Atomic Emission Spectroscopy. Lecture 19. Advantages of Plasma Sources. No oxide formation as a result of two factors including Very high temperature Inert environment inside the plasma (no oxygen) 2. Minimum chemical interferences
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Atomic Emission Spectroscopy Lecture 19
Advantages of Plasma Sources • No oxide formation as a result of two factors including • Very high temperature • Inert environment inside the plasma (no oxygen) 2. Minimum chemical interferences 3. Minimum spectral interferences except for higher possibility of spectral line interference due to exceedingly large number of emission lines (because of high temperature)
4. Uniform temperature which results in precise determinations 5. No self-absorption is observed which extends the linear dynamic range to higher concentrations 6. No need for a separate lamp for each element 7. Easily adaptable to multichannel analysis
Plasma Emission Instruments Three classes of plasma emission instruments can be presented including: 1. Sequential instruments In this class of instruments a single channel detector is used where the signal for each element is read using the specific wavelength for each element sequentially. Two types of sequential instruments are available:
Linear sequential scan instruments where the wavelength is linearly changed with time. Therefore, the grating is driven by a single speed during an analysis of interest b. Slew scan instruments where the monochromator is preset to provide specific wavelengths; moving very fast in between wavelengths while moving slowly at the specific wavelengths. Therefore, a two-speed motor driving the grating is thus used.
Slew scan spectrometer • Two slew-scan gratings • Two PMTs for VIS and UV • Most use holographic grating
2. Multichannel Instruments This class of instruments is also referred to as simultaneous instruments in which all signals are reported at the same time using two types of configurations:
a. Polychromators Multiple detectors, usually photomultiplier tubes are used. Beams of radiation emerging from the grating are guided to exit slits (each representing the wavelength of a specific element) are focused at several PMTs for detection. Detection, thus, takes place simultaneously
b. Array-based systems This multichannel type instrument uses a multichannel detector like a charge injection device or a charge-coupled device. Diffracted beams from a grating pass through a prism where further resolution of diffracted beams takes place by a prism. The prism will disperse the orders of each diffracted beam. The multichannel detector can also be a linear photodiode array as in the figure below:
Diode Array Detector Grating
CCD or CID Detector Grating
3. Fourier transform instruments (FT) Instruments in which the signal is coded will need a decoding mechanism in order to see the signal. FT is a very common technique for decoding time domain spectra. In such instruments, the detector records the change of signal with time, which is practically not useful. However, Fourier transformation of the time domain signal yield a frequency domain spectrum, which is the usual signal, obtained by conventional methods. Instruments that rely on decoding a coded signal is also said to have a multiplex design.
Sample Introduction There are several methods for sample introduction; the most widely used is, of course, the nebulization of an analyte solution into the plasma. However, other methods, as described earlier, are fine where vapors of analyte molecules or atom from electrothermal or ablation devices can be driven into the torch for complete atomization and excitation. For your convenience, sample introduction methods are summarized here again:
Samples in Solution Pneumatic Nebulizers Samples in solution are usually easily introduced into the atomizer by a simple nebulization, aspiration, process. Nebulization converts the solution into an aerosol of very fine droplets using a jet of compressed gas. The flow of gas carries the aerosol droplets to the atomization chamber or region.
Ultrasonic Nebulizers In this case samples are pumped onto the surface of a piezoelectric crystal that vibrates in the kHz to MHz range. Such vibrations convert samples into homogeneous aerosols that can be driven into atomizers. Ultrasonic nebulization is preferred over pneumatic nebulization since finer droplets and more homogeneous aerosols are usually achieved. However, most instruments use pneumatic nebulization for convenience.
Electrothermal Vaporization An accurately measured quantity of sample (few mL) is introduced into an electrically heated cylindrical chamber through which an inert gas flows. Usually, the cylinder is made of pyrolytic carbon but tungsten cylinders are now available. The vapors of molecules and atoms are swept into the plasma source for complete atomization and excitation.
Hydride Generation Techniques Samples that contain arsenic, antimony, tin, selenium, bismuth, and lead can be vaporized by converting them to volatile hydrides by addition of sodium borohydride. Volatile hydrides are then swept into the plasma by a stream of an inert gas.