5 STEPS TO BUILD A HIGH-PERFORMANCE SCIENTIFIC LIBS SETUP

5. How can I further upgrade my new setup?

In part I. dealing with the LIBS setup devices only those building blocks were listed that are necessary for sample analysis by means of LIBS method. Advanced extensions were mentioned in part II dealing with assembling the devices and adjustment of the setup. Here a brief inventory is summarized together with some application examples.

As already mentioned, one of the basic elements of a LIBS setup is a motorized sample manipulator. A complex LIBS setup requires a system that enables mounting all the indispensable optomechanical elements with common intersection of the optical pathways, which lies usually on the sample surface or closely above it – in the spot of plasma creation. With respect to safety and further extensions of the setup an ideal design seems to be a specialized interaction chamber, that seals the laser-matter interaction in an airtight body, offers see-through windows equipped with appropriate filters and above all dispose of sufficient number of mounting ports for additional devices and optomechanics.

LIBS Interaction Chamber by Atomtrace LIBS Interaction Chamber by Atomtrace

Such a chamber enables to realize a more advanced LIBS techniques or to utilize another analytical techniques in one moment, some of them are listed below.

LIBS in low pressure or in He/Ar atmosphere. Vacuum interaction chamber enables LIBS experiments at low pressure with different gases that may intensify emission of particular line and considerably improve signal to noise ratio. Moreover, it is possible to simulate atmospheric conditions specific to particular environments (e.g. extraterrestrial research). Vacuum components, such as various valves, pressure regulators, vacuum fittings and hoses, vacuum pumps, pressure gauges, reduction valves, etc., are necessary. The chamber can be heated or frozen for cryogenic analyses of biological samples.

Double pulse LIBS is a step towards detection limits enhancement [2] while keeping or reducing the size of ablation craters. This procedure is based on utilization of two laser pulses following each other in precisely given time interval. The outcome is a plasma with higher temperature, longer lifetime and higher emission intensity recorded on the analyzer detector. According to the arrangement (collinear or orthogonal) it is necessary to equip the setup by either laser capable of double pulse mode, or two synchronized lasers. Collinear arrangement [3] is simpler as far as the instrumentation is concerned (both laser pulses come from the same direction and use the same focusing optics). The signal is intensified, depending on the element and matrix, up to ten times. Drawback of this geometrical arrangement can be the risk of secondary ablation by the second laser pulse. This can be a problem in applications where the smallest possible ablation crater is desired, e.g. in a case of precious samples or while creating chemical maps with high spatial-resolution. In these cases it is more convenient to employ orthogonal arrangement (laser pulses are perpendicular to each other).

In orthogonal configuration [4] the primary laser pulse can have relatively low energy (thus a small ablation crater is created), while it is possible to obtain high sensitivity, in other words low limits of detection.

LIBS+LIF (Laser-Induced Fluorescence) is a technique basically similar to the DP-LIBS. The primary pulse creates a plasma that becomes a target for the secondary laser pulse (e.g. from a tunable titanium-sapphire laser) tuned to a particular atomic transition [5]. Intensification of emission of a particular transition occurs, leading to significant improvement of a limit of detection of the analyzed element. The setup has to be supplemented with a mentioned second laser, often with its excitation in the form of another pulsed laser.

LIBS+RAMAN (Raman spectroscopy) is a combination of quasi-destructive elemental analysis and well established elemental/molecular analytical technique bringing the best of each together [6]. Utilizing Raman spectroscopy requires just notch filter and a powerful spectrometer.

Analyses of melts can be performed by bringing the laser beam into an electrical furnace with liquid or highly heated solid sample [7], using mirrors: spectra collection is realized in safe distance from the sample with respect to the temperature.

Applying a pulsed electric or transverse magnetic field [8] to confine laser-induced plasma can result in increasing the detected signal intensity and improving the limits of detection.

Acoustic pressure measurement at the moment of plasma creation can be carried out by placing a microphone (even a small electret one) to close proximity of the sample to detect hearable and near ultrasound signal. In order to detect kHz and MHz frequencies it is necessary to attach the microphone to the sample. Generally an oscilloscope or a PC card is needed for making a record. The signal can be used for the purpose of LIBS emission normalization and for fundamental studies.

„White light“ intensity, i.e. the total emission of a microplasma can be detected by a photomultiplier or a photodiode placed close to the microplasma. Again an oscilloscope or a PC card is needed and the acquired signal can serve for LIBS emission normalization and for studies of microplasma extinction.

Analysis of liquids by so called Liquid LIBS technique enables measurement of chemical composition of liquids or chemicals dissolved in them [9] [10]. Analyzed solution circles in the system with a jet. Laser is focused on the stream of liquid coming out the jet. Circulation of the liquid is maintained by a pump. This is the optimal way of liquid samples analysis considering the setup simplicity and quality of obtained data. The setup consists of a pump, a system of pipes, a jet, a capture beaker, a tank and several optomechanical elements. Usually, due to its dimensions, it is placed aside from the chamber with the manipulator. It is, however, possible to design such as a miniaturized module, that it may be placed inside the interaction chamber. In that case, laser pathways alterations and rearranging of the collecting optical system is unnecessary.

Plasma imaging utilizing scientific CCD camera, objective and couple of filters one can imagine the plasma plume. This enables not only proper calibration of focusing/collecting optics, but in combination with externally triggered camera also the possibility to time resolve the plasma plume volume evolution.

Shadowgraphy [11] is the next step after plasma imaging. When collimated light is led through the plasma volume the gradient in optical thickness of plasma causes changes in the light intensity. This way also the acoustic shockwave can be visualized. Considering that the illuminating light is a pulsed laser, no special camera requirements to achieve temporal resolution are demanded.

Interferometry [11] is a clear next step when one handles the imaging and shadowgraphy. By utilizing two beam splitters Mach-Zehnder interferometer can be built which can provide measurements of volume gradients of optical thickness and morphology of plasma plume.

Spectrally resolved imaging – using acusto-optics tunable filters (AOTF) in front of plasma imaging system, one can measure plasma plume with both the spatial and spectral resolution [12].

Autoloader – the final step in laboratory style measurement automation. Equipped with standard method of inspection and a bunch of samples from various sources, the only goal is to process the data and submit a report.

 

References

[2] J. Scaffidi, S. M. Angel, D. A. Cremers, Emission enhancement mechanisms in dual-pulse LIBS, Analytical Chemistry, 78 (2006), 24-32.

[3] V. N. Rai, F. Y. Yueh, J.P. Singh, Study of laser-induced breakdown emission from liquid under double-pulse excitation, Appl Opt, 42 (2003), 2094-2101.

[4] J. Uebbing, J. Brust, W. Sdorra, F. Leis, K. Niemax, Reheating of a Laser-Produced Plasma by a 2nd Pulse Laser, Appl. Spectrosc., 45 (1991), 1419-1423.

[5] X. K. Shen, Y. F. Lu, Detection of uranium in solids by using laser-induced breakdown spectroscopy combined with laser-induced fluorescence, Appl Optics, 47 (2008), 1810-1815.

[6] Lin, Q., et al. Combined laser-induced breakdown with Raman spectroscopy: historical technology development and recent applications. Applied Spectroscopy Reviews, 48(6) (2013), 487-508.

[7] U. Panne, C. Haisch, M. Clara, R. Niessner, Analysis of glass and glass melts during the vitrification process of fly and bottom ashes by laser-induced plasma spectroscopy. Part I: Normalization and plasma diagnostics, Spectrochimica Acta Part B-Atomic Spectroscopy, 53 (1998), 1957-1968.

[8] V. N. Rai, J.P. Singh, F.Y. Yueh, R.L. Cook, Study of optical emission from laser-produced plasma expanding across an external magnetic field, Laser and Particle Beams, 21 (2003), 65-71.

[9] P. Yaroshchyk, et al. Quantitative determination of wear metals in engine oils using laser-induced breakdown spectroscopy: A comparison between liquid jets and static liquids. Spectrochimica Acta Part B: Atomic Spectroscopy, 60(7-8), (2005), 986-992.

[10] A. Kumar, et al. (2003). Double-pulse laser-induced breakdown spectroscopy with liquid jets of different thicknesses. Appl, Opt., 42(30), (2003), 6047-6051.

[11] M. Villagran-Muniz, H. Sobral and E. Camps, Shadowgraphy and interferometry using a CW laser and a CCD of a laser-induced plasma in atmospheric air, in IEEE Transactions on Plasma Science, vol. 29, no. 4, (2001), 613-616. 

[12]  D. N. Stratis, K. L. Eland, J. Chance Carter, S. J. Tomlinson, and S. M. Angel, Comparison of Acousto-optic and Liquid Crystal Tunable Filters for Laser-Induced Breakdown Spectroscopy, Appl. Spectrosc. 55, (2001), 999-1004.

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