ICP-MS (Inductively Coupled Plasma Mass Spectrometry) is an extremely important tool used in modern physics and chemistry. ICP-MS has excellent performance, especially when measuring samples at low concentrations and isotopic abundances, and has a wide range of applications. biological materials, high-purity reagents and metals, atomic nuclear materials, geological samples, and foodstuffs.
In the 1980s, Houk and Gray independently reported the mass spectrometry method using inductively coupled plasma (ICP) as the ion source, by which ICP can provide the ion source required for good inorganic analysis. After the interface problem between ICP and mass spectrometer was solved, the ICP-MS method for ultra-trace analysis of inorganic elements was a great success and developed very rapidly.
Since 1983, when two companies in the UK and Canada simultaneously introduced commercial ICP mass spectrometers, several hundred ICP mass spectrometers have been in use worldwide so far.
The main advantages of ICP-MS are summarized as follows: the sample is introduced at atmospheric pressure with an external ion source, i.e., the ions are not in a vacuum; the temperature of the plasma is high, allowing complete evaporation and dissociation of the sample; the percentage of atoms ionized in the sample is high; the ions produced are mainly monovalent; the ion energy dispersion is small; the ion source is at a low potential and can be Simple mass analyzers can be used. Because of its excellent analytical properties, ICP-MS is one of the most important analytical techniques in elemental analysis.
ICP-MS can be used for qualitative, semi-quantitative and quantitative analysis of substances as well as for isotope ratio determination with a wide range of flexible detection modes.
The working principle of ICP mass spectrometry is that while the instrument is running, The sample is fed into the fog chamber by a peristaltic pump, and the aerosol is evaporated, atomized, and ionized in the ICP channel at atmospheric pressure and high temperature of about 7000 K. The ions are accelerated and focused into the mass spectrometer by the sampling cone and separation cone under the accelerating voltage, and the ions with different mass-to-charge ratios are selectively passed through the four-stage mass analyzer and detected by the ion detector.
ICP-MS inductively coupled plasma mass spectrometry is composed of an inductively coupled plasma light source (ICP), an interface, an optical system, a four-stage rod, a detector and a collision reaction cell.
Inductively coupled plasma light source consists of RF generator and feed system.
Inlet system: It converts the solution sample into an aerosol to make it into the ICP flame. It contains atomizer, fog chamber, moment tube, plasma gas, auxiliary gas, carrier gas, and various gas path device systems.
RF generator: The RF generator delivers energy to the plasma through the working coil to maintain a stable discharge of the ICP light source. Currently, there are two main types of RF generators for ICP oscillation, namely self-excited and it-excited.
The interface is generally composed of 2 to 3 cones, respectively, sampling cone and interception cone. Sampling aperture diameter is generally 0.75 ~ 1mm, after cooling the sampling cone near the plasma moment tube, its inter-cone hole aligned with the center of the moment tube pipe, the cone top and moment tube mouth distance of about 1cm. Behind the sampling cone there is an interception cone, shape than the sampling cone small, cone than the sampling cone large. The interception cone is the same as the sampling cone, with a small hole at the top of the tip, and the installation distance between the two cones is 6-7 mm, and on the same axis.
The ions generated by the ICP enter the vacuum system through the sampling hole, where a supersonic jet is formed and its central part flows into the interception hole. Since the extracted ion-containing gas enters the vacuum chamber at ultrasonic velocity and takes only a few microseconds to reach the interception cone, the composition and properties of the sample ions remain essentially unchanged.
The optical system generally consists of ion lenses, both off-axis and non-off-axis designs, whose main function is to avoid the interference of neutral particles and to achieve the purpose of transmitting ions and focusing to the quadrupole.
A four-stage rod consists of four bar electrodes with a hyperbolic or circular cross section. A certain DC voltage and an AC voltage with a frequency in the RF range are applied between the two sets of electrodes.
If the frequency of the AC voltage is made constant while continuously changing the magnitude of the DC and AC voltages (but keeping their ratio constant, voltage scan), or keeping the voltage constant while continuously changing the frequency of the AC voltage (frequency scan), ions with different mass-to-charge ratios can be made to reach the collector (detector) in turn and a mass spectrometer map can be obtained.
The advantages of the quadrupole are: higher resolution; extremely fast analysis, most suitable for use in conjunction with ICP, GC and HPLC, but less accurate and precise than the magnetic deflection type mass analyzer.
In ICP-MS, the detector is usually a pulse counting detector with an electron multiplier tube.
The collision reaction cell technology is different from each other due to patents, and has become one of the most distinctive parts. There are four-stage rod designs, six-stage rod designs, eight-stage rod designs, and those that use collision cell technology on the interface. The four-stage rod design differs from other designs in that the four-stage rod is capable of mass screening, which means that a range of masses can be passed through. The collision cell can use any of the gases including methane, oxygen, hydrogen and helium, as well as strong reaction gases such as ammonia and nitrogen oxide.
Although the design and concept of the collision reaction cell differs from one to another, the principle of de-interference is basically the same, mainly through the collision and reaction between the ion flow generated by the sample and hydrogen or helium or a mixture of both. Helium is an inert gas and mainly acts as a collisional gas, while hydrogen is a weak reactive gas and removes interference mainly by reaction.
1. Inductively coupled plasma mass spectrometers can have a reduced lifetime if they are not maintained. The following are the maintenance methods for ICP-MS.
2. Gas control system: Whether the gas control system of ICP operates stably and normally directly affects the quality of ICP mass spectrometry data. If there are water droplets or other solid impurities in the gas path, it will cause unstable airflow, so the gas control system should be inspected and maintained frequently. First of all, we should do the gas seal test, open the gas cylinder and pressure reducing valve, make the gas pressure indication on the rated value, then close the gas cylinder, observe the pressure gauge pointer on the pressure reducing valve, there should be no drop or very little drop in a few hours, otherwise it means there is a gas leakage in the gas circuit, which needs to be checked and eliminated.
3. Using environment: Inductively coupled plasma spectrometer, like other large precision instruments, needs to operate under certain environmental conditions, otherwise, it will not only affect the performance of the instrument, but even cause damage and shorten the life, etc. According to the optical instrument characteristics, there are certain demands on the environment temperature and humidity`. If the temperature changes too much, the optical components will be affected by temperature changes will produce spectral drift, resulting in unstable measurement data; and if the environmental humidity is too large, the optical components of the instrument, especially the grating easily damaged by moisture or performance degradation.
4. Monthly cleaning of inductively coupled mass spectrometry with ultrasonic waves
5. The atomization chamber, torch tube, and sample injection tube of the inductively coupled mass spectrometry are cleaned monthly with ultrasonic waves.
6. Replace the fan filter on the ICP-MS every four months.
7. Clean the torch chamber with a soft cloth after testing organic samples to prevent the deposition of organic components.
8. Replace the sample inlet tube, peristaltic pump tube, waste tube, gaskets and other consumables as needed.
a. Sample delivery requirements: liquid, incoming samples marked with the name, main components, the approximate content of the elements to be measured.
b. Sample requirements.
c. Test solution needs to be completely clarified.
d. Test solution needs to be pH ≤ 7, hydrochloric acid or nitric acid concentration ≤ 10%.
e. The content of the base element in the test solution should not exceed 10,000 times the element to be measured.
f. If the test solution contains organic matter must be nitrated and then measured on the machine.
g. Sample volume: set according to ICP-MS instrument requirements