linearity
/ lin-ee-AIR-ih-tee /
Pumping gas into a car, you trust that twice as long on the pump means roughly twice the fuel, and the meter climbs in even steps. Linearity is that same proportional honesty in a measurement: as the amount of analyte doubles, the signal you read should also double, climbing in a straight, predictable line.
Linearity is the ability of a method to give a signal directly proportional to the analyte's concentration across a stated range. You test it by measuring a series of standards of known, increasing concentration and plotting signal against concentration; if the points fall on a straight line, the method is linear and a simple equation can convert any signal into a concentration.
It matters because most quantitative analysis relies on reading concentrations off a straight calibration line, and that shortcut only works where the response really is linear. The caveat is that linearity always has an upper edge: push the concentration too high and the signal bends and flattens — detectors saturate, absorbances stray from Beer's law — so you must know and stay within the range where the line holds.
Measuring six standards of a dye at 1, 2, 4, 6, 8 and 10 mg/L, the absorbances fall on a near-perfect straight line up to 8 mg/L (correlation coefficient 0.9998) but bend low at 10, so linearity is declared only up to 8 mg/L.
Signal rising in a straight line with concentration until the detector saturates.
Linearity is about the shape of the response (does it stay straight?), while the linear dynamic range and working range describe how wide a span of concentrations that straight behaviour covers; a high correlation coefficient supports but does not by itself prove linearity.