Anatomy of Protein Electrospray Ionization Mass Spectra by Superconducting Tunnel Junction Mass and Energy Spectrometry

• Published in: Analytical chemistry (Washington) Vol. 94; no. 13; pp. 5284 - 5292
• Main Authors: Jiang, Li-Xue, Plath, Logan D, Halim, Mohammad A, Friedrich, Stephan, Bier, Mark E
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 05-04-2022
• Subjects: Analytical chemistry; Animals; Bovine serum albumin; Carbonic anhydrase; Carbonic anhydrases; Cattle; Chemistry; Cytochrome; Cytochrome c; Cytochromes; Electrospraying; Energy; Fragments; Horses; IgG antibody; Immunoglobulin G; Ionization; Ions; Low noise; Mass spectra; Mass spectrometers; Mass spectrometry; Mass spectroscopy; Mice; Monomers; Proteins; Quadrupoles; Quantum efficiency; Scientific imaging; Serum albumin; Spectrometers; Spectrometry, Mass, Electrospray Ionization - methods; Spectrometry, Mass, Matrix-Assisted Laser Desorption-Ionization - methods; Spectroscopy; Spectrum Analysis; Superconductivity; Tunnel junctions
• ISSN: 0003-2700; 1520-6882
• DOI: 10.1021/acs.analchem.1c05074

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