Atmospheric samples, heterogeneous

Surface Chemical Analysis. Electron spectroscopy of chemical analysis (ESCA) has been the most useful technique for the identification of chemical compounds present on the surface of a composite sample of atmospheric particles. The most prominent examples Include the determination of the surface chemical states of S and N in aerosols, and the investigation of the catalytic role of soot in heterogeneous reactions involving gaseous SO2, NO, or NH3 (15, 39-41). It is apparent from these and other studies that most aerosol sulfur is in the form of sulfate, while most nitrogen is present as the ammonium ion. A substantial quantity of amine nitrogen also has been observed using ESCA (15, 39, 41). 

Lee, J. H., Y. Chen, and I. N. Tang, Heterogeneous Loss of Gaseous H202 in an Atmospheric Air Sampling System, Environ. Sci. Technol., 25, 339-342 (f99f). 

One interesting potential source of N20 is the heterogeneous oxidation of HONO on surfaces (Wiesen et al., 1995 Pires and Rossi, 1997), which has been observed to form N20. This is likely responsible for the observation of significant amounts of N20 in automobile exhaust, which was shown to be an artifact of sampling (Munzio and Kramlich, 1988). However, it may also occur on aerosol particles in the atmosphere (Clemens et al., 1997), an area that warrants further investigation. 

More ambitious attempts at measuring the heterogeneity of the atmospheric aerosol have been undertaken as well. Single-particle analysis by mass spectrometry was demonstrated by Sinha and co-workers (31, 32). In this technique, an aerosol sample is introduced into a vacuum chamber in the form of a particle beam. The particles are injected into a Knudsen cell oven, where they undergo many collisions with the cell wall and are ultimately vaporized and ionized. The ions are then mass-analyzed with a quad-rupole or sector mass spectrometer. So that individual particles can be analyzed, the flux of particles into the Knudsen cell is limited so that coincidence errors are minimized. Ion pulses from individual particles allow the determination of the amount of the species being analyzed in the particular particle. The sensitivity of the technique is limited. For sodium, the detection... 

A 25-mL round-bottom flask was charged with l-[Val-Thr(Bzl)-Val-Thr(Bzl)-DMEDA]2 (180 mg, 102 pmol) dissolved in MeOH (2 mL) and formic acid (approx. 3 mL). The flask was flushed with N2 and 10% Pd/C (102 mg) was added. The N2 atmosphere was then replaced with an atmosphere of H2. The reaction was monitored by C18 RP-HPLC. After 4-5 h, the MeOH was removed under reduced pressure and the thick, heterogeneous soln was diluted with H20 (10 mL). The pH of the soln was adjusted to approx. 7 and the catalyst filtered using a 0.22-pm acetate membrane. The soln was analyzed by analytical C18 RP-HPLC and purified using semi-preparative C18 RP-HPLC. The MeCN in the column fractions was removed under reduced pressure and the sample lyophilized to afford the side-chain deprotected peptidomimetic 3 yield 58 mg (45%) ESI-MS rn/z- [M+H]+ calcd. 1253.5 found 1254.0. 

Fig. 2. Surface heterogeneity of a powder sample of sodium chloride computed from the adsorption of argon at 76.1 K using a two-dimensional gas model of adsorption, x, Sample prepared by electrostatic precipitation of a NaCl aerosol O, same sample but after annealing at 31(1 315°C in a dry nitrogen atmosphere at 600mmHg. (Reproduced from ref. 4 by courtesty of Academic Press, Inc.)...

FIGURE 2.32 The DSC thermograms under nitrogen atmosphere (50 mL/min), sample mass 3 mg at 5 °C/min for chitin/chitosan samples obtained at different reaction times of thermochemical heterogeneous deacetylation. 

The success of studies on ocean chemistry usually hinges on the availability of adequate analytical methodology and expertise. The nature of the chemical problems involved the size and heterogeneity of the ocean the extremes of concentration, composition, temperature, and pressure and the dynamic interactions at its interfaces with the lithosphere, atmosphere, and biosphere create challenging and often unique analytical problems. Current analytical methods are limited, and improved capability in referee, routine, monitoring, and in situ analyses is needed. Valid sampling, the use of reliable standards in calibration, and the collaborative evaluation of methods under real-world conditions are important. 

The first step in developing accurate IPCs is to obtain a representative sample of the reaction, one that accurately reflects the extent of reaction and the profile of impurities in the reactor at the time the sample was withdrawn. Two sets of parameters must be considered whether the reaction is homogeneous or heterogeneous, and the temperature of the reaction relative to the temperature of the sample preparation. All samples must be taken safely, with precautions where appropriate to protect operators and to exclude atmospheric moisture and 02 from both the reactor and the sample. 

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