LSIMS Ion Sources

NR spectrum together with a significant peak at m/z 17 for reionized ammonia. The protonation-deiodination sequence to the 4-iodoaniline and 3-iodoaniline has been applied using the LSIMS ion source. The high-energy CA spectra (Figure 36) of the protonated para-iodoaniline (4-I-ANI-H+) and meta-iodoaniline (3-I-ANI-H+) forms obtained in these conditions are indeed completely different from the spectra shown in Figure 35, and feature very intense peaks at m/z 76, 74 and 50. These peaks, associated with the dehydroanilinium structure b-d in the Cl experiments, are expected for such a distonic ion structure. 

Instrumentation. HPLC isolations were performed on a Beckman 421A system using a Vydac column (C-18, 4.6 x 250 mm). Liquid secondary ion mass spectra (LSIMS) were recorded in the positive ion mode on a Kratos (Manchester, UK) MS-50S mass spectrometer equipped with a 23 kG magnet and post-acceleration detector. The LSIMS ion source has been described elsewhere (28). A Cs+ ion beam of energy 10 keV was used as the primary beam (21). Spectra were recorded (300 sec per decade) with a Gould ES-1000 electrostatic recorder. Tandem MS experiments were performed on a Kratos Concept IIHH (Manchester, UK) four sector instrument of EBEB... 

In FAB, the sample is usually dispersed in a non-volatile liquid matrix, such as glycerol or diethanolamine, and deposited at the end of a sample probe that can be inserted into the ion source. The sample on the probe is ionised when bombarded by the fast atom beam. However, ionisation of the matrix also occurs, leading to a very large background signal. The technique is thus limited for the analysis of small molecules. Fast-moving ions (Cs+ or Ar+) can be used instead of fast-moving atoms, which is the basis of a technique called liquid secondary ion mass spectrometry (LSIMS). 

Since APCI and ESI interfaces operate at atmospheric pressure and do not depend upon vacuum pumps to remove solvent vapor, they are compatible with a wide range of HPLC flow rates. HPLC methods that have been developed using conventional detectors such as UV/VIS, IR, or fluorescence are usually transferable to LC/MS systems without adjustment. However, the solvent system should contain only volatile solvents, buffers, or ion-pair agents to reduce fouling of the mass spectrometer ion source. In the case of chlorophyll solvent systems, isocratic and gradient combinations of methanol, acetonitrile, water, acetone, and/or ethyl acetate have been used for APCI or ESI LC/MS. Unlike continuous-flow FAB/LSIMS, no sample matrix is necessary. 

Figure 5.13 Sketch of a fast atom bombardment (FAB) ion source. The potential difference between the probe tip and extraction grid (not shown) accelerates the ions (from both the Uquid matrix and analyte) into the mass spectrometer. Since a large quantity of neutral matrix is also sputtered by the fast primary beam, the source region must be provided with adequate pumping and the m/z analyzer region differentially pumped. The fast atom beam can be replaced by a beam of fast primary ions (often Cs ) and the technique is then sometimes referred to as Uquid assisted secondary ion mass spectrometry (LSIMS).

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]. 

The FAB source operates near room temperature, and ions of the substance of interest are lifted out from the matrix by a momentum-transfer process that deposits little excess of vibrational and rotational energy in the resulting quasi-molecular ion. Thus, a further advantage of FAB/LSIMS over many other methods of ionization lies in its gentle or mild treatment of thermally labile substances such as peptides, proteins, nucleosides, sugars, and so on, which can be ionized without degrading their. structures. 

The scope of the matrix is not only to transport and maintain the sample in the high-vacuum region of the source of the mass spectrometer, but also it appears to be necessary for the ion formation process. The matrix is often mixed with small quantities of an electrolyte to improve the results. Matrix and electrolyte tend to complicate FAB and FIB/LSIMS spectra, or at least are viewed by the analyst as essentially complicating the matter. 

Mass spectrometer or tandem mass spectrometer (JEOL, Micromass, MAT from ThermoFinnigan) equipped with direct insertion probe and fast atom bombardment (FAB) or liquid secondary ion mass spectrometry (LSIMS) for LC/MS or flow injection using continuous-flow FAB, mass spectrometer must be equipped with continuous-flow ionization source... 

Of the two related techniques, FAB found far greater use in studies of enantioselective discrimination as compared to other desorption/ionization methods, such as MALDI and secondary ion mass spectrometry (SIMS). Chan and coworkers demonstrated enantiodiscrimina-tion of amino acids by a-, P-, and y-cyclodextrins using matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) [28]. The observed levels of enantioselectivity were found to be dependent on the size of cyclodextrin cavities, as well as on the length and functionality of the amino acid side chain. Vairamani and coworkers demonstrated discrimination of amino acid methyl esters using various monosaccharide hosts by liquid secondary ion mass spectrometry (LSIMS) [29]. It is curious that more work has not been done using these sources. MALDI, in particular, is a simple and straightforward technique. Various researchers have demonstrated the observation of noncovalent complexes [30-32], for example, between peptides and proteins, but relatively little work has been performed that focuses on studying enantioselective noncovalent interactions by MALDI-MS. 

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