A device for separating a liquid sample within a collection container, comprising: a separator having an open through-hole defined therethrough, the through-hole defining a first opening at a first end of the through-hole and a second opening at a second end of the through-hole, the first end of the through-hole substantially opposite the second end of the through-hole, wherein the through-hole is adapted for liquid to pass therethrough, the separator comprising: a float, having a first density; and a ballast, having a second density greater than the first density, wherein a portion of the float is connected to a portion of the ballast; characterized in that the separator further comprises a first sealing perimeter and a second sealing perimeter, the float defines a first exterior surface of the separator extending from a portion of the first opening to a portion of the second opening, the ballast defines a second exterior surface, the second exterior surface substantially opposite the first exterior surface, and the float is adapted to deform upon application of rotational force, upon applied rotational force, the separator is adapted to transition from a first initial position in which the first sealing perimeter engages a first portion of a sidewall of the collection container such that the through-hole is oriented in an open position for allowing fluid to pass therethrough, to a second sealing position in which the second sealing perimeter engages a second portion of the sidewall of the collection container to prevent fluid from passing between the separator and the sidewall and for preventing fluid from passing through the open through-hole, and transition of the through-hole from the first open position to the second closed position coincides with rotation of the mechanical separator from the first initial position to the second sealing position. a separator having an open through-hole defined therethrough, the through-hole defining a first opening at a first end of the through-hole and a second opening at a second end of the through-hole, the first end of the through-hole substantially opposite the second end of the through-hole, wherein the through-hole is adapted for liquid to pass therethrough, the separator comprising: a float, having a first density; and a ballast, having a second density greater than the first density, wherein a portion of the float is connected to a portion of the ballast; a float, having a first density; and a ballast, having a second density greater than the first density, wherein a portion of the float is connected to a portion of the ballast; characterized in that the separator further comprises a first sealing perimeter and a second sealing perimeter, the float defines a first exterior surface of the separator extending from a portion of the first opening to a portion of the second opening, the ballast defines a second exterior surface, the second exterior surface substantially opposite the first exterior surface, and the float is adapted to deform upon application of rotational force, upon applied rotational force, the separator is adapted to transition from a first initial position in which the first sealing perimeter engages a first portion of a sidewall of the collection container such that the through-hole is oriented in an open position for allowing fluid to pass therethrough, to a second sealing position in which the second sealing perimeter engages a second portion of the sidewall of the collection container to prevent fluid from passing between the separator and the sidewall and for preventing fluid from passing through the open through-hole, and transition of the through-hole from the first open position to the second closed position coincides with rotation of the mechanical separator from the first initial position to the second sealing position.