Andrew M. Leach

Methods for shot-to-shot normalization in laser ablation with an inductively coupled plasma time-of-flight mass spectrometer

Single-shot laser ablation (one laser pulse per analysis) combined with inductively coupled plasma mass spectrometry offers tremendous potential for the direct elemental analysis of solid samples with high spatial resolution. However, the precision of the method is usually limited by variations in the laser irradiance and in the specimen being sampled. In this study, two data analysis techniques were evaluated to improve single-shot measurement precision. Both methods exploit the simultaneous full-spectrum acquisition capability of a time-of-flight mass spectrometer. In the first approach, a normalization factor is computed from the summed signal generated by all sample constituents, the reasoning being that the summed spectrum should be proportional to the total mass of sample ablated. This scheme results in a greater than factor of two improvement in precision, moderately better than is possible with a single internal standard. The enhancement in measurement precision was found to be concentration dependent, with the greatest improvement (10–50 fold) experienced by high concentration elements. The second method correlates the attenuation of plasma matrix ions to analyte intensities. The attenuation technique demonstrated no statistically significant improvement in precision, limited by the relatively low signal-to-noise ratio of the attenuated signals.

Gas chromatography–inductively coupled plasma time-of-flight mass spectrometry for the speciation analysis of organometallic compounds

An inductively coupled plasma time-of-flight mass spectrometer was combined with a capillary gas chromatograph for the speciation analysis of organotin and organolead compounds. The ability to produce complete mass spectra at a high frequency (typically >20u2006000 mass spectra per second) makes time-of-flight mass spectrometry nearly ideal for the detection of transient signals such as those produced by high speed chromatographic techniques. In this study, GC-ICP-TOFMS instrumental figures of merit were determined for the separation of tetramethyltin, tetraethyltin and tetraethyllead. Data acquisition speed of the current system was limited to 78 integrated complete mass spectra (255 individual spectra summed) per second. However, because all mass-to-charge ratios are detected, collection of a data point every 12.75xa0ms is sufficient for the measurement of all but the fastest transient signals (peak widths <50xa0ms). A tin isotope-ratio accuracy of 0.28% and a precision of 2.88% (RSD) were calculated for a 1xa0s gas chromatographic peak of tetramethyltin. Limits of detection for all three analytes were found to be in the low femtogram region, with a dynamic range of greater than six orders of magnitude.

Rapid simultaneous multielemental speciation by capillary electrophoresis coupled to inductively coupled plasma time-of-flight mass spectrometry

Rapid simultaneous multielemental speciation was carried out by combining capillary electrophoresis (CE) with inductively coupled plasma time-of-flight mass spectrometry (ICP-TOFMS). Fast electrophoretic separation of a mixture of several anionic species and negative-charged metal–cyanide complexes was achieved by using a linear polyacrylamide-coated capillary. The separation of three arsenic species and two cobalt–cyanide complexes required less than 70xa0s in the presence of other anions and metal cyanides of CuII, CrVI, NiII and VV. Simultaneous element-selective detection was provided by ICP-TOFMS. This method can acquire spectra at 20xa0kHz, a rate fast enough for even the narrow transient peaks generated by CE. In this case, because the CE peaks exhibited a 1–3xa0s full width at half maximum (FWHM), an integration time of 250xa0ms was employed. Thus, each single point in a transient peak results from the average of 5000 mass spectra. Furthermore, the multielemental capability of the ICP-TOFMS enabled mass spectral separation of several metal species that were not electrophoretically resolved. The effect of electric field strength, buffer concentration and applied hydrodynamic pressure at the head of the capillary on migration times and resolution has been examined. Absolute detection limits for the different metal species were 1–20xa0pg for 20xa0nL injected sample volumes. Peak-area and elution-time reproducibility were typically better than 4% RSD and 1% RSD, respectively, for 10 successive injections.

Factors affecting the production of fast transient signals in single shot laser ablation inductively coupled plasma mass spectrometry

Low sample consumption and high achievable spatial resolution combine to make laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) an attractive direct sampling technique for the analysis of solids. These desirable characteristics are best realized when the laser ablation system is operated in a single-shot fashion, in which each laser pulse produces a transient analyte signal. The temporal width of a single-shot LA analyte transient is inversely related to the best achievable signal-to-noise ratio (S/N). Thus, production of fast transient signals is an important consideration in single-shot LA analyses. In this study we explore the effects on transient pulse width of ablation-cell volume, the diameter and length of tubing used to connect the ablation cell to the ICP, and the composition and flow rate of ablation-cell sweep gas. Minimization of ablation-cell volume and the length and diameter of the transfer tube was found to dramatically decrease the peak widths of transient signals. However, use of helium gas to sweep analyte particles from the ablation cell was found to significantly reduce the effect of cell volume on transient width. Use of a cell volume of 0.70 cm3 and optimization of other instrumental parameters produced a transient sample pulse 85 ms in duration and limits of detection in the tens of femtograms range for single-shot laser-ablation events.

Time-of-flight mass spectrometry as a tool for speciation analysis

Abstract Time-of-flight mass spectrometry (TOFMS) has recently been introduced as an alternative to scanning-based mass analyzers for use in elemental analysis. Coupled with an inductively coupled plasma or alternative ion source, TOFMS can produce a complete atomic mass spectrum in less than 50 μs. Because of this high spectral-generation rate, even very brief transient signals can be recorded with high fidelity. Furthermore, each mass spectrum is derived from the same sub-microsecond pulse of ions, so high precision can be achieved by using either isotope-ratioing or internal standardization techniques. All these features make TOFMS attractive for the measurement of transient signals, such as those commonly encountered in speciation analysis. In this paper, the capabilities of TOFMS in speciation will be demonstrated through the coupling of gas chromatography and capillary electrophoresis with an inductively coupled plasma-TOFMS. Additionally, the development of novel switched gas sampling glow discharge (GSGD) ionization sources will be described and their role in chemical speciation will be evaluated. The switched GSGD has the ability to collect both atomic and molecular mass spectra in rapid succession, to provide additional information about chemical species. The coupling of various sample introduction systems (flow-cell, exponential dilutor, capillary gas chromatography and electrothermal vaporization) to the GSGD is outlined.

Identification of alloys using single shot laser ablation inductively coupled plasma time-of-flight mass spectrometry

Single shot laser ablation inductively coupled plasma time-of-flight mass spectrometry (LA-ICP-TOFMS) has been developed as a method for the rapid identification of alloy samples. This technique provides good accuracy for elemental concentrations greater than 0.1 percent by mass from samples with a range of different matrix compositions. Through the simultaneous measurement of all elements present within a cloud of ablated particles, relative percent composition values are measured that can be directly compared with certified concentrations. Cluster analysis has been used to successfully classify 33 metal alloys based on the measurement of 15 elements. Additionally, cluster analysis was used to identify the samples from a library of standard reference values. For the 15 alloys used to train this method, the correct standard was identified in greater than 99% of measurements. The only failed identification in the training set may have been due to elemental fractionation nexperienced with a brass sample. For the 18 standards used to validate the technique, samples were identified with a 93% success rate. Misidentifications may have been caused by inaccurate certified data. Several alloys investigated in this study were less than 500 µm in length at their largest dimension and weighed less than one milligram.

Evolution and revolution in instrumentation for plasma-source mass spectrometry

Plasma-source mass spectrometry, usually in the form of inductively coupled plasma mass spectrometry (ICP-MS), has matured into a widely accepted method for ultra-trace multielemental analysis. However, the method exhibits shortcomings. For example, it does not provide adequate precision for isotope ratio measurements if many isotopes are to be determined. Moreover, isobaric overlaps (spectral interferences) can be very troublesome in some situations. Similarly, matrix interferences can adversely affect many determinations. Yet, it is in the area of high-speed transient measurements that ICP-MS perhaps suffers its greatest weakness. When sampling devices such as flow injection, laser ablation, electrothermal vaporization, or chromatography are employed, the user must choose between broad elemental or isotopic coverage and signal-to-noise ratio (S/N). In turn, compromised S/N means lower precision or poorer detection limits. Here, new instrumentation aimed at overcoming these limitations will be described. One system, based on a time-of-flight mass spectrometer, provides excellent detection limits, resolving power better than commercial quadrupole mass filters, precision of at least 0.02% rsd in a ratioing mode, and extraordinarily high speed for use with transient sampling devices. The second instrument is based on a sector-field mass spectrometer but, unlike other such units, is equipped with a focal-plane array detector. So equipped, the system can detect a broad mass range at once.

Use of an ion guide collision cell to improve the analytical performance of an inductively coupled plasma time-of-flight mass spectrometer

Abstract An axial-acceleration inductively coupled plasma time-of-flight mass spectrometer has been equipped with an octopole ion guide/collision cell. Ion-energy experiments prove that instrument duty cycle improves by up to 100% over values determined for conventional ion optics. Both sensitivity and noise were found to decrease with the ion guide in place. Consequently, limits of detection for most elements were found to be comparable to those calculated for conventional ion optics. The exceptions are low-mass ions that enjoy only a relatively small gain in duty cycle and thus exhibited significantly degraded detection capability. Collisional cooling caused the spectral resolving power to improve by up to 80% compared to the conventional optics. Ion chemistry within the collision cell permitted the determination under robust plasma conditions of potassium, calcium, and iron, elements that usually suffer from isobaric overlaps. Improved resolving power helped reduce the adverse effects of hydrocarbon background ions generated by reactions within the ion guide.

Interactions between Methylene Blue and Sodium Dodecyl Sulfate in Aqueous Solution Studied by Molecular Spectroscopy

The interactions of methylene blue (MB, a cationic redox indicator and biological stain) and sodium dodecyl sulfate (SDS, a micelle-forming, anionic surfactant) in aqueous solution have been examined by using Rayleigh scattering, UV-visible absorption, and fluorescence spectroscopy. At SDS concentrations significantly below the critical micelle concentration (cmc), MB forms noncovalent dimers and aggregates with SDS that scatter light but do not fluoresce. For solutions containing 1 μM MB and < 3–5 mM SDS, shifts in the absorption spectrum characteristic of the formation of MB H-aggregates are noted. There appears to be little effect on the fluorescence emission spectrum, indicating that these MB aggregates do not fluoresce appreciably. At and above the known SDS cmc, MB is observed to interact with the micelles. The MB excited-state fluorescence lifetime (380 ps) remains constant until SDS micelles form, then increases to 615 ps. The MB rotational reorientation time similarly increases from 105 to 500 ps between 6 and 8 mM SDS. This finding suggests that the MB is encountering, on average, a microenvironment in the SDS micelles that is 5-fold more viscous than liquid water or the molar volume of the MB/SDS species that is reorienting is 5-fold larger than MB in water.

Multisize CdSe Nanocrystal/Polymer Nanocomposites for Selective Vapor Detection Identified from High-Throughput Screening Experimentation

We have implemented high-throughput spectroscopic screening tools for the investigation of vapor-selectivity of CdSe semiconductor nanocrystals of different size (2.8- and 5.6-nm diameter) upon their incorporation in a library of rationally selected polymeric matrices. This library of resulting sensing materials was exposed to polar and nonpolar vapors in air. Each of the sensing materials demonstrated its own photoluminescence vapor-response patterns. Two criteria for the evaluation of vapor responses of the library of sensing materials included the diversity and the magnitude of sensing responses. We have found several polymer matrices that simultaneously meet these criteria. Our new sensing materials based on polymer-embedded semiconductor nanocrystal reagents of different size promise to overcome photobleaching and short shelf life limitations of traditional fluorescent organic reagent-based sensing materials.