Advisory Panel Jonathan W . Amy Glenn L. Booman Robert L. Bowman
Jack W . Frazer Howard V. Malmstadt William F. Ulrich
INSTRUMENTATION
Metastable Ions in Mass Spectra J. H. Beynon, Department of Chemistry, Purdue University, West Lafayette, Ind. 47907
ANY OF T H E IONS in the ionization
chamber of a mass spectrometer M are produced with so much excitation
energy that they undergo a series of competing unimolecular decompositions before leaving this chamber. The product ions of these decompositions are drawn out of the ionization chamber and many of them are sufficiently stable to be accelerated and to traverse the flight path of the mass spectrometer without further reaction. These "stable" ions form the conventional mass spectrum. Some of the ions, however, do break down before reaching the ion collector. These are the socalled "metastable" ions. DOUBLE-FOCUSING MASS SPECTROMETER
In a double-focusing mass spectrometer, such as is shown schematically in Figure 1, consider ions which fragment in the field-free region in front of the magnetic sector. When an ion )»!+ breaks into an ion m2+ and a neutral fragment (m1 — m2), the mean velocity through the tube of m2+ and of (m1 — m2) will be the same as that of the initial ion mx+. That is to say, the kinetic energy of the initial ion is shared between the two fragments formed in direct proportion to their masses. When these low energy m 2 + ions pass through the magnetic sector, they are deflected to a greater extent than m 2 + ions having the full energy of acceleration, and it can be shown that they will appear as a weak, diffuse peak on the mass scale centered at an apparent mass m* given by: m* = ^
Figure 1. Schematic diagram of a double-focusing mass spectrometer of modified Nier-Johnson geometry (the diagram refers to Model RMH-2 made by Hitachi— Perkin Elmer)
(1)
The appearance of these "metastable peaks" and the difficulties of measuring their positions and intensities are illustrated in Figure 2. Because the m 2 + ions have less than the full energy of acceleration, eV, they can be prevented from reaching the collector if the potential of this elec-
Figure 2. Partial mass spectrum of naphthalene showing weak "metastable peaks" ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
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INSTRUMENTATION
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trode is raised to a value between m2/m1 eV and eV. Early mass spec trometers were, in fact, fitted with such a "metastable peak suppressor" to avoid any interference with the normal peaks in the mass spectrum. Now, however, such suppressors are seldom employed, and it is more usual to concentrate at tention on the metastable ions because of the analytical information which can be obtained from them. According to the quasi-equilibrium theory of mass spectra, the rate con stant (k), for a particular decomposi tion may be related to the internal energy (E) of the dissociating ion, the activation energy (E0), the effective number of oscillators (s), and a fre quency factor (v). The simple original expression connecting these quantities:
(Ε -
EQY
- !
has been found to be a poor approxi mation near threshold, but the modi fied expression relating k and Ε is mathematically too complex to be con sidered here. We shall use only the simple expression to suggest that for the slow decompositions of metastable ions, the excess energy (E — E0) is likely to be small. Thus, when a mass spectrometer is used to study the decompositions of metastable ions, the instrument is behaving as a "filter," studying the decompositions only of those ions which have little energy in excess of the minimum necessary for a particular reaction path, whatever the activation energy for reaction along that path may be. A low frequency fac tor will also lead to observation of a "metastable peak" in the mass spec trum and will be associated, for ex
ample, with reactions in which rear rangement of the atoms in the transi tion state may be necessary before re action can occur. Bombarding electrons in the source can transfer widely differing amounts of energy to molecules, and many ions are formed with large excess internal energies. The aim of much mass spectrometric work is the correlation of mass spectra with molecular structure. It seems likely that these correlations may be improved and minor differences in structure detected if the amount of excess energy (E — E0) can be kept small. Thus, a great deal of attention is now being directed toward ways of increasing the sensitivity of detection of "metastable peaks" and of observ ing them free from interference by the stable ions which make up the con ventional mass spectrum. A method of doing this is to study the decompositions of metastable ions which occur in the region preceding the electric sector in a double-focusing mass spectrometer. When this is done, the ions making up the "metastable peaks" will be more sharply focused by the combined effects of the electric and magnetic fields. The width of these peaks will thus be narrower with con sequent increase in the peak heights. Indeed, peak heights two orders of magnitude greater than those due to metastable ion decompositions in front of the magnetic sector have been ob tained when the product ions m 2 + are made to pass through the electric sec tor. OBSERVING STABLE IONS
When the mass spectrometer is set to observe stable ions, the field be tween the electric sector plates is such that ions which have received the full energy of acceleration will follow a central path through the electric sec tor as shown by the full Une in Figure 3. A product ion m2+ of a metastable
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Figure 3. Paths of ions through the electric sector at normal voltage. The full line represents the path of the main beam of stable ions, the dotted lines the paths of the product ions from metastable ion decompositions
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ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
ion decomposition will follow the dotted path through the electric sector and will not be transmitted because it possesses only a fraction m2/m1 of the necessary energy. I t can be made to follow the central path either by increasing the ion accelerating voltage by a factor m1/rn,2 (keeping the electric sector voltage constant), or by reducing the electric sector voltage by a factor m2/m1 (keeping the ion-accelerating voltage constant). In either case, the main beam of stable ions is then not transmitted. The first method has the disadvantages that as the ion-accelerating voltage is changed, the optimum "tuning" conditions within the ion source are altered owing to changes in field penetration and also that the voltage cannot be changed by more than a factor of about 4, limiting the ratios of
TO1/TO2
which can be studied.
It
has the advantage that the mass scale of the instrument is unchanged during the scan so that the magnet current can be set to observe m 2 + ions, -and as the accelerating voltage is changed, a series of peaks can be observed, each corresponding to a different metastable ion which decomposes into an m 2 + ion. The second method has the disadvantage that the mass scale changes during the scan of electric sector voltage, but the advantage that all the products of metastable ion decompositions (without limitation on the ratio m1/m2) can be made to follow the central path through the electric sector. A detector placed at the exit of the electric sector will then plot a complete ion kinetic energy (IKE) spectrum of the products of metastable ion decompositions. A typical IKE
INSTRUMENTATION spectrum is shown in Figure 4 and provides a detailed "fingerprint" of the sample, which can be used for its identification. Mass analysis of the ions making up any peak in the I K E spectrum can be effected by setting the electric sector to the appropriate value, raising the detector out of the ion beam and scanning the magnet current. Whatever method is used to study the "metastable peaks" in a mass spectrum, the position, shape, and size of the peaks can be used to obtain useful structural information about the decomposing ions. For example, in the mass spectrum of aniline two of the "metastable peaks" occur at masses m* of 46.88 and 45.96, respectively. These numbers are sufficiently accurate that it is not only possible to show that the transitions concerned are: 93+ -> 66+ + 27 and 92+ -> 65+ + 27 but that on the basis of more accurate masses which take account of the nuclear packing fractions of the elements in the decomposing ion, the decompositions can be written C 6 H 7 N + · - > C 5 H 6 + · + HCN and C e H 6 N+ -> C4H3N+· + C 2 H 3 · In this way, the reaction pathways along which many of the fragment ions are formed can be deduced. Also, when impure samples are being ex-
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Figure 4 .
Partial IKE spectrum of N-carbobenzoxy-L-analyl-L-valine, methyl ester Circle No. 83 on Readers' Service Card
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970 .
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INSTRUMENTATION amined, the "metastable peaks" serve to link together various pairs of ions in the spectrum, thus showing they are due to a particular component pres ent in the sample. It is immediately apparent when mass spectra are examined that the "metastable peaks" are not so sharply focused as the peaks due to stable ions. It can be shown that this is a consequence of the release of a small amount of energy during the decompo sition of the metastable ion, which ap pears as kinetic energy of separation c-f the fragments. Both the mean amount of energy and the range of energies released can be deduced from the resultant broadening of the "metas+able peak." The amount of energy released can be used to obtain informa tion on ion structure because in a u^imolecular decomposition, release of energy must correspond to an increased stability of the products formed. Thus, in the mass spectra of all three iso meric nitrophenols, a "metastable peak" is observed corresponding to loss of neutral NO* from the molecular ions. In the case of the o- and jz-isomers, the peak is considerably broadened. Stable quinonoid structures OH
II
kj
OH
°
and
II
Ο 0
can be visualized for fragment ions formed by loss of NO* from the oand p-nitro phenols. This suggests that in the rearrangement reaction the re maining oxj'gen of the nitro group at taches itself to the same ring position as was previously occupied by the nitrogen atom, perhaps via isomerization to nitrite or by formation of a three-membered ring transition state. Correlation of the above structures with the kinetic energy released aiso lends weight to the hypothesis that charges in these positive ions are localized. INTERCHARGE DISTANCE
Information on the intercharge dis tance in doubly charged ions can also be obtained from the widths f "meta stable peaks." The first such system to be studied was benzene, the mass spectrum of which contains a very broad peak corresponding to the meta stable transition: 2
C 6 H e + -> C5H3+ + CH 3 + The release of 2.7 eV of energv in this 100 A ·
transition is mainly due to the po tential energy released when the two positive charges are separated to in finity. Assuming all the energy re leased to come from this source, the init'al charge separation can be calcu lated. If part of the energy is due to the increased stability of the fragments, the intercharge distance will be larger than the value calculated. The calcu lated minimum distance of 5.8 Â in benzene is much greater than the diameter of the benzene ring and shows t h t j the C e H e 2 + ion has an open chain structure. The method might be used to determine, for a homologous series of molecules containing two oxygen or nitrogen atoms, whether the released energy correlates with the distance apart of the heteroatoms. This would confirm localization of charge on these atoms. The heights of "metastable peaks" have also been used to study the slow fragmentation of ions even when much higher probability fast fragmentations are taking place. For example, the mass spectrum of benzoic acid labeled with deuterium on the carboxylic acid group shows a large peak due to loss of neutral OD* and also a very small peak due to loss of OH·. Study of the "metastable peaks" shows that the peak corresponding to loss of OD* in the slow decomposition of metastable molecular ions is only half as large as the peak corresponding to loss of OH*. This suggests that if the positive charge localizes on the carbonyl oxygen, this might enable transfer of an o-hydrogen to take place and that, ultimately, loss of a neutral OH* or OD* might involve either of the o-hydrogens as well as the deuterium atom. Structures such as
deuterated compounds are then ren dered difficult because of the large, and often uncertain, corrections which have to be made for the presence of the fragment ion. The method has been illustrated by the case of toluene which shows a "metastable peak" in its mass spectrum at an apparent mass of 90 due to the transition: 92+ -» 91+ + 1 To transmit the ion of mass 91 (to give the "metastable peak" at mass 90), the electric sector is "tuned" to a voltage equal to a fraction 91/92 of its normal value. If the "tuning" is successively altered to values of 90/91, 89/90, and so forth, other metastable transitions can be plotted in which ions of mass 91, 90, and so forth lose a single hydrogen. Such a scan is shown in Figure 5 and it can be seen that there are no peaks (except the isotope peak at mass 91) greater in intensity than ~ 2 % of the peak at mass 90. If a monodeutero toluene is examined in the same way, the peak as mass 91 (corresponding to 93+ -» 92+ + 1) is again by far the largest peak. The peaks at masses 90 and 91 (correcting for naturally occurring isotopes) are proportional in the mixture spectrum to the molar amounts of toluene and monodeutero toluene. The propor tionality constants can be simply de termined by adding a known amount of toluene to the mixture and remeasuring the peak heights Of the "metastable peaks" at masses 90 and 91. This
•ο
0Γ
0ϋ
Η are therefore visualized for the molec ular ion. All the above kinds of studies can be carried out at maximum sensitivity by using the technique of I K E spec trometry. The sensitivities which can be achieved are so high that "meta stable peaks" of intensity only a few parts per million of that of the largest "metastable peak" in the spectrum can be studied. This enables metastable ior 1 to be used in analyses often with advantages over stable ions. For ex ample, the mass spectra of many or ganic molocules include both a moleculf- ion peak and a peak due to loss of a single hydrogen atom from the molecular ion. Analyses of partially
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
Figure 5. Focused "metastable peaks" in the spectrum of toluene. All the peaks arise from processes in which a single hydrogen atom has been lost
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Figure 6. The IKE spectrum of benzene at electric sector voltages greater than the voltage Ε at which the main beam of stable ions is transmitted
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method is quick and accurate and is thought to be of wide application in the analysis of deuterated compounds. OTHER IONIC REACTIONS
All of the ions discussed so far have been truly metastable and undergo fragmentation unimolecularly. Many other ionic reactions can be made to occur by causing the ion beam to inter act with a gas in the field-free region in front of the electric sector. Such a "collision gas" can cause charge ex change between the ion beam and neu tral gas molecules and can also lead to fragmentation of the ions in new ways. The study of all these product ions can be carried out using exactly the same methods as for metastable ions. Consider Figure 6 which shows part of the I K E spectrum of benzene in the presence of nitrogen as a collision gas. All the peaks shown in this partial spectrum are transmitted at electric sector voltages above the voltage Ε at which the main beam of stable ions is transmitted. Peak A, for example, is due to all the species of doubly charged ions in the ion beam which, because of their two charges, have received twice the energy of acceleration of the
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singly charged ions. The ions have then lost a single charge by capturing an electron from the collision gas and consequently are transmitted through the electric sector at a voltage 2 E. The mass spectrum of this transmitted peak gives another "fingerprint" for benzene, equally as unique as the normal mass spectrum. Other "finger prints" could be obtained by mass analysis of the other peaks such as Β and C. Peak D is due to the frag ment ion C B H 3 + from C e H 6 2 + which has been discussed above. The measurement of kinetic energy release can be made much more easily in the IKE spectrum where there is no inter ference from stable ions. Thus, the study of metastable ions has become of increasing interest and promises to lead to an improved under standing of fragmentation pathways, the energetics of ion decomposition and the structures and stabilities of positive ions. I t has important applications in analysis, especially of isotopically en riched materials, and the wealth of de tail in the I K E spectra and the various other "fingerprints" may lead to an improved ability to distinguish between structurally similar molecules on the basis of their spectra.
