ICP MS Construction principles and interference

The ICP-MS analysis includes the following steps:

① atomization ② Convert most of the atomized atoms into ions ③ Ions are separated according to their mass-to-charge ratio ④ Count the number of various ions

principle: The nebulizer sends the solution sample inplasmaThe light source vaporizes at high temperature and dissociates into ionized gas. The ions collected by copper or nickel sampling cone form a molecular beam at low vacuum of about 133.322 pa pressure, and then enter through a cutting plate with a diameter of 1~2 mmQuadrupole mass spectrometryAfter the analyzer is separated by the filter, it reaches the ion detector, and the content of an element or the isotope ratio can be measured according to the proportional relationship between the count of the detector and the concentration.

I. Plasma

Plasma refers to a gas mixture containing a certain concentration of positive and negative ions that can conduct electricity. In plasma, the concentrations of positive and negative ions are equal, with a net charge of zero. Argon is commonly used to form plasma. Ar ions and electrons are the primary conductive substances. The temperature can typically reach 10,000K.

Composition of inductively coupled plasma generation components:

① Quartz torch tube (Type Fassel)

It is composed of three concentric quartz tubes, and three argon gas streams enter the torch tube respectively. Cooling gas: plasma support gas, protecting the tube wall Auxiliary air: protect the tip of capillary Nucleation gas: sample and penetrate the center of the plasma

Formation of ICP flame torch

To form a stable ICP torch, three conditions must be met: a high-frequency electromagnetic field, working gas, and a quartz torch tube that can maintain stable discharge of the gas. A water-cooled induction coil is wound around the upper part of the tube. When the high-frequency generator supplies power, a strong oscillating magnetic field is generated along the axis of the coil. Using methods such as high-frequency sparks to ionize the working gas flowing in the middle, the resulting ions and electrons interact with the fluctuating magnetic field produced by the induction coil. This interaction causes the ions and electrons within the coil to flow in a closed loop as shown in the diagram; their resistance to this movement leads to ohmic heating. Due to the intense current generating high temperatures, the gas is heated, thus forming a torch-like plasma.

② Coupled load coil (2~3 water-cooled fine copper tubes) ③ RF generator (provides energy) ④ Tesla coil (ignition device)

Figure 1:The course of the sample solution in ICP

II. ICP and MS interface (Interface)

1. Ion extraction: sampling cone (sampling cone); intercepting cone (skimmer cone)
2. Ion focusing: ion lens group
3. Vacuum system: one mechanical pump; one molecular turbine pump

Ⅲ. Mass spectrometer

Fourier transform mass spectrometry (Quadrupole Mass)

The quadrupole mass spectrometer is a mass filter composed of four parallel conductive rods, only ions with a certain mass-to-charge ratio can pass through, and ions that do not meet the mass requirements or collide with the rod or leave the track between the rod are extracted from the system by a vacuum pump.

##Mass spectrometry interference

1. Spectral interference: When the ion type in the plasma has the same mass and charge ratio as the analyte ion, spectral interference is generated.

There are four types of spectral interference:

① Ions of the same mass type;

Isotope interference refers to isotopes of two different elements with almost the same mass. For atomic mass spectrometers using quadrupole mass spectrometers, isotopic species refer to isotopes with a mass difference less than one atomic mass unit. When using high-resolution instruments, the mass difference can be even smaller. Most elements in the periodic table have one, two, or even three isotopes that overlap in mass.

For example, indium has two stable isotopes: 113In+ and 115In+

The former overlaps with 113Cd+ and the latter with 115Sn+.

Because the mass overlap can be accurately predicted from the abundance table, this interference can be corrected with appropriate computer software, which is now available on many instruments.

② Multiatomic or additive ions;

Multiatomic ions (or molecular ions) are the main source of interference in ICPMS. It is generally believed that multiatomic ions do not exist in the plasma itself, but are formed during the ion extraction process by the interaction between the components in the plasma and those in the matrix or atmosphere.

Hydrogen and oxygen account for about 30% of the total number of atoms and ions in plasma, with the majority of the remainder produced by argon in the ICP torch. The background peaks in ICPMS are mainly due to these polyatomic ions. They come in two groups: one lighter based on oxygen and another heavier based on argon, both including molecular ions containing hydrogen.

Example: 16O2 + interference 32S+

③ Oxide and hydroxide ions;

In ICP-MS, another significant interfering factor is the oxides and hydroxides formed from analytes, matrix components, solvents, and plasma gases, with such interference being more pronounced for analytes and matrix components. They almost all form MO+ and MOH+ ions to some extent, where M stands for the element of the analyte or matrix component, potentially leading to peaks that overlap with those of certain analyte ions.

For example: oxides of the five natural isotopes of titanium The mass numbers are 62,63,64,65 and 66 respectively, Interference analysis 62Ni +, 63Cu+, 64Zn+, 65Cu+ and 66Zn+ The formation of oxides is related to many experimental conditions, such as injection flow rate, radio frequency energy, sampling cone-separation cone spacing, sampling hole size, plasma gas composition, oxygen and solvent removal efficiency, etc. Adjusting these conditions can solve some specific problems of oxide and hydroxide overlap.

④ Interference caused by instruments and sample preparation:

When the plasma gas passes through the sampling cone and separation cone, active oxygen ions will be sputtered out of the nickel ions from the cone nickel plate. Measures can be taken to reduce the potential of the plasma to below the sputtering closure value of nickel, which can weaken or even disappear this effect.

Trace concentration levels often show ion peaks unrelated to the analyte. For example, copper and zinc at levels of a few ng·mL-1 are usually impurities in the solvent acid and deionized water.

Therefore, when performing ultra-pure analysis, ultra-pure water and solvents must be used. It is best to dissolve solid samples with nitric acid, because nitrogen has a high ionization potential and its molecular ions are quite weak, with little interference.

2.matrix effect

The samples analyzed in ICP-MS are generally solid samples with a mass fraction of less than 1% or a mass concentration of about 1000ug·mL-1. When the mass concentration of coexisting substances in the solution is higher than 500~1000ug·mL-1, the matrix effect of ICP-MS analysis will appear.

The coexisting material contains low ionization energy elements such as alkali metals, alkaline earth metals and lanthanides in excess of the limit. The number of electrons provided by them is large, which inhibits the ionization of other elements including the analyte element and affects the analysis results.

A high solid content in the sample can affect the atomization and evaporation of solutions, as well as the generation and transport of plasma. If the sample solution is over-saturated or evaporates too quickly, the temperature of the plasma torch will decrease, affecting the ionization of analytes and reducing the response of the analyte. The impact of matrix effects can be minimized through dilution, matrix matching, standard addition, or isotope dilution.

frequently asked questions

1. The test results do not meet the expectations, and are too large or too small. Where does the error come from?

Under the premise that there are no issues in the overall testing process, it is normal to have some differences. Each test has a certain degree of uncertainty, so the data will definitely have some discrepancies. As long as these differences are not significant, it is considered normal. If the differences are substantial, the reasons need to be analyzed from two aspects. First, the test party needs to analyze by itself which links of improper operation may lead to large data deviation, such as uneven sampling, whether the selection of digestion method is appropriate, whether the dilution factor is too large; Second, the sample should be analyzed by itself to see whether there is any unevenness, moisture absorption, water loss and oxidation that lead to changes in the physical and chemical properties of the sample.

2. Why should the test sample after digestion not contain organic matter, solid impurities, F-and so on?

The presence of organic matter and solid impurities can lead to blockage of the instruments sampling tube, causing instrument failure and damage to the instrument; it can also cause test errors; the presence of F-, alkaline substances, etc., can corrode the internal parts of the instrument, causing instrument failure and damage to the instrument.

3. How effective is ICP-MS to measure Hg? What is the range of detection content?

(1) The range of Hg measured by ICP-MS can be as low as ppt level, but the sample treatment and medium are very important, otherwise the deviation will be very large,memory effectIt is also very large; measuring Hg is very troublesome, mainly memory, and it is only effective when washed with an alkaline solution; (2) Generally speaking, it is better to make it about 10ppb or less, because the memory effect is very large and it takes a long time to clean after completion. It can be made with dilution and it is better to wash with alkali.

4. How to treat samples for measuring heavy metals in seawater with ICP-MS? It includes sample dilution and mass number selection.

(1) Acidification and membrane passing. Note that nitric acid and vessels must be clean. It is recommended to use nitric acid after re-distillation. Generally, it is diluted ten times. (2) You measured heavy metals, whether ORS, DRC or CCT, the effect is not too big. The reaction pool is better for mass numbers below 85. cd111 will be interfered by MOZr and other oxides, so the equation can be edited and corrected. Pb is applied to 206+207+208, Hg202.

5. What are the faster pretreatment methods for ICP-MS detection of environmental samples?

(1) AdoptionHigh pressure microwave digestion system, MILLSTONE or CEM, etc.; (2) microwave digestion or acid leaching, depending on the sample and element. If isotope abundance is used, leaching is sufficient; (3) Depending on the environmental samples, water samples can be fixed with acid, soil is more difficult to do, microwave digestion can also be used, according to the different elements used, different speeds and methods are used.