The use of luminol for direct chemiluminescent detection of NO2 was demonstrated to have greater inherent sensitivity than the indirect ozone chemiluminescence detection, with reported detection limits of 30 ppt (Wendel et al. 1983). The luminol detection system uses a gas-liquid reaction leading to light emission with a maximum at approximately 425 nm. This emission, at the maximum sensitivity for most photomultiplier tubes, is responsible for the increased detection sensitivities. The biggest problem with the luminol method for direct measurement of NO2 has been interference from other soluble oxidants, particularly peroxyacyl nitrates (PANs).
The main problem with this method is inherent difficulty in detecting the emitting species (excited NO2). This species emits over a broad region beginning at approximately 660 nm, with a maximum at 1,270 nm; thus, a red-shifted photomultiplier is required for detection.
We modified the first instrument by replacing the Unisearch (Scintrex) LMA-3 luminol detection system with a newly designed reaction cell and a Hamamatsu photon-counting detection module (HC-135).
The reaction cell has a wick design similar to that of the commercial cell (Unisearch). However, our cell has been reconstructed to reduce dead volume and to deliver the effluent of the capillary column directly to the face of the wick. This increases sensitivity and reduces peak width, enabling shorter analysis times. An electronic sampling valve is used to collect a 5-cm3 air sample and inject it onto a 30-ft-long DB-1 capillary GC column for separation of NO2 and PAN. Synchronous control of the sample injection and data collection is accomplished with Labview 6.0 graphical programming software (National Instruments) operating in a Windows 2000 environment. This software package also allows for real-time integration of the chromatographic peaks and final storage of both the raw data and integrated values in spreadsheet format.2 個解答語言8 年前
The simultaneous measurement of NO2 and PAN enables determination of levels of the peroxyacetyl radical in the air, as PAN is in thermal equilibrium with the peroxyacetyl radical and NO2 (Gaffney et al. 2002). Past measurements of PAN and NO2 with this method were made in Centerton, New Jersey; Deer Park, Texas; and Phoenix, Arizona. We redesigned our previous instrument to make it smaller and lighter for easy deployment on aircraft platforms and modified the software to allow real-time data integration and synchronous operation of sample injection and data collection. In this work, we review the method and highlight the modifications made to the instrument, including a new cell design that enables easy monitoring of luminol detection with a Hamamatsu HC-135 photon counting system. We present data taken onboard the National Oceanic and Atmospheric Administration (NOAA) Twin Otter in the recent Bay Region Atmospheric Chemistry Experiment (BRACE) in Tampa, Florida, in May 2002. The NO2 concentrations measured by the fast GC-luminol technique are compared with simultaneous onboard measurements made by photolytic conversion of NO2 to NO followed by ozone chemiluminescence.2 個解答語言8 年前
Along with NO2, PANs are important trace gas species associated with photochemical air pollution. The PANs are a class of organic oxidants having the general chemical structure RC=OO-O-NO2. Peroxyacetyl nitrate (PAN) is the primary peroxyacyl nitrate produced during the oxidation of hydrocarbons in the presence of NO and NO2. PAN is in thermal equilibrium with the peroxyacetyl radical (RC=O-OO.) and NO2 (Gaffney et al. 1989). Because PAN is a trapped peroxy radical, it is an important indicator species of the photochemical age of an air parcel, as well as a means for long-range transport of NO2 leading to the regional formation of ozone and other oxidants. Typically, PAN is measured by using a gas chromatograph (GC) with electron capture detection (ECD). Once automated, this method has been shown to be reliable and quite sensitive, allowing PAN to be measured in the troposphere at levels of low parts per trillion. Unfortunately, a number of other atmospheric gases (e.g., O2, Freons, H2O) also have strong ECD signals or act as inferences and limit the speed with which the analysis can be completed. Typically measurements are obtained at 15- to 30-min resolution. Currently, the shortest ECD analysis time for PAN is approximately 5 min (Williams et al. 2000).
For x = 0.05 and 0.09 samples, no satellite structure was observed above the Cu 2p3/2 peak, indicating that
the only valence state of copper in these two samples is Cu(I). If Cu(II) is present in these glass samples, then
the amount is too small to be detected by XPS. For the x --- 0.14 and 0.18 samples, there is a weak satellite at a
binding energy ~ 9 eV greater than the main Cu 2p3/2 peak, indicating that both Cu(I) and Cu(II) coexist in
these glass samples although, since the intensity of the satellite is very small compared to that observed in CuO,
the amount of Cu(II) is expected to be small. As can be seen from Fig. 1, the intensity of the satellite for the
x -- 0.14 sample is less than for the x = 0.18 sample. Furthermore, the Cu 2p3/2 peak width for the x < 0.14 samples is also smaller than that of the x = 0.18 sample (Table 2), and the Cu 2p binding energies of these three
samples agree well with previously reported values for compounds containing Cu(I) 1 個解答語言8 年前
2x+y=2*1.5+3.5=6.5 3y2=3*3.5*3.5=36.75 x=1.5 1+6xy=1+6*1.5*3.5=32.5
然後再以x=2.03603和y=2.84388做初始值 反覆的做下去 直到x和y變成2和3為止