Ultramark® 1621 is widely used as a calibration standard in mass spectrometry, particularly for high-resolution instruments like time-of-flight (TOF) and Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometers. Its application ensures accurate mass measurements by providing a set of known mass-to-charge (m/z) ratios, which helps in instrument calibration and performance validation. This chemical is especially valuable in proteomics, metabolomics, and other analytical fields where precise mass determination is critical. Its stability and well-defined mass peaks make it a reliable tool for maintaining the accuracy and reproducibility of mass spectrometry data.