Laser secondary neutral mass spectrometry

FIGURE 40.18 Depth profiles by laser secondary neutral mass spectrometry (laser SNMS), secondary ion mass spectrometry (SIMS) with Ar and 02 primary ions, and Auger electron spectroscopy (AES) of implanted boron. Reprinted from Higashi, Y., Quantitative depth profiling by laser-ionization sputtered neutral mass spectrometry (1999) Spectrochimica Acta Part B Atomic Spectroscopy, 54(1), 109-122. Copyright (1999), with permission from Elsevier Science. 

Resonant and non-resonant laser post-ionization of sputtered uranium atoms using SIRIS (sputtered initited resonance ionization spectroscopy) and SNMS (secondary neutral mass spectrometry) in one instrument for the characterization of sub-pm sized single microparticles was suggested by Erdmann et al.94 Resonant ionization mass spectrometry allows a selective and sensitive isotope analysis without isobaric interferences as demonstrated for the ultratrace analysis of plutonium from bulk samples.94 Unfortunately, no instrumental equipment combining both techniques is commercially available. 

The closely allied topics of secondary neutral mass spectrometry (SNMS), fast atom bombardment (FAB), and laser ablation SIMS are important, but are beyond the scope of this chapter. SNMS is a technique in which neutral atoms or molecules, sputtered by an ion beam, are ionized in an effort to improve sensitivity and to decouple ion formation from matrix chemical properties, making quantification easier. This ionization is commonly effected by electron beams or lasers. FAB uses a neutral atom beam to create ions on the surface. It is often useful for insulator analysis. Laser ablation creates ions in either resonant or nonresonant modes and can be quite sensitive and complex. 

Hi) Methods based on mass spectrometry Spark-source mass spectrometry Glow-discharge mass spectrometry Inductively coupled-plasma mass spectrometry Electro-thermal vaporization-lCP-MS Thermal-ionization mass spectrometry Accelerator mass spectrometry Secondary-ion mass spectrometry Secondary neutral mass spectrometry Laser mass spectrometry Resonance-ionization mass spectrometry Sputter-initiated resonance-ionization spectroscopy Laser-ablation resonance-ionization spectroscopy... 

Nuclear Reaction Analysis and Elastic Recoil [>etection Secondary Ion Mass Spectrometry of Polymers Secondary Neutral Mass Spectrometry Laser Ionization... 

Veryovkin, I.V, Calaway, W.F., Moore, J.F, Pellin, M.J., Lewellen, J.W., Li, Y, Milton, S., King, B., Petravic, M. (2004) A new horizon in secondary neutral mass spectrometry post-ionization using a VUV free electron laser. Applied Surface Science, 231-232, 962-966. 

A. 10.3.3 SNMS and RIMS Secondary Neutral Mass Spectrometry (SNMS), also referred to as Sputtered Neutral Mass Spectrometry, is a destructive technique primarily used for examining elemental constituents within solid samples. This technique is closely related to Dynamic SIMS in that an ion beam is used to sputter the solid of interest. The difference lies in the fact that the sputtered neutral population, once ionized, is passed through a mass spectrometer. Ionization is induced via the action of a laser, an electron beam, or plasma (ionization yields vary from 10% for lasers to 1% for plasmas). As the greatest fraction of the sputtered population departs in the neutral state, this methodology provides the advantage of improved detection limits and reduced matrix effects relative to SIMS. Depth resolution can extend to 1 nm. Spatial imaging is generally not carried out. No prior sample preparation is needed, but HV or better conditions are required. 

In other articles in this section, a method of analysis is described called Secondary Ion Mass Spectrometry (SIMS), in which material is sputtered from a surface using an ion beam and the minor components that are ejected as positive or negative ions are analyzed by a mass spectrometer. Over the past few years, methods that post-ion-ize the major neutral components ejected from surfaces under ion-beam or laser bombardment have been introduced because of the improved quantitative aspects obtainable by analyzing the major ejected channel. These techniques include SALI, Sputter-Initiated Resonance Ionization Spectroscopy (SIRIS), and Sputtered Neutral Mass Spectrometry (SNMS) or electron-gas post-ionization. Post-ionization techniques for surface analysis have received widespread interest because of their increased sensitivity, compared to more traditional surface analysis techniques, such as X-Ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES), and their more reliable quantitation, compared to SIMS. 

Several years later, the next step in the application of MS-MS for mixture analysis was developed by Hunt et al. [3-5] who described a master scheme for the direct analysis of organic compounds in environmental samples using soft chemical ionisation (Cl) to perform product, parent and neutral loss MS-MS experiments for identification [6,7]. The breakthrough in LC-MS was the development of soft ionisation techniques, e.g. desorption ionisation (continuous flow-fast atom bombardment (CF-FAB), secondary ion mass spectrometry (SIMS) or laser desorption (LD)), and nebulisation ionisation techniques such as thermospray ionisation (TSI), and atmospheric pressure ionisation (API) techniques such as atmospheric pressure chemical ionisation (APCI), and electrospray ionisation (ESI). 

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 ion source is an essential component of all mass spectrometers where the ionization of a gaseous, liquid or solid sample takes place. In inorganic mass spectrometry, several ion sources, based on different evaporation and ionization processes, such as spark ion source, glow discharge ion source, laser ion source (non-resonant and resonant), secondary ion source, sputtered neutral ion source and inductively coupled plasma ion source, have been employed for a multitude of quite different application fields (see Chapter 9). 

Boesl, U. Weinkauf, R. Schlag, E. Re-flectron Time-of-Fhght Mass Spectrometry and Laser Excitation for the Analysis of Neutrals, Ionized Molecules and Secondary Fragments. Ird. J. Mass Spectrom. lonProc. 1992,112,121-166. 

Since multiphoton excitation in mass spectrometry takes place in the more or less tight laser focus, which can easily be shifted in space and time or be subject to other variations, it can be combined with different ion optical or mechanical arrangements (e.g. sources of neutral molecular systems) without the need for much additional hardware. Thus, by combination with chromatography (particularly gas chromatography), species selection has successfully been realized. Another very promising combination, which has frequently been applied in the recent past for the study of involatile molecules (e.g. polycyclic aromatics, biomolecules), is that of laser desorption of neutral molecules and resonance enhanced multiphoton ionization. All the benefits of multiphoton mass spectrometry, such as soft ionization, selective ionization, controllable fragmentation or secondary excitation for tandem mass spectrometry, may be used in this field. 

Boesl U, Weinkauf R and Schlag EW (1992) Reflectron time-of-flight mass spectrometry and laser excitation for the analysis of neutrals, ionized molecules and secondary fragments. International Journal of Mass Spectrometry. Ion Processes (Special Edition) 112 121-166. 

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