Field desorption
As heating of the emitter continues, low-vapor pressure materials get desorbed and ionized by alkali metal cation attachment.
The ionization of more polar organic molecules (e.g., ones with aliphatic hydroxyl or amino groups) in FD-MS typically go through this mechanism.
Single metal tips can be made from etching wires either by periodically dipping them into molten salts or by electrolysis in aqueous solutions.
The tungsten wire serving as the field anode is then heated up to about 1500 K with direct current at a potential of about 10 kV with respect to a cathode.
Carbon microneedles can be produced within 8-12 h. HR activation method is to reverse the polarity of the emitter and the counter electrode, which emits a strong electron current.
Instead of carbon microneedles, metallic dendrites (mainly of nickel or cobalt) can be produced on thin wires through electrochemical desorption process.
In this technique, the solid samples are dissolved or suspended in a suitable medium, and then an activated emitter (usually a tungsten wire with many microneedles) is dipped into the solution and drawn out again.
They benefit from both: the soft FD ionization and the safe and simple LIFDI transfer of air/moisture sensitive analyte solution.
This transfer occurs from the Schlenk flask to the FD emitter in the ion source through a fused silica capillary without breaking the vacuum.
LIFDI has been successfully coupled to a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer.
The coupled system enables analysis of sulphur-containing materials in crude oil under extremely high mass resolving power conditions.
[11] A major application of FD is to determine the molecular mass of a large variety of thermally labile and stable nonvolatile, nonpolar, and polar organic and organometallic compounds, and of molecules from biochemical and environmental sources.
[7] Many earlier applications of FD to analysis of polar and nonvolatile analytes such as polymers and biological molecules have largely been supplanted by newer ionization techniques.
However, FD remains one of the only ionization techniques that can produce simple mass spectra with molecular information from hydrocarbons and other particular analytes.
The most commonly encountered application of FD at the present time is the analysis of complex mixtures of hydrocarbons such as that found in petroleum fractions.
