LOD calculation with Xcalibur

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

3 posts Page 1 of 1
Good evening everybody,

We are using in our lab a Thermo GC-MS, with Xcalibur software (quite a little bit old model).

We are validating a method and we have to calculate the Limit of Detection (LOD). I'm supposed to inject ten times a reagent blank and see how much the noise signal is, in order to get the signal where S/N equals or exceeds 3.

Here goes my question: How can I proceed to know the noise at my retention time of interest? (apparently, the Rt where my analite would elute, isn't it?). I have been seeking and the only thing I've found is an option where peaks are labelled with, for example, "17 SN RMS". However, reading the Help guide I've understood that noise is expressed as RMS. But I thought SN meant Signal to Noise ratio. I quite don't understand it, and the Help guide is not too helpful with my queries.

COuld you help me find the light with this?

Thank you very much

Laura
Be extremely cautious with Xcalibur. As I remember, the older versions had three integrators. I've only used it in an LC context so it may be different in GC. Of the three, Avalon didn't quantify S/N at all, but Genesis and Isis used two different approaches, which gave results for the same data that differed by an order of magnitude or more. In my view, given that Xcalibur produces self-contradictory results, and didn't seem to have any documentation about where these S/N values come from, if you're trying to use S/N to estimate LOD according to a reputable source such as ICH, you should probably abandon Xcalibur and check the noise manually. But of course manual data-handling isn't very auditor-friendly. It's not an easy situation. It's just not sensible when you can get a LOD ten times better by using a different integrator, even when the integrator is no better at detecting small peaks!

ICH's definition involves looking at the distance from troughs to peaks in the data around 5 (I think... check that) peak-widths either side of the peak. I don't think there was necessarily an obligation to run blanks; the baseline either side of the peak in a standard was fine. Personally I've rather lost faith in LOD's unless in spiked matrices (i.e. real samples) because you need a lot more of something to be sure it's there, if it's sandwiched between nearly-coeluting contaminants than if it's on the background of a beautiful blank run.
Thankyou for your extended response, lmh, I really appreciate your ideas.
I tried to use Xcalibur software and I couldn't manage to get any values....I think I will have to fight against the softare.... I have only used it to get routine sample results, 'til the date. I'm new at validating.
So I did what you proposed....do it manually. I guess that auditors won't like that, but there are no other means... plus when you say that the 2 integrators algorithms yield so much different results.
On the other hand,I've read in this forum that S/N is not too appropiate to determine LoD. So... what are other options? Linear regression and LOD=3*Sb? I'm using Mass Sprectrometry only for identification, not quantification... is this method still appropiate for my purpose?

Thanks in advance.

Laura
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