You have to keep in mind that when you conduct digestion for ICP analysis in open vessels (i.e., in a hot block or on a hot plate), the boiling point is soon reached. This point limits the maximum temperature in open vessel digestions.
The boiling point of nitric acid (the most commonly used reagent for microwave digestion for ICP analysis), for example, is 110 °C. If you need to reach temperatures above 110 °C, this cannot be achieved in an open vessel system. For this, you need pressurized systems, which are instruments that safely and efficiently heat digestion matrices under closed-vessel conditions and therefore let you reach higher temperatures and pressures.
This is why modern microwave systems have been developed to support digestion for ICP analysis in an optimal way. These systems are effective heating sources, and their design lets you reach temperatures up to 300 °C and pressures up to 200 bar. They have also been designed to be corrosion-resistant as well as safe and convenient to handle.
Figure 2 shows a comparison of closed-vessel and open-vessel heating of nitric acid. The figure focuses on the maximum temperature that can be reached.