McReynolds Classification of Stationary Phases

The most widely used system of classifying liquid phases is the McReynolds sys- tem (28) and has been employed to characterize virtually every stationary phase.

McReynolds selected 10 probe solutes of different functionality, each designated to measure a specific interaction with a liquid phase. He analyzed these probe solutes and measured their I values on over 200 phases, including squalane,

which served as a reference liquid phase under the same chromatographic condi- tions. A similar approach was previously implemented by Rohrschneider (29,30) with five probes. In Table 3.7 the probes used in both approaches and their function are listed. McReynolds calculated for each probe, a I value, where

I =Iliquid phase−Isqualane

As the difference in the retention index for a probe on a given liquid phase and squalane increases, the degree of specific interaction associated with that probe increases. The cumulative effect, when summed for each of the 10 probes, is a measure of overall “polarity” of the stationary phase. In a tabulation of McReynolds constants, the first five probes usually appear and are represented by the symbols X,Y,Z,U,S. Each probe is assigned a value of zero with squalane as reference liquid phase.

There were several significant consequences resulting from these classifica- tion procedures. Phases that have identical chromatographic behavior also have identical constants. In this case the selection of a stationary phase could be based on a consideration such as thermal stability, lower viscosity, cost, or availability.

McReynolds constants of the more popular stationary phases for packed col- umn GC are listed in Table 3.8. Note that the DC-200 (a silicone oil of low viscosity) and OV-101 or SE-30 (a dimethylpolysiloxane) have nearly identical TABLE 3.7 Probes Used in McReynolds and Rohrschneider Classifications of Liquid Phases

Symbol

McReynolds Probe

Rohrschneider

Probe Measured Interaction

X Benzene Benzene Electron density for aromatic

and olefinic hydrocarbons

Y n-Butanol Ethanol Proton donor and proton

acceptor capabilities (alcohols, nitriles) Z 2-Pentanone 2-Butanone Proton acceptor interaction

(ketones, ethers, aldehydes, esters)

U Nitropropane Nitromethane Dipole interactions

S Pyridine Pyridine Strong proton acceptor

interaction

H 2-Methyl-2-pentanol — Substituted alcohol interaction similar to n-butanol

J Iodobutane — Polar alkane interactions

K 2-Octyne — Unsaturated hydrocarbon

interaction similar to benzene

L 1,4-Dioxane — Proton acceptor interaction

M cis-Hydrindane — Dispersion interaction

TABLE3.8McReynoldsConstantsandCross-ReferenceofCommonlyUsedStationaryPhases PhaseTemperature (◦ C)Chemical NatureXYZUS

Phases Similar Structure Squalane20/100Cycloparaffin00000 Polysiloxanes DC2000/200Dimethylsilicone1657456643SP-2100,SE-30, OV-101, DC-7105/250Phenylmethylsilicone107149153228190OV-11 SE-3050/300Dimethyl1553446441SP-2100,OV-101, OV-1 SE-5450/3005%phenyl,1%vinyl3372669967 OV-1100/350Dimethyl(gum)1655446542SP-2100 OV-30/35010%phenylphenyl- methyldimethyl44868112488 OV-70/35020%phenylphenyl- methyldimethyl69113111171128 OV-110/35035%phenylphenyl- methyldimethyl102142145219178DC-710 OV-170/37550%phenyl,50%methyl119158162243202SP-2250 OV-220/35065%phenylphenyl- methyldiphenyl160188191283253 OV-250/35075%phenylphenyl- methyldiphenyl178204208305280

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TABLE3.8(Continued) PhaseTemperature (◦ C)Chemical NatureXYZUS

Phasesof Similar Structure OV-610/35033%phenyldiphenyl- dimethyl101143142213174 OV-730/3255.5%phenyldiphenyl- dimethyl(gum)40867611485 OV-1010/350Dimethyl(fluid)1757456743SP-2100,SE-30, OV-1 OV-1050/275Cyanopropyl- methyldimethyl361089313986 OV-2020/275Trifluoropropyl- methyl(fluid)146238358468310 OV-2100/275Trifluoropropyl- methyl(fluid)146238358468310SP-2401 OV-2150/275Trifluoropropyl- methyl(gum)149240363478315 OV-2250/265Cyanopropyl- methylphenylmethyl228369338492386SP-2300,Silar CP OV-27525/275Dicyanoallyl6298727631106849SP-2340 OV-3300/250Phenylsilicone- Carbowaxcopolymer222391273417368 OV-35150/270Carbowax-nitrotereph- thalicacidpolymer335552382583540SP-1000

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OV-17010/25014%cyanopropylphenyl67170153228171 Silar5CP0/25050%cyanopropyl–50% phenyl319495446637531SP-2300, Silar10CP0/250100%cyanopropyl520757660942800SP-2340 SP-21000/350Methyl1757456743SE-30,OV-101, OV-1 SP-22500/37550%phenyl119158162243202OV-17 SP-230020/27550%cyanopropyl316495446637530OV-225 SP-231025/27555%cyanopropyl440637605840670 SP-233025/27590%cyanopropyl490725630913778 SP-234025/275100%cyanopropyl520757659942800Silar10C SP-24010/275Trifluoropropyl146238358468310OV-210 NonsiliconePhases ApiezonL50/300Hydrocarbongrease3222153242 Carbowax20M60/225Poly(ethyleneglycol)322536368572510Superox4 Superox DEGS20/200Di(ethyleneglycol) succinate496746590837835 TCEP0/1751,2,3-Tris(2- cyanoethoxy) propane 5948577591031917 FFAP50/250Freefattyacidphase340580397602627OV-351 Source:DataobtainedfromReferences8,28,and35.

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constants but also observe that these two polysiloxanes have a more favorable higher temperature limit. Comparisons of this type curtailed the proliferation of phases, eliminated the duplication of phases and simplified column selection.

Many phases quickly became obsolete and were replaced by a phase having identical constants but of higher thermal stability such as a polysiloxane phase.

Today polysiloxane-type phases are the most commonly used stationary phases for both packed-column (and capillary-column) separations because they exhibit excellent thermal stability, have favorable solute diffusivities and are available in a wide range of polarities. They will be discussed in greater detail in Part 3 of this chapter.

There likewise was an impetus to consolidate the number of stationary phases in use during the mid-1970s. In 1973 Leary et al. (36) reported the application of a statistical nearest-neighbor technique to the 226 stationary phases in the McReynolds study and suggested that just 12 phases could replace the 226. The majority of these 12 phases appear in Table 3.8. Delley and Friedrich found that four phases, OV-101, OV-17, OV-225, and Carbowax 20M, could provide satisfactory gas chromatographic analysis for 80% of a wide variety of organic compounds (37). Hawkes et al. (38) reported the findings of a committee effort on this subject and recommended a condensed list of six preferred stationary phases on which almost all gas–liquid chromatographic analysis can be per- formed: (1) a dimethylpolysiloxane (e.g., OV-101, SE-30, SP-210), (2) a 50%

phenylpolysiloxane (OV-17, SP-2250), (3) poly(ethylene glycol) of molecular weight (MW) >4000 (Carbowax), (4) DEGS, (5) a 3-cyanopropylpolysiloxane (Silar 10 C, SP-2340), and (6) a trifluoropropylpolysiloxane (OV-210, SP-2401).

Chemical structures of the more popular polysiloxanes used as stationary phases are illustrated in Figure 3.6.

Another feature of the McReynolds constants is guidance in the selection of a column that will separate compounds with different functional groups, such as ketones from alcohols, ethers from olefins, and esters from nitriles. If an analyst wishes a column to elute an ester after an alcohol, the stationary phase should have a largerZ value with respect to itsYvalue. In the same fashion, a stationary should exhibit a largerYvalue with respect toZif an ether is to elute before an alcohol. The appendixes in Reference 12 list McReynolds constants in order of increasing I for each probe in successive tables that are handy and greatly facilitate the column selection process.