NEWS
Results for Organic Solutions
The ability of the ICP analyst to control the temperature of the spray
chamber improves their ability to analyse elements in matrices
that would otherwise require further chemical treatment and the
potential loss of analyte, for example, the analysis of ruthenium in
an organic solvent matrix. The organic solvent matrix is common
with pharmaceutical samples as these are analysed with liquid
chromatographic techniques using organic solvents as the carrier.
Conventional methods would involve wet oxidising the sample
in a mixture of nitric and sulphuric acid to remove the organic
solvent, however there is the possibility of ruthenium loss during the
oxidation process. Another potential method would be to dry ash
the sample, but loss of ruthenium occurs with the volatile species;
and then there is the problem of dissolving ruthenium metal which
can only be done in alkali hypochlorite solutions. The fused alkali
dissolution process results in a solution containing sodium salts
which unfortunately degrades the detection limits.
It is possible to directly measure precious metals in an organic matrix
using the IsoMist set at low temperature. In this experiment the spray
chamber was set to temperatures between -10°C and +10°C in
order to minimise the amount of vapour going into the plasma and
two solvents, methanol and isopropanol, containing various precious
metals, were analysed. Note that, if a methanol matrix is run at
temperatures above 10°C, then the plasma is extinguished and, for
an isopropanol matrix, the plasma is extinguished at temperatures
above 15°C. Additional steps were also taken to reduce plasma
loading such as increasing the power and argon flow rates, as well as
using smaller diameter peristaltic pump tubing.
Table 1 shows the plasma conditions used with the organic
solutions compared with those for the aqueous solutions.
The results for methanol and isopropanol are shown in Figures
5 and 6. Figure 5 shows that there is a general decrease in
intensity as the temperature is increased. This is most likely due
to an increase in plasma loading. Figure 6 indicates that, as the
temperature is increased from -10°C, there is an initial increase in
intensity due to increased sample transport but, above zero, plasma
loading becomes the dominant factor and the intensity decreases.
These results clearly demonstrate that the IsoMist facilitates
the measurement of analytes in volatile organic solvents at
temperatures below 15°C.
Organic Aqueous
Power (W) 1600 1300
Plasma flow (L/min) 19.5 15.0
Auxiliary flow (L/min) 2.25 1.50
Nebuliser flow (L/min) 0.60 0.60
Integration time (s) 10.0 10.0
Pump tubing ID mm black-black 0.76 grey-grey 1.30
Waste tubing ID mm red-red 1.14 blue-blue 1.65
Injector diameter (mm) 1.5 1.8
Pump rate (rpm) 25 15
Sample uptake (mL/min) 1.0 1.5
Table 1: Plasma conditions for the analysis of precious metals in an organic matrix
compared with those for aqueous solutions.
Figure 5: Average n = 5 intensity of various precious metals in a methanol matrix
versus temperature. All species shown are ionic. Note that at spray chamber
temperatures above 10°C the plasma was extinguished.
Figure 6: Figure 6: Average n = 5 intensity of various precious metals in an
isopropanol matrix versus temperature. All species shown are ionic. Note that at
spray chamber temperatures above 15°C the plasma was extinguished.
Conclusion
The benefits of the IsoMist for the analysis
of precious metals are:
(1) control of the temperature of the spray chamber
to enhance signal intensity;
(2) reduced signal instability and improved measurement
accuracy and precision;
(3) direct analysis of elements in organic solvents
at temperatures below 15°C.
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