Gangqiang Li

An inductively coupled plasma—time-of-flight mass spectrometer for elemental analysis. Part I: Optimization and characteristics

An inductively coupled plasma-time-of-flight mass spectrometer (ICP-TOFMS) has been constructed and evaluated for elemental analysis. The instrument produces analog spectra similar to those from quadrupole inductively coupled plasma mass spectrometers. The large abundance of Ar ions is deflected away from the microchannel plate detector to reduce detector dead time and space-charge complications. The ICP-TOFMS, operated in a linear (nonreflecting) mode, currently has a resolving power of 500 (full width at half maximum). Present ion optics employed in the instrument require a trade-off between signal-to-noise ratio and resolving power. In addition, mass-dependent kinetic energies in the supersonic beam created in the ICP mass spectrometer interface cause a mass bias in the right-angle TOFMS because the ions must be steered to the detector to compensate for their velocity in the supersonic beam direction. In the current design the sampling duty cycle is only approximately 3%, thereby limiting sensitivity. However, positive potentials applied to the right-angle extraction region can increase sensitivity by a factor of 2–4 by slowing down the ions that enter the extraction zone. The transmission efficiency of the TOFMS is approximately 20% and is limited by divergence of the ion packet in the drift tube.

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.

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.

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.

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.

Continuum background reduction in orthogonal-acceleration time-of-flight mass spectrometry with continuous ion sources

The continuum ion background in an inductively coupled plasma time-of-flight mass spectrometer (TOFMS) has been reduced by 2 orders of magnitude by using energy discrimination (ED). A potential barrier placed in front of the ion detector effectively discriminates between higher-energy signal ions and the lower-energy background ions. The signal-to-noise ratio was increased tenfold and detection limits of 0.4–4.2 ppt were achieved for 11 elements ranging from Li to U. The resolving power was observed to degrade from 1520 to 1230 with the addition of the potential barrier. The residual background count rate was found to be limited by neutrals formed after exiting the ion reflectron via charge exchange with the background gas. This ED method can be employed to effectively reduce the continuum ion background in any TOFMS that uses orthogonal acceleration with a continuous ion source.

Constant-Fraction Discrimination/Boxcar Integrator for Plasma Source Time-of-Flight Mass Spectrometry

A constant-fraction discrimination (CFD) system has been combined with a boxcar integrator for detection in inductively coupled plasma/time-of-flight mass spectrometry. The discriminator provides gating logic for the boxcar integrator when an incoming ion signal occurs, but discriminates against electronic or background noise of lower amplitude. As a result, the combination can effectively reject noise and accumulate analyte signal, rather than relying on an averaging process to reduce noise levels. The signal-to-noise ratio is therefore enhanced in this operation compared with the conventional boxcar method. The dynamic range of the detection system is at least five orders of magnitude.