Module 06: The RGA Machine
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Inside the RGA
An RGA is a miniature mass spectrometer bolted directly onto the vacuum chamber. Molecules enter, get ionized, filtered by mass, and counted. Click each zone in the diagram below to learn what it does.
Click a zone
Click on any of the four zones in the diagram above to learn how that component works.
Ion Flight Simulator
Watch ions navigate the quadrupole filter. Only ions matching the tuned mass have stable trajectories. Change the ion mass and see who survives.
Fired: 0
Detected: 0
Rejected: 0
Selected Mass
28 AMU
Tuned For
28 AMU
Stability
STABLE
Oscillation
0.8 mm
This is what happens 1 million times per second in your RGA. The quadrupole sweeps its RF/DC voltages to select each mass in turn. The spectrum you see in the dashboard is the count of ions that survive the crossing at each tuning point. Unit mass resolution means the stable window is about ±0.5 AMU wide.
Faraday vs. Multiplier
The same vacuum system produces very different-looking spectra depending on which detector you use. Faraday is honest but blind to traces. Multiplier sees everything but ages over time.
Faraday Cup
Electron Multiplier
Faraday → absolute pressure measurement, calibration, stable long-term reference.
Multiplier → contamination hunting, trace detection, diagnostic spectra down to 10−14 Torr.
Multiplier → contamination hunting, trace detection, diagnostic spectra down to 10−14 Torr.
Multiplier Aging
The electron multiplier surface degrades as ions bombard it over time. Gain drops, so we increase the high voltage (Mult3) to compensate. When Mult3 goes too high, noise increases and the multiplier must be replaced.
SLAC fleet — current Mult3 voltages:
What Can Go Wrong
Filament Burnout
The ionizer filament breaks from thermal stress or chemical attack (e.g., halogen exposure). No filament = no ions = no signal at all.
Fix: Switch to F2 (backup filament). Order replacement.
Trip (Arc / Overpressure)
An electrical arc in the ionizer, usually caused by operating at too high a pressure (>10−4 Torr). The RGA firmware shuts down to protect the filament and multiplier.
Fix: Wait for pressure to drop. Restart RGA. Check for sudden vents.
Multiplier Saturation
Too many ions flood the multiplier. The output current plateaus and peaks appear clipped. Partial pressures read lower than reality.
Fix: Reduce multiplier gain, or switch to Faraday for dominant peaks.
Mass Calibration Drift
Peaks shift from their nominal integer mass due to aging electronics, temperature changes, or mechanical vibration. M28 might appear at 27.8 or 28.2.
Fix: Recalibrate using Ar (M40) and N₂ (M28) as reference peaks.
Ghost Peaks
Spurious peaks at non-integer masses or unexpected positions. Caused by metastable ions, RF cross-talk, or multiply-charged species (e.g., Ar²⁺ at M20).
Fix: Identify the source. Reduce electron energy or check RF tuning.
Quadrupole Theory
The quadrupole mass filter uses four parallel rods with superimposed DC (U) and RF (V·cos(ωt)) voltages. The motion of an ion of mass m and charge e in this field is described by the Mathieu equation, with stability parameters:
a = 8eU / (mω²r0²) q = 4eV / (mω²r0²)
r0 = inscribed radius of rod assembly, ω = RF angular frequency
r0 = inscribed radius of rod assembly, ω = RF angular frequency
Only ions whose (a, q) parameters fall inside the stability region of the Mathieu diagram have bounded trajectories and reach the detector. By scanning U and V at a fixed ratio (a/q = 2U/V = constant), the filter sweeps through masses sequentially.
Resolution:
Δm / m ≈ 0.5 / n
n = number of RF cycles during ion transit. Longer rods + higher frequency = better resolution. Typical RGA: unit mass resolution (1 AMU).
Δm / m ≈ 0.5 / n
n = number of RF cycles during ion transit. Longer rods + higher frequency = better resolution. Typical RGA: unit mass resolution (1 AMU).
Sensitivity & Detection
Sensitivity:
S = Iion / P [A/Torr]
Depends on ionization cross-section, electron energy (70 eV standard), and ion transmission. Typical: 2×10−4 A/Torr for N₂ (Faraday).
S = Iion / P [A/Torr]
Depends on ionization cross-section, electron energy (70 eV standard), and ion transmission. Typical: 2×10−4 A/Torr for N₂ (Faraday).
| Gas | Srel (vs N₂) | σion (Ų) | Faraday limit (Torr) | Multiplier limit (Torr) |
|---|---|---|---|---|
| H₂ | 0.44 | 0.91 | ~5×10−11 | ~5×10−14 |
| He | 0.15 | 0.31 | ~1×10−10 | ~1×10−13 |
| H₂O | 1.00 | 2.05 | ~2×10−11 | ~2×10−14 |
| N₂ | 1.00 | 2.51 | ~2×10−11 | ~2×10−14 |
| Ar | 1.20 | 2.87 | ~1×10−11 | ~1×10−14 |
| CO₂ | 1.40 | 3.52 | ~1×10−11 | ~1×10−14 |
Multiplier gain:
G = δn
δ = secondary electron emission coefficient per stage (~2–3), n = number of stages (~15–20). δ decreases with cumulative ion dose (aging), requiring higher voltage to maintain G.
G = δn
δ = secondary electron emission coefficient per stage (~2–3), n = number of stages (~15–20). δ decreases with cumulative ion dose (aging), requiring higher voltage to maintain G.
SLAC operating parameters:
Electron energy: 70 eV | Extraction voltage: −112 V (standard), −110 V (SBM exception)
70 eV maximizes ionization cross-section for most gases. Lower energy reduces fragmentation but sacrifices sensitivity.
Electron energy: 70 eV | Extraction voltage: −112 V (standard), −110 V (SBM exception)
70 eV maximizes ionization cross-section for most gases. Lower energy reduces fragmentation but sacrifices sensitivity.
References
[1] P.H. Dawson, Quadrupole Mass Spectrometry and Its Applications, Elsevier, 1976 (reprinted AIP, 1995).
[2] J.F. O'Hanlon, A User's Guide to Vacuum Technology, 3rd ed., Wiley, 2003 — Ch. 6: Residual Gas Analyzers.
[3] MKS Instruments, "MicroVision 2 Residual Gas Analyzer — Operating Manual," 2019.
[4] K. Jousten (ed.), Handbook of Vacuum Technology, 2nd ed., Wiley-VCH, 2016 — Ch. 12.
[5] NIST Electron-Impact Cross Section Database, nist.gov.
[2] J.F. O'Hanlon, A User's Guide to Vacuum Technology, 3rd ed., Wiley, 2003 — Ch. 6: Residual Gas Analyzers.
[3] MKS Instruments, "MicroVision 2 Residual Gas Analyzer — Operating Manual," 2019.
[4] K. Jousten (ed.), Handbook of Vacuum Technology, 2nd ed., Wiley-VCH, 2016 — Ch. 12.
[5] NIST Electron-Impact Cross Section Database, nist.gov.