Liquid N2 consumption

[soccerdad007]

Hi everyone
Due to failure of an air compressor, we are having to ship in liquid N2 tanks for our Agilent 6430 QQQ system. We don't use the system 24/7 rather more than 15-20% of the time at most.

Our tanks are lasting no more than 1 week. To me that seems a very short time and not as long as I recall from the past when we had to use liquid N2. Has anyone got any experience with this type of setup. I would be interested in hearing about typical tank life time and anything found to increase the tank lifetimes

Thanks in advance
Kevin

[bundy5555]

Sounds about right. Get your generator fixed asap

[James_Ball]

A dewar of liquid nitrogen will last at best a few weeks if not used at all. To remain liquid it vents off some of the nitrogen continually.

When not in use you need to close the pressure building valve so that it does not recirculate and warm the liquid and close all valves going to the instrument, because most instruments will pull some nitrogen as a curtain gas over the orifice. Of course doing that will allow room air into the analyzer when idle and will take more time to get rid of any moisture and oxygen when you start back up.

[LALman]

A modern dewar vents about 1.5% per day. I use a 230HP dewar of LN2 for my labs 1. GC-FID makeup gas, 2. GC-MS purge and trap gas, and 3. as an idle GC-MS carrier gas when the instrument is idle. Even with rental (~$204/mo) a new Dewar every two months (~$230) is still cheaper and I think more reliably pure than going with tanks of UHP nitrogen.
If your dewar is evaporating in a month or a couple weeks under light use; you should complain and ask for a credit.

[lmh]

Evaporation in the tank wasn't the original poster's problem; it was simply consuming nitrogen very fast. A modern, high-end instrument will probably consume up to 30L/min nitrogen supplied at 7 bar (hopefully rather less; depending on solvent flow, let's say 15L/min). I never know whether the L is at 7 bar or 1 bar... let's assume the worst, that it consumes 15L/min measured at 7 bar.

1 L of liquid nitrogen is 807g, which is about 29 moles, or 690 L at room temperature and pressure, or about 100L at 7 bar, enough to run an LC-MS for around 6 minutes. It really doesn't make sense to run LC-MS instruments on anything but a nitrogen generator. If you have a 300L tank, running your instrument continuously, you have about 30 hours of operation time, so if the original poster was running their system 15-20% of the time (and nitrogen consumption is much lower when not running), then a week sounds about right.

[bcd_GCLCMSMS]

The rate of consumption sounds about right for a modern Agilent LC-MS/MS. Our dewars of N2 (230L HP) would last only about 4-5 weeks completely sealed due to boil off. They last only 2-5 days when in moderate use.

Just a note, there are very large differences in N2 gas consumption between the modern LC-MS/MS units from different vendors. (Obviously, the consumption differs based on settings and LC flow, but I am talking about similar LC setups.) This equates to larger compressors, more electricity, and larger N2 generators + more heat.

Also, the flow rates in standby mode vary greatly with some vendors dropping to nearly 0 flow and others remaining at considerable flow when in standby with no settings available to decrease it. Most vendors recommend not shutting off the flow off of N2 to MS instruments in standby, so this high standby flows can become costly.

Discuss with with a laboratories running the same MS units to see their real world experience to figure this very significant running cost into your budget.

[LALman]

The rate of consumption sounds about right for a modern Agilent LC-MS/MS. Our dewars of N2 (230L HP) would last only about 4-5 weeks completely sealed due to boil off. They last only 2-5 days when in moderate use.

Just a note, there are very large differences in N2 gas consumption between the modern LC-MS/MS units from different vendors. (Obviously, the consumption differs based on settings and LC flow, but I am talking about similar LC setups.) This equates to larger compressors, more electricity, and larger N2 generators + more heat.

Also, the flow rates in standby mode vary greatly with some vendors dropping to nearly 0 flow and others remaining at considerable flow when in standby with no settings available to decrease it. Most vendors recommend not shutting off the flow off of N2 to MS instruments in standby, so this high standby flows can become costly.

Discuss with with a laboratories running the same MS units to see their real world experience to figure this very significant running cost into your budget.

TIL, I had no idea that LC-MS instuments used gas at this rate. Its similar to the use rate of an ICP-MS, but even higher.

[itspip]

I didn't realize how much a Charged Aerosol Detector uses nitrogen until I had to run it for my work. Only got a little over 24 hours of continuous use out of a cylinder.

I don't think the bean counters really understand the gas usage of equipment. If they did they probably wouldn't balk at the cost of a building nitrogen generator or large LN tank.