Field desorption technique

Alternative ( soft ) ionization techniques are not usually required for aromatic isothiazoles because of the stability of the molecular ions under electron impact. This is not the case for the fully saturated ring systems, which fragment readily. The sultam (25) has no significant molecular ion under electron impact conditions, but using field desorption techniques the M + lY ion. is the base peak (73X3861) and enables the molecular weight to be confirmed. 

Two scientific aspects in cluster science have been investigated using pulsed-laser field desorption technique with a considerable degree of success.85 One is the critical numbers, which are the smallest numbers of atoms in multiply charged cluster ions when the ions can still resist being... 

Mass spectrometry has been an extremely useful tool for the characterization of neutral organometallic clusters, except for those few cases which have extremely high molecular weights or possess such ligands as PPh3 that reduce the volatility of the compound. Mass spectrometry has not been a useful characterization technique for ionic clusters because these compounds are insufficiently volatile to permit study by conventional electron-impact techniques. However, there is some hope that with the development of field-desorption techniques ionic clusters as well as neutrals will be capable of being analyzed by mass spectrometry (146). 

Future improvement needs include increased efforts for calibration work, work to extend volatility limits using the direct insertion probe or the field desorption technique (7), and the development of fast and sharp separation methods for specific compound classes. 

Mass spectroscopy (field desorption technique) reveals an increasing ease of positive charging of these molecules with the extension of their conjugated system. The relative intensities of the molecular ions decrease from 100% (7a, 7b) to 0% (10b), whereas ions of higher charge appear with increasing intensity 8b, 9b = 100%, 10b = 100% and = 4%. The molecular ion... 

Mass spectrometry is an important method in determining carbohydrate structures, and there are excellent reviews of the most recent mass spectrometric methods for complex carbohydrates [16,17]. Among many useful techniques, classical ionization of volatile molecules through electron impact (El) or chemical ionization (Cl) [18,19], electrospray ionization (ESI) [20], and field desorption techniques (ED) [21] are frequently employed in structural analysis. 

Unlike other techniques of mass spectrometry, field desorption does not require vaporization of the sample prior to ionization. The field desorption technique has been suggested as a method to determine the molecular weights of sub-milligram quantities of involatile substances such as sugar phosphates without derivatization. 

In the field desorption technique (FD) the sample is placed on a specially prepared emitter wire and ionized in a high voltage electrostatic field. In this way little or no fragmentation occurs and mass peaks are obtained even from practically non-volatile peptides [34]. 

Since the late 1950s, mass spectrometry (MS) has been used to study peptides. However, until as recently as 15 years ago, mass spectrometers were used almost exclusively by organic chemists to investigate small molecules using field desorption techniques. The reason for this was that there were many problems with derivatization of the analytes and the low mass range was restrictive. But in the 1980s, two techniques became popular, which allowed direct... 

Field ionization (27) and field desorption techniques (28) have also been used for SIM in drug analysis. The examples given above were chosen to indicate the variety of applications of SIM. Comprehensive lists of references on SIM have been published for the period up to to the beginning of 1975 (29, 30). 

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