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Low sensitivity for higher MW components in GC

acraig

Hello,

 

I am using an Agilent 7890B with HP-1MS column SN19091S-913E.  Periodically (~25%), when installing a new column, i run into issue where components eluting >35minutes have severely reduced sensitivity. GC parameters at the bottom using splitless injection.  

 

i.e, one standard will have component not detected, while a standard that is 2x concentrated will have area count 100.  I am using multiple equipment and verify there is no problem in standard prep because the other equipment have normal sensitivity and linearity.

 

I have reviewed common problems and solutions here...

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1) Standard too cold/compounds precipitate.

      - Unlikely to be cause because other equipment is operating fine at the exact same time using same standards.              -Also, previous column installed on the now troublesome equipment showed normal sensitivity.

2) Componds are not properly vaporizing in the injection port.

      -Removed glass wool from the liner and reinjected same standards.  No improvement to sensitivity.  Are there other ways to see if this is happening?

3) Column has contamination.

      -Issue has been found multiple times when installing brand new column, probably not the root cause.

4) There is a cold sport somewhere.

      -Not sure if changing column could introduce cold spot somewhere somehow.

5) High column bleed

      -Not observed

6) Purge valve (split) time set too short.

      -This would be a method problem.  This method shows normal sensitivity on other equipment and has on this equipment before replacing column.

7) Missing insulation around injection port.

      -Insulation present.

 

GC parameters below.

 

Injector temp: 300C

Detector (FID): 350C

Gas Flow: H2 40mL/min

                 Air 400 mL/min

           Makeup (He): 20 mL/min

           Constant Pressure: 20 PSI

 

60C start temperature for 1 min

10C/min to 300C

2.5C/min to 325C

325C hold for 20 mins.

 

I have just reinstalled the same column backwards with the outlet side now at the inlet, and cut off ~1foot of the inlet and installed at the outlet.  I am reinjecting same standards to see if maybe problem was on inlet side of column.

 

Since this problem occurs regularly frequently when installing new columns, does anyone have advice what i could try for troubleshooting next?

 

Thank you in advance.

Parents
  • 0

    Good morning,

     

    Yes there is an issue here with sensitivity.

     

    Have a look at your max temperature at 325 degrees. Do you have a problem with column bleed or raising baselines also? as with a new column this maybe the cause.

     

    at 320 degree C-H bonds start to degrade and in the presence of oxygen start to break down to CO2 and H2O which in turn affect your sensitivity. So not only can column bleed cause it to lower but the change in particles in the mobile phase can too.

     

    Another cause can be a leak in the system allowing your analytes to escape. This can be from a leaky septum, not sufficiently tightened gold seal/inlet ferrule/detector ferrule, can also be the length of the column into the inlet/detector. 

     

    And another thing is ensuring the FID has the correct gas flow and is clean of debris.

  • 0 in reply to gchaplai

    Hello,

     

    There is no issue with column bleed or raising baselines at 325C.

     

    I replace septum regularly and do not see any issue between replacements on a given column so i do not believe that to be the cause.  If a leak is the problem, i am leaning towards the tightening of the ferrule's on the inlet/outlet connection since i only see this issue when installing new columns.  The length of column is 4-6mm on inlet side and about 47mm on the outlet, i believe these are correct measurements.

     

    I confirm column lengths when installing by semi-tightening the connection, and then removing the connection to verify my column distances.  Once verified i retighten using same ferrules.  Is this potentially a bad practice?  I am not sure how to verify my column length into inlet/detector otherwise.

     

    Thank you for responding!  

  • 0 in reply to acraig

    right this maybe where this is going wrong on column length, even 0.5 mm out of the correct zone can cause issues.

     

    normally we use the column to pierce a septum (to set the length into the inlet or detector)

    put the nut on, and then the ferrule (in the correct direction)

    cut the column between the ferrule and the end of the column 

    and then adjust the septum so that the distance between the end of the nut and the column is the correct length

     

    Then tighten the nut into the detector or inlet whilst holding the septum, that way the length is set and not need to check the length afterwards:

Reply
  • 0 in reply to acraig

    right this maybe where this is going wrong on column length, even 0.5 mm out of the correct zone can cause issues.

     

    normally we use the column to pierce a septum (to set the length into the inlet or detector)

    put the nut on, and then the ferrule (in the correct direction)

    cut the column between the ferrule and the end of the column 

    and then adjust the septum so that the distance between the end of the nut and the column is the correct length

     

    Then tighten the nut into the detector or inlet whilst holding the septum, that way the length is set and not need to check the length afterwards:

Children
  • 0 in reply to gchaplai

    This is amazing!  I have followed your practice (on the inlet side) right until the end when it comes to how i measure column distance.  I have always measured 4-6mm from the end of the nut.  I see in your comment you specify to measure from the end of the nut, but your picture suggest you are measuring 6mm from the end of the ferrule.  Could you clarify which is correct?  Since 0.5mm off can cause issues, if i am measuring inappropriately then this could certainly be the cause.  

     

    I will try re-installing column that have had sensitivity issues with correct procedure and see if i notice differences.  

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