Plasma-based instrumental techniques that are useful for pharmaceutical analyses fall into two major categories: those based on inductively coupled plasma, and those where a plasma is generated on the surface of the sample. Inductively coupled plasma (ICP) is a high-temperature excitation source that desolvates, vaporizes, atomizes, excites, and ionizes atoms. The excited analyte ions and atoms are subsequently detected by any of a variety of plasma-based spectrochemical means, including inductively coupled plasma–atomic emission spectroscopy (ICP–AES), also known as inductively coupled plasma–optical emission spectroscopy (ICP–OES) and inductively coupled plasma–mass spectrometry (ICP–MS). ICP–AES and ICP–MS may be used for either single- or multi-element analysis and provide good general-purpose procedures for either sequential or simultaneous analyses over an extended linear range with good sensitivity.

An emerging technique in plasma spectrochemistry is laser-induced breakdown spectroscopy (LIBS). In LIBS, a solid, liquid, or gas sample is heated directly by a pulsed laser, and brought to a transient high-energy plasma state where the sample components are reduced to atoms, molecular fragments, and larger clusters. Emissions from the atoms and ions in the sample are collected, typically using fiber optics, and measured using an array detector such as a charge-coupled device (CCD). LIBS can be used for qualitative analysis or against a working standard curve for quantitative analysis. While LIBS is not currently in wide use by the pharmaceutical industry, LIBS is suited for at-line or on-line measurements in a production setting as well as in the laboratory. Because of its potential, it should be considered a viable technique for plasma spectrochemistry in the pharmaceutical laboratory. However, because LIBS is still an emerging technique, details will not be further discussed in this general chapter.

Sample preparation is critical to the success of plasma-based analysis, and it is the first step in performing any analysis via ICP–AES or ICP–MS. Plasma-based techniques are heavily dependent on sample transport into the plasma, and because ICP–AES and ICP–MS share the same sample introduction system, the means by which samples are prepared may be applicable to either technique. The most conventional means by which samples are introduced into the plasma is via solution nebulization. If solution nebulization is employed, solid samples must be dissolved in order to be presented into the plasma for analysis. Samples may be dissolved in any appropriate solvent. There is a strong preference for the use of aqueous or dilute nitric acid solutions, due to minimal interferences with these solvents, when compared to other solvent choices. Hydrogen peroxide, hydrochloric acid, sulfuric acid, perchloric acid, combinations of acids, or various concentrations of acids may all be used to dissolve the sample for analysis. Dilute hydrofluoric acid may also be used, but great care must be taken to ensure the safety of the analyst, as well as to protect the equipment when using this acid. Additionally, alternative means of dissolving the sample may be employed. These include, but are not limited to, the use of dilute bases, straight or diluted organic solvents, combinations of acids or bases, or combinations of organic solvents.

When analyzing samples that are presented to the plasma via solution nebulization, it is important to consider the potential interferences that may arise from the solvent used. In all cases, when samples are to be analyzed using ICP–MS, use an appropriate internal standard. In cases where sample viscosity differs from the standard viscosity, matrix matching or an appropriate internal standard should also be used for ICP–AES analysis. In either event, the selection of an appropriate internal standard should consider the analyte in question, ionization energy, wavelengths or masses, and the nature of the sample matrix.

Where a sample is not found to be soluble in any acceptable solvent, a variety of digestion techniques may be employed. These include hot-plate digestion, or microwave assisted digestions, including open-vessel and closed-vessel digestions. The decision regarding the type of digestion technique to use is dependent on the nature of the sample being digested, as well as on the analytes of interest. Because some metals are volatile (e.g., mercury and selenium), open-vessel or hot-plate digestions are not appropriate for all analytes.

Use acids, bases, and hydrogen peroxide of ultra-high purity. Deionized water must be at least 18 megohm. Check diluents for interferences prior to their use in an analysis. Because it is not always possible to obtain organic solvents that are free of metals, use organic solvents of the highest quality possible with regard to metal contaminants.

There are two ways to introduce the sample into the nebulizer: using a peristaltic pump or by self-aspiration. The peristaltic pump is used to ensure that the flow rate of sample and standard solution to the nebulizer is the same irrespective of sample viscosity. In some cases, self-aspiration can be used, where a peristaltic pump is not required.

A wide variety of nebulizer types is available, including pneumatic (concentric and cross-flow), grid, and ultrasonic nebulizers. Micronebulizers, high efficiency nebulizers, direct injection high-efficiency nebulizers, and flow-injection nebulizers are also available. The selection of the nebulizer for a given analysis should consider the sample matrix, analyte, and sensitivity desired. Some nebulizers are better-suited for use with solutions containing a high concentration of dissolved solids, while others are better-suited for use with organic solutions.

Once a sample leaves the nebulizer, it enters the spray chamber, which is designed to permit only the smallest droplets of sample into the plasma. The spray chamber functions to remove the larger sample droplets generated during the nebulization process, and as a result, typically only 1% to 2% of the sample aerosol reaches the ICP. As with nebulizers, there is more than one type of spray chamber available for use with ICP–AES or ICP–MS. Examples include the Scott double-pass spray chamber, as well as cyclonic spray chambers of various configurations. The spray chamber must be compatible with the sample and solvent and must equilibrate and washout in as short a time as possible. When selecting a spray chamber, the nature of the sample matrix, the desired sensitivity, and the analyte should be considered.

In addition to solution nebulization, it is possible to perform analyses using solid samples via laser ablation (LA). In such instances, the sample directly enters the torch. LA–ICP and LA–ICP–MS are better-suited for qualitative analyses of pharmaceutical compounds, due to the difficulty in obtaining appropriate standards. Nonetheless, quantitative analyses may be performed if it can be demonstrated that the standards used are adequate. This must be demonstrated through appropriate method validation.

Single- or multi-element standard solutions, whose concentrations are traceable to primary reference standards, such as those of the National Institute of Standards and Technology (NIST), may be purchased for use in the preparation of working standard solutions. Alternatively, standard solutions of elements may be accurately prepared from standard materials and their concentration determined independently, as appropriate. Where possible, standards, blanks, and sample solutions should be matrix matched to minimize matrix interference. In cases where matrix matching is not possible, an appropriate internal standard or the method of standard additions should be used for ICP–AES. Standards and blank solutions to be used for ICP–MS analysis should always contain an appropriate internal standard. In either event, the selection of an appropriate internal standard should consider the analyte in question, their ionization energies, their wavelengths or masses, and the nature of the sample matrix.

The components that make up the ICP excitation source include the argon gas supply, torch, radio frequency (RF) induction coil, and RF generator. Argon gas is typically used in ICP, although other gases may also be used, depending on the instrumentation available. The use of gases other than argon is not common practice. The plasma torch consists of three concentric quartz tubes designated the inner, the intermediate, and the outer tube. The nebulizer gas flow helps to create a fine aerosol of the sample solution, and the sample is then carried through the inner tube of the torch and into the plasma. The intermediate tube carries the auxiliary gas. The auxiliary gas flow helps to lift the plasma off of the inner and intermediate tubes to prevent melting and the deposition of carbon and salts on the inner tube. The outer tube carries the plasma or coolant gas, which is used to form and sustain the plasma. The tangential flow of the coolant gas through the torch constricts the plasma and prevents the ICP from expanding to fill the outer tube, preventing the torch from melting. 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 of the argon. In 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 of the argon ionize and excite the analyte atoms in the high-temperature plasma. The plasma operates at temperatures of 6,000 to 10,000 K, such that essentially all covalent bonds and analyte-to-analyte interactions have been eliminated.

An inductively coupled plasma may utilize either an optical or a mass spectral detection system. In the former case, ICP–AES, analyte detection is dependent on the emission wavelength of the analyte in question. Due to differences in technology, a wide variety of ICP–AES systems are available, each with different capabilities, as well as different advantages and disadvantages. Simultaneous systems are capable of analyzing multiple elements at the same time, thereby shortening analysis time. Sequential systems move from one wavelength to the next to perform analyses, and usually provide a larger number of analytical lines to choose from. Charge-coupled devices and charge injection devices, with detectors on a chip, make it possible to combine the advantages of both simultaneous and sequential systems, providing the rapid analysis of the simultaneous units with a wider selection of analytical lines as found with sequential units.

In addition, the ICP can be oriented in either axial or radial (also called lateral) configurations. The torch is positioned horizontally in axial plasmas, and the sample is viewed “end on”; while it is positioned vertically in radial plasmas, and the sample is viewed from the side. Axial viewing of the plasma can provide a more sensitive signal response; however, in some situations where background or sample interference is significant at the wavelength of interest, radial viewing may yield more reliable results. Because of the wide range of elemental concentrations in some real world samples and because of complex matrix problems, there are many cases where radial is better than axial or vice versa. Methods validated using an instrument with a radial configuration may not be completely transferable to an instrument with an axial configuration, and vice versa.

Additionally, dual-view instrument systems are available, making it possible for the analyst to take advantage of either torch configuration. The selection of torch configuration is dependent on the sample matrix, analyte in question, analytical wavelength used, cost of instrumentation, required sensitivity, and type of instrumentation available in a given laboratory.

Regardless of torch configuration or detector technology, ICP–AES is a technique which provides a quantitative measurement of the optical emission from excited atoms or ions at specific wavelengths. These measurements are then used to determine the analyte concentration in a given sample. Upon excitation, an atom emits an array of different frequencies of light that is characteristic of the distinct energy transition allowed for that element. The intensity of the light is generally proportional to the analyte concentration. It is necessary to correct for the background signal from the plasma. Sample concentration measurements are usually determined from a working curve of known standards in the concentration range of interest. It is, however, also possible to perform a single-point calibration under certain circumstances, such as with limit tests.

Since there are distinct transitions between atomic energy levels, emission lines have narrow bandwidths. Spectral separation of multiple emission lines requires a high-resolution spectrometer. The decision regarding which spectral line to measure should include an evaluation of potential spectral interferences. All atoms in a sample are excited simultaneously, however, so samples containing multiple elements can lead to spectral overlap. Spectral interference can also be caused by background emission from the sample or plasma. Modern ICP's usually have background correction available and a number of background correction techniques may be applied. Simple background correction typically involves measuring the background emission intensity at some point away from the main peak and subtracting this value from the total signal being measured. Mathematical modeling to subtract the interfering signal as a background correction may also be performed.

The selection of the analytical line is critical to the success of an analysis, regardless of torch configuration or detector type. Though some wavelengths are most often considered to be the primary analytical wavelengths, because there can be a tremendous variety of sample matrices, the selection of the analytical wavelength must be considered in the context of the sample matrix, the composition of the sample itself, the type of instrument being used, and the sensitivity required. Analysts might first choose to start with the wavelengths recommended by the manufacturer of their particular instrument and select alternate wavelengths based on manufacturer recommendations or published wavelength tables.

Forward power, gas flow rates, and torch position may all be optimized to provide the best signal. When organic solvents are used, it is often necessary to use a higher forward power setting than would be used for aqueous solutions, as well as a reduction in the nebulizer gas flow. When using organic solvents, it may also be necessary to bleed small amounts of oxygen into the torch to prevent carbon build-up in the torch.

Calibration

Procedure

When an inductively coupled plasma utilizes a mass spectral detection system, the technique is referred to as inductively coupled plasma–mass spectrometry (ICP–MS). In this technique, analyte detection is dependent on the masses of the various elemental components of a sample. ICP–MS is an elemental technique, whereby, due to the heat intensity of the plasma source, a sample is, theoretically, reduced to its ionic components. As is the case with ICP–AES, due to differences in technology, a wide variety of ICP–MS instrumentation systems are available.

The systems most commonly in use are quadrupole-based systems. Gaining in interest is time-of-flight ICP–MS. Although still not in widespread use, this approach may see greater use in the future. Additionally, high-resolution instruments are also available.

Regardless of instrument design or configuration, ICP–MS is a technique that provides a quantitative measurement of the components of the sample. Ions are generated from the analyte atoms by the plasma. The analyte ions are then extracted from the plasma using the sampling cone. The skimmer cone, located behind the sampling cone “skims” the ions as they emerge from the sampling cone, where they are then passed into the mass spectrometer. The mass spectrometer separates the ions in a magnetic field according to their mass-to-charge (m/z) ratios. The ICP–MS has a mass range up to 240 atomic mass units (amu). Depending on the equipment configuration, sample adducts with diluents or their decomposition products, oxides, and multiply-charged element ions produced within the plasma may increase the complexity of the resulting mass spectra. Interferences can be minimized by appropriate optimization of operational parameters, including gas flow (nebulizer, plasma, and auxiliary gas flow rates), sample flow, RF power, extraction lens voltage, etc., or through the use of collision or reaction cells, or cool plasma operation, if available on a given instrument. Unless a laboratory is generating or examining isotopes that are not naturally occurring, a list of naturally occurring isotopes will provide the analyst with acceptable isotopes for analytical purposes. Additionally, tables of commonly found interferences and polyatomic isobaric interferences and correction factors may be used.

ICP–MS is generally more sensitive than ICP–AES. The ability of a mass spectrometer to monitor a single ion of a specific mass/charge ratio is a major advantage of ICP–MS for determination of very low analyte concentrations or when elimination of matrix inferences is required. Analytes can often be detected at the parts per trillion (ppt) level using ICP–MS.

The selection of the analytical mass to use is critical to the success of an analysis, regardless of instrument design. Though some masses are often considered to be the primary analytical masses, because there can be a tremendous variety of sample matrices, the recommendation of a specific mass for a given element is not possible. Selection of an analytical mass is always considered in the context of the sample matrix, the type of instrument being used, and the sensitivity required. Analysts might first choose to start with masses recommended by the manufacturer of their particular instrument and select alternate masses based on manufacturer's recommendations or published tables of naturally occurring isotopes.

Optimization of an ICP–MS method is also highly dependent on the plasma parameters and means of sample introduction. Forward power, gas flow rates, and torch position may all be optimized to provide the best signal. When organic solvents are used, it is often necessary to use a higher forward power setting than would be used for aqueous solutions and to reduce the nebulizer flow rate. Additionally, when using organic solvents, it may be necessary to titrate small amounts of oxygen into the auxiliary gas to prevent carbon build-up in the torch. The use of a platinum-tipped sampling or skimmer cone may also be required to reduce cone degradation with some organic solvents.

Calibration

Procedure