Transmission characteristics of a radio‐frequency‐only quadrupole gas‐collision cell at high axial energies in a hybrid tandem mass spectrometer

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Abstract
Collision induced dissociation (CID) experiments using RF‐only quadrupole collision cells normally employ laboratory‐frame collision energies Elab of a few tens of eV. Difficulties in inducing CID reactions for larger precursors (> 600–800 Da) can be attributed in part to collision energies, in the appropriate centre‐of‐mass reference frame, which are too low. Attempts to overcome this problem by using values of Elab in the range of a few hundreds of eV have been largely unsuccessful. The present work shows that a major reason for this lack of success has been an inability of the instruments used to fulfil the focusing requirements of an RF‐only quadrupole cell for such precursors, and their higher mass fragments, in this range of values for Elab. A simple modification permitting use of much higher values of the RF amplitude has been shown to permit the collision cell to be operated so that the ion‐beam profile at the entrance aperture is imaged at the exit; improvements in precursor transmission efficiency by up to two orders of magnitude are thus obtainable. Examples of corresponding improvements in fragment‐ion spectra of a test octapeptide are also demonstrated. The principal drawback of this approach concerns the poor transmission for low‐mass fragments near the stability cut‐off (Mathieu parameter qu > 0.91); reasons for the non‐zero transmission efficiency for fragments with qu > 0.91 are discussed, together with methods for alleviating the low‐mass transmission problem.

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