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Inductively Coupled Plasma-Atomic Emission Spectrometry

           Inductively coupled plasma-atomic emission spectrometry (ICP-AES); also referred to as inductively coupled plasma-optical emission spectroscopy (ICP-OES), is one of the plasma-based instrumental methods. The inductively coupled plasma (ICP) is a high-temperature excitation source that desolvates, vapourizes, and atomizes aerosol samples and ionizes the resulting atoms. The excited analyte ions and atoms can then subsequently be detected by observing their emission lines, a method termed inductively coupled plasma-atomic emission spectroscopy, or the excited or ground state ions can be determined by a technique known as inductively coupled plasma-mass spectrometry (ICP-MS). The ICP can use either an atomic emission (optical emission) or a mass spectral detection system. In ICP-AES, analyte detection is achived at an emission wavelength of the analyte in question. In ICP-MS, analytes are detected directly at their atomic masses because these masses must be charged to be detected in ICP-MS, the method relies on the ability of the plasma source to both atomize and ionize sample constituents. ICP-AES and ICP-MS may be used for either a single- or multi-element analysis and used for either sequential or simultaneous analyses with good sensitivity over an extended linear range.

ICP formation

           The components that make up the ICP excitation source include the argon gas supply, torch, radio frequency (RF) induction coil, impedance-matching unit, and RF generator. Argon gas is almost universally used in the ICP. The plasma torch consists of three
concentric tubes designated as the inner, the intermediate, and the outer tube. The intermediate and outer tubes are almost universally made of quartz. The inner tube can be made of quartz or alumina if the analysis is conducted with solutions containing hydrofluoric acid. The nebulizer gas flow carries the aerosol of the sample solution into and through the inner tube of the torch and into the plasma. The intermediate tube carries the intermediate (sometimes referred to as the auxiliary) gas. The intermediate gas flow helps to lift the plasma off the inner and intermediate tubes to prevent their melting and the deposition of carbon and salts on the inner tube. The outer tube carries the outer (sometimes referred to as the plasma or coolant) gas, which is used to form and sustain the toroidal  plasma. The toroidal plasma is referred to as the induction region through which the sample is introduced into the centre of the plasma.
           An RF induction coil, also called the load coil, surrounds the torch and produces an oscillating magnetic field, which in turn sets up an oscillating current in the ions and electrons produced from the argon. The impedance-matching unit serves to efficiently couple the RF energy from the generator to the load coil. Within the load coil of the RF generator, the energy transfer between the coil and the argon creates a self-sustaining plasma. Collisions of the ions and electrons liberated from the argon ionize and excite the analyte atoms in the hightemperature plasma. The plasma operates at temperatures of 6000 K to 10,000 K (about 5727° to 9727°), so most covalent bonds and analyte-to-analyte interactions are eliminated.

Apparatus

           The apparatus consists essentially of the following elements:
           (a) a sample-introduction system consisting of a peristaltic pump delivering the solution at constant flow rate into a nebulizer;
           (b) an RF generator;
           (c) a plasma torch;
           (d) transfer optics focusing the image of the plasma at the entrance slit of the spectrometer; radial viewing is better for difficult matrices (alkalis, organics), whereas axial viewing gives more intensity and better detection limits in simple matrices;
           (e) wavelength dispersive devices consisting of diffraction gratings, prisms, filters or interferometers;
           (f) detectors converting radiant energy into electrical energy;
           (g) a data-acquisition unit.

Interference

           Interference is anything that causes the signal from an analyte in a sample to be different from the signal for the same concentration of that analyte in a calibration solution. The well-known chemical interference that is encountered in flame atomic absorption spectrometry is usually weak in ICP-AES.  In rare cases where interference occurs, it may be necessary to increase the RF power or to reduce the inner support-gas flow to eliminate it. The interference in ICP-AES can be of spectral origin or even the result of high concentrations of certain elements or matrix compounds. Physical interference (due to differences in viscosity and surface tension of the sample and calibration standards) can be minimized by dilution of the sample, matrix matching, use of internal standards, or through application of the method of standard additions. Another type of interference occasionally encountered in ICP-AES is the so-called “easily ionized elements (EIEs) effect”.  The EIEs are those elements that are ionized much more easily, for example alkaline metals and alkaline earths. In samples that contain high concentrations of EIEs (more than 0.1 per cent), suppression or enhancement of emission signals is likely to occur. 

            SPECTRAL INTERFERENCE This interference may be due to other lines or shifts in background intensity.  These lines may correspond to argon (observed above 300 nm), OH bands due to the decomposition of water (at about 300 nm), NO bands due to the interaction of the plasma with the ambient air (between 200 nm and 300 nm), and other elements in the sample, especially those present at high concentrations. The interference falls into four different categories: simple background shift, sloping background shift, direct spectral overlap, and complex background shift.

            ABSORPTION INTERFERENCE This interference arises when part of the emission from an analyte is absorbed before it reaches the detector. This effect is observed particularly when the concentration of a strongly emitting element is so high that the atoms
or ions of that element that are in the lower energy state of transition absorb significant amounts of the radiation emitted by the relevant excited species. This effect, known as self-absorption, determines the upper end of the linear working range for a given emission line.

            MULTICOMPONENT SPECTRAL FITTING Multiple emission-line determinations are commonly used to overcome problems with spectral interferences. A better, more accurate method for performing spectral interference corrections is to use the information obtained with advanced detector systems through multicomponent spectral fitting.  This quantifies not only the interference, but also the background contribution from the matrix, thereby creating a correction formula. Multicomponent spectral fitting utilizes a multiple linear-squares model based on the analysis of pure analyte, the matrix and the blank, creating an interference corrected mathematical model. This permits the determination of the analyte emission in a complex matrix with improved detection limits and accuracy.
Procedure

            SAMPLE PREPARATION AND SAMPLE INTRODUCTION

            The basic goal for the sample preparation is to ensure that the analyte concentration falls within the working range of the instrument through dilution or preconcentration, and that the sample-containing solution can be nebulized in a reproducible manner. Several sample-introduction systems tolerate high acid concentrations, but the use of sulfuric and phosphoric acids can contribute to background emission observed in the ICP spectra. Therefore, nitric and hydrochloric acids are preferable. The availability of hydrofluoric acid-resistant (for example, perfluoroalkoxy polymer) sample-introduction systems and torches also allows the use of hydrofluoric acid. In selecting a sample-introduction method, the requirements for sensitivity, stability, speed, sample size, corrosion resistance and resistance to clogging have to be considered. The use of a cross-flow nebulizer combined with a spray chamber and torch is suitable for most requirements. The peristaltic pumps used for ICP-AES usually deliver the standard and sample solutions at a rate of 1 mL per minute or less. In the case of organic solvents being used, the introduction of oxygen must be considered to avoid organic layers.

           CHOICE OF OPERATING CONDITIONS
           The standard operating conditions prescribed by the manufacturer are to be followed. Usually, different sets of operating conditions are used for aqueous solutions and for organic solvents. Suitable operating parameters are to be properly chosen:
           (1) wavelength selection;
           (2) support-gas flow rates (outer, intermediate and inner tubes of the torch);
           (3) RF power;
           (4) viewing position (radial or axial);
           (5) pump speed;
           (6) conditions for the detector (gain/voltage for photomultiplier tube detectors, others for array detectors);
           (7) integration time (time set to measure the emission intensity at each wavelength).

           Control of instrument performance
           SYSTEM SUITABILITY The following tests may be carried out with a multi-element control solution to ensure the adequate performance of the ICP-AES system:
           (1) energy transfer (generator, torch, plasma); measurement of the ratio Mg II (280.270 nm)/Mg I (285.213 nm) may be used;
           (2) sample transfer, by checking nebulizer efficiency and stability;
           (3) resolution (optical system), by measuring peak widths at half height, for example, As (189.042 nm), Mn (257.610 nm), Cu (324.754 nm) or Ba (455.403 nm);
           (4) analytical performance, by calculating detection limits of selected elements over the wavelength range.

Validation of the method

           Satisfactory performance of methods prescribed in monographs is verified at suitable time intervals.

           LINEARITY Prepare and analyze not fewer than four standard solutions over the calibration range plus a blank. Perform not fewer than five replicates.The calibration curve is calculated by least-square regression from all measured data of the calibration test. The regression curve, the means, the measured data and the confidence interval of the calibration curve are plotted. The operating method is valid when:
           (1) the correlation coefficient is at least 0.99;
           (2) the residuals of each calibration level are randomly distributed around the calibration curve. 
           Calculate the mean and relative standard deviation for the lowest and for the highest calibration level.
           When the ratio of the estimated standard deviations f the lowest and the highest calibration level is less than 0.5 or greater than 2.0, a more precise estimation of the calibration curve may be obtained using weighted linear regression. Both linear and quadratic weighting functions are applied to the data to find the most appropriate weighting function to be employed. If the means compared to the calibration curve show a deviation from linearity, two-dimensional linear regression is used.

            ACCURACY Verify the accuracy preferably by using a certified reference material. Where this is not possible, perform a test for recovery.

            RECOVERY For assay determinations a recovery of 90 per cent to 110 per cent is to be obtained. The test is not valid if recovery, for example, for trace-element determination, is outside of the range 80 per cent to 120 per cent of the theoretical value. Recovery may be determined on a suitable standard solution (matrix solution) spiked with a known quantity of analyte (concentration range that is relevant to the samples to be determined).

            REPEATABILITY The repeatability is not greater than 3 per cent for an assay and not greater than 5 per cent for an impurity test.

            LIMIT OF QUANTIFICATION The limit of quantification is estimated by calculating the standard deviation of not less than ten replicate measurements of a blank solution and multiplying by 10. When validating a procedure using the method of standard additions, the slope of standards applied to a solution of the test material is used. Other suitable approaches can be used. 

             Inductively coupled plasma-mass spectrometry 
             Inductively coupled plasma-mass spectrometry (ICP-MS) is a plasma-based instrumental method that uses mass spectral detection system. When using the ICP-MS, analytes are detected directly at their atomic masses. Because these masses must be charged to be detected in ICP-MS, the method relies on the ability of the plasma source to both atomize and ionize sample constituents. The basic principles of ICP formation are described in ICP Formation under Inductively Coupled Plasma-Atomic Emission Spectrometry.  The sample-introduction system and data-handling techniques of an ICP-AES system are also used in ICP-MS.

             Apparatus
             The apparatus consists essentially of the following elements:
              (a) a sample-introduction system, consisting of a peristaltic pump delivering the solution at constant flow rate into a nebulizer;
              (b) an RF generator;
              (c) a plasma torch;
              (d) an interface region including cones to transport ions to the ion optics;
              (e) a mass spectrometer;
               (f) a detector;
               (g) a data-acquisition unit.

               Interference
               Mass interference is the major problem, for example by isobaric species that significantly overlap the mass signal of the ions of interest, especially in the central part of the mass range (for example, 40 to 80 a.m.u.). The combination of atomic ions leads to polyatomic or molecular interferences (i.e., ⁴⁰Ar¹⁶O with ⁵⁶Fe or ⁴⁰Ar⁴⁰Ar with ⁸⁰Se). Matrix interference may also occur with some analytes. Some samples have an impact on droplet formation or on the ionization temperature in the plasma. These phenomena may lead to the suppression of analyte signals. Physical interference is to be circumvented by using the method of internal standardization or by standard addition. The element used as internal standard depends on the element to be measured: ⁵⁹Co and ¹¹⁵In, for example, can be used as internal standards. The prime characteristic of an ICP-MS instrument is its resolution, i.e. the efficiency of separation of two close masses. Quadrupole instruments are, from this point of view, inferior to magnetic-sector spectrometers.

                Procedure
                SAMPLE PREPARATIONS AND SAMPLE INTRODUCTION  The sample preparation usually involves a step of digestion of the matrix by a suitable method, for example in a microwave oven. Furthermore, it is important to ensure that the analyte concentration falls within the working range of the instrument through dilution or preconcentration, and that the sample-containing solution can be nebulized in a reproducible manner.  
                Several sample-introduction systems tolerate high acid concentrations, but the use of sulfuric and phosphoric acids can contribute to background emission. Therefore, nitric and hydrochloric acids are preferable. The availability of hydrofluoric acid-resistant (for example, perfluoroalkoxy polymer) sampleintroduction systems and torches also allows the use of hydrofluoric acid. In selecting a sample-introduction method, the requirements for sensitivity, stability, speed, sample size, corrosion resistance and resistance to clogging have to be considered. The use of a cross-flow nebulizer combined with a spray chamber and torch is suitable for most requirements.  The peristaltic pumps usually deliver the standard and sample solutions at a rate of 20 to 1000 μL per minute. In the case of organic solvents being used, the introduction of oxygen must be considered to avoid organic layers.

                CHOICE OF OPERATING CONDITIONS The standard operating conditions prescribed by the manufacturer are to be followed. Usually, different sets of operatingconditions are used for aqueous solutions and for organic solvents. Suitable operating parameters are to be properly chosen:
                (1) selection of cones (material of sampler and skimmer);
                (2) support-gas flow rates (outer, intermediate and inner tubes of the torch);
                (3) RF power;
                (4) pump speed;
                (5) selection of one or more isotopes of the element to be measured (mass).

                Isotope selection
                Isotope selection is made using several criteria.  The most abundant isotope for a given element is selected to obtain maximum sensitivity. Furthermore, an isotope with the least interference from other species in the sample matrix and from the support
gas should be selected. Information about isobaric interferences and interferences from polyatomic ions of various types, for example, hydrides, oxides, chlorides, etc., is usually available in the software of ICP-MS instrument manufacturers.

                Control of instrument performance
                SYSTEM SUITABILITY
                (1) Tuning of the instrument allows to monitor and adjust the measurement before running samples.  ICP-MS mass accuracy is checked with a tuning solution containing several isotopes covering the whole range of masses, for example, ⁹Be, ⁵⁹Co, ⁸⁹Y, ¹¹⁵In, ¹⁴⁰Ce, and ²⁰⁹Bi.
                (2) Sensitivity and short- and long-term stability are recorded. The instrument parameters (plasma condition, ion lenses and quadrupole parameter) are to be optimized to obtain the highest possible number of counts.
                (3) Tuning for resolution and mass axis is to be done with a solution of Li, Y and Tl to ensure an acceptable response over a wide range of masses.
                (4) Evaluation of the efficiency of the plasma to decompose oxides has to be performed in order to minimize these interferences. The ratio Ce/CeO and/or Ba/BaO is a good indicator, and a level less than about 3 per cent is required.
                (5) Reduction of the formation of double-charged ions is made with Ba and Ce. The ratio of the signal for double-charged ions to the assigned element should be less than 2 per cent.
                (6) Long-term stability is checked by running a standard first and at the end of the sample sequence, controlling whether salt deposits on the cones have reduced the signal throughout the run.

                Validation of the method
                Satisfactory performance of methods prescribed in monographs is verified at suitable time intervals.
                LINEARITY Prepare and analyze not fewer than four standard preparations over the calibration range plus a blank. Perform not fewer than five replicates. The calibration curve is calculated by least-square regression from all measured data of the calibration test. The regression curve, the means, the measured data and the confidence interval of the calibration curve are plotted. The operating method is valid when:
                (1) the correlation coefficient is at least 0.99;
                (2) the residuals of each calibration level are randomly distributed around the calibration curve.
                Calculate the mean and relative standard deviation for the lowest and for the highest calibration level. When the ratio of the estimated standard deviations of the lowest and the highest calibration level is less than 0.5 or greater than 2.0, a more precise estimation of the calibration curve may be obtained using weighted linear regression. Both linear and quadratic weighting functions are applied to the data to find the most appropriate weighting function to be employed.
                 If the means compared to the calibration curve show a deviation from linearity, two-dimensional linear regression is used.

                 ACCURACY Verify the accuracy preferably by using a certified reference material. Where this is not possible, perform a test for recovery. 

                 RECOVERY For assay determinations a recovery of 90 per cent to 110 per cent is to be obtained. The test is not valid if recovery, for example for trace-element determination, is outside the range 80 per cent to 120 per cent of the theoretical value. Recovery may be determined on a suitable standard solution (matrix solution) spiked with a known quantity of analyte (concentration range that is relevant to the samples to be determined).

                 REPEATABILITY The repeatability is not greater than 3 per cent for an assay and not greater than 5 per cent for an impurity test.

                 LIMIT OF QUANTIFICATION The limit of quantification is estimated by calculating the standard deviation of not less than ten replicate measurements of a blank solution and multiplying by 10. When validating a procedure using the method of standard additions, the slope of standards applied to a solution of the test material is used. Other suitable approaches can be used.