1887
Anal. Chem. 1986, 58, 1887-1889
silicone oils with the narrow molecular weight range customary in stationary phases prepared specially for use in gas chromatography.
LITERATURE CITED (1) Keuiemans, A. I. M.; Kwantes, A. Vepour Ph8se Chrometogf8phy; Desty, D. H., Ed.; Butterwofths: London, 1957; pp 22-23. (2) Hawkes, Stephen, J.; Mooney, Eric F. Anal. Chem. 1864, 3 6 , 1473- 1477. (3) Hawkes, S. J.; Carpenter, D. J. Anal. Cheffl. 1967, 39, 393-395. (4) Kong, John M.: Hawkes, Stephen J. Macromolecules 1975, 8, 685-687. ( 5 ) Kong, John M.; Hawkes, Stephen J. J. Chromfogr. Sci. 1976, 14. 279-287.
(6) Mlllen, William; Hawkes, Stephen J. J . Chromtogr. Sci. 1977, IS, 148- 150. (7) W i n g s , J. C. D y ~ f f l l c sof Chromtogaphy; Marcel Dekker: New York, 1985; p 235. (8) Ferry, John E. Visccelestic Pfopeftlesof Po&rnefs, 2nd ed.; Wlley: New York, 1970 pp 332-333, 371-375, 466-480.
Stephen J. Hawkes Department of Chemistry Oregon State University Corvallis, Oregon 97331
RECEIVEDfor review Janumy 27,1986. Accepted April 1,1986.
High-Resolution Detection of Daughter Ions with a Hybrid Mass Spectrometer Sir: The technique known as mass spectrometry/mass spectrometry (MS/MS) or tandem mass spectrometry has become an accepted technique in analytical laboratories for the analysis of complex mixtures and a widely used technique for fundamental studies of gas-phase ion chemistry (1,2).The initial work in MS/MS was done on instruments of "reverse" geometry; i.e., the magnetic sector (B) preceded the electric sector (E) (3,4). As it became apparent that MS/MS was very useful for mixture analysis (5),new instruments were built to try to improve upon the performance of the BE instruments, especially the mass resolution. Daughter ion resolution was the f i t area that was improved upon with the application of triple quadrupole (QQQ) systems (6) to mixture analysis. While the triple quadrupole instrument was being developed, several triple sector instruments were also being constructed (7-10).All the latter had a geometry of EBE in which the EB portion of the instrument was a double-focusing mass spectrometer. Thus,they offered greatly increased parent ion resolution. However, the daughter ion resolution was no better than that of the BE instruments. Shortly thereafter, hybrid instruments, combining quadrupoles and sectors, were introduced to try to take advantage of the best resolution features of both triple quadrupole and triple sector instruments. The first of these was of BQQ geometry (11). Since the introduction of this instrument, other hybrids have been reported in the literature in which the sector portion of the instrument was a double-focusing mass spectrometer of EB geometry (12)or BE geometry (13). These instruments offer high parent ion resolution but still only unit resolution for the daughter ions. To improve the daughter ion resolution further, a magnet was added to one of the previously described triple sector instruments to produce an EBEB geometry, or essentially an instrument consisting of two double-focusing mass spectrometers in tandem (8). It was estimated that this instrument would have a daughter ion resolution of =lo OOO but this has yet to be reported experimentally. Recently a commercial instrument of BEEB geometry has been constructed (14). A daughter ion resolution of ~ 5 5 0 has 0 thus far been reported with this instrument (15). Both four-sector instruments use linked scanning a t a constant B/E (16,17),following high-energy collision activated dissociation (CAD), to obtain daughter ion MS/MS spectra. In taking data in this manner, these instruments do not suffer from the artifact peaks that are commonly observed in linked scan spectra obtained on two-sector instruments (18).This is due to the fact that a stage of mass separation occurs before 0003-2700/86/0358-1867$01.50/0
CAD in the four-sector instruments but not before the CAD process in the two-sector instruments, Another approach has been used to obtain high resolution of MS/MS daughter ions, namely, Fourier transform mass spectrometq (Fl'MS) (19).In the FTMS experiment, a single analyzer is used with the ionization, MS/MS reactions and mass analysis being separated in time. This is in contrast to conventional MS/MS instruments where the ionization, MS/MS reactions, and analysis are separated in space. In the first FTMS MS/MS experiment (191, low-energy CAD was used to form the fragment ions, and a resolution of =3000 was reported. Since then, resolution has been improved by approximately an order of magnitude (20). A variant of the FTMS experiment has been performed where a quadrupole is used as the first stage of mass analysis and a FTMS instrument as the second. With this system a resolution of 140000 (FWHM) has been shown (21). In this report, we describe experiments where high-resolution daughter ion spectra are obtained with a new hybrid MS/MS instrument of QEB geometry. These experiments were done by using low-energy CAD, although a unique feature of this instrument is its ability to also detect high-energy CAD daughter ions at high resolution. Thii latter mode of operation has yet to be implemented, however. The instrument will be described briefly here. A detailed description of the instrument will be reported elsewhere (22). Figure 1shows a block diagram of the instrument. The first stage of mass analysis is performed by a UTI lOOC quadruople mass fiter. The EB portion of the instrument is an AEI MS50 double-focusing, high-resolution mass spectrometer. There are three reaction regions in the instrument where ions can undergo reactions and subsequent mass analysis. The first of these is after the quadrupole but prior to acceleration into the EB portion of the instrument. Therefore, low-energy CAD MS/MS experiments can be performed by use of this reaction region. The other two reaction regions are located in the instrument after the accelerating region, and thus the ions have kiloelectronvolt energies in these regions. It is possible to perform high-energy CAD MS/MS and analyze the daughter ions at high resolution with the QEB by the linked scan procedure used on the four-sector instruments (8,14). This would involve mass selecting the parent ion with the quadrupole, performing collisional activation (kiloelectronvolt ion kinetic energy) in the second reaction region, and then link scanning the EB portion of the instrument at a constant ratio of B/E.Given the excellent transmission and resolution of the MS50, this should give results @ 1986 American Chemlcal Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 8, JULY 1986
e'i
1-
I
keV
m / A m =650
I
m/Am =
Figure 1. Block diagram of the QEB instrument. The nominal kinetic energy range of the ions as they pass through the instrument is shown below the diagram: S, source; Q, quadrupole; RR, reaction region; E, electric sector; B, magnetic sector: D, detector.
a t least as good as those obtained on the four-sector instruments (8, 14). The QEB instrument, however, offers another means of obtaining high daughter ion resolution. This can be done by simply mass selecting the parent ion with the quadrupole, performing collisional activation in reaction region one (electronvolt ion kinetic energy), and scanning the MS50 in its normal manner, i.e., scanning the magnet. The obvious difference between these two methods of analysis is the velocity of the parent ion when it fragments. The linked scan method takes advantage of the fact that velocity is a parameter affecting both electric and magnetic sector analysis (E 0: v2, B a v) and that since the fragmentation occurs in a field-free region, all the daughter ions will have the same velocity as the parent ion (to a first approximation). In the low-energy CAD method, the main acceleration is performed after the fragmentation, and thus, all the daughter ions have nominally (again to a first approximation) the same energy, and thus the EB portion of the instrument can operate in a normal manner. A complication in both these methods comes from the fact that the fragmenting ion converts some of its internal energy into translational energy of separation or the so-called kinetic energy release (23). The kinetic energy release appears as a spread in velocity of the daughter ions. This velocity spread is amplified in the laboratory frame of reference and increases with the square root of the parent ion kinetic energy (23). Therefore, the spread in daughter ion velocity is much greater following high-energy CAD. The spread of velocity results in lower sensitivity, since a linked scan by its very nature detects only a narrow range of velocities. While the amplification of the kinetic energy release is much smaller for the dissociation of ions with low kinetic energies, it still causes a spread in energy of the ion beam, which again reduces the transmission and therefore sensitivity. This, combined with the fact that the partitioning of kinetic energy between the ion and neutral upon fragmentation of the low-kinetic-energy ions can cause a several tenths of a percent spread in final ion energies, makes it unclear as to whether the high- or low-energy CAD method will give the better sensitivity (or resolution). It may be compound dependent. Previous results with high-energy CAD showed a daughter ion resolution of ~ 5 5 0 0(14).Our preliminary results using the low-energy CAD method compare favorably to this. Figure 2 shows the resolution obtained under two different sets of conditions for the fragmentation of m / z 134 from n-butylbenzene to m / z 92 and 91. On the left is the "inherent" resolution of the system, i.e., with the MS50 slits wide open. Thii daughter ion mass resolution is better than that obtained on any sector, quadrupole, or hybrid instrument under normal operating conditions with the exception of the above-mentioned four-sector instruments. On the right-hand side of Figure 2 is the resolution obtained when the MS50 source and collector slits are arbitrarily closed down. It is expected that by appropriate tuning of the MS50, even better resolution is obtainable. Figure 3 shows an example of the separation of isobaric daughter ions from a selected parent ion. The parent ion is the protonated molecular ion of 3-methyl-2-butanone, m / z 87. On the left of Figure 3 is the region of the daughter ion
6000
92
I
91 m / z
92
94 m/z
Flgure 2. Daughter ion resolution obtained on the QEB with the slits wide open (left) and with the MS50 source and collector sUts arbitrarily closed down (right).
(b)
43.400 43.000
I
: H ,CO+
4 43.400 43.000
Flgure 3. Portion of the daughter ion spectrum of protonated 3methyl-2-butanone under similar conditions as described In Figure 2. (The resolution on the right-hand side of the figure is approximately 3550.)
spectrum obtained around mlz 43 when the MS50 slits are wide open. On the right of this f i e is that same region after the source and collector slits have been arbitrarily closed down. The isobaric C3H7+and CH3CO+species are both observed to be daughters of the selected parent ion. Since MS/MS is becoming widely used in the field of mixture analysis, and with hybrid instruments expected to play a major role in this area, it is instructive to consider the pros and cons of where the high-resolution measurement should be made. The sector/quadrupole geometry (EBQand BEQ)offers high-resolution measurement and separation of the parent ions in a typical daughter ion MS/MS experiment with unit mass resolution of the daughter ions. The quadruopole/sector geometry offers high-resolution measurement of daughter and parent ions. However, only unit mass sep-
Anal. Chem. 1988, 58, 1889-1892
aration of parent ions is possible, meaning that isobaric parent ions can not be individually selected for a MS/MS experiment. In addition, the high-resolution measurement can be done on daughter ions formed by either high-energyCAD or low-energy CAD. The ultimate resolution obtainable should be better on the sector/quadrupole geometries than on the quadrupole/sector geometries due to the differences in energy spread in ions formed in an ion source (sector/quadrupole) and by collision-activated dissociation (quadrupole/sedor). However, this is counterbalanced by the fact that the daughter ions have a lower mass and thus fewer empirical formulas within given error limits. Also, since high-resolution measurements are being made on more than one mass in the quadrupole sector geometry, more information may be available to determine structural subunits and to “piece”the parent ion back together. We think that the above arguments suggest that the quadrupole/sector geometry provides more information than the sedor/quadrupole geometry except possibly in cases where isobaric parent ions are present. In these cases, the ability to separate parent ions at high resolution is desirable. Thus, in choosing a hybrid instrument for mixture analysis, the expected nature of the mixtures to be analyzed could be important in deciding which geometry is preferable. Registry No. n-Butylbenzene, 104-51-8;3-methyl-2-butanone, 563-80-4.
LITERATURE CITED (1) Cooks, R. 0.;Glish, 0. L. Chem. Eng. News 1081, 59(48), 40. (2) Tandem Mass Spectrometry; McLafferty, F. W., Ed.; Wlley: New Yo&. 1983. (3) Beynon, J. H.; Cooks, R. G.; Amy, J. W.; Baltlnger, W. E.; Ridley, T. Y. Anal. Chem. 1073, 45. 1023A. (4) Wachs, T.; Bente, P.F. 111; Mclafferty, F. W. I n t . J . Mass Specfrom. Ion Phys. 1072, 9 , 333. (5) Kondrat, R. W.: Cooks, R. G. Anal. Chem. 1078, 5 0 , 81A.
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(6) Yost, R. A.; Enke. C. G. Anal. Chem. 1070, 51, 1251A. (7) Maquestlau. A.; van Haverbeke, Y.; Flammang, R. Avrassart. M.; Finet, D.Bull. SOC. Chlm. Belg. 1078, 87, 765. (8) McLafferty,F. W.; Todd, P.J.; McGilvery, D.C.; Baldwln. M. A. J . Am. Chem. SOC. 1080, 702, 3360. (9) Vrscaj, V.; Kramer, V.; Medved, M.; Kralj, B.; Marsel, J.; Beynon, J. H. Ast, T. Int. J . Mass Spectrom. Ion Phys. 1080, 3 3 , 409. (10) Russell, D. H.; Smith, D. H.; Warmack, R. J.; Bertram, L. K. Int. J . Mass Spectrom. Ion Phys. 1080, 3 5 , 381. (11) Glish, G. L.; McLuckey, S. A.; Rldley, T. Y.; Cooks, R. G. Int. J . Mass Spectrom. Ion phvs. 1082. 4 1 , 157. (12) Green, B. N.; Bateman, R. H.; Tummers, M. H.; Smith, D. C. Fresenulus’ 2.Anal. Chem. 1083. 316, 217. (13) Schoen, A. E.; Amy, J. W.; Ciupek, J. D.;Cooks, R. G.; Dobberstein. P.;Jung, G. Int. J . Mass Spechom. Ion Processes 1085, 65, 125. (14) Hass, J. R.; Green, B. N.; Bott, P. A,; Bateman. R. H. submitted for publication in Int J Mass Specfrom Ion Processes. (15) Bursey, M. M.; Hass, J. R. J . Am. Chem. SOC. 1085, 107. 115. (16) Brulns, A. P.; Jennlngs, K. R.; Evans, S. Int. J . Mass Spectrom. Ion Phys. 1078, 26, 395. (17) Boyd, R. K.; Beynon, J. H. Org. Mass Spectrom. 1077, 12, 163. (18) Biiton, J. N.; KyrlakMis, N.; Walght, E. S. Org. Mass Spectrom. 1978, 13, 489. (19) Cody, R. B.; Freiser, B. S. Anal. Chem. 1082, 5 4 , 1431. (20) Carlin, T. J.; Frelser, B. S. Anal. Chem. 1083, 5 5 , 571. (21) McIver. R. T.; Hunter, R. L.; Bowers, W. D. Int. J . Mass Specfrom. Ion Processes 1085. 6 4 , 67. (22) Gllsh, G. L.; McLuckey, S. A.; McBay, E. H.; Bertram, L. K. Int. J . Mass Specfrom Ion Processes, in press. (23) Cooks, R. G.; Beynon, J. H.; Caprioii, R. M.; Lester, G. R. Metastable Ions; Elsevler: Amsterdam, 1973.
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Gary L. Glish* Scott A. McLuckey Analytical Chemistry Division Oak Ridge National Laboratory Oak Ridge, Tennessee 37831 RECEIVED for review January 21, 1986. Accepted March 18, 1986. This work was supported by the U.S.Department of Energy, Office of Basic Energy Sciences, through Contract DE-AC05-840R21400with Martin Marietta Energy Systems, Inc.
Mass Spectrometric Determination of Amines after Formation of a Charged Surface-Active Derivative Sir: Sputtering of organic molecules from liquid solution is a new and very powerful technique for organic secondary ion mass spectrometry (SIMS) (1). It has been demonstrated that when glycerol is used as solvent, sensitivity for this technique is closely related to analyte polarity and surface activity with the strongest signals being obtained for charged surfactants. Conversely, nonpolar and/or uncharged analytes may require derivatization before useful signals can be obtained. The use of “reverse derivatization“to enhance polarity in relatively nonpolar analytes was first suggested by Cooks et al. (2). This concept has been developed further by Kidwell et al. (3) and DiDonato and Busch (4)who used Girard’s reagent T to introduce a quaternary ammonium function into otherwise uncharged analytes, thereby greatly enhancing sensitivity. Barber et al. (5) have noted the importance of surface activity in analysis of peptides, and Ligon and Dorn (6) have suggested that “reverse derivatization” procedures should be designed to enhance both polarity and surface activity. Ligon (7)has shown that derivatization of small peptides with dodecanal can provide enhanced response. Further, Ligon and Dorn (8-10) have demonstrated the analysis of small inorganic anions through the use of cationic surfactants. In this case, the surfactants serve to bind the 0003-2700/86/0358-188980 1.50/0
anions to the surface of glycerol solutions. Surfactants have also been used to bind organic analytes to the glycerol surface (11). In practice, many classes of organic analytes lack either polarity or surface activity (or both) and can benefit from derivatization before SIMS analysis. For each such class of materials, it is necessary to seek reagents that can confer the required combination of physical properties. In this correspondence, we report the use of 2-dodecen-1-ylsuccinic anhydride as a reagent to confer both a negative charge and surface activity on primary and secondary amines. Busch et al. (12)have described reagents for analysis of amines that impart charge but do not significantlyenhance surface activity.
EXPERIMENTAL SECTION The mass spectrometer used in these experiments was a Finnigan-MAT 731. The operating parameters for the SIMS experiment have been described previously (6). Derivatization Procedure. One millimole of each amine (see below) was combined with 1 mmol of 2-dodecen-1-ylsuccinic anhydride in 10 mL of hexane or hexane/diethyl ether as solvent. A small amount of diethyl ether was added to enhance the solubility of the amine in the case of aniline. The reaction mixtures were then heated to 60 O C for 15 min. The solvent was subse0 1986 American Chemical Society
