LINEAR EXCITATION AND DETECTION IN FOURIER-TRANSFORM ION-CYCLOTRON RESONANCE MASS-SPECTROMETRY

We present the first Fourier transform ion cyclotron resonance (FT-ICR ) ion trap designed to produce both a linear spatial variation of the excitation electric potential field and a linear response of the detec tion circuit to the motion of the confined ions. With this trap, the m agnitude of the detected signal at a given ion cyclotron frequency var ies linearly with both the number of ions of given mass-to-charge rati o and also with the magnitude-mode excitation signal at the ion cyclot ron orbital frequency; the proportionality constant is mass independen t. Interestingly, this linearization may be achieved with any ion trap geometry. The excitation/detection design consists of an array of cap acitively coupled electrodes which provide a voltage-divider network t hat produces a nearly spatially homogeneous excitation electric field throughout the linearized trap; resistive coupling to the electrodes i solates the a.c. excitation (or detection) circuit from the d.c. (trap ping) potential. The design is based on analytical expressions for the potential associated with each electrode, from which we are able to c ompute the deviation from linearity for a trap with a finite number of elements. Based on direct experimental comparisons to an unmodified c ubic trap, the linearized trap demonstrates the following performance advantages at the cost of some additional mechanical complexity: (a) s ignal response linearly proportional to excitation electric field ampl itude; (b) vastly reduced axial excitation/ejection for significantly improved ion relative abundance accuracy; (c) elimination of harmonics and sidebands of the fundamental frequencies of ion motion. As a resu lt, FT-ICR mass spectra are now more reproducible. Moreover, the linea rized trap should facilitate the characterization of other fundamental aspects of ion behavior in an ICR ion trap, e.g. effects of space cha rge, nonquadrupolar electrostatic trapping field, etc. Furthermore, th is novel design should improve significantly the precision of ion rela tive abundance and mass accuracy measurements, while removing spectral artifacts of the detection process. We discuss future modifications t hat linearize the spatial variation of the electrostatic trapping elec tric field as well, thereby completing the linearization of the entire FT-ICR mass spectrometric techniques. Suggested FT-ICR mass spectrome tric applications for the linearized trap are discussed.