Chemistry Reference
In-Depth Information
1) single-drop micro-extraction (SDME) based on the suspension of a micro-drop of a water-immiscible
organic solvent in an aqueous donor solution;
2) dispersive liquid-liquid micro-extraction (DLLME); and
3) hollow-fiber micro-extraction (HF-LPME) [18].
The main difference between SDME and LPME is the form of the analyte-acceptor phase. In SDME, the
solvent is in the form of a suspended drop; whereas, in LPME, the solvent is placed in a hollow fiber. Analytes
are transferred from the donor phase (usually aqueous) to a receptor phase through an organic phase
immobilized in pores in the hollow fiber. In two-phase mode, the solvent in the pores of the fiber is the same
as that present in the fibre. In three-phase mode, the solvent placed in the pores of the fiber differs from that
inside the fiber. The three-phase mode is applied for extracting polar analytes, while the two-phase mode is
applied for extracting non-polar and semi-polar analytes. The amount of solvent in the fiber is approximately
5 or 10-25 or 30
l [7,19]. After extraction, the acceptor solution is directly subjected to a final chemical
analysis by HPLC, GC, CE, or mass spectrometry (MS) [19]. The advantage of LPME over SDME is the
presence of the fiber that supports organic solvents, decelerating the process of dissolution or evaporation (in
HS-SDME mode) of the solvent [7].
In membrane techniques, the samples may be preconcentrated due to passing the analytes through a
polymeric membrane. Some of the techniques require small amounts of organic solvents, while others are
virtually solventless. Membrane extraction may be applied as an efficient tool in many modes, including
microporous membrane liquid-liquid extraction (MMLLE), membrane extraction with sorbent interface
(MESI), membrane-assisted solvent extraction (MASE) and supported liquid membrane extraction
(SLME) [7].
μ
22.2.3 Analysis
After the separation from the sample matrix and other interfering compounds, the detection and identification
of an analyte are areas in which the chemistry can be made green. The selection of the appropriate analytical
method is generally based on the following criteria: (1) expected concentration of the analyte in the sample;
(2) number of samples to be analyzed; (3) time that can be devoted to the analysis; and (4) cost of analysis.
In accordance with the Green Analytical Chemistry criteria, the development of instrumental methods has
generally led to an efficient use of energy, especially when the method is highly automated and uses a minimal
amount of sample. Instrumental methods in environmental analysis generally involve electrochemical,
spectroscopic and chromatographic analyses [1, 2].
Spectroscopic methods used in the analysis of environmental samples include mainly UV-visible molecular
absorption spectroscopy, atomic absorption spectrometry (AAS) and atomic emission spectrometry (AES).
The most widely used techniques are flame atomic absorption spectrometry (F AAS), graphite furnace atomic
absorption spectrometry (GF AAS), inductively coupled plasma optical emission spectroscopy (ICP OES)
and inductively coupled plasma-mass spectrometry (ICP-MS). Atomic spectroscopic methods have been
extensively used for the determination of pollutant elements, such as toxic heavy metals, because they
generate low volumes of residuals and because it is possible perform fast analysis with low sample
consumption [8].
F AAS is a technique that is relatively inexpensive and easy to operate with little interference. When
interferences occur, they are easily identified and usually easily controlled. On the negative side, refractory
elements cannot be determined with good sensitivity because the flame temperatures are not high enough to
atomize a large fraction of these analytes. It is essentially a single-element technique because, in the traditional
equipments, each element requires its own hollow-cathode lamp. The major advantage of GF AAS is that its
detection limits are 10-100 times better than for F AAS or for ICP OES, sharing the limitations of F AAS for
