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
2
-A-R [I]
(wherein R
1
represents an ionic functional group which becomes an ion in a solvent, R
2
represents a structure which can bind to other substance, and A represents an arbitrary spacer moiety).