The Ocean Optics USB 2000 CCD spectrometer
measures wavelengths from 340 to 1020 nm and has
a 200 µm slit width that provides a 10-nm FWHM
resolution. This small mobile spectrometer was configured
for exceptional sensitivity of broad spectral
structures so that phosphorescence decay could be
measured. The accuracy of the spectrometer’s wavelength
position was confirmed using the 435.8 and
546.1 nm lines from a mercury lamp. However, for
the majority of the data, the spectrometer was not
radiometrically calibrated, which would adjust the
relative intensities obtained across the wavelength
range. We have elected to provide the uncorrected
data (i.e., the direct output from the spectrometer), as
these are the spectra most likely to be produced by
others using this type of instrumentation.
The CCD spectrometer was initially selected for
the blue diamond study reported in Eaton-Magaña et
al. (2006b, 2008), because its ability to resolve lowintensity
luminescence allowed the collection of
time-resolved phosphorescence spectra. In addition,
the portability and ease of use enabled us to take the
instrument to the gems, providing access to many
more stones than would otherwise have been possible.
The experimental apparatus shown in figure 2
was easily transported to and quite effectively used in
the vaults of the Smithsonian Institution and in the
business office of a diamantaire. When fitted with
appropriate filters, the spectrometer proved effective
for rapidly recording fluorescence spectra, permitting
the study of a large number of colored diamonds
within the limited time constraints of their availability.
Nevertheless, it should be noted that this CCD
spectrometer might be inappropriate for investigations
that require the higher resolution of traditional
research spectrofluorometers.
An important strength of the Ocean Optics spectrometer
is that the fiber tip, when placed on the
table of the stone, illuminates approximately similar
volumes for each stone. Therefore, comparison of relative
intensities is a reasonable possibility and likely
more so than when luminescence is measured
through a stone or from multiple facets of a stone as
occurs in traditional measurements. We were able to
compare the measured fluorescence intensities of the
diamonds tested with this equipment since we used
a consistent configuration and testing method; however,
we do not feel that the absolute intensities we
obtained could be reliably compared to data collected
using similar experimental setups.
Comparison of the mobile CCD spectrometer
with a standard spectrofluorometer revealed some
distinct advantages and disadvantages. Speed is one
advantage of the CCD instrument: The spectrofluorometer
took four hours to collect the series of spectra
for each sample shown in figures 7 and 11,
whereas the CCD spectrometer generated its spectra
in mere seconds. Additionally, a spectrofluorometer
cannot record time-dependent spectra, such as
phosphorescence spectra, because the instrument
slowly scans across the wavelength range. The
CCD spectrometer is ideal for collecting such spectra,
as data for the entire wavelength range may be
collected simultaneously over integration periods as
short as 0.5 second.
The spectral resolution of the CCD spectrometer
is lower than that of the spectrofluorometer and
most other standard spectroscopy equipment available
in a gemological laboratory, as smaller peaks
typically are obscured by dominant bands, and in
most cases only the general shape of the band is provided.
Consequently, spectra obtained using the
Ocean Optics system may need to be cross-referenced
initially with other spectroscopic methods to
fully analyze the defects causing the observed fluorescence.
Last, but of singular importance for small labs
especially, the CCD spectrometer is quite economical.
The latest model (figure A-1) currently sells for
about $2,300.