EXPERIMENTAL-DETERMINATION OF THE NUMBER OF TRAPPED IONS, DETECTION LIMIT, AND DYNAMIC-RANGE IN FOURIER-TRANSFORM ION-CYCLOTRON RESONANCE MASS-SPECTROMETRY

Determination of the number of ions that produce a detected time-domain signal in Fourier transform ion cyclotron resonance mass spectrometry is fundamental in establishing (a) the detection limit for a single data acquisition, (b) the maximum number of trapped ions, and (c) the mass spectral dynamic range obtained as the ratio of (b)/(a). Moreover, the ability to monitor ion absolute abundances throughout various experimental event sequences provides a direct measure of (e.g.) ionization efficiency, collisional-induced dissociation efficiency, and ion trapping efficiency. In this paper, we determine the absolute number of trapped ions by comparing the experimentally observed signal voltage to that calculated for a single ion orbiting at the (measured) ICR orbital radius of the ion packet, assuming that all ions orbit in a tight coherent packet in the trap midplane. The effects of relaxing these assumptions are estimated. In this way, we determine a detection limit (undamped signal acquired for 1 s, magnitude-mode mass spectral peak height to baseline standard deviation noise ratio of 3:1 measured at a frequency of 600 kHz, excited ICR orbital radius of 50% of the maximum possible cubic trap radius) of approximately 177 singly charged ions and a dynamic range of 32 000 at 3.0 T in a cubic trap. Various assumptions, the detection circuit model, and proposed applications for the determination of the number of trapped ions are discussed.