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).
Mass spectrometry.
All experiments were performed on a Synapt high-definition mass spectrometer, a hybrid quadrupole ion-mobility TOF mass spectrometer
(Waters Corp.). Detailed descriptions of the instrument was found elsewhere (17, 18). A schematic diagram of the instrument
is shown in Figure 1a.
Figure 1a & 1b
|
The instrument was operated in positive-ion mode with a capillary voltage of 3.0 kV. The sampling cone voltage was set at
40 V. The ion source block and nitrogen desolvation gas temperatures were set to 100 °C and 250 °C, respectively. Nitrogen
was used as both the nebulizing gas and the desolvation gas. The desolvation gas was set to a flow rate of 250 L/h. The quadrupole
was operated in nonresolving mode to transmit a wide m/z range.
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.
|
