Often prepared as a representative mixture of several different (usually bulk) samples, and from which the laboratory sample is taken.
Source:
PAC, 1988, 60, 1461. 'Nomenclature, symbols, units and their usage in spectrochemical analysis-X. Preparation of materials for analytical atomic spectroscopy and other related techniques (Recommendations 1988)' on page 1465 (https://doi.org/10.1351/pac198860091461)