Tom L. Ricca

Ion cyclotron resonance excitatio/de-excitation: A basis for Stochastic fourier transform ion cyclotron mass spectrometry

Abstract Previous mass spectrometers based on the ion cyclotron resonance principle have employed continuous excitation (single-pulse or frequency-sweep), With detection during (frequency-sweep) or after (single-pulse or frequency sweep) the excitation. The present paper introduces an experiment in which an ion is first excited to a Larger orbit by continuous excitation, and then “de-excited” back to its starting point. The effect is demonstrated for the C9F20N+ peak (m/z = 502) in the Fourier transform ion cyclotron mass spectrum of perfluorotributylamine. By choosing which ion m/z ratios are “de-excited”, it should be possible to generate mass “windows” within which ions experience no net excitation. Potential applications of the method include the generation of an excitation with sharply defined “windows” or “steps”, with major advantages for MS/MS or multiple-ion-monitoring experiments.

Ultrahigh‐resolution Fourier transform ion cyclotron resonance mass spectrometer

A new Fourier transform ion cyclotron resonance mass spectrometer is described in three sections: magnet, vacuum system, and electronics/data system. Each component is described in detail (e.g., manufacturer’s part numbers, drawings, circuit schematics, etc.). Of special interest is the high‐frequency signal generation and processing required to extend the lower‐mass limit for singly charged ions to 1 u. In particular, two new multiple heterodyne techniques for high‐frequency measurements are described, as well as a method for producing a long‐duration electron beam, and the implementation of z‐axis ejection for selective removal of unwanted ions of various mass‐to‐charge ratios.

Clipped representations of fourier-transform ion-cyclotron resonance mass spectra

Abstract Because of its high mass resolution over a wide mass range, a Fourier transform/mass spectrometry (F.t./m.s.) experiment may generate very large spectral data sets (⩾ 64 K each), creating severe data storage problems for g.c./F.t./m.s. and for mass spectral library storage, search and retrieval. Fortunately, the stored experimental data need not be highly precise. In this paper, it is shown that useful mass spectra can be produced from time-domain F.t./m.s. data that have been clipped to only 1 bit/word, thereby offering a potential reduction by a factor of ⩾ 20 in data storage requirement. In many cases, mass spectral resolution can actually be enhanced by the clipping process. Finally, it is shown tha a compound can be identified quickly and accurately by comparison of its clipped representation with similarly clipped data for library compounds.

Beating the Nyquist Limit by Means of Interleaved Alternated Delay Sampling: Extension of Lower Mass Limit in Direct-Mode Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

According to the Nyquist theorem, the highest signal frequency which can be represented without foldover (aliasing) in a Fourier transform frequency-domain discrete spectrum is one-half of the time-domain sampling frequency. For example, since ion cyclotron resonance (ICR) frequency is inversely related to ionic mass-to-charge ratio, m/z, the highest ICR frequency (corresponding to the lowest correctly represented m/z) in direct-mode Fourier transform ICR mass spectrometry is restricted to one-half of the maximum sampling frequency, or about m/z ≥ 18 at 3.058 tesla (T) for a maximum sampling frequency of about 5.2 MHz. In this paper, we show that interleaved addition of two digitized time-domain transient signals, one of which is delayed by one-half of one sampling period (i.e., half of one cycle of the time-domain sampling frequency) with respect to the other, generates a time-domain discrete waveform which is indistinguishable from a single waveform produced by sampling at twice the original sampling rate. Thus, provided that the two transients have (or have been normalized to) the same magnitude, one can double the Nyquist-limited frequency range. If the sampling period is divided into three or more equal parts, with interleaved addition of three or more correspondingly delayed transients, the same method can further increase the upper frequency limit. The method is applied to the experimental doubling or quadrupling of FT/ICR direct-mode frequency range, as for example in the extension of the lower mass limit to below m/z = 12 at 3.058 T with a sampling rate of only 4.0 MHz.