Patrick P. Mahoney

Laser ablation-inductively coupled plasma mass spectrometry with a time-of-flight mass analyser

Laser ablation has been employed for sample introduction into an ICP-time-of-flight (TOF) mass spectrometer. The transients generated by the ablated material are generously sampled by the 10 kHz repetition rate of the TOF instrument. A detection limit of 10 ppb for Pb in a cast-iron standard is calculated from integration of a 0.3 s transient signal generated by a single laser pulse. By simultaneously acquiring and rationing the signals from two isotopes of Zn, the substantial pulse-to-pulse power fluctuations from the laser are virtually eliminated. Although some of the data are presented in a single- or double-channel acquisition mode, the results demonstrate the sensitivity and rationing abilities available for all elements and isotopes simultaneously from a single laser pulse. Use of a digital oscilloscope provides a full elemental spectrum for each laser pulse as the laser is rastered across a lava sample that contains plagioclase crystals. The relative spatial distributions for 11 elements of interest contained in this sample are plotted over an 11 mm distance. This paper is not intended to be a display of state-of-the-art laser-ablation techniques, as the large beam divergence of the ruby laser limits the spatial resolution to 1 mm. However, the ability of the plasma-source TOF mass spectrometer for analysing transient signals is clearly demonstrated.

Time-of-Flight Mass Spectrometry for Elemental Analysis

Inductively coupled plasma mass spectrometry (ICPMS),1 ® rst introduced by Houk et al. in 1980,2 has become the method of choice for routine elemental analysis in a wide range of applications. ICPMS offers nearly complete elemental and isotopic analysis for most samples over a broad range of element concentrations. Although ICPMS has matured into a powerful technique, several limitations remain; a list of its strengths and weaknesses can be found in Table I. To overcome the shortcomings of ICPMS, a number of researchers have investigated other types of mass analyzers as alternatives to the commonly used quadrupole mass ® lter and sector-® eld analyzer. The advantages of each type of mass analyzer have been reviewed previously,3 and only a short description of recent advances in ICPMS with each type of mass analyzer is given below. A detailed overview of the development and performance of an ICP± time-of̄ ight (TOF) instrument, ® rst described by Myers and Hieftje,4 is then given wherein the

Characterization of a Radio-Frequency Glow Discharge/Time-of-Flight Mass Spectrometer

A radio-frequency glow discharge/time-of-flight mass spectrometer (RFGD-TOFMS) has been developed by simple modification of the interface to an ICP-TOFMS. The work described here evaluates the interface and operating conditions of the RFGD-TOFMS. The ion optics which focus ions toward the entrance of the TOFMS are the same as those used in the original ICP-TOFMS instrument. By means of pin-shaped brass samples of varied lengths, the sample-skimmer distance in the RFGD-TOFMS has been optimized at 4 mm. The discharge pressure and power have been found to be optimal at 50–60 W and 0.3 Torr, respectively. The application of small negative potentials to the skimmer cone (extraction orifice) was found to improve signals marginally. However, higher negative potentials reduced both signal levels and resolving power. The skimmer potential also affects the final kinetic energy of the ions before their extraction into the TOFMS. At 0.3 Torr all ions extracted for mass analysis have approximately the same kinetic energy, which can be estimated in the orthogonal TOFMS. Detection limits for several standard samples are at the single-ppm level, which is not unexpected; with the same ion-optical system, the current ICP-TOFMS also produces detection limits that are 2–3 orders of magnitude worse than those of many commercial instruments.

An inductively coupled plasma-time-of-flight mass spectrometer for elemental analysis. Part III: Analytical performance

A time-of-flight mass spectrometer (TOFMS) was evaluated as a mass analyzer for inductively coupled plasma mass spectrometry (ICP-MS). The long-term drift of signals was in the range of 7–8% relative standard deviation, whereas the short-term precision was between 5 and 20%, somewhat worse than is typically reported for commercial ICP-MS instruments (5%). However, precision can be improved considerably in the TOFMS by ratioing isotopic peaks or through internal standardization, a consequence of its ability to extract all measured ions simultaneously from the inductively coupled plasma. This feature was demonstrated by monitoring the 206Pb/208Pb ratio with boxcar averagers. In this ratioing mode, precision was improved to approximately 0. 5%. Detection limits were measured with two alternative signal processing systems: (1) discriminator-gated integration and (2) integration of digitized spectra. Both methods improved the signal-to-noise ratio by a factor of from 10 to 100, although detection limits were still 1–2 orders of magnitude poorer for most elements than from the best commercial ICP-MS instruments. The dynamic range of the discriminator-gated integration system is over 4 orders of magnitude, but can be extended to 106 with planned increases in primary ion-beam current, which is currently 10–100 times lower than is found in other instruments. Virtually simultaneous multielement and multiisotope analysis is possible for masses from 7Li to 209Bi with minimal mass bias and detection limits on the 0. 4–2-ppb level.

Toward the Next Generation of Atomic MassSpectrometers

Atomic mass spectrometry, embodied principally as ICP mass spectrometry (ICP-MS) and glow discharge mass spectrometry (GDMS), has enjoyed rapid growth during the last decade, yet both methods exhibit shortcomings that would be desirable to reduce or eliminate. Prominent among these shortcomings are drift and limited precision, several troublesome spectral and matrix interferences, and moderate atom-detection efficiency. This last limitation is particularly troublesome when ICP-MS, for example, must be interfaced to analytical systems that deliver extremely small sample volumes or low flow rates or when extremely limited sample sizes must be examined. Such situations are projected to be increasingly common in the next decade because of the importance of biotechnology and nanostructured materials. Overcoming these limitations will require substantial modifications in both sources and mass-spectrometer designs. Sources will be required that are more efficient at sample utilization, aerosol volatilization and atomization and that provide multidimensional information. Similarly, mass spectrometers of the future must be more atom-efficient, should measure all elements and isotopes simultaneously, and must operate on a time scale that is compatible with microsampling and transient-sampling technology. Possible alternative systems that meet these criteria will be outlined and their likely performance assessed. Greatest emphasis is placed on time-of-flight mass spectrometry coupled with an ICP source.

At-Line Quantitative Ion Mobility Spectrometry for Direct Analysis of Swabs for Pharmaceutical Manufacturing Equipment Cleaning Verification

The potential for ion mobility spectrometry (IMS) to provide rapid at-line quantitation of residues on surfaces via direct analysis of swabs is attractive for pharmaceutical manufacturing equipment cleaning verification. In this study, the development of an IMS method to provide acceptable quantitation of active pharmaceutical ingredients and cleaning agents is described. Key modifications to commercially available instrumentation were made to achieve a dynamic range of 5-100 microg per 25 cm2 surface area and acceptable analyte recovery in the presence of ionizable matrix components. The results of this study effectively demonstrate the capability of IMS to serve as an at-line quantitative analytical method.

Isotope ratios and abundance sensitivity obtained with an inductively coupled plasma-time-of-flight mass spectrometer

Isotope ratios and abundance sensitivities have been determined with an inductively coupled plasma-time-of-flight mass spectrometer (ICP-TOFMS). Abundance sensitivities are at least in the 106 range for low abundance ions that precede high abundance ions. Three methods of detection for isotope-ratio measurement have been compared. The three systems involve gated detection followed by analog integration, analog averaging, or ion counting. Gated ion counting offers excellent precision—between 0. 64 and 1. 00% relative standard deviation (RSD). These values approach those predicted from counting statistics and are comparable to those reported for other inductively coupled plasma-mass spectrometry (ICP-MS) instruments. In addition, a greater number of accumulated counts or longer analysis times would afford precisions of 0. 1% with stable gating electronics. The accuracy of the counting method is in the 1–10% range if no correction for mass bias is performed. However, this ion counting method suffers from a limited dynamic range due to pulse pileup. Constant-fraction discrimination gated integration and commercial boxcar averager techniques offer a broader dynamic range because of their analog nature, but the attainable RSD values are limited by drift in the detection systems and by the methods employed to calculate an accurate ratio. Overall, mass bias in the ICP-TOFMS is more severe than previous work in ICP-MS due primarily to detection system bias.

Electrospray Ionization Time-of-Flight Mass Spectrometer for Elemental Analysis

Electrospray ionization (ESI) has been combined with a time-of-flight (TOF) mass spectrometer for elemental analysis. With the use of a heated-capillary interface, the instrument is shown to be stable to within 5% relative standard deviation (RSD) over a 60-min period. The ratio between the isotopes of rubidium is measured with a precision of 0.4% RSD for a 1-min integration time. With the addition of a supporting electrolyte as a spray stabilizer and internal standard, the dynamic range is linear over at least three orders of magnitude. The extent of solvent-cluster fragmentation is found to be governed primarily by the voltage differential between the capillary and skimmer and, to a lesser extent, by the capillary temperature. The capillary voltage also affects the distribution of species among the parent and its fragment ions for ferrocene and tetraethyllead. Under “mild” interface conditions, a resolving power of 1400 is achieved for an organolead complex. A spectrum for a larger molecule, polypropylene glycol, is presented to show the versatility of the ESI-TOF instrument for both atomic and molecular analysis.

Radio-Frequency-Powered Planar-Magnetron Glow Discharge as a Source for Time-of-Flight Elemental Mass Spectrometry

A radio-frequency (rf) planar-magnetron glow discharge was investigated as an ion source for time-of-flight mass spectrometry. The first stage of a conventional ICP-MS interface was modified to create a planar-magnetron glow-discharge cell. The pressure and power of the magnetron source were optimized for ion signal. The perpendicular geometry of the mass spectrometer enables the relative energies of the extracted ions to be determined; the energies of the argon support gas and analyte ions were compared. The figures of merit for the system were investigated, and detection limits of 0.1–1 ppm were achieved for elements in a conducting matrix. Detection limits were an order of magnitude worse for elements in an electrically insulating sample. Along with an initial survey of the major spectral interferents, the relative sensitivities for different elements were determined. The source was also operated at low pressures (0.01 Torr) in order to determine whether operating in this pressure regime can be used to alleviate polyatomic interferences.

Fluorimetric Analysis on Individual Nanoliter Sample Droplets

Fluorescence from rhodamine 6G molecules in 225-μm-diameter (6-nL) droplets of ethanol is measured as they fall through a 514.5-nm Ar-ion laser beam, which is operated at a power density of 250 W/cm2. The fluorescence, measured at 555 nm, is imaged onto the entrance slit of a monochromator and measured with a photomultiplier tube. The droplets are produced by repeatedly plunging a 190-μm glass stylus into and out of a sample solution. The frequency of droplet production (150 Hz) corresponds to a flow rate of about 50 μL/min. The present detection limit of 3.7 fg, or 7.8 amol of rhodamine 6G, is limited by stray light and dark-current fluctuations.