Miniature Ion Research Articles

Evaluation of the potentiometric detection of acetylcholine and other neurotransmitters in capillary electrophoresis.

It is demonstrated that acetylcholine may be determined potentiometrically in capillary electrophoresis down to 6 x 10(-7) M with a miniature coated-wire ion selective electrode. The more established amperometric method is not possible for this compound. Similarly, 4-aminobutyric acid (GABA) is not accessible amperometrically; although potentiometric detection was possible, the sensitivity is not good. Other neurotransmitters may be determined by either method, but in these cases amperometric determination is generally favorable. For serotonin the potentiometric detection limit was 4 x 10(-5) M, while for the amperometric method a detection limit of 10(-7) M was found.

Fundamental considerations for the application of miniature ion mobility spectrometry to field analytical applications

Ion mobility spectrometry (IMS) is a popular field analyzer. The reason for its popularity is its small size, its sensitivity, and the amount of information contained in the ion mobility spectrum. Several research groups are working on smaller IMS cells using microelectromechanical systems (MEMS) technology. New principles of operation will be needed to achieve this goal. The theory supporting the development of the radio frequency (RF) IMS is described. RF-IMS separates ions by applying an asymmetric RF field across two parallel plates and passing the ions through the separator. The performance of the device no longer depends on a simple linear relationship between the drift velocity and an electric field, but rather on a nonlinear relationship between these parameters. Consequently, a more detailed momentum-transfer theory is needed. The required theory is developed and applied to the interpretation of experimental data here. The importance of using momentum transfer theory in combination with the continuity equation is demonstrated. © 2000 John Wiley & Sons, Inc. Field Analyt Chem Technol 4: 255–267, 2000

New noninvasive transcutaneous approach to blood glucose monitoring: successful glucose monitoring on human 75 g OGTT with novel sampling chamber

A novel noninvasive and quasi-continuous method of transcutaneous blood glucose monitoring for use with the human 75 g oral glucose tolerance test (OGTT) has been developed. The effused fluid was obtained by applying suction on the skin surface and labeled suction effusion fluid (SEF). The system consists of two main parts: a suction apparatus and the glucose sensor system. The suction apparatus applies vacuum to the patient's skin at 400 mmHg absolute pressure to collect the SEF. The miniature ion sensitive field effect transistor (ISFET) based glucose sensor can measure glucose in small SEF quantities. The monitoring system is based on the association between the glucose concentration in the SEF and in the serum. During the 75 g OGTT, the glucose change in the SEF was measured every 10 min. Although a response delay of up to 20 min was observed in the SEF glucose change, it was possible to perform the 75 g OGTT by this noninvasive monitoring method.

Evaluation of National Synchrotron Light Source beam line fast valve protection system

A fast valve designed and produced at Brookhaven National Laboratory for the National Synchrotron Light Source (NSLS) was evaluated for operational and system parameters. A functional prototype of a typical beam line was built to measure fast valve protection for simulated experimental beam line catastrophic vacuum failures. The fast valve, which was designed to stop shock wave propagation due to in-rushing gas, yielded a nitrogen leak rate from 0.5 to 0.02 Torr l/s during the first 3 s of beam line vacuum failure. An UHV all-metal electropneumatic valve in series with the fast valve will close and seal in less than 3 s. The fast valve closure time which measured between 4.5 and 6.5 ms from initially sensing the shock wave to closure of the fast valve did not significantly change after vacuum bakeouts to 200 °C, repeated cycling, or shock wave operation. The fast valve firing circuit described sensed shock wave current from a miniature appendage ion pump and fired the fast valve in approximately 0.5 ms. Measurements were made for helium, nitrogen, argon, and air shock waves. Closing time and bounce were measured for several valves by sensing either laser or incandescant light during operation.

Miniature ion mobility spectrometer cell

A miniature ion mobility spectrometer call using resistive coated ceramic electrodes is described. Its construction, performance and results of parametric studies are reported. The cell has a total ion current in the range 1 to 8*10-10 A and a minimum baseline ion current of 1.9*10-11 A. The total ion current less baseline ion current difference affects sensitivity through scaling relationships with the reactant and product ion currents. Because a membrane inlet is used for sample introduction, the carrier and drift gas flow rates both affect sensitivity. Although protonated quasi-molecular ions of sample molecules are found to be stable in the cell, protonated dimer ions dissociate in the higher electric fields which can be applied to the cell. Resolution of ion mobility peaks scales as the two-thirds power of the drift length squared divided by the gate width applied to the shutter grid. A microprocessor-controlled environmental monitoring system has been assembled using the cell as a sensor.

New on-source miniature atomic FAB and alkali ion SIMS sources

Miniature atom and ion sources have been developed which mount directly onto the mass spectrometer ion source, within the vacuum chamber. The new sources are inexpensive to construct and require no external mounting flange. The use of an ion versus atom beam has been found to be unimportant, although the particle mass and impact energy have important effects on the intensity of quasimolecular ions, apparently due to the deposited momentum. A model of ionization of samples suspended in a liquid matrix and bombarded by energetic particles is proposed, in which ionization occurs in the “gas phase” by a charge-cascade mechanism similar to chemical ionization.

Use of an ion gauge beam flux monitor for resistivity control in CuInSe 2 grown by molecular beam epitaxy

We have constructed and utilized a miniature ion gauge to set the copper, indium and selenium fluxes for the growth of CuInSe 2 onto CdS to form solar photovoltaic heterojunctions. Resistivities from 0.01 Ω cm n type to 0.001 Ω cm p type can be reliably achieved to within a factor of 10 of the desired value. The copper-to-indium arrival rate ratio is critical in determining the layer resistivity of p-type material, with a high copper-to-indium ratio generally giving low resistivity. The selenium arrival rate is also important, however, in that low selenium rates always lead to n-type layers, independent of the copper-to-indium arrival rate ratio.

The mechanisms of etch pit and ripple structure formation on ion bombarded Si and other amorphous solids

Quasi-dynamic studies of the ion bombardment induced surface morphological changes on Si single crystals substrates have been made using a miniature ion accelerator located in the target chamber of an SEM. 10 keV Ar + ions were employed over the fluence range 10 16–10 20 cm −2 and following incremental increases in fluence, direct observations of individual surface features were made. For high incidence angles to the normal the first features to appear were etch pits which, with increasing fluence interact to produce corrugated facet structures and ripples. Feature development is interpreted in terms of preferential sputtering at native or irradiation induced defect complexes, which initiate etch pits.

Bremsstrahlung radiators and beam filters for 25‐MeV cancer therapy

Bremsstrahlung spectra and on‐axis depth‐dose distributions due to 25.5‐MeV electrons incident on seven different Ta and one Al radiator were studied, with and without beam‐hardening filters of Al and Pb. The photon spectra were measured with a gamma‐ray spectrometer consisting of a liquid deuterium target viewed by a time‐of‐flight detector of photoneutrons. A remote‐controlled, miniature ion chamber was used to measure depth‐dose profiles in a water phantom. The Ta radiators ranged in thickness from 0.012 radiation length up to 1.0 X0. The thinner the radiator, the harder was the measured spectrum. The maximum forward yield was obtained for a thickness of about 0.3 X0. The forward yield and spectral shape of the photon beam from 0.07 X0 of Al as radiator were very similar to those from 0.06 X0 of Ta. Beams produced in Ta radiators and filtered by 27.28 g/cm2 of Al gave a harder spectrum than those filtered by 15.06 g/cm2 of Pb. Total absorption cross sections of Al and Pb for photon energies between 4 and 24 MeV were obtained which agree with theoretical calculations, within measurement errors. The depths in water at which the dose fell to 0.5 of peak dose was greater when the Al filter was used than when the Pb filter was used. For end point energies greater than 10 MeV, beam‐hardening and field‐flattening filters should be made, if space permits, of low‐atomic‐number materials.

A radiation-activated sensor

A MOSFET discriminator is described as the key circuit element for a recycling ion-chamber discharge system, the power demand of which varies with the signal (radiation) level. The radiation detector is a miniature ion chamber, compatible with the circuit battery voltage. Two discharge methods are discussed, as well as an adaptation of the recycling circuit to photoelectronic sensing.

In-Situ Metal-Gas Reaction Studies

Studies of the kinetic and microstructural aspects of the high-temperature interaction of oxygen and hydrogen with single crystalline {100} nickel films were conducted in-situ in a differentially pumped UHV specimen chamber of a transmission electron microscope. To atomically clean the specimen surface before initiating the reaction process, a miniature argon ion gun was developed and installed in the chamber, allowing sputter etching of the specimen surface under simultaneous electron microscope observation while removing surface material at a rate of up to 50 Å/min. The background pressure in the chamber was in the low 10−9 torr range. During the ion etching and metal-gas reaction processes, the total contribution of all background gases – except for tne particular reaction gas – was kept below 2×10−8 torr and recovered to the low 10−9 torr range within several minutes after closing the gas inlet valve.

A flexible fiber optics scintillation probe for intracavitary dosimetry.

In order to investigate suspected fluctuations in dose rate to the rectum and/or bladder of patients during uterocervical radium therapy, it was desired to make continuous intracavitary recordings in both locations. A number of commercially produced radiation-monitoring instruments with probes slender enough for transurethral insertion were available, but in general they are so handicapped by rigid construction and the weight of attached apparatus that their use would appear likely to introduce considerable anatomical distortion. Some in vivo measurements have also been made with the miniature Sievert type condenser ion chambers, phosphor glass microdosimeter needles, and lithium fluoride thermoluminescent detectors. In most cases such technics have yielded a fair degree of agreement with the theoretically calculated values, but since all are inherently integrating methods they were not suitable for registering the postulated variations of dose rate with time. A continuously recording ratemeter with a detecting probe of a maximum diameter of not more than a few millimeters and sufficiently light and flexible to avoid any mechanical displacement of the surrounding organs seems to be essential for adequate evaluation of this phenomenon. It was therefore decided to undertake modification of equipment already on hand in this laboratory, to permit incorporation of the features considered most important. The Curtis Nuclear Corporation's “Probitron” (Fig. 1) is a scintillation type detector-ratemeter with six-decade ranges of dose-rate sensitivity, with additional adjustments for empirical calibration and background suppression. Recorder operation is provided through a plug connector. The radiation-sensing element is a plastic phosphor, located in the aluminum-capped tip of a cylindrical stainless-steel probe approximately 17 cm long. Light excited by the incident gamma radiation is conducted via a solid Lucite “light pipe” through the central axis of the probe to an attached photomultiplier tube, where it is converted to an electrical analog signal. Connections from the photomultiplier to the main amplifier chassis, and from thence to the recorder, are by shielded coaxial cables. Three interchangeable probes were originally provided: one straight cylinder with a nominal diameter of 1/4 inch (6.4 mm); and two of smaller gauge, one straight and the other with a 45° angulated tip, each 1/8 inch (3.2 mm) in diameter. The probe to be used is secured in a socket fitting, near one end of and perpendicular to the axis of the photomultiplier housing, by means of a threaded metal collar. In the left lower foreground of Figure 1, the larger probe is shown attached ready for use, with the two smaller versions lying on either side of it. It is apparently intended that the photomultiplier housing, which has a diameter of slightly more than 5 cm and a length of 18 cm, be held in a sort of pistol grip technic while measurements are made.

COMPARATIVE EFFECTS OF 50 KVP AND 250 KVP X-RAYS ON THE DOG.

The subject of relative biological effectiveness of various ionizing radiations has been difficult and vexing to handle meaningfully when tissue distribution of dose is not uniform. It has been suggested by some that problems relating to linear energy transfer in tissues should appropriately be divided into two components, that relating to macroscopic energy distribution and that relating to energy distribution in terms of individual ionizing events. The former effect was evaluated by looking at the biological potency of a lower energy x-ray source (50 Kvp) in the dog. Previously it was shown that no single dose parameter is adequate to express the biological potency of lower energy x rays, although it was suggested that, to the extent that it was possible to measure it, the dose to the critical organ, usually bone marrow, would be the most significant. Further studies were completed on dogs exposed to high doses of x radiation from a 50 Kvp beryllium window generator. Doses of 4000 to 10.000 rad (air) at the potential midline of the subject were shown to be as effective as doses of 200 to 300 rad of 250 Kvp x rays. Hematological comparisons of the two radiations show close correlation withmore » mortality and a relative potency factor of 1/30 for the lower energy radiation. The lethal dose of 50 Kvp x rays for the dog is 7500 r (air). Serious lesions of the skin were seen as a complicating factor at all doses in excess of 4000 r (air). Dosimetry was done in tissue equivalent wax phantoms using the rotational exposure method. Phantom depth dose measurements with miniature ion chambers and chemical dosimetry in agar gels yielded essentially identical values of 3% for midline tissue dose. (auth)« less