(736f) Novel Sensor for Measuring Trace Impurities in Ultra Pure Hydrogen

Parts per trillion analysis of ultrapure gases is currently done by Atmospheric Pressure Ionization Mass Spectrometry, APIMS, which requires sophisticated laboratory instrumentation and a specialized operator. When measuring contamination in hydrogen, a chemically pumped dense metal heated palladium membrane can concentrate the impurities, by eliminating most of the hydrogen gas while retaining all of the contaminants, leaving a remainder that can be analyzed with an RGA. To illustrate the method, a one part per trillion contaminant species in an accumulated 5 standard liter of ultrapure hydrogen sample that ends up in the closed 1 liter Residual Gas Analyzer chamber represents a partial pressure of 5*760*E-12 or 3.8E-9 Torr.

Associated manipulations and data interpretation are simple enough for the creation of an online sensor device highly desirable for gas quality monitoring in semiconductor manufacturing or in hydrogen filling stations for cars in the future.

Hydrogen removal with a palladium based membrane is gradient driven and sub-milliTorr hydrogen sample pressures can be achieved by chemical pumping downstream of the membrane. This is achieved by introducing air downstream of the membrane which oxygen irreversibly reacts with hydrogen to form predominantly OH ions. The concentrated 7.5 cc sample is introduced in a 1 liter RGA chamber, priory evacuated by a turbopump and then isolated by a valve causing a very reproducible pressure rise. Filament initiated chemistry and outgassing are slow processes under these low pressure conditions and can easily be distinguished from the actual contribution of the introduced sample by recording the initial step change in the contaminating gas’ partial pressures. Trace contaminants in the UPH stream accumulate linearly with the sample time and are typically generated downstream of the hydrogen purifiers used in semiconductor manufacturing.  Palladium purifier contamination mechanisms were identified and remedied by design and treatment of components downstream the purifier’s membrane and are linked to the steel’s carbon content and oxidation.

Ample experimental data will be provided demonstrating linearity of accumulation, calibration, chemistry in the RGA chamber and ppt level sensitivity for contaminating species.