1524 (17)
Anal. Chem. lQ86,58, 1524-1527
American Chemical Society Committee on Envlronmental Improvernent. Anal. Chem. 1880. 52, 2242-2299.
RECEIVED for review August 19,1985. Accepted January 14, 1986. This research was supported by the US.Environmental
Protection Agency through R810894-010 and CR812366-01-0. However, this report has not been subject to review by the Environmental Protection Agency and therefore does not necessarily reflect the views of the Agency, and no official endorsements should be inferred.
Selective Chlorine Dioxide Determination Using Gas-Diffusion Flow Injection Analysis with Chemiluminescent Detection David A. Hollowell,' James R. Gord, Gilbert Gordon, and Gilbert E. Pacey*
Department of Chemistry, Miami Uniuersity, Oxford, Ohio 45056
An automated chemiluminescent technlque has been developed utlllrlng the advantages of gas-dlffuslon flow lnjectlon analysis. A gas-dlffuslon membrane separates the donor (sampling) stream from the acceptor (detectlng) stream and removes lonlc Interferences. A novel chemllumlnescence flow-through detector cell Is used to measure the concentration of chlorine dloxlde as a functlon of the Intensity of the chemllumlnescence produced from Its reactlon wlth lumlnol. The chemllumlnescent reagent merges wlth the analyte dlrectly In front of the photomuklpller tube In order to maxlmbe the sensltlvlty of the system. The detectlon llmlt for chlorlne dloxlde Is approxlmately 5 ppb. The method Is over 1500 times more selectlve for chlorlne dloxlde than for chlorine on a mole basls. This method ellmlnates Interference from Iron and manganese compounds, as well as other oxychlorlnated compounds such as chlorlte Ion and chlorate Ion.
The primary uses of chlorine dioxide are the disinfection of water and the bleaching of paper pulp (1). The health hazards associated with the byproducts of water chlorination have caused chlorine dioxide to be considered a viable alternative because of two main advantages: chlorine dioxide does not react with ammonia to form chloramines and chlorine dioxide decreases the formation of chlorinated organic byproducts. Minimizing the oxychlorinated species residual in drinking water is important for health reasons, and therefore they must be monitored routinely (2). The target level for research in the determination of chlorine dioxide should be at least 10 times lower than the recommended levels. At this point in time, the lowest recommended level is the 7-day no-adverse-response level (SNARL) of 0.125 mg/L chlorine dioxide that has been recommended by the National Research Council's Safe Drinking Water Committee (3). Spectrophotometric ( 4 4 3 ,iodometric (7),voltammetric (8), and amperometric (9) methods have all been used for the determination of chlorine dioxide. These methods are not capable of selective and reproducible ( 5 % or less) measurement of chlorine dioxide at the SNARL level. All have varying degrees of interferences, with chlorine being the most common and largest interferent in most cases. The chemiluminescent methods for the determination of chlorinated species have exhibited superior detection limits and selectivity (10-14). There are three main reagents that have been used for chemiluminescent detection of chlorine l
Present address: Smith, Kline, Philadelphia, PA, 0003-2700/88/0358-1524$0 1.50/0
compounds in the literature. The first is luminol (&amino2,3-dihydro-l,I-phthalazinedione) with hydrogen peroxide as a catalyst (10-12). The second solution consists of luminol by itself for the determination of free chlorine (13). The final solution consists of hydrogen peroxide by itself (14). Previously (2) we described an absorbance method for the determination of chlorine dioxide utilizing gas-diffusion flow injection analysis (FIA).van der Linden has recently published a definitive paper on the gas-diffusion process in FIA (15). In gas-diffusion flow injection analysis the analyte first must pass through a membrane before the detection process. This separation step removes most of the possible interferents and ultimately improves the selectivity of the method. The shortcoming of the prior method is that its detection limit does not fall below the recommended SNARL level. The current investigation is concerned with evaluating and optimizing the chemiluminescent reactions between chlorine dioxide and various chemiluminescent reagents using gas diffusion for the selective determination of chlorine dioxide in water. The goal of this research was to develop a method that has a detection limit of 0.01 mg/L or less.
EXPERIMENTAL SECTION Apparatus. A schematic of the gas-diffusion flow injection analysis system is shown in Figure 1. The flow system consists
of a Tecator Model 5020 flow injection analyzer with a Tecator Chemifold V gas-diffusion manifold. Figure 2 is a schematic of the T-Spiral flow cell that was mounted directly in front of a photomultiplier tube in order to maximize detection of the light from the chemiluminescent reactions. The membrane used was a 0.45-wm-pore-size Teflon membrane (W. L. Gore and Associates). The detector used consisted of a GCA/McPherson photomultiplier module (Model EU-701-30). The output from the PMT was connected to a Keithly electrometer (Models 601 and 617 were both used at various times). The output from the electrometer was fed to a strip chart recorder, and the resultant peak heighta were measured. The flow rates of the donor and acceptor streams were each 1 mL/min unless otherwise indicated. Replicate injections of at least 6 per sample were made in all cases. The flow system used 0.5-mm-i.d. Teflon tubing. Reagents. The method for generating chlorine dioxide solutions has been described previously (16). The stock solution obtained from this procedure was refrigerated in the dark to avoid decomposition. Chlorine solutions were prepared daily by bubbling chlorine gas into pH 2 triply distilled water (adjusted with nitric acid). 3-Aminophthalhydrazide (luminol) (Aldrich) and hydrogen peroxide (Fischer Scientific) were used as received. Phosphate and borate buffers were used to study the effects of pH on the chemiluminescence. Total phosphate and borate concentrations were 0.025 M in all buffered systems. M) soluSpectrophotometric standardization of dilute ( tions of chlorine dioxide at 359 nm utilized a molar absorptivity 0 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1986 5.5
Luminescent R e agen 1
A
PMT
Acceptor Stream 1 rnllmin
Waste
5i
A
1525
1
A A
A A
I
b
I
A
Cell
\ Donor Stream -I
PYlw
Waste
erislaltic Pump
Flgure 1. Schematic dlagram of FIA system used to study the
chemiluminescence. Luminol Reagenl
Plexiglass Body
Sample Stream
'1
1.5
-
I
-
+
A
&
TO
r
-
Spiral
Photomultiplier Tube
I SIDEVIEW R t b
A t A
d
Figure 2. Schematic of T-spiral flow cell.
value of 1250 M-l cm-l (17,18). Chlorine solutions were stand(DPD) ardized by using the N,N-diethyl-p-phenylenediamine ferrous titrimetric method (19). Monochloramine samples for interference studies were prepared (under basic conditions) with ammonium chloride and chlorine and were standardized by using ferrous titrimetric method the Nfl-diethyl-p-phenylenediamine (19).
RESULTS AND DISCUSSION The optimum injection volume for maximizing the concentration of chlorine dioxide passing through the Teflon membrane without achieving equilibrium has been determined previously (2). Therefore, in order to obtain maximum sensitivity, a volume of 350 p L was chosen as the sample volume for all experiments carried out in this investigation. In order to make a direct comparison of the three chemiluminescent reagents, luminol only, hydrogen peroxide only, and luminol with hydrogen peroxide, each of these solutions was tested in the same system using chlorine dioxide as the oxidizing species. The light output from the three reagents was observed a t different pH values. As can be seen from Figure 3, the peroxide reaction has a maximum light output at pH 8. The luminol reagent light intensity is at its maximum a t pH 13, which is also about the same magnitude of light intensity as the hydrogen peroxide reagent at its maximum at pH 8. The solution containing luminol with hydrogen peroxide as a catalyst appears to be the best reagent for the determination of chlorine dioxide due to the increased intensity of light at the maximum around pH 9. The intensity at this point is approximately 2 orders of magnitude larger than the
PE
Flgure 4. Relative light emission for chlorine dioxide (+) and normalized M chlorine (A) as a function of pH using lo-' M luminol wlth
hydrogen peroxide. maximum intensities for the other two reactions. However, Figure 3 does not take into account the fact that chlorine is a major interferent in almost all methods for the determination of chlorine dioxide. Therefore, each of the three reactions was tested with chlorine to determine the selectivity of each reagent. The results for chlorine were normalized by multiplying the peak light output for chlorine by the ratio of the concentration of chlorine to chlorine dioxide concentration so that a direct comparison to chlorine dioxide could be made. Figure 4 shows the difference in light output between chlorine dioxide and normalized chlorine as a function of pH of the solution containing 1X M luminol in the presence of 1X M hydrogen peroxide. Based on these results, at the optimum pH of this system, a selectivity factor of about 10 could be expected based on the difference in light intensity for chlorine dioxide vs. chlorine. A comparison of the light output between chlorine and chlorine dioxide as a function of pH using 0.1 M hydrogen peroxide as the chemiluminescent reagent gave a slight increase in selectivity and a decrease in the sensitivity compared to the reagent containing luminol with peroxide. The final comparison is made in Figure 5 for the solution containing only luminol. This graph compares the light output
1526
ANALYTICAL CHEMISTRY, VOL. 58, NO. 7, JUNE 1986 Table I. Solutions Tested for Their Possible Interference Levels”Using Gas-Diffusion FIA compd
4
+
+
A
A
Fe(NH4)z(S04)z.6H20 KMn04 H202 NH4Cl NaCl
i i
+
A
A
A
+
tl
d
I
i
0.5
4I
concn, M
0.1 0.5 3b
NaCIOz NaC103 Fe(NO& KNOZ
1 1 0.1 1
1 1
Table 11. Determination of Chlorine Dioxide in the Presence of a Large Excess of Chlorine
I
I
O
I !
A
compd
“The solutions tested gave a signal that could not be distinguished from the background noise. Concentration in percent not molarity.
A
A
concn, M
A
I
L
.
-
solution composition CIOz added, Clz added, mg/L mg/L
-
7
13
I1
g
PB
Figure 5. Relatlve light emission for chlorine dioxide (+) and normalized chlorine (A) as a function of pH using M luminol.
t
6-
0.0160 0.0160 0.160 0.160 0.160
0.42 4.2 0.42 4.2 42
chlorine dioxide found, mg/L f std deviation
error, mg/L
0.0174 f 0.0002 0.0206 f 0.0004 0.165 f 0.002 0.171 f 0.002 0.192 f 0.002
0.0014 0.0046 0.005
0.011 0.032
t t
5t
4-
+ t
3-
d
t
a
b
et
A
J-
0 1 -8
I
I
-6
I
I
I
-4
I
-2
.
I
I 0
Log Molar Concentrition
Figure 8. Concentration dependence of eak height for chlorine dioxide (+) and chlorine (A): [luminol] = 10-P M; pH 13.
as a function of pH for both chlorine and chlorine dioxide. In this system containing only luminol, the sensitivity also decreases compared to the reagent containing both luminol and peroxide, but the selectivity is very high at pH 13. Due to the larger selectivity of the solution containing only luminol compared to the solution containing both luminol and peroxide, combined with a better observed reproducibility of the solution containing only luminol compared to the solution containing only 0.1 M hydrogen peroxide, only luminol was used at pH 13 for all subsequent experiments involving the determination of chlorine dioxide. The concentration of luminol was varied and found to produce the maximum amount of light around M. A decrease in light output a t higher concentrations of luminol M luminol a t pH was observed. A solution containing 13 was used in all subsequent studies. As the total flow rate through the flow cell increased, it was observed that the resultant peak height produced by the chemiluminescence also increased. The peak height is 10 times greater at a total flow rate of 4 mL/min compared to a flow rate of 1 mL/min. At 8 mL/min, the peak height is almost 25 times as large as the height a t 1 mL/min. This phenomenon is due to the design of the flow cell coupled with the very fast reaction of chlorine dioxide with luminol. As the flow rate increases, more of the chlorine dioxide enters the cell per unit time and “instantly” reacts with the luminol, thereby
increasing the resultant peak height and also decreasing the peak width. At the highest of the flow rates tested, the system has a tendency to leak due to the increased pressure. Figure 6 shows the calibration curve for this system. This graph also quantitatively shows the interference of injected chlorine. The detection limit (SIN = 3) for chlorine dioxide is 5 pgIL with a linear range up to 2 mg/L and a usable range up to 74 mg/L. The slope of the least-squares line for chlorine dioxide is 2.32 X lo9 mm M-l with a correlation coefficient of 0.9978. For injected aqueous chlorine, the detection limit is 8 mg/L with a linear range up to 7600 mg/L. The slope of the least-squares line for chlorine is 5.91 X lo3mm M-l with a correlation coefficient of 0.9990. This means that the system described is better than 1500 times more selective for chlorine dioxide than chlorine on a mole basis. Monochloramine in the prescence of chlorine produces no additional signal and therefore is not an interferent. Other compounds that commonly interfere with the determination of chlorine dioxide were tested to check their levels of interference. The results are shown in Table I. In all cases, the response of the solutions could not be distinguished from the base line. Note that these were the highest concentrationstested, not the maximum concentration before interference was observed. These maximum concentrations were much higher than those expected in real samples. In order to check the accuracy of this method, synthetic samples were prepared to contain mixtures of chlorine dioxide with relatively high levels of chlorine. The chlorine dioxide concentrations were chosen to be just above and almost an order of magnitude below the SNARL level. The results of these assays are shown in Table 11. Only a small error is observed when chlorine is in a slight excess. As is expected, the error increases as the chlorine to chlorine dioxide ratio increases to the point where a ratio of 262 gives an error corresponding to 5 ppb chlorine dioxide for the solution containing 0.016 mg/L chlorine dioxide and an error corresponding to 32 ppb chlorine dioxide for a 0.16 mg/L solution of chlorine dioxide. In cases where more selectivity would be needed, oxalic acid can be added to the donor stream to mask (2) the chlorine present in the injected sample. In conclusion, this method can be used to selectivity monitor chlorine dioxide at levels more than 1 order of magnitude below those recommended by the National Research Council Committee on Safe Drinking Water. The estimated sensitivity
Anal. Chem. 1986, 58, 1527-1529
of the previously reported chemiluminescent method (10) for the determination of chlorine dioxide has a detection limit around 50 ppb. The selectivity of this prior method was not quantitatively discussed, but the results reported here using luminol in the presence of hydrogen peroxide with gas diffusion indicate that our method using luminol only is more selective vs. chlorine and other interfering ions. It is shown that this method has a detection limit of 5 ppb, which is an order of magnitude lower. The selectivity of this method is excellent giving no interference from any ions tested and a selectivity factor of over 1500 for chlorine dioxide vs. chlorine (or monochloramine)on a mole-to-mole ratio. Therefore, the method presented here offers significant improvements for the selective determination of chlorine dioxide and can be used to reliably monitor the SNARL level recommended by the National Research Council. Registry No. CIOz, 10049-04-4;C1, 7782-50-5; luminol, 52131-3.
LITERATURE CITED (1) Masschelein, W. J. CHLORINE DIOXIDE: Chemlstry and Environmental Impact of Oxychlorjne Compounds; Ann Arbor Sci-
1527
ence: Ann Arbor, M I , 1979 Chapter 14. Hollowell, D. A.; Pacey, G. E.; Gordon, G. Anal. Chem. 1985, 5 7 , 2851. Drinking Wafer and Health; National Research Council, National Academy Press: Washington, DC, 1980 (Voi. 3), 1982 (Vol. 4). Hodgden, H. W.; Ingols, R. S. Anal. Chem. 1954, 26, 1224. Masscheleln, W. Anal. Chem 1966, 38, 1839. Palin, A. T. J . Am. Water Works Assoc. 1975, 6 7 , 32. Post, M. A,; Moore, W. A. Anal. Chem. 1959, 31, 1872. Smart, R. 6.; Freese, J. W. J . Am. Water Works Assoc. 1982, 7 4 , 530. Haller, J. F.; Lister, S. S. Anal. Chem. 1948, 2 0 , 639. Isacsson, U.; Wettermark, G. Anal. Chlm. Acta 1976, 8 3 , 227. Isacsson, U.; Wettermark, G. Anal. Lett. Part A 1978, A I I ( I), 13. Smart, R. 6. Anal. Lett. Part A 1981, A 14(3), 189. Marino, D. F.; Ingle, J. D. Anal. Chem. 1881, 53, 455. Marino, D. F.; Ingle, J. D. Anal. Chim. Acta 1981, 123, 247. W. E. van der Linden Anal. Chim. Acta 1983, 151, 359. Rosenbiatt, D. H.; Hayes, A. J., Jr.; Harrison, 6. L.; Streaty, R. A.; Moore, K. A. J . Org. Chem. 1963, 28, 2790. Kieffer, R. G.; Gordon, G. Inorg. Chem. 1968, 7 , 235. Gordon, G.; Kieffer, R. G.; Rosenblatt, D. H. The Chemistry of Chlorine Dioxide, Progress In Inorganic Chemlstry; Llppard, S . J., Ed.; Wiley: New York, 1972; Vol. 15, p 201. Standard Methods for the Examination of Water and Wastewater, 15th ed.; American Public Health Association: Washington, DC, 1980; 304.
RECEIVED for review September 3, 1985. Resubmitted February 3, 1986. Accepted February 3, 1986.
Air Sampling of N-Methylmorpholine on Solid Sorbent and Determination by Capillary Gas Chromatography and a Nitrogen-Phosphorus Detector Barbro Andersson* and Kurt Anderss'on National Board of Occupational Safety and Health, Research Department, Chemical Unit in Umea, Box 6104, S-900 06 Umea, Sweden
A method for sampling and determination of N-methylmorpholine In workroom air is presented. Sampling was performed on the adsorbent Amberilte XAD-7 at air levels of 2-100 mg/m3 methylmorpholine, and desorptlon was effected by solvent extraction with ethyl acetate. Analysls of the free amine was carried out by means of capillary gas chromatography with flame lonizatlon or nitrogen-phosphorus detection (FID and NPD, respectively). The recovery tests were performed at 20 % and 85 % relative alr humldlty, giving recoveries of 90-96% with a relatlve standard devlatlon of 2-5%. Afler 6 weeks of storage in a deep freezer at -20 OC, the recovery was even higher than 90%. The gas chromatographic detection limits of methylmorphoiine were 0.5 ng (FID) and 0.05 ng (NPD). The method was applled to the polyurethane industry.
Tertiary aliphatic amines, including N-methylmorpholine, find applications in a variety of industries: as corrosion inhibitors in steam boiler systems and as starting materials and solvents in organic synthesis. The main use of these compounds is, however, as catalysts in polyurethane production. In the vapor phase, methylmorpholine causes irritation in the eyes and the respiratory system. The occurrence of asthma among workers in the polyurethane industry has generally been ascribed to the isocyanates, but methylmorpholine and some other tertiary amines are now suspected as having similar 0003-2700/86/0358-1527$01.50/0
effects (1, 2). Moreover, the amines are present in 10-fold higher concentrations. Air sampling of tertiary amines has been performed on solid sorbents, such as silica gel, as recommended by the National Institute for Occupational Safety and Health for N-ethylmorpholine (3),acid-treated silica gel (4), Chromosorb 103 (5), and acid-treated Tenax (6). The amines are desorbed by solvent extraction except in the case of Chromosorb 103, where thermal desorption has been used (5). The amines are also trapped in impingers containing an acidic water solution (4, 7-10), which can then be analyzed directly. Analysis of tertiary amines has been performed by gas chromatography,usually on special base-treated columns, with flame ionization (4) or nitrogen-selective detection (7-9). Electron capture detection has also been used after derivatization of the amine (11). The opportunity has also been taken of utilizing the bonded-phase capillary columns in amine analysis. Analysis of tertiary amines by liquid chromatography with precolumn (12) or postcolumn (13) derivatization, with UV and fluorescence detection, respectively, has also been reported. An isotachophoretic method was reported recently (10). Some of the references above pertain to N-methylmorpholine (5, 8, 9). In the industrial hygiene sector, air sampling is usually carried out by a safety engineer stationed within the industry. The collected samples are sent to a laboratory for analysis. Accordingly, special procedures have to be observed in collection, transport, and storage of the samples prior to analysis. 0 1986 American Chemical Society
