2312
Anal. Chem. 1988, 60,2312-2314
Self Chemical Ionization in an Ion Trap Mass Spectrometer Sir: In a recent Correspondence in Analytical Chemistry (I) Eichelberger et al. addressed the topic of self chemical ionization (self-CI) in an ion trap detector (ITD) (2,3). The report referred to a recent paper by Olson and Diehl ( 4 ) in which the presence of (M + 1)+ peaks in spectra obtained by using an ITD were attributed to self-CI. Eichelberger et al. demonstrated the ITD system to be subject to space charge effects with large sample sizes (>50 ng) that cause broadening of peaks and misassignment of masses by the data system. They suggested space charge effects, therefore, to be the source of the (M + 1)+ peaks observed by Olson and Diehl. The compounds used to demonstrate the effect of space charge (hexachlorobenzene and anthracene-dlo) were aprotic and therefore could not undergo self protonation. The possibility of space charge causing a mass misassignment by the data system was clearly demonstrated, but the possibility for self-CI, by species inclined to undergo this reaction, under conditions readily obtained in the ITD, was not precluded. The fact that ion/molecule reactions, such as self-CI, can occur in a three-dimensional quadrupole (as in any type of mass spectrometer) is not at issue. Indeed, ion/molecule reactions have been studied in detail in three-dimensional quadrupoles (5). Furthermore, the ITD is often intentionally operated under conditions conducive for the occurrence of ion/molecule reactions, viz. in the CI mode of operation (6). The purpose of this correspondence is to elucidate the conditions under which ion/molecule reactions, and self-CI reactions in particular, are likely to occur in the ITD and the closely related Ion Trap Mass Spectrometer (ITMS). EXPERIMENTAL SECTION Maas spectra and tandem mass spectrometry (MS/MS) spectra were obtained with a Finnigan MAT ion trap mass spectrometer (ITMS) equipped with gas and solids probe inlets (no gas chromatograph). The ITMS is a more versatile and powerful version of the ITD capable, inter alia, of MS/MS experiments. The ion trap electrode structures of the two systems are physically identical, except that the end caps are not grounded in the ITMS, and the two systems share most of the normal operating parameters. A description of the ITMS can be found elaewhere (7). Data were obtained with an experimental version (3.91) of software for the ITMS provided to us by Finnigan. The ion trap was tuned by using the standard procedure and software provided with the instrument. For some experiments a dc voltage was added to the ring electrode after the ionization period to mass selectively store ions of a defined m/z. The dc was then removed and the remainder of the experiment performed by operating the ion trap in the mass selective instability mode (2). Data were collected at various combinations of sample pressure and reaction time. These conditions are noted in the text. For experiments in which a reactant ion was mass selected to determine the rate of reaction, the ionization period was set to give approximately equal initial numbers of the reactant ion. Normal mass spectra and MS/MS spectra were obtained by using an ionization time of 0.1 ms. In all cases the temperature of the system was maintained at 373 K with a background helium pressure reading, unless otherwise Torr, corrected for the sensitivity of the ion noted, of 7 X gauge. Samples used in this study were introduced into the ion trap through a variable leak valve to a constant pressure. RESULTS AND DISCUSSION Self-CI can imply a variety of reactions including, among others, electron transfer and proton transfer, which can occur from the molecular ion or from one or more fragment ions. A general scheme shown in eq 1 indicates that m different X,+ + M Mp,' + neutral products
-
1 Md,+
(1)
+ neutral products 0003-2700/88/0360-2312$0 1.50/0
reactant ion species, X,+, where n is used to denote the particular reactant ion, give parent ions Mp,+ that may fragment to give daughter ions Md,+. For the case when the number density of M is constant, the total number of ions arising from self-CI is given by [Mp,+
+ Md,+]
m
[X,'], (1- d k n ) L M I t )
=
(2)
n=l
n=l
where [X,+Io is the initial concentration of the nth reactant ion, k, is the rate constant for the reaction of the nth reactant ion with M, and t is the reaction time. For the simple but commonly observed case of a single reactant ion that transfers a proton to the neutral molecule and where fragmentation of the protonated molecule does not occur, the extent to which the reaction proceeds is given by
(3) (Note that [X'], does not necessarily include the total number of X+ ions formed initially since many of these ions may fragment. Since fragmentation typically occurs within a microsecond, bimolecular reactions do not compete with unimolecular decay for ions with sufficient energy to fragment.) While the absolute number of MH+ ions formed depends upon the number of X+ ions, the relative amount of MH+ is only a function of the neutral concentration and reaction time, for a given system. Rate constants typically can be from high values of (1-10) X lo4 cm3/(molecule s) to values orders of magnitude smaller. Reaction times can be as short as =l ms, to greater than 1s, and number densities can range typically from 1 x IO4 to 1 x IO1' molecules/cm3. The reaction time referred to here includes the time after the ionization pulse before the mass scan plus the time required during the mass scan for the radio frequency (rf) amplitude to reach a point where the reactant (or product) is ejected from the ion trap. (This distinction can be important when two ions of different mass have nearly the same rate constant for reaction with M. The reaction with the lighter reactant ion is discriminated against relative to that involving the higher mass ion due to a shorter reaction time.) The ionization time may also be included in the reaction time if mass-selective storage is not employed. Reliable rate constant measurements require an accurate pressure measurement. At present we can only measure the pressure in the ITMS vacuum system with an uncalibrated ionization gauge tube and, even when corrected for relative sensitivity, is not expeded to accurately reflect pressure within the ion trap itself. We therefore studied a reaction with a well-known rate constant to determine how accurately the ITMS, with its present approach to pressure measurement, can be used to measure an ion/molecule reaction rate constant. We chose a widely studied example of a self-CI reaction, that of ionized methane with methane, viz. CH4+'
+ CH4
-
CHS+ + CH3'
(4)
The rate constant was obtained in the usual way by measuring the [CH,+'] versus reaction time and dividing the slope of the plot of -In [CH4+']/[CH4+']oversus reaction time by [CH,]. (This procedure is equivalent to plotting -In (1- [CH,+]/ [CH4+'],) vs t to obtain k when, as we have observed in this case, no other reaction channels significantly deplete CH,+'.) It was noted that the measured rate constant was insensitive to the presence of background helium (i.e. the same rate was obtained within experimental error with a pressure of 7 X lo4 Torr of He and without any He in the ion trap). The rate 0 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 20, OCTOBER 15, 1988
1
I
4
100%
'
la)
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1
0.8
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:, I !II ~1 I ,
, ,
,
,
-r--r 60
40
m/z
"
I
'
"
'
~
"
'
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"
k = 3 . I X ~ O -cm3/molec-s ~
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(b)
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"
2313
I
J- 1 ~, >j)\lb'\, 80
72
74
h ,
76
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0
2
4
6
8
10
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Figure 1. ITMS mass spectra of ethyl formate at sample pressures of (a) 1.5 X lo-' and (b) 1.0 X 10" Torr. Parts c and d show the real time ion current profiles in the region of the molecular ion obtained at the pressures in parts a and b, respectively.
Figure 2. Comparison of the calculated value (curve) of (1 - e - k [ M ] f ) and experimental data points as a function of the number density of M. The curve was calculated by using a rate constant determined by varying reaction time at a constant pressure.
constant was also relatively insensitive to the rf trapping level in cases where no obvious mass discrimination was present. With a pressure of 8 X Torr (the ionization gauge reading corrected for the sensitivity for methane) a rate constant of (1.5 f 0.5) X lo4 cm3/(molecules) was obtained. (We believe that the large uncertainty is due to the present inability to precisely measure the pressure in the ion trap.) The generally accepted thermal and near thermal energy rate constant is ~ 1 . X2 cm3/(molecule s) (8)as obtained by a variety of methods including use of a three-dimensional quadrupole (9). Esters are compounds previously noted for undergoing self41 and provided two of the three cases where Olson and Diehl observed enhanced (M + 1) peaks ( 4 ) . We have observed in the mass spectrum of ethyl formate (molecular weight 741, depending upon conditions, a signal at m / z 75 significantly larger than expected from 13C,when operating the ITMS in the nominal electron impact mode and without any CI reagent gas present. The MSIMS spectrum of the (M 1)+ ion confirms that it is (M + H)+ resulting from self-CI and not a result of space charge; i.e. the MS/MS spectrum of the ion at (M + 1)+ matches that of MH+ formed by methane CI of ethyl formate in the ion trap and shows different mass fragment ions than appear in the MS/MS spectrum of M+'. Figure 1 shows two mass spectra acquired for ethyl formate at indicated pressures of 1.5 X lo-' and 1.0 X lo4 Torr, respectively. In both cases the m/z 75 peak is larger than that expected for the 13Cisotope peak and the real-time ion peak profiles in the molecular ion region both show the problem of space charge peak broadening to be absent. Furthermore, at the higher ethyl formate pressure ions at m / z 47 and m / z 29, the major fragment ions observed in the MS/MS spectrum of MH+, increase significantly in relative abundance. This indicates that some fragmentation proceeds after proton transfer. It is possible to determine which ions are protonating the background ethyl formate molecules by mass selectively storing each of the ions in the mass spectrum in turn. This approach confirms that the ions responsible for protonating ethyl formate are derived from the ionization of ethyl formate and are not simply ions from traces of background species such as water. We found that ethyl formate is protonated both by the molecular ion at m / z 74 and by the ion at m / z 31, presumably protonated formaldehyde, H2COH+. The rate constants for protonation by the molecular ion, M+', and by m/z 31 were measured by varying the reaction time and calculated
to be 3.1 X lo4 and 2.8 X lo4 cm3/(molecule s), respectively. The results for the methane self-CI reaction suggest that the actual rate constants are somewhat less than this. These are not unusually high rate constants but are typical for a moderately fast ion/molecule reaction. Figure 2 shows a plot of (1 - e-k[M]t) versus [MI, where M is ethyl formate, over the range of (1.5-6.7) X lo9 (5.8 X to 2.6 X lo-' Torr) predicted for k = 3.1 X lo4 cm3/ (molecule s) and t = 52.5 ms. As expected, the data points taken as a function of pressure at a fixed reaction time correspond well to the curve that was determined from a rate calculated by changing the reaction time at a fixed pressure. The results indicate that, in the absence of instrumental discrimination effects, the extent to which an ion/molecule reaction is likely to occur in the ion trap can be predicted, as expected, from first-order kinetics when the reaction conditions are known and the rate constant is either known or can be predicted. The rate constant in the ITMS can be measured provided an accurate pressure measurement in the reaction volume is made. Under gas chromatography/mass spectrometry (GC/MS) conditions, [MI is not constant but varies with the GC peak profile. Nevertheless, eq 2 can still be used to approximately determine the likelihood for self-CI since typical GC peak widths (full width at half maximum) are on the order of 1-5 s and the reaction time for each individual scan is on the order of 0.05 s or less. For each individual scan, therefore, [MI changes relatively slowly. In order to maintain a pressure of 1 mTorr in the ion trap, a flow rate of helium + sample into the flow restrictor of the connection line to the ion trap from the gas chromatograph is typically 1 mL/min at -473 K. This corresponds to a flow of 4.3 X mol/s. A 50-ng sample of molecular weight 300 entering the ion trap in a GC peak with a full width at half maximum of 1.5 s gives a peak flow rate of 1.0 X 10-lomol/s, assuming a Gaussian peak shape. This corresponds to a sample partial pressure of 2.3 X Torr. During the 1.5 s over which most of the analyte is eluting, the sample pressure would be 11.2 X Torr. At a trap temperature of 373 K this leads to a number density of 12.9 X lo9molecules/cm3. If we assume that M+' protonates M, that the mass range is scanned starting from 200 amu at the usual rate of 5555 amu/s, and the time after ionization and before the mass scan is 0.001 s, the reaction time is 0.019 s. The ratio [MH+]/ [M+'l0 can be calculated for various rate constants by using eq 1 to determine the likelihood that products from
+
Anal. Chem. 1988, 60.2314-2317
2314
self-CI might be observed. For example, a rate constant for the reaction of 1 X lo4 cm3/(molecule s) under these conditions would yield that ratio [MH+]/[M+’], = 0.05. This would lead to a [ (M 1)+]/ [M+’] ratio in the mass spectrum of ~0.05above the fraction expected on the basis of the 13C contribution, provided the ions are trapped and detected with equal efficiency. At longer reaction times, such as would result from a scan starting at 20 amu, the [MH+]/[M+’], ratio would be 0.13. The purpose of this discussion is to show that products from self-CI (or other ion/molecule reaction) can be observed for suitable compounds under conditions that can be readily obtained, in the ITD and ITMS. The likelihood that this may occur can be anticipated in a straightforward manner. As has been demonstrated, space charge can lead to peak broadening which can cause the data system to misinterpret signals as arising from (M + l)+ions in mass spectra obtained from large sample sizes (greater than about 50 ng depending upon the compound (1,lO)). While the automatic gain control mode of operation recently developed by Finnigan (11)addresses the space charge problem, this does not reduce the potential for self41 since it is the concentration of the neutral, not the ions, along with the reaction time that determines the relative degree of self-CI. The segmented scan function used in the ITMS (and ITD) data system reduces the reaction time and thus reduces the likelihood of the ion/molecule reactions being observed. This paper has shown that “large” sample sizes can lead to the observation of significant concentrations of protonated species from self-CI in systems with fast reaction rates. The two experimental variables that can be controlled to avoid self-CI are reaction time and sample concentration. (In GC/MS, the sample concentration is inversely related to the chromatographic peak width, i.e. narrower peaks give higher instantaneous concentrations at the peak maximum than wider peaks, for the same quantity of sample.) The Finnigan data system is designed to reduce the reaction time to the minimum practical so it is up to the experimentalist be aware of the approximate concentration of the analyte, to prevent self-CI, or a t least to be cognizant of its possible occurrence. In general, self-CI or other ion/molecule reactions are expected
+
to be observed in the ITD or ITMS mainly with samples in which it is observed in other types of mass spectrometers, such as, for example, esters. The observation of self41 will likely require a rate constant 21 x cm3/(molecule s). ACKNOWLEDGMENT The authors thank Don Hoekman and Michael WeberGrabau of Finnigan for providing experimental software and for helpful discussions and Henry S. McKown of ORNL for construction of the dc pulsing circuit used in these experiments. LITERATURE CITED (1) Eichelberger, J. W.; Budde, W. L.; Slivon. L. E. Anal. 0”.1987, 59, 2730. (2) Stafford, G. C.; Kelley, P. E.: Stephens, D. R. US. Patent 4540884, 1985. (3) Stafford, G. C.; Kelley, P. E.; Syka. J. E. P.; Reynolds, W. E.; Todd, J. F. J. I n t . J . Mass Spectfom. Ion Processes 1984, 60,85. (4) Olson, E. S.; Diehl, J. W. Anal. Chem. 1987, 59,443. (5) Todd, J . F. J. Dynamic Mass Spectrometry; Price, D., Todd, J. F. J., Eds.; Heyden: London, 1981; Vol. 6, Chapter 4. (6) Kelley, P. E.; Stafford, G. C.; Syka, J. E. P.; Reynolds, W. E.; Louris, J. N.; Todd, J. F. J. Adv. Mass Spectrom. 1986, 106, 869. (7) Louris, J. N.; Cooks, R. G.; Syka, J. E. P.; Kelley, P. E.; Stafford, G. C.; Todd, J. F. J. Anal. Chem. 1987, 59, 1677. (8) Henchman, M. Ion-Molecule Reactions; Franklin, J. L., Ed.; Plenum: New York, 1972; Vol. I, Chapter 5. (9) Lawson, G.; Bonner, R. F.; Mather, R. E.; Todd, J. F. J.; March, R. E. J. Chem. SOC.. Faraday Trans. 1 1978. 72, 545. (10) Eichelberger, J. W.; Budde, W. L. Blamed. Environ. Mass Spectrom. 1987, 14. 357. (11) Stafford, G. C.; Taylor, D. M.; Bradshaw; S. C.; Syka, J. E. P.; Uhrich, M. Presented at the 35th ASMS Conference on Mass Spectrometry and Allied Topics, Denver, CO, May 24-29, 1987.
Scott A. McLuckey* Gary L. Glish Keiji G. Asano Gary J. Van Berkel Analytical Chemistry Division Oak Ridge National Laboratory Oak Ridge, Tennessee 37831 RECEIVED for review April 29, 1988. Accepted July 22,1988. Research sponsored by the U.S. DOE Office of Basic Energy Science under Contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc.
TECHNICAL NOTES Thermospray Liquid Chromatography/Mass Spectrometry in Deuterium Oxide Charles G . Edmonds, Steven C. Pomerantz, Fong Fu Hsu, and James A. McCloskey*
Departments of Medicinal Chemistry and Biochemistry, University of Utah, Salt Lake City, Utah 84112 Deuterium labeling of organic compounds plays an important role in mass spectrometry in studies of ionic reaction mechanisms, and in applications involving structural characterization. In addition to conventional methods of deuterium introduction including direct exchange in the mass spectrometer inlet system (I, Z ) , gas-phase exchange under chemical ionization conditions (3)and exchange in conjunction with fast atom bombardment (FAB) (4)have been studied. The on-line exchange of deuterium for protium during combined chromatography/mass spectrometry, fist reported using GC/MS (5), is advantageous because both there is direct applicability to components in mixtures and high exchange levels of acidic hydrogen atoms with minimum back exchange
can be obtained. Although the introduction of deuterium can be effectively carried out by a variety of novel preparative liquid and gas chromatographic procedures (6),the direct combination with mass spectrometry obviates the necessity of isolation of individual components and permits direct comparison of mass spectra of labeled constituents of a multicomponent mixture with spectra of unlabeled components obtained in conventional fashion from a separate chromatographic analysis. Directly combined liquid chromatography/mass spectrometry (LC/MS) provides particular advantages in the analysis of polar compounds for which GC/MS is less well suited. Deuterium-protium exchange using LC/MS was first reported
0003-2700/88/0360-2314$01.50/00 1988 American Chemical Society
