Kaveh Kahen

Direct injection high efficiency nebulizer-inductively coupled plasma mass spectrometry for analysis of petroleum samples

Abstract Direct injection high efficiency nebulizer (DIHEN)-inductively coupled plasma mass spectrometry (ICPMS) is investigated for analysis of petroleum samples dissolved in volatile organic solvents. To minimize solvent loading, the solution uptake rate is reduced to 10 μl/min, far less than the level (85 μl/min) commonly used for aqueous sample introduction with the DIHEN, and oxygen is added to the nebulizer gas flow and outer flow of the ICP. Factorial design is applied to investigate the effect of nebulizer tip position within the torch and the nebulizer and intermediate gas flow rates on the precision and the net signal intensity of the elements tested for multielemental analysis. Cluster analysis and principal component analysis are performed to distinguish the behavior of different isotopes, oxide species and doubly charged ions. The best operating conditions at a solution uptake rate of 10 μl/min are: RF power=1500 W, nebulizer gas flow rate=0.10–0.12 l/min, intermediate gas flow rate=1.5 l/min and DIHEN tip position=3–4 mm below the top of the torch intermediate tube. Acceptable recoveries (100±10%) and good precision (less than 3% relative standard deviation) are obtained for trace elemental analysis in organic matrices (a certified gas oil sample and a custom-made certified reference material) using flow injection analysis. Because of high blank levels, detection limits are 1–3 orders of magnitude higher for organic sample introduction than those acquired for aqueous solutions.

Nano-HPLC-inductively coupled plasma mass spectrometry for arsenic speciation

A nano-high performance liquid chromatography-inductively coupled plasma mass spectrometry (nano-HPLC-ICPMS) method is developed, using a demountable direct injection high efficiency nebulizer (d-DIHEN), to reduce sample and mobile phase consumption, minimize organic waste generation, decrease analysis time, and enhance separation efficiency. A HPLC column (50 mm × 0.3 mm id), packed with 3.5 μm C18 material, is explored for chromatographic separation of five arsenic species naturally present in the environment or introduced as a pollutant: sodium (meta)arsenite [As(III)], arsenic acid [As(V)], dimethylarsenic acid (DMA), disodium methylarsenate (MA), and p-arsanilic acid (p-ASA). A fast chromatographic separation of five arsenic species is achieved in less than 12 min at a solution flow rate of 0.9 μL min−1 using a 50 nL sample injection. The HPLC-ICPMS interface provides well defined flow injection profiles at various concentrations, giving a correlation coefficient of 0.999 for each individual arsenic species calibration curve. Precision values for peak height and area of five arsenic species range from 0.5 to 6.5% RSD and absolute detection limits are within 0.4 to 5.4 pg arsenic, which are comparable to previously reported data at higher solution uptake rates (20 μL min−1 to 1 mL min−1) and larger sample injection volumes (20–100 μL).

Investigation of the direct injection high efficiency nebulizer for axially and radially viewed inductively coupled plasma atomic emission spectrometry

A direct injection high efficiency nebulizer (DIHEN) is explored for introduction of the sample aerosol into the central channel of the axially and radially viewed inductively coupled plasma (ICP) of a commercial ICP atomic emission spectrometer (ICPAES). The DIHEN is a micro-nebulizer that requires very low solution uptake rates (1–100 µL min−1) and nebulizer gas flow rates (<0.2 L min−1) compared to conventional nebulizer-spray chamber arrangements (∼1.0 mL min−1 and ∼1.0 L min−1, respectively). Signal-to-background ratios (SBRs), detection limits, and precision of the DIHEN are comparable or superior to those of the conventional sample introduction system, but the Mg II 280.270/Mg I 285.213 nm ratios are lower with the DIHEN, indicating that the DIHEN is more susceptible to matrix effects than the conventional nebulization system, for both the axial and radial ICPAES systems. Matrix effects are further investigated by comparing intensity ratios with and without 0.1% and 0.5% Na for several spectral lines having energy sum ranging from 7.93 to 14.79 eV. Replacement of Ar with Ar–O2 and Ar–N2 mixtures in the outer gas flow of the plasma improves SBRs and Mg II 280.270/Mg I 285.213 nm ratios of the DIHEN, and reduces matrix effects. By reducing solution uptake rate from 60 to 30 µL min−1, matrix effect is also reduced. Operation of the radial instrument at 1700 W reduces matrix effect compared to the effect noted for the axial instrument at 1500 W. Finally, the utility of the technique in practical ICPAES studies is demonstrated using a custom made organo-metallic standard for As, Hg, and Pb in xylene.

Modified Nukiyama–Tanasawa and Rizk–Lefebvre models to predict droplet size for microconcentric nebulizers with aqueous and organic solvents

Aerosols characteristic of several organic solvents (hexane, acetone, xylene, toluene, methanol, and ethanol) produced by a direct injection high efficiency nebulizer (DIHEN) are measured using a phase Doppler particle analyzer (PDPA) over a wide range of operating conditions (nebulizer gas = 0.2–1.0 L min−1, solvent flow rate = 10–500 μL min−1). The Sauter mean diameter, defined as the volume-to-surface area ratio (D3,2), and cumulative count percent of organic aerosol are measured and compared with those of aqueous droplets. These parameters are chosen because the performance of inductively coupled plasma (ICP) as an excitation and ionization source is affected by the size of the introduced droplets, particularly in the case of direct injection nebulizers where the primary aerosol is directly introduced to the plasma without being filtered by the spray chamber. The size distribution of the droplets demonstrates a notable shift toward smaller droplets, and is generally narrower when organic solvents are used instead of water. This effect is more pronounced for hexane and acetone, having a considerably lower surface tension and viscosity, respectively, compared with water. A D3,2 of 4.6 μm is obtained for hexane, compared with 7.2 μm for aqueous solutions at a nebulizer gas flow rate of 0.2 L min−1 and a solution uptake rate of 50 μL min−1. This decrease in droplet size is less significant for ethanol, methanol, toluene and xylene. Experimental results are also compared to D3,2 values calculated by the Nukiyama–Tanasawa (N–T) equation and Rizk–Lefebvre (R–L) model. While the cited models correctly predict the trend in size variation as a function of nebulizer gas flow rate, and to some extent solvent characteristics, an overestimation and an underestimation of D3,2 is observed for all tested solvents at low nebulizer gas flow rates for the N–T model and the R–L model, respectively. Modified equations are proposed which are capable of predicting D3,2 values for several solvents with greater accuracy.

Plasma-Assisted Reaction Chemical Ionization for Elemental Mass Spectrometry of Organohalogens

AbstractWe present plasma-assisted reaction chemical ionization (PARCI) for elemental analysis of halogens in organic compounds. Organohalogens are broken down to simple halogen-containing molecules (e.g., HBr) in a helium microwave-induced plasma followed by negative mode chemical ionization (CI) in the afterglow region. The reagent ions for CI originate from penning ionization of gases (e.g., N2) introduced into the afterglow region. The performance of PARCI-mass spectrometry (MS) is evaluated using flow injection analyses of organobromines, demonstrating 5–8 times better sensitivities compared with inductively coupled plasma MS. We show that compound-dependent sensitivities in PARCI-MS mainly arise from sample introduction biases. Figureᅟ

Gas Chromatography Plasma-Assisted Reaction Chemical Ionization Mass Spectrometry for Quantitative Detection of Bromine in Organic Compounds

We have recently introduced plasma-assisted reaction chemical ionization mass spectrometry (PARCI-MS) for elemental analysis of halogens in organic compounds. Here, we utilize gas chromatography (GC) coupled to PARCI-MS to investigate the mechanism of Br(-) ion generation from organobromines and to evaluate analytical performance of PARCI for organobromine analysis. Bromine atoms in compounds eluting from GC are converted to HBr in a low-pressure microwave induced helium plasma with trace amounts of hydrogen added as a reaction gas. Ionization is achieved by introducing nitrogen into the afterglow region of the plasma, liberating electrons via penning ionization and leading to formation of negative ions. We demonstrate that N2 largely affects the ionization process, whereas H2 affects both the ionization process and in-plasma reactions. Our investigations also suggest that dissociative electron capture is the main ionization route for formation of Br(-) ions. Importantly, GC-PARCI-MS shows a uniform response factor for bromine across brominated compounds of drastically different chemical structures, confirming PARCIs ability to quantify organobromines in the absence of compound-specific standards. Over 3 orders of magnitude linear dynamic range is demonstrated for bromine quantification. We report a detection limit of 29 fg of bromine on-column, ~4-fold better than inductively coupled plasma-MS.

Desolvation-induced non-linearity in the analysis of bromine using an ultrasonic nebulizer with membrane desolvation and inductively coupled plasma mass spectrometry

Ultrasonic nebulization coupled with a membrane desolvator is used for the introduction of bromine containing samples in inductively coupled plasma mass spectrometry. Severe non-linearity (R2 = 0.9317) is observed when ammonium bromide (NH4Br, 0–100 ng mL−1) solution is nebulized, suggesting the loss of bromide ions at the desolvation stage. It is proposed that the primary reason for this phenomenon is the formation of gaseous HBr and its subsequent removal in the desolvation unit. In order to preserve the linearity in the calibration curve, excess NaCl (1 μg mL−1 of Na) is added to the solutions, resulting in a perfectly linear (R2 = 0.9999) calibration curve. However, in the presence of a strong acid (2% HNO3), no significant improvement in the linearity is observed with the addition of NaCl. Bromine signal enhancement and improved linearity are also observed when 1 μg mL−1 of Na is added to KBr solutions. Also, addition of 2% HNO3 to KBr solutions results in signal suppression through the formation of HBr.