(Greek letter nu) of electromagnetic radiation is defined byυ=1/λ, with typical
units being cm−^1. Thus, for example, the wavenumberυ= 1500 cm−^1 is equivalent
to a wavelength ofλ= 6.67μm.
Example 9.1What are the wavelength equivalents in nm to the wavenum-
bersυ= 6500 cm−^1 , 15,000 cm−^1 , and 25,000 cm−^1?
Solution: Using the relationship λ(nm) = 10^7 /υ(cm−^1 ) yields 1538 nm,
667 nm, and 400 nm, respectively.9.1 Fluorescence Spectroscopy
Fluorescence spectroscopyinvolves the observation and analysis of thefluores-
cence spectrum either from a naturallyfluorescing molecule or from an extrinsic
fluorophore that is attached to the molecule. The interpretations of these spectra
include studying the characteristics of molecules, identifying infectious diseases,
and performing noninvasive biopsies. In a standard biopsy procedure forsoft tissue
pathology(the diagnosis and characterization of diseases in soft tissue), typically
one or more tissue samples are physically removed for later laboratory evaluation.
In contrast, optical spectroscopic methods reduce the need for surgical removal of
tissue. Instead, by using an optical probe placed on or near the surface of the tissue
to be evaluated, an imaging system records some in vivo form of spectral analysis
of the tissue [ 2 ]. This feature of an optical biopsy enables an immediate diagnosis of
the tissue characteristics instead of having to wait hours or days for a standard
laboratory evaluation of a tissue sample. Depending on the spectroscopic method
used, the diagnostic information obtained from the tissues can be at the biochem-
ical, cellular, molecular structural, or physiological levels [ 5 – 9 ].
An advantage offluorescence spectroscopy for soft tissue pathology is that the
emitted spectra are sensitive to the biochemical composition of the tissue being
examined. This feature is helpful in assessing whether the tissue is in a normal or
diseased state. For example, Fig.9.1shows generic plots offluorescence intensity
as a function of wavelength for spectroscopic measurements of healthy and
malignant samples of a specific tissue. Distinctfluorescence intensity variations are
clearly seen. The wavelength range and the exactfluorescence spectral intensity
response will depend on the tissue type being examined and the nature of the
disease. The curves in Fig.9.1 show that for this particular tissue type the
fluorescence intensity increases with the progression of the tissue disease. In other
cases thefluorescence intensity might decrease with the onset and progression of a
specific disease.
9 Spectroscopic Methodologies 261