Time-delayed mass spectrum

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. ... 

Fig. 1. Typical mass spectrum of S02 clusters taken at zero delay time between the pump and probe. Note the dominant intensity of the (SC>2)nSO+ series is due to an ion-state oxygen atom loss channel.

The energy of the laser pulse is measured using an integrating sphere and a (GeGaAs) photodiode, integrated by a boxcar. To ensure that mass spectra and laser energies are recorded in pairs, a home built amplitude to time converter is used to write the energy as a time delay at the end of each mass spectrum. 

For pump-probe photoionization (PPI, Fig.l) the first laser pulse is tuned into resonance with the (vibrationless) electronic transition of the molecule, the second pulse is red-shifted in wavelength, so that the enhanced (1+1 ) photoion signal can be easily identified. When a time-of-flight mass spectrometer is used for detection the mass-selective photoion signal as a function of time delay can be recorded as the RCS spectrum of the electronically excited state, which is particularly useful for the specific investigation of molecular clusters. 

Bigger clusters have been formed, for instance, by the expansion of laser evaporated material in a gas still under vacuum. For metal-carbon cluster systems (including M C + of Ti, Zr and V), their formation and the origin of delayed atomic ions were studied in a laser vaporization source coupled to a time-of-flight mass spectrometer. The mass spectrum of metal-carbon cluster ions (TiC2 and Zr C j+ cluster ions) obtained by using a titanium-zirconium (50 50) mixed alloy rod produced in a laser vaporization source (Nd YAG, X = 532 nm) and subsequently ionized by a XeCl excimer laser (308 nm) is shown in Figure 9.61. For cluster formation, methane ( 15% seeded in helium) is pulsed over the rod and the produced clusters are supersonically expanded in the vacuum. The mass spectrum shows the production of many zirconium-carbon clusters. Under these conditions only the titanium monomer, titanium dioxide and titanium dicarbide ions are formed. 

Since we are interested in possible formation of hydrogenated carbons in the present work, we now switch the buffer gas from He to H2 expecting formation of polyyenes and other novel hydrocarbons. The mass spectrum at the bottom of Figure 9.4 was obtained in the H2 buffer gas for a delay time of 35 ps. There are several points to note in this spectrum, which are as follows. 

In the (J-LIT hybrid [25] instruments, an ion trap is implemented as the second stage of mass analysis, either for accumulation of ions to achieve improved sensitivity after colhsion cell CID [25,46], and/or to perform MS [25,46,47]. The Q-LIT instrument can either be operated as a conventional TQ instrument or as the hybrid instrument. In TQ mode, the instrument is capable of all acquisition modes of a TQ, including SRM. In the hybrid mode, fidl-spectrum data can be acquired in the enhanced product ion (EPI) mode with up to 60-fold enhaneed sensitivity eompared to TQ instruments. Next to the enhanced multiple-charge sean and the time-delayed fragmentation scan, the system allows the acquisition of MS spectra, with the second dissociation step to be performed in the LIT [46,47]. 

FIGURE 5.6 Liquid SIMS-TOF mass spectrum of a peptide obtained after 20 ps time delay between the ionizing and extraction pulses. The peaks marked by vertical lines belong to the y-ion series. 

More recently, delayed orthogonal extraction has been utilized to observe peptide fragmentation on MALDI time-of-flight instnmients. Figure 7.9 shows the high irradiance MALDI mass spectrum of melittin obtained by Standing et al., following an I8-ps delay. 

Fig. 10.3 Two-step photoionization of benzaldehyde (a) quantum transitions for two-step photoionization (b) mass spectrum of two-step photoionization (solid lines) and single-photon ionization (dashed lines) (c) dependence of molecular and fragment photoion yield on the delay time between the two laser pulses. (Prom Antonov et al. 1978.)...

Figure 2.6 shows a MALDI-FT-ICR-MS spectrum of an ethylene oxide-propylene oxide copolymer in broadband mode. For this spectrum, the trapping delay was optimised to be 900 ps for maximum signal-to-noise ratio (S/N) at the centre of the MWD, i.e., it was not compensated for the flight-time-induced mass discrimination. The spectrum is the sum of the spectra of 250 consecutive laser shots. The expansion of the mass scale shows that the resolution is sufficient to resolve the naturally occurring isotopes of a component molecule. For example, the resolution in broadband mode (m/dml gy is 15000, with an S/N of 330 at mIz 969.6. 

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