Every LC-MS source drinks nitrogen — commonly in the range of 10–15 litres per minute per instrument, all day — and how you supply it is a five-figure decision over an instrument’s life. Cylinders, dewars of liquid nitrogen with gas withdrawal, or an on-site membrane/PSA generator: here is how the economics and practicalities actually compare.
The consumption reality
At ~10 L/min for an eight-hour day, a single LC-MS consumes roughly 5 m³ of nitrogen daily — a standard 50 L cylinder (~9–10 m³ of gas) lasts about two days. Labs running multiple instruments or overnight batches burn through cylinders fast, and every changeover is manual handling, dead time, and a run-abort risk if the cylinder empties mid-sequence.
Cylinders: low entry cost, high running cost
Pros: zero capital, ultimate purity available, no maintenance, no power draw.
Cons: per-m³ cost is the highest of the options once rental, delivery, and handling are counted; storage and transport regulations; changeover interruptions; and the real safety topic of moving high-pressure cylinders through a lab.
Best fit: low or intermittent consumption — one instrument, part-time use, or backup supply.
On-site generator: capital cost, then cheap gas
Membrane and PSA generators separate nitrogen from compressed air on demand. After purchase, the running cost is essentially compressed air and electricity plus periodic filter/maintenance — per-m³ cost falls dramatically below cylinder gas at LC-MS consumption levels.
Payback: for a lab feeding one to three mass specs continuously, generators commonly pay back in the low single-digit years against cylinder supply — faster with more instruments. Model it with your actual consumption and local gas prices; the crossover is volume-driven.
Purity note: LC-MS sources typically want ~95–99% nitrogen — comfortably within membrane/PSA range. Applications needing 99.999% (some GC carrier uses) change the calculus toward PSA with polishing or cylinders.
Plan for: compressor capacity and dryness (the hidden dependency — the generator is only as good as its air feed), service intervals, noise placement, and a small buffer tank.
Reliability and redundancy
A generator failure stops every instrument it feeds, so serious labs keep a cylinder manifold as automatic backup — the generator carries the base load, cylinders carry the exceptions. That hybrid gives generator economics with cylinder resilience, and it is the configuration we recommend for any multi-instrument facility.
The refurbished angle
Nitrogen generators are mechanically simple — vessels, valves, membranes/sieve beds, and a controller — and the used market offers substantial savings. The items to verify on a used unit: sieve bed/membrane condition (ask for outlet purity measured at rated flow), compressor hours if integrated, and service history of filters and valves. A generator tested to deliver rated purity at rated flow is a known quantity; one “working when removed” is not.
PLC North stocks tested lab gas generators and supporting equipment, each verified for outlet purity and flow before sale — ask for the measured figures on any listing, and tell us your instrument count for a sizing sanity-check. Browse our tested gas generators.