Optimization of ion trapping characteristics for studies of ion photodissociation' RICHARD JAMES HUGHES, RAYMOND EVANS MARCH,' A N D ALEXANDER BALDWIN YOUNG'

Dep.part/ne/lt of Cher~~istry, Tre~zt U/live/.si/y. Peterboro~lgh. 012 t . , C C I I ~ C ~ ~ C ~ K9J 7B8

Received June 30, 1982

RICHARD JAMES HUGHES, RAYMOND EVANS MARCH, and ALEXANDER BALDW~N YOUNG. Can. J. Chem. 61, 824 (1983). In ordcr to characterize unan~biguously photochemical energy absorption and subsequent intramolecular relaxation

dissociation, collision-free conditions are required. Isolated molecule photochemistry may be studied in a quadrupole ion store (QUISTOR) under near collision-free conditions. Photodissociation of primary ions, H2' and CHI+, has been described and we have reported on the slow multiphoton induced dissociation of protonated dimers of 2-propanol in the infrared region. Ions may be trapped in a QUISTOR in either a total storage mode wherein ions of different masses are stored simultaneously, or a mass selective mode wherein single ion species are isolated. Utilization of the QUISTOR as an ion/molecule reactor in the total ion storage mode facilitates the formation of secondary ions chosen for study, their isolation, and irradiation. Deter- mination of the ranges of ion masses which may be stored in each mode is described. In the total storage mode, ion mass-to-charge ratios to m/e 455 have been observed. Selective storage to /n/e 219 is reported, though with poor resolution. The effects of variation in trapping parameters including drive frequency and physical dimensions of the device are discussed.

RICHARD JAMES HUGHES, RAYMOND EVANS MARCH et ALEXANDER BALDWIN YOUNG. Can. J. Chem. 61, 824 (1983). Des conditions de collision libre sont nkcessaires pour caractkriser sans ambiguitC I'absorption d'knergie photochimique et

la relaxation et dissociation intramolCculaire subsCquentes. On doit ktudier la photochimie de la molkcule isolCe dans une rCservc d'ions quadrupolaire (QUISTOR) dans des conditions de quasi collision libre. On a dCcrit la photodissociation des ions primaires H2+ et CH,+ et nous avons rapporte la dissociation induite par multiphoton lent des dimkres protonCs de propanol-2 dans la region infrarouge. Les ions peuvent etrc pikgCs dans un QUISTOR soit dans un mode d'enlnlagasinage total ou les ions de masse diffkrente sont emmagasinees silmultanCment, soit dans un mode de selection dc masse oh les ions simples sont isolCs. L'utilisation du QUISTOR comme un rCacteur d'ion/molCcule dans le mode d'emmagasinage total facilite la formation des ions secondaires choisis pour Ctude, leur isolation et leur irradiation. On dkcrit la dktermination des intervalles des masses d'ions qui peuvent Etre emmagasinks dans chacun des modes. On a observt, dans le mode d'emmagasinage total, des rapports masse d'ions/charge de nl/e = 455. On rapporte un emmagasinage sClectif avec un rapport rn/e = 219 mais avec une faible rCsolution. On discute des effets de la variation des paramktres de piCgeage incluant la frCquence de conduite et les dimensions physiques de I'appareillage.

Introduction In a recent review of ion photodissociation by Dunbar ( I ) ,

three phases of the short history of such studies were recog- nized. The initial studies in the 1960's were carried out in the laboratories of Dunn (2) and Dehmelt (3); the second stage began in 1970 as photodissociation became a major area of interest to ion cyclotron resonance (ICR) spectroscopy with at least six ICR laboratories active in such studies; and the third stage, beginning in 1973, saw the renewed application of mass- filter type instruments to these problems. To this historical perspective as of 1977, one may add a fourth stage of slow multiphoton induced ion dissociation studies initiated in 1978 with the work of Beauchamp and co-workers (4). Among the early studies originating from Dehmelt's laboratory were a few studies on the photodissociation of H?.' and CH,' carried out using R F quadrupole traps (5). Recently, we reported (6) on the slow multiphoton induced dissociation of the protonated dimer of 2-propanol using a quadrupole ion store (QUISTOR). While QUISTOR work may be nearly synonymous with that of ICR (7), in that positive and negative ion storage may be effected under virtually collision-free conditions, resonance ejection

[Traduit par le journal]

may be utilized to identify precursors (8), and ion photo- dissociation may be induced upon irradiation in the ultraviolet, visible (5), and infrared regions (6), there are essential differ- ences between the two techniques. The QUISTOR may be operated not only as a total ion storage device (cf. ICR), but as a mass spectrometer (9) and as a specific ion reactor (10) in which the device is operated initially in a mass selective mode with relaxation to total ion storage. Furthermore, collisional focusing of the entire ion cloud concentrated at the centre of the quadrupole field, which has been proposed earlier (1 I) and has been demonstrated by fluorescence observations (12), greatly facilitates ion photodissociation with a single pass of a laser beam through the centre of the device. As it is thus no longer necessary to use mesh electrodes so that the entire storage volume may be illuminated, solid electrodes with concomitant finer control of the quadrupole field may be used to effect mass selectivity of medium resolution, i.e. M / A M ;= 300.

The ultimate objective of the present work is to determine the optimum operating conditions for the consecutive creation and storage (in the total ion storage mode) of a primary or second- ary ion of choice, isolation of the chosen ion species (in the mass selective mode), followed by simultaneous irradiation of

'This paper was presented at the 65th Conference of the Chemical the ion and relaxation of the field (to the total ion storage mode)

Institute of Canada, May 30 - June 2, 1982, in Toronto, under the in order to effect trapping and subsequent mass analysis of the

title Mass storage range of the QUISTOR. ionic photofragments. In practice, mode switching has been 'Adjunct Professor, Department of Chemistry, Queen's University, achieved (10) but the mass ranges of ions which can be manip-

Kingston, Ont. ulated thus have not been explored in detail. W e report here 'Reeistered in the Ph.D. programme in Chemistry of Queen's the present level of optimization of QUISTOR characteristics .., . -

University. with respect to the ranges of ion mass-to-charge ratios which

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HUGHES ET AL.: I 825

may be stored in the total ion storage mode and in the mass selective mode. In an accompanying publication (28) are re- ported the results of multiphoton dissociation in the QUISTOR of gas-phase ions derived from 2-propanol and its deuterated analogues using low-intensity infrared CO? laser radiation at 944 cm-'.

Theory The conventional quadruopole mass filter employs four

stainless steel rods to which is applied a composite DC and RF potential kc$,,. Mass separation is effected according to the criterion of path stability as the ions traverse the length of the rod-electrode assembly from source to detector. With the z- axis as the direction of ion travel, mass separation is achieved by applying ++,, to rods in the s-z plane and -6, to rods in the J J - z plane. For the QUISTOR, the three electrodes (one ring and two end-caps) are in the form of a hyperboloid of one sheet combined with a hyperboloid of two sheets, respectively. Due to the rotational symmetry of the QUISTOR, any point in the inner volume defined by the electrodes can be expressed in terms of a radial coordinate r and an axial coordinate z. The inner radius of the ring electrode is denoted r,, and the distance from the centre of the trap to either end-cap electrode is denoted zo. From the Laplace condition

[I] o 2 + = 0

the relative magnitudes of ro and z,, are thence determined to be

and the entire structure is depicted schematically in Fig. 1. The form of the potential +,, is a combination of a DC

component U and a sinusoidally varying component V,, such that

[3] 4" = U + Vo cos R t

where 0 is the angular frequency (2.rrf) and V,, is the amplitude of the alternating potential developed between oppositely charged electrodes. The potential at any point inside the QUISTOR is given by [4].

The force acting upon an ion in the z-direction is given by

d'z [5] -eE, = m -

dt'

which together with [4] gives

d'z 2e [6] 7 - 7 ( U + V,) cos Rt)z = 0

dt- mro-

Expression [6] is in fact an equation of the Mathieu type for which the general form is

with the transformations

End Cop -

E n d C a p -

FIG. I . Section through the hyperbolic structure of thc QUISTOR showing the dimensions ro and z0.

where U is the DC voltage, Vo is the 0-p RF voltage, m/e is the mass-to-charge ratio, ro is the radius of the ring electrode, and R is the radial frequency of the RF voltage.

An analogous treatment of the motion in the I-direction gives

so that

and

The detailed solution of the Mathieu equation has been presented elsewhere (13). An essential feature is that the tra- jectories of the ions within the device are found to be either "stable", i.e. the value of displacement periodically passes through zero, or "unstable" in which case the displacement increases towards infinity. The stability of the trajectory de- pends upon the numerical values of the parameters a and q and of course, for the practical purpose of trapping ions in a QUISTOR, the trajectories of the ions in the 1.- and z-directions must both be stable simultaneously. This leads to a "stability envelope" for the QUISTOR in a,q space, which is illustrated in Fig. 2. For any particular ion the values of a and q must be within the enclosed area if it is to have a stable trajectory. The other lines crossing the stability envelope are so-called iso-P lines corresponding to loci of a,q coordinates such that the various wave forms of the trajectories of the ions all possess the same frequencies and general character (14), although the am- plitudes of the oscillations may be different. The P values are related to the dominant secular frequencies, or and w,:

As the oscillation frequency is mass dependent, i.e.

and in the z-direction with a; = 0

it is possible to effect resonance ejection of ions of specific mass-to-charge ratio (6).

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CAN. J . CHEM. VOL. 61. 1983

FIG. 2. Stability envelope near the origin for the QULSTOR with the operating line of constant U / V , and showing some iso-P lines

The trapping potential, +,, may be applied in various fash- ions to the QUISTOR electrodes. Bonner (15) has summarized as follows the operating modes according to the application of these potentials. Mode I: Analogous to the operation of the mass filter, the combined RF and DC voltage, $,,, is applied to the ring electrode and -$o to the end-caps. Mode 11: 4, is applied to the ring and the end-caps earthed. Mode 111: The DC component of is applied to the end-caps and the RF com- ponent to the ring.

In this laboratory, the QUISTOR is operated exclusively in Mode 11. Although Mode 1 has the advantage of doubling the values of (I and q at a given U and V and hence doubling the effective mass range, this mode necessitates interfacing the rectangular extraction pulse, which empties the trap, with the applied RF and DC potential.

Operation of the QUISTOR along the q-axis, i.e. with zero applied DC potential (a = 0), corresponds to a total ion storage mode. It is expected from [9] and Fig. 2 that for a fixed value of V, a range of values of m / e will be stable, corresponding to q. (=-29,) lying within the limits 0 to 0.91. It may be noted that, at a given value of V,, ions are stored with different efficiencies so a degree of mass selectivity may result (16).

To operate the QUISTOR as a mass spectrometer, mass selective ion storage is used in which applied potentials are

chosen so as to yield pairs of a,q values close to either the upper or lower apex of the stability diagram in Fig. 2, so that only ions of one m / e value are stable (and hence stored) at a time. As in the conventional quadrupole mass filter, therefore, a mass spectrum may be generated by varying U and V , at a constant ratio of U/Vo such that for each mass the cr,q coordin- ates move along the operating line shown in Fig. 2. Experi- mental results for a limited mass range have been reported by Dawson et al. (17), Harden and Wagner (I8), Mastoris (19), Sheretov et a[. (20), and more recently by Mather et al. (9) operating in Mode I.

The common regions in a,q space for both axial and radial stability give rise to but a single diagram, shown in Fig. 2, which is accessible experimentally. The diagram has two apices of interest, one of which lies above the q-axis and one below. For the mass filter, the stability diagram is symmetric (a, = -(I,.) about the q-axis and either apex may be used for mass selectivity. For the QUISTOR, the diagram is asymmetric (a, = -2a,) and aIthough both apices may be used for mass selectivity the Iower apex is preferred as it is more sharp and affords a higher possible resolution. However, when mode switching is required only the upper apex (q- < 0.91) may be used, wherein relaxation of the DC component determines a point on the q-axis lying within the stabiIity diagram. For the

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HUGHES ET AL.: I

-" I C R E A T I O N P U L S E

D E F O C U S P U L S E I

AM v I

D E T E C T P U L S E I

E X T R A C T P U L S E I 1 I

S T O R A G E TIME-4

I ( V A R I A B L E ) I

FIG. 5. DC pulse sequence for operation of the QUISTOR in the mass selective mode.

T H E O R E T I C A L S C A N L l N E

P R A C T I C A L S C A N L l N E

m /e 71

FIG. 3. Lower apices of experimental stability diagrams for five ions in U, V space. The apices are shifted by space-charge within the trap; a DC offset (AM) is necessary to intersect the stability diagram apex for each mass.

F~lament Suppart \ ,F~lament ___a Electron Gale

/

Exlmclor End-cop' -- Deflector

FIG. 4. A section through the QUISTOR showing the orientation with respect to the quadrupole mass filter. The QUISTOR, which is mounted on a separate flange, abuts the ion source of the mass filter. The ion source, which is not shown here, both facilitates ion trans- mission from QLllSTOR to mass filter and provides mass calibration.

lower apex, q, is always greater than 0.91 and relaxation of the DC component would locate a point on the q-axis lying beyond the stability diagram.

Mass selectivity in the QUISTOR may be visualized more

readily when the stability diagrams are plotted in terms of U and V, whereupon a stability diagram is generated for each value of tnle . In order to generate a mass spectrum, the QLlISTOR is operated at a constant DC/RF ratio (and multi- pulsed fashion) such that each successive tn/e value is stable within the trap as +, is ramped, and an apex of each diagram is intersected so as to yield good peak shapes of adequate resolution. The theoretical stability diagrams are derived from consideration of a single ion in the field of the QUISTOR. In practice, may ions are stored simultaneously and the resulting space charge may induce distortion of the stability diagram (14, 21-25). The diagrams are shifted from their predicted locations, as shown in Fig. 3, so that the practical scan line does not go through the origin and does not have the predicted ratio of DC/RF of 0.28. The effect of space charge may be considered in terms of an additional DC voltage to the elec- trodes leading to a net change in the U coordinate in Fig. 3 and described as A M . Experimentally, AM is available as a vari- able parameter as is the ratio of DC/RF. It should be noted that a negative voltage for A M must be applied to the ring in order to compensate for the positive space charge. A DC voltage of opposite polarity is required when A M is applied to the end-cap electrodes.

From an examination of q, [9], it is seen that the range of n ~ / e values which can be stored within the useful range of q, is inversely proportional to the physical dimensions of the QUISTOR (ro2) and to the drive frequency (a'). Each of these parameters is examined in turn.

Experimental The experimental arrangement is depicted in Fig. 4 wherein the

QUISTOR is operated in a repetitively pulsed fashion as described elsewhere (6). A typical pulse sequence for mass-selective operation is depicted in Fig. 5. The precise separation of the QLllSTOR elec- trodes was maintained with ruby spheres of diameter 9.5 mm counter- sunk into the electrodes; the spheres were obtained from the Specialty Ball Company, Rocky Hill, Connecticut, U.S.A. Electrons are pro- duced from the heated tungsten filament which s biased at -70 V. The electron gate is held at - 100 V to exclude electrons from the trap and is pulsed periodically to approximately - I0 V for 50-200 IJ-s to allow electrons into the trap which then ionize the sample molecules at a pressure of 2-4 mPa. After a predetermined storage time, positive ions are extracted through perforations in the extractor end-cap by a negative rectangular pulse of -60 V amplitude and several RF cycles duration. The ions travel between the deflector plates and then into the mass filter assembly.

In the total storage mode the deflector plates may be held at a low

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CAN. I. CHEM. VOL. 61, 1983

PFK

0 rns

FIG. 6. QUISTOR total storage spectrum of PFK at 3 mPa pressure and 0 ms of storage. RF drive 600 V 0-p at 1.7 MHz. No applied DC (equivalent to e.i. spectrum).

positive potential (2-5 V) to focus the entire ion beam into the mass analyser. In the selective storage mode unstable ions exit the trap spontaneously during the mass sorting process, which takes several hundred microseconds, and a positive pulse (80 V) is applied to one of the deflector plates to defocus these ions and prevent them from reaching the detector. As the mass filter is not actively employed in the selective storage mode, this defocus pulse may be applied to one pair of rods to ensure that unstable ions do not reach the detector (9, 26). The trapping potential +o is supplied to the ring electrode by an Extranuclear Laboratories Quadrupole Power Supply, Model 01 1- 15, which was modified as proposed by Todd and co-workers (9) to enable selection of the correct U / V o ratios to operate at either the upper or lower apex of the QUlSTOR stability envelope.

Results and discussion Gaseous ions are trapped in the QUISTOR for periods of up

to 125 ms which corresponds to the lowest pulse frequency, 8 Hz, at our disposal. Ions may be trapped in the total storage mode (a = 0) or mass selectively. Let us examine first the mass range of ions which may be stored simultaneously, i.e. in the total storage mode. In this mode,stored ions are ejected from the QUISTOR and analysed subsequently with a mass filter, thus determination of the range of m/e values stored simultaneously is dependent upon the mass range of the filter tandem to the QUISTOR.

Until recently the trapped ion of greatest m/e value which could be detected from the QUISTOR was C,Fgt, at m/e 219, which is one of the more abundant ions produced by electron impact of perfluorokerosene (PFK). This mass range deter- mination was restricted by the mass range of the quadrupole mass filter (Vacuum Generator Q7B) which was nominally 200 amu.

A new quadrupole mass filter (Vacuum Generators

QXK400) has been used to extend the detectable limit of the total ion storage mass range. In Figs. 6 and 7 are shown the mass spectra of ions derived from perfluorokerosene in the total ion storage mode, at 0 ms (corresponding to a normal electron impact spectrum) and after 30 ms of total ion storage, re- spectively. The spectrum at zero storage time can be compared with the normal mass spectrum obtained with the quadrupole mass filter as the original filament assembly of the mass filter is intact and in place, and with the calibration mass spectrum of PKF supplied by the manufacturer. Virtually all of the pri- mary ions expected from PFK, up to m / e 455, have been observed with operation of the QUISTOR as an electron impact source. The mass spectrum obtained after 30 ms of ion storage, as shown in Fig. 7, demonstrates the ion storage capability of the QUISTOR up to m/e 455, i.e. C,,F,,', at the extreme mass limit of the QXK400. The relatively high intensities of peaks corresponding to m/e > 380 is extremely encouraging and indicative that the mass range of the QUISTOR in the total ion storage mode may be considerably in excess of the present observation. In this work, the QUISTOR is driven with a mod- ified Extranuclear Laboratories 01 1-15 power supply and the mass filter with the QXK400 power supply. The mass range of the mass filter has been extended to approximately 800 amu when driven by the Extranuclear power supply but it is not possible to use the Extranuclear power supply to drive both the QUISTOR and the mass filter simultaneously.

Comparison of the mass spectra in Figs. 6 and 7 yields a great deal of information on the ion-chemistry of the PFK system at 2 mPa. As there is little information in the chemical literature on high molecular weight fluorocarbon ion/molecule chemistry, a brief commentary may be of interest here. C2F5+ (m/e 119) and C3F7+ (m/e 169) decrease rapidly in intensity

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HUGHES ET AL.: I

PFK

FIG. 7. Total storagc spectrum of PFK after 30 n ~ s with operating conditions as in Fig. 6.

HEXANES

1.4 MHz

FIG. 8. Illustration of partial mass selectivity in the QUISTOR of ions derivcd from ~nixcd hexane isomers. Here, thc ncgativc DC bias (AM) is insufficient to counteract the space-charge and the ramped DC/RF scan line fails to intersect the apices of stability diagrams for ions of low m/e ratio.

with reaction time while C,F5.' (m/e 131) and CJF7' (m/e 181) increase in intensity along with C6Fl l+ (t?z/e 281), C7F13' (tnle 331), C8FI5' (m/e 381), and C9FI7' (m/e 431). Thus there appears to be some degree of enhanced stability (or un- reactivity) for ions of the type c,,F~,,-,. Similarly, examination of the peaks for C7HII+ (m/e 2931, CRF13' (m/e 343), C9FI5+ (m/e 393), and CIOFI7+ (mle 443) indicates a slightly dimin- ished stability for ions of the type c,,F~,-,; a further diminution in stability is observed for ions of the type c,,F~,-,. In sum- mary, the ion-chemistry as elucidated from the mass spectra in Figs. 6 and 7 shows that series of ions are formed with in- creasing stability such that C,,F;,_, > c,,F~,-, > c , , F ~ , - ~ .

Mass selective operation of the QUISTOR has been demon- strated in the range m/e 12 (Ct) to m/e 100 (C7HI6') with heptane (9), and in cylindrical devices to m/e 86 (C6H1dt) with hexane (26). Both the resolution of the mass spectrum, defined as MIAM, and the intensity of each peak are extremely sensi- tive to the ratio of DC/RF and to the applied DC bias, AM. A mixture of hexane isomers was chosen for examination. The gross effects of the magnitude of AM and the DC/RF ratio are shown in Fig. 8; a mass spectrum extending from m/e 25-71

was obtained wherein the lower masses have not been selected. In this case the AM was insufficiently negative (<-0.8 V) and the DC/RF ratio too low (<0.28). The resolution at m/e 29 is - 100 and diminished to -70 at m/e 57. In Fig. 9 , AM is made more negative (-2 V) and the DC/RF ratio in excess of 0.30, only the ions of m/e < 29 are observed with good resolution though here the resolution at m/e 29 is -150 and diminishes with m/e ratio. The diminution of resolution with m/e ratio signifies that the DC/RF is higher than ideal.

For the mass spectrum shown in Fig. 10, where AM was set at -0.8 V and the DC/RF ratio at 0.290, a range of masses from m/e 13-71 was selectively stored. Peaks corresponding to C3 and C4 ions are seen to be of much higher intensity than the remainder of the spectrum. A change in the DC/RF of 0.67% (0.290 to 0.292) produced the mass spectrum shown in Fig. 11, in which normal relative peak intensities have been restored but m/e 7 1 was not observed. Thus the question arises as to whether there are unique settings of AM and DC/RF ratio at which the potentially high resolution of which the QUlSTOR ought to be capable is realizable over a range of masses. Let us examine two factors: (i) the magnitude of AM which should

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CAN. J . CHEM. VOL. 61. 1983

FIG. 9. As for Fig. 8, but here both the AM bias and the ramped DC/RF ratio are too high so that only the apices of stability diagrams for m/e < 30 are intersected.

-

1 .

HEXANES AM -0 .8V 1 14 MHz

D C / ~ ~ 0.290

FIG. 10. Illustration of QUISTOR mass selectivity of ions derived from mixed hexane isomers over the range m/e 13-7 1; 2 mPa, 1.4 Mhz, AM = -0.8 V, DC/RF = 0.290.

balance the space charge of the stored ions, and (ii) the degree values though of equal intensities and stored under similar of distortion of stability diagrams for ions of differing m/e conditions. Firstly, the variation in relative peak intensities in

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HEXANES A M -0 .8V !

FIG. 11. As for Fig. 10 with the DC/RF ratio increased to 0.292. Resolution of the mass spectrum is considerably enhanced (cf. Fig. 10) with accompanying decrease in signal intensity.

HEXANES

1.4 MHz

1 I I i I 1 I I

5 7 I

7 1 I

V O L T S

FIG. 12. Optimum mass spectum obtainable with mixed hexane isomers; 2 mPa, 1.4 MHz, AM = - l .O V , DC/RF = 0.293 , maximum RF was 2000 V p-p, scan rate = 4 s amu'.

a normal mass spectrum would indicate that compensation for experimental conditions. Thus it is concluded that ultimate space charge should vary with m / e value. Secondly, although optimization of AM and DC/RF ratio would require adjustment little experimental work (9, 26) has been carried out in this of these parameters for each ion species. While such optimi- area, there is some evidence to suggest that stability diagram zation could be achieved possibly with microprocessor control distortions for Net and Art are not identical under similar it would yield little of immediate value. Mass selective oper-

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832 CAN. J . CHEivl. VOL. 61. 1983

HEXANES

I MHz

1000 Ill V O L T S

FIG. 13. As for Fig. 12. The drive frequency has been lowered to 1.0 MHz and the maximum RF was 1000 V p-p.

HEXANES

QUAD MASS FILTER

FIG. 14. Electron impact spectrum of mixed hexane isomers obtained from the VG QXK 400 quadrupole mass filter

ation of the QUISTOR for ion photodissociation studies per se frequency of 1.4 MHz is shown in Fig. 12; it should be noted does not necessitate the generation of mass spectra. However, that mle 71 was stored selectively with an RF voltage (0-p) of the optimum mass spectrum of mixed hexane isomers at a drive slightly less than 2000 V.

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HUGHES ET AL.: I 833

In accordance with [7] and [8], the (1: and ql values are inversely proportional to R 2 , therefore when the drive fre- quency is reduced to 1.0 MHz the effective mass range should be doubled for a given applied voltage o r a given ion should be stored selectively at half the previous voltage. This dependance is shown in Fig. 13 where m/e 71 is observed, albeit faintly at slightly less than 1000 V. The relative peak intensities of the spectrum in Fig. 13 are in good agreement with the electron impact spectrum shown in Fig. 14 which was obtained with the mass filter alone. A prominent peak at m/e 18 in these spectra is due to H 2 0 i . The trace of water present served as an indi- cator of ion/molecule reactions during the mass sorting period which could have produced H,Oi at m / e 19; none was ob- served, hence the selectivity process is rapid.

Values for a, and q, are also inversely proportional to r02. All of the above storage spectra were obtained with a QUISTOR for which a value of 1 .0 X lo-' m was chosen for r-,. the ",

internal radius of the ring electrode. While some studies have been carried out with smaller QUISTORS, e.g. ro = 7 .0 x lo-' m (27) with an (mle),,,, = 107 and r.,) = 3.5 X lo-' m (12) with which B a ' ions at m/e 137 were stored, and storage capabilities of quadrupole ion traps of different sizes having cylindrical symmetry have been examined cursorily (26), no systematic examination of the mass storage range dipendance on ro has been made. A new QUISTOR has been constructed recently in this laboratory with ro = 6.0 X 10-3 m . In the total storage mode a mass spectrum of PFK has been obtained for the range m/e 69-381 under the following conditions; 2 ms stor- age, 2 mPa, 1 .8 Mhz, 600 V 0-p. These storage conditions correspond to values for q: of 0 .73 and 0.13 for m/e 69 and m/e 381, respectively. With the small QUISTOR in the mass selec- tive mode, ions derived from m/e 219 are observable though of poor resolution, furthermore the peaks show marked splitting. This work is in progress.

Conclusion W e conclude that the QUISTOR is viable as a photochemical

reactor for photodissociation studies of gaseous ions. T h e ion storage capability in the total storage mode has been shown to extend to m/e 455 and possibly beyond. Though not of direct interest to photodissociation studies, the extended mass storage capability may facilitate low pressure chemical ioniz- ation studies, pursued previously for low molecular weight compounds, to molecules of interest with higher molecular weights. The demonstrated ranges for mass selectivity 'are en- couraging for the isolation of single ion species for irradiation.

Acknowledgements The authors acknowledge with thanks the financial support

of Trent University, the Natural Sciences and Engineering Re- search Council of Canada (for Operating and Equipment Grants), Imperial Oil Company Limited, Air Resources Branch of the Ontario Ministry of the Environment, and Queen's Uni- versity for a Graduate Student Assistantship to A.B.Y. W e acknowledge also the technical assistance of G. Wynn, C. J. S . Stuart, and W . King.

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4. R. L. WOODIN. D.S. BOMSE, and J. L. BEAUCHAMP. J. Am. Chem. Soc. 100, 3248 (1978).

5. E. S. ENSBERG and K . B. JEFFERTS. Astrophys. J. 195, L89 (1975); K. B. JEFFERTS. Phys. Rev. Lett. 20, 39 (1968); 23. 1476 (1969).

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7. R. C. DUNBAR. 111 Chemical reactivity and reaction paths. Editecl by G. Klopman. Wiley-lnterscience, New York. 1974. p. 339.

8. M. A. ARMITAGE, J. E. FULFORD, DUONG-NHU, R. J. HUGHES, and R. E. MARCH. Can. J. Chem. 57. 2108 ( 1979).

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10. J . E. FULFORD and R. E. MARCH. Int. J. Mass Spectrom. Ion Phys. 26, 155 (1978).

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12. W. NEUHAUSER. M. HOHENSTRATT, P. E. TOSCHEK, and H. G. DEHMELT. Appl. Phys. 17, 123 (1978); Phys. Rev. Lett. 41,223 (1978).

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Instrum. 40, 144 (1969). 18. C. S. HARDEN and P. E. WAGNER. Edgewood Arsenal Special

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