2. ENVIRONMENTAL ChEMISTRy & TEChNOLOGy 2.1. Lectures

Chem. Listy, 102, s265–s1311 (2008) Environmental Chemistry & Technology
between acidified and non-acidified samples. Similarly, changes
in retention times were negligible. The remaining issue
with the use of MES was the increase in baseline noise and
subsequent increase in detection limits. This problem was
overcome by pre-cleaning the buffer using a column packed
with Eichrom Diphonix ® resin. The resulting baseline noise
reduced approximately three times and the corresponding
background fluorescence was almost seven times less.
It was thus determined that a pre-cleaned buffer of
0.25M MES adjusted to a pH of 6.05 with naOH, was the
optimum choice for the determination of aluminium in acidified
seawater samples.
Temperature
The response of the reaction between aluminium and
lumogallion has been investigated in both batch techniques
and flow systems. In the batch method, an optimal temperature
of 80 °C is generally accepted 13,15 , whereas FIA methods
tend to use 50 °C. The latter is based on investigations carried
out by Resing and Measures which concluded that most of
the temperature-based reaction rate gain had been achieved
by this temperature 12 . Independent investigation into the effect
of temperature on the rate of reaction was undertaken by
us due to the fact a different buffer was used. It was found
that the highest response, in terms of peak area, was obtained
at temperatures between 65 and 75 °C (Fig. 1.). Based on this
response, 70 °C was chosen as the temperature at which to
operate the post column reactor for all subsequent anlyses.
Fig. 1. Dependence of fluorescence response on temperature
Lumogallion and Reaction Coil
The extent of chemical reaction needs not be complete
for an analytical technique to be valid. However, it is desirable
to obtain as high a reaction yield as possible in order
to ensure the technique has good precision. For the reaction
between aluminium and lumogallion, the concentration of
post-column reagent may be changed, along with temperature
and reaction time, in order to control the extent of reaction.
Three concentrations of lumogallion (0.03, 0.04 and
s321
0.05 mM) were tested in order to exhaust possible improvements
to the system via this approach. The concentrations
chosen were based on those used in flow systems. It was
found that at concentrations higher than 0.03 mM, no significant
improvements were achived. Additionally, the effect of
increasing the length of the post column reaction coil from
2m to 4m was also studied. The result, however, was a slight
reduction in fluorescence. A MES buffer containing 0.03 mM
lumogallion together with a 2m reaction coil were thus used
in all subsequent analyses.
Surfactant
Howard and co-workers reported an increase in the fluorescence
intensity of the aluminium-lumogallion complex
of as much as 5-fold through the addition of a non-ionic
surfactant 13 . Further investigation has been carried out by
Resing and Measures 12 , which showed that Brij-35 enhanced
fluorescence to a greater extent than other surfactants, such as
Triton X-100 and cetylammonium bromide (CTAB). In order
to ensure the lowest limit of detection was achieved for this
system, an investigation into the effect of surfactants was also
carried out. The results differed substantially from those discussed
earlier. It was found that although the addition of Brij-
35 enhanced fluorescence marginally, a simultaneous increase
in baseline noise negated any improvement achieved.
Interestingly, when CTAB was tested, the aluminium peak
disappeared altogether. This was considered to be an effect
of the surfactant adhering to the tubing walls and effectively
stripping the aluminium from the reagent stream. The system
required flushing with methanol in order to resume normal
operation. Consequently, further investigation into the
possible use of surfactants was abandoned, with the decision
to explore other approaches to lowering the detection limit
being deemed more favourable.
S a m p l e V o l u m e
A more attractive approach for achieving a low LOD
was increasing the sample loop volume. All previous experiments
had been carried out using a volume of 20 µl. The
response of the system to higher volumes was investigated
and the results are depicted in Fig. 2. It can be seen that for
volumes between 20 and 500 µl, the system follows a linear
response, as expected. It was also noteworthy that no reduction
in column efficiency was experienced at higher volumes.
The highest efficiency was achieved for a 100 µl sample
loop, which was unexpected considering that band broadening
is generally associated with increased sample size and
is often responsible for an observed reduction in performance
of the chromatographic column as injection volume is increased.
Another unexpected result of increasing the sample
volume was an increase in retention time. Generally, a decrease
in retention time would be expected due to competition
from other analytes for chelation sites, especially in such a
complex matrix as seawater. This was shown not to be the
case for IDAS and may be explained in terms of the forma-