54 May / June 2019
ionRocket: An innovative Thermal Desorption and Pyrolysis Device for Direct Analysis in Real Time-Mass Spectrometry (DART-MS) for Rapid Characterisation of Difficult Samples
by Derek Gonzales, Mike Churchill PhD Biochromato USA, 10060 Carroll Canyon Rd. Suite 100, San Diego, CA 92131
Direct analysis in real time-mass spectrometry († DART-MS) enables rapid analysis of both solid and liquid samples under ambient (open laboratory) conditions without sample preparation. However, certain samples are difficult to analyse using DART-MS due to the varying volatility of substituents in complex samples. Furthermore, fibres and powders can be challenging to keep in place in the DART gas stream. To overcome these challenges, an innovative sample introduction device called ‘ionRocket’ has been developed, which gradually heats a sample placed directly beneath the DART gas stream. Gradient heating of the sample before ionisation creates time/temperature resolved mass spectra, separating species both by their thermal desorption profiles and m/z. Samples are placed in a small single-use copper vessel before heating. This both facilitates thermal transfer and also provides the option to weigh samples prior to analysis. Weighed samples yield reproducible mass spectra intensity, allowing calibration curves of analyte amount vs peak intensity to be calculated. The change in temperature by time separates compounds based on volatility thus providing an additional axis of data similar to a chromatogram, while maintaining minimal sample preparation.
Introduction:
Direct analysis in real time-mass spectrometry (DART-MS) enables nearly instantaneous determination of sample composition using mass spectrometry. Therefore, DART-MS is a powerful method for analysis of sample mixtures. For example, many polymers are difficult to volatilise using DART because of the thermal energy required to break chemical bonds into smaller fragments. Furthermore, the temperature at which compounds vaporise varies based on volatility. If the DART gas temperature is too low, samples will not vaporise efficiently. However, too much heat transfer risks decomposition of compounds and thus cannot be analysed. Samples containing complex matrices can be difficult to resolve desired peaks due to these differences in volatility between compounds. Analysis of oil additives is one example. To enable analysis of difficult samples by using DART-MS, an innovative device for DART-MS, called ‘ionRocket’ was developed, which induces thermal desorption and pyrolysis of samples [1-3]. The vapour phase of polymer samples is generated by applying a temperature-controlled heating gradient, then ionised and introduced into the mass spectrometer.
Liquid or solid samples are introduced just below the DART stream in a small copper vessel where they are heated directly using an electronic resistive heater (Figure 1a-c), in a controlled manner, from room temperature to 600°C typically at a rate of 100°C/min. Compounds are then vaporised and ionised using the DART source
Figure 1a. Diagram of ionRocket between DART ion source and Mass Spectrometer.
and analysed using mass spectrometry. Initially, as the temperature rises, more volatile compounds thermally desorb at relatively low temperatures from around 100-300°C, this can be defined as the thermal desorption region. Then, at higher temperatures around 400-600°C, pyrolysis products of polymers and other compounds can be detected with a specific thermal decomposition pattern based on the material composition, considered the pyrolysis region. Thus, the Thermal Desorption and Pyrolysis (TDP) device ‘ionRocket’ coupled with DART-MS (TDP/DART-MS) offers detailed information on the sample material composition in minutes without any sample preparation.
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