Laser-based Techniques and Applications - LATA

TPCI/NHRF

LIPS (English)

Laser-based Analytical Applications by Laser-Induced Plasma Spectroscopy (LIPS):

Basic principles and examples

A. Introduction

The LIPS technique (Laser Induced Plasma Spectroscopy, LIPS or Laser Induced Breakdown Spectroscopy, LIBS) has a broad applications field and this is due to many reasons: It is a relative simple technique, the samples require minimal or no preparation at all and is considered as (almost) non destructive. The technique can be applied on solid (metals, ceramics, polymers, drugs, wood, paper), liquid (water, colloids, industrial and biological liquids) and gaseous (industrial exhaust gases, air particulates) samples.

The technique is based on the spectroscopic analysis of the optical emission spectrum when a powerful laser pulse (τ =10^-8 sec) is focused on the sample (Fig. 1a).

Figure 1: Principle of the LIPS technique.

A tiny amount of the sample is evaporated and ionized (plasma plume). Recombination of the electrons with the ions follows (Fig. 1b) and the decay to lower atomic states leads to the characteristic emission spectrum of each element (Fig. 1c). Comparison with reference spectra leads straightforward to the assignment of each line to the elements in the plume (qualitative determination) while from the ratio of each line over a reference element leads to quantitative results (Internal standardization method).

B. Advantages of the LIPS technique.

The LIPS technique is characterized by high spatial resolution. Since the laser beam is focused on a very small area of the sample (< 1 mm), the technique also allows the elemental analysis of non-homogeneous samples. Depth profiling is also possible, by recording spectra after a certain number of pulses at the same point. Such possibilities prove the big advantages of LIPS, compared to other analytical techniques, e.g. XRF, atomic absorption etc. that give results average over the whole volume of the sample. Furthermore, it is a typical time-resolved technique (TR-LIPS), because the maximum emission of each element occurs in general at a different time, in respect to the laser pulse arrival at the sample. The latter not only serves as a further criterion for line assignment (e.g. ionic vs. atomic lines), but also allows the determination of low concentration elements that may be blended by the emission of high concentration ones as it is the case in classical spectroscopy.

C. Technical details.

The LATA TR-LIPS setup is presented in Fig. 2. The pulsed Nd:YAG laser is synchronized with the H.V. delay pulse generator and the pC that controls the OMAIII detector head by a 10 oscillator. The Η.V. pulse unit controls the two characteristic times of the LIPS experiment, that is the delay D and the gate G. G is the window duration for the emission signal collection by the ΟΜΑ ΙΙI detector and D is the time distance of the gate G from the laser pulse. Both G και D are in the range nsec to msec.

Figure 2: Block diagram of the LIPS setup in the LATA Lab.

The ΟΜΑ ΙΙΙ detector (EG&G), Fig. 3, is controlled by a pC and is synchronized by the 10 Hz oscillator and the H.V. pulser for opening and closing. The spectral sensitivity extends in the 200-700 nm and in combination with the Jobin-Yvon spectrometer records ~20 nm with every laser shot. The latter is in particular very important for sensitive samples that may be destroyed after a few laser pulses.

Figure3: ΟΜΑ ΙΙΙ Head of the LIPS setup

The sample(s) is (are) set on a moveable ΧΥ table that is controlled by a pC and is mounted in a high vacuum chamber. The pressure and the type of the ambient gas can be selected for the purposes of the experiments. The plasma emission is collected by a quartz optical fiber and focused on the entrance slit of the spectrometer. A typical plasma picture with the characteristic yellow Να lines (sample: ΝαF, Apoflux^TM) as well as the optical fiber collection head are presented in Fig. 4.

Figure 4: Na plasma and collection optical fiber

D. Applications (from LATA Lab.).

The LIPS technique finds in particular application on solid samples, like alloys, ore etc. where the qualitative and quantitative analysis can be performed in situ and in real time. In Fig. 5 the average of 200 spectra from a stainless steel sample is presented (sampling time 20 sec), together with 3 reference spectra of pure Ni, Cr and Fe in the same UV spectral range.

Figure 5: LIPS spectrum of a stainless steel plasma, together with the 3 reference spectra Ni, Cr και Fe.

The stainless steel elements have been identified with the aid of Fig. 5. Using a set of stainless steel samples with known elemental concentration (reference samples), the calibration curve based on the so-called “Internal Standardization” method has been deduced, see Fig. 6. This is the correlation between the ratio of the intensities of two elements Ni/Fe versus the ration of the same elements in the sample, with a correlation factor of r = 0.995. This linearity proves the strength of the LIPS technique in respect to a quantitative measurement under certain and precisely selected conditions during the measurement.

Figure 6: “Internal Standardization” calibration curve Ni/Fe in a stainless steel sample using the LIPS technique.

A further in situ industrial application of the LIPS technique is during the recycling procedure of used automobile catalysts. LIPS allows the rapid determination of the noble metals Pt, Pd και Ph in the catalyst matrix consisting of Al₂O₃in real time, see Fig. 7.

Figure 7: LIPS spectra of a reference Al₂O₃, a reference sample with 5% Pd and a real sample from a used catalyst. Sampling time : 1.5 min.

Reference samples of Pd in Al₂O₃ revealed the calibration curve of Fig. 8 with correlation factor 0.997.

Figure 8: Calibration curve for Pd in used automobile catalysts

The LIPS technique, considered as almost non-destructive, and in combination with its spatial resolution capabilities (spatial resolution), applies to the investigation of small archeological species (e.g. coins): elemental identification by LIPS, combined with historical and other related data, reveals valuable information about the origin of the samples.

Figure 9: Cross section of a Roman coin : LIPS spectra have been recorded at three different positions of the coin.

In Fig. 9, LIPS spectra have been recorded at three different ablated positions. In Fig. 10, a LIPS spectrum is shown which corresponds to the first ablated position from the left, see Fig. 9: from this spectrum, the elemental composition of the coin at this point is revealed. Besides investigating the homogeneity of the coin, the LIPS technique may be used for “profiling”, that is recording spectra from different depths, vertical to the coin surface. Therefore, aging effects as well as environmental influence on the coin can be studied that may aid to the cleaning of the surface coin etc.

Figure 10: LIPS spectrum of the Roman coin where the local elemental composition can be seen.

Further LIPS applications refer to dentistry, to differentiate the healthy part of the tooth from that affected by caries, making again use of the spatial resolution of the technique. It was found that caries results to the increase of the Mg composition in respect to the Ca one in the affected zone. Such a rapid determination becomes possible with the aid of an optical fiber for those tooth positions where the dentist has no or minimal access.

Various portable LIPS setups can be on-line adapted today to monitor many industrial procedures where an in situ and rapid elemental determination is necessary during the fabrication process, e.g. in the pharmaceutical industry.

Finally, an interesting application has been already in use: this relates to the trace element analysis during the mineral excavation process. As an example, we consider the amount of sulfur in coal mineral production: International rules prescribe an amount of not more that 1% in coal. This reduces the amount of SO₂production in thermoelectric power plants that leads to the acid rain polluting the surrounding atmosphere and affecting human health. In Fig. 11 the sulfur LIPS spectrum in coal in the VUV spectral range is presented in ambient conditions that may be used for the on-line quantitative determination of sulfur near thermoelectric power plants.