Clarity discussion forum

Dear Colleagues,
I have for example following sequence:

• Hexane 1
  Sample 1
  Hexane 2
  Sample 2
  Hexane 3
  Sample 3

Is it possible to subtract from each Sample chromatogram previous Hexane chromatogram (sample x – hexane x) with any command or batch and save result as a new file? Or in the other way: From selected samples subtract one selected Hexane chromatogram and save result of each subtraction to a new file? The process that we use for samples evaluation is according to ISO 9377-2 (Water quality - Determination of hydrocarbon oil index). We have to subtract the column background from the sample.

Best Regards
Tomas Paul, Clarity 2.6.6.574

Dear Tomas,
Clarity enables you to subtract the results of another chromatogram from your sample. The subtracted chromatogram file is specified in the method, on the Advanced tab to be precise. However, the subtracted chromatogram is common for all samples measured according to the method, so the automatic subtraction of the previous sample is not supported at the moment. Moreover, the blank which is measured in the same sequence and is measured according to the same method can't be used as a subtraction chromatogram during the acquisition because it is not measured at the time of the measurement of the first sample and would be also subtracted from itself.

On the other hand, I strongly believe (but may be mistaken at that point) that the DIN EN ISO 9377-2 norm does not require the measurement of the blank before each run, only the subtraction of the background, which could be measured just once during the sequence and then subtracted from all samples using the Batch function. Don't hesitate to ask if you would need any further help with that.

If you would be interested in the possibility of subtracting the background sample based on some variable (similar to what can be used in the file name creation in the single run or sequence), please make a thread in the Clarity Wishlist section. We would create a poll for that option and see if the issue is interesting for more users. That could speed the process of the implementation up.

Best regards,
Daniel Mentlik

Dear Daniel, thank you for your quick reply. I works good. I have created method especialy for evaluating and substract first hexane from all samples with batch process (Reprocess by instrument method). Unfortunatelly I have no choice to save reprocessed samples with another file name. Files are overwritten. I will experiment little more with this functions.

On the other hand, sometimes it hapens that measered sample contains heavy hydrocarbons that are eluated to the following sample in sequence. Please see attached chromatograms. So we suppose it is more correct to subtract previous hexane which is influented by heavy fraction of hydrocarbons and increase the baseline. The standard EN 14039 (matrix Waste) is more strict with background correction then EN ISO 9377-2 (matrix Water).

In a few days I will post the request to Clarity Wishlist section as you suggested.

Once more thank you.
Tomas

In fact, the chromatogram files are not overwriten, they are just saved with a link to the subtracted chromatogram. You can get back by opening the saved version from history or just removing the subtracted chromatogram link in the chromatogram, measurement tab.

Regarding the blanks - subtracting the preceding contaminated blank could introduce a significant negative error in the next sample as the contamination will surely be significantly lower in it.

Really, no overwritten files. That's great. I'm still learning new capabilities of this great software. Thanks.

Regarding contaminated blanks by column elution followed with sample. If I subtract from sample first blank in sequence instead of previous blank It leads to the positive error. Some dirty samples containing heavy hydrocarbons are eluated to the next several blanks. In some cases we subtract the blank that is immediately after the sample. The question is which error is smaller. It is individual and it depends on type of contamination. In many cases we have to confirm samples which are influenced by previous very positive samples on another column or after heating contaminated column when oven temperature is set to 345°C for 0.5h.