An assembly for enabling separation of a liquid sample into first and second phases, comprising: a collection container having an open end, a closed end, and a sidewall extending therebetween defining an interior, the collection container further defining a longitudinal axis between the open end and the closed end; a closure adapted for sealing engagement with the open end of the collection container; and a separator disposed within the collection container and sealingly engaged via a first sealing perimeter with a first portion of the sidewall adjacent the open end in an initial position during introduction of the fluid sample into the collection container, the separator having a through-hole defined therethrough for allowing 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, wherein the closure is pierceable by a cannula for introducing the fluid sample into the collection container, wherein the separator is spaced apart from the cannula during introduction of the fluid sample into the collection container, characterised in that upon application of rotational force, at least a portion of the separator deforms and the separator transitions from the initial position to a second position in which the separator is sealingly engaged via a second sealing perimeter with a second portion of the sidewall and when the separator is in the second position, the engagement of the second sealing perimeter with the sidewall prevents fluid from passing between the separator and the sidewall and from passing through the open through-hole wherein transition of the through-hole from the open position to the closed position coincides with rotation of the separator from the initial position to the second position. a collection container having an open end, a closed end, and a sidewall extending therebetween defining an interior, the collection container further defining a longitudinal axis between the open end and the closed end; a closure adapted for sealing engagement with the open end of the collection container; and a separator disposed within the collection container and sealingly engaged via a first sealing perimeter with a first portion of the sidewall adjacent the open end in an initial position during introduction of the fluid sample into the collection container, the separator having a through-hole defined therethrough for allowing 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, wherein the closure is pierceable by a cannula for introducing the fluid sample into the collection container, wherein the separator is spaced apart from the cannula during introduction of the fluid sample into the collection container, characterised in that upon application of rotational force, at least a portion of the separator deforms and the separator transitions from the initial position to a second position in which the separator is sealingly engaged via a second sealing perimeter with a second portion of the sidewall and when the separator is in the second position, the engagement of the second sealing perimeter with the sidewall prevents fluid from passing between the separator and the sidewall and from passing through the open through-hole wherein transition of the through-hole from the open position to the closed position coincides with rotation of the separator from the initial position to the second position.