Sputter atomization

For quantitative analysis, the resolution of the spectral analyzer must be significantly narrower than the absorption lines, which are - 0.002 nm at 400 nm for Af = 50 amu at 2500°C (eq. 4). This is unachievable with most spectrophotometers. Instead, narrow-line sources specific for each element are employed. These are usually hoUow-cathode lamps, in which a cylindrical cathode composed of (or lined with) the element of interest is bombarded with inert gas cations produced in a discharge. Atoms sputtered from the cathode are excited by coUisions in the lamp atmosphere and then decay, emitting very narrow characteristic lines. More recendy semiconductor diode arrays have been used for AAS (168) (see Semiconductors). 

Because SIMS can measure only ions created in the sputtering process and not neutral atoms or clusters, the detection limit of a particular element is affected by how efficiently it ionizes. The ionization efficiency of an element is referred to as its ion yield. The ion yield of a particular element A is simply the ratio of the number of A ions to the total number of A atoms sputtered from the mixing zone. For example, if element A has a 1 100 probability of being ionized in the sputtering process—that is, if 1 ion is formed from every 100 atoms of A sputtered from the sample—the ion yield of A would be 1/100. The higher the ion yield for a given element, the lower (better) the detection limit. 

Figure 1 Schematic of DC glow-discharge atomization and ionization processes. The sample is the cathode for a DC discharge in 1 Torr Ar. Ions accelerated across the cathode dark space onto the sample sputter surface atoms into the plasma (a). Atoms are ionized in collisions with metastable plasma atoms and with energetic plasma electrons. Atoms sputtered from the sample (cathode) diffuse through the plasma (b). Atoms ionized in the region of the cell exit aperture and passing through are taken into the mass spectrometer for analysis. The largest fraction condenses on the discharge cell (anode) wall.

Taking atomic sputtering into account the proportion of the particles emitted as molecules is negligible and the partial sputtering yield for element A in sputter equilibrium can be determined by use of ... 

In a similar study, Lo et al. have compared the characteristics of atoms sputtered from copper surfaces in simulations which used both pair-additive potentials and EAM potentials Significant differences were found for many... 

Although the FABMS (Fast Atom Bombardment Mass Spectrometry) technique has only been in use for a few short years (since 1981), it traces its roots back well over a century (1). It has been observed that the bombardment of a surface by energetic ions produces the desorption of atoms and molecules from the surface of the target. This process, known as Sputtering, produces a yield (number of atoms sputtered per incident ion) which generally increases with the energy, the mass and the incidence angle of bombardment (2). 

For a mechanical mixture, the independent distribution of Mo and A1 makes the intensity ratio IMJIM almost unaffected by the fluence. The steep rise of the initial intensity ratio /Mo//Ai after a prolonged calcination indicates that more Mo03 has dispersed onto the surface of y-Al203. However, after calcining the sample for more than 5 h, the initial intensity ratio remains unchanged. This indicates that an utmost limit of dispersion has been reached. During the ion bombardment, the surface atoms sputter off and the intensity ratio /Mo//A, decreases. 

Brief mention should also be made here of high intensity (also known as boosted output ) hollow cathode lamps.7 In these lamps an auxiliary current of around 200-400 mA is applied to the dilute cloud of atoms sputtered outside the central zone of the normal hollow cathode. The atoms are thus excited and emit intense radiation which may be used in AAS or AFS. Once again an auxiliary power supply is required, and the lamps themselves are more complex and correspondingly more expensive. Such lamps have had a rather chequered history, finding great favour in some environmental analytical laboratories but never being widely used on any routine basis. 

In the AMT Ar sputtering process, gap distance is an important parameter. It influences the pathway of gas or sputtered particles (molecules or atoms). Sputtering rate will change when the gap distance is varied. Figure 17.22 shows the relationship... 

The number of atoms sputtered by a single incident ion varies with an ion s individual trajectory. Figure 15 shows the distribution of sputtered particles per incident ion at four different primary energies. As the collision energy increases from 100 to 1000 eV, the most probable yield shifts from 0 to 5 sputtered atoms per ion impact. The computer program, TRIM, mentioned earlier, provides quantitative simulations of both sputtering thresholds and yields. ... 

Atoms sputtered from Lines from low lying upper levels 20-50... 

Three soft ionisation methods are in use for earbohydrates, fast atom bombardment (FAB), eleetrospray ionisation (ESI) and matrix-assisted laser desorption/ionisation (MALDI). FAB is the oldest and involves directing a high-energy beam of Cs" ions or Xe atoms at the sample dissolved in a nonvolatile solvent such as m-nitrobenzyl alcohol. The atoms sputter the sample and matrix [M + H] or [M + Na]" ions are commonly observed. With an upper limit of M of about 2000, FAB is not that soft, and is usually used for small oligosaccharides it has the further disadvantage that the sample is prepared and then directly introduced into the mass spectrometer, so that it cannot be combined with liquid chromatography. 

If the sample is a bulk solid sample and is an electrical conductor, it is possible to use it as the cathode of a kind of spectral lamp whose functioning principle is identical to that described for a hollow cathode lamp (cf. Section 13.5.1 and Figure 14.5). The atoms sputtered and removed from the surface of the sample are excited by the plasma. This GD-OES technique provides a rapid and accurate surface analysis, less susceptible to matrix effects and sample homogeneity. It has the advantage of yielding spectra with low background levels whose emission lines are narrow since atomization takes place at lower temperatures than that of the previous techniques. 

Directional distribution, cosine law, of Cu atoms sputtered by Ar ions of 20 keV according to G. 

Ion Gun. An ion gun is used in XPS and Auger instruments for two purposes (1) to clean the sample surface of any external contamination layer and (2) to sputter atoms from the surface in order to obtain a depth profile analysis, discussed under applications. One type of ion gun uses a heated hlament to ionize inert gas atoms. The ions are accelerated by a potential placed on the ionization chamber and are focused to strike the sample surface. The ions remove atoms from the sample surface by collision. The rate of removal of surface atoms is controlled by the kinetic energy of the ions and by the namre of the surface atoms sputtering rates may be as high as 10 nm/min. 

The degree of ionization is defined as the ratio of emitted positive M+ or negative M ions to the total amount of the isotope M (expressed as number of atoms) sputtered over the same time period. The product of and S is referred to as the secondary ion yield Sj.+, and relates to the number of secondary ions Tormed per incident ion. Another commonly encountered term is the useful (or practical) ion yield Pu+tvi+j which is the number of ions detected per atom present in the sputtered volume the instrumental detection efficiency for the particular isotope monitored nn + is the ratio of detected to emitted ions from the analyzed area A. 

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