centration levels within which the alloy elements are significant portions of the mass of the plasma, enhancement, nonlinear calibrations, and poor reproduction of response is obtained. Since elements that are normally solutes to the alloy matrix element are now subsolutes to the plasma, analytical curves based on the energy emitted as light in these dilute systems are quite
linear and very reproducible. The dependent relationship of alloy solute elements to the principal alloy element is eliminated. Elements in solution become reasonably independently emitting species in the total plasma. The author recognizes the necessity for considerably more work in substantiating the universality of the example demonstrated here with
manganese. It is hoped that spectrochemists will try this general approach on their individual problems, after first optimizing their plasma arcs, and thus provide additional evidence. RECEIVED for review June 1, 1964. Accepted September 28, 1964. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., 1964.
Infrared Emission Spectra of Solid Surfaces M. J. D. LOW and H. INOUE School of Chemistry, Rutgers, The State University, New Brunswick, Infrared emission spectra were obtained of oleic acid on the surfaces of aluminum plates, as well as of silicone lubricant, paint, paper, rubber, and polyethylene. The exploratory experiments indicate that emission methods could b e applied in special cases for the examination of the surfaces of opaque bulk solids.
S
OMF YEARS AGO
Eischens and Pliskin
( 1 ) described several experiments
debigned to explore the possibility of using infrared emission spectroscopy to study the structure of molecules chemisorbed on the surfaces of bulk metal specimens. The very poor resolution of the spectra obtained and the failure of some experiments cast doubt on the scope of application of the method. However, the potential of the emission method is great, as it could provide a means of studying the surfaces of opaque materials. IVe have conaequently made a series of experiments to explore the application of this method.
N. J .
cause of poor thermal contact between foil and plate. For experiments with oleic acid, the emitter plates were polished with grade 000 sandpaper and heated in air a t 400' C. for about 4 hours prior to each experiment to obtain stable surfaces, except where otherwise noted. A plate or pair of identical plates was then installed and brought to constant temperature, and a background spectrum was obtained. A thin layer of oleic acid was then quickly spread on a plate with a filter paper or glass-wool brush, excess oleic acid was wiped off with filter paper so that the plate surface appeared to be dry, and the emission spectrum was recorded. Samples of materials other than oleic acid were pressed against a n emitter plate. The emitting surfaces were placed about 5 cm. from the PE 112 slits and about 2
EXPERIMENTAL
A11 the emission spectra shown below were obtained with a Perkin-Elmer ?*Iode1 521 spectrophotometer. Satisfactory .spectra could also be obtained with a Perkin-Elmer Model 112 spectrometer using NaC1 optics. I n each case, the original thermocouple detector was replaced by one of higher sensitivity purchased from C. 11. Reeder Co. Spectra were rerorded a t normal ami)lification-i.e., a t 1 x scale expansion excellt where noted. [-nits of emission and absorption ordinates are arbitrary . Figure 1 shows the emission devices. Plain sheets of metal were used, rather than the rods described by Eischens and Pliskin. The emitter plates could be easily replaced or removed for cleaning. Some experiments were made with plates coyered with household foil, but were abandoned because it was very difficult to obtain reproducible background emissions, probably be-
s
Figure 1 .
S
Emission devices
The devices were m a d e o f '/ls-inch aluminum sheet. Dimensions a r e not critical. Device A, used far single-beom operation, could b e positioned b y sliding the plate, P , into the sample holder of the instrument. Device 8 wos used for double-beam operation. A heater, H , directly behind the emitter, S, was m a d e o f insulated Nichrome wire. Temperature was measured b y means of a thermocouple behind S
em. from the PE 521 slits. Variations in sample position did not bring about significant changes in the detected emission. The calculated spectral slit width a t 1600 em.-' with 290- and 500micron slits was 3 and 5 cm.-l, respectively. RESULTS
Most of the work was done with oleic acid on aluminum plates. Representative emission spectra as w-ell as an absorption spectrum of oleic acid are shown in Figure 2. h'oticeable in all spectra are bands near 5.82 microns, the carbonyl C=O stretching mode that is characteristic of C 4 associated with the acid hydroxyl groups. This is the pronounced feature of the oleic acid spectrum. Ionization of the carboxyl group removes the 5.82-micron band and brings about' bands at' higher frequencies, spectra of oleates showing strong bands near 6.4 microns as well as weak ones in the region of 6.8 to 7.0 microns, corresponding to antisymmetrical and symmetrical vibration of the ionized carboxyl group, respectively. The emission spectra also show bands near 6.4 microns. Or, the emission spectra show the pressure of both undissociated and dissociated oleic acid, implying that an interact,ion had occurred between the aluminum emitter plate and the acid. The C-H stretching frequencies of CH3 and CH2 groups of both acid and oleate occur near 3.37 microns with a separate CH, band at 3.52 microns. There are additional bands for oleic acid in the 3- to 4-micron region corresl)onding to OH stretching bands of strongly bonded acid dimers. Broad emission bands centering near 3.4 microns could be detected from plates treated with oleic acid, indicating that the C-H stretching frequencies were observed. Resolution was relatively poor, however, separate bands a t 3.37 and 3.52 microns not being clearly distinguishable, Reproducible spectra could be obVOL. 36, NO. 13, DECEMBER 1964
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,
1 ,
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Figure 2. aluminum
tained with the emitter temperature a t 100" C. d t 150" and 200" C., however, evaporation of oleic acid from the emitter plate occurred quite rapidly for a minute or so after the oleic acid was placed on the plate. Also, changes occurred in the spectia with the passage of time, indicating a reaction of acid with the aluminum surface to form an oleate surface species.
4
5
Figure 4.
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All spectra were obtained a t 200' C., with 290-micron fixed slits. Times elapsed in minutes between acid addition and the recording o f the spectra were: A, none; 8, 13; C, 23; D, none; E, 1 1 ; F, 21. Spectrum G is the background
The emission spectra obtained can be markedly affected by the condition of the surface. Figure 3 gives an example of this. Aluminum plates were heated in air for 2 hours a t 200' C. Oleic acid \Tas then added, and spectra -4,B , and C were obtained. Spectrum D, E , and F were obtained with plates that had been heated a t 300' C. for 61 hours. The spectra obtained \\ith the plates
6
ANALYTICAL CHEMISTRY
,
7.5
7
Figure 3. Effect of surface treatment on oleic acidaluminum interaction
7
Emission spectra
Spectrum A: Unidentified green enamel paint an a piece o f steel plate cut from laboratory furniture Spectrum 6: White bond paper Spectrum C: Red silicone rubber Spectrum D: Daw Corning silicone stopcock grease Spectrum E: 5-Mil polyethylene film, partially pyrolized. Ail spectra were recorded with 290-micron fixed slits. Spectra A, B , and D were obtained a t 200' C., spectra C and E at 150' C.
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,
Differential emission spectra of oleic acid on
Spectra A: Repetitive scans made a t least 80 minutes after application o f acid to one o f the p a i r o f emitter plates, a t 100' C. with 500micron fixed slits Spectra 8: Repetitive scans made after 60 minutes or more after the acid addition, a t 150' C. with 500-micron fixed slits Spectra C: At 203' C., 290-micron fixed slits, with 5 X ordinate scale expansion, recorded 42 minutes after acid addition Spectra D: Conditions as for spectra C, recorded 132 minutes after acid addition Spectra E: Absorption spectra o f oleic acid in hexane
3
,'
subjected to the more severe heat treatment exhibit pronounced bandq near 6.4 microns, suggesting that the higher degree of oxidation of the aluminum surface has brought about a greater interaction with the acid. Several other exploratory experiments were made, the emission spectra being shown in Figure 4. The samples were used "as is!" no attempt being made to prepare the surfaces in any 13 ay. DISCUSSION
-
The infrared emission technique: seem not a t all as versatile as the various and well-known techniques of absorption spectrometry. The instrumentation requirements for emission work are more specific than those for abPorption spectrometry, because the beam chopper of the spectrometer must' be located between the sample and the detector. Also, as the emission of samples amounts to about 2 to 10% of the radiant output of a globar source under normal operating conditions, scanning has to be slower and signal amplification set a t higher gains than with normal absorption work, so that amplifier stability requirements are greater. Doublebeam operation and scale expan sion ' are desirable and useful. but not essential. I t is probable that sensitivity and resolution can be improved through the use of cooled detectors. The results of the prePent exploratory experiments suggest. however, that specialized applications of emission methods are feasible for analytical and resear velopment of method for special cases where advantage can be taken of the possibility of examining
rough surfaces of opaque materials. The emission method could certainly be directly applied to various lubrication lxoblems with existing instrumentation a< it was possible to obtain evidence of interactions between oleic acid and a metal surface. an analytical method, the emission technique offers the advantages of simplicity of sample preparation and examination. Empirical methods could be developed for specialized applicationb for routine work, such as quality control of pigments or coatings. The
samples must be hot, so that heat-sensitive materials could not be reproducibly examined, at least at present. This particular failing can be turned to advantage, however, for studies of thermal degradation of opaque or not easily handled materials such as rubbers, leather, plastics, coatings, or various fabrics. The method has potential application for the study of boundary lubrication, solid lubricants, solid-state reactions, and gas-solid interactions a t the surfaces of wires, ribbons, metal single crystals, and catalysts.
LITERATURE CITED
(1) Eischens, R. P., Pliskin, W. A , , Advan. Catalysis 10, 51 (1958). RECEIVEDfor review July 13, 1964. Accepted September 9, 1964. Report of work done, in part, under contract with the U. S. Department of Agriculture and authorized by the Research and Marketing Act. The contract was supervised by the Northern Utilization Research and Development Division of the Agricultural Research Service. Support from the Petroleum Research Fund of the American Chemical Society by Grant S o . PRF1247-A3 is acknowledged, as is National Science Foundation Instrument Grant No. GP1434.
Analysis of a TrinucIear Aromatic Petroleum Fraction by High Resolution Mass Spectrometry H. E. LUMPKIN Research and Development, Humble Oil and Refining Co., Baytown, Texas
,This paper describes the first use of high resolution mass spectrometry combined with the low ionizing voltage technique for the analysis of a complex mixture. Identifications are achieved by precise mass measurement of the molecular ion, assumption of a reasonable structure, then measurement of fragment ions to prove or disprove the assumed structure. The analysis of a trinuclear aromatic fraction from the 347-60" C. boiling range of a high sulfur and nitrogen crude is used as an example of these methods. In addition to six condensed aromatic hydrocarbon types, five sulfur, two oxygenated, and one nitrogen compound type were found. A quantitative analysis for each of these types based on low voltage sensitivities is given; however, accuracy is limited to some extent by the availability of pure compounds. HE M A J O R use of high resolution mass spectrometry has been the identification and structural determination of pure compounds, or of the major component of a simple misture. Beynon has pioneered this field and his book (1j is re,:lete with esamples of the use of this methcd. High resolution mass spectrometry and organic chemistry are happily joined in the excellent work of Biemann and his associares (2-4, 8), particularly in the structural elucidation of many complex Compounds encountered in natural products. The low ionizing voltage method introduced by Field and Hastings (6) has continued to be developed in this
laboratory (9, 10) as well as in others (5) and is now in widespread use as an analytical technique. One of the major disadvantages of the technique when applied to complex petroleum mixtures is the ability to distinguish only seven classes of compounds. For example, alkyldibenzothiophenes cannot be distinguished from alkylnaphthalenes as both form molecular ions in the C,,H2,-iz mass series. When high resolving power is available, however, this disadvantage no longer esists and the number of compound types which can be determined in admisture is limited only by the resolution and limits of detectability of the instrument. This paper describes the use of high resolution mass spectrometry with the
6 5 6 - 6 8 0 ' F. BOILING RANGE FRACTION
TRI NUCLEAR AROMATIC
Figure 1 . Separation aromatic fraction
of
trinuclear
low ionizing voltage technique for the analysis of a complex petroleum mixture. EXPERIMENTAL
A sample from the trinuclear aromatic portion of a coker gas oil was selected for this study. The sample had been suhjected to distillation, thermal diffusicn, and chromatographic separation (Figure 1) some years ago. Although it was not kept from contact with air during its soi)aration, the fractions were tightly sealed during the interim pericds. =\ double focusing in.trunient having an electrcstatic analyzer radius of 15 inches and a magnetic analyzer radius of 12 inches was used in this investigation. The instrument is calmble of a resolution of over 1 to 10,000 and uses the X e r (12) peak inatrhing system for mass measurement (manufactured by -1ssociated Elect r i d industries, Lt,d,, it is designated as lIS9). Recorded spectra of the sample were obtained a t four operating modes: low resolution high voltage (70-volt ionizing voltage): low resolution low voltage (about 8-volt' imizing yoltage), high resolution high voltagc, and high resolution low vc~ltage. Sc>lection of parent peaks for further inve was made from the lcw resolution low voltage run. Howevcr, mass measurements must be made at high resolution, and both high and low voltage spectra a t high resclution were used to locate and recognize the ma+ drsired and to select the 1)eaks anlong the multiplets which were the molecular ions to be measured. .I minimum amount of low voltage sensitivity data on available ]jure compounds were obtained. These data indicated that the relative sensitivities VOL. 36, NO. 13, DECEMBER 1964
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