INTRODUCTION

The characterization of polymers provides a demanding analytical challenge, due to the potential presence of a large number of polymeric distributions within a single sample. Key areas of interest include understanding the various end groups present, and the backbone architecture. Previous work1 using UHPLC and HR/MS demonstrated the importance of coupling a compositional separation with mass spectrometry, due to the complexity of typical samples that can contain isobaric, or even isomeric, species that are not easily differentiated by mass spectrometry alone.

Andrew J. Hoteling1, Eleanor Riches2, 1Bausch + Lomb, Rochester, NY; 2Waters Corporation, Manchester, UK

METHODS Sample Preparation

PDMS samples from different lots (Lot A and Lot B) were supplied by Bausch + Lomb (Rochester, NY)

PDMS solutions were prepared at a concentration of 0.03 mg/mL in 2:1 IPA:0.01 M aqueous ammonium acetate for analysis by infusion-(IM)-MS

PDMS solutions were prepared at a concentration of 1.0 mg/mL in IPA for analysis by LC-(IM)-MS

MS Conditions

MS system: SYNAPT G2-S HDMS Ionization mode: ESI+ Analyser: High Resolution mode (40K resolution) Capillary voltage: 3.0 kV Source temp: 120 oC Desolvation temp: 300 oC Sampling cone: 50.0 V Mass range: m/z 100 – 5000 Wave velocity: 1000 m/s LockSpray: Leucine enkephalin (m/z 556.2771) LC Information

Mobile phase A: aqueous 10 mM ammonium acetate Mobile phase B: THF:IPA 1:1 (v/v) Column: ACQUITY BEH C18, 2.1 x 150 mm, 1.7 m

Data were acquired using MassLynx v4.1, and processed using

MassLynx v4.1 and DriftScope v2.5.

CONCLUSIONS Infusion-ToF MS shows that the functional PDMS samples

are too complex for simple analysis without separation prior to mass analysis.

Infusion-IM-ToF MS utilizes the separation power of ion mobility. This enabled some degree of isolation and simplification of the complex data. Clear patterns and relationships were easily seen.

LC-MS provides an alternative, orthogonal mode of separation compared with IM. Separation by retention time employs different properties of the components species from those that drive separation in IM. Although the reverse phase LC separation provides a high degree of separation of different oligomer end-group series, there are regions of the chromatograms where there is series overlap and peak co-elution.

LC-IM-ToF MS analysis proved the most powerful approach for more thorough separation and characterization of complex polymeric samples. Different underlying series that were not necessarily clearly resolved with other combinations of techniques were visualized and isolated. The combination of the two dimensions of separation enhanced the visualization of the various oligomer series within the samples. In addition, ions from in-source fragmentation are separated from molecular ions in the IM dimension, but are aligned in the LC dimension, enabling for further structure understanding.

In this work, we evaluate the additional, complementary information obtained from ion mobility mass spectrometry (IM-MS) with electrospray ionization (ESI), both with sample infusion alone and when coupled to liquid chromatography (LC). Polydimethyl siloxane (PDMS) samples from different batches were used as the analytes. Figure 1 shows a schematic of the SYNAPT HDMS ion mobility instrument used in this work. The ion mobility region comprises three consecutive travelling wave sections, known as a TriWave. When analyzed using ion mobility, ions are separated according to their size, shape, and charge state. RESULTS & DISCUSSION

The PDMS samples comprised polymeric dimethylsiloxane species with a monofunctional methacrylamide end group, as first discussed in previous work,1 the general structure of which is shown in Figure 2(a). In addition, an unwanted PDMS species with difunctional methacrylamide end groups was also suspected to exist (Figure 2(b)). Sample complexity can arise from a range of structural features, such as branching or cyclization, fragmentation to form related oligomers, or the presence of different end groups. The ability to deconvolute and understand the species contained within such polymer samples is of vital importance in understanding the polymer’s key properties. A simple 2-minute infusion acquisition, with Time-of-Flight mass spectrometry (ToF MS) analysis, clearly illustrates the level of complexity encountered within these samples.

Figure 3 shows the zoomed mass spectral region over the range m/z 800—2000, with the magnified inset showing detail over the range m/z 1200—1380. Many overlapping related (or possibly unrelated) series can be seen in these regions. The oligomer series with the expected end-groups is labelled as (a) with a spacing of 74 Da between peaks. Note that the series shows a pattern of large-small-small-large, etc., which is indicative of the synthetic reaction used to form the PDMS polymer (i.e., ring opening of D3, a cyclic compound consisting of three dimethylsiloxane units).

Acknowledgements Thanks go to Jennifer Hunt and Ivan Nunez at Bausch + Lomb, Rochester, NY, for providing all PDMS samples for this work. References 1. Hoteling, A. J.; Papagelis, P. T.; Structural Characterization of Silicone Polymers using Compositional UHPLC Separation, Electrospray Ionization, Accurate Mass, and Tandem MS. Proc. 60th ASMS Conf. Mass Spectrom. Allied Topics 2012.

A similar process was carried out to investigate Region B and Region C in more detail. Figures 7 and 8 show the results from Regions B and C respectively. Region B was found to be a doubly charged series related to the primary polymeric material.

The final analysis of the PDMS samples was by LC-IM-ToF MS. Figure 10 shows the mobilograms obtained from these analyses. Here Drift Time is plotted against Retention Time. The spots seen running in curves across the middle of each mobilogram correspond to chromatographic peaks, and the vertical stacks of dots correspond to related fragments under each retention time peak. Based on their drift times, related series of chromatographic peaks can be selected (along the diagonal curves), or related series of precursor and fragments ions can be selected (along the vertical stacks). These selections are shown in Figure 11.

Figure 6 shows Region A isolated from the bulk data. The mass difference for the dimethylsiloxane monomer is clearly seen, however the accurate masses do not match the expected polymer structure. Note that the oligomer series observed in Figure 5 does not have the large-small-small-large, etc. expected pattern of peak intensity. This different pattern suggests that these are fragment ions. The m/z values of this series are consistent with a fragment sequence related to the dimethylacrylamide end-group of the expected polymer.

Observations from previous ion mobility analyses indicate that related species lie along similar drift time diagonals within the mobilogram. Focusing on the more complex Lot B, this suggests that the three regions highlighted in Figure 5 contain some key differences, because they occupy different drift time diagonals. By using drift time as a guide, different regions were selected, isolated, and investigated further.

Figure 1. A schematic of the SYNAPT® G2-S HDMS™ instrument, with travelling wave ion mobility functionality.

Time (min)

%A

%B

Flow Rate (mL/min)

0.00 50 50 0.2

30.0 5 95 0.2

35.0 0 100 0.2

38.1 50 50 0.2

45.0 50 50 0.2

38.0 0 100 0.2

Ion mobility data was viewed in plots showing m/z, ion intensity, retention time (when chromatography was used), along with drift time. The drift plot is sometimes called a “mobilogram” (Figure 4). The mobilograms in Figure 4 show further 2-minute infusion runs of PDMS samples, Lot A and Lot B, this time with ion mobility (IM) separation included (IM-ToF MS). The mobilogram for Lot B looked more complex than that for Lot A.

Figure 6. Region A is isolated based on its drift time pattern. The ion intensity pattern indicates that these are fragment ions related to the expected polymer.

Region A isolated and extracted from the bulk data

74

74 7474 74

Region B isolated and extracted from the bulk data

Doublycharged

3737

37

Figure 7. Region B is isolated based on its drift time pattern. Examination of the ion clusters and the mass differences shows it to be a doubly charged series resulting from the higher mass species of the primary polymeric material.

EVALUATING THE USE OF ION MOBILITY-MASS SPECTROMETRY FOR POLYMER CHARACTERIZATION USING POLYDIMETHYLSILOXANE AS A MODEL

Figure 2. (a) The general structure of the primary polymeric material in the PDMS samples, with a monofunctional methacrylamide end group. (b) The structure of an additional polymeric sub-series in the PDMS samples. The repeat unit nominal mass of the dimethylsiloxane monomer is 74.

(a)

(b)

Figure 8. Region C is isolated based on its drift time pattern. Complex detail related to both series (a) and series (b) is revealed, along with the repeat mass difference for the dimethylsiloxane monomer.

Region C isolated and extracted from the bulk data

7474 74

Protonated(b) series

Protonated(a) series

LC-MS offers a contrasting mode of separation compared with IM-ToF MS. By incorporating chromatography, different species in these complex samples were separated according to their retention times. Figure 9 shows the LC-MS analysis of Lot A and Lot B. More peaks were observed in the chromatogram for Lot B compared with Lot A, particularly from 25 minutes onwards — again indicating the greater complexity of Lot B.

Figure 4. Mobilograms showing infusion-IM-ToF MS data for PDMS samples, Lot A and Lot B. Drift Time is shown on the y-axis and mobility separation is observed in the vertical direction.

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LOT A

LOT B

Figure 9. LC-MS analysis of Lot A and Lot B. Lot B was, again, seen to be more complex than Lot A.

By contrast, Region C was found to be more complex, containing ion clusters from both a polymeric series related to structure (a) and a series related to structure (b) (Figure 2). The monomer mass difference of dimethylsiloxane, mass of 74, was also observed in Figure 8.

Figure 5. Ion mobility separation of an infusion acquisition of PDMS sample Lot B. Three key regions of interest, A, B, and C, are highlighted.

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A

C

B

LOT B

Sodiated (a)series ions

74 74 74

Sodiated (b)series ions

Related fragment ionsRelated (b) series ions

7474

74

A B

Figure 11. Region X and Region Y isolated in PDMS sample Lot B analysed using LC-IM-ToF MS. Region X is a series of sodiated ions related to the difunctional structure (b) in Figure 2. Region Y is a sodiated cluster of precursor ions related to the monofunctional structure (a) in Figure 2, and the associated series of fragment ions.

Figure 3. Magnified mass spectral regions for PDMS sample Lot B, analysed by infusion-ToF MS, illustrating a high degree of sample complexity.

(a)

(a)(a)

(a)

(a)(a)

[(a)+H]+ [(a)+NH4]+

[(a)+Na]+

n = 12Figure 10. Mobilograms for Lot A and Lot B analysed using LC-IM-ToF MS. Drift time is plotted on the y-axis and Retention Time is plotted on the x-axis.

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LOT A

LOT B X

Y