Isotope Ratio Mass Spectrometer (IRMS)

Introduction

Isotope ratio mass spectrometers (IRMS) are precision instruments used to determine isotope ratios. With the improvement of isotope mass spectrometry technology, the research field of IRMS has been greatly expanded. In addition to the well-known application of stable isotope geochemistry, currently, IRMS technology is also applicable to agriculture, medicine and environmental science and other research fields.

Fundamentals of IRMS:

In a closed vacuum system, the sample to be tested is converted into positively charged ions by the ion source in the instrument. These ions obtain energy under the action of a high-voltage electric field, and are focused and shaped into a beam of ions with a rectangular cross-section. The beam is directed into a fixed magnetic field (called a magnetic separator). When the high-voltage voltage and magnetic field strength are constant, the deflection radius of curvature of particles with different charge-to-mass ratios (Q/m) is also inconsistent. The charged ion current with different Q/m is collected by the receiver at the corresponding position of the magnetic field outlet and converted into a voltage signal. The intensity of the ion current actually reflects the number of these particles with different Q/m, and the ratio between various isotopes can be quantitatively measured.

The Structure of IRMS

Like other mass spectrometers, IRMS can be divided into four parts: sampling system, ion source, mass analyzer and detector, in addition to electrical system and vacuum support system.

The sampling equipment mainly includes metal belt racks, sample trays, sample evaporators, etc. Mainly complete the sample preparation and sample delivery. The treated sample is first dissolved and then dropped on the metal sample strip, and the sample is evaporated to dryness by a sample desiccator. The treated samples were first dissolved and then dropped onto a metal sample strip, and the samples were evaporated to dryness through a sample desiccator. For elements with high ionization potential, in order to improve the ionization efficiency, a layer of so-called emitter can be coated on the sample tape first, and then the sample is coated, which can greatly improve the ion emission rate.

The role of the ion source has two aspects, ionizing the sample to be analyzed into positive ions and extracting, accelerating and focusing the ions. It is characterized by small dispersion of ion energy, less sample amount (as low as 1 microgram), stable ion beam current, and high detection sensitivity. The ions generated by ionization are firstly emitted from the slit of the shielding section and enter the electrostatic lens system. After being accelerated in the negative high-voltage electric field, they are focused on the deflection electrode and then enter the analysis orbit. In this way, the ions pass through the electrostatic lens to form an ion beam with a certain energy and a rectangular interface. Often more than one ion source can be used to determine isotopic abundance for an element.

The mass analyzer receives ions with different Q/m from the mass analyzer to separate. The main body is a sector magnet. It is required to have a large separation and a good focusing effect.

Ion detector receives ion beams with different Q/m from the mass analyzer, amplifies and records them. It consists of an ion receiver and an amplifying measuring device. After the ions pass through the magnetic field, the ion beam to be analyzed passes through a special slit and is refocused onto a receiver and collected. The receiver is generally a Faraday cylinder.

Basic Measurement Process of IRMS

In stable isotope analysis, mass spectrometry is performed in the form of gas, so it is often called a gas mass spectrometer. The measurement process of an isotope mass spectrometer can be summarized into the following steps.

1. Send the analyzed sample into the ion source in the form of gas.
2. Convert the element to be analyzed into a cation with charge e, and apply a longitudinal electric field to collimate the ion beam into a parallel ion beam with a certain energy.
3. Use electric and magnetic analyzers to decompose the ion beam into components with different Q/m.
4. Record and measure the intensity of each component of the ion beam.
5. Use a computer program to convert ion beam intensity into isotopic abundance.
6. Compare the sample to be tested with the working standard to obtain the isotope ratio relative to the international standard.

Application of IRMS Analysis Technology

Through isotope analysis, it is possible to know the optimal formula ratio and time for fertilizing crops, understand the composition and origin of items, and infer the characteristics of paleoclimate and environmental conditions. The following are a few examples of the applications.

1. Analysis of Drug Abuse Sources
   The isotopic fractionation effect in nature makes the isotopic composition of carbon, nitrogen and other elements in plants have distinct geographical identification. Therefore, by analyzing the stable isotopic composition of plant-derived drugs, it can be related to the place where plants grow. Heroin is a semi-synthetic drug derived from morphine alkaloids as the starting point for synthesis. Since heroin is a secondary product after acetylation of morphine, and morphine is an alkaloid that can be extracted from opium, the stable isotopic composition of heroin (δ¹³C,δ¹⁵N) can indicate the geographic origin of the poppy plant used for its synthesis.
2. Monitoring and Environmental Protection of Pollutants in Ecosystems
   Under different environmental conditions, the composition of stable isotopes will vary to some extent. For example, nitrogen-containing substances from different sources can have different nitrogen isotopic compositions, so nitrogen isotopes are a good indicator of pollutants. At present, the use of chemical fertilizers is very common. Nitrogen fertilizers and other nitrogen-containing organic matter in the soil flow into rivers, lakes and seas with the loss of soil and water. Therefore, the δ¹⁵N value can be utilized as an indicator of water environmental pollution.
3. Food Quality Control
   According to the difference of δ¹³C value of plant C3 and C4 cycle products, carbon isotope technology can play a special role in food quality control, and can solve the problems that cannot be solved by conventional analysis technology. For example, conventional normalization techniques cannot distinguish beet sugar and sucrose, but beet is a C3 plant, with δ¹³C about -25.5‰, and sucrose is a C4 plant, with δ¹³C about -11.5‰, which can be easily distinguished using carbon isotope techniques. Similarly, maple is a C3 plant, with δ¹³C -22.4‰ to -25.5‰, so if sucrose is mixed in maple syrup, it can be detected by δ¹³C analysis. The same principle can also be applied to archaeology, as the δ¹³C of organic residues can be traced back to the food conditions of ancient civilizations. Combining carbon and oxygen isotopes also allows for finer judgments about the mix of different foods.