Direct and Selective Determination of Atmospheric Gaseous Hydrogen

An on-line method is described for the determination of atmospheric hydrogen peroxide, collected by a diffusion scrubber and detected in a flow system...
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Anal. Chem. 1996, 68, 3945-3950

Direct and Selective Determination of Atmospheric Gaseous Hydrogen Peroxide by Diffusion Scrubber and 1,1′-Oxalyldiimidazole Chemiluminescence Detection Malin Stigbrand, Anders Karlsson, and Knut Irgum*

Department of Analytical Chemistry, Umea˚ University, S-901 87 Umea˚, Sweden

Gaseous hydrogen peroxide and organic peroxides are primarily formed in the atmosphere by the photochemically initiated binary reaction between alkyl-, acyl-, or hydrogen peroxyl components.1 Peroxides have a great influence as oxidation agents in atmospheric reactions, and one of the better known oxidation reactions brought about by peroxides is the formation of sulfate from sulfur dioxide.2 This reaction occurs in the condensed phase and serves as the most important component in the sink process for atmospheric sulfur dioxide. Hydrogen peroxide has further been postulated to have an influence on the atmospheric aqueousphase oxidation of nitrous acid to nitric acid, a reaction that is significant below pH 3.3 In addition to contributing to these important atmospheric reactions, there are indications of a negative effect of hydrogen peroxide on vegetation.4 Determination of hydrogen peroxide selectively in the presence of the other peroxides that are naturally occurring in the atmosphere requires either a specific detection method or a separation step preceding a nonselective detection step. Hell-

pointner and Ga¨b5 were able to identify various peroxide species in an ambient air sample by liquid chromatography. In the air samples, they found methyl hydroperoxide (MHP) to be the most dominating organic peroxide and were able to detect only small amounts of hydroxymethyl hydroperoxide (HMHP). To the contrary, Hewitt and Kok6 found the dominating organic peroxide species to be HMHP. In addition, they were able to measure MHP, 1-hydroxyethyl hydroperoxide (1-HEHP), and ethyl hydroperoxide (EHP), but all were present at concentration levels less than one-tenth that of hydrogen peroxide and HMHP. Recent developments in continuous gas sampling techniques such as the stripping coil7 and the diffusion scrubber8 have simplified the time-resolved determination of hydrogen peroxide. Both these techniques can easily be automated for extended measurement periods. The liquid-phase determination systems for hydrogen peroxide in the two cited works are both of the flow injection configuration, based on the peroxidase-catalyzed generation of a fluorescent product from p-hydroxyphenylacetic acid (PHOPA). A problem that both these groups have identified in direct determination using PHOPA chemistry is the lack of selectivity for hydrogen peroxide, with respect to organic peroxides. To circumvent this problem, Lazrus et al.7 utilized two parallel flow channels, where hydrogen peroxide was selectively destroyed in one of the channels by addition of catalase. Indirect values for the hydrogen peroxide concentrations could then be calculated by alternating determination of the two channels. The other approach, taken by Dasgupta et al.,8 made use of a packed manganese dioxide reactor, which was switched periodically into the analytical flow system to selectively remove hydrogen peroxide. Dasgupta’s group later improved their analytical system in several respects, by substituting bovine hematin for peroxidase.9 The main benefit gained by using hematin was its lower catalytic efficiency for organic peroxides. However, the low molecular weight hydroperoxides still remain problematic; while hematin reduces the fluorescent product yield from MHP to 10% of the signal from the same amount of hydrogen peroxide, the sensitivity for HMHP is identical to that for hydrogen peroxide. This is especially unfortunate, since HMHP is one of the most abundant atmospheric organic peroxides.6 Thus, in order to fully under-

(1) Finnlayson-Pitts, B. J.; Pitts, J. N. Atmospheric Chemistry: Fundamentals and Experimental Techniques; John Wiley and Sons, Inc.: New York, 1986; Chapter 3-D. (2) Calvert, J. G.; Lazrus, A.; Kok, G. L.; Heikes, B. G.; Walega, J. G.; Lind, J.; Cantrell, C. A. Nature 1985, 317, 27-35. (3) Lee Y.-N.; Lind, J. A. J. Geophys. Res. 1986, 91, 2793-2800. (4) Mo ¨ller, D. Atmos. Environ. 1989, 23, 1625-1627.

(5) Hellpointner, E.; Ga¨b, S. Nature 1989, 337, 631-634. (6) Hewitt, N. C.; Kok, G. L. J. Atmos. Chem. 1991, 12, 181-194. (7) Lazrus, A. L.; Kok, G. L.; Lind, J. A.; Gitlin, S. N.; Heikes, B. G.; Shetter, R. E. Anal. Chem. 1986, 58, 594-597. (8) Dasgupta, P. K.; Dong, S.; Hwang. H.; Yang, H.-C.; Zhang, G. Atmos. Environ. 1988, 22, 949-963. (9) Zhang, G.; Dasgupta, P. K.; Sigg, A. Anal. Chim. Acta 1992, 260, 57-64.

An on-line method is described for the determination of atmospheric hydrogen peroxide, collected by a diffusion scrubber and detected in a flow system by using 1,1′oxalyldiimidazole peroxyoxalate chemiluminescence. Interferences from the organic peroxides most abundantly occurring in the atmosphere [methyl hydroperoxide and hydroxymethyl hydroperoxide (HMHP)] were investigated and showed that the method had a selective response for hydrogen peroxide. The pH-dependent dissociation rate of HMHP to hydrogen peroxide and formaldehyde was estimated and could be controlled by a buffered scrubber liquid (pH 5.0) to eliminate the contribution of hydrogen peroxide from dissociated HMHP. The linearity of the response was excellent in the tested interval from the detection limit (23 pptv) to 3.37 ppbv. The time resolution was high, with an injection throughput of 120 h-1. The applicability of the technique was assessed by measurement of the atmospheric hydrogen peroxide concentration outside the laboratory over a period of 22 h.

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stand the peroxide-mediated atmospheric oxidations processes, new methods have to be developed for the selective, real-time determination of the atmospheric peroxide species at trace concentrations. Within this area, the most imminent task is to develop a fast method capable of discriminating between hydrogen peroxide and the more abundant organic peroxides, namely, HMHP and MHP. When a nonquantitative diffusion-based sampling technique is used for sampling of gaseous hydrogen peroxide from the atmosphere, low molecular weight organic peroxides such as HMHP and MHP will be transported to the collection surface with an efficiency close to that of hydrogen peroxide, in spite of the difference in molecular mass for these peroxides and hydrogen peroxide. Moreover, the water solubility of HMHP and MHP can promote significant amounts to be collected by techniques where water acts as the sink. HMHP has a higher Henry’s law constant than hydrogen peroxide,10 which means that it is distributed more strongly to the aqueous phase than the latter and may therefore give notable interference. MHP, on the other hand, is 300 times less soluble than hydrogen peroxide11 but can still be suspected to interfere in areas were it is present at high concentration. For instance, in a study performed by de Serves and Ross12 to investigate the collection efficiency for peracetic acid, which has a Henry’s law constant not widely different from that of MHP,11 it was found that peracetic acid was collected by 9.5% efficiency in the diffusion scrubber compared to 21% for hydrogen peroxide. These factors combined may therefore induce significant errors if nonselective detection methods are used. The formation of a fluorescent reagent by a reaction coupled to the presence of peroxide, as used by previous contenders in this field, is indeed an attractive analytical route. However, an even more appealing chemistry for this particular problem is peroxyoxalate chemiluminescence (PO-CL) (for a review, see ref 13). As opposed to fluorescence, where light is used for excitation, chemiluminescence relies on emission of light from energy-rich chemical reaction products or intermediates, produced against a dark background. This absence of stray light typically leads to detection limits 10-100 times better than fluorescence detection.13 Specific for the PO-CL chemistry, is that it involves a reaction between an activated oxalate and hydrogen peroxide, which results in labile and energy-rich intermediates capable of exciting a suitable luminophore. The PO-CL chemistry has consequently been used for sensitive determinations of hydrogen peroxide in analytical methods.14,15 For the envisioned application, where the sample is determined without any preceding separation, the extent to which organic peroxides will interfere is of paramount importance. The specificity for hydrogen peroxide over organic peroxides in the PO-CL reaction has been tested by Rauhut et al.16 In their study, aimed at finding suitable oxidizers for commercial chemiluminescent devices, rather bulky organic peroxides, e.g., tert-butyl hydro(10) Zhou, X.; Lee, Y.-N. J. Phys. Chem. 1992, 96, 265-272. (11) Lind, J. A.; Kok, G. L. J. Geophys. Res. 1986, 91, 7889-7895. (12) de Serves, C.; Ross, H. B. Environ. Sci. Technol. 1993, 27, 2712-2718. (13) Kwakman, P. J. M.; Brinkman, U. A. Th. Anal. Chim. Acta 1992, 266, 175192. (14) Gu ¨ bitz, G.; van Zoonen, P.; Gooijer, C.; Velthorst, N. H.; Frei, R. W. Anal. Chem. 1985, 57, 2071-2074. (15) Grayeski, M. L.; Woolf, E. J.; Helly, P. J. Anal. Chim. Acta 1986, 183, 207215. (16) Rauhut, M. M.; Bollyky, L. J.; Roberts, B. G.; Loy, M.; Whitman, R. H.; Iannotta, A.V.; Semsel, A. M.; Clarke, R. A. J. Am. Chem. Soc. 1967, 89, 6515-6522.

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peroxide and benzoyl peroxide were investigated. It was concluded that the organic peroxides were active to some extent in the light-generating reaction, but the response for hydrogen peroxide was at least 3 orders of magnitude higher at equal concentrations. Due to the low detection limits attainable with PO-CL, it is not surprising that the chemistry has already been implemented for the detection of gaseous hydrogen peroxide. Jacob et al.17 used a discontinuous cryogenic technique that required sampling for 1 h. Samples were thawed and analyzed in a separate flow system. In view of recent findings in peroxyoxalate chemiluminescence, the instrumental setup can be improved in several aspects, compared to the setup of Jacob et al. A single flow channel eliminates mixing problems and simplifies the instrumental set up. A single flow line can be achieved in two ways. Gu¨bitz et al.14 utilized an immobilized fluorophore in combination with a reagent donor column which was packed with solid bis(2,4,6trichlorophenyl) oxalate (TCPO) and placed in-line. Although this resulted in a single line flow manifold, the limited stability of the packed reagent bed was a major drawback, which made it difficult to inject aqueous samples. A better way to accomplish a simple and reliable single-line system can be realized with the recently introduced PO-CL reagent 1,1′-oxalyldiimidazole (ODI), in combination with an immobilized fluorophore, since ODI does not require a base catalyst for its CL reaction.18 Another advantage of ODI that can be used to implement enhanced reaction schemes for aqueous samples compared to TCPO is a higher solubility in acetonitrile, which provides for a higher sensitivity. In this study, a method concerning collection of gaseous hydrogen peroxide using a diffusion scrubber and detection with ODI peroxyoxalate chemiluminescence was evaluated. Interferences from naturally occurring organic peroxides were also investigated. EXPERIMENTAL SECTION Reagents and Solutions. TCPO was synthesized as described in ref 19 and 1,1′-oxalyldiimidazole as described by Murata.20 Acetonitrile was of Analyzed HPLC Grade (Baker, Deventer, Holland) and was additionally dried by adding at least 100 g of washed (water) and dried (120 °C for 72 h, followed by 380 °C for 24 h) 3 Å molecular sieve (KeboLab, Stockholm, Sweden) per liter of solvent. Withdrawal from the bottle was accomplished by a siphoning system comprising an in-line filter (Solvent IFD, Catalog No. 6725-5002; Whatman, Maidstone, England) to remove fines from the molecular sieve. The contents were protected from ambient water during withdrawal by a drying tube filled with silica gel (KeboLab). Oxalyl chloride (Aldrich, Steinheim, Germany; 98%), 2,4,6-trichlorophenol (Aldrich; 98%), (trimethylsilyl)imidazole (TMSI) (Aldrich; Derivatization grade), triethylamine (Fluka, Buchs, Switzerland; p.a.), hydrogen peroxide (Merck, Darmstadt, Germany; p.a., 30% aqueous), and formaldehyde (Riedel de Hae¨n, Seelze, Germany; 36.5%) were used as received. All other chemicals used were of reagent grade. Water was purified using Super-Q (Millipore, Bedford, MA) equipment to a conductivity of less than 60 nS‚cm-1. This water was thoroughly degassed with He, protected from light, and (17) Jacob, P.; Tavares, T. M.; Klockow, D. Fresenius Anal. Chem. 1986, 325, 359-364. (18) Stigbrand, M.; Ponte´n, E.; Irgum, K. Anal. Chem. 1994, 66, 1766-1770. (19) Mohan, A. G.; Turro, N. J. J. Chem. Educ. 1974, 51, 528-529. (20) Murata, S. Chem. Lett. 1983, 1819-1820.

Figure 1. Experimental setup of the atmospheric hydrogen peroxide determination system and the dynamic gas generation system. The individual components are referred to by letters that are described in the Experimental Section.

continuously recirculated through a column (50 mm long by 10 mm i.d.) containing manganese dioxide granulate (Aldrich; 99+%) at room temperature to reduce and maintain a low background level of hydrogen peroxide.21 A 130 mM hydrogen peroxide intermediate stock solution was prepared, standardized by iodometric titration, and stored in the refrigerator. Dilute solutions of hydrogen peroxide in water were prepared daily in brown highdensity polyethene (HDPE) bottles (Nalgene, Rochester, NY) from the stock. The formaldehyde was standardized by hydrochloric acid titration of the hydroxide remaining after alkaline oxidation by hydrogen peroxide, using phenolphthalein as indicator.22 The reagent carrier solutions were prepared daily, by dissolving ODI and TCPO in acetonitrile to final concentrations of 1 and 0.25 mM, respectively. Sonication for 15 min in an ultrasonic bath was necessary to complete the dissolution of TCPO. Equipment. The experimental setup of the air sampling, calibration, and FIA system is shown in Figure 1. The flow system was propelled by an inert high-pressure pump (Q) (CMA Microdialysis, Uppsala, Sweden), operating at a flow rate of 0.5 or 1 mL/min. The reagent carrier solution was kept in a polypropylene bottle (R) (Nalgene), equipped with a drying tube. The samples from the diffusion scrubber were injected by the means of a pneumatically operated poly(ether ether ketone) (PEEK) six-port injection valve with Tefzel rotor seal (S) (Rheodyne Model 9010, Cotati, CA), fitted with a 10 µL PEEK loop. The injection interval was controlled by a Chron-Trol Timer (Lindburg Enterprises, San Diego, CA). A Gilson Minipuls 2 peristaltic pump (O) was used to pump the scrubber liquid. All interconnections were made with PEEK capillary tubing of 0.17 mm i.d., except for the tubes connecting the chemiluminescence detector, which were 0.25 mm i.d. black-dyed PTFE (Jour Research, Onsala, Sweden) to avoid stray light piping into the detector (T). The detector assembly has been described in detail previously,18 and the voltage across the photomultiplier tube (PMT) was 600 V. A flow cell made from quartz tubing (22 mm long by 2.4 mm i.d.; wall thickness 0.8 mm) was placed in close proximity to the PMT window, with a piece (21) Hwang, H.; Dasgupta, P. K. Anal. Chem. 1986, 58, 1521-1524. (22) Walker, J. F. Formaldehyde; Robert E. Krieger Publishing Co.: Malabar, 1964; Chapter 18.

of specular aluminum foil placed above the cell, acting as a reflector. The flow cell contained an in situ polymerized sorbent rod (corresponding to preparation PGT 9-12, described in ref 23), which was functionalized with 3-aminofluoranthene. Adapters were affixed at both ends of the quartz tubing to keep the rod in place. An OP-amp-based laboratory-constructed current-to-voltage converter was operated at 20 V/mA to convert the PMT output into a signal suitable for recording by a 3396 A integrator (HewlettPackard, Palo Alto, CA) in source signal mode. A diffusion scrubber (DS) (N) was used for the sampling procedure. The sampling membrane was a 40 cm long piece of Celgard X-20 (Hoechst Celanese, Charlotte, NJ), a 0.40 mm i.d. porous polypropene membrane tubing, into which a 0.27 mm diameter PTFE monofilament (Utildi, Laholm, Sweden) had been inserted to accomplish a higher surface-to-volume ratio. Details of the construction, except for the filament insert, have been described in a previous paper.24 The gas flow through the DS was accomplish through suction by an air pump (K) (KNF Neuberger, Freiburg-Munzingen, Germany; Model N022 AN.18), which was set to 2.0 standard liters per minute (SLPM) with a mass flow controller (L) (Tylan, Eching, Germany). A Whatman GF-A glass fiber filter (M) was placed upstream from the controller to prevent mass flow variations due to deposition of particles. A 10 mM acetic acid buffer at pH 5.0 prepared from the MnO2treated water was used as scrubber liquid, if not otherwise noted, and kept in a brown glass flask (P). Before use, the scrubber liquid was degassed with helium and thereafter pressurized to 60 kPa above atmospheric pressure. Calibration Source and Dynamic Gas Generation System. The source used for generating gaseous hydrogen peroxide for calibration was a “Henry’s law device”,25 based on permeation through a 80 cm long by 1.6 mm i.d. porous PTFE membrane tubing (W. L. Gore and Associates, Elkton, MD; Type TA001), immersed in 40 mM aqueous hydrogen peroxide (D). The source was submersed in a thermostatic bath set to 16.4 °C for temper(23) Ponte´n, E.; Viklund, C.; Bogen, S. T.; N-jon Lindgren, A° .; Irgum, K. Anal. Chem. submitted. (24) Lindgren, P. F.; Dasgupta, P. K. Anal. Chem. 1988, 61, 19-24. (25) Hwang, H.; Dasgupta, P. K. Environ. Sci. Tehnol. 1985, 19, 255-258.

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ature control (E). At the nitrogen flow used through the source (69 mL/min), a saturated hydrogen peroxide atmosphere was reached according to literature values25 of the Henry’s law constant for aqueous hydrogen peroxide. The saturated gas thus produced was mixed with zero gas immediately downstream from the source in the dynamic generation system. The hydrogen peroxide concentration of the generated calibration gas was checked by sampling in an impinger filled with deionized water, followed by determination of the impinger solution using the ODI CL system and found to be 269 ( 22 ppbv (95% confidence interval; n ) 4). The source gas flow was regulated by a needle valve (I) and checked with a rotameter (J), while the zero gas dilution flow was set by a mass flow controller (B) (Tylan) that had been calibrated to an accuracy of (1.0%. PTFE tubing was used throughout the entire gas generation system. Zero gas was generated by purifying compressed house air through columns (A) (80 cm long by 35 mm i.d.) filled with soda lime (BHD Chemicals, Poole, England), Amberlyst 15 cation exchanger in the H+ form (Serva, Heidelberg, Germany; analytical grade, 0.4-1.2 mm), molecular sieve (3 and 5 Å mixed in 50:50 proportion; KeboLab), and activated charcoal (KeboLab) in the mentioned order. Particles that may have been released from the purifying column assembly were collected on a 1.6 µm glass fiber filter. The zero gas was supplied in excess, regulated with a needle valve (C). Nitrogen gas (G) (AGA, Sundbyberg, Sweden), purified by silica gel and activated carbon (H) was used to ascertain a high-quality carrier gas for the Henry’s law source. A pneumatic valve (F), controlled from the Chron-Trol device, instantaneously switched between ambient air, calibration gas, and zero gas. Preparation of Methyl Hydroperoxide. The method used for synthesis of MHP was adopted from the literature26,27 and chosen because it is less hazardous compared to other methods described. The distillation step was omitted, as the peroxide will be used in aqueous solution only, and the byproducts either can be easily reduced to an acceptable level or are not expected to interfere. The synthesis mixture contained 2.2 ( 0.1 mM hydrogen peroxide and 38.9 ( 1.7 mM organic peroxide (95% confidence interval), where the total amount of peroxides was determined by iodometric titration and the hydrogen peroxide contents by the titanium(IV) oxalate method.28 WARNING: The synthesis procedure is potentially dangerous because of the acute and chronic toxicity of dimethyl sulfate. Consult all relevant material safety sheets and take the precautionary measures required before embarking upon this synthesis! Preparation of Hydroxymethyl Hydroperoxide. HMHP is an addition compound, formed in an equilibrium reaction between formaldehyde and hydrogen peroxide in aqueous solution. A second reaction step also exists where HMHP and formaldehyde are in equilibrium with bis(hydroxymethyl) peroxide (BHMP). It is thus not possible to synthesize pure HMHP, so in the experiments carried out in order to determine the interference from HMHP, an equilibrated mixture was used where concentrations for the various peroxide species were calculated from literature thermodynamic data. The equilibrium and rate constants for these reactions have been investigated in earlier work,28 and were last revised by Zhou and Lee,10 who determined the (26) Davies, D. M.; Deary, M. E. J. Chem. Soc., Perkin Trans. 2 1992, 559-562. (27) Behrman, E. J.; Biallas, M. J.; Brass, H. J.; Edwards, J. O.; Isaks, M. J. Org. Chem. 1970, 35, 3069-3075. (28) Marklund, S. Acta Chem. Scand. 1971, 25, 3517-3531.

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formation constants to 149.6 M-1 for HMHP (from hydrogen peroxide and formaldehyde), and 11.71 M-1 for BHMP (from HMHP and formaldehyde). The SOLGASWATER software package29 was used to calculate the equilibrium concentrations of the different peroxide species from a series of starting concentrations of hydrogen peroxide and formaldehyde. To an aluminum foil-wrapped polyethene bottle containing hydrogen peroxide-free deionized water, formaldehyde was added to give an 18.0 mM solution, which was injected into the analytical system to check for background effects in the PO-CL reaction. A second solution was prepared containing formaldehyde and hydrogen peroxide to initial concentrations of 18.0 mM and 11.7 µM, respectively. At equilibrium, the computed concentration for formaldehyde will be essentially unaltered, while the concentrations for the peroxide species in the system will be 2.74 µM for hydrogen peroxide, 7.43 µM for HMHP, and 1.57 µM for BHMP. This solution was allowed to equilibrate for 1 h and then used to determine the relative sensitivities of hydrogen peroxide and HMHP in the ODI PO-CL reaction, by relating the signal obtained with that expected for the calculated amount of hydrogen peroxide in the mixture at equilibrium. Kinetics Measurement of the Dissociation of HMHP. The dissociation kinetics experiment was carried out according to the principle described by Zhou and Lee,10 with the exception that the increasing hydrogen peroxide concentration was determined by the automated PO-CL FIA system. HMHP was synthesized by equilibrating a mixture of 49.9 mM hydrogen peroxide and 18.0 mM formaldehyde in water for 1 h at room temperature. The equilibrium concentrations were calculated by SOLGASWATER29 to be 14.4 mM for HMHP, 0.46 mM for BHMP, and 35.0 mM for hydrogen peroxide in the synthesis mixture. The kinetic study was initiated by transferring 200 µL of this mixture to a polyethene bottle containing 100 mL of either 10 mM acetic acid/sodium hydroxide buffer, pH 5.0, or 10 mM phosphoric acid/sodium hydroxide buffer, pH 7.2, under stirring. Samples of the reaction mixture were automatically injected into the FIA system every 60 s, whereby the increase in hydrogen peroxide concentration was monitored as an indicator of the decreasing HMHP concentration. Ambient Air Measurement. The DS-CL-FIA system was operated with a scrubber liquid flow rate of 20 µL/min and a gas sampling flow rate of 2 SLPM. The carrier flow rate in the FIA system was set to 0.5 mL/min during this study in order to save reagents. A scrubber liquid sample was injected into the CL-FIA system every fourth minute, and every third hour the system was recalibrated by switching 0.57 ppbv calibration gas and zero gas into the DS for 16 min each. The signals were evaluated on the basis of their peak heights. RESULTS AND DISCUSSION Responses from MHP and HMHP. Table 1 shows that neither MHP nor HMHP result in PO-CL responses that can be significantly differentiated from the signal that is to be expected from the free hydrogen peroxide contents in the synthesized hydroperoxides. When the HMHP mixture was injected in the FIA system, the signal obtained decreased to 25.7% of that from hydrogen peroxide standard only. This value corresponds well with the fraction of free hydrogen peroxide, predicted to 25.4% when calculated using SOLGASWATER. Blank signals from formal(29) Eriksson, G. Anal. Chim. Acta 1979, 112, 375-383.

Table 1. Results from the Organic Peroxide Interference Studies sample 2.74 µM H2O2b + 7.43 µM HMHP + 1.57 µM BHMP 18.2 µM H2O2c + 323 µM MHP

responsea (% of expected)

no. of expts

101.2 ( 2.4

4

84.6 ( 1.0

5

a The reported response is the signal in percent of the signal from a hydrogen peroxide standard with the same concentration as the hydrogen peroxide present in the interferent samples. See Experimental Section for details on the preparation of these interferent samples and calculation/measurement of their hydrogen peroxide concentration. b The hydrogen peroxide value is calculated from thermodynamic data. c Measured by the titanium(IV) oxalate method.

dehyde corresponding to the concentration present in the HMHP mixture did not differ significantly from water blank signals. Klockow and Jacob30 have investigated the interference from high concentrations of formaldehyde in the chemiluminescence reaction of TCPO. At relatively low concentration (10-3 M), there is no difference in response at a hydrogen peroxide concentration of 150 nM, but there is a dramatic change at a formaldehyde concentration of 1 M, where the response decreases to essentially zero. In view of our findings, this is probably caused by formation of organic peroxides which result in a decrease in the free hydrogen peroxide concentration. The synthesis mixture of MHP was diluted and finally contained 323 ( 13 µM MHP and 18.2 ( 1 µM hydrogen peroxide. The response obtained from this MHP/H2O2 mixture was somewhat lower than expected, but this can be explained by the hydrogen peroxide determination, which was carried out by the titanium(IV) oxalate method according to Marklund.28 His observations show that the reaction of the titanium reagent is not entirely selective for hydrogen peroxide. Both HMHP and BHMP give a response, and one cannot exclude the same behavior for MHP. In kinetic experiments he showed that organic peroxides react at a lower rate compared to hydrogen peroxide. The response for the organic peroxides could not be explained only by the dissociation of HMHP and BHMP, since the complex was formed more rapidly than could be predicted by the hydrolysis. This bias would result in an overestimation of hydrogen peroxide in our synthesis mixture, and consequently the response in the FIA system would become lower than expected. No corresponding investigation was carried out on 1-HEHP and EHP. It is reasonable to assume a similar lack of interference in the peroxyoxalate reaction, not least because of the more sterically hindered configurations of 1-HEHP and EHP. The solubility in the scrubber liquid also determines the collection efficiency. No data of the Henry’s law constants were found in the literature for 1-HEHP and EHP, but Hewitt and Kok6 have presented results from measurements of distributions of peroxides between rainwater and the atmospheric gas phase. The distribution was considerably more shifted toward the aqueous phase for HMHP and MHP, as compared to 1-HEHP and EHP, indicating a lower solubility in water for the latter two compounds. A further discrimination of these peroxides should consequently occur in the scrubber collection procedure. Moreover, the occurrence of these organic peroxides is lower than for HMHP (30) Klockow, D.; Jacob, P. Chemistry of multiphase atmospheric systems Springer: Berlin, 1986; pp 117-130.

Figure 2. Dissociation rate of the HMHP in aqueous solution at different pH, ([, acetate buffer, pH 5.0; 9, phosphate buffer, pH 7.2) compared to the decomposition rate calculated according to data from Zhou and Lee10 (dashed line, pH 5.0; dotted line, pH 7.2).

and MHP.5,6 The likelihood of interference from these compounds in real-life sampling should therefore be essentially nil. Dissociation Kinetics for HMHP. A hitherto ignored problem in water-based sampling methods for hydrogen peroxide concerns the dissociation of HMHP to hydrogen peroxide and formaldehyde. Marklund28 and later Zhou and Lee10 determined the rate constant of the dissociation reaction at different pH, and both found that there is a significant pH dependency in the decomposition rate. Above a pH of approximately 5.5, the dissociation rate increases to levels where significant amounts of hydrogen peroxide will be released within 1 min, the time lapse from the entering of the scrubber liquid at the top of the scrubber until it emerges at the detection cell in our setup. At neutral pH, for example, calculations show that 25% of the initial concentration of HMHP will be dissociated within this time frame. This will induce a positive interference in the determination of hydrogen peroxide if HMHP is present in the sampled air. To ensure an accurate measurement of free hydrogen peroxide, it is thus essential that the sample residence time in the system is minimized or that the scrubber liquid is buffered to a pH where HMHP is not dissociated to a significant degree during the sampling process. Our kinetic experiment of the dissociation rate is displayed in Figure 2 alongside curves calculated from the results of Zhou and Lee. A discrepancy is noticed in the acetate buffer experiment at pH 5.0, where the signal was only 96% of the expected. However, the slopes of the curves at different pH seem to coincide. Use of pH 5.0 acetate buffer as scrubber liquid results in less than 1% dissociation after 200 s, which we consider to be sufficient in our system. System Performance. Several parameters control the sensitivity and time resolution, and these can be balanced to optimal conditions in each application. In practice, two parameters are more important than others to obtain a high sensitivity, namely, the scrubber liquid flow rate and the reagent flow rate in the FIA system. A high scrubber liquid flow rate increases the time resolution while the sensitivity decreases due to a less enriched sample to inject. Consequently, the sensitivity increases and the time resolution decreases with a lower scrubber liquid flow rate. The practical sample capacity of the FIA system is 120 injections/h or one injection every 30 s in the concentration range of interest. The minimum possible time resolution is then dependent on the Analytical Chemistry, Vol. 68, No. 22, November 15, 1996

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Figure 3. Triplicate injections of gaseous H2O2 at mixing ratios of 3.37, 2.09, 1.43, 0.84, and 0.57 ppbv followed by blanks. The final peaks are five additional blank injections expanded by a factor 16. Samples were injected every 4 min, and the reagent flow rate was 0.5 mL/min. The peak height for the highest concentration of H2O2 corresponds to 2.6 µA.

scrubber liquid flow rate, which should be sufficient to fill the injection loop between injections. At this sampling rate, the technique presented here can compete favorably with real-time spectroscopic in situ measurement methods. In addition, the scrubber air flow rate is important for the total amount of analyte to be collected. The collection efficiency of hydrogen peroxide for the diffusions scrubber equipped with a Celgard X-20 membrane tubing has been studied by Dasgupta et al.8 The lower reagent flow rate (0.5 mL/min) was chosen during long runs (24 h) in order to decrease the consumption of carrier. The peak areas obtained are the same at 0.5 and 1 mL/min, but the peak heights decrease by a factor of 2 when the lower flow rate is used. At the two reagent flows tested, the detection limits were estimated to be 23 and 30 pptv, respectively. These values are calculated from measurements of the peak heights from four injections of 570 pptv calibration gas and correspond to the 3σ criterion. The background level with a PMT bias at 600 V was 7 nA above the signal from pure acetonitrile, and the peak-to-peak noise was 1.35 nA (at both 0.5 and 1 mL/min). When aqueous samples were determined, the detection limit was estimated to be 0.1 nM at a reagent flow rate of 1 mL/min, and the blank signal obtained from MnO2-treated water corresponds to a hydrogen peroxide concentration of approximately 5 nM. In gas calibration, the attainable range is usually limited by the operational range of the mass flow controllers. Calibration was carried out with gaseous standards in the range 0.6-3.4 ppbv (Figure 3) and the response was described by a straight line with the slope 775 ( 13 nA/ppbv, with an intercept of 44 ( 20 nA and a regression coefficient of 0.999 (reagent flow rate 0.5 mL/min). Although the reliable experimental range is limited by the gas generation system, the PO-CL reaction detection is known to show excellent linearity, with a linear range often exceeding 4 orders of magnitude. Suitability for Ambient Air Measurements. The system was tested for continuous ambient air measurement over a period of 22 h on October 11-12, 1995, where the air outside the laboratory was determined for its hydrogen peroxide content. Notable in the results presented in Figure 4 is the rapid response to the changes between sampled gas, calibration gas, and zero gas, which allows calibration and blanks for the entire sampling

3950 Analytical Chemistry, Vol. 68, No. 22, November 15, 1996

Figure 4. Continuous determination of gaseous atmospheric hydrogen peroxide over a period of 22 h on October 11-12, 1995, using the DS-CL-FIA system. Sampling took place outside the chemistry building, situated at the campus 3 km from downtown Umeå (63°51′ N; 20°16′ E). Official weather conditions at Umea˚ airport (∼3 km from the laboratory at 3 h) intervals during the actual period are indicated in the figure. The open sectors of the pies indicate percentage clear sky. Open squares (0) represent zero gas, closed diamonds (() ambient gas, and open triangles (4) standard gas. The dotted line is the signal from zero gas at the start of the sampling, indicating that some drift in baseline takes place, which can be corrected for by the blanks that are interleaved in the sampling scheme.

and determination system to be performed en suite in continuous measurements. Only a single measurement datum had to be discarded each time the system switched between sample, calibration gas, and zero gas. Although the concentration of hydrogen peroxide was found to be steadily decreasing from the initial level of 100 pptv during practically the entire sampling period, the signal never went below the system detection limit. No increase was seen in the hydrogen peroxide concentration during the afternoon, in spite of a clear sky during the measurement. It should be kept in mind, however, that Umeå is situated at latitude 63°51′ N. Although the length of the day this time of the year is still 10 h, the solar inclination at noon is only 18° above the horizon. ACKNOWLEDGMENT This work was supported by The Swedish Natural Science Research Council, through Grant K-KU 8724-309 and by The Bengt Lundqvist Memorial Foundation (MS). We express our gratitude to Einar Ponte´n for the synthesis of the in situ polymerized sorbent and to Jan Nordin for help with the SOLGASWATER calculations. Received for review February 14, 1996. Accepted July 29, 1996.X AC960140K X

Abstract published in Advance ACS Abstracts, September 15, 1996.