Hi I am trying to figure out how I calculate percent recovery after we have spiked soil samples with a herbicide, extracted the soil using an ASE instrument, then analyzed using HPLC. I cant find any book or chemistry site which explains the process or gives a staight formula. This is what I have so far:

CF=extract vol(ml)x spike conc(ppm)/hplc inj vol

mass inj x CF ?????

UUUgh.Help!

I think you're psyching yourself out. Problems like this generally yield quite readily to "unit analysis". By "unit analysis" I mean that you carefully watch the units of all values you use and cancel them as you perform the calculations.

Presumably you start with a sample of soil onto which you have spiked some mass "M" of herbicide. (M will be in units of grams.)

You extract this sample and end up with an extraction volume "V". (V will be in units of liters. If in milliliters, convert to liters.)

From these we can calculate the theoretical (100% recovery) concentration "C_theory" of herbicide in the extract as

C_theory = M / V

(Note that the underscore is meant to imply that the letter or word that follows is supposed to be a subscript.)

You inject a volume "v_x" (in microliters) of this extract into the chromatograph. The resulting peak is of area "A_x".

You make a separate injection of volume "v_s" of an herbicide standard made up at a concentration of "C_s" (in grams per liter, or milligrams per milliliter). The resulting peak is of area "A_s".

Now we have all the information we need to calculate the actual concentration, "C_x", of herbicide in the extract:

C_x = C_s * [( A_x / v_x )/( A_s / v_s )]

The percent recovery is then the ratio of the actual concentration in the extract to the theoretical concentration, expressed as a percentage:

Percent Recovery = ( C_x / C_theory ) * 100%

If you examine each equation you will find that the units properly cancel out through the calculations. If not, then you've done something wrong. If this is not clear, ask again.

There are many other ways these equations could be put together, but this should get you started.

WOW! Lotsa math!!

The cheater's method for % recovery is to compare your extracted standard to a neat standard that you've run (same concentration and conditions of course). Then
%Rec=extracted/neat*100%