Gregory C. Eiden

Collision and reaction cells in atomic mass spectrometry: development, status, and applications

The development and utilization of collision and reaction cells in atomic mass spectrometry is reviewed. These devices have been used for decades in fundamental studies of ion–molecule chemistry and have only recently been applied in the GD-MS and ICP-MS fields. Such cells are used to promote reactive and non-reactive collisions, with resultant benefits in interference reduction, isobar separation, and thermalization/focusing of ions in ICP-MS. Novel ion–molecule chemistry schemes, using a variety of reaction gas reagents selected on the basis of thermodynamic and kinetic principles and data, are now designed and empirically evaluated with relative ease. These chemical resolution techniques can avert interferences requiring mass spectral resolutions of >600000 (m/Δm). Purely physical ion beam processes, including collisional dampening and collisional dissociation, are also employed to provide improved sensitivity, resolution and spectral simplicity. Collision and reaction cell techniques are now firmly entrenched in current-day ICP-MS technology, enabling unprecedented flexibility and freedom from many spectral interferences. A significant body of applications has now been reported in the literature. Collision/reaction cell techniques are found to be most useful for specialized or difficult analytical needs and situations, and are employed in both single- and multi-element determination modes.This contribution is a guest editorial statement and technical assessment for a special issue of the Royal Society of Chemistry journal entitled Journal of Analytical Atomic Spectrometry (JAAS). The editorial introduces the subject area of collision and reaction cells in atomic mass spectrometry, reviews current literature and commercial instrumentation trends, and previews four perspective and numerous research articles contained in the special journal issue.

Communication. Selective removal of plasma matrix ions in plasma source mass spectrometry

We report a new method for selective removal of argon ions and other plasma matrix ions in plasma source MS. The method consists of sampling the plasma and reacting the sampled plasma and analyte ions with hydrogen gas. Reactions have been studied in three instruments: in the ion trap of a plasma source ion trap (PSIT) mass spectrometer and in the post-skimmer region of both a conventional ICP mass spectrometer and a second PSIT. In the ion trap, the reaction between Ar+ and H2 proceeds at nearly the collisional rate whereas reaction of most other atomic ions is four to five orders of magnitude slower. For modest H2 pressures and reaction times in the ion trap [10–4 Torr (1 Torr = 133.322 Pa) and 10 ms], the Ar+ signal is reduced by six orders of magnitude. We have examined reactions of H2 with 33 different atomic ions; the only ions for which a reaction was evident were N+, O+, Cl+, and Ar+. The decrease in Ar+ occurs by a sequence of fast reactions resulting in charge transfer from Ar+ to form the low m/z ions H2+ and H3+, which are rapidly ejected from the ion trap. The net effect is the selective removal of Ar+ chemically, not by virtue of its mass-to-charge ratio only, as in resonant ion ejection methods. In the conventional ICP-MS experiments the reaction time is short, limiting the decrease in Ar+ to about 40-fold in preliminary and unoptimized experiments. However, the reaction is still selective: simple scattering by H2 reduces the 45Sc+ signal at only 5% of the rate of reactive loss of Ar+. Production of H2+ and H3+ is observed directly in the conventional ICP-MS experiments, indicating that the chemistry in the post-skimmer region is consistent with that observed in the ion trap. We discuss methods by which the magnitude of Ar+ reduction observed in the ion trap might be realized in conventional ICP-MS, thus possibly allowing a greater analyte ion transmission efficiency and reduced space-charge effects.

Analytical performance of the plasma source RF quadrupole ion trap in elemental and isotopic MS

Recent progress in the development of ion trap ICP-MS instrumentation is presented in terms of performance figures of merit for trace elemental and isotopic analysis. Conventional polyatomic ion interferences are avoided altogether using collisional dissociation techniques inherent to ion trap operation. The current detection limits are in the low pg mL –1 range, as obtained over extended mass scans using multi-element solutions. The dynamic range linearity is nearly six orders of magnitude using variable injection times to control ion density in the trap. The abundance sensitivity is 10 5 to 10 6 using selective ion monitoring. Other performance figures approach or exceed those of conventional linear quadrupole based ICP-MS instrumentation.

Femtosecond laser ablation particle introduction to a liquid sampling-atmospheric pressure glow discharge ionization source

This work describes the use of a compact, liquid sampling–atmospheric pressure glow discharge (LS-APGD) ionization source to ionize metal particles within a laser ablation aerosol. Mass analysis was performed with a Thermo Scientific Exactive Mass Spectrometer which utilizes an orbitrap mass analyzer capable of producing mass resolution exceeding m/Δm > 160,000. The LS-APGD source generates a low-power plasma between the surface of an electrolytic solution flowing at several μl min−1 through a fused silica capillary and a counter electrode consisting of a stainless steel capillary employed to deliver the laser ablation particles into the plasma. Sample particles of approximately 100 nm were generated with an Applied Spectra femtosecond laser located remotely and transported through 25 meters of polyurethane tubing by means of argon carrier gas. Samples consisted of an oxygen free copper shard, a disk of solder, and a one-cent U.S. coin. Analyte signal onset was readily detectable relative to the background signal produced by the carrier gas alone. The high mass resolution capability of the orbitrap mass spectrometer was demonstrated on the solder sample with resolution exceeding 90,000 for Pb and 160,000 for Cu. In addition, results from a laser ablation depth-profiling experiment of a one cent coin revealed retention of the relative locations of the ∼10 μm copper cladding and zinc rich bulk layers.

Plasma source ion trap mass spectrometry: Enhanced abundance sensitivity by resonant ejection of atomic ions

An experimental study of resonant ion excitation in an rf quadrupole ion trap is reported. Atomic ions are generated in an inductively coupled plasma and injected into the ion trap where, after collisional cooling, they are irradiated by a low-voltage, dipole coupled waveform. Single frequency, narrowband, and broadband excitation pulses have been used. Absorption lineshapes (plots of observed ion signal versus excitation frequency) are shown for variations in buffer gas pressure and the amplitude and duration of the single frequency pulses. The absorption lineshapes are usually asymmetric and tail toward lower frequencies. At sufficiently low buffer gas pressure or potential well depth, the lineshapes broaden and become more asymmetric to the point that absorption by ions with adjacent mass-to-charge ratios overlaps. This overlapping absorption reduces the selectivity with which a single mass-to-charge ratio ion can be excited and ejected relative to nearby mass-to-charge ratio ions. The rate of ion ejection is different on the low versus high frequency edges of the absorption lines. This difference in ejection rates provides an important key to understanding the shape of the absorption lines. All of these observations are explained in terms of the known kinematic behavior of ions in real traps, that is, traps with substantial higher order symmetry components in the trapping field (“nonlinear” fields). The importance of the nonlinearity of the trapping field in understanding the observed lineshapes and their time dependencies is discussed. We also report resonant ejection results obtained using multiple frequency (narrow or broad bandwidth) excitation. Multiple frequency excitation allows ions with different mass-to-charge ratio values to be ejected from the trap using one excitation waveform. The finite ion storage capacity of the ion trap is thereby reserved for the ion(s) of interest. We show that ejection of 89Y ions can be ∼ 105 times more efficient than ejection of ions at either m/z 88 or 90.

Isotopic analysis of uranium in NIST SRM glass by femtosecond laser ablation MC-ICPMS

We employed femtosecond Laser Ablation Multicollector Inductively Coupled Mass Spectrometry for the determination of uranium isotope ratios in a series of standard reference material glasses (NIST 610, 612, 614, and 616). The uranium in this series of SRM glasses is a combination of isotopically natural uranium in the materials used to make the glass matrix and isotopically depleted uranium added to increase the uranium elemental concentration across the series. Results for NIST 610 are in excellent agreement with literature values. However, other than atom percent 235U, little information is available for the remaining glasses. We present atom percent and isotope ratios for 234U, 235U, 236U, and 238U for all four glasses. Our results show deviations from the certificate values for the atom percent 235U, indicating the need for further examination of the uranium isotopes in NIST 610-616.

The influence of ns- and fs-LA plume local conditions on the performance of a combined LIBS/LA-ICP-MS sensor

Both laser-induced breakdown spectroscopy (LIBS) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) are well-established analytical techniques with their own unique advantages and disadvantages. The combination of the two analytical methods is a very promising way to overcome the challenges faced by each method individually. We made a comprehensive comparison of local plasma conditions between nanosecond (ns) and femtosecond (fs) laser ablation (LA) sources in a combined LIBS and LA-ICP-MS system. The optical emission spectra and ICP-MS signal were recorded simultaneously for both ns- and fs-LA and figures of merit of the system were analyzed. Characterization of the plasma was conducted by evaluating excitation temperature and electron density of the plume under various irradiation conditions using optical emission spectroscopy, and correlations to ns- and fs-LIBS and LA-ICP-MS signal were made. The present study is very useful for providing conditions for a multimodal system as well as giving insight into how laser ablation plume parameters are related to LA-ICP-MS and LIBS results for both ns- and fs-LA.

Femtosecond laser ablation multicollector ICPMS analysis of uranium isotopes in NIST glass

We utilized femtosecond laser ablation together with multi-collector inductively coupled plasma mass spectrometry to measure the uranium isotopic content of NIST 61x (x = 0, 2, 4, 6) glasses. The uranium content of these glasses is a linear two-component mixing between isotopically natural uranium and the isotopically depleted spike used in preparing the glasses. Laser ablation results match extremely well, generally within a few ppm, with solution analysis following sample dissolution and chemical separation. In addition to isotopic data, sample utilization efficiency measurements indicate that over 1% of ablated uranium atoms reach a mass spectrometer detector, making this technique extremely efficient. Laser sampling also allows for spatial analysis and our data indicate that rare uranium concentration inhomogeneities exist in NIST 616 glass.

Characterization of extreme ultraviolet laser ablation mass spectrometry for actinide trace analysis and nanoscale isotopic imaging

We demonstrate a new technique for trace analysis that has nanometer scale resolution imaging capability: Extreme Ultraviolet Time-of-Flight Laser Ablation Mass Spectrometry (EUV TOF). We describe the characterization of this technique and discuss its advantages. Using the well-standardized NIST 61x glasses, the results show the EUV TOF spectra contain well defined signatures of U, Th, and their oxides, with far fewer spectral interferences than observed in Time-of-Flight Secondary Ion Mass Spectrometry (SIMS TOF). We demonstrate that the ratio of U and Th ions to the oxide ion signatures is adjustable with EUV laser pulse energy. Sample utilization efficiency (SUE) which measures the ratio of detected ions to atoms in the ablated volume was used as a measure of trace analysis sensitivity of EUV TOF. For U and Th, SUE is 0.014% and 0.017%, respectively, which is comparable to SIMS TOF in the same mass range. In imaging mode EUV TOF is capable to map variations in composition with a lateral resolution of 80 nm. Such high lateral resolution enabled mapping of the isotope distribution of 238U and 235U in closely spaced micron-size uranium oxide particles from isotope standard materials. Trace elemental sensitivity and nanometer spatial resolution gives EUV TOF great potential to dramatically improve the state-of-the-art laser ablation/ionization mass spectrometry and elemental spectro-microscopy for applications such as geochemical, forensic and environmental analysis.

RadICalc: a program for estimating radiation intensity of radionuclide mixtures

RadICalc was developed to address the need for a computer program that could calculate the composition, activity, and measurable radiation of arbitrary radionuclide mixtures over time without significant effort from end-users. It provides an interface to perform decay calculations and can search and display the resulting data in graphical or tabular form. RadICalc can also determine radiation expected at specific masses with user-defined molecules in addition to atomic species for use in mass-based isotope separations for radiometric counting applications, a novel method under development at Pacific Northwest National Laboratory.