Chromatography Forum

I am hoping that someone out there can lend me some assistance. I am trying to figure out how totalchrom calculates the raw amount. We recently upgraded to TCWS6.3.1 from TCWS6.2.0(?) and the new version no longer calculates the raw amount for peaks that go off scale (I was told by PE that they took the ability out so that the new version would be better for the user - which it isn't now). So if anyone out there can lend me some help I would really appreciate it. Thanks so much.

I don't know anything about TotalChrom, but your question is a bit confusing. When you say "off-scale" do you mean that the peak is larger than the current screen settings? There shouldn't be any issue with this, because the system should calculate the results from the digital data. The peak can be brought on-scale by changing the display range.

If off-scale means the peak flattens out even at full-scale, then you have a different problem. This is called saturation and it is not a good thing if you want to use any data from the peak. It is unlikely that you can get useful analytical data in such situations.

If your problem is different than these, then please write back with more details.

Thanks for the reply Merlin (by the way I've taken one of your classes at EAS). The second part of your post is what I'm experiencing. In the older version of totalchrom, if a peak went off scale (above 1000mv) then obviously it was flat across the top but the program would still calculate a value for the area and state that it wasn't a "true" value. We would use these values as a way to judge what we might do next, if anything else were possible. The new version no longer does this calculation, simply says off scale. I am working on adding a "custom expression" to my reports to give me an idea of what the area is and what the results might look like if the peak had been on scale. We are only trying to do this to save some data, not as a way to run all of our samples. Can you provide calculations as to how totalchrom, or a similar system, calculates the areas? Thanks so much for your assistance. This is on a GC-FID system.

Ryan:
Good to hear from you.

I'm not sure I can provide a perfect answer to your question. The exact algorithms used in these programs are proprietary. But here are some general comments.

The system collects a data point at whatever sampling rate you have set. This number is associated with the time since injection, and is usually stored in a data array. Once the data are collected, the system will process it, finding the peak start and stop points, along with the max (retention time). A baseline is drawn between the start and stop points (actually, there are several options for doing this, but let's forget that for now). For each data point, the system subtracts the baseline from the raw data point, and adds up all the values. This is the area. It is essentially a histogram sum. You may sometimes see units like mV-sec, indicating that the system calcualted the area of each histogram slice (response multiplied by time).

To do this yourself, you would have to get the raw data values. Maybe they could be exported to a file and imported into a spreadsheet.

But again, the information is only marginally useful because the system is overloaded.

If you have more questions, email or call. Do you still have my contact info?