On-Line Analysis of Nitrogen Stable Isotopes in NO from Ambient Air

A method was developed for the on-line analysis of nitrogen stable isotopes at the natural abundance level in NO in order to study the NO contribution...
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Anal. Chem. 2001, 73, 1126-1133

On-Line Analysis of Nitrogen Stable Isotopes in NO from Ambient Air Samples Jutta Lauf† and Gerhard Gebauer*

Lehrstuhl fu¨r Pflanzeno¨kologie, Universita¨t Bayreuth, D-95440 Bayreuth, Germany

A method was developed for the on-line analysis of nitrogen stable isotopes at the natural abundance level in NO in order to study the NO contribution to the nitrogen cycle in ecosystems and in the atmosphere. The method enables a quick and accurate determination of 15N/14N ratios for NO and consists of the following steps: (a) accumulation of NO from air samples on a molecular sieve of 5 Å, (b) desorption of NO from the molecular sieve during 15 min of heating at 350 °C (an offset of ∆δ 4.6‰ must be corrected for), (c) trapping and cryofocusation of the desorbed NO on a PoraPlot Q matrix at -196 °C during heating, (d) release of the trapped NO from the PoraPlot Q matrix followed by chromatographic separation, reduction to N2, and isotopic composition analysis. A minimum sample size of 125 nmol of NO is recommended. A correction function for the calculation of the δ15N-NO values was introduced for sample sizes from 125 to 220 nmol of NO. Measurements of NO in automobile exhaust have proven the applicability of the developed method. In the atmosphere, nitrogen monoxide is a highly reactive trace gas of mostly anthropogenic origin. Three processes lead to the production of NO: (1) fossil fuel combustion above 1000 °C,1 (2) microbiological nitrification,2 and (3) microbiological denitrification.2 Fertilizer application in agricultural systems further increases NO emissions due to accelerated nitrification and denitrification processes. On a global scale, soils emit ∼20 Tg N yr-1 as NO and thus are an important source for the atmospheric NO budget, since soil NO emissions are similar in magnitude to the amount of NO produced by fossil fuel combustion.3 The presence of NO contributes to the ozone production cycle in urban regions4 and NO is a greenhouse gas.5 It also contributes to the atmospheric input of nitrogen to terrestrial ecosystems. Plants are able to use NO2 as a source of nitrogen for growth.6-8 Nitrogen dioxide can * Corresponding author: (fax) 0049-921-552564; (e-mail) Gerhard.Gebauer@ uni-bayreuth.de. † Present address: Max-Planck-Institut fu ¨r Kernphysik, Bereich Atmospha¨renphysik, Saupfercheckweg 1, D-69117 Heidelberg, Germany. (1) Holleman, A. F.; Wiberg, N. Lehrbuch der anorganischen Chemie; de Gruyter: Berlin, 1985; Vol. 33. (2) Stevens, R. J.; Laughlin, R. J.; Burns, L. C.; Arah, J. R. M.; Hood, R. C. Soil Biol. Biochem. 1997, 29, 139-151. (3) Davidson, E. A.; Kingerlee, W. Nutr. Cycl. Agroecosyst. 1999, 48, 37-50. (4) Warneck, P. Chemistry of the natural atmosphere; International Geophysics Series 41; Academic Press: New York, 1988. (5) Lammel, G.; Grassl, H. Environ. Sci. Pollut. Res. 1995, 20, 40-45. (6) Sommer, S. G.; Jensen, E. S. J. Environ. Qual. 1991, 20, 153-156.

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be produced through the atmospheric oxidation of NO. The input of nitrogen from the atmosphere may induce nutritional imbalances in plants, especially in regions with poor soils. The excessive deposition of atmospheric nitrogen is closely linked to forest decline across large areas in Europe.9 Until now, there have only been a few studies done on 15N/ 14N ratios of NO.8,10,11 In these studies, samples were prepared for isotope ratio mass spectrometry using off-line techniques. The gases were trapped chemically (as salts) from a specified volume of air. These salts were converted to N2 using Cu/CuO in evacuated, sealed glass ampules10,11 and subsequently measured either via a viscous-flow inlet system or by use of an elemental analyzer coupled to an isotope ratio mass spectrometer.8 Recent advances in isotope ratio gas chromatography/mass spectrometry have initiated new studies based on the on-line analysis of isotope abundance in trace gases. Applications for on-line analysis of N2O are already available, since the N2O atmospheric concentration is ∼2 orders of magnitude higher than NO. In addition, N2O is inert under tropospheric and analytical conditions.12,13 The purpose of this report is to present a new sampling technique for the subsequent on-line analysis of nitrogen stable isotopes in NO at the natural abundance level at concentrations found in the atmosphere. First data on nitrogen isotope abundance of NO in automobile exhaust are reported. EXPERIMENTAL SECTION Mass Spectrometer. The 15N/14N ratios of N2 gas generated from NO were measured using a Finnigan MAT delta S (Finnigan MAT, Bremen, Germany) isotope ratio mass spectrometer (IRMS) coupled to a Hewlett-Packard gas chromatograph (GC) 5890 Series II (Wilmington, DE) via a Finnigan MAT combustion interface II (Finnigan MAT). Isotope ratios were compared to those of a N2 reference gas (99.9995%, Linde, Munich, Germany), which was previously calibrated against the reference substances N1 and N2 provided by the IAEA (Vienna, Austria). Results are reported in the δ notation, where δ15N (‰) ) (RSA/RST - 1) × 1000. RSA and RST refer to the ratios 15N/14N in the sample and in air N2.14 The internal reproducibility of the N2 mass spectrometer (7) Segschneider, H.-J.; Wildt, J.; Fo ¨rstel, H. New Phytol. 1995, 131, 109-119. (8) Ammann, M.; Siegwolf, R.; Pichlmayer, F.; Suter, M.; Saurer, M.; Brunold, C. Oecologia 1999, 118, 124-131. (9) Schulze, E.-D. Science 1989, 244, 776-783 (10) Heaton, T. H. E. Atmos. Environ. 1986, 21, 843-852. (11) Heaton, T. H. E. Tellus 1990, 42B, 304-307. (12) Yoshida, N.; Matsuo, S. Geochem. J. 1983, 17, 231-239. (13) Brand, W. A. Isot. Environ. Health. Stud. 1995, 31, 277-284. (14) Mariotti, A. Nature 1983, 303, 685-687. 10.1021/ac0009292 CCC: $20.00

© 2001 American Chemical Society Published on Web 02/13/2001

Figure 1. Schematic diagram of the SICU-GC-combustion interface-IRMS coupling as used in the experiments 1, 2, and 4. The GC mode and the SICU mode are indicated. (A) shows the T-piece for the NO injection as used in the SICU mode in experiments 1, 2, and 4. (B) shows an air sample cylinder as used in experiments 3 and 4. (C) shows the total SICU-GC-combustion interface-IRMS system as used in experiments 1, 2, and 4.

measurement is typically (0.15‰ or better. The intensity of the ion current is given in volts. An integration over time results in volt seconds. These units can be used both to report the absolute recovery of N2 or as a basis for the calculation of the recovery in percent. The methods used to generate N2 from NH3, NO, N2O, and NO2 have already been described by Lauf and Gebauer.15,16 Sample Preparation. Pure NO (99.5%, Linde) purchased in a lecture bottle was used for method development. The NO was flushed for 5 min through a 100-mL glass cylinder with a Teflonsealed rubber septum. The NO samples were removed from the glass cylinder with a gastight 100-µL syringe (Hamilton, Bonaduz, Switzerland). The volume-to-mass relationship based on the daily air pressure and temperature was corrected for by using the gas law. For the analysis of pure NO, a T-piece (Figure 1 A) with shape and surface deactivation as described by Lauf and Gebauer16 was installed into an automated pre-GC concentration device13 (PreCon, Finnigan MAT). In our investigations, the PreCon was used in a modified version, as described in Figure 1 and in the following section. Its main function was to provide a platform for the development of a system for sample introduction and cryofocusation. It has therefore been termed the “sample introduction and cyrofocusation unit” (SICU). The NO was trapped on a molecular sieve in a sample gas cylinder (Figure 1B). The surface was not deactivated due to the limited temperature stability of the chemical cover.17 A 10-g aliqout of molecular sieve was filled into a sample gas cylinder and fixed with quartz wool. The molecular sieve was activated at 450 °C under continuous He (99.996%, Riessner, Lichtenfels, Germany) flow for 6 h.18,19 Afterward, the sample vessel was sealed and was opened only in the SICU or for air sampling. (15) Lauf, J.; Gebauer, G. Deutsche Patentanmeldung, 196 50444.9, 1996. (16) Lauf, J.; Gebauer, G. Anal. Chem. 1998, 70, 2750-2756. (17) Schomburg, G. Gaschromatographie, Grundlagen, Praxis, Kapillartechnik; VCH: Weinheim, 1987; p 74. (18) Grubner, O.; Jiru ¨ , P.; Pa´lek, M. Molekularsiebe; VEB Deutscher Verlag der Wissenschaften: Berlin, 1968; p 46. (19) Leibnitz, E.; Struppe, H. G. Handbuch der Gaschromatographie, 3rd ed.; Akademische Verlagsgesellschaft Geest & Portig KG: Leipzig, 1984.

SICU and GC Setup. Figure 1C illustrates the SICU-GCcombustion interface-IRMS system as used for the subsequent experiments. Carrier gas flow for the GC system was 5 mL of He/min (99.996%, Riessner). All connections within this part of the system were exclusively made by deactivated fused-silica capillaries (0.32-mm i.d.). Carrier gas flow in the SICU was 25 mL/min He (99.9999%, Linde). The carrier gas was cleaned with two chemical traps: (a) He passed a NaOH trap before it entered the SICU. (b) During the freezing procedure, the carrier gas was flushed through a trap that was filled with PoraPack S (Waters, Milford, MA) and was run 20 cm through a bath of liquid nitrogen. Traces of N2 and O2 in the carrier gas were trapped during this procedure. All connections within this part of the system were exclusively made from deactivated fused-silica capillaries (0.53 mm i.d.). This equipment allows two modes of δ15N-NO analysis: (a) the GC mode as a control and (b) the SICU mode for further method development (Figure 1). Within the GC mode, NO was applied via the injector of the GC and then was flushed via the Valco six-port to the GC column where it was separated from other compounds. Afterward, it passed the combustion interface, where the NO was reduced to N2 on a Ni reactor and where H2O and CO2 were removed from the carrier gas stream. Afterward, the N2 was led to the IRMS. Within the SICU mode, the NO could be applied via a T-piece (Figure 1A). It passed a Mg(ClO4)2 trap and was trapped on a matrix which was submersed in liquid nitrogen (see Method Development Strategy). After the matrix was removed from the liquid nitrogen, NO was released and then flushed to the GC and handled as described in the GC mode. The T-piece could be replaced, and a sample gas cylinder with molecular sieve could take its place. The sample gas cylinder could be heated while being installed in the SICU. The heater reached temperatures of up to 1200 °C. Temperatures noted in this report represent the temperatures of the molecular sieve within the sample gas cylinder. Analytical Chemistry, Vol. 73, No. 6, March 15, 2001

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Table 1. Summary of the Experimental Conditions as Used for Experiments 1-4a (a) expt no., (b) gas amounts and types (a) 1, (b) 213 nmol of NO (a) 2, (b) 85-213 nmol of NO (a) 3, (b) 21 µmol of NO, NH3, NO2, or N2O, respectively (a) 4, (b) 164 nmol of NO

experimental conditions SICU sample vessel trapping matrix trapping matrix

purpose

exptl setup

trapping matrix selection for quantitative trapping and release in the SICU limits of the NO sample size for quantitative trapping and release in the SICU molecular sieve selection

Figure 1A

(a) PoraPlot Q (b) PoraPlot S

Figure 1A

PoraPlot Q

desorption temp optimization

Figure 2

Figure 1B

PoraPlot Q

desorption temp (°C)

control expt GC detn GC detn

(a) 3, (b) 4, (c) 5, (d) 10 Å

30-1200

(a) 5, (b) 5 Å + Mo

150-450

SICU detn

a Column 1 indicates (a) the respective experiment number and (b) the amounts of gas as used for the respective experiments. Experiments 1, 2, and 4 were performed with NO; experiment 3 was performed with NO and with NH3, N2O, and NO2. In column 2, the purpose of the experiment is explained. Column 3 refers to the experimental conditions with respect to (a) the experimental setup, (b) the trapping matrix used in the SICU, (c) the trapping matrix used in the sample vessel, and (d) the desorption temperature in the sample vessels. Column 4 indicates the respective control experiment.

Method Development Strategy. The following processes had to be investigated for the new on-line method of δ15N-NO determination from ambient air samples. (a) Molecular Sieve Selection. The NO concentrations in ambient air are too low for direct δ15N-NO determination. Therefore, NO has to be accumulated to increase the sample size. The properties of different moleculare sieves for NO accumulation from air samples were tested. In ambient air, the concentrations of H2O and CO2 are high compared to NO. They must be reduced, since H2O and CO2 also adsorb on the molecular sieve surface and thus block the active sites of the molecular sieve. As H2O and CO2 can easily be removed with either chemical (Mg(ClO4)2) or physical traps (cooling bath) before the air passes the molecular sieve, their properties were not further investigated. Similar procedures have already been described for the isotope measurement of N2O12 and CO2.20 Furthermore, other atmospheric trace gases containing nitrogen will be fixed on the molecular sieve, e.g., NH3, N2O, and NO2. Their desorption temperatures must be noted for the correct δ15N-NO analysis, since these gases can interfere with the δ15N-NO analysis during desorption from the molecular sieve, chromatography, and isotope measurement. Of special interest is N2O, since it is an inert gas,1 which cannot be trapped chemically in front of the molecular sieve and is ∼100 times more concentrated in ambient air than NO.4 (b) Desorption Temperature Optimization. After the NO is accumulated from the air samples on a molecular sieve, it must be desorbed from the molecular sieve in the SICU through heating. A molecular sieve with a low desorption temperature for NO is required, and quantitative NO desorption from the molecular sieve must be achieved. To fulfill these requirements, the desorption conditions and the desorption temperature of NO must be optimized. (c) Quantitative Trapping and Release in the SICU. During thermal desorption of NO from the molecular sieve, which lasts several minutes, the released NO must be trapped again and must be focused for the following injection into the GC-IRMS. Trapping (20) Bol, R. A.; Harkness, D. D. Radiocarbon 1995, 37, 643-647.

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in a fused-silica capillary at -196 °C, as described for N2O,13 does not seem promising due to the low melting (-163.6 °C) and boiling points (-151.8 °C) of NO. A suitable trapping matrix had to be found, where NO can be fixed and released without isotopic fractionations within a suitable sample range. (d) GC Separation, Reduction, and Isotopic Measurement of δ15N-NO was described by Lauf and Gebauer.15,16 The development of the methodology was accomplished with a stepwise analysis from steps c to a. This approach was necessary in order to prevent isotope effects after the analyzation step. Experimental Setup for the Method Development. Four different types of experiments were performed to investigate the conditions required for the δ15N-NO determination (Table 1). As described above, the experiments had to be performed in a sequence that differed from the chronological sampling steps of the method. Experiments 1 and 2 refer to the method development step (c): quantitative NO trapping and release in the SICU. Experiment 1 deals with the selection of a suitable NO trapping matrix in the SICU, whereas in experiment 2 limits of the NO sample range for SICU trapping were investigated. Experiment 1 was performed to select a matrix for NO trapping in the SICU at -195.8 °C (liquid N2). δ15N-NO values were measured using two different freezing matrixes and were compared to the GC determination (control experiment). The trapping matrixes were as follows: (a) PoraPlot Q (ID 0.32 mm, 30 cm long, Chrompack, Middelburg, The Netherlands) and (b) PoraPlot S (i.d. 0.53 mm, 30 cm long, Chrompack). As a result of experiment 1, PoraPlot Q was used as the NO trapping matrix in the experiments 2 and 4 (Table 1). In experiment 2, N2 recoveries and δ15N-NO values were determined in the SICU mode within a specified NO sample range. Again, the results were compared with the GC determination (control experiment). In experiment 3 (Table 1), four different molecular sieves were tested for their NO and also their NH3, N2O, and NO2 sorption and desorption properties: (a) 3 (Roth, Karlsruhe, Germany), (b) 4 (Merck, Darmstadt, Germany), (c) 5 (Roth), and (d) 10 Å (Roth). Experiment 3 corresponds to the method development step a: Molecular Sieve Selection. Water

Table 2. Trapping Matrix Selection for Quantitative Trapping and Release in the SICUa

δ15N value replicates Figure 2. Schematic diagram of the coupling between the air sample cylinder and the combustion interface-IRMS as used in experiment 3.

vapor and CO2 in the air can easily be eliminated before entering the air sample cylinder containing the molecular sieve. Therefore, their properties were not investigated. The air sample cylinders were directly connected to the IRMS via a H2O and CO2 scrubber and the combustion interface (Figure 2). NaOH was used to trap H2O and CO2 generated during the heating process, because molecular sieve destruction occurs at temperatures above 800 °C. The sorption and desorption properties of the respective gases were tested separately. The respective gas was allowed to sorb on to the respective molecular sieves. After 5 min, the air sample cylinders were heated with 10 °C min-1 from room temperature to 1200 °C and the N2 recovery was monitored continuously. The 5-min delay was necessary to allow the sample gas, which was not adsorbed, to leave the system. δ15N values were not measured in this experiment as the high amount of sample gas exhausted the Ni reactor of the combustion interface and made correct quantification and isotopic measurements impossible. As a result of experiment 3, the quantitative NO adsorption and desorption from the 5-Å molecular sieve (Roth) was tested in experiment 4 (Table 1). This experiment corresponds to the method development in step b: Desorption Temperature Optimization. The temperature dependency of the N2 recovery and the δ15N-NO values of the desorbed NO were measured under two different conditions: (a) molecular sieve of 5 Å and (b) molecular sieve of 5 Å + 185 mg of Mo (99.95%, o.d. 0.25 mm, Goodfellow, Cambridge, U.K.). Mo is commonly used to reduce NO2 to NO in environmental analysis.15,16,21 We assumed that its addition to the molecular sieve would reduce the NO2 generation from NO during heating. The SICU mode with PoraPlot Q as freezing matrix was used as a control in this experiment. The N2 recovery from the molecular sieve determination was calculated in percent referring to the N2 recovery from the SICU determination () 100%). The δ15N-NO values obtained from the molecular sieve determination were calculated as absolute deviation from the δ15N-NO value obtained from the SICU determination (∆δ15NNO, ‰). Exhaust Gas Measurements. NO in exhaust gas from three different engines was analyzed: (a) petrol engine without catalytic converter; (b) petrol engine with three-way catalytic converter, and (c) diesel engine with diesel catalytic converter. The exhaust gas was sucked through a sample vessel filled with a molecular sieve of 5 Å (Figure 1B) for 30 or 60 s (1-5 L/min), depending on the NO concentration. The molecular sieve was activated as described above. In front of the sample vessel, two H2O traps were installed: (a) a glass coil, which was submersed into a mixture of 2-propanol/dry ice, and (b) a Mg(ClO4)2 trap. The NO was analyzed as described above. (21) VDI Richtlinien 2453, Blatt 6, 1980.

GC mode

SICU mode PoraPlot S

SICU mode PoraPlot Q

-68.0 ( 0.7 3

-75.6 ( 1.7 * 4

-68.6 ( 1.1 5

a Mean δ15N-NO values (( 1 standard deviation) and number of replicates are shown for the control experiment (GC mode) and for two types of trapping matrixes (PoraPlot S and PoraPlot Q) as used in the SICU mode. The gas amount of each injection was 213 nmol of NO. Significance levels of differences between the control experiment (GC mode) and the differrent trapping matrixes in the SICU mode with two types of trapping matrixes are indicated by asteristks: *, p < 0.05.

Statistics. Means were calculated from three measurements. Error bars represent ( 1 standard deviation from the mean. Differences between means within each experiment were tested for significance with a one-way ANOVA after checking for homogeneity of variances (Levens test). When treatment effects were significant, the LSD test with a significance level p < 0.05 was used to distinguish the means. Mean values between two experiments were tested for significant differences with Student’s t test for paired samples. All statistics were computed using the statistical package SPSS.22 RESULTS AND DISCUSSION Trapping Matrix Selection for Quantitative Trapping and Release in the SICU. During the thermal desorption of NO from the molecular sieve, a quantitative trapping in the SICU must be ensured. The melting (-163.6 °C) and boiling points (-151.8 °C) of NO indicate that quantitative NO trapping in a fused-silica capillary at -195.8 °C (N2liquid) could not be achieved. For this reason, in experiment 1 (Table 1) two trapping matrixes were tested for their NO trapping properties: (a) PoraPlot Q and (b) PoraPlot S. The mean δ15N-NO value of the recovered NO in the SICU mode was compared to the GC measurement as the control experiment (Figure 1A, C). Results are shown in Table 2. The mean δ15N-NO value achieved with the PoraPlot S trapping matrix was significantly more negative (p < 0.05) than the mean δ15N-NO value determined by GC measurement (Table 2). Thus, PoraPlot S appeared unsuitable as a trapping matrix for NO under the conditions of this experiment. The δ15N-NO values obtained from the GC mode and PoraPlot Q as freezing matrix in the SICU mode, however, did not differ significantly (p > 0.05). The standard deviation in the SICU mode was higher than in the GC mode, due to an additional process in the measurement procedure. On the basis of these results, all following experiments were executed with a PoraPlot Q capillary as trapping matrix. Limits of the NO Sample Size for Quantitative Trapping and Release in the SICU. Quantitative trapping and release of NO within a sample range from 85 to 213 nmol NO was tested in experiment 2 (Table 1). The N2 recovery and the δ15N-NO values of the recovered NO in the SICU mode were compared to the GC measurement as a control experiment. As shown in Figure 3, significant NO losses occurred in the sample range from 85 to 125 nmol of NO when compared to the control experiment. These (22) Norusis, M. J. SPSS/PC+; SPSS Inc.: Chicago, IL, 1986.

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Figure 4. Molecular sieve selection. NO desorption profiles from four types of molecular sieves (3, 4, 5, and 10 Å) are shown in dependence on the desorption temperature. A total of 21 µmol of NO was applied to each of the molecular sieves. The molecular sieves were heated at 10 °C min-1, and the NO desorption was monitored continuously as N2 recovery. Due to different adsorption capacities of the respective molecular sieves, the adsorbed NO amounts were different. Thus, the N2 recoveries were expected to be unequal for the four types of molecular sieves.

Figure 3. Limits of the NO sample size for quantitative trapping and release in the SICU. The N2 recoveries and δ15N-NO values are compared in dependence on the NO sample size in a range from 85 to 213 nmol of NO under two experimental conditions: (a) GC mode (control experiment) and (b) SICU mode with PoraPlot Q as trapping matrix. Each data point is the mean of three replicates. Error bars indicate ( 1 standard deviation, which are in some cases smaller than the symbols. Solid lines are regressions and 95% confidence intervals for the GC mode. Dotted lines are regressions and 95% confidence intervals for the SICU mode. Regressions for the N2 recoveries in the range from 85 to 213 nmol of NO: (a) GC mode: y ) 0.20x - 9.10; r 2 ) 0.996; (b) SICU mode: y ) 0.25x - 18.62; r 2 ) 0.997. Regressions for the δ15N values: (a) GC mode in the range from 85 to 213 nmol of NO: y ) 0.01x - 73.11; r 2 ) 0.852; (b) SICU mode in the range from 128 to 213 nmol of NO: y ) 0.05x - 80.27; r 2 ) 0.712).

NO losses resulted in a significant loss in accuracy for the δ15NNO determination. NO losses during the trapping in the SICU may be traced back to several reasons: (a) nonquantitative NO sorption on the PoraPlot Q matrix; (b) NO losses during freezing of NO due to chromatographic eluation from the PoraPlot Q matrix; (c) nonquantitative NO desorption from the trapping material. As losses occurred mainly for low sample sizes, the peak shape may also have a significant impact. Peak width of the NO peaks in the SICU and the GC mode was similar. However, both peaks showed a tailing, which complicates peak integration. On the basis of the results of experiment 2, a minimum NO sample size of 125 nmol of NO is recommended. Furthermore, a correction for the δ15N-NO dependency on the sample size in the sample range from 125 to 220 nmol of NO is required (correction algorithm: y ) -0.04x + 10.00; y ) δ15N; x ) nmol of NO). After this correction, no significant deviation between the GC and the SICU δ15N-NO determination was observed. However, the sensitivity of the correction algorithm to changes in the 15N/14N ratio of the analyzed NO is unknown. Therefore, further 1130 Analytical Chemistry, Vol. 73, No. 6, March 15, 2001

improvement of the NO trapping method toward avoidance of the mathematical correction should be achieved. Molecular Sieve Selection. NO from air samples should be accumulated on a molecular sieve to achieve adequate sample sizes. The most important properties of the molecular sieve were a low NO desorption temperature and quantitative NO desorption from the molecular sieve. The desorption temperature should be low to avoid chemical reactions of NO and thermal destruction of the molecular sieve. Other reactive gases in the atmosphere, especially NH3, N2O, and NO2, will also be fixed on the molecular sieve and they can detrimentally affect a correct δ15N-NO determination. Therefore, the sorption and desorption properties of NO and of NH3, N2O and NO2 from different molecular sieves were investigated in experiment 3 (Table 1). Figure 4 shows the NO desorption profiles from four types of molecular sieves (3, 4, 5, and 10 Å) in dependency on the temperature in a continuous flow and continuous heating system (Figure 2). Due to the different adsorption capacities of the respective molecular sieves for NO, in this experiment, the N2 recoveries achieved with different molecular sieves are not comparable despite application of equal NO sample amounts. Therefore, the N2 recovery is reported as signal height (V) in dependence on the desorption temperature. The lowest NO desorption temperature was found with the 5-Å molecular sieve. Three peaks were obtained with the maximal N2 recovery at desorption temperatures of 400 and 800 °C. The two peaks at 800 °C desorption temperature were not completely separated and are, therefore, handled as one peak in the following discussion. On the basis of the desorption profiles of the four types of molecular sieves, the molecular sieve of 5 Å was chosen as most suitable for the accumulation of NO from air samples. Figure 5 shows the desorption profile of NO together with the desorption profiles of NH3, N2O, and NO2 from the 5-Å molecular sieve. Due to the different adsorption capacities of the 5-Å molecular sieve for the analyzed gases, the N2 recoveries

Figure 5. Molecular sieve section. Desorption profiles from the 5-Å molecular sieve in dependence on the desorption temperature are compared for four gases (NO, NH3, NO2, N2O). A total of 21 µmol of the respective gases was applied to the molecular sieve. The molecular sieve was heated at 10 °C min-1, and the desorption of the respective gases was monitored continuously as N2 recovery. Due to different adsorption capacities of the respective gases on the 5-Å molecular sieve, the amounts of adsorbed gas were expected to be different. Thus, the N2 recoveries were expected to be unequal for the four gases.

are again not comparable despite equal sample sizes analyzed. Therefore, the N2 recovery in this experiment is again reported as signal height (V) in dependence on the desorption temperature. The first NO peak and the N2O peak were obtained with identical methodology in a temperature range from 300 to 500 °C. NO and N2O can easily be separated through gas chromatography. Therefore, the similar desorption temperatures from the 5-Å molecular sieve do not interfere with the other steps of the measuring process. NH3 desorption may interfere with the desorption of the second NO peak in a temperature range from 600 to 900 °C. In future applications of the method, water vapor must be removed from the sample air before it passes the molecular sieve. An acid chemical material (e.g., Mg(ClO4)2) has to be chosen for this purpose, because of the acidic chemical character of NO. This acid material will also trap alkaline NH3. Minor amounts of NH3, which may escape from the acid trap, can easily be separated from NO by GC separation. The second NO peak and the NO2 peak resulted under identical methodology in a temperature range from 600 to 900 °C. It seems probable that the second NO peak results from NO oxidation to NO2 on the oxygen-containing molecular sieve surface. This reaction may be inhibited in a static desorption procedure from the molecular sieve with heating for several minutes at a lower given temperature, because oxidation is more probable at high desorption temperatures. This was tested in the following experiment with a molecular sieve of 5 Å. Desorption Temperature Optimization. Quantitative NO desorption from a 5-Å molecular sieve through heating for 15 min at a certain temperature was tested in experiment 4 (Table 1). The desorbed NO was trapped on PoraPlot Q in the SICU as described above. NO analysis in the SICU mode without previous trapping on a molecular sieve was used as the control experiment. The N2 recovery from the SICU determination was defined as 100%

Figure 6. Desorption temperature optimization. Temperature dependence of the N2 recovery and the ∆δ15N-NO values are compared for three different experimental conditions: (a) SICU analysis as a control experiment; (b) 5-Å molecular sieve; and (c) 5-Å molecular sieve + 185 mg of Mo. N2 recoveries and δ15N-NO values of the SICU control experiment were compared to experiments b and c. The N2 recovery of the control experiment was defined as 100% (n ) 3), and the N2 recoveries found for experimental conditions b and c are shown in percent on this basis. 15N/14N ratios under experimental conditions b and c are shown as relative deviations from the control experiment () ∆δ15N, ‰). The desorption temperatures were equal for all experiments and are only shifted slightly for optical reasons. All data points in experiments b and c are means of three replicates. Error bars are ( 1 standard deviations, which are in some cases smaller than the symbols. Data points in the control experiment are means of three replicates, which are equal for each desorption temperature; i.e., the control experiment had by definition no temperature dependence. Dotted lines represent the ( 1 standard deviations of the control analyses (n ) 3). Different letters denote significantly different groups for the respective temperatures.

(n ) 3), and the N2 recovery from the molecular sieve determinations was calculated in percent on this basis. δ15N-NO values from the molecular sieve determination were calculated as absolute deviation from the δ15N-NO values of the SICU determination (n ) 3; ∆δ15N-NO). The N2 recovery from the molecular sieve was highest between 300 and 400 °C (Figure 6). At lower and higher desorption temperatures, the N2 recovery decreased. ∆δ15N-NO values were highest below 300 °C and above 400 °C and lowest at desorption temperatures between 300 and 400 °C. However, the δ15N-NO values from the molecular sieve determination differed from the SICU control experiment even in the temperature range from 300 to 400 °C. The smallest ∆δ15N-NO value of 4.6‰ was obtained at 350 °C desorption temperature. Analytical Chemistry, Vol. 73, No. 6, March 15, 2001

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∆δ15N may result from NO oxidation to NO2 on the hot, oxygenrich molecular sieve surface. To minimize ∆δ15N-NO values, an experiment with Mo addition to the 5-Å molecular sieve in the reaction vessel was performed. Mo is commonly used to reduce NO2 to NO in environmental analysis.15,16,21 It was presumed that the added Mo would reduce NO2 generation from NO during heating. As shown in Figure 6, the ∆δ15N-NO was smaller with Mo addition than without Mo addition to the molecular sieve at 350 °C desorption temperature, but the N2 recovery was even more variable than without Mo addition. Mo will not only reduce NO2, which was generated from NO oxidation, but also NO2 from the sample air and may react with other nitrogen-containing compounds in the sample gas (e.g., NH3, peracylnitrates). These reactions with Mo all are detrimental to δ15N-NO determination from air samples. For this reason, Mo addition to the molecular sieve is not recommended. In addition to NO2, N2O and N2 could also be generated from NO on hot molecular sieve surfaces.23 These gases were not analyzed in this study. However, they may also contribute to the δ15N-NO offset. The 4.6‰ offset detected for the NO desorption at 350 °C from a 5-Å molecular sieve must be subtracted from the δ15N-NO values of the air samples. Use of a microwave desorber might reduce the heating time and, thus, might be a suitable tool to circumvent the offset correction in the future. Nevertheless, after consideration of all described corrections, the new on-line method results in (0.9 ‰ reproducibility for the δ15N-NO determination from ambient air samples. Exhaust Gas Measurements. The δ15N-NO analysis from exhaust gas was complicated by high amounts of carbon monoxide in the sample. CO has the same mass-to-charge ratio (m/z) as N2, to which NO was converted for isotopic measurement. Therefore, CO interferes with the δ15N-NO analysis if it reaches the mass spectrometer at the same time as NO. Complete chromatographic separation of CO and NO is difficult, but possible and essential for correct δ15N-NO determination. The δ15N-NO values obtained from petrol and diesel engines were completely different and showed different trends with warming of the engine and the catalytic converter (Figure 7). Engines were started at the beginning of the air sampling. In the few minutes of idle running, the engines never reached their usual operating temperature, and therefore, the reported δ15N-NO values are presumably not typical for long-term NO exhaust. This is important for the interpretation of the data. The huge differences between the δ15N-NO values of the petrol and the diesel vehicle shortly after the start of the engine may be due to the different combustion temperatures and pressures.24 The δ15N-NO values from an early working phase of the engines were influenced by the type of the engine and not by the presence or absence of a catalytic converter. The δ15N-NO values showed a trend with the running time of the engine, which was different for the diesel and the petrol vehicle with catalytic converter (Figure 7). This trend may be an effect of the catalytic reaction on the surface of the catalyst, since the diffusion velocity for 14NO is expected to be higher than for 15NO. Therefore, 14NO can react faster with the catalyst and exhaust gas is enriched in 15NO as shown for the diesel vehicle. An increase in the exhaust gas temperature should (23) Chang, Y.-F.; McCarty, J. G. J. Catal. 1997, 165, 1-11. (24) Hamm, G.; Burk, G. Tabellenbuch Kraftfahrtechnik; Holland + Josenhans Verlag: Stuttgart, 1990; p 138.

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Figure 7. Results of the exhaust gas measurements. The δ15NNO values of exhaust gases from three different engines are shown in dependence on the time after the ignition/start of the respective engines. Horizontal lines indicate the sampling interval, which was 1 min for the engines with catalytic converter and 30 s or 1 min for the engine without catalytic converter.

decrease the difference in the diffusion velocity of 14NO and 15NO and make the δ15N-NO values in the exhaust gas less positive. This trend can be seen from the petrol engine. An O2 addition to the burning chamber is needed for the catalytic reaction. The lambda probe, which regulates the O2 addition, requires an operating temperature of 300 °C for optimal function.24 The δ15N-NO values of vehicles with warmed engines may therefore be completely different from the ones measured in this experiment. Warming of the lambda probe usually takes 15-30 min. δ15N-NOx measurements by Heaton10,11 in exhaust gas range from -2 to -13‰ with different engine types. Ammann et al.8 sampled NO and NO2 from air on a transect orthogonal to a highway and found a mean δ15N of +5.7‰. A more detailed investigation is required to describe the temporal development of the δ15N-NO and δ15N-NO2 values emitted in exhaust gases from different types of vehicle engines. CONCLUSIONS This study presents a new method for the sampling of NO from ambient air for the subsequent on-line analysis of its isotopic composition. Almost no data on 15N/14N ratios of this important atmospheric trace gas are available at present, since such methods are not available as yet. First results obtained with the new online method provide new perspectives for future applications, e.g., in ecology (NO emission from soils) and atmospheric chemistry (NO sources and sinks in the stratosphere, tracing of NO pollution back to the respective sources). However, in its present status, the new method still has limitations, especially with respect to the two trapping processes. Two mathematical offset corrections had to be introduced for the two trapping processes to achieve a satisfactory reproducibility. These corrections may not be constant over a wider range of δ15N-NO values. Therefore, future goals

are to circumvent these offset corrections, e.g., by usage of a microwave desorber. ACKNOWLEDGMENT This study was supported by the Bayreuther Institut fu¨r Terrestrische O ¨ kosystemforschung (BMBF PT 51-0339476B) and by the European Council (FAIR3 CT96-1920). Christoph May and Gisela Schmidt helped with the statistical analysis of the data.

Proofreading and language improvement of the manuscript by Chris Tarn and Jens-Arne Subke is gratefully acknowledged.

Received for review August 7, 2000. Accepted December 18, 2000. AC0009292

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