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UV-femtosecond laser ablation

Introduction

The Leibniz University of Hannover has developed a novel laser ablation system to be used as micro-analytical tool in the elemental and isotope analysis of solid materials. The system is based on a femtosecond all solid-state Ti-sapphire based Laser operating at a fundamental wavelength tunable between 775-785 nm coupled to frequency conversion setup to produce deep UV laser radiation at wavelength of 196 and 262 nm. The system is combined with ICP based analytical intrumentation (ICP-OES and MC-ICP-MS).

What is laser ablation?

Laser Ablation uses highly focused laser light pulses to remove (ablate) material at a spatial resolution of a few micrometers in diameter from any solid sample. The aerosol produced during this process is transferred by a continuous stream of gas into any plasma-based analytical instrument. At the Institute for Mineralogy (Hannover) this is either a Varian Vista Pro optical ICP for elemental analysis, or a Finigan Neptune Multicollector ICP-MS for precise isotope ratio measurements The beauty of this technique is that the sample is under atmospheric pressure so that even liquids can be sampled which would be impossible when using micro beam techniques such as Electron Microprobe or SIMS (Secondary Ion Mass Spectrometry), which require a vacuum at the sampling site. That means you could even sample tissue samples, or, if you are interested in food research, the cold cuts of your lunch sandwich.

Why femtosecond laser ablation?

The beauty of this technique is that the sample is under atmospheric pressure so that even liquids can be sampled which would be impossible when using micro beam techniques such as Electron Microprobe or SIMS (Secondary Ion Mass Spectrometry), which require a vacuum at the sampling site. That means you could even sample tissue samples, or, if you are interested in food research, the cold cuts of your lunch sandwich.

Why deep UV laser ablation?

In order to achieve the highest possible level of matrix independency in terms of absorption, the wavelength used for ablation should be in the ultraviolet region. This is important in the earth - and environmental sciences where matrices like carbonates and quartz as well as iron meteorites are investigated. The absorption is a material property. However, many materials show high absorption coefficients below 200 nm. For transparent materials the absorption controls the rate of ablation at a given energy density and in turn influences the size of the particles generated during the ablation process. If the particles generated increase in size the mass spectrometers' inductively coupled plasma may not completely ionised these. This will then lead to a different type of elemental fractionation which is based on energetic differences of the elements and even present on the isotopic scale. The system in Hannover uses 196 nm in order to minimize effects produced by the aerosol size distribution. Therefore, we expect this system to be an ideal technique to measure heavy stable metal isotope fractionation in situ.

Technical specifications of our system

The ablation system is based on a Ti-sapphire regenerative amplifier system producing infrared light pulses having a pulse width of 100 femtoseconds or 0.0000000000001 seconds. By comparison, if traveling at the speed of light for 100 femtoseconds you would only just cover about 30 micrometers in distance. After frequency conversion into the UV spectra the final output of the system comprises two usable laser beams of wavelengths of 196nm and 262nm evenso the fundamental could be used for micromachining, too.

Lasers: A look into the interior of the laser box illustrates the optical complexity of generating femtosecond pulses together with the realization that you need more than a single laser to do so. Just a warning: If you don`t know what you are doing - leave the box closed!!

The interior of the laser box (left) and a close-up of the amplifier (right)

The damage to the sample

The damage to the sample: Always appealing - a simple circular spot ablated by 196 nm laser pulses. A lateral resolution of about 2 micrometers can be reached. A variety of matrices which can be ablated cover quartz, carbonates, diamond, silicates, oxides, sulphides, metals and even materials hard to dissolve such as polymers.

The advantages of deep UV laser ablation

No detectable laser-induced "elemental fractionation"!

Applying UV fs laser ablation to U/Pb dating of minerals the recorded ratios are constant during spot analysis unlike results observe for ns laser ablation. The figure to the right shows the results of spot analyses of 30 microns in diameter over an ablation interval of 60 seconds. The precision of 0.4% in the U/Pb ratio is limited by counting statistics on the faraday cups, only.

 

No detectable laser-induced isotopic fractionation!

The spot analysis shows constant Fe isotope ratios using fs pulses. The reproducibility of 0.1‰ (2SD) obtained is sufficient to detect variation in natural samples.

 

Matrix in-dependency

The good agreement of UV fs laser ablation and conventional solution MC-ICP-MS data illustrates that UV fs laser ablation can be successfully applied to the analysis of stable Fe isotopes for various matrices using a single metal standard (IRMM-014) for calibration. The precision is better than 0.1‰ (2SD), larger errors are due to sample inhomogeities.