Column Flow Rate and Pressure

[Ento_Joe]

Hi all,

I am having some issues regarding one of my methods I am using to analyse the headspace of some weevils.

I am using an Agilent 7890 B Gas Chromatograph coupled to an Agilent 5977 A Mass Spectrometer. I am currently running a dual inlet/column column system as the rear inlet and column are connected to a Markes UNITY2 thermodesorption unit for analysis of VOCs trapped on sorbent tubes, while the front inlet and column are reserved for liquid/SPME injections. Both columns run through a Deans switche before connecting to the MS via a transfer line.

The issue I am having is that when I change the GC method from constant pressure mode to constant flow I get a PCM A not ready error message and my GC goes into a "not ready" state - it doesn't seem to be able to handle the increased flow. In constant pressure mode I operate the column I am using at 10 PSI and the other column at 4 PSI, while the transfer line is set at 2 PSI - these were all the factory default settings.

I can't seem to get the GC to stablise under a constant flow programme, any ideas would be great!

Thanks,

Joe

[rb6banjo]

Please post as much of your method as you can (column dimensions, temperatures, etc.). The more information you give, the better help you will receive.

Does the error occur at the beginning (e.g., you never get the system to equilibrate) or is it during the run that you see the error? To maintain constant flow throughout the run, the inlet pressure must increase. Perhaps you are trying to deliver more pressure than the EPC is capable of delivering? You might also ensure that the delivery pressure on the outlet of your carrier gas tank is larger than the maximum inlet pressure needed during your run (when the oven is warmest).

[EntoJoe]

Please post as much of your method as you can (column dimensions, temperatures, etc.). The more information you give, the better help you will receive.

Does the error occur at the beginning (e.g., you never get the system to equilibrate) or is it during the run that you see the error? To maintain constant flow throughout the run, the inlet pressure must increase. Perhaps you are trying to deliver more pressure than the EPC is capable of delivering? You might also ensure that the delivery pressure on the outlet of your carrier gas tank is larger than the maximum inlet pressure needed during your run (when the oven is warmest).

Hi rb6banjo,

Thanks for the response. I thought I had attached the method I am trying to setup, but it seems it wasn't done correctly - sorry! The system takes a while to equilibrate, but the error doesn't appear until about 4 minutes into the run and I have noticed that my mass spectra look a little sparse...not sure if that's related though. The outlet of the carrier gas tank is able to deliver a pressure of 230 PSI/16 bar, so I don't think that is the issue.

Agilent set the GC-MS up as it now (constant pressure), but a lot of the papers in my field run their systems in constant flow mode at either 1.0 or 1.5 ml/min.

Here is the method that I am trying to set up:

INSTRUMENT CONTROL PARAMETERS: Nema1
---------------------------------------

C:\MassHunter\GCMS\1\methods\ThermoDesorb Constant Flow.M
Tue Dec 20 11:03:27 2016

Control Information
------- -----------

Sample Inlet : GC
Injection Source : External Device
Mass Spectrometer : Enabled


No Sample Prep method has been assigned to this method.


GC
Oven
Temperature
Setpoint On
(Initial) 40 °C
Hold Time 2 min
Post Run 50 °C
Program
#1 Rate 10 °C/min
#1 Value 240 °C
#1 Hold Time 20 min


Equilibration Time 0.5 min
Max Temperature 325 °C
Maximum Temperature Override Disabled
Slow Fan Disabled
Cryo Off

ALS
Front Injector
Syringe Size 10 μL
Injection Volume 1 μL
Solvent A Washes (PreInj) 0
Solvent A Washes (PostInj) 0
Solvent A Volume 8 μL
Solvent B Washes (PreInj) 0
Solvent B Washes (PostInj) 0
Solvent B Volume 8 μL
Sample Washes 0
Sample Wash Volume 8 μL
Sample Pumps 6
Dwell Time (PreInj) 0 min
Dwell Time (PostInj) 0 min
Solvent Wash Draw Speed 300 μL/min
Solvent Wash Dispense Speed 3000 μL/min
Sample Wash Draw Speed 300 μL/min
Sample Wash Dispense Speed 3000 μL/min
Injection Dispense Speed 6000 μL/min
Viscosity Delay 0 sec
Sample Depth Disabled
Injection Type Standard
L1 Airgap 0.2 μL
Solvent Wash Mode A, B

Sample Overlap
Mode Sample overlap is not enabled

ALS Errors Pause for user interaction

Front SS Inlet He
Mode Splitless
Heater On 250 °C
Pressure On 4.3213 psi
Total Flow On 103.1 mL/min
Septum Purge Flow On 3 mL/min
Gas Saver Off
Purge Flow to Split Vent 100 mL/min at 1 min
Liner Agilent 5190-2295: 870 μL (Universal, low pressure drop, ultra i)

Back SS Inlet He
Mode Splitless
Heater On 250 °C
Pressure On 13.376 psi
Total Flow On 19 mL/min
Septum Purge Flow On 3 mL/min
Gas Saver Off
Purge Flow to Split Vent 15 mL/min at 999.99 min

Thermal Aux 2 (MSD Transfer Line)
Temperature
Setpoint On
(Initial) 280 °C
Post Run 0 °C


Column
Column #1
Flow
Setpoint Off
(Initial) 0.1 mL/min
Post Run 0.78872 mL/min

Agilent 19091S-433: 1
HP-5ms
0 °C—325 °C (350 °C): 30 m x 250 μm x 0.25 μm
Column lock Unlocked
In Front SS Inlet He
Out PCM A
(Initial) 40 °C
Pressure 4.3213 psi
Flow 0.1 mL/min
Average Velocity 2.8627 cm/sec
Holdup Time 17.466 min

Column #2
Flow
Setpoint Off
(Initial) 1 mL/min
Post Run 0 mL/min

Agilent 19091S-433: 2
HP-5ms
0 °C—325 °C (350 °C): 30 m x 250 μm x 0.25 μm
Column lock Unlocked
In Back SS Inlet He
Out PCM A
(Initial) 40 °C
Pressure 13.376 psi
Flow 1 mL/min
Average Velocity 22.593 cm/sec
Holdup Time 2.2131 min

Column #3
Flow
Setpoint Off
(Initial) 1.3 mL/min
Post Run 0.84739 mL/min

Agilent
Retention Gap
0 °C—325 °C (325 °C): 2 m x 150 μm x 0 μm
Column lock Unlocked
In PCM A He
Out MSD
(Initial) 40 °C
Pressure 3.032 psi
Flow 1.3 mL/min
Average Velocity 160.13 cm/sec
Holdup Time 0.020817 min

Column Outlet Pressure 0 psi

PCM A
PCM A He
PCM A He Supplies Column 3

Aux PCM A He
Pressure
Setpoint Off
(Initial) 10 psi
Post Run 0 psi

***Excluded from Affecting GC's Readiness State***


Signals
Signal #1: Test Plot
Description Test Plot
Details
Save Off
Data Rate 50 Hz
Dual Injection Assignment Front Sample

Signal #2: Test Plot
Description Test Plot
Details
Save Off
Data Rate 50 Hz
Dual Injection Assignment Back Sample

Signal #3: Test Plot
Description Test Plot
Details
Save Off
Data Rate 50 Hz
Dual Injection Assignment Back Sample

Signal #4: Test Plot
Description Test Plot
Details
Save Off
Data Rate 50 Hz
Dual Injection Assignment Back Sample




MS Information
-- -----------


General Information
------- -----------

Acquisistion Mode : Scan
Solvent Delay (minutes) : 0.00
Tune file : C:\MassHunter\GCMS\1\5977\ATUNE.U
EM Setting mode Delta : 0.000000

Normal or Fast Scanning : Normal Scanning
Trace Ion Detection : On
Run Time (if MS only) : 650 minutes

[Scan Parameters]
Start Time : 0.00
Low Mass : 50.00
High Mass : 550.00
Threshold : 50
A/D Samples: : 4


[MSZones]

MS Source : 230 C maximum 250 C
MS Quad : 150 C maximum 200 C

Timed Events
----- ------
Number Events= 0



END OF MS ACQUISTION PARAMETERS


TUNE PARAMETERS for SN: US1435L416
---------------------------------

Trace Ion Detection is ON.

EMISSION : 34.593
ENERGY : 70.007
REPELLER : 34.899
IONFOCUS : 90.331
ENTRANCE_LE : 17.627
EMVOLTS : 994.140
Actual EMV : 994.1
GAIN FACTOR : 0.36
AMUGAIN : 1510.000
AMUOFFSET : 121.813
FILAMENT : 1.000
DCPOLARITY : 1.000
ENTLENSOFFS : 15.482
MASSGAIN : -475.000
MASSOFFSET : -39.000

END OF TUNE PARAMETERS
----------------------



END OF INSTRUMENT CONTROL PARAMETERS
------------------------------------

Cheers,

Joe

[Peter Apps]

Maybe check with Agilent that their Dean's switch can operate with the inlet pressure set to give constant flow - I have a suspicion that the EPCs that drive the column pressure and the Deans switch pressure are confusing one another.

Peter

[rb6banjo]

For a 30 m x 0.25 mm x 0.25 µm column, at 40 °C, to get a 1.25 mL/min. flow rate, you need a head pressure of 10 psig. At 240 °C to achieve the same conditions, you need a 22.8 psig inlet pressure (using the Restek flow calculator @ http://www.restek.com/ezgc-mtfc).

Are you able to force these types of conditions manually and get a system that is stable?

[EntoJoe]

For a 30 m x 0.25 mm x 0.25 µm column, at 40 °C, to get a 1.25 mL/min. flow rate, you need a head pressure of 10 psig. At 240 °C to achieve the same conditions, you need a 22.8 psig inlet pressure (using the Restek flow calculator @ http://www.restek.com/ezgc-mtfc).

Are you able to force these types of conditions manually and get a system that is stable?

Hi rb6banjo,

I have set all of the columns to constant pressure mode under the following parameters:

Column 1: 20 PSI (originally 10 PSI)
Column 2: 15 PSI (originally 4 PSI)
PCM A: 10 PSI (originally 2 PSI)

The systems appears to be stable, but obviously the flow rate isn't constant. Perhaps I could use a ramped pressure programme? Are there any negative consequences from running the GC-MS at these increased pressures? Obviously more carrier gas will be used.

Cheers,

Joe

[rb6banjo]

Operation of the mass spec. is all about the flow. You can't overwhelm it with flow because you won't be able to maintain an adequate vacuum for ionization/fragmentation of your analytes.

The viscosity of your carrier gas changes with temperature. To maintain a constant flow at high temperature, the inlet pressure must increase. So, if you want 1.25 mL/min at 40 and 240 °C, your system will adjust the inlet pressure accordingly throughout the chromatographic run. In constant-flow mode, you will be able to watch your inlet pressure increase as the oven temperature increases. When you operate in constant pressure mode, the inlet pressure stays the same and you just live with the fact that the flow decreases at elevated temperature.

The advantage of keeping the flow constant is that you can operate at the "optimum" flow rate throughout your run. Your chromatographic efficiencies will be the best - in theory you'll be able better resolve closely eluting components. As the flow decreases, you drift from optimum flow conditions and thus your peak shapes suffer a little. I've never found it to be a huge deal in practical application. So, I operate in constant pressure mode all of the time. Others who operate in constant flow mode will likely have different opinions.

Here you go. Here's a link to a discussion on the Restek blog (no, I don't work for Restek) that addressed optimum carrier gas flows.

http://blog.restek.com/?paged=57

You can see that the optimum for a helium carrier gas is somewhere in the 15-30 cm/s region across the typical column dimensions. Linear velocity (cm/s) is calculated from the length of the column and the time it takes for an unretained material (like methane) to traverse the column from inlet to detector. To get 30 cm/s on a 0.25 mm column, you need a head pressure of 3.64 psig (at 40 °C, vacuum detector). To keep that linear velocity at 240 °C, the inlet pressure must be 11.05 psig.

[Ento_Joe]

Operation of the mass spec. is all about the flow. You can't overwhelm it with flow because you won't be able to maintain an adequate vacuum for ionization/fragmentation of your analytes.

The viscosity of your carrier gas changes with temperature. To maintain a constant flow at high temperature, the inlet pressure must increase. So, if you want 1.25 mL/min at 40 and 240 °C, your system will adjust the inlet pressure accordingly throughout the chromatographic run. In constant-flow mode, you will be able to watch your inlet pressure increase as the oven temperature increases. When you operate in constant pressure mode, the inlet pressure stays the same and you just live with the fact that the flow decreases at elevated temperature.

The advantage of keeping the flow constant is that you can operate at the "optimum" flow rate throughout your run. Your chromatographic efficiencies will be the best - in theory you'll be able better resolve closely eluting components. As the flow decreases, you drift from optimum flow conditions and thus your peak shapes suffer a little. I've never found it to be a huge deal in practical application. So, I operate in constant pressure mode all of the time. Others who operate in constant flow mode will likely have different opinions.

Here you go. Here's a link to a discussion on the Restek blog (no, I don't work for Restek) that addressed optimum carrier gas flows.

http://blog.restek.com/?paged=57

You can see that the optimum for a helium carrier gas is somewhere in the 15-30 cm/s region across the typical column dimensions. Linear velocity (cm/s) is calculated from the length of the column and the time it takes for an unretained material (like methane) to traverse the column from inlet to detector. To get 30 cm/s on a 0.25 mm column, you need a head pressure of 3.64 psig (at 40 °C, vacuum detector). To keep that linear velocity at 240 °C, the inlet pressure must be 11.05 psig.

Thanks for the information, I found that to be a very helpful read. I do have one question. You said that you run your instrumentation in constant pressure mode as I am doing, what pressure do you typically operate at?

Cheers,

Joe

[Peter Apps]

Hi Joe

Keep in mind that if you increase the pressure at the switch (I assume that's what PCM A is) then you reduce the pressure drop (pressure in the inlet minus pressure at the end of the column) across both columns, and decrease the volume flow and linear velocity accordingly. In your set-up you will also be increasing the flow into the MS.

Ideally you want only just enough additional gas going to the switch to push the analytes from one side to the other - if you had a column downstream of the switch (which would be the classic application of a Deans' switch) you would need enough pressure at the switch to drive carrier gas through that column - but you only have an MS downstream so you should need only a whisper of pressure to make the switch work.

I was under the impression that the Agilent Deans' switch ran entirely under the control of their fancy EPC pressure and flow programming, so the instability is a bit of a surprise. Maybe Gasman will chip in.

Peter

[rb6banjo]

My Agilent system that employs their Dean's Switch is the classic mode:

GC: Agilent 7890
First column: ZB-5 (Phenomenex) 30 m x 0.32 mm x 0.25 µm. Inlet pressure is 8.42 psig. Transfer line to detector 1 (FID) is 3.64 m of 0.25 mm deactivated fused silica.
Second column: Simplicity Wax (Supelco) 30 m x 0.32 mm x 0.25 µm. Midpoint pressure 4.32 psig. Outlet of this column goes straight into my 5977 mass spectrometer.

I get what is essentially a boiling-point-type separation on the first column. I can heartcut whatever fraction of that (in time) to the second column so I can get a completely different selectivity for a hopefully more refined separation. It's a very powerful setup for maximizing separation power. The 4.32 psig at the midpoint is sort of "pushing back" against the 8.42 psig at the inlet so the calculations provided by the Restek flow calculator don't really apply. I arrived at these conditions by starting with the Agilent Deans-switch flow calculator and then empirically setting the inlet pressure by successive injections of the same sample to ensure the "burp over" at the switch to one detector or the other was a minimum. I was shooting for the best peak shapes on the first column separation - especially for the early eluting materials.

Here's an example of what can be done with this sort of thing. Separating menthol from camphor in a medicated rub is difficult on any single phase. But with a system like what I describe, you can get it:

https://onedrive.live.com/?authkey=%21A ... ot&o=OneUp

The 4.32 psig that feeds the second column is a little larger than optimum but the EPC modules are not as reliable at those really low pressures. So, I settled on these conditions. I've been running this way for a long time now and it basically works for my applications.

[James_Ball]

With the PCM-A and the long narrow transfer line, I believe the system by default may be set up to do backflushing of the column through the inlet at the end of the run. What may be happening is the pressure at the end of the column is increasing but the inlet pressure is not decreasing to allow backflush and it is causing the flow calculations to be messed up. I believe the Mass Hunter software has a built in calculator for figuring the backflush paramaters, but it may not be able to take into account the second column. If that is what is happening you will need to disable the PCM-A portion.

When I run dual columns I don't have the Deans switch but I do have to run a large bore very short transfer line, so that the end of the column is seeing vacuum and not pressure so that the instrument can calculate proper head pressure for the flow setting requested.

If the switch is confusing the inlet pressure calculations it may be why it works in constant pressure mode and not in constant flow mode.