Module 09: Reading the Spectrum
← Academy The Spectrum
An RGA (Residual Gas Analyzer) ionizes residual gas molecules and sorts them by mass-to-charge ratio (m/z, in AMU). The result is a bar chart of partial pressures. A clean, well-baked system is dominated by H₂ (mass 2) with traces of CO (28) and CO₂ (44). Water (18) and hydrocarbons (39–100) indicate the system needs more bake time or has a contamination source.
Species Explorer
Click a species to highlight its cracking pattern — the set of mass fragments produced when the molecule is ionized. The base peak (100 %) is the most intense line, and is not always the parent (counter-example: ethane C₂H₆, M=30 — base peak sits at M=28, while the parent M=30 is a shoulder). Lighter fragments appear at characteristic ratios.
Cracking Pattern
Click a species to see its cracking pattern and fragment ratios.
The M28 Problem
Mass 28 is the most ambiguous peak in any RGA spectrum. It can be N₂ (air leak), CO (outgassing), or even C₂H₄ (ethylene). Misidentifying it leads to wrong diagnoses. Here is how to tell them apart:
N₂ — Air Leak
If M32 (O₂) is present alongside M28, check the ratio. Atmospheric air has N₂/O₂ ≈ 3.7 by partial pressure. If you see this ratio, it is an air leak. Argon (M40) at ~1% of M28 confirms it.
Look for: M28 + M32 + M40 | N₂/O₂ ≈ 3.7
CO — Outgassing
CO often appears with CO₂ (M44) because both originate from oxide-layer reduction and beam-induced desorption. If M44 is correlated with M28 but M32 is absent, the M28 is predominantly CO.
Look for: M28 + M44, no M32 | CO/CO₂ correlation
C₂H₄ — Ethylene
Ethylene and other light hydrocarbons produce M28 but also have fragments at M27, M26, M25. If you see a cluster of peaks in the 25–27 range alongside M28, suspect hydrocarbon contamination.
Look for: M25 + M26 + M27 cluster near M28
Quick Rules — 5 Spectrum Reflexes
- M2 dominant, nothing else: System is clean. H₂ is the endgame species — it permeates through stainless steel and cannot be fully removed. This is a well-baked system.
- M18 > M2: Water-dominated spectrum. The system needs more bake time or has a virtual leak (trapped volume releasing water).
- M28 strong + M32 present: Air leak. Check all flanges, valves, and feedthroughs. Use helium to locate the leak.
- Forest at M39–100: Hydrocarbon contamination. Identify the source: pump oil backstreaming, dirty components, solvents, or O-ring outgassing.
- M40/M28 ratio ≈ 0.012: Argon relative to nitrogen (0.93 % vs 78 % of air). This confirms an air leak even when M32 is ambiguous due to overlap with methanol (M32) or O₂ being quietly consumed by a getter or ion pump.
Build Your Own Spectrum
Toggle gas sources on and off to build a composite spectrum. Watch how real-world contamination scenarios look on an RGA. The diagnostic engine analyzes your spectrum in real time.
Complete Cracking Patterns
Relative intensities (%) for major fragments at 70 eV electron impact ionization. Parent ion = 100%.
| Species | Formula | Base peak (=100%) | Major fragments (m/z: intensity %) | Sensitivity |
|---|---|---|---|---|
| Water | H₂O | 18 | 17: 25%, 16: 2%, 1: 5% | 1.00 |
| Nitrogen | N₂ | 28 | 14: 7%, 29: 0.7% | 1.00 |
| Oxygen | O₂ | 32 | 16: 11%, 34: 0.4% | 0.86 |
| Carbon monoxide | CO | 28 | 12: 4.6%, 16: 1.7%, 29: 1.1% | 1.05 |
| Carbon dioxide | CO₂ | 44 | 28: 11%, 16: 8.5%, 12: 6%, 45: 1.2% | 1.40 |
| Argon | Ar | 40 | 20: 22%, 36: 0.3% | 1.20 |
| Hydrogen | H₂ | 2 | 1: 3% | 0.44 |
| Methane | CH₄ | 16 | 15: 85%, 14: 16%, 13: 7%, 12: 3% | 1.60 |
| Ethane | C₂H₆ | 28 | 27: 33%, 26: 23%, 29: 22%, 30: 26%, 25: 5% | 2.60 |
| Pump oil (DC-704) | tetraphenyl-tetramethyl-trisiloxane, M=484 | 78, 76 | 77: 25%, 51: 18%, 50: 12%, 39: 8% (parent 484 rarely visible) | — |
Sensitivity & quantification:
Ptrue = Pindicated / Srel
where Srel is the relative sensitivity factor (N₂ = 1.00). Heavier molecules generally have higher sensitivity due to more ionization cross-section.
Ptrue = Pindicated / Srel
where Srel is the relative sensitivity factor (N₂ = 1.00). Heavier molecules generally have higher sensitivity due to more ionization cross-section.
Numbers to Carry
- Air: N₂/O₂ = 3.7, Ar/N₂ = 1.2%
Sources · 6
- P.H. Dawson, Quadrupole Mass Spectrometry and Its Applications, Elsevier, 1976 (reprinted AIP, 1995).
- NIST Chemistry WebBook, Mass Spectra.
- MKS Instruments, MicroVision 2 Residual Gas Analyzer : Operating Manual, 2019.
- J.F. O'Hanlon, A User's Guide to Vacuum Technology, 3rd ed., Wiley, 2003 : Ch. 6, Residual Gas Analyzers.
- J.H. Batey, Quadrupole Gas Analyzers, Vacuum 37 (1987) 659-668.
- K. Jousten (ed.), Handbook of Vacuum Technology, 2nd ed., Wiley-VCH, 2016 : Ch. 12, Partial Pressure Measurement.