Inorganic Oxide Adsorbents and their Applications

The most important adsorbents for extraction and matrix simplification are silica gel, alumina, Florisil and diatomaceous earths. Silica gel, prepared from sodium silicate using the sol-gel procedure, is the most widely used general-purpose adsorbent. Silica gels used for solid-phase extraction have surface areas of about 300-800 m2 g-1, pore sizes from 4-10 nm, and an apparent pH of 5.5-7.5. The apparent sorbent pH is characterized as the observed pH of a 5% (w/w) aqueous suspension. Alumina is prepared by the low temperature dehydration (< 700°C) of alumina trihydrate and is a mixture of y-alumina with small amounts of a-alumina (less active form) and sodium carbonate. Depending on processing conditions, alumina is available as neutral (pH 7.5 + 0.5), weakly acidic (pH 6.0 $ 0.5), acidic (pH 4.5 $ 0.5) and basic (pH 9.5 + 0.5) forms. Adsorbents used for extraction and matrix simplification have a surface area of about 150m2g_1 and a pore size of 6 nm. Florisil is a magnesium silicate prepared by precipitation from a mixture of magnesium sulfate and sodium silicate solutions followed by calcining at about 1200°C. It has a surface area of about 250-300 m2g_1 and an apparent pH of about 8.5. Diatomaceous earths are flux-calcined forms of natural silica with very small surface areas. They are used as a filter aid and as a dispersant for liquid extraction using matrix dispersion techniques (see matrix dispersion).

The general extraction mechanism and applications of the inorganic oxide adsorbents are summarized in Table 1. Adsorbent properties that increase retention are a larger surface area and a high activity. Adsorbent activity is controlled by the intentional addition of water to the dried adsorbent prior to use and by drying extracts with anhydrous sodium sulfate, or a similar drying agent, prior to applying the extract to the adsorbent. A small column of

Table 1 General applications of solid-phase extraction

(1) Inorganic oxide adsorbents

• Isolation of low and medium polarity analytes from non-aqueous solutions

• Isolation of cations (alumina and silica) and anions (alumina) from buffered aqueous solutions

• Matrix simplification by fractionation into groups containing a similar number and type of functional group Examples n Isolation of organochlorine pesticides and polychlorinated biphenylsfrom transformer oil, animal fats and oils, etc. using Florisil. n Isolation of lipids by chromatography over silica gel using chloroform to elute simple lipids, acetone to elute glycolipids and methanol to elute phospholipids. n Group fractionation of polycyclic aromatic compounds (hydrocarbons, A/-containing and OW-containing) in synthetic fuels over alumina using a step solvent gradient. n Isolation of paraquat and diquat from high moisture crops in a pH 9 aqueous extract using silica gel n Mycotoxins in feeds using silica gel n Pesticides in foods, feeds and soil extracts; alkaloids, pigments and flavour compounds from plants; sugars and caffeine in cola beverages, inorganic anions and organic acids in aqueous solution using alumina; steroids and vitamins from creams and oil-based suspensions.

(2) Low specificity sorbents (aqueous solutions)

• Isolation of neutral and ionizable analytes from aqueous solution. Weak acids and bases by ion suppression. Strong acids and bases using ion pair extraction (alternative to ion exchange)

• Retention increases with solute size and is reduced by polar interactions (particularly hydrogen-bonding) and ionization

• Polar bonded phases provide only weak retention and are not particularly useful unless elution of the analyte is a problem from nonpolar sorbents

Examples n Isolation of agricultural and industrial chemicalsfrom surface waters using C18, carbon or poly(styrene-divinylbenzene)(PS-DVB) n Isolation of drugs from biofluids using C18, C8, PS-DVB or cyanopropyl (CN) n Isolation of macromolecules from biofluids and fermentation broth using C4 n Isolation of pigments and colouring materials from beverages and food extracts using C18 n Isolation of carbohydrates and nucleosides from biofluids using AMINO n Isolation of proteins, peptides and surfactants using DIOL

(3) Low specificity sorbents (organic solvents)

• Retention depends on the type and number of functional groups. Solute size is not important CN Strong dipole-type interactions and weak hydrogen-bond acidity

AMINO Strong hydrogen-bond base and weak hydrogen-bond acid. Weak dipole interactions

DIOL Strong hydrogen-bond acid and weak hydrogen-bond base with significant capacity for dipole-type interactions Examples n Isolation of polar pesticides from fats and oils n Isolation of polycyclic aromatic compounds from fuel oils n Active ingredients from ointments and suppositories

(4) Ion-exchange sorbents

• In general strong ion exchangers are used to isolate weak acid/bases of opposite charge and weak ion exchangers strong acid/bases

• Retention selectivity can be adjusted by manipulating the sample pH and ionic strength

• Choice of competing ion, its concentration and eluent pH controls selectivity for matrix simplification and elution

• Isolation of macromolecules in an active form may require special non-denaturing sorbents based on cellulose, agarose or dextran Examples n Isolation of carboxylic, sulfonic and phosphoric acids, phenols, amines and inorganic ions from water n Isolation of amino acids, organic acids, nucleosides and nucleotides from biofluids n Isolation of organic acids and bases from coal-derived and synthetic fuels n Isolation of organic acids, phenols and amines from wine, fruit juices and food extracts sodium sulfate connected in tandem with the adsorbent cartridge can be used as an additional precaution. The Brockmann scale (based on the relative retention of test dyes, see Table 2) provides a widely used standardized scale of adsorbent activity. Adsorbents of defined activity are prepared by adding a known amount of water to the adsorbent, shaking to avoid clumping, and then allowing the adsorbent to equilibrate overnight in a closed container. Analyte properties that increase retention depend on the num ber and type of functional groups present. Hydrogen-bonding functional groups are strongly retained, those with a significant dipole-character are retained to a lesser extent, and polarizable functional groups are the least retained. Irreversible adsorption and catalytic degradation of sensitive analytes can occur on all inorganic oxide adsorbents and is a source of low recovery for some analytes. Alumina and silica can function as selective ion exchange sorbents with buffered aqueous samples (see Table 1).

Table 2 Standardization of adsorbent activity

Brockmann activity grade Percentage of water (w/w)

Table 2 Standardization of adsorbent activity

Brockmann activity grade Percentage of water (w/w)

Alumina

Silica gel

Florisil

I1

0

0

0

II

3

5

7

III

6

15

15

IV

10

25

25

V

15

38

35

1Activate sorbents by heating alumina at 400°C for 8-12 h, silica gel at 180°C for 8-12 h, and Florisil at 130°C for 8-12 h.

1Activate sorbents by heating alumina at 400°C for 8-12 h, silica gel at 180°C for 8-12 h, and Florisil at 130°C for 8-12 h.

Coating silica or alumina with chemical reagents, such as sulfuric acid, sodium hydroxide, alkaline potassium permanganate, silver nitrate, etc., is used to improve the selectivity of the isolation of some analytes from their matrix. Silver nitrate, for example, improves the isolation of olefins from hydrocarbons due to formation of charge-transfer complexes. Acids can be used for the selective isolation of bases and vice versa. Silica impregnated with 2,4-dinitrophenylhydrazine is widely used for the selective isolation of volatile ketones and aldehydes from air for analysis by high pressure liquid chromatography.

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