FOURIER TRANSFORM AND DIODE LASER INFRARED SPECTRA OF FORMALDEHYDE AND ITS APPLICATON TO AIR POLLUTION MONITORING.

摘要

High resolution ((DELTA)(nu)= ('(TURN))0.004 cm('-1)) FT-IR spectra have been recorded from 890 to 1580 cm('-1). Over 3200 transitions in the (nu)(,4), (nu)(,6) and (nu)(,3) bands of H(,2)CO have been assigned. Line were analyzed using a Watson reduced Hamiltonian. The bands were fitted simultaneously, including the extensive Coriolis coupling effects between the upper states. The fit yielded improved molecular constants including, for the first time, all the sixth order coefficients and a third order Coriolis coupling constant for the interaction between (nu)(,6) and (nu)(,3).;The TDL system was also used to study the effects of collision broadening with foreign gases. Pressure broadening coefficients for air, H(,2), O(,2) and N(,2) were determined. These are very similar to theoretical literature values in the cases of air, N(,2) and O(,2). The H(,2)-H(,2)CO values are in good agreement with earlier experimental millimeter-wave results.;A method for the measurement of small amounts of H(,2)CO in air was developed for use in domestic air pollution monitoring. Conventional reduced pressure techniques were found unsuitable due to the adsorptive nature of formaldehyde. Our suggested technique employs the TDL spectrometer coupled with a multipass White cell (4-100 meter) and a cold trap (liq. N(,2)) installed between sample and White cell. The cold trap allows the reduction of pressure within the White cell (to <10 torr) without loss of sample since the H(,2)CO is frozen out in the trap. The H(,2)CO sample is sublimated by heating and then analyzed. This method may employ relatively shorter absorption pathlengths since most of the original H(,2)CO sample (mass) is available for the analysis. Based on the mass of H(,2)CO present in 5 liter air samples in a 100 m optical pathlength this technique demonstrated a limit of detection of 6 x 10('-3) absorption units and a sensitivity of 4.4 x 10('-5) m('-1). For the strongest line in ('Q)Q(,3) pf (nu)(,1) (C-H stretch) this limits the detection to ('(TURN))7 ppbv. This limit can be reduced to <5 ppb by simply increasing the sample volume to 10 or 15 liters.;Standard 5 liter grab bags used for air sampling have proven unsuitable for work with H(,2)CO. It was found that due to adsorption and possibly polymerization these bags show a decreased H(,2)CO content with time. Consequently the construction of a mobile "super sniffer" was suggested since field samples of H(,2)CO could not be collected and analyzed in the laboratory. The design and operational parameters of the apparatus were outlined. This unit may be easily modified (in the field) to allow for the analysis of other air pollutants.;A tunable diode laser (TDL) has been used to determine absolute line strengths for 28 transitions in the (nu)(,4) and (nu)(,6) bands. The line strengths were used to determine dipole moment derivatives and band strengths.