Development of ICP spectrometer instruments

After several decades of development, ICP emission spectrometers have greatly improved in terms of sensitivity and stability, and instrumental analysis functions. The main performance is as follows:

First, the progress in the light source

1. Improvement of the high frequency generator:

Since the ICP electron density and excitation temperature decrease with increasing frequency, the background intensity of the source (continuous spectrum of Ar) is inversely proportional to the square of the frequency and much lower with increasing frequency. Therefore, in order to improve the stability of the high-frequency generator, a solid-state generator with high frequency stability and output power stability is used, the plasma impedance can be automatically adjusted and compensated, and the operating parameters and the ignition program can be controlled by the computer. Automatic ignition. Therefore, the new commercial instruments have used solid-state generators, combined with the introduction of the sample to take a constant temperature, improve the stability of the injection, plus the constant temperature of the optical system, high thermal stability, so that the instrument preheating time is greatly shortened , greatly improving the analysis accuracy and accuracy of the ICP spectrometer analysis method.

2, using end-view technology to improve sensitivity:

In recent years, commercial ICP spectrometers have introduced axial (end-view) ICP with high sensitivity and good detection limit. In the ICP spectrometer, the ICP tube is usually placed vertically, and viewed from the side, it is called radial (side view) ICP. The detection limit of the ICP (horizontal tube horizontal placement) is usually several times higher than the side view by an order of magnitude. This is because only a part of the normal analysis area of ​​the ICP is observed in the side view, and the signal amount is small and the background is high. The end-view can observe the light emission signal of the entire normal analysis area of ​​the ICP, and increase the measurable signal quantity, while the spectral background is low and the signal-to-back ratio is high. Therefore, the detection limit of the end view is significantly higher than the side view. The main disadvantage of looking at ICP is that the linear range is relatively small. Moreover, the matrix effect is more obvious when analyzing complex samples of matrix. This is because when the ICP is used to observe the ICP, the temperature of the tail flame of the plasma moment is low, and electrons and ions are recombined, which may result in self-priming and molecular spectroscopy. For this purpose, it is usually necessary to use a compressed air to "cut" the tail flame, or a "cold cone technique" (a metal cone placed in the ICP tail flame region like an ICP-MS interface cone) to effectively eliminate the interference of the tail flame. Improve the stability of the end ICP and reduce the matrix interference of the end ICP.

Second, the development of detectors

1. The performance of photomultiplier tube (PMT) is further explored:

In order to adapt to the rapid determination of multi-spectral lines, a high dynamic range PMT detector has been developed in recent years, which can be controlled by computer in real time according to the intensity of the optical signal (according to the concentration of the component to be tested), and the gain is automatically adjusted at high speed. More spectral information can be acquired with the fast scan method. With this high dynamic range detector and its fast signal acquisition circuit, the spectrum can be acquired at high speed to obtain the full spectrum of the sample.

2, using optical solid multi-channel detector (CCD, CID), can record all spectral lines:

Charge coupled devices (CCDs) and charge injection devices (CIDs) are cited on ICP spectrometers due to their high photoelectric conversion efficiency and extremely low noise at low temperatures, and their high detection sensitivity. The area array CCD or CID can have many detection units. Combined with the two-dimensional spectrum generated by the Echelle grating, the ICP spectrometer can be superior to the usual multi-channel simultaneous instrument with full spectrum direct reading. In order to make the instrument have higher resolution and wider spectral coverage, CCD or CID needs to have larger size and higher integrated unit; CCD and CID have lower response in the ultraviolet region, and fluorescent coating is required. Enhance the response to ultraviolet light; the noise is high at normal temperature, the CCD needs to use a semiconductor cooler, and cool to -70 ° C; CID needs to be cooled to -80 ° C with liquid nitrogen or low temperature refrigerator.

Third, ICP emission spectrometer to the full spectrum direction

Due to the optical performance and volume limitations of PMT, many simultaneous spectral information is lost, whether it is a simultaneous or sequential ICP spectrometer, which limits the development of spectral instruments to full-spectrum direct reading. Solid-state detectors such as CCD and CID have high quantum efficiency as photoelectric elements, close to the theoretical limit of ideal devices, and are ultra-small, large-scale integrated components that can be used as linear array and area array detectors. It can record thousands of lines at the same time, which greatly improves the multi-element simultaneous measurement function of the spectrometer. With the two-dimensional spectrum generated by the echelle grating and the prism dispersion system, a point-like spectrum is formed on the focal plane, and a CCD, CID type surface array detector is used, so that the spectral information can be obtained to a greater extent, and the instrument has the photoelectric method. The advantages of the spectrograph method are convenient for spectral interference and simultaneous measurement of spectral line intensity distribution, which is beneficial to the adoption of multi-spectral correction technology, effectively eliminating spectral interference and improving selectivity and sensitivity. And there are new changes in the structure of the instrument. The development of spectroscopic instruments with new detectors has become the development direction of a new generation of ICP spectrometers.

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