Sample charge

Designation Capacity Typical sample charge, lb Motor Mill diameter, in... 

Because polymers are typically non-conductive, sample charging can occur and has to be compensated carefully, e. g. by use of a low-energy electron-flood gun, to avoid line-shape distortion and misinterpretation of the measurements. 

For probing the nature of the acid sites by X-ray photoelectron spectroscopy (XFS), the samples were evacuated before gaseous pyridine was adsorbed. Excess pyridine was desorbed at 1S0°C, and then samples were pressed onto a sample stub imder Nj and loaded into the SCIENTA ESCA-300 instrument without exposure to air. Sample charging was minimized by using a Qood gun while acquiring the experimental data. 

One can compensate for charging by using a so-called flood gun, which sprays low-energy electrons onto the sample. Charging can also be minimized by using a beam of atoms instead of ions as primary particles. In this case, kinetic emission of electrons is the only source of charging, if we ignore the low yields of secondary ions. 

Matrix effects and inhomogeneous sample charging seriously hinder quantitative analysis of SIMS on technical catalysts. Although full quantitation is almost impossible in this area, the interpretation of SIMS data on a more qualitative base nevertheless offers unique possibilities. Molecular cluster ions may be particularly informative about compounds present in a catalyst. 

The 5950A ESCA spectrometer is interfaced to a desktop computer for data collection and analysis. Six hundred watt monochromatic A1 Ka X-rays are used to excite the photoelectrons and an electron gun set at 2 eV and 0.3 mAmp is used to reduce sample charging. Peak areas are numerically integrated and then divided by the theoretical photoionization cross-sections (11) to obtain relative atomic compositions. For the supported catalyst samples, all binding energies (BE) are referenced to the A1 2p peak at 75.0 eV, the Si 2p peak at 103.0 eV, or the Ti 2p3/2 peak at 458.5 eV. 

Secondary ion mass spectra were measured using a Perkin-Elmer+PHI 3500 instrument. Experiments were carried out with 4 kV Ar ions at beam currents of 3 and 300 nanoamps. Spectra were measured to at least 500 daltons (d). Samples were prepared in the manner used for the XPS studies. For measurements on the pure complexes, sample charging occurred, as evidenced by the inability to record secondary ion mass spectra. To reduce charging, a low energy electron beam (50-400 eV) was rastered across the sample during SIMS analysis. Positive and negative ion SIMS spectra were recorded however, only positive ion spectra are of interest for this discussion. In the spectra only unipositive ions were detected, so that the mass numbers detected correspond to combinations of the various isotopes of the elements in the ion. Thus an ion at m/z 17 d is assigned to... 

Cowie, G.L., and J.I. Hedges. 1992. Improved amino acid quantification in environmental samples charge-matched recovery standards and reduced analysis time. Marine Chemistry 37 223-238. 

The Sccmning Electron Microscope (SEM) is a standard imaging technique based on electron back-scattering from the sample surface. It analyses the surfaces of solid objects, producing images with the resolution which is about order of magnitude better than that of optical microscopy (typically 10 nm). The SEM avoids the problem of thin samples (TEM) but the SEM observation requires the deposition of a thin conductive metal film on the sample surface to prevent sample charging. 

Detailed analysis of the chemical shifts demands higher resolution than a basic ESCA spectrometer can provide. The needed resolution improvement may be achieved experimentally by monochromatizing the x rays and/or by high-resolution energy analysis of the electrons. Either way, because of the reduced slit widths required, penalties are paid in decreased signal-to-noise ratio and longer observation time. Furthermore, the problem of sample charging (Section IV.D) is typically more severe when monochromatized x rays are used (Brundle and Baker, 1981). 

To improve the charging performance, the grounded electrodes are placed around the corona emitter. The structure is called the corona housing and helps direct the ion current and holds the corona emitter. Screen grids are then inserted between the corona emitter and the sample to provide better control of charging this is the corona grid. Several variables determine the efficiency of sample charging. We note here only the most important ... 

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