In photocatalytic water splitting, the hydrogen produced in a typical lab run sits at the micromole level. At that scale, a single manual sampling step can introduce more gas than the reaction itself produced in an hour. Online detection solves the problem by acquiring production data continuously, without opening the system, so the reaction is never disturbed.
Offline sampling transfers a fraction of headspace gas to a chromatograph. Small absolute errors, a few microliters of air ingress or a partially flushed syringe, translate into large relative errors when the sample itself is small. The result is scattered data that reflects the sampling process more than the catalyst.
An online micro-gas system sweeps the reactor headspace with a carrier stream and routes it to a detector, typically a thermal conductivity detector or electrochemical sensor for H2. Because the carrier flows continuously, each measurement reflects the gas produced in the preceding interval. Integrating the signal over time gives cumulative production; differentiating it gives the production rate.
Detection limit appropriate to micromole-per-hour production; a linear range wide enough to cover both the induction period and peak rate; response time short enough to resolve kinetic features; and baseline stability over multi-hour runs. Verify specifications with your expected matrix, not pure calibration gas alone, because moisture and CO2 influence some sensors.
Continuous rate curves reveal induction periods, deactivation and mass-transfer plateaus that single-point offline measurements hide. They also enable direct comparison between runs, because every data point shares the same measurement chain.
Perfectlight's Ugas series online micro-gas analysis systems are built for exactly this use case; see gas detection products and the water splitting reaction systems they pair with.
