Mahadeva P. Sinha

Laser-induced volatilization and ionization of microparticles

We have developed a method for the laser‐induced volatilization and ionization of individual microscopic particles on a continuous, real‐time basis. A beam of particles is produced by the expansion of an aerosol into a vacuum through a capillary nozzle and skimmer system. The particle in the beam is then hit by a high‐energy Nd‐YAG laser pulse for its volatilization and ionization. Mass spectral measurements using a quadrupole mass spectrometer on the ions thus generated from potassium biphthalate particles (1.96 μm in diameter) have been made. The combined ionization efficiency for K+ ion is found to be ∼10−6. The advantages of laser‐induced volatilization and ionization along with the real‐time capability of the present method make it very useful for the chemical analysis of aerosol particles by mass spectrometry. For the synchronization of the particle with the YAG laser pulse, the velocity of the particles in the beam has also been determined. This was accomplished by measuring the time‐of‐flight of m...

Development of a miniaturized, light‐weight magnetic sector for a field‐portable mass spectrograph

We have designed and fabricated a miniaturized, light‐weight magnetic sector for a focal plane mass spectrograph (Mattauch–Herzog design) by using a new high‐energy‐product magnet material (Nd‐B‐Fe alloy), and a high permeability magnet yoke material (V‐Co‐Fe alloy). The magnetic sector weighs less than 10 kg and has a focal plane of 5.1 cm in length and covers a nominal mass range of 40–240 amu. Such a magnetic sector in conjunction with an array detector and a short microbore capillary column is eminently suited for the development of a field‐portable gas chromatograph‐mass spectrometer instrument of high performance.

Miniature focal plane mass spectrometer with 1000-pixel modified-CCD detector array for direct ion measurement

A high performance, focal plane miniature mass spectrometer (MMS) of Mattauch–Herzog geometry with a CCD-based array detector for the direct and simultaneous measurements of different mass ions is described. Miniaturization (10cm×5cm×5cm,395g) was accomplished by using high-energy-product magnet material (Nd–B–Fe alloy) and a high permeability yoke material (V–Co–Fe Alloy) for the fabrication of the magnetic sector. The electrostatic sector was machined from a single piece of machinable ceramic (MACOR). All the components of the analyzer are mounted on a single plate, which facilitate their alignment and make the instrument rugged. The modified-CCD based ion detector array has 1000 elements (20μm×2mm) and was invented in our laboratory. The photosensitive part of the CCD was replaced with a metal-oxide-semiconductor (MOS) capacitor for ion detection. The ion sensing capacitor plates are connected to the CCD gates that are operated in the fill-and spill mode providing a gain in the charge domain for the si...

Laser ablation-miniature mass spectrometer for elemental and isotopic analysis of rocks

A laser ablation-miniature mass spectrometer (LA-MMS) for the chemical and isotopic measurement of rocks and minerals is described. In the LA-MMS method, neutral atoms ablated by a pulsed laser are led into an electron impact ionization source, where they are ionized by a 70 eV electron beam. This results in a secondary ion pulse typically 10-100 μs wide, compared to the original 5-10 ns laser pulse duration. Ions of different masses are then spatially dispersed along the focal plane of the magnetic sector of the miniature mass spectrometer (MMS) and measured in parallel by a modified CCD array detector capable of detecting ions directly. Compared to conventional scanning techniques, simultaneous measurement of the ion pulse along the focal plane effectively offers a 100% duty cycle over a wide mass range. LA-MMS offers a more quantitative assessment of elemental composition than techniques that detect ions directly generated by the ablation process because the latter can be strongly influenced by matrix effects that vary with the structure and geometry of the surface, the wavelength of the laser beam, and the not well characterized ionization efficiencies of the elements in the process. The above problems attendant to the direct ion analysis has been minimized in the LA-MMS by analyzing the ablated neutral species after their post-ionization by electron impaction. These neutral species are much more abundant than the directly ablated ions in the ablated vapor plume and are, therefore, expected to be characteristic of the chemical composition of the solid. Also, the electron impact ionization of elements is well studied and their ionization cross sections are known and easy to find in databases. Currently, the LA-MMS limit of detection is 0.4 wt.%. Here we describe LA-MMS elemental composition measurements of various minerals including microcline, lepidolite, anorthoclase, and USGS BCR-2G samples. The measurements of high precision isotopic ratios including (41)K/(39)K (0.077 ± 0.004) and (29)Si/(28)Si (0.052 ± 0.006) in these minerals by LA-MMS are also described. The LA-MMS has been developed as a prototype instrument system for space applications for geochemical and geochronological measurements on the surface of extraterrestrial bodies.

Active pixel sensors for mass spectrometry

Abstract Active pixel sensors (APS) are micro-fabricated CMOS amplifier arrays that are rapidly replacing CCD devices in many electronic imaging applications. Unlike the pixels of a CCD device, the sensing elements of the APS will respond to locally situated electrostatic charge, owing to the amplifier present in each pixel. We have built two small test arrays with microscopic aluminum electrodes integrated onto standard APS readout circuitry for the purpose of detecting low-energy gas-phase ions in mass spectrometers and other analytical instruments. The devices exhibit a near-linear dynamic range greater than four orders of magnitude, and a noise level of less than 100 electrons at room temperature. Data are presented for the response of the APS detectors to small ions in a miniature magnetic sector mass spectrometer and in an atmospheric pressure jet of helium. Data for individual highly-charged electrospray droplets are presented as well. Anticipated improvements suggest that in the near future APS ion detectors will posses noise levels approaching 10 electrons and will have a useful dynamic range over six orders of magnitude.

Simultaneous direct detection of sub keV molecular and atomic ions with a delta-doped charge-coupled device at the focal plane of a miniature mass spectrometer

A delta-doped charge-coupled device (CCD) was used for the simultaneous and direct detection of low-energy atomic and molecular ions dispersed along the focal plane of a miniature mass spectrometer (MMS). The measured detection threshold for charged particles with a delta-doped CCD has been extended down to 700eV, representing over an order of magnitude improvement compared to conventional solid-state detectors. We report the direct detection of 700eV energy ions by the mass spectral measurements of species such as iron pentacarbonyl. The combination of delta-doped CCD and MMS enables high-speed, precision mass spectrometry of ions and molecules on a small scale suitable for field and space applications.A delta-doped charge-coupled device (CCD) was used for the simultaneous and direct detection of low-energy atomic and molecular ions dispersed along the focal plane of a miniature mass spectrometer (MMS). The measured detection threshold for charged particles with a delta-doped CCD has been extended down to 700eV, representing over an order of magnitude improvement compared to conventional solid-state detectors. We report the direct detection of 700eV energy ions by the mass spectral measurements of species such as iron pentacarbonyl. The combination of delta-doped CCD and MMS enables high-speed, precision mass spectrometry of ions and molecules on a small scale suitable for field and space applications.

Iron isotope fractionation between liquid and vapor phases of iron pentacarbonyl

Iron isotope fractionation between liquid and vapor iron pentacarbonyl was measured in a closed system at approximately 0 and approximately 21 degrees C to determine if Fe isotope analysis of iron pentacarbonyl vapor is viable using electron-impact, gas-source mass spectrometry. At the 2sigma level, there is no significant Fe isotope fractionation between vapor and liquid under conditions thought to reflect equilibrium. Experiments at approximately 0 degrees C indicate iron pentacarbonyl vapor is approximately 0.05 per mil (per thousand) greater in (56)Fe/(54)Fe than liquid iron pentacarbonyl, which is just resolvable at the 1sigma level. Partial decomposition of iron pentacarbonyl vapor or liquid to an iron oxide or iron metal shows that significant isotopic fractionation occurs, where the decomposed product has a lower (56)Fe/(54)Fe ratio as compared to the starting iron pentacarbonyl. It follows that methods to decompose iron pentacarbonyl must be quantitative to obtain accurate isotope values.

In situ geochronology as a mission-enabling technology

Although there are excellent estimates of ages of terrains on Mars from crater counting, even a few absolute ages would serve to validate the calibration. Results with uncertainties, although much larger than those that could be achieved in labs on Earth, would be extremely valuable. While there are other possibilities for in situ geochronology instruments, we describe here two alternative technologies, being developed in JPL. There are two common features of both. The first is analysis by means of miniature mass spectrometer. The second is use of laser sampling to reduce or avoid sample handling, preparation and pre-treatment and equally importantly, to allow analysis of individual, texturally resolved minerals in coarse-grained rocks. This textural resolution will aid in selection of grains more or less enriched in the relevant elements and allow construction of isochrons for more precise dating. Either of these instruments could enable missions to Mars and other planetary bodies.

The Impact of Astronomy Technologies on Chemical Analysis

The chemical sciences have been profoundly influenced by the development of advanced focal plane array detectors. The incorporation of existing technologies developed for infrared multiplexers has already had a positive effect on the fields of isotope ratio mass spectrometry and ion mobility spectrometry. Multiplexer based ion detectors have improved detection limits by several orders of magnitude over conventional Faraday cups while maintaining a high degree of stability.