Chem 2000 Spectrophotometer
  

Spectrometer. 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 um 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 your source light is on. You than must choose either to Store or Cancel your 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 you must 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 your 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. you must 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 you want to 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 spectra.

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