MW calculations

The method used to obtain the average MW of PEG was adapted from the method published by Bagal et al. (15). Briefly, the experimental data, after processing by MassLynx for smoothing and centering, were fit to a Gaussian function to determine the average MW and the polydispersity of PEGs, expressed as Equation 1 as follows:

f(x) =A exp(–0.5[(x – x _o)/ w]² )

Because full isotopic resolution at baseline was achieved for PEG 4450, the experimental data for PEG 4450 were first smoothed using the Savitzky Golay filter from MassLynx and then centered. The most abundant isotopic cluster peaks (¹³C2) were selected and used to fit the Gaussian function. The final value that corresponds to the average molecular weight was subtracted off two ¹³C to compensate the extra mass account included in the curve-fitting process. For larger MW PEGs, the isotope peaks are either partially or not resolved. In this case, spectra for large PEGs are smoothed using a wavelet thresholding to generate only one centered mass value for each PEG oligomer at each charge state. These mass values were used for the curve-fitting process. The processed data from MassLynx were exported into the data analysis and graphing software OriginPro8 (OriginLab Corporation, Northampton, MA) for plotting and performing Gaussian curve-fitting.

Ion–molecule reactions

The ion–molecule reactions were performed in the trap cell of the Triwave in the mass spectrometer (see Figure 1a). To perform the ion–molecule reactions, some minor modifications to the gas pipelines of Synapt were made so that neutral reagent vapor could be introduced into the trap cell. However, all of the modifications are external to the mass spectrometer. A schematic diagram of the mechanical assembly surrounding the manifold is shown in Figure 1b.

The superbase reagent (liquid) was placed in a custom-made glass tube (6.35 mm [0.25 in. o.d.] x 3.81 mm [i.d.] x 152.4 mm (6-in. length)], which was connected to a needle valve (Catalog No. 6060795, Waters Corp.) through a union connector (Catalog No. SS-400-6-2, Swagelok ). The needle valve was then connected with stainless steel tubing (0.125 in.) to a two-way switching valve (Catalog No. SS-41GS2, SV1, Swagelok), which was linked to a three-way union using the same type of stainless steel tubing. One arm of the three-way union was connected to the trap gas inlet of the analyzer housing, and the third arm was extended to the existing argon collision-gas pipeline of the instrument by means of the second two-way switching valve (SV2, see Figure 1b). The glass tube and the rest of the vapor flow path were heated to 100–150 °C using heating tape (Catalog No. 14-488-28, Fisher Scientific) to ensure rapid evaporation of DBU and to keep the superbase vapor from condensing back to liquid.

The incorporation of the two two-way switching valves in the gas pipelines allows the instrument setup to be readily switched between the standard configuration and a configuration for introducing reagents of interest into the trap cell. An appropriate flow of superbase vapor into the trap was obtained by adjusting the needle valve at a chosen temperature. In a typical charge-reduction operation, both the superbase vapor and argon gas were simultaneously introduced into the trap cell, and the concentration of the reagent introduced into the trap was controlled by changing the argon gas flow rate through a software-controlled flow controller. Under the experimental conditions where only reagent vapor is desired in the trap cell, the two-way switching valve (SV2) was completely shut off. On the basis of the relative flow rates of the superbase vapor and argon gas, the typical pressure of trap cell during the experiments was maintained at 0.008–0.05 mbar (uncorrected, as measured at the vacuum housing ion pirani gauge). After experiments were completed each day, the Triwave section was isolated from the superbase gas line (by switching off the SV1 and turning on SV2) and placed back to the standard configuration.