Fig. 5: Online mass spectrometry from a single Pd nanoparticle.

a Baseline-adjusted (BA) raw QMS counts for CO₂ (gray line) together with the mean CO₂ signal (green line - obtained as average measured BA-count across each pulse) for n = 1 Pd nanoparticle, measured across the entire α_CO range, α_CO ∈ (0, 1), and at 450 ^∘C. b Same as (a) but for an empty nanochannel, i.e., n = 0. Note the very similar and stochastic appearance of apparent CO₂ pulses both for n = 1 and 0 when analyzed in the standard way. c Mean BA-CO₂ counts extracted for each α_CO pulse based on the standard analysis across the entire α_CO range, α_CO ∈ (0, 1), and at 450 ^∘C, for both n = 0 (black lines) and 1 (green lines). For n = 1 we executed 7 consecutive α_CO sweeps, and for n = 0 we executed 5 consecutive α_CO sweeps. As key observation, we note high reproducibility of the α_CO sweeps for both n = 0 and 1, as well as that no clear activity trend as a function of α_CO is resolved for the n = 1 sample, despite a generally slightly higher number of counts compared the n = 0 reference. d Same as (a) but for DAE-denoised BA-QMS CO₂ counts (purple line). Note the significantly smaller but at the same time non-stochastic CO₂ pulses resolved by the DAE compared to the standard analysis. e Same as (d) but for an empty nanochannel, i.e., n = 0. Note that using the DAE, a flat baseline at zero BA-counts is obtained for the empty nanochannel. f Same as (c) but for DAE-denoised data, where distinct reaction rate maxima are resolved for n = 1 (purple curves), with \({\alpha }_{{{{\rm{CO}}}}}^{\max }\) ranging between 0.65 and 0.6 for temperatures between 450 and 410 ^∘C (inset), in good agreement with the earlier results for n = 10 and 1000. For lower temperatures, as well as for n =  0 (black lines representing multiple sweeps at 450 ^∘C), the DAE outputs counts  < 1, which is physically unreasonable and thus defined as the limit of detection. g \({\alpha }_{{{{\rm{CO}}}}}^{\max }\) values for n = 1000 (blue), 10 DAE-denoised (orange) and 1 DAE-denoised (purple) for 400 ^∘C≤ T ≤ 450 ^∘C, i.e., the range for which the n = 1 DAE-signal is above the limit of detection. h CO₂ counts normalized by the number of particles on the respective sample, obtained at \({\alpha }_{{{{\rm{CO}}}}}^{\max }\) for n = 1000 (blue), n = 10 DAE-denoised (orange) and 1 DAE-denoised (purple) for 400 ^∘C ≤ T ≤ 450 ^∘C. Source data are provided as a Source Data file.