Time, electron ionization

For rapid scanning of GC output and to maximize sensitivity, mass analyzers are normally operated in selected ion mode, where only a few pre-selected ions are monitored at any one time. Electron ionization at 70 eV is normal although increased response has been observed in some systems using helium carrier gas when operated at lower ionization energies (10-20 eV). There is a growing interest in the use of negative ion chemical ionization as this technique is very sensitive to certain halocarbons, notably those containing bromine or iodine. 

An attractive reason for introducing Cl was that it provided much softer ionization than the dominant ionization technique available at that time, electron ionization (El). 

A major advantage of the TOF mass spectrometer is its fast response time and its applicability to ionization methods that produce ions in pulses. As discussed earlier, because all ions follow the same path, all ions need to leave the ion source at the same time if there is to be no overlap between m/z values at the detector. In turn, if ions are produced continuously as in a typical electron ionization source, then samples of these ions must be utihzed in pulses by switching the ion extraction field on and off very quickly (Figure 26.4). 

It is worth noting that some of these methods are both an inlet system to the mass spectrometer and an ion source at the same time and are not used with conventional ion sources. Thus, with electrospray, the process of removing the liquid phase from the column eluant also produces ions of any emerging mixture components, and these are passed straight to the mass spectrometer analyzer no separate ion source is needed. The particle beam method is different in that the liquid phase is removed, and any residual mixture components are passed into a conventional ion source (often electron ionization). 

Conditions apparatus, Hewlett-Packard HP5890 equipped with an HP5972 mass-selective ion detector (quadruple) column, PTE-5 (30 m x 0.25-mm i.d.) with 0.25- am film thickness column temperature, 50 °C (1 min), increased at 20 °C min to 150 °C(5 min) and then at 4 °Cmin to 280 °C (30 min) inlet and detector (GC/MS transfer line) temperature, 250 and 280 °C, respectively gas flow rate, He carrier gas ImLmin" injection method, splitless mode solvent delay, 3 min electron ionization voltage, 70eV scan rate, 1.5 scanss scanned-mass range, m/z 50-550. The retention times of benfluralin, pendimethalin and trifluralin are 15.2, 25.1 and... 

Chlornitrofen and nitrofen conditions for GC/MS column, cross-linked methyl silicone capillary (12 m x 0.22-mm i.d., 0.33- am film thickness) column temperature, 60 °C (1 min), 18 °C min to 265 °C inlet, transfer line and ion source temperature, 260, 200 and 200 °C, respectively He gas column head pressure, 7.5 psi injection method, splitless mode solvent delay, 3 min electron ionization voltage, 70 eV scan rate, 0.62 s per scan cycle scanned mass range, m/z 100-400. The retention times for chlornitrofen and nitrofen were 11.8 and 11.3 min, respectively. The main ions of the mass spectrum of chlornitrofen were at m/z 317, 319 and 236. Nitrofen presented a fragmentation pattern with the main ions at m/z 283, 202 and 285. ... 

DGE a AC AMS APCI API AP-MALDI APPI ASAP BIRD c CAD CE CF CF-FAB Cl CID cw CZE Da DAPCI DART DC DE DESI DIOS DTIMS EC ECD El ELDI EM ESI ETD eV f FAB FAIMS FD FI FT FTICR two-dimensional gel electrophoresis atto, 10 18 alternating current accelerator mass spectrometry atmospheric pressure chemical ionization atmospheric pressure ionization atmospheric pressure matrix-assisted laser desorption/ionization atmospheric pressure photoionization atmospheric-pressure solids analysis probe blackbody infrared radiative dissociation centi, 10-2 collision-activated dissociation capillary electrophoresis continuous flow continuous flow fast atom bombardment chemical ionization collision-induced dissociation continuous wave capillary zone electrophoresis dalton desorption atmospheric pressure chemical ionization direct analysis in real time direct current delayed extraction desorption electrospray ionization desorption/ionization on silicon drift tube ion mobility spectrometry electrochromatography electron capture dissociation electron ionization electrospray-assisted laser desorption/ionization electron multiplier electrospray ionization electron transfer dissociation electron volt femto, 1CT15 fast atom bombardment field asymmetric waveform ion mobility spectrometry field desorption field ionization Fourier transform Fourier transform ion cyclotron resonance... 

Chromatographic methods are used to separate the components in a mixture, but in a complex mixture, a single chromatographic method or step many not separate all components. In these cases, using simple retention time to identify the components will not suffice and the identification of components in the mixture will be incorrect. Thus, the addition of a method of identification such as mass spectrometry (MS) or Fourier transform infrared (FTIR) is essential. In some cases, it may even be necessary to confirm either an FTIR or MS identification by the same method applied in a different way. For example, FTIR may be followed by MS, or electron ionization (El) MS followed by chemical ionization (Cl) MS or by an entirely different method. 

From the 1950s to the present mass spectrometry has changed tremendously and still is changing. [12,13] The pioneering mass spectrometrist had a home-built rather than a commercial instmment. This machine, typically a magnetic sector instrument with electron ionization, delivered a few mass spectra per day, provided sufficient care was taken of this delicate device. If the mass spectrometrist knew this particular instrument and understood how to interpret El spectra he or she had a substantial knowledge of mass spectrometry of that time. [14-18]... 

Most of these processes are very fast. Ionization happens on the low femtosecond timescale, direct bond cleavages require between some picoseconds to several tens of nanoseconds, and rearrangement fragmentations usually proceed in much less than a microsecond (Fig. 5.3 and Chap. 2.7). Finally, some fragment ions may even be formed after the excited species has left the ion source giving rise to metastable ion dissociation (Chap. 2.7). The ion residence time within an electron ionization ion source is about 1 ps. [9]... 

Fig. 5.3. Schematic time chart of possible electron ionization processes. Reprinted from Ref. [4] with permission. John Wiley Sons, 1986.

Investigated explosives included 2,4,6-trinitrotoluene (TNT), 2,4,6,N-tetranitro-N-methylaniline (tetryl), l,3,5-trinitro-l,3,5-triazacyclohexane (RDX), 1,3,5,7-tetranitro-l,3,5,7-tetrazacyclooctane (HMX) and pentaerythritol tetranitrate (PETN). The temperature of the injector, cooled with liquid CO2, was —5°C for 0.3 min, programmed from —5 to 250° C, at a rate of 200°C/min, with a final hold time of 8.4 min. The column temperature was 80° C for 2 min, programmed to 250° C at 25°C/min, with a final hold of 2 min. Electron ionization (El) in the positive-ion mode was used. Figure 4 shows the mass chromatograms of a mixture of explosives (lOppb each), extracted from water by Hquid—liquid extraction and X 100 concentration. Identification was based on typical fragment ions for each one of the explosives. 

MS, chemical ionization-MS, and sometimes GC/infrared spectroscopy (IR) have been used with GC/MS to obtain structural information. Examples of the use of GC/ MS for identifying new DBFs include the recent identification of iodo-acids. The iodo-acids were discovered in drinking water treated with chloramination through the use of full-scan GC/MS on the methylated extracts. Empirical formula information for both the molecular ions and the fragment ions was obtained by high-resolution electron ionization (EI)-MS, and the spectra were interpreted to yield tentative identifications of five new iodo-acids (iodoacetic acid, bromoiodoacetic acid, ( )-3-bromo-3-iodopropenoic acid, (Z)-3-bromo-3-iodopropenoic acid, and )-2-iodo-3-methylbutenedioic acid). Structural assignments were then confirmed by the match of mass spectra and GC retention times to authentic chemical standards, several of which had to be synthesized. 

Figure 24-22 Selected reaction monitoring in gas chromatography-mass spectrometry, (a) Reconstructed total ion chromatogram of extract from orange peel with electron ionization, (b) Selected reaction monitoring with the precursor ion m/z 293 selected by mass filter Q1 in Figure 22-21 and product ion m/z 264 selected by mass filter Q3. The chromatogram is a graph of intensity at m/z 264 from Q3 versus time. [Courtesy Thermo Ftnnlgan GC ana GC/MS Division, San Jose. CA.J...

The experiments of Kistiakowsky and Kydd [1] were done by single-pulse photolysis with a 500-J flashlamp, the reaction vessel contents being sampled via a pinhole leak into the electron ionization source of a Bendix time-of-flight (TOF) mass spectrometer. Mass spectra were obtained by pulsed extraction of ions from the ion source at 50-fis intervals after the flash. The signal from the electron multiplier detector was displayed on a cathode ray tube, which was photographed with a rotating drum camera. 

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