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Figure 4 (a) surface scan of a quality cuvette
Figure 4 (b) surface scan of a lower quality cuvette
It’s inevitable that established analytical procedures carried out in a laboratory will migrate over time to become on-line or remote measurements made as close to the sample source or process as possible. For many types of optical analysis this has been a relatively slow process, but driven by recent developments in optical-fibre technology, alternative light sources and CCD array detectors these traditional laboratory techniques are venturing out into the wider-world at an ever increasing rate.
In most cases the transfer from laboratory to on-line analysis requires the change from putting a sample in a container to the immersion of an optical probe into a vessel or flowing stream of sample. The importance of the path-length range and accuracy, as well as total transmission values, remain as important for probes as they are for cuvettes, while a number of other characteristics become key considerations for probe-based measurements as well.
For use in these generally more rugged and demanding situations optical elements of Quartz, Sapphire or even Diamond become the materials of choice, while the body or stems of the probe can be constructed from Quartz, PTFE, Stainless-Steel, Hasteloy, Titanium or even Tantalum for resistance to the widest range of aggressive samples and compatibility to specific conditions of temperature and pressure.
Figure 5: From left to right unmasked and partially masked plastic cuvettes along with a fully self- masking quartz cuvette
surrounding the central chamber (see Figure 5), making them self-masking and suitable for the beam geometry of almost any instrument.
Quartz cuvettes are most often chosen for their superior performance in the UV wavelength range but in many other instances their impressive chemical resistance and physical durability, along with excellent temperature, mechanical and dimensional stability, make them ideal for other applications in both the visible and NIR wavelength ranges too. With adherence to a few simple care and handling guidelines they will give a lifetime of dependable and repeatable measurements.
When it comes to automation and more complex designs, glass and quartz cuvettes come into their own, with many different flow-through configurations and temperature controlled versions, with others for high pressure or vacuum operation, some fully sealed or with a septum for sample injection, as well as demountable cells for path-lengths as small as 0.01mm!
The huge benefit of the continuous monitoring of reactions or processes brings with it another variable, and that is how the probe is mounted into its vessel or pipe-line. There are many standard flanges and compression fittings that can be used in different situations, adding further options to the probe design. To enable the optimum selection to be made from all these choices a probe configurator is now available at
www.mypatprobe.com which enables relevant decisions to be made for Transmission, Transflection, Reflection and ATR probes.
Often the advantages of continuous monitoring can be just as important in the laboratory environment as they are in the process environment, so in the opposite direction process hardware is being adapted for use in the laboratory. Optical immersion probes are being integrated into an ever increasing range of laboratory instruments dedicated to specific measurement techniques; from photometric end-point detection in titrations, to identifying the cloud and crystallisation points in lab-scale reactors, for solubility and dissolution experiments, for colour comparison in quality control, for the remote measurement of hazardous materials in glove boxes and many more.
The rapid growth of MEMS (Micro Electro- Mechanical Systems) in spectrophotometer design has led to many new micro-modular systems being available from a broad range of manufacturers. These have all strongly supported the application-specific approach to satisfying the analytical and measurement requirements of individual customers; creating in effect a growing market for bespoke optical instrumentation. The adaptability and flexibility of optical immersion probes makes them an ideal sample interface for this type of approach to many applications.
Figure 6: Glass and Quartz cuvettes offer solutions to many applications
For many years traditional laboratory spectrophotometers have, in the main, only supported the analysis of your samples when contained in cuvettes; perhaps the next generation will embrace both containment and immersion, and let the routine laboratory access the advantages of discrete measurement and continuous monitoring using optical immersion probe technology.
Figure 7: Industrial probes for process monitoring/control
Figure 8: Experience the potential of using an immersion probe with a lab spectrophotometer
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