DE10324671B4 - Determination of isotopic ratios of ions by mass chromatography - Google Patents

Abstract

A method for determining isotopic ratios of ions eluting from a chromatograph comprising the steps of:
Obtaining a mass chromatogram of a first isotope (main isotope), the mass chromatogram having a first mass chromatogram peak having a first intensity (A) and a first background level (B),
Obtaining a mass chromatogram of a second isotope (side isotopes), the mass chromatogram having a second mass chromatogram peak having a second intensity (a) and a second background level (b),
Determining the ratio R _α between the height (a + b) of the second mass chromatogram peak plus a displacement value (bx) and the height (A + B) of the first mass chromatogram peak,
Determining the ratio R _β between the second background level (b) plus the shift value (bx) and the first background level (B),
Determining a specific displacement value (bx) which causes the ratio R _α to be substantially equal to the ratio R _β , and
Determining the relationship between the frequency of the second isotope (side isotopes) and the frequency of the first ...

Description

It is known, the isotope abundance to measure a particular compound in a mixture by the Burned compound in an oven (for example, in oxygen) or (for example in an inert gas) is pyrolyzed while they from a gas chromatograph ("GC") or a liquid chromatograph ("LC") elutes. These Approach has the advantage that the chemical properties the compound the sample isotope ratio of the products from the Do not influence oven.

The However, known approach has the disadvantage that observed will that the Background increases with the rise in the temperature of the chromatograph. This may be due to the column bleeding be due to the mixture of interest, a large number which may contain low-level components elute and not complete be separated. However, in these mixtures, the compounds have a similar Isotopic ratios, if they have been balanced in their course. The compound to be analyzed or the compounds to be analyzed are facing because of their difference the balanced background of interest.

The Nature of the background makes the exact determination of the beginning and the end of the mass chromatographic peaks normally difficult. The measurement of the isotope ratio based on the areas The mass chromatogram of the main isotope and the side isotope is therefore not accurate under normal experimental conditions.

In the DE 69 016 900 T2 discloses a method for determining the isotopic composition of at least one element selected from oxygen, nitrogen and hydrogen in a sample, wherein the sample is introduced into a carrier gas that does not comprise any of the selected elements, the sample is converted to an analysis gas and the analysis gas is in a mass spectrometer is introduced to determine the isotopic composition of the sample.

It is therefore desirable an improved method for mass spectrometry and an improved mass spectrometer provide.

According to one aspect of the present invention, there is provided a method of determining isotopic ratios of ions eluting from a chromatograph comprising the steps of:
Obtaining a mass chromatogram of a first isotope (main isotope), the mass chromatogram having a first mass chromatogram peak having a first intensity (A) and a first background level (B),
Obtaining a mass chromatogram of a second isotope (side isotopes), the mass chromatogram having a second mass chromatogram peak having a second intensity (a) and a second background level (b),
Determining the ratio R _α between the height (a + b) of the second mass chromatogram peak plus a displacement value (bx) and the height (A + B) of the first mass chromatogram peak,
Determining the ratio R _β between the second background level (b) plus the shift value (bx) and the first background level (B),
Determining a specific displacement value (bx) which causes the ratio R _α to be substantially equal to the ratio R _β , and
_Β determining the ratio between the frequency of the second isotope (In addition to isotope) and the frequency of first isotope (major isotope) either on the basis of the ratio R _α or the ratio R when the shift amount (bx) is such that the ratio R _α is substantially equal to the ratio R is _β .

According to another aspect of the present invention, there is provided a method of determining isotopic ratios of ions eluting from a chromatograph comprising the steps of:
Obtaining a mass chromatogram of a first isotope (main isotope), the mass chromatogram having a first mass chromatogram peak having a first intensity (A) and a first background level (B),
Obtaining a mass chromatogram of a second isotope (side isotopes), the mass chromatogram having a second mass chromatogram peak having a second intensity (a) and a second background level (b),
Determining the ratio R _α between the height (a + b) of the second mass chromatogram peak and the height (A + B) of the first mass chromatogram peak plus a displacement value (bx),
Determining the ratio R _β between the second background level (b) and the first background level (B) with a shift value (bx),
Determining a specific displacement value (bx) which causes the ratio R _α to be substantially equal to the ratio R _β , and
_Β determining the ratio between the frequency of the second isotope (In addition to isotope) and the frequency of first isotope (major isotope) either on the basis of the ratio R _α or the ratio R when the shift amount (bx) is such that the ratio R _α is substantially equal to the ratio R is _β .

The first and second isotopes are preferably the combustion or pyrolysis products of a compound. For example, carbon containing ¹³ C and ¹² C may be burned to form carbon dioxide, which is then ionized so that the major isotope that is mass analyzed is ¹² CO ₂ ⁺ with a mass to charge ratio of 44 and the minor isotope, is mass analyzed, ^{13 is} CO ₂ ⁺ with a mass to charge ratio of 45.

The the first and / or the second mass chromatogram peak can not really occur as a single sharp point. The tips can instead, it can be somewhat noisy and it can be in place a single sharp point, a number of so-called "lower peaks" may be present. The heights the first and / or the second Massenchromatogrammspitze can by Means of the lower peaks are determined.

Of the Chromatograph may comprise a gas chromatograph, preferably via a Combustion or pyrolysis chamber or stage with an ion source in the manner of an electron impact ion source ("EI ion source"). Alternatively, the chromatograph may comprise a liquid chromatograph, preferably over a combustion or pyrolysis chamber or stage and a preceding or upstream solvent removal step with an ion source in the manner of an electron impact ion source ("EI ion source") is connected.

According to another aspect of the present invention, there is provided a method of mass spectrometry in which the sample isotope ratio between a second isotope (minor isotope) and a first isotope (major isotope) is determined by the following steps:
Adding a shift value to the data related to the second isotope, or to the data related to the first isotope (main isotope),

Determining a background isotope ratio and
Changing the shift value until the average sample isotope ratio summed over the period of time in which the first and second isotopes elute is substantially equal to the background isotope ratio.

According to another aspect of the present invention, there is provided a mass spectrometer comprising: an ion source coupled to a chromatography source;
a magnetic sector mass analyzer,
a first detector for detecting ions having a first mass-to-charge ratio and a second different detector for detecting ions having a second different mass-to-charge ratio and
a data processing device that:
(a) obtaining a mass chromatogram of a first isotope (main isotope), the mass chromatogram having a first mass chromatogram peak having a first intensity (A) and a first background level (B),
(b) obtaining a mass chromatogram of a second isotope (side isotopes), the mass chromatogram having a second mass chromatogram peak having a second intensity (a) and a second background level (b),
(c) determining the ratio R _α between the height (a + b) of the second mass chromatogram peak plus a displacement value (bx) and the height (A + B) of the first mass chromatogram peak,
(d) determining the ratio R _{β of} the second background level (b) plus the displacement value (bx) and the first background level (B),
(e) determining a specific displacement value (bx) which causes the ratio R _α to be substantially equal to the ratio R _β , and
(f) the ratio between the frequency of the second isotope and the frequency of the first isotope determined by either the ratio R _α or the ratio R _β when the displacement value (bx) is such that the ratio R _α is substantially equal to the ratio R _β .

According to another aspect of the present invention, there is provided a mass spectrometer comprising:
an ion source coupled to a chromatography source,
a magnetic sector mass analyzer,
a first detector for detecting ions having a first mass-to-charge ratio and a second different detector for detecting ions having a second different mass-to-charge ratio and
a data processing device that:
(a) obtaining a mass chromatogram of a first isotope (main isotope), the mass chromatogram having a first mass chromatogram peak having a first intensity (A) and a first background level (B),
(b) obtaining a mass chromatogram of a second isotope (side isotopes), the mass chromatogram having a second mass chromatogram peak having a second intensity (a) and a second background level (b),
(c) determining the ratio R _α between the height (a + b) of the second mass chromatogram peak and the height (A + B) of the first mass chromatogram peak plus a displacement value (bx),
(d) determines the ratio R _β between the second background level (b) and the first background level (B) with a shift value (bx),
(e) determining a specific displacement value (bx) which causes the ratio R _α to be substantially equal to the ratio R _β , and
(f) the ratio between the frequency of the second isotope and the frequency of the first isotope determined by either the ratio R _α or the ratio R _β when the displacement value (bx) is such that the ratio R _α is substantially equal to the ratio R _β .

The of the preferred embodiment The problem addressed is the determination of exact isotope ratios using gas or liquid chromatography tips in the presence of an indeterminate and possibly changing Background and in the absence of any knowledge of the exact location of the beginning and end points of an observed mass chromatogram peak.

According to the preferred embodiment the experimental data are manipulated in a way that the beginning and end of a mass chromatogram peak no have a strong effect on the isotope enrichment calculation. It is a shift value to the data of the Nebenisotops (or alternatively of the major isotope) so that the particular background isotope ratio between the minor isotope and the major isotope the particular sample isotope ratio between similar to the secondary isotope and the main isotope.

The Sample isotope ratio can initially be determined on the basis of the raw data, so that the zero value is not considered becomes. If the background is low, the sample isotope ratio appears noisy. It can be a value both to the data of the main isotope and to the data of the sub-isotope are added to the noise effects to reduce. However, this is for the present invention is not essential.

The Data is preferably analyzed, and mass chromatogram peaks are identified in the lane of the main isotope. On the track of the main isotope and the trace of the side isotope lying points No peaks are selected, and the ratio between the background of the side isotope and the background of the main isotope is calculated from the data. Values of background isotope ratio are interpolated so that a Background-isotope ratio each of the measured sample isotope tips can be assigned. The beginning and ending point of each mass chromatogram peak is determined in the first data pass, and it becomes the total peak area of the Main isotopes and side isotope, as well as the sample isotope ratio.

The Background-isotope ratio and the sample isotope ratio are therefore calculated using non-zero corrected values calculated. By adding a shift value to the data of the minor isotope (or major isotope), the isotope ratio can be changed until the background isotope ratio essentially equal to the sample isotope ratio. This allows it then that that desired Sample isotope ratio is determined accordingly.

The preferred method is not particularly sensitive to the onset and end of mass chromatographic peaks because there are few differences in the mass chromatogram peak on the flanks of a mass chromatogram peak Isotope ratio occur and the flanks do not contribute significantly to the determination of the sample isotope ratio.

Various embodiments The present invention will now be described by way of example only with reference to FIG the attached drawing, wherein:

1 shows a simplified mass chromatogram peak of the minor isotope and a simplified mass chromatogram peak of the major isotope,

2 Figure 12 shows a part of a mass chromatogram of a major isotope formed by burning a substance eluted from a GC mass chromatograph and the sample isotope and background ratio between the minor isotope and the major isotope, with a first offset value added to the data of the minor isotope;

3 shows a part of a mass chromatogram of a major isotope formed by burning a substance eluted from a GC mass chromatograph, and the sample isotope and background ratio between the minor isotope and the major isotope, with a second offset value added to the data of the minor isotope;

4 shows a part of a mass chromatogram of a major isotope formed by burning a substance eluted from a GC mass chromatograph and the sample isotope and background ratio between the minor isotope and the major isotope, with a third shift value added to the data of the minor isotope;

5A a mass chromatogram of a major isotope shows and 5B a corresponding mass chromatogram of a side isotope shows and

6 a graph of a compensatory shift value as a function of the ratio difference for in the 5A and 5B shown tip 4 shows.

A preferred embodiment of the present invention will now be described with reference to 1 described. As can be seen from this figure, the intensity of the mass chromatogram peak corresponding to a major isotope (that is, the frequency of the main isotope) can be regarded as "A", and the corresponding background intensity can be considered as "B" with an arbitrary baseline. Similarly, the intensity of the mass chromatogram peak corresponding to a minor isotope (ie, the frequency of the minor isotope) may be considered as "a," and the corresponding background intensity with any baseline determined may be considered "b." From these experimental data, the values of B, A + B, b, and a + b can be determined, but it is not trivial to pinpoint a and A because it is difficult to pinpoint where the mass chromatogram peaks begin and end , Because the background continues to change with time, the background at the time the main isotope or side isotope elutes is not well known.

Based on the experimental data, two ratios can be calculated directly. First, the ratio of peak heights (or areas) R ₀ can be determined:

Second, the ratio of backgrounds R _b can be determined:

The ratio to be determined, however, is the ratio R of the isotopes above their respective backgrounds:

According to the preferred embodiment, a compensatory shift value bx is added to the values of the background b of the mass chromatogram peak of the minor isotope so that the background of the mass chromatogram peak of the minor is then b + bx and the height of the mass chrom the matograph tip of the minor isotope a + b + bx.

gegeben.The new ratio R _α of peak heights ("sample isotope ratio") is by

given.

gegeben.Similarly, the new background height ratio R _β ("background isotope ratio") is through

given.

In the preferred embodiment, the value of bx is then adjusted until R α = Rβ = R * applies. Accordingly: b + bx = R * · B and a + b + bx = R * * A + R * * B

Inserting results then: a + (b + bx) = a + (R * · B) = R * · A + R * · B and thus: a = R * · A

Therefore:

Therefore, the required isotopic ratio R is equal to R _α or R _β when R _α = R _β .

In the 2 - 4 typical experimental data are presented and the effect of adding various displacement parameters bx to the background of the side is shown. Part of the mass chromatogram of the main isotope is in 2 shown. A first shift value bx has been added to the background of the slave isotope. The mass chromatogram peak of the minor isotope (not shown) is similar to that of the major isotope found in 2 is shown.

In 2 Also, the ratio between the ions of the minor isotope (with a mass-to-charge ratio of 45) and the ions of the main isotope (with a mass-to-charge ratio of 44) as a function of time, with a first shift value to the data of the Side isos added.

The Mass chromatogram top of the main isotope shows how the background In this particular case, with time, increasing. As shown, increase the background isotope ratio and the sample isotope ratio in the same way as the trace of the main isotope. There may be cases in which the background decreases with time.

A second different shift value bx was then added to the data of the slave isotope, and the resulting curves are in 3 shown. It can be seen that by changing the shift value bx added to the data of the slave isotope ratio as well as the sample isotope ratio the background isotope ratio can be changed. In this case, it can be seen that the peaks of the sample isotope ratio are negative with respect to the background isotopic ratio for the second shift value, while the peak of the sample isotope ratio is negative for that in the 2 used in the example illustrated are positive relative to the background isotope ratio. The slope of the background isotope ratio has also changed from positive to 2 too negative in 3 changed.

In accordance with the preferred embodiment, a shift value may be chosen such that the background isotope ratio equals the sample isotope ratio. This is in 4 shown. This in 8th ^H shown ackground isotope ratio is about -8, and that over the period in which the first and second isotope elute samples summed average isotope ratio is also substantially -8.

For the very accurate measurement of isotopic ratios, where areas under the mass isotope peaks of the major isotope and minor isotope are measured, the normally chosen start and end points, even in cases where individual pure mass chromatogram peaks are measured, can significantly affect the particular sample isotope ratio to have. In the in the 2 - 4 As illustrated, it can be seen that the starting and ending points of the mass chromatograph tips are covered by the background. However, according to the preferred embodiment, the values at the edges of each peak are placed in the vicinity of the ratio of the peak itself, therefore the start and end points are not critical.

In the in the 2 - 4 As shown, the minor isotope with a mass-to-charge ratio of 45 and the major isotope with a mass-to-charge ratio of 44 are not common. This is off 4 see where the sample isotope ratio looks similar to a derivative curve. The minor isotope with a mass-to-charge ratio of 45 elutes a few milliseconds before the main isotope with a mass-to-charge ratio of 44, which is why the in 4 shown sample isotope ratio of the minor isotope with respect to the main isotope has an increase at the beginning of the peak of the sample isotope ratio.

The Mass chromatogram for the major isotope can not easily with the mass chromatogram for the minor isotope be aligned. Because the background isotope ratio according to the preferred embodiment however, has been set equal to the sample isotope ratio the effect of the flanks of the tips very low.

Some typical experimental data are in the 5A and 5B wherein four mass chromatogram peaks of the major isotope and minor isotope, respectively, are shown from four separate experimental runs. Mass chromatograph peak 4 corresponding to the fourth experiment run was analyzed. In the 5A data related to the intensity of the major isotope were analyzed using conventional peak detection algorithms. The pre-peak background start time was determined to be 22117, and the pre-peak background end time was determined to be 22317. The peak start time was 22,359, the peak peak time was 22,397, and the peak end time was 22,444. The top-to-peak background start time was determined to be 22,485 and the peak to end background end time was determined to be 22,685.

The following table shows how the different ratios calculated according to the preferred embodiment change as a function of a compensating shift value bx.

6 FIG. 12 is a graph showing how the compensating shift value bx changes with the ratio difference (R _α -R _β ). The value of the compensatory shift value bx required to achieve a ratio difference of zero was calculated as 462.634055. At this compensating shift value bx, the toe-back-to-background ratio was 0.011829 and the toe-to-back background ratio was 0.011828. The sub-isotope / main isotope peak-to-peak background ratio was 0.011828, the peak ratio was 0.011828, and the ratio difference was 0.000000. Accordingly, the required sample isotope ratio, which is R _α or R _β, is 0.011828. Without a compensatory shift value, the sample isotope ratio R _α would have to be determined to be 0.011211, giving a difference of 0.0006 or about 6. This shows that according to the preferred embodiment, a significant improvement in accuracy is achievable.

Although the preferred embodiment a reasonable one estimate the peak starting points and peak endpoints needed the top start points and the top end points are not with the to be determined with the same accuracy as the conventional ones Ratio determination method. Accordingly, conventional ones Peak detection algorithms (usually on the first and second Derivative) completely appropriate.

In similar Way is the location of the background areas in front of and behind the top not critical. Suitable criteria are that 200 data points for each Background area can be used. The end of the preamble background area should preferably be in about half of a peak width before be the top start position, and the beginning of the after peak background area should preferably be in about half the peak width after be the top end position. The location of the background areas can in the case that within the range other peaks are detected are adapted algorithmically.

wobei i der Anfang des Vorspitzenhintergrunds ist, j das Ende des Vorspitzenhintergrunds ist, b₁ der Intensitätswert der Nebenisotopenspur ist, B₁ der Intensitätswert der Hauptisotopenspur ist und x der postulierte ausgleichende Verschiebungswert ist.The value of the peak-to-background ratio can be defined as follows:

where i is the beginning of the toe background, j is the end of the toe background, b _{1 is} the intensity value of the sub isotope trace, B _{1 is} the intensity value of the major isotope trace, and x is the postulated compensatory shift value.

wobei k der Anfang des Nachspitzenhintergrunds ist, 1 das Ende des Nachspitzenhintergrunds ist, b₂ der Intensitätswert der Nebenisotopenspur ist, B₂ der Intensitätswert der Hauptisotopenspur ist und x der postulierte ausgleichende Verschiebungswert ist.Similarly, the value of the peak to peak ratio can be defined as follows:

where k is the beginning of the after peak background, 1 is the end of the after peak background, b _{2 is} the intensity value of the minor isotope trace, B _{2 is} the intensity value of the major isotope trace, and x is the postulated compensatory shift value.

Of the Value of the background ratio at the peak high is preferably by interpolation between the Vorspitzen- and Post-peak background conditions calculated.

wobei m der Anfang der interessierenden Spitze ist, n das Ende der interessierenden Spitze ist, a der Intensitätswert der Nebenisotopenspur ist, A der Intensitätswert der Hauptisotopenspur ist und x der postulierte ausgleichende Verschiebungswert ist.The value of the chromatograph mass peak ratio can be defined as follows:

where m is the beginning of the peak of interest, n is the end of the peak of interest, a is the intensity value of the minor isotope trace, A is the intensity value of the major isotope trace, and x is the postulated compensatory shift value.

Of the for the range of values used may be initially arbitrary, he can however later algorithmically adapted to parenthesize the leading to a ratio difference of zero To ensure value.

It can at the data other minor Improvements are made. The mass chromatogram peak of the minor isotope can be shifted so that it is directly under the mass chromatogram peak of the main isotope. It can also be the tip shape mathematically be improved to measure the mass chromatogram peaks to improve which are adjacent or which overlap.

Although the present invention with reference to preferred embodiments will be understood by those skilled in the art that various changes can be made on the form and the details, without from that in the appended claims to depart from the scope of the invention.

Claims (19)

A method for determining isotopic ratios of ions eluting from a chromatograph, comprising the steps of: obtaining a mass chromatogram of a first isotope (main isotope), wherein the mass chromatogram comprises a first mass chromatogram peak having a first intensity (A) and a first background level (B) obtaining a mass chromatogram of a second isotope, wherein the mass chromatogram has a second mass chromatogram peak having a second intensity (a) and a second background level (b), determining the ratio R _α between the height (a + b) of the second mass chromatogram peak plus a shift value (bx) and the height (A + B) of the first mass chromatogram peak, Determining the ratio R _β between the second background level (b) plus the displacement value (bx) and the first background level (B), determining a specific displacement value (bx) that results in the ratio R _{α being} substantially equal to the ratio R _β , and determining the ratio between the frequency of the second isotope (In addition to isotope) and the frequency of first isotope (major isotope) either on the basis of the ratio R _α or the ratio R _β, when the shift amount (bx) is such that the ratio R _α is substantially equal to the ratio R _β . A method for determining isotopic ratios of ions eluting from a chromatograph, comprising the steps of: obtaining a mass chromatogram of a first isotope (main isotope), wherein the mass chromatogram comprises a first mass chromatogram peak having a first intensity (A) and a first background level (B) obtaining a mass chromatogram of a second isotope, wherein the mass chromatogram has a second mass chromatogram peak having a second intensity (a) and a second background level (b), determining the ratio R _α between the height (a + b) of the second mass chromatogram peak and the height (A + B) of the first mass chromatograph peak plus a displacement value (bx), determining the ratio R _β between the second background level (b) and the first background level (B) with a displacement value (bx), determining a specific displacement value (bx ), the daz u leads to the ratio R _{α being} substantially equal to the ratio R _β , and determining the ratio between the frequency of the second isotope (secondary isotopes) and the frequency of the first isotope (main isotope) either on the basis of the ratio R _α or the ratio R _β when the displacement value (bx) is such that the ratio R _α is substantially equal to the ratio R _β . The method of claim 1 or 2, further wherein a compound is burned or pyrolyzed. The method of claim 3, wherein the first and the second isotope combustion or pyrolysis products of the compound are. Method according to one of the preceding claims, in the first and / or the second Massenchromatogrammspitze more Sub-tips have. Method according to claim 5, wherein the heights of first and / or second Massenchromatogrammspitze by means the lower peaks are determined. Method according to one of the preceding claims, in the chromatograph is designed as a gas chromatograph. The method of claim 7, wherein the gas chromatograph via a Combustion or pyrolysis connected to an ion source is. Method according to one of claims 1 to 6, wherein the chromatograph as a liquid chromatograph is trained. The method of claim 9, wherein the liquid chromatograph on a Lösungsmittelbefreiungs- and a combustion or pyrolysis stage with an ion source connected is. A method according to claim 8 or 10, wherein the ion source an electron impact ion source ("EI ion source"). Method for mass spectrometry, in which the sample isotope ratio between a second isotope (minor isotope) and a first isotope (main isotope) determined by the following steps: Adding one Shift value to the data, referring to the second isotope (Nebenisotop), or to the data referring to the first Refer to isotope (main isotope), Determining a background isotope ratio and To change of the shift value until the over the period in which the first and second isotopes elute summed average sample isotope ratio substantially equal to the background isotope ratio. A mass spectrometer comprising: an ion source coupled to a chromatography source, a magnetic sector mass analyzer, a first detector for detecting ions having a first mass-to-charge ratio, and a second different detector for detecting ions having a second different mass Charge ratio and a data processing device, comprising: (a) a mass chromatogram of a first isotope (main isotope), the mass chromatogram having a first mass chromatogram peak having a first intensity (A) and a first background level (B), (b) a mass chromatogram of a second isotope, wherein the mass chromatogram has a second mass chromatogram peak having a second intensity (a) and a second background level (b), (c) the ratio R _α between the height (a + b) of the second mass chromatogram peak plus a displacement value (bx) and height (A + B) the first mass chromatogram peak determines (d) the ratio R _{β of} the second background level (b) plus the displacement value (bx) and the first background level (B) determines (e) determines a specific displacement value (bx) which causes the Ratio R _α substantially equal to the ratio R _β , and (f) the ratio between the frequency of the second isotope (Nebenisotops) and the frequency of the first isotope (Hauptisotops) determined either on the basis of the ratio R _α or the ratio R _β , if Shift value (bx) is such that the ratio R _α is substantially equal to the ratio R _β . A mass spectrometer comprising: an ion source coupled to a chromatography source, a magnetic sector mass analyzer, a first detector for detecting ions having a first mass-to-charge ratio, and a second different detector for detecting ions having a second different mass Charge ratio and a data processing device, comprising: (a) a mass chromatogram of a first isotope (main isotope), the mass chromatogram having a first mass chromatogram peak having a first intensity (A) and a first background level (B), (b) a mass chromatogram of a second isotope, wherein the mass chromatogram has a second mass chromatogram peak having a second intensity (a) and a second background level (b), (c) the ratio R _α between the height (a + b) of the second mass chromatogram peak and the height (A + B) of the first mass chromatogram peak resembled determines a shift value (bx), (d) determining the ratio R _β between the second background level (b) and the first background level (B) with an offset value (bx), (e) determining a specific shift amount (bx), the to leads that the ratio R _α substantially equal to the ratio R _β , and (f) the ratio between the frequency of the second isotope (Nebenisotops) and the frequency of the first isotope (Hauptisotops) either on the basis of the ratio R _α or the ratio R _β determined when the shift value (bx) is such that the ratio R _α is substantially equal to the ratio R _β . A mass spectrometer according to claim 13 or 14, wherein the chromatograph is designed as a gas chromatograph. A mass spectrometer according to claim 15, wherein said Gas chromatograph over a combustion or pyrolysis stage connected to an ion source is. Mass spectrometer according to one of claims 13 or 14, wherein the chromatograph formed as a liquid chromatograph is. A mass spectrometer according to claim 17, wherein said Liquid chromatograph over a Solvent removal stage and a combustion or pyrolysis stage with an ion source connected is. A mass spectrometer according to claim 16 or 18, wherein the ion source is an electron impact ion source ("EI ion source").