Direct diffusion mass spectrometry

In order to provide AMS analyses to the broad ocean sciences research community, the National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS) was established at Woods Hole Oceanographic Institution (Massachusetts) in 1989. Studies performed there include identification of sources of carbon-bearing materials in the water column and sediment, dating of sedimentary samples, investigations of paleocirculation patterns (e.g., from observations of differences in 14C relative abundances in planktonic and benthic foraminifera, and coral cores and cross sections), as well as studies of modern oceanic carbon cycling and circulation. In fact, much that is known about advective and diffusive processes in the ocean comes from measurements of chemical tracers, such as 14C, rather than from direct measurements of water mass flow. 

The optimum linear velocity for a capillary column depends on the pressure in the column because Wopt is proportional to the average diffusion coefficient, which varies inversely with pressure. Operation of a short wide-bore column at vacuum outlet conditions results in a significantly faster analysis than would occur if the same column was used under atmospheric outlet pressures. Mass spectrometry (MS) has made vacuum GC very easy to implement, since the mass spectrometer provides both detection and a source of vacuum. Vacuum GC can be achieved practically by incorporating a restriction at the inlet end of a wide-bore capillary column, and interfacing the terminal end of the column directly into the MS. The function of the restriction is to deliver an optimal helium flow for the mass spectrometer, and it can be as simple as a short section of 20 pm i.d. capillary (or a longer section of 100-150 pm i.d. capillary). An optimal carrier gas velocity of 90-100cms can be expected for a 10 m x 50 pm column with a restriction at the inlet, and a speed gain of a factor of 3-5 times can easily be obtained. 

The tracer diffusivities of 1 0, on the ab-plane and in the c-direction, of monocrystalline samples were measured by using secondary ion mass spectrometry, at between 250 and 350C and between 450 and 700C, respectively, under an O partial pressure of latm. It was found that the data could be described by the expressions,... 

Tracer diffusion in the b-direction of single crystals was investigated at temperatures ranging from 1121 to 1313K. The diffiisivities, as found by determining penetration profiles using secondary ion mass spectrometry, could be described by ... 

Bulk self-diffusion coefficients were measured in single crystals by using a methodology which was based upon the use of Fe stable isotopes and depth profiling by secondary ion mass spectrometry. The self-diffusion coefficients measured in the c-axis direction, between 900 and llOOC in an oxygen atmosphere, were described by ... 

It was recalled that the models which were used to describe ZnO-based varistors relied upon diffusion and defect data which had been obtained by using techniques that had been superseded by methods such as secondary ion mass spectrometry. Values were reported here for O diffusivities in undoped monocrystalline ZnO as a function of temperature and orientation. Evaporation was taken into consideration when analyzing the experimental results. It was found that, to within experimental error, the energetics of diffusion were isotropic, but were slightly faster in the c-direction. The present results could be described by ... 

Despite the fact that direct analysis methods exclude a cost-intensive separation step overall analysis cost may still be high, namely by the need for more sophisticated instrumentation (allowing for a physical rather than chemical separation of components) or extensive application of chemometric techniques. The wide variety of additives that are commercially available and employed complicate spectroscopic data analysis. For multicomponent analysis some kind of physical separation of additive signals is often quite helpful, e.g. based on mobility (as in LR-NMR or NMRI), diffusion coefficient (as in DOSY NMR), thermal behaviour (as in a thermal analysis and pyrolysis techniques) or mass (as in tandem mass spectrometry). The power of signal processing techniques (such as multi-wavelength techniques, derivative spectrophotometry) is also used to the fullest extent. 

AES — Auger Electron Spectroscopy DLTS - Deep Level Transient Spectroscopy SEM - Scanning Electron Microscopy SIMS - Secondary Ion Mass Spectrometry D(c) - Concentration Dependent Diffusion Coefficient Maximum Diffusion Coefficient (f) - East Diffusion Component (i) - Interstitial Diffusion Component (s) - Slow Diffusion Component (II) - Parallel to c Direction (JL) - Perpendicular to c Direction... 

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