N. H. Cheung

Resonance-enhanced laser-induced plasma spectroscopy: ambient gas effects

Abstract When performing laser-induced plasma spectroscopy for elemental analysis, the sensitivity could be significantly enhanced if the plume was resonantly rekindled by a dye laser pulse. The extent of the enhancement was found to depend on the ambient gas. Air, nitrogen, helium, argon and xenon at pressures ranging from vacuum to 1 bar were investigated. In vacuum, the analyte signal was boosted because of reduced cooling, but it soon decayed as the plume freely expanded. By choosing the right ambient gas at the right pressure, the expanding plume could be confined as well as thermally insulated to maximize the analyte signal. For instance, an ambient of 13 mbar xenon yielded a signal-to-noise ratio of 110. That ratio was 53 when the pellet was ablated in air, and decreased further to 5 if the dye laser was tuned off resonance.

Sensitive elemental analysis of aqueous colloids by laser-induced plasma spectroscopy

Highly sensitive elemental analysis of lead carbonate colloids was demonstrated by two-pulse laser-induced plasma spectroscopy. The first laser pulse created a vapor plume with the particulates concentrated in space because of their slower propagation. They were then ablated by an ArF laser pulse that efficiently atomized and excited the lead analyte. The lead emissions were much enhanced, while the background continuum interference was minimized. The detection limit for lead was shown to be 14.2 ppb, compared with 13 ppm achieved by conventional laser-induced breakdown spectroscopy of lead ions in water and 210 ppb for lead aerosols.

Resonance-enhanced laser-induced plasma spectroscopy for sensitive elemental analysis: Elucidation of enhancement mechanisms

When performing laser-induced plasma spectroscopy for elemental analysis, the analyte signal-to-noise ratio increased from four to over fifty if the plume was reheated by a dye laser pulse tuned to resonant absorption. Time-resolved studies showed that the enhancement was not due to resonance photoionization. Rather, efficient and controlled rekindling of a larger plume volume was the key mechanism. The signal-to-noise ratio further increased to over a hundred if the atmosphere was replaced by a low-pressure heavy inert gas. The ambient gas helped confine and thermally insulate the expanding vapor.

Adsorption of bovine serum albumin on fused silica: Elucidation of protein-protein interactions by single-molecule fluorescence microscopy.

The adsorption of bovine serum albumin (BSA) on fused silica at pH 4.7 was studied at the single molecules level by total-internal-reflection fluorescence microscopy. This pH value was the isoelectric point of BSA. At low [BSA] of 20pM, protein molecules adsorbed as monomers. At intermediate [BSA] of 500pM, protein molecules adsorbed as clusters of about five monomers on average. Both monomers and clusters had adsorption rate coefficients of the order 10(-7)ms(-1) and desorption rate coefficients of about 2x10(-2)s(-1). The respective steady-state coverage was about 10x higher than that at neutral pH, presumably because of the more favorable BSA-silica electrostatics. At pH 4.7 and with [BSA] higher than 100nM, adsorption begot further adsorption to produce nonlinear isotherms. The coverage at 1microM BSA was 2.5x that of the linearly extrapolated coverage. This suggests that at pH 4.7, solute-adsorbate affinity was the dominant factor that explains the enhanced adsorption observed in ensemble measurements.

Spectroscopy of Laser Plumes for Atto‐Mole and ng/g Elemental Analysis

Abstract Two all‐optical analytical techniques are reviewed. Both are capable of highly sensitive multi‐element analysis. One is by means of resonance‐enhanced plasma spectroscopy. It minimizes the continuum background associated with laser‐induced plasmas. Relative to laser‐induced breakdown spectroscopy, the signal‐to‐noise ratio is improved by orders of magnitude, thus allowing the quantitation of sodium and potassium at the single blood cell level. The other technique utilizes laser‐excited atomic fluorescence. It has been traditionally handicapped by its one wavelength–one transition specificity. We showed, however, that numerous elements could be induced to fluoresce at a single excitation wavelength of 193 nm provided that the analytes were imbedded in dense plumes, such as those produced by pulsed laser ablation. This method eliminates the continuum plasma background and sub‐ppb sensitivity was demonstrated in the analysis of aqueous lead colloids.

Sensitive elemental analysis by ArF laser-induced fluorescence of laser ablation plumes: Elucidation of the fluorescence mechanism

Numerous atomic analytes in plumes produced by pulsed-laser ablation fluoresced upon ArF laser irradiation. The likely mechanism was photoexcitation to levels near the ionization limit. These levels were dense and were probably broadened by the extreme plume density to allow efficient absorption of 193 nm photons. The excited atoms relaxed to intermediate states as the plume expanded. Interparticle interaction weakened and transitions from these states produced sharp spectral lines for elemental analysis. This ArF-induced fluorescence technique was orders of magnitude more sensitive than laser-induced plasma spectroscopy.

Measuring Binding Kinetics of Ligands with Tethered Receptors by Fluorescence Polarization and Total Internal Reflection Fluorescence

Binding kinetics of nuclear receptors and their specific ligands was measured using polarization anisotropy complemented with total internal reflection fluorescence. Binding affinities of tethered full length human estrogen receptor alpha (ERalpha) with 17beta-estradiol, diethylstilbestrol, raloxifene, 4-hydroxytamoxifen, tamoxifen, and genistein were measured to be 100 (as reference), 100, 35, 21, 8, and 1.5, respectively. They agreed with published results. For the first time, rate constants were measured, and off rates were 1.5, 1.5, 1.3, 1.6, 1.7, and 2.3 x 10(-3) s(-1) while on rates were 11, 10, 3.3, 2.4, 1.0, and 0.26 x 10(5) M(-1)s(-1), respectively. For the antiestrogen drugs, their comparable off-rates correlated well with their equally similar potency. Eleven ginsenosides were screened as potential ligands. None were found to bind to ERalpha, but Rb1(S) and 20(S)-Rg3 were shown to bind to peroxisome proliferator-activated receptor gamma. The latter finding corroborated strongly with the therapeutic effects of ginsenosides on diabetic mice observed in a separate study. Our method would complement surface plasmon resonance assay for small ligands in the mass range of tens to hundreds of Daltons.

Multi-element analysis of ceramic and polymeric samples by ArF laser excited atomic fluorescence of ablated plumes

Laser-excited-atomic-fluorescence at the single excitation wavelength of the ArF laser was demonstrated for numerous analytes. These analytes were present in the material plumes produced by pulsed laser ablation of ceramic and polymeric targets. Under minimally destructive conditions, the fluorescence spectra were orders of magnitude brighter than the corresponding laser-induced breakdown spectra. Simultaneous emissions from Al, Ca, Co, Cr, Cu, Fe, In, Mg, Mn, Na, Pb, Sn, and Si were observed. The technique was applied to four analytical problems: the analysis of dried paint for trace lead when μg g−1 detection limits were achieved; the analysis of valuable potteries when two look-alike specimens were differentiated based on practically non-destructive single-shot analysis; the elemental analysis of ink when lines written with different pens could be discriminated without discernable sample destruction even under the microscope; and the analysis of electrode-plastic interfaces when the detection sensitivity was comparable to SIMS. The fluorescence intensity varied with the fluence and the timing of the ArF laser pulse in ways suggestive of particulates in the plume.

Transmittance and resistivity of semicontinuous copper films prepared by pulsed-laser deposition

Thin copper films were grown on glass by pulsed-laser deposition. The simultaneous in situ monitoring of the electrical resistance and optical transmittance of the growing film yielded highly reproducible and consistent data about percolation onset and film conductivity, both being useful indicators of film quality. When prepared under favorable conditions, films as thin as 1.5 nm would percolate, and became fully continuous at 5 nm, with conductivity reaching 30% of that of bulk copper.

Minimally destructive and multi-element analysis of steel alloys by argon fluoride laser-induced plume emissions.

The mechanical properties and corrosion resistance of steel alloys depend on their elemental compositions. Elemental analysis of these alloys is therefore of immense practical importance not only for finished steel products but also for online industrial processing. Since the 1990s, laser-induced breakdown spectroscopy (LIBS) has been proven to be a useful technique for these applications. It delivers better than 1% analytical accuracy and lg/g limits of detection (LOD). In LIBS analysis of steel, the typical mass removal rate was tens of nanograms and the etch depth was tens of nanometers per laser pulse. This kind of target destruction is acceptable for most applications. However, it may not be tolerated for decorative surfaces and precision parts or when high resolution depth profiling of passivation layers is required. We recently showed that the LIBS sensitivity could be significantly enhanced if the ablated plume was induced to fluoresce by an argon fluoride (ArF) 193 nm laser pulse. Aluminum alloys were subsequently analyzed when sample destruction was minimal and the mass LODs for trace elements were in the tens of attomoles and etch depths were only fractions of a nanometer. While the ArF enhanced emissions behaved like laserexcited atomic fluorescence (LEAF), numerous elements were shown to emit when excited at the single wavelength of 193 nm. The technique therefore preserves the universality of LIBS but adds sensitivity. Given the complex elemental composition of steel alloys, the capability to analyze multielements simultaneously is especially critical. We therefore applied the ArF enhanced probe to the elemental analysis of steel alloys. In what follows, we will report its nondestructiveness, multi-element universality, excellent mass LOD, and LEAF-like mechanism. EXPERIMENTAL