1846
Anal. Chem. 1984, 56, 1846-1849
We have shown that NICI mass spectrometry can be used to provide accurate quantitative data for PAH in a crude oil sample with a minimum amount of sample cleanup or pretreatment. The variations in response factors for different PAH in the NICI mode reported by other investigators can be linked to differences in operating conditions. This report has shown how the selectivity of detection for isomeric PAH can change as a function of ion source pressure and temperature. It is possible by carefully adjusting the ion source pressure and temperature to change both the selectivity of detection of BaP to BeP as well as the absolute sensitivity of detection for BaP. Source pressure and temperature and the manner in which they are monitored should be taken into account when comparing NICI results from different instruments. As noted earlier, the reasons for the NICI sensitivity of PAH are not completely understood. Our observed differences in NICI sensitivities for BaP and BeP do not correlate well with reported electron affinities. Other factors such as the reagent gas and electron energy may also show an effect on the selectivity. The qualitative selectivity of NICI mass spectrometry for certain PAH seems to be a general observation in the recent literature. Registry No. Benzo[a]pyrene, 50-32-8;benzo[e]pyrene, 19297-2; indeno[l,2,3-cd]pyrene,193-39-5;benzo[ghi]perylene, 191-
24-2; perylene, 198-55-0;methane, 74-82-8.
LITERATURE CITED (1) Dougherty, R. C. Blomed. Mass Spectrom. 1981, 9 (7),283. (2) Hass, J. R.; Friesen, M. D.; Hoffman, M. K. Pract. Spectrosc. 1980, 3, Part B, 316. (3) Cavallaro, A.; Bandi, G.; Invernizzi, G.; Luciani, L.; Monginl, E.; Gorni, 0. Pergamon Ser. Environ. Scl. 1982, 5 , 55. (4) Dougherty, R. C.; Wander, J. D. Biomed. Mass Spectrom. 1980,7 (Q),
401. (5) Stan, H.J.; Kellner, G. Blom. Mass Spectrom. 1982, 9 (ll),483. (6) Ramdahl. T.; Urdal, K. Anal. Chem. 1982, 5 4 , 2256. (7) Iida, Y.; Daishima. S . Chem. Lett. 1983,273. ( 8 ) Oehme, M. Anal. Chem. lS83, 55, 2290. (9) Zackett, D.; Clupek, J. D.; Cooks, R. G. Anal. Chem. 1981, 53, 723. (IO) Stockl, D.;Budzikiwlecz, H. Org Mass Spectrom. lS82, 77 (e),376. (11) Chen, E. C. M.; Wentworth, W. E. J . Chromatogr. 1981,277, 151. (12) Hilpert, L. R.; Byrd, 0. D., NBS, June 1983,unpublished results. I
RECEIVED for review January 19, 1984. Accepted April 24, 1984. The authors acknowledge partial financial support from the Office of Health and Environmental Research of the Department of Energy. Certain commercial equipment, instruments, or materials are identified in this report to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.
Quaternary Ammonium Salts as Calibration Compounds for Fast Atom Bombardment Mass Spectrometry A. J. DeStefano*' and T. Keough The Procter & Gamble Company, Miami Valley Laboratories, P.O. Box 39175, Cincinnati, Ohio 45247 The fast atom bombardment (FAB) mass spectra of the quaternary-ammonium-salt mixture methyipoiy(oxyethylene)( 15)octadecylammonium chloride (Ethoquad 18/25) have been used to generate nominal-mass computer caiibrations to 1200 amu. Using derivatlves formed by the reaction of Ethoquad 18/25 with perfiuorinated anhydrides, we were able to extend the mass calibration to 1800 amu. The FAB spectra of organic salt mixtures contain high-mass ions (600-1600 amu) of significantly greater lntenslty than the FAB spectra of inorganic salts.
-
Fast atom bombardment (FAB) mass spectrometry (1-3) is a rapidly emerging technique for the analysis of polar (4, 5),labile (6-8) compounds. To date, most of the FAB studies reported in the literature have been performed in the analog mode, with the data being output onto UV-sensitive paper. For most studies in which the mass spectrometer has been operated under computer control, calibration of the mass axis was accomplished in the FAB mode using either Ultramark compounds (9)or alkali halide salts dissolved in glycerol (9). Alternatively, calibration was done in the electron ionization (EI) mode and this E1 calibration was used for FAB analyses (10). We felt it important to operate our spectrometer under computer control for a number of reasons. First was the obvious problem of storing and archiving large quantities of Current address: Norwich Eaton Pharmaceuticals,P.O. Box 191, Norwich, NY 13815. 0003-2700/84/0356-1846$01.50/0
UV-sensitive paper. Second, we wanted to take advantage of some of the capabilities afforded by the modern data systems including background subtraction, spectral averaging, and the displaying of desorption profiles. The latter feature was especially important in light of reports of significant changes in some FAB spectra as a function of sample analysis time (11,12).Since we anticipated performing a large number of FAB analyses, and since switching from E1 to FAB required a source change on our instrument, our interest was in developing a FAB-oriented mass-calibration procedure. Our calibration attempts using inorganic salts dissolved in glycerol were, in general, unsuccessful. We were unable to consistently produce high-mass cluster ions of sufficient intensity to calibrate the mass axis to beyond 1000 amu. In addition, the gaps between reference ions in the spectra of the pure alkali halides were sufficiently large that mass calibration often failed with our standard (low field) magnet when the instrument was operated at full accelerating voltage. The inconsistent calibration results we obtained with the inorganic salts led us to examine a new approach involving organic salts. We reasoned that high-molecular-weightorganic salts could provide an increase in sensitivity over inorganic salts since the need to form large cluster ions would be eliminated. For these studies, we chose the mixture methylpoly(oxyethylene)(15)octadecylammonium chloride (A), (CH~CHZO)~H
I
CI~H~~N-CH~CI
I
(CHZCH~O)XH
A, 3c
+ y = 15 (average)
0 1984 American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 11, SEPTEMBER 1984
1847
Table I. Calibration Table for Ethoquad 18/25 proposed structure
mlz 100,
45 t
200
m
-0
400
800
600
1000
1200
a 0 -
100
[r m
50
0 300
400
600
500
700
800
900
1000
1100
1200
M Z
Flgure 1. Positive-ion FAB spectrum of Ethoquad 18/25.
manufactured commercially as Ethoquad 18/25. With this mixture we obtained a nominal-mass calibration to 1200 m u . Formation of perfluoroalkyl esters allowed us to extend the calibration range to approximately 1800 amu.
EXPERIMENTAL SECTION Instrumentation. All spectra were obtained on a Vacuum Generators ZAB-2F reverse-geometry, double-focusing mass spectrometer operating at an ion accelerating voltage of 8 kV and a mass resolution of approximately 2500 (10% valley definition). Fast atom bombardment was accomplished by using Xe (99.995%, Matheson Gas Products, Inc., Joliet, IL) as the primary particle, with a modified saddle-field ion source (Ion Tech, Ltd., Teddington, UK). Typically, the tube current was 1 mA and the energy 7.5 kV. All mass spectra were acquired and processed with a Vacuum Generators 2350 data system. Reagents. Ethoquad 18/25 was obtained from Armak Industrial ChemicalDivision (Chicago, IL), while the perfluorinated anhydrides were obtained from PCR, Inc. (Gainesville,FL). All reagents were used without further purification. Procedure. Computerized mass calibration was accomplished with FAB of neat Ethoquad 18/25. For these experiments, the spectrometer was scanned over the mass range of 20-1250 amu. The spectrometer was scanned over the range of 20-1950 amu when Ethoquad 18/25 derivatized with perfluorinated anhydrides was used as the calibration material. In both casea, the scan speed was 10 s/decade of mass with a 2-5 interscan delay. Typically, the multiplier gain and zero level of the amplifier were set to ensure that 500-600 peaks were obtained in each time file used for calibration purposes. Since Ethoquad 18/25 and the perfluorinated ester derivatives are viscous liquids, they were applied directly to the stainless steel stage of the sample probe without employing a FAB solvent. Dispersion of Ethoquad or its derivatives in glycerol proved to be of no value since we observed no increase in sample ion current and the glycerol-related ions were completely suppressed by the sample. Derivatization of the Ethoquad was accomplished simply by combining the Ethoquad with an excem of the desired perfluoroalkyl anhydride and stirring for approximately 1 min to assure complete mixing. The exothermic reaction warmed the reaction mixture somewhat, and no additional heating was required to produce the spectra shown. RESULTS AND DISCUSSION The positive-ion FAB spectrum of Ethoquad 18/25 is shown in Figure 1(top). Figure 1(bottom) shows the ions in the mass range 300-1200 amu normalized to m / z 768. The low-mass end of the spectrum is dominated by ions at m/z 45 (C2H60+) and 58 (CH2=N(CH3)2+). These ions are analogous to those formed in the electron ionization (EI) spectra of alkylpoly(oxyethylene glycols) and alkyl amines, respectively. The most intense ion above m / z 58 is the imminum ion C
+
N=CH2
H 37
I
CH3
+ y = 21 + y = 20 a, x + y = 19 a, x + y = 18 a, x + y = 17 a, x + y = 16 a, x + y = 15
1208.882 1164.857 1120.829 1076.803 1032.777 988.7709 944.7247 900.6985 856.6723 812.6461 768.6199 724.5937 680.5675 634.5258 590.4996 546.4734 502.4472 458.4210
a, x
a, x
+ y = 14 + y = 13 + y = 12 + y = 11 + y = 10
a, x a, x a, x a, x a, x a, x
+y =9 =8 =7 =6 =5 =4
b, x b, x b, x b, x b, x
mlz
proposed structure
502.4472 458.4210 414.3948 370.3686 326.3424 296.3316 268.3003 264.1810 220.1548 176.1286 132.1024 114.0919 100.0762 88.0762 72.0812 58.0656 45.0340 29.0391
b, x = 5 b, x = 4 b, x = 3 b, x = 2 b, x = 1 CZoH4,N C18H38N c, x = 5 c, x = 4 c, x = 3 c, x = 2 CeH12NO' C5HloNOt c, x = 1 C4HloN' CSHsN' CZH5,O' CzH5
(CHZCH~OI~H
I I (CHzCHzO1,H
C,~H:,N-CH~
C , ~ H ~ , ~ J C H ~ C H ~ O CH&-ICH~CH~O),H )~H
I
I
CH2
a
CH2
b
C
a t m/z 296. The region above m / z 600 is dominated by the intact molecular cations (B), where x + y is 8-20. Other ion (CHZCHZO),H
I
CI~H~~*N-CH~
I
(CHZCHZO)XH
B series observed include the imminium ions
+
+
C18H37N(CH,CH,0),H
and CH N CHZCH20),H
3 CH, ~1(
II
CH.2
Table I lists one of the calibration tables we constructed for computer calibration with Ethoquad 18/25. Included in Table I are the accurate masses used and the proposed structure of each ion. No attempt was made to experimentally determine the accurate mass of each ion or to determine if mass multiplets were present. Nonetheless, using the masses listed in the calibration table and time files obtained from Ethoquad 18/25 spectra, we obtained nominal-mass calibration of the mass axis of the spectrometer over the range 23-1200 amu. In an effort to extend the calibration to higher mass, we reacted the Ethoquad 18/25 with perfluorobutyric anhydride. This rapidly converted the Ethoquad 18/25 to species of the form C. The spectrum of this mixture is shown in Figure 2. 0 I1 (CHZCHZO)$C~F~
I
C I ~ H ~ ~ N - C H ~0C I I I1 (CHZCHZO)~CC~F~
C
The major ion series observed are the perfluorobutyl ester equivalents of those observed in the spectrum of the underivatized sample. Representative members of these series are labeled in Figure 2. In addition to these ion series we observe intense ions at m/z 169 (presumably C3F7+)and m/z 241. The m / z 241 ion was assigned the structure
K
c ,F,COC
H,CH~+
since we determined from a peak-matching experiment that
ANALYTICAL CHEMISTRY, VOL. 56, NO. 11, SEPTEMBER 1984
1848
100
200
300
400
600
500
200
300
400
500
600
900
1000
1100
1200
+
100
E
2 a
g
50
U
2
600
700
800
1000
900
1100
1200
2a:
0 600
700
600
0 (CH2CH,O),-&CiF,, +I
100,
+V
C,,H,,N-CH,TX
=
1 2 7
(CH,CH,O),EC,F,, 50. 1234
1200
1300
1400
1500
1600
1700
1800
M/Z
1322
1384 ,428 1472
1760
0 1200
1300
1400
1500 M/Z
1600
1700
1600
Figure 2. Positive-Ion FAB spectrum of Ethoquad 18/25 derivatized with perfluorobutyric anhydride.
Figure 3. Posltlve-ion FAB spectrum of Ethoquad 18/25 derivatized with perfiuorooctanoic anhydride.
Table 11. Calibration Table for Ethoquad 18/25 Derivatized with Perfluorobutyric Anhydride
In an attempt to extend the calibrated mass range still further, we reacted the Ethoquad 18/25 with perfluorooctanoic anhydride to form derivatives D. The FAB spectrum of this
proposed structure
mlz
a , x + y = 22 a, x + y = 21 a, x + y = 20 a,x+y=19 a,x+y=18 a, x + y = 17 a,x+y=16 a, x + y = 15 a, x + y = 14 a, x + y = 13 a, x +.y = 12 a, x + y = 11 a, x + y = 10 a,x + y = 9 a,x + y = 8 a, x = 12 b, x = 11 b, x = 10 b,x=9 b,x=8 b,x=7 b,x=6
1644.862 1600.836 1556.809 1512.783 1468.757 1424.731 1380.705 1336.678 1292.652 1248.626 1204.600 1160.574 1116.547 1072.521 1028.495 1006.606 962.5802 918.5540 874.5278 830.5016 786.4754 742.4492
proposed structure
mlz
698.4230 680.2879 636.2617 592.2355 548.2093 504.1831 460.1569 416.1307 372.1045 328.0783 296.3316 284.0521 268.3003 24 1.0099 168.9888 100.0762 88.0762 72.0812 58.0656 45.0340 29.0391
II
('$HzCH201,CC3F, CIBHIIN-CHJ
0
I
II
(CHZCH~OI,CC~F,
a
!!
I1
+
c, x = 10 c,x=9
c,x=8 c,x=7 c,x=6 c,x=5 c,x=4 c,x = 3 c,x = 2 C2oH42N' c,x = 1 Cl8H38N+ CsF7H402' C3F7+ CSH10NO' C4HloNO' C4H10N'
CSHBN' CZH60' C2HS'
8ll
+
C I ~ H ~ , N - ( C H ~ C H ~ O I , C C ~ F ~ CH3N~CHzCHZOI,CC3F7
I/
ll
CH2
CHZ
b
II
(CHZCHZO)~CC~FI~
b,x=5
0
+I
0
C
the elemental composition of this ion was C6H4F702.The pesk-matching experiment was used to ensure that the m / z 241 ion was related to the derivatized Ethoquad and not to an ion such as 0 0
It Ill+
CJF.ICOC
formed from residual derivatizing reagent, Table I1 lists the calibration table constructed from the derivatized Ethoquad spectrum, including the accurate masses and proposed structures of the ions selected. Calibration against the time files generated from the positive-ion FAB analysis of mixture C enabled us to obtain a nominal mass calibration over the mass range 23-1600 amu.
I
C I ~ H ~ ~ N C H ~0C I
I
I/
(CHZCHZO)XCC~FE
D mixture is shown in Figure 3. The ion series observed are completely analogous to those described above. Using this mixture we were able to extend the calibration range to 1800 amu. However, it was more difficult to obtain consistently good calibrations with this derivative than it was with the other compounds. Since the species
-
0
Il
+
c7F1Sc
has the same nominal mass as the species (CH2CH20)9H+ (397 amu), the major ion series in the spectrum overlap. At low resolutions the computer-generated time centroids of several peaks are actually weighted averages of mass doublets of comparable intensities. This can produce calibrations which are not dependable. By judiciously choosing ions for our calibration table, we were able to calibrate over the mass range 23-1800 amu, but we do not recommend this derivative for day-to-day use unless the mass range 1600-1800 amu is important. To compare the sensitivities of the organic salts used in this study to other common FAB calibration standards, we obtained the positive-ion FAB spectra (Figure 4) of glycerol, KI in glycerol, and Ethoquad 18/25 derivatized with perfluorooctanoic anhydride under identical spectrometer conditions and electron-multiplier detector sensitivities. At the multiplier setting used, neither the glycerol nor the KI in glycerol produced ions above mlz 1000 at intensities detectable by the data system. In contrast, the derivatized Ethoquad 18/25 clearly showed ions to 1736 amu. Finally, we compared the FAB behavior of Ethoquad derivatized with perfluorobutyric anhydride (Figure 2) with that of Ultramark 1621, both run neat. The Ultramark spectrum exhibited intense ions between m/z 766 and 1490. The most abundant series (990,1090,1190, 1290,1390, and 1490) reached a maximum intensity of 11% (relative to the base peak CF2H+)at m / z 1190. However,
1849
Anal, Chem. 1984, 56, 1849-1852 Glycerol
0 0
200
400
600
r 4 0 X
0
200
400
100- 45
600
800
-
1000
800
1000
1200
1400
1600 KI
1200
-4OX-+
1400
+
1800
Glycerol
1600
1600
Derivatlzed Ethoquad
50
0 0
200
400
600
800 MIZ
1000
1200
1400
1600
mately an order of magnitude less intense than those observed in the spectra of the derivatized Ethoquad 18/25. The derivatized Ethoquad 18/25 offers the advantages of higher sensitivity and closely spaced reference ions without the need to precisely mix a number of inorganic salts. It is noteworthy that given a suitable lock-mass compound, these FAB-generated computer mass calibrations can be used in other ionization modes as well. We have used these calibrations to obtain spectra in the negative ion FAB, positiveion EI, and positive and negative ion field desorption (FD) modes (12).These calibrations can be especially useful in the FD mode where fleeting signals and a different desorption behavior for each component in a mixture make the construction of a calibration mixture extremely difficult. Registry No. Ethoquad 18/25, 28724-32-5.
1800
LITERATURE CITED
Flgure 4. Comparison of the positive ion spectra of glycerol (top), K I plus glycerol (center), and Ethoquad 18/25 derivatized with perfiuorooctanoic anhydride (bottom) under identical instrument conditions.
Ultramark did not exhibit any significant ions (21%)between m / z 313 and 766. We felt that such a large spacing between reference ions could cause problems in calibrating the spectrometer over the entire mass range. The high mass ions (1072, 1116, 1160, 1248, 1292, 1336, etc.) in the spectrum of derivatized Ethoquad (Figure 2) reached a maximum intensity of 6% (relative to the base peak at m / z 58) which is comparable to that of the high mass ions observed in the FAB spectrum of Ultramark 1621. Furthermore, there is a series of comparably intense ions between m / z 328 and 680 that facilitate computer calibration over the entire mass range. Subsequent to this work, we have achieved a computerized nominal-mass calibration to -2000 amu using NaI as the reference material (9) and an ion accelerating voltage of 6 kV. The lower accelerating voltage was necessary to circumvent magnet saturation effects which were causing the calibration to fail above 1300 amu. This was in part due to the large spacing (150 amu) between ions in the calibration table. In addition, the ions observed above 1000 amu were approxi-
-
(1) Barber, M.; Bordoll, R. S.; Sedgwlck, R. D.; Tyler, A. N. J. Chem. Soc., Chem. Common. 1981, 325-327. Tyler, A. N., Nature (Lon(2) Barber, M.; Bordoll, R. S.; Sedgwlck, R. 13.; don)1981, 293, 270-275. (3) Surman, D. J.; Vlckerman, J. C. J. Chem. Soc., Chem. Commun. 1981, 324-325. (4) Wllllams, D. H.; Bradley, C.; Bojesen, G.; Santlkarn, S.; Taylor, L. C. E. J. Am. Chem. SOC.1981, 103, 5700-5704. (5) Monaghan, J. J.; Barber, M.; Bordoll. R. S.; Sedgwick, R. D.; Tyler, A. N. Org. Mass Spectrom. 1982, 17, 569-574. (6) Fenwlck, G. R.; Eagles, J.; Self, R. Org. Mass Spechom. 1982. 17, 544-546. (7) Barber, M.; Bordoll, R. S.; Sedgwick, R. D.; Tyler, A. N. Blomed. Mass Spectrom. 1981, 8, 492-495. (8) Dell, A.; Morris, H. R. Blochem. Blophys. Res. Commun. 1981, 702, 730-738. (9) Aberth, W.; Straub, K. M.; Burllngame, A. L. Anal. Chem. 1982, 54, 2029-2034. IO) Baczynskyj, L.; Nielsen, J. W. Presented at the 30th Annual Conference on Mass Spectrometry and Allied Topics, Honolulu, HI, June 6-11, 1982. 11) Bone, W. M.; Hunt, D. F.; Marasco, J. M. Presented at the 30th Annual Conference on Mass Spectrometry and Allied Topics, Honolulu, HI, June 6-11, 1982. 12) DeStefano, A. J.; Keough, T. Presented at the 1983 Pittsburgh Conference and Exposition on Analytical Chemistry and Applied Spectroscopy, Atlantic Cky, NJ, paper no. 367.
RECEIVED for review July 21,1983. Resubmitted and Accepted April 23, 1984.
Differentiation of Monoepoxide Isomers of Polyunsaturated Fatty Acids and Fatty Acid Esters by Low-Energy Charge Exchange Mass Spectrometry Thomas Keough,* Edward D. Mihelich,' and David J. Eickhoff T h e Procter and Gamble Company, Miami Valley Laboratories, P.O.Box 39175, Cincinnati, Ohio 45247 Low-energy charge exchange mass spectrometry Is a convenient technique for the unambiguous differentation of monoepoxide isomers of polyunsaturated fatty acids, fatty acid methyl esters, and fatty acetates. This technique eiimlnates mutlistep derivatlzatlon procedures which are typically utilized prior to characterization of these Isomers wlth conventional electron Ionization mass spectrometry,
Arachidonic acid monepoxides have drawn recent attention due to successes in their regiospecific synthesis (1-3), conversion to biologically important HETE's (3),and identifi'Present address: Eli Lilly a n d Co., D e p t MC705, Indianapolis,
IN 46285.
0003-2700/84/0356-1849$01.50/0
cation as cytochrome P-450 oxidation products of arachidonic acid (4) that appear to have physiological importance (5,6). These monoepoxide isomers have been previously characterized on the basis of conventional 70-eV electron ionization (EI) mass spectrometry (7). However, the characteristic fragment ions observed in the E1 spectra are extremely weak (relative abundance less than 5% of the base peak). Thus, multistep derivatization procedures have been utilized to convert the monoepoxide isomers into molecules which yield more definitive E1 spectra (4, 7). These derivatization procedures are difficult to apply to trace quantities of material isolated from biological sources since they typically involve several sample handling steps (such as catalytic hydrogenation of the carbon-carbon double bonds, epoxide ring opening, and derivatization) prior to characterization using GC/MS. We 0 1984 American Chemical Society
