Laser Enables Inexpensive Elemental Analysis

New Laser Enables Inexpensive but Flexible Elemental Analysis

Laser induced breakdown spectroscopy, or LIBS, is a type of atomic emission spectroscopy. LIBS operates by focusing the laser onto a small area at the surface of the specimen; when the laser is discharged it ablates (vaporizes) a very small amount of material, in the range of nanograms to picograms, which generates a plasma plume with temperatures in excess of 10,000 K. There is a short delay of approximately 10 µs while the plasma cools before the characteristic atomic emission lines of the elements can be observed, making necessary a shutter delay on the detector.

LIBS can analyze solids, liquids and gases and can return results rapidly with very little damage to the sample. LIBS requires minimal sample preparation and due to the fact that such a small amount of material is consumed during the LIBS process, the technique is considered essentially non-destructive. LIBS is a very rapid technique giving results within seconds, making it particularly useful for high volume analyses or on-line industrial monitoring. LIBS is an entirely optical technique and requires only optical access to the specimen. Fiber and telescopic optics can be employed to allow for remote analyses, unlike analytical tools that require samples to be brought to the lab.

The image shows two versions of passively Q-switched lasers. The larger laser nearer the top of the photo is an end-pumped excitation laser and the laser nearer the bottom is a side-pumped version used for LIBS analysis. At the top of the photo is the power supply.

When taking a measurement with LIBS, the output of a pulsed laser is directed either onto the surface of a solid or focused into a fluid such as air or water. When the laser strikes the surface of the solid or is focused within the fluid a spark is produced. If you gather some of the light from the spark and analyze it by breaking it up into its individual wavelengths then you can determine what elements were in the spark and their concentration within the sample. Although a LIBS measurement system can offer a relatively low-cost and highly versatile sampling modality, it is plagued by its requirements for precise control of laser pulses in conjunction with precise timing of the optical spectrometer. Current LIBS systems use actively Q-switched lasers, due to their precision timing. Q-switching uses an attenuator in the laser resonance cavity that acts as a shutter. In an actively Q-switched laser an input signal is used to control the laser output and to time the measurement within the spectrometer. Passively Q-switched lasers do not have a highly precise timing signal, making them unsuitable for LIBS to those in the art. However, if you develop a way to either independently measure the output timing of the laser and use this information to control a timing and measurement system then the cost of a LIBS measurement system can be reduced significantly. 

LIBS systems are used for many applications. NETL scientists have invented a method that simplifies the current state-of-the-art LIBS systems while also reducing the component cost and complexity while maintaining measurement integrity. The precise timing and control between the laser and the spectrometer detector is the key to producing the best possible data with a LIBS system. Traditionally, an actively Q-switched laser was used because the time between the initiation of the Q-switch and the opening of the shutter of the detector are fairly simple and well known. This invention provides the same degree of precision timing as the actively Q-switched output but with fewer components and a lower cost laser system. This is accomplished by detecting the output of the passively q-switched laser and directing a signal to a timer so that after a prescribed amount of time the shutter of the spectrometer is opened, just as it is in the traditional system. 

This invention will allow for precise control with fewer control lines and less expensive parts. The laser can be miniaturized into a monolithic optical unit while still providing sufficient output to produce laser sparks. The laser can be pumped with a bank of diode lasers or a vertical-cavity surface-emitting laser (VCSEL) to maintain good mode and output beam quality. The use of the passively q-switched laser will reduce system cost by thousands of dollars. 

This invention can also be used to report the output pulse energy of the laser system to the LIBS analysis system. Maintaining consistent output pulse energy levels and/or at least accurately reporting the variations can help reduce errors within the LIBS analysis process. The measurement of Q-switch delay can also be used to calibrate and report output pulse energy. 

Contact: Dustin McIntyre

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