The microwave accelerated dissolution has become the main sample pre-treatment method for elemental analysis in different kind of environmental samples¹. The microwave based methods are powerful but those methods are usually time consuming and tedious. An alternative and faster sample pre-treatment method is an ultrasound-assisted extraction which has been validated also for several sample matrices.²

In this study the ultrasound-assisted extraction followed with ICP-OES is validated for the determination of element concentrations in pine needles. The microwave digestion procedure (BCR 100, Beach leaves) was used as a reference method for ultrasound-assisted extraction procedure. The need for analysis of pine needles is based on their ability to be good indicators of both environmental contamination and available nutrient content of soil. The increased interest on the analysis of pine needles in Finland is due to the use of energy wood ash as a fertilizer in forests.

Efficient and cost effective method of extraction for the determination of element concentrations in pine needles was developed. A standard reference material (SRM) or real sample of about 250 mg was accurately weighed into a 100 ml plastic screw-top bottle into which 5 mL of water and 5 mL of aqua regia was added. The bottle was closed and placed into a 650 W, 35 kHz, Model Transsonic 820/H ultrasonic water bath (ELMA, Singen, Germany). The optimized sonication procedure lasted 9 minutes and was carried out at a temperature of about 50 ⁰C.

The determined element (Al, Ca, Cd Cu, Fe, K, Mg, Mn, Na, P, and Zn) concentrations were highly comparable with the certified concentrations of the SRM 1575 (Pine needles). The element concentrations determined with both ultrasound-assisted extraction and microwave digestion followed with the ICP-OES were also statistically tested and significant differences were found for Fe, Mg and P only.

The main advantages of the ultrasound-assisted extraction, when compared to conventional methods, are the speed of extraction (9 min), high sample treatment capacity (50 samples at the same time) and low reagent usage resulting in low nitrate and chloride concentrations when aqua regia is used. The ultrasound-assisted extraction procedure is carried out at low temperatures and pressures, which resulting in capability to extract sample without loss of easily volatile elements such as arsenic and mercury.² When centrifuge tubes are used, the sample solutions can be centrifuged instead of filtering, which is necessary for the sequential extraction procedures.³

1. Lamble, K. & Hill, S., Analyst, 1998, 123, 103-133.
2. Ashley, K., Sonication as a sample preparation method for elemental analysis, in Wilson & Wilson`s Comprehensive Analytical Chemistry, Volume XLI, Sample Preparation for Trace Element Analysis, Eds. Z. Mester and R. Sturgeon, Elsevier, Amsterdam, 2003, p. 353-366.
3. Väisänen, A. & Kiljunen, A., Intern. J. Environ. Anal. Chem., 2005, 85, 1037-1049.

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