Detecting trace species at buried interfaces (e.g. active sites on an operando electrocatalyst) is challenging. While vibrationally resonant sum frequency generation spectroscopy offers, in principle, an attractive approach for characterising such weak interfacial species, in measuring signals from such species and understanding the manner in which their spectral response may be distorted by other, unavoidable, surface and bulk contributions, is extremely challenging. Martin’s new, collinear, time-domain, heterodyned, shot-to-shot referenced, balance-detected SFG spectrometer, debuted recently in a paper in Phys Chem Chem Phys, offers unmatched signal to noise and phase stability. When you absolutely, positively, without a doubt, need to characterise small interfacial signals accept no substitutes.