Disclosed is a probe for mass spectrometry of liquid samples, which may effectively ionize the sample without adding a protic solvent to the mobile phase in the ionization method in mass spectrometry of liquid samples. The probe according to the present invention has a structure represented by the Formula [I]: R.sup.2-A-R.sup.1[I] (wherein R.sup.1 represents an ionic functional group which becomes an ion in a solvent, R.sup.2 represents a structure which can bind to other substance, and A represents an arbitrary spacer moiety).