TECHNOLOGY
GLC Analyzes Aromatic Impurities in Styrene Triode cell detector and capillary columns enable gas-liquid chromatograph to separate trace aromatics Gas-liquid chromatography (GLC) has taken on still another diflScult chore—analysis of trace aromatic impurities in styrene. Behind the method lies development of the Lovelock triode cell. The cell provides a detector sensitive enough to make capillary columns useful for GLC analysis of impurities in styrene. When it is calibrated for low concentrations, the cell will quantitatively determine impurities in concentrations of a few parts per million. In the Lovelock detector, argon is excited to its metastable state by electron bombardment. When a vapor sample is added, the vapor molecules collide with metastable argon atoms, pick up energy, and are ionized. The resulting current increase under an applied potential is a function of vapor concentration. Dow says this detector is more sensitive than flame detectors and other ionization detectors it has tried. Xylenes are of primary interest in the analysis because they are present as minor impurities and prove diffficult to separate from styrene and major impurities such as ethylbenzene and cumene. Several different GLC capillary columns have been prepared in Dow Chemical's laboratories for styrene analysis, O. L. Hollis of Dow told the gas chromatography symposium at the Southwest Regional Meeting of the ACS held in Oklahoma City, Okla. Depending on the column, practically all known impurities in styrene can be determined. The columns are made of stainless steel, either 0.010-in. i.d. tubing or 25 gage needlestock. Stationary liquid phases in the columns are bis(phenoxyphenyl) ether in diethyl ether, a Ucon oil in absolute methanol, or Apiezon L in methylene chloride. Columns are 200 ft. long for the bis(phenoxyphenyl) ether and Apiezon L phases, and 125 ft. long for the Ucon phase. Elution Time Fitted to Analysis.
Operating conditions were selected 50
C&EN
DEC. 19, 1960
specifically to give satisfactory separation of ethylbenzene from m- and p-xylene and cumene from o-xylene. Temperatures and flow rates were selected to give elution times less than 30 minutes when impurities of interest appear before styrene. For other impurities, conditions were adjusted to give as rapid elution as possible consistent with desired separation, says Mr. Hollis. All columns worked well up to 200° C , thus allowing examination of components boiling up to about 300° C. Mr. Hollis uses a sample splitter before the columns. Normally the splitter holds total sample to a column to less than 0.005 microliter out of a 0.5 to 1.0 microliter sample introduced to it. The various columns separate the impurities in different ways, Mr. Hollis points out. The Apiezon column gives a separation similar to boiling point separation, but does not separate pfrom m-xylene, or o-xylene, styrene, and cumene from each other. The
Ucon column generally retains compounds with unsaturated side chains longer than it retains those with saturated side chains. It also leaves both m- and p-ethyltoluene obscured by styrene. The bis (phenoxyphenyl) ether column separates most of the components in finished styrene well. The p- and m-xylenes are separated from each other and from ethylbenzene; oxylene, cumene, and styrene give distinct peaks; and m- and p-ethyltoluene can be distinguished. This column is unusual, says Mr. Hollis, in that mand p-ethyltoluene, structurally very similar, are eluted from it in opposite order of their boiling points and of their elution from the Apiezon column. Mr. Hollis made no effort to determine minimum detectable limits of various styrene impurities. Order of magnitude detectable limits range between less than 5 p.p.m. for benzene and toluene up to less than 25 p.p.m. for ethyltoluenes, butylbenzenes, or /J-methylstyrene.
CHROMATOGRAM OF STYRENE ON BIS (PHENOXYPHENYL) ETHER COLUMN AT 150° C, 40 p.s.i.g.
STYRENE PROFILE. Chromatogram made from styrene resolved by a bis(phenoxyphenyl) ether column, then detected by the Lovelock triode cell, shows separation of the xylenes, ethylbenzene, styrene, and the ethyltoluenes
