Chemistry Reference
In-Depth Information
Ultrasound energy can also stimulate sample digestion, that is, when a substance is dissolved in the
presence of moisture and heat. This process is often used to convert the matrix into one that is more
conveniently handled by analytical apparatus and ultrasound substantially shortens the digestion time. In
addition, combining ultrasound with microwave energy gives better results than if these agents are used
separately to digest food and mineral samples [99].
In ultrasound-assisted methods of cleaning glassware, the ultrasonic energy causes trace contaminants to
be desorbed from container or sampler walls. In another application of this kind, contaminants are removed
from the surfaces of the membranes used for separating components from mixtures or for purifying analytical
reagents [100].
Ultrasound significantly increases the rate of sample dissolution: mass transfer coefficients are greater
in its presence than in its absence [101]. Dissolution of solid samples is necessary mainly in food analysis
(i.e. powdered milk and other powdered food products [97]).
Ultrasound can also be used to homogenize or emulsify a sample (if it consists of two immiscible liquid
phases): it can alter its physical properties without altering its chemical composition. In this way its chemical
structure is made uniform, even though two phases are still present (one phase dispersed in the other). The
relevant physical processes are mixing and stirring, which are caused by the propagation of ultrasonic waves
through the liquid medium [102]. Many samples of food containing fats require homogenization before
chemical analysis can begin.
Ultrasound is frequently used to speed up chemical reactions (the term sonochemistry has been introduced),
for example, the derivatization of inorganic and organic analytes in sample preparation. But this application
is rare; ultrasound is, however, commonly applied to reactions like depolymerization, esterification, alkylation,
redox reactions and complex formation, to facilitate the production of target derivatives [102]. The ultrasound-
assisted combination of extraction and derivatization, for example, in the extraction of haloacetic acids from
vegetables and their simultaneous derivatization to methyl esters, enables both processes to be performed in
less than 15 min [103].
An important aspect in process analysis is on-line sampling, which makes frequent use of membranes.
These have the disadvantage of fouling up, which impairs their quality and shortens their operational
lifespan [104].
Some liquid samples need to be degassed before analysis: with ultrasound this is done quickly and
efficiently.
7.4.3
Pressurized liquid extraction
Pressurized liquid extraction (PLE) (or pressurized fluid extraction or enhanced solvent extraction) is
considered to be the next generation of extraction procedures after Soxhlet extraction. The main advantages
of PLE over Soxhlet extraction are the smaller volumes of solvent required (reduced from up to 200 ml to
1-20 ml) and the shorter extraction time (reduced from 24 h to 30 min). PLE uses a variety of environmentally
friendly solvents like water, ethanol and methanol, but also traditional organic solvents [105]. The high
pressure applied during extraction enables work at temperatures well above the boiling point at standard
pressure. Elevated temperatures increase analyte solubility and diffusion rates, but decrease viscosity and
surface tension of the solvent. Elevated pressure improves the penetrability of the extractant into the pores of
the target material [106].
PLE is a common method for extracting organic analytes from environmental [107], food [108] and
biological [109] samples; it has also been used to extract trace elements from food samples [110].
Finally, it should be mentioned that the term 'accelerated solvent extraction (ASE
®
)' is often misused as a
synonym for PLE. Although its principle of operation is similar to that of PLE, ASE is the registered name of
a method and devices designed by a certain company.
