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Instrument Database:
PerkinElmer Inc. - Spectrum GX Optica System
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| Year of introduction |
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| Status |
available |
| Company |
PerkinElmer Inc.
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| Categories |
Spectrometer ( Molec. ): FTIR
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KEY BENEFITS
Absolute transmission accuracy
Optical baffles block stray reflections
High degree of control over source image
Abscissa precision of ±0.01 cm-1
PerkinElmer has used its extensive experience with the optical industry to develop Spectrum™ GX Optica; an FTIR spectrometer designed specifically for applications demanding very high levels of transmission accuracy. Scientists can now enjoy all the benefits of FTIR while ensuring photometric accuracy that enables them to guarantee their product against specifications.
The essential difference Despite the universal adoption of FTIR spectrometers for other applications, doublebeam dispersive instruments are widely preferred for measurements ot optical components. Conventional FTIR spectrometers show ordinate errors from stray light produced by multiple reflections from highly reflective surfaces typical of optical samples, and also the non-linearity of the detector and electronics. Additionally, every optical component in the IR beam, including the source and detector, can contribute to stray reflections. Multiple reflections through the interferometer produce double modulation of the IR beam. This results in false energy being folded back into the spectrum. This is often apparent as artifacts in regions of the spectrum blocked by the filter being measured. Consequently, the true characteristics of the filter are prevented from being recorded.
This is not the case however, in the Spectrum GX Optica. With this specially upgraded version of the Spectrum GX FTIR spectrometer, all possible stray reflections are prevented from reaching the detector. Typical ordinate accuracy for transmission measurements is better than 0.25%T with uncoated germanium (nominal 47%T) and better than 0.01%T with a typical cut-off filter (nominal O%T).
Spectrum GX Optica benefits from abscissa calibration being tied to the wavelength of an internal HeNe reference laser (the Connes advantage). This provides abscissa precision of ±0.01 cm-1. Abscissa accuracy can be validated relative to a traceable reference material (TRM) by using PerkinElmer's Automatic Precision Validation (APV™ ) kit.
New optical design The unique PerkinElmer dual iris assembly contributes to the high performance levels seen in the Spectrum GX Optica. It comprises two variable apertures, a Jaquinot stop (J-stop) and a beamsplitter stop (13-stop). Independent software control of the aperture sizes is provided. These apertures control beam divergence and source intensity, providing users with a high degree of control over the size and quality of the source image at the sample. In conventional FTIR spectrometers, small samples sometimes present problems because they obscure the beam. The dual iris assembly completely overcomes this.
Optical baffles are arranged along the IR beam path to block stray reflections. A customized detector mount ensures that IR radiation reflected from the detector is not co-linear with the incoming beam. This stray light is prevented from returning to the interferometer and producing double modulation artifacts. The spectra shown illustrate the performance of Spectrum GX Optica against that of a standard FTIR spectrometer.
Figure 1 shows two spectra of a sample of germanium measured in transmission. The upper spectrum was measured using a standard FTIR spectrometer and the lower was measured with a Spectrum GX Optica.
The spectrum measured with the standard FTIR spectrometer shows a higher apparent transmission and artifacts due to reflections within the spectrometer. These reflections cause some of the IR radiation to be double modulated (pass through the inter-ferometer twice) before reaching the detector. Hence the residual CO2 band at 2350 cm-1 is also apparent at approximately 4700 cm-1.
Figure 2 shows two spectra of a filter which transmits IR radiation at less than 1500 cm-1. The important function of the filter is to block the radiation at greater than 1500 cm-1. These spectra are similar, except that the spectrum measured using the standard FTIR spectrometer has some small features evident at greater than 1500 cm-1.
When the range at greater than 1500 cm-1 is expanded, as shown in Figure 3, it is clear that the standard FTIR spectrometer shows artifacts and false energy that the Spectrum GX Optica does not. This is again due to double modulation causing the energy at less than 1500 cm-1 to be folded over and appear at greater than 1500 cm-1.
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