Anatomy of Protein Electrospray Ionization Mass Spectra bySuperconducting Tunnel Junction Mass and Energy SpectrometryLi br

Cryogenic superconducting tunnel junction (STJ)detectors have the advantage of single-particle sensitivity, highquantum efficiency, low noise, and the ability to detect the timeand relative impact energy of deposited ions. This makes themattractive for use in mass spectrometry (MS) and as a form ofenergy spectrometry. STJ cryodetectors have been coupled totime-of-flight (TOF) mass spectrometers equipped with a matrix-assisted laser desorption ionization (MALDI) source and to anelectrospray ionization (ESI) TOF mass spectrometer. Here, a lab-made linear quadrupole ion trap (LIT) mass spectrometer systemwas coupled to an ESI source and a 16-channel Nb-STJ array with improved readout electronics. The goal was to investigatefundamentals of ESI-generated protein ions by further exploiting the advantage of resolving these ions in a third dimension of therelative energy deposited into the STJs. The proteins equine cytochrome c, bovine carbonic anhydrase, bovine serum albumin, andmurine immunoglobulin G were studied using this ESI-LIT-STJ-MS instrument. Multiply charged monomers, multimers, andfragments from metastable ions were resolved from monomer peaks by differences in ion deposition energy even when these ionshave the same mass-to-charge ratio as the corresponding monomer. The determination of a fragment mass from metastabledecomposition is accomplished without knowing the charge state of the fragment. The average charge state of the multimers isreduced with each addition of a protein which is presumed to be a direct reflection of the surface area available for charging. Multiplycharged in-source fragments have also been observed and distinguished in the mass spectrum of carbonic anhydrase by using thedifferences in the energy deposited in the STJs.