Tools · Reference

RGA Spectrum Reading Abacus

Every peak, every signature, every ratio. What a residual gas spectrum is telling you, and what to do about it.

Part 1 — Mass table

What every peak means, in an accelerator UHV context. Click a row to expand. Click a header to sort. Filter by mass, ion, or text.

Part 2 — Diagnostic signatures

A single peak is never enough. The pattern makes the diagnosis. Click a row to expand.

2.1 — The 12 key signatures

2.2 — Fluorine decision tree: "I see M69, what is it?"

Part 3 — Diagnostic ratios

The numbers that decide.

3.1 — Air leak vs outgassing

M32 + M40 in atmospheric ratio = leak. M28 without M32 and M40 = CO (normal outgassing). Edge case: scroll-pump backdiffusion shows M32 but O₂/N₂ < 0.20.

3.2 — Separating N₂ from CO at M28

M14/M28 > 5% = significant N₂ (leak). M12/M28 > 2% = significant CO. In a clean baked system with no leak, M28 is essentially 100% CO.

3.3 — HC vs natural isotope at M29

M29 is always present (~1.3% of M28 = ¹³CO + C¹⁸O). Only a problem if >> 1.3%. Confirm HC with M15 and M43.

3.4 — PFPE ratios (Fomblin)

M69 rising over days/weeks = active backstreaming (turbo bearings failing). M69 constant = legacy contamination.

3.5 — PDMS ratios (silicone)

3.6 — Methanol vs O₂ at M32

M32 = O₂ is the default assumption. After cleaning with methanol, M32 can be methanol parent. If M31 is strong and M40 absent: methanol, not a leak.

Part 4 — Rapid decisions

4.1 — "Is the spectrum clean?" 7-point checklist

4.2 — "Pressure is too high" 60-second triage

4.3 — HC envelope: 14 AMU pattern (CₙH₂ₙ₊₁ series)

Each 14 AMU apart (CH₂ unit). The further the envelope extends to high mass, the heavier the source (oil > solvent > fingerprint). Second series CₙH₂ₙ₋₁: M27, M41, M55, M69, M83. Note: M69 in this HC series is C₅H₉⁺, not CF₃⁺.

Part 6 — Reference spectra

6.1 — Clean baked system (target)

 
H₂ towers over everything. M28 is 10-100x lower than M2, and it is CO (verified by absence of M32/M40). M32, M40, M69 absent. Everything above M47 at the noise floor. This is what GO looks like.
 
6.2 — Contaminated system (common problems)
 
 
M18 dominant = water. M32 + M40 = air leak. M69 = fluorine. HC peaks beyond M47 = contamination. M2 not dominant = dirty. Fix in order: leak first, then bake (water), then contamination.
 
 
 
Part 7 — Cracking patterns
 
Fragment fingerprints at 70 eV electron impact. Fixed by physics, independent of RGA brand.
 
 
7.9 — RGA sensitivity factors
 
 
True partial pressure = displayed PP / sensitivity factor. H₂ is always underrepresented (reads 0.44x), CO₂ overrepresented. Most RGA software does not correct automatically.
 
 
 
Part 8 — Advanced diagnostics
 
8.1 — Quantitative N₂/CO separation
 
f(N₂) = (R − 0.009) / 0.063, where R = M14/M28 measured.
 
 
8.2 — ESD artifact protocol
 
 
Degas protocol: run filament degas at high emission (5-10 mA) 30-60 min, wait 10 min, rescan. Peaks that decreased = ESD artifact (ignore). Peaks that stayed = real chamber gas. Always degas before a qualification scan.
 
8.3 — Ion pump specific signatures
 
 
M40 alone (without M32) is not an air leak. It is almost certainly ion-pump Ar re-emission. Confirm by checking M32.
 
8.4 — Expected baseline partial pressures (clean baked system)
 
 
 
 
Sources & references
 
Compiled and cross-referenced against vacuum-physics literature and field experience. Facility-specific qualification specifications (Part 5 of the field edition) are omitted from this public document.