Systematic factors affecting high mass-resolution and accurate mass assignment in a quadrupole ion trap

ELSEVIER

and Ion Processes

International Journal of Mass Spectrometry and Ion Processes 144 (1995) 87 103

Systematic factors affecting high mass-resolution and accurate mass assignment in a quadrupole ion trap F.A. Londry, R.E. M a r c h * Trent University, Peterborough, Ont., K9J 7B8 Canada Received 22 September 1994; accepted 5 January 1995

Abstract

The separate and combined influences of a phase-locked relationship between the drive (r.f.) and excitation (a.c.) frequencies, and the noise level on the drive voltage, on the ejection of ions from a quadrupole ion trap have been investigated at each of two scanning rates, 10 and 5000 u s -l . The half-width of an individual mass peak, which has been shown previously to vary directly with the mass scanning rate, is dependent also on the a.c.-r.f, phase relationship, and can be minimized. However, it is not practical to resolve an ion-intensity signal beyond that point where the peak halfwidth becomes significantly less than the uncertainty in its position. It has been observed that the position of mass peaks with half-widths of 1 or 2mu can vary by as much as 0.25% as the a.c.r.f, phase difference is varied over one complete cycle. Furthermore, the noise level present on the amplitude of the r.f. drive of the ion trap used in this study (primarily 60 and 120 Hz) can cause an uncertainty in peak positions of as much as 0.1%. With the a.c.-r.f, phase difference locked at a favorable value and the scan function triggered by the phase of the line voltage, it is possible to achieve mass-peak stability to within 5 mu for m/z 264 corresponding to a resolution of 50 000. The results of a simulation study of peak position as a function of the a.c.-r.f, phase difference at a mass-scanning rate of 5000 u s i are in good agreement with experimental results. Simulated trajectories were used also to determine the effect of the a.c.-r.f, phase relationship on the phase-at-ejection of the a.c. and r.f. potentials. Keywords: Axial modulation; High mass-resolution; Mass scanning rate; Phase-locking; Quadrupole ion trap

I. Introduction While the high-resolution mass-selection ejection capability of the quadrupole ion trap [1] has been demonstrated by a number of investigators [2-8], accurate mass assignment of the highly resolved ion-intensity signals remains problematic. Although many factors have been found which affect the position of peaks on the mass axis [9], multiple scans of a * Corresponding author.

single ion species with the conditions of number of ions, helium pressure, a.c. amplitude and frequency held constant can yield individual mass measurements which are dispersed over several hundred milli-a.m.u. (mu). Two criteria essential to reproducing peak positions to within a few mu have been investigated. The first of these, suggested recently by Julian et El. [10], requires that a phaselocked relationship exists between the frequency of the r.f. drive and that of the axial modulation voltage (a.c.) used for resonant

0168-1176/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0168-1176(95)04153-2

88

F.A. Londry, R.E. March~International Journal of Mass Spectrometry and Ion Processes 144 (1995) 87-103

ejection during the mass-selective instability scan. The second criterion requires that the noise level on the amplitude of the r.f. drive be reduced dramatically below that present in many contemporary commercial instruments. The effects of a phase-locked relationship between the a.c. and r.f. potentials as well as 60Hz (and higher harmonic) noise on the amplitude of the r.f. drive on selected ionintensity signals have been investigated by monitoring peak intensity, peak position and peak half-width under various conditions. Several a.c. frequencies were employed so as to span a range of values for the qz, eject trapping parameter [1]. Scan rates of 10 and 5000us -1 were selected as being the most appropriate for study, bearing in mind that resolution varies inversely with scan rate [7]: a scan rate of 10us -1 yields mass peaks with half-widths of less than 2 m u (which corresponds to a resolution of 130000 for m / z 2 6 4 ) , adequate for practical chemical applications and readily obtainable; a scan rate of 5000us -l yields ion signals which can be compared with those produced by commercial instruments. These experiments were performed using stable ions of perfluorotributylamine (PFTBA) which span the mass range of commercial ion traps, specifically, m / z 6 9 , 131,264, 414 and 614.

2. Theory of a.c.-r.f, phase locking The a.c.-r.f, phase relationship can be characterized by specifying an effective initial phase, ~b, for the r.f. drive potential, u, and none for the a.c. potential, Ul, such that u = Vsin(ZTrft - ~)

(1)

and Ul = V1 sin(ZTrflt)

(2)

where V and V1 are amplitudes, and f and fl are frequencies.

In these experiments, the phase of the r.f. was fixed, while the phase of the a.c. was controlled through software such that Eqs. (1) and (2) could be rewritten as u = Vsin(ZTcft)

(3)

and ul = V1 sin(27rflt + 01 + 0)

(4)

where 01 is a constant phase angle added to the a.c. waveform to bring the first positive-going zero-crossing of the a.c. into coincidence with that of the r.f. when 0 = 0, and 0 is an additional phase angle which was used to vary the phase difference between the a.c. and r.f. signals. Although Eqs. (3) and (4) describe most accurately the experimental situation, all a.c.r.f. phase relationships have been expressed in terms of ~b, referring to Eqs. (1) and (2). This approach is justified because the r.f. frequency is a constant of the system and, as discussed in more detail below, all possible combinations of the r.f. and a.c. phases can be spanned by varying ~ through 360 °. Throughout this text, the term a.c.-r.f, phase difference refers specifically to ~ of Eq. (1) on the assumption that the r.f. and a.c. potentials are described by Eqs. (1) and (2). In order for a practical phase-locked relationship to exist between r.f. and a.c. waveforms, the period of (u + ul) must be small relative to the time required to eject a particular mass, typically 60-120 #s. This condition is achieved when the ratio of the r.f. and a.c. frequencies can be represented by small integers. The period is shortest when the frequency of the r.f. drive is a small integer multiple (3 or 4) of the a.c. frequency. For example, if fl is chosen to be 1/3 of a 1.00 MHz r.f. drive, the full range of a.c.-r.f. phase relationships which can occur for a particular value of ~b would be repeated 20-40 times during the time required to eject a specific mass. By retarding the phase of the r.f. potential over a range between 0 and

F.A. Londry, R.E. March~International Journal of Mass Spectrometry and Ion Processes 144 (1995) 87-103

360 ° or, equivalently, advancing the phase of the a.c. signal t h r o u g h f l / f × 360 ° = 120 °, all possible combinations of the a.c. and r.f. phases can be spanned. Similarly, for an a.c. frequency of 250.0 kHz, advancing the phase of the a.c. by 90 ° is equivalent to retarding that of the r.f. by 360 °. Regardless of the values o f f and j] all possible combinations of the a.c. and r.f. phases can be spanned by varying ~b through 360 °. However, if the period of (u + Ul) is not relatively small, the particular range of a.c.-r.f. phase relationships which is spanned during the ejection of a particular mass would be only a fraction of the range of phase relationships possible for any fixed value of ~. Consequently, if the period of (u + ul) is large, the particular range of a.c.-r.f, phase relationships which was in effect during the ejection of a particular mass would depend on the starting point of the mass axis of the final segment of the scan function.

3. Experimental 3.1. Apparatus Experiments were carried out using an extensively modified Varian Saturn I ion trap which had been converted to a drive frequency of 1.00 MHz. All aspects of the scan function were controlled from a 80 486-based host computer using modified commercially available data-acquisition equipment [6]. The computer code which was used to control the system was written in a combination of C and Assembler. Potentials were applied to the end-cap electrodes in dipolar mode using a custom-designed PCB called the waveboard. Single- and multiplefrequency waveforms were generated in digital form, stored on disk in the host computer, and downloaded, subsequently, to waveboard RAM. A specific waveform was selected by specifying a particular range of waveboard

89

R A M to be clocked out by a high-speed DAC. The final amplitude of the waveform was controlled by setting a reference voltage at an amplification stage before being coupled to the end-cap electrodes through a transformer. The signal-to-noise ratio of mass spectral data can often be improved by co-adding several individual scans, obtained in rapid succession, which span the same mass range. Following the jargon of the principal North American vendors, a scan of a particular mass range is usually composed of several co-added microscans, each of which spanned the mass range of interest. In this work, the term microscan has been used to identify unambiguously a single r.f. voltage ramp spanning a specific mass range. 3.2. Phase locking The r.f. and a.c. signals were phase-locked by using a single 16MHz crystal oscillator to provide the frequency source for both the r.f. drive and the waveboard clock. Dividing the 16MHz signal appropriately resulted in a 1.00MHz r.f. drive frequency replacing the original 1.05MHz signal and the digitized waveforms stored in waveboard R A M being clocked out at 4 MHz. To ensure a consistent phase relationship between the a.c. and r.f. signals, the waveboard was prevented from generating a signal until a rising edge of the r.f. occurred. The phase difference between the a.c. and r.f. signals was established by measuring the time between the trigger pulse on the waveboard which clocks out the first word of the digitized waveform and the first positive-going zero-crossing of each (a.c. and r.f.) waveform using a digital storage oscilloscope. The r.f. drive was monitored, with the a.c. disabled, by inserting a divide-by-ten probe between the plates of the capacitor which provides feedback from the r.f. coil to detection circuitry. The phase of the a.c. was

90

F.A. Londry, R.E. March/International Journal o f Mass Spectrometry and Ion Processes 144 (1995) 87 103

measured, with the r.f. disabled, by monitoring directly the signal on the upper end-cap electrode. It was by this technique that values for 01 of Eq. (4), appropriate for each a.c. frequency, was determined. 3.3. Line locking Line locking was introduced to investigate the impact on mass spectral data of 60 Hz, and higher harmonic, noise. A technique was developed which allowed a scan function to be synchronized with the phase of the line voltage. The output from a low-voltage transformer was divided and used as input to a Schmitt trigger whose digital output switched high at 2/3 maximum and low again at 1/3 maximum. By gating each microscan with the falling edge of the Schmitt trigger output, it was ensured that identical scans would always bear the same phase relationship to the line voltage. To determine the effect of varying this phase relationship, a variable delay, specified in degrees of the line-voltage phase, was inserted following the gate which initiated each microscan. Unless indicated otherwise, all data were collected under line-locked conditions with the scan delay fixed at a value chosen to yield stable peak positions. 3.4. The scan function The scan function employed in these experiments consisted of the following five segments. (1) The r.f. amplitude was ramped quickly to the ionization storage level such that the value of the trapping parameter qz [1] for the ion species of interest was qz = 0.20. (2) Gas molecules were ionized by electron bombardment. During ionization, a notched broad-band waveform was applied in dipolar mode to the end-cap electrodes such that only those ions for which qz = 0.20 were allowed to accumulate while all others were ejected reasonantly from the ion trap. The

broad-band waveform consisted of components which ranged in frequency from 17 to 500kHz at 500Hz intervals with an 8kHz notch centred at qz = 0.20 corresponding to a frequency of 71.5 kHz. (3) Following the ionization period, with the r.f. amplitude held constant, the same broadband waveform, which was used during ionization, was applied, at increased amplitude, for 500-2000 #s in order to remove all traces of ions other than the one of interest. (4) The r.f. amplitude was ramped at the rate of about 2 x 105us -~ to bring the mass of interest close to the point of ejection. (5) The electron multiplier and the a.c. ejection signal were enabled and, following a waiting period of 500 #s to allow multiplier transients to die out, the mass of interest was ejected during a mass-selective instability scan.

3.5. Procedure Peak or ion-signal area, peak position and peak half-width were measured as functions of the a.c.-r.f, phase difference under linelocked conditions; the same parameters were measured as a function of the delay imposed at the beginning of each microscan, or scan delay (measured in degrees of the line-voltage phase), while the a.c.-r.f, phase relationship was held constant. A.c. frequencies of 200.0, 250.0, 333.3, 397.0 and 462.0kHz were used to obtain values for qz, eject of 0.53, 0.64, 0.78, 0.86 and 0.90, respectively. The first three of these frequencies were chosen because their periods are simple integer multiples of the period of the r.f. drive, a prerequisite for a useful phase-locked relationship. The a.c. frequency of 397.0kHz was chosen for comparison with previous work, before line locking and phase locking were implemented, and to investigate the consequences of phaselocked a.c.-r.f, relationships when the ratio of r.f. and a.c. frequencies cannot be represented

F.A. Londry, R.E. March~International Journal of Mass Spectrometry and Ion Processes 144 (1995) 87-103

by a small integer. Finally, fAC = 462.0kHz was chosen as this frequency yields the value of qz, eject c o m m o n to most commercial instruments. Analytic scan rates of 10 and 5 0 0 0 U S - 1 were employed to obtain ion intensity signals for some stable ions of PFTBA, specifically, m/z69, 131, 264, 414 and 614. To maintain a c o m m o n linkage throughout the discussion of experimental results we have presented only those results obtained with m/z264. These results typify the behaviour of the other ion species studied except where noted.

91

06 0s ~ 0.4 ~ 03 ~ ~ 02 0 t ~L J~AL~

0.0 "J~' ~ . A - J ~ .

263,60

(a)

263.65 263.70 263.75 !63.80 Mass Number

L ~LILAJ • * I . ~

263.85

263.90

05 0.4 "2"

4. Results and discussion

4.1. Peak dispersion

0.3

c~

0.2

H

The impact on peak-position stability of both phase locking and line locking is demonstrated by the degree of peak dispersion when the signal intensities of ten high-resolution microscans of a particular ion species are coadded. Fig. 1 shows the results of co-adding ten scans of m/z264 w i t h qzejec[ = 0 . 7 8 obtained at a scan rate of 1 0 u s - . With both phase locking and line locking disabled (Fig. l(a)) individual microscan-peaks with half-widths less than 1.5mu were dispersed over a range of about 300 mu. With line locking still disabled but phase locking enabled (Fig. l(b)) individual peaks were dispersed over 65 m u only, indicating that the degree of dispersion due to a random a.c.-r.f, phase relationship is significantly greater than the dispersion caused by 60 (and 120)Hz noise on the r.f. amplitude. When both phase locking and line locking were enabled (Fig. l(c)) the ten co-added scans yielded a single peak, broader by a factor of 2 than the individual microscan peaks. Let us now examine in detail, the separable effects of, firstly, locking the start of each

0.I

0.0 ..,,.l,k 263.60 263.65

263.70

(b)

263.75

263.80

263.85

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Mass Number

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263.60 (c)

L. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

263.65

263.70

263.75

263.80

263.85

263.90

Mass Number

Fig. 1. The result of co-adding ten microscans o f m/z 264 with qz, eject = 0.78: (a) with both phase locking and line locking disabled; (b) with phase locking enabled but line locking disabled; (c) with both phase locking and line locking enabled.

F.A. Londry, R.E. March/International Journal of Mass Spectrometry and Ion Processes 144 (1995) 87-103

92

microscan to a fixed point in the line-voltage phase and, secondly, locking the a.c.-r.f. phase relationship.

262.g5 262.90

.^

~

~

,,

i

~

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."~262.85

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~262.80 4-'

~_262.75

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if~i

:~"

•"

i

~

i

600

800

~262.70

i

if~i

ii:

262.65

262.60

200

400

(a)

Scar~-delay

1000

1 2 0 0 1400

1600

(degrees)

263.68 zx

~

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?

3"

.~263.60

if& i::~ii::ii~iii~ii':i ~iii%ii~ i ii~ iiii~?:i

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263.52

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400

600

800

1000

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(b) 263.660 2631659

v~263.658

~ 263.657 g ~ 263,656 ii i ! i~ 263.655 263,654 (c)

20

40

60

80

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Real Time (s)

Fig. 2. Peak position of m/z264 with qz, e j e c t = 0.64 plotted as a function of (a) scan delay expressed in degrees of the line-voltage phase using a scan rate of 5000 u s 1, (b) scan delay expressed in degrees of the line-voltage phase using a scan rate of 1 0 u s - l , and (c) real time with the scan delay held c o n s t a n t using a scan rate of 1 0 u s -~ .

To demonstrate more clearly the extent to which peak position is affected by the presence of line-voltage noise in the r.f. circuit, a variable scan-delay was inserted following the gate which initiated each microscan at a particular phase of the line voltage. With the a.c.-r.f. phase difference fixed at a value about which peak positions remained reasonably stable, the scan delay was varied over four complete cycles of the line voltage. The results obtained for scan rates of 5000 and 10 u s -1 are shown in Figs. 2(a) and 2(b), respectively, where the scan delay has been expressed in degrees of the 60Hz line-voltage phase. Note that there is a shift in apparent mass of about + l u between Figs. 2(a) and 2(b). In general, with all other parameters held constant, the amplitude of the a.c. signal used for resonant ejection must be reduced by about an order of magnitude when the scan rate is reduced from 5000 to 10 u s -1 in order to obtain acceptable mass spectra. This combination of reduced scan rate and a.c. amplitude results in a shift of ion-intensity signals to higher apparent mass unless the calibration of the instrument is adjusted to compensate. At each a.c. frequency studied, the calibration of the instrument was held constant while other parameters were changed so that apparent masses measured under different conditions could be compared. Although it is more obvious from Fig. 2(a) than Fig. 2(b), both show a periodicity of approximately 120 Hz. The 120 Hz periodicity is manifest in Fig. 2(b) by virtue of the fact that datum points are quantized to fall approximately on lines of negative slope which intersect the abscissa at intervals of 180 ° . This periodicity is seen more clearly when the

F.A. Londry, R.E. March~International Journal of Mass Spectrometry and Ion Processes 144 (1995) 87 103

figure is viewed from the lower right-hand corner. Also, the total variation in peak position observed in Fig. 2(b) is about 180mu. However, only about 2/3 of this variation appears to be scan-delay sensitive, that is, about 1/3 of the variation is a much more slowly (than 120Hz) changing function of time. Consequently, when one considers that the ten ion-intensity signals shown in Fig. 1(b) were collected at pseudorandom phases of line voltage over a time period corresponding to about 1/15 the collection time of Fig. 2(b), the degree of peak dispersion represented by those two figures is consistent. The total time required to collect the ~145 data points in Fig. 2(b) was about 100 s. With the system both phase locked and line locked at values chosen to minimize the variation in peak position, peak position was recorded over the same time period as that required to accumulate the data of Fig. 2(b). The observed variation in peak position, over the data collection period of 100s, for m/z 131, 264 and 414 at each of a.c. frequencies of 200.0, 250.0 and 333.3 kHz was only a few mu. Typical of these results are the data obtained for m/z 264 with fAC = 333.3 kHz shown in Fig. 2(c). In the absence of other extraneous sources of noise, line locking and phase locking would be adequate to reproduce the positions of mass peaks with half-widths of ,,~ 5 mu, corresponding to a resolution greater than 50000 for m/z264. The quantization apparent in these results is due to the limited resolving power of 4-byte reals. At the faster scan rate of 5000us -~, about 84u are scanned during one period of the 60Hz line voltage; that is, the total change in the line-voltage phase during the time required to eject a particular m/z with a halfwidth of 400 mu is about 2 °. Therefore, at this higher scan rate, one would expect that peak position would vary with scan delay in the same manner that the 60Hz noise (and its higher harmonics) varies with time. In other

93

words, the data of Fig. 2(a) provide a fairly accurate representation of the 60Hz (and higher harmonic) noise which was present on the amplitude of the r.f. drive voltage. In this case, the dominant component of the noise appears to be a 120Hz signal characteristic of full-wave rectification. Similar data were collected for other mass peaks in the P F T B A spectrum using a.c. frequencies of 200.0, 250.0 and 333.3kHz. These data showed that the degree of variation in peak position with line-voltage phase increased with both the m/z ratio and qz, eject" These observations are consistent with 120Hz noise present on the amplitude of the r.f. voltage at a level of about 0.1% (peak to peak). Because the noise present on the r.f. amplitude can have a significant effect on the apparent mass of ions, it is worthwhile examining, in some detail, how mass spectra obtained at different scan rates are affected by this phenomenon. During the final segment of the scan function used to collect mass spectra, the amplitude of the r.f. voltage on the ring electrode is ramped linearly in time in order to bring ions of successively higher mass into reasonance with an a.c. ejection signal which is applied to the end-cap electrodes. To a good approximation, the relationship between the amplitude of the r.f. voltage and the mass/ charge ratio at the ejection point in stability space is linear. Therefore, the r.f. voltage ramp can be expressed conveniently in mass units per second (us-l). If the electronics of an instrument were noise free then the amplitude of the r.f. drive voltage during the final segment of a scan function could be represented, accurately, by a straight line, the slope of which determines the scan rate. Because electronic systems are less than ideal, a certain amount of noise is superimposed on the linear r.f. voltage ramp and causes a degree of peak dispersion. At the higher scan rate, when the slope of the linear voltage ramp is significantly greater than the maximum slope

94

F.A. Londry, R.E, March~International Journal of Mass Spectrometry and Ion Processes 144 (1995) 87-103

2h : t,

'i :'Ei ~ !1

i

i

i

Fig. 3. The addition of a linear ramp corresponding to 10 u s -1 to the data of Fig, 2(a) and expressing the abscissa in milliseconds. This representation shows an approximation to the amplitude of the r.f. drive voltage during the analytic segment of the scan function which was used to obtain the data of Fig. 2(b). The inset corresponds to the shaded area with the aspect ratio changed to expand the abscissa.

of the noise component, the degree of peak dispersion will depend directly on the amplitude of the superimposed noise. In contrast to the fast scan rate (5000us -1) at which ~ 84u are scanned during one period of a 60Hz signal, only ~ 0 . 1 7 u are scanned over the same period at the slow scan rate of 10us -1. Consequently, at the lower scan rate, the rise time of the positive-slope portion of the 120 Hz noise is considerably faster than that of the linear ramp. Under these conditions, the degree of peak dispersion depends more on the frequency of the noise component than on its amplitude. As discussed above, the temporal variation of the data of Fig. 2(a) provides a good approximation to the 120Hz noise which is present on the amplitude of the r.f. drive voltage independent of the scan rate. Therefore, if the abscissa is converted to time, rather than degrees of the 60 Hz line-voltage, these data can be superimposed upon a linear 10 u s -1 ramp to obtain a representation of the time variation in the amplitude of the r.f. drive voltage, Fig. 3, during the final segment of the scan function which was used to obtain the

data of Figs. l(a), l(b), l(c), 2(b) and 2(c). The inset in Fig. 3 corresponds to the small shaded area with the aspect ratio changed for greater clarity. The horizontal dis particular mass/charge ratio is ejected is fixed, it is clear from Fig. 3 that, at a scan rate of 10 u s-l, ions would have been ejected most often in response to a change in the r.f. level due to the superimposed noise rather than by the r.f. voltage ramp itself. Also, it can be seen from Fig. 3 that, at scan rates at which the slope of the linear ramp is much less than the rise time of superimposed noise, the degree of peak dispersion will be determined by the slope of the ramp and the frequency, rather than the amplitude, of the dominant noise component. Also, it can be seen from Fig. 3 that a relatively large shift in apparent peak position would occur in response to a small change in the scan delay (phase of the noise) when that change caused the r.f. ejection level to be reached by a noise peak adjacent to one which had caused the previous ejection. This effect is manifested in Fig. 2(b) in which peak positions are quantized to fall on straight lines of negative slope which intersect the x axis at 180 ° intervals. Furthermore, the maximum step-size between adjacent noise peaks in Fig. 3 is on the order of 100mu, comparable to the vertical separation of the quantized levels evident in Fig. 2(b). An additional consequence of this effect, commonly observed by the authors in highly resolved mass spectra, is peak splitting which results when only some ions of a particular m/z ratio are ejected by one noise peak while the rest are ejected by an adjacent noise peak which occurs later in time. When this happens, the magnitude of the splitting in the time domain is equal to the period of the noise which caused the split, in this case, 120 Hz. Finally, Figs. 1(c) and 2(c) show ion-intensity signals obtained by successive microscans to

F.A. Londry, R.E. March~International Journal of Mass Spectrometry and Ion Processes 144 (1995) 87-103

be stable within about 5 mu over more than 60s. However, the data for these figures were collected with the magnitude of the scan delay and the a.c.-r.f, phase difference chosen to stabilize peak position. Because other sources of electronic instability are overwhelmed by the 120 Hz noise, and because ions are ejected by the rising edges of 120 Hz noise superimposed on the amplitude of the r.f. voltage, peak positions may actually become less stable than indicated by Figs. l(c) and 2(c) if the 120Hz noise were to be removed. Work is currently under way to identify and to eliminate this source of electronic instability. 4.3. Phase locking In these experiments peak intensity, peak position and peak half-width were recorded as a function of the phase difference between the a.c. and r.f. signals. All a.c.-r.f, phase differences are expressed in terms of degrees of the r.f. drive, ~ of Eq. (1). Data were collected for m / z 6 9 , 131, 264, 414 and 614 using each of the a.c. frequencies 250.0 and 333.3kHz and at each of the scan rates 10 and 5000us -l. For comparison with commercial instruments, some data were collected using an a.c. frequency of 462.0kHz which resulted in qz, eject = 0 . 9 0 . (This value of qz, eject is employed in many commercial instruments which have an r.f. frequency of 1.05 MHz and use an a.c. ejection frequency of 485.0kHz.) Ion ejection at an a.c. frequency of 462.0 kHz was investigated only at the higher scan rate of 5000us -~ since it was found that high-resolution mass spectra could not be obtained when ions were ejected resonantly at a working point this close to the flz = 1 stability boundary. As the amount of data generated by these experiments was large, only those results which best illustrate salient features are presented here.

95

M/z264 ejected at 10 us -1 with qz, ejea = 0.64 Phase-lock experiments carried out with an a.c. frequency of 250.0 kHz, that is, one-fourth of the drive frequency, resulted in ion ejection at a working point of qz, eject= 0.64. Figs. 4(a)-4(c) show peak intensity, peak position and peak half-width, respectively, recorded as a function of the phase difference between the a.c. and r.f. signals over four complete cycles of the r.f.-drive phase-offset, 4~. While there is only weak evidence of periodicity in the peak area (or intensity) as shown in Fig. 4(a), the periodic dependence of peak position (Fig. 4(b)) and peak half-width (Fig. 4(c)) are more pronounced. Note the apparent quantization of peak position values in Fig. 4(b). These quanta appear on a time scale axis at a frequency of 120 Hz as a consequence of full-wave rectified line-voltage noise on the r.f. amplitude as discussed previously. Again, this quantization can be understood with reference to Fig. 3. Imagine that the r.f. ejection level was moving down from some initial position near the top of a noise peak to successively lower values in response to changes in the a.c.-r.f, phase difference. This reduction in the r.f. level required for ejection would result in a monotonic and smoothly varying decrease in the time of ejection until the ejection line intersected the next lower noise peak. When this occurred, there would be an abrupt stepwise decrease in the time of ejection which would result in an abrupt decrease in apparent mass. Therefore, the quantization of apparent mass in Fig. 4(b) is a consequence of noise present on the r.f. amplitude and the spacing between groups of quantized values is consistent with a noisepeak spacing of 120 Hz. The data presented in Fig. 4(c) have obvious implications for high-resolution applications; as peak half-widths are such a strong periodic function of ~, it is important to select judiciously an a.c.-r.f, phase difference with which peaks of small half-width may be

F.A. Londry, R.E. March/International Journal of Mass Spectrometry and Ion Processes 144 (1995) 87-103

96 2800

obtained so as to optimize the mass resolution of the instrument.

2600

2400 ~ o 2200

2000

180( 0 (a)

200 400 600 800 1000 1200 1400 1600 AC-RF Phase Difference (RF degrees)

264,40 264.35 264.30 .~z264.25

t

.~ 264.20 4~

~ 264.15 6

264.10 264,05 264.00 263,95

/ i

i

i

i

i

i

400 600 800 I000 1200 1400 1600 AC-RF Phase Difference (RF degrees)

200 (b)

10

JJ__

t~ O-

0 (c)

600 800 1000 1200 1400 1600 200 400 AC-RF Phase Difference (RF degrees)

Fig. 4. Data obtained for m/z264 with qz, eject= 0.64 at a scan rate of 10 u s -~ which show variation with respect to the a.c.-r.f. phase difference, 8, of (a) peak area, (b) peak position, and (c) peak half-width.

M/z 264 ejected at 5000 u s- 1 with qz, eject = 0.64 The variations of peak area, peak position and peak half-width as a function of the a.c.r.f. phase difference at the higher scan rate of 5000us -1 are shown in Fig. 5 with datum points connected by solid lines. Parenthetically, the dotted line in Fig. 5 shows data obtained at 5000us -1 with qz, eject=0.90 which will be discussed in the next section. Although the solid line results are similar to those shown in Fig. 4, some significant differences are evident. The dependence of peak area on the a.c.-r.f, phase difference (Figs. 4(a) and 5(a)) is more clearly defined at the higher scan rate. Also at the higher scan rate, the variation in peak position over 360 ° of (Fig. 5(b)) is nearly double that observed at 10 u s-~ (Fig. 4(b)). Although the relative variation in peak half-width is less dramatic at the higher scan rate (Fig. 5(c)) than at the lower scan rate (Fig. 4(c)) once more it is seen that peak half-width can be minimized by choosing the a.c.-r.f, phase difference from a favourable range of values. Comparison of the scales on the ordinates of Figs. 4(c) and 5(c) indicates that much higher mass resolution (mass divided by peak width at half-maximum) obtained at the lower scan rate. M/z 264 ejected at 5000 u s -1 when the ratio f/fl cannot be represented by small integers For the sake of comparison with commercial instruments, data collected at a scan rate of 5000us -1 with qz, eject= 0.90 (flz,eject = 0.924) have been displayed in Fig. 5 as dotted lines. These results do not show the strong dependence of ~b demonstrated by the data for which the ratio f / f l can be represented by small integers. Although the a.c.-r.f. phase difference, ~b, was varied through four complete cycles for both values of qz, eject shown in Fig. 5, the time required to span

F.A. Londry, R.E. March/International Journal of Mass Spectrometry and Ion Processes 144 (1995) 87-103 2600 2500 2400 v

2300 2200 2100

2000 IgO0 (a)

i

i

,

,

i

,

,

200 400 600 800 1 0 0 0 1 2 0 0 1 4 0 0 1600 AC-RF Phase Difference (RF degrees)

264.8 264.7

264,6 .~z264, 5

~ 264.4 ~264.3

~264.2 264.1 264.0 263.9 (b)

200 400 600 800 1 0 0 0 1 2 0 0 1400 AC-RF Phase Difference (RF degrees)

1600

600 550 "~ 5o0 45o

400 350 300

250 200

(c)

i

i

i

L

i

,

i

200 400 600 800 1 0 0 0 1 2 0 0 1400 AC-RF Phase Difference (RF degrees)

1600

Fig. 5. Data obtained for m/z264 at a scan rate of 5000us -1 which show variation with respect to the a.c.-r.f, phase difference, ~b, of (a) peak area, (b) peak position, and (c) peak halfwidth. The solid and dotted lines represent data obtained with qz, eject= 0.64 and 0.90, respectively.

97

one complete cycle of the a.c.-r.f, relationship with qz, eject =-0.90, is relatively long. Consequently, even though the a.c.-r.f, phase relationship is fixed, it is unlikely that it repeats in the 60-100#s required to eject a particular mass. Results very similar to those observed with qz, eject = 0.90 were obtained also with qz, eject = 0.72 (f/fl = 3.41). However, even when the ratio f / f l cannot be represented by small integers, the characteristics of ion-intensity signals may still show a marked dependence on q~. Both peak position and peak halfwidth of re~z264 were found to be periodic functions of q~ when qz, eject= 0.71 ( f / f l = 3.48). For this case, as before, the time required to span one complete cycle of the a.c.-r.f, relationship was relatively long. Consequently, as discussed earlier in the section on theory, the phase relationships in effect during ejection of a particular mass depended as much on the starting point of the analytic scan-segment (on the mass axis) as on the value of q~. As a result, the phase of the periodic functions of ~b, peak position and peak half-width, were also initial-mass dependent.

General observations The data presented in Figs. 4 and 5 represent only a very small fraction of those collected in this investigation. The characteristics of these data are influenced, sometimes strongly, by many factors including pressure of the helium buffer gas, scan rate, frequency and amplitude of the axial modulation signal, and the mass/ charge ratio and number density of ions in the trap. For example, the intensity of the ion signal may drop to near zero over specific ranges of the a.c.-r.f, phase difference if the number density of those ions in the trap is too low. Conversely, the range of the a.c.-r.f. difference which yields acceptable ion-signal intensities can often be extended by increasing ionization time. Further evidence of interdependence among parameters is provided by

98

F.A. Londry, R.E. March~International Journal o f Mass Spectrometry and Ion Processes 144 (1995) 87-103

the observation that the range of values for the axial-modulation amplitude over which highresolution mass spectra can be obtained increases dramatically with ion number density. Consequently, due to strong interrelationships among many parameters, it is possible to specify suitable values for the a.c.-r.f, phase difference only under very specific conditions. Therefore, rather than attempting to summarize large amounts of data, a brief discussion is given of factors which should be considered when choosing an a.c.-r.f, phase difference for a specific application. Highly resolved mass intensity signals cannot be obtained when the value of qz, eject is close to the /3z = 1 stability boundary. Although high-resolution results can be obtained for m/z values greater than 200u w i t h qz, eject z 0.86, the value of qz, eject must be reduced further still to obtain consistently high-resolution mass spectra of m/z69 and 131; for example, when qz, eject is reduced to 0.78, the half-widths of m/z69 and 131 become comparable to those obtained with higher masses under the same conditions. Therefore, if high-resolution spectra are to be obtained over the normal mass range of commercial ion traps, ions must be ejected resonantly at values of qz at which peak intensity, peak position and peak half-width may be affected by the phase relationship which exists between the a.c, and r.f. potentials. These mass-peak characteristics are affected strongly by the a.c.-r.f, phase difference when the ratiof/f1 can be represented by small integers, and less dramatically so when it cannot. Although values of qz, eject c a n be found at which mass-peak characteristics are insensitive to the a.c.-r.f, relationship, a relatively minor change in some system parameters can result in a marked increase in sensitivity to the a.c.r.f. phase relationship, even when none was apparent before. Furthermore, if the ratio f/f1 cannot be represented by small integers,

the phase of the periodic functions of q~, such as peak position and peak half-width, are usually initial-mass dependent. Therefore it is preferable that the phase relationship between the a.c. and r.f. potentials be locked at some value which stabilizes peak position and minimizes peak half-width, and that the a.c. frequency be chosen such that the ratio f/f1 can be represented by small integers. Because some calibration procedure must always be invoked in order to assign mass/ charge values to particular ion-intensity signals, the actual peak-position of an ion signal is of less consequence than the reproducibility of that position. Therefore, data such as those represented in Figs. 4(b) and 5(b) are useful in the determination of the value, or range of values, of the a . c . r . f , phase difference for which the peak position varies most slowly. Furthermore, it is usually desirable to choose an a.c.-r.f, phase difference which results in a small peak half-width from data such as those presented in Figs. 4(c) and 5(c). Therefore, when choosing an a.c.-r.f, phase difference, it is desirable to maximize signal intensity, stabilize peak position and minimize peak half-width. Unfortunately, the range of a.c.-r.f, phase differences which meet these three criteria do not always overlap. Consequently, under some conditions, it is necessary to compromise performance with respect to one parameter in order to optimize performance with respect to another. For example, in order to minimize peak halfwidth at a relatively stable peak position it may be necessary to settle for reduced signal intensity.

4.4. Simulation results Many of the experiments described above are amenable to computer simulation. Using a computer program described elsewhere [11], single-ion trajectories were simulated under the same conditions used to collect the data

F.A. Londry, R.E. March/International Journal of Mass Spectrometry and Ion Processes 144 (1995) 87-103

265[ 264-

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263 262

261 260 259 (a)

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264.8 264.6 264.4 "2"264,2

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laboratory experiments. After allowing sufficient cooling time for the ion to be focused near the centre of the trapping volume, the r.f. and a.c. levels were manipulated in order to simulate accurately the fourth and fifth segments of the experimental scan function described earlier. Segments four and five of the scan function were simulated repeatedly for each ion while the a.c.-r.f, phase difference, ~, was varied, in steps of 20 °, through four complete cycles. During the final two segments each ion was subjected to collisions with the helium buffer gas as described previously [11]. Simulations were not carried out at the lower scan rate of 10us -1 because the computational time required would have been excessively long.

1264.0

i 263.8 ~263.6 263.4 263.2 263.0 (b)

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i

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200 400 600 800 I 0 0 0 1200 1400 AC-RF Phase Difference (RF degrees)

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Fig. 6. The apparent mass at ejection obtained from simulated trajectories plotted against the a.c.-r.f, phase difference, ~b, for m / z 264 at a scan rate of 5000 u s I with qz, eject 0.64: (a) all the data obtained from the simulated trajectories of ten different ions; (b) triangles show the results of averaging the mass at ejection of those trajectories from (a) for which the mass at ejection fell between 262.2 and 264.5 at each value of the a.c. r.f. phase difference; the dotted line shows the result of applying a cubic-spline smoothing algorithm to the average values and the solid-line data represent the experimental data of Fig. 5(b), reproduced here for ease of comparison with the simulation results. =

shown in Fig. 5, corresponding to m/z 264 at a scan rate of 5000us -1 with qz, eject = 0.64. In these simulations each ion was created at a r a n d o m position within a relatively small volume near the centre of the ion trap and given a r a n d o m velocity which was consistent with a Maxwell-Boltzmann distribution whose average was 220°C. The initial r.f. storage level was chosen to correspond to the r.f. level in effect during ionization in the

Apparent mass at ejection The position of the mass axis of an ion intensity signal obtained experimentally can be correlated with the r.f. amplitude at which a simulated ion-trajectory terminated at an end-cap electrode. All the data obtained from simulation for peak position as a function of the a.c.-r.f, phase difference for each of ten ions are shown in Fig. 6(a). Clearly, there are several datum points in this figure which would contribute to a general background only, having been ejected at an r.f. amplitude too far from the mean to contribute to an ionintensity signal. In order that the simulation data be representative of ions which would have contributed to an ion-intensity signal, those simulated trajectories which resulted in peak positions which fell outside of the range 262.2 < meject < 265.0 were culled from the simulation results. This reduced the number of datum points from the 730 shown in Fig. 6(a) to 627. The numbers of d a t u m points which remained at each increment of the a.c.r.f. phase difference (10 or less) were averaged and then level-shifted by about 0.7 u for ease of comparison with laboratory results. Each triangle in Fig. 6(b) is the result of averaging

100

F.A. Londry, R.E. March/International Journal of Mass Spectrometry and Ion Processes 144 (1995) 87 103

the apparent mass at ejection of ten (or fewer) ions subjected to a particular a.c.-r.f, phase difference; the dotted line is the result of smoothing the simulation data using a cubicspline algorithm. The experimental data of peak position vs. a.c.-r.f, phase difference from Fig. 5(b) have been reproduced in Fig. 6(b) with a solid line. Considering the small number of simulated trajectories used to obtain the average peak position, these simulation data show remarkably good agreement with experiment in both the phase and amplitude of the dependence of peak position upon the a.c.-r.f, phase difference. Phase at ejection In the simulation program, an ion was deemed to have been detected at the instant it struck an end-cap electrode. However, calculations show that in the experimental situation, the time which elapses after an ion passes through a hole in the detection end-cap and before it is detected electronically is of the order of 1 #s. Consequently if these data were to be compared with results obtained experimentally it would be necessary to take into account the significant delay between ejection and detection. The phase of each of the r.f. and a.c. potentials at the time of ejection for each of the ten simulated trajectories over four periods of a.c.-r.f, phase difference were recorded. These data showed that ions are ejected most often during a relatively small fraction of the r.f. period when the r.f. phase is between about 240 ° and 270 °, with a few ejections occurring at slightly higher values. This value is in agreement with Julian et al. [10] who reported that ions are ejected only when the r.f. is negative with negative slope, just prior to reaching the minimum. The phase of the a.c. potential at the time of ejection is plotted in Fig. 7 as a function of the a.c.-r.f, phase difference. These simulated results correspond to ejection through the lower end-cap (detection)

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~ 140 %

120

~ ~00 80

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I O0 200 300 AC-RF Phase Difference

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400 500 (RF degrees)

600

Fig. 7, The phase of the a.c. potential at the time of ejection as a function of the a.c.-r.f, phase difference for simulated trajectories.

electrode while the phase of the a.c. potential is specified at the upper end-cap electrode. Although the data were collected for Fig. 7 over four complete cycles of the a.c.-r.f. phase difference, it was possible to simplify Fig. 7 by either adding or subtracting an integral multiple of 360 ° to the a.c.-r.f, phase difference of each datum point such that all points fell on a single straight line. From this figure it can be seen that ions were ejected at the lower end-cap electrode only when the a.c. phase was between 90 ° and 190 ° . Again, these data are consistent with those of Ref. [10] where it was reported that positive ions are ejected via the lower (detector) end-cap electrode during the positive half of the a.c. cycle. These phase-at-ejection results are illustrated in Fig. 8 where a.c. and r.f. waveforms of 250.0 kHz and 100 MHz, respectively, are plotted as a function of time, with the a.c.r.f. phase difference fixed at 0 °. The bold section of each curve indicates the range of each cycle which is favourable to ejection at the lower end-cap electrode. Clearly, detection can occur only when the bold sections of the a.c. and r.f. waveform overlap in time. Since the time duration of the favourable r.f. range is only about 10% of that of the a.c. range, the linear nature of Fig. 7 is easily understood;

F.A. Londry, R.E. March~International Journal of Mass Spectrometry and Ion Processes 144 (1995) 87-103 1.2 0.8

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Fig. 8. The broken and solid lines show a.c. and r.f. waveforms of 250.0 kHz and 1.00 MHz, respectively, plotted as a function of time, with the a.c.-r.f, phase difference fixed at 0 °. The bold section of each curve indicates the range of each cycle which is favourable to ejection at the lower end-cap electrode as determined from Figs. 7 and 8.

that is, the time at which overlap of the favourable ranges occurs will correspond to a relatively small fraction of the a.c. range favourable to detection and will depend linearly on the a.c.-r.f, phase difference. Furthermore, with the a.c.-r.f, phase difference fixed, it would be expected that the a.c. phase at ejection would vary over a range comparable to the 30 ° span of the r.f. phase over which ejection occurs. This effect is observed in Fig. 7 where, for most values of the a.c.-r.f, phase difference, the a.c. phase at ejection varies over about 8 ° of the 250.0 kHz a.c. potential (8 ° of a 250.0 kHz signal spans the same time interval as 32 ° of a 1.00 MHz signal). The range of the a.c. phase favourable to detection as indicated in Fig. 8 merits further consideration. Fig. 8 shows that ions are ejected at a time when the detection end-cap is becoming less negative. If the secular motion of the ion was in phase with the a.c. field, it would be moving towards the centre of the trap in this quadrant, rather than towards the lower end-cap electrode, and would not be detected. However, detailed examination of simulated-ion trajectories indicates that the secular motion of the ions usually lags

101

the phase of the a.c. potential just prior to detection. There are two obvious reasons why an ion's motion is only in phase with the a.c. field for relatively short periods of time. The first of these is that the secular frequency of the ion is increasing continuously as the r.f. amplitude is ramped; in fact, during the time required to eject a particular mass at 5000us -1 when qz, eject ~-0.64 (about 100#s) the theoretical secular frequency of that mass changes by about 600Hz. Secondly, as the ion comes into resonance, the amplitude of its axial motion increases, resulting in an additional increase in the secular frequency of the ion due to the influence of higher order terms in the electric potential inside a trap of stretched geometry. As the secular frequency of the ion changes, its motion moves in and out of phase with the driving force so that the interaction of the a.c. field with the ion alternates between driving and damping. Amplitude enhancement will begin when the phase of the motion leads that of the driving force by 90 ° and will continue until the motion lags that of the driving force by the same amount. During the time preceding ejection, an ion's motion is subjected to alternate periods of destructive and constructive interference. This effect is illustrated by the simulated trajectory shown in Fig. 9 where the axial component of an ion's motion during the final (fifth) segment of the experimental scan function is plotted as a function of time. This particular trajectory, for which the a.c.-r.f, phase difference was fixed at 0 °, terminated at the lower end-cap with an apparent mass at ejection of 262.5 u consistent with Fig. 6(a). The r.f. and a.c. phases at ejection were 242.1 o and 150.5 °, respectively. Since amplitude growth (or decay) is approximately linear in response to a (primarily) dipole field [12] it would be expected that an ion would be ejected with greater probability during the latter half of a period of amplitude growth

102

F.A. Londry, R.E. March~International Journal of Mass Spectrometry and Ion Processes 144 (1995) 87-103

g 4~

Q-

2.0

2.2

2.4

Time

2.6

2 8

3.0

(ms)

Fig. 9. F r o m simulation, the axial component of an ion's motion during the final (fifth) segment of the experimental scan function plotted as a function of time. This particular trajectory, for which the a.c.-r.f, phase difference was fixed at 0 °, terminated at the lower end-cap with an apparent mass at ejection of 262.5 u consistent with Fig. 6(a).

when the phase of the motion lags that of the driving force, as observed.

5. Conclusions

An experimental study has been carried out of the mass-selective ejection of ions from an extensively modified commercial quadrupole ion trap. All aspects of the scan function were controlled from a 80 486-based host computer using modified commercially available data-acquisition equipment [6]. While the r.f. drive frequency has been changed modestly, the mass scanning rate may be varied, and the a.c. and r.f. potentials may be phase locked and line locked, the instrument and its power supplies were basically unchanged. Potentials were applied to the end-cap electrodes in dipolar mode using a customdesigned PCB called the waveboard. The modest change in the r.f. drive frequency was made in order to facilitate phase locking of the a.c. and r.f. potentials. When ions are ejected resonantly during a mass-selective instability scan, peak intensity, peak position and peak half-width are

relatively strong functions of the phase relationship between the a.c. and r.f. signals, provided that the ratio f i l l can be represented by small integers. Values of qz, eject c a n be found at which mass-peak characteristics are relatively insensitive to the a.c.-r.f, phase relationship. However, because the degree of sensitivity to ~ can change significantly with other trapping parameters, a fixed-phase relationship between a.c. and r.f. signals provides a reliable technique for achieving peak-position stability. Because the half-widths of ion-intensity signals can be strong periodic functions of the a.c.-r.f, phase difference, it is important to select judiciously an a.c.-r.f, phase difference with which peaks of small half-width may be obtained so as to optimize the mass resolution of the instrument. With a fixed and optimized phase difference between the a.c. and r.f. signals, the positions on the mass axis of highly resolved ionintensity signals can be reproduced within 2 5mu, corresponding to a mass resolution between 50000 and 130000 for m/z264. In order to obtain this degree of reproducibility with the level of electronic noise present on the r.f. drive of the instrument used in these investigations, it was necessary to synchronize each scan function with the phase of the line voltage. Ironically, it is suspected that the degree of mass-peak stability so achieved may be a consequence of the dominant, 120Hz, noise component itself; that is, if other sources of electronic instability were not so overwhelmed by the 120Hz component, it is possible that peak positions may become less stable. Work is currently under way to reduce this noise level by an order of magnitude from its current value of about 0.08% (peak to peak), while still retaining the use of the commercial power supplies and amplifying circuit. The variation in peak intensity, peak position and peak half-width with the a . c . r . f .

F.A. Londry, R.E. March/International Journal of Mass Spectrometry and Ion Processes 144 (1995) 87-103

phase difference are influenced, sometimes strongly, by many factors including the pressure of the helium buffer gas, scan rate, frequency and amplitude of the axial modulation signal and, the mass-to-charge ratio and number of density of ions in the trap. Consequently, it is possible to specify suitable values for the a.c.-r.f, phase difference only under very specific conditions. Considering the small number of simulated trajectories used to obtain the average mass at ejection, these simulation data show remarkably good agreement with the experimental results in both the phase and amplitude of the dependence of peak position upon the a.c.-r.f, phase difference. It was determined from simulations that, at a scan rate of 5000us -1, the phase of the r.f. drive at the time of ion ejection varies between 238 ° and 278 ° which corresponds to the region where the r.f. phase is negative with negative slope as the potential approaches its minimum, in agreement with the findings of Julian et al. [10]. Simulated trajectories also showed that the phase of the a.c. potential at ejection varies linearly from about 90 ° to 190 ° as the a.c.r.f. phase difference is advanced through a full cycle. In general, the phase of an ion's secular motion at the time of ejection lags that of the a.c. potential.

Acknowledgements The authors gratefully acknowledge the

103

financial assistance of Varian Associates Inc., the Natural Sciences and Engineering Research Council of Canada, and Trent University. For his outstanding technical support and significant contributions to this work, the authors would like to express their appreciation to Allan Kift.

References [1] R.E. March and R,J. Hughes, Quadrupole Storage Mass Spectrometry, Chemical Analysis Series, Vol. 102, Wiley Interscience, New York, 1989. [2] R.E. Kaiser, Jr., R.G. Cooks, G.C. Stafford, Jr., J.E.P. Syka and P.H. Hemberger, Int. J. Mass Spectrom. Ion Processes, 106 (1991) 79. [3] J.C. Schwartz, J.E.P. Syka and I. Jardine, J. Am. Soc. Mass Spectrom., 2 (1991) 198. [4] D.E. Goeringer, S.A. McLuckey and G.A. Glish, Proc. 39th ASMS Conf. Mass Spectrometry and Allied Topics, Nashville, TN, 1991, p. 532. [5] J.D. Williams, K. Cox, K.L. Morand, R.G. Cooks, R.K. Julian and R.E. Kaiser, Proc. 39th ASMS Conf. Mass Spectrometry and Allied Topics, Nashville, TN, 1991, p. 1481. [6] F.A. Londry, G.J. Wells and R.E. March, Hyperfine Interactions, 81 (1993) 179. [7] F.A. Londry, G.J. Wells and R.E. March, Rapid Commun. Mass Spectrom. 7 (1993) 43. [8] R.E. March, F.A. Londry and G.J. Wells, Proc. 39th ASMS Conf. Mass Spectrometry and Allied Topics, San Francisco, CA, 1993, p. 392a. [9] C.D. Cleven, K.A. Cox and R.G. Cooks, Rapid Commun. Mass Spectrom., 8 (1994) 451. [10] R.K. Julian, Jr., H. Reiser and R.G. Cooks, Int. J. Mass Spectrom. Ion Processes, 123 (1993) 85. [11] F.A. Londry, R.L. Alfred and R.E. March, J. Am. Soc. Mass Spectrom., 4 (1993) 687. [12] J. Franzen, Proc. 42nd ASMS Conf. on Mass Spectrometry and Allied Topics, Chicago, IL, 1994, p. 228.