Kevin P. Pfeuffer

Visual observations of an atmospheric-pressure solution-cathode glow discharge

The solution-cathode glow discharge (SCGD) is an optical emission source for atomic spectrometry comprised of a moderate-power atmospheric-pressure DC glow discharge sustained directly upon the surface of an electrically conductive solution. The SCGD boasts a simple, inexpensive design and has demonstrated detection limits similar to those of more conventional excitation sources used in atomic spectrometry. Although the analytical performance of the SCGD as an optical emission source is well characterized, the mechanism through which the discharge atomizes and excites analyte from the sample solution remains a point of debate. The current paper presents visual observations of the SCGD from a variety of imaging techniques. The implications of the images regarding the mechanism of analyte solution-to-plasma transport and excitation in the SCGD are discussed.

Visualization of mass transport and heat transfer in the FAPA ambient ionization source

Ambient desorption/ionization mass-spectrometry (ADI-MS) has shown tremendous potential for the direct analysis of materials in the open atmosphere. Unfortunately, processes governing analyte desorption and transport into the mass spectrometer, which ultimately limit sampling reproducibility and quantification, have not been investigated for most ADI-MS sources. For plasma-based ADI-MS sources, such studies are further complicated because the discharge support gas is optically transparent. Here, two methods were employed to probe these otherwise invisible phenomena. Specifically, schlieren imaging and infrared (IR) thermography were utilized to visualize plasma-gas flow and heat transfer, respectively, from a pin-to-capillary geometry flowing atmospheric-pressure afterglow (FAPA) ambient ionization source. The influence of operating conditions was investigated, including plasma-gas flow rate and source capillary diameter. Interactions of the desorption/ionization beam with a sample probe or a sample surface before it is captured by a mock mass spectrometer interface were also explored. These experiments showed that schlieren imaging is a viable means for visualization of the plasma transport gas, and that coupling with IR thermography yields information on gas and temperature distributions. Schlieren images revealed that the existence of a discharge can alter by up to 8 cm the location where the flowing afterglow transitions from laminar to turbulent flow. Addition of a sample-introduction probe into the plasma gas perturbed the width of the gas beam by 1.5 cm. Additionally, helium impinging on a surface expanded rapidly, unless an interface was present to capture the gas, while local heating was confined to a small area (<1 cm2), based on 75% of the maximum temperature, compared to background.

Halo-shaped flowing atmospheric pressure afterglow: a heavenly design for simplified sample introduction and improved ionization in ambient mass spectrometry.

The flowing atmospheric-pressure afterglow (FAPA) is a promising new source for atmospheric-pressure, ambient desorption/ionization mass spectrometry. However, problems exist with reproducible sample introduction into the FAPA source. To overcome this limitation, a new FAPA geometry has been developed in which concentric tubular electrodes are utilized to form a halo-shaped discharge; this geometry has been termed the halo-FAPA or h-FAPA. With this new geometry, it is still possible to achieve direct desorption and ionization from a surface; however, sample introduction through the inner capillary is also possible and improves interaction between the sample material (solution, vapor, or aerosol) and the plasma to promote desorption and ionization. The h-FAPA operates with a helium gas flow of 0.60 L/min outer, 0.30 L/min inner, and applied current of 30 mA at 200 V for 6 W of power. In addition, separation of the discharge proper and sample material prevents perturbations to the plasma. Optical-emission characterization and gas rotational temperatures reveal that the temperature of the discharge is not significantly affected (<3% change at 450 K) by water vapor during solution-aerosol sample introduction. The primary mass-spectral background species are protonated water clusters, and the primary analyte ions are protonated molecular ions (M + H(+)). Flexibility of the new ambient sampling source is demonstrated by coupling it with a laser ablation unit, a concentric nebulizer, and a droplet-on-demand system for sample introduction. A novel arrangement is also presented in which the central channel of the h-FAPA is used as the inlet to a mass spectrometer.

Effect of internal and external conditions on ionization processes in the FAPA ambient desorption/ionization source.

Ambient desorption/ionization (ADI) sources coupled to mass spectrometry (MS) offer outstanding analytical features: direct analysis of real samples without sample pretreatment, combined with the selectivity and sensitivity of MS. Since ADI sources typically work in the open atmosphere, ambient conditions can affect the desorption and ionization processes. Here, the effects of internal source parameters and ambient humidity on the ionization processes of the flowing atmospheric pressure afterglow (FAPA) source are investigated. The interaction of reagent ions with a range of analytes is studied in terms of sensitivity and based upon the processes that occur in the ionization reactions. The results show that internal parameters which lead to higher gas temperatures afforded higher sensitivities, although fragmentation is also affected. In the case of humidity, only extremely dry conditions led to higher sensitivities, while fragmentation remained unaffected.

Chapter 7:Flowing Atmospheric-Pressure Afterglow (FAPA), the Plasma-based Source for your ADI-MS Needs

Plasma-based ambient mass spectrometry sources are a very promising group of sources that, despite having a similar basic mechanism, all utilize very different geometries and discharge types to achieve desorption and ionization. A particularly promising source is the flowing atmospheric-pressure afterglow (FAPA) that has been developed within the Hieftje lab at Indiana University. In this chapter we explore the development, characterization and applications of the FAPA source. Fundamental examinations into reagent formation and optical-emission characterization provide insight into matrix effects and the He discharge itself. Schlieren imaging is also used to better understand ambient mass transport. A wide variety of FAPA practices are also presented, including the successful coupling of gas chromatography, capillary electrophoresis, laser ablation and a droplet-based sample-introduction system. Applications for detection of pesticides, explosives, drugs and atomic species are shown as well; additionally, chemometric methods coupled with the FAPA source successfully identified polymer types and counterfeit electronic components.