Single Scan Two-Dimensional Fourier Transform Ion Cyclotron Resonance Mass Spectrometry – ONE_SHOT_FT-ICR_MS_2D

Bi-dimensional Fourier Transform Mass Spectrometry (2D FT-ICR MS) has received little attention relative to 2D NMR since its introduction in 1987. The main drawbacks of the original version of the 2D FT -ICR MS were: the loss of resolution caused by in ICR-cell CID, the difficulty of processing data without sacrificing resolution and the intense noise due to ion fluctuations. In recent years, in the frame of a preliminary ANR project entitled ‘FT-ICR 2D’, we have revisited 2D FT-ICR and presented solutions to these problems by using methods of fragmentation such as infrared multi-photon dissociation (IRMPD) and electron-capture dissociation (ECD), by improving the 2D pulse sequence, by developing software that can handle files of several gigabytes, and by introducing innovative algorithms based on mathematical theory of sparcity for noise reduction. With these tools we have been able to obtain ECD 2D spectra of molecules such as insulin (5.7 kDa).
Our main objective is to decrease the acquisition time, which today is comparable to liquid chromatography analysis. So far, we have acquired 2D spectra with a moderate resolution similar to quadrupole MS in the first dimension, and high resolution as good as in FT -ICR in the second dimension. To achieve a resolution similar to quadrupole MS in the first dimension, one must record about two thousand spectra, which typically requires about two hours.
A first improvement that will be explored in the proposed project ‘ONE_SHOT_FT-ICR_MS_2D’ will be to introduce non-uniform sampling (NUS) in FT-ICR. In multidimensional NMR, non-uniform sampling (NUS) is used routinely, and in particular to speed up the analysis of protein spectra. New algorithms must be developed for FT-ICR spectra, since the data matrices are about one thousand time larger in ICR than in NMR spectra, and the known algorithms such as Maximum Entropy which strongly depend on data size cannot be applied.
A second improvement is to transpose ‘single scan 2D NMR’ to FT-ICR. In this NMR method, the sample is placed in a non-uniform magnetic field with a gradient along the z axis. This allows a double temporal and spatial encoding of the phase of the magnetization of the nuclei. Whereas chemists tend to focus on the detection of cyclotron motions perpendicular to the magnetic field in ICR cells, physicists often prefer to detect magnetron motions along the magnetic field. We plan to use a ‘compensated’ cell with an additional magnetic field gradient in the direction of the main field to enable ‘single scan 2D FT-ICR’. Obviously this part of the proposal is very challenging.
To develop these improvements, several tools have to be developed or improved. Firstly we plan to improve pulse sequences which are at the heart of FT-ICR. We will investigate ‘chirp pulses’ with non-constant durations and intensities in view of obtaining a uniform initial phase of all ions after their excitation. This should pave the way to novel techniques in FT-ICR that have similar advantages as spin echoes in NMR. We also plan to continue developing algorithms for noise reduction and processing non-uniformly sampled data. The new algorithm urQRd developed during our ANR project FT-ICR 2D can efficient de-noise FT-ICR spectra that are not too complex (ca 1000 peaks.) The requirements in terms of time and memory are proportional to the size of the matrix and the number of peaks. The algorithm has to be improved for handling complex spectra (over 1 million peaks).
Finally, examples of real-world samples will be chosen from the top-down proteomics and metabolomics platforms of IBISA. 2D FT-ICR MS will be applied to samples that are quite abundant and cannot easily be separated by chromatography. We have already shown in our laboratory that 2D FT-ICR MS can be applied to the analysis of triacylglycerols (TAG) isolated from human blood.