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Qalitative mass-spectrometric analysis of types of compound in oil fractions and products of refining
QALITATIVE MASS-SPECTROMETRIC ANALYSIS OF TYPES OF COMPOUND IN OIL FRACTIONS AND PRODUCTS OF REFINING* YE. S. BRODSKII (Received 27 Augus$ 1976) TH~ possibility of identifying the molecular structure of organic compounds using mass-spectra is determined by the selectivity of decomposition in ionization and the formation of ions with weights corresponding to given structural fragments of molecules. In mass-spectra of composite mixtures such as petroleum fractions, products of processing oils, shales, charcoals, etc. including a large number of individual compounds, ions corresponding to mass-spectra of individual compounds cannot normally be isolated because of mutual superimposition. Furthermore, the variety of methods of molecular breakdown of various compounds reduces the selectivity of mass-spectra of composite mixtures. Conventional methods of identifying individual compounds according to ions with given weights or weight differences of ions, corresponding to structural fragments of molecules, are inapplicable in this case. Characteristics of types of compound in composite mixtures are normally based on the presence in mass-spectra of organic compounds of homologous series of ions, corresponding to most probable processes of decomposition [1]. Alkylbenzenes are characterized by Cull+n_7 ions with masses of 91, 105, 119, etc., alkylnaphthalenes, by CnH2n-lS + ions with masses of 141, 155, 169, etc. However, mass-spectra of low resolution only contain seven pairs of different homologous series, corresponding to molecular and basic fragmented ions. Many types of compound therefore form series of isobar molecular and characteristic fragmented ions. Furthermore, characteristic ions may sometimes be in several and not one homologous series. Thus, mass-spectra of diphenyl paraffins contain characteristic fragmented CnH~+,_15 ions, which correspond to separation of alkyl radicals from molecular ions and intens~ peaks of fragmented C~H2+_7 ions, typical of alkylbenzenes. Even in relatively simple mixtures containing not more than seven types of compound, types of compound cannot always be identified with sufficient accuracy simply because some homologous ionic series are present in the mass-spectrum. A whole number of reliable analytical criteria have to be used for reliable identification. Criteria ensuring the identification of types of compound in composite mixtures may be the intensity distributions of peaks between homologous series of ions and inside each series. Groups of ions and not individual ions are * Neftekhimiya 17, No. 3, 473-480, 1977. 115
116
Y~.. S. BRODSKII et al.
therefore the characteristic elements of mass-spectra of composite mixtures. Each type o f compound is normally characterized by the existence of certain groups of ions situated in one or several homologous series and corresponding to the most probable process of molecular breakdown. The main groups of ions are molecular ions of M +, dehydrogenated molecular ions (M--H) +, (M--R) + ions, where 1% is the substituent and other ions correspond to given structural fragments. Each type of compound has a typical group of fragments and therefore a given number of characteristic groups of ions. Tricyclic naphthene hydrocarbons are characterized by the following groups of ions: molecular M + in the homologous series CnH~_4, of which the position is determined b y the molecular weights of these compounds in the mixture; (M--I~) + ions (R--alkyl) in the homologous series of CnH2n-5, + the weights of which are determined by the condensed ring system and the number of carbon atoms in the remaining substituents; Cn]-I2n-5 + ions with lower weights than the previous ones formed during the decomposition of the cyclic system, etc. (Fig. 1). Thiamononaphthenes are characterized b y the existence of groups of ions: M +, Cntt2n-lS+, [(M--R)-H~S] +, [(M--R)-HS] +, etc.
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FIe. 1. Distribution of peak intensities in characteristic groups of ions in average massspectra of tricyclic naphthene hydrocarbons (a) and thiamonocyclanes (b). Differences in the fine structure of molecular fragments of individual compounds of each type are reflected in some redistribution of integral intensities of peaks of individual groups of ions and in a variation of the distribution of peak intensities inside the groups. Characteristic groups of ions in mass-spectra of composite mixtures, reflecting main trends of molecular decomposition of types of compound form
Types of compound in oil fractions
117
"group mass-spectra" [2]. They are determined by the position of characteristic groups of ions in homologous series, their integral intensities and the individual distribution of peaks in these groups. Transition from the conventional to the group mass-spectrum corresponds to transition from the idea of mass-spectrum in the form of several individual ions with given weights to the idea represented by a number of groups of ions corresponding to average structural fragments of molecules. On the other hand, this idea is similar to the mass-spectra of individual compounds, the difference being that elements of the group mass-spectrum correspond to average molecular fragments of individual compounds of a given type. As with mass-spectra of individual compounds, structural fragments enable average molecules of compound types analysed to be formed. On the other hand, each of the groups of ions is characterized by the distribution of peak intensities, the profile of which depends both on the type of compound and the individual components of the composite mixture. Distributions of characteristic ionic groups according to homologous series and inside homologous series form the main characteristics for the identification and qualitative analysis of types of compound in composite mixtures. The mass-spectrum of a composite mixture is analysed by comparing distributions of overall peak intensities of characteristic groups of ions and positions of their centres and the type of individual distribution of peaks in characteristic groups of ions. Centres of distributions of characteristics groups of ions and corresponding overall peak intensitie~ form a linear spectrum, which may be compared directly with the same spectra of standard mixtures of known composition. Figure 2 shows overall peak intensities of characteristic groups of ions in the mass-spectrum of a fraction isolated from diesel fuel and massspectra of individual types of saturated sulphur compound. The latter were obtained by averaging mass-spectra of individual compounds described in the literature [3] and obtained by the authors. The presence in the mass-spectrum of the mixture of characteristic groups of molecular ions in homologous series CnH2nS+, CnH2n_2S+, CnH2~_4S+ and even fragmented ions CnH2n_IS+ (n-~4-9), CnH2n_aS+ (n=8-12), CnH2n_~S+ (n=13-17) in ranges corresponding to the separation of alkyl chains from corresponding molecular ions is evidence of the the presence of thiamono-, thiabi- and thiatricyclanes in the mixture. The presence of thiabi- and thiatricyclanes is also confirmed by groups of ions CnH2,~-3S+ ( n z 5 - 8 ) , CnH2n_sS+ (n=10-13) in the mixture. Groups of + ,, CnH2~_ + 8H CnH 2n-~ + are typical of all thiacyclanes hydrocarbon ions C~H2nand provide little information in this case. A group of CnH2~-sS+ ions ( n = 8-11) points to the possible presence of thiadecalins or thiaindanes in the mixture. Analysis of the distribution of peak intensities in characteristic groups of ions may provide further information for the identification of types of compound. Types of compound of different structures with ions of isobar characteristics often differ in the distribution of peak intensities in these groups of
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Types of compound in oil fractions
119
ions. For example, aromatic compounds in oil fractions normally have asymmetrical distribution of peaks of characteristic fragmented ions (M--R) + with a steep front in the range of lower mass numbers. This is due to the high stability of the aromatic system so that ions formed b y the separation of alkyl radicals are less likely to break down further. Naphthene-aromatic compounds are characterized b y a more even distribution of peak intensities of ions (M--R) +, which is close to symmetrical distribution, as a result of further breakdown of the saturated part of the naphthene-aromatic nucleus of the molecule. Peaks of molecular ions of mixtures containing the first representatives of corresponding homologous series are distributed in a similar way. Molecular ions of the first members of homologous series of aromatic compounds are very stable and their peaks are of relatively high intensity. The distribution o f peaks of molecular ions in this case has a steep front in the region of lower mass numbers. Molecular ions of naphthene-aromatic compounds are less stable and are very likely to break down during the separation of naphthene rings. The distribution of peaks of molecular ions is therefore more symmetrical than that of aromatic compounds [2]. Although the distribution of peak intensities in characteristic groups of ions is most often represented b y a smooth curve with one maximum, for some types of compound, of which the structure accounts for the formation of certain ions, which are relatively very stable, distribution follows a different pattern. Sterane compounds in oil fractions are characterized b y well expressed peaks of ions with masses of 203, 217, 218, and 259, which are well separated on the background of other peaks in corresponding homologous series. Tetracyclic naphthene hydrocarbons of another structure have smooth distribution of peaks of ions in these homologous series with one broad maximum. The presence of several maximum characteristics in the range of mass numbers indicated is a proof of the presence of sterane hydrocarbons in the mixutre. Triterpane hydrocarbons m a y be identified b y a similar method in the mixture according to the unusually high peak intensity of ions with masses of 191 and 192 isolated on the background of other peaks in corresponding homologous series. The identification of isoprenoid paraffin hydrocarbons is based the same w a y on the unusually high peak intensity of ions with masses of 113, 183, 253, etc. andions with masses 112, 182, 252, etc., when no molecular ions of other types of compound are superimposed on them. Analysis of distributions of peak intensities in characteristic groups of ions not only provides further information for the identification of types of compound, but also enables some structural characteristics of molecules such as the degree of condensation of rings, average length of substituents, average degree of substitution, etc. [4] to be evaluated. To isolate and analyse characteristic groups of ions in mass-spectra of composite mixtures, the theory of mass-spectrum m a y be used in the form of graphical distribution of peak intensities of ions in each homologous series
120
YE. S. BRODSKII
or in the form of a square table of homologous series of ions. Masses of adjacent. ions in horizontal lines of the Table differ b y one atomic unit of mass (a.u.m.), while in vertical columns, by 14 a.u.m. Tabulated or diagrammatic representation of mass-spectra of composite mixtures in the form of homologous series of ions enables all most typical features of mass-spectra to be readily dealt with and peaks to be isolated which are most prominent on the general background of the group and correspond to most stable ions formed.~s a result of specific processes of decomposition for certain types of compound. This group is formed first of all by ions M+, (M--H)+, (M--R)+, the overall intensity of which gives a first idea about the types of compound in the mixture. A more detailed identification is made using other characteristic groups of ion particles and the form of curves of peak intensity distribution in characteristic groups of ions. It is essential to bear in mind that some groups of ions may be due to isotope superimposition of ions of other elementary composition and not the formation of characteristic ions in molecular decomposition. To exclude possible errors in identification, tabulated or diagrammatic representations of mass-spectra may be used after considering the natural occurrence of isotopes. The Table shows the mass-spectrum of one of the fractions obtained b y adsorption separation of a sulphur-aromatic concentrate isolated from diesel fuel of Western Surgut petroleum. The maximum in the homologous series of + CnI'I2n- 7 ions starts from mass number 77 and not from mass number 91, as is typical of alkylbenzenes with a primary alkyl chain, whereby on transition to higher mass number of 91 and 105, peak intensity decreases. This is evidence of the absence of alkylbenzenes from the mixture with primary alkyl substituents. This type of distribution of these peaks together with the high peak intensity of the ion with a mass number of 78 indicates the presence of diphenylmethane type compounds and other phenyl-substituted compounds. The maximum distribution of peak intensities in this homologous series of ions in the range of mass numbers 119, 133 and 147 proves the possible presence of alkylbenzenes with secondary, or tertiary alkyl substituents or branched diphenylparaffins. Maximum distribution of peak intensities in this homologous series in the range of high mass numbers and the type of distribution in this region indicate the presence of alkylbenzthiophens. Similarly, the characteristic group of ions in the region of mass number 159, 173, etc. identifies naphthene benzothiophens, while the group of ions in the range of mass numbers 157, 171, etc., dinaphthene benzothiophens indicated. A group of ions in the range of mass numbers 135, 149, etc. points to the possible presence of thiaindanes and (or) thiadekalins in the mixture. It should be considered tha~ the most intensive peaks in this group of ions with masses 163, 177 and 191 are due to the isotope superimposition of CntI2n-11S+ ions containing a 34S isotope. A group of ions in the range of mass numbers 141, 155, etc. characterizes alkylnaphthalenes, in the region of 139, 153, etc.-- naphthenonaph-
Types of compound in oil fractions
121
¢ha]enes or diphenyls, 151, 165, etc. -- dinaphthenonaphthalenes, fluorens or dihydroanthracenes. Finally, the intensive peak of an ion with mass number 178 indicates the presence of unsubstituted phenanthrene or anthracene hydrocarbons. Overlapping isobar ions of different elementary compositions m a y be separated using the model distribution of peak intensities obtained either b y using simple mathematical models [5, 6], or standard mass-spectra of simpler mixtures to obtain these distributions for given compounds of this t y p e without noticeable distortions involved b y the superimposition of isobar ions. Figure 3 shows an example of the separation of isobar ions of CnH+n_9, CnH~,_laS+ and CnH2,-~s. + I 150
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Fro. 3. Distributions of peak intensities of isobar ions of CnH=n-9+ (1), C,H=,-1,sS+ (2) ~nd CnI-Ia.-~s + (3) and their overall distribution (4) in the mass-spectrum of a fraction obtained from adsorption separation of diesel fuel. Numerous series of isobar ions of different elementary composition m a y be ,separated b y high resolution mass-spectrometry. To separate C2Its-S, I-Ii=--(J doublets in the range of mass numbers of about 300 a resolution of about 3000-5000 is required, for CIId--O , NH2--O, O~--S, N20--CS doublets, 10,000-15000, for Ctt2--N, CO--N2 doublets, 20,000-30,000, while for Cs--HdS , Ctt--13C doublets, 100,000. Mass-spectrometers normally used for chemicM analysis cannot always ensure the requisite resolution for the separation o f all ions in the multiplets. In these cases the series of ions m a y be shown separately using model distributions. Mass-spectra of isobar ions of different elementary composition, obtained b y high-resolution mass-spectrometry or using model distributions are shown in various lists of the Table, in order to obtain simpler mass-spectra from a complex mass-spectrum with separated lines of isobar ions, each of which m a y be analysed separately. Distribution of peak intensities in homologous series is considerably simplified and subsequent separation and identification of types of compound are more accurate. The numerical representation of the mass-spectrum in tabulated homologous series of ions of the same elementary composition enables an efficient selection of analytical characteristics to be made for quantitative analysis.
|22
YE. S. BRODSKII
Thus, for types of compound, characterized by the presence of long chains, the overall peak intensity of (M--R) + ions is a fairly accurate analytical characteristic [7]. For compounds with comparatively short alkyl substituents, the overall peak intensity of ~ + and (I~--R) + ions should be used as analytical characteristic. For compounds with a system of condensed saturated rings it MASS-SPECTRUM
OF A THIOPHEN
CONCENTRATE
ISOLATED
FROM
DIESEL
FUEL
-*/e Intensity 17
18
19i
20
21
22
23
24
25
31 5 45 140 59 74 73 29 87 90 101 38 115 700 129 380 143 230 157 240 171 320 185 170 199 120 213 62
32 140 46 7 60 8 74 63 88 50 102 80 116 190 130 I00 144 80 158 100 172 140 186 95 200 80 214 41
33 12 47 9 61 9 75 62 89 L20 [03 90 [17 HO :31 ~60 .45 ~80 .59 ~70 .73 L40 .87 ~50 ~01 .80 ~15 .50
34
35
36
37 13 51 120 65 100 79 140 93 50 107 27 121 52 135 100 149 110 163 240 177 3OO 191 22O 2O5 llO 219 62
38 30 52 140 66 20 80 80 94 38 108 17 122 14 136 17 150 38 164 67 178 170 192 80 206 40 220 23
39 240 53 80 67 62 81 72 95 60 109 37 123 26 137 18 151 100 165 340 179 160 193 90 207 57 221 29
48 -62 27 76 140 90~ 30 104 48 118 100 132 110 146 ll0 160 440 174 400 188 190 202 110 216 67
49 50 11 120 63 64 110 67 77 78 400 840 91 92 320 57 105 106 260 58 119 120 800 170 133 134 1100 680 147 148 580 300 161 162 1950 810 175 176 2600 1400 189 190 1400 88o 203 204 560 520 217 218 210 310
is also essential to include ions of the subsequent ions in analytical Types istic
in
analysis according
groups
intensities
208 46 222 22
breakdown
28 860 42 34 56 31 70 41 84 42 98 14 112 l0 126 58 140 70 154 280 168 150 182 100 196 90 210 90 224 95
of primary
29 llO 43 160 57 100 71 90 85 55 99 19 113 31 127 230 141 1200 155 1280 169 900 183 420 197 150 211 65 225 58
30 14 44 360 58 25 72 11 86 35 100 15 114 37 128. 720 142 70o ]56 900, 170 800 184 420 198 220 212 110 226 75
(M--R) +
characteristics.
of compound
qualitative
27 140 41 260 55 150 69 110 83 8O 97 43 111 26 125 25 139 2O0 153 460 167 260 181 220 195 2OO 209 170 223 130
of ions
in
composite
mixtures
may
thus
to analytical" criteria, which
certain
of ions of these groups
homologous
series,
be
subjected
include
relative
and the form of distribution
to
character-
integral
peak
of peak inten-
,Types of compound in oil fractions
123
sities inside characteristic groups of ions. Types of compound in composite mixtures m a y be identified by using suitable and readily automated (by a computer) methods for the analysis of individual organic compounds according to mass-spectra on the one hand and methods of analysis of the form of undivided peaks in gas chromatography, or absorption bands in adsorption spectra -- on the other. To analyse mixtures of certain origin, for example petroleum fractions, a catalogue m a y be compiled of the type distributions o f characteristic groups of ions in mass-spectra and peaks of ions in these groups in mass-spectra of various types of compound. SUMMARY
]. Types of compound m a y be identified in composite mixtures from a combination of analytical data including the presence of typical groups of ions in given homologous series, relative integral peak intensities of ions o f these groups and the type of distribution of peak intensities in typical ionic groups. 2. Overlapping homologous series of isobar ions of different elementary compositions m a y be classified using high resolution mass spectrometry or model distributions of peak intensities in characteristic groups of ions. REFERENCES
1. A. A. POLYAKOVA and R. A. KHMEL'NITSKII, Mass-spektrometriya v organicheskoi khimii (Mass-Spectrometry in Organic Chemistry). Khimiya, Leningrad, 1972 2. Yo. S. BRODSKII and Yu. M. GOL'BERG,Zh. analit, khimii 31,565, 1976 3. E. STENHAGEN, S. ABRAHAMSONand F. W. MCLAFFERTY,Atlas of Mass Spectial Data; 1969 4. Ye. S. BRODSKH, Zh. analit, khimii 31, 2216, 1974 5. Ye. S. BRODSKII, I. M. LUKASHENKO, Yu. M. GOL'BERG mid V. G. LEBEDEVSKAYA, Zh. analit, khimii 29, 2026, 1974 6. C. J. ROBINSON, Analit. Chem. 41, 1549, 1969 7. Yo. S. BRODSKII, I. M. LUKASHENKO and Yu. A. VOLKOV, Khimiya i tekhnol. topliv i masel, :No. 2, 56, 1976
116
Y~.. S. BRODSKII et al.
therefore the characteristic elements of mass-spectra of composite mixtures. Each type o f compound is normally characterized by the existence of certain groups of ions situated in one or several homologous series and corresponding to the most probable process of molecular breakdown. The main groups of ions are molecular ions of M +, dehydrogenated molecular ions (M--H) +, (M--R) + ions, where 1% is the substituent and other ions correspond to given structural fragments. Each type of compound has a typical group of fragments and therefore a given number of characteristic groups of ions. Tricyclic naphthene hydrocarbons are characterized by the following groups of ions: molecular M + in the homologous series CnH~_4, of which the position is determined b y the molecular weights of these compounds in the mixture; (M--I~) + ions (R--alkyl) in the homologous series of CnH2n-5, + the weights of which are determined by the condensed ring system and the number of carbon atoms in the remaining substituents; Cn]-I2n-5 + ions with lower weights than the previous ones formed during the decomposition of the cyclic system, etc. (Fig. 1). Thiamononaphthenes are characterized b y the existence of groups of ions: M +, Cntt2n-lS+, [(M--R)-H~S] +, [(M--R)-HS] +, etc.
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FIe. 1. Distribution of peak intensities in characteristic groups of ions in average massspectra of tricyclic naphthene hydrocarbons (a) and thiamonocyclanes (b). Differences in the fine structure of molecular fragments of individual compounds of each type are reflected in some redistribution of integral intensities of peaks of individual groups of ions and in a variation of the distribution of peak intensities inside the groups. Characteristic groups of ions in mass-spectra of composite mixtures, reflecting main trends of molecular decomposition of types of compound form
Types of compound in oil fractions
117
"group mass-spectra" [2]. They are determined by the position of characteristic groups of ions in homologous series, their integral intensities and the individual distribution of peaks in these groups. Transition from the conventional to the group mass-spectrum corresponds to transition from the idea of mass-spectrum in the form of several individual ions with given weights to the idea represented by a number of groups of ions corresponding to average structural fragments of molecules. On the other hand, this idea is similar to the mass-spectra of individual compounds, the difference being that elements of the group mass-spectrum correspond to average molecular fragments of individual compounds of a given type. As with mass-spectra of individual compounds, structural fragments enable average molecules of compound types analysed to be formed. On the other hand, each of the groups of ions is characterized by the distribution of peak intensities, the profile of which depends both on the type of compound and the individual components of the composite mixture. Distributions of characteristic ionic groups according to homologous series and inside homologous series form the main characteristics for the identification and qualitative analysis of types of compound in composite mixtures. The mass-spectrum of a composite mixture is analysed by comparing distributions of overall peak intensities of characteristic groups of ions and positions of their centres and the type of individual distribution of peaks in characteristic groups of ions. Centres of distributions of characteristics groups of ions and corresponding overall peak intensitie~ form a linear spectrum, which may be compared directly with the same spectra of standard mixtures of known composition. Figure 2 shows overall peak intensities of characteristic groups of ions in the mass-spectrum of a fraction isolated from diesel fuel and massspectra of individual types of saturated sulphur compound. The latter were obtained by averaging mass-spectra of individual compounds described in the literature [3] and obtained by the authors. The presence in the mass-spectrum of the mixture of characteristic groups of molecular ions in homologous series CnH2nS+, CnH2n_2S+, CnH2~_4S+ and even fragmented ions CnH2n_IS+ (n-~4-9), CnH2n_aS+ (n=8-12), CnH2n_~S+ (n=13-17) in ranges corresponding to the separation of alkyl chains from corresponding molecular ions is evidence of the the presence of thiamono-, thiabi- and thiatricyclanes in the mixture. The presence of thiabi- and thiatricyclanes is also confirmed by groups of ions CnH2,~-3S+ ( n z 5 - 8 ) , CnH2n_sS+ (n=10-13) in the mixture. Groups of + ,, CnH2~_ + 8H CnH 2n-~ + are typical of all thiacyclanes hydrocarbon ions C~H2nand provide little information in this case. A group of CnH2~-sS+ ions ( n = 8-11) points to the possible presence of thiadecalins or thiaindanes in the mixture. Analysis of the distribution of peak intensities in characteristic groups of ions may provide further information for the identification of types of compound. Types of compound of different structures with ions of isobar characteristics often differ in the distribution of peak intensities in these groups of
118
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Types of compound in oil fractions
119
ions. For example, aromatic compounds in oil fractions normally have asymmetrical distribution of peaks of characteristic fragmented ions (M--R) + with a steep front in the range of lower mass numbers. This is due to the high stability of the aromatic system so that ions formed b y the separation of alkyl radicals are less likely to break down further. Naphthene-aromatic compounds are characterized b y a more even distribution of peak intensities of ions (M--R) +, which is close to symmetrical distribution, as a result of further breakdown of the saturated part of the naphthene-aromatic nucleus of the molecule. Peaks of molecular ions of mixtures containing the first representatives of corresponding homologous series are distributed in a similar way. Molecular ions of the first members of homologous series of aromatic compounds are very stable and their peaks are of relatively high intensity. The distribution o f peaks of molecular ions in this case has a steep front in the region of lower mass numbers. Molecular ions of naphthene-aromatic compounds are less stable and are very likely to break down during the separation of naphthene rings. The distribution of peaks of molecular ions is therefore more symmetrical than that of aromatic compounds [2]. Although the distribution of peak intensities in characteristic groups of ions is most often represented b y a smooth curve with one maximum, for some types of compound, of which the structure accounts for the formation of certain ions, which are relatively very stable, distribution follows a different pattern. Sterane compounds in oil fractions are characterized b y well expressed peaks of ions with masses of 203, 217, 218, and 259, which are well separated on the background of other peaks in corresponding homologous series. Tetracyclic naphthene hydrocarbons of another structure have smooth distribution of peaks of ions in these homologous series with one broad maximum. The presence of several maximum characteristics in the range of mass numbers indicated is a proof of the presence of sterane hydrocarbons in the mixutre. Triterpane hydrocarbons m a y be identified b y a similar method in the mixture according to the unusually high peak intensity of ions with masses of 191 and 192 isolated on the background of other peaks in corresponding homologous series. The identification of isoprenoid paraffin hydrocarbons is based the same w a y on the unusually high peak intensity of ions with masses of 113, 183, 253, etc. andions with masses 112, 182, 252, etc., when no molecular ions of other types of compound are superimposed on them. Analysis of distributions of peak intensities in characteristic groups of ions not only provides further information for the identification of types of compound, but also enables some structural characteristics of molecules such as the degree of condensation of rings, average length of substituents, average degree of substitution, etc. [4] to be evaluated. To isolate and analyse characteristic groups of ions in mass-spectra of composite mixtures, the theory of mass-spectrum m a y be used in the form of graphical distribution of peak intensities of ions in each homologous series
120
YE. S. BRODSKII
or in the form of a square table of homologous series of ions. Masses of adjacent. ions in horizontal lines of the Table differ b y one atomic unit of mass (a.u.m.), while in vertical columns, by 14 a.u.m. Tabulated or diagrammatic representation of mass-spectra of composite mixtures in the form of homologous series of ions enables all most typical features of mass-spectra to be readily dealt with and peaks to be isolated which are most prominent on the general background of the group and correspond to most stable ions formed.~s a result of specific processes of decomposition for certain types of compound. This group is formed first of all by ions M+, (M--H)+, (M--R)+, the overall intensity of which gives a first idea about the types of compound in the mixture. A more detailed identification is made using other characteristic groups of ion particles and the form of curves of peak intensity distribution in characteristic groups of ions. It is essential to bear in mind that some groups of ions may be due to isotope superimposition of ions of other elementary composition and not the formation of characteristic ions in molecular decomposition. To exclude possible errors in identification, tabulated or diagrammatic representations of mass-spectra may be used after considering the natural occurrence of isotopes. The Table shows the mass-spectrum of one of the fractions obtained b y adsorption separation of a sulphur-aromatic concentrate isolated from diesel fuel of Western Surgut petroleum. The maximum in the homologous series of + CnI'I2n- 7 ions starts from mass number 77 and not from mass number 91, as is typical of alkylbenzenes with a primary alkyl chain, whereby on transition to higher mass number of 91 and 105, peak intensity decreases. This is evidence of the absence of alkylbenzenes from the mixture with primary alkyl substituents. This type of distribution of these peaks together with the high peak intensity of the ion with a mass number of 78 indicates the presence of diphenylmethane type compounds and other phenyl-substituted compounds. The maximum distribution of peak intensities in this homologous series of ions in the range of mass numbers 119, 133 and 147 proves the possible presence of alkylbenzenes with secondary, or tertiary alkyl substituents or branched diphenylparaffins. Maximum distribution of peak intensities in this homologous series in the range of high mass numbers and the type of distribution in this region indicate the presence of alkylbenzthiophens. Similarly, the characteristic group of ions in the region of mass number 159, 173, etc. identifies naphthene benzothiophens, while the group of ions in the range of mass numbers 157, 171, etc., dinaphthene benzothiophens indicated. A group of ions in the range of mass numbers 135, 149, etc. points to the possible presence of thiaindanes and (or) thiadekalins in the mixture. It should be considered tha~ the most intensive peaks in this group of ions with masses 163, 177 and 191 are due to the isotope superimposition of CntI2n-11S+ ions containing a 34S isotope. A group of ions in the range of mass numbers 141, 155, etc. characterizes alkylnaphthalenes, in the region of 139, 153, etc.-- naphthenonaph-
Types of compound in oil fractions
121
¢ha]enes or diphenyls, 151, 165, etc. -- dinaphthenonaphthalenes, fluorens or dihydroanthracenes. Finally, the intensive peak of an ion with mass number 178 indicates the presence of unsubstituted phenanthrene or anthracene hydrocarbons. Overlapping isobar ions of different elementary compositions m a y be separated using the model distribution of peak intensities obtained either b y using simple mathematical models [5, 6], or standard mass-spectra of simpler mixtures to obtain these distributions for given compounds of this t y p e without noticeable distortions involved b y the superimposition of isobar ions. Figure 3 shows an example of the separation of isobar ions of CnH+n_9, CnH~,_laS+ and CnH2,-~s. + I 150
L
100 50 3 103 131 159 I87 2/5 2/¢3 271 rn/e
Fro. 3. Distributions of peak intensities of isobar ions of CnH=n-9+ (1), C,H=,-1,sS+ (2) ~nd CnI-Ia.-~s + (3) and their overall distribution (4) in the mass-spectrum of a fraction obtained from adsorption separation of diesel fuel. Numerous series of isobar ions of different elementary composition m a y be ,separated b y high resolution mass-spectrometry. To separate C2Its-S, I-Ii=--(J doublets in the range of mass numbers of about 300 a resolution of about 3000-5000 is required, for CIId--O , NH2--O, O~--S, N20--CS doublets, 10,000-15000, for Ctt2--N, CO--N2 doublets, 20,000-30,000, while for Cs--HdS , Ctt--13C doublets, 100,000. Mass-spectrometers normally used for chemicM analysis cannot always ensure the requisite resolution for the separation o f all ions in the multiplets. In these cases the series of ions m a y be shown separately using model distributions. Mass-spectra of isobar ions of different elementary composition, obtained b y high-resolution mass-spectrometry or using model distributions are shown in various lists of the Table, in order to obtain simpler mass-spectra from a complex mass-spectrum with separated lines of isobar ions, each of which m a y be analysed separately. Distribution of peak intensities in homologous series is considerably simplified and subsequent separation and identification of types of compound are more accurate. The numerical representation of the mass-spectrum in tabulated homologous series of ions of the same elementary composition enables an efficient selection of analytical characteristics to be made for quantitative analysis.
|22
YE. S. BRODSKII
Thus, for types of compound, characterized by the presence of long chains, the overall peak intensity of (M--R) + ions is a fairly accurate analytical characteristic [7]. For compounds with comparatively short alkyl substituents, the overall peak intensity of ~ + and (I~--R) + ions should be used as analytical characteristic. For compounds with a system of condensed saturated rings it MASS-SPECTRUM
OF A THIOPHEN
CONCENTRATE
ISOLATED
FROM
DIESEL
FUEL
-*/e Intensity 17
18
19i
20
21
22
23
24
25
31 5 45 140 59 74 73 29 87 90 101 38 115 700 129 380 143 230 157 240 171 320 185 170 199 120 213 62
32 140 46 7 60 8 74 63 88 50 102 80 116 190 130 I00 144 80 158 100 172 140 186 95 200 80 214 41
33 12 47 9 61 9 75 62 89 L20 [03 90 [17 HO :31 ~60 .45 ~80 .59 ~70 .73 L40 .87 ~50 ~01 .80 ~15 .50
34
35
36
37 13 51 120 65 100 79 140 93 50 107 27 121 52 135 100 149 110 163 240 177 3OO 191 22O 2O5 llO 219 62
38 30 52 140 66 20 80 80 94 38 108 17 122 14 136 17 150 38 164 67 178 170 192 80 206 40 220 23
39 240 53 80 67 62 81 72 95 60 109 37 123 26 137 18 151 100 165 340 179 160 193 90 207 57 221 29
48 -62 27 76 140 90~ 30 104 48 118 100 132 110 146 ll0 160 440 174 400 188 190 202 110 216 67
49 50 11 120 63 64 110 67 77 78 400 840 91 92 320 57 105 106 260 58 119 120 800 170 133 134 1100 680 147 148 580 300 161 162 1950 810 175 176 2600 1400 189 190 1400 88o 203 204 560 520 217 218 210 310
is also essential to include ions of the subsequent ions in analytical Types istic
in
analysis according
groups
intensities
208 46 222 22
breakdown
28 860 42 34 56 31 70 41 84 42 98 14 112 l0 126 58 140 70 154 280 168 150 182 100 196 90 210 90 224 95
of primary
29 llO 43 160 57 100 71 90 85 55 99 19 113 31 127 230 141 1200 155 1280 169 900 183 420 197 150 211 65 225 58
30 14 44 360 58 25 72 11 86 35 100 15 114 37 128. 720 142 70o ]56 900, 170 800 184 420 198 220 212 110 226 75
(M--R) +
characteristics.
of compound
qualitative
27 140 41 260 55 150 69 110 83 8O 97 43 111 26 125 25 139 2O0 153 460 167 260 181 220 195 2OO 209 170 223 130
of ions
in
composite
mixtures
may
thus
to analytical" criteria, which
certain
of ions of these groups
homologous
series,
be
subjected
include
relative
and the form of distribution
to
character-
integral
peak
of peak inten-
,Types of compound in oil fractions
123
sities inside characteristic groups of ions. Types of compound in composite mixtures m a y be identified by using suitable and readily automated (by a computer) methods for the analysis of individual organic compounds according to mass-spectra on the one hand and methods of analysis of the form of undivided peaks in gas chromatography, or absorption bands in adsorption spectra -- on the other. To analyse mixtures of certain origin, for example petroleum fractions, a catalogue m a y be compiled of the type distributions o f characteristic groups of ions in mass-spectra and peaks of ions in these groups in mass-spectra of various types of compound. SUMMARY
]. Types of compound m a y be identified in composite mixtures from a combination of analytical data including the presence of typical groups of ions in given homologous series, relative integral peak intensities of ions o f these groups and the type of distribution of peak intensities in typical ionic groups. 2. Overlapping homologous series of isobar ions of different elementary compositions m a y be classified using high resolution mass spectrometry or model distributions of peak intensities in characteristic groups of ions. REFERENCES
1. A. A. POLYAKOVA and R. A. KHMEL'NITSKII, Mass-spektrometriya v organicheskoi khimii (Mass-Spectrometry in Organic Chemistry). Khimiya, Leningrad, 1972 2. Yo. S. BRODSKII and Yu. M. GOL'BERG,Zh. analit, khimii 31,565, 1976 3. E. STENHAGEN, S. ABRAHAMSONand F. W. MCLAFFERTY,Atlas of Mass Spectial Data; 1969 4. Ye. S. BRODSKH, Zh. analit, khimii 31, 2216, 1974 5. Ye. S. BRODSKII, I. M. LUKASHENKO, Yu. M. GOL'BERG mid V. G. LEBEDEVSKAYA, Zh. analit, khimii 29, 2026, 1974 6. C. J. ROBINSON, Analit. Chem. 41, 1549, 1969 7. Yo. S. BRODSKII, I. M. LUKASHENKO and Yu. A. VOLKOV, Khimiya i tekhnol. topliv i masel, :No. 2, 56, 1976