CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to United Kingdom Patent Application No. GB2213500.8, filed Sep. 14, 2022, which is incorporated herein by reference in its entirety.

FIELD

The present disclosure relates generally to mass spectrometers with a static field mass filter.

BACKGROUND

Multicollector mass spectrometers, such as inductively coupled plasma mass spectrometers (MC-ICP-MS), are instruments used to investigate small differences in the abundance ratios of analysed isotopes.

For example, Strontium (Sr) has 4 isotopes with the following masses and abundances:

Isotope
84Sr
86Sr
87Sr
88Sr

Abundance
0.56%
9.86%
7%
82.58%

With improvements in the accuracy of mass spectrometers during the last century, it became clear that the isotope ratios (87Sr/86Sr≈0.7103 or 84Sr/87Sr≈0.0535) are not identical across different samples. Furthermore, it became apparent that the accurate determination of Sr isotope ratios is a powerful tool for archaeologists. Since the isotopic composition of the diet of an individual is preserved in the bones of an individual, a statement about the place of birth of an individual can be made when matching the Sr isotope ratios of the bones of a dead body to the Sr isotope ratios of the soil in a specific region.

Besides Sr, there are many other isotope systems which are of interest for scientific or technical questions. Another example is the Rubidium-Strontium (Rb—Sr) dating which makes use of the fact that 87Rb decays into 87Sr with a half-life of about 50 billion years: by determining the 87Sr/86Sr ratio as well as the Rb/Sr ratio of different minerals of a sample, the time elapsed since the sample crystallised can be calculated.

However, the 87Sr cannot be easily mass resolved from the 87Rb (86.909180527 amu to 86.9088774970 amu would require a mass resolving power of more than 200,000, while the upper limit of commercially available isotope ratio mass spectrometers is below 50,000). Isotopic methods that suffer from isotopic interferences as the Rb/Sr method therefore require complex chemical cleaning steps prior to the actual measurement with a mass spectrometer, which makes these methods time consuming and limits them to samples that are available in relatively high quantities.

A solution for this problem is the use of a collision reaction cell: the ions are guided through a cell which is filled with a reactive gas. With an appropriate choice of the gas, one can obtain that the analyte ions are mass-shifted (by forming molecules when reacting with the gas), while the interfering ions are not. For example, the analyte ions become 16 atomic mass units (amu) heavier when reacting with oxygen, while the mass of the interfering ions remains the same.

By doing this, the mass difference of sample ions and interfering ions, which was marginal before entering the collision cell, becomes large enough to be easily resolved by the mass spectrometer downstream of the collision cell.

To avoid the problem of elements that interfere with the mass shifted analyte ions, a pre-mass-filter with a bandpass characteristic can be used, such that only the masses of interest reach the collision cell while ions that interfere with the mass shifted ions are not transmitted. In EP 3 769 334 B1, the entire contents of which are herewith incorporated by reference in this document, a pre-filter comprising a combination of two Wien filters is disclosed. Such a pre-filter can have a mass-independent transmission.

A pre-filter comprising a combination of two Wien filters is suitable for blocking the intense Ar beam caused by the plasma source of ICP-MS instruments as early as possible in the ion optics. By blocking the Ar beam, the total ion load of the ion beam is greatly reduced. This is beneficial for the resolving power of the instrument and, in particular, for the abundance sensitivity. However, optimizing the settings of a pre-filter comprising a double Wien filter is not intuitive.

SUMMARY

Aspects of the present disclosure are defined in the accompanying independent claims.

Overview of Disclosure

A method of analysing a field of a mass spectrometer comprising a mass analyser and a static field mass filter having a first Wien filter and a second Wien filter is disclosed herein. The method comprises:

- for each of a plurality of predetermined strengths of one of an electric field or a magnetic field of the first and second Wien filters:
  - setting the one of the electric field or the magnetic field of the first and second Wien filters to the predetermined strength;
  - causing a beam of ions comprising one or more ion species to be injected through the static field mass filter; and
  - measuring, using the mass analyser, a respective intensity of ions of each of the one or more ion species in the beam.

Optionally, the one or more ion species comprise a plurality of ion species, and the method further comprises:

- determining a ratio of the intensity of ions of a first one of the plurality of ion species and the intensity of ions of a second one of the plurality of ion species.