Chromatography Forum

By definition Limit of Detection (LoD) is described as the smallest concentration of a measurand that can be reliably measured by an analytical procedure. But this can change over time.

And although a validated analytical procedure studies variations over short time, it never evaluate long term changes.

Keeping in mind that we have to garantee the LOD to our client, how do you deal with this? Go for the worst case samples in validation? Give some extra limit to be sure? And if so, how much?

Once a audit expert told us that it is expected a procedure to performe up to 3x times worse what we see in the validation. But this realy depends what samples we have to validate, doesnt it?

Once a mesurement expert told us that in the US people usually buy an equipment that is able to do 10x more than they realy need, so they are sure no matter what that it will be able to performance ok.

Once a positive case is detected, a second analysis can be asked by a court of law several months after. So you can understand how it can be a problem if we arent able to verify the result few months after because our LOD got worse.

PS. Controlling forbiten and cancerogenic residues coumponds in food.

Thank you all for your time and knowledge!

One thing you can do is to analyze a duplicate sample spiked at 1-2X the LOD to be certain you are seeing that level in each sample. Make the quality control statement that you must have a detect for the spiked sample for the target analyte to assure all samples meet the required LOD.

You can also use an internal standard and require all samples to have a minimum response for that internal standard that would show that the instrument is still giving the same performance. Gain on the detector would be adjusted to keep the internal standard at the same levels on a long term basis(or instrument maintenance if that is what is needed).

It looks like maintenance is what is needed although we are short of money. But yes, gain in also an option. We used to use it on GC-MS equipment but we hadnt used it in our LC-MS/MS although there is an control in the software.

Thanks very much for your sugestions.

Increase detector gain do help to keep the detector respose in area and height, but in our case it doesn't do anything about S/N. Signal is just enhanced as much as noise.

So we do keep missing detection cabability with time.

By definition Limit of Detection (LoD) is described as the smallest concentration of a measurand that can be reliably measured by an analytical procedure. But this can change over time.

What you have described there is Limit of Quantitation (LOQ) not LOD.

In analytical chemistry, LOD (limit of detection), is the lowest quantity of a substance that can be distinguished from the absence of that substance (a blank value) within a stated confidence limit (generally 1%).

By definition Limit of Detection (LoD) is described as the smallest concentration of a measurand that can be reliably measured by an analytical procedure. But this can change over time.

What you have described there is Limit of Quantitation (LOQ) not LOD.

In analytical chemistry, LOD (limit of detection), is the lowest quantity of a substance that can be distinguished from the absence of that substance (a blank value) within a stated confidence limit (generally 1%).

True. Thanks for seeing that.