Improvements on an out-of-plane MEMS quadrupole for portable mass spectrometry

Quadrupoles are mass filters composed of a set of four cylindrical rods in a symmetrical setup that are biased with a combination of DC and RF potentials. There has been an active interest in developing scaled-down quadrupole technology for over a decade. This has led to remarkable results in both miniaturized and MEMS-based quadrupoles. This paper reports the design, fabrication, and characterization of a second generation of MEMS out-of-plane quadrupoles mass filter, part of a portable mass spectrometer for gas sensing. The mass spectrometer is composed of a CNT-based ionizer array, a MEMS quadrupole mass filter, MEMS electrometer for charge sensing, and a MEMS displacement pump that provides the required vacuum level. Two key advantages of scaling down quadrupoles are device portability and the relaxation on the vacuum level required for operation. For a fixed rod aspect ratio the quadrupole weight roughly scales to the cube of the rod diameter. Also, scaled down quadrupoles can work at higher pressure provided the gas mean free-path is larger than the quadrupole length. We have experimentally demonstrated that higher working pressure, up to the mTorr level, is compatible with our CNT-based electron impact and field ionizers with no loss in ionizer performance, thus setting the maximum quadrupole length at 5 cm.