Richard A. Fass and Steven G. Kendall
Pornono College Clarernont, California 91711
I
I I
Appearance Potential Measurements by Mass Spectrometry A physical chemistry experiment
The uuse of mass spectrometry for the study of the properties of ions in the gas phase is often not given much attention in physical chemistry courses at the undergraduate level. The current interest in the mechanisms and kinetics of ion-molecule reactions (1) and in the technique of photo-electron spectroscopy (Ic) makes this topic an important part of the curriculum. This paper describes an experiment used in our physical chemistry laboratory course, designed to introduce junior or senior students to the mass spectrometer. The appearance potentials of the CH4+ parent ion and the CH,+ fragment ion from methane are measured, and the heats of formation of these ions are calculated. The technique could be applied to many other systems as well. I t is not the purpose of this paper to provide an introduction to the principles and practical considerations in mass spectrometry; these subjects are adequately treated elsewhere (8, 3). The book by Kiser (2) is a particularly lucid and informative introduction. Similar experiments involving the measurement of appearance potentials have been briefly described in a t least two instrumental analysis texts (4, 5). In this paper some student results are presented and some of the limitations of appearance potential data obtained by electron impact methods are briefly discussed. Part of the reason for the absence of mass spectrometry from the undergraduate physical chemistry laboratory is undoubtedly a reluctance to expose a very expensive and very delicate instrument to a group of students with very little training or instruction. We have found, however, that very satisfactory results can be obtained with very little risk after only 1 hr of on-site instruction to a small group. The instrument is put in a "ready" condition for each student (a 10min job) before he begins a run, and he is instructed to touch only the controls which are directly involved in the experiment. Most students feel that this limited contact with a major instrument is a very informative and satisfying experience. The time required to complete the laboratory work is about 5 hr, including 1 hr of instruction in a small group. Theory
,The appearance potential of an ion in a mass spectrum is defined as the minimum ionizing voltage required to obtain a detectable ion current at the collector. It is important to keep in mind that this definition implies that the appearance potential depends on instrument sensitivity and other instrumental parameters, and on the method used for extrapolation of the data to "zero" ion current. Notwithstanding this qualifica-
tion, the appearance potential can often be related in a very simple way to the enthalpy change of some reaction. For example, for the reaction A+E--B++C+2e-
(1)
AH . , = appearance potential of B+ [AP(B+)]. Here B+ is a fragment ion produced by bombardment of A molecules in the ionization chamber, and C is a neutral fragment. For a reaction in which no neutral fragment is formed D
+ e--D++
2e-
(2)
AH ....ti, = AP(D+) = the ionization potential of D [IP(D)]. These interpretations of appearance potential data require three major assumptions 1) The product ions and fragments as well rts thereactant molecules are in their ground states 2) The electron beam is monoenergetic 3) The ionization chamber is free of electrical or magnetic fields which might change the effective electron energy.
The first assumption is valid in many instances for studies a t low electron voltages but, in general, the appearance potentials obtained by electron impact methods must be considered upper limits to the "true" appearance potentials corresponding to ground state products. Studies in which excited states are involved can often lead to some interesting information about the energetics of electron impact phenomena (6). The second assumption is obviously not valid, since the electrons produced from a hot filament must exhibit a t least the Maxwell-Boltzmann thermal energy distribution characteristic of the filament temperature. The use of an internal standard such as argon for appearance potential measurements can largely eliminate errors due to this effect in some cases, and this technique is used here. More elegant methods have been developed for producing more monochromatic electron beams (8). The validity of the third assumption will vary from one instrument to another. I n many instruments the effectsof additional fields can be minimized by working at low ion repeller voltages; in this experiment we find that +2 V on the ion repellers is a convenient setting. The use of an internal standard also tends to cancel errors from this effect, hut more sophisticated techniques are required to eliminate it completely (7). With the qualifications mentioned above, heats of formation of gaseous ions can be obtained from these appearance potential measurements. For example, from eqn. (1) AHlm(B+)= AH,'(A) - AHlo(C)
+ AP(B+)
Temperature corrections can be ignored in these measurements since they would be considerably smaller Volume 48, Number 8, August 1971
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545
than the expected uncert,ainty (+0.5 eV mole-'). Similarly, from eqn. (2)
=
12 kcal
The heats of format,ion of the neut.ral species have been measured by independent methods and tabulations are available in many physical chemistry text,s and chemistry handbooks. Experimental
The indmnnent which is available at Pomona College is an Hitachi Perkin-Elmer RMU-6D single-focusing 90' sector mass spectrometer. The output is displayed on a Honeywell Model 1508 "Visicorder" oscillographic recorder. The uv-sensitive chart paper used with this recorder amounts to a significant expense for class use, but the experiment would work just as well with a conventional external potentiometric recorder. I n the latter case it mould not he feasible to record as many points because of the much slower response time of such a recorder. We found it desirable to improve the reproducibility of the chamber voltage measurements by installing a more readable external meter to monitor the ionizing voltage in the 10-20 eV range. However, the "chamb~r vo!tage" meter supplied with the instrument would be adequate, although not as accurate or convenient. Samples of methane and argon are introduced successively on a conventional metal inlet system (CEC Model 21-OslA), expanding from a measured pressure in a 3-ml calibrated volume to a 1-1 expansion flask before introduction to the analyzer through a pinhole leak. Sample pressures are adjusted so that the m/e 16 -peak from methane (CH.,+) and the m/e 40 peak from argon (Ar+) are similar in size (within about 20y0 of each other). On our instrument this requires a 2: 1 ratio of argon: methane. The various sensitivity controls (electron multiplier sensitivity, multiplier high voltage supply, slit widths, dc amplifier sensitivity) are then adjusted to provide near full-scale peak heights a t 70-eV chamber voltage for these peaks (and the m/e 15 peak due to CHa+) on the least sensitive of the four galvanometers of the oscillographic recorder. These galvanometers have nominal relative sensitivities of 1:3 :10 :30. For accurate quantitative measurements these galvanometers must he calibrated, but in this experiment we are concerned only with relative peak heights for different peaks measured on the same galvanometers, so absolute calibration is not necessary. The three peaks (m/e 15, 16, and 40) are then scanned a t 0.2 eV intervals from 16 eV to 14 eV chamber voltage, and then a t 0.1 eV intervals from 14 eV to 10 eV. 'It should he noted that these voltages are only 'approximate and actually represent the average of a distribution of electron energies. It is not surprising, therefore, that the m/e 40 peak is still detectable far below an indicated chamber voltage of 15.8 eV, the accepted value for the ionization potential of argon. The use of argon as an internal standard obviates the need for a direct measurement of the absolute energy of the electron beam. After the series of low-voltage measurements is Data obtained by Donald Guiney.
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Journol of Chemical Education
completed, the three peaks of interest are again scanned a t 70 eV. Since the contents of the 1-1 expansion volume on the inlet system are somewhat depleted during the 1 hr required to make low voltage measurements, the peak heights are not expected to agree with those obtained initially. The procedure used to analyze the data is fortunately quite insensitive to variations in the sample pressure (and thus in the 70 eV peak heights) for each ion of interest. Calculations
A typical ionization efficiency curve (peak height versus chamber voltage) is s l ~ o ~inmthe text by Iciser (7). It is apparent that a simple extrapolation of such data t o zero peak height in order to determine the ap-. pearance potential of an ion is extremely difficult. Several empirical methods have been devised for treating ionization efficiency data to yield accurate appearance 'potentials. One such method was described by Lossing, et al. (S), and is used in this experiment: A semilogarithmic plot is made of the logarithm of the peak height (8s a percentage of the 70 eV peak height) versus the nomiqal electron (chamber) voltage. Lossing and co:workers found that such plots have identical shapes for many different ions a t peak heights below about ly0of the 70 eV peak heights. This parallelism of the ionization efficiency curves' on a semi-logarithmic plot is a fortuitous circumstance, and permits the appearance potentials of many ions t,o be measured with reference to one standard. The displacements along the voltage axis of the curves for the unknowns are measured relative to the standard in the region where all of the curves are nearly parallel. It is not necessary to accurately know the absolute values of electron voltages; only diierences in nominal chamber voltages over the region from 10 to 16 eV must he known since the absolute value of the appearance potential of the standard is known. Results
A typical set of student results1 are shown in the figure. The curves are clearly parallel below peak heights of 1% of the 70 eV peak heights. This paral-
.,
,+, 10
,
14 16 CUAMBER VOLTACIE(eV)
Semilogarithrnic plot of ionization efficiency curve*.
A,
+.
,
12
A, CH4+;
m, CHs+;
Table 1.
Voltage Differences between Curves of Figure
Peak height I% of 70 eV value)
Distance from Ar + curve (eV) CHA+ CTT.+
this table are the heats of format,ion of CH3+and CHI+ derived from these appearance potentials IT-ith the corresponding literature values. Conclusion
Table 2.
Appearance Potentials and Heats of Formation Appearrtnee potential*
Ion
CH&+ CHJ+
Calcd. (eV) Lit,L 13.3 14.6
13.6 14.3
Heat of formation (kcal mole-') Calcd. Lit? 289 268
274 260
Based on a value of 15.8 eV for the ionization potential of arson. Values taken from Ref. (9). See dih~saionin text.
lelism is further illustrated by the data of Table 1, taken from the curves in the figure.. These data show that the appearance potential of CH4+ is 2.5 i 0.1 eV less than that of argon, and the appearance potential of CH,+ is 1.2 + 0.1 eV less than argon. These results are compared with literature values (9) in Table 2, where the calculations are based on a value of 15.5 eV for the appearance potential of argon. There is a great disparity of values for appearance potentials reported in the literature, and the choices made in Table 2 are somewhat arbitrary. Also included in
The experiment described above has proved to be a valuable addition to our physical chemistry laboratory. Although it requires somewhat more faculty attention than many other experiments, it is a very reliable and satisfying experiment from the student's point of view. We hope to proceed to develop more experiments for the mass spectrometer and for other major research instruments in an effort to expose our students as much as possible to modern techniques in chemistry. Litemfure Cited (1) (a) "Ion-Molecule Rssctiona in the G&sPhase:' Advances in chemist^ Series No. 58. American Chemical Society. Washington, D. C., 1966. (b) LLMPE,F. W.. FRANILIN,J. L.. AND FIELD.F. H.. "Kinetlos of the Reactions of Ions with Moleouler." in "~rogreadin Rsaotion Kinetics." Vol. 1. (Editor: Porter, G.) Penaman Presa. New York. 1961, PP. 6 6 1 0 3 . (4 Cxmn*, W. A., hsn RUSSELL, M. E.. . I Chen. .
- ,.*"., .*,".-" \-""",. W., "Introduction to Mass Spectrometry and Its AppliPA.,.
4.3
'.d.,P.
IIORP,
(2) KIBER.Robert
N. J., 1965. Instruments and Techniques," Intersaienoe Publishem, - ~ e w ~ o r k 196s. ; (4) MELOAN,C. E., AND KIFER.R. W.. "Problems and Experiments in Instrumental Anslyaie," Charles E. Merrill Books. Inc.. Columbus. cations," Prentioe-Hall. Ino., Englewood Cliffs,
(3) Ronaz. JOHN,"Introduction to Mass Soeotrometrv.
~~
~
", ,062
nhi-
(5) GULGBAULT, G. G.. AND Hmars. L. G.. "Instrumental Analysis Msnual."
Marcel Dekker, Inc., New York, 1970. (6) L I ; w I ~D.. . AND H A ~ L C W.. H.. J . Chem. Phua.. - 52..6348 (19701. . . (7) K r s ~ nap. . cif.. p. 168. (8) Lossme, F. P., TICKNER. A. W., AND BRYCE.W. A.. J . C h m . P ~ Y B 19. .. (9) "Ionization Potentials, Appearance Potentials, and Heats of Formation
of Gaseous Positive Ions," NSRDS-NBS 26, Office, Washington, D. C., 1969.
U. S. Gov't. Printing
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