Saturation correction for ion signals in time-of-flight mass spectrometers

Abstract

The invention relates to time-of-flight mass spectrometers in which individual time-of-flight spectra are measured by detection systems with limited dynamic measurement range and are summed to sum spectra. The invention proposes a method to increase the dynamic range of measurement of the spectrum. To achieve this, those ion signals whose measured values display saturation of the analog-to-digital converter (ADC) are replaced by correction values, particularly if several successive measured values are in saturation. The correction values are obtained from the width of the signals, preferably simply from the number of measured values in saturation.

Claims

1. A method for increasing a dynamic measurement range of a spectrum acquisition of a time-of-flight mass spectrometer, comprising: acquiring a single individual time-of-flight spectrum at uniform amplification containing ion signals, each ion signal having a multitude of measured values, and at least a first one of said ion signals having been driven into saturation; replacing those ion signals that were driven into saturation with correction values, wherein an intensity of the correction values for the first ion signal is determined from a width of the measured values at a saturation upper intensity limit of the first ion signal; and adding the single individual time-of-flight spectrum, corrected accordingly, to a sum time-of-flight spectrum.

2. The method according to claim 1, wherein the correction values are provided in a memory device and are ordered according to the number of measured values of an ion signal in saturation.

3. The method according to claim 2, wherein the memory device comprises a table, and the correction values in the table are obtained by calibration measurements of the isotope patterns of substance ions in time-of-flight spectra.

4. The method according to claim 1, wherein for determining the correction values the times of flight of the ions in the ion signal are used additionally.

5. The method according to claim 4, wherein the correction values are provided in a memory device and are ordered according to the number of measured values of an ion signal in saturation and according to time-of-flight ranges.

6. The method according to claim 1, wherein corresponding correction values replace each measured value at the upper intensity limit.

7. A method for increasing a dynamic measurement range of a spectrum acquisition of a time-of-flight mass spectrometer, comprising: acquiring a single individual time-of-flight spectrum at uniform amplification containing ion signals, each ion signal having a multitude of measured values; replacing those ion signals that were driven into saturation with correction values, wherein an intensity of the correction values is calculated from a signal shape in a part of an ion signal in saturation, which is not at an upper intensity limit, in the same single individual time-of-flight spectrum; and adding the single individual time-of-flight spectrum, corrected accordingly, to a sum time-of-flight spectrum.

8. The method according to claim 7, wherein the correction values are provided in a memory device and are ordered according to the number of measured values of an ion signal in saturation.

9. The method according to claim 8, wherein the memory device comprises a table, and the correction values in the table are obtained by calibration measurements of the isotope patterns of substance ions in time-of-flight spectra.

10. The method according to claim 7, wherein for determining the correction values the times of flight of the ions in the ion signal are used additionally.

11. The method according to claim 10, wherein the correction values are provided in a memory device and are ordered according to the number of measured values of an ion signal in saturation and according to time-of-flight ranges.

12. The method according to claim 7, wherein corresponding correction values replace each measured value at the upper intensity limit.

13. A method for increasing a dynamic measurement range of a spectrum acquisition of a time-of-flight mass spectrometer, comprising: acquiring a single individual time-of-flight spectrum at uniform amplification containing ion signals, each ion signal having a multitude of measured values, and at least a first one of said ion signals having been driven into saturation; replacing those ion signals that were driven into saturation with correction values, wherein an intensity of the correction value for the first ion signal is determined from a value equal to a total number of the measured values for the first ion signal that are at a saturation upper intensity limit; and adding the single individual time-of-flight spectrum, corrected accordingly, to a sum time-of-flight spectrum.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates the statistical relationship between the true maximum of the ion signals beyond the saturation value of 255 and the number of measurements in saturation (black dots).

(2) FIG. 2 demonstrates that there is a wide range for the signal strengths before one measured value in saturation becomes a sequence of two measured values in saturation. An approximate correction of the signal overshoots can only be achieved by means of a large number of corrections for ion signals of the same mass.

(3) FIG. 3 presents the calculated isotope group of signals for a peptide with mass 2000 atomic mass units. In the upper diagram, the intensities are displayed in a linear mode, in the bottom diagram, they are displayed logarithmically. If, for instance, the signals 1, 2, 3, and 4 are saturated in a measured spectrum, their true height can be calculated from the signals 5, 6, 7, and 8.

(4) FIG. 4 is a flow diagram that illustrates an exemplary method for increasing a dynamic measurement range of a time-of-flight mass spectrometer.

DETAILED DESCRIPTION OF THE INVENTION

(5) As already briefly described above, the invention provides a method to increase the dynamic measurement range for the spectrum acquisition. To achieve this, those ion signals which drive the analog-to-digital converter (ADC) into saturation in an individual time-of-flight spectrum are replaced by correction values, particularly if the saturation values extend over several successive measurements. The corrections are derived from the width of the signals, preferably simply from the number of measured values in saturation. Since the signal forms can change as a function of the mass of the ions, the correction values can additionally depend on the time of flight. The correction values can be stored in a table, ordered according to signal widths and time-of-flight ranges, for example. The table values are obtained from large numbers of calibration measurements or calculated using the measured or calculated signal shapes for ions of the same mass.

(6) This presupposes that the SEM is adjusted in such a way, as described in the introduction, that a maximum dynamic measurement range is obtained without losing individual ion signals.

(7) In a very simple, but already very effective embodiment, all measured values which are sampled in an ADC at a rate of eight gigasamples per second and with eight bit depth, for example, are investigated immediately as usual in an FPGA or a DSP for the presence of a signal maximum. If a signal maximum is present, the maximum measured value and the corresponding time of flight, which exists as a counting index with at least 24 bit depth, is sent to a special arithmetic unit, which adds the measured value at the position of the time of flight of the signal maximum to a sum spectrum. Since around a million values are measured in an individual time-of-flight spectrum, but there are at most only a few thousand ion signals, the special arithmetic unit can also operate more slowly than the FPGA; a simple PC can be used here, for example.

(8) If the search algorithm for signal maxima used in the FPGA determines that the saturation value 255 was transmitted by the ADC, the counting of the measured values in saturation begins. As soon as a measured value is no longer in saturation, the FPGA sends the time-of-flight index together with the number of measured values in saturation to the special arithmetic unit. The special arithmetic unit takes a corrected measurement value from a table, which is structured according to the number of the saturated measured values and the time-of-flight ranges, and adds it to the sum spectrum, at the position of the time of flight which corresponds to the center of the saturation range.

(9) The table values for the corrections can be obtained by statistical averages from large numbers of calibration measurements. For these calibration measurements it is necessary to know the true signal intensities at the positions of saturation. Particularly suitable for this purpose are the isotope patterns of organic substances, which contain signals with widely differing, but known intensities. FIG. 3 shows an example of the isotope pattern of a peptide with mass 2000 atomic mass units. The high-intensity ion signals beyond the saturation limit, which are not directly measurable, can be calculated from low-intensity isotope signals which are still in the unsaturated measurement range. It is then possible to determine the statistical relationships between intensity beyond the saturation and number of successive measured values in saturation. All the measurements are carried out with many individual time-of-flight spectra. Using appropriate substances of different masses, which each supply sufficiently high ion currents, it is also possible to take measurements in different time-of-flight ranges. The calibration measurements can be carried out automatically with suitable programs and provide the above-mentioned tables with correction values as a function of the number of successive measured values in saturation and as a function of the time-of-flight range.

(10) The corrected measured values from the table will not correspond, in individual cases, to the true intensity values of the ion signals; but in the statistical average over thousands of individual spectra, a quite good approximate value results if the method is well calibrated. With this method it is possible to extend the dynamic range of measurement by two orders of magnitude and more, which at the same time also means an increase in ion sensitivity by two orders of magnitude. This sensitivity increase is, of course, primarily brought about by improvement of the ion source and ion transmission in the mass spectrometer; but without the application of this invention, it cannot be exploited with customary detection systems.

(11) The method only works, however, as long as neighboring ion signals do not overlap in the saturation region. This requires the time-of-flight mass spectrometer to have a good mass resolving power itself, i.e. without the computational improvement of the mass resolution. This is usually the case in the lower mass range, where the extension of the dynamic measurement range is particularly desirable.

(12) In addition to this very simple method, which only ever adds one single correction value to the sum spectrum, more complex methods can be used. It is entirely possible to add correction values at the times of flight of all measured values in saturation. These correction values can also be obtained by calibration measurements using isotope patterns and stored in suitable tables. There is then no increase in the mass resolution, but it is possible to obtain more quantitatively accurate measurements.

(13) The development of transient recorders is targeted not only at faster acquisition rates, but also at higher data depths for the analog-to-digital conversion. The aim is to achieve 10 or even 12 bit data depth. Even when these transient recorders are on the market, the problem with saturated measured values will soon reappear as a result of the continued development of ion sources with better yield and mass spectrometers with better transmission. It will then again be possible to replace saturated measurement values by correction values according to this invention.

(14) FIG. 4 illustrates a flow chart of an exemplary method according to the invention. In step 100, an individual time-of-flight spectrum containing ion signals is acquired wherein each ion signal has a multitude of measured values. In step 102, those ion signals that were driven into saturation are replaced with correction values. In step 104, the individual time-of-flight spectrum, now containing measured values and correction values, is added to a sum time-of-flight spectrum.

(15) While various embodiments of the present invention have been disclosed, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible within the scope of the invention. Accordingly, the present invention is not to be restricted except in light of the attached claims and their equivalents.