Desorption by ionic bombardment SIMS

This method is already relatively old and has been used for the past several years in organic chemistry and biochemistry. High kinetic energy (several keV) primary ions, e.g., Ar, bombard a surface on which the sample has been deposited. Under these conditions, ions are extracted from the surface and can be analyzed, Benninghoven and co-workers [102] presented a number of examples carbohydrates, alkaloids, amino acids (and derivatives) and peptides. As with the other methods, both positive and negative ionization modes are possible (Fig. 14). More recently, the same author [103] demonstrated the possibility of studying non-volatile nucleic acids and compared the results obtained with the other desorption methods. Sensitivity limits are on the order of ng. 

We have summarized in the previous pages the recent developments (prior to 1980 ) of new soft ionization techniques and of various peripherals. These aspects often responded to the need to obtain molecular ions, in order to determine the molecular masses of these often thermo-labile biological molecules, leading to the development of soft ionization methods and to obtain sufficient vaporization (or desorption) of the sample studied. Among other methods, field desorption (and DCI), the use of lasers and radioactive decomposition of Cf have been introduced. 

Mass spectrometry may be characterized by successful results in the field of biochemistry this success is due above all to the development of GC/MS (and LC/MS), combined with a computer for rapid data processing. The use of mass fragmentometry (SIM, MID) has resulted in assays of very small quantities in mixtures (currently on the order of a pg, and even a fg, scale). 

Clearly, it is very difficult to obtain an exhaustive list of limitations, which in fact may only apply to individual cases. Nevertheless, mass spectrometry offers other 

This analytical field no longer utilizes fragment ions formed in the source, but rather ions characterized with a greater lifetime ( 10 second) and which decompose in the field free regions (FFR) of the mass spectrometer. These ions are termed metastable ions and are quite useful for demonstrating the existence of characteristic uni-molecular (or bimolecular) decompositions. 

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