Laser Field Desorption

Two scientific aspects in cluster science have been investigated using pulsed-laser field desorption technique with a considerable degree of success.85 One is the critical numbers, which are the smallest numbers of atoms in multiply charged cluster ions when the ions can still resist being... 

In pulsed-laser field desorption of helium from rhodium surfaces, if the... 

Saponins are too polar, too thermally unstable and not volatile enough to directly provide ions that are suitable for analysis. Until recently, derivatives (Me or TMS ethers) had to be prepared in order to obtain spectra. In the past ten years however, new techniques of soft ionization have emerged which have proved useful in the analysis of saponins. They are Field Desorption (FD), Fast Atom Bombardment (FAB), Laser Field Desorption (LD) and Californium Plasma Desorption (CPD). 

Mobility of this second kind is illustrated in Fig. XVIII-14, which shows NO molecules diffusing around on terraces with intervals of being trapped at steps. Surface diffusion can be seen in field emission microscopy (FEM) and can be measured by observing the growth rate of patches or fluctuations in emission from a small area [136,138] (see Section V111-2C), field ion microscopy [138], Auger and work function measurements, and laser-induced desorption... 

Mass Spectrometry. Field desorption mass spectrometry has been used to analy2e PPO (179). Average molecular weight parameters (M and could be determined using either protonated (MH + ) or cation attachment (MNa + ) ions. Good agreement was found between fdms and data supphed by the manufacturer, usually less than 5% difference in all cases up to about 3000 amu. Laser desorption Fourier transform mass spectrometry was used to measure PPG ion and it was claimed that ions up to m/2 9700 (PEG) can be analy2ed by this method (180). 

Recent attention has focused on MS for the direct analysis of polymer extracts, using soft ionisation sources to provide enhanced molecular ion signals and less fragment ions, thereby facilitating spectral interpretation. The direct MS analysis of polymer extracts has been accomplished using fast atom bombardment (FAB) [97,98], laser desorption (LD) [97,99], field desorption (FD) [100] and chemical ionisation (Cl) [100]. 

As field desorption (FD) refers to an experimental procedure in which a solution of the sample is deposited on the emitter wire situated at the tip of the FD insertion probe, it is suited for handling lubricants as well as polymer/additive dissolutions (without precipitation of the polymer or separation of the additive components). Field desorption is especially appropriate for analysis of thermally labile and high-MW samples. Considering that FD has a reputation of being difficult to operate and time consuming, and in view of recent competition with laser desorption methods, this is probably the reason that FD applications of polymer/additive dissolutions are not frequently being considered by experimentalists. 

Over the years, a lot of desorption ionization techniques have been introduced to MS, such as plasma desorption, field desorption, laser desorption, secondary ion mass spectrometry, fast atom bombardment, matrix assisted laser desorption and desorption electrospray ionization. Most of them are actually no longer used. In the following paragraphs, both matrix assisted laser desorption (MALDI) and desorption electrospray ionization (DESI) will be discussed. 

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... 

Frauenkron, M. Berkessel, A. Gross, J.H. Analysis of Ruthenium Carbonyl-Porphyrin Complexes a Comparison of Matrix-Assisted Laser Desorp-tion/Ionization Time-of-Flight, Fast-Atom Bombardment and Field Desorption Mass Spectrometry. Eur. Mass Spectrom. 1997, 5,427-438. 

Very large branched alkanes, such as 24,24-diethyl-19,29-dioctadecylhepta-tetracontane, CgvHng, for example, pose difficulties to obtaining useful mass spectra and even 15 eV El does not anymore allow for the detection of their molecular ions. [80] Beyond C40 alkanes, especially in case of mixtures such as hydrocarbon waxes or polyethylenes of low molecular weight, field desorption and matrix-assisted laser desorption/ionization are the ionization methods of choice (Chaps. 8, 10). 

Cl in conjunction with a direct exposure probe is known as desorption chemical ionization (DCI). [30,89,90] In DCI, the analyte is applied from solution or suspension to the outside of a thin resistively heated wire loop or coil. Then, the analyte is directly exposed to the reagent gas plasma while being rapidly heated at rates of several hundred °C s and to temperatures up to about 1500 °C (Chap. 5.3.2 and Fig. 5.16). The actual shape of the wire, the method how exactly the sample is applied to it, and the heating rate are of importance for the analytical result. [91,92] The rapid heating of the sample plays an important role in promoting molecular species rather than pyrolysis products. [93] A laser can be used to effect extremely fast evaporation from the probe prior to CL [94] In case of nonavailability of a dedicated DCI probe, a field emitter on a field desorption probe (Chap. 8) might serve as a replacement. [30,95] Different from desorption electron ionization (DEI), DCI plays an important role. [92] DCI can be employed to detect arsenic compounds present in the marine and terrestrial environment [96], to determine the sequence distribution of P-hydroxyalkanoate units in bacterial copolyesters [97], to identify additives in polymer extracts [98] and more. [99] Provided appropriate experimental setup, high resolution and accurate mass measurements can also be achieved in DCI mode. [100]... 

Fig. 2.5 An ion kinetic energy distribution of field desorbed He ions taken with a pulsed-laser time-of-flight atom-probe. In pulsed-laser stimulated field desorption of field adsorbed atoms, atoms are thermally desorbed from the surface by pulsed-laser heating. When they pass through the field ionization zone, they are field ionized. Therefore the ion energy distribution is in every respect the same as those in ordinary field ionization. Beside the sharp onset, there are also secondary peaks due to a resonance tunneling effect as discussed in the text. The onset flight time is indicated by to, and resonance peak positions are indicated by arrows. Resonance peaks are pronounced only if ions are collected from a flat area of the...

Fig. 2.22 In [UP - 1 )/T] vs. 1 IT plot for field adsorption of He on the W (112) surface, obtained from a pulsed-laser stimulated field desorption experiment. In the high temperature region, the data points fit well into a straight line of H = 0.17 eV, as expected from a theoretical analysis. In the low temperature region, the data points deviate significantly from the linear plot of the solid line. This deviation indicates that there may be another binding state with a much smaller binding energy of 0.08 eV.

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