Liquid Calibration Unit (LCU)

Setup for Efficient Evaporation

The LCU uses an optimized evaporation technique that can be used for volatile as well as semi-volatile compounds. They key component is the purpose-built nebulizer: a gas and liquid flow mix at the tip of the nebulizer producing a fine mist. These micro-droplets are ejected into a heated (up to 250°C) evaporation chamber (see figure above), which facilitates the evaporation process.

This system has several advantages:

• The evaporation of compounds with a slow evaporation rate is enhanced by the enlarged surface area of the droplets.
• And, compounds which are dissociated in water, such as acids (or bases), experience a decreasing pH value when the water evaporates from a droplet. This in turn reduces the dissociation and leads to a total evaporation of the compound.

Range of Compounds and Concentration

The LCU can be employed for calibration of trace gas analyzers over an extensive range of compounds and concentrations. 

Solubility

For trace concentrations, a very low solubility of a compound in water is sufficient to produce an aqueous standard. As an example, this has successfully been achieved for propofol which has a solubility of only 0.12 g/l (25 °C) in water.

Vapor pressure

The efficient evaporation mechanism enables calibration of compounds even with very low vapor pressures (i.e. semi-volatiles). This has successfully been demonstrated for caffeine and indole (vapor pressure of only 1.6 Pa at 25 °C), which set the current benchmark.

The LCU has successfully been used with a growing variety of compounds, including ketones, aldehydes, alcohols, amines, terpenes, organic acids, and esters. 

Concentration Range

The LCU has a dynamic range of 100. The resulting concentration in the gas phase is directly dependent on the concentration in the aqueous standard. This allows to explore concentrations up to ‰ or even % and down to the pptv range and below.