Table 111.
Kinetic Results for CO Loss
Na ion, p.p.t.
Activation energy, Kcal./mole
>0.01 ~. -
74
1.3 2.2 3.2
70
Arrhenius factor, sec.-l 1 x ,. in18 -i.5 x 1018
58 38
5 1
x
x
1014 10'0
tion prior to precipitation, the small peak was observed in the material thus prepared. A series of samples were prepared from solutions of calcium and sodium nitrate of various mole per cent compositions to investigate t h e effect of sodium ion concentration on the decomposition rate. Oxalic acid was used t o precipitate the samples of calcium oxalate. Neutron activation analysis was used to determine the total sodium content of each sample and will be reported as parts per thousand (p.p.t.). The results obtained for the decomposition temperatures for these samples are given in Table 11. The sodium content of the sample prepared from solutions without the addition of sodium ion was found to be less than 10 p.p.m. ,411 samples were then decomposed on the thermobalance under identical conditions. Figure 3 shows the portion of the actual thermogram corresponding to the carbon monoxide loss for these samples. Not only do the decomposition temperatures decrease lvith increasing sodium ion content over the 10-90~o range,
but the curves also change shape indicating different decomposition rates. Kinetic analyses were made on these runs using a method based on that suggested by Newkirk ( 6 ) , and developed more fully by Freeberg ( 2 ) . The curves exhibiting precipitous rates of decomposition were not amenable to such studies. Kinetic results calculated from seven temperature programmed runs on reagent grade CaC204 gave a deviation from the mean activation energy of 1 6 Kcal./mole a t the 99% confidence limits. Kinetic activation energies calculated for runs made on samples containing added sodium ion are listed in Table 111. It should be noted that the activation energ) shows a decrease with increasing sodium content. Angeloni (1) has shown by the DTA method that the activation energy corresponding to the reaction responsible for the small peak was approximately 4 Kcal./mole. This supports the above observation that the lowering of the TGA activation energy is a result of sodium ion irnpurity. The results obtained from the samples that gave very rapid weight loss indicated that the sodium may catalyze the oxalate decomposition, thereby changing the kinetics of the reaction. CONCLUSIONS
This study has s h o m the usefulness of thermal methods to determine the effect of impurities on solid state decomposition reactions. Additional work would be required to determine whether the effect of the impurity results from the production of a crystal defect in the
calcium oxalate monohydrate or through an adsorption phenomenon. However, the results indicate that such impurities have a measurable effect on TGA curves. Although TGA constants and temperatures are not measured at equilibrium conditions, the trends observed appear to be real as verified by results obtained by independent techniques. The DTA results indicated that the effects produced can be isolated, thus giving insight into the cause of the observed behavior. Since sample preparation affects both the shape of the reaction curves and temperatures of reaction a t selected points for the decomposition of calcium oxalate, this strongly points u p the necessity for ascertaining the purity of the material in addition to other experimental variables as shown by previous investigations (5, 6) before it can be used as a standard. LITERATURE CITED
(1) Angeloni,, F. M., Ph.D. thesis, The Pennsylvania State University (1965).
(2) Freeberg, F. E., Ph.D. thesis, The
Pennsylvania State University, University Park, Pa., 1965. (3) Hartman, K. 0 Hisatsune, I. C.,
J . Phys. Chem., 69,'583 (1965). (4) Murphy, C. B., AKAL. CHEM.36, 347 R (1964). (5) Newkirk, A. E., Ibid., 30, 162 (lY58). (6) Ibid., 32, 1658 (1960). (7) Simons, E. A,, Newkirk, A. E., Talanta 11, 549-71 (1964). RECEIVEDfor review June 29, 1965. Accepted September 22,1965. Abstracted
in part from Ph.D. thesis of Fred E. Freeberg and presented at 16th Annual Pittsburgh Conference on Analytical Chemistry and -4pplied Spectroscopy, hlarch 1965.
A Windowless Photoionization bource for High Resolution AnaIyticaI Mass Spectrometers C. E. BRION Department o f Chemistry, University o f British Columbia, Vancouver 8, B.C., Canada
b A windowless light source has been constructed for use with the Associated Electrical Industries high resolution mass spectrometer (Type M.S.9). Photons from the helium 584 A. (21.21 e.v.) emission are used for the photoionization of atoms and molecules. The photon flux is adequate for operation of the mass spectrometer at high resolving power to permit the resolution of mass multiplets and the accurate measurement of mass, The possible uses and applications of the device are discussed. Evidence is presented for the occurrence of thermal decomposition in the electron impact ion source and it is shown that these effects can b e minimized by the use of photoionization. 1706
ANALYTICAL CHEMISTRY
I
ANALYTICAL mass spectrometers ions are generally produced by electron impact. Electron impact sources are simple in operation and have the important property that a high ionizing flux is easily obtained. This results in large and easily measured ion currents. However despite its general usefulness, ease of operation, and appeal for a wide variety of problems there are some disadvantages of the electron impact source for certain applications. Due to the proximity of the hot tungsten filament (-2000°K.) the ion source usually operates a t 80-200' C. above room temperature depending on the construction of the particular source. I n addition to the general field of organic chemistry, mass spectrometry is
N
assuming an increasing importance in biochemistry. Here many delicate molecules may be subject to thermal decomposition in the mass spectrometer ion source and pyrolysis on the filament could be even more serious. I n addition, in physical chemistry and allied subjects, there are many problems where thermal decomposition is an operational hazard, frequently of an unknown magnitude since it is often difficult to ascertain whether a given fragment has been produced by thermal or electron impact induced dissociation. Numerous problems immediately suggest themselves where it would be of advantage to be able to operate a mass spectrometer ion source a t a series of closely controlled temperatures. All these applica-
tions are, at best, difficult using the conventional electron impact ion source. Photoionization sources overcome many of these problems t o a large extent ( 2 ) . I n particular, it is possible to run the ion source a t any desired temperature. h further advantage is that the ionizing energy can be precisely defined whereas in electron impact spectra there is not only a Xaxwellian thermal spread of electron energies but also space charge and rontact potentials which modify the nominal electron energy. This leads to lack of reproducibility in low voltage mass spectra. An error in the calibration of the electron energy scale may be the reason for the consistently lower abundances in the electron impact spectrum in Table I. Photoionization spectra are also more amenable to comparison with theories of mass spectra since the energy of the ionizing radiation is closely defined. However photoionization sources have received relatively little attention probably due to the technical difficulties involved in their operation and construction. Since the cross section for photoionization is much smaller than for electron impact the use of such sources has awaited the ready availability of high sensitivity detectors. The existing woik on photoionization has recently been reviewed by Elliott (4). Typical light sources utilize an electrical discharge in a capillary containing helium or a mixture of gases at relatively high pressure (-1 mm.). Unless very efficient differential pumping is employed it is newssary to isolate the light source from the high vacuum of the mass spectrometer with a lithium fluoride window. Such an arrangement has been used by Inghram and his coworkers (8) and also by Lossing and Tanaka (9). However the use of a LiF window restricts the photon beam to wavelengths above 1050 -1.-Le., energies below 11.8 e.v. A windowless differentially pumped system places no restriction on the energy of the transmitted photon beam. This technique has been used by Al-Joboury and Turner ( I ) for photoelectron spectroscopy and in far ultraviolet spectroscopy by Weissler et al. (12) and also by Watanabe ( I I ) . It has not been exploited in mass spectrometry except for the work of Frost, hlak, and klcDomel1 (5) and more recently that of Dibeler and Reese (3). However these instruments have been designed for the study of threshold ionization phenomena and not mass spectral fragmentation patterns. They employ grating monochromators to vary the wavelength in a precise manner. The resulting photon fluxes are very low due to the high resolving power and low reflectivity of the diffraction grating. Such a device is not suitable for the ionizing source of an analytical
Table I. High Resolution Mass Spectra of 2-Pentanone at 2 1.2 1 E.V. 245C I.l/cs
m/e 86 71 58 43 43 42 42 41 29
Relative Abundance Photon Electron Ion 24.2 19.1 P
13.0 9.6 P-CH, - ~" 10.7 10.2 P-CZH~ 20.6 19.2 CsH? 100.0 100.0 CH3CO 1.7 0.6 C3He 2.4 1.1 CHiCO 8 3 3.2 C3H5 1.2 0.5 CzH5 Conditions for spectra, zero repeller potential, 8-kv. ion accelerating potential, sample pressure 2 X 10-6 mm. Hg; electron impact, 10 PA. trap current, 1kv. multiplier; photoionization, 2.5-kv.
multiplier.
Figure 1.
Photoionization source
mass spectrometer for which a high ionizing flux a t a single and preferably high energy is required. -1 microwave discharge in helium produces a line emission spectrum and in the far ultraviolet the 584 A. (21.21 e.v.) helium resonance line accounts for the majority of the radiation, A light source of this type has been successfully used in photo-electron spectroscopy (6) and should therefore provide a satisfactory mass spectrometer light source if adequate differential pumping is achieved to exclude helium from the ion source. At first it might appear that 21 volts is a rather low ionizing energy in that electron impact spectra at this energy are of relatively low intensity. However the differing nature of the threshold law for photoionization (7) should compensate for this to a large extent. Spectra at lower energy are also somewhat simpler since highly excited states of the ion are not accessible. I n view of the potential usefulness of photoionization in analytical work it was decided to construct a suitable light source for the lI.S.9 mass spectrometer. EXPERIMENTAL
The light source is shown in Figure 1.
A needle valve controls the flow of
tank helium into the quartz tube where the discharge is produced by a Raytheon 2450 Mc./s. unit coupled to a microwave cavity which is a modified form of that described by Zelikoff et al. (IS). The discharge is initiated with a tesla coil. The discharge tube is cooled with compressed air. To prevent excessive helium from entering the mass spectrometer the lower portion of the source consists of a fine quartz capillary
(0.5 mm. diameter) which transmits a very narrow light beam but severely impedes the flow of helium. A 4-cm. pumping line is situated close to the mouth of the capillary and helium is pumped away as rapidly as possible by a 100 liters/sec. oil diffusion pump backed by a 400 liters/min. mechanical pump. With this arrangement and the optimum helium pressure (that giving the maximum light intensity), the mass spectrometer operating source pressure was lo-' mm. Hg. Under these conditions and using 70-volt electrons only a very small peak due to He+ was detected in the mass spectrometer. The photon beam passes into the ion source through a small hole in the ion chamber wall and passes out through a similar hole in the far side. This should eliminate the liberation of photo electrons. The photon beam is perpendicular to the long axis of the ion source exitslit. This transverse mounting allows use of a n electron beam in the same source. To obtain optimum stability i t is necessary to allow the helium to flow for at least 1/4 hour to purge out any residual air. DISCUSSION
Since photoionization cross sections are approximately two orders of magnitude less than for electron impact the resulting photo-ion currents are relatively small. Satisfactory spectra were nevertheless obtained with a n electron multiplier voltage of 2.5 kv. for sample pressures of 10-6 mm. Hg. From a comparison of multiplier voltages it is estimated that the relative intensity is given by the ratio: Electron impact ion current (10 pamp. trap current) = 350 Photoionization current Table I compares the photoionization high resolution mass spectrum of 2pentanone with that obtained with 21.2 e.v. electrons. The electron energy scale was calibrated by measuring the appearance potential of the VOL. 37, NO. 13, DECEMBER 1965
1707
m/e = 58 fragment from 2 pentanone. A value of 9.87 e.v. was taken as the absolute value of this -4.P. from the
sample was placed in a tube outside the source and the vapor allowed to flow directly into the ion chamber. A work of hfurad and Inghram (10). suitable source pressure was obtained I n general, only ion abundances greater without any heating. However the than 1% of the base peak are recorded. mass spectrum was still dependent on The identity of ions was checked by the length of time the filament had been mass measurement. Ion source condiswitched on and even a t thermal equitions were essentially the same for both librium it was suspected that thermal spectra and it can be seen that they are decomposition had taken place. Using very similar. The intensity of the photoionization with the source at photoionization current was quite sufroom temperature a satisfactory and ficient to permit operation a t high rereproducible mass spectrum was imsolving powers. The mass ratio of the mediately obtained. .A series of experim/e = 43 doublet from 2-pentanone ments have been carried out to ascertain produced using the light source was the extent and nature of the thermal measured by the oscilloscope peak effects. Figure 2 shows the appearance of the matching method. A ratio of 1.000 848 was found (an error of only 2 p.p.m. mass spectrum in the parent peak region from the true ratio of 1.000 846). as a function of ion source conditions For some molecules it is highly and ionizing mode. The photoionization probable that thermal decomposition spectrum (a) shows the molecular ion occurs a t the ion source ambient unambiguously. Case (b) shows a temperature (-225' C. for the X S . 9 ) spectrum obtained when a scan is made and also pyrolysis may occur on the as soon as the filament is switched on filament. The following examples taken with the Source relatively cool, Already from recent work in this laboratory the parent peak has slightly decreased clearly demonstrate these effects and and the P-H20peak increased suggesting their subsequent elimination using thermal decomposition on the filament photoionization. as does the appearance of a large P-2 I n studying osmium tetroxide ( 0 ~ 0 ~ peak. ) To ensure that the differences ina mass spectrometric analysis was respectra (a) and (b) are not due to difquired. This extremely volatile solid is ferences in ionization probability for stable a t room temperature but is now photoionization and electron impact known to decompose to a black solid spectrum (e) mas obtained using photoionization with the electron beam fila(OsOJ a t high temperatures. On introducing OsOl into the spectrometer, ment switched on just before the scan high voltage breakdown rapidly enand a t zero electron energy. A11 ions sued and it was impossible to make the must be produced by photoionization. analysis. Subsequent dismantling of The nerv peaks which appear in (b) are the ion source for cleaning showed quite also present in (e) and therefore clearly that thermal decomposition had originate from pyrolysis on the filament occurred particularly in the hotter and not from direct ionization and disregions of the source around the filasociation of the sample. Spectrum (d) ment. S o problem was encountered is the result of electron impact ionization using photoionization and a satisfactory with the source a t thermal equilibrium mass spectrum was obtained with no ( N 225' (3.). Equilibrium is only attained after approximately 2 hours and detectable source contamination. I n it is important to note that a conthis way it is now possible to study tinuously changing mass spectrum is remany other compounds which decomcorded during this time. Large changes pose under electron impact source have occurred in (d) probably due to conditions. decomposition on the hot ion chamber Thermal decomposition effects are walls. The parent peak has almost dirrather more spectacular in the study of trans-4-tertiary butylcyclohexanol appeared while the P-2 peak remains the same as in (b) and (c). I n an unahich is a stable molecule by normal known compound the identity of the standards. I t was suspected that parent could be subject to some unthermal decomposition was occurring certainty. Spectrum (e) shows the when this sample was introduced via a photoionization mass spectrum with conventional high temperature inlet the source heated to 250" C. but with the system into the electron impact ion filament off, The P-2 peak is no longer source. On the other hand, use of a direct insertion probe was undesirable observed and therefore arises from filament pyrolysis as previously desince the compound was so volatile at duced. The parent peak has increased the ion source ambient temperature that a steady vaporization was imslightly over (d) and m/e 138 and 123 remain similar. The source was then possible. I n this situation it is often the practice to switch on the filament imallowed to cool to room temperature and mediately before a scan is made. For the photoionization spectrum taken a t quantitative work this practice is various temperatures. The ratio P/ undesirable and irreproducible spectra P-H,O increased until a t room temperaare often obtained. Subsequently the ture spectrum (a) was re-obtained. 1708
ANALYTICAL CHEMISTRY
(0) Photoionization
(b) Electron Impact
I
I Photoionization
l i
(filament on)
120
I
(d) Electron ImDact
,
(source hot, 25O'C.l
(source 250°C)
,I I
1.M
I
M
These experiments clearly indicate some of the possible thermal hazards associated with the use of electron impact for the study of fragmentation. It should be stressed that the normal electron impact operating conditions have been used in this study of what is by normal standards a stable molecule. It is evident that caution must be used in the interpretation of spectra and for reproducible quantitative work it is necessary that the source acquires an equilibrium temperature. The photoionization method can remove many uncertainties and ensures that the source temperature need not be in excess of that necessary for vaporization of the sample. I n addition a relatively larger parent peak can be expected for those compounds subject to thermal decomposition. The conventional electron impact source can be modified to minimize the effects of filament pyrolysis and ion chamber decomposition if the filament is placed in a separate differentially pumped box a t some distance from the ion chamber. This is desirable in all analytical mass spectrometers but entails major reconstruction of the source and its housing. Photoionization offers a simple alternative. ACKNOWLEDGMENT
The author expresses his thanks to Dr. D . C. Frost and Mr. D. Vroom for useful discussions and to Dr. C. A. JIcDowell for his interest and encouragement in this work.
(5) Frost, D. C., Mak, D., McDowell, C.A,, Can. J . Chem. 40, 1064 (1962). (6) Frost, D. C., McDowell, C. A., Vroom, (1) Al-Joboury, hl., Turner, D. W., J. D. A., unpublished experiments,1964-65. Chem. SOC.1963, 5141. (7) Geltaman, S., Phys. Rev. 102, 171 (2) Beynon, J. H., “Mass Spectrometry (19.56). and Its Applications to Organic Chem~ _ _ _ . (8) Huizeler, H., Inghram, M. G., Moristry,” p. 117, Elsevier, Amsterdam, rison, J. D., J. Chem. Phys. 27, 313 1960. (3) Dibeler, V. H., Reese, R. RII., J. Chem. (1957). (9) Losing, F. P., Tanaka, Y., Ibid., 2 5 , Phys. 40, 2034 (1964). (4) Elliott, R. M., Ch. 4, “Mass S ec1031 (1956). trometry,” C. A. McDowell, ed., ~ I C - (10) Murad, E., Inghram, M. G., Ibid., 40, Graw Hill, New York, 1963. 3263 (1964). LITERATURE CITED
(11) Watanabe, K., Ibid., 26, 542 (1957). (12) Weissler, G. L., Sampson, J. A. R., Ogawa, &I., Cook, G. R., J. Opt. SOC. Am. 49, 338 (1959). (13) Zelikoff, AI., Wychoff, P. H., Auschenbrand. C. XI.. Loomis. R. S..Ibid.. 42, 818 (1952). ’
RECEIVEDfor review >lay 6, 1965. Accepted October 4, 1965. The National Research Council of Canada provided financial assistance in this work.
Determination of Carboxylic Acids Present as Esters in Plasticizers and Polymers by Transesterification and Gas Chromatography STANLEY J. JANKOWSKI and PATRICIA GARNER Celanese Corp. o f America, Central Research laboratories, Summit, N. 1.
b A
gas chromatographic procedure
is proposed for the determination of dicarboxylic and monocarboxylic acids present as ester functionalities in plasticizers and polymers. These carboxylic acids are converted to their methyl esters by transesterification using a sodium methoxide-methanolmethyl acetate reagent, extracted with benzene containing diphenyl ether, as an internal standard, and separated and determined by gas chromatographic techniques. Conversion to the methyl esters has been quantitative for a variety of aliphatic and aromatic carboxylic acid esters of aliphatic and aromatic mono- and polyhydroxy compounds. Acid contents in the range 3 to 81% have been studied. Free carboxylic acids are not converted to their methyl esters by this technique.
C
are present as ester functionalities in an extensive variety of materials which are of importance in the chemical industry. Coatings, fats and oils, fibers, films, and plasticizers are areas where ester functionalities are frequently encountered. The complexity of the material varies from simple esters, such as dioctyl phthalate or polyethylene terephthalate, to highly complex mixtures such as alkyd resins. Wet chemical analysis (6) of ester functionalities has consisted of saponification of the material with aqueous or alcoholic caustic followed by recovery and separation of the liberated carboxylic acids or their salts. The procedures are laborious and time consuming, and quantitative separation of the individual acids present is seldom achieved. Infrared absorption spectrophotomARBOXYLIC ACIDS
etry has been used to determine ester functionalities. However, its application is limited to very simple mixtures. Because of similarity of the infrared spectra of aliphatic carboxylic acid esters, the infrared approach also lacks sensitivity for the analysis of small amounts of one ester in the presence of large amounts of another ester. Thus, as much DS 10% of an unknown ester present in a polyester polymer or resin will not be detected unless further wet chemical operations are undertaken. Ultraviolet absorption spectrophotometry has been applied in the analysis of esters containing aromatic or unsaturated acids. This method is very sensitive but its application is limited to the most simple mixtures. The presence of several aromatic and/or unsaturated functionalities requires separation of the components, prior to measurement of the specific components by ultraviolet absorption techniques. Gas chromatography has increased the possibilities of analyzing esters rapidly with excellent precision. Currently, direct determination of esters by gas chromatographic techniques is limited to those compounds having sufficient vapor pressure a t the operating temperature of the analytical column to permit elution of the ester in a reasonable period of time. The esters must be stable a t the temperatures required for introduction of the sample onto the chromatographic columns. Esters with boiling points as high as 400°C. have been analyzed in this laboratory by gas chromatographic techniques. Esposito and Swann (2) have reported the use of a transesterification technique for identification of some 19 carboxylic acids used in the production of synthetic resin. No quantitative
results were presented by these workers, and the work was limited to alkyd and polyester coating resins. Percival ( 7 ) has extended this work to include semiquantitative results. The work was limited to polyester resins, and reaction times of 18 to 42 hours are required for transesterification. Work in this laboratory has shown these techniques are not applicable to the analysis of high molecular weight polyester polymers such as polyethylene terephthalate fiber and film or spandex fibers. Reflux of these materials with up to a 100-fold excess of 0.5N lithium or sodium methoxide in methanol or 10% boron trifluoride in methanol for periods of up to 8 hours gave no significant amounts of the corresponding dimethyl esters. Conversion was less than 1%. Use of pressure equipment to increase the reaction temperature t o 100°C. did not significantly improve the results. EXPERIMENTAL
Apparatus and Materials. CHROThe instrument used to obtain the chromatograms was a Model -4-700 Aerograph Autoprep (Wilkens Instrument and Research, Inc.) equipped with a brown Electronik Recorder (Minneapolis-Honeywell Regulator Co.). Operating conditions were detector cell temperature, 250” C.; detector cell current, 175 ma.; injection port temperature, 250” C.; helium flow a t exit, 70 cc. per minute; column temperature, Ucon-50HB280X, 170” C., and Bentone 34-Carbowax 20bl, 195’ C. COLUMNPREPARATION. The Ucon column packing was made with 15% by weight of liquid phase on 60- to 80-mesh Chromosorb W. The BentoneMATOGRAPHIC U K I T .
VOL. 37, NO. 13, DECEMBER 1965
0
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