Laser Assisted Microwave Plasma Spectroscopy (LAMPS)

 

LAMPS is a technology designed to improve the sensitivity and reproducibility of LBS analysis.

 

LIBS is based on the generation of a plasma by a high power density pulse of laser energy focused on a small spot on solid or liquid samples. A small amount of material is ablated into the plasma, atomized and ionized by the plasma energy. As the plasma cools, electrons return to the atoms of the various elements in the ablated sample and emit light at quantum defined emission wavelengths. The wavelength is diagnostic of the presence of a particular element. The intensity is related to its abundance in the plasma.

 

The plasma created by the laser pulse persists for 10 – 20 msec. Its intensity, shape, temperature distribution and location vary considerably from laser pulse to laser pulse. This leads to a fairly high variability in the intensity levels observed by the LIBS spectrometers. LAMPS uses an intense, shaped field of microwave radiation to confine and add energy to the plasma created by the laser pulse. LAMPS plasmas persist for msec, compared to msec without microwaves. This results in higher signals. The confinement allows the optics to be pointed more precisely at the plasma locus, resulting in more uniform signals.

Here are images of a plume on an Al target. In the first image, the LAMPS microwave cavity is off. In the second image the cavity is turned on

  

 

In the figures above, an Al target was sampled with a 30 mJ laser. The figure on the left shows a plume photographed at 394nm. This plume was approximately 1 mm in length and persisted for 20 msec.  In the figure on the right, the LAMPS microwave cavity was turned on. The plasma grew to 5mm in length and persisted for 600 msec. The plasma growth is also shown in the photographs below.

 

The increase in signal is directly related to the persistence of the plume, as the electrons gain energy from the microwaves and continue to emit light as they return to electronic orbitals of the atoms. Eventually the atoms escape the cavity and the plasma diminishes. In practice the cavity is pulsed to permit dissipation of the plasma materials and introduction of the next plasma pulse from the laser.

 

The spectra above compare LAMPS to LIBS. The spectra on the left shows a 100X improvement in signal. The spectrum on the right shows CN- radicals in explosives. The improvement was 250X.

 

LAMPS Microwave Cavity

 

 

 

The LAMPS cavity is the cylindrical hole in the black plate on the left side. The fiber bundle which detects the plasma emissions is coming in from the back and left of the cavity. The apparatus is installed in the sample chamber so that the cavity is vertical over the sample. Laser energy is focused from the top through the cavity to the sample stage below. Focus can be as fine as ~40 mm.

 

The sampling chamber provides critical Eye Safety by using >OD 6 laser safety shielding on the windows, and safety interlocks on the door. The sample stage is a manual x,y positioner. A light and a USB camera provide imaging of the sample. Gas purge ports and valves and exhaust fans complete the sample chamber.

 Spectrometers

The key to LIBS and LAMPS measurements is the ability to acquire high resolution spectra over a broad wavelength range of a transient event. This is accomplished by coupling 7 HR2000+ spectrometers into one system, the LIBS2500-PLUS.  Each spectrometer covers ~140nm starting at 200nm for Channel A and ending with 980nm got channel G. Optical resolution is ~ 0.1nm FWHM. Pixel resolution is ~ 0.05 nm/pixel. In all there are 14,336 pixels acquiring synchronous spectra. The detector integrates for 1 ms. The maximum data transfer rate to the PC is about 500 spectra (14,336 values) per second over the USB2.0 port.

 

 

LIBS Lasers

We provide Big Sky CFR Pulsed Nd:YAG Lasers with our systems. The CFR design is especially rugged as both resonator mirrors are on the same rigid metal plate. A 50 mJ laser is used with LAMPS. For LIBS a 200 mJ laser is recommended. For double pulsed LIBS, a 200 mJ primary laser and 50 mJ secondary laser are used. The lasers are mounted to the sample chamber and aligned to the focusing optics. An external power supply and water chiller connects to the laser.

 Software

The standard operating software is OOILIBS. This provides control of the lasers, has user selectable delay for the Q switch, acquires, plots and saves spectra, and provides a spectral line library and correlation function for identification of peaks.

Spectra may be exported to EXCEL. Application specific libraries of peaks can be loaded. You can zoom into a wavelength range to look for a peak in the library by double clicking on the line in the table. Or, the correlation routine will match peaks to wavelengths in the library and rank the elements by how many of the emission lines it found. The user can set baseline and peak width filters for the search.

  

Quantification can be approached by analyzing samples with known concentrations of the elements of interest, or by measuring peak ratios of two elements. In general the sample matrix, laser power and sample presentation will affect peak heights for LIBS analysis. These affects are more controlled in LAMPS, because the plasma conditions are most strongly controlled by the microwave energy. 

 

SpecLine
Software for Peak finding and line identification

SE-SPECLINE.....  $4,150

This high end software is for all scientists and engineers working in the field of spectroscopy, such as astrophysics,

plasma science or plasma processing. This tool supports and

makes it easy to evaluate spectral data, i.e. finding specific

lines in spectra, identifying unknown peaks, identify atomic

lines and molecular bands or comparing data from different

measurements in spectral data. Almost instantly line peaks

and bands will be found automatically using several powerful

filter functions and the extensive data base for atoms and

molecules.

Comparing of Spectral Data

Spectra comparison is easily possible in a single diagram. Up to twelve spectra – even with different file formats – can be opened in one single diagram. But still all the spectra may be evaluated by each other independently.

Database Search

The extensive database could be used for simulation too. After molecules and or atoms are selected, the line positions will be shown in the spectrum.

Peak Finding and Line Identification

In the special Line Identification dialog all the necessary parameters for peak finding and line identification can be easily defined and the line identification process can be started by a single click. In addition noise smoothing and specific filtering is possible to further improve automatic peak finding.