Apparatus
To homogenize the samples an Ultra-Turrax T25 (Janke and
Kunkel) and an Omni-Mixer and Homogenizer (Omni International)
were used. The homogenized samples were freezedried
with a Bench Top Shell Freezer, Model 77570, 7757-01
(Labcondo). The digestion was carried out in a Parr 4782
Microwave Digestion Bomb (Parr Instrument), which is
designed specifically for microwave heating. The bomb is made
from a high-strength microwave-transparent material to enclose a chemically inert Teflon sample cup of 45 ml. A Portland
Model KOR-1137 microwave oven was used throughout.
A Perkin-Elmer 5000 atomic absorption spectrometer furnished
with a cadmium hollow cathode lamp was used. The
instrument was set at 228.8 nm. The acetylene flow rate was 2.0
l min21 and an air flow rate of 17.0 l min21 was employed to
ensure a clear blue (oxidizing) flame. The aspiration flow rate of
the nebuliser was adjusted to be the same as the flow rate of the
FI channel. The spectrometer output was connected to a Perkin-
Elmer 50 servograph recorder with a range of 5 mV. The signals
measured were the heights of the absorbance peaks.
A Crison standard pH meter with an Ingold U 455 electrode
was used.
The flow system consisted of a Gilson Minipuls-3 peristaltic
pump, fitted with poly(vinyl chloride) tubes, three Rheodyne
(Models 5041 and 5301) four-way or switching valves, and an
ion-exchange minicolumn manufactured from Tygon tubing
(85 31.6 mm id) packed with 0.1 g of 20 mesh PAPhA (a slurry
of resin beads was injected into the minicolumn with a syringe);
the ends of the tube were fitted with cottonwool to retain the
resin beads in the tube. The connection between the 0.8 mm id
PTFE tubing was achieved by using two other Tygon tubes (0.6
mm id and about 10 mm long). The resin bed was 68 mm long
for a total volume of solid substrate of 170 ml. The subsequent
use of 0.5 mol l21 hydrochloric acid as eluent in each
operational cycle was sufficient to make the resin ready for reuse.
PTFE tubing (0.8 mm id) was employed.
Materials and vessels used for trace analysis were kept in
10% nitric acid for at least 48 h and subsequently washed three
times with ultrapure water before use.