Fundamentals of Sample Preparation for Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 171

The above equations indicate that extraction efficiency increases with increasing
analyte's partition coefficient (D) and with the decreasing water to solvent ratio. We
use the example below to further illustrate the general principles of solvent
extraction.

EXAMPLE 7.1. Liquid-liquid Extraction. A 100-mL aqueous sample containing
naphthalene is extracted using octanol as the solvent. Naphalene has a low water solubility
of 31 mg/L at 20 C, and an octanol-water partition coefficient (K ow ) of 1950. Calculate:

(a) The extraction efficiency with one 100-mL octanol;
(b) The extraction efficiency with two 50-mLs of octanol.
(c) If the initial concentration of naphthalene is 25 mg/L, what is the residual
concentration after four successive extractions using four 25-mLs of octanol?

SOLUTION:
(a) V w ¼V s ¼100 mL, D¼K ow ¼1950. Use Eq. 7.9, we calculate E¼1/[1þ(100/
100)(1/1950)]¼99.95%.
(b) For the first 50-mL, we have E¼1/[1þ(100/50)(1/1950)]¼99.90%. For the
second 50-mL, we have accumulative E¼99.90%þ(10099.90%)99.90%¼
99.99%, meaning only 0.09% is additionally extracted from the second 50-mL.
(c) C w ¼25 mg/L, V s ¼25, V w ¼100, n¼4, we use Eq. 7.11 to calculate C w ¼25
(1/(195025/100þ1) 4 ¼4.3910 10 mg/L.

Further generalizations about L-L extraction can be inferred from the above
example: (a) Given a total volume of extracting solvent, extraction efficiency
increases as the number of extraction increases. The extracted amount acquired by
2nd and subsequent extractions might be negligible if the compound is very
hydrophobic (large D). (b) For one-step L-L extraction, D must be large, that is
greater than 10, for a quantitative recovery ( 99%) of the analyte in the solvent

phase. This is the consequence of the phase ratio within the practical range of
0.1

>

10. (c) Normally two to three repeated extractions are required with
fresh solvent to achieve quantitative recoveries.

< V s /V w <

7.3.2 Solid Phase Extraction

Solid phase extraction (SPE) is the technique that retains analyte from a flowing
liquid sample on solid sorbent and subsequently recovers analyte by elution from the
sorbent. In principle, SPE can be based on a number of solute-sorbent interaction
mechanisms. SPE has various phase types, which include reverse phase, normal
phase, ion exchange, and adsorption. The reverse phase C 18 (octadecyl bonded
silica) and C 8 (octyl bonded silica) are the most commonly used for the extraction of
hydrophobic environmental analytes. Normal phase SPE uses cyanopropyl bonded,
diol bonded, or aminopropyl bonded silica. They are used for polar contaminants
such as cationic compounds and organic acids. The ionic exchange SPE is based on
the electrostatic attraction of a charged group on the compound to a charged group