Applied Spectroscopy

A medium resolution mid-infrared FT-IR instrument (IBM Instruments IR 44) has been modified to do step scanning; this has been done with the use of concepts previously applied to both near- and far-infrared instruments. In this paper we illustrate the method used for driving the mirror in the step scan mode and present some preliminary results from using the instrument with photothermal detection. At the current state of development, results obtained with the use of phase modulation indicate that this method produces significantly higher signal-to-noise ratios than does the use of amplitude (chopper) modulation to generate the photothermal signal.

A typical pharmaceutical tablet is too small for analysis in an ordinary NIRA instrument. An intact tablet will not begin to fill a solid-sample holder designed for use with a powdered sample. Grinding the sample is likewise unproductive, as a single tablet also does not provide enough powder for the sample cup. Potential NIRA applications such as the detection of product tampering and even routine quality control are unnecessarily complicated by this grinding requirement. A method of analyzing single, intact tablets using a double-reflecting aluminum sample holder is described in this report. The integrity of the sample is preserved during the analysis, allowing the tablet to be sold or consumed after the procedure.

Emission spectra in the UV-visible and near-infrared (NIR) spectral regions are presented for several nonmetals introduced as aqueous solutions by a glass-frit nebulizer. Detectability improves dramatically for some nonmetals with the use of a desolvation system to reduce the amount of water vapor entering the plasma. Detection limits for P, I, S, Cl, Br, and C are in the low- to sub-ppm range. Good precision (RSD <5%) and a dynamic range of 2–3 decades were observed for I and P. Easily ionizable elements such as K, Na, and Li can enhance nonmetal emission by as much as 30%.

A progammable, high-resolution liquid-handling device, suitable for both aqueous and organic solvents, is described. This device, called an isolated-droplet generator (IDG), is based on the vibrating-capillary principle of droplet production. The unit described is capable of converting a liquid stream, such as that produced by an HPLC, to a subnanoliter-sized monodisperse droplet stream generated at rates of up to 50 kHz. Charging and deflection circuitry can be used to select individual droplets or droplet packets from the main stream. The instrument is an improvement over older designs, in that computer control imparts sufficient flexibility to make the device useful as a general-purpose high-resolution liquid-handling system. Droplet production parameters can be automatically altered to compensate for changing liquid streams, such as occur in liquid chromatography with gradient elution. The waveforms for droplet production and charging are produced entirely in hardware by a programmable counter/timer integrated circuit (IC).

The intensity of the hyperfine doublet of the Cu(I) 324.8-nm transition was observed as a function of current, fill gas, and fill gas pressure for a new design of hollow cathode lamp incorporating a positive column and an enlarged volume cathode cavity. The intensity ratio of the two components, which reflects the degree of self-absorption, and the flame absorption sensitivity were compared with those of a conventional hollow cathode lamp. Higher intensity, better component ratio, and increased absorption sensitivity were found in the new lamp.

Current-voltage characteristics and spatially resolved atomic emission data are used to describe the basic operation of a magnetron glow discharge plasma device. The low-pressure glow discharge lamp uses a center-post cathode and a concentric ring-shaped anode. A coaxial magnetic field of a few hundred Gauss is used to achieve magnetron operation where plasma electrons are trapped in closed paths which are concentric with the electrode structure. This results in dramatic changes in the radiative and electrical properties of the device. With constant current, lamp operating voltage may be reduced by more than a factor of two when the magnetic field is present. The effects of filler gas pressure and magnetic field strength on the current-voltage characteristics are presented. The presence of the magnetic field results in a radial contraction of the plasma. This contraction increases with increasing field strength and with decreasing pressure. Ion lines from the Ar filler gas are more affected by the field than are neutral-atom lines from the cathode material.

A simple method is proposed for short fall-time dye laser pulse generation. The pulse tailoring is demonstrated by an excimer laser pumped double cavity dye laser. The achieved fall time is 280 ps with the use of a 7 ns long pump pulse.

Individual PCB congeners have been quantitated at ppm levels, with an average error of ±3.2%, with the use of a helium discharge detector (HDD) for element-selective detection of Cl emission. Chlorinated internal standards of known concentrations were added to each solution determined to establish the relative peak areas per unit concentration of Cl present. No detector precalibration or response factor formulations were required, since the detector response is based solely on the moles of Cl present. The same methodology was utilized to determine the % Cl in Aroclor samples without prior identification of the PCB congeners present.

Blood plasma protein infrared spectra, while qualitatively very similar, display subtle differences in the frequencies and intensities of absorption bands. These small differences are sufficient to permit an accurate quantitative analysis of mixtures of these proteins. In this paper we examine the performance of some alternative methods of spectroscopic quantitative analysis in determining the concentrations of proteins in aqueous solutions. The widely-used K matrix method, using sloping baselines and intercept functions, was found to be inadequate for these determinations. In contrast, a method based on the little-known Q matrix approach, augmented by a robust equation solver, yielded results with a sufficient degree of accuracy to make it a viable tool for use in the study of proteins at solid interfaces and for more general applications in the field of protein chemistry.

The problem of incomplete atomization when nonmetals are being determined in aspirated organic solutions was studied. Abel inverted spatial profiles of C₂, CN, C(I) and S(I) emission were acquired at several heights. It was found that molecular emission caused by the analyte is restricted to the lower aerosol channel. Nonmetal atomic emission is found in the toroidal region and higher up in the aerosol channel. Compounds that contained nonmetals comprised of either oxygen, nitrogen, or sulfur, which were dissolved in xylene, exhibited nonmetal emission intensities independent of the structure of the compound.

Infrared reflectance spectra of a thermally grown 30-nm SiO₂ film on a Si wafer were measured as a function of incident angle and polarization. Spectra measured with s-polarized light resemble the published extinction coefficient for SiO₂. The p-polarized spectra show significant distortions at all incident angles. Bands change in frequency and intensity and can even invert as the incident angle increases beyond the Brewster angle of the Si substrate. Spectral simulations using the classical electromagnetic equations reproduce these distortions.

Fluorescence was collected from cyanine-dye-impregnated arachidic acid monolayers at the air/water interface with the use of a fiber optics configuration and a Langmuir film balance. Fatty-acid-to-dye molar ratios in the monolayers ranged from 99:1 to 1:1. The monolayers were compressed in a step-wise manner, with sampling of cyanine fluorescence after each compression step. A drop in fluorescence intensity ranging from 20 to 80% was observed between the uncompressed and compressed monolayers. The observed fluorescence decrease appeared to be a function of barrier pressure rather than molecular area and dye concentration.

Measurements of emission intensities from spatially inhomogeneous sources often require faithful image reproduction at the entrance slit of a monochromator or spectrograph, or at some intermediate aperture. In fundamental studies, the spatial variations in intensity contain information about the basic nature of the source. For analytical measurements, the spatial resolution is required for the selection of a region in the source which exhibits the optimum analytical characteristics. Refractive optics can be used for this type of image transfer, but only if proper attention is given to the minimization or elimination of lens aberrations. We have developed a simple lens system that allows for the elimination of chromatic aberration and the control of spherical aberration. Two plano-convex fused-silica lenses with nominal focal lengths of 300 mm are mounted on translation stages that are driven in opposite directions by a right-hand/left-hand leadscrew. The performance of this system will be discussed and compared with that predicted by ray-tracing calculations. The magnitudes of various lens aberrations will be compared, and their effect on spatially resolved measurements of emission from an inductively coupled plasma will be examined.

Under certain conditions, highly specific fluorescence spectra, showing molecular vibrations as in infrared spectroscopy, can be obtained for the analysis of compounds separated in TLC and microbore LC. Two techniques, both employing low-temperature solid samples, are available: Shpol'skii spectroscopy and Fluorescence Line-Narrowing (FLN) spectroscopy. The former utilizes n-alkane solvents, which form regular, crystalline solids; the latter is based on selective laser excitation. Both techniques can be applied for detection in TLC. FLN is more feasible, since the Shpol'skii method requires spraying of the TLC plate, which leads to significant spreading of the spots. FLN can also be used for offline detection in micro-LC after immobilization of the column effluent on a TLC plate. In addition, some exploratory experiments on the applicability of FLN for on-column detection in LC are presented.

The solid-surface fluorescence and phosphorescence quantum yield values and phosphorescence lifetime values were obtained for p-aminobenzoic acid (PABA) and 4-phenylphenol adsorbed on α-cyclodextrin/NaCl mixtures. From the luminescence quantum yield and phosphorescence lifetime data, the triplet formation efficiency values, the phosphorescence rate constants, and the rate constants for radiationless transition from the triplet state were determined for PABA and 4-phenylphenol on α-cyclodextrin/NaCl mixtures. In addition, the percentages of radiative and nonradiative transitions were calculated for the two compounds. The various luminescence parameters obtained in this work provided important insights into the analyte/α-cyclodextrin substrate interactions responsible for the observed solid-surface room-temperature fluorescence (RTF) and phosphorescence (RTP). Furthermore, the RTF and RTP intensities of PABA and 4-phenylphenol were obtained from several α-cyclodextrin/NaCl mixtures. These data showed the importance of the initial wet chemistry in the sample preparation procedure.

IR and/or NMR spectra-structure correlations have been developed which aid in the elucidation of molecular structure of organonitrogen-containing compounds.

Fourier transform infrared (FT-IR) and Fourier transform nuclear magnetic resonance (FT-NMR) methods were used to examine the adsorption and reaction of diisopropyl fluorophosphate (DFP) on various solid adsorbents. Static and flow system experiments were monitored with the use of FT-IR to determine DFP adsorption rates and isotherms on silica, coated silicas, γ-alumina, coated aluminas, and activated charcoal. The adsorption of DFP(g) onto the solid adsorbents was generally very rapid, with a half-life of 20 s for 1 mg DFP onto 25 mg of 350 m²/g silica. The DFP adsorption isotherm on silica indicated chemisorption to a monolayer at P/P₀ < 0.6, followed by increased coverage that appears to be physical adsorption. Diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, photoacoustic spectroscopy (PAS), and solid-state ³¹P NMR of adsorbed DFP showed chemisorption on silica and on alumina. Bonding at the P=O of DFP was indicated by a −41 cm⁻¹ shift in the v(P=O) and a 1-ppm upfield shift in the ³¹P resonance. DRIFT, PAS kinetics, and ³¹P NMR showed that DFP hydrolyzed after the initial adsorption on alumina and some coated materials but not on silica or activated charcoal. The rate of hydrolysis increased on alumina with addition of water and varied with different aluminas and coated silicas.

Reference infrared vapor-phase spectra of 15 polychlorinated dibenzodioxin-p-dioxin (PCDD) isomers were recorded at low microgram concentrations. Ether linkage (COC) bond angles for these isomers and for the 22 tetrachlorodibenzodioxin (TCDD) isomers were calculated from infrared data, with the use of mass approximations for the terminal atom in a nonlinear XY₂ model and by neglecting the valence force equation symmetric stretch bending term. Calculated bond angles show a good correlation with x-ray diffraction (XRD) and carbon-13 nuclear magnetic resonance (¹³C NMR) relaxation results. Molecular geometries in PCDD isomers, as defined by the COC bond angle and the COC stretching frequencies, were found to range from near planar, in laterally substituted isomers with high electron withdrawing capacity aromatic rings, to tetrahedral, for isomers with low electron withdrawing capacity rings. Non-bonded interactions were also found to influence molecular geometry. Molecular geometry was used to assign structures for the 1,2,3,6,7,8- and 1,2,3,7,8,9-hexachlorodibenzodioxin isomer mixture and was found to be an important factor in estimations of COC bond strength from empirical data. Correlations between infrared data and PCDD LD₅₀ values suggest that molecular geometry, polarizability, and steric PCDD/receptor interactions are associated with isomer toxicity.

A method is shown for the determination of kinetic parameters from dynamic FT-IR experiments. The effect heating rate has on the reproducibility of the calculated activation energy is discussed. The curing of PMDA/ODA polyimide at several heating rates is given as an example.

Nickel metal with an unoxidized and an electrochemically oxidized surface can be transferred considerable distances between vacuum systems, without significant surface change, in an anaerobic cell for study by various surface science methods (XPS, Auger, ISS, and SIMS). The SIMS and ISS spectra are very sensitive to hydrocarbon contamination, which can be nearly eliminated by using the transfer cell. Complementary information is obtained using these surface analytical methods. Electrochemically oxidized nickel metal samples show the presence of oxide and hydroxide species. The results are compared with spectral data for a number of nickel compounds.

A series of β-diketones and alkenes have been examined by matrix isolation Fourier transform infrared spectroscopy. The matrix experiment readily detects the presence of extensive keto-enol tautomerization in selected β-diketones. Certain absorption bands in the IR could be used to estimate the extent of the tautomerization. The data gathered on the alkenes found their absorption bands to occur in the same regions (±5 cm⁻¹) as those found for VP and SS phases. Minimal nearest-neighbor (aggregation) interactions were found. These results are in contrast to those found for aldehydes, ketones, and acids of similar chain lengths.

The characteristics of the OH stretching absorptions in a series of catechols, resorcinols, hydroquinones, and diols have been documented with the use of matrix isolation Fourier transform infrared spectroscopy. Steric and electronic effects were described and found to agree with published results on the vapor-phase studies on the same compounds. The positions of the OH absorptions were shown to fall within a window set on the high energy side by the vapor-phase results and on the lower energy side by solid-state/solution results. The very low full width at half-height values unique to the matrix experiment allowed for the observance of absorption bands not yet seen before in the vapor-phase or solid-state studies. These new bands confirm the presence of intramolecular interactions not previously documented. The data as a whole do indicate that extensive intermolecular interactions do occur at very low loadings on the cryogenic disk (10 ng) for compounds containing polar substituents.

Qualitative and quantitative studies of the reaction of black carbon with the oxides of nitrogen, including NO, NO₂/N₂O₄, N₂O, and N₂O₃, have been carried out with the use of Fourier transform infrared spectroscopy (FT-IR). The active reactant is shown to be NO₂, whether it acts as a disproportionation product or as an impurity in the gas under study. FT-IR spectra of the surface species identify them as resulting from reaction of carbon with NO₂. For paraffin candle soot which was exposed simultaneously to oxygen atoms, and nitric oxide at 298 K, the surface species also are due to NO₂, formed by oxidative adsorption of NO on the soot surface.

A PX1 multilayer pseudocrystal is compared with a TlAP crystal for the analysis of light elements by means of two different excitation sources, i.e., a chromium and a scandium target tube. Peak and background intensities for Cl, S, P, Si, Al, Mg, Na, F, and O are compared with the use of both a fine and a coarse collimator. The most suitable working conditions for the analysis are pointed out.

A variable-length plasma source has been developed for analytical atomic emission spectroscopy. The length of the plasma, adjustable during plasma operation, provides a means of adjusting the sample residence time in the plasma. The plasma operates from three compact, simple, and inexpensive direct-current power supplies. Three concentric quartz tubes supply argon and sample aerosol to the plasma. The modular design allows the quartz tubing to be easily replaced and a variety of electrode distances to be established. The argon consumption, typically 7.4 L/min, is comparable to, or less than, that for commercial DCP and ICP systems. Many of the vertical spatial characteristics of the plasma described here are similar to those documented for the ICP. Changes in the nebulizer gas flow rate produced spatial shifts in the maximum of the vertical, spatial analyte emission profiles of Mg. Increases in the signal-to-background ratios at common analytical wavelengths were observed with increasing plasma length. The movement of the region of maximum emission to positions below the top of the outer quartz tube, which occurs as a result of increasing the plasma length, is thought to be the main reason for the worse detection limits. The addition of 10% nitrogen to the outer argon gas flow caused a 35% enhancement in the Ca ion and a 28% decrease in the Ca atom signal-to-background ratios.

Vertical spatial profiles of a six-electrode, direct-current plasma emission source were used to study the influence of plasma length on the interference of Na on Ca and Zn and P on Ca. For the shortest plasma (11.5 mm), depression of the Ca atom line is observed in the region up to 10 mm above the tip of the sample bullet when Na is present. A cross-over to enhancement occurs for higher regions. This is directly opposite to the observations in the ICP for the Na interference on Ca. Increasing the plasma length causes downward shifts in the cross-over point which are not a simple effect of the lowering of the sample bullet. When P is present, the Ca atom emission is initially depressed in the region directly above the sample bullet. At higher observation heights, little effect is observed. Similar behavior is observed for the Ca ion line in the presence of P, suggesting the possibility of a classical vaporization-type interference mechanism. The interference effects studied here can be virtually eliminated by careful selection of plasma length and observation height.