A retention clip for attachment to a sample rack adapted to hold one or more sample collection units, the retention clip comprising: an engagement head for attachment to the sample rack, the engagement head having: a) an upper engagement member extending from a bridge along a transverse direction that is perpendicular to the vertical direction, wherein the upper engagement member includes a first flange and a first protrusion that extends from the first flange along the vertical direction, wherein b) the bridge extends along a vertical direction, and a curved tail extends from the engagement head, characterized in that the engagement head further comprises: c) a lower engagement member extending from the bridge along the transverse direction and spaced from the upper engagement member so as to face the upper engagement member, wherein the upper engagement member and the lower engagement member attach the engagement head to the sample rack, wherein the first protrusion of the upper engagement member extends toward the lower engagement member and the lower engagement member includes a second flange and a second protrusion that extends from the second flange along the vertical direction toward the upper engagement member, wherein the curved tail extends downwardly from the lower engagement member of the bridge to a terminal end of the curved tail and wherein the curved tail is curved in a direction opposite to a direction that the upper engagement member and the lower engagement member extend, and wherein the curved tail defines the terminal end that includes a curved contact surface, wherein the terminal end is offset with respect to a vertical plane that extends through the bridge along the vertical direction. an engagement head for attachment to the sample rack, the engagement head having: a) an upper engagement member extending from a bridge along a transverse direction that is perpendicular to the vertical direction, wherein the upper engagement member includes a first flange and a first protrusion that extends from the first flange along the vertical direction, wherein b) the bridge extends along a vertical direction, and a curved tail extends from the engagement head, a) an upper engagement member extending from a bridge along a transverse direction that is perpendicular to the vertical direction, wherein the upper engagement member includes a first flange and a first protrusion that extends from the first flange along the vertical direction, wherein b) the bridge extends along a vertical direction, and a curved tail extends from the engagement head, characterized in that the engagement head further comprises: c) a lower engagement member extending from the bridge along the transverse direction and spaced from the upper engagement member so as to face the upper engagement member, wherein the upper engagement member and the lower engagement member attach the engagement head to the sample rack, wherein the first protrusion of the upper engagement member extends toward the lower engagement member and the lower engagement member includes a second flange and a second protrusion that extends from the second flange along the vertical direction toward the upper engagement member, wherein the curved tail extends downwardly from the lower engagement member of the bridge to a terminal end of the curved tail and wherein the curved tail is curved in a direction opposite to a direction that the upper engagement member and the lower engagement member extend, and wherein the curved tail defines the terminal end that includes a curved contact surface, wherein the terminal end is offset with respect to a vertical plane that extends through the bridge along the vertical direction. c) a lower engagement member extending from the bridge along the transverse direction and spaced from the upper engagement member so as to face the upper engagement member, wherein the upper engagement member and the lower engagement member attach the engagement head to the sample rack, wherein the first protrusion of the upper engagement member extends toward the lower engagement member and the lower engagement member includes a second flange and a second protrusion that extends from the second flange along the vertical direction toward the upper engagement member,