Download (direct link):
8.2.1 Example 8.1: Liquid-Liquid Extraction of various Polycyclic Aromatic Hydrocarbons from Water
188.8.131.52 Extraction Conditions:
These were as follows:
• Sample volume: 25 ml
• LLE conditions: sample extracted with 2 x 3 ml of dichloromethane plus 1 g of salt (NaCl)
• Extracts shaken for 5 min each
Comments Combined extracts placed in a volumetric flask, and internal standard added, prior to analysis.
184.108.40.206 Analysis by GC-MS
Separation and identification of the individual PAHs was carried out on an HP 5890 Series II gas chromatograph, fitted to an HP 5971A mass spectrometer.
Methods for Environmental Trace Analysis
Figure 8.3 Typical procedure employed for liquid-liquid extraction, using a separating funnel.
A 30 m x 0.25 mm id x 0.25 ^m film thickness HP-5ms capillary column was used, with temperature programming from an initial temperature held at 90oC for 2 min before commencing a 7°Cmin-1 rise to 285°C, with a final time of 20 min. The split/splitless injector was held at 280°C and operated in the splitless mode, with the split valve closed for 1 min following sample injection. The split flow was set at 40 ml min-1, and the mass spectrometer transfer line was maintained at 280°C. Electron impact ionization at 70 eV, with the electron multiplier voltage set at 1500 V, was used, while operating in the single-ion monitoring (SIM) mode.
220.127.116.11 Typical Results
These are shown in Figure 8.4 .
~±r fin ± rh
X 1 t —
1 2 3 4 5 6 7 8
Polycyclic aromatic hydrocarbon
Figure 8.4 Results obtained from the liquid-liquid extraction of various polycyclic aromatic hydrocarbons from water: 1, naphthalene; 2, acenaphthylene; 3, acenaphthene; 4, fluorene; 5, phenanthrene; 6, anthracene; 7, fluoranthene; 8, pyrene  (cf. DQ 8.4).
Comment on the results obtained in this study (see Figure 8.4).
It can be seen that the maximum recovery does not exceed approximately 80%, with a minimum recovery of approximately 55%. The precision of the method, based on three replicate samples, is adequate and reflects the manual nature of the LLE process used in this analysis.
8.3 Solid-Phase Extraction
Solid-phase extraction (SPE), which is sometimes referred to as liquid-solid extraction, involves bringing a liquid or gaseous sample into contact with a solid phase or sorbent whereby the analyte is selectively adsorbed onto the surface of the solid phase. The latter is then separated from the solution and other solvents (liquids or gases) are added. The first such solvent is usually a wash to remove possible adsorbed matrix components; eventually, an eluting solvent is brought into contact with the sorbent to selectively desorb the analyte. The focus of this present chapter will be on SPE involving liquid samples and solvents. The solid-phase sorbent is usually packed into small tubes or cartridges, with the system resembling a small liquid chromatography column. The sorbent is also available in round, flat sheets which can be mounted in a filtration apparatus (Empore discs™). In this case, the sorbent resembles that of the commonly used filter paper. Whichever design is used, the sample-containing solvent is forced by pressure or vacuum through the sorbent. By careful selection of the sorbent, the analyte should be retained by the sorbent in preference to other extraneous material present in the sample. This extraneous material can be washed from the sorbent by the passing of an appropriate solvent. Subsequently, the analyte of interest can then be eluted from the sorbent by using a suitable solvent. This
Methods for Environmental Trace Analysis
solvent is then collected for analysis. Obviously, further sample clean-up or preconcentration can be carried out, if desired.
From this brief introduction, what would you consider to be the important parameters for SPE?
The choice of sorbent and the solvent system used are of paramount importance for effective pre-concentration and/or clean-up of the analyte in the sample.
The process of SPE should allow more affective detection and identification of the analyte.
8.3.1 Types of SPE Media
Generally, SPE sorbents can be divided into three classes, i.e. normal phase, reversed phase and ion-exchange. The most common sorbents are based on silica particles (irregular shaped particles with a particle diameter between 30 and 60 ^m) to which functional groups are bonded to surface silanol groups to alter their retentive properties (it should also be noted that unmodified silica is sometimes used). The bonding of the functional groups is not always complete, so some unreacted silanol groups remain. These unreacted sites are polar, acidic sites and can make the interactions with the analytes more complex. In order to reduce the occurrence of these polar sites, some SPE media are ‘end-capped’, that is, a further reaction is carried out on the residual silanols using a short-chain alkyl group. End-capping is not totally effective. It is the nature of the functional groups which determines the classification of the sorbent. In addition to silica, some other common sorbents are based on florisil, alumina and macroreticular polymers.