Initially i Think your detection limit too high for ECD. You use GC-MS just to confirm some compounds, am I right? 17-19% to expanded uncertainly in water matrix is a robust result to low concentration spikes. More concentrated samples (500 ng / L), as yours, should have lower values. However sample extration and manipulation become a huge source of deviation.

Sometimes despite our efforts we notice that calculated standard deviation from samples replicates is quite low but expanded uncertainty assumes high values. Is this pattern to your data?

It is normally derived from uncertainty sources. This pattern can be found mainly in volumetric analysis. I assume you to quantify samples using internal calibration procedures.

More information would help a lot to provide a better answer

Hope its useful.

I thanks Carlos for your concerned reply.

Yes GC-ECD were use for quantitative and GC-MS for qualitative confirmation. the value 500 ng/L is purposely done, since our field samples are supposed to analyse at detection limit of 0.5 ppb.

Relative std deviation vary from 0.005 to 0.146 for all analytes.

please elaborate the point of quantifying samples using internal calibrated procedures.

Reply will be appreciated.

Initially my apologies for delay in answer. I asked about internal standard procedures because some scientists prefer use it before extraction as surrogate standard. This way allows to correct any losses in extraction and manipulation. Some americans and europeans laboratories analyse PAH this way.

Perhaps you can do the same to your samples to reduce variation. Personally I think it is not the solution, just something to future considerations.

Hope its useful.

Best Regards

Carlos German Massone

cgmassone (Skype)

Oh k, well thanks for your answer carlos, i ll look in details of using internal standards.