Chemistry Reference
In-Depth Information
N
2
N
2
Injection
port
Tr a p
Sample
(water bath 50°C)
Figure 8.5
Microdistillation apparatus.
8.2.4
Distillation and microdistillation
Distillation, which is based on the compositional difference between a liquid and the vapour it forms, is an
old separation technique the polluting effects of which can be minimized by using an appropriate sample
volume and vapour-collection medium. The efficiency of this technique depends on the differences in
volatility (vapour pressure) between the species present in the liquid. Similarly to other gas-liquid techniques
discussed in this chapter, the greenest distillation approaches are those involving miniaturization and dynamic
manifolds in which the vapour formed is retained in the tubing system in order to avoid losses of environmental
pollutants. To this end, the vapour is trapped - usually by a liquid - without user intervention and partial or
total automation introduced as a result.
Miniaturization of distillation has led to microdistillation. A continuous analyser with a built-in
microdistillation systems usually comprises; (1) a propulsion module with the two-fold purpose of continuously
feeding the microdistiller with liquid samples - optionally preconditioned with either an additional buffer or a
reagent stream - and removing the outgoing streams from all other units, also in a continuous fashion; (2)
a suitable microdistillation module for gas-liquid separation; (3) a condenser and/or absorption module to
transfer the gaseous analyte to an outgoing liquid stream; and (4) a continuous detection module.
Microdistillation has been successfully applied to the analysis of biological samples such as blood and gut
contents for the determination of hydrogen sulfide without the typical problems posed by the volatility of this
analyte and viscosity of the samples in other techniques [25]. Figure 8.5 shows the microdistiller used, where
detection was done by ion chromatography. Finally, Figure 8.6 shows the experimental set-up used for the
determination of fluoride ion in water samples, based on a flow system combining microdistillation and
spectrophotometric detection [26].
8.2.5 Head-space separation
Head-space separation is probably one of the greenest separation techniques, particularly when the target
analyte is volatile enough to be separated from the sample matrix simply by heating (i.e. without a reagent).
Current head-space devices, which are usually coupled to a gas-chromatograph, consist of an oven, a sampling
device and a transfer line to the separation-detection instrument. The system is internally equipped with a
