University of California, Riverside

High Resolution Mass Spectrometry Facility



Liquid Introduction Field Desorption Ionization


The UCR MS Facility recently acquired the ability to perform Field Desorption Ionization (FD). We are one of the few MS facilities in the world to offer access to this technique to the general scientific community.

Origins

Many years ago a technique called Field Ionization Mass Spectrometry (FI) was invented by R. Gomer and M.G. Inghram ( J. Am. Chem. Soc., 1955, 77, 500). The hallmark of the technique was the observation of a molecular ion (M+.) as the base peak in the mass spectrum for all samples. This gas phase ionization (FI) was later extended to other samples via deposition onto a wire followed by desorption and ionization (FD).

Why Have I Not Heard of This Technique?

The techniques of FI/FD were difficult to use routinely because of the need for very high voltages and very delicate emitter wires. Emitter wires broke often, in part, because of the requirement to remove the probe after every sample. Once electrospray and MALDI were invented, FI/FD and FAB use virtually disappeared.

Why is it Available Now?

FI/FD were never really dead thanks to the efforts of Dr. Bernhard Linden of Germany. Dr. Linden developed a new way to deposit samples onto an FD emitter and made the FD technique more reproducible.

In his new approach, a sample in solution is aspirated into the mass spectrometer via a narrow fused silica capillary column. The sample is deposited onto the FD emitter in the process and then can be analyzed. Once the emitter is inside the vacuum of the instrument, it does not have to be removed after each sample, thus extending its useful life. Since samples are introduced as liquids through a capillary into the mass spectrometer the technique has been called Liquid Introduction Field Desorption Mass Spectrometry (LIFDI ).

Why Use LIFDI?

Electrospray, FAB, and MALDI are currently used to obtain MW information for molecules. They are most useful for polar molecules. LIFDI is not restricted by polarity of the sample. LIFDI is another tool in the arsenal of mass spectrometry to obtain MW information. It produces M+ ions in a very soft ionization process, leading to mass spectra with little or no fragmentation of the molecular ion. When conventional techniques do not produce a molecular ion, LIFDI is an alternative method that can be used for this purpose. LIFDI will ionize all molecules, unlike ESI or MALDI, which may not protonate a molecule. At UCR, we have used LIFDI to ionize molecules with molecular weights ranging from 300 to 2400 (see Figure 1 below).

What are the limitations of LIFDI analysis?

Every analytical technique has limitations. LIFDI is no exception. The limits of LIFDI are:

  1. Water and acetonitrile are a problem. These solvents will freeze out as they elute from the capillary and into our vacuum in the mass spectrometer. This leads to poor sample transfer.
  2. FD requires an electrical current be ramped up through the emitter wire. This current can cause some thermal decomposition for very labile species.
  3. Samples must be completely dissolved. Incomplete solubility or particulates could clog the capillary line used to transfer samples to the emitter.
  4. LIFDI detection limits are compound dependant primarily due to desorption differences from the emitter.
  5. Because LIFDI produces M+ ions, we can not analyze negative ions in this technique.

NOTE : As of the summer 2012, the previous limitation for nominal mass data only has been removed for molecules under MW 1250. Accurate mass data to determine molecular formula can now be provided.

 

Submitting Samples

Samples can be sent neat. Provide us with 200ug of material.  For air sensitive samples that you have prepared in a glove box under inert atmosphere, dissolve 200ug of your sample into 1 ml of solvent and send that to us for analysis.

 

Summary

If other methods have failed to provide MW information for your sample, contact us to arrange a LIFDI analysis. It may be possible to get good results with this technique.

Note: At UCR, LIFDI is done on our GCMS. To do LIFDI requires that we remove the GC connection and replace it with our LIDFI equipment. Because of this, we schedule LIFDI work ONLY when there are enough samples to justify the change.

If you are interested in getting some LIFDI analyses, check our Update page about the next scheduled LIFDI dates for analyses.

Example of LIFDI Capability

lifdi

Figure 1. LIFDI mass spectrum of a mixture of cyclic, multi chlorinated oligomers. Each cluster represents the molecular ion (M+.) of one component in the mixture.
Sample courtesy of Dr. C. Reed of UCR.

More Information

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Riverside, CA 92521
Tel: (951) 827-1012

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High Resolution Mass Spectrometry Facility
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Tel: (951) 827-5287
Fax: (951) 827-4713
E-mail: Jie.Zhou@ucr.edu

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