Diode-Array Spectrophotometer

.The HP 8452A Diode-Array Spectrophotometer is a single-beam, microprocessor-controlled spectrophotometer. Visible/UV - range of 190 to 820 nm with 2 nm resolution. It is used when speed of measurement is essential. It is faster, more sensitive and has more precision than a conventional spectrometer due to the photo diode-array detection system. See below for detail of the operation. This spectrometer is ideal for kinetic

Background for Diode array -- Diode arrays are assemblies of individual photo diodes in a linear array. Self-scanned arrays have the read-out electronics included on the chip with the array. When read out, all elements of the array must be read out in series. The array has 1024 elements. Light of all wavelengths falls on the diode-array and is measured simultaneously, that is, data acquisition is done in parallel. speed is the best known advantage of diode-array spectroscopy. Data is acquired in parallel, the detectors are read-out by "electronic scanning", and microprocessors and computers are used to process data; Measurements are made at different wavelengths at the same time. Conventional spectrophotometers can make multi-wavelength measurements but there is a time differential between each measurement. The diagram below illustrates what happens in a diode array spectrophotometer. First, a light source generates light at a specific wavelength or wavelengths. Normally, a UV/Vis spectrophotometers utilize two light
sources: a deuterium arc lamp for consistent intensity in the UV range (190 to 380 nm) and a tungsten- halogen lamp for consistent intensity in the visible spectrum (380 to about 800 nm). Some spectrophotometers use xenon flash lamps, which offer decent intensity over the UV and visible regions.

The source light is then directed to a dispersion device that causes different wavelengths of light to be dispersed at different angles. Two common dispersion devices used in UV/Vis spectrophotometers are prisms and holographic gratings. The angle of dispersion with a prism, however, can be nonlinear and sensitive to changes in temperature. Holographic gratings are glass blanks with narrow ruled grooves. The grating itself is usually coated with aluminum to create a reflecting source. Holographic gratings eliminate nonlinear dispersion and are not temperature sensitive. They do require filters, though, since light is reflected in different orders with overlapping wavelengths.

Once the light has been passed through the dispersion device and the sample of interest it reaches a
detector. Detectors in UV/Vis spectrophotometers come in a variety of shapes and sizes.
Photomultiplier tubes are common; they provide good sensitivity throughout the UV/Visible spectral
range and are highly sensitive at low light levels. Photodiodes have seen increasing use as detectors
in spectrophotometers, bringing to the table a wider dynamic range. A photodiode is generally made
up of a semiconductor and a capacitor to charge the semiconductor. As light hits the semiconductor,
electrons flow through it, thereby lowering the charge on the capacitor. The intensity of light of the
sample is proportional to the amount of charge needed to recharge the capacitor at predetermined
intervals. As opposed to having single photodiodes, some spectrophotometers are composed of a
photodiode array. Here, several photodiode detectors are arranged on a silicon crystal. The
advantage of an array is the ability to do side-by-side readings, thus increasing speed.

The entrance slit, dispersion device, and exit slit are referred to as the monochromator. Light passing
through the monochromator exits as a band. The width of this band of light at half the maximum
intensity is the spectral bandwidth. Bandwidth comes in to play with regard to accuracy, since the
accuracy of any absorbance measurement is dependent on the ratio of the spectral bandwidth to the
natural bandwidth of the substance being measured. The natural bandwidth is the width of the
absorption band of the sample at half the absorption maximum. As a rule, a ratio between spectral
bandwidth and natural bandwidth of 0.1 or less will generate absorbance measurements 99.5 percent
accurate or better.

In a conventional spectrophotometer, polychromatic light from the monochromator is transmitted
through the sample, and the sample absorbance is determined by comparing the intensity of the light
hitting the detector with just a sample blank with the intensity of light hitting the detector with the
sample in place. With a diode array spectrophotometer, polychromatic light passes through the
sample and is focused on the entrance slit of the polychromator (the entrance slit and dispersion
device). The light is then dispersed onto the diode array with each diode measuring a portion of the
spectrum.

Changing Default Settings

The devault setting do not allow the user to compare one spectra to the next. Select option F5 to change for Single Spectra to Overlay Spectra.

Procedures to Set Directories, Save, and Copy Files
1. Use Esc or F10 to find the Files menu (F6) and select Files
2. First note and record the current directory, it should be C:\HPUV\DATA. The name of this directory is HewittPackardUltraViolet). If the default directory is changed it must be returned to the default directory by the user.
3. Users should create their own data directory under their own name, e. g. C:\HPUV\DATA\Name. Do not use long directory or file names. The DOS protocol does not allow files names to exceed 8 characters and may contain alpha and numeric characters but not any reserved DOS characters. The DOS extension for a data files (.WAV) is automatically added. The DOS *.WAV files are relatively small, between 600 and 4,400 bytes.
4. It is also possible to save data directly to the B drive if that directory is selected. The B drive uses a 3 1/2" floppy disk. When changing the directory type B:
5. It is also possible to save data to the C drive and subsequently copy files to a floppy. After the files have been saved exit the spectrometer menu and select the Utility Menu. Enter the proper password and select Valet to copy files.

Valet is a DOS directory program that makes it easy to copy files. Use the space bar to tag a few files to be copied or if all the files are to be copied use the "T" key to tag all files. Similarly, use the "U" key in Valet to untag all files which have been tagged. Select each file to be copied than type C to copy files and enter the destination. Normally the destination will be B: for 3 1/2" floppy, note the colon must be used. Use Valet to check the presence of the copied file on the floppy disk before deleting the original file.