Assessing Impurities with ChemStation/ChemStore Software.

[Anthony Crawshaw]

As a relatively new-comer to ChemStation (Previously, about 6 years experience with Chromeleon) I have one major frustration with the system that no-one seems to have mentioned.

As far as we can see, the ChemStation system is fine, if all you intend to do is simple one peak assays. Anything more than that and it really does struggle. Take for example the quantification of multiple impurities or degradation products in a pharmaceutical product.

There is no way to quantify them!!

The problems seem to be as follows:

1) In a dual assay/degs method. The most common approach is to quantify the imps against the main band assay peak. This is all well and good if all the imps have the same response factor. But how often is this true?? ChemStation can not assign individual response factors to individual peaks unless they are entered into the calibration table. This leads onto..

2) If the imps are entered into the calibration table, then any impurities in sample injections are quantified against the impurity peaks that may be present in the standard injections, rather than being quantified from the main band as desired. This also leads onto...

3) The only way to actually name the peaks (apart from going into each individual chromatogram) is to name them in the calibration table.

4) And if you are unfortunate to have dual actives, then you really do have problems!

This really is a vicious circle. The worrying thing is, that from recent training training with Agilent. They don't seem to know how to get around this.

Surely this is a basic requirement! Its such a shame that Agilent Hardware is of such a high standard, but is only let down with a legacy product such as ChemStation. This is only one of many limitations we have discovered using ChemStation...but I don't want to go on for ever!

Having said all this...I'm hoping that I'm completely wrong and someone out there perhaps has an answer?

[AICMM]

Mr. Crawshaw,

If I understand you correctly, you want to quantify the impurities relative to some other component? I am assuming the active ingredient, right? Doesn't that make your main component an internal standard, at least for the purpose of measuring the impurities against it?

Is it possible to add another component as an internal standard?

I am not sure I understand your problem as written.

Best regards.

[Anthony Crawshaw]

No, this implementing external standards ie seperate solutions/injections to the samples. These standards contain the pure active ingredient at a known amount/purity.

The sample is then assayed against this calibration for both main band assay and any impurities present.

As I mentioned. The main problem is that in many cases, if there are multiple impurites they all have different responses compared to the main band. These obviously need to be taken into account in the calculation. However there is no facility in ChemStation to do this that I am aware of.

[Anthony Crawshaw]

No, this is implementing external standards ie seperate solutions/injections to the samples. These standards contain the pure active ingredient at a known amount/purity.

The sample is then assayed against this calibration for both main band assay and any impurities present.

As I mentioned. The main problem is that in many cases, if there are multiple impurites they all have different responses compared to the main band. These obviously need to be taken into account in the calculation. However there is no facility in ChemStation to do this that I am aware of.

[gcguy]

We run many methods as described using chemstation. It is all a question about where you enter the values, ie. in the sequence table or the calibration table.

For impruities which use the same RF as the main band, the easiest way to assign the RFs is to run the calibration and then copy and paste the calibrant RF into the lines on the calibration table for the impurities.

For impurities that use a relative reponse to the calibrant a correction factor must be put into the calibration table and the above repeated.

For impurities which are calibrated for using a standard simply use the weight and strengths in the appropriate place.

GCguy

[Anthony Crawshaw]

Thanks GCGuy,

Unfortunately I think I am more confused than ever now!!

We understand how to enter a relative response factor for impurities if all the impurites present happen to have the same relative response....that part's easy!

Its when you have multiple relative response factors.

For impruities which use the same RF as the main band, the easiest way to assign the RFs is to run the calibration and then copy and paste the calibrant RF into the lines on the calibration table for the impurities.

Our understanding is, is that if we enter impurities into the calibration table then the impurities in the calibration standard injections will essentially create individual calibration curves for each imp. The imps in the samples will then be quantified from these curves.

ie. Area of Imp in Sample/Area of Imp in standard

What we want is:

Area of Imp in Sample/Area of Main band in Standard

As soon as anything is entered into the calibartion table, as the name suggests it is used to calibrate. We do not want the imps calibrated from themselves.



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[Bruce Hamilton]

Can I clarify whether you are trying to do this on a Chemstore and/or Chemstation system?. I suspect Chemstore may make the system less flexible...

Bruce Hamilton

[gcguy]

If you put the correction value for each impurity into the calibration table as a multiplier you can then use the same RF for the impurities as the calibrant but the results will reflect the differances in response.

Or you could do what I do when Chemstation is confusing me and set up a spread sheet....

GCguy

[Anthony Crawshaw]

Can I clarify whether you are trying to do this on a Chemstore and/or Chemstation system?. I suspect Chemstore may make the system less flexible...

As we are a GMP facility we bought the additional ChemStore/Security Pack. I don't think that is the issue however, as all the upfront calculations are done in ChemStation. However, do know what your saying.

If you put the correction value for each impurity into the calibration table as a multiplier you can then use the same RF for the impurities as the calibrant but the results will reflect the differances in response.

After re-reading that several times, it finally clicked and think I know what your saying. It's definitely worth a try!

[Anthony Crawshaw]

PS...Spreadsheets!! The bain of every analytical chemist!

[Anthony Crawshaw]

GCGuy

Tried it out and think it works!! So big thanks there!

For anyone else out there suffering the same problem, here's what we did:

The main problem had been that as soon as an impurity is added into the calibration table, any impurities present in the samples are then calibrated essentially against themselves rather than the main band. This is a problem. However, its actually the solution as well. What needs to be done is to make ChemStation think it is calibrating impurities against the main band. So...

1) Enter all the known impurities into the calibration table. If you are working via a calibration curve i.e. multiple levels, each of these impurities will need to be entered in at the same levels.

2) Enter the amount for the main band in the calibration table. This can be a weight or concentration depending on how you are calculating.

3) For each of the impurities multiply the amount of the main band by the appropriate response factor, and enter into the amount for that impurity.

4) Note the area (or mean area) of the main band peak in the calibration standards (for each level present).

5) Here's the cleverly simple bit... Force the area of each impurity to be the same as the figure noted in 5)

6) Re-calibrate.

Although ChemStation is still calibrating the impurities against themselves, it is actually using the area from the main band.

If all the figures are correctly entered into the sequence table i.e. sample weights, sample types, and other multipliers such as dilution factors, fingers crossed you should get a result for each impurity.

Although we didn't look into it, I think that any peaks that are not in the calibration table i.e. unknowns, will default to a correction factor of 1 and automatically be quantified off the main band.

[Bruce Hamilton]

5) Here's the cleverly simple bit... Force the area of each impurity to be the same as the figure noted in 5)

Rather circular , and thanks for detailing the solution so others can use it.

Please keep having fun,

Bruce Hamilton

[gcguy]

If you copy the response factor for the calibrant peak and put it into the amt/area column of the impurities and change the multiplier to reflect the difference in response factor you will get the same effect.

GCguy