Es
--
Table 11. Results Showing Improved Precision in Determination of Calcium and Phosphorus in Titanium Dioxide Pigments as Frequency of Ion Beam Chopping Is Reduced P 2 0 jppm weight CaO ppm weight Beam chopping Rel. std. Rel. std. frequencyKc/sec. Result dev. Result dev. Without beam chopping 1300 11.5 650 13.7 X-ray spectrometric method 1400 5.0 700 5.0 1370 1350 1400 1350 1390
110 50 10 2 0.5
10.9 14.4 9.4 5.9 4.8
660 710 770 680 710
15.9 11.7 5.8 5.9 4.8
factors of calcium and phosphorus to silicon from previous work (8) were used. The exposures were calculated as a ratio as follows : In Figure 4 linear portions of curves relating peak height and exposure for standard and unknown elements are shown. From this it can be seen that
M=
Yu - Ys log ES - log EU
(4)
where M is the previously calculated slope for the batch of plates; Yu - Ys is the difference in densities (peak heights) of the unknown and standard elements; Es is the actual exposure of a singly charged isotopic line of the standard element at a fixed density; and EM is the actual exposure of a singly charged isotopic line of the unknown element a t the same density. Then by rearranging Equation (4)
Eu
= antilog
[TI Yu - Ys
from which the required ratio Eu/Es can be calculated, The density of each spectral line was measured in three places-top, middle, and bottom and the results averaged to produce 40 separate determinations for each element at each beam chopping value. The results, which have been CORfirmed by x-ray spectrometric analysis, are summarized in Table I1 and compared to those previously obtained without beam chopping. The results are shown in Figure 5 in which the relative standard deviation is plotted against a function of T/t in which T = time interval between the start of adjacent beam suppressing pulses and t = duration of each pulse. The function i"/t is directly related to the weight of sample consumed in each determination and is used to indicate the variation in relative standard deviation as the weight of the sample per determination is increased. The cube root of this function was selected to facilitate graphical representation. The results demonstrate that by introducing the technique of ion beam chopping the precision of the method can be improved to a value equivalent to that found for isotope abundance measurements (12, 13). This value is also very close to that attainable by x-ray fluorescence analysis. Of greater significance, however, is the fact that the improved precision is attainable at relatively high concentrations thereby greatly extending the useful range of the technique of spark source mass spectrography. RECEIVED for review February 27, 1967. 21, 1967.
Accepted August
Fluoroalcohol Esters as Derivatives for Mass Spectrometry Richard M. Teeter Chewon Research Co., Richmond, Gal$ 94802
A new series of esters prepared from a,a,Q-trihydroperfluoroalcohols is described for use as derivatives to aid in the mass spectrometry of carboxylic acids. They are stable and volatile. Their mass spectra resemble those of methyl esters except that the ions containing the fluoroalcohol and carbonyl portions of the molecule are removed to high mass, facilitating easy identification in the presence of interfering impurities.
plification into the mass spectrum. The esters are stable and volatile and yield mass spectra which greatly resemble those of methyl esters with the striking feature of having many of the ions displaced to higher mass where they can be examined without interference from other compounds.
1965, Barber, Jollk, Vilkas, and Lederer ( I ) determined the structure of the acyl nonapeptide fortuitine by mass spectrometry. One feature of the compound that simplifies the interpretation of its mass spectrum is the presence of a large acyl group of 20 or 22 carbons, which moves the molecular weight and the significant fragments to high mass where there is little interference from competing fragmentation paths. This concept has been used to develop a new series of derivatives to aid the mass spectroscopist in the study of carboxylic acids. Conversion of carboxylic acids to their trihydroperfluoroalcohol esters is a practical way to introduce the desired sim-
The most convenient technique for preparing the esters is the direct reaction between the acids and excess alcohol (available from Pierce Chemical Co., Rockford, Ill,), catalyzed by BF3.Etz0. Reaction mixtures were warmed for 10 minutes on a hot plate, then dissolved in dichloromethane and washed with aqueous sodium bicarbonate and water. After evaporation of solvent, each ester was purified by trapping from a gas chromatograph. The column was 22 feet of lIa-inch stainless steel tubing packed with Carbowax 20M terephthalate (Varian Aerograph, Walnut Creek, Calif.), 5z on Fluoropak 80 (The Fluorocarbon Co., Anaheim, Calif.). The terephthalate ester was prepared from terephthaloyl chloride in dry pyridine solution, then isolated and purified as above. The mass spectra were all run on an AEI MS-9 doublefocusing instrument.
IN
(1) M. Barber, P. .Toll&, E. Vilkas, and E. Lederer, Biochem. Biophys. Res. Commun., 18,469 (1965).
1742
e
ANALYTICAL CHEMISTRY
EXPERIMENTAL
100-
ga.
,? I 60
100
I 140
U
! o
! Y I
220
180
300
260
380
340
m/e Figure 1. Spectrum of butyl decanoate .4 .5
2 0 -I
a
IQ I-
% +=.
z
Y a B w
m/e
Figure 2. Spectrum of dibutyl terephthalate
J’*.3y 9,,
H(CH,),COOCH,
MW 186
-
lbo
I40
Ne
le0
220
z v)
0 -I
5 I-
B I-
z
W
a V
2 Figure 3. Spectra of decanoate esters
RESULTS AND DISCUSSION
Because they are highly polar, carboxylic acids are strongly adsorbed on the glass surfaces of mass spectrometer inlet systems. Inasmuch as the molecular ions are of low stability, the parent peaks are small. These two factors, when reinforced by the low vapor pressure due to dimerization, are responsible for the difficulties encountered by the mass spectroscopist when he tries to analyze carboxylic acids. The task becomes formidable when a mixture of acids is diluted with hydrocarbons or alcohols of similar molecular weight. The adsorption
and volatility problems are customarily attacked by esterification to either methyl or trimethylsilyl esters. The former have received considerable attention, and a major review of their mass spectra is available (2). Each, however, has serious drawbacks. Methyl esters are only 14 amu higher in molecular weight than acids. Consequently, in a complex mixture (2) R. Ryhage and E. Stenhagen in “Mass Spectrometry of Organic Ions,” F. W. McLafferty,Ed., Academic Press, New York,
1963, p. 399. VOL. 39, NO. 14, DECEMBER 1967
0
1743
9. I v)
z P 2
s 0 I-
b c
z w
2
W
o‘. 100
r
I40
‘II
180
220
280
300
m/e 340
380
420
460
500
540
580
620
In
z 0
r. 50 w
0,
A
E
40
30
e
20 20
W
B b-
z
u a
Mf
10
(M-lS)+
0
420
Figure 4.
4b
500
W
P
540
5bO
--0 020
Spectra of homologous esters
60 Y
2
50
MW 586
s-40
g: y 30 85 2o
G
10
0
Figure 5. Spectra of fluoroalcohol decanoates
with hydrocarbons or alcohols, methyl ester molecular ions are little more outstanding than those of acids. Trimethylsilyl esters are less well known (3)and offer one advantage over methyl esters. They move the molecular weight up by 72 amu, which, in some cases, is enough to eliminate (3) R. M. Teeter, “Mass Spectrometry of Trimethylsilyl Esters” in “Mass SpectrometryConference, June 3-8, 1962, New Orleans, La.,” ASTM Committee E14, p. 51.
1744
e
ANALYTICAL CHEMISTRY
interferences. These esters have their own set of disadvantages. The reagents used to prepare them also convert alcohols to trimethylsilyl ethers if alcohols are present. The alcohols and the ethers have the same nominal molecular weights as the acids and esters. Also, the ester parent peaks are small. Even more serious for structure determination is the relative insensitivity of the fragmentation pattern to changes in structure. The use of some higher aliphatic alcohol, such as butanol, to esterify unknown acids or mixtures is another alternative to the
y
100
CH~=CH-?O
4
W
a
BO-
55
LQ W %8
60-
~
HOxCH2CH2L
mle ioi
H3CCHzOOCCH,CH2COOCHz(CF2)6H
m/e
415
MW 460
LLlo
0-40-
s w
2
u a
200
' 1 '
1
,
0
601
H(CF,),CH,COCCH,CH,COOCH,(CF~)~H
MW 746
t.
H(CF,),CH,OOCCH,CH,~O
m/e
2
20-
W
k2 2
10CSH2F9
415
(M-19)'
methyl ester, These esters, however, introduce further complexity into the molecule; and the result is a more complex spectrum which is more difficult to interpret. This is illustrated by a comparison of Figure 1, the spectrum of butyl decanoate, with the upper spectrum of Figure 3, methyl decanoate. In all of the spectra the largest peak was chosen as the base peak for the figure. The spectrum of butyl decanoate shows that we have achieved the desired result of raising the molecular weight. However, it has been at the cost of adding a peak to the spectrum (m/e 173) that is the result of alkyl group cleavage with concurrent rearrangement of two hydrogen atoms to the charged acyl residue (4). Even more complex is the spectrum shown in Figure 2, that of dibutyl terephthalate, where this same rearrangement results in the addition of two peaks to the spectrum, at mle 223 and mle 167. Esterification of an acid with an ar,a,&trihydroperfluoroal-
coho1 leads to a derivative which has enhanced molecular ion stability and greatly increased molecular weight without a great decrease in volatility. Moreover, the fragments that contain the fluoroalcohol moiety and are, therefore, in the high molecular weight region show relative intensities that are very similar to those shown by the methyl ester. They are characteristic of the structure of the original acid and can be very useful in structure studies. The change in spectrum in going from a methyl ester to a fluoroalcohol ester is shown in Figure 3, where the spectra of methyl decanoate and 1,1,7-trihydroperfluoroheptyldecanoate are compared. The parent peaks are lined up, rather than the mass scales, in order to emphasize the similarities in pattern. In each case, one of the more prominent ions results from the very common rearrangement involving a six-membered cyclic transition state which was first postulated by McLafferty (5).
(4) C. Djerassi and C. Fenselau, J. Am. Chem. SOC.,87, 5756 (1965).
( 5 ) J. A. Gilpin and F. W. iMcLafferty, ANAL.C H E M 29, . , 990 (1957). VOL. 39, NO. 14, DECEMBER 1967
1745
Methyl ester
186 -
74 -
112
Fluoroalcohol ester
CH
/3
13\
O+
CH
466 -
112
Almost as large, in both spectra, are the peaks due to the fragment ions resulting from loss of a CiHls radical. They appear at rnje 87 and mje 387, respectively. The acyl ion at rnje 155 is present in both spectra; but with the methyl ester it falls in the middle of a group of peaks that contains the acyloxy group, -COOCHI. There is, of course, no peak at the corresponding place, mje 455, in the fluoroalcohol ester spectrum as the acyl ion is still at rnje 155. The only peak in the fluoroester spectrum above rnje 374 that is larger than 1% of the base peak and that does not have a counterpart in the methyl ester spectrum is the small rnje 467 peak due to loss of F*from the parent ion, This fact points up the property of these esters that is responsible for the striking appearance of their spectra and, consequently, for the utility of the esters as derivatives for mass spectrometry. Loosely speaking, the fluoroalcohol chain does not fragment. There are only a few low intensity fragments, one of which was examined in detail. A small peak at mje 385 is the largest peak in the spectrum above m/e 374 exclusive of the parent ion and the fragments formed by loss of F. or R -and their isotope peaks. It is 0.22% of the largest peak (m/e 43) and 0.58% of the mje 374 peak. It is a triplet consisting of 12% C16H&F8, 85% CllH90F12, and 3 %
3 74
_.
C I O H ~ O ~ FThe I ~ . first of these is probably a simple cleavage of the fluoroalcohol chain with the loss of HCFzCFz-; but the other two, involving loss of CeHI3O. and C7HI1(C7Hl5. Hz?), respectively, are more involved. As noted, however, the total intensity of the rnje 385 peak is very low. The other peaks in this high mass end of the fluoroalcohol ester spectrum are all due to loss of alkyl radicals from the acid portion of the ion. This, of course, is true also for esters of acids homologous with decanoic acid. Figure 4 shows the spectra of the esters, with the same alcohol, of the CS and Clz straight chain acids. The products of the six-membered cyclic rearrangement appear, in each case, at rnje 374; the acyl ions are at mle 127 and rnje 183 and the molecular ions are at rnje 458 and rnje 514. The fluoroheptanol is not the only alcohol that is useful in this context. There is a choice of 2,4, 6 , 8 , or 10 CF2units in HOCHt(CFz),H giving increases in molecular weight above that of the methyl ester of 100, 200, 300, 400, or 500. The molecular weight range of the acid sample is the governing factor, a wide range requiring a large molecular weight change in order to lift the lowest acid beyond the highest nonacid. Generally, the heptanol, HOCH2(CF2)6H,has proved useful;
+
__
I -
CH3 LL
0
ill 60
Is ,l,I IMI 100
, 111 140
1
,
I
180
220
1
260
340
300
,I!
360
420
460
500
540
W
COOCH,(CFz)6
H
pm/e
q30-
z w
8
I.2
COOCHg(CF2leH
50-
-
620
4--I/
I
5
580
w e
463
8
MW 194
c
2010-
mle 437
W
CL
0,
1 ,
,
60
100
140
IbO
220
2bO
Figure 8.
1746
a
ANALYTICAL CHEMISTRY
300
3hO
380
420
w e Spectra of aromatic esters
460
500
540
?SO
600
METHYL ESTERS
41
e9
d5
65
61 lis
m/e
129
-
I
FLUOROALCOHOL EST ERS
L
I
I
41
55
69
85
m/e-
101 115 129
Figure 9. Spectra of esters of mixed acids
I
313
I
I
I
I
I
I
359 374 387 415 443 500 Figure 10. Spectrum of esterifled acid mixture
but several others were examined. The esters of decanoic acid prepared with the Ca,Cayand Ce fluoroalcohols give the spectra shown in Figure 5. Here, again, the molecular ions have been Iined up rather than the mass scales so that the very similar patterns at the high mass end are emphasized.
1
I
542
598
With the esters, methyl and fluoroheptyl, of cyclohexanecarboxylic acid, Figure 6, no six-membered ring can form to permit the previously mentioned cyclic rearrangement; so it does not appear in either spectrum. As expected for a cyclic compound, the molecular ion is a little more prominent than beVOL. 39, NO. 14, DECEMBER 1967
rn
1747
fore; and fragmentation of the ring leads to the loss of C4H7 (M-55 ion). The acyl ion at mje 111 and the cyclohexyl ion at m/e 83 both give relatively larger peaks in the fluoroalcohol ester spectrum than in the methyl ester spectrum. Diethyl succinate was treated under esterification conditions; and two products were isolated, the mixed ester and the di(fluoroalcoho1)ester. Their spectra, given in Figure 7, both show molecular ions and a large peak from a fragment at m/e 415. Being symmetrical, this is the only acyl ion that the di- ’ (fluoroalcoho1)ester shows; but the mixed ester also has an acyl ion at mje 129. The loss of C2H4from this ion leads to a very abundant fragment at m/e 101. There is no corresponding loss from the acyl ion at mje 415 because the fluorine-containing chain cannot undergo the rearrangement. The molecular ion of the mixed ester loses C2H3. to form the fragment at mle 433 in a manner analogous to the two-atom rearrangements previously shown in the spectra of butyl decanoate and dibutyl terephthalate. Figure 8 shows the mass spectra of Auoroalcohol esters of two aromatic acids, mesitoic acid, and terephthalic acid. In each case, the largest peak of the spectrum is due to the acyl ion formed by loss of a RuoroaIkoxy radical.
The last examples to be described are the two spectra shown in Figure 9. The upper is the spectrum of the methyl esters prepared from a mixture of acids ranging from C 5through CIS. The Cll methyl ester parent ion and acyl ion are marked to aid orientation. Acyloxy, alkyl, and acyl ions from the different acids overlap with the parent ions, making a molecular weight distribution very difficult to obtain. Below is the spectrum of the fluoroheptanol esters of the same mixture. As before, the cyclic six-membered rearrangement product is at mje 374 ; but above that, the only even-mass peaks are parent ions augmented a calculable amount by C13peaks from acyloxy ions of known composition. The high mass region of the lower spectrum is shown, amplified, in Figure 10. The marked similarity of the fragment-ion profile from mje 374 through mfe 443 to that shown by the dodecanoate ester (Figure 4) is evidence in favor of a straight chain structure for the components of the mixture. It is also clear that with calibration, good molecular weight distribution data could be obtained simply. RECEIVED for review July 7, 1967. Accepted September 13, 1967. Presented at the meeting of ASTM Committee E-14 on Mass Spectrometry, Denver, Colo., May 14-19, 1967.
Measurement and Interpretation of Metastable ectrometry T. W. Shannon,’ T. E. Mead,2 C. G . Warner,’ and F. W. McLafferty Department of Chemistry, Purdue University, Lafayette, Ind. 47907
A number of techniques for studying metastable ion transitions in mass spectra are compared. Advantages of the method proposed by Barber and Elliott for producing pure metastable spectra in a doublefocusing mass spectrometer include a sensitivity increase of a factor of 50 over conventional spectra and exact determination of the mass of the daughter ions. However, to identify and measure all of the metastable transitions in an unknown spectrum using electrical recording requires a separate scan of the accelerating voltage for each fragment ion in the normal spectrum. The use of the photoplate makes possible the recording of all ions simultaneously during this voltage scan, which greatly simplifies the routine collection of such pura metastable spectra. The availability of such data and its display as a metastable map aid in the elucidation of molecular structures, allow significant new correlations of mechanisms with structures, and make possible much more extensive use of the new technique of metastable ion characteristics. THEIDENTIFICATION of metastable ion decompositions is of considerable importance for structure determination by mass spectrometry. Peaks resulting from such decompositions can serve to identify particular reaction paths (1-3). (These peaks will be referred to as “metastables,” although the quotation marks formerly used to avoid ambiguity will be omitted.) (1) J. H. Beynon, “Mass Spectrometry and its Application to Organic Chemistry,” Elsevier, Amsterdam, 1960. (2) F. W. McLafferty, R. S. Gohlke. and R. C. Golesworthy, ASTM E-14 Conference on Mass Spectrometry, June 1964. (3) F. W. McLafferty, “Interpretation of Mass Spectra,” W. A. Benjamin, Inc., New York, 1966, p. 64.
1748
a
ANALYTICAL CHEMISTRY
Although such evidence is often cited in mechanistic studies. it does not appear to be generally recognized that reaction pathway identification can also indicate a particular arrangement of atoms. For example, in a hypothetical spectrum the presence of ions corresponding in mass to AB and ABC could indicate either of the molecular structure possibilities ABCBA or ACBBA. However, a metastable decomposition of ABC -+ AB would be possible, barring rearrangements, only for the structure ABCBA (3). In addition, recent work indicates that metastable ions can be used as a characteristic property of a particular ion structure (4), and applications to a variety of systems indicate this to be a valuable new tool for the elucidation of ion decomposition reactions (5-10). These potentialities of metastables prompted our search for a more sensitive, unambiguous, and convenient method for their identification. Narrowing the recorded width of the ordinary ion peaks using high resolution greatly facilitates recognition and measurement of metastable peaks, and Postdoctoral Research Fellow. Visiting Scientist, 1965; permanent address, Research Laboratories, American Cyanamid Co., Stamford, Conn. 1
2
(4) T. W. Shannon and F. W. McLafferty, J. Am. Chem. SOC.,88, 5021 (1966). (5) F. W. McLafferty and W .T. Pike, Ibid..89, 5951 (1967). (6) F. W. McLafferty, M. M. Bursey, and S. M. Kimball, Ibid..88,
5022 (1966).
(7) M.M. Bursey and F. W. McLafferty, Ibid.,p. 5023. (8) Peter Brown and Carl Djerassi, Ibid.,89,2711 (1967). (9) F. W. McLafferty and W. T. Pike, Ibid.,p. 5953. (IO) W. T. Pike and F. W. McLafferty, Ibid.,p. 5954.
