A “Time-of-Flight Mass Spectrometer” (hereinafter, TOFMS) is a type of device used for performing a mass analysis by measuring the time of flight required for each ion to travel a specific distance and converting the time of flight to the mass-to-charge ratio. This analysis is based on the principle that ions accelerated by a certain amount of energy will fly at different speeds corresponding to their mass-to-charge ratio. Accordingly, elongating the flight distance of ions is effective for enhancing the mass resolving power (or resolution of the mass-to-charge ratio m/z values). However, the elongation of a flight distance along a straight line requires an enlargement of the device. Given this factor, Multi-Turn Time-of-Flight Mass Spectrometers (Multi-Turn TOFMS; hereinafter, MT-TOFMS) have been developed in which ions are made to repeatedly fly along a closed orbit such as a substantially circular shape, substantially elliptical shape, substantially “8” figure shape, or other shapes, in order to simultaneously achieve the elongation of the flight distance and the downsizing of the apparatus (refer to Patent Documents 1 and 2, and other documents).
Another type of device developed for the same purpose is the multi-reflection time-of-flight mass analyzer, in which the aforementioned loop orbit is replaced by a reciprocative path in which a reflecting electric field is created to make ions fly back and forth multiple times and thereby elongate their flight distance. Although the multi-turn time-of-flight type and the multi-reflection time-of-flight type use different ion optical systems, they are essentially based on the same principle for improving the mass resolving power. Accordingly, in the context of the present description, the “multi-turn time-of-flight type” should be interpreted as inclusive of the “multi-reflection time-of-flight type.”
As previously described, a MT-TOFMS can achieve a high level of mass resolving power. However, it has a drawback due to the fact that the flight path of the ions is a closed orbit. That is, as the number of turns of the ions increases, an ion having a smaller mass-to-charge ratio and flying faster overtakes another ion having a larger mass-to-charge ratio and flying at a lower speed. If such an overtaking of the ions having different mass-to-charge ratios occurs, it is possible that some of the peaks observed on an obtained time-of-flight spectrum correspond to multiple ions that have undergone a different number of turns, i.e. traveled different flight distances. This means it is no longer ensured that the mass-to-charge ratio and the time of flight uniquely correspond, so that the time-of-flight spectrum cannot be directly converted to a mass spectrum.
Because of the aforementioned drawback, in conventional MT-TOFMSs, ions are selected in advance among the ions that originate from a sample generated in an ion source so that their mass is limited to a range (i.e. range of mass-to-charge ratio m/z values) where the aforementioned overtaking will not occur. The selected ions are made to fly along the loop orbit to undergo a predetermined number of turns and then be detected. Although a mass spectrum with a high mass resolution can be obtained with such a method, the range of the mass spectrum is significantly limited. This is contrary to the advantage of TOFMSs that a mass spectrum with a relatively wide mass range can be obtained by one measurement.
Given this factor, a variety of methods have been conventionally proposed for deducing the number of turns of the peaks appearing on a time-of-flight spectrum in order to convert the time of flight to the mass-to-charge ratio. For example, Patent Document 3 proposes a method in which the results obtained by performing a plurality of mass analyses of the same sample under different conditions are compared to deduce the number of turns of the peaks appearing on a spectrum. Although such a method is effective, the data processing will be inevitably complicated. Moreover, the deduction of the number of turns is difficult particularly when the number of components contained in the sample is large.
Patent Document 4 proposes a method in which a multi-correlation function of plural time-of-flight spectra taken at different timings of deviation of ions from the loop orbit is computed to reconstruct a time-of-flight spectrum for a single turn. In this method, the following formula (1) is used to obtain the intensity G(T) of an ion with a flight time T on the loop orbit from plural sets of time-of-flight spectrum data obtained by performing a plurality of mass analyses under ejection timings that give different numbers of turns:
                                          G            ⁡                          (              T              )                                =                                    ∫              yl              yu                        ⁢                                          H                ⁡                                  [                                                            F                      ⁢                                                                                          ⁢                      1                      ⁢                                              {                                                                              N                            ⁢                                                                                                                  ⁢                            1                            ⁢                                                          (                              T                              )                                                        ×                            T                                                    +                          y                                                }                                                              ,                                          F                      ⁢                                                                                          ⁢                      2                      ⁢                                              {                                                                              N                            ⁢                                                                                                                  ⁢                            2                            ⁢                                                          (                              T                              )                                                        ×                            T                                                    +                          y                                                }                                                              ,                    …                    ⁢                                                                                  ,                                          Fr                      ⁢                                              {                                                                              N                            ⁢                                                                                                                  ⁢                                                          r                              ⁡                                                              (                                T                                )                                                                                      ×                            T                                                    +                          y                                                }                                                                              ]                                            ⁢                              ⅆ                y                                                    ,                            (        1        )            where Fj (j=1, 2, . . . , r) is the intensity of an ion with the number of turns Nj retrieved from the measurement data, y is the deviation of flight time, yl is the lower limit value of the deviation time, yu is the upper limit value of the deviation time, and H is a function determined by the values of the variables Fj. As specific examples of the function H, the arithmetic mean, the minimum value, the geometric mean, the harmonic mean, and other values are proposed. However, it is suggested that, in order to eliminate a pseudo peak which happens to have a large Fj value, the definition of the function H is preferably determined so that, among a variety of magnitudes of Fj, smaller values are more significantly reflected in the function H than larger values.
Patent Document 4 points out that, an insensitive period in which some ions travelling on the loop orbit are not detected may occur depending on the timing at which ions are ejected from the loop orbit. However, this document fails to propose any measures against it. This insensitive period occurs due to the fact that the gate electrode (ion mirror) for deviating ions from the loop orbit has a finite length and therefore an ion passing the gate electrode at the point in time when the turning ions are made to deviate is not ejected in an appropriate direction (i.e. the direction in which the ion can be detected by the detector). In the case where a processing is performed in accordance with H which is defined in such a manner as to lay weight on smaller values of Fj rather than larger values as previously described, in particular, in the case where the geometric mean or the harmonic mean is used, if any peak having an intensity of 0 is contained in peak intensities Fj, the peak will be excluded from the reconstructed time-of-flight spectrum. That is, the peaks of ions which are not observed in a plurality of mass analyses for different numbers of turns due to the insensitive period as previously described will not appear on the reconstructed spectrum. This may cause the failure of ion detection, resulting in the erroneous determination that a component that should be contained in the sample is not contained.
[Patent Document 1] JP-A 2006-228435
[Patent Document 2] JP-A 2008-27683
[Patent Document 3] JP-A 2005-116343
[Patent Document 4] JP-A 2005-79049