Experiment

Chemicals and reagents. Methanol (CH₃OH) and isopropanol (IPA) were purchased from Fisher Scientific (Pittsburgh, PA). Ultrapure water (18.2 M) was obtained from a Milli-Q water purifier (Millipore Corporation, Medford, MA). Ammonium acetate, cesium iodide and superbase, 1,8-diazabicyclo[5.4.0]-undec-7-ene (DBU), were purchased from Sigma (St. Louis, MO). PEG 4450 was obtained from Waters Corporation (Milford, MA). PEG-aldehyde 20K was purchased from NOF Corporation (Tokyo, Japan). All materials were used without additional purification.

Sample preparation. All polymer samples were prepared at analyte concentrations of 1–2 mg/mL (10^-4 M based on the nominal molecular mass) either in a 50/50 (v/v) solution of water and methanol or 10 mM ammonium acetate and methanol. The solutions were directly infused into the ESI source at 10 µL/min by a syringe pump (Harvard Apparatus, Holliston, MA).

Figure 1a & 1b

The instrument can be operated in two modes: TOF mode and TOF-mobility mode. For TOF-mobility experiments, the ion mobility cell pressure was maintained at 0.5 mbar (N₂) and separations were optimized by tuning the wave pulse height from 5 to 14 V. The transfer cell pressure was maintained at 0.02 mbar unless stated otherwise. Pressures in the trap and IMS cell were the direct readbacks from the active pirani gauges equipped with the instrument. The RF amplitude used on the trap, ion mobility cell, and transfer cell was 320 V peak-to-peak. Ion mobility cell traveling wave velocities of 240 m/s were used.

The instrument was externally mass calibrated with cesium iodide (1.0 mg/mL in 50% IPA–50% water). Data acquisition and processing were controlled by MassLynx 4.1 software (Waters Corp). Mass spectra were acquired every 1 s in TOF mode and every 2 s in TOF-mobility mode from m/z 100 to m/z 6000 for PEG 4450 samples and from m/z 500 to m/z 10,000 for PEG-aldehyde 20K for 6 min. Additional data processing to yield the information on the average molecular weight and the polydispersity of the PEGs followed the methods described below.