Atomic and molecular spectra of vapors evolved in graphite furnace. Part 7: Alkaline metal sulfates and sulfides

Thermal behavior and vapor absorption spectra of Li, Na and K sulfates and Li, Na sulfides in graphite vaporizers for atomic absorption spectrometry are investigated using a UV spectrometer with a charge-coupled device (CCD) detector. Fifty spectra in the wavelength range 190–380 nm are collected during the vaporization of 1 μg and 10 μg of metal as sulfide and sulfate, respectively, simultaneously to the increase of vaporizer temperature from 700 to 2700 K during 10 s. The behavior of transient spectra is compared for the pyrocoated tube and filter furnace. It is found that Na and K sulfates in the pyrocoated tube evolve, first, metal bound sulfur–oxygen compounds, while Li sulfate emanates S2 vapor. For all substances studied, the vaporization involves active interaction with graphite, suggestively with formation of graphite–metal and graphite–sulfur intercalation compounds. Those compounds retained metal and sulfur until above 2000 K. Both vapor dissociation and decomposition of intercalation compounds are accompanied by appearance in gas phase of CS molecules. In the filter furnace at high temperature the structured spectrum between 190 and 210 nm is characteristic for all sulfur containing compounds. It is suggestively attributed to CnSm (n>m) molecule. The samples of Li and Na sulfides do not retain initial chemical composition after dissolving, sampling from solution, drying and pyrolysis at 700 K. Their vapor spectra show the presence of sodium sulfate (or sulfite) and lithium oxide in the respective dry residues. The diffuse absorption bands at 190–230 nm with maxima at 200–210 nm precede or appear together with molecular spectra. The phenomenon is suggestively assigned to the light scattering on specific molecular clusters.