Detection of lanthanum on stainless steel and aluminum with LIBS
Goal:
Evaluate the performance of the HR2000 spectrometer for the LIBS detection of lanthanum on stainless steel and aluminum
Method:
LIBS
Introduction:
The target analyte for the samples is the element Lanthanum (La). There are two spectral regions of interest. In the region centered at 324 nm, there is a La doublet at 317 nm and an emission line at 332 nm. The second spectral region contains La emission lines at 427 nm and 433 nm.
Hardware Used:
LIBS2000+ broadband high-resolution spectrometer
50 mJ Big Sky Nd:YAG laser
LIBS sampling chamber with 125 mm focal length lens
Acquisition Parameters:
2 microsecond Q-switch delay between laser event and data acquisition
Measurement Mode:
LIBS
Experiment Conditions:
Samples were prepared by evaporation of a series of 3 microliter aliquots of La at three different concentrations. Concentrations were 10,000, 1,000 and 50 ppm, labeled 10k, 1k, and 0.05, respectively. Two different substrates were used, 6061 aluminum (0.125″ thick), and 304 stainless steel (0.025″ thick). The discrete spots where La was deposited on the metal surfaces were easily visible (with the naked eye) on the aluminum substrate at all concentrations, and on the stainless steel substrate at the 10k concentration. The La spots were not visible (with the naked eye) on the stainless steel substrate for the 1k and 0.05 La concentrations.
The La deposition pattern for the stainless steel samples were all the same, a 5 x 5 matrix, with one spot missing at the label. The 10k aluminum sample had a 4 x 5 spot pattern with one spot missing at the label, while both the 1k and 0.05 concentrations had a full 5 x 5 spot pattern. Due to differences in spot diameter these samples were not suitable for development of a traditional calibration curve.
Results:
Single and multi-shot LIBS spectra were acquired for several La spots on each sample. For the 10k La samples, extra data was acquired to profile the effect of subsequent shots at the same spot. The 10k samples were analyzed the most extensively.
Results are shown in the plots below. The LIBS analysis of La deposited on 6061 aluminum is shown in Figure 1. Each spectrum shown is the average of multiple single shot spectra (see legend for number of single shot spectra averaged). Two spectral regions that contain the La lines of interest are shown.
LIBS analysis of La deposited on 304 stainless steel is shown in Figure 2. Each spectrum shown is the average of multiple 3-shot spectra. As shown for the aluminum substrate, the spectral regions containing the La lines of interest are shown. In Figure 3, the effect of taking multiple shots at the same location is shown. Note that the richest spectrum is achieved with the second shot.
In Figures 4 and 5, the LIBS spectra for the 6061 aluminum and 304 stainless steel substrates are shown, respectively. Each spectrum represents the average of three 3-shot spectra. Note that the spectrum for stainless steel has more elemental lines indicative of differences in the plasmas obtained for the different materials.
Conclusions:
For La deposited on 6061 aluminum, 3 of 6 La peaks included in the Elemental Identification library were detected at the 50 ppm La concentration. At the 1000 ppm La concentration, 4 out of 6 peaks were detected with 5 of 6 La peaks detected in the 10,000 ppm concentration. With a 304 stainless steel substrate, 5 of the 6 La lines were detected at all La concentrations. The data show that La deposited on aluminum and stainless steel substrates is detectable down to the 50 ppm level with more peaks detected at higher La concentrations. Improved detection on the stainless steel substrate illustrates the impact of the matrix on detection. Differences in the plasma characteristics for the two substrates may have resulted in the detection of additional elemental lines (example of matrix effects on detection).
For the elemental lines of interest, the La doublet around 317 nm was detected on both the aluminum and stainless steal substrates at the 10,000 and 1000 ppm La concentrations. The elemental line around 332 nm was not detected at any La concentration on either substrate. The peaks around 427 and 433 nm were detected on the stainless steel substrate at all three La levels. For the aluminum substrate, the peak around 433 nm was detected at all three La levels with the peak around 427 nm detected at the 1000 and 10,000 ppm levels.
Tags: LIBS, stainless steel
