Quadrupole trap improved technique for ion isolation

A method for isolating an ion in a QIT (1) employing values from a mass axis calibration chart to establish the maximum DAC value to scan to in order to scan out m(p)-1 and less during ramp up of RF trapping field while applying a specifically selected fixed supplemental frequency applied during said calibration; and employing values from the calibration curve to establish the DAC value to scan out m(p)+1 and greater during ramping down of RF trapping field, while applying a previously determined fixed broadband spectrum to the QIT end caps.

FIELD OF THE INVENTION 
This invention relates to an improved method and apparatus for isolating an 
ion of interest in a quadrupole ion trap. 
BACKGROUND OF THE INVENTION 
The quadrupole ion trap (QIT) was first disclosed in the year 1952 in a 
paper by Paul, et al. This paper disclosed the QIT and the disclosure of a 
slightly different device which was called a quadrupole mass spectrometer 
(QMS). The quadrupole mass spectrometer was very different from all 
earlier mass spectrometers because it did not require the use of a magnet 
and because it employed radio frequency fields for enabling the separation 
of ions, i.e. performing mass analysis. Mass spectrometers are devices for 
making precise determination of the constituents of a material by 
providing separations of all the different masses in a sample according to 
their mass to charge ratio. The material to be analyzed is first 
dissociated/fragmented into ions which are charged atoms or molecularly 
bound groups of atoms. 
The principle of the quadrupole mass spectrometer (QMS) relies on the fact 
within a specifically shaped structure radio frequency (RF) fields can be 
made to interact with a charged ion so that the resultant force on certain 
of the ions is a restoring force thereby causing those particles to 
oscillate about some reference position. In the quadrupole mass 
spectrometer, four long parallel electrodes, each having highly a precise 
hyperbolic cross sections, are connected together electrically. Both dc 
voltage, U, and an RF voltage, V.sub.0 cos .omega.t, can be applied. When 
an ion is introduced or generated within the spectrometer, if the 
parameters of the quadrupole are appropriate to maintain the oscillation 
of those ions, such ions would travel with a constant velocity down the 
central axis of the electrodes at a constant velocity. Parameters of 
operation could be adjusted so that ions of selected mass to charge ratio, 
m/e, could be made to remain stable in the direction of travel while all 
other ions would be ejected from the axis. This QMS was capable of 
maintaining restoration forces in two directions only, so it became known 
as a transmission mass filter. The other device described in the above 
mentioned Paul, et al. paper has become known as the quadrupole ion trao 
(QIT). The QIT is capable of restoring forces on selected ions in all 
three directions. This is the reason that it is called a trap. Ions so 
trapped can be retained for relatively long periods of time which supports 
separation of masses and enables various important scientific experiments 
and industrial testing which can not be as conveniently accomplished in 
other spectrometers. 
The QIT was only of laboratory interest until recent years when relatively 
convenient techniques evolved for use of the QIT in a mass spectrometer 
application. Specifically, methods are known for creating ions of an 
unknown sample after the sample was introduced into the QIT, and adjusting 
the QIT parameters so that it stores only a selectable range of ions from 
the sample within the QIT. Then by linearly changing, i.e., scanning, one 
of the QIT parameters it became possible to cause consecutive values of 
m/e of the stored ions to become successively unstable. The final step in 
a mass spectrometer was to sequentially pass the separated ions which had 
become unstable into a detector. The detected ion current signal 
intensity, as a function of the scan parameter, is the mass spectrum of 
the trapped ions. 
U. S. Pat. No. 4,736,101 describes a quadrupole technique for performing an 
experiment called MS/MS. In U. S. Pat. No. 4,736,101, MS/MS is described 
as the steps of forming and storing ions having a range of masses in an 
ion trap, mass selecting among them to select an ion of particular mass to 
be studied (parent ion), disassociating the parent ion by collisions, and 
analyzing, i.e. separating and ejecting the fragments (daughter ions) to 
obtain a mass spectrum of the daughter ions. To isolate an ion for 
purposes of MS/MS the '101 patent discloses a method of scanning (ramping 
up) the RF trapping field voltage according to known equations to eject 
ions having atomic mass up to the m/e of ion of interest. Then, the RF 
trapping field voltage is lowered and the ions remaining are disassociated 
by collision. Finally, the RF trapping voltage is scanned up again and a 
mass spectrogram of the ejected daughter ions is obtained. One technique 
for obtaining collision induced disassociation (CID) to obtain daughter 
ions is to employ a second fixed frequency generator connected to the end 
plates of the QIT which frequency is at the calculated secular frequency 
of the retained ion being investigated. The secular frequency is the 
frequency in which the ion is periodically, physically moving within the 
RF trapping field. 
The '101 patent also discloses use of a supplementary RF field voltage 
applied to the end cap electrodes of a QIT containing daughter ions while 
the RF trapping field is being scanned as a means of successively ejecting 
increasing mass ions to obtain a spectrum. In this instance, the patent 
employs a reduced maximum magnitude of the RF trapping field voltage. 
The difficulty with the technique of the '101 patent is that after the 
ionization step, the parent ion, m(p), is selected for MS/MS using the so 
called mass instability method. This is where one of the quadrupole 
parameters, i.e. the RF field voltage, is varied to move the ions having 
M/e outside the range of interest into the instability region, i.e. 
q.sub.z &gt;0.908. In the '101 patent this was accomplished by ramping up the 
RF trapping field voltage to cause those ions having M/e less than the 
selected parent ion, m(p), to be ejected. Ions of mass greater than m(p) 
are retained in the trap. The voltage level of the RF trapping field is 
then lowered and CID accomplished. This means that ions having greater 
than the M/e of the selected m(p) were present during CID. These ions can 
cause interference and/or unwanted reactions or daughter ions. 
The problem of incomplete isolation in MS/MS of the parent m(p) ion is 
addressed in U. S. Pat. No. 4,749,860. In this prior patent, a second, 
supplemental RF field is applied to the end caps. The frequency of this 
supplemental RF field corresponds to the secular frequency of a specific 
ion having a M/e value which is one M/e unit greater than the selected 
parent ion, i.e. m(p)+1. The '860 patent applied this supplement RF field 
to the end caps simultaneously with the application of the ramping of the 
voltage of the RF trapping field to the ring electrodes. There are at 
least three problems with this '860 approach. First, the use of mass 
instability scanning to eject ions of mass less than m(p) suffers from 
poor mass resolution and thus results in significant loss in the intensity 
of the m(p) ion while attempting to completely move the m(p)-1 ion out of 
the stability region. Second, the stability boundary on the high side is 
flat so that this procedure also suffers significant loss of the m(p) ion 
when trying to eliminate the m(p)+1. 
Finally, to use the '860 technique, it is essential to know the precise 
value of the trapping field operating on the ions in order to calculate 
the precise frequency to apply to the supplementary field. This precise 
frequency is difficult to know because of mechanical or electrical 
imperfections and because of space charge effects which act to 
significantly shift the stability region. The equation used to calculate 
the supplemental frequency which is given in the '860 patent is 
W=1/2.beta..sub.z W.sub.o, where W.sub.o is the frequency of the RF trap 
field. 
The value .beta..sub.z is known to be defined by several approximating 
formulas, each of which are known to be accurate only for regions of the 
stability chart for lower values of the q.sub.z. Accordingly, it has 
become common to apply the supplemental frequency to eliminate the high 
m(p)+1 values at low values of q.sub.z parameter. In this low q.sub.z 
region, the relationship between the mass and resonant frequency is 
non-linear and the resolution at usual scan speed is poor. Furthermore, 
there is a limit to the maximum mass which can be ejected by this 
technique. To increase the value of the RF field beyond this value will 
also eject the parent ion of interest. To reach these higher mass value 
ions, the '860 patent adds an additional step of frequency scanning the 
supplemental frequency downward to low frequencies. This frequency 
scanning technique requires complex equipment and also introduces 
undesirable additional process time into the isolation process. 
U.S. Pat. No. 4,762,545 discloses a technique called tailored excitation 
ion spectroscopy for employing Fourier synthesized excitation to create a 
time domain excitation waveform to cause tailored ejection of specific 
bands or ranges of ions. As pointed out in the '545 patent, the tailored 
FT method requires an extremely high power amplifier with high voltage 
output unless phase scrambling is employed. U.S. Pat. No. 4,945,234 
discloses that phase scrambling distorts the excitation spectrum so that 
it is not possible to achieve arbitrary excitation frequency spectra at 
suitable low peak excitation voltages at the same time and that 
corrections are required for certain so called Gibbs oscillations. FT 
tailored excitation requires very expensive computational and RF 
synthesization equipment in order to be capable of tailoring to any 
desired frequency components. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide an improved method for 
isolating an ion, particularly useful for MS/MS requiring simpler and less 
expensive equipment. 
It is a further object to provide ion isolation methods and apparatus 
having high resolution, permitting isolation of a parent ion without loss 
of the parent ion intensity. 
It is a feature of my invention that it uses a calibration of the mass axis 
of the trap along with specifically selected supplemental generator 
frequencies to eject ions above and below the selected ion. 
It is a feature of my invention that my method employs a single, 
specifically fixed frequency supplemental field which is used to 
efficiently eject all ions of lower mass number than m(p) without 
requiring calculations by the user of the secular frequency for each m(p). 
It is a further feature of my invention that it employs a broad band 
generator having a fixed spectra for resonance ejection of all ions having 
mass numbers greater than m(p).

BRIEF GENERAL DESCRIPTION OF THE INVENTION 
I have devised a technique using an empirical calibration procedure 
combined with one of the known techniques for sequentially scanning ions 
out of a QIT to precisely eject all ions up to and including the ion one 
atomic mass units, that is m(p)-1, less than the ion mass m(p) which is 
selected to be isolated. My technique exhibits both efficiency and high 
resolution so that substantially no m(p) ions are lost when ejecting the 
m(p)-1 ions using my procedure. This can be critical when the selected ion 
is very low concentration. 
As described in U.S. Pat. No. 4,736,101, a supplemental oscillator at a 
fixed frequency connected to the end caps of a QIT will sequentially 
resonantly eject ions from the QIT to a detector when the RF trap field 
voltage is scanned upward according to a linear ramping function of time. 
The RF scanning also produces scanning of the secular frequencies of each 
ion species in the QIT and when that secular frequency matches the 
frequency of the supplementary oscillator, the particular species will 
resonantly absorb energy and become ejected from the trap. 
I have discovered a novel way to use this previously known sequential QIT 
ejection processes and the known mass calibration procedure to precisely 
and efficiently eject the ions up to and including the ion one atomic mass 
unit, less than e.g., m(p)-1, a selected parent ion m(p) which parent is 
previously selected for isolation storage in the QIT. 
First, I use a particularly selected supplemental fixed frequency. The 
selection process will be explained subsequently. 
Second, as known in the art, I establish the calibration curve for the 
particular QIT to create a precise empirical relationship between the 
setting of the digital to analogue converter (DAC) 10 for the RF trapping 
voltage and the mass of the ion which is resonantly ejected and detected 
at the selected fixed supplemental field for the particular values of DAC 
setting, i.e. RF trapping field. The calibration curve is established 
using a calibration gas (PFTBA) which has masses at well known values 
distributed across the mass regions of interest. 
After obtaining the calibration, one is prepared to run the experiment and 
to eliminate all ions of an ionized sample of m/e less than and including 
m(p)-1. From the calibration chart prepared above, I can now selected the 
value of the DAC which will cause ejection of any selected m/e value. 
Since I know the parent ion, m(p), that I wish to isolate, I ramp the DAC 
value up to the value for the DAC from the calibration curve for the 
m(p)-1 ion while the supplemental generator is enabled at the selected 
frequency for which the calibration curves were developed. 
When the RF storage field potential 11 is ramped up to the m(p)-1 value 
commanded by the value of the DAC set in the above step, this will cause 
ejection of ions m(p)-1 and lower mass, and leave remaining all ions m(p) 
and greater in the trap. 
My technique for selecting the fixed supplemental frequency to be used 
above is important. It can be shown that any frequency can be selected as 
the supplemental frequency and as the RF voltage is ramped, the various 
masses will increase in value of q until their secular frequency equals 
the supplemental frequency resulting in ejection. However, the resolution, 
i.e., ability to selectively resonant one ion value m/e without exciting 
m/e+1, depends on the number of cycles of the supplemental field that the 
ion experiences during the excitation process. Accordingly, at a given 
scan rate, dv/dt, it follows that the maximum number of cycles of 
interaction will be obtained at the maximum frequency of the supplemental 
field. 
The maximum limit of the secular frequency occurs when .beta..sub.z =1. 
This is where q.sub.z =0.908 which is the stability boundary for all the 
ions. In practice, I have discovered an undesirable beat phenomena occurs, 
when .beta..sub.z =1. Accordingly, the actual supplemental frequency is 
selected to be somewhat less than 1/2 the trapping frequency. I have found 
that .beta..sub.z =0.923 results is no beating and provides good 
resolution at reasonable scan speeds. 
The next steps in my procedure to isolate the selected m/e ion in the QIT 
is to remove the ions having m/e values greater than the selected ion. 
At the previously calibrated value of the RF field voltage, V.sub.m-1 for 
which the m(p)-1 ion was ejected, there will be the corresponding 
calibrated value of q for those ions of m/e greater than m(p)-1. In 
general, for V.sub.m-1 there are masses (m+i) and corresponding 
(q.sub.m+i) and thus (.beta..sub.m+i) and (W.sub.m+i) for all such masses. 
Since (m+1) ion is close to the m-1 ion for which the relationship between 
RF trapping field voltage (and thus DAC value) and mass had been 
established by calibration, the relationship between the secular resonant 
frequencies can be expressed as W.sub.m+1 =W.sub.m-1 +.DELTA.W. I have 
discovered that this expression is independent of the exact value of m/e 
in the regions for which the mass axis has been calibrated. Accordingly, 
once the resonance frequency corresponding to (m+1) is found by 
calibration at any mass (m), the system is piecewise calibrated exactly at 
all masses (m+i) displaced from a mass m for which the mass axis had been 
calibrated. 
In theory, it is possible to determine .DELTA.W and calibrate the (m+1) ion 
resonance by varying the frequency, but it is more straight forward and 
easier to first fix the frequency of the supplemental field at a value 
corresponding to ion (m+j) for V.sub.m-1 where j=2, 3 or 4. 
Then, the trapping field is iteratively decremented, i.e., scanned down, by 
a small value (.DELTA.V) until the ion m+1 is observed to disappear. The 
final calibrated value of the trapping field is thus V=V.sub.m-1 
-.DELTA.V. 
While the value of .DELTA.V could be determined for each calibration ion 
that was used to create the piecewise linear calibration curve, in 
practice the same offset has been found adequate for most all mass values. 
The commonly used calibration gas in PFTBA (perfluorotribulylamine) since 
it has several well known intense ions at masses from 31 up to 614 and 
each has an isotope at (m+1). Thus the nearby major ion can be used for 
calibration of the mass axis and the isotope is ion at (m+1) can be used 
for determining the trapping field offset voltage (.DELTA.V). 
This procedure provides the precise control required to resonantly eject 
(m+1), ions without loss of the selected parent ion (m). To eject any 
other ion of m/e greater than (m+1) does not require as much care. By 
providing a plurality of frequencies in a composite broadband waveform 
when the frequencies are spaced less than the width of the ion resonance, 
the remaining ions can be ejected. If the trapping voltage offset begins, 
as described above, at a value less than .DELTA.V and increases to 
.DELTA.V, then all the resonant frequencies corresponding to higher masses 
will be swept by the frequencies that are in the composite waveform. The 
scanning reduces the need to have the frequency spacing in the broadband 
waveform less than the width of the resonance. 
DETAILED DESCRIPTION OF THE INVENTION 
With reference to FIG. 1, the quadrupole ion trap 1 employing a ring 
electrode 2 of hyperbolic configuration is shown connected to a radio 
frequency trapping field generator 7. The digital-to-analogue converter 
(DAC) 10 is connected to the RF trapping field generator 7 for controlling 
the amplitude of the output voltage 11. In this schematic, the hyperbolic 
end caps 3 and 3' are connected to winding 4 of a coupling transformer 8 
having a center tap 9 connected to ground. The transformer 8 secondary 
winding is connected to a fixed frequency generator 5 and to a fixed 
broadband spectrum generator 6. Controller 12 is connected to DAC 10 via 
connector 18 and the three generators 5, 6 and 7 via connectors 13, 14 and 
19 respectively to manage the timing of the QIT sequences. 
With reference to FIG. 2 timing diagrams, the inventive method of using the 
apparatus of FIG. 1 is described. In FIG. 2(b), there is shown the RF 
trapping field waveform 11 representative of the change as a function of 
time of the RF storage field potential output (v) of the trapping field RF 
generator (7) used as part of the process to isolate a selected parent ion 
of mass/charge ratio m(p). The sample material to be analyzed is 
introduced into the trap and caused to be ionized in the trap by electron 
impact or chemical ionization by ionization apparatus (not shown). The 
ionization takes place during the time B-1, FIG. 2(b), during which time 
the RF voltage (v) is raised a small amount to a voltage level V.sub.1, 
selected to cause the trap to store a selected range of masses including 
m(p), as will be explained subsequently. Immediately after ionization, the 
RF trapping field is ramped from V.sub.1 to V.sub.2. During at least a 
portion of the ramping time, the fixed frequency generator 5 is turned on, 
FIG. 2(a), to induce resonant ejection of all the ions of mass/charge 
ratio less than and including m(p)-1. As stated earlier, the frequency of 
generator 5 should be slightly less than 1/2 the frequency of RF trapping 
field generator 7. It was known in the prior art to ramp increase the RF 
trapping field to sequentially eject, in ascending order the low mass to 
high mass ions by the so called destabilizing technique known as mass 
instability scanning. In my method, in addition to the RF trapping field 
ramp, I simultaneously apply a fixed frequency to the end caps equal to 
approximately 1/2 the RF trapping field frequency as the RF voltage 
supplemental frequency from generator 5 to resonant with the secular 
frequency of the ions. 
In my invention, after calibration of the mass axis of the QIT is 
completed, no calculations are necessary to determine the secular 
frequency and the fixed frequency generator 5 does not need to be adjusted 
in frequency during an experiment. In fact, the fixed frequency generator 
5 should be set at approximately 485.0 KHz for and RF Trapping frequency 
of 1.05 MHz. This single fixed frequency RF generator can be used for 
ejection of ions m(p)-1 for all m(p) up to greater than 700. This 
significantly simplifies both the quadrupole apparatus and the method of 
using such apparatus. 
According to the theory, for a fixed radius trap operating at a fixed RF 
frequency, F, the relationship of the RF trapping field voltage, V, the 
mass/charge ratio and the parameter q.sub.z are related as follows: 
##EQU1## 
For a device where r=1.times.10.sup.-2 meters and F=1.0 MHz 
##EQU2## 
where m is in atomic mass units and V is in volts. 
The equation to determine the secular frequency of resonance is: 
##EQU3## 
FIG. 4 illustrates the relationship between the parameter .beta..sub.z and 
q.sub.z. There are several approximating equations which have been used to 
relate .beta..sub.z to q.sub.z, as shown in FIG. 4. Equation (1) FIG. 4 is 
accurate for q.sub.z &lt;0.4. Equation (2) FIG. 4 is accurate for q.sub.z 
&lt;0.6. Equation (3), is derived by the method of successive approximations 
and is accurate in the region near q.sub.z =0.9. At q.sub.z =0.908, it is 
known that theoretically .beta..sub.z =1. The relationship between 
.beta..sub.z and q.sub.z is highly significant in the context of this 
invention. Until my invention, one needed to determine the secular 
resonance frequency for any ion to be ejected by calculation. In order to 
determine the secular frequency for exciting a particular ion, one needed 
to first determined the precise value of .beta..sub.z. However, even 
without considering the shifts due to space charge or mechanical effects, 
it is extremely difficult to determine .beta..sub.z theoretically near 
q=0.908. 
Equations (1), (3) and those equations on FIG. 4, show the relationship 
between the fundamental parameters of the trap and the secular resonant 
frequencies. For a given value of q from equation (1), it can be seen that 
by increasing V, sequential values of M are brought to the same value of 
q. From equation (3), the resonant frequency W.sub.s of the ion depend on 
.beta. and .beta. is also a function of q. Thus by choosing a value of the 
supplemental frequency W.sub.s applied to the end caps and by ramping V, 
the various masses will increase in their of q and W.sub.s until W.sub.s 
equals the supplemental frequency and the ion absorbs energy and is 
ejected. 
The mass axis has been calibrated as shown in FIG. 3 for a fixed value of 
supplemental frequency. Ideally, m is linearly related to V and to the DAC 
control value. Using a calibration gas (PFTBA) with masses at well known 
values distributed across the mass range of interest, a piecewise linear 
calibration curve is determined between the DAC value and the mass of the 
ion that is resonantly rejected for the fixed supplemental field. This 
curve establishes the DAC values to bring a given mass into resonance with 
the fixed supplemental field. With the mass axis calibration established 
for resonance ejection, to isolate any particular mass (m), i.e. mc3, FIG. 
3 within the calibrated range, the DAC value corresponding to the mass 
(m-1), i.e., DAC 2 for mc2 is taken from the calibration curve and set 
into the DAC 10 (FIG. 1) as the maximum value of the RF voltage ramp 
during portion 22, FIG. 2(b). As the RF voltage 11 ramps up, the ions up 
to and including (m-1), i.e., mc2 are ejected from the trap. 
It is next necessary to eject those ions having mass numbers greater than 
m(p). To eject those ions near m(p), I use a similar concept. I determine 
another calibration for the QIT. By setting the frequency of the 
supplemental frequency generator connected to the end caps to a value 
corresponding approximately to the secular frequency for one of the close 
ions, (m+j), where j=1, 2 or 3 for the same value of maximum DAC used 
earlier to eject (m-1), and by decrementing RF trap voltage (DAC) until 
the ion at m+1 is ejected, I can calibrate the value .DELTA.V or 
.DELTA.DAC to eject the m+1 ion. I have determined that .DELTA.DAC so 
determined is adequate for all values of mass to eject the (m+1) ion. 
In my preferred procedure, when the supplemental broadband generator 6 
waveform which includes composite frequencies, one of which is the secular 
frequency for resonating (m+j), is exciting the QIT and by ramping the RF 
field voltage the amount .DELTA.V, down, i.e., decrementing the DAC to the 
previously calibrated value .DELTA.V, those ions (m+j) to (m+1) will be 
ejected. As shown in FIG. 2(c1), a broadband supplementary AC field 
supplied by broadband frequency generator 6 is switched on and applied to 
the trap end caps. This field corresponds to frequencies for resonance of 
m(p)+3 in the range of 420-460 KHz down to 10-20 KHz for masses 600-700. 
The broadband frequency distribution could be a series of discrete 
frequencies equally spaced as in FIG. 2(c1) or can be continuous as in 
FIG. 2(c2), or it could be non-uniformly spaced in the frequency domain. 
Alternatively, the ejection of ions m(p)+1 and greater could be effected by 
using a fixed supplemental generator waveform which contains a discrete 
collection of frequencies which are spaced apart less than the width of 
the ion secular resonance, or a continuum of frequency as depicted in FIG. 
2(c2) such as would be obtained by filtering random noise with a low pass 
filter so as to provide a sharp frequency cut-off at the desired 
frequency, corresponding to M+1. For these closely spaced supplemental 
frequencies, the RF trapping field could remain at a constant value as 
depicted by 22-2 in the waveform of the RF storage field potential, FIG. 
2(b). 
FIG. 5 is a frequency spectrum of the broadband waveform of generator 6 
which has been used to resonantly eject all the ions of mass number 
greater than m(p)+1. This spectrum was created by summing 1000 discrete 
frequencies, between 20 KHZ and 420 KHZ, that were equally spaced with 
their phases calculated by a random number generator. The cut-off at high 
frequencies in the frequency spectrum is very sharp, such as -26 db in 2.5 
KHz. Alternatively, the broadband waveform could be obtained by means of 
digitally filtered noise which contains no gaps or notches in the 
frequency spectrum created. Additionally, as described in combination with 
the ramping down voltage of FIG. 2(b), 22-1, the ensemble of frequencies 
could be wider apart than the width of the resonance line, FIG. 2(c1) 
because the RF trapping fields voltage is decremented which causes the 
intermediate ions to come into resonance with the applied frequencies. 
The invention herein has been described with respect to specific figures. 
It is not my intention to limit my invention to any specific embodiment, 
but the scope of my invention should be determined by my claims. With this 
in view,