Troubleshooting front inlet flow problems

Hi mj.hazlett,

Can you tell me a little more about your inlet parameters? Column dimensions and approximate temp that the inlet shuts down would be useful. Oven ramp rate would be beneficial as well.

Usually a shut down at higher oven temps indicates that you may have marginal total flow set for the inlet. Split/splitless inlets are designed to work with a minimum of 20ml/min. You can sometimes get away with lower total flow with isothermal or small rates of temp increase.  You are correct that one flow channel (inlet) shouldn't affect the other as they are on separate EPC modules. A supply of 95 psi should be fine as long as its 95psi to the back of the instrument and not meeting a restricted gas trap that would lower the pressure under flow conditions. 

The parameters are below; the flow issues start when the oven reaches 200C where the flow is half the setpoint, and the shut down occurs shortly after when the oven is around 220C.

OVEN
   Initial temp:  50 'C (On)               Maximum temp:  300 'C
   Initial time:  4.00 min                 Equilibration time:  0.50 min
   Ramps:
      #  Rate  Final temp  Final time
      1 20.00      250        6.00
      2   0.0(Off)
   Post temp:  0 'C
   Post time:  0.00 min
   Run time:  20.00 min

FRONT INLET (SPLIT/SPLITLESS)           BACK INLET (SIM DIST)
   Mode:  Split                            Pressure:  15.00 psi (On)
   Initial temp:  260 'C (On)              Gas type:  Nitrogen
   Pressure:  10.00 psi (On)
   Split ratio:  50:1
   Split flow:  115.7 mL/min
   Total flow:  120.5 mL/min
   Gas saver:  Off
   Gas type:  Nitrogen

COLUMN 1                                COLUMN 2
   Capillary Column                        Capillary Column
   Model Number:  Agilent 19091J-413       Model Number:  Agilent 19091P-Q04
   HP-5  5% Phenyl Methyl Siloxane         HP-PLOT Q
   Max temperature:  325 'C                Max temperature:  290 'C
   Nominal length:  30.0 m                 Nominal length:  30.0 m
   Nominal diameter:  320.00 um            Nominal diameter:  320.00 um
   Nominal film thickness:  0.25 um        Nominal film thickness:  20.00 um
   Mode:  constant pressure                Mode:  constant pressure
   Pressure:  10.00 psi                    Pressure:  15.00 psi
   Nominal initial flow:  2.3 mL/min       Nominal initial flow:  2.3 mL/min
   Average velocity:  38 cm/sec            Average velocity:  43 cm/sec
   Inlet:  Front Inlet                     Inlet:  Back Inlet
   Outlet:  Front Detector                 Outlet:  Back Detector
   Outlet pressure:  ambient               Outlet pressure:  ambient

FRONT DETECTOR (FID)                    BACK DETECTOR (TCD)
   Temperature:  270 'C (On)               Temperature:  250 'C (On)
   Hydrogen flow:  40.0 mL/min (On)        Reference flow:  20.0 mL/min (On)
   Air flow:  450.0 mL/min (On)            Mode:  Constant column+makeup flow
   Mode:  Constant makeup flow             Combined flow:  7.0 mL/min
   Makeup flow:  45.0 mL/min (On)          Makeup flow:  On
   Makeup Gas Type: Nitrogen               Makeup Gas Type: Nitrogen
   Flame:  On                              Filament:  On
   Electrometer:  On                       Negative polarity:  Off
   Lit offset:  2.0

Ok, this is interesting as you are working in constant pressure and not constant flow as I assumed. Usually constant pressure takes oven temp and pressure changes during the run out of the equation.What shutdown message are you seeing on the front panel of the GC or in the logbook when it stops during your run?

The flow decreasing behavior you're noticing is in line with expectation. If the inlet is set to constant pressure, then the column flow will indeed decrease with increasing oven temperature. The total flow values you list in your method are really only applicable when you first start the run, when the oven is only 50 degrees C. Over the course of the run, that total flow value will decrease precipitously down to about 54.5 mL/min by my calculations. Besides the linear velocity being too high for Nitrogen (and therefore a not optimal Van Deemter efficiency), the GC can have trouble maintaining that low a flow.

So, the question is -- does the mode have to remain in Constant Pressure or can the method be changed to Constant Flow? If you're able to change this parameter, your flow will remain stable (as will the linear velocity) and the carrier gas requirement will go up instead of down.

You can use the GC Pressure/Flow Calculator (found here: Agilent | GC Calculators) to determine the column flows, inlet pressures, and linear velocities at different temperatures and for different column dimensions and gases.

The flow decreasing behavior you're noticing is in line with expectation. If the inlet is set to constant pressure, then the column flow will indeed decrease with increasing oven temperature. The total flow values you list in your method are really only applicable when you first start the run, when the oven is only 50 degrees C. Over the course of the run, that total flow value will decrease precipitously down to about 54.5 mL/min by my calculations. Besides the linear velocity being too high for Nitrogen (and therefore a not optimal Van Deemter efficiency), the GC can have trouble maintaining that low a flow.

So, the question is -- does the mode have to remain in Constant Pressure or can the method be changed to Constant Flow? If you're able to change this parameter, your flow will remain stable (as will the linear velocity) and the carrier gas requirement will go up instead of down.

You can use the GC Pressure/Flow Calculator (found here: Agilent | GC Calculators) to determine the column flows, inlet pressures, and linear velocities at different temperatures and for different column dimensions and gases.

If you have the same symptom during constant flow mode, I would first guess that the pressure at the source (95 psi) is not sufficient. Do you have Nitrogen tank feeding this system? Are there any other lines (either for other instruments or any other device that consumes Nitrogen) being fed by the same source? Try increasing the delivery pressure from 95 to 105 psi but don't exceed 120 psi. If you do that, does the oven temperature at which the shutdown occurs increase?

The other possibility is that you have an inlet leak. You hinted at the fact that you tried an inlet test. If you did the "Inlet Leak Check," can you specify where in the process the inlet flow went to zero? I'm including the document for reference:Manual Steps for Inlet Leak Check, Pressure Decay Test, and Split Vent Restriction Test for 6890/7890.

valentinrusu,

I understand that the flow will decrease over the temperature ramp, the method compensates for this by adjusting the flow rate setpoint as the temperature ramp increases... the issue is that the instrument cannot reach this decreased setpoint and does a front inlet flow shutdown when it can't reach this setpoint.  I have tried also with a constant flow program and the same front inlet flow shutdown occurs as the temperature increases.

Replacement cartridges in a two pack G1544-80530, for flushing, I have just adapted a syringe and forced solvent (your choice) through once the lines have been removed.Flushed into a beaker in a hood.  Part of the lines are copper and can be replaced if needed.

That's the interesting part as they shouldn't depend on each other, two flow channels that are separate with seperate EPC modules. You have good supply pressure to the instrument so unless there is a restricted external gas trap its interesting that you see the effects that you do.

6890/7890 Split Vent Restriction Test

1. Set the mode to split
2. You can either have the column installed or the inlet capped.
3. De-configure the column from the inlet.
4. Set the inlet temperature to a constant value.
5. Set the inlet pressure to 0 PSI (this is not the same as turning the pressure OFF).
6. Set the inlet total flow to 400 ml/minute.
7. Monitor inlet actual pressure.  Typical Values for a new, clean GC are, for split liner 1-2 PSI, and for a splitless liner, 3-10 PSI.  Much higher values indicate condensed sample contamination in the injection port split vent tube, the copper split vent line or the split vent trap, or possibly a defective EPC.

There shouldnt be an issue with setting headpressure to zero and total flow to whatever you like, are you doing this from the keypad of the GC?

valentinrusu,

Thanks for including the document for reference, I did try that inlet leak check; after I complete step 6 the total flow rate is zero. 

The GC is the only instrument connected to the nitrogen at the moment.  I used to have the source set to 80 psig and increased it to 95 psig when I noticed the issue and it did not seem to impact it.  The max pressure on the FID and TCD are 100 psig, so I don't feel comfortable going much higher...