Adsorbents for GSC: Porous Polymers, Molecular Sieves, Carbonaceous Materials

nature and properties of a support in order to generate meaningful retention data and compare separations.

3.5.1.2 Teflon Supports

Although diatomite supports are widely used support materials, analysis of cor- rosive or very polar substances requires even more inertness from the support.

Halocarbon supports offer enhanced inertness, and a variety have been tried, including Fluoropak-80, Kel-F, Teflon, and other fluorocarbon materials. How- ever, Chromosorb T, made from Teflon 6 powder, is perhaps the best material available because high column efficiencies can be obtained when it is coated with a stationary phase having high surface area such as polyethylene glycols.

Chromosorb T has a surface area of 7–8 m²/g, a packing density of 0.42 g/mL, an upper coating limit of 20%, and a rather low upper temperature limit of 250^◦C. Applications where this type of support is recommended are the analyses of water, acids, amines, HF, HCl, chlorosilanes, sulfur dioxide, and hydrazine.

Difficulties in coating Chromosorb T and packing columns may be encountered as the material tends to develop static charges. This situation is minimized by using (1) plasticware in place of glass beakers, funnels, and other components;

(2) chilling the support to 10^◦C prior to coating; and (3) also chilling the column before packing. References 12–16 yield further information for successful results with this support. However, preparation of columns containing Teflon-coated sta- tionary phases is best performed by the column manufacturers.

The interested reader desiring further details about solid supports is urged to consult the comprehensive reviews of Ottenstein (10,11) and the benchmark book, The Packed Column in Gas Chromatography, written by Supina (12).

3.5.2 Adsorbents for GSC: Porous Polymers, Molecular Sieves,

number of porous polymers, these adsorbents may be employed for the anal- yses of aqueous solutions and the determination of water in organic matrices.

There are three separate product lines of commercially available porous poly- mers, namely, the Porapaks (Millipore Corp.), the Chromosorb Century Series (Johns-Manville), and HayeSep (Hayes Separation) polymers. Within each prod- uct line there are several members, each differing in chemical composition and, therefore, exhibiting unique selectivity, as may be observed in Table 3.5. On the other hand, some adsorbents are quite similar such as is the case with Pora- pak Q-Chromosorb 102 (both styrene –divinylbenzene copolymers) and HayeSep C-Chromosorb 104 (both acrylonitrile –divinylbenzene copolymers). We should expect to see in the future new polymers addressing old separation problems as was the case with the arrival of HayeSep A, which can resolve a mixture of nitrogen, oxygen, argon, and carbon monoxide at room temperature (19).

3.5.2.2 Molecular Sieves

These sorbents are also referred to as zeolites, which are synthetic alkali or alkaline-earth metal aluminum silicates and are utilized for the separation of hydrogen, oxygen, nitrogen, methane, and carbon monoxide. These substances are separated on molecular sieves because the pore size of the sieve matches their molecular diameter. There are two popular types of molecular sieves used in GSC, Molecular Sieve 5A (pore size of 5 ˚A with calcium as primary cation) and Molecular Sieve 13X (pore size of 13 ˚A with sodium as primary cation).

At normal column temperatures, molecular sieves permanently adsorb carbon dioxide, which gradually degrades the O2–N2 resolution. The use of a sil- icagel precolumn that adsorbs carbon dioxide eliminates this problem. Molecular sieve columns must be conditioned at 300^◦C to remove residual moisture from the packing; otherwise the permanent gases elute too quickly, with little or no resolution, and coelution or reversal in elution order for CO methane may occur (20).

3.5.2.3 Carbonaceous Materials

Adsorbents containing carbon are commercially available in two forms: carbon molecular sieves and graphitized carbon blacks. The use of carbon molecular sieves as packings for GSC were first reported by Kaiser (21). They behave sim- ilarly to molecular sieves because their pore network is also in the angstrom range.

Permanent gases and C1–C3 hydrocarbons may be separated on carbonaceous sieves such as Carbosphere and Carboxen.

Graphitized carbons play a dual role in GC. They are a nonspecific adsor- bent in GSC having a surface area in the range of 10–1200 m²/g. Adsorbents such as Carbopacks and Graphpacs may also serve as a support in GLC and in GLSC where unique selectivity is acquired and a separation is based on molecular

TABLE 3.5 Porous Polymeric Adsorbents for GSC

Adsorbent

Polymeric Composition or Polar Monomer

(PM)

Maximum

Temp (^◦C) Applications HayeSep A DVB-EGDMA 165 Permanent gases, including

hydrogen, nitrogen, oxygen, argon, CO, and NO at ambient temperature; can separate C2 hydrocarbons, hydrogen sulfide, and water at elevated

temperatures

HayeSep B DVB-PEI 190 C1 and C2 amines; trace amounts

of ammonia and water

HayeSep C ACN-DVB 250 Analysis of polar gases (HCN,

ammonia, hydrogen sulfide) and water

HayeSep D High purity DVB 290 Separation of CO and carbon dioxide from room air at ambient temperature; elutes acetylene before other C2 hydrocarbons; analyses of water and hydrogen sulfide

Porapak N DVB-EVB- EGDMA

190 Separation of ammonia, carbon dioxide, water, and separation of 165 acetylene from other C2 hydrocarbons

HayeSep N EGDMA (copolymer)

Porapak P Styrene-DVB 250 Separation of a wide variety of alcohols, glycols, and carbonyl analytes

HayeSep P Styrene-DVB 250

Porapak Q EVB-DVB copolymer

250 Most widely used; separation of hydrocarbons, organic analytes in water, and oxides of nitrogen

HayeSep Q DVB Polymer 275

Porapak R Vinyl pyrollidone (PM)

250 Separation of ethers and esters;

separation of water from chlorine and HCl

HayeSep R 250

Porapak S Vinyl pyridine (PM)

250 Separation of normal and branched alcohols HayeSep S DVB-4-

vinylpyridine

250

TABLE 3.5 (Continued )

Adsorbent

Polymeric Composition or Polar Monomer

(PM)

Maximum

Temp (^◦C) Applications Porapak T EGDMA (PM) 190 Highest-polarity Porapak; offers

greatest water retention;

determination of formaldehyde in water

HayeSep T EGDMA Polymer 165

Chromosorb

101 Styrene-DVB 275 Separation of fatty acids, alcohols, glycols, esters, ketones, aldehydes, and ethers and hydrocarbons.

102 Styrene-DVB 250 Separation of volatile organics and permanent gases; no peak tailing for water and alcohols 103 Cross-linked PS 275 Separation of basic compounds, such as amines and ammonia;

useful for separation of amides, hydrazines, alcohols, aldehydes, and ketones

104 ACN-DVB 250 Nitriles, nitroparaffins, hydrogen

sulfide, ammonia, sulfur dioxide, carbon dioxide, vinylidene chloride, vinyl chloride, trace water content in solvents

105 Crosslinked polyaromatic

250 Separation of aqueous solutions of formaldehyde, separation of acetylene from lower hydrocarbons and various classes of organics with boiling points up to 200^◦C

106 Crosslinked PS 225 Separation of C2 – C5 alcohols;

separation of C2 – C5 fatty acids from corresponding alcohols 107 Crosslinked

acrylic ester

225 Analysis of formaldehyde, sulfur gases, and various classes of compounds

108 Crosslinked acrylic

225 Separation of gases and polar species such as water, alcohols, aldehydes, ketones, glycols Key: DVB—divinylbenzene; EGDMA—ethylene glycol dimethacrylate; PEI —polyethyleneimine;

ACN—acrylonitrile; EVB—ethylvinylbenzene.

Source: Data obtained from References 8, 15, and 16.

geometry and polarizability considerations. Coated graphitized carbons can tol- erate aqueous samples and have been used for the determination of water in glycols, acids, and amines by DiCorcia and co-workers (22–24). In the latter roles, since graphitized carbon has a nonpolar surface texture, it must be coated with a stationary phase for deactivation of its surface. The resulting packing reflects a separation that is a hybrid of gas–solid and gas–liquid mechanisms.

Frequently, the packing is further modified by the addition of H₃PO₄ or KOH to reduce peak tailing for acidic and basic compounds, respectively. Separations of alcohols and amines are displayed in Figure 3.3. The USP (United States Phar- macopoeia) support designations specified in many gas chromatographic methods appear in Table 3.6.

TABLE 3.6 USP Designations of Popular Supports and Adsorbents USP

Nomenclature USP Support Description

S1A Siliceous earth (Chromosorb W; see method 1 for details on treatment)

S1AB Siliceous earth, treated as S1 and both acid- and base-washed S1C Crushed firebrick, calcined or burned with a clay binder above

900^◦C, acid-washed, may be silanized (i.e., Chromosorb P) S1NS Untreated siliceous earth (i.e., Chromosorb W)

S2 Styrene – divinylbenzene copolymer with nominal surface area of<50 m²/g and an average pore diameter of 0.3 – 0.4µm S3 Styrene – divinylbenzene copolymer with nominal surface area

of 500 – 600 m²/g and an average pore diameter of 0.0075µm

S4 Styrene– divinylbenzene copolymer with aromatic –O and –N groups having a nominal surface area of 400 – 600 m²/g and an average pore diameter of 0.0076µm

S5 High-molecular-weight tetrafluoroethylene polymer, 40/60-mesh

S6 Styrene – divinylbenzene copolymer having a nominal surface area of 250 – 350 m²/g and an average pore diameter of 0.0091µm

S7 Graphitized carbon having a nominal surface area of 12 m²/g S8 Copolymer of 4-vinylpyridine and styrene– divinylbenzene S9 Porous polymer based on 2,6-diphenyl-p-phenyl oxide S10 Highly crosslinked copolymer of acrylonitrile and

divinylbenzene

S11 Graphitized carbon having a nominal surface area of 100 m²/g, modified with small amounts of petrolatum and

poly(ethylene glycol) compound

S12 Graphitized carbon having a nominal surface area of 100 m²/g Source: USP Column Cross-reference Chart, Restek Corporation.

FIGURE 3.3 Separation of C1 – C5 alcohols (a) and aliphatic amines (b) on graphitized carbon. (Reproduced from Reference 20: W. A. Supina, in Modern Practice of Gas Chro- matography, 2nd ed., R. L. Grob, ed., copyright 1985, John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.)