Spike volume

Discussions about sample preparation: extraction, cleanup, derivatization, etc.

5 posts Page 1 of 1
In our lab, we adhere to the rule that we do not spike samples (wholeblood) with methanolic standards by more than 8% of the samplevolume. This is mostly in order to keep the matrix as similar as possible, however nobody knows where the 8% is based upon. After some reading you find that people use different mixima for this, ranging from 5-10%.

Does anyone here have an opinion on this and maybe some references?
The only thing I can think of is what you cast in your allusion. Someone "once-upon-a-time" studied it and determined that somewhere slightly north of 8%, they found that there was a measureable difference in how the matrix behaves in the analysis.

The other consideration might be the dilution factor. Adding 8% MeOH changes an analyte concentration that is really 50 ppm to 46 ppm. Could it be, that's within the error of the measurement and someone didn't want to worry about dealing with dilution factors?

A little more detail on what you're actually trying to accomplish in your analysis might help you get better answers than what I've provided here.
rb6banjo wrote:
The only thing I can think of is what you cast in your allusion. Someone "once-upon-a-time" studied it and determined that somewhere slightly north of 8%, they found that there was a measureable difference in how the matrix behaves in the analysis.

The other consideration might be the dilution factor. Adding 8% MeOH changes an analyte concentration that is really 50 ppm to 46 ppm. Could it be, that's within the error of the measurement and someone didn't want to worry about dealing with dilution factors?

A little more detail on what you're actually trying to accomplish in your analysis might help you get better answers than what I've provided here.


I am thinking the dilution factors is most likely the culprit. I worked with IC samples in the past where 10ml samples are spiked with 1.2ml of spiking solution. Recoveries work well for samples with no target analytes, but for samples with target analytes the recoveries were off, because we weren't initially taking into account that instead of the final volume having 10ppm from the initial sample it was now down to 8.3ppm and if spiked at 5ppm you would then have 13.3ppm instead of 15ppm so the recovery always came out at 88.7% instead of 100%.

I most prefer to spike microliters of spike into 10's of ml of sample so the dilution is not noticeable in the final solution.
The past is there to guide us into the future, not to dwell in.
dp_kloos wrote:
In our lab, we adhere to the rule that we do not spike samples (wholeblood) with methanolic standards by more than 8% of the samplevolume. This is mostly in order to keep the matrix as similar as possible, however nobody knows where the 8% is based upon. After some reading you find that people use different mixima for this, ranging from 5-10%.

Does anyone here have an opinion on this and maybe some references?


One possibility is that above 8% you get some precipitation of denatured proteins.

Peter
Peter Apps
The problem arises when analyzing wholeblood; normally small molecules partition themselves a bit between freely dissolved and bound to proteins/ into cells.

When spiking some analytes in an organic solvent could cause a different partitioning and therefore the spiked sample, does not represent the original matrix any longer. When more organic solvent is added, more analyte will be unbound in solution.
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