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Drift times in ion cyclotron resonance spectrometers
Volume 36, number 2
DRI?3
TIMES IN ION CYCLOTRON
W.J. VAN
DER HART
1 November 1975
CFiEhIICAL PHYSICS LETTERS
and H.A. VAN
RESONANCE
SPECTROMETERS
SPrWNG
Depmtmerlr of Theoretical Organic Chernistv, Univemity ofLeyden, Leyden, The Xetherbnds Received 30 June 1975
Drift times in an ICR spectrometer ran be determined in a simple way by the use of source modulation of the phase shift of the rmrginal oscillator output signal.
and obs-ervauon
1, Introduction In the derivation of rate constants for ion-molecule reactions from ion cyclotron resonance (ICR) spectra [l-j] it is essential to know the time delay between the formation of ions in the source region and the beginning and end of their detection by the marginal oscillator. These drift times can in principle be calculated from the magnetic field strength and the applied drift voltages but especially at low drift velocities, this procedure may easily result in substantial errors in the rate constants. McMahon and Beauchamp [5] therefore described the measurement of drift times by pulse methods. Ions are formed by a 50 PS pulse of the ionizing electron beam. After a time delay At the trapping voltage in one of the sections of the cell is reversed in sign, so the ions, if in this section, are I-IOlonger trapped and will be lost at the walls. As a consequence the total ion current vanishes. By variation of A! it thus be:omes possible to measure the time the ions need to drift to the end of the separate sections of the ICR cell. Unfortunately, t&is method besides being laborious is not very well suited for accurate measurements because of the following ieasons: (i) Total drift times are of the order of a few milliseconds. Therefore the 50 fls pulse of the electron beam can at most be repeated once each 5 ms. This means that, in order to have the same conditions as iri .normaI ICR measurements where the total ion current is about 5 X lo-l2 A, drift times should be measured
modulation
Fig. 1. Phase relation between gnal osciuator output.
source modulation and mu-
with a total ion current
of 5 X lo-l4 A. In this case rather low. (ii) Because of fringing of the electric fields, ions start to be detected by the marginal oscillaror when in a 4-section cell, say, they are still in the reaction region. On the other hand a reversal of sign of the trapping voltage in the reaction region will cause ions in the analyzer section to get lost at the walls. McMahon and Beauchamp’s method therefore ovcrestimates the time the ions need to drift to the point where they start to be detected. These two problems would be solved if it were passible to measure drift times by means of the marginal oscillator signal during normal operating conditions. We will show that at least a partial solution can be obtained in a rather simple way. If the ICR signal is modulated in the source region, e.g, by souice drift modulation, sours trapping ,moduIation oi e!ectron beam modu!ation, the drift of the ions from the source to the detection region leads to ;Lshift of the phase of the modulated margina!
the signal to noise ratio becomes
Vo!ume 36, number 2
CHEMICAL PHYSICS LJZTERS 36-l
oscillator signal with respect to the modulation (see fig. 1). Suppose in the case of electron beam modulation, t&e electron beam is switched on at time t = fO. After a time delay (tl - f,,) the fast ion reaches the detection region. Then the marginal oscillator signal rises proportional to (t - p1)2-until the first ion leaves the detection retion. _4fter this the signal remains constant until the last ion produced during the electron beam open period reach,:s the analyzer. The signal then decreases quaclraticaily with time. It foilows that the drift of the.ions through the cell results Ln a phase shift, Q (in degrees), Q= -361) “,,;
{(Q- f,,) + 2-“’
1 November 1975
‘observed
I
.
-
1
1
(tZ-tl ) + fe)
where vrnod is the modulation frequency and t, has been added to account for the additional phase shift due to the band width of the electronic detection system. We have investigated whether this phase shift can be used for the determination of drift times. Measurements were performed on the ArC ion at 8Cl!JOG with a flat 4-section cell equipped with a 4-grid R.P.G. electron gun. This cell is part of a home-built ICR spec-
OOr
08
OL
1.2
1.6
2.0
2.L
28
' calculated
Fig. 2. Variatioil of drift times with drift voltages (see text).
plot of the observed values of (tl-f,,)
i2-1’2(t2-~I)
+ te against the calculated
values of (tl- to) + 2-l’* (r2- fl) and extrapolation to zero drift times we obtained t, = 0.50 ms (dashed line in fig. 2).
.trometer, the details of which will be described elsewhere. PIiase shifts between rrargiqal clscillator output and modulation were measured either with an Ad-Yu phase angle meter or with the phase shifter of a PAR
2.2. Determiualion
122 lock-in
several values of the reaction drift voltage, the drift voltage in the analyze: region was varied from 0.2 to 2.5 V. By plotting the observed values of (rl - to) +
amplifier.
In all cases 9 was exactly
pro-
portional t0 vmoC (Up t0 Ymod = 200 Hz). Measurements of this type requir? a careful adjustment of the ICR instrument. It is especially important to observe the marginal oscillator output on an oscilloscope. In
some cases, e.g., at high electron beam currents, we observed distortions with respect to the shape depicted in fig.1. This not only results in an incorrect phase
shift but as a consequent: signal ti;ensities in an ICR spectrum becorn? meankgless when these distortions depend on the magnetic field strength.
of (tl -to)
Using a constant source drift voltage of 0.8 V and
2-112 (r2-- tl) f t, against the calculated value of the analyzer drift time, extrapolation to zero drift time in the analyzer region and subtraction of the value of t, it then becomes
Table 1 (tI--to) es a hnction
possible
to obtain the value of
of the renction
drift voltage
01 -to) hs)
:p Reaction
2. &s&s 2.1. Determinetion
-
present method
0.2 CL3 OS
1.78 1.48 1.10
0.7 1.0
method
Vd anal = 0.3 v
vd a,-Ed = 2.5 v
2.00 1.64
1.90 i.55
0.92
1.22 0.95
1.16 0.96
0.75 0.52
O.&l 0.56
0.77 0.54
oft,
The time delay due to the band width of the electronic detection system was measured bi using a common, drift voItage for source, reaction and analyzer sections which was varied from 0.5 to 2.5 V. From a 216
McM&on-Beauchamp
drift voltage
2.0
Volume 36, number 2
1 November 1975
CHEMICAL PHYSICS LETTERS
Table 2 Analyzer and total drift times
-_
-~-__--
Reaction drift voltage. 0.20
--
0.30
0.50 -..
0.70
1.00 --..-
.._--.
2.00
annlyzer drift voltage 0.3 v
trr-re1+ (fz-f1) (rz-ro) (fz-ro),
2-1’2 (1a-Tr)f
fe
3.38
3.06
hlchlahon-Beauchamp
1.56 3.34. 3.50
1.53 3.01 3.13
2.65 1.49 2.59 2.71
2.43 1.43 2.35 2.48
2.24 1.40 2.15 2.25
1.97 1.41 1.86 1.99
2.98 0.99 2.17 2.85
2.65 0.95 2.43 2.49
2.26 0.93 2.03 2.09
2.05 0.89 1.81 1.86
1.85 0.85 1.60 1.67
1.60 0.82 1.34 1.41
annlyzcr drift voltage 0.6 V (Cl -Co) f 2-l’* (f2-t (rz-tl)
,) f te
02-to)
(tz-rc),
McXlahon-Bcauchamp
(tl- to) as a function of the reaction drift time (fig. 1). The results are collected in table 1 together with values
lems and the value of (tl -fo) + 2-“2 (t2-tl) for each peak. In the calculation of rate constants we need,
obtained by McMahon and Beauchamp’s method for
depending on the expression used [l-5],
analyzer
of(tl-to)+
drift voltages
of 0.3 and 2.5 V.
2.3. Determir;ation of (t,-t,) From
together obtained
(tl -to) given above with the values of (tl-to)+ 2-“2 (f2-tl)+tc from the phase shifts, we have calculated
(tl-t0)+:(t2-tl)
the values or
(tl-t0)+$(t2-tl).
ad (t2-to)
the values of C, and
analyzer drift time (r2-tl)
$(t2--tt),
the
and the total drift time
(t2-to) for a constant source drift voltage of 0.8 V, varying reaction region drift voltages and analyzer drift voltages of 0.3 and 0.6 V. The results are given
in table 2.
3. Conclusions From the values given in the tables together with the general behav-iour depicted in fig. 2, it appears that the use of phase shifts results in a reliable estimation of drift times. It is true that the experiments described above are rather time consuming. However, there is a very useful and simple alternative. When the phase of. the !ock-in amplifier is referenced to the modulation itself, measurement of both the in-phase and the 90’ out-of-phase ICR spectrum gives the values of I cos& and I sin+ for each peak in the spec-
trum. Because the value of C, is a constant of the instrument which can be measured once-for-all, we thus obtain both the value of I without phase shift prob-
Especially in a 4-section cell the differences between these values are relatively small. We therefore think that a reliable correction can be made, e.g., by assuming a proportionality of these values with the calculated ones. In this way we obtain by simply recording the ICR spectrum twice both the intensity and the drift time for each ion. Acknowledgement The investigations were supported by the Netherlands Foundation for Chemical Research (S.O.N.) with financial aid from the Netherlands Organization for the Advancement of Pure Research (Z.W.O.).
References [l] [2] [3]
[4] [5] [6]
NT. Bowers, 0.0. Ellrman and J.L. Beauchamp, J. Phys. Chem. 72 (1968) 3599. S-E. Buttrill Jr., I. Chem. Phys. 50 (1969) 412.5. G.C. Goode. R.M. O’R:ally. A.J. FerrerCorrcia, R.I. Massey, K.R. Jenning, J.H. Futtrell and P.M. Llewellyn. Interu. J. Mass Spectrom. Ion Phys. 5 (1970) 393. MB. Conisarow,.J. Chem. Phys:55 (1971) 20.5. W.J. van der Hart, Chem. Phys. Letters 23 (1973) 93. T.B. htchiahon and J.L. Beauchamp, Rev. Sci.Instr. 42 (1971) 1632.
DRI?3
TIMES IN ION CYCLOTRON
W.J. VAN
DER HART
1 November 1975
CFiEhIICAL PHYSICS LETTERS
and H.A. VAN
RESONANCE
SPECTROMETERS
SPrWNG
Depmtmerlr of Theoretical Organic Chernistv, Univemity ofLeyden, Leyden, The Xetherbnds Received 30 June 1975
Drift times in an ICR spectrometer ran be determined in a simple way by the use of source modulation of the phase shift of the rmrginal oscillator output signal.
and obs-ervauon
1, Introduction In the derivation of rate constants for ion-molecule reactions from ion cyclotron resonance (ICR) spectra [l-j] it is essential to know the time delay between the formation of ions in the source region and the beginning and end of their detection by the marginal oscillator. These drift times can in principle be calculated from the magnetic field strength and the applied drift voltages but especially at low drift velocities, this procedure may easily result in substantial errors in the rate constants. McMahon and Beauchamp [5] therefore described the measurement of drift times by pulse methods. Ions are formed by a 50 PS pulse of the ionizing electron beam. After a time delay At the trapping voltage in one of the sections of the cell is reversed in sign, so the ions, if in this section, are I-IOlonger trapped and will be lost at the walls. As a consequence the total ion current vanishes. By variation of A! it thus be:omes possible to measure the time the ions need to drift to the end of the separate sections of the ICR cell. Unfortunately, t&is method besides being laborious is not very well suited for accurate measurements because of the following ieasons: (i) Total drift times are of the order of a few milliseconds. Therefore the 50 fls pulse of the electron beam can at most be repeated once each 5 ms. This means that, in order to have the same conditions as iri .normaI ICR measurements where the total ion current is about 5 X lo-l2 A, drift times should be measured
modulation
Fig. 1. Phase relation between gnal osciuator output.
source modulation and mu-
with a total ion current
of 5 X lo-l4 A. In this case rather low. (ii) Because of fringing of the electric fields, ions start to be detected by the marginal oscillaror when in a 4-section cell, say, they are still in the reaction region. On the other hand a reversal of sign of the trapping voltage in the reaction region will cause ions in the analyzer section to get lost at the walls. McMahon and Beauchamp’s method therefore ovcrestimates the time the ions need to drift to the point where they start to be detected. These two problems would be solved if it were passible to measure drift times by means of the marginal oscillator signal during normal operating conditions. We will show that at least a partial solution can be obtained in a rather simple way. If the ICR signal is modulated in the source region, e.g, by souice drift modulation, sours trapping ,moduIation oi e!ectron beam modu!ation, the drift of the ions from the source to the detection region leads to ;Lshift of the phase of the modulated margina!
the signal to noise ratio becomes
Vo!ume 36, number 2
CHEMICAL PHYSICS LJZTERS 36-l
oscillator signal with respect to the modulation (see fig. 1). Suppose in the case of electron beam modulation, t&e electron beam is switched on at time t = fO. After a time delay (tl - f,,) the fast ion reaches the detection region. Then the marginal oscillator signal rises proportional to (t - p1)2-until the first ion leaves the detection retion. _4fter this the signal remains constant until the last ion produced during the electron beam open period reach,:s the analyzer. The signal then decreases quaclraticaily with time. It foilows that the drift of the.ions through the cell results Ln a phase shift, Q (in degrees), Q= -361) “,,;
{(Q- f,,) + 2-“’
1 November 1975
‘observed
I
.
-
1
1
(tZ-tl ) + fe)
where vrnod is the modulation frequency and t, has been added to account for the additional phase shift due to the band width of the electronic detection system. We have investigated whether this phase shift can be used for the determination of drift times. Measurements were performed on the ArC ion at 8Cl!JOG with a flat 4-section cell equipped with a 4-grid R.P.G. electron gun. This cell is part of a home-built ICR spec-
OOr
08
OL
1.2
1.6
2.0
2.L
28
' calculated
Fig. 2. Variatioil of drift times with drift voltages (see text).
plot of the observed values of (tl-f,,)
i2-1’2(t2-~I)
+ te against the calculated
values of (tl- to) + 2-l’* (r2- fl) and extrapolation to zero drift times we obtained t, = 0.50 ms (dashed line in fig. 2).
.trometer, the details of which will be described elsewhere. PIiase shifts between rrargiqal clscillator output and modulation were measured either with an Ad-Yu phase angle meter or with the phase shifter of a PAR
2.2. Determiualion
122 lock-in
several values of the reaction drift voltage, the drift voltage in the analyze: region was varied from 0.2 to 2.5 V. By plotting the observed values of (rl - to) +
amplifier.
In all cases 9 was exactly
pro-
portional t0 vmoC (Up t0 Ymod = 200 Hz). Measurements of this type requir? a careful adjustment of the ICR instrument. It is especially important to observe the marginal oscillator output on an oscilloscope. In
some cases, e.g., at high electron beam currents, we observed distortions with respect to the shape depicted in fig.1. This not only results in an incorrect phase
shift but as a consequent: signal ti;ensities in an ICR spectrum becorn? meankgless when these distortions depend on the magnetic field strength.
of (tl -to)
Using a constant source drift voltage of 0.8 V and
2-112 (r2-- tl) f t, against the calculated value of the analyzer drift time, extrapolation to zero drift time in the analyzer region and subtraction of the value of t, it then becomes
Table 1 (tI--to) es a hnction
possible
to obtain the value of
of the renction
drift voltage
01 -to) hs)
:p Reaction
2. &s&s 2.1. Determinetion
-
present method
0.2 CL3 OS
1.78 1.48 1.10
0.7 1.0
method
Vd anal = 0.3 v
vd a,-Ed = 2.5 v
2.00 1.64
1.90 i.55
0.92
1.22 0.95
1.16 0.96
0.75 0.52
O.&l 0.56
0.77 0.54
oft,
The time delay due to the band width of the electronic detection system was measured bi using a common, drift voItage for source, reaction and analyzer sections which was varied from 0.5 to 2.5 V. From a 216
McM&on-Beauchamp
drift voltage
2.0
Volume 36, number 2
1 November 1975
CHEMICAL PHYSICS LETTERS
Table 2 Analyzer and total drift times
-_
-~-__--
Reaction drift voltage. 0.20
--
0.30
0.50 -..
0.70
1.00 --..-
.._--.
2.00
annlyzer drift voltage 0.3 v
trr-re1+ (fz-f1) (rz-ro) (fz-ro),
2-1’2 (1a-Tr)f
fe
3.38
3.06
hlchlahon-Beauchamp
1.56 3.34. 3.50
1.53 3.01 3.13
2.65 1.49 2.59 2.71
2.43 1.43 2.35 2.48
2.24 1.40 2.15 2.25
1.97 1.41 1.86 1.99
2.98 0.99 2.17 2.85
2.65 0.95 2.43 2.49
2.26 0.93 2.03 2.09
2.05 0.89 1.81 1.86
1.85 0.85 1.60 1.67
1.60 0.82 1.34 1.41
annlyzcr drift voltage 0.6 V (Cl -Co) f 2-l’* (f2-t (rz-tl)
,) f te
02-to)
(tz-rc),
McXlahon-Bcauchamp
(tl- to) as a function of the reaction drift time (fig. 1). The results are collected in table 1 together with values
lems and the value of (tl -fo) + 2-“2 (t2-tl) for each peak. In the calculation of rate constants we need,
obtained by McMahon and Beauchamp’s method for
depending on the expression used [l-5],
analyzer
of(tl-to)+
drift voltages
of 0.3 and 2.5 V.
2.3. Determir;ation of (t,-t,) From
together obtained
(tl -to) given above with the values of (tl-to)+ 2-“2 (f2-tl)+tc from the phase shifts, we have calculated
(tl-t0)+:(t2-tl)
the values or
(tl-t0)+$(t2-tl).
ad (t2-to)
the values of C, and
analyzer drift time (r2-tl)
$(t2--tt),
the
and the total drift time
(t2-to) for a constant source drift voltage of 0.8 V, varying reaction region drift voltages and analyzer drift voltages of 0.3 and 0.6 V. The results are given
in table 2.
3. Conclusions From the values given in the tables together with the general behav-iour depicted in fig. 2, it appears that the use of phase shifts results in a reliable estimation of drift times. It is true that the experiments described above are rather time consuming. However, there is a very useful and simple alternative. When the phase of. the !ock-in amplifier is referenced to the modulation itself, measurement of both the in-phase and the 90’ out-of-phase ICR spectrum gives the values of I cos& and I sin+ for each peak in the spec-
trum. Because the value of C, is a constant of the instrument which can be measured once-for-all, we thus obtain both the value of I without phase shift prob-
Especially in a 4-section cell the differences between these values are relatively small. We therefore think that a reliable correction can be made, e.g., by assuming a proportionality of these values with the calculated ones. In this way we obtain by simply recording the ICR spectrum twice both the intensity and the drift time for each ion. Acknowledgement The investigations were supported by the Netherlands Foundation for Chemical Research (S.O.N.) with financial aid from the Netherlands Organization for the Advancement of Pure Research (Z.W.O.).
References [l] [2] [3]
[4] [5] [6]
NT. Bowers, 0.0. Ellrman and J.L. Beauchamp, J. Phys. Chem. 72 (1968) 3599. S-E. Buttrill Jr., I. Chem. Phys. 50 (1969) 412.5. G.C. Goode. R.M. O’R:ally. A.J. FerrerCorrcia, R.I. Massey, K.R. Jenning, J.H. Futtrell and P.M. Llewellyn. Interu. J. Mass Spectrom. Ion Phys. 5 (1970) 393. MB. Conisarow,.J. Chem. Phys:55 (1971) 20.5. W.J. van der Hart, Chem. Phys. Letters 23 (1973) 93. T.B. htchiahon and J.L. Beauchamp, Rev. Sci.Instr. 42 (1971) 1632.