Well-established procedures for the characterization of contamination during outgassing usually involve total mass measurements through quartz crystal microbalance (QCM). Recently, the addition of mass spectrometry (MS) measurements to these data has become more common. The combination of both high sensitivity QCM and MS data may lead to a better understanding of the physics taking place during outgassing contamination processes. The way to do so is to complement the basic measurements of total mass loss on QCMs by the identification of each species and the quantitative determination of each species contribution.In a first characterization step, the thermogravimetric analysis of contaminants deposited on QCMs allows a partial species separation that helps exploiting mass spectrometry data. In return, these data permit a finer species separation. The key to these measurements is to obtain sufficient signal to noise ratio in the mass spectrometer. Though outgassing of space materials is not done the same way in Europe (multi-temperature steps, ECSS-Q-TM-70-52A) and in the US (isothermal, ASTM E-1559-09), both tests could be used to perform a first species separation, as reported here. Most species outgassed by a few common materials were identified (and quantified) through TGA and MS coupling. As reported in a companion paper, the knowledge of these species' spectra then allows the analysis of the MS data during the initial outgassing phase, determining the quantitative outgassing of each species and leading to the improved comprehension of the physical laws ruling outgassing.

The prediction of contaminant levels is paramount to controlling and reducing their impact on space missions. In recent years, it has become clear that a real breakthrough could only be achieved through a change of paradigm, namely, by going beyond the classical characterization of total contaminant mass and instead characterizing the various emitted chemical species individually: both quantitatively and chemically. This paper first reviews the methodology proposed to achieve this objective and then its implementation on two examples of materials (Black Kapton® and NuSil CV4-2946) on the basis of existing ASTM-E-1559 outgassing data (Garrett, J. W., Glassford, P. M., and Steakley, J. M., “ASTM-E-1559 Method for Measuring Material Outgassing/Deposition Kinetics,” Journal of the IEST, Vol. 38, No. 1, 1995, pp. 19–28) including mass spectrometry (MS) data. We show that the thermogravimetric analysis performed on the contaminant deposits (heating at 1 K/min) allows a good enough time separation of chemical species to analyze and often identify them through their mass spectra. In turn, the knowledge of the fragments constituting their spectra allows an improved analysis of the MS data collected during the initial outgassing phase. The outgassing time profiles of each of these chemical species then tells a lot about their actual outgassing physical laws. On the two studied materials, outgassing physics were found to be consistent with Fickian or non-Fickian diffusion rather than with residence time desorption. After confirming these findings with more specific and more sensitive experiments, the door will be open to greatly improve assessments of the contaminant amounts and nature in flight through realistic multispecies physical models.

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A new approach to material outgassing modelling based on chemical species separation was applied to Bake-Out modelling. The outgassing of the Scotchweld EC-9323-2 epoxy glue was first characterized experimentally, both before and after Bake-Out. Based on the former, the latter was modelled, following the classical approach on the one hand and the new approach on the other hand. The MS-based species separation of this new approach allowed measuring the emitted flux of each species individually, even during the outgassing phase. The evolution of these fluxes all along the outgassing tests, during the five classical 24 h plateaus was successfully modelled by diffusion-limited outgassing laws, while it proved inconsistent with a desorption-limited outgassing. Their modelling by diffusion laws allowed a very consistent modelling of the total mass measurements in TML and CVCMs, with a small number of chemical species of different volatilities. The effect of a preliminary Bake-Out on this outgassing was finally modelled and compared to the experimental characterization of the baked material. The modelling accuracy still remains comparable to that of the traditional modelling based on mathematical species. Yet, the realistic species separation of the new approach allows simpler assumptions on water regain (its first species) than the traditional one, which needs to assess the amount of water contained in each mathematical species. Progress in the new method should come from an improved species separation and a direct characterisation of the water flux.

Molecular contamination can be detrimental to spacecraft life expectancy as it may induce changes to the properties of the surfaces on which it deposits. In order to control these potential issues, manufacturers need to assess in-flight contamination levels in the quite early design phases, possibly leading to design changes or extra material bake-out to fulfil requirements. The accuracy of model assessments may thus have direct impacts on missions and costs. Prediction of outgassing and deposition at mission timescale is done by extrapolating short-term on-ground experimental results, by fitting the latter with some empirical or physical laws. Common methods are the power law interpolation (in isothermal conditions) or the residence time approach (with thermally activated time constants), which leads to exponential decays in isothermal conditions. Although they allow fitting the measured outgassed flux over few-day-long experiments, their extrapolation to ten or fifteen-year-long missions is more challenging. Validating empirical laws might involve long-term empirical comparisons, while obviously the validation of physical models should start with checking their physics. In spite of many years trying, a basic difficulty was yet found to block significant progress on physical validation. As long as measurements are limited to the total mass on QCMs it looks possible to fit data as a sum of almost any law, such as residence time or diffusion models e.g. for outgassing. Following this observation, a conclusion was drawn that the only way to discriminate between two different physics was to characterize contamination at chemical species level. Such characterizations are a prerequisite to unravel the physics at play during several phenomena: outgassing, deposit films dynamics, photochemistry, environment/ATOX interactions, etc. This innovative approach, presented herein, aims at identifying outgassed contaminants by means of coupled thermogravimetric analysis and mass spectrometry. For this purpose, this paper reports the successful species separation and identification of contaminants outgassed from a material used in space applications: the epoxy adhesive Scotchweld EC-9323-2.