Understanding the similarities and differences between these three instrumental analysis methods can help you choose a more suitable analysis method according to different analysis requirements.
The injection section and plasma of ICP-AES and ICP-MS are very similar. ICP-AES measures the optical spectrum (165-800nm), and ICP-MS measures the ion mass spectrum, which provides information on each atomic mass unit (amu) in the range of 3-250 amu. Therefore, ICP-MS can also measure isotopes in addition to elemental content determination.
The detection limit
The detection limit of ICP-MS is very impressive, and most of the detection limits of its solutions are at the ppt level (it must be remembered that the actual detection limit cannot be better than the cleaning conditions of your laboratory), The detection limit of graphite furnace AAS is sub-ppb level, and the detection limit of most elements in ICP-AES is 1-10ppb, and some elements can also achieve impressive sub-ppb level detection limits in clean samples. It must be pointed out that the ppt-level detection limit of ICP-MS is for a simple solution with few dissolved substances in the solution. If the detection limit of the concentration in the solid is involved, the advantage of the ICP-MS detection limit will be as much as 50 times worse due to the poor salt tolerance of ICP-MS, and some common light elements (such as S, Ca, Fe , K, Se) have serious interference in ICP-MS, which will also deteriorate the detection limit.
Interference
The above three technologies present different types and complex interference problems. To this end, we discuss each technique separately. Interference in ICP-MS:
1. Mass Spectrometry Interference
The interference of the mass spectrum in ICP-MS (isobaric interference) is predictable, and its number is less than 300, and the mass spectrometer with a resolution of 0.8 amu cannot distinguish them, for example, the interference of 58Ni on 58Fe, 40Ar on 40Ca, 40Ar160 on 56Fe or 40Ar-Ar on 80Se (mass spectrum overlay). The element correction equation (the same principle as the interference line correction in ICP-AES) can be used for correction, selectively select some isotopes with low natural abundance, and use "cold plasma torch flame shielding technology" or "collision cell technology" Can effectively reduce the impact of interference.
2. Matrix acid interference
It must be pointed out that HCI, HCIO4, H3PO4 and H2S04 will cause considerable mass spectral interference. Cl+, P+, S+ ions will combine with other matrix elements Ar+, O+, H+ to form polyatoms, for example, the superposition interference of 35Cl 40Ar on 75As, 35Cl160 on 51V. Therefore, avoiding the use of HCl, HClO4, H3PO4, and H2SO4 in many analyzes by ICP-MS is critical, but this is impossible. The methods to overcome this problem include "collision cell technology", using chromatographic (micro plug) separation, electrothermal evaporation (ETV) technology, etc. before the sample is introduced into ICP. Another more expensive option is ICP-MS using a high-resolution sector magnetic field, which has the ability to resolve less than 0.01 amu and can remove many mass spectral interferences. The test solution for ICP-MS analysis is usually prepared with nitric acid.
3. Doubly charged ion interference
The mass spectrum interference produced by doubly charged ions is half of M/Z of single charged ions, such as 138Ba2+ to 69Ga+, or 208pb2+ to 104Ru+. Such interferences are rare and can be effectively eliminated by optimizing the system prior to analysis.
4. Matrix effect
The difference in the viscosity of the test solution and the standard solution will change the efficiency of each solution to generate aerosol, which can be effectively eliminated by the matrix matching method or the internal standard method.
5. Ionization interference
Ionization interference is caused by the high concentration of group 1 and group 1I elements in the sample. It is effective to use matrix matching, dilute sample, standard addition method, isotope dilution method, extraction or chromatographic separation to solve it.
6. Space charge effect
Space charge effects mainly occur behind the skimmer cone, where the net charge density deviates significantly from zero. High ion densities lead to interactions between ions in the ion beam, resulting in the loss of light ions first in the presence of heavy ions, eg, Pb+ to Li3+. Matrix matching or careful selection of internal standards within the mass range of the analyte can help to compensate for this effect, but this is difficult in practice. Although the isotope dilution method is effective, it is expensive. The simple and most effective method is to dilute the sample.
lCP-AES interference
1. spectral interference
The number of spectral interferences of ICP-AES is large and difficult to solve. There are more than 50,000 spectral lines of ICP-AES recorded, and the matrix can cause quite a lot of problems. Therefore, high-resolution spectrometers must be used for the analysis of certain samples, such as steel, chemical products, and rocks. Interfering element correction, which is widely used in fixed-channel ICP-AES, can be achieved with limited success. The background in ICP-AES is high, and offline background correction is required. The application of dynamic background correction is very effective for improving accuracy. Spectral peaks or bands of various molecular particles (eg, OH) will cause some analytical problems for some analyte elements with low content, affecting their detection limit in actual samples.
The background in ICP-MS is quite low, typically less than 5 C/S (counts/second), which is one of the main reasons why ICP-MS has excellent detection limits.
2. Matrix effect
Like ICP-MS, ICP-AES can apply internal standards to account for matrix effects such as spray chamber effects and viscosity differences between sample and standard solutions.
3. Ionization interference
Careful selection of analytical conditions for each element or addition of ionization buffers (eg, excess group I elements) can reduce the effect of easily ionizable elements.
GFAAS interference
1. Spectral interference
GFAAS with deuterium lamp background correction has a little spectral interference, but GFAAS with Zeeman background correction can remove these interferences.
2. Background interference
During the atomization process, for different substrates, the conditions of the ashing step should be carefully set to reduce the background signal. The use of matrix modifiers helps to increase the allowable ashing temperature. In many GFAAS applications, Zeeman buckle backgrounds give better accuracy than deuterium light buckle backgrounds.
3. Gas phase interference
This is formed when the atomic vapor of the substance being measured enters a cooler gaseous environment. Now using isothermal graphite tube design and platform technology, the sample is atomized and then enters a hot inert gas environment, which can effectively reduce this interference.
4. Matrix effect
The matrix effect is generated by the different residues of the measured substance on the graphite tube, which depends on the type of sample. The application of matrix modifiers and hot injection can reduce these effects very effectively.
Easy to use
In daily work, ICP-AES is the most mature in terms of automation, and can be used by unskilled personnel to apply the methods formulated by ICP-AES experts. The operation of ICP-MS is still relatively complicated until now. Since 1993, although there has been great progress in computer control and intelligent software, it still needs to be fine-tuned by technicians before routine analysis, and the method research of ICP-MS is also very complicated and time-consuming work. Although the routine work of GFAAS is relatively easy, formulating the method still requires quite skilled technology.
Total dissolved solids TDS in the sample
In routine work, ICP-AES can analyze 10% TDS solutions, even as high as 30% saline solution. ICP-MS can analyze 0.5% solutions for short periods of time, but most analysts are happy with solutions up to 0.2% TDS. When the original sample is solid, ICP-MS requires higher dilution than ICP-AES and GFAAS. It is not surprising that the detection limit converted to the original solid sample does not show much advantage.
Linear Dynamic Range LDR
ICP-MS has an LDR that exceeds the lower fifth power, and various methods can develop its LDR to the eighth power of ten, but for ICP-MS anyway: High matrix concentration will cause many problems, and the best solution to these problems is dilution. For this reason, the main field of ICP-MS application is trace/ultra-trace analysis.
The LDR of GFAAS is limited to the order of 2-3 years. If a sub-sensitive line is selected, higher concentration analysis can be carried out. ICP-AES has an LDR of more than 5 orders of magnitude and strong salt resistance. It can measure trace and major elements. ICP-AES can measure the concentration up to a percentage. Therefore, ICP-AES plus ICP-MS, or GFAAS can well meet the needs of the laboratory.
Precision
The short-term precision of ICP-MS is generally 1-3% RSD, which is obtained in routine work using the multiple internal standard method. Long-term (several hours) precision was less than 5% RSD. Good accuracy and precision can be obtained using isotope dilution, but the cost of this method is too expensive for routine analysis.
The short-term precision of ICP-AES is generally 0.3-2% RSD, and the long-term precision of several hours is less than 3% RSD. The short-term precision of GFAAS is 0.5-5% RSD, and the long-term precision depends not on time but on the number of times the graphite tube is used.
Sample Analysis Capabilities
ICP-MS has the amazing ability to analyze a large number of samples for the determination of trace elements, with typical analysis times of less than 5 minutes per sample, and in some cases as little as 2 minutes. Consulting Laboratories believes that the main advantage of ICP-MS is its analytical capabilities.
The analysis speed of ICP-AES depends on whether the full-spectrum direct-reading type or the single-channel scanning type is used. The time required for each sample is 2 or 6 minutes. The full-spectrum direct-reading type is faster, and it usually takes 2 minutes to measure a sample.
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