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?

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.

When the instrument is used in constant pressure mode the column flow rate is not controlled, but rather will go up or down based on the temp of the oven and changes in temperature . Is it the the total flow that is dropping off of the set point? The reason I ask this is that it looks like your method is Simulated Distillation and its possible this is an issue with restriction in the split vent line , split vent cartridge or both. 

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.

James,

The shutdown message just says "Front Inlet Flow Shutdown", and sometimes has said "Back Inlet Flow Shutdown" in addition.  The front inlet flow rate is always below the setpoint on the instrument display when it happens.  I have noticed that if I increase the split ratio at the front inlet, the temperature where the shutdown occurs is lower.  I have also tried with constant flow to see if this helps, however it does not. I notice when setting up the method, the back inlet pressure always needs to be about 5psig higher than the front inlet in order to be able to get the right flow rate.

It sounds like you have some issues with the split lines and split vent cartridges. I would suggest cleaning or replacing the lines, but more importantly, the split vent cartridges should be replaced. In one of the above posts, valentinrusu has a link for manually checking for a split vent restriction. You could start there.

James