Using the Multimode Sample Introduction System (MSIS) for Low Level Analysis of Arsenic and Selenium in Water J. L. Schroder* H. Zhang
SOIL CHEMISTRY
Dep. of Plant and Soil Sciences Oklahoma State Univ. Stillwater, OK 74078
In recent years, the problems associated with the measurement of low concentrations of arsenic (As) and selenium (Se) using conventional nebulization and inductively coupled plasma–atomic emission spectroscopy (ICP–AES) have been largely overcome by using hydride generation. In 2002, a radically new design for the combined nebulizer/gas liquid separator referred to as the Multimode Sample Introduction System (MSIS, Marathon Scientific, Niagara Falls, Ontario, Canada) was introduced. The feasibility and detection limits of combining the MSIS with a Spectro Ciros CCD (axial) ICP–AES for the determination of low concentrations of As and Se in water were examined. Overall, the system was inexpensive, easy to install, accurate and precise, and lowered the quantification limits by approximately 100-fold for As and by 20-fold for Se as compared with a conventional nebulization. Unlike other conventional hydride generators, the MSIS does not need to be removed from the ICP when analyzing other elements without hydride generation. Therefore, the MSIS is recommended to laboratories seeking low detection limits for As and Se with existing or new ICP instruments. Abbreviations: ICP–AES, inductively coupled plasma-atomic emission spectroscopy; ICP–HG, inductively coupled plasma-hydride generation; ICP–MS, inductively coupled plasma-mass spectroscopy; LOD, limits of detection; MCL, maximum contaminant levels; MSIS, Multimode Sample Introduction System.
I
n the United States, the Safe Drinking Water Act (SDWA) is the main federal law ensuring the quality of drinking water (USEPA, 2008a). Under the authority of SDWA, the U.S. Environmental Protection Agency has established drinking water regulations and has set maximum contaminant levels (MCLs). The MCLs are maximum concentrations of different chemicals that are allowed in public drinking water systems (USEPA, 2008b). In 2006, the MCL for As was lowered from 50 to 10 μg L−1 to protect human health (USEPA, 2008b). The current MCL for Se is 50 μg L−1 while the current ambient water quality criteria for surface water is set at 5 μg L−1 to be protective of aquatic organisms (USEPA, 2008c). Thus, the need to accurately measure low concentrations of As and Se exists. Inductively coupled plasma–atomic emission (ICP–AES) is a robust technique for elemental analysis that is widely used for the analysis of drinking waters and is capable of providing the needed detection limits and linear dynamic ranges for most elements. However, there are exceptions and certain elements known as the hydride forming elements (i.e., As, Sb, Bi, Ge, Sn, Pb, Se, Te, and Hg) which have high ionization potential and low level analyses of Soil Sci. Soc. Am. J. 73:1804-1807 doi:10.2136/sssaj2009.0033 Received 26 Jan. 2009. *Corresponding author (
[email protected]). © Soil Science Society of America 677 S. Segoe Rd. Madison WI 53711 USA All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.
1804
these elements using ICP–AES and conventional pneumatic nebulization have proven to be problematic. Many of these problems may be overcome by using hydride generation in which the sample is acidified with HCl and then reacted with sodium borohydride (NaBH4). The reaction produces a volatile hydride that is introduced into the plasma for analysis. The introduction of the sample as a gas results in significant increases in analyte intensities and improved detection limits (Wiltsche et al., 2008). Hence, hydride generation has become widely used in ICP–AES analyses for certain elements (Dedina and Tsalev, 1995; Pohl, 2004; Sturgeon et al., 2005). Traditionally, hydride generation utilized a separate gas–liquid separator. Often times the system is costly and cumbersome, is difficult to attach the hydride generation unit to the ICP, and requires attaching the unit every time it is used for As and Se analysis. However, over the last couple of decades, several designs have evolved that combine the nebulizing and vapor generation function for the simultaneous determination of hydride and non-hydride forming elements. McLaughlin and Brindle (2002) advanced a radically new design for the combined nebulizer/gas liquid separator referred to as the MSIS. The MSIS utilizes thin film flowing technology to eliminate signal noise that has traditionally been a problem where the reductant and sample have been mixed in a confined tube resulting in increased pressure and bubble formation. McLaughlin and Brindle (2002) showed the MSIS was far superior to the traditional separate frit-based gas– liquid separator because of a dramatic reduction in relative standard deviation (RSD). The MSIS uses thin film flowing technology to deliver the analyte solution and reductant with the overall effect of minimizing frothing (McLaughlin and Brindle, 2002). The objective of this study was to evaluate the feasibility of combining the MSIS with a Spectro Ciros CCD (axial) ICP–AES SSSAJ: Volume 73: Number 6 • November–December 2009
Fig. 1. The Multimode Sample Introduction System (MSIS) (R.L.J. McLaughlin, I.D. Brindle U.S. Patent no. 6.891.605).
for the determination of low concentrations of As and Se in water samples.
MATERIALS AND METHODS Multimode Sample Introduction System Arsenic Analysis To determine low levels of As and Se and to remove potential interferences found on direct analysis of samples, inductively coupled plasma–hydride generation (ICP-HG) was used. For As analysis, hydride generation is achieved by reacting a sample acidified with HCl with sodium borohydride (NaBH4). The reaction produces a volatile hydride (arsine, AsH3) which is transferred to the plasma for analysis. Inorganic As, arsenate (AsO43−) and arsenite (AsO33−) can be reduced to arsine gas through the hydride process, although reduction of arsenate is more time consuming (Chen et al., 1992; Dedina and Tsalev, 1995; McLaughlin and Brindle, 2002). Therefore, samples are normally chemically treated with a prereductant such as potassium iodide, thiourea, or l-cysteine to reduce all As to arsenite (AsO33−) before analysis. For our study, we utilized the recently developed MSIS (McLaughlin and Brindle, 2002). The MSIS utilizes a cyclonic spray chamber that has been modified by the addition of two vertically opposite tubes located in the center of the spray chamber (Fig. 1). The MSIS was equipped with a Burgener Mira Mist nebulizer. The system is designed so that mixing of the acidified sample or calibration standard with the reductant (NaBH4) occurs in the gap (1–3 mm) approximately 1/3 of the way from the top of the spray chamber (McLaughlin and Brindle, 2002). The lower tube’s diamond-ground surface serves to promote the reaction by providing a large surface area for the gaseous products to migrate to the gaseous phase (Fig. 2). Argon gas was introduced to the system via the nebulizer. The MSIS was operated with the introduction of HCl from the top and NaBH4 from the bottom. Although, the MSIS accommodates a dual mode operation where direct nebulization and hydride generation occur at the same time, the vapor generation mode only was used for the determination of As and Se. The instrument configuration and settings used for the experiment are shown in Table 1. Calibration standards of 0.0, 2.0, 5.0, 10.0, and 20.0 μg As L−1 were prepared in a matrix of 2% (v/v) trace metal grade HCl (Thermo Fisher Scientific Inc.) and 1% (m/v) l-cysteine using a stock As solution of 1000 μg As L−1 purchased from Environmental Resource Associates
SSSAJ: Volume 73: Number 6 • November–December 2009
Fig. 2. Photograph of MSIS equipped with Burgener Mira Mist nebulizer. (R.L.J. McLaughlin, I.D. Brindle U.S. Patent no. 6.891.605).
(ERA, Arvada, CO). The concentration of l-cysteine was optimized by using different concentrations of l-cysteine and comparing the response of a 10 μg L−1 As calibration standard. All calibration standards were prepared in Class A volumetric glassware using deionized– distilled water that had a minimum resistivity of 18 MΩ cm. The same ratio of l-cysteine was added to the samples as to the calibration standards. A solution of 1.5% (m/v) sodium borohydride (NaBH4) (98%, Acros Organics, catalog # 189305000) solution in 0.05 M reagent grace NaOH (Fisher Scientific) was prepared. L-Cysteine (99%, Acros Organics, catalog # 173601000) was added to the calibration standards as a prereductant and the standards were allowed to react for 60 min before analysis. Although many prereductants have been utilized during the development of hydride generation, we chose to use l-cysteine because it has been shown to significantly reduce interferences and improve the recovery of As (Brindle and Le, 1989; Chen et al., 1992). When Chen et al. (1992) used l-cysteine and evaluated 50 ug L−1 As (III) with 200 mg L−1 of Fe (II), Fe (III), Mn, and Hg, recoveries of As were ≥ 99%. Additionally, they reported a recovery of 100% for As when they evaluated the affect of 20 mg L−1 Pb in conjunction with lcysteine. Concentrations of these elements in water are typically much less than the concentrations evaluated by Chen et al. (1992). For example, concentrations of Fe in groundwater range from 1.0 to 10.0 mg L−1 while concentrations of Fe in the surface range from 0.05 to 0.2 mg L−1 (Lee and Lin, 2007). Typical concentrations of Mn in natural waters Table 1. Instrument conditions used in As and Se analysis of water samples. Nebulizer Torch RF Power, W Plasma Ar Flow, L min−1 Auxillary Ar Flow, L min−1 Nebulizer Ar Flow, L min−1 Delay time, s Sample Uptake Rate, mL min-1 Number of Integrations Integration Period, s ICP Wavelength for As Wavelength for Se
Burgener Peek Mira Mist EOP Flared End Torch, 2.5 mm 1400 14.0 1.0 1.0 90 1.0 3 12.0 Spectro Ciros CCD (axial) 189.042 nm 196.090 nm 1805
Table 2. Analytical parameters obtained by various methods standardized by the USEPA for As and Se analysis of water samples. Element
Detection
LOD ††
EPA Method
Reference
µg L−1 As ICP-AES† 35.0 6010 U.S. EPA, 2007a ICP–MS‡ 1.0 6020 U.S. EPA, 2007b GFAAS§ 1.0 7010 U.S. EPA, 2007c 0.1 7063 U.S. EPA, 1996 ASV# Se ICP-AES† 50.0 6010 U.S. EPA, 2007a ICP–MS‡ 1.0 6020 U.S. EPA, 2007b GFAAS§ 2.0 7010 U.S. EPA, 2007c ASV# NA 7063 U.S. EPA, 1996 † Inductively Coupled Plasma-Atomic Emission Spectrometry. ‡ Inductively Coupled Plasma-Mass Spectrometry. § Graphite Furnace Atomic Absorption Spectrometry. # Anodic Stripping Voltammetry. †† Limit of detection.
range from 0.1 to 1.0 mg L−1 (Lee and Lin, 2007). Additionally, lead (Pb) ranges from 5 to 30 ug L−1 in both groundwater and surface waters (United States Department of Health and Human Services, Agency for Toxic Substances and Disease Registry, 2007a). Freshwaters without known sources of contamination usually contain
