Analyzing a sample generates a chemical or physical signal proportional to the amount of analyte in the sample. This signal may be anything we can measure, such as mass or absorbance.
Consider the two graduated cylinders shown here, each containing a solution of 0.010 M Cu(NO3)2. The cylinder on the left contains 10 mL, or 1.0 × 10-4 moles of Cu2+, and the cylinder on the right contains 20 mL, or 2.0 × 10-4 moles of Cu2+.
If a technique responds to the absolute amount of analyte, as is the case when we measure mass, then the signal depends on the amount of sample we analyze; thus, if we determine the amount of Cu2+ in the two cylinders by precipitating it as Cu(OH)2 and isolating it and weighing it as CuO, then the mass we obtain for the cylinder on the right is twice that for the cylinder on the left. A technique responding to the absolute amount of analyte is a total analysis technique. Mass and volume are the most common signals for a total analysis technique, and the corresponding techniques are gravimetry and titrimetry.
On the other hand, if a technique responds to the relative amount of analyte, as is the case when we measure a solution’s absorbance, then the signal does not depend on the amount of sample analyzed; thus, if we determine the concentration of Cu2+ in the two cylinders by measuring each solution’s absorbance, we obtain identical absorbances for the two solutions. We describe a technique responding to the relative amount of analyte as a concentration technique. Spectroscopy and electrochemistry are examples of concentration techniques.
