Chromatography Forum

We are using ChemStation V. 08.03 so this may have been fixed already. Can anyone explain how to calculate ESTD results for samples using the average response of standards that bracket it in the sequence table?

We have a policy that an intermediate standard must be run after every 10 sample injections. On occasion, we need to average the response of the bracketing standards and use that value to calculate the results of the samples in that segment of the sequence table.

I've tried following the manual (such as it is) and I just can't get it.

Sorry no one has responded. I just joined and found your post.

First, are you using the "Bracketing" feature in the sequence table? You need to set the injection type to Bracket rather than Calibration. With Bracketing, the system will automatically inject a sample at the end of the sequence, then average the results, and calculate for all samples in between. If you want this done during the run, then use a value in the Interval column, which will then automatically inject the standard after that many sample injections.

This approach is not very flexible, but may give you what you want.

Thank you for your response.

If I have 35 samples and I want to inject standard after every 10, how would I set up my sequence? I understand that I would have 3 segments with 10 samples each, but what would I do to include the last 5 ?

Do I actually include the intermediate calibration vials in the sequence table, or will the instrument automatically go back to the same vial that was injected before the samples?

Would I just put my calibration vial first and then list all 35 samples behind it? Would the instrument automatically make a standard injection after the first 10 and then at the end of the run also?

It's exactly like you said in your last paragraph:

Put your calibration vial first and then list all 35 samples behind it. The instrument automatically make a standard injection after the first 10, after the 20th et 30th and then at the end of the run for the last 5 samples.

The interval is for the number of injections not for the number of samples if you make more than one injection for each sample.

A few comments on using the bracketing feature:

1. The Sample Type must be set to "Calibration." Only then can you select Bracketing in the Update RF column.
2. As stated, a value in the interval column will repeat this standard after the indicated number of "injections" not samples. And, yes, it will inject from the same vial.
3. The system starts a new calibration table at the start of the run, then injects the samples, followed by the standard again. THEN, the system averages the two standards and calculates results for the samples already injected. You won't get any reports until the second calibration is complete.
4. Now the system resets the calibration to the most recent standard, and continues on.
5. The only problem with this approach is that the calibration is changing throughout the run, and some people don't like that approach. You can track the values used in the calibration by looking carefully at the reports, remember that the calibration info at the end of the sequence is not the same as that during the sequence.

If you want to see the actual sequence of injections, go to Sequence, Partial Sequence, and you will see what will happen.

Good luck.