Chemistry Reference
In-Depth Information
SPE, in all its modes and formats, is arguably the preferred method for preparing liquid samples, especially
those containing more polar analytes. On-line and automated solid-phase extraction-liquid chromatography
(SPE-HPLC) is a fully mature technique that can easily be miniaturized. In addition to the effectiveness of
the extraction technique, many other factors can affect the analytical plan [38]. New alternative solvents
(fluorous, polymeric, ionic liquids etc.) can make conventional liquid-liquid extraction systems more efficient
by reducing the volume of solvent required. However, the equipment and operating costs, complexity of the
method, volume of organic solvent and level of automation must also be considered.
All of the above processes consume additional energy to conduct the analysis, for such requirements as
using additional equipment or processes, evaporating solvent, drying the sample, and so on. Traditional
sample preparation techniques such as liquid-liquid extraction (LLE) and Soxhlet extraction (SOX) rely on
solvents for extraction. They often use large amounts of organic and chlorinated solvents, and have long
extraction times (usually 6-24 h), which makes them unfavourable from an environmental and energetic
perspective. Pressurized-liquid extraction (PLE) in its static (SPLE) and dynamic (DPLE) forms was
developed to replace traditional sample pre-treatment techniques. It is evident that applying high temperatures
to conserve energy shortens the time and increases the efficiency of the process. PLE, also known as
pressurized fluid extraction (PFE), pressurized solvent extraction (PSE), accelerated solvent extraction (ASE)
and enhanced solvent extraction, generates faster extractions because of higher diffusivity, improved
solubilization capability and more efficient analyte interactions in liquid solvents at temperatures above their
boiling points. PLE provides quantitative extractions with less solvent consumption and substantially shorter
extraction times than the Soxhlet method. For a number of organic trace pollutants in various solid and semi-
solid matrices, recoveries with PLE are comparable or even better than those using Soxhlet extraction.
Solvents similar to those used for Soxhlet extraction usually yield good results, so it is relatively straightforward
to replace old methods with PLE. Mixtures of low-polar and high-polar solvents generally provide more
efficient extractions of analytes than single solvents. The temperatures are usually in the range of 60-200°C.
Except for labile analytes or samples, higher extraction temperatures will increase the efficiency of PLE as a
result of enhanced sample wetting and better penetration of the extraction solvent, and also because of higher
diffusion and desorption of the analytes from the matrix to the solvent. It is possible to use purely aqueous
solvents in PLE, (i.e. pressurized hot water extraction (PHWE), also called sub-critical water extraction
(SWE), hot-water extraction, high-temperature water extraction, superheated water extraction and hot liquid-
water extraction). This technique uses water as an extraction solvent at temperatures of 100-350°C and at
pressures high enough to keep it liquid. Typical extraction times for PLE are 10-30 min, which substantially
reduces the energy requirement per sample. However, PLE instruments can seldom be used for PHWE
because the sealing materials do not withstand the high temperatures.
15.6
Effects of automation and micronization on energy consumption
As mentioned above, the demand for chemical analysis is growing rapidly, especially for environmental
purposes. Real-time field measurement capability is needed for continuous environmental monitoring on land
and - even more importantly - in oceans and seas. Process analytical chemistry, for obtaining analytical data
on large-scale production operations, is another swiftly expanding discipline [39,40]. These requirements are
putting pressure on labs to increase their throughput and shorten response times to produce data for decision-
making. In addition, laboratories must be able to determine whether the planned procedure will be unique or
used routinely. There is growing demand for miniaturized measurement devices, scanning sensor devices and
rapid data handling with high-throughput and intensive experimentation platforms. The objectives are
changing from simply proving the feasibility of a chemical reaction to undertaking more in-depth scientific
studies and industrial piloting. The quality of the data must simultaneously be maintained or improved.
