A flow cell (250; 350) for a fluorescence spectrometer, the flow cell comprising: a flow channel (254; 354) to receive a flow of a liquid sample; an excitation light entrance window to receive excitation light from a light source; and an emission light exit window to transmit fluorescent emission light from the liquid sample in the flow channel from the flow cell; wherein the excitation light entrance window includes an excitation light entrance window waveguide including: a waveguide core (261; 361) formed of a core material and having an inner end face; and a cladding medium (274; 374) surrounding a portion of the waveguide core, wherein the cladding medium has a refractive index less than the refractive index of the core material; and wherein the inner end face of the excitation light entrance window waveguide: defines a portion of the flow channel; serves as a light input port for the excitation light into the flow channel; and contacts the liquid sample in the flow channel; and wherein the emission light exit window includes an emission light exit window waveguide including: a waveguide core (262; 362) formed of a core material and having an inner end face; and a cladding medium (274; 374) surrounding a portion of the waveguide core, wherein the cladding medium has a refractive index less than the refractive index of the core material; and wherein the inner end face of the emission light exit window waveguide: defines a portion of the flow channel; serves as a light output port for the fluorescent emission light out of the flow channel; and contacts the liquid sample in the flow channel; wherein the inner end face of the excitation light entrance window waveguide and the inner end face of the emission light exit window waveguide abut one another at the flow channel to form an axially extending corner of the flow channel, wherein the waveguide core of the excitation light entrance window waveguide includes an end portion extending beyond the cladding medium of the excitation light entrance window waveguide to the inner end face of the waveguide core of the excitation light entrance window waveguide; and the flow cell includes an end pad of opaque, light absorbing material surrounding the end portion of the waveguide core adjacent the flow channel and adjacent the inner end face of the excitation light entrance window waveguide, wherein the end pad absorbs light from the waveguide core of the excitation light entrance window waveguide and prevents light from propagating directly from the excitation light entrance window to the emission light exit window. a flow channel (254; 354) to receive a flow of a liquid sample; an excitation light entrance window to receive excitation light from a light source; and an emission light exit window to transmit fluorescent emission light from the liquid sample in the flow channel from the flow cell; wherein the excitation light entrance window includes an excitation light entrance window waveguide including: a waveguide core (261; 361) formed of a core material and having an inner end face; and a cladding medium (274; 374) surrounding a portion of the waveguide core, a waveguide core (261; 361) formed of a core material and having an inner end face; and a cladding medium (274; 374) surrounding a portion of the waveguide core, wherein the cladding medium has a refractive index less than the refractive index of the core material; and wherein the inner end face of the excitation light entrance window waveguide: defines a portion of the flow channel; serves as a light input port for the excitation light into the flow channel; and contacts the liquid sample in the flow channel; and defines a portion of the flow channel; serves as a light input port for the excitation light into the flow channel; and contacts the liquid sample in the flow channel; and wherein the emission light exit window includes an emission light exit window waveguide including: a waveguide core (262; 362) formed of a core material and having an inner end face; and a cladding medium (274; 374) surrounding a portion of the waveguide core, a waveguide core (262; 362) formed of a core material and having an inner end face; and a cladding medium (274; 374) surrounding a portion of the waveguide core, wherein the cladding medium has a refractive index less than the refractive index of the core material; and wherein the inner end face of the emission light exit window waveguide: defines a portion of the flow channel; serves as a light output port for the fluorescent emission light out of the flow channel; and contacts the liquid sample in the flow channel; defines a portion of the flow channel; serves as a light output port for the fluorescent emission light out of the flow channel; and contacts the liquid sample in the flow channel; wherein the inner end face of the excitation light entrance window waveguide and the inner end face of the emission light exit window waveguide abut one another at the flow channel to form an axially extending corner of the flow channel,