CRM-GTX1&4-c (replacement for CRM-GTX1&4-b) is a calibration solution CRM designed for use in chemical analysis methods. Each ampoule contains 0.5 mL solution of a mixture of gonyautoxin-1 at a concentration of 60.4 μM and gonyautoxin-4 at a concentration of 19.7 μM in 0.003 M hydrochloric acid plus 0.01 M acetic acid.