Elsevier

Microchemical Journal

Volume 97, Issue 2, March 2011, Pages 154-159
Microchemical Journal

Removal of spectral interferences and accuracy monitoring of trace cadmium in feeds by dynamic reaction cell inductively coupled plasma mass spectrometry

https://doi.org/10.1016/j.microc.2010.08.003Get rights and content

Abstract

In the direct ICP-MS determination of Cd in feed samples, significant spectral interferences caused by high concentrations of Mo can play an important role. In the present study, Mo based oxide or hydroxide polyatomic interferences were eliminated by dynamic reaction cell (DRC) with O2 as reaction gas. Some other oxide or hydroxide interferences (i.e. Zr and Ru) were simultaneously reduced by this technology. These potentially interfering polyatomic ions 95Mo16O+, 94Mo16OH+, 94Zr16OH+, 98Mo16O+, 98Ru16O+ and 97Mo16OH+ on 111Cd+ and 114Cd+ were oxidized to higher oxides MoO2+, MoO3+, MoO4+, RuO3+, RuO4+, MoO2H+, MoO3H+, ZrO2H+ and ZrO3H+. The rejection parameter q (RPq) of DRC and the flow rate of O2 were optimized and set at 0.75 and 2.0 ml min 1, respectively. In addition, the residual isobaric interference of 114Sn on 114Cd was corrected using a mathematical correction equation. The limit of quantitation (LOQ) for 111Cd or 114Cd was 0.8 or 1.0 ng g 1 and the analysis results of NIST 1567a wheat flour and 1568a rice flour standard reference materials were in good agreement with the certified values. As the routine cadmium monitoring method in our laboratory, the proposed method was applied to the accuracy determination of 562 pig feed samples for the Monitoring of Central Meat Reserves (CMR) of China.

Introduction

Under the Monitoring of Central Meat Reserves (CMR) of China [1], 30 parameters are controlled for the livestock and frozen meat, including heavy metals, and one of them is cadmium, an undoubtedly toxic element, having cumulative and irreversible effect on human and animal organisms causing various toxic effects like renal disfunction and osteomalacia [2], [3].

The technique of microwave-assisted acid digestion procedure followed by inductively coupled plasma mass spectrometry (ICP-MS) provides high analytical throughput and simultaneous determination of trace toxic elements in human or animal food samples [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. However, one of the most significant limitations of this technology is isobaric or polyatomic interference, the potential interferences on two commonly used isotopes of cadmium (111Cd and 114Cd) as 95Mo16O+, 94Mo16OH+, 94Zr16OH+, 98Mo16O+, 96Mo18O+, 98Ru16O+, 97Mo16OH+, and 114Sn [14]. Therefore, when these interference elements are tens to hundreds of times higher than the Cd concentration in the real feed samples, it could significantly lead to biased results.

The two strategies generally employed for reducing these spectral interferences were separation methods and non-separation methods. Suzuki et al. [15] used a commercially available chelating resin NOBIAS PA-1 for SPE to remove Mo from urine samples for determination of Cd by ICP-MS. Although this off-line SPE-ICP-MS method gave 0.012 μg Cd l 1 as the method limit of quantification, the residual Mo concentration in the sample solution was found to be as high as 2.521 μg l 1 in the urine samples after SPE separation. Recently, a technology combined two separation methods was used to eliminate the spectral interferences and matrix effects in the soil geological samples [16]. After solvent extraction with an acidic extractant, interferences from Sn and Zr were reduced over 1000 fold, and Mo about 250 fold, meanwhile the other matrix elements could be effectively removed by a SPE column. Some other separation methods such as chromatography [17], precipitation [18], extraction [19], volatile species generation [20] and electrothermal volatilization [21] have been proposed for eliminating these interferences. Membrane desolvation sample introduction techniques could reduce oxide and hydroxide interferences by introducing the sample into the plasma as a dried aerosol [22], [23]. A few studies have used membrane desolvation-ICP-MS for Cd in biological and environmental samples [24], [25], [26], and the MoO+ interferences were sufficiently reduced to allow the accurate determination of Cd (0.005% 95Mo produces 3 cps from 95Mo16O at 111Cd) [26]. However, these above methods are either time consuming or requiring expensive regents and additional equipment. A simple non-separation method based on addition of acetonitrile to reduce the MoO interference was reported by Karunasaga and Arunachalam [27]. The researchers explained that the reduction in MoO ion interference may apparently involve a suppression of the oxide ion formation due to the oxygen scavenging capacity of the cyanogens radicals. However, the ZrO or RuO interferences were not reported. Mathematical correction is another choice, however, it was reported that this method was not sufficient for low Cd samples with high Mo, Zr and Sn concentrations [28].

An alternative method based on the dynamic reaction cell (DRC) techniques has proved to be effective for the alleviation of these metal oxide or hydroxide interferences [29], [30]. A few studies [31] for eliminating the interferences using O2 as reaction gas DRC method have been published since the first systematic study carried out in noble metals determination by Simpson et al. [32]. The O2 as reaction gas DRC-ICP-MS method was also successively employed to eliminate the Mo or Zr oxide interferences for Cd in urine [33], rice and soil samples [34].

The aim of this paper is to describe a simple method to reduce the spectral interferences and accuracy determination of Cd in the feed samples by DRC-ICP-MS. O2 as reaction gas was utilized actively to promote oxidation of interfering species (MoO, MoOH, ZrOH and RuO) to higher oxides, resulting in separation of analytes from interfering oxides or hydroxides. The optimization of the technique and its analytical performance, as well as its applications to the determination of Cd in NIST SRMs (1568a rice flour and 1567a wheat flour) and 562 feed samples from the Monitoring of Central Meat Reserves (CMR) of China, are discussed in detail in this work.

Section snippets

Instrumentation

A PerkinElmer SCIEX ELAN DRC-e (dynamic reaction cell) ICP-MS instrument was used a gem tipped cross-flow nebulizer interfaced with a (Ryton) Scott double pass spray chamber, using a 2.0 mm id alumina injector tube, comprised the sample introduction system. The operating parameters of the DRC-ICP-MS used for this work are summarized in Table 1. The DRC gas O2 was purchased from Praxair (China) Investment Co., Ltd. (99.999% purity). A CEM MARS X-press (CEM, Matthews, NC, USA) microwave apparatus

Spectral interferences checking

It was well known that accuracy determination of trace or ultra-trace levels of cadmium in the presence of high Mo, Zr, Ru is difficult due to the formation of the oxides or hydroxides, such as 95Mo16O+, 94Mo16OH+, 94Zr16OH+, 98Mo16O+, 96Mo18O+, 98Ru16O+, 97Mo16OH+, whose isotopes overlap the two commonly used isotopes of cadmium (111Cd and 114Cd). In this experiment, the spectral interferences were checked under the standard mode ICP-MS (DRC OFF), and the signal contribution on m/z 111 and 114

Conclusions

The spectral interferences of Mo-oxides or hydroxides, Zr-hydroxides, Ru-oxides and Sn were found to cause a significant positive bias to Cd monitoring by ICP-MS. The O2 DRC technology successfully reduced the Mo, Zr and Ru-based interferences on 111Cd and 114Cd, and the residual isobaric interference of 114Sn on 114Cd was corrected using correction equation. This relatively simpler non-separation approach has been used as the laboratory routine method to determinate trace cadmium in pig feeds

Acknowledgments

This work is supported by the Monitoring Program for the Central Meat Reserves (CRC) from the Ministry of Commerce of the People's Republic of China and the grants from the National Nature Science Foundation of China (No. 40973021 and 40821061).

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