A collection media assembly for receiving one or more particles from a volume of fluid within a fluid composition sensor, the collection media assembly comprising: a housing; a particle collection slide comprising a transparent substrate and a collection media disposed upon an upper surface of the transparent substrate, the collection media configured to receive one or more particles from a volume of fluid received through a fluid inlet; and at least one alignment feature (113, 123); wherein the housing defines an open lower end configured to interact with an imaging device, such that the one or more particles received by the collection media are visible through the transparent substrate from the open lower end of the housing; wherein each of the at least one alignment features (113, 123) is configured to engage a corresponding element disposed within the fluid composition sensor so as to constrain relative movement between the collection media assembly and the corresponding element in at least a first direction; wherein the housing comprises an upper cassette portion and a lower cassette portion; wherein the upper cassette portion is configured to direct the volume of fluid toward a receiving area of the collection media; wherein an opening extends through the lower cassette portion; and characterized in that the opening is arranged to define a line of sight to the one or more particles received by the collection media for the imaging device, the line of sight extending through the lower cassette portion, and in that the lower cassette portion comprises one or more features configured to receive and secure the particle collection slide across the opening, so that the opening defines distinct first and second opening portions (124A, 124B) located on either side of the particle collection slide, the distinct first and second opening portions (124A, 124B) configured to enable the volume of fluid to flow through the collection media assembly. a housing; a particle collection slide comprising a transparent substrate and a collection media disposed upon an upper surface of the transparent substrate, the collection media configured to receive one or more particles from a volume of fluid received through a fluid inlet; and at least one alignment feature (113, 123); wherein the housing defines an open lower end configured to interact with an imaging device, such that the one or more particles received by the collection media are visible through the transparent substrate from the open lower end of the housing; wherein each of the at least one alignment features (113, 123) is configured to engage a corresponding element disposed within the fluid composition sensor so as to constrain relative movement between the collection media assembly and the corresponding element in at least a first direction; wherein the housing comprises an upper cassette portion and a lower cassette portion; wherein the upper cassette portion is configured to direct the volume of fluid toward a receiving area of the collection media; wherein an opening extends through the lower cassette portion; and characterized in that the opening is arranged to define a line of sight to the one or more particles received by the collection media for the imaging device, the line of sight extending through the lower cassette portion, and in that the lower cassette portion comprises one or more features configured to receive and secure the particle collection slide across the opening, so that the opening defines distinct first and second opening portions (124A, 124B) located on either side of the particle collection slide, the distinct first and second opening portions (124A, 124B) configured to enable the volume of fluid to flow through the collection media assembly.