Vessel-Inside-Vessel Technology

Analytical chemists are being required to measure lower and lower levels of trace metals in samples. This frequently requires the analyst to make measurements near the method detection limit which usually result in decreased accuracy and precision. Closed-vessel microwave digestion techniques require a minimum volume of 10 mL to achieve accurate temperature monitoring of the reaction conditions. Ideally, modern spectroscopic techniques require samples submitted for analysis to have acid concentration of 1 to 5% v/v. This requires samples that were digested with 10 mL of acid to be diluted by a factor of 500 to achieve the 1 to 2% acid content necessary for ICP analysis. These large dilution factors often result in some of the analytes becoming non-detectable.
These large dilutions are often overcome by maximizing the sample size relative to the amount of digestion acid. This approach works well for samples that do not contain a large amount of organic material. For samples with high organic content this is usually not an option because the reaction gases (CO₂ and NO_x) produced during the digestion can cause the microwave vessel to vent when larger sample sizes (greater than 0.5 grams) are used. Recent advances in understanding of microwave-assisted sample preparation have led to the development of vessel inside vessel technology as a means to improving method detection limits for high organic samples.
The Vessel inside vessel technology provides better reaction control. Controlling reaction kinetics is especially important when trying to digest large quantities (0.5 to 1.0 g) of organic material because the potential for auto-catalytic decomposition increases. Moreover, it helps control these self-sustaining auto-catalytic reactions by providing a heat sink for the energy liberated during oxidization.