Total Flow When Internal Pressure is Zero

[Biochrom]

Hi I am stoked that there is a forum for this. I did a quick search and couldn't find an answer, sorry for the huge post. I am a student trying to learn to be lab tech and fix our Shimadzu GC2010-FID which sees a lot of culture supernatant samples. Any help is greatly appreciated.

The problem: If we lower the split ratio below 1:20 the internal pressure drops slowly until a "CAR1: Purge Leaks" error is displayed. We used to run 1:5 until this started, at which point 1:10 worked, then only 1:15 worked, now anything below 1:20 will not work. We are using glass inserts for split use with glass wool (we put the wool in ourselves), and an HP-chiral 20b column (30 m, 0.32-mm internal diameter, 0.25-mm film thickness; Agilent Technologies), He carrier gas.

Question: Assuming no real leaks, is this a problem in the Math that the TFC and APC are calculating from the sensors or settings, or is this a broken TFC resistor, valve, etc. Is it possible to have a bad setting somewhere?

What I have done: Snooped all lines and found no leaks. I followed the manual protocol for detecting leaks:
(In short) Set Flow control to 'Continuous', Turn off, All temps < 40 C,
Remove column and seal injector and detector with graphite ferrule. Blank nut Purge and Split outlets.
Cylinder pressure at 400 kPa,
Set Split mode to 'Direct', Control mode to 'Pressure'.
Set Inlet Pressure to 150 kPa,
Set Purge flow to 500 ml/min,
Press [On/Off] to start AFC control.
Result: Inlet pressure stays at 150 kPa. Total Flow is 20 ml/min (Indicates Leak). Snooped all connections no leaks detected, including injection port connections, septum, O-ring (flouride). (We don't have an electronic detector, I do my best to prevent drips etc).
When I Turn the system off again: Inlet pressure climbs very slowly (1kPa/5min) It never decreases (Indicating no negative leak). Total flow remains constant at 20 ml/min (Why? How?).
When I release the pressure: Inlet pressure drops to 0 [System is open at the column nut], Total flow remains at 20 ml/min. Is the TFC control valve leaking? Or is it reporting a confused number?
When External pressure is increased or decreased, Total Flow at zero inlet pressure changes roughly according to the following:
{Inlet Pressure}, External Pressure, [Total Flow]
{0 ml/min}, 900 kPa, [30 ml/min]
{0 ml/min}, 400 kPa, [20 ml/min]
{0 ml/min}, 180 kPa, [10 ml/min]
(Is external pressure leaking into the system dependent on it's pressure?)
I also removed the injection port lower conections to check for broken glass, cleaned and reinstalled: No leaks before or after.

So does this sound like broken parts or a problem with the way we are using the machine? Thank you for your help!

John

[Peter Apps]

HI John

Welcome to the forum.

Having to progressively increase the split ratio in order to maintain inlet pressure is a classic symptom of a large and increasing leak at the inlet. By far the most likely place for that leak is the septum - when did you last replace it ? What happens when you put a new one in ?

And you really are taking risks by leak checking with snoop - use a 50:50 mix of water and isopropanol if you really cannot get hold of a proper leak seeker - at least then when it gets inside your plumbing you have some chance that it will evaporate away in time.

Peter

[Biochrom]

Hi Peter, Thank you for your reply. The septum and O-ring are changed often (every 3 weeks or so), and I used new ones to start looking for the problem. The good news is that when I started up the machine this morning after making all those changes last night, it started working again, at least at split ratio 1:10. I am guessing that there may have been a leak on the bottom side of the injector port, which I opened to look for glass shards. I must have missed the leak before, and now it is tighter. I can use 1:5 for split injection, but only by increasing the purge flow from 3ml/min to 40 ml/min.

I will stop using snoop! Thank you for the suggested alternative. So far everything looks ok.

John

[Peter Apps]

Hi John

If you have to set the septum purge to 40 ml/min you still have the original problem - and the gas flows in the inlet will be going in very strange directions because the main flow is now going out at the top instead of the bottom.

If you set the septum purge to 40 ml/min and the split to 10:1, what flow (ml/min) actually comes out of the septum purge line, and how much comes out through the split ?

Have you leak-checked upstream of the inlet towards the gas contoller ?

Peter

[AICMM]

Biochromm

You can hold the higher split but not the lower split? When you go for a 1:5 split by raising the purge flow from 3 to 40, that means that your column flow is now 8 mL's/min? Do you have the means to measure the carrier flow? This is made harder by the fact that I do not know Shimadzu flow control paradigm, sorry.

Best regards,

AICMM

[GCPRO]

Sounds like you had a leak in the inlet. The trickiest leaks are the hidden ones. Depending on the manufacturer, there are several possibilities under the cover, so to speak. For example, with Agilent S/Sl inlets, there is a baseplate seal and washer at the bottom. This is typically covered by the aluminum insulator cup. So you have to take that off and the leak might not still not be noticed becuase it comes out the top threads. Anyway, the basplate disks should be replaced on occassion and certainly should not be reused (they don't re-seal well) - and one should use a new washer underneath it (on the column connection side, opposite the liner side).

One could also have a leak at the top (inet top weldment cocked or not night) and at cracked braising points (tubing connections to inlet parts, like on the inlet top welment that you flex back to change the liner on an Agilent GC). Cracks are more likely to developed on lines that are moved or flexed.