caCD ‘alignment’ cell

With these principles in mind, the second generation of the caCD cell was designed and fabricated. Using a modified Jasco base-plate, this cell was built around the principle of minimising optical components whilst taking advantage of the unique capillary optics to refocus the light beam.

The initial work on this new design was to test the suitability of a single focusing lens as a means of aligning the light beam effectively. Through the use of Blu-tack™, different lenses were substituted into the light path to test their effectiveness. However, in order for the caCD cell to be used as a routine spectroscopic method, a more user-friendly design was required. To this end, a Teflon lens holder was designed which could both accommodate the lens and fit well inside the slide-way of the Jasco base-plate. This design was both simple and inexpensive (figure 2.7).

Figure 2.7: The alignment caCD cell, including lens holder. The design for this cell was simple and very cost effective. However, difficulties with data reproducibility marred the cell design.

After several experiments using different lenses at different focal lengths, it became clear that a DCX lens of focal length 100 mm was ideal for the caCD application. A lens of this focal length brings several advantages. Firstly, the level of light focusing

is to a good standard, the beam creating a band 200 µm wide at the focal point of the lens. This is within the sample region of the capillary, enabling the majority of the light produced by the spectrophotometer to pass directly through the sample, rather than the quartz walls of the capillary.

The greatest advantage of the 100 mm focal length lens, however, is the low level of divergence that the lens creates in the light beam after it has passed the focal point of the lens. By placing the focal point of the lens 2 mm before the capillary, the light ray becomes slightly divergent (figure 2.8). However, due to the long focal length, this does not create a large light beam – in reality the light beam remains small enough to enter the sample area of the capillary. The capillary then refocuses the light beam, generating a light beam that exhibits a high degree of collimation after the capillary. It then enters the detector.

Figure 2.8: Diagram showing the effect on the light beam as it interacts with the capillary.

This design was then tested using Na [Co (Edds).H2O] to see what effect these

-40 -30 -20 -10 0 10 20 30 40 240 320 400 480 560 640 720 Capillary CoEdds R,R Capillary CoEdds S,S Cuvette CoEdds R,R Cuvette CoEdds S,S Cuvette water Capillary water C D / m d e g Wavelength (Nm)

Figure 2.9: Na [Co (Edds)].H2O 0.55 mM spectra acquired through both cuvette CD and caCD. The caCD spectra were acquired using the alignment cell design of caCD base plate. The path-length of the capillary was 1.12 mm, determined via analysis with potassium dichromate. Spectra scaled using a 0.892 correction factor to facilitate comparison between the spectra. The path-length of the cuvette was 1 mm, verified with potassium dichromate.

In figure 2.9, the spectra overlay fairly well, but the longer wavelength light suffers from a poor baseline. We believe this is due to the effect of chromatic aberration on the focusing lens. Chromatic aberration (illustrated figure 2.10) can be defined as ‘the failure of a lens to focus different wavelengths of light to the same convergence point’. In the case of UV light, the chromatic aberration of the lens occurs in a

longitudinal manner, i.e. the focal point moves at different distances away from the

lens. (Transverse chromatic aberration, i.e. the movement of the convergence point

to different positions within the focal plane, was not observed within the caCD setup). Chromatic aberration has been a known cause of difficulty in optical applications since the 17th century, where it was traditionally minimised by increasing the focal length of lenses within an optical system. This approach proved quite effective, as the focusing ‘gradient’ of the lens would be significantly reduced in a longer path- length lens.

Table 2.2: Table and graph showing the effect of chromatic aberration on the focal length of a 100 mm DCX lens. A J-600 spectropolarimeter was used as the light source for these measurements. Focal point determined using white fluorescent paper.

* As wavelength decreases, the width of the light beam increases. This is very closely related to the slit size of the spectrophotometer at that wavelength. It must also be noted that as wavelength decreases, the light beam becomes more diffuse and may scatter more. Masking of the capillary is therefore vitally important to ensure spectral quality.

** Taken as a midpoint of the focusing/diverging light cone

Wavelength /nm

Focal Point** of 10 cm DCX lens/cm

700 10 650 10 600 10 550 10 500 10 450 10 400 10 375 9.75 350 9.5 325 9.25 300 9 275 9 250 8.5 225 8

Chromatic aberration in a 10 cm focal length

lens

Figure 2.10: Diagram to illustrate the effect of longitudinal chromatic aberration on a light beam. (Image taken from http://en.wikipedia.org/wiki/File:Lens6a.svg)

Some optical applications do not allow for a simple increase in the focal length of lenses used and so a number of specialist lenses have been developed to counter this problem throughout the history of optics. Achromatic lenses were first developed in the 1750’s, followed by the invention of the superior apochromatic lens in 1763. Current lenses devised to reduce chromatic aberration are based on the superachromatic design, invented by Max Herzberger in 1963 13. However, these lenses are only available in the IR and visible light regions. A UV superapochromatic lens has unfortunately not yet been developed. This is due in part to the difficulty of developing a composite UV lens with the very limited materials available that can successfully transmit UV light, but also because the applications of such a lens are a lot more limited than for visible region applications. Chromatic aberration in UV applications therefore remains an issue to be addressed.

In the case of caCD, the change in wavelength has a big impact on the focal length of the lens, shifting the convergence point by as much as 20% at the lowest observable wavelength (data shown in table 2.2). As the spectrophotometer moves into the UV region, the focal length decreases as the focusing effect of the lens becomes less pronounced. As a result, less light reaches the detector overall since the light beam diverges more as the energy of the light beam increases.

In this instance, the interesting optics of the capillary has proven advantageous. Taking advantage of the net focusing effect of the capillary, the lensing effect observed can help refocus the light – the greater the chromatic aberration, the greater the focusing effect of the capillary.

The issue with the alignment cell was found to be the lack of reproducibility and reliability of the method when compared to a ‘fixed’ optical path. We postulate that the movement of the lens itself was generating a degree of transverse aberration at a given wavelength, resulting in data that were not of sufficient quality.

This meant that the data were neither repeatable nor reliable, both of which are absolutely essential in a scientific quantitative instrument. Therefore a new design was made, building on the successes of the previous design whilst improving upon its weaknesses.