UV-visible spectroscopy systems
To record the UV-visible spectrum of a gaseous pollutant, two experimental systems are available in our group. They consist in a deuterium-tungsten lamp with an emission spectrum between 200 and 800 nm, an absorption cell of 107.15-cm optical path and a spectrograph connected through fiber optics and a CCD detector.
UV-visible spectroscopy system: 1. StellarNet UV-Vis light source; 2. Absorption cell; 3. Fiber optics; 4. StellarNet UV-Vis Spectrograph.
UV-visible spectroscopy system: 1. Oriel UV light source; 2. Absorption cell; 3. Filter and fiber optics; 4. CCD detector; 5. Chromex spectrograph.
Fourier transform IR spectroscopy systems
Our group has two FTIR spectroscopy systems that measure the IR spectra in the range of 500 to 4000 cm-1. The first one has a Tensor 27 FTIR spectrometer (Bruker) with an MCT (mercury cadmium and tellurium) detector cooled with liquid nitrogen, and a 10-cm optical path cell with ZnSe windows and a multipass cell (up to 800 cm path length). This FTIR spectrometer records IR spectra in the range of 500 to 4000 cm -1.
FTIR spectrometers. A) Tensor 27 (Bruker) with the 800-cm cell; B) Thermo Nicolet coupled to the 96-m multipass cell.
Absorption gas cells of 10-cm and 800-cm path lengths.
The second FTIR spectrometer is a commercial equipment (Thermo Nicolet, model Nexus 870) that has an MCT detector cooled with N2(l) that, together with the KBr optics, allows the detection of IR radiation in the range of 4000-650 cm-1 . It is coupled to the 96-m multipass gas cell. The IR radiation source consists of a Globar lamp that emits radiation in the 6000-50 cm-1 range. The detector directly transforms the analog signal to a digital one. Finally, the infrared spectrum is obtained by applying the Fourier transform to the interferogram.
Gas chromatography coupled to mass spectrometry (GC-MS)
This technique is mainly employed for the detection of gaseous reaction products. A commercial GC-MS (Thermo Electron, Trace GC Ultra and DSQII models) is used. This equipment is coupled offline to the reaction chambers. The introduction of gas samples into the chromatograph is done using the solid phase microextraction (SPME) technique with a fiber composed of 50/30 μm divinylbenzene/carboxy/polydimethylsiloxane (DVB/CAR/PDMS). The separated products in the GC are detected by a quadrupole-type mass spectrometer with electron impact ionization at 70 eV.
Proton Transfer Ionization Time-of-Flight Mass Spectrometry (PTR-ToF-MS)
This technique is also used for the detection of gaseous reaction products. A commercial PTR-ToF-MS equipment (IONICON, model PTR-TOF 4000) is coupled online to the reaction chambers and allows to monitor the temporal evolution of both reactant and products.
PTR-ToF-MS coupled to the 264-L smog chamber.
Fast Mobility Particle Size Analyzer Spectrometer (FMPS)
The formation of ultrafine particles (aerosols) and their size distribution is monitored online by means of an FMPS equipment connected to the 264-L smog chamber. This commercial equipment (TSI mod. 3091) measures the number of particles formed with diameters between 5.6 to 560 nm, with a temporal resolution of one second, allowing the visualization of changes in the particle size distribution in real time. The equipment is connected to the 264-L reactor, from where the aerosols are formed, and then the gas containing the aerosols are transfered to the 16-L glass multipass cell to monitor the unreacted reagent concentration by FTIR.
FMPS coupled to the 264-L smog chamber.
Slow flow reaction cell
For the kinetic study of the OH-reactions as a function of temperature (263 – 358 K) and pressure (50 – 600 Torr), the diluted reactant, the carrier gas (main flow) and the OH radical precursor are flown through a Pyrex reaction cell. Calibrated mass flow controllers are employed for that purpose. The temperature of the gas mixture is measured by a thermocouple inserted on the top of the cell and placed a few centimeters above the reaction zone. To keep the total pressure constant in the cell, the total flow is regulated by a needle valve located at the exit of the cell. The reactor has four arms sealed with quartz windows that allow the perpendicular photolysis and excitation laser beams to cross it. Perpendicular to both laser beams, the optical system and the photomultiplier tube are placed to collect the fluorescence emitted by the OH radical.
Reaction cell: Entrance of bath gas (He or N2) with OH-precursor; 2. Entrance of diluted pollutant; 3. Outlet of cooled/heated liquid; 4. Quartz windows; 5. Optical system.
The desired temperature is achievable by flowing through the external jacket of the reactor heated water (T > 278 K) or a cooled mixture of ethanol and water (T < 278 K) from a thermostatic bath.
Thermostatic bath to control the temperature inside the reaction cell during the kinetic studies.
Atmospheric simulation chambers or smog chambers
For kinetic and mechanistic studies of the gas-phase reaction of a pollutant with the main tropospheric oxidants, at room temperature and atmospheric pressure, two Pyrex reactors act as atmospheric simulation chambers: one of 16 L of capacity and another one of 264 L. The 16-L reactor is a multipass cylindrical cell that has three mirrors inside to allow a maximum optical path length of 96 m. The 264-L reactor has 4 quartz windows that allow working with radiation between 200 and 300 nm. Both chambers are surrounded by several actinic and germicidal lamps that produce Cl atoms or OH radicals by continuous UV photolysis of Cl2 or H2O2, respectively. These two chambers are coupled to different analytical techniques such as FTIR, GC-MS or PTR-ToF-MS spectroscopy in order to simultaneously monitor the concentration of pollutant and the gaseous products over time.
Photograph of the atmospheric simulation chambers used to study the kinetics and formation of gaseous products and particulate matter from reactions of pollutants with OH, Cl, and ozone.
The gases are introduced into the chambers by expansion through a 1-L glass bulb in a vacuum line.
Vacuum line. (1) Expansion bulb; (2) Pressure gauge; (3) Synthetic air cylinder; (4) Chlorine cylinder Cl2; (5) Liquid N2 trap; (6) Outlet to ¼”; (7) Rotulex-type that connect the cold fingers containing the liquid reagents to the vacuum line.
Solar photolysis cell and solar simulator
To investigate the atmospheric degradation of a pollutant initiated by sunlight, a 20-cm Pyrex cell (4 cm of diameter) sealed with quartz windows is used. This cell is coupled to the FTIR spectrometer to monitor the concentration of reagent and gaseous products as a function of time. The photolysis cell is covered with a black cloth to avoid interference from photodissociation of the sample by ambient radiation.
Solar simulator and absorption cell used to study the kinetics and formation of gaseous products during the UV photolysis of pollutants.
The solar simulator (SunLiteTM Solar Simulator, model 11002) consists in a Xe lamp – which does not generate ozone – that irradiates downward the sample at λ≥290 nm with an emission spectrum similar to that of sunlight. It includes an adjustable height support that allows varying the irradiance. The gas temperature (25ºC) is regulated by means of a thermostatic bath.
Thermostatic bath to control the temperature of the gas in the solar photolysis cell.
Ozone is generated in an ozone generator (Ozogas, mod. T.R.C.E. – 500) by electric discharge of synthetic air at a pressure of 1 bar using a gas flow of 5 L/min.
O2 + discharge → 2 O
O2 + O + M → O3 + M M = N2, O2
Photograph of the ozone generator
Photograph of the 248-nm excimer laser (Coherent, ExciStar 200).
Nd-YAG laser (InnoLas, SpitLight 1200) that generates 532 nm radiation from the frequency doubling of fundamental wavelength, 1064 nm.
Dye laser (LiopTec, LiopStar) with an ethanolic solution of Rhodamine 6G (emission at 564 nm).
Frequency doubling unit to generate ~ 282nm radiation from 564nm (LiopTec LiopStar).
Nd-YAG laser (Continuum, NY 81 CS-10) that generates 532 nm radiation by frequency doubling of 1064 nm.
Dye laser (Continuum, ND60) with an ethanolic solution of Rhodamine 6G (emission at 564 nm).
Frequency doubling unit to generate ~ 282nm radiation from 564nm (Continuum).