Optical Rotation of Dextromethorphan Hydrobromide Measured using a Polarimeter with 365nm Mercury Lamp
August 25, 2022
Introduction
In accordance with the U.S. Pharmacopeia (USP), the measurement of optical rotation must be measured at 325 nm, however there is no emission line at 325 nm available in a polarimeter using a Na or Hg lamp as the light source. Therefore, for the measurement of optical rotation of Dextromethorphan Hydrobromide, a polarimeter with halogen lamp as a light source and interference filter (325 nm) has been typically used, but as shown in Fig. 1, the optical rotatory dispersion (ORD) spectrum, has very steep slope in the short wavelength range near 325 nm, where even a very small error in the wavelength of interference filter may cause a large error in the optical rotation measurement, which can make accuracy quite difficult to achieve. On the other hand, if measurement using the Hg 365nm emission line were permitted, since no wavelength error is expected, the measurement of optical rotation can be implemented with much greater accuracy. In this experiment, dextromethorphan hydrobromide was measured using a P-2000 polarimeter with a Hg lamp at 365 nm, with a halogen lamp and interference filter at 325 nm and with the P-2000 polarimeter using its ORD optional attachment.
Results
Measurement using P-2000 Polarimeter
| Optical rotation (365 nm, 20°C): | +2.5655° (P-2000 with Hg lamp) |
| Specific rotation [α]36520: | +142.53 |
| Optical rotation (325 nm, 20°C): | +4.7034° (P-2000 with halogen lamp & interference filter) |
| Specific rotation [α]32520: | +261.30 |
ORD attachment
| Optical rotation (365 nm, 20°C): | +2.5706° (P-2000 with ORD attachment) |
| Specific rotation [α]36520: | +142.81 |
| Optical rotation (325 nm, 20°C): | +4.8726° (P-2000 with ORD attachment) |
| Specific rotation [α]32520: | +270.70 |
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