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.
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 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.