Mid-point backflush Retention Time Locking Calculator

Retention Locking using a mid-point backflush column configuration with GCMS or GCMS QQQ systems


Currently both GCMS Chemstation and GCMS QQQ Masshunter have a built in feature to perform retention locking. However this feature will not work with all column configurations. In particular the now increasingly popular mid-point backflush arrangement cannot be retention locked using the built in utility. It is necessary to use the RT locking calculator spreadsheet.


A typical mid-point configuration consists of two identical 15m x 250u x 0.25u columns. The 7890A Configuration should reflect the following:

    • Column 1    Inlet -------?         Aux EPC  Constant flow 1.2 ml
    • Column 2    Aux EPC------?   Vacuum    Constant flow 1.4 ml


With this as a starting point, create 4 additional methods using the identical oven program and zone temperatures while setting the column flows as follows.

     Column 1     Column 2

     1.0               1.2

     1.1               1.3

     1.2 (already created as the base method)

     1.3               1.5

     1.4               1.6


Once these methods have been created, run a standard containing the compound that is to be used as the RT locking compound. Upon doing so there will be 5 different elution times for this compound.


Using the attached excel spreadsheet calculator, fill in the Ret Time columns with the elution times that were achieved and fill in the Desired Locking RT elution time for this compound in place of the x.x minutes.


Fill in the field labeled Pressure (or flow) you just ran: with the flow rate that provided the closest RT to the desired RT.


Fill in the field RT you just got: with the RT closest to the desired locking RT in place of the x.x minutes.


This will create a curve from which you can determine the best flow rate for column 1. That flow rate is displayed in the yellow field

Set your pressure or Flow to:

Once this flow has been determined Column 2 is always set precisely 0.2 ml higher.


See the additional instructions for relocking the method built into the spreadsheet.

rt lock

mid column concurent






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