Fig. 1. Quadrupole mass spectrometry with superconducting single particle detection.

(A) Proteins are volatilized using electrospray ionization charge-reduced in bipolar air by a corona discharge. Further charge reduction by photochemistry is enabled by a high-power ultrafast ultraviolet laser interacting with the molecules in the entrance chamber. The ions are filtered in a quadrupole mass selector and pass through a radio frequency hexapole guide toward a quadrupole deflector. The ions are then steered to either a TOF-MS with multichannel plate (MCP), a phosphor screen with photo-multilpier (PhS), or the SSPD array. A Faraday plate can be shifted into the position of the SSPD to calibrate the incident ion current at high flux. Panel (B) shows an electron micrograph of two pixels (SSPDs), each with a size of 20 × 20 μm² (D₁), while (C) shows a close-up image of a detector pixel (D₂) that has a 100 times larger area and a 500-nm line width, as also described in the text.