Mass Spectrometric Analysis of High Molecular Weight, Saturated Hydrocarbons R. J. CLERC, ARCHIE HOOD, and M. J. O'NEAL, JR. Houston Manufacturing-Research Laboratory, Shell O i l Co., Houston, Tex.
based 011 the observation that for any one hydrocarbon type, a large fraction of the total ion intensity is distributed over a small number of large peaks, which are thus characteristic of that hydrocarbon type, and that when the peaks in each of these groupings of characteristic peaks are mathematically combined and treated as one peak, they provide distinctive pattern coefficients for the determination of the corresponding hydrocarbon types. The saturate types in the gasoline range are limited principally t o alkanes and one-ring cycloalkanes, but in the heavy oil fractions there are many multiring cycloalkane types that must be considered. Similarly, the number of possible isomers increases tremendously in the heavier fractions. Thus, the simple classification of alkanes and cycloalkanes used for gasoline-range hydrocarbons must be expanded to include all possible molecular structures and configurations. These considerations led to a reinvestigation of the mass groupings commonly associated nith hydrocarbon-type determinations.
A mass spectrometric method has been developed for the hydrocarbon type analysis of high boiling, predominantly saturated petroleum fractions. The following hydrocarbon types are determined: alkanes (iso-plus normal), noncondensed cycloallranes (mono- plus polyring compounds in which no rings are in a condensed structure), condensed cycloalkanes (two or more rings in condensed configuration), monoaromatics, and naphthalene derivatives. In addition, the relative abundance of cyclopentyl and cyclohexyl rings as free structures is estimated. The method is based on the mass spectra of over 60 pure hydrocarbons and several narrow hydrocarbon-type concentrates. Average spectra of all compounds of each of the several hydrocarbon types were used to formulate the calculation matrix. Use of the method to analj-ze each of these pure compounds shows that structural variations within each hydrocarbon type generally do not greatly affect the determination of that type. On the average, pure compounds of a single hydrocarbon tj-pe are analyzed with an accurac>-of about 95% (theoretical = 100%). The consistency of the method for different hydrocarbon mixtures has been determined b?; application to synthetic blends and to the anal>-sisof separated petroleum fractions.
Table I.
Comparison of Alkane Mass Series % ' Alkane Peaks 243-99
Structure
c6-C-cS
270-113
% of total
% Of total
226
summations 75
268
63 (low)
35
282
77 (high)
36
324 338 366
69 68 65
30 36 34 (low)
366 394 436
69 68 06
37 36 35
450
64
35
1101.
wt.
ions 38 (high)
A 3
CS--c-C6
T
HE mass spectral analysis of high boiling oils is dependent
CS
upon the solution of problems which have previously limited the application of mass spectrometry t o the range of lo\%-molecular -eight components. Such problems fall into two categories: instrumental and interpretational. As previously reported ( 6 ) , the most important instrumental difficulties concerned with obtaining mass spectra of heavy materials have been largely overcome. For example, the vapor pressure requirement has been met by using a heated inlet system, and the resolving power of a commercial mass spectrometer has been increased by decreasing the slit widths and increasing the amplifier sensitivity. Therefore, the present investigation is concerned with the interpretation of mass spectra in terms of the chemical components in unknown high boiling oil samples. Preliminary work on the interpretation of mass spectral data of high boiling oils showed that the heavier fractions (lubricating oil distillates, etc.) contained 80 many different types of compounds that a reliable analysis of the unseparated mixtures would he extremely difficult in most cases. I t has been found convenient t o make a preliminary chromatographic separation into a saturate concentrate and one or more aromatic concentrates, and then to analyze the concentrates individually by means of the mass spectrometer. This paper deals with a method of interpreting the mass spectra of the saturate fractions. Monoaromatic and naphthalene components are included in this method in order to detect aromatic impurities that may be due to incomplete chromatographic separation.
C2-C-Cia
c2 7t-Cz2
n-Cu
C,-c-cn-c-ca c5 k5 n-cze n-Cza Cio-C-Cla
I
ClO ri-C32
.4r.variation from iiiean ( % of mean value)
4.7
2.2
~
It was found that when typical gasoline-range groupings of mass spectral peaks were applied to the analysis of alkane molecules in the lubricating oil range, there was considerable variation with respect to the nature and extent of branching of the alkanes. Several groups of peaks were investigated in an attempt t o minimize this effect. It was found, for example, that the effects of alkane subtype variations were smaller when the 70, 71, 84, 85, 99, and 113 peaks were used than when typical gasoline method groupings such as the 43, 57, 71, 85, and 99 peaks were wed. Tahle I Phew a comparison of results obtained when the above two mass groupings are used as a measure of various pure heavy alkanes. An average variation of 2.2y0of the mean was obtained with the former mass group as compared with 4.770 obtained with the latter grouping. Examination of cycloalkane pure compound spectra showed that multiring compounds could be differentiated according t o whether or not the structure contained a condensed ring system-
EXPERIMENTAL
Determination of Characteristic Mass Groups. Several mass spectrometric analytical methods have been reported for the determination of hydrocarbon types in the low boiling distillate range (1-3, 5 ) . In general, these hydrocarbon-type methods are
868
V O L U M E 27, NO. 6, J U N E 1 9 5 5
869
Le., two carbon atoms common to two rings. Mass groups could be selected so that the fragment patterns were about the same regardless of the number of rings in the molecule, the classification being only a matter of ring condensation. Consequently, the noncondensed cycloalkane grouping would classify the following structures together:
The condensed grouping would include systems containing two carbon atoms common to two ring structures, such as:
5% saturates, and the mass spectral cracking patterns of aromatic concentrates from different sources proved to be similar. The mass groupings most characteristic of these aromatic types are tabulated in Table 11. The number of spectra used for each hydrocarbon type in determining the mass groupings is shown in Table 111. Application of Total C4 to CIl Ion Intensity. There were several difficulties associated u-ith the use of sensitivities (peak height per unit charge) in the development of the method. For example, the exact amount of sample in the high temperature mass spectrometer could not be determined with sufficient accuracy. This caused inconsistent and unreliable calibration patterns for the mass groupings. In addition, when the summations of peaks in each particular mass grouping were based on sensitivities, the patterns varied with molecular weight and, in some cases, with subtype differences. These effects are shown in Figure 1 for the alkane mass groupings.
Although the number of compounds available for study was limited, the relative abundance of cyclopentyl and cylohexyl rings in the noncondensed cycloalkane grouping could be determined. The basis of this differentiation lies in t h g a c t that
of the two comparable structures, a - C n and i>-Cn, the substituent groups under electron bombardment usually dissociate a t the ring leaving a CsH:, ion and a CsHi ion, respectively, which can be detected in the mass spectrum. In order to use mass groupings which mould give the best differentiation between the cyclopentyl and cyclohexyl types and a t the same time be suitable for the determination of the total noncondensed cycloalkanes, it was necessary to use overlapping groupings as shown in Table 11.
Table 11. Characteristic Mass Groupings Alkanes ( 4 L K ) Noncondensed cyclopentanes (NCC.4-Ca) Noncondensed cyclohexanes (NCCA-Ce) Condensed cycloalkanes (CCA) Monoaromatics (RIA) Xaphthalenes (NAPH)
Table 111.
70, 71, 84, 85, 99, 113 69, 83, 97, 111, 125, 139 83, 97, 111, 125, 139, 153 67, 81, 95, 109, 121, 122, 123, 135, 136, 137, 149, 150, 151 77, 78, 90, 91, 92, 104, 105, 106, 117, 118, 119, 129, 130, 131, 132, 133, 143, 144, 145, 146 128, 141, 142, 155, 156
Calibration Spectra for Matrix Development
Pure comoounds Alkanes' Sonconden-rd c yrIb1~eiitnries Soncondr u r d cyr1ohe.usr.c. Condensed cycloalkancHydrocarbon-type concentrates Monoaromatics Xaphthalenes
No. of Spectra 29 4 12 17 5 2
The determination of the mass groupings characteristic of mono- and diaromatic types was accomplished without having to resort to pure compounds, since narrow monoaromatic and naphthalene concentrates typical of the aromatics occurring as impurities i n chromatographic saturates were available for calibration. These aromatic concentrates were shown by ultraviolet absorption and sulfuric acid extraction to contain less than
0
n-Alkonr
0
I,Wlb".,
1 1
m-
j
I
I
I
I
1
1
The above difficulties were largely eliminated when the mass patterns were expressed as percentages of the total ion intensity from m/e 45 to 156 (C, to C1l). For example, it can be seen from a comparison of Figures 1 and 2 that the use of the total Ca t o C1l ion intensity has decreased markedly the effect of molecular weight on the alkane grouping, and at the same time has improved the precision of the data considerably. The use of percentage of total C4 to Cn ion intensity also makes it possible to use spectra which have been obtained under radically different instrumental conditions-e.g., from other laboratories as in the iZmerican Petroleum Institute catalog of mass spectra. This approach is based on the assumption that the total C4 to Cn ion intensity per unit volume of sample charged to the mass spectrometer is independent of hydrocarbon type. Although there is appreciable variation in data from pure compounds, the best available data from narrow petroleum fractions indicate the following approximate relative sensitivities: alkanes 1.0, noncondensed cycloalkanes 1.2, and condensed cycloalkanes 1.O. However, the relative sensitivities vary with such factors as the source of the crude oil and the techniques used in obtaining the concentrate, so that until such factors can be evaluated completely, it is difficult to say that a correction for sensitivity would actually represent an improvement. Accordingly, equal sensitivities have been assumed for the Cd to CIl portion of the spectrum of all hydrocarbon types considered in the method. Development of Matrix, Inverse, and Corrections. The average mass grouping patterns of each of the hydrocarbon types described above were used to construct the matrix shown in
ANALYTICAL CHEMISTRY
870 Table IV.
3Iatrix
con-
Noncondensed Cycloalkanes CycloCyclopentanes hexanes 9.3 14.4 35.0 33.j 21.7 5.2 7.3 0.3 0.4 0.4 0.2
Alkanes 35.8 270 269-139 9.7 283-153 5,Q 1 2 267 277 0 1 2128 1.7
densed Cycloalkanes 2.7 9.4 5.5
30.0
Table V.
jo.6 3.2 0.1
267 + O 123 - 0 727 + O 161
577 -0.003 -1.012 +0.360
2128 -0.046 -0.199 -0,287
496 0 000 0 017
+ 2 047 - 0 158 + O 009
-0.281 +2.501 -0.16G
-0.150 -0.265
0 768
1 455
+
-
+ +
~
Inverted 3Iatrix
270 269-139 583-153 2 509 + 3 196 - 3 029 - 1 099 +13 379 -15 OF8 4-0 183 - 9 094 +13 840
Alkanes Cyclopentanes Cyclohexanes Condensed cycloalkanes + O 021 Monoaromatics $ 0 011 Saphthalenes - 0 121
MonoNaphthaaromatics lenes 3.6 5.1 8.4 10.1 5.5 6 1 5 1 7.1 407 4.7 2.8 41.2
0 002 0 002 0 033
- 0 -
~
Total
2 191
1 289
*__ +
1.399
1,521
1
I
j
0
n-*tWI I k a l M
-
c
I
I
I
I
I
I
Table IV. For convenience and ease of solution, the matrix was inverted by conventional algebraic methods to the form shown in Table V. In the application of the inverted matrix there is a considerable effect of alkane molecular weight on the apparent noncondensed cycloalkane concentration. This effect is shown in Figure 3, in which the apparent concentrations of cyclopentanes and cyclohexanes in the alkanes are plotted as functions of alkane carbon number. It can be seen that the apparent cyclopentane concentration is a direct function of alkane molecular weight, and that isoalkanes give higher cyclopentane values than do n-alkanes. The apparent cyclohexane concentration reaches a minimum value at an alkane C-number of about 16, and there is no obvious difference between n- and isoalkanes in this respect. The concentrations of condensed cycloalkanes, monoaromatics, and naphthalenes were found to be practically independent of
'
I
1
!
MS NO
6
8 -30
I
I 1
0
4
A m n n t wcpatm
E
,
2
,
I
I6
,
,
I , ( , 2Z 24
,
, 28
ca-c4mtm n
1
,
1
32
Figure 3.
n-C4(43/581 DATE
-
lMRParnHMBER COMMENTS
Mass Series ( 2 in C n H e n t z )
-c
d
1 0 c
5 0
C,
CS
* 270-113 = L 69-139
.
40
44
the alkane molecular weight and therefore are not shown. The curves shown in Figure 3 can be used to correct the analyses of both pure compounds and mixtures on the basis of the amount, the average molecular weight, and the type of alkanes. For mixtures, this information can be determined from parent-peak analysis (6). The use of "grids" is convenient for the tabulation of mass spectral data for analysis where mass groupings are employed. The grid consists of an array of mass series (z in C,,HZ~+*) us. carbon number (n). An example of such a grid is shown in Figure 4, which is a copy of the calculation sheet devised for this type of analysis. The grid is simply a convenient method of arranging data for ( 1 ) the summation of each grouping, (2) determination of the total ion intensity, (3) solution by means of the inverted matrix, and (4)correction of the solution to obtain final compositional results. The cyclopentyl-cyclohexyl determination cannot be expressed in terms of a molecular analysis because both structures can exist in the same molecule. Therefore, the amounts of cyclohexanes and cyclopentanes are reported in terms of a relative-abundance value, which is a ratio of noncondensed cyclopentyl to cyclohexyl rings. Although the amounts of cyclopentyl and cyclohexyl
PICKED _ _ CAXULATED-
ALK
,
Effect of alkane molecular weight on apparent cycloalkane concentration
DATE
-r,
1
36
;:;/y;
A M E C4RBON NUMEER
SPhtPLE
VOL
1
.me
183-ISJ
Figure 4.
CCA
= 267-151
MA
L77-146
NAPH
Z 128-156
TOTAL INTENSITY l C 4 - C l l l
Calculation sheet for inverse analysis
~
V O L U M E 2 7 , N O . 6, J U N E 1 9 5 5
871
types are not given in final form as percentage composition, the numerical values (see line marked “corrected” in Figure 4) are combined to give total noncondensed cycloalkanes (XCC.4). Thus, the final analyeis includes percentage values for alkanes (ALK), noncondensed cycloalkanes (SCCA), condensed cycloalkanes (CCil), monoaromatics (MA), and naphthalenes (KA4PH) plus a ratio of noncondeneed cyclopentyl to cyclohexyl rings. -c,,-q For example, a cyclopentyl cyclohexyl alkane,
‘
\ Rz
/
\
Rz
Rz
respectively, in xhich R is a relatively long straight chain and
R,and Rzare relatively short straight alkyl chains. From Table V I 1 it can be seen that all of these types were found to give 99 to 100% alkanes, except in those cases in which the sum of the carbons in R1 and RZ is 9 or 10. In such cases a large spectral peak equivalent to an alkyl (C,H,,+,) fragment of ten or eleven carbon atoms occurs, and because these peaks fall in the mass-equivalent naphthalene grouping (see m/e 141 and 155 in Figure 4),values of 2 to 3% naphthalene were obtained. Table VI1 also includes an example of a highly branched isoalkane which was found to give only 80% alkanes on analysis. Since this highly branched molecule is considered to be nonrepresentative of isonlkanes occurring in heavy petroleum fractions, it has not been included in the summary in Table XII. Of the cyclopentanes analyzed (Table VI11 ), the mono-n-alkyl
() bJ
~ o u l dtheoretically be analyzed as 1 0 0 ~ o noncondensed cycloalkanes n ith a relative abundance of 50i.50 cyclopentyl-cyclohexyl rings.
Table YI. .4nalysis of n-.-ilkanes Number of pure ii-alkanea Carbon number range Composition, % .4lkanes Noncondensed cycloalllanes Condensed cycloalkanes Monoaroniatics Naphthalene-
Table VII.
18 14-43 59-100 0-0 0-0 0-0 0-0
5
5 6
5
Table VIII.
-4nalysis of Isoalkanes Length of
+ Rz (C-Atoms) RI
Type
‘
/RI
2-8
9-10 ,11-20
R- C
d
\R* RI
‘C-R--C
fr
99-100
5-15
83-95
NCCA
Ck/C6 100/0
CCA 0-1
55145-
0
Q1 ’
60/40
/R’
R2’
Error, % -Cls
Analysis of Cyclohexanes .4LK CdCs NCCA
CC.4
0
100
0/100
0
0
94
25/75
6
53
0/10015/85
7
ANALYTICAL CHEMISTRY
872 cyclopentanes,
bR,
exhibit mass spectra which are the
most typical of spectra of KCCA fractions obtained from petroleum. All of the mono-n-alkyl cyclopentanes analyzed were found to give a t least 99 % noncondensed cycloalkane content (NCCA), with all of the rings appearing as cyclopentanes. The effect of branching of the alkyl chain near the ring, which certainly does not appear to be very extensive in high boiling cycloalkanes from petroleum, is t o give a high apparent value for alkanes ( 5 to 15%). The branched compounds shown,
pentyl-to-cyclohexyl ring ratio obtained for the compound 1,ldicyclopentylethane results from the fact that the major peak for this nontypical compound is a C, fragment falling in the cyclohexyl grouping. All of the four polycyclopentanes shown in Table VI11 contribute erroneously t o the condensed cycloalkane content (CCA). I n fact the two which, simply on the basis of the presence of a long alkyl chain, would appear to be most typical of heavy petroleum molecules-Le., CIS
/R
, also
(i
-C-C
contribute erroneously to the cyclohexane
R'
types because of the large Cs and C, fragments produced in the ionization chamber. Similarly, the very low value for the cyclo-
Table X.
C-"n
Analysis of Mixed Cycloalkanes NCC.4
C6/C6
CCA
11.4
84
40/60
16
1
93
26/75
7
0
[,()-i;C,,.
and
iTn
-give
the largest
condensed cycloalkane
contribution If, however, these two compounds were made still more representative t o the extent of adding even one methyl group per ring, the condensed cycloalkane values would be expected to fall from 21 and 55%, respectively, to values well below lo%, because the major fragments would no longer fall in the condensed cycloalkane mass grouping. The analysis of cyclohexane types is shown in Table IX. The results are generally similar t o those noted for the cyclopentanesLe., mono-n-alkyl cyclohexanes are all analyzed with good accuracy, the effect of less typical branching near the ring is to give high values for alkanes, and the polycyclohesanes (of nontypical types) contribute someivhat to condensed cycloalkanes. The only dialkyl cyclohexane analyzed was the di-n-decyl derivative, which was found to contribute someffhat t o both the cyclopentane and condensed cycloalkane groupings.
Table XI. Analysis of Condensed Cycloalkanes ALK
05
!II' SI!
SCCA
CCA
MA
ALK
NCCA
CCA
MA
3
0
96
1
0
0
91
6
0
0
97
3
0
0
94
6
0
1
92
7
0
11
0
89
0
0
28
72
0
22
15
63
0
0
0
96
4
R
R-C-R
2
1
97
0
cr,3
0333 cs
0
C
c A-cs-c /c c'
/-J4 0
1
98
1
dbL1 v
V O L U M E 2 7 , NO. 6, J U N E 1 9 5 5 Table XII. Pure Saturate Type .4lkane Koncondensed cycloalkanes Condensed cycloalkanes
873
Summar?-of .4nalyses of Pure Saturates .Iv. Interference No. of Av. Coni- Analysis, pounds % 29 95
Range,
~
%
(Contributions to Other Types) Type
57-100
cc
a
70-100 Alkanes 6 Noncondensed 92 63-100 cycloalkanes S .I1.. 95 a Contributions from alkanes are taken into account by means of corrections shown in figure 3. 16
91
15
Table XIII. ALK
c,r-c=c CLa-c=C-c
Analysis of Alkenes 11.4
X.4PH
0
0
100/0
CCA 0 2
0
0
85/15
10
1
3
Ca/Cc 73/25
0
XCCA 76 58
0
86
24
t:
ci-c-c-cs
6, The analysis of mixed noncondensed cycloalkane types is shown in Table X. Although neither of the two compounds in the table is typical of heavy petroleum molecules, i t is possible to get an idea of the accuracy of the cyclopentyl-cyclohexyl ratio. In both compounds this ratio is slightly lower than the theoretical values of 50/50 and 33/67, respectively. Table X I contains the analyses of pure condensed cycloalkanes. Of the dicycloalkanes, the mono-n-alkyl Decalins all were found to give analyses of 98% condensed cycloalkanes or better. However, the corresponding hydrindanes gave about 28y0 noncondensed cycloalkanes on analysis. Although this represents an undesirable limitation of the method, the explanation of the error is that for this type of compound one of the largest peaks of the spectrum is that equivalent to a C7 one-ring fragment resulting from the loss of the alkyl side chain and the nonsubstituted methylene groups of the cyclopentano ring. The same type of fragmentatiqn occurs to some extent for the a-mono-n-alkyl Decalins, but the extent to which it does occur was taken into consideration in the matrix. For both the Decalins and hydrindanes, this error is diminished by branching of the alkyl side chain near the ring, but such branching greatly increases the contribution to alkanes, as noted in the previous tables. The condensed tricycloalkanes all give satisfactory analyses. For the four compounds shown, the worst error was 4% monoaromatics obtained for butyl perhydroanthracene, which is probably the least typical tricycloalkane because of the shortness of the alkyl substituent. The condensed tetracycloalkanea, which include a rather wide variety of structures, were all found to give good results except for a persistent small error in monoaromatics content ( 3 to 7 % ) . This error results from the tendency of these four-ring molecules t,o give fragments containing only 6 to 11 carbon atoms but retaining the same order of hydrogen deficiency-or therefore the same apparent number of saturated rings-as the original molecules. -4lthough alkenes are not generally expected components of heavy oil fractions, their presence can affect t,he mass spectronieter method to a considerable extent. The simple monoalkenes would be determined by this method primarily as noncondensed cyclopentanes (Table XIII). ;\lore complicated alkenyl structures might be expected to give different types of interferencese.g., cycloalkenes might be expected to interfere with condensed cycloalkanes. Firm conclusions regarding the alkene analysis, however, cannot be made until a greater variety of pure conipounds of this structure becomes available. The analysis of pure compounds has revealed some of the limitations which might be expected with this method. Because the method i p based on average spectra of pure compounds,
certain molecular structures can lead to negative values for cornponents that are not present. Pure alkanes seldom indicate negative values greater than 1%, but certain of the cycloalkane structures were found to show negative values as high as 26% in isolated cases. Thus the applicability of the method T O natural petroleum fractions would depend upon how well the average patterns represent the actual compounds present in the sample. Actual applications to narrow petroleum fractions have failed to show negative values in cases where a given component was thought to be absent. The usefulness of this method for the analysis of isolated pure compounds, however, is strictly limited because of the above variations. When a sample contains only a limited number of compounds, a parent-peak analysis generally is more useful both in applicability of the method and in terms of the amount of information gained by the analyfi. Severtheless, the above analyses by the fragment method have indicated the major type of subtype variations which might be expected. Cyclopentyl-Cyclohexyl Distribution. A method which has been used to determine the relative amounts of cyclopentyl and cyclohexyl rings is that of Lipkin and Kurtz (4). I n this method the average number of rings in a completely saturated sample is determined accurately by carbon-hydrogen ratio, and then the average number of carbon atoms in the ring is determined by means of a density correlation. In the mass spectrometric method the ratio of cyclopentyl to cyclohexyl rings is determined for only noncondensed cycloalkanes, while in the method of Lipkin and Kurtz, the average number of carbon atoms per cycloalkyl ring is determined for the entire saturate fraction (including the condensed structures). Therefore, the two methods are not strictly comparable. However, the data of Lipkin and Kurtz ( 4 ) show that aromatic extracts after hydrogenation had the same average number of carbon atoms per ring as the saturate fraction obtained from the original oil sample. If the average number of carbon atoms per ring is the same for the entire saturate fraction as for the aromatics, it is also possible that it is the same for both the noncondensed and the condensed ring structures. By making such an assumption, the mass spectrometric and density methods can be compared.
Table XIV. Hydrocarbon Type Analysis of Saturates from Dewaxing of Lubricating Oil Raffinate Hydrocarbon Type hlkanev Noncondensed cycloalkanes Condensed cycloalkanes Monoaromatics Total
Saturates in Charge Exptl. Calcd. 37 37 40 40 21 22 2 - 1 100 100
-
Saturates in Products Oil Wax 10 27 33 7 21 1 1 -0 65 35
-4 “homogeneous” saturate fraction from Ponca City, Okla., lubricating oil n-as obtained through the Advisory Committee for API Project 6. This fraction (.4PI designation .4,-5) is nearly identical with one which had been analyzed by Lipkin and Kurtz (API designation A1-6). The Lipkin and Kurtz method results in values of 5.3 to 5 4 carbon atoms per cycloalkyl ring, while the mass spectrometer method indicates an abundance ratio of 70/30 cyclo-Cj to cyclo-C~,or therefore 5.30 carbon atom per cycloalkyl ring. The agreement of these two entirely different methods of analysis lends some weight to both methods, although conclusive proof of the methods is still lacking. If these data are meaningful, it appears that there are more cyclopentyl rings that cycloheuyl ring,i in many high boiling petroleum fractions, and that this predominance of cyclopentyl rings is equally prevalent in the aromatic, the noncondensed cycloalkane, and the total saturate fractions. Internal Consistency. The consistency of a method for the analysis of different types of mixtures may be evaluated con-
an
ANALYTICAL CHEMISTRY
veniently by application to the charge and products of a separation based upon some physical property. For example, the present mass spectrometric method of analysis may be applied to the charge and products of a dewaxing operation on a lubricating oil raffinate. Such analyses are shown in Table XIV, and compositions are given on a yield basis, so that the sum of the crude wax and dewaxed oil components (shown in the table as "calculated saturates in charge") should equal the experimentally determined composition of the original raffinate saturates. The agreement of the sum of the products with the charge indicates that this method consistently groups the same components irrespective of the type of mixture analyzed. A similar comparison involving the products of urea extraction of a paraffin wax is shown in Table XV.
Table XV. Hydrocarbon Type Anal)-sis of Products from Urea Extraction of a Paraffin Wax Hydrocarbon Type Alkanes Noncondensed cycloalkanes Condensed cycloalkanes Jlonoaromatics Total
Charge Exptl. Calcd. a7 88 13 12 0 0 0 - 0 100 100
-
Products UreaNonreactive reactive 85 3 5 7 0 0.3 - 0 - 0 1 90 10
Table XVI. Analysis of Alkane-Cycloalkane Blends Wax Cycloalkane concentrate Blend 1 Experimental Calculated Blend 2 Experimental Calculated Blend 3 Experimental Calculated
ALK 94 2
NCCA 6 50
CCA 0 43
MA 0 5
28 27
37 38
33 31
4 4
49 50
27 27
22 21
3 3
71 72
16 16
11 10
2 2
.
Another method of testing the consistency of an analytical procedure is to blend analyzed hydrocarbon-type concentrates and then to compare the analysis calculated from the concentrates with that determined by analysis of the blend itself. Accordingly, blends were made with a predominantly alkane petroleum fraction and a predominantly cycloalkane fraction. The alkane used in this series of blends was an Indonesian wax which had a melting point of 129" F. and contained only 6% nonalkanes. The cycloalkane used was the saturate fraction from a chromatographic separation of a California lubricating oil distillate and contained only 2% alkanes. The mass spectrometric analyses of the two starting materials are tabulated in Table XVI along with the experimental and calculated values for the blends. It may be seen that the deviation between calculated and determined values is no more than 2 percentage units in any case. The calculated values for the compositions of the blends in Table XVI are given on a volume basis. Therefore, the values obtained from the saturate method generally agree within 1% by volume of the calculated values. A closer study of blend 2, however, showed that the calculated alkane content is 49.8% on a volume basis and 47.3% on a weight basis (and also on a mole basis, since the molecular weights of the two concentrates are nearly identical), while the actual experimental value was found to be 48.6%. The saturate method therefore has given a value about halfway between volume per cent and weight per cent or mole per cent and within 1 to 2 percentage units of the values expressed on any of the three bases. Limitations on Monoaromatic Concentration. In the analysis of pure compounds, aromatics have been found to have a strong tendency to exhibit negative values for cyclopentanes. Therefore, the cyclopentyl-cyclohexupl ratio is questionable when the
aromatic content exceeds 5%. With that exception, however, hydrocarbon-type analysis by the saturate method appears to be reasonable even in the presence of large concentrations of monoaromatics. The amounts of aromatics which can be tolerated in the simplified hydrocarbon-type analysis may be seen by reference to Figure 5, which gives a series of analyses of narrow fractions taken during a chromatographic separation of dewaxed, deresined lubricating oil distillate. Only the portion from 65 to 85% by weight yield is shown for the chromatographic separation, as in that range the alkanes gradually fade out completely, the condensed and noncondensed cupcloalkanes drop sharply as the nonaromatics concentration increases rapidly to nearly loo%, and finally the naphthalene content begins to increase. Bctually, the condensed cycloalkanes should reach zero in this separation a t about 77% elution, but the method continues to show a spurious small content of this type, M hich becomes larger as the naphthalene content begins to rise and as the monoaromatics become more naphthenic in character. For narrow chromatographic fractions, therefore, the molecular-type analysis appears to be satisfactory even in predominantly monoaromatic fractions, as long as the naphthalene content is negligible. For total eluted saturate concentrates, however, little over-all error would result from using the total eluted material up to 857, yield, which would mean in this case a saturate concentrate containing 15% aromatics. Instrumental Consistency. The data used in developing this analytical method were obtained with a Consolidated Engineering Corp. (CEC) Model 21-1C2 mass spectrometer modified for high-temperature operation (5). Recently, this instrument was remodeled by conversion to CEC Model 21-103. I t was found that the original inverted matrix (Table V) was still applicable, but that the molecular weight effect of the alkanes (Figure 3) had changed somewhat. Therefore, it was necessary to determine new correction curves. I t appears likely that other mass spectrometers will require separate correction curves, although the inverted matris terms should be applicable. I
C H ~ T ~ R b W InE CLO. Xr
Figure 5.
Chromatographic separation of lubricating oil
Table XVII. Composition' of Saturates from JIedium Viscosity Lubricating Oils East Texas
Gulf Coast
26
RlidContinent 19
1:
9
California 3
51
54
54
23
27
31
53 38
53 44
60'40
GD/4O
60/40
65/35
70/30
Pennsylvania Alkanes Noncondensed cycloalkanes Condensed cycloalkanes Cyclopentyl-cyclohexyl ring ratio
V O L U M E 2 7 , NO. 6, J U N E 1 9 5 5
875
APPLICATIONS
In addition to the applications included in the evaluation of accuracy and internal consistency of the method, one more application is given in Table XVII to provide a better idea of the usefulness of the method in the study of composition of lubricating oils. The table gives the analysis of saturate concentrates from medium-viscosity lubricating oils from various crude sources. The data show that although the noncondensed cycloalkanes and cyclopentyl-cyclohexyl ratio remain fairly constant, the alkane content drops from a high of 26% in the Pennsylvania saturates to a low of 3% in the California saturates while the condensed cycloalkanes vary from a low of 23% to a high of 44%, respectively.
ect 42, and especially to R. R. Schiessler for the loan of many of the pure hydrocarbons employed in this work. The help of RI. L:4ndrB, H. J. Cannon, C. K. Hines, and Jan Samson is also gratefully acknowledged. LITERATURE CITED
(1) Brown, R. 8.,ANAL.CHEM., 23, 430 (1951). ( 2 ) Brown, R. A . Doherty, W., and Spontak, J.. Consolidated Engineering Corp., Mass Spectrometer Group, Report 84, 1951. (3) Friedel, R. A., A p p l . Spectroscopy, 6, 24 (1952).
(4) Lipkin, ill. R., and Kurtz, S. S., Division of Petroleum Chemistry, 100th Meeting, ACS, Detroit, Mich., September 1940. (5) Lumpkin, H. E., Thomas, B. FV., and Elliott, A , , ~ N A L .CHEM., 24, 1389 (1952). ( G ) O'Seal, 11.J., and Wier, T. P.. Ibid., 23, 830 (1951).
ACKNOWLEDGMENT
The authors wish to express their appreciation to the Advisory Committee of the American Petroleum Institute Research Proj-
RECEIVED for review August 2 , 1954. Accepted February 7, 1955. Presented a t the Second Annual Meeting of ASTM E-14 Committee o n Mass Spectrometry, M a y 1954.
Analysis of Fluorinated Polyphenyls by Mass Spectrometer PAUL BRADT and FRED L. MOHLER National Bureau o f Standards, Washington,
D. C.
A mass spectrometric method has been used to investigate the molecular weight and chemical composition of some fluorinated polyphenyls. The polymers were evaporated from a tube furnace directly into the ionization chamber of a mass spectrometer and mass spectra recorded as the furnace temperature w-as increased step by step. Mass spectra of polymers made from p dibromotetrafluorobenzene showed molecules of formula (CeF&+Brz with n ranging from 3 to 6. Polymers made from the diiodo compound gave ions with as many as eleven phenylene rings with or without one or two iodine atoms attached to the chain. RIolecule ions are predominant in these mass spectra; this simplifies the interpretation of results in terms of molecular weight distribution.
T
H E mass spectrometric analysis of heavy nonvolatile compounds by the use of a heated reservoir on the high pressure side of the inlet leak has become a standard procedure ( 1 , 5 ) , and mass spectra of mixtures containing hydrocarbons with as many as 45 carbon atoms (molecular weight approximately 630) have been published. I n exploratory research there are some advantages in evaporating molecules or thermal degradation products from a small tube furnace directly into the ionization chaniher of a mass spectrometer. This arrangement is more flexible, as the furnace can easily be adapted for use under a wide range of conditions and it is a simple matter to clean out nonvolatile residues. There is an important difference b e b e e n the two techniques. Degradation products from a tube furnace enter the ionization chamber after relatively few collisions and mag include radicals and other unstable configurations. Molecules from a reservoir are the products in thermal equilibrium a t the reservoir temperature and pressure. The application of the tube furnace technique to a study of pyrolysis products of polystyrene has been described, including some experimental details (2). EXPERIMENTAL PROCEDURE
Hellmann ( 3 ) has synthesized chains of fluorinated phenylene rings by heating p-dibromotetrafluorobenzene or diiodotetrafluorobenxene in the presence of hot copper. The products are granular light gray solids, presumably of the structure Br-CEFd-
CsF4-C6F4-Br, and the corresponding compounds terminated by iodine rather than bromine. This solid material is partially soluble in benzene and was separated into two fractions on this basis. These polymeric materials were submitted for mass spectrometric study, to determine the number of phenylene rings in the material and to investigate the chemical composition and thermal degradation. The samples were placed in a small metal or glass sample holder in a tube furnace which extended to the inlet port of the ionization chamber of a 60" Nier-type mass spectrometer ( 4 ) . A thermocouple in contact mith the sample holder measured the temperature of evaporation. The mass spectrum was recorded with a pen recorder by varying the magnetic field. Exceptionally heavy ions were encountered and the mas9 range was extended by lowering the ion-accelerating voltage from 2500 volts to less than 500 volts. The resolving power under these conditions was less than 200 and there is an uncertainty of nearly 1% in the mass scale. However, these compounds give mass peaks which are widely separated on the mass scale and the identification of the ions is probably reliable. In the case of the bromine compounds the triple isotope structure of the molecular ion3 containing two bromine atoms and the double structure of fragment ions containing one bromine atom were conspicuous in spite of incomplete resolution and this aided in identifying the ions. The procedure was to increase the sample temperature a t a slo~vrate until the sample began to evaporate, and hold the temperature constant while the spectrum was recorded. The temperature was increased step by step and the spectrum recorded at each increment in temperature. The rate of evaporation did not remain constant over the time required to record the spectrum, and after 4 or 5 hours insulating films formed in the ionization chamber and the evperiment had to be interrupted to clean the electrodes. RESULTS
Table I gives the larger mass peaks in the polymerized bromine compounds. The first column identifies the ion, the second column gives the molecular weight (the median value is given for polyisotopic ions), and the third column gives the mass spectrum of the fraction soluble in benzene. The compound began to evaporate a t about 100' C. and the spectrum was recorded at 1 2 i " C. This is an unusual spectrum, for all fragment ions
