Lisa K. Olson

Chromium speciation by anion-exchange high-performance liquid chromatography with both inductively coupled plasma atomic emission spectroscopic and inductively coupled plasma mass spectrometric detection

Development of a new method for the determination of Cr(III) and Cr(VI) is described. Anion-exchange high-performance liquid chromatography (HPLC) was used to separate Cr(III) and Cr(VI) with on-line detection by inductively coupled plasma atomic emission spectroscopy (ICP-AES) at 2766 A in preliminary studies, and inductively coupled plasma mass spectrometry (ICP-MS) with single-ion monitoring at m/z 52 and m/z 53 for final work. A mobile phase consisting of ammonium sulfate and ammonium hydroxide was used, and a simple chelation procedure with EDTA was followed to stabilize the Cr(III) species in standard solutions. ICP-MS results indicated the feasibility of using chromium isotope m/z 53 instead of the more abundant m/z 52 isotope due to a high mobile-phase background most significantly from the SO+ polyatomic interference. The absolute detection limits based on peak-height calculations were 40 pg for Cr(III) and 100 pg for Cr(VI) in aqueous media by HPLC-ICP-MS. The linear dynamic range extended from 5 ppb (ng/ml) to 1 ppm (micrograms/ml) for both species. By HPLC-ICP-AES, detection limits were 100 ng for Cr(III) and 200 ng for Cr(VI). Cr(III) was detected in NIST-SRM 1643c (National Institute of Standards and Technology-Standard Reference Material, Trace Elements in Water) by HPLC-ICP-MS at the 20 ppb level.

Radiofrequency glow discharge mass spectrometry for gas chromatographic detection: a new departure for elemental speciation studies

An rf glow discharge source operating at 30 W power and about 0.6 mbar pressure was utilized as a detector for tetraalkyltins separated by gas chromatography. These were the optimized power and pressure values. Detection levels of about 1 pg were comparable to some of the lowest reported literature values, even with a non-vented chromatographic experiment. NIST SRM 2715 Lead in Reference Fuel was used to test the analytical capabilities of the technique with good results. In an exciting new departure for elemental speciation studies, structural information can be obtained by scanning the mass spectrum at each chromatographic peak. Fragment information was also available from the SRM.

Radiofrequency Glow Discharge as an Ion Source for Gas Chromatography With Mass Spectrometric Detection

A radiofrequency (rf) powered glow discharge (GD) has been used as an ion source for GC–MS. The improved GC–GD interface has allowed separation and detection of five organotin compounds with better figures of merit than previously reported. Limits of detection for four compounds were from 0.6 to 13 picogram (as tin). Optimization of the discharge pressure and sampling distance with respect to the primary analytical species of interest, elemental tin, has been performed. Characterization of the source has been made by studying the effect of the cell pressure, applied rf power and sampling distance on analyte signal and organotin fragmentation patterns. The results obtained indicate that the mass spectra generated do not differ significantly with the varying source pressure and rf power as they exhibit the same features in terms of parent-molecular peaks and fragment ions.

Feasibility study of low pressure inductively coupled plasma mass spectrometry for qualitative and quantitative speciation

Low-pressure ICPs (LP–ICPs) formed with helium have been utilized as ion sources for MS. In the first part of the paper a description is given of a plasma operated at 90 W forward power, used to ionize organotin and organolead compounds introduced by GC, with detection limits of 11, 2 and 14 pg obtained for tetraethyltin (TEtSn), tetrabutyltin (TBuSn) and tetraethyllead (TEtPb), respectively. The same plasma operated at 45 W forward power yielded fragment ions of TEtSn and TBuSn. In the second part, a customized mass spectrometer for use with an LP–ICP is described. The optimum power for the generation of fragment ions of perfluorotributylamine (PFTBA) was found to lie between 5 and 8 W. Two optima for skimming distance were observed, at 6 and 8 mm downstream of the sampler orifice. Mean ion kinetic energies for fragment ions of PFTBA at 69,219 and 502 m/z were 1.2, 1.7, and 2.1 eV, respectively, at 6 W forward power, but increased when the power was increased to 8 W. Molecular and other fragment ions were observed for chloro-, iodo- and dibromobenzenes introduced using GC.

Elemental speciation for chromium in chromium picolinate products

Chromium picolinate products have been examined for different forms of chromium, using chromatographic separation and inductively coupled plasma mass spectrometric detection. The brands we evaluated contained no detectable amount of elemental chromium(VI), the toxic form. Since chromium picolinate might have other chromium forms as impurities, different products may contain different forms of chromium species. Compared with ion-exchange, reversed-phase chromatography showed excellent chromium recovery based on the amount stated on the product label.

Determination of halogenated compounds with supercritical fluid chromatography–microwave-induced plasma mass spectrometry

Helium microwave-induced plasma mass spectrometry (MIP-MS) has been evaluated as an element selective detector for supercritical fluid chromatography (SFC). In this study the determination of halogenated hydrocarbons at trace levels is demonstrated by using two test compounds, 1-chloronaphthalene and 1-bromo-2-methylnaphthalene. The SFC–MIP-MS interface was developed and optimized. Chromatograms obtained for the two halogenated compounds show detection levels in the low picogram range with absolute minimum detectable quantities of 100 and 25 pg for Cl and Br, respectively. Absolute detection limits were found to be 15 pg for Cl and 0.75 pg for Br. The linear ranges are over 3 orders of magnitude (50 pg–50 ng) and reproducibilities for the sample injections are of the order of 5% relative standard deviation. A preliminary chromatogram for a standard pesticide mixture is also presented.

Hydride generation, electrothermal vaporization and liquid chromatography as sample introduction techniques for inductively coupled plasma mass spectrometry☆

Although the most common method of sample introduction for inductively coupled plasma-mass spectrometry (ICP-MS) used is pneumatic nebulization, it suffers from limitations particularly with respect to the transport efficiency. In an effort to improve the analyte transport to the plasma, many alternative techniques have been studied, including electrothermal vaporization (ETV) and hydride generation, among others. Flow injection and liquid chromatography, although they employ pneumatic nebulization, have also been studied as sample introduction techniques due to the ability to analyze small volume samples and obtain speciation information. Here, some advances in sample introduction for ICP-MS will be discussed including hydride generation and ETV techniques. In addition, recent studies in HPLC-ICP-MS will also be described. Advantages and disadvantages of each method will be reviewed together with the application to some representative samples.

Comparison of electrospray and inductively coupled plasma sources for elemental analysis with mass spectrometric detection

Both the qualitative and quantitative aspects of electrospray (ES) and ICP sources were investigated using the same mass spectrometer (originally a VG PlasmaQuad I instrument). While it is well established that ICP-MS is a powerful tool for elemental analysis, electrospray ES-MS has recently become popular as a promising elemental analysis technique that can complement the detection capabilities of ICP-MS. The in-house constructed ES source efficiently produced the bare singly charged metal ion and the optimum ES source conditions varied depending on the charge reduction required to attain the singly charged state. The day-to-day signal reproducibility was excellent (5% RSD), and the removal of the photon stop increased ion transport to the detector. A comparison of the figures of merit for Rb, Cs, Ba, V, Cr, Ni, Co, Cu, Zn and U showed that the detection limits obtained by ES-MS (ng ml–1) are only 2–3 orders of magnitude higher than those found using the ICP source. Additionally, Ca was determined in NIST SRM 1643c (Trace Elements in Water) by ES-MS.

Utilization of metallic platforms in electrothermal vaporization inductively coupled plasma mass spectrometry

A method to improve the analytical performance of hydride trapping on Pd inside a graphite furnace and subsequent determination by ICP-MS was investigated. Strips of tungsten, tantalum, molybdenum and rhenium were coated (electroplated/sputtered) with palladium and used as platforms inside the graphite furnace. A typical pyrolytic graphite-coated graphite platform was also coated with Pd in the same manner, for performance comparisons. Scanning electron microscopy (SEM) and X-ray fluorescence spectrometry were used to characterize the surfaces. SEM showed a smoother Pd layer covering the metallic substrates when compared with the graphite. An analyte signal reproducibility study revealed that Ta, Re and Mo were not ideal substrates for this purpose. The As peaks obtained from trapping on the graphite platform were broadened with a decrease in intensity after a few firings. The Pd-sputtered tungsten platform showed superior performance over the other platforms. The linear dynamic range obtained using this platform was improved by one order of magnitude in the upper limit (from 1 to 100 ng ml–1) over that reported using a graphite platform in previous work. The limit of detection of arsenic was found to be 0.01 ng ml–1, within the same range obtained from the graphite platform.

Reduced-pressure microwave-induced plasma mass spectrometric detection of phosphorus and sulphur in gas chromatographic eluates

The determination of phosphorus and sulphur at trace levels is not possible by conventional atmospheric pressure plasma mass spectrometry because of high background signals at m/z= 31 (14N16OH+) and m/z= 32 (16O2+). The reduced-pressure plasmas decrease the background at the major isotopes of phosphorus and sulphur such that sub-nanogram detection is possible. The use of reduced-pressure helium and nitrogen microwave-induced plasmas for the determination of phosphorus and sulphur is discussed. With a helium plasma, the detection limit for phosphorus in triethyl phosphite ranges from 1 to 90 ng depending on the amount of torch cooling. Using a nitrogen plasma, sub-nanogram detection limits for phosphorus and sulphur in two organophosphorus pesticides were obtained.