ICP-OES principle and pre-treatment technique

Inductively coupled plasma atomic emission spectrometry (ICP-OES) is a spectral analysis method using inductively coupled plasma moment as the excitation light source, which has the advantages of high accuracy and precision, low limit of detection, fast determination, wide linear range, and simultaneous determination of multiple elements, etc. It has been widely used for the determination of dozens of elements in environmental samples and samples of rocks, minerals, metals and so on.

Principle of ICP-OES

ICP-OES is a common method for the determination of elemental content in samples, and its working principle is: the sample to be tested enters the plasma centre channel of the quartz torch tube through the aerosol formed by the sprayer, and the spectral lines radiated after excitation by the light source are directed to the dispersive grating of the dispersive system through the incident slit, and then the characteristic spectral lines of the element to be tested are projected to the CCD after spectroscopic separation, and then processed through the circuit, and then the data are processed by the computer to determine the elemental content. content.

background

ICP-AES is known as Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES), also known as Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES). It is mainly used for qualitative (presence or absence) and quantitative (amount) analysis of elements in samples, and can analyse more than 70 elements in the periodic table.

Figure 1: Periodic Table of the Elements, showing detection limits for useable elements

The powerful quantitative function of ICP-AES is widely used in the elemental analysis of samples in the fields of nano, catalysis, energy, chemical industry, biology, geology, environmental protection, medicine, food, metallurgy, agriculture and so on.

the principle of the instrument inductively coupled plasma emission spectrometer (ICP-OES/AES)

The use of plasma excitation light source so that the sample evaporation vapor, dissociation or decomposition of the atomic state, the atoms can be further ionized into an ionic state, the atoms and ions in the source of excitation luminescence. The light emitted by the light source is decomposed into spectra arranged by wavelength using a spectroscopic system, after which the spectra are detected using an optoelectronic device. The sample is analysed qualitatively according to the measured spectral wavelengths and quantitatively according to the intensity of the emitted light. The working mode is as follows: the sample to be tested enters the plasma centre channel of the quartz torch tube through the aerosol formed by the sprayer, and the light is radiated through the heating and excitation of the light source, and then diffracted by the grating, and then the grating is rotated by the stepping motor, so that the characteristic spectral line of the element is accurately located at the exit slit, and the photomultiplier tube transforms the intensity of the spectral line into the photocurrent, which is then processed through the electric circuit, and then the computer carries out the data processing to determine the element's content. The data is processed by the computer to determine the content of the element.

Figure 2: Basic Instrumental Components of ICP-OES

The injection system and ion source of ICP-MS are basically the same as those of ICP-OES. Only after most of the atoms are converted into ions, the ions are separated according to the charge-to-mass ratio and the number of ions is counted.

Figure 3: Basic Instrumental Components of ICP-MS

The ICP-OES method is mainly composed of two parts: sample pre-treatment and on-board detection, of which sample pre-treatment accounts for 90%-95% of the workload, and on-board detection accounts for 5%-10%. Thus, the difficulty of this method lies in the sample pre-treatment. Here are some tips and precautions for sample pre-processing, taking soil samples as an example.

ICP-OES pre-treatment

Processing container

Nowadays, we usually use 30mL PTFE crucible as the container for dissolving samples. Before use, firstly, scrub it with decontamination powder, and then put it into the hot nitric acid solution with a concentration of 20%-30% for 2-3 hours. Next, it is soaked in slightly boiling water for 2 hours. Finally, it is removed and washed 3 times with secondary deionised water. After the above operations, ensure the cleanliness of the PTFE crucible to avoid sample contamination.

Weigh the sample

Before weighing the sample, number the crucible. When weighing the sample, weigh accurately according to the method requirements, pay attention to the order of the sample, remember not to invert the number, tamper with the number. After weighing, rinse the crucible wall with a washing bottle (rinse the sample stuck on the crucible wall during weighing to the bottom of the crucible to avoid sample loss).

Dissolution of the sample

We detect copper, lead, zinc, manganese, vanadium, titanium, chromium, cobalt, nickel and other elements in the soil, generally using the "four acid" dissolution of the sample method, the so-called "four acid" is nitric acid, hydrofluoric acid, perchloric acid and hydrochloric acid. Usually, the first to add nitric acid, hydrofluoric acid and perchloric acid, add the amount of 5mL, 5mL, 2mL. add a good acid, cover the crucible lid, the temperature of the hot plate is set to 180 ℃, placed in the electric heat from the low temperature to start heating. Here need to pay attention to be heated from low temperature, because the temperature of the electric hot plate is too high, the confusion in the crucible is consumed too quickly, the sample can not be fully eliminated. Add the lid to ablate for about 2 hours, close the electric heating plate, and leave it to rest overnight. After that, open the lid and heat the ablation openly until the mixed acid solution bubbles dry. Then 10 mL of aqua regia at a concentration of 1:1 was added, and after the salts in the sample were dissolved and the solution was clear, the volume was fixed. In the condensation, the crucible should be rinsed with deionised water for 2-3 times.

static sample

After the sample is fixed, shake it well, so that the components to be measured are evenly distributed in the solution. After 3-4 hours of resting, and then on the machine for measurement. The purpose is to let the salt precipitate at the bottom of the solution to be measured, so as to avoid clogging the ICP-OES injection system during the test. If there is an urgent need for individual samples to be analysed, they can be filtered and measured directly on the machine after the stationary phase.

Pre-treatment is the most tedious and time-consuming step in the analysis of a sample, which directly affects the detection results. Therefore, we should master and optimise the sample pre-treatment process in the actual analysis and testing work, so as to improve the production speed and ensure the quality requirements.