Low-cost tungsten coil atomic emission spectrometry for portable testing
Fresh Fuel is Better than Fossil
As global energy demands increase, energy conservation and alternative energy sources become increasingly important. Biodiesel is a popular alternative, produced via conversion of vegetable oils. While some argue that farming of biodiesel crops contributes to deforestation and competes with food production, others agree it is still appealing due to its sustainability, relatively low emissions, and ability to be used directly in standard diesel engines.
To maximize the benefits of biodiesel, it is essential to optimize and validate its quality. High levels of sodium (Na) and potassium (K) originating from the catalysis process can impact engine performance and lifetime, while trace metals increase harmful emissions. Techniques like flame atomic absorption spectrometry (FAAS) and inductively coupled plasma optical emission or mass spectrometry (ICP OES or ICP-MS) have traditionally been used, but involve considerable sample preparation and analysis in a lab. Tungsten coil atomic emission spectrometry (WCAES) offers a simpler solution, and when powered by an Ocean Optics spectrometer, it is also more economical and portable.
Harnessing the Right Emissions
A collaboration between Wake Forest University and the Federal University of São Carlos, Brazil, has shown atomic emission spectrometry to be both sensitive and accurate for quantitative measurements of sodium and potassium in biodiesel fuel. Samples are simply diluted with methanol and placed in μL quantities on the tungsten filament of a standard microscope light bulb. Temperatures of up to 2500 K successively decompose, atomize and excite the biodiesel, resulting in atomic emission at characteristic lines for minerals and metals present within the sample.
In a recent experiment, light from a tungsten filament was imaged onto the entrance slit of a USB-series spectrometer and spectra were recorded at 0.8 ms intervals for 20 seconds. The intensity of the sodium line at 589.0 nm and the potassium lines at 766.5 and 769.9 nm were plotted as a function of time. By calibrating against commercially available biodiesel reference solutions, the integrated signal for each element could then be correlated directly to its concentration.
Elegance in the Field
With a limit of detection of 20 μg/kg for sodium and 80 μg/kg for potassium, this technique is more than adequate to test for the combined limit of 5 mg/kg established by North American (ASTM D6751), European (EN 14214) and Brazilian (ANP 07|2008) standards. It also offers the repeatability and linear dynamic range needed, accurately measuring a 20 mg/kg reference sample. Capable of multi-element analysis, emission spectrometry is a shining example of how elegant optical design and use of a compact spectrometer can enable rapid and inexpensive field test methods to make biodiesel a more viable energy alternative.
Optical System – Emission
- USB4000 spectrometer (350–1000 nm)
- Fused silica lens (25 mm diameter, 75 mm focal length; used to image 1:1 coil image directly onto slit)
Reference: Direct determination of sodium, potassium, chromium and vanadium in biodiesel fuel by tungsten coil atomic emission spectrometry. Dancsak, S.E.; Silva, S.G.; Nobrega, J.A.; Jones, B.T.; Donati, G.L. Analytica Chemica Acta, 2014, 806, pp 85–490.