Honestly, a mass spectrometer is not always the best instrument for quantification, but it is a highly sensitive instrument. So if low levels need to be detected, a mass spectrometer can quantify based on a proper commercial QC standard. Ideally this is an isotope labelled variant of your target molecule. If the concentration is sufficiently high to detect the compound by UV or visible light, this method is preferred in view of costs considering also the selectivity of the wavelength. Mass spectrometry is prone to ion suppression by other competing molecules in the sample, which requires the need to include proper controls. Of course we will always discuss with you the optimal design of the experiment first.