Molecular SIMS

The first TOF-SIMS instrument [98] employed a pulsed alkali primary ion source. The primary ions were created from alkali aluminosilicate by thermionic emission, accelerated by up to 25 keV and focused onto the monomolecular sample layer on gold or silver surfaces in bursts of 5-50 ns. (This type of Cs primary ion sources is still in use for LSIMS.) Useful spectra of amino acids and nucleosides [98,99] and even peptides [100] were obtained in this way. Soon, TOF-SIMS became a major competitor of PD-MS [101-103] and the development of FAB and LSIMS led to fruitful approaches such as TOF-LSIMS [104]. 

Note In the 1980s FAB, LSIMS and PDMS were the methods of choice for the analysis of biomolecules, and there was mutually fostering research on these methods. After the advent of MALDI and ESI in the late 1980s, they were rapidly superseded, and by now, have been almost quantitatively replaced. 

Modem TOF-SIMS applications are often in the field of synthetic polymer analysis, but rarely for the determination of molecular weight distributions which is the domain of MALDI. More frequently, low-molecular-weight additives or surface modifications are analyzed as there is no interference with a matrix as would 

Example Additives are intentionally added to a polymer to stabilize or improve different polymer properties. They act as antioxidants, UV stabilizers, antistatic agents, flame retardants, etc. However, their migration towards the surface may result in blooming, thereby negatively affecting surface properties that are required for specific applications. TOF-SIMS is ideally suited to characterize surface compositions because it offers molecular information on the uppermost monolayers at high sensitivity. Here, the antioxidant Irgafos 168 (0.3 wt.%) has been analyzed on the copolymer PETi, a poly(ethylene terephthalate (60%) isophthalate (40%)) (Fig. 15.16) [109]. 

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It is a remarkable feature of secondary ion mass spectrometry (SIMS) that considerable chemical information is accessible through the procedurally simple physical technique of sputtering. SIMS--espec ia 11 y under low primary ion flux conditions ("static SIMS," a 1 s o known as "molecular SIMS" when applied to compounds)—provides information on molecular weight and molecular structure and allows isotopic analysis. The surface sensitivity of SIMS permits its use in imaging, in monitoring of surface... 

This review focuses on the phenomenon of molecular SIMS—that is, the physical and chemical bases for its many analytical applications. The applications themselves are also reviewed. The coverage is somewhat historical, emphasizing progress which has come out of this and other laboratories in the past five years. SIMS is discussed in the context of experiments using related desorption ionization (DI) methods, especially laser desorption (LD) and fast atom bombardment (FAB). 

Three types of ionization processes were distinguished ( 1) as contributing to the ions observed in molecular SIMS spectra direct desorption of precharged materials, cat ionization/anionization, and electron ionization (Equations 1-3, respectively). The equations illustrate overall reactions and do not attempt to explain detailed mechanistic steps. 

In the discussion of mechanism given below, support for the concepts just outlined will be marshalled from newer experimental results which are accomodated by the above model and which call for intact emission of molecules. It should be noted, however, that in the case of a strong molecule-substrate interaction, fragmentation probably occurs directly at the surface. Clearly this is the case when polymers are examined by bombardment techniques. Almost all molecular SIMS experiments employ static conditions in which virgin... 

The techniques of field desorption (FD) and electrohydrodynamic ionization (EHD) differ from SIMS, LD, and FAB in their physical basis and in features seen in the spectra. For example, the diquaternary ammonium salts discussed above yield intact doubly-charged ions, and fragmentation is less extensive. Nonetheless, the three classes of ions described for molecular SIMS are generally seen in FD (50). An EHD study of a series of diquaternary ammonium salts (51) led to the conclusion that the amount of internal energy deposited in EHD is less than in SIMS and FAB. The same study also indicated that FAB (liquid matrix) deposits less energy than SIMS (solid matrix), so in this case the order of energy deposition is SIMS > FAB > EHD. 

Summary of mechanism. Taking into account the data in hand (and that which follows), it is possible to classify the many processes which occur in molecular SIMS in a qualitative manner. Table II, while not complete, illustrates many of the processes which contribute to molecular SIMS, FAB, and LD spectra. The following synopsis of the mechanism complements the material in Table II ... 

A study of silver carboxylates (56) employing evaporation followed by electron ionization (El) in conjunction with MS/MS allowed structures to be formulated for certain ions observed in SIMS. The SIMS spectrum of silver propionate, for example, contains unusual ions indicative of high energy bond cleavages, in contrast to most other molecular SIMS spectra. These ions are... 

In addition to molecular imaging and such obvious advances as increased mass range and higher transmission analyzers, there are a number of new topics in molecular SIMS emerging as areas of interest. Not least of these is the study of chemical reactions at surfaces, which may be one source of the beam damage seen at high primary ion fluxes. A spectacular example of ion-beam-induced... 

As a sub-element of the static SIMS methodology, SIMS has become (most recently) a new ionization source for the analysis of nonvolatile and thermally labile molecules including polymers and large biomolecules such as proteins. Since most of these latter studies deal with the emission of polyatomic or molecular ions, the name "molecular SIMS" has been applied [1B-21],... 

The application of molecular SIMS as a sensitive ionization source for nonvolatile and thermally labile molecules compares favorably with other new ionization methods in mass spectrometry such as field desorption (FD), Californium-252 plasma desorption (PD), fast heavy ion induced desorption (FHIID), laser desorp-... 

This paper discusses SIMS as a multi-dimensional technique for the analysis of inorganic and organic materials. The paper is divided into two parts inorganic and organic (or molecular) SIMS. The inorganic SIMS part focuses on the methods of quantitative analysis and depth profiling applications. In particular, SIMS matrix effects are defined and the physical models and empirical methods used to quantify SIMS results are reviewed. 

The emission of molecular ions in organic SIMS is discussed with respect to the method of ionization and the various sample preparation and matrix-assisted procedures used. The matrices include various solid-state and liquid matrices such as ammonium chloride, charcoal, glycerol, and galliun. A neutral beam source is described to analyze thick insulating films. Various chemical derivatization procedures have been developed to enhance the sensitivity of molecular SIMS and to selectively detect components in mixtures. 

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