Development of an instrument for LIBS and imaging

Summary
Objectives
Applications
Standardisation
Optical arrangements
Parts list
References

Summary

In the first instance, this instrument will be used to determine the photosynthetic performance and elemental composition of individual cells within mixed populations of microalgae. This will involve imaging of chlorophyll a fluorescence, measurement of chlorophyll a excitation characteristics and the application of LIBS.
 

The system (August 2006):

Included in this view are a Big Sky 200 mJ laser (right) and a high-resolution imaging system based around an AVT Dolphin F145b camera. The blue light is provided by two Luxeon star LEDs (peak at 465 nm), which provide up to 12 000 µmol m^-2 s^-1. The camera is connected to an InfiniTube, Uniblitz shutter and Mitutoyo 10 x objective. This combination provides a working distance of 33.5 mm and a field of view of approximately 880 µm x 660 µm; making each pixel equivalent to approximately 0.43 µm².
 

Click here for a complete parts list.

Detail of fibreoptic arrangement Unbranched fibres (front) are used to transmit light to the two spectrometers that cover the UV. A multibranch fibre (back) serves the other five spectrometers. This arrangement makes it possible to optimise the signal strength across the entire spectral range (from 200 to 960 nm).

Chlorophyll a fluorometry is an established, non-invasive technique for investigating oxygenic photosynthesis. A number of chlorophyll a fluorescence imaging systems have already been developed at the University of Essex. Of particular relevance to this proposal are microscope-based systems, used to determine PS II photochemical efficiencies from microphytobenthos within intact biofilms and pelagic microalgae within mixed oceanic and estuarine populations.

LIBS is an extremely versatile method for the determination of elemental composition, which has been used in a wide range of applications – from the detection of trace amounts of explosives on luggage in airports to the detection of sub-pg amounts of Ca and Mg within sub-µm diameter aerosols (Hahn & Lunden, 2000). With this technique, a brief, high powered laser pulse is focused onto a sample to create a plasma, which results from vaporisation and atomisation of a small amount of target material. Spectroscopic analysis of the light emitted as the plasma cools allows for the identification of elements within the sample from their unique spectral signatures. The spectral region of interest is from 180 to 850 nm. It includes several peaks per element, which allows for cross-checking against interference. After calibration, it can often be used in a quantitative manner, with an accuracy that is typically better than 15%. A much higher level of accuracy can be achieved when LIBS is used to determine ratios of elements within a sample.

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Objectives:

1. To combine an Ocean Optics LIBS 2000+ spectrometer, a Big Sky CFR200 Pulsed Nd:YAG Laser and a high resolution, chlorophyll a fluorescence imaging system of in-house design
2. To calibrate the LIBS measurements against independent, macroscopic chemical determinations of elemental composition, using cells that range from a few µm to hundreds of µm in size
3. To use the combined LIBS / chlorophyll a fluorescence imaging system to derive physiological and elemental composition data from a range of types of benthic and pelagic algae