By Ely Silk
So, youâ€™ve just come back from your nature walk carrying some interesting looking rock samples.Â You probably figured that with all those field guides and charts at home, identification will be a breeze.Â Rummaging around your closet, you find that hardness of minerals kit you got as a kid on one of your museum field trips. Â The kit often included a streak plate.Â Assuming the kit and plate are properly used, my experience indicates that you can narrow the approximately 6,000 possible choices of rocks and minerals down to 5,980 or so.
If youâ€™re planning to run chemical analyses on your samples, you will first need to finely crush representative parts of the unknown rock and subject the resulting powder to extremely corrosive acidsâ€”if you are able to buy them anymore. Â This is followed by a series of qualitative tests for different cations. Â These tests are a chore to set up and also involve chemicals that are getting tougher to acquire. Â You then have to dispose of the spent chemicals.
Traditional spectral analysis using a carbon arc and an associated spectrometer is certainly possible, but it requires lots of experience to get accurate results. Â Some elusive components in your specimens may be quite volatile, and their spectral lines could vanish before you spot themâ€¦or think you spot them.
If only there were a better way to quickly and reliably determine the unknownâ€™s chemical composition, identification could be simplified andâ€¦fun!
With the advent of pulsed lasers, in 1962/63 a new technique was introduced:Â laser-induced breakdown spectroscopy (LIBS).Â Though it got off to a slow start, it is now the subject of intense investigation and application in analytical labs worldwide.Â NASA has placed a LIBS system aboard its Mars Science Laboratory Curiosity rover. Â Typically, commercial systems range in price from $50k on up.
Why so expensive? Â A typical LIBS apparatus consists of a series of narrow-band spectrometers with resolutions of 0.1nm or less. Â Each spectrometer is equipped with a highly sensitive photodetector, such as a high-resolution CCD chip.Â Enough spectrometers are used to adequately cover the spectral range of interest.Â As an example, to cover the range from 200nm in the ultraviolet to 800nm in the near infrared may require six or seven spectrometers. Â Alternatively, a single echelle spectrometer can be used.Â This type of spectrometer can image the entire spectrum on one detector with very high resolving power. Â You will also need a powerful pulsed laser usually emitting at 1064nm, focused to produce a plasma plume on the analyte, the material being analyzed.Â The laser pulse duration should be on the order of 10 nanoseconds or less. Â Add to this, broad-range fiber optics, standard optical components and mounts, electronics to delay the detected light pulse, sample mounts, a computer with spectroscopy software, etc., etc., etc.
What is a citizen scientist to do? Â Multiple spectrometers are used because the resolution is inversely proportional to the spectral range covered.Â You can, of course, purchase a standard USB spectrometer that covers the entire range from 200nm to 800nm or beyond, but its resolution may be no better than 1nm. Â Nevertheless, I decided to try that approach to see what could be done within its limitations. Â The spectrometer I use has a stated resolution of 0.8nm. Â But then thereâ€™s the laser requirement. Â I happened to have one lying around that could do the job. Â I bought the system years ago for some experiments with dye lasers. Â However, that laser is getting old and less energetic.Â Besides, this wonâ€™t help experimenters who need to buy a laser.
I hunted around and finally decided to evaluate the relatively inexpensive imported cosmetic or tattoo lasers. Â Whereas a commercial pulsed Nd:YAG laser marketed for LIBS work can run upwards of $25k, the imports are priced at under $3000â€”often way under. Â I acquired or built the necessary electronics for handling the pulses produced by the plasma plumes.
The following chart shows a sample spectrum.
The total cost for the system shown here is under $7000.Â This is for brand-new, off-the-shelf components direct from the manufacturers.Â Just think of the possibilities of being able to perform analyses with no muss, no fuss.Â Place your specimen and zap it.Â The spectrum immediately appears on the screen!
I just added a section to my Web site that goes into greater detail on my experiments with both electric spark spectroscopy and LIBS.Â The section includes a very brief history of spectroscopy as well. Â I also touch upon the nature of the analysis that the experimenter must perform to figure out what those peaks on the computer graph are saying.Â Hereâ€™s the link:
One of my main objectives in building my Web site is to develop techniques and lower-cost procedures for different areas of science to meet the needs of citizen scientists, as well as professional scientists, researchers in developing countries, small labs, and educational centers.
I hope that this discussion helps convince you to get involved with a technology that could make your chemical analyses enjoyable, if not a breeze.Â LIBS is a fantastic and fun way to perform chemical analysis on materials ranging from glass and ceramics to rocks, meteorites, works of art (arrivederci, Mona Lisa), and archaeological treasures. Â And thatâ€™s just for openers. Â The applications are endless.
Finally, let me leave you with this warning: Always use proper eye protection when using any laser, especially a pulsed Nd:YAG laser.Â It can permanently blind you before you can say, â€œWho turned out the lights?â€
I wish you happy (and careful) experimenting!