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55


Traditional pyrolysis-gas chromatography/mass spectrometry (Py- GC/MS) obviates the need for compatible solvents, but frequently depends on the searching of spectra from py-GC/MS databases for identification of unknowns. Molecular weight distribution data can also be lost, since only pyrolysis products are seen. (TDP/DART)- MS does not require dissolution or other preparation of the sample prior to introduction to the Mass Spectrometer. It also permits the individual measurement of both low molecular weight pyrolysis products and thermally desorbed (but not pyrolysed) products, as well as potential for molecular weight distribution analysis. This combination provides valuable information for elucidating the chemical structure of unknown compounds, including polymers with higher dispersity and mixtures containing reactants and degradants. Both TDP/DART-MS and MALDI-MS can also be used for examining terminal functional groups in unknown polymer samples.


As a comparison with other analytical techniques for polymers, we outline 3 methods, namely 1) py-GC/MS, 2) MALDI-MS, and 3) TDP/ DART-MS, for analysis of the homopolymer Poly (butyl acrylate), PBA. In this work, the resulting data from these analytical approaches are compared by looking at the repeating unit, molecular weight distribution, and terminal functional groups for a sample of PBA.


Results and Discussion Nylon-6


Figure 1b. ionRocket coupled with DART placed directly on the Mass Spectrometer front end.


As an illustration of analysis using TDP/DART-MS, a small sample of nylon-6 was placed in the copper sample vessel. The sample was heated from room temperature to 600°C at a rate of 100°C/min. and the total ion current (TIC) was measured over time.


Figure 1c. Top view of the copper vessle ‘Sample Pot’ inserted into ionRocket heater between the DART source and MS front end.


Application challenges


Lubricating oil is composed of base oil and additives. Synthetic base oils and various functional additives (e.g. antioxidants, detergent- dispersants, etc.) are used in the development of lubricants for machinery, automobiles, and other industries. Thus, it is important to obtain detailed information on base oils and additives, for market research, research and development, and quality control. In order to analyse the base oils and additives of lubricating oils, in traditional analysis, complicated pretreatment may be required, which takes time and effort. Thermal Desorption and Pyrolysis combined with Direct Analysis in Real Time- Mass Spectrometry (TDP/DART-MS), is a useful method for qualitative and quantitative analysis of lubricating oil.


Polymers and resins (e.g. Nylon-6) present a unique challenge for many mass spectrometry systems. MALDI permits the direct characterisation of repeating units, but requires identifying a solvent/ matrix system which allows both the polymer and the matrix to first dissolve and then subsequently co-crystallise. Furthermore, polymers with higher polydispersity (Mw / Mn > 1.2) can be extremely difficult to analyse, due to both MS ionisation and instrumentation limitations.


Figure 2.


Mass spectra was separated by time (temperature). The additives Dibutyl Phthalate (DBP), Trimethylolpropane, and Tris (2-chloropropyl) phosphate could be detected at relatively lower temperatures (~200- 300°C) in the thermal desorption range. Then, as the temperature increased, polymer fragments of nylon could be detected at higher temperatures, along with a specific pyrolysis pattern of nylon-6 (Figure 2).


Determination of Polymers


Polymer materials can quickly be identified using TDP/DART-MS. As an example of this, various polymer samples were separately placed in the copper vessel, where they were heated from room temperature to 600°C at a rate of 100°C/min.


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