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. <EOS>