Anal. Chem. 1981, 53, 183-187
183
Comparisoin of Mass Spectrometry and Nuclear Magnetic Resonance! Spectrometry for Determination of Hydrocarbon R. S. Orubko, D. M. Clugston, and Edward Furlmsky" Energy Research Laboratories, Department of Energy, Mines and Resources, Ottawa, Ontarlo, Canada K 1A OG 1
The hydrocarbon-type content of petroleum fractions has tradltlonally been determlned by uslng different techniques but often wlth poor agretsment among the varlous results for a glven sample. In thls study, three different fractlons were analyzed by using mass spectrometric (MS), nuclear magnetlc resonance (NMR), and fluorescent Indicator analysis (FIA) techniques. The lacand 'H NMR results were flrst converted from an atomic to a molecular bask. The MS, NMR, FIA, and bromllne number results are compared, wlth special attention given to the olefinlc contents. The assumptlons Involved In, and lirnltations of, each technlque are Identified. The analysis of llght fractions free of dlenes, olefins, and heteroatom-ccontalningspecies is best performed by MS methods. The atomic hydrogen and carbon dlstrlbutlon from the NMR method Is found to be applicable to all samples examined. Whlle these atomic data can be correlated with the properties of the fractlons, the NMR resuns on a molecular basis are uncertain because of the number of assumptions Involved.
Performance characteristics and other properties of petroleum products depend on their chemical compositions. Among the determining factors is the content of aromatic, olefinic, and saturated hydrocarbons. For example, the amounts of aromatic and saturated hydrocarbon affect the combustion properties of fuels, while olefins have a marked effect on a fuel's stability. Petroleum fractions must be subjected to various refining treatments to meet final product specifications. Such treatments always lead to a change in the constituent hydrocarbons. A rapid method of determining the proportions of the hydrocarbons would be a valuable tool for controlling technological parameters during production. Mass spectrometric (MS) techniques have been extensively applied, for many years to various types of hydrocarbon analysis, and new developments have been periodically reviewed (1). A number of MS analytical methods were standardized (2). ThLese can be applied with good precision to samples which fall within the limitations of the methods. Two important limitations are that there be very low amounts of olefins and of heteroatom (i.e., S-, N-, 0-) containing compounds. Both these classes of compounds are a source of interference in the calculations based on the spectral information. It appears,,therefore, that MS methods are suitable only for analyzing refined products from which these interferences are usually absent. Nuclear magnetic resonance spectrometry (NMR) is a powerful tool in fuel analysis as well (3). Preferential use of this technique in analysis of coal-derived liquids, i.e., liquids with high content of heteroatoms, suggests that the interference of the latter i8 less critical. An additional advantage of NMR over MS is NMR's ability to analyze nonvolatile samples, including semisolids and solids. The gas chromatographic-mass spectrometric (GC/MS) technique acts at a molecular level; that is, there is a molecular
separation followed by detection and analysis of molecules. The NMR method, on the other hand, observes the carbon or hydrogen on an atom-by-atom basis and is therefore classified as an "atomic" analysis. In a I3C spectrum, one observes and distinguishes carbons primarily on the basis of carbon type (i,e,, sp3, or sp) and secondarily on the environment in which the carbon is found. In a lH spectrum, NMR separates hydrogen by the type of carbon to which it is bonded and also by the environment to which it is located. To compare these results with those from MS techniques, one must convert the contents of aromatic, olefinic, and saturate carbon and hydrogen atoms from the NMR analyses to a molecular basis. Interpretation of NMR data on a molecular basis and the subsequent comparison of these results with those derived from MS and fluorescent indicator analysis (FIA) are the subjects of this report. Three samples with widely different compositions were chosen for this study, and light fractions boiling under 200 O C were selected to minimize the interferences. This report gives the results for these samples and points out the complications and assumptionsinherent in each technique when used for the quantitative determination of hydrocarbon types.
EXPERIMENTAL SECTION The three samples used in this work were petroleum fractions boiling below 200 O C and were selected to show variability in chemical composition. Detailed descriptions of methods used for the fluorescent indicator analysis (FIA) as well as determination of bromine numbers (Br No.) and olefin contents were published elsewhere (4). Average molecular weights were determined from the freezing point depression with a Cryette A automatic cryoscope. Elemental analysis was performed on a Perkin-Elmer 240 analyzer. Mass Spectrometric Procedure. A Finnigan GC/MS with an INCOS data system was used. The samples were injected onto a 1.83-m (6ft) column packed with 3% Dexil300 on acid-washed Chromosorb W and temperature programmed from 40 to 250 O C at 10 OC/min. This procedure accomplished two purposes; it removed any involatile impurities and it achieved some separation of the sample components to simplify individual mass spectra. The mass spectrometer was operated in chemical ionization (CI) mode with methane as the reagent gas. Successive3-s scans were acquired as long as material eluted from the GC column. The sums of a series of peaks characteristic of a given class of compounds used in the calculations are summarized in Table I. Empirical correctionsbased on the spectra of pure olefins were then applied for the contributionsof olefm to monocyclic, dicyclic, and saturate summations. A synthetic mixture containing 10% olefins, 19% aromatics, 12% rnonocyclics, and 59% paraffiis was used to obtain sensitivity factors for the various classes of compounds and to monitor day-to-dayperformance of the instruments. NMR Procedure. Individual samples were prepared for the lH and 13C NMR analyses in 5- and 10-mm tubes, respectively. The reference compound, hexamethyldisiloxane (Merck Sharp and Dohme), and the material to be analyzed were weighed directly into the NMR tube. For the 'H NMR analysis, deuteriochloroform (CDCld was added as a solvent and internal lock, the tube was sealed with a rubber stopper and Parafilm, and the
0003-2700/81/0353-0183$01.00/0 Published 1981 by the American Chemlcal Society
184
ANALYTICAL CHEMISTRY, VOL. 53, NO. 2, FEBRUARY 1981
Table I. Summations of Mass Spectroscopic Peaks components summation equation saturated hydrocarbons monocyclic saturated hydrocarbons dicyclic saturated hydrocarbons monoaromatics naphthalenes
2 7 1 = H71 + H85 + . . . + H169 2 69 = H69 + H83 t . * . + H167 2 6 7 = H67 t H81 t * * + H165 2 7 9 = H79 t H93 + . . . + H177 2129 = H129 + H143 + . . . t H185
contents were mixed thoroughly. In the 13C NMR analysis, chromium tris(acety1acetonate) (Cr(ACAC)3)was added to the sample to give a concentration of 0.080.15 M in the final solution. This concentration decreased all the I3C Tl's to the point that a rapid pulse repetition rate could be used without severe line broadening or shifting (5). Finally, for 13Canalysis, CDC13was weighed into the sample tube to serve as solvent, secondary reference compound, and internal lock. The tube was corked tightly and sealed with Parafilm and the contents were mixed thoroughly. In the 13Cspectra there was close agreement between the absolute carbon analysis derived from the two reference compounds. Also, results of the carbon and hydrogen analyses by a combustion technique (Perkin-Elmer 240 analyzer) were in good agreement with those obtained from the 13Cand 'H analyses. The 'Hand NMR spectra were obtained at 80 and 20 MHz, respectively, on a Varian CFT-20 pulse Fourier transform spectrometer. For the 'H spectra the following parameters were used transmitter positioned ca. 2.3 ppm upfield from tetramethylsilane, sweep width of 1202 Hz, Butterworth filter bandwidth of 2 kHz, 8192 data points, 3.407-5 acquisition time, a 42O rf pulse of 12 ps duration, a 300-s delay after each transient, and five transients for each spectrum. The 13C spectra were obtained with a transmitter positioned ca. 14 ppm upfield from Me,Si, a sweep width of 5000 Hz, a Butterworth filter bandwidth of 8 kHz, 8192 data points, a 0.819-s acquisition time, a 90° rf pulse of 17 ws duration, a 3.6-5 delay after each transient, ca. 15000 transients accumulated for each spectrum, and gated proton noise decouplii (to eliminate residual nuclear Overhauser effect differences) (5). Each free induction decay was transformed withouth sensitivity enhancement. For the lH spectrum, three spectral regions were integrated: 6 = 0.2-3.5 for all saturated hydrogen, 6 = 4.3-6.5 for olefinic hydrogen, and 6 = 6.5-8.5 for aromatic hydrogen. Typically,the olefinic hydrogen continuum returns to base line by 6 = 6.1, but one of the samples used in this work gave resonances in the region 6 = 6.0-6.5. It was felt that these represented the hydrogen of conjugated dienes and thus the integral over this region was included with that of the olefinic hydrogen rather than with that of the aromatic hydrogen. Use of the reference compound data allowed for the calculation of moles of aromatic hydrogen, olefinic hydrogen, and saturate hydrogen per gram of sample. This was then converted to the percentage of aromatic, olefinic, and saturate hydrogen, HA,Ho,and Hs, respectively. In the 13Cspectrum only two regions were integrated 6 = 5-70 for saturate (sp3)carbons and 6 = 100-170for aromatic plus olefinic carbons. Use of the reference compound allowed a simple calculation of the number of moles of saturate carbons per gram of sample and the number of moles of aromatic plus olefinic carbon per gram of sample. Since there is no ready way of separating the aromatic and olefinic carbons in the 13C spectrum, it was necessary to estimate the amount of olefinic carbon per gram of sample from the lH results. This value was then substracted from the 13Caromatic plus olefinic carbon result to give the amount of aromatic carbon per gram of sample. Estimating the amount of olefinic carbon per gram of sample involved assuming that for every olefinic carbon there is one olefinic hydrogen. Thus the value obtained for moles olefinic hydrogen per gram in the 'H analysis was the value used for moles olefinic carbon per gram of sample. Taking into consideration all possible basic olefinic structures (Figure 1) the 1:l assumption is quantitative for structures I and 11,underestimates in the case of structures I11 and IV, and overestimates in the case of structure V. The validity
R,-HC =CH-R2
R
4C=CH2
R\
/
R2
/
R2 111
1
Rl,
C = CH- R~
,R3
R - HC= CH2
c=c
\
R2
R4
IV
Y
Figure 1. Possible olefinic structures.
of this assumption is difficult to assess and so it must be considered as first approximation. Intuitively, it would seem to be a reasonable determination of a sample's olefinic carbon, as well as leading to a more accurate estimate of its aromatic carbon. It should be accurate enough to allow comparison of results between samples and development of trends in a series. Once the moles of aromatic, olefinic, and saturate carbon per gram of sample were determined, the percentage of aromatic carbon (CAI,olefinic carbon (Co), and saturated carbon (Cs) were calculated. The accuracy of this method was determined by analyzing two synthetic mixtures, one containing 71% of saturates, 10% of olefins, and 19% of aromatics and the other prepared by adding 3% of thiophene, 1% of pyridine, 2% of dibenzofuran, and 1% of tetrahydrofuran to the above mixture. The repeatibility in analyzing the saturate hydrogen was better than *0.5%, for the aromatic hydrogen was better than &2%, and for the olefinic hydrogen was better than *5% of the actual amounts present in the mixtures. The same repeatibilities were observed for corresponding carbon types. The NMR analysis of these two standard mixtures showed an increase of aromatic hydrogen content from 0.011 51 to 0.013 11 mol/g, a decrease of saturate hydrogen content from 0.1267 to 0.1129 mol/g, and a decrease of olefinic hydrogen from 0.00226 to 0.002 10 mol/g. These results reflect accurately the changes in concentration caused by the addition of the compounds to the original standard mixture; Le., the inclusion of tetrahydrofuran hydrogens in the saturated portion end of all heteroring hydrogens in the aromatic portion and all hydrogens of the heterorings in aromatic portion was confirmed.
RESULTS AND DISCUSSION T o compare hydrocarbon type contents determined by the different techniques, one must make sure that the comparisons are made on the same basis. At first approximation the information obtained from MS, FIA, and Br No. analyses is based on the relative numbers of molecules while the results from NMR methods are based on the relative numbers of atoms. The MS methods are straightforward when applied to the light fractions such as the samples used in this work. However, the content of olefins and of molecules containing heteroatoms must be low. Otherwise, in the case of olefins, predeterminations must be performed by other methods or estimated by reference to other samples in a series of similar analyses This value, which is entered into the program analyzing the raw MS data, markedly affects the results subsequently determined by the MS method. Failure to directly obtain the olefin content is a weakness of the MS method. The accuracy of the olefin results derived from the Br No. depends on the deviation between the experimental and the theoretical Br No. values. The deviations for alkanes, cycloalkanes, aromatics, alkenes, and cycloalkenes are very small (4).Fractions which contain only these hydrocarbon groups are well suited for this analysis, and reliable results should be obtainable. Complications arise when dienes and S- and
ANALYTICAL CHEMISTRY, VOL. 53, NO. 2, FEBRUARY 1981
some N-containing compounds are present because for these there is a very large deviation between experimental and theoretical Br No. v,alues. Results obtained by the FIA method suffer because of the incomplete separation of the hydrocarbon groups. This is easily confirmed by an MS investigation of the separated hydrocarbon fractions. For example, a series of paraffinic (saturate) hydrocarhlons is clearly observed in the collected aromatic fraction, while at the same time aromatics can be detected in the collected saturate fraction. The “olefinic” fraction is even more complex. Polar (Le., S, N, and 0 containing) compounds are eluted very slowly and are usually included in the aromatic fraction, regardless of the nature of the molecules in which they are found. In contrast, NMR techniques do not suffer from many of these interferences. However, it is essential to acknowledge the nature and the basis of the results. The 13C NMR technique gives relative percentages of aromatic, olefinic, and saturate carbon atoms (CA,CO,and CS),while the lH NMR gives the percentage of aromatic, olefinic, and saturate hydrogens (HA, Ho, and Hs). These results might be misleading to those who are used to thinking in molecular terms. For example, a sample having CA = 12% and CO = 6% might, at f i t glance, seem to contain twice as many aromatics as olefins. It is important, however, to remember that it takes at least six aromatic carbon13to make one basic aromatic ring, or molecule, but only two olefinic carbons to give one olefinic molecule assuming similar average molecular weight (AMW). With these factors applied to a sample with the above CAand COvalues, the content of aromatics and of olefins on a crude molecular basis will be 2% and 3%, respectively. There are, therefore, more olefinic molecules than aromatic molecules-the reverse of what the carbon atom distribution results indicate. Small values of CO,then, must not be ignored. The following attempt is made to convert the results from an atomic basis to a molecular basis to emphasize the significance and to clarify the meaning of the NMR results. Some assumptionri must be made to obtain the amounts of aromatic, olefinic, and saturate molecules. First, once a molecule contains an olefinic double bond or an aromatic ring it is defined as an olefinic molecule or an aromatic molecule, respectively. Second, there is only one double bond per olefinic molecule, and only one aromatic ring per aromatic molecule. , Third, the remainder of the aromatic or olefinic molecule is made up of alkyl groups containing only saturate carbon and hydrogen The boiling point range of the samples used in this study ensures that the number of molecules containing two or mare aromatic rings is negligible. Fourth, a saturate molecule contains only saturate carbons and hydrogens. These assumptions become tenuous at higher olefin concentrations because of the increasing chance of finding olefinic and aromatic functions, or two or more olefinic groups, in the same molecule; whereupon a given molecule would be counted more than once. Calculating Procedure of NMR Results. The procedure combines quantitative data on hydrogen and carbon distribution obtained experimentally. This differs from previous studies in which carbon distribution was usually derived from hydrogen distribution (6, 7). This makes it possible to express compositions of liquid fractions in terms of aromatic, olefinic, and parafinic molecules contents. With the weight piercent of carbon (C,) and of hydrogen (HT)as determined by elemental analysis and the percentages of carbon and hydrogen atoms, corresponding carbon contents (CA, Cb,and Cl,) and hydrogen contents (HA, H b , and Hl,) on a weight basis are derived from the formula
x: = XTXi/lOO X=CorH
(1)
185
i = A, 0, S This formula gives absolute amounts of hydrogens and carbons and is an extension of the Brown and Ladner equation which defines some parameters, e.g., the aromaticity as the ratio of aromatic carbon to total carbon content. The sum of C i + HA reflects, then, the amount of carbon and hydrogen in aromatic rings, while C b + H’o is the amount of carbon associated with double bonds and of hydrogen attached to these carbons. The sum of Cl, and Hl,represents the amount of carbon and hydrogen associated with the saturated alkyls, as well as including the alkyl substituents on aromatic rings and olefinic double bonds. Because the NMR technique does not include alkyl substitutes on the aromatic rings, the following formula has to be applied to obtain the content of aromatic molecules % aromatics = ( C l ,
AMW +H i ) (c6H6-n)
(2)
where AMW is an average molecular weight of the sample and n the number of ring hydrogens substituted by alkyl groups. The value of n can be derived from the equation
n=78-
7 2 ( C ’ ~+ Hi)
cl,
(4)
The amount of olefinic molecules is obtained in a similar manner with the formula % olefinics =
AMW (CZHZ)
(Cb + Hb)-
(5)
The C2H2 (gram molecular weight of 26) in the equation reflects the assumptions that there is one double bond per molecule and that one hydrogen atom is attached to each olefinic carbon. The amount of alkyl substituents attached to aromatic molecules and olefinic molecules (as indicated in the definitions and assumptions above) must be subtracted from the Cl, Hl,sum to get the content of saturate molecules. The following formula is used
+
% saturates = ( C g
+ Hg)- % arom. -
( C l , + H i ) - % olef. - (Cb + H’o) (6)
The results of these calculations performed on the three samples are summarized in Table 11. The assumptions introduced during these calculations inevitably affect the final results. For example, the average molecular weight is not necessarily the same for the aromatic, olefinic, or saturate hydrocarbons. Also as previously discussed and illustrated, the NMR determination of olefinic carbon is based on the assumption that there is only one olefinic hydrogen attached to each olefinic carbon. As compensation for these it is important to remember that while other techniques are limited to light fractions with low olefin and heteroatom contents, the NMR analysis is suitable even for samples as heavy as coals. To minimize the number of assumptions used in an NMR analysis it might be desirable to use the carbon and hydrogen contents as an atomic basis for correlation with the properties of the various samples. The results of the hydrocarbon content analyses as determined by the different methods are presented in Table 111. From the chemical composition point of view, sample 1 is representative of many commercial products in that it is a refined fraction. Species which would interfere with the analyses are not present and thus an estimate of the content
186
ANALYTICAL CHEMISTRY, VOL.
53, NO. 2, FEBRUARY 1981
Table 11. Recalculations of NMR Results to Molecular Basis sample no. 2
no. 3
84.9 14.0 0.09 1 (PPm) 0 110
85.0 13.9 0.58 0.083 42 111
85.5 12.2 1.72 0.01 121 102
8.5 0.0 91.5 2.2 0.0 97.8
8.6 4.0 87.4 2.5 2.0 95.5
15.4 20.8 63.8 6.3 11.1 82.5
H‘A,% H’o,% H‘s, %
7.2 0.0 77.7 0.31 0.0 13.69
7.3 3.4 74.3 0.35 0.28 13.30
13.2 17.8 54.5 0.77 1.35 10.06
aromatics, % olefins, % saturates, %
11.0 0.0 89.0
11.3 15.7 73.0
18.7 75.1 3.9
no. 1 CT, % HT,%
s, %
N , % (PPm) Br No.
AMW
CA, % C O , 7% cs, ?6 H A , 9%
Ho, Hs, % C ’ A ,% C’O, % C’S, %
Table 111. Hydrocarbon Group Contents (Percent) as Determined by the Different Methods sample no. 1 no. 2 no. 3 aromatics from FIA NMR MS olefins from FIA NMR MS Br No. saturates from FIA NMR MS
9.6 11.0 8.6 0.5 0.0 0’
0 89.9 87.9 91.3
8.4 11.3 7.7 25.1 15.7 20‘ 20.9 66.5 72.0 72.3
25.7 18.7 14.0 57.1 75.1 50.0’ 58.8 17.2 3.9 36.0
a The M S technique is incapable of independently determining an olefin concentration, These reported values (which were also used in the program for calculating the aromatics and saturates from the M S data) are estimates made with knowledge of the sample history and its FIA and Br No. analyses.
of each hydrocarbon type can be performed relatively easily by MS as well as NMR techniques. Also, the absence of olefii and the very small amounts of S-, 0-, and N-containing compounds make this sample ideal for FIA analysis. However, an MS investigation of the fractions from the FIA separation clearly shows the same overlap of aromatic and saturate molecules previously mentioned. Therefore, the FIA method may only be a useful tool in establishing trends within a series of samples rather than in determining exact values of content of hydrocarbon types. In the case of sample 2, some complications may arise for FIA and MS methods because of the presence of olefins and S- and N-containing compounds. For the MS analysis, a predetermination of the olefins must be made by another technique. The amount of olefins obtained from the Br No. is believed to be a maximum value because an additional consumption of bromine by S- and N-containing compounds cannot be excluded. In comparison with the other methods, FIA gives a lower content of saturates, and this suggests that an incomplete separation (Le., an overlap of saturate fraction with the olefinic fraction) is the reason for the higher olefin
value determined by the FIA. Depending on whether the FIA or Br No. olefin content is used in the MS analysis, the results will subsequently be affected and the accuracy of the MS method becomes questionable. In the case of sample 2, an olefin content of 20% was assumed for the MS calculations. From an analytical point of view sample 3 is an example of a very complex fraction, with special problems arising because of the presence of dienes. The estimate of olefins from the Br No. is affected in particular. If one attempts a recalculation of the NMR results from an atomic to a molecular basis (vide supra), logical results are not obtained. For example, the molecular olefin content (Table 111) from the NMR analysis is unquestionablytoo high. The value may be affected by the assumption that each double bond is on a separate molecule, which is not the case of a diene. Also, aromatic ring substituents containing double bonds, if present, are counted twice, once as an aromatic molecule and once as an olefinic molecule. The interference of the dienes can be corrected and accounted for by estimating the diene content. However, the diene correction will only add to the uncertainty of the NMR results expl;essed on a molecular basis because of the number of previous and current assumptions and corrections needed for the calculation. The presence of dienes will markedly affect MS results as well. The tendency of dienes to be strongly adsorbed on a solid surface will result in a slow elution during the FIA analysis and most likely lead to their inclusion in the aromatic fraction. It appears that a quantitative analysis on a molecular basis for hydrocarbon types in refined, low olefin content products can be performed quickly and accurately by MS techniques. The evaluation of samples containing dienes or large amounts of olefins and heterocyclic compounds can be performed by MS and NMR methods but with a sacrifice in accuracy as is clearly illustrated by sample 3. The interference of heteroatoms restricts the use of MS techniques in hydrocarbon group determinations only to commercial fuels (2). In the case of synthetic liquids derived from coal and tar sands, the heteroatom content is usually high. Heterocyclic rings, aryl ethers, phenols, and anilines represent a substantial portion of heteroatom-containing species (8), as other heteroatom-containing structures will hardly survive conditions applied during processing of synthetic fuels. In lH and 13CNMR analysis such molecules are included in aromatic fractions (9). This makes NMR techniques more suitable for the analysis of synthetic materials compared to MS techniques. The NMR results at the atomic level, Le., the percentages of aromatic, olefinic, and saturate carbons and hydrogens, appear to be the least affected by various interferences and involve the fewest assumptions. The ease with which these results can be obtained and their accuracy make the NMR method a promising one. The ability to analyze the heaviest fractions, including coals, is of extraordinary importance as well. Correlating these NMR results with various parameters and properties of the products may therefore be very helpful. The marked difference in properties of the three samples analyzed in this work can, for example, be clearly recognized from either the NMRs atomic carbon or hydrogen distributions shown in Table I. ACKNOWLEDGMENT The authours wish to acknowledge P. Lafontaine and D. Sorensen for their technical assistance. LITERATURE CITED (1) Fraser, J. M. Anal. Chem. 1977, 49, 231R. (2) “Petroleum and Lubricants (11)”; American Soclety for Testing Materials: Phlladelphia, PA, 1976; p 24. (3) Clutter, D. R.; Petrakis, LeonMas; Stenger, R. L.; Jensen, R. K. Anal. Chem. 1972, 44, 1395.
187
Anal. Chem. 1981, 53, 187-189 (4) "Petroleum Products and Lubricants (I)"; American Society for Testing Materials: Philadelphia, PA, 1976; p 24. (5) Ozubko, R. Anal. Cbem., in press. (6) Brown, J. K.; Ladner, W. R. Fuell960,39, 87. (7) Williams, R. 8.; Chiamberlaln, N. F. "Proc.-WorM Pet. Congr., 6th 1963, Section V, 17. (8) Sternberg, H. W.; Raymond, R.; Akhtar, S. ACS Symp. Ser. 1975,
No. 20, 111. (9) Snape, C. E.; Ladner, W. R.; Battle, K. D. Anal. Chem. 1979, 57, 2189.
RECEIVED for review April 2,1980. Accepted October 20,1980.
Medium-Resolution Mass Spectrometry as a Nitrogen Compound Specific Detector Emilio J. Gallegos Chevron Research Conrpany, Richmond, Callfornia 94802
A gas chromatograph or a direct-insertion probe interfaced to a medlum-resolution mass spectrometer set at about 3000 resolution is used in the multiple Ion detection (MID)mode
to monltor the Intensity of the CH2N+ ion at m / q 28. This conflguratlon converts the system Into a nltrogen compound specific detector. Practicability of this system in terms o? levels of detection, quantitation, etc., is demonstrated by use of authentic nitrogen compound mixtures, gasoiines, and coals. The significance of CO+ and C2H,+ Ion monitoring, also found at m / q 28, Is dlscussed.
Nitrogen compounds contribute to gum formation in fuels and on the pistons, valves, and rings of internal combustion engines. They are ah0 process catalyst poisons. The use of increasingly heavy fractions of petroleum and, in the near future, the refining of coal and shale oils which introduce considerably higher levels of nitrogen will increase the importance of isolating and identifying nitrogen compounds both in the finished product and in the feedstocks to refinery processes. A number of element-specific detectors have been described ( I , 2). They use a bead of silica doped with an alkali metal. Nitrogen, phosphorus, and sulfur compounds have been detected, although the mechanism of the detector is not well understood, and quantitative reproducibility is difficult to achieve. The method descrilbed here depends upon the detection of the CHzN+ion using a medium-resolution mass spectrometer. This will allow detection of all volatile nitrogen-containing compounds with N-C-H bonding.
EXPERIMENTAL SECTION Two different gas chromatograph-mass spectrometer-computer (GC-MS-C)systems were used in this work. One used a 305-m squalane-coated 0.5 mm i.d. stainless steel capillary column coupled to an AEI M13-9 Nier Johnson double focusing mass spectrometer through a 0.0025-mm vinyl methyl silicon membrane interface. The other system used a 75-m, Dexil400 coated 0.25 mm i.d. glass capillary column coupled directly to the source of a Nuclide 12-90-Gsingle focusing mass spectrometer. The mass resolution of both systems was set to about 3000. An all-quartz direct-insertion probe described elsewhere (3) was used in conjunction with the MS-9 mass spectrometer for analysis of the solid coal samples. Data were gathered at 5-s intervals in the multiple ion detection, MID, mode for the probe work. The data taken from GC-MS-C were gathered at 2-s intervals in the MID mode. 0003-2700/81/0353-0187$01.00/0
Magnetic scanning for fuel mass spectra were gathered at 4.553 intervals on the Nuclide-Dexil system.
RESULTS AND DISCUSSION Figure 1is reproduced from the mlq 28 profile for pyridine. Nz+and CO+ are due to background. This neverthelessshows all possible fragment ions at nominal mass mlq 28. These are CO+, N2+,CH2N+,C13CH3+,and CzH4+taken at about 5000 resolution. At a resolution of 3000, CHZN' and C2H4' may be monitored independently without interference from the others. Table I gives names, boiling point, structural formula, and percent of total ionization of the CHzN+ ion in the mass spectra of 21 of nitrogen-containing compound types expected in gasoline range samples. They are numbered in order of elution time through a Dexil 400 glass capillary column. Figure 2 shows the CHzN+,CzH4' mass chromatograms, and the reconstructed gas chromatograms (RGC) of a standard 21-nitrogen compound mix. These data were generated from the Nuclide-Dexil system. The oven was heated from 50 to 300 "C at 4 OC/min. The numbers on the RGC trace refer to the standard nitrogen compounds listed in Table I. The RGC represents a separate run in which total mass spectra were acquired in the magnet scan mode. The CH2N+and C2H4+ mass chromatograms were acquired by scanning only over the mlq 28 multiplet. Note that the C2H4+ ion intensity for all of the nitrogen compounds is 25% or less than that of CHZN' ion a t mlq 28. Figure 3 shows the CH2N+and CzH4+mass chromatograms and the inverted gas chromatograph flame ionization detector trace of gasoline distillation cuts 20-23. These data were generated from the 305-m MS9-squalane system. Note that the position of nitrogen compounds (see arrow) changes with respect to the hydrocarbon going from cut 20 through cut 23. These data suggest that there is mainly one nitrogen component and that it is the same one in the four cuts. The compound was identified to be Cmethylbenzenamineat a level of about 400 ppm in cut 21. Subsequent dilutions of this cut indicate that the lower limit of detection for this nitrogen compound by the method described here is approximately 5 PPm. A similar analysis was made on the pyrolytic effluent of six United States coals. The coals were finely powdered and introduced into the mass spectrometer via an all-quartz, direct-insertion probe, which places the sample 1cm from the electron beam. They were heated from approximately 100 "C to approximately 600 "C at 10 "C/min. 0 1981 American Chemical Soclety
