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


Spectroscopy
Volume 29, Issue 4, pp. 26-31

Detectors

CCD Detectors

Inexpensive CCD detectors were covered in some detail above. Beyond these very low-end detectors, there are numerous variations of CCD detectors that are more sensitive than the basic detector. Back-thinned detectors can have enhanced sensitivity, while high-quality CCDs that are cooled with multistage cooling can have signal-to-noise ratios that exceed those of an intensified CCD. The main drawback of these detectors is that the exposure cannot be gated (turned on and off) as can the other detectors. These are available in square (for example, 2048 2048) and "spectroscopy" (for example, 1340 100) formats.

Intensified CCD Detectors

Intensified charge-coupled devices (ICCDs) have long been the mainstay for LIBS applications. As described above, ICCDs can be gated (some as fast as 2 ns) and have a dynamic range exceeding 104. To accomplish this, the output of a photocathode is amplified (under vacuum) by a microchannel plate amplifier. The resulting electrons strike a phosphor, which is fiber-coupled to a CCD detector. The variable voltage that can be applied to the microchannel plate provides both the high gain potential and the gating capability of the device. Largely because of the uncertainties in the microchannel plate amplifier, ICCDs can be quite noisy at high gain settings. In addition, images are not as sharp as a typical high-quality CCD camera because there can be some "smearing" caused by crosstalk between the fibers coupling the light from the phosphor to the CCD. These cameras are quite expensive.

Electron-Multiplying CCD Detectors

Among the new kids on the block, electron multiplying charge-coupled device (EMCCD) cameras have generated some interest in the LIBS community because of their high sensitivity and much lower cost than ICCDs. In EMCCD cameras, the readout register has multiple preamplification stages before A/D conversion. This allows low-light signals to be enhanced before conversion, thus moving a low-light signal above the A/D noise floor. The gain possible in EMCCDs is about 103. Precision and speed of gating, however, has been a development issue for LIBS. Because of the coupling of the preamplifier and the A/D conversion, the gating has an inherent minimum delay and the actual gate open time may vary across the chip because the chip is read out. Efforts are being made to address these issues to make EMCCDs more on par with ICCD gating performance. EMCCD cameras, as of this writing, are approximately 30–40% the cost of ICCDs, so they are worth watching as they mature.

Photomultiplier Tubes

Photomultiplier tubes (PMTs) are detectors of the last century, correct? For the students reading this, these were the detectors that "old" guys like this author used with a scanning monochromator to obtain broadband emission spectra in 20 min flat. (Yes, one spectrum in 20 min. Today's students are spoiled!) Not so fast. With gains of up to 107 and fast readout electronics available, in some situations they may be the most versatile detectors for LIBS. They can be used for either emission-line-selective filters or in a spectrometer configuration in conjunction with a Paschen-Runge spectrometer. Don't count these detectors out for the ultimate in speed (kilohertz acquisition possible) and sensitivity.


Figure 3: Relative performance of detection system components.
Figure 3 summarizes the salient features of detection systems. Spectrometers or detectors are graded on a relative scale for broadband nature, sensitivity, triggering and gating, and cost. It is obvious, as we stated at the outset, that there are few clear winners. The one statement that can be made with confidence is that if one has a single, known emission line to measure, a PMT system is worthy of consideration. Presently, many OES systems still use PMT arrays for line detection for this reason.


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