| CPC G01N 21/031 (2013.01) [G01J 3/0291 (2013.01); G01J 3/42 (2013.01); G01N 21/55 (2013.01); G01N 2021/551 (2013.01)] | 32 Claims |

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1. A multipass cell comprising:
a first reflector arrangement; and
a second reflector arrangement, the first and second reflector arrangements defining an optical cavity therebetween and the cell;
wherein the first reflector arrangement is configured such that light incident on the first reflector arrangement is at least partially retroreflected towards the second reflector arrangement, wherein the second reflector arrangement comprises a concave surface that is reflective, wherein the first reflector arrangement comprises an aperture for allowing light to enter and exit the optical cavity.
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