I'm using ammonia as a reagent gas for chemical ionization MS. Set at 2 bar, the pressure on the pressure reducer drops during a series of analyses to -1 bar, resulting in a drop from 1.5 mL/min flow to a steady 0.4 mL/min. Of course, this non-optimal flow yields serious losses in sensitivity and inconsistent measurements.
The pressure reducer is suited for ammonia and two types have been tested, the tubing has a large amount of windings stacked in a vertical position and was replaced by a longer one, and even the entire module with mass flow controller and valves inside the MS was replaced. No results. The technicians from both the MS company and from the gas supplier do not see the cause.
Has anyone experienced similar issues, or does anyone have an idea about potential solutions to this problem?
I had a similar experience with NH3. You run 1-2 samples and then the cylinder looks almost empty. I found the following advice in http://ww2.chemistry.gatech.edu/~bostwick/stms/ammonia.txt. "If you are using a small ammonia cylinder - stick it in a thermostatically controlled bath and you will hold the vapour pressure constant and make it easy to regulate with a needle valve."
Hope this will be of help to you.
I had a similar experience with NH3. You run 1-2 samples and then the cylinder looks almost empty. I found the following advice in http://ww2.chemistry.gatech.edu/~bostwick/stms/ammonia.txt. "If you are using a small ammonia cylinder - stick it in a thermostatically controlled bath and you will hold the vapour pressure constant and make it easy to regulate with a needle valve."
Hope this will be of help to you.
Als de druk op de ontspanner naar -1 valt, is de afname groter naar de MS dan de toevoer vanuit de cylinder om de druk constant op 2 bar te houden. (te sterk vacuüm?)
Als u een 2-trapsonspanner met 2 manometers heeft kan u alles makkelijk controleren (controleer ook alles op lekken).
As Manolis said the cylinder size can be the culprit. As the gas expands leaving the cylinder, the temperature drops and so does the pressure (especially considering NH3 boiling point of −33.34 °C at 1 atm). A larger cylinder could improve your situation (more thermal inertia: larger mass-less cooling given the same flow) and a thermostatic bath should solve it. Unless it's another problem.
Good luck
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