Detectors for LIBS

The subject of detectors for LIBS is quite complicated, encompassing cost, sensitivity, and range. Having used nearly all types of LIBS detectors, unfortunately we can say that there is no magic silver bullet that is useful in every situation. In most cases, there are tradeoffs that need to be made. This short post should suffice in helping you choose a detector for your application.

We will consider simple CCD-based spectrometers, broadband array spectrometers, echelle spectrometers with intensified CCDs, Czerny-Turner spectrometers with intensified CCDs, and PMT-based spectrometers. These have the following attributes:

CCD-based spectrometers: Used in many applications, CCD-based spectrometers are inexpensive and have fairly broad wavelength coverage, depending on the CCD. Typically these cannot be gated faster than a few ms, and because of this, it is difficult to control the data acquisition time or make acquisition very short, which can influence the repeatability of the measurement. Cooled CCDs can have very low dark noise, and thus a very low overall noise floor.

Broadband diode array spectrometers: These spectrometers are arrangements of CCD-based detectors designed to cover a broad wavelength range. Typically uncooled, with multiple-CCD spectra stitched together to form larger broadband spectrum, these systems can provide relatively low cost with multi-element sensitivity. Depending on the configuration, detection limits with these spectrometers can be surprisingly low. At best, these systems generally provide a 0.1 nm resolution.

Echelle spectrometers: Echelles provide broadband coverage which is usually combined with a cooled, intensified CCD that allows gating of spectral acquisition to 10s of nanoseconds. In an echelle, the incoming light is dispersed by a prism and a grating onto the detector, resulting in a 2-dimensional spectral field, the various orders (typically the relatively weaker higher orders) of which are sorted by software to assemble an entire spectrum. Depending on the wavelength region of the emission line, echelle spectrometers with intensified cameras can outperform broadband diode arrays in detection sensitivity by an order of magnitude or more. Resolution is typically 0.05 nm (50 pm) or better. The intensified camera readout noise typically limits the signal-to-noise ratio.

Czerny-Turner spectrometers: These disperse light on a single dimension using a grating, generally using the first order of the grating. On a typical 256 x 1064 intensified CCD array, one thus collects the strongest dispersion of the light (the first order from the grating) and can chose to collect (“bin”) from 1 to 256 rows high of data on the intensified CCD. The strong first order light combined with the choice of binning from 1-256 rows and variation of the gain on the camera intensifier of 1-256 results in an instrument with incredible dynamic range. Unfortunately, limits on the width of the intensified CCD chip are such that gratings with dispersion of about 0.01 – 0.2 nm in a 0.25-meter spectrometer yield only 20-30 nm of spectral range. The grating can typically be rotated on a turret to cover a broad range, but the single-shot spectral range is limited, with the result that simultaneous measurements of multiple elements is typically not possible. With this limitation in mind, the Czerny-Turner with an intensified CCD is one of the most sensitive configurations for LIBS.

Photomultiplier Tube (PMT)-based spectrometers: Photomultipliers are point detectors with much greater potential gain than even an intensified CCD, due to the multiple gain stages – typically they can provide 10e7 signal amplification to the iCCD’s 5 x 10e4 – more than two orders of magnitude improvement. However, as point detectors they need to be implemented either in broad arrays or in with multiple detectors in a Paschen-Runge spectrometer configuration. Hence PMT-based detectors work well once the analyst knows what they are looking for. Another benefit is that PMTs are analog devices that can be continuously read (very fast), providing a dynamic signal that can be particularly useful in understanding events in a laser plasma.

How to sort this out? Obviously there is no clear winner in every category of cost, sensitivity, triggerability, and broadband nature. In these categories the various choices are ranked:

Cost: CCD < Broadband CCD < Paschen-Runge PMT < Czerny-Turner iCCD < Echelle iCCD

Sensitivity: CCD < Broadband CCD < Echelle iCCD < Czerny-Turner iCCD < Paschen-Runge PMT

Triggerability: CCD = Broadband CCD < Echelle iCCD = Czerny-Turner iCCD < Paschen-Runge PMT

Broadband nature: CCD = Czerny-Turner iCCD < Paschen-Runge PMT (depending on # channels) < Broadband CCD = Echelle iCCD

Hopefully this gives the reader a sense for the choices. Please comment here or feel free to contact me if you need help deciding about detectors for a particular application!