White light polarimetry for pharma applications

Peter Marriot examines the reasons why the evolution in light sources has happened and explains why a tungsten halogen light still provides the same accuracy as a more traditional high-pressure discharge lamp

Traditionally optical rotation measurements in the pharmaceutical industry have been performed with polarimeters using single wavelength light sources such as sodium lamps (589nm) and mercury lamps (546nm). In the last decade, the US and European Pharmacopoeias have stopped mentioning the actual lamps and now only mention the wavelength required. In fact, US Pharmacopoeia goes at far as to state, ‘‘it is now common practice to use other light sources, such as xenon or tungsten halogen.’’

Advantage of tungsten halogen lamps

Wide availability: Tungsten halogen lamps are used in a huge variety of equipment apart from polarimeters. This includes laboratory equipment like microscopes and non laboratory equipment like projectors. This wide availability means that many places stock them and ensures continued supply even if the original manufacturer of the polarimeter can no longer be contacted.

Low price: Since tungsten halogen lamps are a commodity item that can be purchased from several vendors the price is many times lower than that of a high-pressure lamp that can usually only be purchased from the polarimeter manufacturer.

Long life: Most tungsten lamps have a lifetime measured in thousands of hours, this contrasts with high pressure lamps which are generally measured in hundreds of hours.

Ease of use: To change wavelengths with a white light polarimeter the operator simply has to move a filter. Changing wavelengths with a high pressure discharge lamp polarimeter means changing the lamp, in some cases this is a simple movement of a mirror but it some cases this can be a complex process. A high-pressure discharge lamp also needs to warm up and this can make multi wavelength measurements very time-consuming.

Perceived disadvantage of tungsten halogen lamps

Despite the benefits of tungsten halogen lamps there is a school of thought that high-pressure discharge lamps must be better. Perhaps some think that since they are so much more expensive they must have advantages. The main argument put forward here is that a tungsten halogen polarimeter will have an error due to optical rotary dispersion effects. It is worth examining thought this in detail.

Dispersion effects: Traditional polarimetry avoids optical rotatory dispersion (ORD) effects by performing the measurement at a single wavelength. In white light polarimeters the measurement is made over the narrow frequency pass band of the interference filter. For most common materials where the ORD is effectively linear over filter pass band, there is no error introduced through ORD, and the results are identical to a single frequency measurement at the central frequency of the filter pass band.

The elimination of linear ORD effects is easily demonstrated. Consider a normalised symmetric filter pass band function:

F (*),

and a local linear approximation to the ORD function:

D(*) = m* + b,

where * is the translated wavelength such that * = 0 represents the centre of the pass band. The measured rotation will be integral of the product of these functions over all *:

* (m*+b) F(*) d* = * m*F(*) d* + * b F(*) d*

The first integral on the right vanishes because it is odd function evaluated over even limits, and the remaining integral gives the value of rotation at the centre of the pass band. Thus there is no error introduced by linear ORD.

Typical white light polarimeters use filters with a FWHM of 10nm. This is sufficiently narrow to justify a linear approximation to the ORD over the bandwidth of the filter for pharmaceutical applications. In general, it can be assumed that ORD effects will be negligible when the ORD does not have a complex structure over the pass band of the filter.

Peter Marriott is sales manager, Rudolph Research Analytical, USA. Email: pmarriott@rudolphresearch.com