reciprocating piston pump - the most common HPLC pump design which is available in single or multiple-piston arrangements forces mobile phase through the column on a forward piston stroke, and refills the cylinder with mobile phase on the recycle stroke.

See reciprocating piston pump.

recorder - electromechanical instrument which transforms the chromatographic signal into a graphical record.

reduced plate height (h) - Used to measure efficiencies of columns. An HPLC column with an h value 2 is considered to be well-packed, h = H/dp.

refractive index detector - HPLC detector which monitors the changes of the refractive index of the column effluent. The only universal detector in HPLC, and the less sensitive one.

See refractive index detector.