*3.1.7. Laser secondary neutral mass spectrometry*

tions of SIMS can be broadly subdivided into static and dynamic SIMS. Static SIMS (SSIMS) is used to investigate the composition of the outermost monolayer on any solid. Dynamic SIMS (DSIMS) examines the concentration profile as the function of depth [68],[69],[70].

118 Apatites and their Synthetic Analogues - Synthesis, Structure, Properties and Applications

**Fig. 3.** Main components of SIMS [68] and schematic representation of ion microprobe imaging time-of-flight secon‐

The mass filter of mass spectrometer of SIMS instruments defines the type of instruments [68],

**1.** Magnetic sector SIMS (M-SIMS) instruments (**Fig. 3**(**b**), similar to those used in original

**2.** Quadrupole SIMS (Q-SIMS) instruments (**Fig. 4**(**a**), first appeared in the 1970)

**Fig. 4.** Schematic representation of magnetic sector and quadrupole SIMS instruments [70].

Secondary ion emission begins when the primary ions energy exceeds some threshold level. This energy ranges from 30 to 80 eV and is much higher than the energies for sputtering of

**3.** Time-of-flight SIMS (ToF-SIMS) instruments (**Fig. 4**(**b**), first appeared in the 1980)

dary ion mass spectrometry with reflectron-based mass analyser [69].

[70]:

mass spectrometer)

Laser secondary neutral mass spectroscopy (laser-SNMS) can be further divided to nonreso‐ nant laser-SNMS (NR-laser-SNMS) and resonant laser-SNMS (R-laser-SNMS). In NR-laser-SNMS (**Fig. 5**(**b**)), an intense laser beam is used to nonselective ionization of all atoms and molecules within the volume intersected by the laser beam. Sufficient laser power density, which is necessary to saturate the ionization process, is typically achieved in a small volume. It limits the sensitivity of the method and leads to the problems with quantification due to the differences between effective ionization volumes of different elements. Laser-SNMS method has significantly improved ionization efficiency over SIMS (**a**) [78],[79].

**Fig. 5.** Comparison of ToF-SIMS and laser-SNMS: (a) direct analysis of secondary ions (ToF-SIMS), (b) nonresonant laserpost-ionization of secondary neutrals (NR-laser-SNMS) and (c) resonant post-ionization of secondary neutrals (R-laser-SNMS) [78].

Resonant laser-SNMS uses a resonance laser ionization process, which selectively and efficiently ionizes atoms and molecules over a relatively large volume (**Fig. 5**(**c**)). The meth‐ od has unit ionization efficiency for over 80% of elements in periodic table, i.e. the overall efficiency is greater than that for NR-laser-SNMS, the quantification is also simpler and extremely high selectivity prevents almost all isobaric and molecular interferences [78],[79].

The method of NR-laser-SNMS was used by DAMBACH et al [80] to investigate different states of biomineralization in vitro. The results indicate that in the vicinity of single osteoblasts, extracellular enrichment of potassium typically occurs during initial stages of mineralization. Potassium may interact with matrix macromolecules and prevent an uncontrolled apatite deposition. However, apatite biomineral formation is correlated with a potassium release. In conclusion, potassium seems to be involved in the process of extracellular matrix biomineral‐ ization.
