Patent Publication Number: US-2021175060-A1

Title: Mass Spectrum Processing Apparatus and Method

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2019-221008 filed Dec. 6, 2019, the disclosure of which is hereby incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     Field of the Disclosure 
     The present disclosure relates to a mass spectrum processing apparatus and method, and in particular to a technique for evaluating an estimated composition. 
     Description of Related Art 
     A mass spectrum processing apparatus has a function to estimate a composition of a sample (more accurately, a composition or computational formula). In the estimation of the composition, a molecular ion peak is selected on a mass spectrum by an analyzer or automatically. The composition of the sample is theoretically estimated based on a precise mass of the molecular ion peak. Normally, a plurality of composition candidates are estimated, and, among these candidates, probable composition candidates are narrowed down. As a method of narrowing down, there is known a method which uses an isotope pattern. In this method, an isotope pattern theoretically calculated from a composition candidate (theoretical isotope pattern) is matched to a molecular ion peak group in the mass spectrum (measured isotope pattern). 
     When the sample is an organic compound, due to the presence of isotopes, the molecular ions of the sample are in general constituted from first molecular ions having a smallest mass, second molecular ions which are heavier than the first molecular ions by 1 u, third molecular ions which are heavier than the first molecular ions by 2 u, . . . and the like. In correspondence to this, the molecular ion peak group in the mass spectrum are constituted from a first molecular ion peak, a second molecular ion peak, a third molecular ion peak, . . . and the like. In general, for molecules having molecular weights of 500 or smaller, the first molecular ion peak is a monoisotopic peak, and an intensity of the first molecular ion peak is the largest. For certain organic compounds, an intensity of the second or later molecular ion peak may be the largest. 
     As a soft ionization method, various ionization methods are known. For example, there are known a field ionization method (FI), a field desorption ionization method (FD), a photoionization method (PI), and the like. According to the soft ionization method, an isotope pattern of a molecular ion [M] +  tends to be observed. With this observation, an isotope pattern of a molecular ion [M+H] +  to which a proton is added may be observed. On the mass spectrum, an overlap exists between a first molecular ion peak of the molecular ion [M+H] +  to which the proton is added and a second molecular ion peak (that is, a second isotope peak) of the molecular ion [M] +  to which the proton is not added, and a peak occurs in which these peaks are added. In reality, such overlaps occur at a plurality of peak positions. 
     As a representative ionization method of a hard ionization method, there is known an electron impact ionization method (EI). When the hard ionization method is used, fragment ions are primarily observed. On the mass spectrum, an isotope pattern of the molecular ion [M] +  may or may not appear. For certain organic compounds forming the sample, an isotope pattern of a molecular ion [M−H] +  from which hydrogen is desorbed may be observed. In this case, on the mass spectrum, an overlap exists between a second molecular ion peak of the molecular ion [M−H] +  from which hydrogen is desorbed, and a first molecular ion peak of the molecular ion [M] +  from which hydrogen is not desorbed, and a peak occurs in which these peaks are added. In reality, such overlaps occur at a plurality of peak positions. 
     JP H3-285158 A discloses a mass analysis apparatus which can switch between the EI method and the CI method. 
     When the composition is to be evaluated or narrowed down based on the isotope pattern matching, if the isotope pattern of the molecular ion [M+H] +  to which the proton is added and the isotope pattern of the molecular ion [M−H] +  from which hydrogen is desorbed are detected at the same time with the isotope pattern of the molecular ion [M]+, the isotope pattern matching cannot be appropriately performed. 
     An advantage of the present disclosure lies in enabling correct composition evaluation using the isotope pattern matching, in a situation in which there is a possibility of detection of an isotope pattern of a molecular ion to which the proton is added or a possibility of detection of an isotope pattern of a molecular ion from which hydrogen is desorbed. 
     SUMMARY 
     According to one aspect of the present disclosure, there is provided a mass spectrum processing apparatus comprising: a composition estimator that estimates a composition based on at least one of a first monoisotopic peak included in a first peak group in a first mass spectrum produced through soft ionization of a sample, or a second monoisotopic peak included in a second peak group in a second mass spectrum produced through hard ionization of the sample; a primary pattern matching unit that attempts primary pattern matching between the first peak group and the second peak group; and a first secondary pattern matching unit that attempts, when the first pattern matching is successful, secondary pattern matching between a representative peak group which represents the first peak group and the second peak group, and an isotope pattern theoretically derived from the estimated composition, to thereby evaluate the estimated composition. 
     According to another aspect of the present disclosure, there is provided a method of processing a mass spectrum, comprising: a composition estimating step in which a composition is estimated based on at least one of a first monoisotopic peak included in a first peak group in a first mass spectrum produced through soft ionization of a sample, or a second monoisotopic peak included in a second peak group in a second mass spectrum produced through hard ionization of the sample; and a step of evaluating the estimated composition, wherein, in the step of evaluating, there are utilized primary pattern matching between the first peak group and the second peak group, and secondary pattern matching between a measured isotope pattern which is the first peak group or the second peak group, and a theoretical isotope pattern theoretically derived from the estimated composition. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Embodiment(s) of the present disclosure will be described based on the following figures, wherein: 
         FIG. 1  is a block diagram of a mass analysis system according to an embodiment of the present disclosure; 
         FIG. 2  is a diagram showing a first chromatogram and an EI mass spectrum; 
         FIG. 3  is a diagram showing a second chromatogram and an SI mass spectrum; 
         FIG. 4  is a diagram showing a plurality of types of measured isotope patterns which may be included in an SI mass spectrum and an EI mass spectrum; 
         FIG. 5  is a diagram showing an example of pattern matching between a measured isotope pattern and a theoretical isotope pattern; 
         FIG. 6  is a diagram showing another example of the pattern matching between the measured isotope pattern and the theoretical isotope pattern; 
         FIG. 7  is a diagram showing pattern matching between a measured isotope pattern of fragment ions and a theoretical isotope pattern of fragment ions; 
         FIG. 8  is a diagram showing a relationship between a plurality of types of isotope patterns which may be included in an SI mass spectrum and a plurality of types of isotope patterns which may be included in an EI mass spectrum; 
         FIG. 9  is a diagram showing a plurality of functions of the data processing unit shown in  FIG. 1 ; 
         FIG. 10  is a diagram showing a part of a mass spectrum processing method according to an embodiment of the present disclosure; 
         FIG. 11  is a diagram showing other parts of the mass spectrum processing method according to an embodiment of the present disclosure; 
         FIG. 12  is a diagram for explaining comparison of ratios; 
         FIG. 13  is a diagram showing a calculation method of a partial composition; and 
         FIG. 14  is a diagram showing an evaluation method of a composition candidate using fragment ions. 
     
    
    
     DESCRIPTION OF THE DISCLOSURE 
     An embodiment of the present disclosure will now be described with reference to the drawings. 
     (1) Overview of Embodiment 
     A mass spectrum processing apparatus according to an embodiment of the present disclosure comprises: a composition estimator; a primary pattern matching unit; and a first secondary pattern matching unit. The composition estimator estimates a composition based on at least one of a first monoisotopic peak included in a first peak group in a first mass spectrum produced through soft ionization of a sample, or a second monoisotopic peak included in a second peak group in a second mass spectrum produced through hard ionization of the sample. The primary pattern matching unit attempts primary pattern matching between the first peak group and the second peak group. The first secondary pattern matching unit attempts, when the primary pattern matching is successful, secondary pattern matching between a representative peak group which represents the first peak group and the second peak group, and an isotope pattern theoretically derived from the estimated composition, to thereby evaluate the estimated composition. 
     The soft ionization and the hard ionization are relative concepts, distinguished in the viewpoint of ease of production of fragment ions. In an embodiment of the present disclosure, the soft ionization is ionization in which a proton adding phenomenon may occur. The first peak group basically corresponds to one of a simple isotope pattern of a molecular ion [M] + , a simple isotope pattern of a molecular ion [M+1] + , or a combined isotope pattern in which these isotope patterns are overlapped. Here, “+1” means an addition of a proton H. The simple isotope pattern means a non-combined isotope pattern. 
     On the other hand, the hard ionization is ionization in which a hydrogen desorbing phenomenon may occur. The second peak group basically corresponds to one of the simple isotope pattern of the molecular ion [M] + , a simple isotope pattern of a molecular ion [M−1] + , or a combined isotope pattern in which these isotope patterns are overlapped. Here, “−1” means desorption of hydrogen, H. 
     The primary pattern matching is pattern matching between a measured isotope pattern which is the first peak group and a measured isotope pattern which is the second peak group. With the attempt of the primary pattern matching, a relationship between the first peak group and the second peak group may be identified. More specifically, when the primary pattern matching is successful, it is highly likely that both the first peak group and the second peak group are simple isotope patterns. On the other hand, when the primary pattern matching is not successful, it is highly likely that at least one of the first peak group or the second peak group is a combined isotope pattern. 
     The secondary pattern matching is pattern matching between the measured isotope pattern and a theoretical isotope pattern. When the representative peak group which is set as the matching target is the simple isotope pattern, and the estimated composition has a certain appropriateness, the secondary pattern matching will be successful. 
     The composition estimation is basically performed based on the first monoisotopic peak, but alternatively, the composition estimation may be performed based on the second monoisotopic peak, or based on two monoisotopic peaks. In general, the first peak group is a molecular ion peak group with the first monoisotopic peak as a head peak, and the second peak group is a molecular ion peak group with the second monoisotopic peak as a head peak. There may be cases in which the second monoisotopic peak cannot be clearly identified (or, even further, when the second peak group cannot be clearly identified), and, in such cases, other evaluation methods are applied. 
     When the elements of a compound which is a mass analysis target are all primary isotopes (isotopes having the largest natural abundances), a peak corresponding to the compound is called a monoisotopic peak. Normally, a peak group having a molecular ion peak serving as the monoisotopic peak at a lowest-mass-side is used in the isotope pattern matching. The first peak group or the second peak group normally serves as the representative peak group, but alternatively, a representative peak group may be produced based on these peak groups. 
     In an embodiment of the present disclosure, the mass spectrum processing apparatus further comprises a mass difference calculator. The mass difference calculator calculates a mass difference between the first monoisotopic peak and the second monoisotopic peak. An influence of a difference of one hydrogen or two hydrogens on the isotope pattern of the molecular ion is small. Thus, the primary pattern matching unit relatively shifts one of the first peak group or the second peak group based on the mass difference, and then attempts the primary pattern matching. Either one of the peak groups may be shifted. Alternatively, the peak group may be logically shifted, without being actually shifted. 
     When the first monoisotopic peak is a peak corresponding to the molecular ion [M] +  or a peak corresponding to the molecular ion [M+1] + , and the second monoisotopic peak is a peak corresponding to the molecular ion [M] +  or a peak corresponding to the molecular ion [M−1] + , 0, 1, or 2 (in units of u) is determined as the mass difference. When one of the peak groups is shifted by an amount corresponding to the mass difference, it becomes possible to correctly correlate the first peak group and the second peak group on a mass axis (or on an m/z axis). That is, the primary pattern matching between the two peak groups can be enabled. 
     When a numerical value other than 0, 1, and 2 is determined as the mass difference, it is likely that the second peak group does not occur. Thus, application of the primary pattern matching is put off, and other evaluation methods are applied. More specifically, as will be described below, the composition is evaluated using isotope pattern matching based on the first peak group and isotope pattern matching based on a fragment peak group. 
     In an embodiment of the present disclosure, the mass spectrum processing apparatus further comprises a selector and a second secondary pattern matching unit. The selector selects, when the primary pattern matching is unsuccessful, a peak group, of the first peak group and the second peak group, having a higher likelihood of being a peak group derived from a single molecular ionic species, as a peak group of interest. The second secondary pattern matching unit attempts the secondary pattern matching between the peak group of interest selected by the selector and the isotope pattern theoretically derived from the estimated composition. 
     When the primary pattern matching is unsuccessful, it is estimated that at least one of the first peak group or the second peak group is likely the combined isotope pattern. At the same time, there is a possibility that one of these peak groups is a simple isotope pattern (a possibility that one of these peak groups is a peak group derived from a single molecular ionic species). In consideration of this, in the above-described configuration, a peak group of interest which is likely a simple isotope pattern is selected, and the secondary pattern matching is attempted on the peak group of interest. 
     In an embodiment of the present disclosure, the selector calculates a first ratio based on an intensity of a head peak in the first peak group, and a sum of intensities of a plurality of succeeding peaks in the first peak group. In addition, the selector calculates a second ratio based on an intensity of a head peak in the second peak group, and a sum of intensities of a plurality of succeeding peaks in the second peak group. The selector then selects the peak group of interest by comparing the first ratio and the second ratio. 
     In the case of the simple isotope pattern, the sum of the intensities of the succeeding peaks is relatively small as compared to the intensity of the head peak. On the other hand, in the case of the combined isotope pattern, the sum of the intensities of the succeeding peaks is relatively large as compared to the intensity of the head peak. Based on such a property, the peak group of interest may be selected. Alternatively, the peak group of interest may be selected by other methods. 
     In an embodiment of the present disclosure, the mass spectrum apparatus further comprises a fragment evaluator. The fragment evaluator evaluates, when the secondary pattern matching by the second secondary pattern matching unit is unsuccessful, the estimated composition based on a fragment ion peak group included in the first mass spectrum or the second mass spectrum. 
     In an embodiment of the present disclosure, the fragment evaluator comprises a fragment composition estimator, and a fragment pattern matching unit. The fragment composition estimator estimates a fragment composition based on a head peak in the fragment ion peak group. The fragment pattern matching unit attempts fragment pattern matching between the fragment ion peak group and a fragment isotope pattern theoretically derived from the fragment composition. 
     In general, a plurality of peak groups obtained by detection of fragment ions may include a peak group corresponding to the simple isotope pattern. By performing fragment isotope pattern matching using such a peak group, the composition may be evaluated. When, for example, a fragment composition estimated from the head peak is C 2 H 4 O 2 , and the composition to be evaluated is, for example, C 10 H 20 O, a number of oxygen atoms in the fragment exceeds a number of oxygen atoms in the molecular ion. When such a non-matching or contradictory relationship is found, the composition may be removed from the evaluation target before the isotope pattern matching is performed. 
     In an embodiment of the present disclosure, the mass spectrum processing apparatus further comprises a pattern matching unit. The pattern matching unit attempts, when the second mass spectrum does not include the second peak group, pattern matching between the first peak group and the isotope pattern theoretically derived from the estimated composition. 
     In the above-described configuration, pattern matching similar to the related art is performed between the first peak group and the isotope pattern, when the first mass spectrum includes the first peak group, and the second mass spectrum does not include the second peak group. Then, the composition is evaluated using the fragment ion peak group as described below, as necessary. 
     In an embodiment of the present disclosure, the mass spectrum processing apparatus further comprises a fragment evaluator. The fragment evaluator evaluates, when the pattern matching by the pattern matching unit is unsuccessful, the estimated composition based on a fragment ion peak group included in the first mass spectrum or the second mass spectrum. 
     In an embodiment of the present disclosure, the fragment evaluator comprises a fragment composition estimator and a fragment pattern matching unit. The fragment composition estimator estimates a fragment composition based on a head peak in the fragment ion peak group. The fragment pattern matching unit attempts fragment pattern matching between the fragment ion peak group and a fragment isotope pattern theoretically derived from the fragment composition. 
     A method of processing a mass spectrum according to an embodiment of the present disclosure comprises a composition estimating step, and a step of evaluating the estimated composition. The composition estimating step is a step in which a composition is estimated based on at least one of a first monoisotopic peak included in a first peak group in a first mass spectrum produced through soft ionization of a sample, or a second monoisotopic peak included in a second peak group in a second mass spectrum produced through hard ionization of the sample. The step of evaluating the estimated composition utilizes primary pattern matching between the first peak group and the second peak group, and secondary pattern matching between a measured isotope pattern which is the first peak group or the second peak group, and a theoretical isotope pattern theoretically derived from the estimated composition. 
     The above-described method may be realized as a function of hardware or a function of software. In the case of the latter, a program executing the method is installed to an information processor via a network or via a transportable recording medium. The concept of the information processor includes a mass spectrum processing apparatus, and also a mass analysis apparatus or a mass analysis system. 
     (2) Details of Embodiment 
       FIG. 1  shows an example configuration of a mass analysis system according to an embodiment of the present disclosure. A mass analysis system  10  temporally separates a plurality of compounds included in an original sample, and performs mass analysis on each of the compounds. The mass analysis system  10  shown in  FIG. 1  more specifically comprises a gas chromatograph apparatus  12 , a mass analysis apparatus  14 , and an information processor  16 . Alternatively, the gas chromatograph apparatus  12  may be omitted. Alternatively, in place of the gas chromatograph apparatus  12 , a liquid chromatograph apparatus or the like may be used. 
     When the original sample is introduced to the gas chromatograph apparatus  12 , a plurality of compounds which are temporally separated appear on an output side of the gas chromatograph apparatus  12 . The plurality of compounds are samples serving as analysis targets from the viewpoint of the mass analysis apparatus  14 . The plurality of compounds thus separated are sequentially introduced to the mass analysis apparatus  14 . In the illustrated example configuration, the mass analysis apparatus  14  comprises an ion source unit  20 , a mass analyzer  26 , and a detector  28 . In the present embodiment, two measurements on the same original sample are repeatedly executed. One measurement includes composition separation and mass analysis. 
     In the present embodiment, the ion source unit  20  includes a first ion source  22  and a second ion source  24  which are selectively used. The first ion source  22  is a hard ion source. More specifically, the first ion source  22  is an ion source which follows the electron ionization (EI) method. According to the first ion source  22 , in general, a relatively large number of fragment ions can be produced. 
     The second ion source  24  is a soft ion source. That is, the second ion source  24  is an ion source which follows, for example, the field ionization method. According to the second ion source  24 , fragment ions tend to be not produced, but the molecular ions can be clearly detected. Examples of a second ionization method include, in addition to the field ionization method, a chemical ionization method, a field desorption ionization method, a photoionization method, and the like. For the measurement of two times, the ion source to be actually used is sequentially selected through electric switching, mechanical switching, or manual switching. An order of selection of the ion sources may be arbitrarily determined. 
     In the selected ion source, ions are produced from the sample introduced thereto. The ions are guided by an action of an electric field to the mass analyzer  26 . Alternatively, two lines of mass analysis subsystems may be placed in parallel to each other. In this case, the two ion sources may be operated in parallel to each other. In this case, the individual mass analysis subsystem is formed from the mass analyzer and the detector to be described below. 
     The mass analyzer  26  performs mass analysis on ions generated by the ion source unit  20  based on a mass-to-charge ratio (m/z) of each ion. For example, when the mass analyzer  26  is a time-of-flight type mass analyzer, the individual ions are detected by the detector  28  after a time of flight corresponding to the mass-to-charge ratio of each of the ions. Alternatively, a mass analyzer of other types (such as, for example, a magnetic field sector type mass analyzer, a quadrupole type mass analyzer, or the like) may be used. The detector  28  detects ions, and specifically has an electron multiplier tube. A detection signal  28 A is output from the detector  28 . The detection signal  28 A is sent to the information processor  16  via a signal processor circuit (not shown). 
     The information processor  16  corresponds to a mass spectrum processing apparatus, and comprises a processor  18 , an inputter  32 , a display  34 , or the like, and also a memory (not shown). The processor  18  is formed from, for example, a CPU which executes a program. Alternatively, in place of or along with the processor  18 , various calculation devices may be used. Alternatively, the information processor  16  may be formed from a plurality of computers. Alternatively, functions of a part of the computers may be present on a network. 
     The processor  18  functions as a calculator, a controller, and a processing unit. A data processing function of the processor  18  is shown on  FIG. 1  as a data processing unit  36 . As will be described below, the data processing unit  36  functions as a spectrum producer, a composition estimator, and a composition evaluator. In addition, in evaluating the composition, the data processing unit  36  functions as a mass difference calculator, a primary pattern matching unit, a first secondary pattern matching unit, a ratio calculator, a selector, an isotope pattern producer, a second secondary pattern matching unit, a fragment pattern matching unit, or the like. 
     The memory is formed from a semiconductor memory, a hard disk drive, or the like. On the memory, a plurality of programs executed by the CPU are stored. These programs include a spectrum processing program and a composition estimating program. On the memory, a composition estimation database is constructed. Alternatively, the composition estimation database may be constructed on a storage connected to the information processor  16  via a network. The composition estimation database has information which is referred to when a composition of molecular ions (overall composition) is estimated and a composition of fragment ions (partial composition) is estimated. 
     The inputter  32  is formed from a keyboard, a pointing device, or the like. Using the inputter  32 , for example, a user selects a peak and inputs a processing condition. In the present embodiment, using the inputter  32 , the user designates a composition search range, as a composition estimating condition. The composition search range more specifically includes a plurality of atomic number ranges corresponding to a plurality of elements. Each atomic number range is defined by a lower limit and an upper limit. The composition estimating condition which is input is displayed on the display  34  to be described below, and is also registered in the memory. When the composition estimating condition is changed as will be described below, the composition estimating condition after the change is displayed on the display  34  and is registered in the memory. 
     The display  34  functions as a display unit, and is more specifically formed from a liquid crystal display, an organic EL device, or the like. On a display screen of the display  34 , a chromatogram, a mass spectrum, the composition search range, difference information (such as neutral loss or the like), a composition estimation result, a composition evaluation result, or the like is displayed. 
     A composition evaluation method according to the present embodiment will now be described in detail with reference to figures from  FIG. 2  and on. 
       FIG. 2  shows a chromatogram  64  produced using the first ion source; that is, produced through hard ionization of the sample. In the chromatogram  64 , a vertical axis shows TIC (Total Ion Current), and a horizontal axis shows RT (Retention Time). The chromatogram  64  (and a chromatogram  75  shown in  FIG. 3 ) is produced from a mass spectrum array based on a detected signal, by the data processing unit shown in  FIG. 1 . 
     In the chromatogram  64 , a plurality of compound peaks  67  appear at different times on the retention time axis. Windows  66 ,  68 ,  70 , and  72  are set for the plurality of compound peaks  67 , and a mass spectrum is accumulated in each of the windows  66 ,  68 ,  70 , and  72 . A parameter w shows a width of the window  66 . The parameter w is set in advance or is set individually or dynamically. The mass spectrum is repeatedly produced by the data processing unit shown in  FIG. 1 , based on the detected signal, and in a certain time interval. 
     For each of the windows  66 ,  68 ,  70 , and  72 , a plurality of mass spectra observed in the corresponding period are accumulated, to produce an accumulated mass spectrum.  FIG. 2  exemplifies an accumulated mass spectrum  74  produced by accumulating a plurality of mass spectra observed in the window  66 . In the illustrated example configuration, in the accumulated mass spectrum  74 , a molecular ion peak group is not clear. On the other hand, as shown by reference numeral  73 , a plurality of fragment ion peak groups appear. Depending on the conditions, the molecular ion peak group may clearly appear also on the accumulated mass spectrum  74 . 
       FIG. 3  shows the chromatogram  75  produced using the second ion source; that is, produced through soft ionization of the sample. Similar to the chromatogram  64  shown in  FIG. 2 , in the chromatogram  75  also, a plurality of compound peaks  77  appear at different times on the retention time axis. For these peaks, windows  76 ,  78 ,  80 , and  82  for accumulation are set. 
     Between the chromatograph  64  and the chromatogram  75 , positions of occurrence of the plurality of compound peaks  67  and  77  are slightly deviated in the retention time axis direction. In the present embodiment, in consideration of such a deviation, compound peak pairs are correlated; that is, pairing is executed, between the chromatogram  64  and the chromatogram  75 . In this case, for example, with a peak top in one chromatogram as a reference, a search range is set on the other chromatogram, and a peak top may be searched in the search range. In the pairing, a method of using a center of gravity, a method of using waveform fitting, or the like may be used. 
     In the plurality of windows  76 ,  78 ,  80 , and  82 , the mass spectra are accumulated to produce a plurality of accumulated mass spectra.  FIG. 3  exemplifies an accumulated mass spectrum  84  produced by the accumulation of mass spectra in the window  76 . In the accumulated mass spectrum  84 , there is almost no fragment ion peak, but a molecular ion peak group  85  is clear. Depending on the conditions, a plurality of fragment ion peak groups may clearly appear in the accumulated mass spectrum  84 . 
     In this manner, between the two chromatograms, an accumulated mass spectrum pair is produced for each peak pair which is paired. For each compound separated by the gas chromatograph apparatus, the accumulated mass spectrum pair as described is produced. For each accumulated mass spectrum pair, a composition of the compound is estimated, and the estimated composition is evaluated. Alternatively, from a plurality of composition candidates which are estimated, a most probable composition candidate is selected. In the following, the accumulated mass spectrum produced through the soft ionization of the sample will be referred to as an “SI mass spectrum”, and the accumulated mass spectrum produced through the hard ionization of the sample will be referred to as an “EI mass spectrum”. 
     At an upper part of  FIG. 4 , a plurality of types of first peak groups which may be included in the SI mass spectrum are shown. At a lower part of  FIG. 4 , a plurality of types of second peak groups which may be included in the EI mass spectrum are shown. Each peak group is formed from a first monoisotopic peak (head peak) and a plurality of succeeding peaks succeeding the first monoisotopic peak (a plurality of isotope peaks of second and later orders). The illustration of  FIG. 4  is merely exemplary, and does not show a form of each peak group in an exact shape. 
     More specifically, at the upper part of  FIG. 4 , three peak groups  90 A,  90 B, and  90 C which may be included in the SI mass spectrum are shown. The peak group  90 A is a simple isotope pattern which occurs when the molecular ion [M] +  is detected, the peak group  90 B is a simple isotope pattern which occurs when the molecular ion [M+1] +  is detected, and the peak group  90 C is a combined isotope pattern which occurs when the molecular ion [M] +  and the molecular ion [M+1] +  are detected. As shown by reference numeral  94 , the peak group  90 C corresponds to an overlap of two isotope patterns. 
     Similarly, at the lower part of  FIG. 4 , three peak groups  92 A,  92 B, and  92 C which may be included in the EI mass spectrum are shown. The peak group  92 A is a simple isotope pattern which occurs when the molecular ion [M] +  is detected, the peak group  92 B is a simple isotope pattern which occurs when the molecular ion [M−1] +  is detected, and the peak group  92 C is a combined isotope pattern which occurs when the molecular ion [M] +  and the molecular ion [M−1] +  are detected. As shown by reference numeral  96 , the peak group  92 C corresponds to an overlap of two isotope patterns. 
       FIG. 5  shows isotope pattern matching. A plurality of composition candidates are estimated based on a precise mass of one of monoisotopic peaks. The composition candidates form a composition candidate list  100 . The monoisotopic peak is selected by the user or automatically. For each composition candidate, the composition evaluation of the present embodiment is applied. 
     As shown by reference numeral  101 , based on each composition candidate, an isotope pattern  102  is theoretically produced. Here, pattern matching will be considered between the three peak groups  90 A,  90 B, and  92 B shown in  FIG. 4  (measured isotope patterns) and the isotope pattern  102  (theoretical isotope pattern). 
     Because the peak group  90 A is the simple isotope pattern, the peak group  90 A and the theoretical isotope pattern  102  match each other. Although the peak group  90 B is the simple isotope pattern, there is a mass difference of 1 u between the peak group  90 B and the theoretical isotope pattern  102 . Thus, the mass difference is first resolved, and then, the pattern matching is applied. Specifically, the peak group  90 B is shifted to a low-mass side by −1 u as shown by reference numeral  104 , and the pattern matching is then performed between the shifted peak group  90 B′ and the theoretical isotope pattern  102 . The shifter peak group  90 B′ and the theoretical isotope pattern  102  match each other. Although the peak group  92 B is also the simple isotope pattern, there is a mass difference of 1 u between the peak group  92 B and the theoretical isotope pattern  102 . Thus, the mass difference is first resolved, and the pattern matching is then applied. Specifically, the peak group  92 B is shifted to a high-mass side by +1 u as shown by reference numeral  104 , and then, the pattern matching is performed between the shifted peak group  92 B′ and the theoretical isotope pattern  102 . The shifted peak group  92 B′ and the theoretical isotope pattern  102  match each other. When the isotope pattern matching is successful, appropriateness of the estimated composition can be evaluated. For example, when a similarly of two patterns is greater than or equal to a predetermined threshold, matching of the two patterns is determined. 
     On the other hand,  FIG. 6  shows another type of isotope pattern matching. In  FIG. 6 , elements shown in  FIG. 5  are assigned the same reference numerals, and their descriptions will not be repeated. The peak group  90 C is the combined isotope pattern, and does not match the theoretical isotope pattern  102  with or without a shift. Similarly, the peak group  92 C is also the combined isotope pattern, and does not match the theoretical isotope pattern  102  with or without a shift. Therefore, when the composition evaluation is to be performed using the isotope pattern matching, desirably, a type of the peak group to be considered is identified prior to the isotope pattern matching. 
     As shown in  FIG. 7 , the above description is also applicable to a case when the isotope pattern matching is to be applied on a peak group which occurs in detection of fragment ions. In  FIG. 7 , a mass spectrum shown at the upper part includes a plurality of peak groups. A peak group shown by reference numeral  112  includes a molecular ion peak  110  serving as the monoisotopic peak. A peak group shown by reference numeral  118  includes a fragment ion peak  116  serving as the monoisotopic peak. 
     When the peak group  112  corresponds to the combined isotope pattern, in the evaluation of an estimated composition  114 , a mass difference (neutral loss) is calculated between the molecular ion peak  110  and the fragment ion peak  116 . Based on the mass difference, a partial composition  120  is estimated. By subtracting the partial composition  120  from the composition  114 , a composition  122  corresponding to the fragment ion peak  116  (fragment composition) is estimated. 
     As shown by reference numeral  123 , a theoretical isotope pattern  124  is produced based on the fragment composition  122 . When the peak group  118  is a peak group  118 A, because the peak group  118 A corresponds to the simple isotope pattern, the peak group matches the theoretical isotope pattern  124 . On the other hand, when the peak group  118  is a peak group  118 B, because the peak group  118 B corresponds to the combined isotope pattern, the peak group does not match the theoretical isotope pattern  124 . 
     In fact, normally, the plurality of fragment peak groups included in the mass spectrum include a certain number of fragment peak groups corresponding to the simple isotope pattern. Therefore, in performing the isotope pattern matching, desirably, the plurality of fragment peak groups are referred to. 
       FIG. 8  shows a relationship between a plurality of types of the peak groups which may be observed under the soft ionization and a plurality of types of the peak groups which may be observed under the hard ionization, summarized two-dimensionally. Under the soft ionization, a proton adding phenomenon may occur. Thus, under the soft ionization, a peak group which is an isotope pattern of the molecular ion [M] + , a peak group which is an isotope pattern of the molecular ion [M+1] + , and a peak group corresponding to an overlap of these isotope patterns may be observed. 
     On the other hand, under the hard ionization, a hydrogen desorbing phenomenon may occur. Thus, under the hard ionization, a peak group which is an isotope pattern of the molecular ion [M] + , a peak group which is an isotope pattern of the molecular ion [M−1] + , and a peak group corresponding to an overlap of these isotope patterns may be observed. 
     As shown in  FIG. 8 , theoretically, 16 combinations (observation forms) #00˜#33 may be considered. However, under the soft ionization, because the molecular ion [M] +  (or the molecular ion [M+1] + ) is basically detected, #00, #10, #20, and #30 may be removed. Thus, in reality, 12 observation forms, #01˜#03, #11˜#13, #21˜#23, and #31˜#33 may occur. 
     Of the 12 observation forms, a group  130  is formed from observation forms #01, #02, and #03. These observation forms correspond to cases in which the molecular ion [M] +  (or the molecular ion [M−1] + ) is not detected under the hard ionization. 
     Whether a current observation result corresponds to an observation form, among the 12 observation forms, belonging to the group  130 , or corresponds to one of the others, 9 observation forms, #11˜#13, #21˜#23, and #31˜#33, may be distinguished by calculating a difference in two monoisotopic masses corresponding to the two monoisotopic peaks included in the two mass spectra; that is, a mass difference. Specifically, when the mass difference is a number other than 0, 1, or 2 (when the molecular ion peak serving as a monoisotopic peak is not included in the mass spectrum produced under the hard ionization), the current situation belongs to the group  130 . When the mass difference is 0, 1, or 2, the current observation result corresponds to one of the 9 observation forms, #11˜#13, #21˜#23, and #31˜#33. 
     Four observation forms, #11, #12, #21, and #22, forming a group  132  presume that two peak groups are both simple isotope patterns. Therefore, when the mass difference is 0, 1, or 2, and the primary pattern matching (matching between two measured isotope patterns) is successful, normally, the secondary pattern matching (matching between a measured isotope pattern and a theoretical isotope pattern) will also be successful. 
     5 observation forms, #31, #32, #13, #23, and #33, presume that at least one peak group is the combined isotope pattern. Therefore, when the mass difference is 0, 1, and 2, and the primary pattern matching is unsuccessful, the current observation result corresponds to one of the 5 observation forms, #31, #32, #13, #23, and #33. 
     Of the 5 observation forms, #31, #32, #13, #23, and #33, groups  134  and  136  (more specifically, 4 observation forms, #31, #32, #13, and #23) presume that a peak group corresponding to the simple isotope pattern and a peak group corresponding to the combined isotope pattern are observed. Therefore, the former peak group is selected, and the secondary pattern matching is attempted on the selected peak group. From a selection target of the peak group, the group to which the current observation result belongs (group  134  or  136 ) is identified. 
     When the mass difference is 1, and the primary pattern matching and the secondary pattern matching are both unsuccessful, the current observation result corresponds to the observation form #33. In this case, the fragment pattern matching is applied. Similarly, when the mass difference is a number other than 0, 1, or 2, and the pattern matching is unsuccessful, the current observation result corresponds to the observation form #03, and in this case also, the fragment pattern matching is applied. 
     As described, according to the present embodiment, by applying the primary pattern matching and the secondary pattern matching stepwise, an evaluation method conforming to the current observation result can be selectively applied. Details will now be described. 
       FIG. 9  shows a plurality of functions of the data processing unit  36  shown in  FIG. 1 , as a plurality of blocks. The data processing unit  36  comprises a mass spectrum producer  210 , a composition estimator  212 , and an evaluator (narrowing-down unit)  214 . In  FIG. 1 , illustration of a total ion current chromatogram (TICC) producer or the like is omitted. 
     The evaluator  214  has a first classifier  216  which functions as a first classifying unit. The first classifier  216  has a mass difference calculator  218  as a mass difference calculating unit. The mass difference calculator  218  calculates a mass difference between two molecular ion peaks (two monoisotopic peaks) included in two mass spectra. The two molecular ion peaks may be designated by the user or automatically. 
     The evaluator  214  has a second classifier  220  which functions as a second classifying unit. The second classifier  220  has a pattern comparator  222  which functions as a pattern comparing unit or a first pattern matching unit. With the pattern comparator  222 , two peak groups included in two mass spectra; that is, two isotope patterns, are compared to each other. From a comparison result, an observation form or a group corresponding to the current observation result may be identified. 
     The evaluator  214  has a third classifier  224  which functions as a third classifying unit. The third classifier  224  also functions as a selector. The third classifier  224  has a ratio calculator  226  which functions as a ratio calculating unit. The ratio calculator  226  calculates a predetermined ratio for each of the peak groups when the primary pattern matching is unsuccessful, and a peak group corresponding to the simple isotope pattern is selected from two peak groups. The ratio indicates a degree of possibility that the peak group is the simple isotope pattern. The details of this process will be described later. 
     The evaluator  214  has an isotope pattern producer  228  which functions as an isotope pattern producing unit. The isotope pattern producer  228  produces an isotope pattern corresponding to the composition candidate. The isotope pattern is the theoretical isotope pattern. 
     The evaluator  214  has a pattern comparator  230  which functions as a pattern comparing unit, a first secondary pattern matching unit, and a second secondary pattern matching unit. The pattern comparator  230  performs the secondary pattern matching between the theoretical isotope pattern produced by the isotope pattern producer  228 , and the measured isotope pattern which is the observed peak group. 
     The evaluator  214  has a composition estimator  232  which functions as a composition estimating unit, a composition difference evaluator  234  which functions as a composition difference evaluating unit, and a fragment pattern comparator  236  which functions as a fragment pattern comparing unit (or a fragment pattern matching unit). 
     The composition estimator  232  estimates a partial composition based on a mass difference (neutral loss) between a mass of the molecular ion peak (monoisotopic peak) and a mass of the fragment ion peak (monoisotopic peak). The estimated partial composition is compared to the composition candidates, so as to evaluate the appropriateness of the composition candidate. 
     The fragment pattern comparator  236  performs pattern matching between a theoretical isotope pattern based on the fragment composition estimated for the fragment peak, and the fragment ion peak group (measured isotope pattern), to evaluate the appropriateness of the composition candidate. 
     Next, with reference to  FIGS. 10 and 11 , an evaluation method according to the present embodiment will be described in detail. In S 10 , two molecular ion peaks (two monoisotopic peaks) included in two mass spectra SI and EI produced under two ionization methods are identified, and masses thereof, PM SI  and PM EI  are calculated. In S 12 , the two masses PM SI  and PM EI  are compared to each other. If PM SI ≥PM EI , S 16  is executed, and, if this is not the case, an error is judged in S 14  and the process is completed. S 12  is a step for checking that the molecular ion peak is included at least in the mass spectrum SI. 
     In S 16 , the composition is estimated based on the mass PM SI . Normally, a composition candidate list formed from one or more composition candidates is formed. Alternatively, the composition candidate list may be produced based on PM EI . In S 18 , it is checked whether or not there is at least one composition candidate, and, if not, an error is judged in S 20 , and the process is completed. For each composition candidate, a sequence of processes from S 21  is executed. A process shown by reference numeral  216 A corresponds to the first classifier shown in  FIG. 9 , and a process shown by reference numeral  220 A corresponds to the second classifier shown in  FIG. 9 . 
     In S 21 , the mass difference is calculated by finding PM SI -PM EI . By calculating the mass difference, it is possible to identify that the two peak groups both exist, in particular, that a peak group exists in the mass spectrum EI. In  FIGS. 10 and 11 , a matrix shown in  FIG. 8  is schematically drawn at each branch. A gray portion in each matrix shows a narrowed-down portion. 
     In S 22 , it is judged whether or not the mass difference is one of 0, +1, or +2. When judged NO, a process from S 34  shown in  FIG. 11  is executed. When judged YES, in S 24 , a shift corresponding to the mass difference is applied to the peak group included in the mass spectrum EI, and the primary pattern matching; that is, matching between two measured isotope patterns, is performed between two peak groups. When the primary pattern matching is successful (S 26 ), S 28  is executed, and when the primary pattern matching is unsuccessful, a process from S 40  shown in  FIG. 11  is executed. 
     In S 28 , a representative peak group which represents the two peak groups is selected from the two peak groups. Basically, either peak group may be selected. A selection rule may be defined in advance. Alternatively, the two peak groups may be combined. In S 28 , pattern matching is performed between the representative peak group which is the measured isotope pattern, and the theoretical isotope pattern produced from the estimated composition. This pattern matching corresponds to first secondary pattern matching. If the estimated composition candidate is appropriate, the secondary pattern matching will be successful. If the secondary pattern matching is unsuccessful, an error may be judged and the process may be completed. Alternatively, as shown in  FIG. 10 , the process from S 40  may be executed, via S 32 . 
     In  FIG. 11 , in S 40 , the ratio is calculated for each of the two peak groups. 
       FIG. 12  exemplifies a method of calculating the ratio. A first peak group  200  is formed from a head peak  202  and two succeeding peaks  204 , and a second peak group  206  is formed from a head peak  208  and two succeeding peaks  209 . 
     Based on the first peak group  200 , a ratio ε SI  (=A1/A0) between an intensity A0 of the head peak  202  and a sum A1 of intensities of the two succeeding peaks  204  is calculated. Similarly, based on the second peak group  206 , a ratio ε EI  (=B1/B0) between an intensity B0 of the head peak  208  and a sum B1 of intensities of the two succeeding peaks  209  is calculated. When the peak group corresponds to the simple isotope pattern, the ratio ε is small, and, if the peak group corresponds to the combined isotope pattern, the ratio ε is large. Thus, two ratios ε SI  and ε EI  are compared to each other, to identify the larger of the two ratios. The peak group with the smaller ratio ε is likely to correspond to the simple isotope pattern. Here, a number of the succeeding peaks to be included in the calculation of the intensity sum may be designated by the user in advance. 
     In  FIG. 11 , in S 42 , a peak group having the larger ratio is selected. That is, the first peak group in the SI mass spectrum or the second peak group in the EI mass spectrum is selected. When the first peak group is selected, S 44  and S 46  are executed. When the second peak group is selected, S 50  and S 52  are executed. In  FIG. 11 , a process shown by reference numeral  224 A corresponds to the third classified shown in  FIG. 9 . 
     In S 44 , pattern matching (second secondary pattern matching) is performed between the measured isotope pattern which is the selected first peak group, and the theoretical isotope pattern derived from the composition candidate. If the two isotope patterns match (yes in S 46 ), it is judged that the estimated composition is appropriate, and the process is completed. If the two isotope patterns do not match (no in S 46 ), a process from S 56  is executed. 
     In S 50 , pattern matching (second secondary pattern matching) is performed between the measured isotope pattern which is the selected second peak group, and the theoretical isotope pattern derived from the composition candidate. If the two isotope patterns match, it is judged that the estimated composition is appropriate (yes in S 52 ), and the process is completed. If the two isotope patterns do not match (no in S 52 ), similar to the above, the process from S 56  is executed. 
     In S 34 , pattern matching is performed between the measured isotope pattern which is the first peak group in the SI mass spectrum, and the theoretical isotope pattern derived from the composition candidate. In S 36 , if it is judged that the pattern matching is successful, it is judged that the composition candidate is appropriate, and the process is completed. 
     On the other hand, if the pattern matching is unsuccessful in S 36 , a fragment process from S 56  is executed. More specifically, the fragment process from S 56  is individually applied to each of a plurality of fragment ion peak groups included in a certain mass range in the two mass spectra. 
     In S 56 , a mass difference (neutral loss) between the molecular ion peak (monoisotopic peak) and a head peak (monoisotopic peak) in the fragment ion peak group is calculated, and the partial composition is estimated from the mass difference. In S 58 , the partial composition is subtracted from the estimated composition candidate, to determine a composition corresponding to the head peak of the fragment ion peak group. If, as a result of the subtraction, a contradiction or a non-match occurs such as that a number of a particular atom is negative (ng in S 60 ), an error is judged in S 62 , and the process is completed. 
     On the other hand, when the subtraction is successful in S 60 , in S 64 , pattern matching (third secondary pattern matching) is performed between the measured isotope pattern which is the fragment ion peak group, and a theoretical isotope pattern derived from the fragment composition identified by the subtraction. If the measured isotope pattern and the theoretical isotope pattern do not match (no in S 66 ), an error is judged in S 68 , and the process is completed. If the measured isotope pattern and the theoretical isotope pattern match (yes in S 66 ), it is judged that the composition candidate is appropriate, and the process is completed. 
     The fragment process from S 56  will now be described with reference to  FIGS. 13 and 14 . 
     In  FIG. 13 , the SI mass spectrum is shown at an upper part, and the EI mass spectrum is shown at a lower part. The SI mass spectrum includes a molecular ion peak group, and a mass  140  of a molecular ion peak  138  serving as the monoisotopic peak included in the molecular ion peak group is identified. A composition candidate list  142  is formed based on the mass  140 . The composition candidate list  142  is formed from a plurality of records, each record including a composition formula  144  and an estimated error  146 . 
     The EI mass spectrum includes a plurality of peak groups as a plurality of peak collections. Head peaks  148 ,  150 ,  152  of the respective peak groups correspond to the monoisotopic peaks. By subtracting each of the head peaks  148 ,  150 , and  152  from the mass of the molecular ion peak  138 , mass differences  154 ,  156 , and  158  are calculated. From these mass differences  154 ,  156 , and  158 , a partial composition corresponding to the neutral loss is estimated. 
     Alternatively, the fragment ion peak group included in the EI mass spectrum may be referred to. Alternatively, the fragment composition may be directly estimated based on the mass identified from the fragment ion peak. However, with the above-described structure, an estimation precision of the fragment composition can be improved, or the number of fragment composition candidates can be narrowed down. An upper limit and a lower limit of a range for searching the fragment ion peak group may be set by the user or automatically. 
       FIG. 14  summarizes a result of the fragment process. Along a vertical axis, a plurality of composition candidates  164  are listed. Along a horizontal axis, a partial composition candidate  166 , appropriateness/inappropriateness of the subtraction result  168 , a result of isotope pattern comparison  170 , and a result of evaluation  172  are shown. 
     In the illustrated example, for each composition candidate  164 , three evaluations using three partial composition candidates  166  are performed. In the case in which the three partial composition candidates  166  are respectively subtracted from a composition candidate of interest, for example, when a number of oxygen atoms O or a number of carbon atoms C is negative, the combination of the composition candidate and the partial composition candidate is judged as inappropriate. In this case, the isotope pattern matching is not performed. 
     If the subtraction result is appropriate, pattern matching (third secondary pattern matching) is performed between the measured isotope pattern which is the fragment ion peak group, and the theoretical isotope pattern, and match/non-match is judged. In the illustrated example, match is judged for a combination shown with reference numeral  174 , and a composition candidate corresponding thereto is evaluated as OK. 
     As described, according to the present embodiment, the isotope pattern matching can be correctly applied in a situation in which proton addition or hydrogen desorption may occur.