A system comprising: a mass spectrometer; a microfluidic device comprising a microchannel having an exit point leading and connected to the mass spectrometer; and a means for applying pressure to fluids in the microchannel; wherein the microchannel comprises the following fluids: a first plurality of plugs of aqueous fluid flowing in a stream of a carrier fluid immiscible with the aqueous fluid, wherein each of the first plurality of plugs is substantially surrounded by the immiscible carrier fluid, wherein the mass spectrometer is configured to detect at least a first plug of the first plurality of plugs of aqueous fluid following its exit through the exit point, wherein the first plug comprises one or more components and wherein the mass spectrometer is configured to detect one or more mass spectra of the components, characterised in that: said means for applying pressure to fluids in the microchannel are configured to: generate a constant known flow rate of the fluids in the microchannel; and drive the first plug by pressure through the microchannel toward the exit point, and in that the system comprises means for determining the distance travelled by said at least first plug inside the microchannel, wherein the mass spectrometer is configured to obtain time-resolved mass-spectra of the at least first plug by means of the value of the constant flow rate of the fluid in the microchannel and the value of a first distance traversed by the first plug from a first point in the microchannel upstream of the exit point and through the exit point to the mass spectrometer, measured by said means for determining the distance travelled by said at least first plug inside the microchannel. a mass spectrometer; a microfluidic device comprising a microchannel having an exit point leading and connected to the mass spectrometer; and a means for applying pressure to fluids in the microchannel; wherein the microchannel comprises the following fluids: a first plurality of plugs of aqueous fluid flowing in a stream of a carrier fluid immiscible with the aqueous fluid, wherein each of the first plurality of plugs is substantially surrounded by the immiscible carrier fluid, wherein the mass spectrometer is configured to detect at least a first plug of the first plurality of plugs of aqueous fluid following its exit through the exit point, wherein the first plug comprises one or more components and wherein the mass spectrometer is configured to detect one or more mass spectra of the components, characterised in that: said means for applying pressure to fluids in the microchannel are configured to: generate a constant known flow rate of the fluids in the microchannel; and drive the first plug by pressure through the microchannel toward the exit point, and in that the system comprises means for determining the distance travelled by said at least first plug inside the microchannel, wherein the mass spectrometer is configured to obtain time-resolved mass-spectra of the at least first plug by means of the value of the constant flow rate of the fluid in the microchannel and the value of a first distance traversed by the first plug from a first point in the microchannel upstream of the exit point and through the exit point to the mass spectrometer, measured by said means for determining the distance travelled by said at least first plug inside the microchannel. generate a constant known flow rate of the fluids in the microchannel; and drive the first plug by pressure through the microchannel toward the exit point, and in that the system comprises means for determining the distance travelled by said at least first plug inside the microchannel, wherein the mass spectrometer is configured to obtain time-resolved mass-spectra of the at least first plug by means of the value of the constant flow rate of the fluid in the microchannel and the value of a first distance traversed by the first plug from a first point in the microchannel upstream of the exit point and through the exit point to the mass spectrometer, measured by said means for determining the distance travelled by said at least first plug inside the microchannel.