Systems and methods for dynamically adjusting sampling rates of mass spectrometers

A mass spectrometer includes an ion detector, an analog-to-digital (A/D) converter, and a decimator. The analog-to-digital (A/D) converter is configured to receive and sample an analog signal from the ion detector thereby providing a first plurality of samples at a first rate. The decimator is configured to receive the first plurality of samples and to transmit, at a second rate, a second plurality of samples that are based on the first plurality of samples. The decimator is further configured to dynamically adjust the second rate so that memory requirements for the mass spectrometer are reduced.

RELATED ART

In time-of-flight mass spectrometers (TOFMS), a mass specimen 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.

The amount of data required to accurately define the mass spectra measured by the mass spectrometer can be significant requiring a large amount of memory, which can be expensive and prohibitively complicated. Moreover, reducing the memory requirements of a mass spectrometer is generally desirable so that the cost and complexity of the mass spectrometer can be reduced.

SUMMARY OF THE DISCLOSURE

Generally, embodiments of the present disclosure provide mass spectrometers and methods for dynamically adjusting sampling rates for signals from ion detectors.

A mass spectrometer in accordance with one exemplary embodiment of the present disclosure comprises an ion detector, an analog-to-digital (A/D) converter, and a decimator. The analog-to-digital (A/D) converter is configured to receive and sample an analog signal from the ion detector thereby providing a first plurality of samples at a first rate. The decimator is configured to receive the first plurality of samples and to transmit, at a second rate, a second plurality of samples that are based on the first plurality of samples. The decimator is further configured to dynamically adjust the second rate so that memory requirements for the mass spectrometer are reduced.

A method in accordance with another exemplary embodiment of the present disclosure comprises: detecting ions; sampling an analog signal indicative of the detected ions thereby providing a first plurality of samples at a first rate; transmitting, at a second rate, a second plurality of samples of the analog signal to a summer, the second plurality of samples based on the first plurality of samples; storing in memory values defining a mass spectrum; summing, via the summer, the second plurality of samples with the values; and dynamically adjusting the second rate.

A method in accordance with yet another exemplary embodiment of the present disclosure comprises: detecting ions; sampling an analog signal indicative of the detected ions to provide a first plurality of samples at a first rate; sampling the analog signal to provide a second plurality of samples at a second rate that is lower than the first rate; and summing the first and second plurality of samples with values stored in memory, the values defining a mass spectrum.

DETAILED DESCRIPTION

The present disclosure generally relates to mass spectrometers and methods for dynamically adjusting an effective sampling rate of a signal from an ion detector so that memory requirements can be reduced. A time-of-flight mass spectrometer in accordance with one exemplary embodiment of the present disclosure, for each mass scan, ionizes a mass specimen, 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, the effective sampling rate of the analog signal is changed. In one embodiment, the analog signal is effectively sampled at a relatively high rate at the beginning of the mass scan as compared to later in the mass scan. Thus, as the mass scan progresses, the number of digitized samples provided for summing per unit of time decreases thereby reducing the number of memory locations required to store the resultant mass spectrum as compared to an embodiment in which the analog signal is effectively sampled at the same high rate throughout the entire mass scan.

FIG. 1illustrates a conventional time-of-flight mass spectrometer10. A mass specimen to be analyzed is introduced into an ion source11that ionizes the specimen. The ions so produced are accelerated by applying a potential between the ion source11and an electrode12. The measurement of the mass specimen is composed of multiple mass scans. At the beginning of each mass scan, a controller15causes a short pulse to be applied between the electrode12and ion source11by sending the appropriate control signal to a pulse source17. The controller15also resets the contents of a write address register21. On subsequent clock cycles, the address register21is incremented by a signal from a clock24, and an analog signal generated by an ion detector25is digitized by an analog-to-digital converter (A/D)27. The value stored in memory29at the address specified in the address register21is applied to an adder33, which adds the stored value to the value provided by A/D converter27. The summed value is then stored back in memory29at the address in question.

As noted above, the time required by an ion to traverse the distance between the electrode12and the detector25is a measure of the mass of the ion. This time is proportional to the value in address register21when the ion strikes the detector25. Hence, memory29stores 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 memory29defines a mass spectrum of the mass specimen 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 detector25. The signal generated by the ion detector25depends on the number of ions striking the detector25during the clock cycle in question. In general, this number is relatively small, and hence the statistical accuracy of the measurements obtained in any single mass scan is usually insufficient. In addition, there is a significant amount of noise in the system. The noise is generated in the detector25, analog path, and in the A/D converter27.

To improve statistical accuracy, the data from a large number of mass scans are summed. At the beginning of the measurement process, the controller15stores zeros in all of the memory locations in memory29and initiates the first mass scan. When the first mass scan is completed, the controller15resets the address register21and initiates another mass scan by causing the pulse source17to pulse the electrode12. 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.

Thus, each memory address stores a sum of corresponding samples from different mass scans. As used herein, samples are “corresponding” if they are taken at the same time after the start of their respective mass scans. For example, a sample taken at a time, t1, after the start of a first mass scan corresponds to a sample taken at the same time, t1, after the start of another mass scan. The sum of corresponding samples from each of the mass scans is stored in one of the addresses in memory29and represents a data point of the resultant mass spectrum. Note that the start of a mass scan refers to the generation of the pulse that ionizes the mass specimen being analyzed by the mass scan.

Further, in embodiments for which the write address register21is incremented for each clock cycle, contiguous memory addresses store samples that are consecutive in terms of time. For example, assuming that the address register is four digits and is reset by the controller15at the beginning of a mass scan, as described above, the data value stored at address0000represents the sum of digital samples taken during the first clock cycle of each mass scan. The data value stored at address0001represents the sum of digital samples taken during the second clock cycle of each mass scan, and so on. Moreover, the foregoing is illustrated byFIG. 2.

In this regard, address (n) of memory29stores the sum (samplen+1) of digital samples taken during the (n+1)thclock cycle after the start of each mass scan, where n is a positive integer less than the total number of addresses in memory29. Address (n−1) of memory29stores the sum (samplen) of digital samples taken during the (n)thclock cycle after the start of each mass scan, and address (n−2) of memory29stores the sum (samplen−1) of digital samples taken during the (n−1)thclock cycle after the start of each mass scan. Further, address (n−3) of memory29stores the sum (samplen−2) of digital samples taken during the (n−2)thclock cycle after the start of each mass scan, and address (n−4) of memory29stores the sum (samplen−3) of digital samples taken during the (n−3)thclock cycle after the start of each mass scan.

Unfortunately, the amount of memory29required to store all of the data points of the resultant mass spectrum can be quite large thereby increasing the cost and complexity of the mass spectrometer10. In this regard, the memory29has a number of addresses equal to or greater than the number of data points used to define the resultant mass spectrum. Most time-of-flight mass spectrometers sample the analog signal from the ion detector25at a very high rate to achieve a desired resolution and, therefore, create an extremely large number of data points.

FIG. 3depicts a time-of-flight mass spectrometer50in accordance with an exemplary embodiment of the present disclosure. To simplify the description ofFIG. 3and subsequent drawings, those elements that serve functions analogous to elements discussed above with reference toFIG. 1have been given the same numeric designations.

As shown byFIG. 3, the mass spectrometer50comprises an ion source11, a pulse source17, a write address register21, a clock24, an ion detector25, memory29, an adder33, and a sampling system51. As shown byFIG. 4, the sampling system51comprises an A/D converter27. The elements17,21,24,25,27,29, and33, operating under the direction and control of a controller52, perform essentially the respective functions as the elements of the same reference numerals inFIG. 1. The controller52can be implemented in hardware, software, or a combination thereof. As an example, the controller52may be implemented in software and executed by a digital signal processor (DSP), a central processing unit (CPU), or other type of apparatus for executing the instructions of the controller52. In other embodiments, the controller52can be implemented in firrnware or hardware, such as logic gates, for example.

As described above with reference toFIG. 1, a mass specimen to be analyzed is introduced into the ion source11that ionizes the specimen. A pulse from the pulse source17causes the ions in the ion source11to be accelerated toward the ion detector25, which detects the accelerated ions. The ion detector25outputs an analog signal indicative of the detected ions.

As inFIG. 1, the analog signal output by the detector25ofFIG. 3is sampled by the A/D converter27ofFIG. 4. Referring toFIG. 4, the digitized samples from the A/D converter27are processed by a digital filter65and a decimator66, which will both be described in more detail below. Similar to the conventional mass spectrometer10ofFIG. 1, digital samples from the decimator66ofFIG. 4are summed by a summer33(FIG. 3) with samples from previous mass scans, and the results of the summing are stored to memory29.

Thus, once the spectrometer50ofFIG. 3takes a measurement, which preferably includes a large number of mass scans, the memory29is storing measurement data similar to the embodiment depicted byFIG. 1. Each address in memory29is storing a running sum of digitized samples and represents a data point of the resultant mass spectrum defined by the measurement data in memory29.

In a time-of-flight mass spectrometer, heavier mass ions arrive at the ion detector after lighter mass ions. The analog signal from the ion detector25as 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 detector25over a small duration of time. Ions of the same mass are generally bunched together as they travel toward and strike the ion detector25and 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 detector25will be referred to hereafter as “analog pulses.”

The ions of lighter mass ion packets tend to be bunched closer together than the ions of heavier mass ion packets. Indeed, for lighter mass ion packets, there is statistically less separation in time between multiple ion strikes from the same packet as compared to the statistic separation time for heavier mass ion packets. Moreover, the width of an analog pulse from the ion detector25for a lighter mass ion packet is usually smaller than the width of an analog pulse from the ion detector25for a heavier mass ion packet. Thus, although a high sampling rate may be desired to adequately sample the analog pulse for a lighter mass ion packet, a lower sampling rate may be adequate to sample the analog pulse for a heavier mass ion packet.

The decimator66ofFIG. 4uses the relationship between mass and peak width to dynamically control the effective sampling rate of the sampling system51. As used herein, the “effective sampling rate” refers to the rate at which samples are provided by the system51for summing with the values in memory29that define the resultant mass spectrum. As is apparent from the following description, the effective sampling rate of the system51can be different than the actual rate at which the A/D converter27samples the analog signal from the ion detector25. Further, the term “dynamically” is used herein to describe an event or action that occurs as a mass scan is progressing.

In at least one embodiment, the decimator66allows a higher effective sampling rate for analog pulses corresponding with lighter mass ion packets and lowers the effective sampling rate for analog pulses corresponding with heavier mass ion packets. In this regard, the lighter mass ion packets arrive at the ion detector25before the heavier mass ion packets and, therefore, are sampled first. Thus, for each mass scan, the decimator66initially outputs samples at a high rate. However, over time, heavier mass ion packets begin to arrive at the ion detector25and are sampled. Moreover, after a predefined amount of time has elapsed, the decimator66lowers its rate of outputting samples such that digitized samples for the heavier mass ion packets are output at a lower rate as compared to the samples for the previously sampled lighter mass ion packets. In other words, the decimator66decimates the effective sampling rate of the system51. The decimating of the effective sampling rate reduces the number of samples to be stored in memory29and, therefore, reduces the required memory size of memory29.

In order to further conserve memory space as the mass scan progresses and, therefore, the desire for higher sampling rates abates, the effective sampling rate can be decimated more than once during the same mass scan as the ions in the packets reaching the detector25become heavier. However, once a mass scan is complete, the effective sampling rate is set to the initial rate that is to be used for the lighter mass ion packets to be received for the next mass scan.

In one embodiment, which will be described in more detail hereafter for illustrative purposes, the decimator66ofFIG. 4reduces its output rate by a factor of two (2) each time it decimates the effective sampling rate.FIG. 5shows an exemplary graph of the decimator's output rate versus time for such an embodiment. In this regard, assume that time, t0, represents the beginning of a mass scan of a duration, T. Initially, the decimator's output rate is y. Once one-fourth of the mass scan is complete (i.e., at time T/4), the decimator's output rate is reduced by one-half to y/2. Note that the ions being sampled after time T/4are heavier than the ions sampled for the same mass scan prior to time T/4such that a lower output rate can be used after time T/2. Once one-half of the mass scan is complete (i.e., at time T/2), the decimator's output rate is reduced by one-half to y/4. Further, the ions being sampled after time T/2are heavier than the ions sampled for the same mass scan prior to time T/2such that a lower output rate can be used after time T/2. Once three-quarters of the mass scan is completed (i.e., at time3T/2), the decimator's output rate is again reduced by one-half to y/8. Note that the ions being sampled after time3T/4are heavier than the ions sampled for the same mass scan prior to time3T/4such that a lower output rate can be used after time3T/4. Once the mass scan is complete (i.e., at time T), the decimator's output rate is reset toy for the next mass scan.

In the example shown byFIG. 5, the decimator's output rate is reduced by one-half after completion of each quarter of a mass scan. However, rate adjustments other than one-half are possible, and the timing of when adjustments occur may be different in other embodiments as well. Further, it is unnecessary for the decimator's output rate to be reduced by the same amount for each rate adjustment, and any number of rate adjustments per scan are possible.

Note that it is possible for the sampling rate of the A/D converter27to be adjusted as is described above for the decimator's output rate. In such a case, implementation of the digital filter65, which will be described in more detail hereafter, and the decimator66would be unnecessary. However, changing the sample rate of a high speed A/D converter can be problematic. Moreover, using a decimator66to adjust the effective sampling rate of the system51is generally desirable so that the sampling rate of the A/D converter27may remain constant.

Reducing the output rate of the decimator66reduces the effective sampling rate of the system51even though the actual sampling rate of the A/D converter27remains unchanged. In this regard, the number of samples used to define the resultant mass spectrum is generally equal to the number of samples output by the system51per mass scan, not the number of samples output by the A/D converter27per mass scan. Moreover, the Nyquist criterion specifies that a sampling rate must be twice the highest frequency component of the sampled signal in order to generate an accurate representation of the signal. If the sampling rate of the A/D converter27is close to the minimum Nyquist rate (i.e., half of the highest frequency component of the analog signal from ion detector25), then a reduction of the decimator's output rate is likely to reduce the effective sampling rate of the system51below the minimum Nyquist rate for the analog signal output by the ion detector25. Thus, the digital filter65preferably filters the digitized samples from the A/D converter27such that the Nyquist criterion is not violated due to an adjustment to the system's effective sampling rate by the decimator66.

For example, for each adjustment of the effective sampling rate by the decimator66, the digital filter65may be configured to filter the samples from the A/D converter27such that its bandwidth is reduced by at least the same percentage as the adjustment to the effective sampling rate. Thus, in the embodiment described above referring toFIG. 5in which the decimator66reduces its output rate by one-half for each adjustment, the digital filter65may be configured to reduce its bandwidth by at least one-half each time the decimator66reduces its output rate. For example, referring toFIG. 5, at time T/2, the digital filter65may reduce its bandwidth to one-half of its original bandwidth (i.e., the filter's bandwidth at time t0through T/2). At time T/2, the digital filter65may reduce its bandwidth to one-quarter of its original bandwidth, and at time3T/4, the digital filter65may reduce its bandwidth to one-eighth of its original bandwidth. At time T, the digital filter65may reset its bandwidth to the original bandwidth for the next mass scan. Other algorithms for controlling the bandwidth of the digital filter65to prevent violation of the Nyquist criterion are possible in other embodiments.

If the output rate of the digital filter65is higher than the output rate of the decimator66, such as may initially be the case when the decimator66reduces its output rate and the sampling rate of the A/D converter27remains constant, then at least some of the data from the digital filter65may be lost. For example, assume that the output rate of the decimator66equals the output rate of the digital filter65before an adjustment to the output rate of the decimator66. If the decimator's output rate is reduced by one-half, as described above, then the decimator66may be configured to output every other sample received from the digital filter65. In such an embodiment, one-half of the filtered samples may be discarded or lost.

Alternatively, to provide a more accurate resultant mass spectrum, the decimator66may be configured to combine samples to reduce or eliminate the number of samples discarded. For example, if the decimator's output rate is reduced by one-half, as described above, the decimator66may be configured to take an average of every two consecutive samples from the A/D converter27and to output each of the calculated averages as the digitized samples to be summed by summer33. An exemplary embodiment implementing such an approach will be described in more detail below with reference toFIGS. 4 and 6.

In this regard,FIG. 4shows a detailed view of the decimator66for an exemplary embodiment in which the decimator66averages samples instead of discards samples after a reduction in the system's effective sampling rate by the decimator66. InFIG. 4, the decimator66comprises a buffer77and a sample combiner78. The buffer77may comprise registers (not specifically shown) or other memory devices for buffering the filtered samples from the digital filter65. The sample combiner78may be implemented in hardware, software, or a combination thereof. As an example, the sample combiner78may be implemented in software and executed by a digital signal processor (DSP), a central processing unit (CPU), or other type of apparatus for executing the instructions of the sample combiner78. In other embodiments, the sample combiner78can be implemented in firmware or hardware, such as logic gates, for example. Note that other configurations of the decimator66are possible, and it is unnecessary for such other configurations to include a sample combiner78, such as is depicted inFIG. 4.

Once the buffer77has accumulated a set of consecutive samples to be averaged, the sample combiner78sums the samples within the set and divides the sum by the total number of samples of the set. The result is the average of the samples in the set, and this average is output by the signal combiner78to the summer33(FIG. 3). Moreover, an exemplary operation of the sample combiner78will be described in more detail below.

In the following description, assume for illustrative purposes that the output rate of the decimator66is to be controlled in accordance with the graph depicted byFIG. 5for each mass scan. Initially, as shown by block105ofFIG. 6, the controller52(FIG. 3) initializes a variable x to zero (0) and a constant a to the number of rate adjustments to be performed by the decimator66during the mass scan. In the instant example in which the decimator's output rate is to be adjusted four times for each mass scan, a is equal to four (4).

In block108, the controller52begins a mass scan by causing the pulse source17to generate a pulse, which causes ions in the ion source11to be accelerated toward the ion detector25. In block111, the controller52provides control signals to the decimator66such that the output rate (RD) of the decimator66is equal to the sampling rate (RA/D) of the A/D converter27divided by 2x. Initially, x is zero (0), and RDis, therefore, equal to RA/D. Thus, the controller52increments the address register21each clock cycle similar to the conventional controller15ofFIG. 1. Further, the sample combiner78allows the samples from the digital filter65to pass through the decimator66unchanged. Accordingly, until the decimator's output rate is later adjusted, a sample is provided each clock cycle to the summer33, which sums the sample with a running sum in memory29, similar to the conventional spectrometer10depicted byFIG. 1. In addition, as shown by block111ofFIG. 6, the controller52also provides control signals to the digital filter65such that the bandwidth (BWfilter) of the filter65is initially the same as the bandwidth (BWanalog) of the analog signal from the ion detector25.

InFIG. 6, the expression t represents the amount of time that has elapsed since the start of the current scan, and the expression T represents the total amount of time that elapses during the mass scan. Thus, t is preferably 0 at the start of the mass scan and equals T at the end of the mass scan. When t is greater than or equal to T/a, one-quarter of the mass scan has been completed. Noting that x is initially set to a value of zero (0), the controller52increments x to a value of one (1) once one-quarter of the mass scan is complete, as shown by blocks116and118.

In block121, the controller121compares t to T. Until the end of the mass scan, t will be less than T, and the controller52, therefore, returns to block111. Now that x equals one (1), the controller52provides control signals to reduce the decimator's output rate to one-half of its original output rate or, in other words, to reduce the decimator's output rate to one-half of RA/D. The controller52also provides control signals to the digital filter65to reduce the filter's bandwidth to one-half the bandwidth of the analog signal from the ion detector25.

When the output rate of the decimator66is reduced to one-half of RA/D, the sample combiner78of the decimator66takes an average of every two consecutive samples received from the A/D converter27and outputs such average in lieu of the two consecutive samples. Note that, unless otherwise indicated herein, samples are “consecutive” if they are successively taken by the A/D converter27. In addition, the controller52begins to increment the address register21every other clock cycle rather than every clock cycle. Further, a value is read out of the memory29and summed by summer33with the output of decimator66only when the address register21is incremented. The result of foregoing changes is illustrated byFIG. 7.

In this regard, assume that address (n−4) is storing the running sum for the last sample output by the decimator66just before the decimator's output rate is reduced from RA/Dto one-half of RA/D. In such an example, address (n−4) is storing Samplen−3as it does in the embodiment depicted byFIG. 1, as can be seen by comparingFIGS. 2 and 7. However, after outputting samplen−3, the sample combiner78begins to average every two (2) consecutive samples from the A/D converter27. Thus, address (n−3) stores the average of samplen−2and samplen−1, and address (n−2) stores the average of samplenand samplen+1, as shown byFIG. 7. Accordingly, as can be seen by comparingFIGS. 2 and 7, the amount of memory used to store the data for samplen−3to samplen+1, is reduced as compared to an embodiment that does not adjust the effective sampling rate of the system51. In this regard,FIG. 2shows that five memory addresses are used in the conventional spectrometer10ofFIG. 1to store the data for samplen−3to samplen+1, but only three addresses are used in the spectrometer52of the current example.

After one-half of the mass scan is complete, the controller52makes a “yes” determination in block116ofFIG. 6and, therefore, increments x to a value of two (2) in block118. Now that x equals two (2), the controller52provides control signals to reduce the decimator's output rate to one-fourth of its original output rate or, in other words, to reduce the decimator's output rate to one-quarter of RA/D. The controller52also provides control signals to the digital filter65to reduce the filter's bandwidth to one-quarter of the bandwidth of the analog signal from the ion detector25.

When the output rate of the decimator66is reduced to one-quarter of RA/D, the sample combiner78of the decimator66takes an average of every four consecutive samples received from the A/D converter27and outputs such average in lieu of the four consecutive samples. In addition, the controller52begins to increment the address register21every four clock cycles rather than every other clock cycle. Thus, an average for four samples from A/D converter27is summed every four clock cycles with a corresponding running sum in memory29.

After three-quarters of the mass scan is complete, the controller52makes a “yes” determination in block116and, therefore, increments x to a value of three (3) in block118. Now that x equals three (3), the controller52provides control signals to reduce the decimator's output rate to one-eighth of its original output rate or, in other words, to reduce the decimator's output rate to one-eighth of RA/D. The controller52also provides control signals to the digital filter65to reduce the filter's bandwidth to one-eighth of the bandwidth of the analog signal from the ion detector25.

When the output rate of the decimator66is reduced to one-eighth of RA/D, the sample combiner78of the decimator66takes an average of every eight consecutive samples received from the A/D converter27and outputs such average in lieu of the eight consecutive samples. In addition, the controller52begins to increment the address register21every eight clock cycles rather than every four clock cycles. Thus, an average for eight samples from A/D converter27is summed every eight clock cycles with a corresponding running sum in memory29.

From the foregoing description it can be seen that controller52increments address register21at a rate equal to the output rate of decimator66.

At the end of the mass scan, the controller52makes a “yes” determination in block116, as well as in block121. Thus, in block125, the controller52determines whether another mass scan is to be run. If the desired number of mass scans have yet to be performed to achieve the desired statistical accuracy for the resultant mass spectrum, the controller52resets x to a value of zero (0) in block128and repeats the aforedescribed process for the next mass scan. Once the desired number of mass scans have been performed, the process depicted byFIG. 6ends, and the memory29is storing data points defining the resultant mass spectrum.

Since the effective sampling rate of the system51is reduced during the mass scan, the total number of data points in the memory29for the spectrometer52is less than the total number of data points in memory29for the conventional spectrometer10ofFIG. 1. However, even though the effective sampling rate is ultimately reduced, the lighter mass ions in the spectrometer52can be effectively sampled at a relatively high rate as compared to the heavier mass ions, which arrive at the ion detector25later in the mass scan. Moreover, any adverse impact to the performance of the mass spectrometer52due to the reduction in the effective sampling rate can be minimized by strategically selecting the timing and the amount of the sampling rate adjustments such that lighter mass ions are effectively sampled at a higher rate as compared to heavier mass ions, as described herein.

It should be noted that it is unnecessary for the effective sampling rate to be reduced by a factor of two for each rate reduction or for the effective sampling rate to be periodically reduced at equal time intervals. Further, it is unnecessary for the bandwidth of the filter65to be reduced by the same percentage or at the same times as the effective sampling rate reductions provided that the Nyquist criterion remains satisfied.