Cryogenic FTMS
FTMS has traditionally been constructed on large, horizontal, room temperature bore magnets, but if it is possible to construct the FTMS in the 4.2K cold bore of a superconducting magnet, several important advantages will be achieved.
First, the vacuum system will cryopump itself so the base pressure will be in the <10e-12 mbar range and the pumping speed will be in the 10e+5 l/sec range.
Second, the cold vacuum system walls can be placed in thermal contact with the input resistors of a preamplifier. This will reduce thermal Johnson noise by a factor of 8.4 relative to room temperature operation which increases sensitivity and dynamic range by the same amount.
Third, the vacuum system can be constructed in a 1″-2″ diameter bore which allows the use of NMR-style magnets for FTMS. Thus, the high field NMR magnets up to 23.3T can be used for FTMS without any change to the fundamental coil design.
There are substantial, but solvable, engineering problems involved in the construction of a cryogenic FTMS. They are detailed in the reference below, but primarily involve design of the system to minimize heat transfer into the liquid helium dewar and thereby control the helium boiloff rate.
- O’Connor, P. B. Considerations for the design of a Fourier Transform mass spectrometer in the 4.2K cold bore of a superconducting magnet Rapid Commun Mass Spectrom 2002, 16, 1160-1167. Link
If you cannot access this paper from the publisher, please email me and I will email you a copy of the pdf file. poconnor@bu.edu
To Download some recent pictures of the cryogenic FTMS vacuum chamber, click here.
