Pyrolysis-Gas Chromatography-High Resolution Mass Spectrometry with Soft Ionization for Increased Confidence of Polymer Characterization

This application note highlights a comparison between classical pyrolysis-GC with EI-Tandem Quadrupole MS and pyrolysis-APGC coupled to a QToF MS. The analytical advantages of soft ionization and high resolution accurate mass acquisition were explored for the analysis of polymer standards and biobased plastic bags. Additionally, software tools to assist putative identification of compounds are highlighted. This instrument configuration demonstrates an effective tool for the characterization of complex post consumer, industrial or biobased polymeric materials.

Benefits

• Pyrolysis-APGC-QToF MS is an effective analytical tool for laboratories focusing on compositional analysis of complex polymer materials using gas phase separation
• Soft ionization using APGC allows for molecular ion detection to assist with the confirmation of chemical elemental composition, structural elucidation, and ultimately, for compound identification
• The soft ionization can be achieved in a more time efficient workflow than is possible with traditional vacuum GC-MS instruments
• The MS^E data acquisition mode allows for the collection of both the accurate mass of precursor and fragment ions, key for the structural elucidation of unknowns within a sample

Scientific interest is growing in sustainable polymers and natural materials to increase recycling and develop biobased plastics to replace single use oil-based plastics. Polymers derived from natural sources, such as feedstocks, however, are complex and require characterization prior to use. Furthermore, increased use of post-consumer recyclates and biobased plastics may lead to an increase in regulations around the presence of additives and potential contaminants. This could potentially require more in-depth evaluation of the chemical make-up of the polymer formulations.¹ 

In the field of polymer research, pyrolysis coupled to gas chromatography-mass spectrometry (GC-MS) has been extensively used and is becoming increasingly prominent² for samples that are not amenable to solubilization.³

However, the high energy of electron ionization (EI) leads to insufficient sensitivity and selectivity for undertaking the characterization of polymers and the identification of additives and potential contaminants by this technique. Pyrolysis-GC with soft ionization high resolution mass spectrometry (HRMS) is a complementary technique used in this field to help address some of these limitations. Atmospheric pressure gas chromatography (APGC) is an ionization technique similar to atmospheric pressure chemical ionization, which uses a corona discharge, enabling softer ionization. This results in molecular ion detection which can help with the confirmation of a molecular formula for identification. APGC can be coupled to a quadrupole time-of-flight mass spectrometer (QToF MS) which can acquire data in MS^E mode, whereby both low and high collision energy spectra are simultaneously acquired. Using this technique, the accurate mass of both precursor and fragment ions are available to provide information for structural elucidation and ultimately aid compound identification.⁴ 

In the experiments described, a comparison between classical pyrolysis-GC with EI-Tandem Quadrupole MS and pyrolysis-APGC coupled to a QToF MS was performed. The analytical advantages of soft ionization and accurate mass acquisition were explored. Additionally, MassFragment, a software tool to assist putative identification of compounds, is highlighted.

Sample Description

A selection of polymer standards and commercially available biobased plastic bags (BIOTEC GmbH & Co. KG) were weighed to approximately 0.1 mg and loaded into glass capillaries between two plugs of quartz wool and placed into the pyrolyzer autosampler. All samples were acquired in triplicate on two instruments: Xevo™ TQ-GC with an EI source and Xevo G2-XS QTof with an APGC™ source.

Data Management

Data acquisition, processing and reporting were performed using Waters MassLynx™ 4.2. Software. Putative identifications were made using MassFragment.

The total ion pyrograms from the polymer standards acquired with the two different instruments were compared. From an initial appraisal, it was clear that the py-APGC-QToF MS technique was more information rich. This can be seen here in the case of the polystyrene standard shown in Figure 1. The region highlighted from 12 to 17.5 minutes shows the presence of a greater number of peaks. There are differences in retention times between the two data sets due to the presence of a vacuum at the outlet of the column on the TQ-GC, whereas the APGC source is at atmospheric pressure.

Soft ionization is critical when undertaking characterization due to the presence of the molecular ions. On traditional vacuum GC-MS instruments, soft ionization is normally achieved by chemical ionization (CI) in positive or negative ionization mode. This often requires venting the MS, exchanging the ion source with a dedicated CI source followed by conditioning of the source, and using additional chemical reagent gases, which all add to the analysis cost, time, and complexity of the analysis. With APGC, soft ionization can be achieved without these steps, providing a more time efficient workflow.

Interrogation of the peaks within the pyrograms of the two techniques highlighted more benefits of using APGC. In the case of the polystyrene standard, the mass spectrum was investigated for the presence of the masses that should typically be detected by pyrolysis-GC-MS.¹ Whilst the styrene monomer, dimer, and trimer could be detected via both ionization techniques, the tetramer at m/z 416.2504 could only be detected via APGC (Figure 2) due to the reduced ionization-induced fragmentation.

For some compounds, the use of the softer ionization of APGC results in the presence of the molecular ion, which is critical when detection and confirmation of the molecular ion is required. This is demonstrated here for the polymer, polycaproamide (Nylon 6). In Figure 3, a compound identified at m/z 194.2374, (assigned as N-(5-cyanopentyl)pent-4-enamide, C₁₁H₁₈ON₂)² was found in the APGC pyrogram with the protonated molecular ion present. The corresponding chromatographic peak in the EI pyrogram had the appropriate mass fragments but without the presence of the molecular ion. 

Another advantage of the APGC-QToF MS is the use of MS^E where high and low collision energies are alternated so that the accurate mass of both precursor and fragments ion can be simultaneously acquired. This assists with the structural elucidation of unknowns. For the compound identified at m/z 194.2374, Figure 4 demonstrates the high and low energy spectra for the peak, showing increased fragmentation with the higher collision energy. 

Identification

Using APGC-QToF MS for polymers, soft ionization results in detection of higher masses and gives rise to the presence of the molecular ion which assists with elemental composition prediction. Additionally, if MS^E is implemented, the accurate mass of both the precursor and fragment ions are acquired. Combining these tools is critical for the elucidation of unknowns where compounds cannot be found in a library. Commercially available bioplastic bags (BIOTEC GmbH & Co. KG) were analyzed. An example pyrogram for a plastic bag sample (Bioplast 500) is shown in Figure 5 using the using the APGC-Xevo G2-XS QTof with MS^E.

The high and low energy mass spectrum were generated for the peak at retention time 19.62 minutes (Figure 6). An elemental composition was predicted using the accurate mass of the molecular ion, and the peak was putatively assigned as erucamide. MassFragment, a software tool to assist identification of compounds, was employed directly from the spectra. The *mol file for this compound (obtained from Chemspider) was searched against the accurate mass fragments of the high energy spectra with many matches made, increasing confidence in this identification. MassFragment assigns the proposed *.mol chemical structure of erucamide to the measured fragment ions by applying novel algorithms to known precursor structures.⁵ This approach is based on systematic bond disconnection of the precursor structure instead of the traditional rule-based approach. For a 100% match a reference sample would be needed, however, this tool saves analyst’s time in the elucidation process.

The results obtained in the experiments described in this application note demonstrate that pyrolysis-APGC-QToF MS is an effective analytical tool for laboratories focusing on compositional analysis of complex polymer samples using gas phase separations and for materials that are not amenable to solubilization.

Typical py-GC-EI-MS is challenged by insufficient sensitivity and selectivity to undertake characterization of polymers and the identification of additives and potential contaminants. APGC uses soft ionization reducing fragmentation and promoting the presence of the molecular ion assisting with elemental composition prediction. This technique eliminates the need for time consuming chemical ionization that is usually employed by dedicated vacuum GC-MS systems, a necessary step that would be needed to confirm the molecular ions of fragmented electron ionization compounds. This will require instrument venting, source replacement and conditioning, and optimization of reagent gas, all adding to the time and cost of the analysis.

Further to this, MS^E data acquisition allows for the collection of both the accurate mass of precursor and fragment ions which is key for the structural elucidation of unknowns within a sample. Together this technology offers a unique solution for increasing confidence in the data for the characterization of unknowns in post consumer, industrial or biobased polymeric materials.

1. Welle F., Franz R. Recycling of Post-Consumer Packaging Materials into New Food Packaging Applications—Critical Review of the European Approach and Future Perspectives. Sustainability 2022;14, 824.
2. Tsuge S., Ohtani H., Watanabe C. Pyrolysis-GC/MS Data Book of Synthetic Polymers. 2011.
3. Peacock P. M., McEwen C. N. Mass Spectrometry of Synthetic Polymers. Anal. Chem. 2006;78(12): 3957–3964.
4. Stevens DM, Cabovska B, Bailey AE. Detection and Identification of Extractable Compounds from Polymers. Waters Application Note 720004211, 2012.
5. Hill AW., Mortshire-Smith RJ. Automated Assignment of High-Resolution Collisionally Activated Dissociation Mass Spectra Using a Systematic Bond Disconnection Approach. Rapid Commun. Mass Spectrom. 2005,19(21): 3111–3118.