David H. Tracy

Improved performance using internal standardization in inductively-coupled plasma emission spectroscopy

Abstract Significantly improved performance in ICP emission spectroscopy is possible using internal standardization given the proper choice of operating conditions. A dual monochromator detection system was used to study the correlation between two emission signals recorded simultaneously as several ICP parameters were varied. The behavior of the signal formed by dividing the analyte and reference signals was also measured. The noise behavior of twenty analytically important elements was compared to that of a manganese ion line used as the internal standard for a single, compromise choice of ICP operating parameters (RF power = 1250 W, carrier gas flow rate = 0.7 or 0.91 min , and viewing height = 20mm). The elements differed widely in excitation and ionization energies. Sample concentrations were chosen so that the analyte emission signals were sufficiently intense that the noise and drift contributions due to plasma background and shot noise could be ignored. Under these conditions very good signal correlation (r ⩾ 0.95) and similar noise proportionality factors were observed leading to noise reductions of ⩾ 10 × after signal division in almost every instance. After signal division the noise, defined as the relative standard deviation of the emission signal, (time constant = 1.0 s, n = 20) was less than 0.1 %. The closely similar noise behavior is attributed to sample density fluctuations arising from corresponding fluctuations of the sample aerosol density. Good correlation is also observed between fluctuations in the plasma background simultaneously detected at two widely separated wavelengths when appropriate steps are taken to reduce shot noise. At carrier gas flow rates above 1.01 min and at viewing heights below 15mm or above 20mm, the signal correlation is reduced or absent, yielding little or no improvement using internal standardization. The use of internal standardization leads to other improvements in analytical performance in addition to noise reduction. Drift is reduced, and nebulizer performance is improved in that it is less sensitive to variations in liquid sample uptake rate, or nebulizer instabilities associated with high solids content sample introduction. The divided signal takes much less time to reach equilibrium following introduction of a new sample.

Multivariate methods for interpretation of emission spectra from the inductively coupled plasma

Abstract Multivariate methods are described which improve the quality of the information which is extracted from inductively coupled plasma (ICP) atomic emission spectra. The enhancements provided by these methods result from using more of the spectral information than is used by simpler peak height measurement schemes. The detection limit of an analyte is significantly improved when mathematical treatment is applied to emission spectra at multiple wavelengths near the peak maximum compared to peak height measurements which make use of only one bandpass of information. The advantages of these multivariate methods are greatest in situations where spectral interferences are present at the analyte wavelength. These algorithms replace auxiliary background correction routines since a background profile can be fit to the unknown spectrum. Even very small wavelength registration errors can be corrected using numerical derivatives of the unknown spectrum as components in the linear model. In this paper, multivariate methods for ICP are developed using only the method of least squares (MLS). The method is applied to simulated spectra and to real spectra collected on the Plasma 2000 spectrometer. A trace level determination of cobalt in the presence of an interfering concentration of iron is demonstrated at the 238.892-nm cobalt line.

Absolute spectral radiance of 27 MHz inductively coupled argon plasma background emission

Abstract The spectral radiance of the plasma continuum of a 27 MHz argon ICP has been measured for two sets of operating conditions in the wavelength range from 192 to 600 nm. At an rf forward power of 1250 W and a viewing height of 15 mm above the load coil, a maximum spectral radiance of (1.5±0.3) × 1012 photons s−1 mm−2 sr−1 nm−1 was observed, at a wavelength of approximately 450 nm. At 200 nm, the spectral radiance is 60 times lower. For wavelengths below 400 nm, the absolute plasma emission spectrum is well represented by a graybody function with a temperature of 5480 K and an emissivity of 4 × 10−5.

Exposure Dose Control Techniques For Excimer Laser Lithography

Measurements of pulse-to-pulse energy fluctuation statistics were made at several wave-lengths for a commercial discharge pumped excimer laser, under a variety of operating conditions. Various approaches to achieving accurate exposure dose control in microlithographic exposure tools using noisy pulsed sources are discussed. A pulse-by-pulse active control technique for static field lithography is described in detail, together with measured performance data. Alternative techniques suitable for one dimensional scanning slit field exposure tools are presented.

Effects of permeation of helium through the walls of fused silica capillary GC columns

Hold-up times and peak widths have been measured in long fused silica GC capillary columns at high temperature with helium as the carrier gas. The results lead to the conclusion that the helium permeates through the column walls. The conventional Poiseuille theory of carrier gas flow has been extended to include this phenomenon. The resulting “loss modified Poiseuille” model, which uses literature values for the permeability of fused silica to helium, has been used to simulate the observed behavior. Good agreement between simulation and experiment validates the model. Simulations have been used to explore the effect of column permeability on hold-up times, peak widths, and velocity profiles over a broad range of column geometries.

Signal instability in inductively coupled plasma emission due to electrostatic effects in the nebulizer spray chamber

Abstract Electrostatic fields created by the charge on the spray in the pneumatic nebulizer are found to be an intermittent source of several types of ICP emission instability. The electrostatic effects act to reduce the sample aerosol density leaving the spray chamber. Simple diagnostics allow unambiguous confirmation of the presence of electrostatic signal depression and suggest techniques for its control.

Precision of the petrochemical process analysis using NIR spectroscopy

Quantitation of benzene in gasoline is becoming important due to new federal and state regulations. To meet these regulations an on-line analysis method for benzene in gasoline is needed. It has been shown that NIR is an ideal instrument for on-line analytical measurements. We report in this paper an analysis of benzene in synthetic gasoline with a standard error of prediction of 0.22 wt%. Both principal component analysis and partial least squares chemometric calibration methods were used. Both methods gave the same results for the prediction of the validation sample set.