Fundamentals of Sample Preparation for Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 180

Alumina Column Alumina is a highly porous granular aluminum oxide
(Al 2 O 3 ). It is prepared from Al(OH) 3 , which is dehydrated and calcined at
900 C with a layer of aluminum oxycarbonate, [Al 2 (OH) 5 ] 2 CO 3 . This allows the
pH adjustment to the acidic (pH 4-5), neutral (pH 6-8), or basic (pH 9-10)
grades for a particular application. The alumina solid is packed into a column
topped with a water-absorbing substance over which the sample is eluted with
a suitable solvent, which leaves interference on the column. Alumina is used
to separate analytes from interfering compounds of a different chemical
polarity.

Florisil Column Florisil is a registered trade name of U.S. Silica Co. It is an
activated form of magnesium silicate with basic properties. Florisil is more acidic
and milder (with regard to compound decomposition) than alumina and silica gel.
Florisil has been used for the clean-up of pesticide residues and other chlorinated
hydrocarbons, for the separation of nitrogen compounds from hydrocarbons, and
for the separation of aromatic compounds from aliphatic-aromatic mixtures.
Florisil is also good for separations with steroids, esters, ketones, glycerides,
alkaloids, and some carbohydrates.

Silica Gel Column Silica gel is a regenerative adsorbent of silica with weak
acidic amorphous silicon oxide. It is the precipitated form of silicic acid (H 2 SiO 3 ),
formed from the addition of H 2 SO 4 to sodium silicate. It forms very strong
hydrogen bonds to polar materials and can lead to analyte decomposition. It is
somewhat soluble in methanol, which should never be used as an elution solvent.
Silica gel can also remove slightly polar substances from hexane solutions and
serves to isolate the strictly petroleum hydrocarbon analytes.

Gel Permeation Chromatography (GPC) GPC is a size-based separation
using a hydrophobic gel—a cross-linked divinylbenzene-styrene copolymer.
Organic solvents are used to selectively pass large macromolecules, such as
protein, phospholipids, resins, lignins, fuvic, and humic acids, whereas smaller
molecules of target analytes are retained in the pores of gel. Methylene chloride is
then used to displace the analytes. The GPC is the most universal clean-up
technique to remove high molecular weight and high boiling point material from
matrices for the analysis of a broad range of SVOCs and pesticides.

Acid-base Partition Clean-up Unlike all the above methods, this is a liquid-
liquid partition clean-up to separate acid analytes (e.g., organic acids and phenols)
from base/neutral analytes (e.g., amines, aromatic hydrocarbons, and halogenated
organic compounds), using pH adjustment in a liquid-liquid extraction. In a
separatory funnel extraction, the sample is first adjusted with NaOH to pH 12

andthenextractedwithCH 2 Cl 2 , referred to as the base/neutral fraction.This
sample is then adjusted to pH

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2 with H 2 SO 4 and extracted with CH 2 Cl 2 to obtain
the acid fraction (see also Section 2.1.2).

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