Residual Gas Analysis (RGA) for Vacuum Leak Detection

In vacuum processes a total pressure gauge only reflects the overall pressure change; it cannot separate background outgassing from a genuine leak, nor answer the question of which gas is getting in. A residual gas analyzer (RGA) ionizes the gas and separates it by mass, turning the chamber content into a partial-pressure spectrum, so maintenance teams move from asking whether the pressure rose to asking which species rose. This information is critical for locating leak paths, evaluating seal materials and tracing process contamination.

A typical RGA uses a quadrupole mass analyzer. Gas molecules are ionized by a filament, separated in a quadrupole field by mass-to-charge ratio, and scanned point by point by a detector to produce a partial pressure for each mass number. Water (18), nitrogen (28), oxygen (32), carbon dioxide (44) and hydrocarbon fragments usually make up the background spectrum; when a specific mass peak rises in step with equipment actions, it usually means the corresponding gas is entering from outside or being released by the process. Comparing the current spectrum with a stored baseline is the first step in diagnosis.

In leak detection, the most common RGA method is the tracer-gas spray technique. After recording a stable baseline, the operator slowly sprays a tracer gas along flanges, welds, valve seats and other suspected leak sites while watching the target mass peak. Common tracer gases include helium (mass 4), hydrogen (mass 2) and argon (mass 40). When the spray reaches a real leak, the peak rises clearly and reproducibly; the response time and amplitude also give a rough indication of the leak-rate order of magnitude.

The key advantage of RGA-based leak checking is selectivity: it can distinguish a real leak from a virtual one. Common background sources—water-vapor outgassing, elastomer seal outgassing or gas trapped in dead volumes—raise the total pressure as well, but they do not make the tracer's characteristic peak rise in step. By locking onto a tracer mass peak instead of total pressure, false positives drop significantly and teams avoid repeatedly disassembling hardware for what is only outgassing.

When choosing a tracer gas, mass interference and process safety both matter. Mass 28 corresponds to both nitrogen and carbon monoxide, and mass 44 to both carbon dioxide and nitrous-oxide fragments, so spectra must be read with context; an atmosphere containing hydrogen also needs compatibility checks. RGA is well suited to locating mid-range leaks and diagnosing gas composition, while very fine leaks should still be confirmed quantitatively with a helium mass-spectrometer leak detector—the two tools complement each other.

A repeatable leak-check workflow often delivers more than buying a more expensive instrument: run a rate-of-rise test to size the overall leak level, use RGA with segmented spraying to localize the leak, then re-test to confirm the repair. Teams should also archive the baseline spectra, events and conclusions of each check so the records form traceable trend data for preventive maintenance.

HNR (HaiNa Precision) offers RGA residual gas analyzers, vacuum gauges and gas mixing systems, covering everything from total-pressure monitoring to composition diagnostics for vacuum coating, semiconductor and photovoltaic processes. For RGA selection, leak-check procedures or integration with an existing gas train, please contact HNR for engineering support.