If I want to detect a substance which is below the detection limit of my instrument, can I just add a known quantity and then quantify it?
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1$\begingroup$ To see, why this is impossible, try to make a step by step plan for a specific example. $\endgroup$– Paul KolkCommented May 3 at 17:07
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$\begingroup$ If the limit is due to noise then averaging may help and one way to do this is to use a Hadamard weighting scheme as this will reduce noise from the instrument. This has recently been applied in xray diffraction to detect weak signals., see Yorke et al. Nature Methods volume 11, pages1131–1134 (2014). The supplementary has details of the method. $\endgroup$– porphyrinCommented May 4 at 7:49
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I will rather ask you, what is a detection limit? Chemists don't agree among themselves about the definition because they do not know enough statistics and statisticians do not know about much about chemical analysis. Hard to find the optimum combination among humans. There is a book author on this topic in SE Chemistry!
The standard procedure when something is not detectable (practically) is to take a large sample, say for water, you take several liters, and keep concentrating. This is called sample pre-concentration.
What you suggest is called spiking, this is also done but when the concentration is too low, it becomes useless. Preconcentration is the correct solution.
