Influence of Droplets with Net Charge in Inductively Coupled Plasma Atomic Emission Spectroscopy and Implications for the Easily Ionizable Element Chemical Matrix Effect

Abstract

In the generation of an aerosol by pneumatic nebulization, concurrent with the production of a distribution of droplet sizes is a second distribution of net charge on the droplets. The influence that the droplets with net charge exert, spatially, on Ca(II) emission in the inductively coupled plasma (ICP) has been examined in the absence and presence of NaCl, an easily ionized element chemical matrix. Tertiary aerosol droplet size distributions were also measured. A mesh positioned in the spray chamber was used to establish an electric field between it and the grounded solution being nebulized. A 200 V dc potential applied to the mesh discriminated against the transport of droplets with net charge to the plasma. For samples that contained 100 mM NaCl, the full width at half-maximum of the Ca(II) emission profile viewed at 10 mm above the load coil was significantly reduced with 200 V applied to the mesh. The source of off-axis analyte emission enhancement low in the plasma in the presence of a NaCl matrix has been attributed to desolvation of the droplets with net charge beyond the coulomb fission limit, causing progeny droplets to be created within the aerosol. Droplets with net charge comprise ∼ 50% of the aerosols studied in this work, and these droplets do have a role in determining the analyte emission profiles in the inductively coupled plasma.

The role of the nebulizer on the sodium interferent effects in inductively coupled plasma atomic emission spectrometry

“…Firstly, the presence of charged species in an aerosol droplet produces repulsion forces that can result in a droplet break-up, thus giving rise to finer droplets. 33,34 This fact could explain the lower tD 3,2 provided by the sodium chloride solutions. Secondly, it must be considered that evaporation of pure water aerosols is more severe than sodium chloride aerosols.…”

Improved Monochromatic Imaging Spectrometer

“…For example, it has long been known that sources for atomic spectroscopy-including flames, inductively coupled plasmas, microwave plasmas, and glow discharges-exhibit such heterogeneity. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] A number of approaches have been used to obtain spatial resolution with optical probes. Point-by-point measurements can be made with relatively simple, sensitive, and inexpensive instrumentation.…”

“…It has been used to examine inductively coupled plasmas (ICP) [8][9][10][11][12][14][15][16][17][18] and glow discharges. 13 It has been used both to study the fundamental properties and processes of plasmas 8,10,11,13,14,[16][17][18] and to optimize the performance of plasmas. 9,12,15 In the MIS described by Olesik and Hieftje, 19 displayed schematically in Fig.…”

Use of Laser Light Scatter Signals to Study the Effect of a Direct-Current Biased Mesh Screen in a Spray Chamber on Aerosols Generated for Use in Atomic Spectroscopy

“…The ability to effect changes in the droplet size distribution in tertiary aerosols using an electric ® eld located inside a spray chamber has been reported. 1 An electric ® eld between a direct-current biased mesh screen inserted into the spray chamber and the sample solution at ground potential was used to adversely affect the trajectories of droplets with net charge, effecting their rem oval from the secondar y aerosol stream. The origin of droplets with net positive and negative charge are from statistical ion pop-ulation¯uctuations and/or surface effects during the instant of droplet form ation in a pneumatic nebulizer.…”

“…4 ±6 Introduction of a NaCl chemical matrix to a solution being nebulized causes an increase in the m agnitude of the net charge on the droplets and an increase in the size distribution of droplets that carry net charge in the aerosol. 1 Each droplet with net charge can lead to enhancement of analyte emission, off-axis, low in the plasma by way of evaporating to the Coulom b ® ssion limit, and fragm enting, 7 introducing sm all progeny droplets into the aerosol stream. 8 The Coulom b ® ssion limit is the minimum droplet size that can contain a speci® ed net charge, and details of this phenomenon are provided in the Discussion section of this report.…”