Electron ionization interfaces using

Some analyte fragmentation can be induced with APCI-MS and ESI-MS by collision-induced dissociation (CID) on octapole, hexapole, or cone devices at the input of the mass spectrometer. The newly developed direct electron ionization interface (DEI) involves the direct introduction of a nano-LC system working with a mobile phase flow rate of between 0.3 and 1.5 /rl/min into a mass spectrometer equipped with an electron ionization interface. It has been used to determine and identify several OPPs in water samples. Electron ionization generates spectra that can be interpreted using commercially available documentation (Wiley or NIST). 

The mass spectrometer should provide structural information that should be reproducible, interpretable and amenable to library matching. Ideally, an electron ionization (El) (see Chapter 3) spectrum should be generated. An interface that fulfils both this requirement and/or the production of molecular weight information, immediately lends itself to use as a more convenient alternative to the conventional solid-sample insertion probe of the mass spectrometer and some of the interfaces which have been developed have been used in this way. 

Arguably the ultimate LC-MS interface would be one that provides El spectra, i.e. a spectrum from which structural information can be extracted by using famihar methodology, and this was one of the great advantages of the moving-belt interface. There is, however, an incompatibility between the types of compound separated by HPLC and the way in which electron ionization is achieved and therefore such an interface has restricted capability, as previously discussed with respect to the moving-belt interface (see Section 4.2 above). 

The range of compounds from which electron ionization spectra may be obtained using the particle-beam interface is, like the moving-belt interface, extended when compared to using more conventional methods of introduction, e.g. the solids probe, or via a GC. It is therefore not unusual for specffa obtained using this type of interface not to be found in commercial libraries of mass spectra. 

Following separation on conventional gas chromatographic columns, electron-capture detector (402) has been used for the determination of the hydroxy metabolite of dimetridazole in swine muscle with good sensitivity and specificity. To confirm the presence of lasalocid residues in bovine liver, gas chromatography coupled with mass spectrometry via a chemical ionization interface (387) has been successfully applied. 

Conventional high pressure NICI spectra were obtained using a Hewlett-Packard 5985B quadrupole GC/MS, as described previously (1). Methane was used as the Cl reagent gas and was maintained in the source at 0.2-0.4 torr as measured through the direct inlet with a thermocouple gauge. A 200 eV electron beam was used to ionize the Cl gas, and the entire source was maintained at a temperature of 200° C. Samples were introduced into the spectrometer via the gas chromatograph which was equipped with a 25 meter fused silica capillary column directly interfaced with the ion source. For all experiments, a column coated with bonded 5% methyl phenyl silicon stationary phase, (Quadrex, Inc.) was used and helium was employed as the carrier gas at a head pressure of 20 lbs. Molecular sieve/silica gel traps were used to remove water and impurities from the carrier gas. 

Volatiles isolated by the purge-and-trap method were analyzed by GC-MS using a Varian 3400 gas chromatograph coupled to a Finnigan MAT 8230 high resolution mass spectrometer equipped with an open split interface. Mass spectra were obtained by electron ionization at 70 eV and an ion source temperature of 250°C. The filament emission current was 1 milliampere and spectra were recorded on a Finnigan MAT SS 300 Data system. 

An ideal interface should not cause extra-column peak broadening. Historical interfaces include the moving belt and the thermospray. Common interfaces are electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCl). Several special interfaces include the particle beam—a pioneering technique that is still used because it is the only one that can provide electron ionization mass spectra. Others are continuous fiow fast atom bombardment (CF-FAB), atmospheric pressure photon ionization (APPI), and matrix-assisted laser desorption ionization (M ALDl). The two most common interfaces, ESI and APCI, were discovered in the late 1980s and involve an atmospheric pressure ionization (API) step. Both are soft ionization techniques that cause little or no fragmentation hence a fingerprint for qualitative identification is usually not apparent. 

GC/MS analyses most often employ one of two complementary ionization processes, electron ionization (El) or chemical ionization (Cl). This is because both El and Cl are gas phase ionization phenomena and are therefore well suited to interface with a separation technique (GC) that is also accomplished in the gas phase. The extractables profiles shown in Figs. 3-5 along with the Abietic Acid GC/MS analysis shown in Fig. 1, were acquired using GC/MS with El. The El ionization process is based on the interaction of an energetic electron beam (70 eV) with neutral analyte molecules in the gas phase, producing a radical cation, or molecular ion (M+ ) that can undergo fragmentation in the gas phase after redistribution of excess... 

Various types of HSCCC-MS have been developed using frit fast-atom bombardment (FAB) including continuous flow (CF) FAB, frit electron ionization (El), frit chemical ionization (Cl), TSP, atmospheric pressure chemical ionization (APCI), and electrospray ionization (ESI). Each interface has its specific features. Among those, frit MS and ESI are particularly suitable for directly interfacing to HSCCC, because they generate low back-pressures of approximately 2 kg/cm, which is only one-tenth of that produced by TSP. 

Several interfaces have been used for CCC-MS (mass spectrometry). The first employed is thermospray (TSP). When a column is directly coupled with TSP MS, the CCC column often breaks due to the high back-pressure generated by the thermospray vaporizer. By contrast, other interfaces, such as fast atom bombardment (FAB), electron ionization (El), and chemical ionization (Cl), have been directly connected to a CCC column without generating high back-pressure. Such interfaces can be applied to analytes with a broad range of polarities. As it is suitable to introduce effluent from the column CCC into MS at a flow rate of only between 1 and 5 L/min, the effluent is usually introduced into the MS through a splitting tee, which is adjusted to an appropriate ratio. 

The instrument was operated in the electron ionization (El) mode with 70-eV electrons, a source temperature of 200 °C, the conversion dynode at -5000 V and the secondary electron multiplier at 2400 V. The source and the collector slit widths were adjusted to obtain trapezoidal peaks with flat tops. The GC-MS interface was at 280 °C and high-purity He was used as a carrier gas. Data were acquired in the selected-ion monitoring (SIM) mode using voltage peak switching and the quantitation was based on peak areas. 

A schematic of a particle beam interface is shown in Figure 21.13. The eluent from the HPLC column is nebulized using helium gas to form an aerosol in a reduced pressure chamber heated at 70°C. A cone with a small orifice is at the end of the chamber, which leads into a lower pressure area. The difference in pressure causes a supersonic expansion of the aerosol. The hehum and the solvent molecules are lighter than the analyte molecules and tend to diffuse out of the stream and are pumped away. The remaining stream passes through a second cone into a yet lower pressure area, and then the analyte vapor passes into the ion source. The particle beam interface produces electron ionization (El) spectra similar to those of GC-MS, so the vast knowledge of El spectra can be used for analyte identification. 

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