I. Factors Affecting Plasma Temperature
The temperature of plasma is comprehensively influenced by multiple factors, and each factor plays a unique role in the state change of plasma.
1. **Carrier gas flow rate**: There is a delicate relationship between the carrier gas flow rate and the temperature at the central part of the plasma. When the carrier gas flow rate increases, it is like suddenly injecting a large amount of cold air into a thermal field, which will cause the temperature at the central part of the plasma to drop. This is because the increase in the carrier gas flow rate accelerates the heat dissipation, making it difficult for the central region to maintain a relatively high temperature.
2. **Carrier gas pressure**: The carrier gas pressure has a significant impact on the excitation temperature. The excitation temperature increases as the carrier gas pressure decreases. This can be understood as follows: when the carrier gas pressure decreases, the distance between gas molecules increases, and the collision frequency decreases. As a result, energy is more likely to be concentrated, which in turn promotes an increase in the excitation temperature.
3. **Frequency and Input Power**: The effects of frequency and input power on the excitation temperature each have their own characteristics. The excitation temperature increases with the increase of power, showing an approximately linear relationship. The higher the power, the more energy is provided to the plasma, and naturally, the higher the temperature. However, when other conditions remain the same, increasing the frequency will actually lead to a decrease in the discharge temperature. This may be because the increase in frequency makes the energy distribution more dispersed, failing to concentrate in a local area to generate high temperature.
4. Introducing a releasing agent with a low ionization potential (such as Tl) will have an impact on the electron temperature of the plasma. When such a releasing agent is introduced into the plasma, the electron temperature will increase. This is because the releasing agent with a low ionization potential can lose electrons more easily, which increases the number of electrons in the plasma and thus raises the electron temperature.
II. Elimination and Suppression of Ionization Interference
Ionization interference is an interference phenomenon that occurs when atoms are ionized in the vapor phase of a flame or plasma, and it can have an adverse impact on the analysis results.
When atoms are ionized in a flame or plasma, the number of neutral atoms of the analytical element in the flame will decrease. Since neutral atoms are crucial for generating analytical signals, a decrease in the number of neutral atoms means a reduction in the analytical signals, thereby affecting the accuracy of the analysis.
To eliminate or suppress this ionization interference, two effective methods can be adopted.One method is to add an excessive amount of easily ionizable elements to both the standard and the analytical sample. These easily ionizable elements will be rapidly ionized in the flame or plasma, stabilizing the concentration of free electrons at a relatively high level. As a result, the ionization of the analytical element will be suppressed, ensuring a relatively stable number of neutral atoms of the analytical element in the flame and enhancing the intensity of the analytical signal.The other method is to lower the temperature. Since the higher the temperature, the greater the degree of ionization, reducing the temperature can decrease the ionization degree of atoms, thereby reducing ionization interference.
3. Interference effects of reagent acidity on the ICP - AES method
The acidity of reagents has various interference effects on the ICP - AES method, which are mainly reflected in the following important aspects.
First of all, the acidity of the reagent will affect the aspiration rate and the spectral line intensity of the elements in it. Under normal circumstances, both the aspiration rate of the solution containing the reagent acidity and the spectral line intensity of the elements in it are lower than those of the aqueous solution. As the acidity increases, the spectral line intensity will decrease significantly. This is because the change in acidity will alter the physicochemical properties of the solution, affecting the atomization process of the elements and the generation of the emission spectrum.
The effects of different types of inorganic acids on the spectral line intensity are not the same. Arranged in the order of increasing influence degree: HClLess than nitric acid. In addition, the change in spectral line intensity is directly proportional to the change in the enhancement rate. That is to say, any alteration in the enhancement rate will directly lead to a corresponding change in the spectral line intensity. This provides an important reference for our research on the interference of reagent acidity on the ICP - AES method. 4. Main types of spectral interferences in the ICP - AES method
In the ICP - AES method, spectral interference is an important factor affecting the accuracy of the analysis results, and there are mainly the following types.
1. **Spectral line interference**: Spectral line interference refers to the situation where other spectral lines overlap or are close to the analytical spectral line, thus affecting the accurate measurement of the analytical spectral line. This interference may come from other elements in the sample, or it may be the stray spectral lines generated by the instrument itself.
2. **Interference of the band system with the analytical spectral lines**: A band system is a banded spectrum composed of a series of spectral lines. When the band system overlaps with the analytical spectral lines, it can mask the signals of the analytical spectral lines, resulting in inaccurate measurement results.
3. **Interference of continuous background on analytical spectral lines**: The continuous background refers to the continuously distributed background signal present in the spectrum. This background signal will be superimposed on the analytical spectral lines, making it difficult to accurately measure the intensity of the analytical spectral lines, thus affecting the sensitivity and accuracy of the analysis.
4. **Interference Caused by Stray Light**: Stray light refers to the light that is not part of the analytical spectral lines and is generated in the optical system of the instrument due to scattering, reflection, and other reasons. Stray light can enter the detector, interfere with the measurement of the analytical spectral lines, and cause errors in the measurement results.
5. Correction of Sensitivity Drift in ICP - AES Analysis
During the analysis process using the ICP - AES method, sensitivity drift is a common problem, which can affect the accuracy and reliability of the analysis results. Multiple factors can cause the sensitivity to drift.
Changes in gas pressure can affect the atomization efficiency and the distribution of ground - state atoms. Variations in gas pressure can alter the state of the plasma, thereby influencing the atomization process of elements and the quantity of ground - state atoms, which in turn leads to the drift of sensitivity.
Problems such as capillary blockage and poor waste liquid drainage can affect the solution uptake rate and atomization efficiency. If the capillary is blocked, the solution cannot enter the plasma normally. Or, if the waste liquid is not drained smoothly and accumulates in the system, it will affect the introduction of elements and the atomization effect, thus causing changes in the sensitivity.
Factors such as voltage changes can also affect the sensitivity. Unstable voltage can influence the energy supply of the plasma, resulting in fluctuations in the excitation temperature and atomization efficiency, which in turn causes the drift of sensitivity.
To correct the sensitivity drift, a method can be adopted where one quality control sample with a composition close to that of the samples is measured after every 10 samples are measured. By comparing the measurement results of the quality control sample, changes in sensitivity can be detected in a timely manner, and corresponding adjustments can be made. At the same time, appropriately shortening the time interval of standardization according to the age of the instrument in use can ensure the stability of the instrument during the measurement process and reduce the influence of sensitivity drift.
VI. How to avoid cross - contamination between samples in ICP analysis
In ICP analysis, cross - contamination between samples can seriously affect the accuracy of the analysis results. Therefore, effective measures need to be taken to avoid it.
During measurement, samples with significantly different concentrations should not be measured sequentially. If samples with large concentration differences are measured one after another, elements in high - concentration samples may remain in the instrument's sample introduction system, thus contaminating the subsequent low - concentration samples to be measured. To avoid this situation, samples with similar concentrations can be grouped together for measurement. This can reduce the mutual interference between samples of different concentrations and lower the possibility of contamination.
Between the measurements of samples, the injection system should be rinsed with distilled water or a solvent. Rinsing can effectively remove the residual samples in the injection system and prevent the residual samples from affecting the measurement of the next sample. By rinsing with distilled water or a solvent, it can be ensured that the samples for each measurement are pure, thereby improving the accuracy of the analysis results.
VII. Conditions that the acids used for sample decomposition in the ICP - AES method must meet
In the ICP - AES method, it is crucial to select appropriate acids for sample decomposition. These acids must meet a series of strict conditions.
First of all, the acid should decompose various elements as quickly and completely as possible. Only by fully decomposing the elements in the sample can the accuracy of subsequent analysis be guaranteed. If the decomposition is incomplete, some elements may not be detected, resulting in deviations in the analysis results.
Secondly, the amount of the element to be measured in the acid should be negligible. If the acid contains a relatively large amount of the element to be measured, it will increase the background value in the sample, affecting the sensitivity and accuracy of the analysis.
When decomposing the sample, the elements to be measured should not be lost. This requires the acid to have mild properties and not react chemically with the elements to be measured, which could lead to the volatilization or precipitation of the elements. In this way, it can be ensured that all the elements to be measured can enter the subsequent analysis process.
No insoluble substances should be formed between the acid and the element to be measured. If insoluble substances are formed, the element to be measured cannot be effectively detected, which will affect the analysis results.
During the determination, the influence of co - existing elements should be minimal. When acids are used to decompose samples, excessive co - existing elements should not be introduced, so as to avoid interference of these co - existing elements with the analysis results.
Finally, acids should not damage instrument components such as the nebulizer and the torch tube. If an acid is highly corrosive, it will damage the instrument, affecting its service life and the accuracy of analysis.
VIII. In the ICP - AES method, special attention must be paid to the preparation of standard solutions
In the ICP - AES method, the preparation of standard solutions is a crucial step and must be given special attention.
Incorrect preparation methods will lead to the occurrence of systematic deviations. Standard solutions are the basis for establishing calibration curves. If the preparation method is inaccurate, it will cause deviations in the calibration curves, thus affecting the accurate determination of the elemental content in samples.
If the medium and acidity are not appropriate, precipitation and turbidity will occur. Inappropriate medium and acidity can change the chemical properties of the solution, causing certain components in the solution to undergo chemical reactions, resulting in the formation of precipitates or turbidity. This affects the uniformity and stability of the solution, and further impacts the analysis results.
Improper grouping of elements can cause spectral line interference among elements. The spectral lines of different elements may overlap or interfere with each other. If the elements are not grouped reasonably, it will lead to spectral line interference, resulting in inaccurate analysis results.
Insufficient purity of reagents and solvents can lead to an increase in the blank value, a deterioration of the detection limit, and an enlargement of errors. Reagents and solvents with insufficient purity may contain impurities. These impurities can increase the background value of the sample, lower the detection limit, and at the same time introduce additional errors, thus affecting the accuracy of the analysis.
IX. Precautions for Preparing Multi-element Stock Standard Solutions for ICP Analysis
Preparing multi-element stock standard solutions for ICP analysis requires attention to multiple details to ensure the quality and stability of the solutions.
High-purity acids or ultrapure acids should be used as solvents. The impurity content in high-purity acids or ultrapure acids is extremely low, which can reduce the interference of impurities on the analysis results and ensure the purity of the standard solution.
Double-distilled ion-exchanged water should be used. Double-distilled ion-exchanged water has undergone multiple distillation and ion-exchange treatments, resulting in extremely low levels of impurities and ions in the water. It can provide a pure solvent environment for the standard solution.
Spectroscopic pure, high-purity or reference materials should be used. These high-purity substances can ensure the accurate content of elements in the standard solution and improve the accuracy of the analysis results.
Divide the elements into several groups for preparation to avoid spectral line interference or precipitation. Chemical reactions may occur between different elements, resulting in the formation of precipitates, or their spectral lines may interfere with each other. Through reasonable grouping, these problems can be avoided, ensuring the stability of the standard solution and the accuracy of the analysis.
10. Special requirements when using organic reagents for ICP analysis
When using organic reagents for ICP analysis, there are some special requirements to be met compared with aqueous solution samples.
The high - frequency power generally should be higher than that for aqueous solution samples. This is because the properties of organic reagents are different from those of aqueous solutions, and higher energy is required to excite and atomize the elements in them. A relatively high high - frequency power can ensure that the elements in organic reagents are fully excited, thereby improving the sensitivity of the analysis.
The flow rate of the cooling gas should be increased, while the flow rate of the carrier gas should be decreased. At the same time, a relatively high - flow auxiliary gas should be introduced. The combustion of organic reagents in the plasma generates a large amount of heat. Increasing the flow rate of the cooling gas can effectively remove the heat and protect the instrument components. Reducing the flow rate of the carrier gas allows the organic reagents to stay in the plasma for a longer time, thus improving the atomization efficiency. Introducing a high - flow auxiliary gas can improve the stability of the plasma and ensure the accuracy of the analysis.
There are also certain special requirements for the structure and installation of the torch tube. Since the combustion characteristics and physicochemical properties of organic reagents are different from those of aqueous solutions, the structure and installation method of the torch tube need to be adjusted accordingly to meet the analysis requirements of organic reagents.
Organic solvents with chain structures are often used as diluents. Organic solvents with chain structures have low viscosity and good volatility, which can better meet the requirements of ICP analysis. They can disperse the sample more uniformly in the solution, improving the atomization efficiency and the accuracy of analysis.
11. Definition of diluents and requirements for diluents used in ICP - AES method
In the ICP - AES method, the diluent is an important auxiliary reagent, which plays a crucial role in sample analysis.
Generally speaking, it is relatively difficult to use pneumatic nebulization for sample introduction of samples with high viscosity. To overcome this problem, low - viscosity organic solvents are commonly used to dilute the samples, and such organic solvents are called diluents.
There are several requirements for diluents as follows:1. First, the viscosity of the diluent should be low. A low - viscosity diluent enables the sample to pass through the pneumatic nebulization injection system more easily, improving the injection efficiency and nebulization effect.2. Second, the number of carbon atoms in the molecule should be small. Diluents with fewer carbon atoms have lower boiling points and better volatility. They can volatilize rapidly in the plasma, reducing the impact on plasma stability.3. Third, the diluent should have moderate volatility. A diluent with moderate volatility neither volatilizes too quickly, which would cause sample loss, nor too slowly, which would affect the analysis speed.4. In addition, the diluent should produce little or no toxic gases. This is to ensure the safety of operators and environmental friendliness.5. Meanwhile, the diluent should allow a relatively high injection volume without extinguishing the plasma. A higher injection volume can improve the sensitivity of the analysis. However, if the diluent cannot withstand a high injection volume, it will cause the plasma to go out, affecting the normal progress of the analysis.6. Finally, the carbon deposition produced by the diluent at the torch nozzle should be minimal. Carbon deposition can affect the performance and service life of the torch tube. Reducing carbon deposition can ensure the stability of the instrument and the accuracy of the analysis.
12. Influence of diluents on ICP analysis
Diluents have various impacts on ICP (Inductively Coupled Plasma) analysis, and these impacts cover all aspects of the analysis process.
The viscosity of the diluent affects the atomization and sampling rate. A diluent with a relatively high viscosity will increase the resistance of the sample in the sampling system, reducing the sampling rate and atomization effect. On the other hand, a diluent with a relatively low viscosity allows the sample to pass through the sampling system more smoothly, improving the sampling efficiency and atomization quality.
The density, viscosity, and surface tension of the diluent affect the initial diameter of the formed droplets. Appropriate density, viscosity, and surface tension can make the initial diameter of the droplets more uniform, which is beneficial for the atomization and excitation of elements. If these parameters are not appropriate, it will lead to non - uniform droplet sizes and affect the accuracy of the analysis.
The boiling point of the diluent affects the evaporation of droplets and the evaporation amount of the organic solvent entering the ICP channel, thereby influencing the stability of ICP. A diluent with a lower boiling point is more likely to volatilize in the plasma and can quickly carry the sample into the plasma for analysis. However, if the boiling point is too low, the organic solvent may evaporate too rapidly, which will affect the stability of the plasma. On the other hand, a diluent with an excessively high boiling point may lead to incomplete evaporation of the organic solvent. The residual organic solvent will affect the state of the plasma and the analysis results.