ISQ · EI Tune Evaluation
Drop in an ISQ tune report (or enter the numbers) for an instant read on the air/water, leak, vacuum, calibration and detector checks – and, if anything fails, what it means and what to do.
Enter each ion as the % of m/z 69 (from the Air/Water Check column, not the raw Abundance column).
The MS analyzer runs under high vacuum. A vacuum leak is any path that lets atmospheric air into that space – most often the column / transfer‑line fitting, the vent‑valve knob, the source or manifold seals, the top‑cover O‑ring, or the GC inlet.
Air floods the spectra with nitrogen (m/z 28), oxygen (32), argon (40), CO₂ (44) and water (18). Beyond bad data, a leak oxidizes and shortens the filament, drives the multiplier voltage up, kills sensitivity, raises chemical noise, and in a bad case stops the system reaching vacuum at all.
Both raise the background, but they mean different things. Water‑dominated (high m/z 18, low O₂, leak check OK) is residual moisture – it pumps away over hours to a day. An air leak shows O₂ (32) climbing, N₂:O₂ trending toward atmospheric ~4:1, argon (40) appearing, and a leak‑check number that rises and won't fall.
The single most telling ion is O₂ (m/z 32): gas filters scrub oxygen out of the carrier, so oxygen in the spectrum almost always means fresh air is getting in.
Leak check over limit and not dropping · rising foreline pressure or won't reach vacuum · O₂ and N₂ climbing in the air/water scan · short filament life · multiplier voltage creeping up over time · loss of sensitivity / rising detection limits · noisy baseline.
Spray a leak‑detector gas (difluoroethane / tetrafluoroethane / canned duster) around suspected joints while watching the real‑time scan. When the trace spikes, the spot you just sprayed is the leak.
Common fixes, in order: tighten the transfer‑line nut / union, tighten the vent‑valve knob, replace cracked column / inlet ferrules, confirm clean He flow from the GC, and clean the top‑cover O‑ring. For high foreline pressure that won't fall, check the vent valve, foreline hose/clamps, and pump oil level.
Procedure: ISQ Series (LT) Hardware Manual p.108–109, "Investigating Vacuum Issues – An air leak has been detected."
The ISQ 7000 is the next‑generation ISQ‑LT (same single‑quad EI core, FC43 cal gas, and air/water & leak diagnostics), so its published limits apply to the LT. Reference thresholds only – not a substitute for the instrument's AutoTune pass/fail or your lab SOP. FWHM is stored as a peak image in the report and is entered manually.