I NST R UME NTAT I 0 N
Advisory Panel Jonathan W. Amy Glenn L. Booman Robert L. Bowman
Jack W. Frazer Howard V. Malmstadt William F. Ulrich
Metastable Ions in Mass Spectra J. H. Beynon, Department of Chemistry, Purdue University, West Lafayette, Ind. 47907
ANY OF THE IONS
M chamber
in the ionization
of a mass spectrometer are produced with so much excitation
DOUBLE-FOCUS1NG MASS SPECTROMETER
I n 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 m,+ breaks into an ion mz+ and a neutral fragment (ml - m 2 ) , the mean velocity through the tube of m2+ and of (ml - m 2 ) will be the same as that of the initial ion m,+. That is to say, the kinetic energy of the initial ion is shared between the two fragments formed in direct proportion t o their masses. When these low energy m2+ ions pass through the magnetic sector, they are deflected to a greater extent than m2+ 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 a t an apparent mass m* given by:
The appearance of these “metastable peaks” and the difficulties of measuring their positions and intensities are illustrated in Figure 2. Because the m2+ ions have less than the full energy of acceleration, eV, they can be prevented from reaching the collector if the potential of this elec-
-
Second Field -Free Region
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.
c
_
I #, I
Electric Sector
/
‘////
I
590 MM
Beta Slit
--- -.-. *.
Magnetic Sector
1500 MM
+
To Detector
From Ion Chamber
Figure 1. Schematic diagram of a double-focusing mass spectrometer of modified Nier-Johnson geometry (the diagram refers t o Model RMH-2 made by HitachiPerkin Elmer)
+-(lo23
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39+)++(126+& 39-t--* 28
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Figure 2. Partial mass spectrum of naphthalene showing weak “metastable peaks” ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
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trode is raised to a value between ma/ml eV and eV. Early mass spectrometers were, in fact, fitted with such a “metastable peak suppressor” t o 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 attention on the metastable ions because of the analytical information which can be obtained from them. ilccording to the quasi-equilibrium theory of mass spectra, the rate constant ( k ) , for a particular decomposition may be related to the internal energy ( E ) of the dissociating ion, the activation energy (E,,), the effective number of oscillators (s), and a frequency factor (”). The simple original expression connecting t,hese quantities: k
=
+)E - Ea
s-l
has been found to be a poor approximation near threshold, but the modified expression relating k and E is mathematically too complex to be considered here. We shall use only the simple expression to suggest that for the slow decompositions of metastable ions, the excess energy ( E - E,) 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 factor will also lead to observation of a “metastable peak” in the mass spectrum and will be associated, for ex-
ample, with reactions in which rearrangement of the atoms in the transition state may be necessary before reaction 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 - E,) 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 observing them free from interference by the stable ions which make up the conventional 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 consequent 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 obtained when the product ions m2+ are made to pass through the electric sector. OBSERVING STABLE IONS
When the mass spectrometer is set to observe stable ions, the field between the electric sector plates is such that ions which have received the full energy of acceleration will follow a central path through the electric sector as shown by the full line in Figure 3. A product ion ma+ 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. It can be made to follow the central path either by increasing the ion accelerating voltage by a factor m,/m2 (keeping the electric sector voltage constant), or by reducing the electric sector voltage by a factor m,/m, (keeping the ion-accelerating voltage constant). I n 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 ml/m2 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 m2+ 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 a n m2+ 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 t o follow the central path through the electric sector. A detector placed a t the exit of the electric sector will then plot a complete ion kinetic energy ( I K E ) spectrum of the products of metastable ion decompositions. A typical I K E
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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. V7hatever 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 a t masses mn of 46.58 and 45.96, respectively. These numbers are sufficiently accurate that it is not only possible t o show that the transitions concerned are: 93+ + 66+
+ 27
92+ + 65+
+ 27
and
but that on the basis of more masses which take account nuclear packing fractions of ments in the decomposing decompositions can be written CGH7N+* +CjHG+*
+ HCN
and I n 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.
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Partial IKE spectrum of N-carbobenzoxyL-analyl-L-valine,methyl ester ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
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amined, the “metastable peaks” serve to link together various pairs of ions in the spectrum, thus showing they are due to a particular component present 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 t o stable ions. It can be shown that this is a consequence of the release of a small amount of energy during the decompositiou of the metastable ion, which appears as kinetic energy of separation ci’ the fragments. Both the mean amount of energy and the range of energies released can be deduced from the resultant broadening of the “metastdde peak.” The amount of energy released can be used to obtain information on ion structure because in a L.,imolecular decomposition, release of energy must correspond to an increased stability of the products formed. Thus, in the mass spectra of a11 three isomeric nitrophenols, a “metastable peak” is observed corresponding to loss of neutral 90‘ from the molecular ions. I n the case of the o- and p-isomers, the peak is considerably broadened. Stable quinonoid structures
0
can be visualized for fragment ions formed by loss of NO* from the oand p-nitro phenols. This sugpests that in the rearrangement reaction the remaining oxygen of the nitro group attLches 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 structves with the kinetic energy released also lends weight to the hypothesis that charges in these positive ions are localized. INTERCHARGE DISTANCE
Information on the intercharge distance in doubly charged ions can also be obtained from the widths f “metastable peaks.” The first such system t o be studied was benzene, the mass spectrum of which contains a very broad peak corresponding to the metastable transition:
CGHG2+ +=
C,H,+
+ CH,+
The release of 2.7 eV of e n e r p in this 100A
transition is mainly due to the potential energy released when the two posAtive charges are separated to infinity Assuming all the energy released to come from this source, the init a1 charge separation can be calculated. If part of the energy is due to the increased stability of the fragments, the intercharge distance nil1 be larger than the value calculated. The calculated minimum distance of 5 8 A in benzene is much greater than the diameter of the benzene ring and shows th: the C6HG2+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 at,oms. 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 euample, 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 a n 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 rendered 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 a t a n apparent mass of 90 due to the transition: 92+ + 91+
+1
To transmit the ion of mass 91 (to give the “metastable peak” a t mass go), 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 a t mass 91) greater in intensity than -2% of the peak a t 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 a t 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 proportionality constants can be simply determined 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
+
> E
%
are therefore visualized for the molecular ion. 411 the above kinds of studies can be carried out a t maximum sensitivity by using the technique of I K E spectrometry. The sensitivities which can be achieved are so high that “metastable 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; to be used in analyses often with advantages over stable ions. For example, the mass spectra of many organic molecules include both a molecu k - 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
2;
J 92
90 88 91 89
85 86
83
84
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 E at which the main beam of stable ions is transmitted
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 t o interact with a gas in the field-free region in front of the electric sector. Such a “collision gas” can cause charge exchange between the ion beam and neutral gas molecules and can also lead t o 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 a t electric sector voltages above the voltage E a t which the main beam of stable ions is transmitted. Peak A, for example, is due t o 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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ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
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 a t 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 “fingerprints” could be obtained by mass analysis of the other peaks such as B and C. Peak D is due to the fragment ion C,H,+ from CsHs2+ which has been discussed above. The measurement of kinetic energy release can be made much more easily in the I K E spectrum where there is no interference from stable ions. Thus, the study of metastable ions has become of increasing interest and promises to lead to an improved understanding of fragmentation pathways, the energetics of ion decomposition and the structures and stabilities of positive ions. It has important applications in analysis, especially of isotopically enriched materials, and the wealth of detail 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.
