Biomedical Engineering Reference
In-Depth Information
16.3 Motivation for Raman Spectroscopy
Multiwavelength spectroscopy of biofluids provides several advantages over
chemical assays that are not particular to Raman spectroscopy. First, all mea-
surements are performed on the same sample volume, since multiple chemi-
cals' concentrations can be computed from a single spectrum. There is typ-
ically just one optical “sensor unit” or cartridge required. In multi-chemical
assays, the sample must be separated into subvolumes that are sent to differ-
ent single-chemical sensor units. This increases the volume of sample needed,
the complexity of the sample's path through the analyzer, and the number of
sensor units needed.
Another general advantage is that optical sensing requires no physical
or chemical contact with the sample. After a spectroscopic measurement is
performed, the sample can be sent on to other diagnostic instruments, recycled
into the body (e.g., dialysis), or archived for future analysis. Such options do
not exist when chemical tests are used. The non-contact nature of optical
spectroscopy also eliminates the need for reagents and reduces the number of
components that need to be cleaned (for a multi-use instrument).
Raman spectroscopy has some particular advantages for biofluid analysis
as well. The sharpness of fundamental vibrational peaks enables dense packing
of information into a spectral interval, much more so than for the broader
peaks typical of fluorescence or visible/NIR absorption. These extra degrees of
spectral freedom are important when it comes to measuring the concentration
of minor contributors above the various noise sources.
Mid-IR absorption spectroscopy probes the same energy regime of fun-
damental vibrations and provides comparable quantitative accuracy (to be
discussed below). However, a Raman advantage for biofluids is that water
is a weak Raman scatterer but a strong mid-IR absorber. Thus, mid-IR ab-
sorption requires samples to be dried prior to analysis, which requires extra
preparation steps and time, and does not permit the sample to be saved or
passed along in its original liquid state. Another advantage is that Raman
spectroscopy can be performed in a backscattering direction, thereby permit-
ting analysis of thick and/or turbid samples (including in vivo applications),
whereas absorption spectroscopy is intrinsically a transmissive approach.
16.4 Experimental Setups and Data Processing
16.4.1 Equipment and Geometries
Overwhelmingly, the published research on Raman-based biofluid analysis
uses excitation in the near-infrared regime in order to reduce fluorescence.
Standard linewidth-narrowed diode lasers at 785 or 830 nm are most common.
As noted in Chap. 1, several hundred milliwatts of multiple-spatial mode light
can routinely be obtained from diode lasers, making them economical choices
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