SQ RMs are used for ongoing drift correction. These "SQ" RMs do not correspond to any particular alloy matrix, and their structures may not match that of chill cast disks. Their recommended use is to provide reproducible spectral intensities for drift correction and their relationship to analytical curves must be determined by the user under the particular conditions of use. The compositions are designed to provide convenient reference points for a large number of analytical curves with a minimum number of RM’s. The RMs are checked carefully for reproducibility of spectral response but are not certified with respect to true composition. Only approximate values are issued with these RMs. As with all RMs used specifically for drift correction, a specific procedure defined by the spectrometer vendor must be followed when replacement SQ RMs are put into service to establish the correct relationship to previously defined analytical curves. Failure to follow the correct replacement practice may compromise analysis accuracy.