EI/CI ion source

With the advent of capillary GC, [50-54] the need for separators and the concomitant risk of suppression of certain components vanished. Capillary columns are operated at flow rates in the order of 1 ml min and therefore can be directly interfaced to EI/CI ion sources. [48,49] Thus, a modem GC-MS interface basically consists of a heated (glass) line bridging the distance between GC oven and ion source. On the ion source block, an entrance port often opposite to the direct probe is reserved for that purpose (Chap. 5.2.1). The interface should be operated at the highest temperature employed in the actual GC separation or at the highest temperature the column can tolerate (200-300 °C). Keeping the transfer line at lower temperature causes condensation of eluting components to the end of the column. 

In a particle-beam interface (PBI), the column effluent is nebulized, either pneumatically or by TSP nebulization, into a near atmospheric-pressure desolvation chamber, which is connected to a momentum separator, where the high molecular-mass analytes are preferentially transferred to the MS ion source, while the low molecular-mass solvent molecules are efficiently pumped away. The analyte molecules are transferred in small particles to a conventional EI/CI ion source, where they disintegrate in evaporative collisions by hitting a heated target, e.g., the ion source wall. The released molecules are ionized by El or conventional CL... 

Figure 20-51. Transfer line connection between the GLC and the EI/CI ion source of the mass spectrometer detector. (Courtesy - ThermoQuest Finnegan Corp., Austin, TX)...

The GCQ deteaor consists of an EI/CI ion source (exchangeable volumes) and operate in either of two ion polarity modes positive or negative (ECD-MS). System can operate in the following scan modes ... 

Fig. 5.3. Filaments for EI/CI ion sources. A coiled filament of the VG ZAB-2F (left) and a straight wire filament of the JEOL JMS-700 (right). The shields behind the filament are at the same potential as the wire itself and the white parts are made of ceramics for insulation.

The historic moving belt interface seems rather a curiosity. The LC effluent is deposited onto a metal wire or belt which is heated thereafter to desolvate the sample. Then, the belt traverses a region of differential pumping before it enters an EI/CI ion source where the analyte is rapidly evaporated from the belt by further heating (Fig. 5.14) [50-54]. 

Some unusual adduct ions, corresponding to [M + NO] and [M + N02], appear in the mass spectra of nitrate ester explosives, such as NG and PETN, and nitramines, such as RDX and HMX [38, and references cited therein]. Their formation, which is favored by GI MS conditions, especially in high sample pressures, is attributed to ion-molecule reactions between NO and N02 ions and the neutral molecules of the explosives. These adduct ions were also reported to be highly abundant in the El spectra of nitrate esters [19], when El was carried out in a tight, dual EI/CI ion source. 

FAB has been coupled (both on-line and off-line) with several separation techniques to improve FAB analysis of mixtures (e.g. TLC-FAB-MS, TLC-SIMS). Because FAB and FD provide highly complementary data for many sample types, combination FAB/FD sources have been reported. EI/FD/FAB and EI/FI/FI) ion sources have also been mentioned [22], as well as a CI/FAB source [80]. 

GC-MS analysis used a Finnigan 4000 quadrupole EI/CI mass spectrometer. Electron Impact spectra were recorded continually using an Incos Nova 4 data system. Ion source temperature was 250°C and the ionisation energy 70 eV. 

The occurrence of [Mh-H] ions due to bimolecular processes between ions and their neutral molecular counterparts is called autoprotonation or self-CI. Usually, autoprotonation is an unwanted phenomenon in EI-MS. [M-i-1] ions from autoprotonation become more probable with increasing pressure and with decreasing temperature in the ion source. Furthermore, the formation of [M-i-1] ions is promoted if the analyte is of high volatility or contains acidic hydrogens. Thus, self-CI can mislead mass spectral interpretation either by leading to an overestimation of the number of carbon atoms from the C isotopic peak (Chap. 3.2.1) or by... 

The particle-beam interface is an analyte-enrichment interface in which the column effluent is pneumatically nebulized into a near atmospheric-pressure desolvation chamber connected to a momentum separator, where the high-mass analytes are preferentially directed to the MS ion source while the low-mass solvent molecules are efficiently pumped away (71, 72). With this interface, mobile phase flow rates within the range O.l-l.O ml/min can be applied (73). Since the mobile phase solvent is removed prior to introduction of the analyte molecules into the ion source, both EI and CI techniques can be used with this interface. 

Mass Spectrometer. The mass spectrometer was a Hewlett-Packard 5988A quadrupole mass spectrometer with a dual EI/CI source and positive and negative ion detection. The system was controlled by a Hewlett-Packard 1000 computer. The mass spectrometer was periodically tuned manually using perfluorotributylamine (PFTBA) on ions m/z 69, m/z 214, and m/z 502 in El and PCI modes, and on ions m/z 245, m/z 414, and m/z 633 in NCI mode. 

The proton transfer reaction shown in equation 14b represents a general class of reactions (equation 17) known as self CF reactions in which a compound acts as its own Cl reagent gas Such reactions can be observed in EI/CI sources of conventional mass spectrometers, and are important in ICR mass spectrometers. The rates of formation of MH ions from M (equation 17) ions of organophosphorus species are shown in Table 5 It should be pointed out that although the structures of the M ions in these experiments were assumed to have the same connectivity as in the neutral precursors M, distonic ions may also be responsible for the proton-transfer reactions observed, based on the work of Kenttamaa and coworkers ... 

The ion source is a custom designed variable temperature EI/CI source. Metal containing precursor ions are formed by electron impact (150 eV) ionization and fragmentation of volatile precursors such as Fe(CO)5 Co(CO)3NO. T ical source pressures are lO torr, and source temperatures are kept below 280 K to minimize decomposition of the organometallics on insulating surfaces. Adduct formation results from reaction of an atomic metal ion or metal containing species with small molecules. The ion source is operated under nearly field free conditions to prevent translational excitation of the ions, which are accelerated to 8 kV before mass analysis. 

Although the ion trap is considered by many analysts to be the most or one of the most sensitive instruments for GC/EI MS and positive ion GC/CI MS, new techniques to improve instrument performance are being developed. We briefly describe here new scanning sequences, the fitting of external ion sources, and applications of GC/MS/MS procedures. 

Although ions can be generated by El and Cl of compounds introduced directly into an ion trap, it might be desirable to apply other methods of ionization. Moreover, the use of an external ion source may solve the problem of self-CI during GC/ EI-ITMS analyses. Such considerations have been the basis of studies on the injection of externally generated ions into an ion trap [8]. 

The detectors used in HTGC are, apart from the highly versatile flame ionization detector (FID), the phosphorus/nitrogen-selective alkali-flame ionization detector (AFID), the atomic emission detector, the inductively coupled plasma (ICP)-mass spectrometer, and, last but not least, mass spectrometers with electron impact and chemical ionization ion sources (EI/CI-MS). 

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