Direct determination of phosphorus in biological samples using a solid sampling-high resolution-continuum source electrothermal spectrometer: comparison of atomic and molecular absorption spectrometry

Phosphorus is an element that is difficult to determine accurately by most analytical techniques. In particular, phosphorus determination by means of electrothermal atomic absorption spectrometry (ETAAS) has been traditionally hampered by (i) the practical inability of using phosphorus resonance lines; (ii) its complex chemical behaviour (e.g., formation of volatile compounds and a tendency to interact with graphite); and, particularly, (iii) the overlap observed between molecular and atomic phosphorus lines, which makes it very difficult to obtain reliable results when using line source-ETAAS. In this work, the potential of high resolution-continuum source ET spectrometry for the direct determination of phosphorus in biological solid samples is demonstrated. Two different methodologies, based on the monitoring of either atomic or molecular lines, were compared, paying special attention to the judicious selection of the chemical modifier and the temperature program that will be most adequate to the intended purpose. In order to enable atomic phosphorus monitoring, a high atomization temperature (2650 °C) and a combination of chemical modifiers consisting of palladium, ascorbic acid and tungsten (as permanent modifier) were used. On the other hand, when monitoring PO lines, tungsten coating and vaporization at only 1900 °C proved to be sufficient for attaining reliable results, although attention has to be paid to the possible effect of spectral interferences (e.g., NO lines). Both methods permitted the direct analysis of the samples under investigation, offering a high sample throughput (about 15-20 minutes per determination), good sensitivity (m ₀ around 5 ng and LODs at the µg g^–1 level), RSD values in the 5-10% range, and straightforward calibration with aqueous standards.