J. Phys. Chem. 1995, 99, 5001-5008
5001
Reactions of SFs- and I- with Atmospheric Trace Gases L. Gregory Huey,*tTDavid R. Hanson: and Carleton J. Howard NOAA, Aeronomy Laboratory, Boulder, Colorado, 80303 Received: November 3, 1994; In Final Form: January 27, 1995@
The rate constants for the reactions of SF6- with ClN03, HNO3, HC1, N2O5, NO;?, 0 3 , C12, c120, HOCl, CF20, CSCFO, and SO2 were measured at 293 K using the flowing afterglow technique. The rate coefficients range from 2 x lo-'' to 2 x cm3 molecule-' s-', Most of the SF6- reactions give multiple products except for C12, N02, and Os, which react only by charge transfer, and CF20 and CF3CF0, which react only by fluoride transfer. The reactions of SF6- with Br20, BrN03, and HOBr were studied qualitatively. All of the SF6- reactions studied provide a method for selectively detecting the neutral reactant by chemical ionization mass spectrometry. The reactions of I- with ClN03, N2O5, HN03, NO2, and O3 were also studied. The reactions with ClN03 and N205 are fast (k 2 9 x cm3 molecule-' S - I ) and produce only N03-. I- is unreactive with HN03, NO2, and 03.The SF6- reactions with HCl and 03 and the I- reaction with N2O5 were found to be significantly faster than reported previously.
Introduction Chemical ionization mass spectrometry (CIMS) has been used in our laboratory to detect many atmospherically important species.'-4 Recently, Hanson and Ravishankara studied the heterogeneous chemistry of HNO3, ClN03, N2O5, HC1, HOCl, HOBr, BrN03, and CF2O using CIMS.5-9 Selective detection of these compounds was accomplished by reaction with either SF6- or I-. These ion-molecule reactions have not been studied previously except for the reactions of sF6- with HCl and I- with N2O5; thus we set out to measure the rate constants and product yields for these reactions as well as other reactions of SF6- with species such as 0 3 and C120. This study clarifies the interaction of these ions with many atmospheric species.
t TURBO PUMP
TURBO PUMP
*
TURBODWG PUMP
Figure 1. Overview of flowing-afterglow/CIMS apparatus.
Experimental Section The ion-molecule reaction rates were measured using the flowing-afterglow technique, which was originally developed in this laboratory and has been widely used for measurements of this type.Io We only briefly discuss this technique as it has been described thoroughly elsewhere.' The new apparatus used for these studies is shown in Figure 1. It consists of a flow tube, where the ion-molecule reactions are carried out, and three differentially pumped chambers, where the ions are sampled and mass analyzed. The flow tube is a 130 cm long x 7.30 cm i.d. stainless steel tube through which a large flow of carrier gas is maintained by a 500 W s Roots blowerlmechanical pumping system. Ions generated at the upstream end of the flow tube are transported down the tube with the carrier gas. A small fraction of the total flow is sampled through a 0.5 mm diameter orifice. The sampled ions are focused with ion optics through the first two chambers and mass analyzed with a quadrupole mass filter and detected with an ion multiplier located in the third chamber. The fist chamber is pumped by a 1000 L/s turbomolecular drag pump which can be operated at pressures up to 0.01 Torr. The next two chambers are both pumped with 500 Us turbomolecular pumps. This differential pumping scheme allows the operation of the flow tube at pressures up to 100 Torr, with the 0.5 mm diameter Also affiliated with: Cooperative Institute for Research in Environmental Science, University of Colorado, Boulder, CO 80309. @Abstractpublished in Advance ACS Ah.structs, March 15, 1995.
0022-365419512099-5001$09.00/0
sampling orifice. The apparatus is operated as a flowingafterglow at flow tube pressures of 0.4-1.0 Torr and as a chemical ionization mass spectrometer (CIMS) at flow tube pressures of 0.4- 100 Torr. Helium was used as the carrier gas in the ion flow tube at flow rates of 100-180 STP cm3 s-I and at pressures of 0.400.80 Torr. This yielded average canier gas flow velocities of (0.3- 1.2) x lo4 cm/s and reaction times of the order of 10 ms. The I- and SF6- reactant ions were made by electron attachment to CF3I and SF6, respectively. The electrons were generated from an electrically heated, thoriated-iridium filament biased at -50 eV. The heating current was regulated to produce a constant emission current of 5 PA. CF31 or sF6 was added in trace amounts to the flow tube just downstream of the filament. The ion-molecule reactions were carried out by adding the neutral species to the flow tube through either of two inlets corresponding to reaction distances of 55 and 80 cm. The first inlet was located 25 cm downstream of the filament. The reactant ion signal was monitored as a function of neutral reactant concentration for both reaction distances and the rate constants were calculated using the method described by Ferguson et al.'I This method derives the reaction time from the average flow velocity multiplied by a factor of 1.6 to account for the radial velocity and ion concentration profiles in the flow tube. The use of two reaction distances provided a consistency check of the calculated reaction time and demonstrated that the carrier gas flow and reactant mixing were well behaved. Figure 0 1995 American Chemical Society
5002 J. Phys. Chem., Vol. 99, No. 14, 1995 I
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TABLE 1: Summary of the W Absorption Cross Sections Used for Each Species
l
z=55 cm k=1.16~10-~ z=80 cm k = 1 . 1 8 ~ 1 0 - ~
species d = 214 nm
u (cm2 molecule-') d = 185 nm d = 254 nm
HCI
3.3 x 10-19
3.87 x IO-'' N205 3.94 x lo-'' CINO? 3.60 x c120 HNO, NO^ 4.0 x 10-19
19 56 19 19 14 57 19
SO2
0 3
3.46 x IO-'* 1.63 x 10-17 1.137 x lo-''
c12
'
[CINO,] (10' molecule cm-,) Figure 2. SF6- signal vs [CIN03]: the open circles correspond to a reaction distance of 55 cm and a measured k = 1.16 x cm3 molecule-' s-'; the solid circles correspond to a reaction distance of cm3 molecule-' s-]. 80 cm and a measured k = 1.18 x 2 shows a sample decay plot for the reaction of sF6- with ClN03, the extracted rate coefficients for both reaction distances are within 2% of one another. The accuracy of our analytical procedure was verified by comparing our derived rate coefficients with values derived by others for the well-characterized ion-molecule reaction: He'
+ N, - N2+ + He
-.N+ + N + He
(1)
which has been measured in many different laboratories.I2 The rate constant measured for reaction 1 in the present work is k = (1.2 f 30%) x cm3 molecule-' s-'. This compares favorably to the values from ref 12, which are, in units of cm3 molecule-' s-l, 1.23 f 20%, 1.2 f 30%, and 1.25 f 20%, respectively. The apparatus was operated as a CIMS to measure the impurities in many of the compounds studied. The major difference between operation of the apparatus as a flowingafterglow or as a CIMS is the concentration of the neutral reactant in the flow tube. When the apparatus is operated as a flowing-afterglow a relatively large concentration of neutral reactant ( lolo- l o t 4molecules ~ m - is ~ added ) to the flow tube to decrease the reactant ion signal by a factor of 1.1-1000. For operation as a CIMS, the concentration of the neutral reactant is kept below 1Olo molecules cm-3 for reaction times on the order of 10 ms and for a neutral-reagention rate constant of 1 x cm3 molecule-' s-I. Under these conditions, the reactant ion signal (Le., the reagent ion) is essentially unchanged by the addition of the neutral reactant and the product ion signal is linear with respect to the neutral c~ncentration.'~ The product ion signal is proportional to the reaction time, neutral concentration, and the rate constant for the reaction of the neutral with the reactant ion. Therefore, impurity levels can be determined from the relative product ion signal levels, the relative reaction rates, and product yields for reactions of the reagent ion with the species of interest. It should be noted that low concentration impurities are not depleted by reaction in the ion flow tube
1 = 330 nm ref
58 2.6 x 10-19 59
because the product of the reaction time, rate coefficient, and reagent ion concentration is always held to less than 0.01, For the present system, typical detection limits for species with large rate constants with the reagent ion are on the order of lo6 molecule cm-3 ( S / N = 2, 1 s integration time) which allows for identification of impurities at the 0.01% level. The major sources of error in our experiments are inaccuracies in the measurement of the concentration of the neutral reactant and the presence of reactive impurities. Many of the species used are difficult to handle because they are both reactive and thermally unstable. Therefore, special care was taken in the preparation, handling, and concentration measurements of the reactants as described in the following paragraphs. The concentrations of HCl, SO;?,N205, C1NO3, C120, NO;?, and HN03 were measured by optical absorption in a 2.5 cm diameter, 50 cm long, glass absorption cell, fitted with quartz windows. The absorption cell was connected to the flow tube by a short length of 6 mm diameter glass tubing. The absorption was measured at either the 214 nm atomic zinc line or the 185 nm atomic mercury line. The 214 nm light was detected with a phototube (Hamamatsu R765) equipped with a 214 nm bandpass filter. The detector for the 185 nm absorption measurements was a true solar blind phototube with a Cs-I photocathode (Hamamatsu R1371) with a spectral response of 140-200 nm. This limited spectral response allows for the isolation of the 185 nm line without the use of filters. The absorption cross sections used are listed in Table 1. The concentration of species X in the flow tube, [XI,, in molecule cm-3 was determined using eq i, where UX,J is the absorption -ln(Z/Zo) FAcP, [XI, = 4 . n FFTPAC cross section for species X at wavelength 1 (cm2 molecule-'), /is the length of the absorption cell (50 cm), FFT and FAC are the total flows through the flow tube and absorption cell (STP cm3 s-l), Pm and PAC are the pressures in the flow tube and absorption cell (Torr), and I and IO are the measured light intensities with and without the reactant present, respectively. The ClzO was synthesized by passing a small flow of electronic grade Clz in UHP helium through a trap at 273 K filled with freshly recrystallized and dried mercuric oxide. The C12 reacted with the HgO to form Cl2O.I4 The effluent from this trap passed through another trap at 195 K where a mixture of C120 and unreacted Cl2 was collected. The Cl2 impurity, as determined by CIMS, was reduced to 3 x cm3 molecule-' s-'). All of the possible products of the reactions of the bromine species with SF6- could not be identified due to interferences. However, the identified products were determined to be the major products of the reactions. For the Br2O reaction with SF6-, the charge transfer product Br2O- was not observed and an upper limit for this channel of less than 2% of the fluoride transfer channel was determined. The possible Brchannel for this reaction was obscured due to the production of this ion as a minor channel of the reaction of SF6- with Br2 (which was in 50-fold excess in these experiments), but we could estimate this channel was less than 20% of the fluoride transfer channel. This is in marked contrast to the SF6- reaction with C120 which produces C1- and C120- with approximately the same yield. The reaction of SF6- with BrNO3 has a yield of less than 3% for the charge-transfer product, BrN03-. Other possible minor product channels of this reaction such as NO3-, SF~NOJ-,and SF5- could not be ruled out due to the presence of a large HNO3 impurity that reacts with SF6- to produce these ions. However, the yields of these channels were found to be < 10%of the fluoride-transfer channel. The formation of SF5from the reaction of HOBr with SF6- could not be ruled out due to a large background of this ion from dissociative electron attachment to SF6 and the small [HOBr] which could be obtained. An upper limit for the yield of SF5- was determined to be
