In analytical chemistry, sensitivity is the slope of the calibration curve. For ICP applications, it is expressed in counts/concentration units. When analysts are thinking about sensitivity, they use a language that expresses sensitivity in terms of detection limits (DL) rather than sensitivity units (S = counts /conc. units). DL is used because S has little value in making relative comparisons. It is much more to the point to speak in terms of DL rather than S. However, there is a relationship between S and DL. The relationship between the two is dependent upon the definition of DL. A popular definition of DL for ICP that incorporates sensitivity (S) is:
DL = (3(std. dev. of noise in counts)) / S
Using the above equation, the DL is expressed in the concentration units of choice. Using ICP as an example, a common concentration unit is ppm = micrograms of analyte/gram solution. Therefore, an analyst will state that a technique that has a DL of 0.001 ppm is 100 times more sensitive than a technique with a DL of 0.1 ppm. On a practical level, we think of sensitivity in terms of DL, even though we need to measure S to calculate the DL.
