1440
Anal. Chem. 1983, 55, 1440-1442
ions, with high efficiency. Thus, one can easily separate and distinguish the I3C isotope peak at M+ + 1even for fairly large organic molecules. The ratio of (M+ + 1)/M+should provide the number of carbons in the compound. Thus, even if other elements are present in an organic compound, the number of carbons can be calculated. In addition, efficient and selective ionization spectroscopy can be used for isotope discrimination (24) and a high-resolution reflectron can add further to that preselected discrimination. A microchannel plate was used as the detector. The great advantage of this device is the high gain (lo'), speed (subnanosecond risetime), and large active detector area (1 in. in this study). A number of other electron multipliers are available and can be used. A channeltron electron multiplier should not be used because of its cone-shaped detector surface. The cone shape will cause different flight times depending on which point on the surface the ion arrives, thus decreasing the resolution. Activated Cu-Be devices can be used if fast enough; however, the gain is degraded when exposed to water vapor in the air. Some of the Cu-Be devices are only marginally fast enough. The simplicity of the TOF device makes it inexpensive to build. It is mechanically much simpler than a quadrupole or double focusing field mass spectrometer and, thus, can be built in any lab. The cost of building the reflectron in parts and machining time was on the order of $2200, not including the detector or power supply. The vacuum feedthroughs which are commercially ~ $ 1 0 each 0 can be replaced by crude inhouse feedthroughs to reduce the cost. Eight feedthroughs were used so that the saving is significant. The cost of the electron multiplier can vary from $400 up to $5000, depending on the type and quality of the multiplier used.
ACKNOWLEDGMENT We wish to thank R. J. Rorden and D. Billings for design
and construction of the pulsed electronics circuit.
LITERATURE CITED (1) Why, W. C.; McLaren, I. H. Rev. Scl. Instrum. 1955. 26, 1150, (2) Mamyrin, B. A.; Karataev, V. I.; Shmikk, D. V.; Zagulin, V. A. Sov. fhys.-JETP (Engl. Transl.) 1973, 3 7 , 45. (3) Mamyrin, 5. A.; Shmikk, D. V. Sov. Phys.-JETf (Engl. Transl.) 1979, 49, 762. (4) Kaufmann, R.; Hillenkamp, F.; Wechsung, R. Med. frog. Tecbnol. 3979, 6 , 109. (5) Denoyer, E.; Van Grieken, R.; Adams, F.; Natusch, D. F. S. Anal. Chem. 5982, 5 4 , 26A. (6) Boesi, U.; Neusser, H. J.; Weinkauf, R.; Schlag, E. W. J . fhys, Chem. 1982, 86, 4857. (7) Lubman, D. M.; Kronick. M. N. Anal. Chem. 1982, 5 4 , 660. (8) Dietz, T. G.; Duncan, M. A.; Liverman, M. G.; Smalley, R. E. Chem. Phys. Lett. 1980, 7 0 , 246. (9) Frueholz, R.; Wessel, J.; Wheatley, E. Anal. Chem. 1980, 52, 281. (10) Lubman, D. M.; Naaman, R.; Zare, R. N. J. Chem. fhys. 1980, 72, 3034. (11) Seaver, M.; Hudgens, J. W.; DeCorpo, J. J. I n t . J . Mass Spectrom. Ion. fhys. 1980, 34, 159. (12) Zandee, L.; Bernstein, R. B. J . Chem. Phys. 1979, 70,2574. (13) Zandee, L.; Bernstein, R. B. J . Chem. Phys. 1979, 77, 1359. (14) Llchtln, D. A.; DattaGhosh, S.; Newton, K, R.; Bernsteln, R. B. Chem. fhys. Lett. 1980, 7 5 , 214. (15) Boesl, U.; Neusser, H. J.; Schlag, E. W. J . Chem. fhys. 1980, 72, 4327. (16) cooper, C. D.; Williamson, A. D.; Miller, J. C.; Compton, R. N. J . Chem. Phys. 1880, 73,1527. (17) Fisanick, G. J.; Eichelberger, T. S., IV; Heath, B. A,; Robin, M. B. J . Chem. Pbys. 1980, 72, 5571. (18) Fisanick, G. J.; Elchelberger, T. S.,I V J . Chem. fhys. 1981, 74, 6692. (19) Reilly, J. P.; Kompa, K. L. J . Chem. fhys. 1980, 73, 5468. (20) Antonov, V. S.; Knyazev, I. N.; Letokhov, V. S.; Matuik, V. M.; Moshev, V. G.; Potopov, V. K. Opt. Lett. 1978, 3 , 37. (21) Antonov, V. S.; Letokhov, V. S. Appl. Phys. 1981, 2 4 , 89. (22) Kiimcak, C.; Wessei, J. Anal. Chsm. 1980, 52, 1283. (23) Parker, D. H.; El-Sayed, M. A. Chem. Pbys. 1979, 42, 379. (24) Lubman, D. M.; Zare, R. N. Anal. Chem. 1982, 54, 2117.
RECEIVED for review October 25, 1982. Accepted March 7, 1983. This work received partial financial support from the U.S. Army Research Office, Contract No. DAAG-29-81-C-0023.
Generation of Formaldehyde in Test Atmospheres with Low Concentrations of Hydrogen and Carbon Monoxide Roland E. Muller and Ulrlch Schurath" Institut fur Physikalische Chemie der Universitat Bonn, Wegelerstrasse 12, 0-5380Bonn I, West Germany
Considerable progress has recently been made in the analysis of very low formaldehyde concentrations in tropospheric air (1-3). Formaldehyde is also an indoor air pollutant, due to outgassing of plastic materials and adhesives, which is suspected to be carcinogenic (4). Formaldehyde test gases for calibration purposes cannot be prepared from the pure monomer which has a strong tendency to polymerize. Geisling et al. (5) describe a dynamic procedure for the generation of formaldehyde in a test atmosphere by the depolymerization of trioxane on a carborundum catalyst at 160 "C. The trimer is continuously evaporated into a carrier gas from a thermostated diffusion cell. We needed about 100 ppm formaldehyde in air for a continuous actinometer, designed to monitor the photolysis frequency of formaldehyde in daylight. In order to measure the very low photochemical CO and H2 yields with a specific detector (6), the background concentrations of these gases in the unexposed test gas had to be a t most in the low parts-per-billion range. We have tested carborundum catalysts under conditions suitable for the depolymerization of trioxane and found that they produce 2180 ppb CO and 240 ppb H2 by the thermal decomposition of 90 ppm formaldehyde in a test gas. We have therefore developed a permeation source 0003-2700/83/0355-1440$01.50/0
which eliminates the problem of thermal decomposition of formaldehyde.
EXPERIMENTAL SECTIQN The aldehyde source consists of a gastight permeation cell, either a stainless steel cylinder of 60 mL volume, or a sealed 300-mL Pyrex flask to further reduce heterogeneous decomposition, which is loosely packed with paraformaldehyde and quartz wool (Merck "Paraformaldehyd reinst", Erg B 6), as shown in Figure 1. A Teflon tube (3.2 mm o.d., 2 mm id.) is tightly sealed into the cell. Aldehyde vapor in thermal equilibrium with paraformaldehyde diffuses through the Teflon tube into the carrier gas which flows through the loop. Active tube lengths up to 266 cm were used. Constant flow rates are maintained by means of ASM flow controllers. The permeation cell is contained in a thermostated brass box (temperature control better than h0.1 " C ) , which is thermally isolated with mineral wool. The box temperature is digitally displayed and can be varied continuously over the ranges 0-60 " C or 80-140 "C. For the lower temperature range, the isolated box is cooled externally. Long path UV absorption is used to measure the aldehyde concentration in the test gas, as shown schematically in Figure 1. The absorption cell is a Pyrex tube of 248 cm length, 2.0 cm 0 1983 American Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 55, NO. 8, JULY 1983 ~
1 . 2 7 -
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1j
_
/
1.0 0,9 -
:I441 _7
/ I I
0,8 -
c
/// q //J/
-.0,70,6 --co 0.5-
-
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Figure 1. Schematics of permeation system and optical absorption cell for aldehyde measurements In test gases: (1) gastighl stainless steel cylinder, or sealed Pyrex cell; (2) Teflon permeation tube; (3) paraformaldehyde and quartz wool; (4) air space between permeation cell and thermostated brass container; (5) stonewool packing.
i.d., which is mounted on an optical bench. Treating the cell with 1,1,1,3,3,3-hexamethyldisilazane resulted in an "e-folding time" for the polymerization of 1.5 torr pure formaldehyde of 250 h in the dark. The cell transmits UV radiation from a highly stabilized deuterium lamp (Cathodeon C 70-3V-H). The absorption path length is doubled by reflection, using a quartz triple prism as end window. The transmitted light is dispersed in a Jobin-Yvon H.20 monochromator with a holographic grating for the range 200-800 nm. The transmitted intensity is measured with a solar blind photomultiplier (Hamarnatsu R166) and a Keithley 155 microvoltmeter. The low output impedance of this instrument allows its output at full light intensity to be compensated to zero, so that very weak: absorptions can be displayed with increased sensitivity on a chart,recorder. The stability of the optical setup was checked repeatedly by flushing the optical cell with pure synthetic air between the concentration measurements. The Iolevel waa rapidly established, and the level dropped by typically 1% in 10 h. The detection limit of the optical system was 2 ppm for formaldehyde and other volatile aldehydes. Monomeric formaldehyde was prepared from paraformaldehyde loosely covered with quartz wool and Pz05in a Pyrex flask, following the procedure of Bass et al. (7). The flask was pumped on the valcuum line and slowly warmed in an air bath until the monomer evolved. The vapor was collected in a cold trap, purified by trap-to-trap distillations, and stored at liquid nitrogen temperature. According to Bass et al., this procedure yields 99.9% pure monomeric formaldehyde (7). Synthetic air (FDquality) was used directly for the permeation system. When the CO and Hzyield of the permeation system was to be measured, the air was stripped of these trace compounds before use by reaction with AgzO and hopcalite in a purification train, as described in the literature (8). As the aldehyde would have interfered with the CO and Hz analysis, most of it was condensecl in a cold trap filled with glass beads (40/60 mesh) at -179 OC,slightly above the condensation temperature of air. The 50-90 ppti (of 90 ppm) formaldehyde which was not retained by the cold trap was absorbed by molecular sieve 13X in a short glass tube at room temperature.
RESULTS AND DISCUSSION Absorlption Cross Sections. Measuring formaldehyde concentrations by UV absorption requires the absorption cross section t o be known at a suitable wavelength. For this purpose, the absorption spectrum was investigated in the range 270-290 nm, where the overlap with the intensity distribution of the deuterium lamp and the spectral sensitivity of the photomultiplier are optimal. The spectral resolution 11nthese and all further measurements was 2.0 nm. The prominent 2641absorption band of formaldehyde at 285 nm was selected for the coincentration measurements. The aldehyde was admitted to the evacuated absorption cell in pure monomeric form and measured with a capacitance manometer (MKS Baratron Model 221 AHS, 0-10 torr range). A plot of In (Io/I) vs. formaldehyde pressure a t 295 K is shown in Figure 2. It
0.2
o , o - L J , 0
0.2
0,L
0.6
I
,
0.8
1.0
,
,
,
,
,
,
1,2 1,L 1.6 1.8 2,O 2,2 HCHO pressure [Torr at 295 K 1
;?,A
Flgure 2. Beer's law plot for pure monomeric formaldehyde in the long path absorption cell at 295 K, k = 285 nm, spectral resolution 2 nm.
exhibits marked curvature, due to the discrete line struc1:ure of the absorption which remains unresolved in the measurements. By comparison, acetaldehyde which has a nearly continuous absorption spectrum in the wavelength range, obeyed Beer's law ulp to at least 1.2 torr, yielding an absorption cm-2 molecule-' at 283 nm. A cross section of 4.4.7 X polynomial fit to the formaldehyde data in Figure 2 yielded the following expression, which was used to calculate formaldehyde concentraitions from the measured intensity ratios
Io/I In ( I o / l ) = cb(ao
_
+ ac)
where c is the concentration in (molecules ~ m - ~b) is , .the optical path length, 496 cm, uo is the limiting low pressure cm2 absorption cross section of formaldehyde, 3.63 x molecule-l, and a is 7.45 x cm5 molecule-*. The ,absorption cross section of the pure aldehyde is unsignificariitly affected by air pressure (9) and can thus be used for cloncentration measurements in test gases at 1 atm total pressure. Test gases prepaired by continuous depolymerization of paraformaldehyde in a permeation source may be contaminated by water vapor, formic acid, methyl formate, and methanol (5). These gases do not absorb at 285 nm (10). The absorption spectrum of the test gas above 260 nm agreed with the reported low-resolution spectrum of formaldehyde (9). In particular, measurements at 240 nm, where formic acid and methyl formate start to absorb significantly, yielded only 6% of the test gas absorbance at 285 nm, which can be attribukd to formaldehyde (IO). This implies that measurements of formaldehyde are essentially free of optical interference a t 285 nm, although this test gas was not analyzed for all possible contaminants. Stability o f Permeation Rates. Paraformaldehyde i,s a polymer of variable chain length, and literature data on the monomer pressure in thermal equilibrium with the polymer are sparse and in poor agreement (11). It was therefore necessary to prove that constant permeation rates could be maintained with paraformaldehyde. The cell waR filled with 10 g of the polymer and thermostated a t 121.8 O C . The permeation tube length was 266 cm, and the air flow rate 50100 mL h-'. The formaldehyde concentration in the effluent gas was measured at intervals over a period of 9 days and plotted vs. time, as shown iin Figure 3. The formaldehyde concentration decreases slightly during the first 40 h. After this induction period, t:he stability of the permeation rate is probably better than the observed scatter of f2% in Figure 3, which includes errors in In ( l o / I )as well as possible fluctuations of the flow rate. After the cell had been stored at room temperature for 118 days, measurements under tbe
1442
ANALYTICAL CHEMISTRY, VOL. 55, NO. 8, JULY 1983
0
20
4b
60
80 100 I20 l i 0 160 180 200 220 time [ h i
Figure 3. Time evolution of formaklehyde concentration in the test gas: permeation cell temperature 121.8 OC, tube length 266 cm, air flow rate 5000 mL h-'.
conditions of Figure 3 yielded 97% of the average permeation rate previously observed. The expected inverse dependence of the formaldehyde concentration on flow rate was confirmed over the range 3000 to 9000 mL h-l. Temperature Dependence of the Permeation Rate. The permeation rate of formaldehyde through a Teflon tube of 266 cm length was optically measured at eight equally spaced temperatures in the range 353-402 K. For comparison, another permeation cell of the design shown in Figure 1, Teflon tube length 122.6 cm, was fiied with pure liquid acetaldehyde, and the permeation rate was determined at five temperatures in the range 285-321 K, using the same optical method at 283 nm. When the logarithms of the specific permeation rates per centimeter tube length, R in (molecule cm-ls-l), were plotted against 1/ T, excellent straight lines were obtained. Linear fits through the data points yielded the following temperature dependences of the specific permeation rates R (two standard deviations in parentheses): Formaldehyde: In R = (59.186 f 0.920) - (10107 f 1 5 2 ) / T
E, = (84.03 f 1.26) kJ molw1 Acetaldehyde: In R = (50.728 f 1.862) - (5871 f 191)/T
E, = (48.81 f 1.59) kJ mol-' The small standard deviations of the linear fits suggest that these formulas may be used with some confidence to calculate specific permeation rates outside the temperature ranges studied. The activation energy of permeation may be interpreted as follows: The diffusion rate of an aldehyde through the Teflon tube is proportional to the vapor pressure inside the cell, and to its diffusion coefficient in Teflon at the specific temperature. The vapor pressure of acetaldehyde at room temperature is given by (12) In p(torr) = 18,011 - 3332.2/T
which yields an enthalpy of vaporization of A", = 27.7 kJ mol-l. The difference, Ed = E, - A H v = 21.1 kJ mol-l, is the activation energy of diffusion for acetaldehyde molecules in Teflon. On the assumption that Ed of acetaldehyde and formaldehyde are approximately equal, an enthalpy of sublimation of paraformaldehyde is obtained, A", = E, - Ed = 63 kJ mol-l, in excellent agreement with the reported enthalpy of polymerization of monomeric formaldehyde (11). CO and H2 Contents of Test Gases. Decomposition of formaldehyde into CO and H, is nearly thermoneutral and occurs heterogeneously at moderate temperatures, particularly on metal surfaces. The CO and H, contents of formaldehyde containing test gases were measured, after trapping the formaldehyde as explained in the Experimental Section. The specific CO and Hz detector of Seiler (6) was used in the GC mode. A stainless steel permeation cell and an all Pyrex cell were tested, both at 90 "C and with a Teflon tubing of 90 cm, yielding 170 ppm formaldehyde at a flow of 3000 mL h-l purified air. The following concentrations, corrected for a low background of CO and Hz in the carrier gas, were measured: Stainless steel cell:
GO
= 30 ppb;
HZ = 50 ppb
= 10 ppb;
Hz = 20 ppb
Pyrex cell:
GO
As expected, the concentrations were lower in the effluent of the Pyrex cell. They decreased slowly with time, indicating that a lower steady state can be reached. The concentrations of CO and H2,although not negligible, are low enough to allow the photochemical yields of these gases to be measured in the projected actinometer. ACKNOWLEDGMENT We thank Wolfgang Seiler of the Max Planck Institute, Department of Atmospheric Chemistry, for the opportunity to use his CO/H2 detector, and his co-workers for their valuable assistance. Registry No. Paraformaldehyde, 30525-89-4;formaldehyde, 50-00-0. LITERATURE CITED ( 1 ) Neitzert, V.; Seller, W. Geophys. Res. Lett. 1981, 8 , 79-82. (2) Lowe, D. C.; Schmidt, U.; Ehhalt, D. H.; Frlschkorn, C. 6. B.; Nurnberg, H. w. ~nvlron.sci. rechnol. 1981, 75,819-823. (3) Groslean, D. Mvlron. Sci. Technol. 1982, 16, 254-262. (4) Perera, F.; Petito, C. Science 1982, 216, 1285-1291. (5) Geisling, K. L.; Miksch, R. R.; Rappaport, S . M. Anal. Chem. 1982, 54, 140-142. (6) Seiler, W.; Glehl, H.; Roggendorf, P. Atm. Technol. 1980, 12, 40-45. (7) Bass, A. M.; Glasgow, L. C.; Jesson, J. P.; Filkin, D. L. Planer. Space Sci. 1980, 2 8 , 675-679. (8) Bullister, J. L.; Guinasso, N. L.; Schink, D. R. J. Geophys. Res. 1982, 8 7 , 2022-2034. (9) Moortgat, G. K.; Kllppel, W.; Mobus, K. H.; Seller, W.; Warneck. P. Report No. FAA-EE-80-47, US. Department of Transportatlon, 1980. (10) Caivert, J. G.; Pitts, J. N. "Photochemistry"; Wiley: New York, 1966; Chapter 5.
(11) Kirk, R. E., Othmer, D. F., Eds. "Encyclopedia of Chemical Technology": The Interscience Encyclopedia: New York, 1951; Vol.
6, 657-675. (12) "Handbook of Chemistry and Physics", 52nd ed.; The Chemical Rubber Co.: Cleveland, OH, 1971.
RECEIVED for review September 13, 1982. Accepted March 1, 1983.
