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F C Walls

Publications and source records attributed to F C Walls.

5 recordsLinked to original sources
PubMed1999

Insight into absorption of radiation/energy transfer in infrared matrix-assisted laser desorption/ionization: the roles of matrices, water and metal substrates.

Although the ionization/desorption mechanisms in matrix-assisted laser desorption/ionization (MALDI) remain poorly understood, there is a clear difference between the energy absorption processes in the ultraviolet (UV) and infrared (IR) modes of operation. UV-MALDI demands an on-resonance electronic transition in the matrix compound, whereas results presented here support earlier work showing that a corresponding resonant vibrational transition is not a requirement for IR-MALDI. In fact, data from the present study suggest that significant absorption of radiant energy by a potential matrix impairs its performance, although this observation is at variance with some other reports. For example, sinapinic acid, with an IR absorption maximum close to the 2.94 micrometer wavelength of the Er-YAG laser, has been little used as an IR-MALDI matrix. By contrast, succinic acid, with much lower IR absorption and no history of use in UV-MALDI as it has no UV absorption at the wavelength of common UV lasers, has become widely recognized as a good general purpose matrix for IR-MALDI. Despite reports by others that glycerol is an effective matrix for IR-MALDI, we find that glycerol, which also absorbs strongly at 2.94 micrometer, is useful only if applied as a very thin film. Thus the cumulative evidence for the role of the matrix in IR-MALDI appears confusing and often contradictory. Water has been postulated to be a major contributor to the absorption of energy in IR-MALDI. Consistent with this, we find that samples dried from D(2)O, which does not absorb at 2.94 micrometer, give spectra of inferior quality compared with the same samples from H(2)O. Similarly, samples dried under vacuum, that probably contain less water than those dried in the open laboratory, give weaker and more erratic spectra. Another potential participant in energy absorption and energy transfer is the surface of the metal support, an alternative mechanism for IR-MALDI, for which some evidence is presented here.

Chemical Phenomena↗
PubMed1990

Artifacts in four-sector tandem mass spectrometry.

Several types of artifacts were shown to be present in 4-sector tandem collision-induced dissociation (CID) mass spectra. In CID spectra of protonated peptides produced by liquid secondary-ion mass spectrometry (LSIMS), peaks corresponding to successive losses of matrix molecules from the precursor ion were observed. In addition, peaks corresponding to MH+ ions of smaller peptides that were also present in the sample/matrix mixture in greater abundance than the selected precursor ion were observed. Both of these types of artifact peaks were shown to originate from the 'peak-at-every-mass' chemical noise at the same nominal mass as that selected by the first 2 sectors (MS1). These noise ions are transmitted through to the collision cell and produce fragments that are analysed and detected in the next 2 sectors (MS2). A second, unrelated, kind of artifact was found to be due to decompositions in the second field-free region of MS2 in an EBEB geometry machine. These artifacts, which are detectable over only a very limited mass range when using a conventional single-point detector, can be present over a much greater mass range when an array detector is used and when the collision cell is floated above ground potential. A clear understanding of the origins of all peaks in a CID spectrum is important in order to have a firm foundation for interpretation, manual or computer-aided, of the spectra of unknown compounds.

Mass Spectrometry↗
PubMed1986

Cooled sample introduction probe for liquid secondary ionization mass spectrometry.

The design and performance of a cooled sample introduction probe for fast atom bombardment or other liquid secondary ionization mass spectrometric studies are described. Cooling relatively volatile matrix materials (sulfolane, thioglycerol, and tetraglyme, for example) in situ in the ion source can increase the duration of the sample spectrum by a factor of 10. Cooling also permits a much wider range of matrix materials to be used. As an example, the spectrum of the carbohydrate, peracetyl [Glu(beta 1----3)]7 Glucitol in tetraglyme matrix, is shown to give an excellent spectrum including cleavage peaks corresponding to losses of the first five sugar residues. The spectrum lasted approximately 10 times longer when the probe tip was cooled to 9 +/- 1 degrees C than when no cooling was used, corresponding to a 10-fold increase in integrated sensitivity.

Cold Temperature↗