MS/MS analysis, which is one type of mass spectrometric technique, has been widely used for the identification, structural analysis or quantitative determination of a substance in a sample as well as for other analytical purposes. There are various configurations of mass spectrometers for performing MS/MS analyses. One type which is comparatively simple in structure as well as easy to operate and handle is a triple quadrupole mass spectrometer including a collision cell sandwiched between quadrupole mass filters located before and after the collision cell.
Commonly known modes for the MS/MS measurement by triple quadrupole mass spectrometers include a multiple reaction monitoring (MRM) measurement, product-ion scan measurement, precursor-ion scan measurement and neutral-loss scan measurement. In the MRM measurement mode, the mass-to-charge ratio of an ion which is allowed to pass through is fixed in each of the front and rear quadrupole mass filters so as to measure the intensity of a specific product ion for a specific precursor ion. In the MRM measurement, ion intensity signals can be obtained with high signal-to-noise ratios, since the two-stage mass separators remove ions or neutral particles which originate from compounds that are not the measurement target or originate from foreign components. Accordingly, the MRM measurement is particularly effective in such analyses as a quantitative determination of a trace amount of a component.
A triple quadrupole mass spectrometer configured in the preciously described manner may be used independently, although it is often used in combination with a liquid chromatograph (LC) or gas chromatograph (GC). A system which employs a triple quadrupole mass spectrometer as a detector for a GC or LC (such a system may hereinafter be called the GC-MS/MS, LC-MS/MS, etc. according to conventional usage) is commonly used for a quantitative analysis of a compound in a sample containing a considerable number of compounds or a sample containing foreign substances, as well as for similar analyses.
When an MRM measurement for a target sample is to be performed in a GC-MS/MS or LC-MS/MS, it is necessary to set an MRM transition as one of the measurement conditions and relate it to the retention time for a compound (component) as a measurement target in advance of the measurement of a target sample. An MRM transition is the combination of the mass-to-charge ratio of a target precursor ion and that of a target product ion. By setting the most suitable MRM transition for each target compound, the signal intensity of an ion originating from each target compound can be obtained with a high level of accuracy and sensitivity, so that the quantity of the compound can also be determined with a high level of accuracy and sensitivity.
In a normal situation, dissociating an ion derived from one compound yields a plurality of kinds of product ions. Accordingly, there are a plurality of possible combinations of the precursor ion and product ion for the MRM measurement of one compound, among which an appropriate combination should preferably be selected as the MRM transition, such as the one which yields the highest level of signal intensity. Accordingly, in a conventional measurement procedure, a product-ion scan measurement for a sample containing a target compound is initially performed, and MRM measurement conditions including the MRM transition are determined based on the result of this measurement. An MRM measurement for the sample containing the target compound is subsequently performed under the determined MRM measurement conditions, and the quantity of the target compound is determined based on the result of this measurement.
For example, Patent Literature 1 discloses a device which is configured to simplify the task of setting measurement conditions for a product-ion scan measurement (e.g. the mass scan range for the product ions). Specifically, an operator specifies a target compound by a clicking operation (or the like) on a compound information table displayed on a display screen. In response to this operation, the device automatically creates measurement conditions for the product-ion scan measurement for the specified compound. While such efforts have been made to simplify the task to be performed by operators, it is necessary to perform two measurements for one sample, i.e. the product-ion scan measurement and the MRM measurement, in order to acquire quantitative information which corresponds to the content of a target compound in a sample. Therefore, the measurement task is cumbersome, and it is difficult to improve the efficiency of the analysis.
In the case where the compound contained in a sample is unknown, it is impossible to specify a target compound on the compound information table mentioned earlier. In such a case, it is necessary to initially acquire a mass spectrum by a normal scan measurement (i.e. a measurement with no dissociation of an ion) in order to determine the kind of compound contained in the sample. Therefore, the measurement task will be even more cumbersome since there are three measurements to be performed for one sample, i.e. the normal scan measurement, product-ion scan measurement and MRM measurement.
As described earlier, a product ion which satisfies a specific condition, e.g. which yields the highest signal intensity, is selected as the MRM transition from the result of the product-ion scan measurement performed on the target compound. However, this MRM transition is not always the most suitable MRM transition for the quantitative determination of the target compound due to the following reason: When a calibration curve for the quantitative determination is prepared, a result of an MRM measurement performed for samples which respectively contain the target compound at different levels of concentration is used. The error of a calibration curve prepared from a result of an MRM measurement performed under the MRM transition determined in the previously described manner is not always smaller than the error of a calibration curve prepared based on a result of an MRM measurement performed under a different MRM transition.