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Chem 2000 Visible Spectrophotometer
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The Chem 2000
is a PC plug-in, 2048- element linear diode-array fiber optic
spectrometer. It has an optical range from 350 to 900 nm and is
mounted on a durable, half-length, 1 MHz analog to digital (A/D)
card. The digital card fits easily into a slot in a personal computer.
The photo to the right shows the board on the left with the light
source in the blue case on the right. Note the delicate optical
fiber cable going from the light source and sample curvette to
the A/D card.
Light source/sampling optics. The tungsten-halogen light
source is with integrated with a 1-cm square cuvette holder is
a versatile white-light source connected to a 1-cm path length
chamber with a 5 mm diameter f/2 collimating lens. The lamp offers
high color temperature and an efficient output. Also included
are built-in filter slots and an on/off switch on the rear
of the blue case.
Optical fiber. The Chem 2000 comes with a 400 µm
diameter, single-strand optical fiber that couples easily via small connectors to the spectrometer and to the light source-cuvette
holder. This silica-core and silica-clad fiber is 2 meters in
length. A black arrow points to the optical fiber.
Care and Use of Optical Fiber Cable
| When the 400 µm diameter optical fibers break they stop transmitting
light. |
| Do not allow the fiber to be bent at a sharp angle. A
bending radius of less than 1" is dangerous. |
| Keep connectors and probe covered with the fibers are
not being used. |
Software. The Chem 2000 uses an OOIChem Operating Software
(Oceans Optics, Inc.). -- It is a Windows-based program
that provides users with a real-time interface to a variety of
sophisticated acquisition, signal processing, display, and measurement
functions. The software supports Windows 95/98 and Windows NT.
An example of the software window software
window is shown.
The software provides users with five different modes of operation:
Scope, Absorbance, Transmission, Relative Irradiance, and Concentration.
In addition, the software allows users to control data acquisition
features such as integration period, averaging, and boxcar smoothing
-- directly from the spectral graph display. Users can acquire
data by taking manual single scans or by running continuous scans,
and add into the spectral window as many as 8 previously saved
overlay spectra.
Users can also perform time acquisition experiments, which
provide the data necessary for kinetics applications. The kinetics
function allows users to monitor and report up to 4 single wavelengths
or up to 2 calculated values from a combination of wavelengths
-- for example, an absorbance value of 0.400 minus an absorbance
value of 0.700. A kinetics strip-chart displays the time series.
When the experiment is complete, the data can be exported to an
ASCII file for additional processing.
OOIBase is a derivation of Ocean
Optics, Inc. Basic Acquisition Software. Another feature of
the software is that users can control the parameters for all
system functions such as acquiring data, designing the graph display,
and configuring the cursor -- and then save and retrieve all of
these system parameters for future experiments.
Additional features of the software include the ability to
save data as ASCII files; to independently store and retrieve
dark, reference, and sample spectra; and to correct for the electrical
dark signal, which compensates for any temperature drift in the
detector. Data can be transferred to spreadsheets or programs
like Grams.
Scope
Mode -- the spectrometer operating status in which raw data
(signal) is acquired by the diode-array
detector -- allows the user to establishes the experiment's signal
conditioning parameters. Most spectrometer-system operating software
does not allow such signal conditioning flexibility. It allows
users to perform the three basic spectroscopic experiments: absorbance,
reflectance and emission. Other functions include electrical dark-signal
correction, dynamic range setting, stray light correction, boxcar
pixel smoothing, and time series acquisition. In addition, it
has various triggering options for spectrometer control of, or
control by, external events such as the firing of a laser or the
pulse of a light source.
Acquisition Parameters
Integration Period. Enter a value to set the integration
time in milliseconds for an active spectrometer channel. The integration
period of the spectrometer is analogous to the shutter speed of
a camera. The higher the value specified for the integration period,
the longer the detector 'looks' at the incoming photons. If the
intensity is too low, increase the time. Or if the intensity is
too high, decrease the time.
Average. Enter a value to implement a sample averaging
function. The higher the value entered the better the signal-to-noise
ratio (S/N). The S/N ratio improves as the square root of
the number of scans averaged.
Boxcar Smooth. Enter a value to implement a boxcar smoothing
technique that averages across the spectral data. This method
averages a group of adjacent detector elements. A value of 5 will
average 5 points to its left and 5 points to its right. The greater
this value the smoother the data and the higher the S/N ratio.
Reference Scan. Selecting the Reference
Button activates a prompt to make sure the source light is on.
You than must choose either to Store or Cancel the
reference scan. A reference spectrum is taken with the light source ON and a blank or solvent in the sample cell. Storing
a reference spectrum is required to before the software can calculate
absorbance, transmission, and relative irradiance spectra.
This command only stores the spectrum. To permanently save the
spectrum to disk select File|Save Spectral Values.
Dark Scan Selecting the Dark
button activates a prompt to make sure the light path is blocked.
You then must choose to either Store or Cancel the
dark scan. A dark spectrum is taken with the light path to the
spectrometer blocked. Storing a dark spectrum is a requisite
before the software can calculate absorbance, transmission and
relative irradiance spectra. This command only stores dark
spectrum, it does not save it.
Absorbance
Experiments
The same general procedures is also followed for obtaining
transmission, concentration, or irradiance spectra. Remember if at any time any sampling variable changes, e.g. integration time, averaging, boxcar smoothing, etc.
first store a new reference and dark spectrum.
1. Select Scope under Mode of Operation. Make
sure the signal is on scale. Adjust acquisition parameters so
the peak intensity of the reference signal is about 3500 counts.
Take a reference spectrum by first making sure nothing is blocking
the light path going to the spectrometer. The analyte to be measure must be absent while taking a reference spectrum. Take
the reference reading by clicking the Reference button,
which stores the spectrum. Storing a reference spectrum is required
before the software can calculate absorbance
spectra.
2. While still in the Scope Mode, take a dark spectrum by first completely blocking the light path going to the spectrometer.
Place a 1-cm black cell into the cell holder. Take the dark reading
by selecting the Dark button to store the spectrum. Storing
the dark spectrum is a requisite before the software can calculate
absorbance measurements.
3. Begin absorbance measurements by placing the 1-cm cell containing
the sample into the light path. Then select Absorbance under Mode of Operation. Click the Scan button.
If Single is selected, only one scan will be taken. If Continuous is selected, the spectrometer will continuously
take scans until the Stop button is selected. To save the
spectrum, select File|Save Spectral Values.
The following shows the basic window from Chem 200 Spectrometer
in the Scope Mode, with a spectrum.
