Field ionization (FI) and field desorption (FD) mass spectrometry rely on activated emitters: thin wires covered with microneedles that concentrate the electric field to the ionization threshold. Bursey, Rechsteiner, Sammons, Hinton, Colpitts and Tvaronas (1976) showed that cobalt dendrites electrochemically deposited on tungsten wire are superior to nickel dendrite emitters for field ionization and simpler to manufacture than carbon microneedle emitters, and they defined an electrical activation step that improves sensitivity. They left four linked questions unresolved: the mechanism by which cobalt emitters degrade after only two or three FD experiments under heating with organic material; the physical origin of the activation-induced sensitivity improvement; the absence of any predictive model connecting electrochemical deposition parameters to needle morphology and mechanical robustness; and the absence of statistically characterized quantitative benchmarking of cobalt against nickel and carbon emitters. These four questions remain unanswered: later work developed alternative emitters, and the field standardized on high-temperature-activated carbon-on-tungsten emitters, bypassing rather than resolving the 1976 questions. The problem is open, but its scientific significance today is low.

Background

Field ionization (FI) is a soft ionization method for mass spectrometry in which gas-phase molecules are ionized in electric fields of order one volt per angstrom at the surface of a sharp metal tip; ionization deposits little internal energy, so FI mass spectra are dominated by molecular ions rather than fragment ions. Field desorption (FD) is the variant in which a nonvolatile sample, for example an organic salt, is deposited directly onto the tip and desorbed as an ion under the combined action of the field and gentle resistive heating of the tip; in the 1970s FD made the soft-ionization analysis of polar and ionic compounds possible. The central component of both techniques is the emitter: a thin tungsten wire (typically about 10 micrometres in diameter) held at roughly 10 kV opposite a counter-electrode. Because the field at a bare wire is far below the ionization threshold, practical emitters are activated, meaning they are covered with thousands of dendritic microneedles (also called whiskers), whose very small radii of curvature concentrate the field. The standard activation method, due to Beckey's group, is high-temperature activation: carbon microneedles are grown by pyrolysis of a carbon-rich vapour such as benzonitrile on the hot wire in a directing electric field (Beckey, Hilt and Schulten 1973). These carbon emitters perform well but take hours to grow in a vacuum system, use a toxic precursor, and were error-prone in many hands, which motivated the search for alternatives. One alternative is electrochemical deposition: metal dendrites, meaning branched metal microcrystals grown from a metal-salt solution under an applied potential, can be grown on a wire in minutes at negligible cost, and the approach had been demonstrated for nickel. Bursey, Rechsteiner, Sammons, Hinton, Colpitts and Tvaronas (1976) systematically developed the cobalt version. Working on a modified AEI MS702 mass spectrometer (a magnetic-sector instrument, which separates ions by their deflection in a magnetic field), they mapped how the deposition parameters (cobalt salt identity, solution concentration near 4 to 6 molal, i.e. moles of cobalt salt per kilogram of water, bath temperature between 65 and 100 degrees Celsius, delivered potential between roughly 20 and 38 V, and the timing and repetition rate of voltage pulses) control needle length, density and mechanical quality; they showed that growth can be localized by masking the wire with an acetone-soluble polymer; they defined a pre-use activation step (about 10 kV applied between emitter and counter-electrode while heating the emitter wire at 40 mA for about 3 minutes) that improves ion-current sensitivity in subsequent use; and they reported that cobalt emitters are superior to nickel emitters for field ionization and simpler to manufacture than carbon emitters, while for FD of organic salts cobalt emitters gave lower currents than nickel and became unusable after only two or three FD experiments. All of these findings were empirical. The study established no mechanism for the FD degradation, none for the activation improvement (the authors only speculated that it may be related to the creation of surfaces on which adsorption occurs more easily, and their micrographs were not of the same tip before and after activation), no model predicting morphology or robustness from the deposition parameters, and no statistically characterized quantitative comparison against nickel and carbon emitters.

Problem Statement

Bursey et al. (1976) left the cobalt dendrite emitter as an empirically working but scientifically unexplained device. The problem is to establish cobalt dendrite emitters as a mechanistically understood and quantitatively characterized FI/FD emitter technology, by jointly resolving the following four linked open questions. (1) Degradation mechanism: what physical and chemical processes cause cobalt dendrite emitters to lose performance after roughly two or three field desorption experiments, when the needles are resistively heated in the presence of deposited organic material? The 1976 study could not characterize the same tip before and after FD use, so candidate mechanisms such as needle blunting, sintering, chemical conversion of the cobalt surface, or carbonaceous contamination remain undistinguished. (2) Activation mechanism: what is the physical origin of the ion-current sensitivity improvement produced by the defined activation step (about 10 kV applied between emitter and counter-electrode while heating the emitter wire at 40 mA for about 3 minutes)? The 1976 authors only speculated that the improvement may be related to the generation of surfaces on which adsorption occurs more easily; competing hypotheses, such as field-induced morphology change, surface cleaning, or chemical alteration of the cobalt, remain untested. (3) Predictive growth control: how do the electrochemical deposition parameters (cobalt salt identity, solution concentration around 4 to 6 molal, bath temperature between 65 and 100 degrees Celsius, delivered potential between roughly 20 and 38 V, and voltage-pulse timing and repetition rate) determine needle length, sharpness, density, coating quality and mechanical robustness (sharpness is the tip radius of curvature of the needles, coating quality the fraction of the wire surface covered by adherent dendrites, and mechanical robustness the fraction of the initial ion current retained after a stated number of heating and handling cycles)? (4) Quantitative benchmarking: how do cobalt dendrite emitters compare with nickel dendrite emitters and with high-temperature-activated carbon emitters in field-ionization ion current, quantitatively and with statistical characterization, across a defined set of compounds and operating conditions? The 1976 comparison (superior to nickel for field ionization, simpler to manufacture than carbon) was qualitative observation on a single instrument. All four questions are jointly required. Each is resolved only by an answer that gives a definite result for the cobalt-on-tungsten system of the 1976 study: (1) and (2) require the operative mechanism to be identified and supported by evidence that the competing candidates cannot equally explain; (3) requires the deposition parameters to be related to measured values of all five of needle length, sharpness, density, coating quality and mechanical robustness; and (4) requires the ion currents of cobalt, nickel and carbon emitters to be reported quantitatively, with statistical characterization (mean and standard deviation over a stated number of independently prepared emitters), over a stated set of compounds and operating conditions.

References

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