5 STEPS TO BUILD A HIGH-PERFORMANCE SCIENTIFIC LIBS SETUP

Brief introduction to the world of LIBS systems

Spectrometer

Plasma emission induced by the laser pulse is collected by the optical system and dispersed by a spectrometer. Spectrometers operate within certain spectral ranges and with a given resolution. Other parameters of concern are sensitivity (throughput or etendue ) and, last but not least, mechanical durability and cost. Czerny-Turner spectrometers (Andor website, 2016) Generally, spectral range of 200-900 nm is quite sufficient for the most of LIBS applications.  Simultaneous measurement of this whole spectral range is possible using echelle type spectrometers that are furthermore distinctive by an excellent resolution that enables detecting neighboring line interferences in the spectrum. Speaking of sensitivity, however, a classic – single disperse element configuration (such as Czerny-Turner configuration), either with reflection or transmission diffraction grating, performs much better. In this case the resolution improves for the cost of the spectral range and vice versa. However, a rotating tower with several gratings compensates for this limitation to some extend and allows choosing the most suitable configuration for particular experiment. It should be also noted that spectrometers filled with noble gases (e.g. argon) allow to extend the spectral range down to approx. 170 nm and thus making it possible to detect e.g. low concentrations of carbon by LIBS.

Detector

A detector can easily become the most expensive item of the whole LIBS setup and that is why its selection has to be done with great attention. Laser-induced plasma lifetime is relatively short, so the detector must be sensitive enough (even in the spectral region bellow 300 nm) and must have the trigger input. Compact Czerny-Turner spectrometer including CCD detector (Ocean Optics website, 2016) Some of the less expensive and more compact spectrometers have integrated simple CCD detector. For some applications this solution may be satisfactory; however a powerful scientific setup must contain an advanced cooled detector of EMCCD or even better: the iCCD or emICCD type. The last two mentioned has an integrated MCP unit that combines the function of a fast electronic shutter and a photomultiplying of the input signal. This kind of detector is thus sensitive to low levels of signal and enables fundamental time resolved studies of the emitted signal with resolution in nanoseconds. Various scientific detectors/cameras (Princeton Instruments website, 2016) For certain purposes a standard photomultiplier coupled to a monochromator`s output slit can be used. Although sensitivity of this configuration is high, only one particular line can be detected at the same time, which makes it usually unsuitable for common LIBS analysis.