Liquid targets

A big step forward came with the discovery that bombardment of a liquid target surface by abeam of fast atoms caused continuous desorption of ions that were characteristic of the liquid. Where this liquid consisted of a sample substance dissolved in a solvent of low volatility (a matrix), both positive and negative molecular or quasi-molecular ions characteristic of the sample were produced. The process quickly became known by the acronym FAB (fast-atom bombardment) and for its then-fabulous results on substances that had hitherto proved intractable. Later, it was found that a primary incident beam of fast ions could be used instead, and a more generally descriptive term, LSIMS (liquid secondary ion mass spectrometry) has come into use. However, note that purists still regard and refer to both FAB and LSIMS as simply facets of the original SIMS. In practice, any of the acronyms can be used, but FAB and LSIMS are more descriptive when referring to the primary atom or ion beam. 

When the liquid target is not a static pool but, rather, a continuous stream of liquid, the added description of dynamic is used. Thus, dynamic FAB and LSIMS refer to bombardment of a continuously renewed (flowing) liquid target. 

A gun is used to direct a beam of fast-moving atoms or ions onto the liquid target (matrix). Figure 4.1 shows details of the operation of an atom gun. An inert gas is normally used for bombardment because it does not produce unwanted secondary species in the primary beam and avoids contaminating the gun and mass spectrometer. Helium, argon, and xenon have been used commonly, but the higher mass atoms are preferred for maximum yield of secondary ions. 

When the incident beam of fast-moving atoms or ions impinges onto the liquid target surface, major events occur within the first few nanometers, viz., momentum transfer, general degradation, and ionization. 

The momentum of a fast-moving atom or ion is di.ssipated by collision with the closely packed molecules of the liquid target. As each collision occurs, some of the initial momentum is transferred to substrate molecules, causing them in turn to move faster and strike other molecules. The result is a cascade effect that ejects some of the substrate molecules from the surface of the liquid (Figure 4.2). The process can be likened to throwing a heavy. stone into a pool of water — some... 

Aberth, W. Straub, K.M. Burlingame, A.L. Secondary Ion-MS With Cesium Ion Primary Beam and Liquid Target Matrix for Analysis of Bioorganic Compounds. Anal. Chem. 1982, 54, 2029-2034. 

Barber et al. introduced FAB in 1981. In this technique, bombardment of a liquid target surface by a beam of fast atoms such as xenon or argon, causes the continuous desorption of ions that are characteristic of the liquid. In a typical FAB spectrum, the analyte ion is usually formed as protonated or cationized ions in positive FAB, and deprotonated ions in negative FAB mode. A few fragmented ions may also be formed. The spectrum usually contains peaks from the matrix, such as protonated matrix clusters of glycerol if it is used as the matrix solvent. FAB utilizes a liquid matrix such as glycerol. The matrix is used to enhance sensitivity and ion current stability. 

The table shows that the specific activity of the powder target was much lower than that of the liquid target. The target of Sm(NO3)3 film prepared by evaporation in a quartz ampoule also showed higher specific activity than did the powdered form. The specific activity of increases in the following order Sm2O3 powder < Sm(NO3)3 film < Sm(NO3)3 liquid. 

Liquid targets are used in the production of many PET radionuclides, particularly 18F and 13N. Fluorine-18 is produced by using a liquid target of lsO-enriched water and so is 13N by using 5 nM ethanol in water. The target volume is small in the range of 3-15 ml under high pressure. Since lsO-water is expensive, it is customary to recover it for subsequent irradiation and the method of recovery is described in the later section. 

Aberfh, W., Straub, K. M., and Burlingame, A. L., Secondary ion mass spectrometry with cesium primary beam and liquid target matrix for analysis of bio-organic compoimds. Anal. Chem., 54, 2029, 1982. 

Large-sized liquid targets are not common, mainly on account of hydrolysis and radiation chemical effects. In contrast, small-sized water targets for production of and have attracted considerable attention. Molten salt and flowing-loop liquid targets have found only limited application. 

Gaseous and liquid targets are commonly used for the production of short-lived radionuclides, especially positron emitters. The chemical separation techniques of some important positron emitters are described in the next section. It should be pointed out here that the methods used have to be fast and efficient, in terms of the purity of the product as well as the recovery of the enriched target material. Furthermore, the chemical form of the separated radionuclide has to be well defined to allow a convenient synthesis of the desired radiopharmaceutical. 

XIX-10] DEDUL, A.V., et al. Conceptual design of the heavy-liquid target complex of the integrated type for multipurpose ADS, CD-ROM, Russian Scientific and Technical Forum. Fast Neutron Reactors (To commemorate the 100 birthday of A. I. Leypunsky). Heavy Liquid Metal Coolants in the Nuclear Technologies HLMC-2003 (Proc. of Conf Obninsk, Russia, December 11-12, 2003), Paper 2308. 

Inrush of liquid from one side of a collapsing bubble produces powerful jet of liquid targeted at the surface... 

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