Pulsed mass spectrometry

Most ion-molecule techniques study reactivity at pressures below 1000 Pa however, several techniques now exist for studying reactions above this pressure range. These include time-resolved, atmospheric-pressure, mass spectrometry optical spectroscopy in a pulsed discharge ion-mobility spectrometry [108] and the turbulent flow reactor [109]. 

The pump-probe concept can be extended, of course, to other methods for detection. Zewail and co-workers [16,18, 19 and 2Q, 93] have used the probe pulse to drive population from a reactive state to a state that emits fluorescence [94, 95, 96, 97 and 98] or photodissociates, the latter situation allowing the use of mass spectrometry as a sensitive and selective detection method [99, 100]. 

For stand-alone or hybrid TOF mass spectrometry, the ions examined must all start from some point at the same instant. From this zero time, the ions are accelerated through a short region by applying a short pulse of electric potential of several kilovolts. The acceleration gives the ions velocities that vary in proportion to the square root of their m/z values. 

Ions for TOF mass spectrometry must be extracted from the ion source in instantaneous pulses. Therefore, either ions are produced continuously but are extracted from the source in pulses, or ions are produced directly in pulses. 

TOF mass spectrometry is ideally suited to those ionization methods that inherently produce ions in pulses, as with pulsed laser desorption or Cf-radionuclide ionization. 

After acceleration through an electric field, ions pass (drift) along a straight length of analyzer under vacuum and reach a detector after a time that depends on the square root of their m/z values. The mass spectrum is a record of the abundances of ions and the times (converted to m/z) they have taken to traverse the analyzer. TOP mass spectrometry is valuable for its fast response time, especially for substances of high mass that have been ionized or selected in pulses. 

In Laser Ionization Mass Spectrometry (LIMS, also LAMMA, LAMMS, and LIMA), a vacuum-compatible solid sample is irradiated with short pulses ("10 ns) of ultraviolet laser light. The laser pulse vaporizes a microvolume of material, and a fraction of the vaporized species are ionized and accelerated into a time-of-flight mass spectrometer which measures the signal intensity of the mass-separated ions. The instrument acquires a complete mass spectrum, typically covering the range 0— 250 atomic mass units (amu), with each laser pulse. A survey analysis of the material is performed in this way. The relative intensities of the signals can be converted to concentrations with the use of appropriate standards, and quantitative or semi-quantitative analyses are possible with the use of such standards. 

A somewhat related technique is that of laser ionization mass spectrometry (LIMS), also known as LIMA and LAMMA, where a single pulsed laser beam ablates material and simultaneously causes some ionization, analogous to samples beyond the outer surface and therefore is more of a bulk analysis technique it also has severe quantiBaction problems, often even more extreme than for SIMS. 

Laser ionization mass spectrometry or laser microprobing (LIMS) is a microanalyt-ical technique used to rapidly characterize the elemental and, sometimes, molecular composition of materials. It is based on the ability of short high-power laser pulses (-10 ns) to produce ions from solids. The ions formed in these brief pulses are analyzed using a time-of-flight mass spectrometer. The quasi-simultaneous collection of all ion masses allows the survey analysis of unknown materials. The main applications of LIMS are in failure analysis, where chemical differences between a contaminated sample and a control need to be rapidly assessed. The ability to focus the laser beam to a diameter of approximately 1 mm permits the application of this technique to the characterization of small features, for example, in integrated circuits. The LIMS detection limits for many elements are close to 10 at/cm, which makes this technique considerably more sensitive than other survey microan-alytical techniques, such as Auger Electron Spectroscopy (AES) or Electron Probe Microanalysis (EPMA). Additionally, LIMS can be used to analyze insulating sam-... 

Matrix-assisted laser desorption mass spectrometry (MALDI-MS) is, after electrospray ionization (ESI), the second most commonly used method for ionization of biomolecules in mass spectrometry. Samples are mixed with a UV-absorbing matrix substance and are air-dried on a metal target. Ionization and desorption of intact molecular ions are performed using a UV laser pulse. 

CA 73,100610 (1970) A pulsed ruby laser-mass spectrometry technique was developed and applied, wherein granular mixts of AP and lightabsorbing substrate materials were rapidly flash py roly zed (0.8 msec) within the low-pressure lon-source chamber of a Bendix TOF mass... 

Reactions of Thermal Energy Ions by Pulsed Source Mass Spectrometry... 

Pulsed source mass spectrometry 150 Pulsing technique.132, 139... 

From a mass spectrometry perspective, the pump must be pulse free, i.e. it must deliver the mobile phase at a constant flow rate. Pulsing of the flow causes the total ion current (TIC) trace (see Chapter 3) - the primary piece of information used for spectral analysis - to show increases in signal intensity when analytes are not being eluted and this makes interpretation more difficult. 

The laser desorption experiments which we describe here utilize pulsed laser radiation, which is partially absorbed by the metal substrate, to generate a temperature jump in the surface region of the sample. The neutral species desorbed are ionized and detected by Fourier transform mass spectrometry (FTMS). This technique has... 

ToF analysers are able to provide simultaneous detection of all masses of the same polarity. In principle, the mass range is not limited. Time-of-flight mass analysis is more than an alternative method of mass dispersion it has several special qualities which makes it particularly well suited for applications in a number of important areas of mass spectrometry. These qualities are fast response time, compatibility with pulsed ionisation events (producing a complete spectrum for each event) ability to produce a snapshot of the contents of the source volume on the millisecond time-scale ability to produce thousands of spectra per second and the high fraction of the mass analysis cycle during which sample ions can be generated or collected. 

In ToF-MS, the ion source is pulsed to create packets of ions. In the conventional procedure, the system waits for all the ions in a packet to reach the detector before injecting the next packet of ions. Complications arise when ToF-MS is coupled to a continuous ion source. Such coupling is therefore often accomplished by the orthogonal extraction approach, in which a segment of the ion stream is accelerated orthogonally by a push-out pulse. However, in this process, up to 95 % of the information contained in the ion steam is lost. Recently, Hadamard transform time-of-flight mass spectrometry (HT-ToFMS) was developed to couple continuous ion... 

Tables 8.57 and 8.61 compare the performance of GD-MS to other inorganic mass-spectrometry techniques, including LA-ICP-MS which acts as a strong competitor. GD and laser ablation techniques offer the possibility to obtain information about the distribution of analyte within the sample, but on quite a different scale. Pulsed GD requires ToF-MS [374].

A different strategy has been applied in our work, that emphasizes the importance of DNA stability on hole transfer within double-stranded DNA. This work is based on determination of the overall yield of oxidized nucleosides that arise from the conversion of initially generated purine and pyrimidine radical cations within DNA exposed to two-photon UVC laser pulses. On the one hand, this work benefits from the excellent current knowledge of chemical reactions involving the radical cations of DNA bases, and on the other hand, from major analytical improvements that include recent availability of the powerful technique of high performance liquid chromatography-electrospray ionization-tandem mass spectrometry (CLHP-ESI-MS/MS) [16-18]. 

Figure 2 Mass spectrometry profiles of pulsing 1 ml 02 into the propylene/He flow.

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