Investigation of the formation of MH+ and other ions in the mass spectrum of methyl decanoate using specifically deuterated decanoates

Chemistry and Physics o f Lipids, 37 (1985) 271-281

271

Elsevier Scientific Publishers Ireland Ltd.

INVESTIGATION OF THE FORMATION OF MH + A N D O T H E R IONS IN THE MASS SPECTRUM OF M E T H Y L D E C A N O A T E U S I N G S P E C I F I C A L L Y D E U T E R A T E D DECANOATES*,**

A.P. TULLOCH and L.R. HOGGE

Plant Biotechnology Institute, National Research Council o f Canada, Saskatoon, Saskatchewan, S 7N 0 W9 [Canada) Received January 29th, 1985 accepted April 8th, 1985

revision received April 8th, 1985

Intensities of [M + 1 ] ÷ ions in mass spectra of methyl esters of C s C14 acids exceeded theoretical values, deviation was greatest for the shortest chain esters and for methyl decanoate was four times the calculated intensity. Similar effects were also observed in spectra of decanoates of C : - C 6 alcohols. The abnormal results were due to the presence of MH + ions and mode of formation of this ion was investigated using all the methyl gem dideuterated decanoates, methyl[10-2H3] - and [3,5,6-2H6] decanoates and [2H3]methyl decanoate. A hydrogen radical migrates from one of the carbons in the chain to the ionized carbonyl oxygen and the resultant ion, on collision with a molecule, transfers the hydrogen, as a proton, giving MH ÷. The transferred hydrogen was derived from those same carbons that provide hydrogen in the first stages of fragmentation to the mass spectrum of methyl decanoate. Thus 37% came from C-4, 15% from C-5,9% from C-6 and lesser percentages from the other carbons. The results do not exclude possible formation of MH + by hydrogen transfer from ions m/z 74 and 87. Modes of formation of ions m/z 103, 117 and 131 were also studied using the deuterated decanoates. They were formed by a mechanism similar to that involved in production of ions m/z 87 and 143 but less specifically. Hydrogen transfer from C-3, C-4 or C-5 was followed by cleavage of the bond between the carbons c~- and 13- to that carbon. Some initial migration from carbons 5, 6 and 7 also occurred.

Keywords: mass spectroscopy; ion-molecule reaction [M + 1] + ions; methyl deuterated decanoates. Introduction A s a t i s f a c t o r y m e t h o d o f m e a s u r i n g d e u t e r i u m i n c o r p o r a t i o n is a n essential part o f an i n v e s t i g a t i o n o f t h e s y n t h e s i s o f d e u t e r a t e d c o m p o u n d s . 1H-NMR is s o m e t i m e s applicable b u t for l o n g - c h a i n m e t h y l a l k a n o a t e s , w h i c h have m a n y p r o t o n s w i t h similar c h e m i c a l shifts, mass s p e c t r o s c o p y is usually t h e o n l y m e t h o d possible [1 ]. D e u t e r i u m i n c o r p o r a t i o n is generally e s t i m a t e d f r o m t h e relative i n t e n s i t i e s o f t h e M ÷ i o n o f t h e required p r o d u c t a n d o f M ÷ ions f r o m i n c o m p l e t e l y d e u t e r a t e d species. This p r o c e d u r e gave s a t i s f a c t o r y results w h e n it was applied t o C16 a n d C ls m e t h y l e s t e r s [ 2 - 4 ] . *NRCC No. 24525. **Presented at the Canadian Chemical Conference in Montreal, Canada, 1984. 0009-3084/85/$03.30 © 1985 Elsevier Scientific Publishers Ireland Ltd. Published and Printed in Ireland

272 When a number of specifically deuterated methyl decanoates were prepared [5], however, measurement of deuterium content from the intensity of the M ÷ ion was complicated by the presence of an abnormally large [M + 1] ÷ ion. In some preparations, deuteration at a site remote from the carboxyl was intended but additional deuteration by partial exchange of protons a- to the carboxyl with deuterons from the solvent was also possible. Since the reaction could not be followed by mass spectroscopy if other factors were increasing [M + 1] +, a study of conditions affecting the intensity of [M + 1] ÷, in the spectrum of methyl decanoate was undertaken.

Experimental Mass spectroscopy GC-MS data were obtained using a model 4000 Finnigan GC-MS system interfaced to a model 2300 Finnigan lncos data acquisition system. The GC column was a 60M X 0.32MM fused silica column, coated with DB-5 (methyl(5%)diphenylsilicone), connected directly to the ion source. The linear velocity of the helium carrier gas was 40 cm/s. Samples were injected in the splitless mode at an initial temperature of 50°C, the temperature was immediately raised ballisticaly to 150°C and programmed at 4°C/rain to 225°C. MS were obtained using multiple ion detection (MID) in the appropriate mass range. Percentage of [M + 1] ÷ in methyl decanoate was determined using an MID program that scanned from mass 179.554 to mass 200.560 in 0.138 s. Proportions of other selected ions were measured in a similar way and isotopic purity of TMS esters of deuterated decanoic acids was determined from the intensities of the ions in the vicinity of the [M-15] ÷ ion. All analyses were performed at 70 ev, intensities were the average of 3 - 5 measurements.

Methyl deuterated decanoates Syntheses of methyl[7-ZH2] -, [9-2H2] - and [10-2H3]decanoates were described previously [5] and methyl[8-2H2]decanoate was synthesized in the same way as methyl[7-2H2Jdecanoate. The preparation of [2-2H2]decanoic acid was as described before [2] and [3-2H2]- and [4-2H2]decanoic acid were prepared in the same way as [3:H2]and [4-2H2]hexadecanoic acids [6]. [5-2H2]Decanoic acid was prepared by two malonate extensions [1,2,7], starting from [1-ZH2]hexanol and [6-2H2]decanoic acid was obtained in the same way from [2-2H2]hexanol (obtained by reduction of [2-ZH2]hexanoic acid [2]). Synthesis of [3,5,6-2Ho]decanoic acid also started from [2-ZH2]hexanoic acid [2], reduction with lithium aluminum deuteride [2] gave [ 1,2-2H4]hexanol and one malonate extension, followed by another lithium aluminum deuteride reduction gave [1,3,4-2H6] octanol. A second malonate extension yielded [3,5,6-ZH6]decanoic acid. Methyl esters of the above acids were prepared by reaction with methanol containing 3% hydrogen chloride in the usual way, all methyl esters were distilled and boiling points per 16 mm were l l0 114°C. Isotopic compositions of the acids, determined as described above were: [2-2H2]decanoic, 94% 2H2, 6% 2H; [3-2H2]decanoic, 98.5% EH2, 1.5% 2H; [4-ZH2]decanoic, 98.8% 2H2, 1.2% 2H; [5-2H2]decanoic, 99.3% 2H2, 0.7% ~H; [6-2H2]decanoic,

273 98.6% 2H2, 1.4% 2H; [7-2H2]decanoic, 96.6% 2H 2, 3.4% 2H; [8-2H2]decanoic 96.7% 2H2, 3.3% 2H; [9-2H2]decanoic, 98.3% 2H2, 1.7% 2H; [10-2H3]decanoic, 97.1% 2H3, 2.9% 2H2; [3,5,6-2H~]decanoic, 95.7% 2H 6, 3.3% 2H,, 1.0% 2H 4. Deuterium content of the methyl esters was assumed to be the same as that o f the corresponding acids. Propyl, butyl, pentyl and hexyl decanoates and [2H3]methyl decanoate were prepared as follows: alcohol (0.0183 mol) and triethylamine (0.027 mol) in methylene chloride (25 ml) were treated with decanoyl chloride (0.0167 mol) in methylene chloride (5 ml) and the solution refluxed for 2 h. The solution was washed with water, dried over sodium sulphate and solvent removed. Crude ester was chromatographed on Biosil A (100 g activated at 120°C) and pure ester was eluted with hexane/ether (99: 1) and distilled. Methyl esters of octanoic, nonanoic, undecanoic and dodecanoic acids and ethyl decanoate were obtained from Nu Chek Prep., Inc. Elysian, MN. Results

and Discussion

The relative intensity of [M + 1 ] ÷ increased with increase in size of sample introduced into the GC-MS instrument as shown in Table 1. As has been noted before, M ÷ is not an abundant ion in mass spectra of C5-C1o esters [8], and as shown in Table II, it formed only 3% of the base peak in the spectrum of methyl decanoate. Due to this low relative intensity of M ÷, 10 ng was the smallest sample which gave an M ÷ ion of sufficient intensity for accurate measurement and this still gave a high value for [M + 1]. Intensity of M ÷ increased with increasing chain-length and at the same time the abnormal intensity of [M + 1] ÷ decreased (Table 1I). Thus instability o f M ÷ and intensity of [M + 1] ÷ are both associated with short chain lengths. The values for observed/calculated [M + 1] ÷ (Table II) also show that, at a sample size of 320 ng, errors due to high values for [M + 1] ÷ would affect results for all methyl esters shorter than methyl hexadecanoate. Samples smaller than 320 ng could be used as chain-length increased but in practice it TABLE I VARIATION OF INTENSITY OF [M + 1] ÷ WITtt SAMPLE SIZE IN THE MASS SPECTRUM OF METHYL DECANOATE Sample s i z e (ng)

Intensity of [M + II ÷a'b

640 320 160 80 40 20 10

62 56 37 30 21 19 15

100. b Value calculated from isotopic contributions for C and H is 12.1% [13 ].

a M÷ =

274 TABLE II VARIATION IN INTENSITY OF M÷ AND [M + 1] + WITH CHAIN LENGTH IN MASS SPECTRA OF METHYL ALKANOATES a No. of carbons in acid

Intensity of M÷ (relative to base peak)

Intensity of [M + 1 ]* (M÷ = 100)

[M + 1 ]÷ observed/ calculated b

8 9 10

1.9 2.2 3.2

165 96 56

16.7 8.7 4.6

11

4.6

35

2.7

12 13 14 15

5.7 6.2 6.8 6.7

29 25 21.5 21

2.0 1.6 1.3 1.2

16

7.2

20.2

1.08

18

8.3

22.2

1.06

a Sample size was 320 ng. b Calculated from isotopic contributions for C and H [ 13 ].

was o b s e r v e d t h a t p o t e n t i a l l y m i s l e a d i n g [M + 1 ] ÷ i n t e n s i t i e s were o b t a i n e d in e s t i m a t i n g d e u t e r i u m i n c o r p o r a t i o n in C~2 a n d C~4 esters as well as in C l o esters. It has n o w b e e n f o u n d t h a t TMS esters o f acids give s p e c t r a w h i c h are a q u i t e satisf a c t o r y m e a n s o f e s t i m a t i n g d e u t e r i u m c o n t e n t in b o t h C16 [9] a n d C1o acids [5]. S o m e o t h e r t y p e o f ester c o u l d , h o w e v e r , still b e useful since TMS esters are sensitive to m o i s t u r e a n d t h e i r p r e p a r a t i o n r e q u i r e s p r e l i m i n a r y h y d r o l y s i s o f m e t h y l esters t o acids d u r i n g w h i c h d e u t e r i u m m i g h t b e lost. D e c a n o a t e s o f C : - C 6 alcohols were p r e p a r e d a n d i n t e n s i t i e s o f M* a n d [M + I ] ÷ are s h o w n in Table 1II. C o m p a r i s o n o f these d e c a n o a t e s

TABLE IlI INTENSITY OF [M + 1 ] + IN MASS SPECTRA OF DECANOATES OF METHYL TO HEXYI., ALCOHOLS a Alcohol chain length

Intensity of [M] + (relative to base peak)

Intensity of [M + 1 ] ÷ (M* = 100)

[M + 1 ]+ observed/calculated b

1

3.2 2.8 2.1 1.2 0.8 0.9

56 46 53 46 42 40

4.6 3.5 3.7 3.0 2.5 2.3

2 3 4 5 6

aSample size was 320 ng. bCalculated from isotopic contribut ions for C and H [ 13 ].

275 and methyl esters (Table II) with the same molecular weight, for example hexyl decanoate and methyl pentadecanoate, showed that intensity of [M + 1 ] ÷ was about twice as large in the spectrum of the hexyl ester as it was in that of the methyl ester. Also intensity of M ÷ relative to the base peak decreased steadily to 0.8% and 0.9% in spectra of pentyl decanoate and hexyl decanoate respectively. Thus esters of Clo acid of these chain-lengths at least are not useful alternatives to methyl esters. The [M + 1] ÷ ion, which has previously been observed in mass spectra of a variety of short chain esters [10], undoubtedly resulted from an ion-molecule reaction since it became more prominent as the sample size increased (Table I). It can be assumed that the ion is formed in a self protonation reaction: M ÷" + M-~MH+ + [M-H]" which could take place either by transfer of an H atom from M to M ÷ or of H ÷ from M ÷" to M [11]. Formation of MH ÷ ion has been observed quite frequently in mass spectra of compounds containing a heteroatom, such as ethers, esters, nitriles and alcohols [ 1 1 - 1 3 ] . Though there have been many investigations of these ion-molecule reactions [11,12], esters such as methyl decanoate do not seem to have been examined. The origin of the hydrogen in MH ÷ is of particular interest. To determine the source of the hydrogen and how it might be transferred, the spectrum of methyl decanoate (Fig. 1) must be considered. The ions and intensities were:

74

I00-

87

>FZ hi I-Z

50-

55

9

143

69 I01

.11

l~l,I .i,h i 6o

,111 I

I00

Fig. 1. Mass spectrum of methyl decanoate.

115 ,ll [

155

129 J I

m/z

I

140

J],

186 I,

276 186(3), 155(11), 143(22), 129(7), 115(3), 101(12), 87(76), 74(100). The principal ions in the spectrum, m/z 74, 87 and 143, are formed in processes which involve transfer of a hydrogen atom to the carbonyl oxygen. Thus the base peak, m/z 74 is fo~xned by the McLafferty rearrangement [13,14] in which the hydrogen from C-4 is transferred and the C-2 to C-3 bond is broken. In formation of ion m/z 87, hydrogen is transferred from C-5, C-6 and to some extent from C.7, followed by reciprocal hydrogen transfer from C-2 and cleavage of the C-3 to C-4 bond [ 1 4 - 1 6 ] . Ion m/z 143 is formed in part by a process again starting with transfer from C-6 but followed by cleavage of the C-7 to C-8 bond [ 15]. In the spectrum of methyl decanoate ion m/z 143 is also the M-43 ion which is formed by expulsion o f C-2, C-3 and C-4 and a hydrogen [14,17]. The specific hydrogen migrations involved in these processes were established by examining mass spectra of methyl esters of specifically deuterated octadecanoic acids [14,18] and transfer of specific hydrogens to form MH ÷ can be investigated in the same way. Thus transfer of a specific deuterium of a deuterated ester should cause an increase in intensity of [M + 2] ÷ (formation of MD +) and a decrease in the intensity of [M + 1] ÷ (MH+). Syntheses of methyl esters o f deuterated decanoic acids, described recently [5] (see Experimental), made it possible to carry out the investigation. The intensities of the [M + 1] ÷ and [M + 2] * ions, relative to that of M ÷, were measured in spectra of all the methyl gem dideuterodecanoates and of [10-2H3] and [3,5,6-2H6] decanoates and of [2H3] methyl decanoate. Relative percentages of MH ÷ and MD ÷ were then calculated, after subtracting the isotopic contribution of carbon and hydrogen, and gave the results listed in Table IV. The amount of deuterium which origiTABLE IV RELATIVE PERCENTAGES OF [MH]+ AND [MD] ÷ IN MASS SPECTRA OF METHYL DEUTERODECANOATESa,b Deuterium position

[MH] + (%)

[MD] + (%)

[2-2H2] [3-2H2] [4-2H2] [5-2H2] [6-2H2] [7-2H2] [ 8-2H:] [9-2H2] [ 10-2H3] [~H31 [ 3,5,6-2H6]

94.2 94.9 62.9 84.7 91.6 94.4 95.7 97.3 96.7 97.1 71.1

5.8 5.1 37.1 15.3 9.4 5.6 4.3 2.7 3.3 2.3 28.9

aSample size was 320 ng. bCalculated from intensities of [M + 1] ÷and [M + 2 ] ÷ after subtraction of theoretical isotopic contr~utions for carbon and hydrogen in a Cll compound (M + 1 = 12.1; M + 2 = 0.7 [13]).

277 nated from C-4 was 37% and lesser amounts came from C-5 and C-6, small proportions probably also came from C-7, C-2 and C-3. The percentage of MD ÷ in the spectrum of methyl [3,5,6-2H6] decanoate was 28.9% agreeing with the total of 29.8% for methyl [3-2H2], [5-2H:] and [6-2H2] decanoates. Deuterium is discriminated against by a factor of 0.88 [ 19], so that in any particular deuterated ester migration of hydrogen rather than deuterium would be preferred to some extent. Thus the total percentage of MD÷ in Table IV is only 90.9%. The values in Table IV show that there is a preference for the transfer of deuterium from those same positions from which hydrogen is transferred in formation of the major fragment ions (see above). The relative percentages are clearly related to the proportions of ions in the spectrum of methyl decanoate (Fig. 1). The base peak is ion m/z 74 and results from hydrogen transfer from C-4 and ion m/z 87, generally about 75% of the base peak, is formed by transfer from C-5, C-6 and C°7 corresponding to the sum ofMD ÷ percentages for isomers with deuterium at these positions (30.3% compared to 37.1% from C-4). Thus MD ÷ is most probably formed in the spectrum of methyl [4-2H2]decanoate as in Fig. 2. A deuterium radical migrates from C-4 to the ionized carbonyl oxygen,

D ~ D .0÷ C6H13~L ~ O C H

3

D,,,~. D~O÷

C6H'3/ L

L "OCH3

D . C6HI5 ~ .

o

+

D

CH30~

O\o. 0 4J~ CH30/~ ~OCH5

~

"C6HI3

[-M+ 2~]* Fig. 2. Formation of [M + 2]+ ion in mass spectru~ of methyl[4-2H2]decanoate.

278 as in the first stage of the McLafferty rearrangement [13], this molecular ion then collides with an ester molecule and a deuteron is transferred from the ion producing [MD] ÷. The CI spectra of esters obtained using methane as reagent gas, in which [MH] ÷ is a prominent ion, have also been explained as attack at the carbonyl group by the reactant ions [20,21]. The results in Table IV, however, do not preclude the possibility that the MH ÷ ion is formed by reaction between the unionized molecule and ions m/z 74 and 87 since in the appropriate deuterated esters these ions are also deuterated. The hydrogen in MH÷ ion, though, is clearly derived from certain carbons only and not randomly from any carbon in the 9 carbon chain. The relationship between intensity of [MH] ÷ and chain length (Table II) is probably related to the ability of the alkyl chain to bend round and protect the ionized carbonyl with its attached hydrogen. A chain length of at least 14 carbons is necessary for a chain to be able to bend back on itself by 180 ° [22] and a still longer chain would be required for carbonyl protection. The Clo chain is too short for this type of protection to be effective. Complete mass spectra were also measured for all of the specifically deuterated decanoates and proportions of labelled fragments formed were compared with those previously reported in spectra of the corresponding methyl deuterated octadecanoates [14,18,23]. This comparison was useful since the isotopic purity of the deuterated esters used previously was relatively low [23]. The base peak, m/z 76, in the spectrum of methyl[2-2H2]decanoate was as expected from previous work with esters [14,19]. Contribution to the base peak by deuterium at other sites was not high but was not investigated because of possible formation of'Mckafferty + 1' ions in methyl esters [13]. The relative percentages of ions m/z 87, 88 and 89 are listed in Table V and are similar to those previously reported [23], that is ion m/z 88 was relatively abundant in spectra of [2-2H2] -, [5-2H2] -, [6-2H2] - and [7-2H2] decanoates and ion m/z 89 very prominent in the spectrum of [3-2H2]decanoate. Again deuterium at other sites made little contribution to ions m/z 88 or 89. The intensities of these 3 ions are included here for comparison with those of the following ions. Ions m/z 101-103, 115-117 and 129-131 are also listed in Table V; ions m/z 101, 115 and 129 are relatively prominent minor ions in the spectrum of methyl decanoate (Fig. 1) but possible modes of formation of these ions do not seem to have been discussed previously. The relative intensities of these ions have been examined since a more complete knowledge of fragmentation patterns would be useful in locating deuterium in methyl esters particularly where deuterium might be located at several carbons. The pattern of intensities of these ions in the spectrum of methyl [2-2H2]decanoate suggested considerable deuterium scrambling such as must also occur in formation of ion m/z 87. Scrambling was also evident in formation of ions m/z 103 and 117 in the spectrum of methyl [3-2H2]decanoate even though these ions include carbons 1 - 4 and 1 5. respectively, ion 131 was very prominent, however, indicating that there was probably no transfer from C-3 in formation of this ion. Ions withm/z 103, 117 and 131 were relatively intense in the spectrum of methyl[4-2H2]decanoate and the mode of formation is proposed in Fig. 3. The bond separated from the radical site by one carbon

279 TABLE V RELATIVE INTENSITIES OF SOME IONS IN MASS SPECTRA OF DEUTERATED DECANOATESa'b Deuterium position

[2-2H2] [3-2H:] [4-2H2] [5-2H2] [6-2H:] [7-~H2] [8-2t4:] [9-2H:] [10-2H3] c

METHYL

Ions m/z 87

88

89

101

102

103

115

116

117

129

!30

131

45 5 84 52 54 67 90 92 91

49 5 11 42 41 30 9 7 8

6 90 5 6 5 3 1 1 1

38 46 24 66 61 73 70 93 80

24 19 7 26 26 21 13 5 2

38 35 69 8 13 6 17 2 3

23 28 34 17 65 63 75 60 73

19 1 7 11 22 13 18 15 15

58 71 59 72 13 24 7 25 4

17 3 16 18 9 63 74 69 75

29 3 5 22 26 31 11 4 10

54 94 79 60 65 6 15 27 2

aSample size was 320 ng. bCalculated after subtracting the appropriate isotopic contributions of carbon and hydrogen. Intensities are expressed as relative percentages within each group of three ions: 87-89, 101-103,115-117 and 129-131. Clons m/z 104, 118 and 132 were 15, 8 and 13%, respectively.

is broken in a manner analogous to that which occurs on formation of ions m/z 87 and 143 [15,16]. Formation of ion m/z 117 (Fig. 3(2)) is particularly interesting since the first stage is the same as that of the McLafferty rearrangement, that is transfer of a deuterium radical from C-4 to carbonyl oxygen. Fragmentation as in Fig. 3(2) is then an alternate pathway to this rearrangement; it is a relatively minor one, though, since in the spectrum of methyl [4-2Hz]decanoate ion m/z 75 is the base peak and ion m/z 117 forms only 2.3% of it. To determine whether the radical sites shown in Fig. 3 are the initial sites or whether there is evidence for internal hydrogen migrations from carbons beyond the bond which is broken, such as occur in formation of ion m/z 87 [15,16], mass spectra of the other deuterated decanoates have to be examined. If analogous transfers occurred ion m/z 102 would be expected to be relatively prominent in spectra of methyl[52H2]-, [6-2H2]- and [7-2H2]decanoates and in fact the relative percentages of this ion in these spectra were 26, 26 and 21% whereas the values for the analogous ion m/z 88 in the same spectra were 44%, 41% and 30% showing that reciprocal transfer probably occurs but to a lesser extent than in formation of ion m/z 87. Similarly ion m/z 116 would be expected in spectra of [6-2H2] and [73H2]decanoates and ion m/z 130 also in the spectrum of the latter ester. These ions were found in comparable proportions. Other prominent ions were m/z 117 in the spectrum of methyl[5-ZH2]decanoate andm/z 131 in the spectrum of methyl[6-2H2] decanoate and these are no doubt formed in the same way as ion rn/z 103 in the spectrum of methyl [4-2H2] decanoate (Fig. 3).

280

C'~H9, D/

H

H

D

+0/

÷0/

C6HI3" D

OCH3

OCH3 m/z 103

H9 C%

D

D

*0 /

÷0/

C5HI i" D

OCH3

D

2

OCH3 mlz I17

H

H

÷0/

*0/ +

D

OCH3

D

OCH3

m/z 131 Fig. 3. Formation of minor ions in mass spectrum of methyl[4-2H2]decanoate. 1. Formation of ion

m/z 103.2. Formation of ion m/z 117.3. Formation of ion m/z 131.

Ion m/z 131 in the spectrum of methyl[5-2H2]decanoate is most probably formed in the same way as ion m/z 117 in Fig. 3, that is both deuteriums are retained because one is transferred to the carbonyl oxygen. The homologous ion in the spectrum of methyl[6-2H2]decanoate is the more prominent ion m/z 145 which forms 8% of the base peak and is accompanied by lesser proportions of ions m/z 144 and 143. As mentioned earlier this ion is also formed by expulsion of C-2, C-3 and C-4 and a hydrogen and the two routes cannot be separated (using deuterated esters). The spectrum of methyl[6-ZH2] octadecanoate [23], however, shows that considerably more deuterium scrambling occurs in formation of the M-43 ion than ofionm/z 145 so that most of this ion may in fact be formed in a similar way to that shown in Fig. 3(2). Ions rn/z 117 and 131 were more intense than would have been expected in the spectrum of methyl[9-2H2]decanoate but could be formed by expulsion of C-2 to C-6 and a hydrogen and of C-2 to C-5 and a hydrogen respectively. The corresponding ions in the spectrum of methyl[10-2H3]decanoate, ions m/z 118 and 132, are less prominent but the

281 intensities of ions in the region of the M-43 ion in spectra of all the methyl gem dideuterated octadecanoates [23] shows that the expulsion pathway seems to involve more scrambling of deuterium at some carbons than at others. Thus examination of the intensities of these minor ions provides more evidence for the importance of hydrogen migrations in mass spectrum of the long-chain methyl esters. They also provide useful confirmatory evidence for the location of deuterium on specific carbons.

Acknowledgements The authors thank L.L. Hoffman, H.M. Harris and D.J. Olson for experimental assistance.

References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

A.P. TuUoch, Prog. Lipid Res., 22 (1983) 235. A.P. TuUoch, Lipids, 12 (1977) 92. A.P. TuUoch, Chem. Phys. Lipids, 18 (1977) 1. A.P. TuUoch and L. Bergter, Chem. Phys. Lipids, 28 (1981) 347. A.P. TuUoch, Chem. Phys. Lipids, 37 (1985) 197. H. Gorrissen, A.P. TuUoch and R.J. Cushley, Biochemistry, 19 (1980) 3422. F. Spener and H.K. Mangold, Chem. Phys. Lipids, 11 (1973) 215. R. Ryhage and S. Stenhagen, Arkiv Kemi, 13 (1959) 523. A.P. TuUoch, Chem. Phys. Lipids, 30 (1982) 325. J.H. Beynon, R.A. Saunders and A.E. Williams, Anal. Chem., 33 (1961) 221. A.G. Harrison, Chemical Ion Mass Spectrometry, CRC Press Inc., Boca Raton, Horida, 1983. C.E. Melton, in: F.W. McLafferty (Ed.), Mass Spectrometry of Organic Ions, Academic Press, New York, 1963, p. 65. F.W. McLafferty, Interpretation of Mass Spectra, 2nd edn,, W.A. Benjamin Inc., Reading, Massachusetts, 1973. Ng. Dinh-Nguyen, R. Ryhage, S. Stallberg-Stenhagen and E. Stenhagen, Arkiv Kemi, 18 (1961) 393. G. SpiteUer, M. SpiteUer-Friedmann and R. Houriet, Monatshefte, 97 (1966) 121. M. Kraft and G. SpiteUer, Org. Mass Spectrom., 2 (1969) 541. H. Budzikiewicz, C. Djerassi and D.H. Williams, in: Interpretation of Mass Spectra of Organic Compounds, Holden-Day Inc., San Francisco, California, 1964. R. Ryhage and E. Stenhagen, in: F.W. McLafferty (Ed.), Mass Spectrometry of Organic Ions, Academic Press, New York, 1963, p. 399. D.H. WiUiams,H. Budzikiewicz and C. Djerassi, J. Am. Chem. Soc., 86 (1964) 284. M.S.B. Munson and F.H. Field, J. Am. Chem. Soc., 88 (1966) 4337. C.W. Tsang and A.G. Harrison, J. Chem. Soc. Perkin II, (1975) 1718. M.A. Winnik, Chem. Rev., 81 (1981) 491. Ng. Dinh-Nguyen, Arkiv Kemi, 28 (1968) 289.