Symposium reveals new GC methods Although it has been 18 years since the ap pearance of the pioneer ing paper of James and Martin, "The Analysis of Fatty Acids and Amines by Gas Chro matography," scientists are still de vising a myriad of new ways of using the valuable technique of gas chroma tography ( GC ). Evidence of the con tinued high activity in GC research was two information-packed sessions of a symposium on gas and liquid chromatography held by the Analyti cal Chemistry divisions (ACS/CIC). Among highlights of the symposium: • Use of "molecular probes" with GC to study crystallinity in polymers. • Trace gas analysis by Plasma Chromatography. • GC determination of dissolved hy drocarbons in aqueous solution by re peated equilibration of sample with gas. The GC technique can be used to obtain information on crystallinity in polymers, Dr. J. E. Guillet and A. N. Stein of the University of Toronto have found. In their experiments they send a pulse of molecules along a narrow tube that has a thin coating of the polymer to be studied covering the inner wall or dispersed on an inert support. These "probe" molecules undergo random diffusional motion in all directions. Upon this motion is superimposed a velocity in the for ward direction maintained by a flow of inert carrier gas such as nitrogen. Each probe molecule will have a velocity component perpendicular to the flow direction that will cause it to impinge on the polymer surface at the wall. If there is no interaction with the polymer, there will be no change in the component of velocity in the for ward direction. Any interaction, however, will cause a drop in the net translational velocity of the probe
R. F. Wernlund (left) and Dr. Cohen ex amine Plasma Chromatography tube
molecules along the tube direction. The nature of the interaction, the Toronto chemists explain, can be learned from this change in velocity by applying simple theoretical con siderations. In earlier work Dr. Guillet, Dr. Olav Smidsrod, and Andjelka Lavoie showed that a "molecular probe" could be used to study glass transitions and thermodynamic interactions in polymer systems [Macromolecules, 2, 272, 443 (1969)]. Their recent work indi cates that this technique may be a powerful new method for studying crystallinity in macromolecules. Crystallinity. With the molecular probe technique, Dr. Guillet and Mr. Stein have studied crystallinity in poly ethylene and polypropylene coated on Chromosorb G. In an experiment a uniform flow of nitrogen (about 40 cc. per minute) is maintained through the column and a 10-nanoliter pulse of probe molecules (dodecane or hexadecane) is injected at one end and detected at the other by a hydrogen flame detector. A small pulse of noninteracting gas (methane) is injected with the probe molecule to aid in detecting the car rier gas front. The retention time is determined from the positions of the peak maxima for methane and for the probe molecule at various tempera tures. The Toronto workers express their experimental results in terms of the retention volume V r . This is defined as the volume of carrier gas required to elute the probe molecules. A fundamental quantity from which other important interactions are de duced is the specific retention volume Vg, which is defined by the equation: Vff = ( 2 7 3 / T ) · ( V r / w ) . Τ is the temperature of the system in °K., V r is the retention volume at that tem perature corrected for pressure drop along the column, and w is the weight of the polymer film. Features. The Toronto pair de termines the value of V g for a par ticular probe molecule such as dodec ane at a series of temperatures. They plot the data in the form of log V g as a function of 1/T. In an experiment on typical low-density polyethylene, two features of the plot are notice able. Above the melting point, the curve is remarkably linear. And sec ond, there is a sharp transition at the melting point, 109° C , corresponding to the complete disappearance of crystallinity. The identity of this point was confirmed by differential scanning calorimetry. The Toronto workers assume that
the probe molecules interact only with the amorphous regions of the poly mer. The extrapolation of the linear portion of the curve to the lower tem perature region should represent the theoretical retention curve for the 1007c amorphous polymer below the melting point. The deviation from linearity would then stem from a drop in the amount of amorphous polymer. This corresponds to an increase in the amount of impenetrable crystalline polymer. Dr. Guillet and Mr. Stein can estimate the amount of amorphous polymer at any temperature below the melting point by comparing the experimental value of V g with the the oretical value V g ' obtained from the extrapolated curve at the same tem perature. The percentage of crystal linity then equals 100 (1 - V g / V g ' ) . Using this expression, the Toronto chemists can determine the degree of crystallinity of the polymer at various temperatures. The Toronto pair obtained data on polyethylene using two different probe molecules, decane and dodecane. The two curves were quite similar and approximated parallel straight lines in the lower temperature regions, where no further crystallization or melting is taking place during the experiment. The melting points estimated from the curves were similar—108° to 109° C. The Toronto scientists also carried out similar experiments using polypro pylene columns and hexadecane as the molecular probe. From their data the Toronto work ers suggest that their assumption re garding the nonpermeability of the crystalline regions to the hydrocarbon probe molecules is a reasonable one. Supporting this hypothesis is the fact that the values of the melting point are not decreased from the expected values for the polymers used. If there were any appreciable solution of the probe molecules in the crystalline regions, some depression of the melt ing point would be expected, they point out. A disadvantage to the molecular probe technique is that the polymer must be in the form of a thin film to be studied. In spite of this, the method's simplicity has much to com mend it. And the additional informa tion on thermodynamic interactions and on structural changes such as transitions which may be derived from the same data are added features, Dr. Guillet points out. Further work dis closed by Dr. Derek Gray and Dr. Guillet by the same method shows that the results obtained on degrees of crystallinity in olefin polymers agree
Dr. Clayton McAuliffe samples gas after equilibration with hydrocarbon in water
well with those obtained by differen tial scanning calorimetry. Dr. Guillet suggests that the "molecular probe" technique will become a standard pro cedure in the study of melting transi tions in macromolecules. Plasma Chromatography. Organic compounds in concentrations less than 0.1 p.p.b. (1:10 1 0 ) can be detected and determined by Plasma Chroma tography [RID, 2 1 , 34 (March, 1970)]. Plasma Chromatography, a trademark of Franklin GNO Corp., has been under development at the company's West Palm Beach, Fla., laboratories for the past five years. Those taking part in the work include Dr. Martin J. Cohen, company vice president, Henry C. Gibson, Jr., presi dent of the company, D. I. Carroll, W. D. Kilpatrick, and R. F. Wernlund. Work on Plasma Chromatography (PC) stemmed from the Franklin GNO scientists' interest in ion-mole cule reactions. In PC, a sample gas containing organic compounds comes in contact with ions that convert each organic molecule to a very stable ionmolecule. These ion-molecules are then separated by injecting them into a tube filled with a nonreactive gas through which they are drifted by a strong electric field. The ion mole cules arrive at a collector as ion peaks at times characteristic of their structure. A plasmagram results from a re cording of detector output versus time. This resembles a chromatogram with a millisecond time scale. Like a chromatogram, the peaks can be used for qualitative and quantitative analysis. The ions of plasmagram peaks are directed into a mass spec trometer with an interfacing technique similar to that used for molecules in gas chromatography to provide posi
tive mass identification for the sample under study. The Franklin GNO team has de veloped an instrumental system for PC called the Alpha/II PC-MS. The system is a combination of a Plasma Chromatograph and a modified quadrupole mass spectrometer. The Finnigan Model 1015 quadrupole mass spectrometer (C&EN, March 16, page 44) is specially interfaced to the PC. The combination of these two instru ments allows the measurement of the molecular ions, both positive and nega tive, formed at atmospheric pressure and, normally, in air ( air is usually the ionizing gas but others can be used). Plasma. Heart of the system is a special SIFAD (Separate Ion Forma tion And Drift) tube. A plasma of both positive and negative ions is gen erated in the ionizer region of the SIFAD drift tube at atmospheric pres sures. The ions are formed by a se quence of steps starting with the emis
or selective reactions may be chosen by the introduction of outside reactant gases into the reaction region. Once introduced into the drift region, the ions are separated by their intrinsic differences in mobility. Either positive or negative ions can be measured simply by reversing the potentials. The process, being re peated every 50 milliseconds, can be considered continuous. In practice, the response time is limited to the several seconds required to get sam ples through the cell. The recorded result is a plasmagram that plots in tensity against mobility, with each type of ion forming a separate peak. A sample of the ion swarm is then injected into the mass spectrometer. The pressure is reduced from the PC pressure, usually atmospheric, to the standard mass spectrometer operating pressure, about 10 - 5 torr. Here, a fairly complete identification of the ions can be made.
Dr. J. E. Guillet (left) and A. N. Stein study polymers with "molecular probes"
sion of a beta ray electron from a radioactive source, 63 Ni. The beta rays create secondary positive ions and low-energy electrons as they col lide with the molecules of the gas. The low-energy electrons quickly attach to the oxygen in air to form 0 2 ~ ions. The water vapor in the air then reacts with the 0 2 ~ ions to form ( H 2 0 ) n 0 2 ~ ions, with η dependent on the relative humidity. The N 2 + posi tive ions start a sequence of ion-mole cule reactions which eventually yield ( H 2 0 ) n H + . These hydrated ions un dergo ion-molecule reactions with the trace compound to form the ion meas ured in the drift tube. The ion-molecule reaction with the millions of collisions possible at at mospheric pressure accounts for the great sensitivity of PC to trace gases. The reactions may be those normally occurring in the air with trace gases,
A typical example of the use of PC for trace analysis is the measurement of dimethyl sulfoxide (DMSO) in air. At a DMSO mole fraction of less than 10~12, no ion-molecule reactions in volving DMSO occur and the plasma gram shows only a peak for ( H 2 0 ) n H + . At a mole fraction of 1 0 - 1 0 , separate peaks are obtained for ( H 2 0 ) n H + and ( D M S O ) H + (verified by the mass spectrometer). At this concentration 7% of the water ions are converted to ( D M S O ) H + . At a DMSO mole fraction of 10~8, 76% of the water ions react, and peaks are ob tained for ( H 2 0 ) n H + , ( D M S O ) H + , and ( D M S O ) 2 H + . At a mole frac tion of 1(H, 100% of the water ions react, and a plasmagram peak consist ing mainly of ( D M S O ) 2 H + forms. The Franklin GNO group can detect as little as 2 Χ 10" 12 mole fraction of DMSO in one minute using Plasma JUNE 8, 1970 C&EN
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Chromatography. Because of its speed in detecting and determining small traces of compounds in air, ap plications of PC include: • Determination of insecticides such as malathion, parathion, and triethyl phosphite in the atmosphere. • Determination of odors caused by animals or humans in the air. • Determination of atmospheric pol lutants. •Determination of odors stemming from illicit drugs or explosives. Equilibrations. Several equilibra tions of a gas such as helium with an aqueous sample permits the accurate GC determination of alkane, cycloalkane, olefin, acetylene, cycloolefin, and aromatic hydrocarbons dissolved in water, according to work by Dr. Clayton McAuliffe of Chevron Oil Field Research Co., La Habra, Calif. In his technique, he plots gas chro matographic data on the successive equilibrium gas phases and back-ex trapolates the curve to the hydrocar bon concentration in the original aqueous sample. For hydrocarbons such as alkanes, which partition 96% or greater into the gas phase, it's more convenient and accurate to sum the amount extracted for the first two equilibrations instead of back-extrapolating, Dr. McAuliffe says. The method gives qualitative sepa ration of hydrocarbons from highly water-soluble compounds, he adds. Thus, the method can be used to deter mine dissolved hydrocarbons in differ ent aqueous media, such as fresh water, sea water, subsurface brines, and biological fluids. In a typical determination, Dr. McAuliffe shakes 25 ml. of the aqueous sample with 25 ml. of helium (nitro gen or other hydrocarbon-free gases may also be used) in a 50-ml. hypo dermic syringe. The gaseous phase is then separated, and an aliquot of the gas is introduced into the gas chro ma tograph for analysis. Any gas remaining in the syringe is carefully discharged and 25 ml. of fresh helium is added. The equili bration process is repeated as many times as is required for the specific ap plication. A plot of the log of the hydrocarbon concentration in the gas phase versus the number of equilibrations gives a straight line. From the straight line fitted through the points, division of the hydrocarbon concentration in the gas phase, for any given equilibration, by the hydrocarbon concentration in the gas phase of the previous equilibra tion gives the percentage of hydro carbon in the water phase. Subtract ing this result from 100 gives the per centage of hydrocarbon in the gas phase. The concentration "of hydro-
carbon in the water phase for the first equilibration and the last equilibration can be calculated, plotted on semilog paper, and extrapolated back to zero equilibration. Dr. McAuliffe has found that the first equilibration removes the majority of alkane hydrocarbons, and three equilibrations remove the majority of cycloalkanes. This leaves predominantly aromatic hydrocarbons in the aqueous phase. Olefin and acetylene hydrocarbons distribute between the water and gas phases with different distribution coefficients from alkane, cycloalkane, and aromatic hydrocarbons. If present in the water sample, they can also be identified and measured. For example, the percentages in the gas phase, calculated from solubilities and vapor pressures (for equal volumes of gas and distilled water), for 1-pentene, 1,4-pentadiene, and 1-pentyne are 94, 83, and 5 1 % , respectively. One of the principal advantages of the method is its ability to determine hydrocarbon concentrations accurately in solutions of varying ionic composition, Dr. McAuliffe points out. It doesn't matter whether the water is fresh or brackish, or is sea water, or subsurface brine. Within 1%, the same final value is obtained from waters of widely varying salinity, even though the distribution coefficients are changed markedly. Separation. The method gives good separation of hydrocarbons from highly water-soluble organic compounds, such as alcohols, aldehydes, ether, and acids, the Chevron scientist says. Because of the high water solubilities of these organic compounds, the distributions are highly favored toward the water phase, and little or none of these compounds is found in the gas phase. Dr. McAuliffe says that the method can detect alkane and cycloalkane hydrocarbons in water if they are present in amounts of one to three parts in 10 12 of water by weight (parts per trillion, p.p.t. ). Because of their lower partitioning into the gas phase, aromatic hydrocarbons can be detected in concentrations of 4 to 12 p.p.t. Methane partitions between the atmosphere and surface waters with about 3.5% in fresh waters, and is present in open ocean waters in amounts of 28 to 36 p.p.t. With the Chevron scientist's procedure, methane can be detected at 1 p.p.t. or less. Sensitivity of the method can be increased by analyzing a larger sample of the gas phase and by increasing the ratio of water to gas, Dr. McAuliffe says. The method should be valuable in water pollution studies in which a complex mixture of hydrocarbons is dissolved in fresh or salt water.
Hydrogen polyoxides found at low temperatures Infrared and Raman spectroscopy used to identify H203 and possibly H204 The existence of hydrogen polyoxides—including hydrogen trioxide, H203, and possibly H 2 0 4 —has been reported by Dr. Paul A. Giguère, Laval University, Quebec. Dr. Giguère related his findings—based on evidence from infrared absorption and Raman spectroscopy—to the Physical Chemistry Divisions ( A C S / C I C ) . Controversy and doubt have long swirled around the existence of these exotic higher hydrogen oxide species, and a few investigators have claimed to observe them, particularly a group of Moscow chemists led by L. I. Nekrasov. Now, a lengthy and difficult series of experiments, carried out over several years by Dr. Giguère and coworker K. Herman, has thrown new light on the higher oxides of hydrogen. The Laval chemists measure infrared absorption, between 300 and 4000 c m . - 1 , of the products from electri-
cally dissociated H 2 0 or D 2 0 vapor, or related systems, trapped at liquid nitrogen temperature. They use the isotope effect of deuterium to overcome blocking of O-O vibration bands by strong libration bands of H 2 0 and H 2 0 2 in the 600 to 900 cm.-^region. Frequencies shifted. Three new absorption bands are thus obtained in the deuterated systems, at 440, 760, and 820 c m . - 1 . By a second isotopic substitution, of 1 8 0 to get D 2 0 1 8 , these frequencies are shifted to about 420, 717, and 775 c m . - 1 , precisely as Dr. Giguère expects for 0 3 skeletonstretching (760 and 820) and chainbending (440) modes. He therefore identifies the IR spectra as those of H 2 0 3 molecules—or rather D 2 O a here—trapped in the frozen products of dissociated water vapor. Mixtures of the two oxygen isotopes indicate, moreover, that the molecule responsible for the IR absorption must contain more than two oxygen atoms. Additional identification of the new IR bands is provided by following their disappearance with time on raising the temperature of the sample. For example, at - 6 5 ° C. the 760 c m . - 1 band drops in five hours to half its original intensity, showing breakdown of H 2 0 3 .
Deuterium reveals three new IR bands, attributed to D2O3 g 0.00
JUNE 8, 1970 C&EN 73
