Calculating the Headspace Sampler GC Cycle Time

Answer

This Information Applies To: Agilent headspace samplers (HS)

Issue

GC cycle time needs to be calculated correctly. Too short a GC cycle time will reduce sample quality and possibly stop the analysis prematurely. Too long GC cycle time can cause reduced sample throughput as vials wait longer than needed before processing.

Background

GC cycle time is the total time required for the GC (or GC/MS) system to return to a ready state after a sample run. HS uses the GC cycle time to calculate throughput and timing. An accurate GC cycle time is crucial to optimize throughput and for correctly processing samples. 

Tips: It is better to enter a longer time than needed than to enter too short a time and possibly reduce sample quality.

Steps to follow

• Manual calculation
• For Agilent GCHS systems controlled by Agilent software (Note: Applicable to OpenLab CDS 2.x, OpenLab ChemStation C.01.X, and MassHunter Acquisition only)

Manual calculation

1. Check GC Run time.

2. Check GC Oven equilibration time. The default value is 3 minutes.

3. Check GC Cooling time. The cooling time is the time to go down to the original temperature after your analysis is over. It is recommended to calculate this parameter manually.

4. If needed, check GC prep run time and inlet cooling time.  However, these parameters are not commonly used and should be added only when necessary.

5. Next, add all timings consolidated from Step 1 to 4. 

6. Finally, with the total time calculated, do an addition of 3~5 minutes. The reason for adding this time is that GC is highly influenced by the surrounding environment, so there may be variables. Therefore, this addition of time is required for that variable.

Caution: This calculation can be flexible because it does not always come out with the same value. The time may increase or decrease depending on the laboratory environment or changes in ambient temperature.

Follow these steps for Agilent GC/HS systems controlled by the following software products:

• OpenLab CDS 2.x 
• OpenLab ChemStation C.01.X
• MassHunter Acquisition

To track GC cycle time via OpenLab CDS 2.x

1. To edit the headspace sampler configuration, click Control > Configuration. See Figure 1.
2. The Configuration-Agilent 7697A window will be displayed, click the Preference Tab. 
3. Make sure to tick the Track GC cycle time check box under Method Development. 
4. Click OK to set the HS configuration. 
5. Run a sequence of at least two samples with and GC cycle time setting of at least 5 minutes greater that the GC oven run time plus the time needed for the oven to return the initial setpoint.
6. When the sequence is completed, enable the Activity Log in Acquisition. Scroll down the activity list and find the Calculated GC cycle time. See Figure 2.

 
Figure 1 OpenLab CDS headspace sampler configuration page
1.Control menu, 2. Configuration tab, 3. Preferences tab, 4. Option to tick Track GC cycle time, 5. OK option 

 
Figure 2. Activity log under Home of OpenLab CDS
1.Home tab, 2. Activity log option, 3. Calculated GC cycle time shown in Activity log

To track GC cycle time via OpenLab ChemStation 

1. In the instrument control view, click Instrument > Agilent 7697A Configuration. See Figure 3.
2. The Agilent 7697A Configuration window is displayed. See Figure 4. 
3. Click the Preferences tab, and tick the Track GC cycle time check box under the Method Development section. 
4. Click OK. 
5. Run a sequence of at least 2 samples with and GC cycle time setting of at least 5 minutes greater that the GC oven run time plus the time needed for the oven postrun and return to the initial setpoint.
6. When the sequence is complete, you can find the Calculated GC Cycle Time by clicking in the sequence name.log file in the sequence folder or by looking in Run.log file in each date file folder. See Figure 5 and 6. 

 
Figure 3. Instrument menu under Method and Run Control 
1. Instrument menu, 2. Agilent 7697A Configuration

 
Figure 4. Agilent 7697A Configuration page 
1. Preference tab, 2. Tick box for Track GC cycle time

 
Figure 5. Example of sequence log file
1. Sequence name.log file, 2. Calculated GC cycle time

Figure 6. Example of Run.log file 
1. Run.log file, 2.Calculated GC cycle time 

To track GC cycle time via Masshunter Acquisition

1. Click Instrument > HS Edit Configuration menu. See Figure 7.
2. Click on Preferences tab and check the Track GC Cycle Time box under Method Development. See Figure 4.
3. Run a sequence of at least two samples with and GC cycle time setting of at least 5 minutes greater that the GC oven run time plus the postrun time and time needed for the oven to return the initial setpoint.
4. The Calculated GC cycle time will be recorded in the mslogbook_history folder in {date and time} mslogbk.htm file. Example is shown in Figure 8. 

Figure 7. Instrument Menu
1. Instrument tab, 2. Option for HS Edit Configuration Second label

 
Figure 8. Overview of mslogbook_history folder
1.Directory of mslogbook_history folder, 2. MSlogbk.htm files for sequence run on specific date and time

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