Supplementary Material
Detailed Description of AP-ECD Source
The AP-ECD source consists of three interconnected sub-assemblies: a sprayer plug with an integrated nanospray source, a spray chamber with a central channel and an inlet for an auxiliary (AUX) gas flow, and the source block with photoionization lamp of an AB SCIEX PhotoSpray APPI source.
The sprayer plug and spray chamber assemblies were custom designed for the AP-ECD source, with machining performed in-house (Mechanical Engineering Services, Department of Chemistry, UBC). The sprayer plug is comprised of a polyimide flange, shaped like a top hat, fitting in the upstream end of the spray chamber to make a hermetic seal, and a nanospray source which screws into a hole on the central axis of the flange. The sprayer plug is secured in the spray chamber with a thumbscrew, allowing for easy removal of the plug when the nanospray source requires servicing. The nanospray source has several parts, including a main body made of brass, with a central bore for a replaceable nanospray emitter and a side bore for a nebulizer (NEB) gas inlet. A stainless steel tube for guiding the NEB gas around the emitter is fastened to the downstream side of the source body with a stainless steel nut and ferrule. The fused silica emitter (SilicaTip™, part # FS360-50-15-N-C15; New Objective, Inc.; Woburn, MA) passes through the NEB gas tube and the body of the nanospray source, positioned so that its tip extends several mm past the end of the NEB gas tube; the emitter is fastened and hermetically sealed at the upstream side of the source body with a stainless steel nut and a polyimide ferrule. In operation, the high-velocity flow of NEB gas helps stabilize the spray by preventing the accumulation of liquid on the emitter tip; the NEB gas also cools the emitter, preventing boiling of the liquid sample at low flow rates when the spray chamber is heated.The liquid sample is coupled to the emitter via an upstream stainless steel union (not shown), through which electrical contact from the electrified body of the nanospray source to the liquid sample is also made.
The spray chamber assembly is comprised of a first brass cylindrical section with a 10mm internal diameter (ID) central channel, and a second section with a central channel tapered from 10 to 7 mm ID. Both spray chamber sections fit into the APPI source block, in the space originally occupied by the PhotoSpray source’s heated nebulizer and its polyimide sleeve. In the first section there is a side channel for the AUX gas inlet, a stainless steel union with a 1/8” outer diameter (OD) compression fitting on one end and pipe threads on the other. The main function of the AUX gas is to transport ions rapidly from the nanospray source through the remainder of the spray chamber and then the APPI source block. The ID of the first spray chamber section is relatively large to facilitate installation of the sprayer plug (without destroying the emitter tip through accidental contact with the walls of the spray chamber), while the second section is tapered to match the ID of the APPI source block, to minimize turbulence in the gas flow between the spray chamber and the source block. The two sections of the spray chamber are in electrical contact with each other and with the APPI source block.
The spray chamber may be heated via an attached band heater and thermocouple, and a temperature controller (model 6100; Omega; Stamford, CT). Since all the heater components are in electrical contact with the spray chamber, and thus at high voltage during operation, the temperature controller is powered through an isolation transformer. For operator safety, both the temperature controller and the isolation transformer are housed in a plexiglass box, and the heater and thermocouple cables are provided with appropriate electrical insulation.
To promote photoelectron production within the APPI source block, a photoionizable dopant is added to the AUX gas. The dopant is introduced to the AUX gas via a 1/8” OD brass union tee (not shown) upstream of the spray chamber. Liquid dopant is delivered to the tee from a syringe pump through a length of fused silica capillary tubing (20 m ID, 360 m OD; Polymicro Technologies; Phoenix, AZ); the capillary is secured in one arm of the tee with a nut, a 1/8” to 1/16” OD reducing ferrule, and an FEP tubing sleeve (model F-242; Upchurch Scientific; Oak Harbor, WA). The liquid dopant is evaporated and swept from the tee by the AUX gas flow entering and exiting through the other arms of the tee. [For stable, pulse-free dopant delivery at the normal flow rate of 0.1 l min–1, it is important to use both a narrow ID capillary (≤ 20 m) and a low volume syringe (≤ 100 L).] Lastly, PTFE tubing is used between the tee and the spray chamber inlet, because it does not decompose or outgas significantly when the spray chamber is heated.
To shield the photoelectron generation region of the APPI source block from the electric field of the nanospray emitter, a thin wire-mesh screen (92% transmission; Unique Wire; Hillside, NJ) is situated between the spray chamber and the source block. The screen is at the same electric potential as the spray chamber and source block, and it confines the electric field of the nanospray emitter to the spray chamber, helping to maintain the source block substantially free of external electric fields. Without the screen, the electric field of the nanospray source may extend into the photoelectron generation region, causing the photoelectrons to be drawn upstream towards the emitter tip instead of the gas-phase peptide ions, preventing ECD.
The APPI source block and photoionization lamp are unmodified, except for the addition of screw holes to the bottom of the source block for mounting purposes.
The AP-ECD source is mounted to the mass spectrometer using the stripped-down frame of the instrument’s original Nanospray™ source. The position of the AP-ECD source relative to the inlet of the mass spectrometer’s atmosphere-vacuum interface is adjustable via a home-built translation stage attached to the rails of the frame; the source is fastened to the translation stage through the screw holes added to the source block.
1