Mass-selective axial ejection linear ion trap

A mass spectrometer using mass selective axial ejection, comprising:

• an ion source configured to supply ions in an axial direction and located at one end of the axis;
• an ion detector located at the other end of the axis;
• a linear ion trap comprising two overlapping parallel pairs of conductive rods of substantially the same length located between the ion source and the ion detector along the axis of the ion trap, the two overlapping parallel pairs comprising a first pair of continuous rods and a second pair of segmented rods, the segmented rods each having a long segment and a shorter segment that is located closer to the ion detector end of the ion trap axis relative to the long segment, wherein each rod in the two pairs is located diagonally from the other rod in that pair, such that each rod in a pair is adjacent to the rods of the other pair;
• an RF signal generating source configured to supply an RF signal, having a first voltage and a first frequency, to each rod in first pair of continuous rods and to the long segments of the segmented rods of the second pair;
• and
• a pair of capacitors that electrically couples the RF signal from the long segments to the shorter segments of each of the segmented rods of the second pair at a second voltage reduced by at least 1% relative to the first voltage of the RF signal.

A method for operating a mass spectrometer to facilitate mass selective axial ejection of ions comprising steps of:

• providing a linear ion trap comprising two axially aligned, interleaved pairs of parallel conductive rods that define an axial direction having an upstream end and an exit end, the pairs including a first pair of continuous rods and a second pair of segmented rods, each segmented rod having a long segment and a shorter segment that is located proximate to the exit end, wherein each long segment of each rod is electrically coupled to the shorter segment via a capacitor, such that an RF voltage applied to the long segments will result in a lower RF voltage applied to the shorter segments, where the lower RF voltage is at least 1% less than the RF voltage applied to the long segments;
• creating a DC well in the axial direction by
• applying a first DC voltage to the first pair of continuous rods and the long segments of the second pair of segmented rods,
• applying a second DC voltage, higher than the first DC voltage, to the shorter segments of the second pair of segmented rods,
• applying a third DC voltage, higher than the second DC voltage, to an exit lens located at the exit end of the linear ion trap, and
• applying a fourth DC voltage, higher than the first DC voltage, to electrodes located upstream of the shorter segments of the second pair of segmented rods;
• trapping ions in the linear ion trap by
• applying a first RF voltage to the first pair of continuous rods and a second RF voltage of the same frequency and substantially the same voltage to the long segments of the second pair of segmented rods and
• injecting ions from an ion source upstream of the linear ion trap;
• and
• ejecting ions axially in a mass dependent manner by applying a third auxiliary AC voltage at a lower voltage and frequency than the first RF voltage to the first pair of continuous rods, such that the third auxiliary AC voltage is of opposite phase at each continuous rod.