Three ICP light source observation methods, which one do you choose?
There are three observation methods for the ICP light source generated in the ICP spectrometer torch assembly, namely: vertical observation (Radial), horizontal observation (Axial) and two-way observation (DUO).
Plasma is one of the cores of ICP-AES. Its structure is shown in the figure. The work coil uses high-frequency (27.12MHz/40.68 MHz) and high-power energy to form a strong alternating electromagnetic field in the quartz torch, ionizing the argon (Ar) in it to form a stable High-temperature discharge torch – plasma. The aerosol of the sample is sprayed into the plasma through the central tube to form an analysis channel of about Φ2mm. Due to the different temperature distributions in different areas of the plasma, the entire analysis channel is divided into: atomization area (lower temperature), atomic emission area (higher temperature) and ion emission area (high temperature).
Scientists have long confirmed that the ion emission area has the best signal-to-background ratio and the best stability, and have proposed the concept of the best observation height (generally 12-16mm above the working coil). Vertical (Radial) observation measures the signal in this area; horizontal (Axial) observation measures the signal of the entire analysis channel except the tail flame.
ICP spectrometer vertical observation
Also known as vertical observation or test observation, it uses a vertically placed ICP spectrometer torch. The direction of the “flame” air flow is perpendicular to the direction of the lighting path; the spectrometer can receive all signals in the entire analysis area.
Vertical observation is a classic ICP observation method. It collects light from the side of the plasma and measures the signal in the ion emission area (the best observation area). Obviously, the signal in this area is not as strong as the signal of the entire analysis channel during horizontal observation, so its sensitivity is not as high as that of horizontal observation. However, the signal in this area can provide the best signal-to-background ratio, especially under complex substrates, the signal-to-background ratio is even more outstanding.
At the same time, vertical observation does not collect optical signals from the atomization region and atomic emission region in the plasma, so it does not have easy ionization interference (EIE) at all, and has a very good linear range (105-106), very small matrix effect and very Low background, flexible and convenient. The vertical observation torch is very short, not easy to stain, and not easy to heat up under high power. It is more suitable for direct injection analysis of high matrix samples, oil samples, and organic samples.
ICP spectrometer horizontal observation
Also known as axial observation or end-view observation, it uses a horizontally placed ICP spectrometer torch. The direction of the “flame” air flow and the direction of the lighting path are horizontally coincident; this allows all the light in each part of the entire flame to pass through the slits.
As early as the early 1980s, our country’s scientific and technical personnel began to try to place plasma horizontally. The signal of the entire analysis channel was collected axially to improve sensitivity, and some application reports were published. However, due to the limitations of tail flame cutting technology and high matrix background, this technology was not actually adopted. Until the early 1990s, scientists continued to develop optical collection, focusing methods, tail flame removal, plasma stabilization, and sample introduction systems. Carry out a series of innovations.
Horizontal observation collects the signal of the entire analysis channel except the tail flame, so its sensitivity is better than vertical observation that only collects signals in local areas, and the detection limit is reduced by 5-10 times. However, because horizontal observation collects signals from the entire analysis channel including the atomization area and atomic emission area, and these two areas are not the best observation areas for ICP, so while horizontal observation improves sensitivity, it also increases background noise and matrix effects, and introduces easy ionization interference (EIE). When the matrix is simple, the background and matrix effects are relatively small, and horizontal observation has a good signal-to-background ratio, so that the instrument has a very good detection limit. However, for situations where the matrix is complex, the background noise and matrix effect are very prominent. At this time, the signal-to-background ratio may not be as good as vertical observation. The matrix effect and easy ionization interference increase the measurement error.
In addition, horizontal observation increases the observation area and increases the self-absorption phenomenon of the emission spectrum line, so its linear range will be affected, especially when measuring high-content elements, which will cause large measurement errors due to the bending of the standard curve. Horizontal observation requires certain techniques to remove the tail flame. In order to obtain stable plasma, the quartz torch for horizontal observation needs to be much longer than for vertical observation, so that the high-temperature area of the entire plasma is included in the torch. Under complex substrates, the torch is easily stained, and under high power, the torch is also easily damaged. In addition, the effective analysis channel collected by horizontal observation is only Φ1~2mm wide, so horizontal observation requires very strict axial alignment and focusing of the external optical path. Otherwise, once it deviates slightly from the center of the analysis channel, the sensitivity will be greatly affected.
It can be seen that horizontal observation is very suitable for areas with relatively simple substrates such as environmental protection, water quality, food, and health inspection. It is not suitable for fields with complex substrates such as metallurgy, petrochemicals, and geology. Easy ionization interference (EIE): refers to the phenomenon that the measurement results of alkali metal elements (K, Na, etc.) are high in the presence of alkaline earth metal elements (Ca, Mg, etc.), that is, alkaline earth metals produce positive interference to alkali metal elements.
Research shows that this interference originates from the lower-temperature atomization zone in the plasma. In the past, people generally used the vertical observation method, which completely avoided the atomization zone in signal collection, so there was no problem of easy ionization interference. With the application of horizontal observation, since horizontal observation collects the signal of the entire analysis channel including the atomization zone, people have realized the seriousness of easily ionization interference (EIE).
The advantages of horizontal observation are:
Since the light from all parts of the entire “flame” can be collected, the sensitivity is high and the detection limit for simple samples is better;
Its disadvantages: large matrix effect and ionization interference, small linear range, the torch solution is contaminated by carbon and salt deposits, and needs timely cleaning and maintenance. The RF power setting cannot generally exceed 1350W; it is used in spectrometer water quality analysis.
Traditional two-way observation is based on the horizontal observation of the ICP light source, adding a set of lateral lighting light paths to achieve vertical/horizontal two-way observation, that is, placing three reflectors in sequence in the direction of the vertical observation of the torch. When vertical observation is used, the light in the vertical direction of the torch is reflected into the original optical path through three reflectors, and the first reflector in the original optical path is rotated so that the light in the vertical direction is consistent with the original horizontal direction. The incoming light coincides with the entire light path.
The switching mirror of this observation method is controlled by a computer. This method combines the characteristics of axial and radial directions, has a certain degree of flexibility, and enhances the ability to measure complex samples. Changing the observation method can achieve the following three methods of measurement:
- Horizontal measurement of spectral lines of all elements;
- All element spectral lines are measured vertically;
- Some elemental spectral lines are measured horizontally, and some elemental spectral lines are measured vertically.
Bidirectional observation can effectively solve the electronic interference existing in horizontal observation and further broaden the linear range. However, this observation method requires constant switching of reflectors, which may cause the stability of the instrument to deteriorate. Due to the need for radial observation, the side of the torch must be opened, which greatly reduces the life of the torch and also changes the shape of the torch flame. The opening of the torch must be strictly aligned with the optical path, otherwise salt will easily accumulate on the wall of the torch, which will cause serious errors in the detection results. At t