Matrix Spike Dilution

Basic questions from students; resources for projects and reports.

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I'm trying to do a matrix spike and I'm getting mixed up with the dilution and was wondering if this forum can help.

The calculation I am using for the spike is:

%R = ((Spiked sample result - unspiked sample result)*100%)/ (known spike added concentration * DF)

DF = Sample volume/total volume of sample plus spike

When I made my standards for 2 g/l I added 200 microL to 800 microL of water. (10 g/l stock solution).

For the spike I added 200 microL to 800 microL of beer (beer is sample matrix)

Doing the spike this way is there a dilution factor that comes into play?

The actual data I am using is as follows:
Spiked sample - 2.15
Unspiked - 0
Expected Spike - 2.8735
I'm not exactly clear on what you're doing. Generally when you fortify your sample with your analyte, you want to do so such that you don't alter the size (usually volume) of the sample. For instance, if you want to determine methanol in an early fraction from a distillation (e.g., making moonshine whiskey) and you're looking for 100 parts-per-million in the sample, you might analyze your sample and then add 1 µL of pure methanol to 4-5 mL of the sample and analyze again. 0.001 mL in 5 mL does not change the volume of the sample appreciably but it does significantly alter the methanol concentration in the sample.

0.001 mL x 0.791 g/mL x 10^6 µg/g/5 mL = 158 µg/mL added

Say your sample gives a response for methanol of 1,200 cts. Now, you add your analyte and you measure 3,600 cts. The unknown concentration is:

Cunk = 158 x 1,200/(3,600 - 1,200) = 79 ppm

I generally like to do more than 1 addition to verify at least some degree of linearity but that's basically the classic "method of standard addition" technique.
I'm not quite sure what's happening here either.

I don't think you need to take into account any dilution factor for your "spike".

You've prepared your standard the same way as your spike using the same volumes and concentrations. If your standard gives you a result of 100 whatevers and your "spike" 86 whatevers then your recovery will be 86% I suppose.

You should however prepare your unspiked sample the same way though. 800 ul of beer plus 200 ul of water. Then as you suggested subtract your unspiked sample from your spiked sample. Then you'll have no need to muck about with DF's and whatevers.
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