Qualitative vs Quantitative
Qualitative measurements by LIBS
Qualitative information on the elemental composition of a material using LIBS is relatively straightforward subject to certain conditions and constraints discussed in Instrumentation & Methodology
. The recorded LIBS spectra may then be analysed to identify the relevant emission lines of interest. Clearly the resolving power and wavelength calibration of the spectrometer will have an impact on the quality of the recorded spectra and the ability to identify emission lines which are largely free of spectral interference. The emission lines may be identified manually by referring to tables of atomic / ionic emission line data (e.g. the NIST database available here
) or semi-automatically using appropriate software. There will always be a degree of “detective work” on the part of the LIBS spectroscopist when attempting to identify the emission lines in any given LIBS spectrum and so it can be useful to record LIBS spectra of a high-purity sample of the element of interest so that a comparison with the recorded spectra of the unknown sample can be made. We supply with our LIBS instruments a sample set containing a range of high-purity elemental samples for this purpose. It should be pointed out, however, that even the most experienced LIBS spectroscopist can make mistakes when identifying emission lines. A very high resolving power spectrometer (e.g. resolving power of 30,000 or more) is clearly very useful in this regard but such spectrometers are often beyond the budget of most customers of LIBS equipment.
The following two superimposed spectra of high-purity zirconium show how higher resolving power is useful in correctly identifying emission lines, especially in materials which have a complex emission spectrum (e.g. steels and certain other alloys such as zircaloy, high-Z elements such as uranium, etc). The spectrum coloured blue was recorded using one of our SpectroModule-6 multi-channel LIBS spectrometers which has a resolving power of approx. 5,000 over this wavelength range. The spectrum coloured red was recorded using an EMU 120/65 echelle spectrometer
(Catalina Scientific, USA) equipped with an EMCCD camera
(Raptor Photonics, UK) configured for a resolving power of approx. 30,000 over this wavelength range (the EMU can be configured for a resolving power of up to 60,000). So at first sight it would seem to make perfect sense to go for a spectrometer of the highest resolving power available, however, there is a significant cost penalty in doing this! The spectrum coloured blue (below) can be achieved with a spectrometer costing as little as GBP 5k whereas the spectrum coloured red requires a spectrometer costing around GBP 65k. So like most things in life, there has to be a compromise!
Comparison of Zr spectra obtained using a spectrometer of resolving power 5,000 (blue line) and a spectrometer of resolving power 30,000 (red line)
The guide we have written on qualitative analysis by LIBS is currently in draft form but we expect to update this webpage and the guide before the end of 2022.
Quantitative measurements by LIBS
Quantitative measurements by LIBS are possible but a number of conditions must be met…
- The sample must be homogenous on a scale comparable to the sampling volume of the LIBS laser beam, which is very small given LIBS is a micro-analytical (surface) technique.
- The LIBS instrument will require calibration using matrix-matched reference materials (preferably Certified Reference Materials or CRMs) which are relevant to the material for which LIBS analysis is required (such materials are sometimes not easy to obtain).
- The LIBS measurement methodology must be appropriate for quantitative analysis (see Instrumentation & Methodology)
- There must be a high degree of control of the LIBS measurement conditions and they must be replicated between conducting the calibration using reference materials and conducting measurements on an unknown sample.
Our discussion here is restricted to adopting a traditional approach to calibration, however, there are many reports in the literature on using a chemometric approach to calibration and/or using the so-called “Calibration Free” approach. To be honest, we at APL are not big fans of either of these approaches, preferring to use traditional calibration techniques for all our work involving quantitative measurements by LIBS.
An example calibration plot is given below. The application relates to the measurement of the chromium content of specific steels over the chromium concentration range 0% to 0.5%. This was required for a “real world” application of LIBS within the nuclear power generation industry.
Cr calibration curve in steel and Cr content of the reference samples employed
(LOD = Limit of Detection, R² = closeness of trend line fit to data, where an R² = 1 indicates a perfect fit)
The guide we have written on quantitative analysis by LIBS is currently in draft form but we expect to update this webpage and the guide before the end of 2022.