Joachim Franzke

Scaling and the design of miniaturized chemical-analysis systems.

Micrometre-scale analytical devices are more attractive than their macroscale counterparts for various reasons. For example, they use smaller volumes of reagents and are therefore cheaper, quicker and less hazardous to use, and more environmentally appealing. Scaling laws compare the relative performance of a system as the dimensions of the system change, and can predict the operational success of miniaturized chemical separation, reaction and detection devices before they are fabricated. Some devices designed using basic principles of scaling are now commercially available, and opportunities for miniaturizing new and challenging analytical systems continue to arise.

The dielectric barrier discharge — a powerful microchip plasma for diode laser spectrometry

The dielectric barrier discharge plasma is presented as a powerful microchip source for analytical spectrometry. The dielectric barrier discharge is characterized by small size, low electric power (<1 W), low gas temperature (approx. 600 K) and excellent dissociation capability for molecular species, such as CCl2F2, CClF3 and CHClF2. It has been used here in the plasma modulation diode laser absorption spectrometry of excited chlorine and fluorine in noble gases as well as in air/noble gas mixtures. The analytical figures of merit of diode laser absorption spectrometry obtained with the dielectric barrier discharge are comparable with the results found earlier with dc and microwave induced plasmas of larger size with much higher plasma powers. Detection limits of 400 ppt and 2 ppb for CCl2F2 in He were found using the Cl 837 nm and the F 685 nm line, respectively.

Dielectric barrier discharge ionization for liquid chromatography/mass spectrometry.

An atmospheric pressure microplasma ionization source based on a dielectric barrier discharge with a helium plasma cone outside the electrode region has been developed for liquid chromatography/mass spectrometry (LC/MS). For this purpose, the plasma was realized in a commercial atmospheric pressure ionization source. Dielectric barrier discharge ionization (DBDI) was compared to conventional electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), and atmospheric pressure photoionization (APPI) in the positive ionization mode. Therefore, a heterogeneous compound library was investigated that covered polar compounds such as amino acids, water-soluble vitamins, and nonpolar compounds like polycyclic aromatic hydrocarbons and functionalized hydrocarbons. It turned out that DBDI can be regarded as a soft ionization technique characterized by only minor fragmentation similar to APCI. Mainly protonated molecules were detected. Additionally, molecular ions were observed for polycyclic aromatic hydrocarbons and derivatives thereof. During DBDI, adduct formation with acetonitrile occurred. For aromatic compounds, addition of one to four oxygen atoms and to a smaller extend one nitrogen and oxygen was observed which delivered insight into the complexity of the ionization processes. In general, compounds covering a wider range of polarities can be ionized by DBDI than by ESI. Furthermore, limits of detection compared to APCI are in most cases equal or even better.

Microplasmas for analytical spectrometry

Recent developments of miniaturized powerful and robust plasmas for analytical applications are reviewed. The plasmas described and discussed may be used for analyte detection in chromatographic or electrophoretic “lab-on-a-chip” systems.

Plasmas for lab-on-the-chip applications

Abstract Two small-size plasmas used as detectors of halogenated hydrocarbons and suitable for miniaturized instrumentation are discussed. A reduced pressure dielectric barrier discharge was integrated in a diode laser atomic absorption spectrometer and the already reported chlorine detection limits of 5 ppm (v/v) can be improved with one order of magnitude by spatially resolved measurements. A microstructured electrode discharge at atmospheric pressure was coupled with a miniaturized Echelle spectrometer and detection limits were found to be 20 ppb for chlorine as well as for fluorine.

The dielectric barrier discharge as a detector for gas chromatography

Abstract The capability of the small-sized dielectric barrier discharge as an element-selective diode laser atomic absorption detector for gas chromatography is investigated. Detection limits have been determined for halogenated and sulfured hydrocarbons in the low to the high pg/s-range dependent on the element measured. Furthermore, the effect of doping gas (oxygen) and make-up gas (argon, helium) on the chromatograms is studied.

Diode laser-aided diagnostics of a low-pressure dielectric barrier discharge applied in element-selective detection of molecular species

Abstract A small, low-pressure dielectric barrier discharge used as a detector for the analysis of halogenated hydrocarbons was studied by diode laser absorption spectroscopy of excited plasma atoms. The distribution, as well as diffusion of the excited atoms, was measured with high spatial and temporal resolution. The major part of the excited atoms was found in a very narrow discharge volume, where the maximum gas temperature and electron density, determined from broadening of the absorption line profiles, were approximately 1000 K and greater than 10 15 cm −3 , respectively.

Diagnostics and application of the microhollow cathode discharge as an analytical plasma

This paper focuses on the diagnostics and applications of the microhollow cathode discharge operated in Ar and He at high pressures. The gas temperature and electron number density in Ar and He are deduced from absorption and emission spectroscopy employing line profile analysis. Additionally, a collisional radiative model is used for the estimation of the electron number density in He. The gas temperature increases in Ar from 500 K at 100 mbar to 2000 K at 1000 mbar. The electron density also increases from 2 × 1015 to 9 × 1015 cm−3 in the same pressure range. In He, the gas temperature reaches values up to 800 K and the electron density does not exceed 5 × 1014 cm−3 at atmospheric pressure. The discharge was coupled with emission and mass spectrometry for analytical applications. The detection of organometallic compounds (ferrocene) reveals good detection limits of about 500 ppb for Fe.

Micro-plasma: a novel ionisation source for ion mobility spectrometry

AbstractIon mobility spectrometry is an analytical method for identification and quantification of gas-phase analytes in the ppbv-pptv range. Traditional ionisation methods suffer from low sensitivity (UV light), lack of long-term stability (partial discharge), or legal restrictions when radioactive sources are used. A miniaturised helium plasma was applied as ionisation source in an ion mobility spectrometer (IMS). Experiments were carried out to compare plasma IMS with β-radiation IMS. It could be demonstrated that the plasma IMS is characterised by higher sensitivity and selectivity than β-radiation ionisation. Plasma IMS is approximately 100 times more sensitive than the β-radiation IMS. Furthermore, variable sensitivity can be achieved by variation of the helium flow and the electric field of the plasma, and variable selectivity can be achieved by changing the electric field of the IMS. The experimental arrangement, optimisation of relevant conditions, and a typical application are presented in detail. FigureMicro-plasma used in ion mobility spectrometry

Ambient diode laser desorption dielectric barrier discharge ionization mass spectrometry of nonvolatile chemicals.

In this work, the combined use of desorption by a continuous wave near-infrared diode laser and ionization by a dielectric barrier discharge-based probe (laser desorption dielectric barrier discharge ionization mass spectrometry (LD-DBDI-MS)) is presented as an ambient ionization method for the mass spectrometric detection of nonvolatile chemicals on surfaces. A separation of desorption and ionization processes could be verified. The use of the diode laser is motivated by its low cost, ease of use, and small size. To achieve an efficient desorption, the glass substrates are coated at the back side with a black point (target point, where the sample is deposited) in order to absorb the energy offered by the diode laser radiation. Subsequent ionization is accomplished by a helium plasmajet generated in the dielectric barrier discharge source. Examples on the application of this approach are shown in both positive and negative ionization modes. A wide variety of multiclass species with low vapor pressure were tested including pesticides, pharmaceuticals and explosives (reserpine, roxithromycin, propazine, prochloraz, spinosad, ampicillin, dicloxacillin, enrofloxacin, tetracycline, oxytetracycline, erythromycin, spinosad, cyclo-1,3,5,7-tetramethylene tetranitrate (HMX), and cyclo-1,3,5-trimethylene trinitramine (RDX)). A comparative evaluation revealed that the use of the laser is advantageous, compared to just heating the substrate surface.