LIBS Basics, Part II: Hardware - - Spectroscopy
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LIBS Basics, Part II: Hardware

Volume 29, Issue 4, pp. 26-31

Detection System

The detection systems used in LIBS can be parameterized by wavelength range, sensitivity, and speed and ability to gate the detector exposure. Overall, the number of choices and range of performance is continually expanding. Given the number of parameters, including cost, that are a consideration in a LIBS system, it's usually true that there is no one-size-fits-all magic bullet that is useful in every situation. Here, we'll cover the main points associated with each of the detector types to provide readers with an overall perspective to make an assessment.

Spectrometer Systems

Charge-Coupled Device–Based Spectrometers

Used in many applications, charge-coupled device (CCD) spectrometers are inexpensive and widely available from many manufacturers. These spectrometers typically use a Czerny-Turner or crossed Czerny-Turner design. To be suitable for LIBS, they must be outfitted with a trigger so that the measurement can be initiated at a known time with no more than a 20–50 ns delay. Presently, none of the CCD systems on the market have a gating feature that allows the detection to be terminated at a known time; these systems are typically measuring for 1 ms or longer following the trigger. Detectors on the standard CCD systems (for example, Toshiba TCD-1304 or Sony ILX511B) are linear arrays and have a typical dynamic range of about 260–300. These chips or similar ones are used in all of the basic CCD systems, and thus all of them have a similar dynamic range. Different manufacturers use various analog-to-digital (A/D) converters with 12-, 14-, and 16-bit ranges, giving the appearance of additional sensitivity, but in reality the higher A/D ranges are simply window dressing and not performance-related. Spectrometers used in the UV region of the spectrum are typically enhanced with a lumogen coating.

Broadband CCD Spectrometers

These spectrometers are arrangements of CCD spectrometers with a common triggering system. Accompanying software can be used to stitch together the output of these systems into a single broadband spectrum. At best, these systems generally provide 0.05 nm full width at half maximum (FWHM) resolution.

Echelle Spectrometers

Echelle spectrometers combine two dispersive elements, typically a prism and a grating, to disperse light in two dimensions on a square detector. This results in a two-dimensional spectral field with various orders in one dimension and wavelength in the other. Software is used to sort the various orders and construct a continuous spectrum, which can range from 200–900 nm or more. Light throughput in these spectrometers is not stellar, typically they have an effective F-number of 7 or 8. Depending on the wavelength of the emission line in question, echelle spectrometers coupled with intensified cameras (see below) can easily outperform CCD spectrometers by an order of magnitude or more. Resolution is typically 0.05 nm or better, but cost, particularly when considering the cost of the camera, can be substantial.

Traditional Czerny-Turner Spectrometers

These more traditional, large Czerny-Turner spectrometers may be 0.25 m in pathlength or more. Typically outfitted with a turret with several diffraction gratings, users can dial the center wavelength, wavelength range, and associated resolution to their liking. The light throughput on these systems is very high and light is generally dispersed using the (very strong) first order of the grating. On a typical intensified camera detector, the light is dispersed across the detector and users can choose to collect ("bin") the camera from one to the maximum number of rows on the camera. Because the gain on the camera can also be adjusted, this combination results in an instrument with incredible dynamic range (104 –105). The downside is that the limitations on the width of the detector result in the collection of only a narrow window of light (30–50 nm) with each shot, if 0.1 nm resolution or better in the spectrum is desired. Hence, associations between multiple elements may be difficult to obtain because the emission lines of the particular elements of interest may be in different parts of the spectrum. This limitation understood, the combination of components is one of the most sensitive LIBS detection schemes, surpassed only by the photomultiplier tube discussed below.

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