Laser-induced breakdown spectroscopy (LIBS) analysis begins with a focused laser beam that ablates the specimen (sample),
generating a small spark. The light, optical emission from the spark, which contains spectral information about the composition
of the sample, is collected by an optical system. These spectral data are analyzed for chemical identification. Although the
LIBS spectra comprise mostly atomic emission lines, relative intensities and shapes of these lines depend upon the nature
of the sample, and thus, LIBS can discriminate even compositionally similar molecular compounds. Such complex substance identification
is achieved using chemometric algorithms to process and compare spectra against a preestablished database.
Despite the significant advantages of LIBS over other analytical techniques, it is used mainly by academia and research institutions
largely due to the unavailability of commercial instruments dedicated to LIBS. However, this situation is changing swiftly
as reliable, compact lasers and spectrometers have become readily available. Several commercial LIBS systems have entered
the analytical instrumentation marketplace recently. Our data illustrate how LIBS and LIBS–Raman analyzers can answer a multitude
of real-world needs for rapid chemical analysis of various substances.
Experimental
The experiments were performed using Applied Spectra's (Fremont, California) RT100-HP and RT100-B commercial LIBS systems.
These similar instruments use different optical spectrographs. Both systems employ programmable intensified CCD cameras with
timing control to select the gate width and delay for optimal instant detection of a range of spectrum acquired from a single
laser shot. The RT100-HP system incorporates a high-performance Czerny-Turner spectrograph. This spectrograph has a dual grating
turret, which allows users to choose a spectral window with relevant spectral resolution (200 nm at resolution of 0.8 nm;
or 40 nm at resolution of 0.1 nm). The second instrument, RT100-B, incorporates a broadband echelle spectrograph that instantly
acquires a wide span of spectrum covering 200–900 nm at resolution ~0.05 nm. Both systems are housed in similar frames with dimensions 740 mm × 810 mm × 1370 mm and weight of 158 kg. Both have an autofocus
function for focusing the optics on the sample surface for ablation and light collection. They include an automated xyz stage for sample positioning at a speed that ranges from 1 μm/s to 40 mm/s with resolution steps and repeatability of 0.1
μm. The main laser delivers 5-ns pulsed radiation up to 90 mJ (user-controlled optical energy per pulse) at 1064 nm. Operation
at 532 nm and higher harmonics can be customized. The laser beam energy can be focused to a spot ranging from 10 μm to 500
μm in size, depending upon the desired application. A red diode laser beam serves as a visible guide, indicating the spot
to be ablated. These instruments are rated as "Class I" laser products, and therefore, contain multiple safety features, such
as an electronic interlock and an eye-protective filtering window that shields the ablation chamber.
Both LIBS models run integrated application software that features an intuitive user interface. The operator views an image
of the analyzed samples on a computer monitor that receives data from an electronic color camera installed inside the sample
load compartment. Operation of the hardware components (laser, spectrograph, ICCD camera, imaging camera, and sample stage)
is controlled through the application software interface. An automated laser shutter ensures the operator's eye safety as
well as facilitating exceptional stabilization of laser pulse energy (before ablating the sample, if so required) in a pulse
train mode. The software includes an automated calibration program for accurate quantitative analysis. To assist with measurement
decisions, the system provides a choice of recipes optimized for different sample matrices. Immediately after each test, the
acquired spectra and measured data, along with the results of calculations (including statistics), are displayed on the screen.
The overall time required to complete an analysis is less than 10 s per sample. This time includes the quantification and
statistical analysis of the spectroscopic data.
The chemometric algorithms tailored to the specifics of the LIBS spectra were developed at Applied Spectra and are available
as separate supporting software.
