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History -- In 1969, Spectrametrics introduced the first commercially available direct current plasma system which used a right angle plasma as an excitation source and a crossed prism echelle spectrometer as the analytical detector. This system, known as the SpectraJet I, suffered from poor light though put as well as a substantial interference of the analyte signal by the plasma background column due to the right angle nature of the arc geometry.
In 1974, Spectrametrics replaced the right-angle plasma with an inverted "V" configuration plasma known as the SpectraJet II. The DC argon plasma is initiated by arcing a high-voltage spark from cathode to anode. Once ignited, the plasma is sustained by a relatively low voltage, about 50 volts with a current of about seven ampers. Samples are nebulized and introduced into the excitation area in aerosol form. Supplementary argon flows around the cathode and anode to maintain a stable discharge. It is important to note that when this source is used with the echelle system, only light from the excitation region is focused into the spectrometer. The intense continuum from the plasma column and the plume is not observed.
In 1977, Spectrametrics introduced the SpectraJet III. In this
three-electrode direct current argon plasma the plasma jet is
formed between two spectrographic carbon anodes and a tungsten
cathode in an inverted
"Y" configuration.
This design resulted in improved stability and better detection
limits than previously reported for dc plasmas. Of particular
importance is its stability in the presence of varying solvent
types such as those containing large amounts of dissolved solids,
organics, or high acid/alkaline concentrations. The sample excitation
region and photometric observation area of the SpectraJet III
is centered uniquely in the crook of the "Y "where spectral
contribution from the plasma continuum is minimal. This zone reaches
an excitation temperature in excess of 8000 degrees centigrade
and is evident in the above photograph where strontium (reddish) is being excited in an area distinct
from the plasma continuum. The plasma requires less than 1000
watts of power and consumes approximately 8 L/min of welder's
grade argon.
Detectors -- The Spectraspan III, V,
and VII echelle spectrometers are able to operate in either the
sequential or simultaneous mode. The spectral output of these
spectrometers consists of approximately 100 spectral orders compressed
into a 4 inch by 5 inch (10 cm by 12 cm) area. A kinematically
mounted cassette is located at the focal plane of these spectrometers.
The cassette is a 6 inch square by 1 inch thick aluminum assembly
which has a locating pin on one side. The exit slits are located
under a protective cover. In the sequential mode, the cassett
e contains one exit slit and the prism/grating
assembly is scanned to allow the appropriate wavelength to hit
the exit the slit. In the simultaneous mode, the prism/grating
assembly is fixed and the cassette contains up to 24 exit slits
for the selected emission lines. This ability to be replaced allows
the laboratory to have a cassette for each application so that
the best emission line for each element can be used. Behind each
exit slit, a periscope is used to redirect the emission light
to a photomultiplier tube where the quantity of light is detected
and the concentration of the associated element is quantitated.
The mirrors used in these periscopes have been optimized for 45
degree incidence angle and have better than 91 percent reflectivity
at 190 nm and nearly 95 percent in the visible region of the spectrum.
Grating -- The basis of the method is
the measurement of atomic emission by an optical spectroscopic
technique. Samples are nebulized and the aerosol that is produced
is transported to the plasma where e
xcitation
occurs. Characteristic atomic-line emission spectra are produced
by a direct current plasma (DCP). Spectra are produced by an echelle
grating spectrometer and the intensities of the lines are moitored
by film or photomultiplier tubes. The photocurrents from the photomultiplier
tubes are processed and controlled by a computer system. A background
correction technique may be required to compensate for variable
background contribution to the determination of trace elements.
Background must be measured adjacent to analyte lines on samples
during analysis. The position selected for the background intensity
measurement, on either or both s
ides
of the analytical line, will be determined by the spectrum adjacent
to the analyte line. The position used must be free of spectral
interference and reflect the same change in background intensity
as occurs at the analyte wavelength measured. Background correction
is not required in cases of line broadening where a background
correction measurement would actually degrade the analytical result.
Physical interferences are generally considered to be effects associated with the sample nebulization and transport processes. Such properties as change in viscosity and surface tension can cause significant inaccuracies especially in samples which may contain high dissolved solids and/or acid concentrations. The use of a peristaltic pump may lessen these interferences. If these types of interferences are operative, they must be reduced by dilution of the sample and/or utilization of standard addition techniques. Also, it has been reported that better control of the argon flow rate improves instrument performance. This is accomplished with the use of mass flow controllers.
Chemical interferences are characterized by molecular
compound formation, ionization effects and solute vaporization
effects. Normally these effects are not pronounced with the DCP
technique, however, if observed they can be minimized by careful
selection of operating conditions (that is, observation position,
and so forth), by buffering of the sample, by matrix matching,
and by standard addition procedures. These types of interferences
can be highly dependent on matrix type and the specific analyte
element.
Elements such as lithium, sodium, potassium, rubidium, cesium,
calcium, magnesium, etc. can cause enhancement of the net signl-to-noise
ratio for many of the elements in this method. This effect can
be both controlled and utilized advantageously by the addition
of lithium, or cesium at a final concentration of 1,000 mg/L,
to the blank, the standards, and the samples.
Echellogram -- The spectral output of a
crossed dispersed echelle spectrometers consists of about 100
spectral orders compressed into a 4 inch by 5 inch (10 cm by 12
cm) area. The vacuum ultraviolet light (below 180 nanometers)
occurs in orders 124 and above and is seen at the top of the spectral
window while the infrared light (above 850 nm) begins in order
23 at the lower left. Visible and ultraviolet light which is used
for most atomic elemental analysis, occurs between these two extremes.
If laid end to end, the useful segments of the echellogram would
be equivalent to a spectrum 19 feet long (6 meters). The resolution
of an echelle spectrometer is typically 10 to 16 times that of
a conventional spectrometer of equivalent focal length. In this
particular case, the spectral bandwidth is 0.0038 mn. and reciprocal
linear dispersion is 0.06 nm/mm in the 112th order 
The two bright spots in the top center are the dual emission
images of the 253.6519 nm
mercury registration lines while the bright blue-white streaks
nearer the bottom are caused by the black body emission of the
source and occur at approximately 400 nm. Each elemental
emission image is approximately 50 microns wide and 300 to 500
microns high, however they
can be broadened by spectral interference or temperature
and doppler effects.
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