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tokens: sensitivity of COSIMA to D and H, as described in the next section. 2.2 Instrument mass fractionation The effect of instrument mass fractionation (hereafter IMF) varies from element to element. For sulphur isotopes, the effect was too small to be measured with COSIMA. For oxygen isotopes, the effect was a few per cent. Because D and H have the largest relative mass difference of any isotopes of any element, the effect of IMF is expected to be quite strong. This effect must be measured, using a suitable analogue for the cometary dust. The measurement was undertaken with the COSIMA reference model (RM) using samples of insoluble organic matter (IOM) from carbonaceous chondrites. Because no evidence for hydrated minerals has been seen in the cometary dust particles analysed by COSIMA (Bardyn et al. 2017 ) and because water is unlikely to remain in dust particles collected by COSIMA grains (as discussed above), it is expected that the H measured with COSIMA will be coming primarily from the high molecular weight organic matter of the sort discussed in Fray et al. ( 2016 ). Meteorite IOM is also macromolecular, with a complex structure. There is a similarity in SIMS response between the two substances, although the H/C ratio in the cometary matter tends to be higher than that in IOM (Fray et al. 2016 ; Isnard et al. 2019 ). Such organic matter has been estimated to constitute 45 per cent of the cometary dust particles by weight (Bardyn et al. 2017 ). Because of this, IOM seems to be the best available choice as an analogue material to measure IMF. IOM samples provided by the Carnegie Institution of Washington were subjected to TOF-SIMS analyses in the COSIMA RM. The D − /H − ratios measured for the various samples in COSIMA are plotted versus the D/H isotopic ratio measured at Carnegie by elemental analysis (Alexander et al. 2010 ) in Fig. 1(a) , while Fig. 1(b) shows the same data with a line that is forced to pass through the origin fitted to it. The inverse of the slope of that line gives the IMF for D/H in COSIMA – a factor of 3.19 ± 0.86. The uncertainty in this result – which is the dominant source of systematic error in the D/H isotopic ratio – is considered in the next section. Figure 1. Panel (a) shows the D − /H − ionic ratio measured in the COSIMA reference model for samples of insoluble organic matter (IOM) from a selection of chondrites provided by the Carnegie Institution plotted versus the D/H isotopic ratio measured in the lab at Carnegie. The difference between the two measurements is due to the COSIMA instrument mass fractionation (IMF). Panel (b) shows the same data, fit with a line forced to pass through the origin. The inverse of the slope of the line gives the IMF for the D/H ratio in COSIMA: a factor of 3.19. 2.3 The particle Jessica Lummene.2 The particle Jessica is different from the other particles in that a very long measurement was done upon it – over 12 h at each location for four locations, when for other particles