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Chromatografy Methods for Environmental - Ando D.J.

Ando D.J. Chromatografy Methods for Environmental - Wiley publishing , 2003. - 265 p.
Download (direct link): chromatography2003.pdf
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DQ 8.8
What do you think the main effects are for ion-exchange sorbents?
Answer
Solvent strength is not the main effect in this case; pH and ionic strength
are the main factors governing analyte retention on the sorbent and its
subsequent elution.
Liquids
155
Table 8.2 Solvent strengths for normal- and reversed-phase sorbents. From Dean, J. R., Extraction Methods for Environmental Analysis, Copyright 1998. © John Wiley & Sons Limited. Reproduced with permission
Solvent strength for Solvent strength for normal-phase sorbents reversed-phase sorbents
Weakest Hexane Strongest
Isooctane
Toluene
Chloroform
Dichloromethane
Tetrahydrofuran
Ethyl ether
Ethyl acetate
Acetone
Acetonitrile
Isopropyl alcohol
Methanol
Strongest Water Weakest
As with the choice of sorbent, some preliminary work is required to affect the best solvents to be used. A typical procedure for SPE is described in Figure 8.9.
8.3.5 Factors Affecting SPE
While the choice of SPE sorbent is highly dependent upon the analyte of interest and the sorbent system to be used, certain other parameters can influence the effectiveness of the SPE methodology. Obviously, the number of active sites available on the sorbent cannot be exceeded by the number of molecules of analyte or otherwise ‘breakthrough’ will occur. Therefore, it is important to assess the capacity of the SPE cartridge or disc for its intended application. In addition, the flow rate of sample through the sorbent is important; too fast a flow and this will allow minimal time for analyte-sorbent interaction. This must be carefully balanced against the need to pass the entire sample through the cartridge or disc. It is normal, therefore, for an SPE cartridge to operate with a flow rate of 3-10 mlmin-1, whereas rates of 10-100 mlmin-1 are typical for the disc format.
Once the analyte of interest has been adsorbed by the sorbent, it may be necessary to wash the sorbent of extraneous matrix components prior to elution of the analyte. Obviously, the choice of solvent is critical in this step, as has been discussed previously. For the elution step, it is important to consider the volume of solvent to be used (as well as its nature).
DQ 8.9
What are the main reasons for the use of SPE for quantitative analysis, for example, HPLC or GC?
156
Methods for Environmental Trace Analysis
Figure 8.9 Typical procedure used for the solid-phase extraction of liquids.
Answer
The main reasons are (a) pre-concentration of the analyte of interest from a relatively large volume of sample to a small extract volume, and
(b) clean-up of the sample matrix to produce a particle-free and chro-matographically clean extract.
All of these factors require some method development, either by using a trial-and-error approach or by consultation of the existing literature. It is probable that both are required in practice.
Liquids
157
8.3.6 Example 8.2: Solid-Phase Extraction of various Phenols from Water
8.3.6.1 Extraction Conditions These were as follows:
• Sample volume: 25 ml
• SPE sorbent: PS-DVB, 230 mg
• SPE conditions: conditioning, 5 ml of acetonitrile followed by 5 ml of water; sample loading; interference elution, 2 ml of water; analyte elution, 4 ml of acetonitrile
Comments Sample extract made up to 10 ml with water.
8.3.6.2 Analysis by HPLC
Separation and quantitation was achieved by using a 25 cm x 4.6 mm id ODS2 column with UV detection at 275 nm. The mobile phase was acetonitrile-H2O-acetic acid (40:59:1), operating under isocratic conditions, at a flow rate of 1 mlmin-1. A 100 ^l ‘Rheodyne’ injection loop was used to introduce samples and standards onto the column (at 35°C).
8.3.6.3 Typical Results
These are shown in Figure 8.10 [2].
DQ 8.10
Comment on the results obtained in this study (see Figure 8.10).
Answer
It can be seen that effective pre-concentration of the three different phenols has been demonstrated with adequate precision.
g 150 | 100 50
<D
0
Figure 8.10 Results obtained from the solid-phase extraction of various phenols from river water, at a ‘spike level’ of 20 ngml-1: A, phenol; B, 4-nitrophenol; C, 2-methylphe-nol: calibration range, 0-400 ngml-1: correlation coefficient(s), 0.9993-0.9979 [2] (cf. DQ 8.10).
B
Phenol
158
Methods for Environmental Trace Analysis
SAQ 8.1
Suggest suitable procedures for the SPE of (a) organochlorine pesticides from drinking and surface waters, (b) carbamate pesticides from river water, and
(c) acid herbicides from surface water.
8.4 Solid-Phase Microextraction
Solid-phase microextraction (SPME) is the process whereby an analyte is adsorbed onto the surface of a coated-silica fibre as a method of concentration. This is followed by desorption of the analytes into a suitable instrument for separation and quantitation. The theory of SPME is discussed in Box 8.1.
Box 8.1 Theoretical Considerations for SPME
The partitioning of analytes between an aqueous sample and a stationary phase is the main principle of operation of SPME. A mathematical relationship for the dynamics of the absorption process was developed by Louch et al. [3]. In this situation, the amount of analyte absorbed by the silica-coated fibre at equilibrium is directly related to its concentration in the sample, as follows:
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