange tubing used was Nafion 811 (DuPont Polymer Products, Wilmington, Del.) of dimensions 0.025-inch i.d. and 0.035-inch 0.d. Nafion is a sulfonated fluorocarbon polymer of equivalent weight 1100 in the protonated form. Figure 1 shows the dialysis assembly. Generally 245 cm of the Nafion 811 is coiled around a 3-cm diameter perforated polyethylene tube. The ends of the cation-exchange tubing are tied to the support with nylon thread. Connections to the peristaltic pump (Buchler Instrument Company) are made with 0.8-mm i.d. Teflon tubing. The cation-exchange tubing coil is completely immersed in the sample solution. The latter is magnetically stirred. Up to lo00 mL of sample is used. The cation-exchange tubing volume was 0.77 mL, and the total volume of the receiver system was 2.17 mL in the typical experiment. The dialysis experiments were performed in two ways. One approach was to continuously pump receiver electrolyte through the cation-exchange tubing into a collection chamber. In this method whenever the nature of the sample was drastically changed, the dialysate was diverted to a waste vessel for 5 min prior to collection in order to condition the membrane. Alternatively, 5 mL of the receiver electrolyte was circulated through the tubing for a prescribed time, diverted into a 10-mL volumetric flask, and diluted to volume. After each set of experiments, the ion exchanger was cleaned by dipping in 1 M HCl while the same concentration of the acid was circulated through the system. After 1 h, the HC1 solutions were replaced with 2 M MgS04. The latter solutions were replaced after an additional 2 h with ones of the same composition as the receiver electrolyte. The system was stored in the electrolyte for at least 2 h prior to initiation of a new series of experiments. I t should be noted that for routine work a few rinses with receiver electrolyte in both the sample chamber and the tubing is sufficient. The receiver electrolyte was generally a 0.2 M MgS04,5 X lo4 M A12(S0& mixture (2). The MgS04 was purified by controlled potential electrolysis of a 1 M solution at a stirred Hg pool (%: 1980
American Chemical Society
