1. COMPARISON OF EXCITATION CONDITIONS IN
ICP-OES AND THEIR USE FOR OPTIMIZED ANALYSIS
Björn Knauthe, Matthias Otto
TU Bergakademie Freiberg, Institute of Analytical Chemistry, Freiberg, Germany
Introduction
In this study it is demonstrated that determination of physical plasma properties can be carried out by means of a routine plasma spectrometer enabling prediction of optimum mesaurement conditions for analysis of metals and nonmetals. The plasma power has been varied in the practically important range from 1100 to 1700 W in combination with observation height in the plasma as well as nebulizer gas flow.
Rotational temperatures, electron number densities and electron temperatures have been invesigated. Of special interest was the extension of the usually applied analytical ares to lower electron temperatures. The investigations are mainly based on the approach by H.R. Griem, Principles of Plasma Spectroscopy, Cambridge University Press, 2001.
Operating parameters (PE Plasma 2000)
Rf power …1700 W
Rf frequency MHz
Plasma gas flow l/min
Auxiliary gas flow l/min
Nebulizer gas flow 0.2…2.0 l/min
sample pump rate ml/min
Observation height 1…20 mm
Results
Rotational temperatures were determined on the basis of Boltzmann plots according to:
whith line strength S, wavenumber , rotational Energy Erot.
As probe for the rotational temperatures several lines of OH-bands were used as given in Fig. 1.
The electron density was deduced from seven Mg I lines of different excitation energy, i.e nm, nm, 277,983 nm, nm, nm, nm, nm. A typical Boltzmann plot is shown in Fig. 2. The required atomic data were retrieved from the NIST data base. The good linearity of the plots enabled electron temperatures to be estimated at mean relative errors of 2 to 3%. The highest deviations were observed at low nebulizer gas flows because of the lower sample amounts reaching the plasma.
The dependence of rotational temperatures and electron densities on the nebulizer gas flow and the observation height is given in Fig. 3. Also given are plots for electron temperatures at rf power values between 1100 and 1700 W (Fig. 4).
Finally, the signal-to-noise ratios for elemental lines of different excitation energies could be estimated and are exemplified in Fig. 5 for the elements K, Na, Ni, Mn, P and S.
Fig. 1. OH-bands as probe for estimation of
rotational temperatures
Fig. 2. Boltzmann plot for estimation of
electron temperatures
Fig. 5. Signal-to-noise ratios for element lines of different excitation energies at 1300 W Rf power
Conclusions
Electron temperatures are especially useful for predicting the optimum experimental conditions (areas) for the lines under consideration
Rotational temperatures are important in order to guarantee approriate atomization, excitation and ionization conditions
Three areas were found from the electron temperatures related to certain excitation energies :
i. area of lower temperature (yellow-red) below 7000 K,  3 eV
ii. area of the usual analytical area (green) at 7000 K to 9000 K, 3 to
6 eV
iii. area of higher temperature (blue) at K and higher, > 6 eV
The optimum areas for different lines correlate well with those of electron temperatures
Predictable:
Optima of detection limits for individual elements
Optimum compromise detection limits for several elements
Fig. 3. Rotational (top) and electron (bottomt) temperature Fig. 4. Electron temperatures at different rf power values
dependences at an rf power of 1300 W.

2. COMPARISON OF EXCITATION CONDITIONS IN
ICP-OES AND THEIR USE FOR OPTIMIZED ANALYSIS
Björn Knauthe, Matthias Otto
TU Bergakademie Freiberg, Institute of Analytical Chemistry, Freiberg, Germany
Introduction
In this study it is demonstrated that determination of physical plasma properties can be carried out by means of a routine plasma spectrometer enabling prediction of optimum mesaurement conditions for analysis of metals and nonmetals. The plasma power has been varied in the practically important range from 1100 to 1700 W in combination with observation height in the plasma as well as nebulizer gas flow.
Rotational temperatures, electron number densities and electron temperatures have been invesigated. Of special interest was the extension of the usually applied analytical ares to lower electron temperatures. The investigations are mainly based on the approach by H.R. Griem, Principles of Plasma Spectroscopy, Cambridge University Press, 2001.
Operating parameters (PE Plasma 2000)
Rf power …1700 W
Rf frequency MHz
Plasma gas flow l/min
Auxiliary gas flow l/min
Nebulizer gas flow 0.2…2.0 l/min
sample pump rate ml/min
Observation height 1…20 mm
Results
Rotational temperatures were determined on the basis of Boltzmann plots according to:
whith line strength S, wavenumber , rotational Energy Erot.
As probe for the rotational temperatures several lines of OH-bands were used as given in Fig. 1.
The electron density was deduced from seven Mg I lines of different excitation energy, i.e nm, nm, 277,983 nm, nm, nm, nm, nm. A typical Boltzmann plot is shown in Fig. 2. The required atomic data were retrieved from the NIST data base. The good linearity of the plots enabled electron temperatures to be estimated at mean relative errors of 2 to 3%. The highest deviations were observed at low nebulizer gas flows because of the lower sample amounts reaching the plasma.
The dependence of rotational temperatures and electron densities on the nebulizer gas flow and the observation height is given in Fig. 3. Also given are plots for electron temperatures at rf power values between 1100 and 1700 W (Fig. 4).
Finally, the signal-to-noise ratios for elemental lines of different excitation energies could be estimated and are exemplified in Fig. 5 for the elements K, Na, Ni, Mn, P and S.
Fig. 1. OH-bands as probe for estimation of
rotational temperatures
Fig. 2. Boltzmann plot for estimation of
electron temperatures
Fig. 5. Signal-to-noise ratios for element lines of different excitation energies at 1300 W Rf power
Conclusions
Electron temperatures are especially useful for predicting the optimum experimental conditions (areas) for the lines under consideration
Rotational temperatures are important in order to guarantee approriate atomization, excitation and ionization conditions
Three areas were found from the electron temperatures related to certain excitation energies :
i. area of lower temperature (yellow-red) below 7000 K,  3 eV
ii. area of the usual analytical area (green) at 7000 K to 9000 K, 3 to
6 eV
iii. area of higher temperature (blue) at K and higher, > 6 eV
The optimum areas for different lines correlate well with those of electron temperatures
Predictable:
Optima of detection limits for individual elements
Optimum compromise detection limits for several elements
Fig. 3. Rotational (top) and electron (bottomt) temperature Fig. 4. Electron temperatures at different rf power values
dependences at an rf power of 1300 W.