Chromatography Forum

hello all,

I am generally pretty good with Empower Custom Fields, but I am terrible with inter sample calculations.

Here is the calculation I need to do. It is for calculating % Related compounds for each peak in a sample.

it is

100*Area/(Sum_of_Area_of_all_peaks_in_the_Sample -Sum_of_Area_of_all_peaks_in_the_blank)

This seems like a pretty straight forward calculation, but I have been wracking my brain over it the past few days, and I decided to come here for help.

I have tried numerous Iterations on the custom fields using a single custom field and multiple custom fields to try and accomplish the calculation.

Here is my current custom field
It is peak and real type, with results in the sample set only. I made sure that the blank is labeled blank (all lower case), and I added summarize custom fields to the end of the sample set. I also created a group peak, which is the sum of all the peaks in the chromatogram, the group peak is SumAll.

100*Area/(SumAll[Area]-blank.%.%.SUM(Area))

The portion labeled blank.%.%.SUM(Area) seems to be returning 0, and it should be ~54,000

100*Area/SumAll[Area] works perfectly and returns the correct number.

Any suggestions or help would be greatly appreciated. This is not the only attempt I have made, I have tried many other approaches, it is just my current one.

The peak areas between peaks in you sample solvent from a blank to actual samples are going to vary slightly. This is one of the reasons why people sort of frown on a blank subtraction in chromatography, which is sort of what you are getting around to doing.

What about creating a named group in your PM with all of your blank peaks. You can then do a per sample area correction by calling the following:

100*Area/(SumAll[Area]-BlankPeakNamedGroup[area])

Ought to get it done for you. Just a different way to skin the cat.

Shaun78,

Thank you very much for the reply and the suggestion. While I agree with you completely and I would never have developed the method like this, I am unfortunately working with a validated method, and it was validated with the subtraction of the blank area from the sample area.

My hands are tied, I am just trying to have a custom field that can do the calculation.

Well, unfortunately my main reason for suggesting that was it is the easiest way to solve your problem.

I need to do some custom field work on an impurity/LIMS project for the company that I work for today. While I am at it I will play around with this as it closely matches something I need to accomplish today.

I will let you know if I am able to come up with anything.

Unfortunately, I am assuming an inter-sample calculation of this nature is going to be difficult because you always need the values from your blank tied to your present sample regardless of system suitability and sample bracketing.

What you have in your initial post ought to work. However, it is going to get knocked out every time you have a clear calibration, which of course you need for your bracketing...

Grrrr.... I'll let you know if I am able to come up with anything.

Shaun78 thanks for the help.

Anyone that is interested I was able to resolve the problem.

Apparently, there is a function in Empower called Total Area, that returns the total area for a chromatogram.

Using this I was able to accomplish the custom calculation with the following custom field.

100*Area/(Total Area-blank.1.SAME(Total Area))

This takes 100 times the area of the current peak, and divides it by the total area of the current chromatogram - the total area of the blank chromatogram.

Thanks for the help and input.

Heh... you beat me to it... I discovered that yesterday as I was nearly out the door.

Sorry I could not be of more assistance, but I am glad you got your problem sorted!