The interferences caused by chloride have been discussed. Inasmuch as this anion must be renio\.ed by volatilization, it was not deemed necessary to test a large list of volatile anions. Nitrate absorbs strongly a t 315 mp but presents no problem, as it does not follow Iialladiuni in the extraction, even if it were not removed by volatilization. Perchlorate causes no interference and the other common nonvolatile anions, sulfate and phosphate, are used in the procedure.
LITERATURE CITED
(1) Ayres, G. H., Alsop, J. H., ANAL. CHEY.31, 1135 (1959).
12) \ , Avres. G. H.. Janota. H . F.. Ibid.. 31. 198s (1959). ’ (3) Ayres, G. H., Narang, B. D., Anal. Chinz. Acta 24, 241 (1961). (4) Beamish, F. E., LIcBride, W. A. E., Ibid., 9, 349 (1953); 18, 551 (1958). (5) ~, Clem. R. G.. Huffman. E. H.. J . Inora. ,\’ucl. bhem., in press. (6) Feigl, F., “Spot Tests in Inorganic Chemistry,” 5th English ed. (translated by R. E. Oesper), p. 328, Elsevier, New York, 1958. ( 7 ) Hillebrand, W. F., Lundell, G. E . F., Bright, H. A,, Hoffman, J. I., “ilpplied Inorganic Analysis,” 2nd ed., p. 347, Wiley, New York, 1953. (8) Jacobs, W. D., ANAL.CHEM.33, 1279 ( 1961). (9) Jacobs, W. D., Wheeler, C. M., Vi aggoner, W. H., Talanta 9, 243 (1962). (10) Janota, H . F., Ayres, G. H., ANAL. CHEM.36, 138 (1964). (11) Kolthoff, I. AI., Elving, P. J., I
,
“Treatise on Analytical Chemistry,” Part 11, 1’01, 8, pp. 429, 434, Interscience, New York, 1963. (12) Pepkowitz, L. P., ANAL. CHEM. 24, 900 (195‘2). (13) Pyle, J. T., Jacobs, W. I)., Talanta 9 , 761 (1962). (14) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” 3rd ed., p. 711, Interscience, New York, 1Q.W
(1 ij-sherif, F . G., >Tichail, K. F., J . Inorg. Xucl. Chem. 25, 999 (1963). (16) Sundarum, A. K., Sandell, E . B., J . Am. Chem. SOC.77, 855 (1955). (17) Templeton, I). H., Watt, G. W.. Garner, C. S., Ibzd., 6 5 , 1608 (1943).
RECEIVEDfor review August 28, 1964, Accepted October 22, 1964. Work done under the auspice of the U. S. Atomic Energy Commission, AEC Contract W7405-eng-48.
Correlations and Anomalies in Mass Spectra Lactones W. H. McFADDEN, E. A. DAY,’ and M. J. DIAMOND Western Regional Reisearch laboratory, Western Utilization Research and Development Division, Agricultural Research Service,
U . S. Department of Agriculture, Albany, Calif. The mass spectra of 17 lactones are presented. Laciones with an aliphatic chain Cz or greater a t the point of ring closure show significant ion current due to loss of the hydrocarbon chain. For 5- and (5-membered rings these ions will geneirally be the most abundant. For larger rings it defines the point of ring closure or the number of carbons in, or at, the ring. With a chain Ca or greater, lactones rearrange to give ions due to loss of H20; with four or more carbons in the chain, an ion current due to loss of a second water molecule is common. These rearrangements aid in identification, particularly with low intensity spectra that may not yield a significant amount of the parenf ion. Rearrangement ions isobaric with esters were not observed in any abundance except for the terminally-closed cyclopentadecanolide which gives several such ions. Knowledge 0.F these anomalous rearrangements is necessary to avoid confusion in analysis of unknowns.
L
AXALOGS of aliphatic hydroxy acids are amenable to identification by infrared spectrophotometry ( I S ) , gas chromatography ( l e ) , and chromatography of derivatives (3, 8). The utility of combinations of these methods for unequivocal identification decreases, however, as one approaches the concentration levels of flavor constituents in foods. A case in point is milk fat, in which the lactones ACTONE
’ O n leave from -the Department of Food Science and Technology, Oregon State University, Corvallis, Ore.
occur in the parts-per-million range (3, 8). To identify these compounds it is necessary to resort either to elaborate, large scale extraction and distillation or to micro-qualitative methods such as mass spectrometry. The latter approach is increasingly attractive, especially with recent developments in combination of capillary gas chromatography with rapid-scan mass spectrometry (4, 6, 10, 11, 1 5 ) . The mass spectra of lactones have been discussed only briefly in previous work ( 2 , 7 , ) , which did not describe effects encountered as the side chain increases. Because these effects have important analytical implications, the mass spectra of 17 available lactones are presented. EXPERIMENTAL
The y-valerolactone was supplied by the Quaker Oats Chemical Co. The other ylactones were obtained from K and K Laboratories except for y-palmitolactone and y-stearolactone which were synthesized by Dr. J. S. Showell of the Eastern Regional Research Laboratory, Cnited States Department of Agriculture. &Lactones were supplied by Unilever Company, England. The 12-stearolactone was prepared from 12-hydroxystearic acid according to the procedure of Stoll and Gardner (14). The t-caprolactone was obtained from K and K Laboratories and the w-lactone, cyclopentadecanolide, was obtained from Aldrich Chemical Co. Impurities in the ylactones (up to CIJ were less than 2% by gas chromatography. The cyclopentadecanolide was checked by gas chromatography, infrared (IR) and nuclear magnetic
resonance ( S M R ) to confirm the struc ture and no significant impurities were detected. The other lactones were used as obtained. The mass spectra were obtained with a Bendix Time-of-Flight mass spectrometer, Model 12, operating with continuous ionization. Samples were introduced from a stainless steel inlet system a t 150” C. RESULTS A N D DISCUSSION
The mass spectra of the lactones are presented in Figures 1 to 3. A tabulation of these data will be circulated in the ASTM E-I4 Committee file of uncertified mass spectra. Background corrections were made in the usual way except when the air background fluctuated significantly. I n such cases, the established Oz/S2 ratio was used to correct the mass 28 ionization. Determination of mass 28 was not possible for y-stearolactone. Parent Ion. Only a small traction of the total ionization is collected as the parent ion. Table I shows t h a t the proportion of ions remaining as t h e parent ion decreases t o a minimum for Cloor CI1lactones and increases beyond that. This small percentage is sufficient with normal sample amounts to obtain the parent mass, but with small samples such as may come from capillary chromatographic columns the parent ion would not always be detectable. Ions Due to Loss of the Side Chain. The most important feature of the mass spectra of the lactones i i the amount of ion current due to the lactone ring-i.e., loss of the side chain. VOL. 37, NO. 1 , JANUARY 1965
89
Figure 1 .
90
ANALYTICAL CHEMISTRY
Mass spectra of y-lactones
80
100
I20
75
1
751-
1
I
50
F
,
c'
\c
180
160
140
200
M A S S
/ L O
CCC\
S C A L E
Figure 3. Mass spectra of e-caprolactone, cyclopentadecanolide, and 12-stearolactone
0 25 I./ - 1
,
.
M-84 .I
I,,
,.,Ill
II ,, ",,I& I
1 .I,.., I
I
I
I
For the y-lactones from Cs to C18 and the &lactones from C8 to C ~ Zthis . breakdown gives the base peak and readily characterizes the number of carbons within, or a t , the ring. When R = H or CHs, this reaction path is less important, but smaller lactones can be distinguished from aldehydes, hydrocarbons, and other isobaric compounds by other features. Terminally closed lactones show very little ion current due to loss of hydrogen and with larger ring sizes, this mode of decomposition is absent. The structure of lactones with larger rings can also be identified by this fragmentation rule. The structure of the 12-stearolactone was determined in part this way and in part by comparison with the y-stearolactone. For the 12-stearolactone, ion current a t mass 197 (10s of CSHI3)is only 2.4% of the total (28.67, of the base peak). However, the possibility that this might be ClrHZ9+ from y-,stearolactone was eliminated by comparison with the mass spectrum of authentic y-stearolactone. Furthermore, as the chain length increases, ion:5 due to Rf become negligible (Table I). The ion current due to loss of the side cahain a t the point of ring closure is plotted in Figure 4 as a function of the number of rarbons. The curves for the y- and &lactones can be interpolated and extrapolated to predict other members. and the espected curves for other ring sizes are suggested. The amount of fragmentation for this mode will decrease with increasing ring size as is i,ndirated by the point for 12-stearolactone. The Ion . ' R F o r several of the iiiash siicctra presented, R+ cannot be distinguibhed from other possible ions oc'curriiig at the same mass. However,
.. I
M-16
M=WS
M-I8
I
I
I
I
I
I
40
I
Number of carbon atoms Figure 4. Ionization due to lactone ion, CH(CH2),COC=0 ---o--J
as a function of the number of carbons in the molecule 0
Table I.
X
y-lactones.
8-loctoner.
0
12-stearolactone
Partial Mass Spectra of Seventeen Lactones
(Percent of Total Ionization) Parent ion y-But yrolactone y-'ialerolactone 7-Caprolactone y-Heptalactone y-Octalactone y-Sonalactone ?-Decalactone y-Undecalactone ?-Palmitolactone 7-Stearolactone 8-Octalactone &Decalactone 6-Undecalactone &Dodecalactone t-Caprolactone 12-Stearolactone Cy clopentadecanolide
4 6 1 7 1 0
1 0 0 9 0 4 0 1 0 1 0 5 0 8 1 2 0 7 06 0 7 3 4 1 0
2 6
Minus. R 1.2 5,s 21.3 __ 34.7 ___ 36.8 34.6 ____ 30.6 21.8 10.3 ___ 7.8 ___ 18.3 20.0 18.5 16.2 0.3 2.4
c
d e
, . .
2.7 (16.9)c 3 4
52d 1 1 (30 6 p 06 0 2
~
~
6 7 82d 0 8 (16 2 ) .
~
Minus Hz0
Minus 2H20
...
... ...
, . . , . .
2 6 1 4 0 9 0 5
1 2 3 2 1
1 3 5 3 7
1 2
14
... , . .
...
0.2 0.4 0.7 1.1
2.0
... 1.1 1.5 1.4
8% ... ... , . .
1.8 2.5 2.0 1.8 1.3 1.0 0.9 2.5 3.2 3.2 3.1
~
1 1
, . .
1-nderlined entries are base peaks. r--OH-T
* Rearrangement
R'
Lactone ring
ion CH~-CH-(CHz),--C Principally due to CHO +. Partly due to ,( CHz),,C,O+. R isobaric with ( P minus R ) +.
2 9 1 9
...
... 0.5
I
+
VOL. 37, NO. I , JANUARY 1965
91
it is clear from Table I that R f is not an important mode of ionization and 3hould not interfere with determining the ring size of an unknown lactone. Ions Due to Loss of Water. This mode of fragmentation is not observed from the smaller lactones, but as the rhain length increases to three or more carbons it becomes significant. Pwsumably, t h k length permits a steric configuration that is favorable t o transfer of a hydrogen in the fayhion :
c
\
//'
H
c
e
c
c
(+I
Such a mechanism is consistent with the accepted fact that one of the more important electronic configurations of the excited niolecule ion involves loss of a *-electron f m n the oxygen ( 5 ) and the established fact that transfer of a hydrogen to an oxygen favors a 6membered intermediate (1, 9 ) as shown. K h e n this transfer has occurred, the lactone ring is less stable, permitting additional hydrogen transfer and subsequent loss of water. I n many cases the ion current due to loss of 18 mass units is several tinies more intense than that due to the parent ion'and may often be uPeful to establish the molecular weight in weak siiectra. It is interesting that as the chain length increases, ions formed by loss of a second water molecule also occur. To establish that this second water loss vias not a result of thermal decomposition in the 150' C. inlet system, ypalmitolactone was introduced directly into the ionization chamber without raising the temperature. The same mass spectrum was obtained in this fashion as with the 150" C. inlet. Mass 28 Ions. For smaller ylactones, it has been establishrd that this ion current is primarily due to C2H,+ from the lactone ring ( 7 ) . Figures 1 to 3 show that this cracking mode is less significant for y-lactones as the chain length increases. For the &lactones, mass 28 ions are relatively insignificant but the ion current a t mass 42 is more intense. The single example of the e-lactone gave only a modest increase iu ion current a t mass 56, had mass 42 as its base peak (20y0of the total), and had significant ionization a t mass 28. Ions Due to Loss of 44 Mass Units. This mode of ionization is noted from smaller lactones and is attributed to loss of COp ( 7 ) . It is insignificant for lactones of more than 7 or 8 carbons. Other Fragment Ions. Rearrangements leading to ion peaks a t masses 92
31, 45, 59, etc., are not observed in significant amounts except for cyclopentadecanolide. This compound is discussed separately. Transfer of one hydrogen to the lactone moiety (as described for the rearrangement leading to loss of HpO) occurs if the chain contains 3 or more carbons. This can lead to ions at masses isobaric with saturated hydrocarbon molecule-ions, but the only significant (1 to 37, of total ion current) species of this type is that formed by
ANALYTICAL CHEMISTRY
transfer of the hydrogen and break of the alkyl chain a t the bond to the ring closure thus forming the ion
r--OH--7 -CH*--CH-(CHn) I
+ ,,--C=O
(This structure is drawn for convenience; the excited ion is lirobably too dynamic to be so simply illustrated.) However, ions due to this inode of breakdown were not observed in the mass spectrum of the Ci2-stearolactcne. As chain length increases, ion masses corresponding to conimon hydrocarbon ions-e.g., 71,70,69, etc.-are observed, but the resolution of the mass spectrometer used is not sufficient to distinguish whether these contain oxygen. For the 12-stearolactone, such ions were observed over a considerable range which accounts for the low intensity (8.37' of the total ion current) of the base peak at mass 55. An ion peak appeared at mass 168 in the spectrum of 12-stearolactone. Presumably it corresponds to loss from the lactone of R-CH-0 which forms the icn CIoHpsCO'. This type of break was established by isotopic labeling in the small lactones studied by Friedman and Long ( 7 ) , but this mode of decomposition is obscured by other ion peaks for most of the lactones studied here. The mass 168 ion gave additional evidence for the structure of the 12stearolactone. Anomalous Observations in M a s s Spectra of Cyclopentadecanolide. The single examljle of an w-la,ctone, cyclopentadecanolide, showed several unexpected rearrangement peaks. Subsequent to mass analysis, this sam1,le was checked for purity by IR, gas chromatography (packed column) , and NMR. All methods showed impurities less than 1% and confirmed the structure. Losses of 18 and 36 mass units were observed for the other lactones only when a side chain of three carbons was
present. In the spectra of cyclopentadecanolide significant ions at masses 222 and 204 (11-18 and 11-36) respectively) indicate that the ring is readily broken in the excited parent ion, thus permitting a transfer of hydrogens from hydrocarbon portions to the oxygens and resulting in the observed loss of HsO and 2H20. An ion peak at 11-60 mass units (180) seems unusual for a lactone system. Presumably, rearrangement results in a 1 0 s equivalent to an acetic acid niolecule thus forming the ion C13Hs4+. This ion further appears t o lose H P to give C13Hn2+at mass 178. The rearrangement leading to loss of mass 60 (C2H402)can also form ions isobaric with esters at masses 172, 158, 130, 116, and 102. Ions one mass unit lower are also commonly observed (masses 73, 101, 115, etc.). Thus, after rearrangement, the molecule-ion can lose a di-unsaturated hydrocarbon molecule or mono-saturated radical. The smallest such loss observed was C5H8. I t is not obvious whether the lower mass ions of this type cccur by secondary fragmentation from this ion (CioHzoO2',mass 172) or by ejection of a larger hydrocarbon group from the rearranged parent ion. Such ions could cause serious confusion in analysis if prior knowledge has not been obtained and purity is uncertain. LITERATURE CITED
(1) Benz, W., Biemann, IC., J . .4m. Chem. SOC.86,' 2375 (1964). ( 2 ) Bie,rpnn, Klaus, ">lass Spectrometry, AIrGraw-Hill, Sew York, 1962. (3) Boldingh, J., Taylor, It. J., Suture 194, 909 (1962).
(4) Brunnee, C., Jenkel, L., Kronenberger, K., 2. .4nal. C'heni. 197, 42 ilC)A.?> - --,
(5) Cummings, C. S., 11, Bleakney, W., Phus Reo. 5 8 , 787 (1940). (6) Ijorsey, J. A , , Hunt, R. H., O'Neal, 11. J.. AKAL.CHEM.35. 511 11963'1. ( 7 ) Friedman, L., Long, F. It., J . Am. Cheni. SOC.75, 2832 (1953). (8) Keeney, P. G., Patton, S., J . Dazry Sc7. 39, 1104 (1956). ( 9 ) IIcFadden, LV. H., Black, I). R., Corse, J. W..J . Phus C'hern. 67.
1517 (1963).
( 1 0 ) AIcFadden, W. H., Teranishi, It., .Yature 200, 329 (196:j). ( 1 1 ) AIcFadden, iV, H., Teranishi, R.,
Black L). R., Day, J. C., Paper presented at the 10th Annual hleeting of .4STAI Committee E-14 on Alass Spectrometry, Xew Orleans, La., June 1962. (12) Pat,ton, S., J . Dairy Sci. 44, 207 (1961). (13) Rasmussen, R. S., Brattain, R. R., J . A m . Chem. SOC.71. 1073 (1943). (14) Stoll, 11.) Gardner,'R. E., hela.?him. -Acta 17, 1609 (1934). ( 1 5 ) Teranishi, R., Buttery, I?. G., hIcFadden, LV. H., Mon, T. R., bVasserman, J., . ~ N A I . .CHEM.36, 1509 (1964). RECEIVEDfor review July 20, 1964. Accepted September 25, 1964. Reference to a conipany or product name does not imply approval or recommendation of the I:. S. Department of Agriculture to the exclusion of others that may be suitable.
