Page 430 - Tandem Techniques

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Basically, the system consisted of a frequency-doubled YAG laser, operated at 10 Hz, which pumped
two dye lasers whose beams were directed into the analytical flame. The first dye laser contained
Rhodamine 6G and was tuned to 568 nm. This light was frequency doubled to give ultraviolet light at
284 nm, the wavelength necessary to excite a tin atom from the 5p level at 3428 cm to the 6s level at
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38629 cm-1, with an energy of 0.086 mJ/pulse.
The second dye laser was tuned to a tin transition, that further excited the tin atoms from the 6s level to
a level just below the ionization limit, from which rapid collisional ionization occurs. The 603.77 nm
line, with a laser energy of 1.8 mJ/pulse, connects the 6s level with the 7p level at 55187 cm which is
-1
only 4045 cm- below the tin ionization limit. The dye used was Rhodamine 640 with 0.1% sodium
1
hydroxide in ethanol. A water cooled cathode, set at a potential of 1000 V, was immersed 2 cm into a
premixed air-acetylene flame which was burnt at a 5 cm slot. The two laser beams were co-linearly
aligned, 3 mm below the cooled cathode. The ionic current was processed electronically with standard
amplifiers.

The assay used to test the method was the determination of tin in drinking water. The separation was
carried out on an ion exchange column (Whatman Partisil-10ZSCX cation exchanger) using a mobile
phase that consisted of 75% methanol and 25% water, containing 0.05M ammonium acetate buffer (pH,
5.1). The flow rate was 2 ml/min and the sample volume was 20 µl. A normal nebulizer was employed
for spraying the sample into the gas stream.

The sample contained tributyltin, tripropyltin and triethyltin, at a concentrations of 260 ng/ml, 310
ng/ml and 300 ng/ml respectively. Peaks were obtained that corresponded to masses of 5.2 ng, 6.2 ng
and 6.0 ng of tributyltin, tripropyltin and triethyltin respectively. Sodium and calcium peaks were also
disclosed that arose from some naturally occurring impurities in the tap water. The authors reported a
detection limit of 3 ng/ml of tin, as tributyltin, which was claimed to be equivalent to a mass detection
limit of 0.06 ng of metallic tin.