CD Instrumentation

A schematic of an Olis CD instrument is shown in Figure 2. A detailed description of each component follows.

Fig. 2
(click to open larger image)

Lamp, Power Supply, Coolant

To produce the requisite UV light, an ozone-producing 150-watt xenon arc lamp is used. A continuous flow of nitrogen gas is required to remove oxygen from the housing (see Nitrogen Purging). The lamp housing used in Olis CDs maintains a cool lamp temperature with continuous flow of water from the Olis cooling box. The cooling box providing the flow contains a sensor, which cuts power to the lamp in the event of water flow disruption, thus providing assurance against overheating while also extending the useful life of the lamp.

When starting the lamp, the electronics and computer should be off to prevent possible damage due to power surges. In addition, the lamp should not be restarted when hot; wait at least 15-20 minutes with the cooling box on before restarting a xenon arc. A lamp can last 2000 hours of run time with proper care albeit losing UV light intensity as the lamp ages.

Monochromator

The function of the monochromator is to select the wavelength(s) of light for the measurement. The output wavelength is determined by the rotation of two gratings (in a double grating monochromator) or rotation of a prism/grating (in a Cary 17, for instance). At a given wavelength, the slit width will determine the bandwidth (wavelength distribution of the output) and the intensity. Thus, relatively wide slits give low wavelength resolution and high intensity, and thus, high signal to noise ratio. Conversely, narrow slits give sharp resolution and relatively lesser signal to noise.

There are three Olis CD instruments—models 20, 17, and 1000—which use three different monochromators. In the DSM 20, a compact, double grating monochromator is used to give a spectral output of 170-700 nm. In the DSM 17, a Cary 17 monochromator is used with a spectral output of 184-2600 nm. The DSM 1000 uses an Olis RSM monochromator with a spectral range of 167-1700 nm. As an F/4.4, the model 1000 provides the highest throughput of light of the three. An additional difference among the monochromators is that the DSM 17 has variable slit widths during data collection, while the DSM 20 and DSM 1000 have fixed slits doing scanning. The DSM 1000 also possesses the possibility to rapid scan (up to 62 scans/sec), best used in the spectral ranges above 400 nm.

Photoelastic Modulator

The PEM contains a photoelastic material that changes its optical properties under pressure. By carefully modulating the pressure on the material with a piezoelectric control, linearly polarized light is converted to circularly polarized. In the Olis dual beam CD, both beams produced by the polarizer pass through the PEM; since the beams are orthagonally polarized; the output of the PEM is two beams that are modulated 180° out of phase. Upon detection by the PMTs the signal is digitally subtracted to give the CD signal. The monochromatic beam passes through a linear beam splitting polarizer and a photoelastic modulator (PEM) to produce two beams of RCPL and LCPL.

Sample Chamber

The sample chamber contains the cuvette holder through which LCPL and RCPL beams pass simultaneously. Typically, cylindrical cells with a very short pathlength (see Sample Concentration Effects) are used. Either a Peltier controller or a water bath, each under optimal computer control, controls the temperature of the cuvette holder.

The detectors are two photomultiplier tubes (PMTs) that contain dynode chains of photosensitive material that emits electrons when a photon is absorbed. This electron flow is amplified by the application of a voltage across the chain (PMT High Volts). Increasing this voltage increases the sensitivity of the detector and, consequently, the noise. The current from the detector is converted to a voltage and sent to the electronics box. Note: Care must be taken not to expose the PMTs to room light when a high voltage is applied to them. Permanent damage to the phototube may result.

Electronics Box and Computer

The power supplies and drivers are contained in a single electronics box. The electronics box also contains control boards that provide for motor movements and data collection. The computer contains a twin 14-bit analog to digital (A/D) converter board that digitizes the analog signals from both beams simultaneously. Communication between the electronics box and the computer occurs through the Com Port 1. Any automated temperature control occurs through Com Port 2.

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