A system for analyzing a chemical composition of a specimen, the system comprising: a substrate sampling probe comprising: an inner tube (234, 334, 434, 534) extending from a proximal end (234a, 334a) to a distal end (234b, 334b), and, said inner tube defining a sampling conduit (236, 336) extending from a distal inlet to a proximal outlet configured to fluidly couple to an ion source probe for discharging desorption solvent in an ionization chamber in fluid communication with a sampling orifice of a mass spectrometer; an outer tube (232, 332, 432) extending from a proximal end (232a, 332a) to a distal end (232b, 332b) and disposed about said inner tube, said outer tube defining a desorption solvent conduit (238, 338) extending from a proximal inlet configured to be in fluid communication with a desorption solvent source to a distal outlet, wherein the distal end of the inner tube is disposed relative to the distal end of the outer tube so as to define a fluid chamber (235, 335) extending distally from the distal end of the inner tube to a liquid/air interface (235a, 335a), wherein the fluid chamber is in fluid communication with the distal outlet of the desorption solvent conduit and the distal inlet of the sampling conduit, wherein the sampling conduit (236, 336) comprises a sampling volume (237, 337) within the distal end of the inner tube adjacent to the fluid chamber, the sampling volume exhibiting a cross sectional area greater than a cross sectional area of a proximal portion of the sampling conduit, the sampling volume being configured to receive a coated surface (222, 322) of a sample substrate (220, 320) having one or more analyte species adsorbed thereto such that the coated surface is in contact with the desorption solvent within the sampling volume as the desorption solvent flows from the inlet of the desorption solvent conduit through the fluid chamber to the outlet of the sampling conduit; characterized in that the inner tube (234, 334, 434, 534) comprises a coupling mechanism by which a distal portion of the inner tube comprising the sampling volume is replaceable by a second distal portion exhibiting a second sampling volume that is different in at least one of shape and cross-sectional area from the sampling volume. an inner tube (234, 334, 434, 534) extending from a proximal end (234a, 334a) to a distal end (234b, 334b), and, said inner tube defining a sampling conduit (236, 336) extending from a distal inlet to a proximal outlet configured to fluidly couple to an ion source probe for discharging desorption solvent in an ionization chamber in fluid communication with a sampling orifice of a mass spectrometer; an outer tube (232, 332, 432) extending from a proximal end (232a, 332a) to a distal end (232b, 332b) and disposed about said inner tube, said outer tube defining a desorption solvent conduit (238, 338) extending from a proximal inlet configured to be in fluid communication with a desorption solvent source to a distal outlet, wherein the distal end of the inner tube is disposed relative to the distal end of the outer tube so as to define a fluid chamber (235, 335) extending distally from the distal end of the inner tube to a liquid/air interface (235a, 335a), wherein the fluid chamber is in fluid communication with the distal outlet of the desorption solvent conduit and the distal inlet of the sampling conduit, wherein the sampling conduit (236, 336) comprises a sampling volume (237, 337) within the distal end of the inner tube adjacent to the fluid chamber, the sampling volume exhibiting a cross sectional area greater than a cross sectional area of a proximal portion of the sampling conduit, the sampling volume being configured to receive a coated surface (222, 322) of a sample substrate (220, 320) having one or more analyte species adsorbed thereto such that the coated surface is in contact with the desorption solvent within the sampling volume as the desorption solvent flows from the inlet of the desorption solvent conduit through the fluid chamber to the outlet of the sampling conduit; characterized in that the inner tube (234, 334, 434, 534) comprises a coupling mechanism by which a distal portion of the inner tube comprising the sampling volume is replaceable by a second distal portion exhibiting a second sampling volume that is different in at least one of shape and cross-sectional area from the sampling volume.