Positive displacement device assembly, the assembly comprising: - a tip intended to support a capillary of a plunger-capillary system, the tip being hollow, centred on a longitudinal central axis of the tip, and including an outer surface provided with an axial retention member of the capillary, - an ejector intended to eject the plunger-capillary system, the assembly being designed so as to enable an axial ejection travel of the ejector relative to the tip, between a top resting end position and a bottom ejection end position, the ejector comprising an ejection end intended to be in axial contact with a capillary end during the ejection travel, the tip including a body as well as a movable capillary retention portion connected to the body by a link (52, 52, 66), the movable retention portion including on its outer surface at least a part of said axial retention member of the capillary, characterised in that the tip also includes a lever rigidly connected to the movable retention part, such that a tilting of the movable part radially inwards, via the link (52, 52, 66) and relative to the tip body, actuates a tilting of the lever radially outwards, the ejector being equipped with a locking member cooperating with the lever such that: - in the top resting end position of the ejector, the locking member occupies a locking position wherein it holds the movable portion of the tip in an active holding position of the capillary, by preventing the lever from tilting radially outwards; and - during the ejection travel of the ejector, the locking member: - allows the tilting of the lever radially outwards, resulting from a tilting of the movable portion radially inwards from its active holding position of the capillary, to a retracted release position of the capillary, or - actuates the tilting of the lever radially outwards, resulting in a tilting of the movable portion radially inwards from its active holding position of the capillary, to its retracted release position of the capillary. - a tip intended to support a capillary of a plunger-capillary system, the tip being hollow, centred on a longitudinal central axis of the tip, and including an outer surface provided with an axial retention member of the capillary, - an ejector intended to eject the plunger-capillary system, the assembly being designed so as to enable an axial ejection travel of the ejector relative to the tip, between a top resting end position and a bottom ejection end position, the ejector comprising an ejection end intended to be in axial contact with a capillary end during the ejection travel, the tip including a body as well as a movable capillary retention portion connected to the body by a link (52, 52, 66), the movable retention portion including on its outer surface at least a part of said axial retention member of the capillary, characterised in that the tip also includes a lever rigidly connected to the movable retention part, such that a tilting of the movable part radially inwards, via the link (52, 52, 66) and relative to the tip body, actuates a tilting of the lever radially outwards, the ejector being equipped with a locking member cooperating with the lever such that: - in the top resting end position of the ejector, the locking member occupies a locking position wherein it holds the movable portion of the tip in an active holding position of the capillary, by preventing the lever from tilting radially outwards; and - during the ejection travel of the ejector, the locking member: - allows the tilting of the lever radially outwards, resulting from a tilting of the movable portion radially inwards from its active holding position of the capillary, to a retracted release position of the capillary, or - actuates the tilting of the lever radially outwards, resulting in a tilting of the movable portion radially inwards from its active holding position of the capillary, to its retracted release position of the capillary. - in the top resting end position of the ejector, the locking member occupies a locking position wherein it holds the movable portion of the tip in an active holding position of the capillary, by preventing the lever from tilting radially outwards; and - during the ejection travel of the ejector, the locking member: - allows the tilting of the lever radially outwards, resulting from a tilting of the movable portion radially inwards from its active holding position of the capillary, to a retracted release position of the capillary, or - actuates the tilting of the lever radially outwards, resulting in a tilting of the movable portion radially inwards from its active holding position of the capillary, to its retracted release position of the capillary. - allows the tilting of the lever radially outwards, resulting from a tilting of the movable portion radially inwards from its active holding position of the capillary, to a retracted release position of the capillary, or - actuates the tilting of the lever radially outwards, resulting in a tilting of the movable portion radially inwards from its active holding position of the capillary, to its retracted release position of the capillary.