Application of Vapor Phase Chromatography to Mass Spectrometer Analysis CHARLES M. DREW, JAMES R. MCNESBY, S. RUVEN SMITH, Chemistry Division,
U. S.
ALVIN S. GORDON
Naval Ordnance Test Station, China Lake, Calif.
Carrier Gas. Helium was chosen as an ideal carrier gas for the following reasons : I t s high thermal conductivity improves the sensit'ivity of the thermal conductivity cells for detection of the carried gases; it is inert and it will not adsorb or condense a t liquid nitrogen temperatures, so that it is easily separated from the fractions of the sample gas; and its mass spectrum does not interfere with other mass spertra. Even the parent peak of 11% may be resolved from the helium parent in most analytical mass spectrometers. Detector Arrangement. The tu-o-pass thermal conductivity detector is constructed of Pyrex KO. 3050 glass w-ith %mil platinum spiral filaments. The detectors are two arms of a conventional Wheatstone bridge circuit. Pure helium passes through the reference side before entering the column, and chromatogram gas and helium pass through the measuring side after emerging from the column. Further details of similar arrangements, as well as other detectors, ran be foiind in the references cited (1-19> 23-26).
A technique is described for the analysis of gaseous mixtures bj- vapor phase chromatography with simultaneous recovery of a portion of each separated fraction for mass spectrometer analysis. This procedure is valuable for positive identification of fractions and permits the analysis of mixtures having identical emergence times. Both vapor-liquid and vapor-solid type systems were employed. A mixture of the five isomers of hexane was separated on a paraffin oil-Celite column into four fractions. ,Mass spectrometer analysis indicated that one fraction contained both 2,3-dimethylbutane and 2methylpentane. The other three fractions contained pure 2,2-dimethylbutane, 3-methylpentane, and nhexane, respectively. Similarly, a mixture of propylene, propgne, and allene was separated. Propyne and allene have almost identical cracking patterns, so that mass spectrometer analysis of the above mixture has not been possible. The utility of the method in the analysis of very small samples was demonstrated by the quantitative separation of 0.05 cc. (STP) of a mixture of ethane, propane, and butane.
I
and
Solid Adsorbents cs. Liquid Partition Columns. I n this work both solid adsorbents, such as silica gel and activated charcoal, and partition columns (paraffin oil supported on Celite 503) were used. The solid-type adsorbents exhibit a very high temperature coefficient of adsorption, making it difficult, to handle a mixture having a wide range of boiling points or molecular weights. The resolution of such column? is high for short lengths. h serious limitation of such columns is their selectivity for certain niaterials. This is especially troublesome, for example, n.ith silica gel which shows great affinity for unsaturated compounds. Thus a IOK boiling unsaturated hydrocarbon such as propylene is carried through a silica gel column a t the same rate as the much higher boiling butane, and separation of the two on silica gel is virtually impossible.
T THE analysis of complex mixtures of volatile materials by
mass spectrometry, one of the most serious limitations is the determination of the number of components present and their identity Very often the mass spectra of various compounds will have contributions a t the same mass peak; and where relatively 1:tr ge molecules cannot be identified the mass spectrum may not be resolvable. By the use of vapor phase chromatograph> such mixtures can be broken down into either pure components or groups of several components which are readily analyzed b\ the mass spectrometer. Vapor phase chromatography has been used to obtain complete qualitative and quantitative information, but the method has limitations where fractions overlap. If vapor phase chromatogr aphy is t o be employed as a general-purpose analytical tool of high precision in nonroutine applications, it is expedient to handle the analysis of overlap fractions by mass spectrometry rather than t o develop specialized columns. Because some compounds have identical emergence times, the qualitative identification of ronstituents of unknown mixtures cannot be unequivoca111 established by chromatography alone.
REFERENCE GAS
PRESSURE REGULATED
S I L I C A - G E L DRYING TR
APPARATUS AWL) PROCEDURE
The general application of gas chromatography to the analysis of volatile materials has been treated extensively in the recent literature (I-19,23-25). The present work treats the special case of applying the method t o mass spectrometer analysis of some unusually difficult systems. The apparatus employed, shown in Figure 1, differs in some respects from that described by others. One important feature is that the main stream of exit gas from the column goes directly to a manifold, where each fraction is collected separately A small portion is bypassed through the measuring side of the thermal conductivity cell and discarded. This is necessary because the relatively hot wire in the thermal condiirtivitv cell catalytically pyrolyzes some of the fractions.
Figure 1.
Schematic diagram of vapor phase chromatography apparatus
On the other hand, this feature can be used to advantage in certain instances-e.g., the separation of ethane and ethylene ( 2 3 ) . The partition columns (6-7, 12-14, 1 7 , 23) exhibit less tailing, permit elution a t considerably loa er column temperatures, and handle a much wider range of materials a t a given temperature than will the solid adsorbents; however, for comparable resolution a much longer column is necessary. The tailing observed on solid adsorbents is probably related to the high heat
979
ANALYTICAL CHEMISTRY
980
Preparation of Long Columns. The Celite-paraffin mixture was prepared by weighing out 2 parts of Celite to 1 part of white paraffin oil, then mixing the materials in a large excess of petroleum ether. The petroleum ether was then pumped off ahile the slurry was shaken. The resulting material is an apparently dry, free-flowing powder having a uniform deposit of paraffin oil over the Celite surface. The copper tubing was packed by sifting the Celite-oil mixture into the top end while holding the tube in a vertical position with a mechanical vibrator attached to the bottom end. After packing, the tubing was wound into a close helical spiraI, so that it could be conveniently fitted into a thermostated bath. The silica gel column used for the separation illustrated in Figure 1 was a glass tubing spiral filled with Davidson 40-mesh PA-100. silica gel 30. Separation and Collection of Fractions. Prior to the appearance of the first fraction, the exit gas stream was diverted through the first of a series of liquid nitrogen cooled U-traps, one of vihich is shown in Figure 1. After the first fraction was collected and the recorder indicated that the exit stream was again pure helium, 400 the first trap was bypassed and the second trap was opened to collect the second fraction, and so on. I n cases where complete separation was not obtained, the traps were changed at the minimum position between peaks, or only a portion of each peak was collected in regions where the best representative sample could be taken. For example, the first part of the first peak and the 300 last portion of the second peak of a two-component mixture having partial but incomplete separation will give almost pure samples of each component. Even where complete separation ia not accomplished, the resulting fractions can be more easily analyzed by the mass spectrometer. 200 ; The helium carrier gas can be pumped off while holding the collecting U-tube at liquid nitrogen temperature. However, in the case of light gases, such as hydrogen, carbon monoxide, and methane, some losses will be encountered. The U-tubes for collecting these light fractions should contain a small quantity of outgassed activated charcoal or other adsorbent. After the helium is pumped off through the vacuum outlet illustrated in Figure 1, the mass spectrometer sample flask is openad to the trap. If the U-tube is 71-armed and the sample flask cooled with liquid nitrogen, all of the volatile material in the U-tube 13 ill dis, _ _ I till over into the sample tube. 0 $5 a0 Sample Introduction. The removable sample inlet tube (Figure RELATIVE TIME MINUTES 1) was filled to the desired pressure with the gas mixture to be analyzed, then attached to the system as shown. The inlet Figure 2. Partial separation of the isomers system was constructed of Hoke high vacuum Sylphon valves of hexane by vapor phase chromatography with Kel-F seats. In this way samples were easily introduced a t paraffin oil-Celite-503 column any desired inlet pressure. h vacuum outlet was provided to Column length, 32 feet; inside diameter, 7 m m . ; remove air betveen the valves before introducing the sample. column temperature, 135' C.; helium flow rate, 17
of desorption from the most active sites which are the last to be vacated. In this, as in previous work (67,i t was noticed that the chromatogram peaks of low boiling materials were sharp and symmetrical when eluted from a column maintained a t constant temperature, but as components of increasing molecular weight or boiling point were eluted, the peaks became broader and developed tails. It was found that gradually increasing the temperature during the run gave equally good separation and sharpened the peaks of the higher boiling material, so that each fraction could be removed in a reasonable time.
ED.
per minute
Column Length. James ( 1 0 ) studied the efiect of changing column length up to 11 feet, and states that the resolution of such columns increases directly as a function of column length. The authors have used paraffin-Celite columns up to 32 feet in length and find that increasing length gives better separation. However, a general spreading out of the fractions on the column bed is encountered, making it difficult to elute a given material over a short time interval at the optimum temperature for separation. This may be due in part to the somewhat larger diameter of the column. The broadening of the peaks lowers sensitivity, making detection of small fractions difficult. The most serious disadvantage of such long columns is the difficulty of achieving adequate carrier gas flow rate (8). In order to drive 7 5 (STP) cc. per minute of helium through a 32-foot Celite-paraffin column in 3/8-inch outside diameter copper tubing, a pressure drop of i 5 pounds per square inch across the column \vas necessary. Janak ( 1 7 ) states that maximum resolution is attained when the ~ the column flo~ rate is increased, so that turbulent f l o through is reached; resolution decreases at still higher rates. The authors confirm the observation that there is an optimum flow rate above which sensitivity is decreased and resolution not greatly improved, and below which resolution is diminished. If this effect is due to turbulence, then it may not be possible to operate a long column having a large pressure drop a t optimum performanre over the entire length because of the difficulty of maintaining turbulence throughout its length. With more permeable packing materials this difficulty vr.ould be overcome.
Materials. I n order to demonstrate the utility of the method, an analysis of a mixture of the five isomera of hexane v a s chosen as a good example of a difficult mass spectrometer analysis that might be handled by vapor phase chromatography. I n the mass spectrometer analysis of the isomers of hexane the cracking patterns of several of the isomers are similar enough to make the analysis very difficult, especially where they are admixed n-ith other hydrocarbons of higher or lower molecular weights. -4 mixture of the five liquid isomers was prepared from Sational Bureau of Standards standards and the vapor phase from this mixture was introduced as described above. A second system-propylene, propyne, and allene-was chosen because the mass spectrometer cracking patterns of the latter two compounds are practically identical, the only significant difference being in the magnitude of the 15 mass peak for the two compounds. Propylene was present as an impurity in the allene sample which was prepared in this laboratory by debromination of 2,3-dibromopropylene with zinc. RESULTS AND DISCUSSION
Analysis of Hexane Isomers. Figure 2 shows the partial but incomplete separation of the five isomers of hexane. The shaded areas under the four peaks represent the regions that were collected and analyzed with a Consolidated 21-103 mass spectrometer. The qualitative identification of each peak of the chromatographic separation was easily made by comparing the mass spectra of each fraction with standards which had been mass spectrometrically analyzed The quantitative analysis of each fraction could be readily accomplished by matrix solution using no more
V O L U M E 28, NO. 6, J U N E 1 9 5 6
981
than a three-element niatris. The order of appearance in the clii,omatogram as determined from the mass spectrometer amlysis was: 2,2-dimethylbutane; 2,3-dimethylbutane 2niethylpentane; 3-methylpentane, and n-hexane. In a recent paper (SO) the hexane isomers \\-ere separated on an Octoil S column a t 65' C. with the same degree of separation as in this Tvork. Here again no separation was achieved between 2-methvlpentane and 2,3-dimethylbiitane. If an unknown mixture contained one or both of the pair 2,3-dimethylbutane and 2-methylpentane, examination of the chromatogram almve could not distinguish between the two isomers nor tell u-hetlier both vere present. An earlier paper ( 1 ) describes the srparation of the saturated constituents of gasoline on a large dinonyl phthalate column, which does give a small amount of separation between this pair. However, these workers relied on nims spectrometer analysis for the identification of the various fractions i n this complicated mixture. Their chromatogram s1iovl.s only I T recognizable fractions, many of which are so misshapen or overlapping that quantitative analysis by vapor phase clironiatography is not possible, while the mass spectrometer an:ilysis reveals 25 compounds with as many as four compounds in a single fraction. \lass spectrometer analyses of the isolated fra(.tioiis provide a rapid quantitative estimation of the composition of the fraction.
Table I. Mass Spectrometer Analysis of Fractions in Chromatographic Separation of Hexane Isomers Sample Component 70 .\
B
C' D
2,P-Dimethylbutane 2,S-Dimethylbutane 2-Methylpentane 3-Methylpent ane 2-llethylpentane n-Hexane
not separate. According to the mechanism presented, the heats of solution of these two compounds in paraffin oil must be closely equal. For the analysis given in Table I all the material in each of the four fractions was collected separately and analyzed with the mass spectrometer. Solution by a matris in the case of the binary mixture (peak 2) was not necessary. The small amount of contamination of the 3-methylpentane with 2-methylpentane may have been due to carry-over in the mass spectrometer inlet system from the 3-methylpentane standard which was analyzed prior to this sample. I n the cases where 100% pure components in the first and last peaks were obtained, the mass spectra of the samples were identical to standard hexanes obtained from the National Bureau of Standards. If a component comes through the column slo\\ly, giving a long front and a long tail in the chromatogram, it is apparent that i t is soluble in the stationary phase. The solubility may be decreased by raising the temperature and causing the coniponent to pass through the column at a faster rate. This principle has been employed in the separation of a mivture having a wide range of boiling points by continuous elevation of column temperature during the run. The t x o effects of changing the column temperature must be employed with discretion in each individual problem. Apparently, lowering the temperature has a greater relative effect on increasing the time between maxima than it has on broadening the peak, so that better separation results. Other work (IO,2%) supports the contention that the velocity of travel through the column is an approximate inverse function of the solubility.
100.0 51.8 48.2 97.3 2.7
400
1 I
100.0 ,300 C
2.3 DIMETHYL BUTANE 2 METHYL PENTANE
2 2 DIMETHYL BUTANE
-
I
i\
A
I
> Since H A - HB is nearly temperature independent, a lowered temperature increases the value of RAIRB,thus increasing the separation. Janali (f6)developed a more detailed theory of the mechanism of chromatographic separation in a column consisting of a gas and a solid adsorbent with a similar result for the effect of temperature The separation of the hexanes was improved considerably by increasing the carrier gas flow rate, but the most striking result was obtained by 1oLvering the column temperature. Figure 3 s h o w the much improved separation, but the second peak containing the 2,3-dimethylbutane and 2-methylpentane still did
01
60
1
120
90
1
I50
TIME, MINUTES
Figure 3.
Separation of hexane isomers attained by lowering column temperature 70" C. Helium flow, 40
CO.
per minute
By changing the stationary phase from aromatic to aliphatic, James ( I O ) was able to reverse the order of appearance of benzene and cyclohexane. I n principle, it should be possible to separate 2,3-dimethylbutane and 2-methylpentane by using a stationary phase which is structurally more nearly like 2methylpentane than 2,3-dimethylbutane. Analysis of Propyne-Allene Mixtures. The compound C3H, has two isomers: methplacetylene, or propyne CH?-C=CH, Since the mass spectra and propadiene, or allene CHB=C=CH*. of these t n o compounds are practically indistinguishable, this system off ers a unique analytical problem. Table I1 illustrates the similaiity of the mass spectral pattern coefficients of allene and propyne in the mass regions of major importance. Any attempt to resolve a mixture of these constituents in a mass spectrum would lead to a pair of simultaneous equations represented by lines with almost identical slopes. This would yield an analysis which is neither accurate nor sensitive. Figure 4
982 PROPYNE AND ALLENE FRACTIONS
> d
l
i
ANALYTICAL CHEMISTRY
j.l
I
.
I
0 > 0
ETHANE
K
PROPANE
0
................................
.. ... I
0 0
1
15
I
30
1
I
45
\_iI
BUTANE
r
2 5 t
I
I\
60
TIME, MINUTES
Figure 4.
Separation of propylene, allene, and propyne on paraffin oil-Celite-503 column
Column length, 32 feet; inside diameter. 7 mm.; column temperature 0' C.; helium flow rate, 40 cc. per minute
shov s the chromatographic separation of a mixture of propyne, allene, and propylene. With the degree of separation obtained i t is possible to handle the analysis of such a mixture with very little difficulty. Since the niass spectrometer is capable of handling very sm:ill samples, it is important that any method used for preliminary separation also be capable of handling minut,e quantities. Unlike fractional distillation, overlap between fractions can be avoided and there is no loss due to column holdup because all t'he sample is driven through. Thus the limit of sample size depends primarily on the sensitivity limit of the detector used. The sample size, therefore, can be small. Figure 5 illiistrates the complete separation of 0.05 cc. (STP) of a mixture of ethane, propane, and butane into three pure fractions. Mass spectrometer analysis of such fractions indicates that the separation is complete, each fraction analyzing pure within the limit, of sensitivity of the mass spectrometer. Because chromatographic separation gives fractions of high purity, the application of the technique to the preparation of high quality standards for the mass spectrometer offers great promise. Tt is also possible t o prepare samples of sufficient size for use as standards in infrared spectroscopy. From the previous discrission on the propyne-allene problem it is apparent that the only means of establishing the puritj- of either propyne or allene relative to each other would involve some method independent oE mass spectral analysis. By chromatographic treatment one could prepare reliable standards from these compounds, even though the original samples were mixed. I n addition to the preparation of pure standards for mass spectrometer calibration, t,he apparatus is useful, in conjunction with the mass spectrometer, in identifying and determining the concentration of trace impurities where the ion currents due to the impurity component are completely masked by the major constituent and their magnitude is within the error limit of reading t,he peak heights of the mass spectra. I n this case it is necessary only t,o collect and measure the minor components in the chromatographic separation. I n studies of gas phase reactions, particularly in photocheniistrj(2), it is frequently necessary to run a reaction to only a few per cent conversion and analyze the products qualitatively and quantitatively. If reaction vessels are about 100 cc. in volume and the initial reactant pressures are of the order of 100 mm., the total amount of products will be somewhat less than 1 cc. a t STP. It is in this area that gas chromatography promises to be of great importance. The complete vapor phase chromatographic separation of ethane, ethylene, carbon monoxide, butane, and diethyl ketone has been accomplished in the preparation of 2,3-butane-d4 (21). Gas chromatography, combined with mass spectrometry, is virtually the only hope in certain stLtdies involving deuterated
0 I5 TIME, MINUTES
0
30
Figure 5 . Separation of 0.05 cc. (STP) of mixture of ethane, propane, and butane on silica gel Column length, 8 feet; inside diameter, 3 mm.; column temperature range 0-100' C.; helium flow rate, 50 CC. per minute
Table 11.
.\lass Spectra of Allene and Propyne Relative Intensities ________~_
M/e 2 12 13 14 15
18 18.5 19 19.5
Allene 0.42 2.29 2.20 3.04 0.25 0.03 0.08
-. :.;;
Proiisne
0.59 2.26 2.21 1.56 1.13 0.25 0.04 2.72 1.93 0.64 0.02 1.38 3.08 2.88 0 67 (0.32) (0.29) 0.07
20 3.06 20,s 0.12 24 1.28 25 .3 07 26 3 83 27 0 68 28 ( 0 12) 29 0 08 35 36 7.17 7.28 20.23 25.81 37 38 37.50 33,82 89 87.17 88.03 40 100.00 100.00 3.29 3.36 41 Sensitivity of propyne = 40.28 div./p. Sensitivity of allene = 36.71 div./p. Sensitivity of n-butane a t mass43 = 50.00 div./p. nButane-pattern mass 15, 4.80; mass 29,41.52, maw 43, 100.00; mass 58, 12.26 Accelerating voltage, mass 28 = 1730 volts.
compounds, the available amounts of which are usually small, and mass spectrometer standards for which are unavailable. A parent hydrocarbon and its various deuterated derivatives cannot be completely separated by vapor phase chromatography and the mass spectrometer analysis of the recovered fraction is necessary for complete identification. I n some specialized cases, some separation of heavy and light hydrogen isotopes has been observed. Melkonian and Reps ( 2 2 ) found that the vapor phase of a mixture of hydiogen and deuterium over silica gel was considerably enriched in the light isotope. Similarly, CH, and CD, mixtures w r e found to be somewhat richer in methane in the vapor phase over silica gel. ACKNOW LEDGSIENT
The authors \?-ish t o acknowledge the assistance of Andreas lr. Jensen and Helen R . l-oung in obtaining the mass spectra, and to thank Charles Staff for pieparation of allene. LITERaTURE CITED
(1) Bradford, B. W., Harvey. D., Chalkley, D. E., J . I?&. Petroleum 4 1 , SO (1955).
V O L U M E 28, NO. 6, J U N E 1 9 5 6 ( 2 ) Callear, A. B., Cvetanovic, R. J., Can. J . Chem. 33, 1256 (1955). (3) Cropper, F. R., Heywood, A , , .Tuture 172, 1101 (1953). (4) Ibid., 174, 1063 (1954). (5) Glueckauf, E., Barker, K. H., Kitt. G. P., Discussions Faradall SOC.7, 199 (1949). (6) Griffiths, J. H., James, D. H., Phillips, C . 9. G., Analyst 77, 897
(1952).
( 7 ) Griffiths, J. H., Phillips, C. S. G . , J . C h e m . SOC.1954, 3446. ( 8 ) Harvey, D., Chalkley, D. E., Fuel 34, 191 (1955). (9) James, A . T., Biochem. J . 52, 242 (1952). (IO) James, -1.T., Research 8 , 8 (1955). (11) James, A. T., Martin, A . J. P., Analyst 77, 915 (1952). (12) James, A. T., Martin, A. J. P., Biocheni. J . 50, 679 (1951). (13) James, D. H., Phillips, C. S.G., J . Chem. Soc. 1953, 1600. (14) Ibid., 1954, 1066.
983
(15) James, D. H., Phillips, C . S.G., J . Sci. Inst. 29, 362 (1952). (16) Janak, J., Collection Czechosloc. Chem. Conimuns. 18, 798 (1953). (17) Ihid., 19, 684 (1954). (18) Ihid., p. 700. (19) Kremer, E., Aluller, R., Mikrochini. Acta 36, 533 (1951). (20) Lichtenfels, D. H., Fleck, S. d.,Borow, F. H., A x . 4 ~ .CHEX 27, I510 (1955). (21) AIcXesby, J. It., Drew, C . 31., Gordon, -1.S.,J . P h y s . Chern. 59, 988 (1955). (22) Nelkonian, G. A , , Reps, B., 2 . Elektrockem. 58, 616 (1954). (23) Patton, H. W., Lewis, J. S., Kaye, W. I., ANAL.CHEW27, 170 (1955). (24) Phillips, C. S.G., Discussions Faraday S G C .7, 241 (1949). (25) Ray, K.H., J . A p p l . Chem. 4, 21 (1954). RECEIVED for review September H , 19.55. Accepted February 27, 1956.
Polarographic Determination of Methyl Methacrylate Monomer in Polymers RENE J. LACOSTE, ISADORE ROSENTHAL, and CARL H. SCHMITTINGER Rohm
&
Haar Co., Philadelphia, Pa.
A polarographic method is described for the determination of methyl methacrylate monomer in polymers and poly-esters at concentration levels as low as 0.1% relative. The method is based on the reduction of the a$unsaturation of the acrylate in a benzene-ethyl alcoholwater solvent system with a tetraalkylammonium salt aslsupporting electrolyte. Some problems of technique which might deter the application of polarography to this type of problem are discussed. Data are also presented to show the applicability of the procedure to monomer determinations in methacrylate-styrene copolymers.
D
OUBLE bonds which are activated by electronegative groups or conjugation are, in general, polarographically
reducible. The potentials a t which reduction takes place vary from about -0.4 volt for compounds such as fumaric acid to about -2.5 volts for butadiene. The authors were confronted with the problem of determining residual amounts of acrylatetype monomers in polymers and of analyzing these compounds in the presence of materials that interfere with conventional assay procedures such as bromination. The pyridine sulfate-dibromide method or the mercaptan addition method ( 3 ) can be used to determine residual monomer in terms of total unsaturation. However, these methods are subject t o interferences from some inhibitors, catalysts, and plasticizers, and are not specific for the niononier iinsaturation. A polarographic method which is based on the reduction of the a,p-iinsaturation in methyl methacrylate has been successfully applied to this problem. Amounts of monomer as little as 0.17; of the polymer have been determined with a precision within =t3'C relative. The limiting factor in sensitivity is the slight solubility of the polymer. This method is applicable to other acrylate esters and the procedure is suitable for determining other reducible groups in polymers. This paper reports on the procedure and the selection of reagents that can be convenientl?. used in a variety of situations for the analysis of acrylates in the presence of other monomers and polymers, and indicates the solution to some problems of technique Tvhich might deter the :ipplication of polarography to this type of problem. Experiments by the authors have shown that the douhle bond in xci,ylic nrids, unlike the eaters, is not reducible poinrographi-
cally. The explanation for this is similar to that given for chloroacetic acid ( 4 ) . The reduction of the double bond 111 acrylate esters is independent of p H over the range from 6 to 10. This indicates, therefole, that hydiogen ions do not enter into the potential-determining step. Studies of other acid-ester pairs (4)indicate that the ester and flee acid should reduce at about the same potential, a i t h the ester being slightly easier t o reduce. However, the acrylate anion could be expected to reduce as much as 1 volt more negative. Consequently, at p H values low enough for the acrylic acid to be undissociated, the hydrogen wave discharges before the double bond wave ( -2.0 volts); at p H values where the acrylic species is present predominantly as the anion, the double bond viave is shifted to potentials beyond the discharge of the alkyl ammonium salts. The net effect is that a wave for free acrylic acid cannot be obtained under an) of the various conditions tried. This can be used to advantage if one wishes to analyze for the ester in the presence of the acid. Then, by running a bromination in addition to the polarographic determination, both components can be determined. Sieman and Shubenko ( 2 ) described a polarographic method which they used for the determination of methacrylic ester in polymerization studies. Their data r e r e obtained in 0 . 1 S lithium chloride and in 0.1S tetramethylammonium iodide with 25% alcohol. Although the3 did not specify the remaining 75yo of the solvent, it is assumed to have been water. These authors reported a half-xive potential of - 1.92 volts us the normal calomel electrode in lithium chloride. Hon ever, this solvent system could not be adapted to a situation in which the polymer is the predominant constituent. Consequently, a benzene-ethgl alcohol system, which ie more amenable for dissolving polymers arid polvesters, \vas investigated and subsequently adopted. The method has also been applied to the determination of monomeric material in methacrylate-styrene copolymers and is applicable to the determination of residual monomer in other acrylic-type polymers and copol) mers. Although the problem of quantitatively handling 1,3-butadiene under these conditions has not been specifically woiked out, this compound should also be determinable in copolymers with the acrylates. APPARATUS AYD REAGENTS
A Leeds & Sorthrup Model E Electro-Chemograph was used a t a damping of 1; average currents were used throughout The dropping mercury electrode (Corning marine barometer tubing)
