Quantitative Characterization of Polymer Blends by Pyrolysis-Gas Chromatography-Time of Flight Mass Spectrometry (Pyr-GC-TOFMS)

Pyrolysis GC–MS is a valuable tool for the material scientist. It is used principally for qualitative identification of polymers and their breakdown products. This application note displays how this technique can also be used for quantitative determination of polymer compositions by comparing peak area ratios of monomers and/or primary pyrolysis products. This application note highlights the use of Pyrolysis-GC-TOFMS to quantitatively determine the concentration of polyethylene (PE) in polyethylene/polypropylene (PP) copolymer blends.

A CDS 5150 Pyroprobe was coupled to a LECO TruTOF HT GC-TOFMS system.

Experimental

The samples were prepared by cutting a portion of pelletized polymer using a razor blade. Sample weights of approximately 0.5 mg were utilized for these analyses. The sample was placed inside quartz pyrolysis tubes with quartz wool to keep the sample position consistent within the tubes. Consistent sample size is required to achieve reproducible quantitative pyrolysis results. Samples were analyzed under the following system conditions:

Pyrolyzer: CDS 5150 Pyroprobe

Interface/Transfer Line Temp: 250 °C

Probe Temp: 0 °C to 750 °C @ 10 °C/mS, hold 10 s

GC: Agilent 7890

Column: Rtx-5, 30 m X 0.25 mm X 0.25 °m

Injection: Split 300:1 at 250 °C

Carrier: helium at 1.0 ml/min, constant flow

Oven: 40 °C hold 2 min, 20 °C/min to 240 °C

MS: LECO TruTOF HT

Saved mass range: 45-650 m/z

Acquisition rate: 20 spectra/s

Source temperature: 250 °C

Transfer Line: 250 °C

Samples

A calibration curve was generated using PP standards containing known levels of PE copolymer.

Results and Conclusions

The ratio of two primary pyrolysis products that are unique to PE and PP were used to generate a calibration curve. The 2-methyl-1-heptene and 2,4-dimethyl-1-heptene are pyrolysis products of PE and PP, respectively. Figure 1 shows an extracted ion chromatogram (EIC) displaying the quantitation masses for these analytes. The peak area ratios for these analytes were plotted against the known concentration of PE in the copolymer blends. The calibration curve is shown in Figure 2. The correlation of determination (r² ) for this calibration was 0.9938. Once a calibration curve is generated, the equation for the slope of the resulting curve can be used to quantitate PE in PE/PP copolymers that have unknown concentrations.

Figure 1: EIC showing the quantitation masses for 2-methyl-1-heptene (m/z 56) and 2,4-dimethyl-1-heptene (m/z 70). The Peak True (deconvoluted) and library match spectra are also shown.

TOFMS is well suited for analysis of complex pyrograms, as it acquires non-skewed full mass range spectra without sacrificing speed or sensitivity. This allows mass spectral deconvolution algorithms to perform optimally, and significantly reduce the data interpretation time required of the analyst.

Figure 2: Calibration curve plotting the ratio of characteristic ions of 2-methyl-1-heptene and 2,4-dimethyl-1-heptene (pyrolysis products of PE and PP, respectively) against the concentration of PE present in PE/PP copolymer blends.

The data generated for this application note was provided by William Gorman of Firestone Building Products Company.

LECO Corporation

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