In recent years, microwave plasma atomic emission spectroscopy (MP AES) has emerged as a powerful sub-variant of Atomic Emission Spectroscopy, offering several advantages over traditional flame and inductively coupled plasma (ICP) excitation sources. But how exactly does MP AES work and what makes it so useful for chemical analysis?
MP AES utilizes microwave energy to generate a plasma (generally Nitrogen) that excites atoms for compositional analysis.
MP AES instruments consist of three main components – the microwave generator, plasma torch, and spectrometer. The microwave generator produces electromagnetic radiation in the microwave frequency range of 300 MHz to 300 GHz. This microwave energy is fed into the plasma torch, which couples the energy to gas flowing through the torch to create a plasma.
The plasma contains a mixture of ions, electrons, and neutral atoms or molecules. Applying the microwave energy heats the plasma to temperatures as high as 10,000 K. As the various species in the plasma collide and interact, the atoms become excited. When these excited atoms relax to lower energy states, they emit photons at characteristic wavelengths depending on the element.
Key Components of an MP-AES Setup
The microwave generator is the power supply for the technique. Magnetrons, klystrons, or solid-state sources provide microwave energy at frequencies typically between 900 MHz to 3 GHz. Adjusting the frequency and power of the microwave radiation allows control over the plasma conditions.
The plasma torch couples the microwave energy to the gas flow to ignite and sustain the plasma. Torch designs include a quartz tube surrounded by a microwave coupling coil. Gases like argon flow through the tube while nitrogen or air flows on the outside to cool the torch. The design allows efficient transfer of microwave energy to the gas.
The spectrometer disperses the light emitted from the excited atoms in the plasma. Optical gratings or prisms spatially separate the wavelengths, allowing detection at specific elemental emission lines by the spectrometer’s detector. Advanced spectrometers allow simultaneous multi-element analysis.
Liquid samples are typically introduced into the plasma as an aerosol via a nebulizer and spray chamber. Solid samples require digestion before nebulization. For direct solids analysis, laser ablation can introduce solid particulates into the plasma.
The Operating Principle of MP-AES
The fundamental principle behind MP AES is the excitation of electrons to higher energy states and the subsequent relaxation and emission of photons.
Unlike ICP AES which uses an argon plasma, MP AES typically utilizes a nitrogen plasma. Nitrogen plasma provides excellent sample atomization and ionization efficiency while also being less expensive than argon. The nitrogen plasma does require some matrix matching between samples and standards to account for any matrix effects. However, the nitrogen plasma performance, lower gas costs, and reduced power requirements make it an advantageous choice for MP AES systems. The ability to use nitrogen instead of argon is a notable distinction between MP AES and ICP AES excitation sources.
Here is an overview of the key processes in sequence:
Atomization and Excitation
Introduced sample material is decomposed into free atoms in the plasma through thermal processes like evaporation, decomposition, and dissociation. Collisions between electrons, ions, and atoms in the plasma transfer energy, exciting outer electrons of the atoms to higher energy levels.
Relaxation and Photon Emission
The excited electrons are unstable and quickly drop back down to lower energy levels, emitting photons in the process. The energy of the emitted photons corresponds to the energy difference between the excited and lower state.
Dispersion and Detection
The photons are sorted by the spectrometer based on wavelength. Element-specific emission wavelengths pass through exit slits to solid-state CCD photodetectors that convert the photons into measurable electrical signals. The intensity of the signal at each wavelength provides the required quantitative information about the elemental composition of the sample.
Benefits of MP-AES Over Flame-AES and ICP-AES
MP AES provides several benefits over traditional atomic emission techniques:
Higher Temperatures and Power Density
The plasma reaches temperatures up to 10000 K, higher than flame (~2000 K) and ICP plasmas (~7000 K). This provides more atomization and excitation and better tolerance of complex matrices.
Faster Analysis Times
Analysis times are reduced from minutes per sample by ICP AES to just seconds by MP AES due to continuous signals and shorter washout times between samples. Throughput can exceed hundreds of samples per day.
Lower Operating Costs
MP AES consumes less argon gas and eliminates expensive radiofrequency generators needed for ICP systems. This significantly reduces operating expenses.
Magnetrons provide microwave energy in a more compact form than large radiofrequency generators. This allows MP AES units to have a smaller footprint in the lab.
Limitations and Potential Areas Of Improvments
While powerful, MP AES does have some limitations:
- Poorer detection limits for some elements like As, Se, and Hg compared to ICP techniques
- Matrix interferences still possible for complex samples
- A smaller linear range than ICP AES may require more dilutions
- Spectral interferences between certain elements need correction
- Requires digestion for solid samples instead of direct analysis
Common Applications of MP-AES
1. Detection Limits and Linear Dynamic Range
Typical detection limits for MP AES range from low μg/L to ng/L levels for many elements. The excellent detection capabilities coupled with a wide linear dynamic range from 4-6 orders of magnitude allow accurate analysis of major, minor, and trace constituents in a single measurement.
2. Minimal Chemical Interferences
The high plasma temperatures of MP AES provide excellent dissociation of molecular species and ionization of elements, minimizing chemical interferences. The wet plasma conditions also promote charge transfer reactions that reduce matrix interferences.
3. Rapid Analysis of Difficult Matrices
Samples with complex or difficult matrices, like organics, can be analyzed directly by MP AES due to the robust plasma conditions. Less sample preparation is required compared to other techniques, facilitating faster sample throughput.
4. Ideal for Quality Control and High Sample Loads
The combination of straightforward sample preparation, fast analysis times, and automated operation make MP AES well-suited for high-volume quality control applications. It can handle several hundred samples per day in fields like chemical and pharmaceutical manufacturing, food and agriculture, and environmental monitoring.
5. Applicable to Liquid and Solid Samples
The plasma’s tolerance for liquids and particulates means MP AES can directly analyze solutions, slurries, and solids via laser ablation with minimal sample preparation. This expands the range of sample types amenable to analysis.
MP-AES Has Revolutionized Spectroscopy
The unique ability of microwave plasma atomic emission spectroscopy to rapidly measure multi-element composition with minimal sample preparation has secured its place as a workhorse analytical tool. Its excellent detection power, wide dynamic range, tolerance of complex matrices, and relatively low operating costs make it ideal for high-volume applications like quality control and process monitoring. Continued development of microwave plasma sources and spectrometer technology will further strengthen MP AES as a leading atomic spectroscopy technique.
Frequently Asked Questions
What types of samples can be analyzed by MP AES?
MP AES can analyze aqueous solutions, acids, slurries, and digested solids. With accessories like a laser or spark ablation system, direct solids analysis is also possible. Organic liquids and biological samples can also be measured as long as they are solubilized or digested first.
What wavelength range is measured?
Most MP AES instruments cover wavelengths from 120-800 nm, capturing the UV, visible, and near-IR regions where many elements have emission lines. This wide wavelength coverage allows simultaneous multi-element analysis.
How does MP AES compare to ICP-OES?
ICP-OES offers better detection limits for some elements and easily handles solid samples. However, MP AES has faster analysis times, lower operating costs, smaller footprints, and higher tolerance of organics and solids. MP AES also provides higher power density for better atomization.
What are the maintenance requirements?
MP AES requires minimal maintenance besides occasional torch cleaning and nebulizer replacement. The solid-state microwave generator provides consistent power and doesn’t need tuning like ICP generators. Overall maintenance needs are low.