The graphite furnace atomic absorption spectrophotometer offers a cost-effective way of carrying out elemental analysis of metals in liquid samples with excellent sensitivity. The liquid samples are injected into a graphite cuvette which is heated electrically following an appropriate temperature program to dry the sample and vaporise it for analysis. In general, the larger the sample injection volume, the better the sensitivity that can be obtained. However, if the sample volume is too large, it heats unevenly leading to the formation of gas bubbles (‘bumping’), which in turn leads to poor reproducibility in the measurement results.
Twin injection technology
Hitachi has overcome this problem in the ZA3000 graphite furnace atomic absorption spectrophotometer by using twin injection technology. Traditional graphite cuvettes have a single hole for sample injection. If a large sample volume is injected into the cuvette, there is a relatively small contact area between the bulky sample droplet and the surface of the cuvette, as shown in the diagram. This results in uneven heating which produces bumping. Hitachi’s twin injection approach uses specially designed cuvettes with two sample injection holes. Half of the sample is injected through the first hole and the remainder through the second hole. This gives two much flatter samples in the cuvette, with a larger overall contact area with the cuvette, as shown in the diagram. This increases the efficiency of the heat conduction to the sample which reduces drying times, minimises bumping and allows larger sample volumes to be used for enhanced sensitivity. Twin injection technology allows the measurement of 0.2μg/L Sb in environmental water without the need for in-furnace concentration. Minimising bumping also improves data reliability.
Automation of bumping monitoring
As mentioned above, if sample bumping occurs during the drying process, measurement accuracy will be compromised. Traditionally, the only way of telling if sample bumping has occurred is to use a built in camera system to watch the sample in the cuvette as it is heated. This relies on the judgment of the human observer and may differ from one observer to another. Hitachi has developed a bumping monitoring system that will automatically detect bumping based on the measurement and reference signals obtained during the drying stage. It will flag any results so that reproducibility can be checked, removing the reliance on human interpretation. By using this automated monitoring system, bumping can be eliminated by adjusting the drying temperature program.
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