| While some progress in laser-based polarimetry has been achieved in the past decade, the MOPED system contains several novel and innovative concepts: the use of a sinusoidally varying magnetic field applied directly to the sample, optical heterodyne mixing at the detector, and double reference lock-in detection on the resulting inter-modulated sidebands An illustrative example is shown in Figure 1 indicating how one might compare MOPED to a modern polarimeter. The analogy illustrates that observing signals at wavelengths or frequencies different from the excitation signals can lead to greatly enhanced sensitivity as intrinsic background and noise pickup are absent. |
Figure 1. Comparison of absorbance and polarimetry to fluorescence and MOPED.
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| Historically the last modifications realizing an order of magnitude improvement for a general-use analytical polarimeter was the introduction of a Faraday modulation by Gillham in 1957 and the introduction of a laser source in 1980 by Yeung. Despite more recent improvements in laser optics, audio electronics, digital signals processing (DSP), and telecommunications these technology advances have not been applied to the problem of chiral sample detection in analytical HPLC systems. MOPED makes use of these recent advances to improve chiral measurement. A conceptual drawing of a MOPED instrument is show below in Figure 2. |
Figure 2. Instrument setup. The layout of instrument components is indicated with the differential photodiode circuit conditioning the signal prior to processing in the lock-in detector. The two polarizers are aligned such that both beams exiting the analyzing polarizer are ±45° from the null point. A driving current modulates the solenoid coil, producing a magnetic field whose amplitude varies sinusoidally in time. The sinusoid also provides the reference wave for a lock-in amplifier.
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Lock-in detection achieves sensitivity many orders of magnitude better than a simple dc measurement. In this case, lock-in detection involves detecting at the mixing frequency of the applied magnetic field and a second modulation on the beam, which has the additional benefit of avoiding leakage noise and any noise in the driving magnetic field applied to the sample.
Modulating the light source intensity of the laser yields additional noise rejection capabilities to the MOPED system. This technique is known as “optical heterodyne detection,” and this additional modulation of the system yields inter-modulated sideband frequencies due to the square law nature of the photodetector. Optical heterodyne detection is often used in conjunction with polarization spectroscopy to yield spectroscopic measurements of unrivaled sensitivity.
In addition, detecting the “phase” of the modulation of the transmitted light (the shift of the sinusoidal modulation of the transmitted light relative to that of the applied magnetic field) yields additional information on the optical activity. In this case the detection of the phase determines which enantiomer is in excess. The phases of different enantiomers will differ by 180 degrees, and thus the sign of the amplitude detected with the lock-in can be determined. |
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