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International Journal of Analytical Chemistry
Volume 2015, Article ID 156509, 10 pages
http://dx.doi.org/10.1155/2015/156509
Research Article

Some Rare Earth Elements Analysis by Microwave Plasma Torch Coupled with the Linear Ion Trap Mass Spectrometry

1Jiangxi Key Laboratory for Mass Spectrometry and Instrumentation, East China Institute of Technology, Nanchang 330013, China
2College of Chemistry, Jilin University, Changchun 130021, China

Received 22 May 2015; Revised 11 August 2015; Accepted 12 August 2015

Academic Editor: Josep Esteve-Romero

Copyright © 2015 Xiaohong Xiong et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

A sensitive mass spectrometric analysis method based on the microwave plasma technique is developed for the fast detection of trace rare earth elements (REEs) in aqueous solution. The plasma was produced from a microwave plasma torch (MPT) under atmospheric pressure and was used as ambient ion source of a linear ion trap mass spectrometer (LTQ). Water samples were directly pneumatically nebulized to flow into the plasma through the central tube of MPT. For some REEs, the generated composite ions were detected in both positive and negative ion modes and further characterized in tandem mass spectrometry. Under the optimized conditions, the limit of detection (LOD) was at the level 0.1 ng/mL using MS2 procedure in negative mode. A single REE analysis can be completed within 2~3 minutes with the relative standard deviation ranging between 2.4% and 21.2% (six repeated measurements) for the 5 experimental runs. Moreover, the recovery rates of these REEs are between the range of 97.6%–122.1%. Two real samples have also been analyzed, including well and orange juice. These experimental data demonstrated that this method is a useful tool for the field analysis of REEs in water and can be used as an alternative supplement of ICP-MS.

1. Introduction

Rare earth elements (REEs), referred to as industrial vitamins, are of particular importance and have extensive applications in various fields, especially in material science and industry, mainly due to their special chemical properties and luminous characters based on their 4f shell electrons [1]. Recently, along with large amounts of exploitation of rare earth resources and the discard of the REEs contained materials, REEs accessed inevitably environment, food chain, and then the human body. Despite this, the positive or negative effect of REEs on the biological body is completely unclear till now. Clearly, a high-sensitive analysis method will promote the researches in this field and give an explicit pattern. Similarly, the potential application fields of REEs are also increasing in current society, and the demand of high-sensitivity analysis methods for REEs is rising gradually. For instance, for biological samples, the sensitivity is general at the level of ng/g. In environmental analysis field, for soil, air, lake water, and so forth, the requirement detection level must at least achieve the magnitude of μg/L, while for sea water and other high technique REEs-containing materials, the according detection level should be the level of ng/L or pg/g [2]. Therefore, developing the sensitive analytical methods for REEs is of vital significance and becomes one of the hot points in analytical science. Because the chemical properties of the 17 REEs are similar, the key point of REEs analysis is the mutual separation of REEs, thus the detection of REEs is usually accompanied by extraction and preseparation [3]. The development of the high-sensitive analytical methods also advances these new separation techniques.

Currently, numerous analytical techniques have been applied in the detection of REEs with the aid of extraction and separation [4], including atomic absorption/fluorescence spectroscopy (AAS/AFS), atomic emission spectroscopy (AES) [5], X-ray fluorescence spectroscopy (XFS) [6], neutron activation analysis (NAA) [7], inductively coupled plasma optical emission spectrometer (ICP-OES) [8], and inductively coupled plasma mass spectrometer (ICP-MS) [9, 10]. Among them, ICP-MS has become one of the most commonly used techniques because of its multiple advantages, including extremely high sensitivity, high analytical speed, broad dynamics range, and synchronous analysis of various elements. The sensitivity of ICP-MS has reached the level of 0.01 ng/g for most REEs and only 10 ng/g for ICP-OES under the same conditions [10]. However, restricted by its large and expensive equipment, the sample injection technique, matrix effect, and the difficulty in the direct analysis of solid sample, ICP-MS is difficult to be applied in the fast field analysis of REEs.

Ambient ionization techniques are a revolution in the mass spectroscopy, which enabled the ionization of samples in their native surrounding with minimum sample pretreatment and brought the breakthrough in the application of MS to high-throughput analysis [11, 12]. There are more than 10 direct ionization techniques based on the atmospheric plasma that have been invented [13], including dielectric barrier discharge ionization (DBDI) [14], desorption atmospheric pressure chemical ionization (DAPCI) [15], desorption corona beam ionization (DCBI) [16], plasma-assisted desorption ionization (PADI) [17], flowing atmospheric pressure afterglow (FAPA) [18, 19], and microplasma discharge ionization (MDI) [20]. Usually, in these methods, the electronic field employed to produce plasma under atmospheric pressure is high discharge voltage (usually several kilovolts) and low frequency (up to 850 Hz) [21]. The temperature of plasma is low (usually several hundred Kelvin in excitation temperature), so that the plasma is weak and even invisible.

The microwave plasma torch (MPT) is a kind of plasma generator at atmospheric pressure by means of the high-frequency (2450 MHz) microwave field, which was developed initially at Jilin University [22], and substantial improvements were made at Indiana University [23]. The MPT can be sustained in a variety of supporting gases, including Ar, He, N2, Ne, and air and produce a stable flame-like plasma, similar to ICP. Based on its multiple merits, for instance, small size, low power consumption, low cost, easy operation, and high excitation efficiency, MPT has broad applications in AES portable spectrometer, supercritical fluid chromatography (SFC), and liquid chromatography (LC) as an excitation light source for elemental analysis [24, 25]. Owing to the excellent ionization efficiency, MPT has also been employed as the ion source in mass spectrometry mainly for the elements analysis and the detection of halogenated hydrocarbons separated by a capillary gas chromatography [26]. Recently, Zhang et al. studied the direct desorption/ionization approach of MPT on a linear ion trap (LTQ) mass spectrometer to analyze a series of small organic molecules and showed that MPT is a useful alternative ambient ion source [27]. However, MPT has been rarely used as the detection of metal elements in mass spectrometry. Presumably ICP is dominated in this field. On another aspect, when the analyte is inorganic molecule, the desorption or ionization efficiency is bare due to the ultralow vapor pressure. Moreover, in the usual direct desorption/ionization approach, plasma crossing sideling the sample surface restricts the efficient transformation of the energy from the plasma to the analytes due to the short interaction length. But this obstacle can be overcome by the approach of sample injection via the central tube of MPT. Some previous researches [28] had demonstrated that the MPT-MS is sensitive enough for the demand of field analysis of metal elements in aqueous liquid at the level of 0.02–1 ng/mL.

In this study, an Ar-assisted MPT coupled with the LTQ mass spectrometer was used for the sensitive detection of trace levels of REE ions in water. The nebulized REE aqueous solution, without any pretreatment, flowed into the plasma through the central tube of the MPT to increase the interaction time of the plasma and the samples. For the slat solution of single rare earth elements, including yttrium, lanthanum, cerium, praseodymium, neodymium, samarium, and europium, the generated complex ions in plasma were detected in both positive and negative ion modes and further characterized in collision induced dissociated (CID) experiments. We further examined the semiquantitative capability of this method for the direct detection of REEs in different aqueous liquids.

2. Materials and Methods

2.1. Materials and Reagents

The 2450 MHz microwave generator (YY1-50 W-2450) and the coaxial line (SFCJ-50-9) were purchased from the Nanjing Electronic Technology Co., Ltd. (Nanjing, China). The MPT was provided by Aimin Yu’s group at Jilin University, China. High purity argon (99.999%) was purchased from the Guoteng Special Gas Ltd. (Nanchang, China). LaCl3, CeCl3, PrCl3, EuCl3, and NdCl3 (analytical grade) were purchased from J&K Chemical Technology Co., Ltd. Yttrium, lanthanum, cerium, praseodymium, neodymium, samarium, and europium standard substance (103 mg/L in 1.0 mol/L HNO3) were purchased from Sinopharm Chemical Reagent Co., Ltd. The water used was deionized water provided by the chemistry facilities in the East China Institute of Technology (ECIT). We used pure water and REEs standard substance to prepare a series of solutions with concentrations from 0 ng/mL to 500 ng/mL for the calibration curve measurements. For these REEs, the maximum concentrations for the calibration curve measurements are different based on the practice signal intensities; that is, the linear ranges are discriminating. For instance, for samarium, the maximum concentration is 500 ng/mL, but for yttrium and praseodymium the corresponding values are 100 ng/mL. The practical water sample was gathered in a well in Ganzhou, Jiangxi Province, PR China. Orange juice was gotten by squeezing an orange also produced in Ganzhou. This practical aqueous was directly analyzed with any other pretreatment, except for the necessary dilution with deionized water.

2.2. Instrumental Setup

Two mass spectrometers were employed in this study, including a commercial linear ion trap mass spectrometer (Thermo Fisher Scientific, San Jose, CA, USA) and a home-built quadrupole mass filter mass spectrometer. Most of MPT-MS experiments were carried out by the LTQ XL mass spectrometer (see [27]), and the quadrupole mass spectrometer is only used in some assisted experiments. To detect the metal ion in solution, a desolvation unit is combined with the MPT ion source and the LTQ mass spectrometer.

The Desolvation Unit. Water samples, without any pretreatments, were pneumatically nebulized and generate the aerosols. The aerosols flow by carrier gas through a desolvation system to reduce the loss of the microwave power due to the absorption by water. The desolvation system comprised a heated tube and a desiccation chamber. The heated tube is 25 cm long and is wrapped with heating tape which is monitored with a thermocouple and a heater controller (XMT61X, Beijing Huibang Science Technology Co. Ltd., China). The heater controller was set at 140°C ± 5°C. A 500 mL flask was filled with concentrated sulfuric acid (98%) to further remove the H2O molecules from the aerosol. At last, the dry aerosols were introduced into the plasma through the central tube of the MPT (see Figure 1).
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