ANALYTICAL CHEMISTRY
306 toward naphthenes, the 1.5% squalane on Pelletex column is more nearly like a solid adsorbent than like a liquid-type column which retards naphthenes. Although the liquid-modified Pelletex column has proved useful for analysis of CS and c 6 saturates, probably the chief value of this type of column is that it separates naphthenes from paraffins in a manner opposite to gas-liquid partition chromatography columns, and therefore offers new possibilities for analysis of saturates. Thus, a “carbon number” analysis is possible, a t least through C,, according to the authors’ experience, and probably higher. When a further breakdown or type analysis is desired, the carbon number cuts can be separated into naphthenes and paraffins by means of a highly polar gas-liquid partition chromatography column, such as glycol, which greatly retards naphthenes. A few olefins and benzene were tested as to emergence time through the squalane on Pelletex column. These results are listed below, together with those for saturates of similar boiling point. Hydrocarbon 1-Pentene n-Pentane 1-Hexene n-Hexane Benzene Cyclohexane Cyclohexene
Boiling Point,
c.
30 36 64 67 80 81 83
Emergence Time, Min. at 20 Ml./Min. 38 36 91
102 114 113 132
With the liquid-modified Pelletex column unsaturation has little effect on the emergence time. All the c6 ring compounds emerge ahead of the C; band, which begins a t about 150 minutes, as shovm in Figure 3. Therefore this column is suitable for carbon number analysis of a mixture containing all three types of hydrocarbons. Interfering olefins and aromatics can be removed and a representative sample of saturates obtained for gas chromatographic analysis by liquid phase fluorescent indicator chromatography using a column similar to that described by Criddle and Le Tourneau ( 2 ) . I t consists of four sections of inside dimensions (top to bottom) 150 X 25, 350 X 10, 350 X 5 , and 700 X 2 mm., the three lower sections containing Davison’s grade 923 silica gel. Gsing a syringe to reduce evaporation losses, 2 to 3 ml. of sample are introduced below the gel surface and eluted with isopropyl alcohol under a pressure of 1 to 2 pounds per square inch. The saturates are collected in a chilled vial, protected from air, and the cut is taken when the first drop of yellow-green fluores-
cent dye forms a t the tip of the column as observed in ultraviolet light in a dark room. This technique was tested with B blend of CS-G saturates with olefins from a catalytically cracked gasoline; gas chromatographic analysis showed a maximum change of 1% in the composition of the recovered materials. For the analysis of CSand C6 saturates the gas chromatographic method described has certain advantages over spectrometric methods. Because saturates outside the C& range do not interfere, as they do in spectrometrv, extreme care in fractionation is not required. Furthermore, the presence of such higher or lower boiling constituents can be detected. However, in the absence of C7’s the infrared method is roughly comparable with the gas chromatographic method in accuracy and time per analysis. Another important advantage of the gas chromatographic method is that the apparatus is relatively simple and does not require a highly skilled operator. I t is concluded that liquid-modified adsorbents, such as the squalane-Pelletex column employed here, constitute a useful addition to the media available for gas chromatographic separations. Columns of this type differ markedly from the more familiar gas-liquid type columns in respect to the sequence of resolution of saturates. They are also “flexible,” for by varying the amount of liquid the naphthene peaks can be moved relative to the paraffin peaks almost a t will. The resolution is excellent and comparable with that of the best gas-liquid type columns tested. LITERATURE CITED
(1) Cremer, E., Muller, R., Mikrochemie 36, 553 (1951). (2)
Criddle, D. W., Le Tourneau, R. L., ANAL. CHEM.23, 1620
(3)
Dimbat, M., Porter, P. E., Stross, F. H., Ibid., 28, 289 (1956). Fredericks, E. M.,Brooks, F. R., Ibid., 28, 2 9 7 (1956). Griffiths, J., James, D., Phillips, C., Analyst 77, 897 (1952). Hirschler, A. E., Amon, S., I n d . Eng. Chem. 39, 1585 (1947). James, A. T., M f g . Chemist 26, 5 (1955). Keulemans, A. I. M., Kwantes. A., “Analysis of T’olatile Organic Compounds by Means of Vapor Phase Chromatography,” World Petroleum Congress, Rome, June 1955. Keulemans, A. I. M., Kwantes, A., Zaal, P., A n a l . Chim. A c t a
(1951).
(4) (5) (6) (7)
(8)
(9)
13, 357 (1965). (10) Patton, H. W., Lewis, J. S., Kaye, W.I., ANAL.CHEY.27, 170 (1955). (11)
Ray, N. H., J . A p p l . Chern. 4, 21, 8 2 (1954).
RECEIVED for review September IS.
1955.
Accepted January 6, 1956.
Mass Spectrometric Analysis Broad Applicability to Chemical Research FRED W. McLAFFERTY The Dow Chemical Co., Midland, Mich. The heated-inlet mass spectrometer, applied in the past mainly to petroleum compounds, is an analytical tool of broad applicability to the whole field of chemistry. Quantitative analysis approaching the high accuracy and number of components found previously with light hydrocarbons is possible. Positive identification of unknown components of complex mixtures is illustrated. Unique information on molecular structure is provided, and complete structure determination is often possible without standards. It is hoped to show that the mass spectrometer provides a valuable complement to the rapidly growing field of instruments capable of analyzing a wide diversity of samples.
T
HOUGH the primary use of the analytical mas8 spectrom-
eter has been in routine quantitative analysis of light hydrocarbons, a number of investigators have reported on the unique qualitative and quantitative applications to the general field of chemistry, especially organic chemistry. The analytical advantages of the mass spectrometer for volatile oxygenated compounds (14, 18, 21, 22, 3.5,36), thiophenes (IQ), aromatic hydrocarbons (IQ), lactones ( I S ) , acids (16), haloalkanes (1, 26, S5), amines ( 7 ) , and metallo-organic compounds (8, 9) have been described. Significance of the spectra of such compounds as ketene dimer (M), pentaborane (IO),diborane (28),dimethylphosphinoborine trimer ( 1 2 ) -phenol, thiophenol, and aniline ( 2 7 ) has also been discussed.
V O L U M E 28, N O . 3, M A R C H 1 9 5 6 Table 1. n/e
105 119 120 132 133 146 147 160 161 162 174 175 188 189 202 203 204 216 217 230 23 1 244 245 246 SP/STol
307
Mass Spectra of Polyisopropylbenzenes
Tetraisopropylbenzene 11
8 1
1
6 3 14 1 4 0 6 0 2 0 7 i:7 24 4 0 1.3 0.69 36 301 6 9 5.7 100 0.23
Triisopropylbenzene 31.4 25.4 2.6 1.7 18.3 2.3 23 3 1.8 109 14 6 1.8 6.7 2.7 338 0.19 3.0
Diisopropylbenzene 61 105 10.6 13.2 4.8 2.6 344 0.47 2.4
Isopropylbenzene 368 3.31 100
100
100
0.26
0.28
0.39
The mass spectrum indicates the amount and mass of the positive ions formed by electron bombardment of a sample. This unique information concerning the concentration and kind of molecules in a sample has been of only limited use in these general fields of chemistry because relatively few compounds of these types give sufficient vapor pressure (at least 0.05 mm.) to be introduced as a gas into the ionization chamber of the ordinary mass spectrometer. The development of heated-inlet systems (S,6,29, S 6 ) has made possible the analysis of compounds of much lower volatility by heating the sample a t 200” to 400’ C. to obtain the necessary vapor pressure. The few instruments of this type described have been used mainly for hydrocarbons of high molecclar weight, though applications to ketones ( 3 2 ) and alcohols ( 4 ) have been demonstrated. This paper points out the value and uniqueness of the quantitative and structural information obtainable with the mass spectrometer on the broad range of compounds that can nom be analyzed v,ith the heated-inlet mass spectrometer. This has apparently been little appreciated up to the present because of the unavailability of heated instruments and the high interest in application of most of these to petroleum hydrocarbons. EXPERIMENTAL
The spectra given were obtained on two 90” sector-type mass spectrometers ( 6 ) . Their inlet systems are heated a t 100’ and 200” C., respectively, and solid samples placed in Teflon capsules can be directly introduced into the 200” system through a vacuum seal ( 5 ) . ?To effort was made to run all the spectra given in this work on the same instrument or under exactly the same operating conditions, as they can be only of semiquantitative use for comparison with other instruments. However, comparison with spectra from a Consolidated Engineering Corp. Model 21-103 mass spectrometer in this laboratory shows striking similarity for many compounds such as aromatic hydrocarbons. Peaks below 1% of the height of the highest have been omitted in the graphicallv presented spectra of pure compounds because of their doubtful structural significance. However, the instruments used are in general capable of recording peaks less than 0.01% of the highest. Evacuation of the sample-handling system for 10 to 20 minutes is usuallv sufficient to lower peaks from the previous sample below this level. QUANTITATIVE ANALYSIS
The high accuracy obtainable in the mass spectrometer analysis of mixtures containing as many as 30 components has been uti-
lized for years for light hydrocarbon and inert gases. A natural extension of this with the heated instruments in this laboratory has given very useful quantitative analyses of mixtures not easily resolved by other methods. The analysis of the reactor products from the alkylation of benzene with propylene is an example of this. A typical ‘‘bottoms” sample might contain 15% mono-, 70% di-, 15% tri-, and 5% tetraisopropylbenzene and higher. Table I shows the relative heights of the important peaks in the mass spectra of the principal constituents. The parent peaks (molecular ions) have been designated as 100 for convenience in calculation. “ S ~ / S T ~refers I ” to the relative sensitivity of the compound, in scale divisions of the parent peak per milligram of sample, as compared to the sensitivity of the m / e 92 peak of toluene. Minor components, such as substituted styrenes and indanes, are also detected and reported. Calculation for this simple type of analysis is carried out by the usual stepwise subtraction of component spectra (37). The mass spectra of the products of the ethylation of dichlorobenzene are given in Table 11. Halogen isotope peaks here and in Table Is’ have been omitted for brevity. Snalyses of synthetic blends of these components, shown in Table 111, illustrate that accuracies are obtainable approaching those found with light hydrocarbon analysis by the mass spectrometer. The crude alkylate was calculated on the weight of sample introduced, so that the amount of material with insufficient vapor pressure to register on the spectrum (tars) could be estimated by difference. An example of the spectra used in analyzing perhalogen compounds is shown in Table Is’. The sensitivities are calculated for the highest, or base, peaks. The-e compounds are products of
Table 11. m/e
139 145 146 I59 167 173 174 187 195 201 202 215 223 229 230 243 258 SP/ST’l
Mass Spectra of Ethylated p-Dichlorobenzenes
Tetraethyldichlorobenzene 4.31 1.34 0.32 1.53 2 46 2 65 0 35 6 23 0 80 2 52 0 22 3 67 20 8 8 69 1.21 127
Triethyldichlorobenzene 7.53 1.71 0.20 3.46 3.18 5.98 0.97 12.7 41.0 13.6 1.53 151
Diethsldichlorobenzene 12.8 0.04 0.33 9.0 72.2 10 6 3 67 164
Ethyldichlorobenzene 128 0.41 2 32 208
Dichlorobenzene 0:40 100
2 : 16 100
1’39 100
i:44 100
100
0 20
0 22
0.24
0.25
0 75
Table 111. Analysis of Known !Mixtures of Ethylated Dichlorobenzene Flashed Alkylate Synthesis By M.S. 63 2 63 2
Dichlorobenzene
Table IV. m/e
Crude Alkylate Synthesis By M.S. 58.8 59.3
Mass Spectra of Bromochloromethanes
Ion CChCCl4 CRrCICBrCls CBrzClCBrzCh CBnCBrsCl CBr4 SB/ST~
CBra
CBrsCl
CBrzCI?
.. .. , . .. ..
..
..
0:56 100’
..
..
5.5 0.09
0.25
0 20
100’
100’
CBrClr 100
CCh 100 0.00
67:2
* 7 0 60W
; 'c; ; ; (
50-
* C d
-
:&C
'; &+:c::l:lj $, '; 11)I, i l l I i fz:::r &\I
-
2 40c
5
30-
w LL
20
c&;'
C5CI+
I
l0CC,"
0
'
,
' /
I
-
f ; ; ; l ;
"
,,""
'
c,cl;
c6c1:
1
C,CI,'
C6 cl;
11
?
11
I
/I
-
1 1 ,
I
I
I
I
The detection of impurities of high chemical similarity in purified samples is especially difficult by methods based on the chemical properties of a compound, The spectrum of the isopropyl ester of 2,4dichlorophenoxyacetic acid (Table VIII) shows the presence of a compound of molecular weight greater by 14 mass units, or CH2. This would correspond t o t h e butyl ester in roughly 1% concentration. I n the same way the m/e 228 peak is probably the monochloroester. I n general, geometric isomer8 and compounds differing only in the position of substitution on an aromatic ring give very similar mass
bility of the fragments that are thus formed. The further papers in this series are planned as detailed studies of the modes of fragmentation of various classes of chemical compounds. -4liphatic hydrocarbons show greater bond cleavage a t the more highly branched points on the carbon chain. Long unbranched chains yield their most abundant ions in the C2 to C6 range. Unsaturation in a molecule usually decreases the amount of fragmentation. Vinylic bonds are strengthened, while allylic bonds are weakened. Aromatic and other resonance compounds are highly stabilized, as would also be ex-
V O L U M E 28, NO. 3, M A R C H 1 9 5 6
311
benzene, can be ruled out withTable VIII. Mass Spectrum of Isopropyl Ester of 2,4-Dichlorophenoxyacetic Acid out reference standards, as it 0 should give large C&CHC1- and C6H&HCH*OHci~--o--cHr-8-o-cH ion fragments. c1 'CH: Amines, alcohols, ethers, Relative Relative mercaptans, and other elecm/e Intensity Ion m/e Intensity Ion tron-releasing groups typi161 10.9 Clz-Ph-0227 1.9 C1-Jh-OCHzCOOCiHy cally caufie cleavage a t the 162 79.4 ClaPh-OH (ream.) 228 1.5 C1-Ph-OCHaCOOC:Hy (imp.) 163 1 2 . 4 229 0 . 7 carbon-carbon bond beta to 164 50.0 230 0.5 Clz-Ph-OCHz233 0.39 Cl-Ph-OCHzCOOCHz175 145.2 this group. -4liphatic amines 176 27.4 Clz-Ph-OCH: (rearr.) 235 0.24 are especially specific in this Cl-Ph-OCHzCOOCzHr177 95.8 247 0.15 178 17.3 249 0.09 regard, so that the most I 185 44 8 C1-Ph-OCHzCOOH 262 100 Ch-Ph-OCHGOOCzH, abundant ion in their spectra 187 14 8 264 65 0 is usnally the fragment from 219 9 5 Clz-Ph-OCH&OO276 1 0 Cl~-Ph-OCHzCOOC4Hs (imp.) 220 35 9 C12-Ph-OCHzCOOH (rearr.) 278 0 6 this rupture which contains 221 9 4 8m/8Tol 0 095 222 27 3 the nitrogen atom. If there is more than one beta bond, Table IX. Mass Spectra of Isomeric Methoxypropanols cleavage of the one holding Mass Ion CHrCHOHCHzOCHs CHaCH(0CHa)CHzOH the largest group is favored. 27.4 Unsaturation on the a-carbon 5.9 0.06 atom can override this tend100 ency, and alpha substitution 1.5 0.6 usually causes niajor ion fragSB/STOI 2.3 2.0 ments vihich are not easily explained by simple bond cleavages. The isomers, l-iiiethoxy-2propanol and 2-methoxv-1-propanol (Table IX), contain a carhon-carbon bond that is heta to both the alcohol .$ 80 and ether groups. The isomers can be distinguished readily, as cleavage of . 70this bond gives the largest peaks in t 60the spectra: CH&H(OH)- and CH3W OCH2--- (both mass 45) in the 150methoxyd-propanol, and CHSCHW 40(OCH3)- (mass 59) and -CH*OH 2 c (mass 31) in 2-methoxy-1-propanol. 5 300 Halogen, nitro, and most all carW I1 a 2 0 bonyl-type groups tend to weaken the OH alpha bond for cleavage by electron loC,H3 impact. This does not apparently exc4 H 3 0 tend t o all electron-attracting groups, ;I" 'TIl I 1 1 I I ' I I' however, as some, like nitriles, tend to break a t the beta bond. The nitro, carbonyl, and other unsaturated groups are complicated by rearrangement tendencies, as described later. With halogenated molecules, fragnientation increases as the halogen atom is made successively iodine, bromine, chlorine, and fluorine. Loss of a halogen atom with formation of an abundant organic ion is favored for bromo and iodo hydrocarbons, though the chloro and especially the fluoro compounds lose a hydrogen atom with the halogen in many cases. When a neighboring point in the molecule ,i,,,~, ,,,] I , , I ,s-H ;;r I , I I) is weakened, as by chain branching or an aromatic group, the inductive effect of the halogen usually enhances IO this cleavage instead of showing the rupture of the carbon-halogen bond. 0 The probable impurities causing the 3 0 40 5 0 6 0 70 80 90 I00 I10 120 130 140 150 I60 170 I80 I90 200 210 22C /e anomalous peaks in the spectrum of Figure 8. Mass spectrum of benzyl sulfide a crude sample of 2,3-dibromocyclohexyl ethyl ether (Figure 3) can be Soi/STol 0.76
"1
0""'-
-
GC-0-CH2
4.
ji;j
,
1, 1, 1
,
,I
,
-
ANALYTICAL CHEMISTRY
312
deduced assuming such behavior of halogen compounds. The characteristic 1 to 1 natural abundance ratio of the bromine isotopes of masses 79 and 81 shows the number of these atoms in the various fragments. The first C-Br bond is cleaved very readily, giving thus the largest peaks (m/e 205 and 207) from the dibromocyclohexyl ethyl ether. Similar fragmentation of the postulated compounds shown would give the anomalous peaks indicated. The loss of the bromine atom from bromophenol is comparatively small, as this involves rupture of the stable a-phenyl bond. Standards for this compound and two of the others confirm their identification@.
>* 70 I-
6ol
Et
- NI
Cd, P h
50
10-
0
",
I(
i].
'
I
'I
PhEt-?-
1 I
"
I
1
Ih',['
I , ,1 Ph
I '
,
-N-CU2Ph
Ph
1
Pn
MOLECULAR REARRANGEMENTS IN MASS SPECTRA
I00
I
I
I
I
I
t
I
I
I
I
I
I
I-
f
>* 70
70
80
90
I00
110
I20
130
I40
150
Figure 11. illass spectrum of 2,2'-dipyridylamine Sl7o/STal =
1.63
I
I
1
The main difficulty encounteied in structure assignments from the ion fragments in the mass spectrum is that sometimes fragments occur that cannot arise from simple cleavage of bonds in the molecule. Several sizable peaks of this kind (m/e 220, 176, and 162) occurred in the spectrum of the isopropyl ester of 2,4-D (Table VIII). The largest peak in the cracking pattern of phenoxy-pniethoxyphenetole, (Figure 4) must be due to such a rearrangement. The inam 186 peak can be shown not to lie due to an impurity such as phenosyphenol by its rate of decay during effusion (II), by its high appearance potential (34), by comparison with the reference spectrum of pure phenoxyphenol, by additional sample purification, or by independent purity determination (as freezing point depression) on the sample. Much of the analytical disadvan. tage incurred by these rearrangements would be removed if they could be predicted from the molecular structure of the compound. Yo such generaliaations have been reported, though evidence has been collected for various mechanisms of rearrangements in hydrocarbons (13, 17, 20, 21, 33). Rearrangements appear to occur so a~ to give more stable products from a particular bond cleavage or cleavages. For example, production of enialler fragments from a cyclic molecule requires breaking two bonds, so that an ion thus produced contains an odd number of electrons. Intra-molecular hydrogen migration when breaking these bonds gives a more stable "even-electron" ion (13, 91). Similarly, the large amount of CCIF; ion in the mass spectrum of CCIF=CF, can be explained by halo-
V O L U M E 28, NO. 3, M A R C H 1 9 5 6 gen migration to stabilize the "oddelectron" ion formed by cleavage of t,he doable bond. Rearrangements producing odd-electron ions, such as niass 60 from butyric and higher acids ( 1 6 ) , can be explained ( 1 5 ) by the formation of a stable, "even-electron" neutral fragment by the intramolecular hydrogen atom migrat,ion. T h u s a p p r e c ia b 1e r e a r r a n g e m e n t usually occurs in a compound that hear3 a hydrogen a t least two carbon atonis (or often carbon and a n o t h e r a t o m ) r e m o v e d f r o m :in "allylic" bond (thought of here as a bond beta to double bonds such as carbonyl, nitrile, and phenyl also). The presence of oxygen and other het'ero atoms near the allylic bond iri the fragment to which the h?-drogen migrates helps also. Thus the 186 peak in the phenoxy-p-methosyphenetole is fornied from hydrogen migration to the aromatic fragment after cleavage of the allylic osygencxrhon bond. Similarly, the largest peaks in the spectra of such diverse compounds as butyric acid ( l e ) , nbutyraldehyde, n-butyronitrile, n-butyramide, phenetole, and d i b u t y l phthalate can be accounted for b y t h i s mechanism. N o appreciable amount of this rearrangement takes plare in the spectra of propionic acid, propionaldehyde, propionitrile, propionaniide, anisole, or dimethyl phthalate, as there ip no hydrogen two chain atonis removed from the allylic, bond. Both steric. factors and the formation of the stalile even-electron ethylene fragment are thought to contribute to this requirement. RIany of the major peaks of triethyl phoephate (Table X ) must be due to the rearrangement of more than one hydrogen atom, giving an even-electron ion as the stable produrt. The presence of extra nonbondirig electrons on the phosphorus atom, 3 s well as the probable st,ability of t,he rearranged ions, may cause this. Similar "double rearrangenients" are shown by cyclohexylacetic acid and isocrotonic acid (though not appreciably by crotonic acid). Correlations appear possible for other rearrangements, such as the large C2C12F3- ion in the spectrum 0
i1
of CC13--C--CFB, C7H8in the s p e c t r u m of C ~ H & H Z O C H Z C ~ H ~ , CH2F- in the spectrum of CHzClCF,, CH50in the spectrum of (CH,)&HCH?OH, C7H80- in the 0
I1
s p e c t r u m of CeH,CHZOCCH,, a n d
313
'7 ' .
I
1
I
1
85
95
1
1
II
90
25
I
0
HC N C ti 3 -
35
55
45
65
75
105
m/e
Figure 12.
Mass spectrum of diethylformamide S d S T o l = 0.28
-90
8
80
Figure 13.
Mass spectrum of n-capronitrile S u / S T o I = 0.44
CH,CH,CH,CH,CH&N
i
t-
z
50,
P
3c.
0160 70
I
I
I 1
I
-- -- z - '
80
90
100
Figure 14.
-- yI10
1
I20
1
I30
I40
150
160
170
180
I90 200
m/e Mass spectrum of thenoyltrifluoroacetone S n d S T o I = 0.14
210 220 2 3 0
ANALYTICAL CHEMISTRY
314 Table X.
Mass Spectrum of Triethyl Phosphate 0 '
I1
P C z H s b /'OC2Hs OCzHs m/e
%
29 45 81 82 83 97 98 99 100 109 110 111
19.5 14.6 43 31 11.1 0.37 0.24 93 0.23 41 8.4 155
100,
!
%
Probable Ion
Probably I o n
18.4 5.9 56 11.4 10.9 10.4 5.2 2.78 100 4.2 3.3 13.G
I
I
00
It /I
c3H&- in the spectrum of C2H50CCOCsHa. S u c h c o r r e l a t i o n s s h o u l d
,\" -*
1
70
'
DIVERSITY OF 4VALYZABLE CO\.IPOUVDS
CHz-CH-
AH
I I I I i
50
Figure 15.
60
m/e
80
70
90
I00
Mass spectrum of glycerol
-
0.23 CHz--CH--CHz
SdSTol
AH &I AH
C r z M e 2 P n~
~
I
I
~
I
+ 30
,
0
'
1 Yle2PnS
05 P n M e 2
1
80 90 100 I10 I20 I30 140 150 160 170 I80 190 200 210 220 230 240 2 5 0 260 270 2 8 0 2 9 0
/e
Figure 16.
3lass spectrum of tetramethyldiphenj ldisiloxane Smr/STol
greatly reduce thie major drawback to the use of the mass spectrometer in determinations of molecular structure.
-
0.52
The heated-inlet mass spectrometer has been applied to a wide range of chemical compounds in this laboratory. Some random examples are shoii n to illustrate the information available in the mass spectra of various chemical types. The mass spectrum of octachloro-1,3-5-hexatriene (Figure 5 ) shows a sizable molecular ion, C&18+, because of the resonance stabilization of the molecule and vinylic attachment of the chlorine. However in the case of the isomeric octachloro - 3 - methylenecyclopentene (Figure 6), there is no molecular ion, and there is a large loss of the allylic chlorine atoms. The conjugated and cyclic carbon skeleton shows little degradation. Another isomer, octachlorohexene-5-yne, s h o w a little less chlorine loss, but more carbon-carbon scission than the methylenecyclopentene. Benzyl salicj late (Figure 7 ) gives the parent and benzyl ion as the major peaks in its spectrum. Benzyl sulfide (Figure 8) also illustrates the ease of breaking bonds beta to the phenyl ring in preference to the alpha bonds. This overshadows the tendency for cleavage a t bonds beta to a sulfur atom. Ethylbenzylaniline (Figure 9 ) also gives its largest peaks as the benzyl and molecular ions. The tendency in amine compounds for rupture of bonds beta to the nitrogen atom accounts for the significant ions a t m / e 196 and 134, and possibly 77.
V O L U M E 28, NO. 3, M A R C H 1 9 5 6
315
Table XI. m/e
%
69 100 119 131 169 181 219 224 23 1 236 243 25: 202 2A3
100 6 3
Ion
%
281 286 293 305 312 317 319 3 24 331 336 343 348 355 362 367 369 374 379
5.2 0.30 5.7 1.2 0.12 0.47 1.0 0.23 3.4 0.11 4.1 0.04 1.1 0.12 0.38 0.34 0.14 0.09
25 25 13
li.0 0 34 7 8 0 63 5 4 1 9 0 23 0 18 0 68 2 4 0 27 0 19
268
269 274 27;
Mass Spectrum of Perfluorokerosine
m/e
Ion
m/e
%
462 467 469 474 481
2.3 0.09 2.3 1.4 0.07 0.31 0.09 0.30 1.2 0.09 1.8 0.03 0.87 0.06 0 23 0.05 0.16 0.88
Ion
I I1
i
m/e
Figure 17.
3Iass spectrum of titanium tetrachloride Siar/STal
90
-
0.23
t -
W
? 40c
-
4
30U
]
20 Fe
I
-
C 5 H5
1,
I 1
-Fe
+
100.
C5H5
l
l
Figure 18.
1
I
1'
1
I
-
0.93
-
.i Fe
FO
,
I
I
I
Mass spectrum of ferrocene SlsdSTol
- - C 5 H5
I
I
I
-
m/e
%
486 493 505 512 517 524 531 536 543 555 562 567 574 581 593 605 617 63 1 643
0.08 1.4 0.45 0.11 0 13
Ion
0.04 0.66 0.06
0 77
0 24 0 06 0.06 0.02 0 33 0 '4 0 08 0 04 0 08 0 05
A similar cleavage of the bond beta to an amino nitrogen atom gives the largest peak in the spectrum of nicotine (Figure 10). This bond is also weakened by its attachment to the saturated ring. Though the fragmentation of this ring can be closely followed in the spectra, no appreciable peaks are found from breakdown of the resonance-stabilized pyridine nucleus. With two pyridine nuclei, 2,2'-dipyridylamine (Figure 11) is so stabilized that it shows little fragmentation. T h e large loss of one hydrogen atom is very probably from cleavage of the hydrogen-nitrogen bond that is beta t o both rings. Diethylformamide (Figure 12) shows stepwise loss of methyl and methylene groups, similar to aliphatic hydrocarbons. The large 30, 44, and 58 peaks are probably from rearrangements of the molecule to the empirical formulas shown. The largest peak in the spectrum of n-capronitrile (Figure 13) a t m/e 41 can b r explained by the hydrogen rearrangement after beta-bond cleavage described previously. This can also be due to a C3H6+ion. It should be possible to determine the relative amounts of these t F o fragments a t high resolution, ou-ing to their slight differences in mass (2). Thenoyltrifluoroacetone (Figure 14) shows the favored cleavage of bonds to carbonyl groups. Of the two possible fragments accounting for the 111 peak, the thenoyl one is indicated by the 113 peak. Sulfur has 32, 33, and 34 isotopes in a natural abundance ratio of 95.1:0.7:4.2. Absolute mass determination ( 2 ) could also differentiate this. Both carbon-carbon bonds in glycerol (Figure 15) are beta to two hydroxyl groups, leading to a large amount of fragmentationand lack of molecular ion,
316
ANALYTICAL CHEMISTRY
The largest peak in the spectrum of tetramethyldiphenyldisiloxane (Figure 16) is formed from scission of carbon-silicon bonds beta to the phenyl groups. The number of silicon atoms in a particular ion is readily determined from the characteristic abundances of the mass 28, 29, and 30 silicon isotopes. Volatile metal halides such as titanium tetrachloride (Figure 17) are becoming common industrially. The overlapping isotopes of titanium and chlorine make a large family of peaks for each ion formula. There is an appreciable parent peak, in contrast to carbon tetrachloride. This is probably due to greater ease of ionization caused by loss of nonbonding electrons from the titanium atom. Ferrocene (Figure 18) is an interesting metallo-organic compound whose high stability lies in the conjugation of the cyclopentadiene rings through bonding to the iron atom in the middle of the “sandwich.” The almost total lack of fragmentation of the rings in the spectrum gives further proof of this concept. The partial spectrum of perfluorokerosine (Table XI) shows how an apparently complex spectrum can be elucidated assuming simple bond cleavages. This material, obtained from the Organic Chemicals Department, E. I. du Pont de Nemours & Co., is very useful as an internal standard for absolute determination of the mass of a particular peak in another sample, because of both its multiplicity of identifiable peaks and its high volatility. CONC LUSIOW s
The mass spectrometer provides an abundance of useful and unique information about a rompound or mixture. Because of its broad applicability to all types of chemicals, it seems surprising that most emphasis has been on petroleum compounds. Of the 10,000 samples per year analyzed by inass spectrometry in this laboratory, the great majority are of a chemical nature. It is hoped that this paper has helped show that the mass spectrometer is a general tool for both qualitative and quantitative analysis in chemical research and production. ACKNOWLEDGMENT
The author wishes to acknowledge the help and advice of
V. J. Caldecourt and J. L. Saunderson, who pioneered in this laboratory the methods and instruments described above, and R. M. A4bernathey and R. S. Gohlke, who developed many of the applications. LITERATURE CITED
(1) Bernstein, R . B., Semeluk, G. P., .bends, C. B., ; ~ K A L .CHEM. 25, 139 (1953). (2) Beynon, J. H., .Vatuie 174, 735 (1954).
(3) Brown, R. A., Melpolder, F. W.,Young, W. 9.. Petroleum Processing 7, 204 (1952). (4) Brown, R. A., Young, W.S., Sicolaides, S., ASAL. CHEM.26, 1653 (1954). (5) Caldecourt, V. J., Ibid., 27, 1670 (1955). (6) Caldecourt, V. J., ASTM Committee E-14 Conference on Mass Spectrometry, Xew Orleans, May 1954. (7) Collin, J., Bull. SOC. m y . sci. Liege 21, 446 (1953). (8) Dibeler, V. H., J . Research Natl. B u r . Standards 49, 235 (1952). (9) Dibeler, V. H., Mohler, F. L., Ibid., 47, 337 (1951). (10) Dibeler, V. H., Mohler, F. L., Williamson, L., Reese, R. AI., Ibid., 43, 97 (1949). (11) Eden, AI., Burr, B. E., Pratt, A. W.,ANAL.CHEM.23, 1735 (1951). (12) Florin, R. E., Wall, L. d.,Mohler, F. L., Quinn. E., J . Am. Chem. SOC. 76, 3344 (1954). (13) Friedman, L., Long, F. rl., Ibid., 75, 2832 (19531. (14) Gifford, A. P., Rock, S. XI., Comaford, D. J., -1s.i~.CHEM.21, 1026 (1949). (15) Gohlke, R. S., McLafferty, F. W,,Division of Gaj. and Fuel Chemistry, 127th Meeting, ACS, Cincinnati, Ohio, 1955. J . Am. Chem. SOC.74,4404 (1952). (16) Happ, G. P., Stewart, D. W., (17) Honig, R. E., P h y s . Rea. 75, 1319(A) (1949). (18) Kelley, H. Al., ANAL.CHEM.23, 1081 (1951). (19) Kinney, I. W., Cook. G. L., Ibid.,24, 1391 (1952). (20) Langer, 8., ASThI Committee E-14 Conference 011 Mass Spectrometry, Sew Orleans, May 1954. (21) Langer, A., J. Phus. Colloid Chem. 54, 618 (1950). (22) Langer, A., Fox, R. E., ASAL. CHEM.21, 1033 (1949). (23) Long, F. A., Friedman, L., J . Am. Chem. Soc. 75, 2837 (1953). (24) AIcLafferty, F. W., Gohlke, R. S., ASTRI Committee E-14 Conference on Mass Spectrometry, New Orleans. 1 I a y 1954. (25) RIcLafferty, F. W., Gohlke. R. S..Clock, G. E., Conference on Analytical Chemistry and Applied Spectroscopy. Pittshurgh, Pa., March 1953. (26) Rlohler, F. L., Bloom, E. G., Lengel, J. H., Wi3e. C . E., J . A m . Chem. SOC. 71, 337 (1949). (27) Mornigny, J., Bull. soc. roy. sci. Li6ge 22, 541 (1953). (28) Korton, F. J., J . Am. Chem. Soc. 71, 3488 (1949). (29) O’Keal, RI. J., Jr., W e r , T. P., ANAL.CHEX.23, 830 (1951). (30) Rock, S. JI.,Ibid.,23, 261 (1951). (31) Rosenstock. H. M., Wallenstein, M. B., Wahrhaftip, .1. L., Eyring, H., Proc. Natl. Acad. Sci. U . S. 38, 667 (1952). (32) Sharkey, A. G., Jr., Schultz, J. L., Friedel, R. A , ASTJI Conimittee E-14 Conference on Mass Spectrometry. Sew Orleans, May 1954. (33) Stevenson, D. P., Hipple, J. A., J . Am. Cheni. Soc. 64, 1588 (1942). (34) Stevenson, D. P., Wagner, C. D., Ibid., 72, 5812 (1950). (35) Taylor, R. C., Brown, R. A,, Young, W. S., Headington, C. E., Ax.4~.CHEM.20, 398 (1948). (36) Thomas, B. W.,Seyfiied, W.D., Ibid.,21, 1022 (1949). (37) Washburn, H. W., Wiley, H. F., Rock, S. AI.. ISD. ENG.CHEM., AXAL.ED. 15, 541 (1943). RECEIVED for review .4pril30, 1955. dcoepted December 10, 1055. ASThl Committee E-14 Conference on Mass Spectrometry, New Orleans, May 1954.
