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How to choose ICP-MS/ICP-AES or AAS?

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How to choose ICP-MS/ICP-AES or AAS?

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For those with a technical background in ICP-AES, ICP-MS is a plasma (ICP) with a mass spectrometer as a detector, while mass spectrometers think of ICP-MS as a mass spectrometer with ICP as a source. In fact, 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, in addition to the determination of element content In addition, isotopes can also be measured.


01 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), graphite furnace The detection limit of AAS is sub-ppb level, and the detection limit of most elements of ICP-AES is 1-10ppb, and some elements can also obtain remarkable 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 it involves the detection limit of the concentration in the solid, due to the poor salt tolerance of ICP-MS, ICP-MS The advantage of MS detection limit will be worse by up to 50 times, and some common light elements (eg, S, Ca, Fe, K, Se) have serious interference in ICP-MS, which will also deteriorate their detection limit.



02 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, 58Ni to 58Fe, 40Ar to Interference of 40Ca, 40Arl60 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 not possible. Methods to overcome this problem include “collision cell technology”, the use of chromatographic (micro-plug) separation before the sample is introduced into ICP, electrothermal evaporation (ETV) technology, etc. Another more expensive option is ICP using a high-resolution sector magnetic field -MS, it has the ability to resolve less than 0.01 amu, and can remove the interference of many mass spectra. 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 relatively 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.


ICP-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. It depends on the type of sample. The application of matrix modifiers and hot injection can reduce these effects very effectively.


03 Ease of use

In daily work, ICP-AES is the most mature in terms of automation, and the method formulated by ICP-AES experts can be used by unskilled personnel. 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. The method of ICP-MS Research is also complex and time-consuming. Although the routine work of GFAAS is relatively easy, formulating the method still requires quite skilled technology.


04 Total dissolved solids in the sample

In routine work, ICP-AES can analyze solutions of 10% TDS and even saline solutions as high as 30%. 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, compared with ICP-AES and GFAAS, ICP-MS requires higher dilutions, It is not surprising that the detection limit converted to the original solid sample does not show much advantage.


05 Linear Dynamic Range

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 concentrations cause many problems, and these problems The best solution is dilution, and for this reason, the main field of application of ICP-MS is in trace/ultratrace 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.


06 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.


07 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. 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.


The analysis speed of GFAAS is 3-4 minutes for each element in each sample, and it can work automatically at night, so as to ensure the analysis ability of samples.


According to the concentration of the solution, the following is an example for reference:
1. Measure 1 to 3 elements for each sample, and the element concentration is sub- or lower than ppb level. If the requirements of the elements to be measured can be met, GFAAS is the most suitable.
2. For 5-20 elements in each sample, the content is sub-ppm to %, and ICP-AES is the most suitable.
3. Each sample needs to measure more than 4 elements, in sub-ppb and ppb content, and the amount of sample is quite large, so ICP-MS is more suitable.


08 Unmanned operation

ICP-MS, ICP-AES, and GFAAS can be run unattended overnight due to the modern automation design and the safety of using inert gas. In order to efficiently analyze production, it is advisable to work overnight.


09 Running costs

 The start-up cost of ICP-MS is higher than that of ICP-AES because some parts of ICP-MS have a certain life span and need to be replaced. These parts include turbomolecular pump, sampling cone and skimmer cone and detector. For ICP-MS and ICP-AES, the lifetime of the nebulizer and the torch are the same. If the laboratory chooses ICP-AES instead of ICP-MS, it is best to have GFAAS in the laboratory. GFAAS shall calculate the charges for its graphite tubes. Among the above three technologies, the cost of Ar gas is quite a budget, and the Ar cost of ICP technology is much higher than that of GFAAS.


10 Basic cost

This is a difficult item to pin down because the cost is based on the degree of automation, accessories and suppliers. It is roughly estimated that ICP-AES is twice as fast as GFAAS, and ICP-MS is twice as fast as lCP-AES. It must be noted that the configuration of accessories will disrupt the cost estimates. In addition, it must be considered that ultratrace analysis requires a clean laboratory and ultrapure chemical reagents, which are not cheap.


11 Appendix

Because it is a fast scanning measurement method, ICP-MS can measure instantaneous signals in multi-element mode, which opens the way for a large number of accessories, and techniques such as electrothermal steaming, laser ablation, glow discharge and spark ablation can be exempted. The dissolution process of the sample. Some accessories can separate or pre-enrich the matrix substances in the sample, for example, hydrogenation method, chromatography (high pressure liquid phase HPLC, ion chromatography, micro plug), etc.


The benefits of separation by chromatography are fully realized in ICP-MS, which is suitable for low-concentration analytes in environmental protection, toxicology, pharmaceuticals and food.


Although ICP-AES can also use some of the above-mentioned accessories, due to the price and limited benefits of these accessories, they are rarely used in the routine analysis of ICP-AES.


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