Wei-Lung Shen

Elimination of the argon chloride interference on arsenic speciation in inductively coupled plasma mass spectrometry using ion chromatography

Ion chromatography has been combined with inductively coupled plasma mass spectrometry for the speciation of arsenic compounds commonly found in urine. Ion chromatography was used to eliminate or reduce the mass spectral interference formed from chloride, namely argon chloride, by resolving chloride chromatographically from the arsenic compounds. A 20-fold dilution of the urine samples was necessary in order to avoid column overloading from chloride and subsequent argon chloride interference. Detection limits in urine of 3.4 p.p.b. AsIII, 4.2 p.p.b. AsV and 7 p.p.b. dimethylarsinic acid were obtained. Three commercially available freeze-dried urine standards were analysed using the standard additions method. Good agreement was obtained for total arsenic content, which was calculated from the sum of the species with accepted arsenic concentration. The relative standard deviation of peak height for each of the species was approximately 5% in urine.

Electrothermal vaporisation interface for sample introduction in inductively coupled plasma mass spectrometry

An electrothermal vaporisation (ETV) device was developed for inductively coupled plasma mass spectrometry by modifying a commercial graphite furnace unit. A detailed description of the modifications are presented. The new ETV interface is designed to increase sample transport efficiency by converting analyte vapour into micro-particles in the furnace. The characterisation and optimisation of the device are also discussed. Ten consecutive firings of a 30-pg sample of Pb resulted in relative standard deviations (RSD) of 4% for peak-height and 1.6% for peak-area measurements. The absolute detection limit, determined by comparing the response obtained for a 3-µl sample of a 10 ng ml–1 Pb solution in 1% HNO3 with that obtained for blanks containing 1% HNO3 only, was 10 and 14 fg, for a dwell time of 10 µs, for peak area and peak height, respectively. The linear dynamic range for Pb extends over three orders of magnitude, from 300 fg to 300 pg.

Helium microwave induced plasma mass spectrometry for detection of metals and nonmetals in aqueous solutions

Abstract A helium microwave induced plasma is evaluated as an ion source for plasma mass spectrometry. Analyte position studies were performed to assess background interferences and the distribution of analyte ions within the helium plasma as a function of nebulizer flow rates. Helium microwave induced plasma mass spectrometry provides detection of the halides in aqueous samples as positive ions at the low to sub-ppb levels. Detection limits for metals are in the sub-ppb range. Determination of selenium and bromide isotope ratios are possible down to the low ppb level. Flow injection was used in a preliminary matrix study. A sodium concentration of 2000 ppm was found to suppress the analyte signals by 15 to 50%. This suppression can be partially corrected for by using internal standards.

A Moderate-Power Nitrogen Microwave-Induced Plasma as an Alternative Ion Source for Mass Spectrometry

The moderate-power nitrogen plasma is evaluated as an alternative ion source for plasma mass spectrometry. The nitrogen MIP-MS exhibits linear dynamic ranges of about 3–4 orders of magnitude and low ppb to sub-ppb detection limits for most elements under multielement scanning conditions. Nitrogen MIP-MS is capable of determining K, Ca, Cr, As, and Se at their major isotopes. In addition, nitrogen MIP-MS provides lower detection limits than argon ICP-MS for K (0.48 ppb) and Ca (0.24 ppb). The nitrogen MIP-MS system allows the determination of isotope ratios for 10 ppm of Ca (40/44) and 100 ppb of K (39/41), Cr (52/53), and Se (80/78) with less than 5% error.

Minimisation of sample matrix effects and signal enhancement for trace analytes using anodic stripping voltammetry with detection by inductively coupled plasma atomic emission spectrometry and inductively coupled plasma mass spectrometry

An on-line flow system for anodic stripping voltammetry has been investigated for the pre-concentration of cadmium and copper and the separation of copper from a sodium matrix for detection by inductively coupled plasma atomic emission spectrometry and inductively coupled plasma mass spectrometry (ICP-MS). Good signal enhancement was obtained with pre-concentration of moderate sample volumes. Matrix elimination of more than 99% was possible for reasonably high concentrations of undesired matrix elements, namely Na and U. Isotope ratio data for ICP-MS indicated that the interference of ArNa+ on Cu+ at m/z 63 could be minimised.

Effect of ion-lens tuning and flow injection on non-spectroscopic matrix interferences in inductively coupled plasma mass spectrometry

The effects of various matrix constituents, Co, Pb and synthetic ocean water, on analyte signal have been studied. Suppression of the 138Ba+ signal was observed in the presence of each matrix, with the Pb matrix having the greatest effect. These matrix-induced suppressions of analyte signal were compensated for in part by the use of In as an internal standard, and by initially tuning the ion lenses for maximum analyte signal in the presence of the matrix, rather than for a simple acidified standard solution. The severity of matrix-induced suppression was compared for flow injection and continuous sample introduction. No significant differences in suppression of analyte signal between the two methods were observed because of the relatively large sample volume used for flow injection. The experimental procedure adopted, continual reference of analyte signals with and without the matrix present, effectively eliminated any bias in the results due to nebuliser and cone blockage.

Background Spectral Features for Moderate-Power Nitrogen Microwave-Induced Plasma-Mass Spectrometry

The mass spectral background features of a moderate-power nitrogen microwave-induced plasma are presented. In general, the mass spectral background obtained from a nitrogen plasma is less congested at low mass than is the case with argon ICP-MS. In addition, there are no significant (less than 20 counts per second) background species above mass 60. At normal multiplier voltage, the major background species found in a dry nitrogen plasma include 14N+, 14NH+, 14N2+, 14N2H+, 14N16O+, 14N16OH+, 14N3+ and 14N4+. This background is unchanged by the nebulization of deionized water, 5% (v/v) acids (nitric, hydrochloric, and sulfuric acids), and 5% (v/v) methanol solutions. These background species do not interfere with the determination of K, Ca, and Se at their major isotopes, which commonly have interfering background species in Ar ICP-MS. On the basis of the nebulization of 5% HCl and methanol in the nitrogen plasma, As and Cr do not have isobaric interferences at their major isotope masses 75 and 52. In addition, preliminary studies on the effect of Na as a concomitant element on analyte signals indicate severe enhancement or suppression of analyte signals.

Low-pressure helium microwave-induced plasma mass spectrometry for the detection of halogenated gas chromatographic effluents

A low-pressure helium microwave-induced plasma was interfaced to a mass spectrometer and the system was utilised as a detector for gas chromatography. The over-all system was optimised with respect to microwave power and the first-stage pressure of the mass spectrometer. The linear dynamic range for iodobenzene extended over three orders of magnitude starting at 1 pg. The absolute 3σ detection limit for iodine was 0.1 pg. The evaluation of the interface for the detection of bromine indicated an interference at mass 79 due to the formation of a polyatomic nickel species. A preliminary detection limit of 3.5 pg was calculated for bromine. A second interface design was used in a preliminary investigation of the backgrounds at masses 35 and 37. The background 35/37 ratio was correct, indicating a source of chlorine within the system. The detection limit for chlorine was 24 pg.

Helium-argon inductively coupled plasma for plasma source mass spectrometry

An He-Ar mixed gas inductively coupled plasma (ICP) was investigated for use as an ion source for plasma mass spectrometry (MS). The addition of He to Ar produces a plasma capable of ionising high ionisation potential elements more efficiently than the pure Ar plasma. The best signal to noise ratios for metallic elements were obtained with a 20% He plasma, and for halides with a 30% He plasma. The He-Ar mixed gas plasma shows an improvement of one order of magnitude in the detection limits of several metallic elements studied and an improvement of two orders of magnitude for non-metals in comparison with Ar ICP-MS. Figures of merit, including detection limits, linear dynamic ranges, metal oxide ratios and doubly to singly charged ion ratios, are reported and compared with those for Ar ICP-MS.

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.