FUNDAMENTALS OF TRANSMISSION FLUCTUATION SPECTROMETRY WITH VARIABLE SPATIAL AVERAGING

Transmission signal of radiation in suspension of particles performed with a high spatial and temporal resolution shows significant fluctuations,which are related to the physical properties of the particles and the process of spatial and temporal averaging.Exploiting this connection,it is possible to calculate the particle size distribution (PSD) and particle concentration.This paper provides an approach of transmission fluctuation spectrometry (TFS) with variable spatial averaging,The transmission fluctuations are expressed in terms of the expectancy of transmission square (ETS) and are obtained as a spectrum,which is a function of the variable beam diameter.The reversal point and the depth of the spectrum contain the information of particle size and particle concentration,respectively.     

Gas-phase anionic complexes of alkali metal ions and peptides: Structure and collision activated decompositions

Alkali metal ions and anionic peptides can be desorbed into the gas phase to give metal-bound peptides and bis(peptide) complexes bearing a ? 1 charge. Although amide nitrogens of peptide bonds are deprotonated in the gas phase by alkali metal ions, this reacion does not occur in solution. Metal-bound dipeptide anions exist as a single structure, whereas those of tripeptide complexes have three structures as revealed by tandem mass spectrometric studies. Ions of bis(peptide) complexes of alkali metals decompose upon collisional activation principally to form deprotonated peptides, in contrast to bis(peptide) complexes of alkaline earth metal ions, which undergo elimination of a neutral peptide.     

Fast-atom bombardment and tandem mass spectrometry of macrolide antibiotics

Molecular weights of macrolide antibiotics can be determined from either (M + H)⁺ or (M + Met)⁺, the latter desorbed from alkali metal salt-saturated matrices. The ion chemistry of macrolides, as determined by tandem mass spectrometry (MS/MS), is different for ions produced as metallated than those formed as (M + H)⁺ species. An explanation for these differences is the location of the charge. For protonated species, the charge is most likely situated on a functional group with high proton affinity, such as the dimethylamino group of the ammo sugar. The alkali metal ion, however, is bonded to the highly oxygenated aglycone. As a result, the collision-activated dissociation spectra of protonated macrolides are simple with readily identifiable fragment ions in both the high and low mass regions but no fragments in the middle mass range. In contrast, the cationized species give complex spectra with many abundant ions, most of which are located in the high mass range. The complementary nature of the fragmentation of these two species recommends the study of both by MS/MS when determining the structure or confirming the identity of these biomaterials.     

Ion cloud manipulation using the radiofrequency-only-mode as an improvement for high mass detection in fourier transform mass spectrometry

It has been difficult to achieve the expected high resolving power for high-mass biomolecule ions in Fourier transform mass spectrometry. Our hypothesis is that ion clouds produced by laser desorption or injection are diffuse and produce poor signals. To test the hypothesis, clouds of benzene molecular ions produced by electron ionization were purposefully expanded via magnetron mode excitation and characterized by a new experimental sequence for cloud sectional analysis. The expanded cloud was then successfully focused to the trap center by using a high-pressure dynamic event (radiofrequency-only mode). The expanded cloud in a conventional cubic trap produces no detectable signal, whereas the focused cloud in a compensated trap yields a high-resolution signal with good signal-to-noise ratio.     

High pressure trapping in Fourier transform mass spectrometry: A radiofrequency-only-mode event

A new event for the Fourier transform mass spectrometry (FTMS) sequence is developed and demonstrated. During this event, called a radiofrequency (RF)-only mode event, the typical passive cubic trap of a Fourier transform mass spectrometer is made to operate as an active quadrupole ion trap. The transition between active and passive modes is developed so that ion loss as a consequence of the transition can be held to 15% or less. The adduct of the ion-molecule reaction of the 1,3-butadiene radical cation and methyl vinyl ether was detected during the Rf-only-mode event at a helium pressure of ～1×10^?3 torr even though this adduct is not detectable under standard FTMS operating conditions.     

A Mechanism for Poor High Mass Performance in Fourier Transform Mass Spectrometry

The discovery of a mass-dependent electrical mechanism for signal loss of high mass ions in Fourier transform mass spectrometry is reported. Theoretical calculations and experimental evidence show the existence of resonances involving the z-motion and radial motion of the ion. The resonances are intrinsic in the cubic trap. This results in energy transfer between modes, expansion of the ion cloud, and a corresponding loss of signal. Quadrupolar trapping potential wells are proposed as a solution.     

A computational approach to calculating frequency-dependent structural operators using the spectral finite element method and a wavenumber-based gridding technique

Spectral finite element methods are used to compute exact vibration solutions of structural models at specific frequencies. The applicability of these methods to certain areas of structural dynamics is limited by two major factors: the lack of separate structural operators (mass, damping, and stiffness matrices), and the subsequent difficulty in computing mode shapes via eigenvalue decomposition. In the work presented in this article, a method is investigated to accurately calculate spectral finite elements while overcoming these limitations. The approach incorporates a two-dimensional, discrete solution utilizing a wavenumber-based gridding technique to compute frequency-dependent local mass, damping, and stiffness matrices which can be assembled into the global structural operators. Computed models are able to be used for precise vibration analysis as well as modal analysis via eigenvalue decomposition of the structural operators.