Biomedical Engineering Reference
In-Depth Information
cuvette
to spectrograph
Fig. 16.2.
An example of an all-reflective, non-common-path geometry. Light is
delivered by a weakly focused beam, and large-angle scattering is gathered and col-
limated by the paraboloidal reflector; see [2] for an application and further references
is to collect the scattering at 90
◦
from excitation. At low numerical aperture,
the excitation in this case looks like an extended line or cylinder, which can
be imaged onto a spectrograph slit or captured by an array of optical fibers.
In the published work on Raman spectroscopy of biofluids, there are no
present examples of placing optical fibers themselves directly within, or ad-
jacent to, a biofluid; there is always a free-space region immediately adjacent
to the sample. As such, although the field of fiber-based biomedical Raman
probes is certainly an active area (see, for example, Motz et al. [3]) as noted
in Chap. 2, the studies on biofluids cited in this chapter used optical fibers
only for convenient piping of light from source to sample to spectrograph.
16.4.2 Liquid Containers
Ex Vivo
For ex vivo applications, the fluid sample can reside in any sort of container
into which light can penetrate. The simplest, if there is at least a sizeable
fraction of a milliliter of sample, is probably a 1-10 mm pathlength cuvette,
irradiated from the side as depicted in Fig 16.1. Plastic cuvettes, while least
expensive, give rise to much stronger Raman bands than glass or quartz and
are therefore avoided in the epi-collection mode unless there is a compelling
reason (e.g., high-volume cost).
The liquid nature of blood and urine samples lends itself to flow-in or flow-
through containers. One approach, used by Qu, is to suck the sample into a
capillary tube, insert an excitation optical fiber at one end of the tube, and
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