GC Control
Determining a Gradient

A Gradient for a gas chromatograph can be either a temperature gradient, for example, for the GC oven, or a pressure or flow gradient, for example, for a GC inlet. There are two ways how to determine a gradient. You can do this either manually on a control panel or automatically via a program:

On a control panel

Different dialog boxes are provided for each device for which a gradient can be determined. Use these dialog boxes to enter the device-specific settings.

  1. Open a control panel for the device for which you want to define a gradient.

  2. Select Flow on the Control menu
    - or -
    Press <CTRL> + <F>.

  3. Determine the desired flow, pressure, or temperature gradient on the GC tab page and further tab pages for the columns and/or inlets. Continue as described under "Entering Basic Points" below.

In a program

The easiest way to create a new program is to use the  Program Wizard.

  1. On the File menu, click New, and then select Program File.

  2. On the GC Options page (or the other pages that support a temperature gradient), click the arrow next to the Type field, and then select Ramped Temperature from the list. This setting allows you to program a temperature gradient. In the same way, you can program flow and pressure gradients by selecting Ramped Flow or Ramped Pressure on the pages for columns and/or inlets. Continue as described under "Entering Basic Points" below.

Entering "Basic points"

This is a description of how to define a temperature gradient:

  1. Select the initial temperature in the Init line.

  2. In the Iso Time column, determine for how long this temperature is valid.

  3. In the next line, enter the slope of the ramp (Rate in °C/min) for changing the temperature. In the middle column, type the Temperature to which the GC willl be raised with this first ramp. Return to the Iso Time column and determine for how long this temperature will remain valid.

In this way, program multiple temperature ramps. (The maximum number of temperature ramps depends on the installed device. Usually, there are three ramps; for the oven of the Agilent 6890 GC, there are six.)

 Tip: 

After the program run, each sample is automatically reset to the original state, i.e., for example, the initial temperature is restored. Restoring the initial temperature in this way is usually the quicker way, compared to setting the corresponding cool-down command in the program.

Direct Input into a Program

If you know the Program syntax, you can directly determine the gradient in the program. Entering the Flow, Pressure and Temp. commands in the program allows you to change the flow rate, pressure, and temperature at a precise time. The gradient profile results from the change in value of a certain quantity at the time t.

For information about how to determine a gradient for HPLC/IC, refer to  Pump and Flow Control: Determining a Gradient.