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DCM only; chloroethene is/are required OCPs, semi-volatile
toluene: methanol used as the to be able to organics, PCBs or
(10:l(vol/vol)); collection absorb OPPs; acetone:
alternatively, a solvent for microwave DCM:phosphoric acid
solvent system TPHs for energy (250:125:15(vol/vol))
of your own determina- for chlorinated
choice tion by FTIR; otherwise, DCM herbicides
236 Methods for Environmental Trace Analysis
Table SAQ 7.3 (continued)
Feature/ conditions Soxhlet Soxtec Ultrasonic MSPD SFE MAE PFE
Solvent consumption (ml) per extraction 150-300 40-50 50-100 20-50 10-20 25-45 25
Sequential or simultaneous sequential (butxs multiple assemblies can operate simultaneously) Systems available for two or six samples simultaneously Sequential Sequential Sequential Simultaneous (up to 14 vessels can be extracted simultaneously) Sequential
Equipment cost Low Low-moderate Low-moderate Low-moderate High Moderate High
Level of automation Minimal Minimal Minimal Minimal Minimal to high Minimal Fully automated - up to 24 samples can be extracted
EPA method 3540 3541 3550 None 3560 for TPHs, 3561 for PAHs and 3562 for PCBs and OCPs 3546 3545
a DCM, dichloromethane; FTIR, Fourier-transform infrared (spectroscopy); TPHs, total petroleum hydrocarbons; PAHs, polycyclic aromatic hydrocarbons; OCPs, organochlo-rine pesticides; PCBs, polychlorinated biphenyl; OPPs, organophosphate pesticides.
Responses to Self-Assessment Questions 237
Methods for Environmental Trace Analysis
Suggesting a procedure for the solid-phase extraction of any of the compounds mentioned is not straightforward. It is always a good starting point to consider the main variables in any SPE procedure. These include the following: nature of the analyte, sorbent to be used, wetting/conditioning of the sorbent, loading the sample, and analyte elution conditions. In considering the responses to the above problems, some examples from the literature have been chosen as illustrations.
(a) Organochlorine pesticides from drinking and surface water 
Analyte: organochlorine pesticides.
Sorbent: C18 cartridge.
Wetting/conditioning the sorbent: 2 x 5 ml of methanol, followed by 2 x 2 ml of water.
Loading: 1 l volumes of drinking and surface water samples from the Gdansk district, Poland, were passed at a flow rate of approximately 10 mlmin-1. Elution: 2 x 2.5 ml of n-pentane and dichloromethane (1:1(vol/vol)).
Other comments: Both extracts were combined and evaporated to 0.5 ml, prior to GC-ECD analysis.
(b) Carbamate pesticides from river water 
Analyte: 10 carbamate pesticides.
Sorbent: C18, ‘Empore’ discs.
Wetting/conditioning the sorbent: 10 ml of methanol under vacuum, followed by 10 ml of acetonitrile (after drying). Subsequently, 30 ml of water, ensuring that the disc does not become dry prior to addition of sample.
Loading: 2 l of water from the River Erbo (Spain).
Elution: After drying for 1 h under vacuum, elution was carried out by using 2 x 10 ml of acetonitrile.
Other comments: Extract was evaporated to dryness and the residue diluted in 500 ^l of methanol prior to analysis by HPLC-MS.
(c) Acid herbicides from surface water 
Analyte: 8 polar acid herbicides.
Sorbent: C18 cartridge.
Wetting/conditioning the sorbent: 10 ml of methanol, followed by 10 ml of deionized water acidified to pH 2.4-2.6 with HCl. The cartridge was not allowed to run dry during this process.
Loading: 500 ml of a surface water sample, acidified to pH 2.4-2.6 with 2 M HCl, and containing 5 ml of methanol and 1.0 ml of internal standard, was percolated through the cartridge under vacuum at a rate of approximately 15 mlmin-1. Cartridge was then air-dried under vacuum for 5 min and centrifuged for 15 min at 500g to remove the remaining water.
Responses to Self-Assessment Questions
Rinsing: The cartridge was then washed with 0.75 ml of methanol and the solution discarded.
Elution: 2 x 2 ml of methanol, under vacuum, at a rate of approximately 2 mlmin-1.
Other comments: Extract was then adjusted to 10 ml with methanol, followed by derivatization and column chromatographic clean-up, prior to GC analysis.
A detailed evaluation of the various techniques, based on the suggested guidelines, is presented below in Table SAQ 8.2. Hopefully, your own survey will have contained most of the information given in this table.
Table SAQ 8.2 Comparison of the various techniques used for the separation and/or pre-concentration of organic compounds in solution
Features/conditions Liquid-liquid SPE SPME
Description of Sample is Analyte retained on Analyte retained on a
method partitioned a solid absorbent; sorbent-containing
between two extraneous fibre attached to a
immiscible sample material silica support. Fibre
solvents; washed from protected by a
continuous and sorbent. syringe barrel when
discontinuous Desorption of not in use. Most
operation possible analyte using organic solvent commonly found for GC applications