Dynamic linear range is the concentration range that can be measured by the instrument with a linear response. This is different from the dynamic range that is the concentration range that can be measured without saturation of the detection system. Dynamic linear range is very different according to the systems. It ranges from 3 decades of measurement to 10 decades of measurement. The mode of observation of the plasma and the detection system have a great influence on the dynamic linear range.
Photomultiplier tubes allow reaching a large dynamic linear range, typically 8 decades and up to 10 decades with the HDD® device from HORIBA Scientific. Solid-state detection devices have limited linear dynamic range due to the limited size of the pixel, thus the limited electron capacity, to the read-out noise and to the A/N converter that limits dynamic range to 16 bits. Some saturation effects occur when the pixel begins to be full. The mode of observation of the plasma has a great influence on the linearity. With axial viewing mode, linear dynamic range is limited by self-absorption effects.
Limitation in performance is mainly linked to the need for extended sample preparation to adapt the sample to the limitation of the instrument, or to the extended time required to find alternative wavelengths to perform the measurement.
Resolution is the ability of the dispersive system to separate narrow peaks. It is defined as the full width at half maximum of the emission line measured (FWHM). High resolution has many benefits as it minimizes spectral interferences with linerich matrices (U, W, Co, Fe…) and it improves the signal-tobackground ratio (SBR) as it reduces the part of background measured with the peak intensity. Thus improved Limits of Detection as LOD is inversely proportional to the SBR.
Experimental resolution depends on the physical width of the line, due to Doppler and Stark effects, and on the instrumental width of the line, due to the optical system (groove density of the grating, focal length, order of diffraction) and the bandpass (product of the width of the widest slit used times the linear dispersion). Optical aberrations may degrade resolution but can be minimized if optical design and assembly are performed with great care. The band-pass is usually the limiting parameter of resolution and it has to be optimized.
Optimization can be done using high groove density for gratings and/or using higher order of diffraction and/or longer focal length and/or using finest slits. This optimization has to be balanced with the need to cover a large wavelength range (high groove density or high orders of diffractions limit the wavelength range) and the need to measure weak signals (finest slits means less light entering the optics and then reduced sensitivity). For solid-state detectors, each pixel has a given band-pass. Resolution is defined by the combination of several pixels’ band-passes, explaining a degraded resolution.
According to the optical mounting and the detection system, resolution varies considerably. Constant resolution is observed for Czerny-Turner and Paschen-Runge optics, while it varies for Echelle optics where resolution increases with the wavelength.
Echelle optics is again a specific case as resolution varies according to the position of the wavelength on the detector. Resolution is better in the center of the detector than on the edges of the detector.
Constant resolution has a huge advantage as it allows the user to know exactly the achieved resolution in all parts of the spectrum, thus facilitating method development.
ICP-OES spectrometer performance can be optimized mainly through the following parameters:
Nebulizer and sheath gas flow rates
The nebulizer and the sheath gas flows control the amount of sample introduced into the plasma, as well as the speed of the sample, and thus the residence time of the sample into the plasma. A low flow leads to a reduced amount of sample when a high flow leads to a high amount of sample and a shorter residence time. In both cases, sensitivity will be degraded. An optimum value has to be found according to the application. For difficult matrices, optimization is usually done based on signal; the maximum of signal being obtained for the best residence time/amount of sample combination.
For simple matrices, or when calibration and samples are identical, optimization can be done on Signal-to-background ratio to obtain better detection limits with a slightly degraded energy transfer.
RF power is the energy that is given to the plasma. It is expressed as the power sent to the RF generator and is generally in the range 800 to 1500W. Low RF Power leads to better SBR as background is low when high RF Power leads to reduced sensitivity but better energy transfer. The effect of the RF Power on sensitivity is less important than nebulization and sheath gas flow rates effect. The optimization of the RF Power usually depends on the nature of the sample. Higher RF Power values are used for difficult matrices or organic solvents.
Speed of peristaltic pump
The speed of the peristaltic pump drives the amount of sample transported to the nebulizer. It has to be optimized so the quantity of sample allows a good sensitivity and a good stability for aerosol generation. A low speed may lead to a low quantity of sample, and then a low sensitivity while a high speed may lead to a noisy aerosol generation and decreased sensitivity. The pump speed has to be defined for each nebulizer/peristaltic pump tubing combination. It has also to be optimized according to the volatility of the sample.
Time of integration
The integration time is defined as the time used to measure the signal. The shorter the time, the noisier the measurement is. Increasing the integration time will reduce the background noise and as the limit of detection is defined as the signal that is statistically different from the noise, decreasing the noise level means improving the detection limit.
Plasma gas and auxiliary gas do not have a great influence on performance from a detection limit point of view. The aim of plasma gas is to provide Argon so the plasma can be sustained. The use of very low plasma gas flow may lead to unstable signals or may increase matrix effects. Typical plasma gas flow for aqueous samples is 12 L/min. This flow has to be increased for high salt concentration, organics, volatile solvents or when high power settings are used.
The auxiliary gas flow is mainly used to avoid contact with the plasma and the inner tube of the torch for high salt contents or organics. With such matrices, an ionization zone exists just before the plasma and may be in contact with the inner tube. This contact may limit performance for elements such as Ca, Si, B and may decrease the lifetime of the tube. With organics, some carbon deposit may also be observed on the inner tube. Increasing the auxiliary flow helps to improve performance and extend lifetime of the tube for such matrices. For volatile solvents, increased auxiliary flow helps in isolating the sample into the Argon flow between the injector and the central channel of the plasma.
Spectral interference is an interfering element that shows up while measuring the signal of a wavelength of an element of interest. Signal is then not only due to the element that should be measured and then a bias is observed on the final result. Spectral interferences can be avoided using high resolution ICP-OES spectrometers, by using an alternative line if possible or by using Inter-Element Correction that is a mathematical procedure to compensate for the contribution of the interfering element on the element of interest. High resolution is preferred to solve this potential issue, as the use of alternative wavelength is not always possible, according to the element and to the required sensitivity. IEC is also a long and complex procedure to establish.
The figure below shows the bias that may be induced by a low resolution compared to a high resolution system that provides a result without bias. The example was Cd analysis in a sample containing high concentration of Fe and very high concentration of As.
Matrix effects are defined as the effect of the composition of the sample on the signal of analytical lines compared to the same signal of the analytical lines without the concomitant elements. Matrix effects result from a change in the plasma conditions and/or a change on the aerosol transport and filtration.
For example, the figure below shows the difference of signal between elements in deionized water and the same concentration of the same elements in NaCl 6 and 10 g/L (sea water).
When matrix effects occur, a bias will be observed on the final result. If a calibration curve is performed in water and in a given sample where matrix effects occur, the bias is easily seen.
Robustness is the ability of the ICP-OES to provide accurate results even with variations of the composition of the sample. A robust ICP-OES is an instrument able to minimize matrix effects. Robust conditions can be obtained through design of the instrument and optimization of the operating conditions.
It has been shown in the literature (“Use of magnesium as a test element for inductively coupled plasma atomic emission spectrometry diagnostics”, J.M. Mermet, Anal. Chim. Acta, 250, 85 (1991)) that the robustness can be checked using the ratio of an ionic to an atomic Mg lines (Mg II 280.270 nm / Mg I 285.213 nm)
This is what is called the Mg ratio, often written Mg II / Mg I. The higher the Mg ratio is, the more robust the instrument is.
Robust conditions can be obtained using high power settings and low nebulization flow rate as shown by Fig.42.
Matrix effects can be minimized by using robust conditions as shown in the figure on right.
The design of the instrument has a great influence on the robustness. Using a radial viewed ICP-OES instrument allows enhanced robustness compared to axial view instruments. Reduced matrix effects are thus observed, simplifying analysis and improving accuracy.
For some particular samples, significant matrix effects can be observed even with a radial view ICP-OES instrument and even with robust conditions. To compensate for these effects, matrix matching, internal standardization or standard additions may be used.
Radial view is known to provide reduced matrix effects where axial view is highly affected by these effects. Reduced matrix effects means that the signal obtained for an element will not change too much according to the matrix. That means that sensitivity will be really close for all kinds of matrices and that no systematic internal standard correction is needed to correct for a potential matrix effect. Moreover, as radial view uses vertical torch, there are fewer issues of deposits in the injector.
The uniqueness of HORIBA Scientific ICP-OES instruments is the association of the total plasma view to the radial view. This feature is due to the optics that allow measuring the whole normal analytical zone where atoms and ions emit their photons. With this total plasma view feature, and the unique 3 mm i.d. injector, detection limits are really close to axial detection limits for water, and better when the sample is more difficult. That makes our radial instrument unique in terms of detection limits.
Horizontal torches for axial viewing ICP-OES instruments are subject to deposition with high total dissolved solids contents. It limits the use of these axial viewing ICP-OES instruments to simple matrices or implies dilution of samples.
As extended torches are used with all kind of axial view ICPOES instruments, to limit the presence of oxygen emission bands, degradation of the torch may be observed for some matrices such as samples prepared through alkali flux. Analysis of organics solvents, such as kerosene or xylene, will also require continuous addition of oxygen with axial viewing instruments to avoid carbon deposits, whereas radial viewing instruments can handle such samples without any oxygen. Dual-view ICP-OES instruments share the design of axial viewing ICP-OES instruments. Thus, the same limitations occur.
The ICP-OES technique is widely used in many fields for many type of analyses. The list below summarizes briefly the main application areas.
Geology / Mining
Pharmaceuticals / Cosmetics