Applied Spectroscopy

Matrix-isolation spectroscopy is particularly useful for the study of transient species and molecular ions. New methods for producing and trapping reactive molecules for spectroscopic characterization are described. Matrix investigations of charge-transfer anionic species, free radicals, positive ions, and van der Waals molecules are discussed.

The prominent lines of 70 elements as emitted in an inductively coupled plasma excitation source have been identified. The lines are listed alphabetically by element and numerically by wavelength. Detection limit capabilities are estimated for each spectral line.

Induction-coupled plasma atomic-emission spectrometry offers an ideal method for simultaneous multielement analysis of natural water samples. The Water Resources Division of the U.S. Geological Survey currently employs this technique for quantitative analysis of 17 major and trace constituents. These include analysis of Ba, Be, Cd, Co, Cu, Fe, Pb, Li, Mn, Sr, Mo, V, Zn, Ca, Mg, Na, and SiO₂ in a routine production mode, in which an excess of 1000 determinations can be made in a normal working day. Comparability studies with conventional single-element methods of analysis, such as atomic absorption spectrometry and colorimetric techniques, show essentially equivalent accuracy and precision, frequently at much higher sensitivity.

The application of electrothermal atomization techniques for atomic absorption spectroscopy has increased many fold in recent years. Along with this widespread application, matrix interferences in pulse-type electrothermal atomizers have been observed. Until recently, very few interference studies have involved constant temperature electrothermal atomizers. In this paper a comparison is made of interferences in pulse-type vs constant temperature atmoziers for various metal chlorides on the Mn atomic absorption signal. The results show no interferences in the constant temperature system while significant matrix interferences are observed in the pulse type unit. It is proposed that the observed differences are due mainly to the rate at which volatilized analyte attains atomization temperature and the residence time in the hot environment.

Details of the processes occurring during sample atomization from a “microarc” discharge have been studied photometrically, by use of high-speed color cinematography and through current-voltage waveforms. The microarc studied here is an atmospheric-pressure inert-gas glow discharge supported between 0.25 mm diameter tungsten wires; quiescent argon-1% H₂ provides reactive-sputtering conditions and improved behavior in the presence of oxygen impurities. Excitation temperatures of ca. 5000°K are measured for the argon glow. Samples of Na, Al, and Sr illustrate the influence of volatile, refractory, insulating, and electron-emitting sample properties on the temporal-spatial-electrical behavior of the discharge. The step-by-step events occurring in the discharge are described qualitatively and a variety of processes are invoked to explain sample volatilization, including sputtering, chemical reactions, and purely thermal effects. In the first stages of the discharge, instabilities are related to the placement and insulating character of deposits. With heating, electron emission becomes important in directing the discharge to or away from the sample; abnormal glow wandering and glow-to-arc transitions can ensue. Improved stability is achieved by uniformly depositing multi-element samples along the electrode, which localizes the initial discharge and promotes ablative cooling of the sample and electrode.

Recently there has been a great deal of interest in the use of solid-state multichannel detectors as sensors for rapid-scan spectroscopy. The charge-coupled device (CCD) photoarray, not previously characterized for this application, is described. The unit has 1728 elements, arranged linearily, which integrate light over the same time frame; the photo signals are then read serially. The CCD was found to produce signals which are linear with both intensity and integration time. The dark signal at −10°C is 0.1% of saturation for a 10 ms integration time, and the noise level is less than 0.08% of saturation. The wavelength response is in the visible region. The application for rapid-scan spectrophotometry is also discussed.

While atomic fluorescence is attractive as a sensitive multi-element technique, lack of a commercial instrument and the need for a dedicated computer system have discouraged its use. A system is described which incorporates atomic emission and fluorescence while using only commercially available instrumentation. A programable calculator operates the system and performs all data manipulation and reduction. The result is an instrument capable of doing routine multi-element analyses without the need for a highly trained operator.

A new digital computational method of spectral line shape analysis is reported. In the present method, the parameters of object functions are directly found to fit the experimental values on the basis of a nonlinear least squares method. The present method is available for curve fitting of experimental results if the functions are expressed in analytical form. As an example, the results of resolving the recorded Raman spectra into the component line shapes is presented. The agreement of the recorded Raman spectra with that composed of the resolved lines is excellent.

A mirror mount adjustable along three Cartesian axes and about two angular axes is described. The mount can support mirrors up to 8 in. (20.3 cm) in diameter, can be positionally set to high tolerances, and is stable over a period of years. Examples of mounting a variety of mirrors are given.

Spacially and temporally resolved spectra are described from explosions of Al foil strips in Ar at 100 and 700 Torr. Spectra were obtained photographically with a time resolution of 0.3 μs using a rotating mirror streaking camera. Measurements were made at 1, 2, 5, 10 and 15 mm above the foil surface with a spacial resolution of 0.1 mm. The early appearance of Ar ion lines for explosions in Ar at 100 Torr implies initial current conduction following foil vaporization via dielectric breakdown of the Ar. At 700 Torr, the complete absence of radiation from Ar ion lines suggests that gas-phase current conduction begins by dielectric breakdown of the metal vapor. The short duration of Ar ion radiation in any observation zone at 100 Torr coupled with the first appearance of radiation from metallic species during the period of decaying Ar intensity implies that the metal vapor is collisionally excited and ionized as it passes through the Ar current sheath. From the results of these studies, a dual-channel, photoelectric, gated integration procedure was used to obtain greater analysis line/background ratios. A two- to four-fold increase in line/background ratios is obtained when radiation from 15 mm above the foil is integrated during the interval from 25 to 125 μs after foil vaporization.

CARS spectral profiles are functionally described by the variation of the square of the total third-order, nonlinear susceptibility with the anti-Stokes frequency shift. This inherent, squared susceptibility signal dependence, when weighted by an experiment dependent convolution, produces a CARS spectral profile which is a problem specific to the experiment design.

In many types of spectral analysis, the quantitation of results is often complicated by the presence of overlapping peaks. We have treated deconvolution as a parameter estimation problem and have applied an optimization procedure called simplex to resolve the true area of overlapping peaks such as those frequently encountered in energy dispersion analysis spectra. This technique overcomes many of the limitations of present deconvolution techniques such as resolution enhancement and digital filter-correlation and has the added advantage of not involving the use of partial derivatives as encountered in gradient-type optimization techniques. This procedure has been applied to a series of artificially generated overlapping Gaussian peaks which are close analogs to actual peaks. The results of this study have shown that this function converges and the correct area of those overlapping peaks can be estimated by this deconvolution.

A Raman multichannel system is described which is feasible for the on-line identification of fractions separated by gas chromatographic columns. In Raman multichannel systems, the sensor, i.e., the laser focus, is small enough and the possible recording times are short enough to avoid any affliction of the separation process, even with capillary columns in use. At the measurements described, capillary columns with a length of 15 m and an inner diameter of 0.25 mm were employed. The detection limit was at about 1 to 10 nl of one fraction corresponding to about 1 to 10% of the total amount of sample suited for capillary columns.

Several operating and sample handling conditions were used to study their effect on spectral subtraction results using Fourier transform infrared spectroscopy. Purge rate, orientation of a heterogeneous sample and sample pathlength were varied to determine under what conditions and to what degree subtraction artifacts can be produced. To some degree, all of the conditions evaluated produced spectral subtraction artifacts although sample pathlength was found to have the greatest effect. Also under certain conditions, concentration differences taken from identical pathlength cells could not be totally nulled by subtraction manipulation.

An ESCA and AES study of the corrosion of 1:1 Pb-Sn solder is reported. The solder was reacted at room temperature with O₂ (1 Torr), H₂O (10 Torr), 100 ppm of NO₂ in N₂ (1 atm), and the latter at 50% relative humidity. Pb and Sn foils were examined under similar conditions. The Pb/Sn ratios observed by ESCA and AES for unmelted solder are close to those expected from bulk composition. After melting, a 50% decrease in Pb/Sn was obtained by ESCA; no AES signal for Pb was observed. Before melting, 90% of the Pb and 100% of the Sn were oxidized; after melting 18% Pb and 70% Sn were oxidized. Exposure of solder to O₂ (1 Torr) resulted in a Pb/Sn ratio which decreased as a function of time to a minimum value, rapidly increased through a maximum around 60 min and then decreased to an equilibrium value. The O/(Pb + Sn) ratio increased rapidly to an equilibrium value. The interaction of solder with 10 Torr of H₂O vapor for 5 min resulted in surface enrichment of Sn with the formation of Sn and Pb oxides. The results obtained with dry NO₂ were similar to those with O₂; Pb/Sn decreased rapidly to a minimum and went through a small maximum to an equilibrium value. The distribution of species determined by ESCA before and after the reaction were essentially the same as for O₂ except the oxidation of Pb was suppressed. No nitrogen species were observed. Solder exposed to at 100 ppm of NO₂ in N₂ 50% relative humidity gave similar results to O₂ and dry NO₂. The Pb/Sn maximum occurred at a reaction time approximately 2 to 3 times that of O₂ and dry NO₂ (40 to 60 min). Two nitrogen species were observed for solder treated with 100 ppm of NO₂ in N₂ at 50% relative humidity; binding energies were ∼404 and 400 eV. These have been attributed to NO₂⁻ and NO⁻, respectively. Mechanisms are proposed for reaction of solder with O₂, dry NO₂, and NO₂ at 50% relative humidity.

A method is presented for the analysis of mycotoxins using acetonitrile (90) water (10) extraction, petroleum ether defatting and ferric gel decolorization, chloroform partitioning, and a 1000-fold concentration. Gas chromatography and thin layer chromatography are used to separate and analyze the three most common thichothecene mycotoxins. The analysis has been applied to animal feeds, human foods, and animal tissues. Confirmation of the presence of the mycotoxins is made by IR, NMR, and/or mass spectrometric analysis and/or the chicken embryo test. The sensitivity of the method is 1 ppm for the three trichothecenes, the level of biological effectiveness. The gas chromatographic analysis technique is sensitive to 10 ng of each mycotoxin; the thin layer analysis is sensitive to 100 ng of each compound. Mass spectra may be obtained from 100 ng of each toxin taken from a thin layer chromatogram and the LD₅₀ for chicken embryos is 100 ng/egg for T-2 toxin and diacetoxyscirpenol. Vomitoxin (deoxynivalenol) has not been subjected to the chicken embryo test due to the lack of authentic pure vomitoxin.

Chloroform (CHCl₃) solutions of tris(8-quinolinolato) Al(III) (AlQ₃), when excited at 395 nm, fluoresce at 520 nm. Addition of tris(8-quinolinolato) Fe(III) (FeQ₃) to these solutions decreases the fluorescence: the ratio of the fluorescent intensity in the presence of FeQ₃, to that in its absence, is exponentially related to [FeQ₃] and independent of [AlQ₃]. We hypothesized that the main cause of this effect was FeQ₃ absorption of the exciting and emitted radiations. The hypothesis was tested in two ways: first, by calculating fluorescence losses using FeQ₃ absorbance measured at 395 and 520 nm; and second, by measuring fluorescence losses when the AlQ₃ was in one optical cell, and the FeQ₃ in others, which were located in either the excitation or emission paths, or both. Comparison between all the results showed that they were not significantly different (P_0.01), thus verifying the hypothesis.

The increased awareness of the deleterious effects of polynuclear aromatic (PNA) compounds emphasizes the need for instruments that are capable of monitoring these compounds and also are suitable for use in health protection programs. Second-derivative UV-absorption spectrometry with least-squares spectral analysis shows promise as a method for analyzing vapor phase PNA's, as well as solutions containing these compounds. Detection limits of a few parts per billion are attainable for many PNA compounds. Examples are given showing the analytical capabilities of this procedure.