Atomic biodiesel assessment

Flame atomic absorption spectrometry (FAAS) can be used effectively as an alternative method for the determination of calcium and magnesium ions in biodiesel and fuels derived from vegetable oil, according to researchers in Brazil. The technique precludes certain preparative steps and avoids the need for some toxic organic solvents.

Chemists Alexandre de Jesus, Ariane Zmozinski, and Janaína Barbar of the Universidade Federal do Rio Grande do Sul (UFRGS), in Porto Alegre, Brazil and Maria Vale and Márcia Silva also at UFRGS and the Universidade Federal da Bahia, in Salvador, explain the advent of biodiesel as a fuel in terms of growing consumption of petroleum-derived products and efforts to reduce carbon emissions and pollution in general. "Biodiesel is not only eco-friendly, because it reduces the current levels of environmental pollution both qualitatively and quantitatively, but is also a strategic source of renewable energy as a substitute for diesel oil and other crude oil products," they say.

However, fuels derived from biological sources can only be used in vehicle engines safely and without damaging the engine if they adhere to as strict a set of quality parameters as conventional fuels. The concentration of calcium and magnesium in a fuel is an important factor in biodiesel quality control. The metals are present as part of the production process and are involved in the washing water where they can form an insoluble soap, which leads to deposits in the engine affecting performance and ultimately engine longevity.

The team explains that there are several methods for determining levels of trace metals in biodiesel that use standard, certificated techniques, including inductively coupled plasma-optical emission spectrometry (ICP-OES). These approaches in practice require the dilution of samples with organic solvents, such as xylene, cyclohexane, or petroleum ether. ICP-OES is expensive because of the high cost of instrumentation and the need to use argon. There are also various technical issues that have not been fully addressed by manufacturers for biodiesel analysis.

The Brazilian team suggests that when only a limited number of elements are to be analysed, then FAAS, despite being a mono-element technique, could be a less costly and easier alternative. Other researchers have investigated alkali elements and phosphorus in biodiesel using FAAS. Various solvents and oil in water methods have emerged to help stabilise the trace elements so that accurate concentrations can be obtained. However, despite the advantages there are still some problems of cost and solvent use.

The team has now investigated a simple, fast, and low-cost method for calcium and magnesium determination using microemulsions as the sample preparation method and FAAS as the quantification technique. The researchers investigated analyte stability in the microemulsions, the optimization of the instrumentation, and recovery tests to compare efficacy against standard methods for metal determination in biodiesel.

They found that in a microemulsified form, the samples remained stable for at least five days. They compared their results to the Brazilian Standard Method ABNT NBR 15556, which consists of the sample dilution with an organic solvent and a determination by FAAS. They obtained limits of detection of 0.04 and 0.1 micrograms per gram and characteristic concentrations of 30 and 6 micrograms per litre for calcium and magnesium, respectively.

"The results obtained were statistically in agreement with those obtained with the Brazilian Standard Method at a 95% confidence level, and the recovery tests presented values between 92 and 117% for biodiesel samples," the team says. "The main advantages observed in relation to the dilution method were the use of inorganic standards in the microemulsions for calibration, no use of carcinogenic organic solvents, and the higher stability of the analytes, besides the higher sensitivity. These advantages make the proposed method more appropriate for routine biodiesel analysis.