Sometimes you find yourself stymied because the thing you are looking for in an analytical test is already present in the solvent you would use to prepare a series of standards. An example might be calcium background in your tap water supply, or a trace of organic A in your organic B solvent.
The way to establish the amount of analyte present is to prepare a series with 0, 1, 2, etc. ADDED analyte, then fit a curve, on a graph or mathematically. You should be looking at a positive response/signal in the test at zero ADDITION, and have to use a graph or fit equation to extrapolate backwards to zero response/signal at what mathematically would correspond to a negative addition. This negative addition amount (absolute value) is exactly what is in your solvent to begin with.
If all you want to do is correct for this background, you could of course simply subtract off the response of a blank, as is often done. This technique is used to find out what is in the blank itself.
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