Patent Publication Number: US-7908093-B2

Title: Mass spectrometer and method for enhancing resolution of mass spectra

Description:
RELATED ART 
     This application is a continuation of U.S. patent application Ser. No. 11/412,887 filed on 27 Apr. 2006 now U.S. Pat. No. 7,412,334, the entire disclosure of which is incorporated into this application by reference. 
    
    
     In time-of-flight mass spectrometers (TOFMS), a mass sample to be analyzed is ionized, accelerated in a vacuum through a known potential, and then the arrival time of the different ionized components is measured at a detector. The larger the particle, the longer the flight time; the relationship between the flight time and the mass, m, can be written in the form:
 
time= k√{square root over (m)}+c  
 
where k is a constant related to flight path and ion energy, c is a small delay time, which may be introduced by the signal cable and/or detection electronics. When the term “mass” is used herein in the context of mass spectrometry of ions, it usually is understood to mean “mass-to-charge ratio.”
 
     An ion detector converts ion impacts into electrons. The signal generated by the detector at any given time is proportional to the number of electrons. There is only a statistical correlation between one ion hitting the detector and the number of electrons generated. In addition, more than one ion at a time may hit the detector due to ion abundance. 
     The mass spectrum generated by the spectrometer is the summed output of the detector as a function of the time-of-flight between the ion source and the detector. The number of electrons leaving the detector in a given time interval is converted to a voltage that is digitized by an analog-to-digital converter (A/D). 
     A mass spectrum is a graph of the output of the detector as a function of the time taken by the ions to reach the detector. In general, a short pulse of ions from an ion source is accelerated through a known voltage. Upon leaving the accelerator, the ions are bunched together but travelling at different speeds. The time required for each ion to reach the detector depends on its speed, which in turn, depends on its mass. Consequently, the original bunch is separated in space into discrete packets, each packet containing ions of a single mass, that reach the detector at different times. 
     A mass spectrum is generated by measuring the output of the A/D converter as a function of the time after the ions have been accelerated. The range of delay times is divided into discrete “bins.” Unfortunately, the statistical accuracy obtained from the ions that are available in a single packet is insufficient. In addition, there are a number of sources of noise in the system that result in detector output even in the absence of an ion striking the detector. Hence, the measurement is repeated a number of times (“multiple scans”) and the individual mass spectra are summed to provide a final result having the desired statistical accuracy and signal-to-noise ratio. 
     Unfortunately, small variations in the mass scans degrade resolution of the resultant mass spectra. Improving the resolution of the resultant mass spectra is generally desirable. 
     SUMMARY OF THE DISCLOSURE 
     Generally, embodiments of the present disclosure provide mass spectrometers and methods for enhancing resolution of mass spectra. 
     A mass spectrometer in accordance with one exemplary embodiment of the present disclosure comprises an ion detector, an analog-to-digital (A/D) converter, a sample adjuster, and an adder. The analog-to-digital (A/D) converter is configured to receive and sample an analog signal from the ion detector thereby providing a plurality of samples. The sample adjuster is configured to identify a peak defined by the samples and to adjust at least one of the samples based on the identified peak. The adder is configured to sum the samples. The summed samples define a mass spectrum and include a result of summing the at least one sample adjusted by the sample adjuster with a running sum of other ones of the samples. 
     A mass spectrometer in accordance with another exemplary embodiment of the present disclosure also comprises an ion detector, an A/D converter, a sample adjuster, and an adder. The A/D converter is configured to receive and sample an analog signal from the ion detector thereby providing a plurality of samples. The adder is configured to sum the samples, and the summed samples define a mass spectrum. The sample adjuster is configured to identify a peak defined by the samples and to suppress at least one of the samples of the identified peak such that a resolution of a peak within the mass spectrum is enhanced. 
     A method in accordance with an exemplary embodiment of the present disclosure comprises: detecting ions; transmitting an analog signal indicative of the detecting; sampling the analog signal thereby providing a plurality of samples; identifying a peak defined by the samples; summing the samples thereby defining a mass spectrum; and suppressing at least one of the samples based on the identifying such that a resolution of the mass spectrum is enhanced. 
     A method in accordance with yet another exemplary embodiment of the present disclosure comprises: detecting ions; transmitting an analog signal indicative of the detecting; sampling the analog signal thereby providing a plurality of samples; identifying a peak defined by the samples; summing the samples thereby defining a mass spectrum; and enhancing a resolution of a peak of the mass spectrum, wherein the enhancing comprises preventing, based on the identifying, at least one of the samples defining the identified peak from affecting the mass spectrum. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure can be better understood with reference to the following drawings. The elements of the drawings are not necessarily to scale relative to each other, emphasis instead being placed upon clearly illustrating the principles of the disclosure. Furthermore, like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1  is a block diagram illustrating a conventional mass spectrometer. 
         FIG. 2  is a graph illustrating an exemplary analog pulse output by an ion detector, such as is depicted in  FIGS. 1 and 11 , for a first mass scan. 
         FIG. 3  is a graph illustrating a representation of an exemplary analog pulse output by an ion detector, such as is depicted in  FIGS. 1 and 11 , for a second mass scan and corresponding to the analog pulse of  FIG. 2 . 
         FIG. 4  is a graph illustrating a representation of an exemplary analog pulse output by an ion detector, such as is depicted in  FIGS. 1 and 11 , for a third mass scan and corresponding to the analog pulses of  FIGS. 2 and 3 . 
         FIG. 5  is a graph illustrating a representation of an exemplary analog pulse output by an ion detector, such as is depicted in  FIGS. 1 and 11 , for a fourth mass scan and corresponding to the analog pulses of  FIGS. 2-4 . 
         FIG. 6  is a graph illustrating a representation of exemplary digital samples of the analog pulse of  FIG. 2 . 
         FIG. 7  is a graph illustrating a representation of exemplary digital samples of the analog pulse of  FIG. 3 . 
         FIG. 8  is a graph illustrating a representation of exemplary digital samples of the analog pulse of  FIG. 4 . 
         FIG. 9  is a graph illustrating a representation of exemplary digital samples of the analog pulse of  FIG. 5 . 
         FIG. 10  is a graph illustrating a representation of an exemplary pulse defined by the mass spectrometer of  FIG. 1  in summing the digital samples of  FIGS. 6-9 . 
         FIG. 11  is a block diagram illustrating a mass spectrometer in accordance with an exemplary embodiment of the present disclosure. 
         FIG. 12  is a block diagram illustrating an exemplary sampling system, such as is depicted in  FIG. 11 . 
         FIG. 13  is a flowchart illustrating an exemplary architecture and functionality of a sample adjuster depicted in  FIG. 12 . 
         FIG. 14  is a graph illustrating a representation of an output of the sample adjuster of  FIG. 12  upon processing, as input, samples in accordance with  FIG. 6 . 
         FIG. 15  is a graph illustrating a representation of an output of the sample adjuster of  FIG. 12  upon processing, as input, samples in accordance with  FIG. 7 . 
         FIG. 16  is a graph illustrating a representation of an output of the sample adjuster of  FIG. 12  upon processing, as input, samples in accordance with  FIG. 8 . 
         FIG. 17  is a graph illustrating a representation of an output of the sample adjuster of  FIG. 12  upon processing, as input, samples in accordance with  FIG. 9 . 
         FIG. 18  is a graph illustrating a representation of an exemplary pulse defined by the mass spectrometer of  FIG. 11  in summing the digital samples of  FIGS. 14-17 . 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure generally relates to mass spectrometers and methods for enhancing resolution of mass spectra. A time-of-flight mass spectrometer in accordance with one exemplary embodiment of the present disclosure, for each mass scan, ionizes a mass sample, and an ion detector provides an analog signal indicative of detected ion abundance as a function of time. The analog signal is sampled, and digitized samples from different mass scans are summed to define a resultant mass spectrum. The number of mass scans is selected to provide a desired statistical accuracy for the resultant mass spectrum. 
     During each mass scan, a sampling system samples the analog signal from the ion detector to provide digitized samples representative of the analog signal. The sampling system detects peaks in the digitized samples and, for each detected peak, identifies one sample representing the maximum sampled point of the detected peak, referred to hereafter as the peak&#39;s “maximum sample.” All of the samples of the peak, except for the maximum sample, are suppressed so that the peak&#39;s maximum sample is the only unsuppressed sample representative of the detected peak. In particular, the sampling system sets all of the other samples of the detected peak to a value of zero. The digitized samples from the sampling system for the current mass scan are then summed with corresponding digital samples from previous mass scans. By suppressing at least some of the samples of the detected peaks other than the maximum sample for each peak, the resolution of the resultant mass spectrum is improved. 
     In other embodiments, more than one sample for each peak may be unsuppressed by the sampling system. For example, the three samples of each peak having the highest values may be unsuppressed by the sampling system. Other numbers of samples per peak, may be unsuppressed in other embodiments. Further, it is unnecessary for the same number of samples for each peak to be unsuppressed by the sampling system. For example, the sampling system may allow only one sample of a first peak to pass unsuppressed but allow three samples of another peak to pass unsuppressed. 
       FIG. 1  illustrates a conventional time-of-flight mass spectrometer  10 . A mass sample to be analyzed is introduced into an ion source  11  that ionizes the sample. The ions so produced are accelerated by applying a potential between the ion source  11  and an electrode  12 . The measurement of the mass sample to be analyzed is composed of multiple mass scans. At the beginning of each mass scan, a controller  15  causes a short pulse to be applied between the electrode  12  and ion source  11  by sending the appropriate control signal to a pulse source  17 . The controller  15  also resets the contents of a write address register  21 . On subsequent clock cycles, the address register  21  is incremented by a signal from a clock  24 , and an analog signal generated by an ion detector  25  is digitized by an analog-to-digital converter (A/D)  27 . The value stored in memory  29  at the address specified in the address register  21  is applied to an adder  33 , which adds the stored value to the value provided by A/D converter  27 . The summed value is then stored back in memory  29  at the address in question. 
     As noted above, the time required by an ion to traverse the distance between the electrode  12  and the detector  25  is a measure of the mass of the ion. This time is proportional to the value in address register  21  when the ion strikes the detector  25 . Hence, memory  29  stores data that can be used to generate a graph of the number of ions with a given mass as a function of the mass. In other words, the data stored in memory  29  defines a mass spectrum of the sample being analyzed. 
     Various devices, such as a Faraday cup, multichannel plate (MCP), electron multiplier (continuous structure as well as dynode structure), conversion dynode, Daly detector, and combinations thereof, may be used to implement the ion detector  25 . The signal generated by the ion detector  25  depends on the number of ions striking the detector  25  during the clock cycle in question. Moreover, in a time-of-flight mass spectrometer, heavier mass ions arrive at the ion detector  25  after lighter mass ions. The analog signal from the ion detector  25  as a function of time exhibits peaks that can be identified as originating from ions of specific masses. A pulse in the analog signal is due to ions of a particular mass striking the ion detector  25  over a small duration of time. Ions of the same mass are generally bunched together as they travel toward and strike the ion detector  25  and will be referred to hereafter as an “ion packet.” Thus, ions within the same “packet” have the same mass. Further, pulses of the analog signal from the ion detector  25  will be referred to hereafter as “analog pulses.” 
     In general, the number ions in an ion packet is relatively small, and hence the statistical accuracy of the measurements obtained in any single mass scan is usually insufficient. In addition, there can be a significant amount of noise in the system. The noise is generated both in the detector  25 , analog path, and in the A/D converter  27 . 
     To improve statistical accuracy, the data from a large number of mass scans are summed. At the beginning of the measurement process, the controller  15  stores zeros in all of the memory locations in memory  29  and initiates the first mass scan. When the first mass scan is completed, the controller  15  resets the address register  21  and initiates another mass scan by causing the pulse source  17  to pulse the electrode  12 . The data from the second mass scan is added to that from the previous mass scan. This process is repeated until the desired statistical accuracy is obtained. 
     Unfortunately, small variations in the mass scans degrade resolution of the resultant mass spectrum defined by the data in memory  29 . For example, clock jitter may cause small timing variations in the mass scans, and the effect of these small timing variations to the resultant mass spectrum can become significant as the output of the detector  25  for many different mass scans is summed. Further, variations in the pulse source  17  may cause the electrodes  12  to ionize the mass sample of the ion source  11  such that some ions of the same mass have slightly different initial energies. As a result, some ions of the same mass may strike the detector  25  at slightly different times. In addition, the detector  25  has finite rise and fall times. Thus, even if ions of the same mass were to strike the detector  25  at exactly the same time, the resulting pulse output by the detector  25  would have a width spanning over a finite range of time. The analog path, including the A/D converter  27 , may further increase the width of the analog pulses output by the detector  25 . These and other variations can significantly degrade the resolution of the resultant mass spectrum. 
     To better illustrate the foregoing, refer to  FIGS. 2-5 , which depict exemplary analog pulses  41 - 44  output by the detector  25 . As shown by these figures, each pulse  41 - 44  has a finite width, which is related to the rise and fall times of the detector  25 . Further, ions of the same mass may strike the detector  25  at different times due to certain variations, as described above, thereby increasing the finite widths of the pulses  41 - 44 . 
     For illustrative purposes, assume that the pulses  41 - 44  depicted by  FIGS. 2-5 , respectively, are corresponding analog pulses output by the detector  25  for different mass scans. As used herein, pulses are “corresponding” if they are representative of ions of the same mass. Thus, each of the pulses  41 - 44  shown in  FIGS. 2-5  ideally would occur at the same time (x) after the start of its respective mass scan, and these pulses are digitized and summed to define a single digital pulse in the resultant mass spectrum. However, as can be seen by comparing  FIGS. 2-5 , there can be slight timing offsets between the pulses  41 - 44  due to variations in the pulse source  17  and/or the detector  25 . In this regard, assume that  FIGS. 2-5  depict corresponding pulses  41 - 44  of four consecutive mass scans. The absolute peak of the pulse  41  shown by  FIG. 2  occurs at time x after the start of the first mass scan, but the absolute peak of the pulse  42  shown by  FIG. 3  occurs at a time greater than x after the start of the second mass scan. Further, the absolute peak of the pulse  43  shown by  FIG. 4  occurs at a time less than x after the start of the third mass scan, and the absolute peak of the pulse  44  shown by  FIG. 5  also occurs at a time less than x after the start of the fourth mass scan. 
     Each of the pulses  41 - 44  is digitized by the A/D converter  27  ( FIG. 1 ), which outputs digital samples of the pulses  41 - 44 . In this regard,  FIGS. 6-9  respectively depict digital pulses  45 - 48  that are defined by sampling the analog pulses  41 - 44  of  FIGS. 2-5 . Each point of the pulses  45 - 48  represents a sample of one of the analog pulses  41 - 44 . In particular,  FIG. 6  depicts a digital pulse  45  that is formed by digitally sampling the analog pulse  41  ( FIG. 2 ), and  FIG. 7  depicts a digital pulse  46  that is formed by digitally sampling the analog pulse  42  ( FIG. 3 ). Further,  FIG. 8  depicts a digital pulse  47  that is formed by digitally sampling the analog pulse  43  ( FIG. 4 ), and  FIG. 9  depicts a digital pulse  48  that is formed by digitally sampling the analog pulse  44  ( FIG. 5 ). 
       FIG. 10  depicts a digital pulse  49 , referred to as the “resultant pulse,” resulting from the summation of the pulses  45 - 48  in  FIGS. 6-9  as would be performed by the conventional mass spectrometer  10  ( FIG. 1 ). The resultant pulse  49  has a relatively large width (z−y) in the time domain based on the widths of the pulses  41 - 44 . Moreover, the aforedescribed offsets in timing of the analog pulses  41 - 44  increase, not only the widths of pulses  41 - 44 , but also the overall width of the resultant pulse  49 . 
       FIG. 11  depicts a time-of-flight mass spectrometer  50  in accordance with an exemplary embodiment of the present disclosure. To simplify the description of  FIG. 11  and subsequent drawings, those elements that serve functions analogous to elements discussed above with reference to  FIG. 1  have been given the same numeric designations. 
     As shown by  FIG. 11 , the mass spectrometer  50  comprises an ion source  11 , a controller  15 , a pulse source  17 , a write address register  21 , a clock  24 , an ion detector  25 , memory  29 , an adder  33 , and a sampling system  51 . As shown by  FIG. 12 , the sampling system  51  comprises an A/D converter  27 . The elements  17 ,  21 ,  24 ,  25 ,  27 ,  29 , and  33  perform essentially the respective functions as the elements of the same reference numerals in  FIG. 1 . 
     As described above with reference to  FIG. 1 , a mass sample to be analyzed is introduced into the ion source  11  that ionizes the mass sample. A pulse from the pulse source  17  causes the ions in the ion source  11  to be accelerated toward the ion detector  25 , which detects the accelerated ions. The ion detector  25  outputs an analog signal indicative of the detected ions. 
     As in  FIG. 1 , the analog signal output by the detector  25  of  FIG. 11  is sampled by the A/D converter  27  of  FIG. 12 . Referring to  FIG. 12 , the digitized samples from the A/D converter  27  are buffered by a buffer  77  and then processed by a sample adjuster  78 , which will be described in more detail hereafter. Similar to the conventional mass spectrometer  10  of  FIG. 1 , digital samples from the sample adjuster  78  of  FIG. 12  are summed by a summer  33  ( FIG. 11 ) with samples from previous mass scans, and the results of the summing are stored to memory  29 . 
     Thus, once the spectrometer  50  of  FIG. 11  takes a measurement, which preferably includes a large number of mass scans, the memory  29  is storing measurement data similar to the embodiment depicted by  FIG. 1 . Each address in memory  29  is storing a running sum of digitized samples and represents a data point of the resultant mass spectrum defined by the measurement data in memory  29 . 
     The controller  15  and the sample adjuster  78  can be implemented in hardware, software, or a combination thereof. As an example, the controller  15  and/or the sample adjuster  78  may be implemented in software and executed by a programmable logic array, a digital signal processor (DSP), a central processing unit (CPU), or other type of apparatus for executing the instructions of the controller  15  and/or the sample adjuster  78 . In other embodiments, the controller  15  and/or the sample adjuster  78  can be implemented in firmware or hardware, such as logic gates, for example. 
     The sample adjuster  78  is configured to identify peaks in the samples received from the A/D converter  27 . Further, for each identified peak, the sample adjuster  78  designates at least one sample as being an “active sample.” As used herein, an “active sample” refers to a sample that is not to be suppressed by the sample adjuster  78 . 
     Preferably, the sample adjuster  78 , for each peak, is configured to identify a predefined number of the samples having the highest values as the peak&#39;s active samples. Thus, the active samples for a given peak represent the peak&#39;s maximum samples. In one embodiment, as will be described in more detail hereafter, the sample adjuster  78 , for each peak, only identifies the sample having the highest value (i.e., the peak&#39;s maximum sample) as an active sample such that each peak has only one active sample. Further, the sample adjuster  78  allows all active samples to pass unsuppressed but suppresses all of the other samples (i.e., each sample not identified as an “active sample” by the sample adjuster  78 ). As used herein, a sample is “suppressed” when it is assigned a value lower than its actual value, as determined by the A/D converter  27 , or it is prevented from affecting the data defining the resultant mass spectrum. In a preferred embodiment, the sample adjuster  78  suppresses a sample by assigning such sample a value of zero (0). Thus, each suppressed sample does not affect the resultant mass spectrum defined by the data stored in memory  29 . 
     There are various techniques that may be employed by the sample adjuster  78  to identify peaks. In one embodiment, the sample adjuster  78  identifies a peak when a string of at least a minimum number, s, of consecutive samples having increasing values is immediately followed by a string of at least a minimum number, t, of consecutive samples having decreasing values. Note that the numbers s and t may be specified by a user or predefined within the sample adjuster  78 . Further, s and t may be equal. 
     When a peak is detected, the maximum sample is the sample within the foregoing two strings having the highest value. Such a sample is preferably identified by the sample adjuster  78  as an active sample for the identified peak. Moreover, the sample adjuster  78  allows each sample identified as an active sample to pass unchanged through the sample adjuster  78  and suppresses each of the other samples. 
     To better illustrate the foregoing, assume that the ion detector  25  of spectrometer  50  outputs the corresponding analog pulses  41 - 44  in consecutive mass scans, as described above for the conventional spectrometer  10 . In such an example, the A/D converter  27  receives the pulses  41 - 44  and outputs the digital pulses  45 - 48  shown by  FIGS. 6-9  in response to the pulses  41 - 44 , respectively. Assume that samples  85 - 88  are the maximum samples of the pulses  45 - 48 , respectively, and that the sample adjuster  78  is configured to identify, for each peak, only the peak&#39;s maximum sample as an active sample. In such an example, the sample adjuster  78 , upon identifying the peak of pulse  45 , suppresses all of the samples of the pulse  45  except the maximum sample  85 . 
     Various techniques may be used to identify the peak of the pulse  45  and to suppress all of the samples of the pulse  45  except the maximum sample  85 .  FIG. 13  illustrates an exemplary process that may be used to achieve the foregoing. In this regard, samples are written to and read out of the buffer  77  ( FIG. 12 ) on a first-in, first-out (FIFO) basis. During the first mass scan, samples of the pulse  45  are written to the buffer  77  by the A/D converter  27  as the converter  27  is sampling the analog pulse  41 . As shown by block  112 , the sample adjuster  78  analyzes the samples stored in the buffer  77  to determine whether these samples define a peak. In the instant embodiment, the sample adjuster  78  compares the samples in the buffer  77  and determines that these samples define a peak if such samples include at least s number of consecutive samples of increasing values followed by at least t number of consecutive samples of decreasing values. 
     Other techniques for identifying a peak of the pulse  45  are also possible in other embodiments. As an example, the sample adjuster  78  may identify any sample as being a peak if it is immediately preceded by a sample of lower value and immediately followed by a sample of lower value within the next two samples. 
     If the samples in buffer  77  do not define a peak, then the sample adjuster  78  reads and suppresses the next sample to be read out of the buffer  77 . In particular, the sample adjuster  78  reads the next sample from the buffer  77  and outputs a value of zero, as shown by blocks  120  and  122 , effectively replacing the sample&#39;s actual value with the value of zero (0). The value output by the sample adjuster  78  is then summed by summer  33  with a running sum from memory  29  at the address specified by the address register  21 . Note that, as a sample is being read out of the buffer  77  by the sample adjuster  78 , a new sample is being written to the buffer  77  by the A/D converter  27 . If the current measurement being performed by the spectrometer  50  is not yet complete, then the sample adjuster  78  makes a “no” determination in block  124  and analyzes, in block  112 , the samples, including the new sample written to the buffer  77 , currently stored in the buffer  77 . 
     Once the sample adjuster  78  determines that the buffer  77  is storing a peak of a pulse  45 , then the sample adjuster  78  identifies the active samples of the peak, as shown by block  133 . In the instant example, assume that the sample adjuster  78 , for each peak, only identifies the peak&#39;s maximum sample as an active sample. Thus, the an active sample is determined to be the highest value stored in the buffer  77  when the sample adjuster  78  makes a “yes” determination in block  115  assuming that the buffer  77  is small enough such that it is unlikely that multiple peaks representing different ion packets can be simultaneously stored in the buffer  77 . Thus, the sample adjuster  78  can compare each of the samples in the buffer  77  to find the sample with the highest value and identify this sample as the peak&#39;s “active sample,” which represents the peak&#39;s maximum sample. Other techniques for identifying the active sample or samples of a peak may be employed in other embodiments. 
     In block  136 , the sample adjuster  78  reads the next sample from the buffer  77  on a FIFO basis and, in block  138 , determines whether this sample is an active sample. If not, the sample adjuster  78  suppresses this sample. In particular, upon reading the next sample in block  136 , the sample adjuster  78  outputs a value of zero, as shown by block  141 , effectively replacing the sample&#39;s actual value with the value of zero (0). 
     However, if the value read from the buffer  77  in block  136  is an active sample, then the sample adjuster  78  outputs the sample without changing its value, as shown by block  144 . The value currently output by the sample adjuster  78  in either block  141  or block  144  is then summed by summer  33  with a running sum from memory  29  at the address specified by the address register  21 . Further, in block  145  the sample adjuster  145  determines whether there are any additional active samples for the peak identified in block  133 . In the instant example, there is only one active sample per peak. Thus, a “yes” determination should be made in block  145 , and the sample adjuster  78  goes to block  124 . However, in other examples for which there are more than one active sample per peak, it is possible for a “no” determination to be made in block  145 . In such a case, the sample adjuster  78  returns to block  136 . 
     Moreover, in the instant example, rather than outputting the digital pulse  45  to the summer  33  as is done in the conventional spectrometer  10 , the sample adjuster  78  outputs the samples shown by  FIG. 14 . As shown by  FIG. 14 , all of the samples of the pulse  45 , except for the maximum sample  86 , are suppressed by the sample adjuster  78 . Thus, only the maximum sample  86  of the identified peak actually changes any of the running sums stored in the memory  29  and, therefore, affects the resultant spectrum defined by the data in memory  29 . 
     Moreover, the aforedescribed process is repeated for the digital pulses  46 - 48  output by the A/D converter  27  for subsequent mass scans. In particular, in the next consecutive mass scan, the A/D converter  27  outputs the digital pulse  46  shown by  FIG. 7 . The sample adjuster  78 , however, suppresses all of the samples defining pulse  46  except for the maximum sample  86 . Thus, the sample adjuster  78  converts the digital pulse  46  of  FIG. 7  into that shown by  FIG. 15 . In the next consecutive mass scan, the A/D converter  27  outputs the digital pulse  47  shown by  FIG. 8  and suppresses all of the samples defining pulse  47  except for the maximum sample  87 . Thus, the sample adjuster  78  converts the digital pulse  47  of  FIG. 8  into that shown by  FIG. 16 . Further, in the following mass scan, the A/D converter  27  outputs the digital pulse  48  shown by  FIG. 9  and suppresses all of the samples defining pulse  48  except for the maximum sample  88 . Thus, the sample adjuster  78  converts the digital pulse  48  of  FIG. 9  into that shown by  FIG. 17 . 
       FIG. 18  depicts an exemplary resultant pulse  149  defined by summing the samples shown by  FIGS. 14-17 . As a result of the processing performed by the sample adjuster  78 , as described above, the resultant pulse  149  has a width (b−a) that is more narrow than that of the resultant pulse  49  defined by the conventional spectrometer  10 . Accordingly, the processing performed by the sample adjuster  78  enhances the resolution of the resultant mass spectrum defined by the data stored in the memory  29 . 
     Note that it is possible for multiple samples of the same peak to have the same value. For example, a sample on the leading edge of a peak may have the same value as a sample on the trailing edge of the same peak. If more than one sample of the same peak are equal and have the highest sampled value for the peak, then the sample adjuster  78  may be configured to select any of the equal samples as the peak&#39;s active sample in block  133  of  FIG. 13 . 
     For example, when the two highest samples for a given peak are equal, the sample adjuster  78  may always select the earliest of the two equal samples or, in another embodiment, may always select the latest of the two equal samples. In another embodiment, the sample adjuster  78  may select the earliest and latest samples per peak in an alternating fashion. For example, for the first peak for which the highest two samples are equal, the sample adjuster  78  may select the earliest of the two equal samples as the first peak&#39;s maximum sample. For the second peak for which the highest two samples are equal, the sample adjuster  78  may select the latest of the two equal samples as the second peak&#39;s maximum sample. For the next peak for which the two highest samples are equal, the sample adjuster  78  may select the earliest of the two equal sample as the peak&#39;s maximum sample, and so on for the remaining peaks. 
     In addition, as described above, it is unnecessary for the sample adjuster  78  to allow only one sample to pass unsuppressed. For example, the sample adjuster  78  may allow the three highest samples per peak to pass unsuppressed. Other numbers of samples may be allowed to pass unsuppressed through the sample adjuster  78  per peak in other examples. 
     Generally, increasing the number of samples per peak allowed to pass unsuppressed decreases the resolution of the peaks of the resultant mass spectrum defined by the data stored in memory  29  but increases the accuracy of the peak centers for this resultant mass spectrum. Thus, a trade-off between peak resolution and center-of-peak accuracy exists when selecting the number of samples per peak that the sample adjuster  78  is to pass unsuppressed. 
     Indeed, to enhance peak resolution for the resultant mass spectrum thereby reducing center-of-peak accuracy, fewer samples per peak should be allowed to pass through the sample adjuster  78  unsuppressed. For example, to maximize peak resolution, one sample per peak may be allowed to pass through the sample adjuster  78  unsuppressed, as described above. However, to enhance center-of-peak accuracy for the resultant mass spectrum thereby reducing peak resolution, more samples per peak may be allowed to pass through the sample adjuster  78  unsuppressed. For example, to maximize center-of-peak accuracy thereby reducing peak resolution, all of the samples per peak may be allowed to pass through the sample adjuster  78  unsuppressed. Moreover, the number of samples per peak allowed to pass through the sample adjuster  78  unsuppressed may be selected to optimize peak resolution and center-of-peak accuracy considerations. 
     The number of samples per peak identified as active samples and, therefore, allowed to pass through the sample adjuster  78  unsuppressed may be predefined in at least some embodiments. For example, a user may specify such number prior to a measurement of a mass sample. Alternatively, the sample adjuster  78  may store a default number that is used unless the user specifies another number to be used for a measurement. In another embodiment, the sample adjuster  78  may be hardcoded to allow a certain number of samples to pass unsuppressed for each peak. Other techniques for controlling which samples are suppressed and unsuppressed are possible in other embodiments. 
     Regardless of the number of samples to be selected as “active samples” that are to pass through the sample adjuster  78  unsuppressed for a given peak, it is generally desirable for the highest sample values to be so selected. For example, if only one sample is to be selected as an active sample for a peak and, therefore, to remain unsuppressed, then it is desirable for the selected sample for the peak to be the one with the highest value (i.e., the peak&#39;s maximum sample). If three samples are to be selected as active samples for a peak, then it is desirable for the selected samples for the peak to be the ones with the three highest values. Ensuring that the highest values are selected as the active samples generally increases the accuracy of the resultant spectrum stored in memory  29 .