1. Field of the Invention
The present invention relates to the calibration of a mass spectroscope that uses an ion trap.
2. Description of Related Art
An ion trap mass spectroscope comprises a ring electrode having a hyperboloid of revolution of one sheet on the inner surface, and a pair of end-cap electrodes disposed opposite each other across the ring electrode and having a hyperboloid of revolution of two sheets on the inner surface. The space surrounded by the ring electrode and the end-cap electrodes forms an ion trapping area. When a predetermined high-frequency voltage is applied to the ring electrode and the pair of end-cap electrodes, a three-dimensional quadrupole electric field is formed in the ion trapping area, in which ions that are either internally generated or introduced from the outside can be trapped. The thus trapped ions move around in the space inside the electrodes at a frequency specific to their mass. In the ion trap apparatus, by applying an auxiliary AC voltage with a frequency corresponding to the mass of each ion to the end-cap electrodes while holding the trapped ions, a target ion can be caused to resonate and the amplitude of its motion can be increased, thereby allowing the ion to be discharged from the ion trap.
On the other hand, by applying a wideband noise that does not include the frequency components corresponding to the ion with a specific mass, end-cap electrode ions of other masses can be discharged from the electrodes while leaving the ion of the specific mass behind. Thereafter, if a weak resonance frequency voltage corresponding to the selected ion is applied, the motion energy of the ion can be increased while the selected ion is kept inside the ion trap. Consequently, the ion in the ion trap repeatedly collides with the helium gas or the like introduced in the ion trap, resulting in the dissociation of the ion (CID). This series of operations is referred to as MS/MS. By making a comparison between the original spectrum and the spectrum obtained after ionic dissociation, structural information about an organic compound can be obtained. Thus, MS/MS is a very important analysis technique in a variety of fields including pharmacy, biochemistry, and environment.
JP Patent Publication (Kokai) No. 7-14540 A (1995) (Patent Document 1) discloses an example of the conventional technique.
With regard to the target ion of a specific mass, it is necessary to accurately determine the value of a corresponding resonance frequency under specific conditions.
When a DC voltage U and a high-frequency voltage V cosΩt are applied between the individual electrodes, a three-dimensional quadrupole electric field is formed in the space between those electrodes. The orbital stability of an ion trapped in this electric field is determined by the values a and q (Equations (1) and (2)) given by an internal radius r0 of the ring electrode, DC voltage U applied to the electrodes, amplitude V of the main high-frequency voltage and its angular frequency Ω, and the mass-to-charge ratio m/z of the ion.
where z is the valence of the ion, m is mass, and e is elementary charge.
In conventional products, the DC voltage U is often not used, so that a=0. As a result, only Equation (2) becomes important. The angular frequency ω of the vibration specific to the ion of a particular mass can be calculated as follows.
ω≈qΩ/2√{square root over (2)} (3)
Thus, if r0 and Ω are fixed values, the vibration frequency of the ion of a specific mass in the ion trap can be uniquely determined by setting the amplitude (voltage) V of the high-frequency voltage at a certain value.
Actually, however, subtle deviations are produced in the actual ion and its resonance frequency from the calculated values due to factors such as subtle variations in the high-frequency voltage applied to the ring electrode and the pressure in the ion trap. Accordingly, the resonance frequency or the value of the main high-frequency voltage must be corrected periodically and for each apparatus. In other words, calibration must be performed, which generally involves the following operations.
At first, a standard sample of which the observed mass is known in advance is prepared, and the sample is introduced into an ion source at a fixed flow rate with use of a sample introducing device such as a liquid feed pump. The sample, which is fed continuously, is ionized in the ion source and introduced into a vacuum system via a sampling unit, before it is introduced into an ion trap via an ion transport unit. After the introduced ion has been trapped, a fixed main high-frequency voltage is applied to the ring electrode and, in this condition, an auxiliary AC voltage of a frequency that in calculation corresponds to the mass of the standard sample ion is applied to the end-cap electrodes, thereby causing the target ion to resonate and to be discharged from the ion trap. If there is any deviation, no resonance would occur at the calculated value setting, and the ion would not be discharged.
Therefore, the frequency of the main high-frequency voltage applied to the ring electrode or that of the auxiliary AC voltage applied to the end-cap electrodes is shifted slightly each time ions are fed to the detector, and a change in ion intensity is detected. When a condition under which resonance occurs is satisfied finally, ions are discharged from the ion trap and the ion intensity of the resultant spectrum decreases, thereby allowing the amount of difference between the calculated value and the actual value to be determined. Based on this result, the auxiliary AC frequency applied to the end-cap electrodes or the main high-frequency voltage applied to the ring electrode is corrected, thus completing the calibration process.
[Patent Document 1] JP Patent Publication (Kokai) No. 7-14540 A (1995)
In the above calibration operation, the condition under which ions actually resonate and are discharged from the ion trap is determined by finely adjusting either the frequency of the auxiliary AC voltage applied to the end-cap electrodes that has a resonance frequency corresponding to the ion in the standard sample to be calibrated, or by finely adjusting the amplitude (voltage) of the main high-frequency voltage applied to the ring electrode. The ion intensity of the observed ion is recorded under varying conditions, until a resonance point is determined at which the lowest ion intensity is obtained. The standard sample is introduced into the ion source at a fixed flow rate during calibration such that a constant amount of ions can be supplied to the ion trap stably.
Actually, however, it is very difficult to keep supplying ions stably into the ion trap due to such troubles as deterioration of the performance of the pump that introduces the sample into the ion source, choked pipe, unstable operation of the ion source itself, and decrease in the efficiency of ionization caused by the contamination of the ion source, for example. In many cases, the ion intensity tends to change periodically or decrease with time.
It is therefore an object of the invention to provide a mass spectroscope capable of accurately and highly reliably determining the ion resonance condition, and a method of calibrating the spectroscope.
The present invention is characterized in that, during the process of acquiring spectrum data continuously, measurements are made while alternately applying and not applying a resonance frequency voltage. Data obtained in the absence of application of the resonance frequency voltage is used as reference data to correct a resonance condition setting data.
According to the present invention, each time ions are introduced into the ion trap, data corresponding to the total amount of ions introduced to the ion trap is measured and is then used as a reference for correction purposes. It is thus possible to find such a set condition under which ions can be discharged from the ion trap with highest efficiency by taking into consideration the ion intensity changes (fluctuation) due to such factors as problems in the pump, the ion source, etc. In other words, the condition under which ions are actually resonant is observed more accurately to realize highly reliable calibration.
Hereunder, the preferred embodiments of the present invention will be described with reference to the accompanying drawings.
The mass spectroscope in this first embodiment, as shown in
The mass spectroscope in this embodiment further includes a control unit 18 and a data processing unit 20. Signal lines 19 are used for connections between the ring electrode 15 and end-cap electrodes 16 of the ion trap, the ion source 12, and the control unit 18; between the detector 17 for detecting an ion intensity for each mass and the data processing unit 20; and between the control unit 18 and the data processing unit 20.
The data processing unit 20 sends an ion trap control condition to the control unit 18 according to an input from the user. The data processing unit 20 is capable of controlling the ion trap at high speed of the μsec (microsecond) order. The data processing unit 20 receives, through the signal line 19, mass spectrum data detected by the detector 17 as a result of controlling the ion trap, and then processes the data before recording or displaying it.
The mass spectroscope in this embodiment uses an ion trap having a mass analyzing unit consisting of the ring electrode 15 and the pair of end-cap electrodes 16. The mass analyzing unit applies a main high-frequency voltage to the ring electrode 15 to form a three-dimensional quadrupole electric field in a space enclosed by the ring electrode and the pair of end-cap electrodes. A sample ionized in the ion source 12 is introduced into the space enclosed by the ring electrode and the pair of end-cap electrodes and is once held there by the formed three-dimensional quadrupole electric field. After that, when obtaining mass spectrum data, the applied main high-frequency voltage is scanned, thereby ions are discharged to and detected by the detector 17 in an ascending order of the mass. The detected ion current signal is sent to the data processing unit 20 and recorded as mass spectrum data in such a form that the mass-to-charge (m/z) ratio is shown on the horizontal axis at certain time intervals.
A single set of data is normally obtained in about several milliseconds. Samples can be introduced continuously in units of 10 minutes to one hour. Data can thus be obtained repetitively while changing conditions.
The present invention makes good use of such features of the ion trap capable of discharging ions selectively by controlling the electric field as described above.
Next, a description will be made of the flow of a calibration process in the first embodiment with reference to the accompanying drawings.
At first, “1” is set in the algebraical symbol N in the data processing unit 20 (or control unit 18) to start the processing. At the same time, a standard sample of which the observed mass is known in advance is caused to flow into the ion source 12 at a fixed flow rate, using the sample introducing device 10, so that the sample is ionized. This ionization of the standard sample with the known observed mass is continuously performed until the end of the calibration process.
Thereafter, a spectrum 1 is obtained.
After that, a spectrum 2 is obtained.
After that, the intensity ratio of the standard sample ions detected in each of the spectra 1 and 2 is calculated and the resultant data is recorded.
After that, the value in the algebraical symbol N is incremented, and the process then returns to the step for obtaining the spectrum 1. Measurements are made by repeating this series of operations while varying the resonance condition little by little according to the set condition data of
This series of operations is repeated until the value in the algebraical symbol N reaches the number corresponding to the condition preset in the set condition data. When the predetermined count is reached, the set condition of the measurement number (N) with the lowest intensity ratio is searched from among the standard sample ion intensity ratio data that have been recorded. The set condition thus retrieved is assumed to be a true resonance condition for discharging the standard sample ions by resonance.
Next,
As a result, the resonance condition of the ion trap can be calibrated by correcting the 10th condition of the calculated resonance point to the 17th condition.
The calibration in the first embodiment enables both the voltage applied to the ion trap and its frequency to be changed to the optimum condition. And, as a result of the calibration, graphs as shown in
In the first embodiment, the calibration result is obtained as shown in
When the predetermined measurement count N is reached, an average value of all the ion intensity values of the spectrum 1 is obtained. The ion intensity of each data in the spectrum 1 is then divided by this average value to find a change rate of the ion intensity (rate of deviation from the average value). After that, each ion intensity value of the spectrum 2 is divided by this ion intensity change rate to correct the ion intensity value, thereby obtaining the final determined data. The 12th measurement condition having the lowest ion intensity value is thus set as a resonance condition here.
If standard samples are used for a mass spectroscope under a specified condition, it is usually possible to predict roughly what the ion intensity value would be. Therefore, by storing the determined data in this second embodiment, it can be determined whether or not the ion intensity during calibration was stable, as well as whether or not the ions of substantially normal intensity were observed.
In the configuration of the mass spectroscope in this third embodiment, ions discharged from the ion trap travel through the ion transport unit 21, then they are deflected and converged through a deflector 22 and a convergence lens 23. The ions are then accelerated in the orthogonal direction by an ion acceleration unit consisting of a pushing-out electrode 24 and an extraction electrode 25. The accelerated ions are reflected by a reflectron 26, and then reach the detector 27 where they are detected.
Mass spectral data obtained by the time-of-flight type mass spectroscope is superior to that obtained by mass separation with an ion trap in terms of mass accuracy and mass spectral resolution. Because the time-of-flight type mass spectroscope is disposed just after the ion trap as described above, the MS/MS operation can be performed using the ion trap and the generated ions can be analyzed using the time-of-flight type mass spectroscope. Although the size of the apparatus increases, an MS/MS spectrum that has high mass accuracy and high resolution can be obtained.
The calibration operation for the ion trap in this third embodiment is the same as that in the first embodiment except that the mass spectral data is obtained by the time-of-flight type mass spectroscope rather than by the ion trap. Therefore, calibration can be performed by carrying out the processes shown in the flowchart shown in
Even in this third embodiment, the resonance condition used between t2 and t3 in
Number | Date | Country | Kind |
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2003-426103 | Dec 2003 | JP | national |
Number | Date | Country | |
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Parent | 11018375 | Dec 2004 | US |
Child | 11508323 | Aug 2006 | US |