The quadrupole ion store (QUISTOR). Part VI. Studies on phase-synchronised ion ejection: The effects of ejection pulse amplitude

International Journal of Mass Spectromehy @ Ekvier Scientific Publishing Company,

THE QUADRUPOLE ION PHASE-SYNCXRON-ISED

and Ion Physics,

Amsterdam

31 (1979)

-Printed

15-29

in The NetherIands

15

STORE (QUISTOR), PART VI. STUIXES ON ION EJECTION: THE EFFECTS OF EJECTION

PULSE AMPLITUDE

R.M. WALDREN Chemical

and J.F.J. TODD

Laboratory,

University

(Received 11 September

*

of Kent,

Canterbury,

Kent

CT.2 7NH

(Ct.

Britain)

1978)

ABSTRACT Three types of experiment have been performed, one in which the ion signal was continuously monitored as a function of the phase angIe of the r.f_ drive potential at which the ion ejection pulse was applied, another in which the ion intensity was determined as a function of ejection pulse amplitude at a given phase angle and a third in which ion arrival time profiles (for a fixed phase angle) were recorded for different ejection puke amplitudes. From the last-mentioned series of measurements, observed ion velocities at the moment of extraction have been found to agree well with values calculated using the methods of phasespace dynamics.

In previous papers in this series we have reported investigations inSo the effects of the synchronisation of the ejection of ions from the quadrupole ion store (QUISTOR) with the phase angle of the r-f, drive potential coupled to the ring electrode. These studies included experiments in which the QUISTOR was operated as a mass spectrometer [ 11, or as the ion source of a quadrupole rn,%s filter [2,3], and in particular involved the determination of ion arrival tknes for different operating points within the (CL,4) stability diagram f4] and for different widths of the ion ejection pulse [Z]. The results reported in this present communication, namely an examination of the effect of varying the ejection pulse amplitude upon the intensity of the detected ion signal, form a natural extension of the investigation into the inaportance of phase-synchronised ejection. They also contribute to the basis of an attempt to develop a theoretical model intended to simtite the ejection process and which is currently nearing completion in this laboratory. -EXPERIMENTAL

As with the previous investigation into the effects of ejection pulse width [Z], two types of experiment were performed. In the first, various operating * Author to whom correspondence should be addressed.

16

conditions of the QUESTOR were selected and the ion signal recorded aa the phase of ion ejection was scanned over four complete cycles of ~the drive potential. Four specific phases of ejection were then chosen and the ion signal (taken as the average height from three separate recordings of the 4oAr* peak obtained by Scanning the mass filter) plotted as a function of the amplitude of the ion ejection pulse. In the second series of experiments the “ion arrival time profile” for a fixed phase angle of ejection was recorded (by scanning the delay of a trigger pulse applied to open a linear gate amplifier so as to vary the interval between ejection and detection) for different ejection pulse amplitudes. A description of the system employed in this work has previously been given in detail [2]. Argon at a pressure of ca. 10W5 torr was used exclusively as the sample. The QUISTOR was mounted so that the Ar’ ions could be mass-analysed either externally by ejection into a EAI Quad 250A mass filter, or internally by application of the appropriate r-f. and d-c. voltages to the QT.ISTOR before ejection directly into an electron multiplier. RESULTS

XND DISCUSSION

Pulse amplitude variation Ion intensity-ejection phase scans The aim of this investigation was to make a qualitative assessment of how the ion signal varied as a function of the phase angle of the r.f. potential fed to the ring electrode when ejection pulses of various amplitudes were applied to one of the end-cap elcL_? des. These pulses were either negative-going so as to cause ion withdrawal by “suck-out” (SO) or positive-going so as to effect ejection by “puJse-out”. Comparisons between these two modes of operation may be seen from the data presented below, which in a limited number of cases includes runs at two ejection pulse widths. The operating conditions are summarised in Table 1 and the phase angle, $, accords with the definition given previously [ 5]_ Fig. 1 contains three traces obtained for Ar’ ions stored in the “total pressure mode” (a, = 0) at different values of qr and then ejected by “suck-out” TDLE Operating

1 parameters

Parameter

Unit

Value

Drive frequency (S2/2a) Ion creation pulse width Ion ejection pulse width Storage time Repetition rate

MHZ

0.813 20 0.6 or 1.0 1.1 800

z ms Hz

17

line

,

Fig. 1. Scans of the (suck-out) ejected AI-+ ion signal versus phase angle (T/I,) of the r-f. drive potential applied to the ring electrode for stated pulse amplitudes (width 0.6 w) with a, = 0 and vaIues of qz equal to (a) 0.40, (b) 0.64, (c) 0.91.

(pulse width 0.6 fis) into the.mass filter; for comparison the scans obtained under equivalent conditions hut using “pulse-out”’ ejection ace given in Fig. 2. Evidently, with suck-out at qr = 0.4 and 0.64 (Fig. la, b) the traces are

18

(0)

I

90 v

I +i ?OY \ 7pY 2

~ /

\ A\ tine

1 ~ A

_/

i

tase linf

Fig. 2. Scans of the (pulse-out) ejected Ar+ ion signal versus phase angle (&) of the r-f. drive potential applied to the ring electrode for stated pulse amplitudes (width 0.6 &s) with a, = 0 and values of qt equal to (a) 0.40, (h) 0.64, (c) 0.91.

relatively simple and increasing the pulse amplitude merely has the elect of enhancing signal intensity. By contrast, at qz = O-91 (Fig. lc) the traces are much more complex and exhibit subsidiary maxima and minima which change in relative intensity as the pulse amplitude is va&d. This increased

19

complexity close to the stability boundary accords with our previous observations in which pulse width was the variable parameter 121. Ejection by “pulseout” gives noticeably different families of curves. Thus, at CJ== 0.4 and 0.64, Figs. 2(a) and (b) reveal a more complex variation than that in Figs. l(a) and (b), with a shift in phase angle of ca. ‘ITand 7r/2 respectively in the relative positions of the maxim a and minima. On the other hand, at CJ== 0.91, Fig. 2(c), to be compared with Fig. l(c), has marked simplicity. The picture changes with increasing pulse-width (Fig. 3) in that at the low value of qz (Figs. 2a, 3a) the general character of the traces is maintained on going from 0.6 to 1.0 ~.ls, whilst at high values of qz (Figs. 2b, c and 3b, c) increasing the pulse amplitude causes the maximum to break up into two “wings” withatroughat$=O. The effect of changing the amplitude for suck-out pulses of two different widths with the QUISTOR operating in the mass-selective mode (a, = -0.60, qz = 1.26) may be seen in Fig. 4. Evidently, at 0.6 E.CS (Fig. 4a) there is a complete change in the character of the phase-dependence which occurs with pulse amplitudes in the region 40-70 V, and as the pulse width is increased to 1 PS the point of the change moves to the interval between 20 and 30 V. This illustrates how critical is the correct choice of the instrumental parameters when the QUISTOR is being operated as a mass spectrometer. Ion intensity--ejection

pulse amplitude

plots

In order to quantify the types of effect noted above, the ion signal intensity was plotted against the ejection pulse amplitude for four different phase angles of ejection and four sets of operating conditions of the system. The parameters common to all the experiments are listed in Table 2. Plots of the Ar’ signal intensity versus amplitude for storage at a, = 0, qr = 9.45 are shown for suckout and pulse-out ejection in Figs. 5(a) and 5(b) respectively and for suck-out at a, = 0, qz = 0.80 in Fig. 6; the corresponding data for suck-out ejection in the mass-selective mode are given in Fig. 7. A number of striking features emerge. In each case the minimum pulse height necessary for any ions to be detected is fairly substantial compared with the depths of the pseudo-potential wells, Dz, (Table 3) calculated according to the Dehmelt model [ 6,7]. This is particularly marked when comparing ejection by suck-out and by pulse-out at the same value of qr (Figs. 5a and b). The relative differences between the shapes of the curves for the four phase angles selected become quite pronounced as one moves through the series from Fig. 5(a) to Fig. 7 * and it is clear that, especially at higher values of some phases of ejection lead to particularly poor signal levels. The 4 L’yood7’ phase angle 9 = ‘ITis the same as that found to gl_-e the optimum performance (maximum sensitivity, least mass discrimination) of the QUISTOR

* The other.

intensity

axes

of

Figs.

5-7

are

scaled

so as to be directly

comparable

with

each

Fig. 3. Scans of the (pulse-out) ejected Ar+ ion signal versus phase angle drive potential applied to the ring electrode for stated puke amplitudes with oz = 0 and values of qz equal to (a) 0.40, (b) 0.64, (c) 0.91.

(&) of the r-f. {width 1.0 ,%)

both as a mass spectrometk [ 11 and as an ion source 133. This corresponds to the application of the ejection- pulse at the phase angle at which the drive potential applied to the ring electrode is passing from a positive value through zero to a negative value. It has already been demonstrate& qualitatively in terms of phase-space ellipses [S] that at this phase angle the ions

21

Fig. 4. Scaus of the (suckxmt)

ejected

Ar’ ion signal versus phase angle (J/,)

or the r-f.

drive potential applied to the ring electrode for the stated pulse amplitudes and c, = -0.6, qz = 1.26; pulse widtk (a) 0.60 ~.rs, (b) 1.0 w_ In these runs the ions were ejected directly onto the first. dynode of an electron multiplier.

have displacements and velocities in both the T- and z-directions favourable for efficient ejection along the z-axis, and that, conversely, conditions are unfavourable at phase angles 9 = 0 and 7r/2. One aspect of the data in Figs. 5-7 worthy of note is that there is little sign of the curves levelling off with pulse amplitude, even for quite large pulses. In fact, with the phase angle JI = x and 3n/2 in the mass-spectrometric niode (Fig. 7) there appear to be distinct regions of ejection. With pulse amplitudes of ca. 90 V there is “saturation” for the most easily removed ions, and as the strength of the ejection field is increased further, other ions, more “difficult” to eject, are drawn from the trap into the multiplier. It TABLE

2

Operating parameters Parameter

Unit

Drive frequency (aL/2m) Ion creation pulse width

MHZ us

Ion ejection pulse Storage time Repetition rate

crs rns HZ

width

Value 0.82 50 0.4 1.0 800

40 ttc, 3:

(

(a)

5.

30 0. 21 5 20

I5

0,

10 55

75

sue k-out EjccIion pulse . PmpIiludc 100 Ivolts)

L

Fig. 5, Plots of Ar* ion signal intensity versus chronised at the stated phase angles and a, = 0, 0.40 m, (b) pulse-out pulse width = 0.40 m.

Fig. 6. Plots of Ar* ion signal intensity chronised at the stated phase angles and O-40 /.I%

versus a, = 0,

ejection

pulse

height

for

qz = 0.45 with (a) suck-out

ejection

pulse

q2 = 0.80 with

ejection synpulse width =

height for ejection syna suck-out pulse width =

23

Fig. 7. Plots of Ar+ ion signal intensity versus ejection pulse height for ejection synchronised at the stated phase angles and a, = -0.62, qz = 1.28 with a suck-out pulse width = 0.40 w_

should be borne in mind, however, that the pulse widths used in these phasesynchronised experiments are extremely short (less than one r-f. period). One might expect that the efficiency of ejection and detection is more likely to be a function af the area of the ejection pulse and for practical applications of the QUISTCR [9,10] pulse widths of ca. 20 11shave been employed. It is perhaps worth noting that we are concerned here only with the detected ion signal and therefore have no direct knowledge at this stage of whether application of the ejection pulse at a “pc3r” phase successfully empties the ions from the trap, albeit with relatively few of them travelling along the z-axis towards the mass filter/muItipl.ier. Complete removal of the ions during the ejection process is of course an essential prerequisite for the acquisition of unambiguous rate constant data when using the QUISTOR for ion-molecule reaction studies [ 11-133. TABLE

3

Depths of pseudo-potential

0.45 0.64 0.8 0.91

6.99 14.16 22.12 28.62

wells for Ar+ ions (conditions

as listed in Table 2)

24

Ion arrival time profiles

Following the observations of ion arrival time profiles reported earlier [ 23, four of the ejection phase angles which gave the narrowest velocity distributions were selected and reexamined for a number of different amplitudes of “suckout” pulse height under the conditions noted in Table 4. The relevant traces are reproduced in Fig. 8. As might be expected, for each phase angle the profiles shift to longer arrival times as the amplitude of the ejection pulse is reduced. At the same time the total ion signal, as represented by the area under each profile (Fig. 9), decreases with pulse amplitude in a manner analogous to that seen in Figs. 5-7. In the earlier investigation [2] we derived the ion kinetic energies corresponding to the shortest arrival times (leading edges of the traces) and the most probable arrival times (maximum ion intensity) and plotted these as a function of the experimental variable. In the present instance it is more instructive to consider how the velocity might be related to the amplitude, W, of the ejection pulse. Thus, if we assume that the final velocity i’ is related to the initial velocity 2 and that the acceleration f is acting linearly over a distance z we have from Newtonian mechanics z

“2

=

i2

Since

+

2fz

(1)

f should be directly related to W we would expect to observe a rela-

tionship of the type 2“Z=i2+kW

(2) where k is a constant. Extrapolation of a plot of 2’2 versus W to W = 0, should therefore yield the square of the initial velocity of the ions in the trap immediately prior to ejection. Four such plots (with Z’ calculated from the arrival time, and assuming a (mean) distance between the centre of the QUISTOR and the detector equal to 15.0 cm) are shown in Fig. 10, and the relevant intercepts derived by a least-squares treatment listed in Table 5. AIso given are the observed initial velocities (iobs ) and the corresponding values (&,,) calculated by the methods of phase space dynamics 18,141 together with ob-

TABLE

4

Operating parameters Parameter

Unit

Value

Drive frequency (aL/2n) Ion creation pulse width Ion ejection pulse width Storage time Repetition rate

MHz

0.81 20 0.75 1.1 800

K K

served initial kinetic energie:,. For the fastest ions ZCalCis equated with the maximum velocity in the z.direction whilst for the “most probable” ions I=is taken to be the average velocity (2,“) at that phase. We see that there is extremely good agreement between the observed and

(a)

r~cclion

pulse

ARRIVAL

h

,cJrctron

TIME

CpSl

p”Ife

Fig. 8. -Arrival time profiles for Ar+ ions ejected at phase angles 9, equal %o (a) 37rJ4, (b) m, (c) 5~14, (d) 37if2 by suck-out pulses (width 0.75 ~_ts) of stated amplitudes after storage at a, = 0, qr = 0.64. The leading edge of the ejection pulse (of the’shape illustrated) is taken as zero time.

27

1.2

D.1

10

20 30 Suck-Out

Fig. 9. Plots amplitude.

of

LO ejection

areas

so

under

60 pulse

70 Omplitude

arrival

00

time

90 (VOltS)

profiles

shown

in Fig.

8 versus

suck-out

pulse

calculated initial velocities, which provides confirmation of the general validity of this simple model and of our application of phase space dynamics to the QUISTOR- The fact that in each case the observed velocity exceeds the calculated value by a small margin may be because the model neglects TABLE

5

Intercepts

from

Fig.

10

Figure

Phase

angle

Ion sample

(rL) 10(a) 10(b) 10(c) 10(d)

?r n .3x/4 5s/4

* Corresponding

Fastest Most probable Fastest Fastest to the maxima

l

of the arrival

Intercept

_ =obs

( 108m2s2)

(lo%ls-‘)

1.76 1.02 1.44 1.13

1.33 1.01 1.20 1.06

time profiles.

_ =cAc

Obs. (ev)

1.11 0.47 0.96 0.84

36.4 21.1 29.9 26.7

KE

Fig.

10. Plots of (velocity)2 versus suck-out pulse amplitude using arrival times taken from $, = z, extrapolation of leading edges (fastest arrival times); (b) $I,. = X, most probable arrival times (maximum signal intensity); (c) & = 3n/4, leading edges; (d) $, = 57rr/4,leading edges.

Fig. 8: (a)

any contribution to the acceleration from the r-f. drive potential during the actual ejection process. Figures 8 and 10 therefore contain the first direct experimental observations on the energetics of ions within the QUISTOR, a subject which has hitherto only been accessible indirectly through the determination of ion-molecule reaction rate constants [ 121. From Tables 3 and 5 we see that each of the kinetic energy values derived by this method (for the phase angles considered) is significantly in excess of the value of the well depth, oz. The latter should, of course, equal the maximum kinetic energy in the z-direction but is derived from a model which essentially ignores the microscopic motion induced by the r-f. drive potential and considers only the macroscopic “secular” motion [ 7 3. ACKNOWLEDGEMENTS

We are grateful to Mr. R.E. Mather for helpful discussions. One of us (R.M.W.) thanks the Science Research Council for the award of a studentship.

29 REFERENCES Iori Phys., 1 J.F.J. Todd and R_M. WaIdren, Part II of this series, Int. J. Mass Spectrom. 29 (1979) 301. Ion Phys., 2 R_M. Waldren and J.F.J. Todd, Part III of this series, Irk. J. Mass Spectrom. 29 (1979) 315. Ion Phys., 3 R&l. WaJdren and J.F.J. Todd, Part IV of this series, Int. J. Mass Spectrom. 29 (1979) 337. of the theory of r-f. quadrupole devices see P-H. Dawson (Ed.), 4 For a general account Quadrupole Mass Spectrometry and Its Applications, Elsevier, Amsterdam, 1976. 5 Ref. 1, Fig. 4. Part V of this series, Int. J. Mass Spectrom. Ion Phys., 6 R.E. Mather and J.F.J. Todd, 31 (1979) 1. Todd, G. Lawson and R.F. Banner. in PH. Dawson (Ed.), Qua&pole Mass 7 J.F.J. Spectrometry and Its Applications, Ch. VIII, Elsetier, Amsterdam, 1976, p. 181. and J.F.J. Todd, in D. Price and J.F.J. Todd (Eds.), Dynamic Mass 8 R_M. Waldren Spectrometry, Vol. 5, Heyden, London 1978, p. 14. Dynamic 9 G. Lawson, J.F.J. Todd and R.F. Bonner, in D. Price and J.F.J. Todd (Eds.), Mass Spectrometry, Vol. 4, Heyden, London, 1975, p_ 39. R.M. WaIdren and J.F.J. Todd, in D. Price and J.F.J. Todd (Eds.), Dy10 R.E. Mather, namic Mass Spectrometry, Vol. 5, Heyden, London, 1978, p_ 71. G. Lawson, J.F.J. Todd and R.E. March, in A.R. West (Ed.), Advances 11 R.F. Bonner, in Mass Spectrometry, Vol. 6, Applied Science, London, 1974, p. 377. 12 G. Lawson, R.F. Bonner, R.E. Mather, J.F.J. Todd and R.E. March, J. Chem. SOC., Faraday Trans. 1.72 (1976) 545. and R.E. March, Int. J. Mass Spectrom. Ion Phys., 27 (1978) 155. 13 J.E. Fulford 14 J.F.J. Todd, R.M. Waldren and R.F. Bonner, to he published.