Photoionization analysis

Lykke K R and Kay B D 1990 State-to-state inelastic and reactive molecular beam scattering from surfaces Laser Photoionization and Desorption Surface Analysis Techniquesvo 1208, ed N S Nogar (Bellingham, WA SPIE) p 1218... 

In Surface Analysis by Laser Ionization (SALI), a probe beam such as an ion beam, electron beam, or laser is directed onto a surfiice to remove a sample of material. An untuned, high-intensity laser beam passes parallel and close to but above the sur-fiice. The laser has sufficient intensity to induce a high degree of nonresonant, and hence nonselective, photoionization of the vaporized sample of material within the laser beam. The nonselectively ionized sample is then subjected to mass spectral analysis to determine the nature of the unknown species. SALI spectra accurately reflect the surface composition, and the use of time-of-flight mass spectrometers provides fast, efficient and extremely sensitive analysis. 

Si 2p line, at about 100 eV BE, is also easily accessible at most synchrotron sources but cannot, of course, be observed using He I and He II radiation. On the other hand, the Zn 3d and Hg 4f lines can be observed quite readily by He I radiation (see Table 1) and the elements identified in this way. Quantitative analysis using relative peak intensities is performed exactly as in XPS, but the photoionization cross sections a are very different at UPS photon energies, compared to A1 Ka energies, and tabulated or calculated values are not so readily available. Quantitation, therefore, usually has to be done using local standards. 

Relative photoionization cross sections for molecules do not vary gready between each other in this wavelength region, and therefore the peak intensities in the raw data approximately correspond to the relative abundances of the molecular species. Improvement in quantification for both photoionizadon methods is straightforward with calibration. Sampling the majority neutral channel means much less stringent requirements for calibrants than that for direct ion production from surfaces by energetic particles this is especially important for the analysis of surfaces, interfaces, and unknown bulk materials. 

The discrete line sources described above for XPS are perfectly adequate for most applications, but some types of analysis require that the source be tunable (i.e. that the exciting energy be variable). The reason is to enable the photoionization cross-section of the core levels of a particular element or group of elements to be varied, which is particularly useful when dealing with multielement semiconductors. Tunable radiation can be obtained from a synchrotron. 

The primary methods of analyzing for lead in environmental samples are AAS, GFAAS, ASV, ICP/AES, and XRFS (Lima et al. 1995). Less commonly employed techniques include ICP/MS, gas chromato-graphy/photoionization detector (GC/PID), IDMS, DPASV, electron probe X-ray microanalysis (EPXMA), and laser microprobe mass analysis (LAMMA). The use of ICP/MS will become more routine in the future because of the sensitivity and specificity of the technique. ICP/MS is generally 3 orders of magnitude more sensitive than ICP/AES (Al-Rashdan et al. 1991). Chromatography (GC,... 

LC-MS-MS was also the method of choice for the analysis of UV filters in solid matrices. Both LC and UPLC have been applied in three out of the four methods available for the determination of UV filters in sludge. Separation was performed on C8 and C18 LC-chromatographic columns (Zorbax, Eclipse, Vydac, and Purosphere) using binary gradient elution of mobile phases consisting of water/ methanol or water/acetonitrile. MS-MS detection was performed in SRM with ESI and atmospheric pressure photoionization (APPI) in both positive and negative modes. For each compound, two characteristics transitions were monitored. In addition to MS and MS-MS, diode array detection (DAD) was occasionally applied to the determination of OT. Spectra were recorded between 240 and 360 nm and discrete channels at 310 nm. 

The study of the photoionization cross section as a function of photon energy for the different orbitals of Me4Sn, which can be a powerful tool for the assignment of the spectra and the analysis of the contribution of the various atomic orbitals to the molecular orbitals, has been carried out by the authors of References 11 and 12 by using He I and He II as ionizing source, and of Reference 13 by using synchrotron radiation. Bertoncello... 

An ultasensitive simultaneous multi-element method of determination for As, Se, Sb and Sn in aqueous solution, consists of hydride generation, collection in a cryogenic trap and end analysis by GC-PID (photoionization detector) LOD ca 1 ng Sn/L for a 28 mL sample. No drying or CO2 scrubbing is necessary before the cold trap35. 

DGE a AC AMS APCI API AP-MALDI APPI ASAP BIRD c CAD CE CF CF-FAB Cl CID cw CZE Da DAPCI DART DC DE DESI DIOS DTIMS EC ECD El ELDI EM ESI ETD eV f FAB FAIMS FD FI FT FTICR two-dimensional gel electrophoresis atto, 10 18 alternating current accelerator mass spectrometry atmospheric pressure chemical ionization atmospheric pressure ionization atmospheric pressure matrix-assisted laser desorption/ionization atmospheric pressure photoionization atmospheric-pressure solids analysis probe blackbody infrared radiative dissociation centi, 10-2 collision-activated dissociation capillary electrophoresis continuous flow continuous flow fast atom bombardment chemical ionization collision-induced dissociation continuous wave capillary zone electrophoresis dalton desorption atmospheric pressure chemical ionization direct analysis in real time direct current delayed extraction desorption electrospray ionization desorption/ionization on silicon drift tube ion mobility spectrometry electrochromatography electron capture dissociation electron ionization electrospray-assisted laser desorption/ionization electron multiplier electrospray ionization electron transfer dissociation electron volt femto, 1CT15 fast atom bombardment field asymmetric waveform ion mobility spectrometry field desorption field ionization Fourier transform Fourier transform ion cyclotron resonance... 

Multiphoton ionization MPI Photoionization Atomic and molecular ions Resonance-enhanced MPI is highly selective Trace analysis... 

By employing a laser for the photoionization (not to be confused with laser desorption/ ionization, where a laser is irradiating a surface, see Section 2.1.21) both sensitivity and selectivity are considerably enhanced. In 1970 the first mass spectrometric analysis of laser photoionized molecular species, namely H2, was performed [54]. Two years later selective two-step photoionization was used to ionize mbidium [55]. Multiphoton ionization mass spectrometry (MPI-MS) was demonstrated in the late 1970s [56—58]. The combination of tunable lasers and MS into a multidimensional analysis tool proved to be a very useful way to investigate excitation and dissociation processes, as well as to obtain mass spectrometric data [59-62]. Because of the pulsed nature of most MPI sources TOF analyzers are preferred, but in combination with continuous wave lasers quadrupole analyzers have been utilized [63]. MPI is performed on species already in the gas phase. The analyte delivery system depends on the application and can be, for example, a GC interface, thermal evaporation from a surface, secondary neutrals from a particle impact event (see Section 2.1.18), or molecular beams that are introduced through a spray interface. There is a multitude of different source geometries. 

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. 

Photoionization, as already pointed out, is characterized by a step function for ionization probabiUty versus energy. The change in mode of ionization is thus much more easily detectable than for electron impact which produces only changes of slope. The combination of photon impact ion sources with mass analysis has been a major advance in technique since it has allowed the direct study of formation and breakdown of excited ions. The first account of such an experiment was given by Hurzeler, Inghram and Morrison (1958) who employed the especially convenient Seya-Namioka type of monochromator, which had then just been described, in conjunction with a conventional magnetic sector mass... 

---