Qinghai Guo

Determination of cadmium in geological samples by aerosol dilution ICP-MS after inverse aqua regia extraction

The determination of cadmium (Cd) in geological samples by inductively coupled plasma mass spectrometry (ICP-MS) suffers from significant Mo and/or Zr based oxide interference. We have developed a valid method for Cd determination using Ar aerosol dilution ICP-MS after extraction with inverse aqua regia. Over 90% of the Zr was removed in the extraction procedure, and the residual Zr-hydroxides and Mo-oxides or hydroxides were successfully eliminated by adding an amount of Ar to the sample aerosol prior to the plasma. Compared to the conventional mode without adding Ar, the amount of oxide and hydroxide ions formed in the plasma was reduced by up to 90%. The relative yields of the interfering oxides and hydroxides were as low as 0.005% (MoOH/Mo or MoO/Mo) and 0.007% (ZrOH/Zr). Under the optimized dilution gas flow rate (0.85 L min−1) and carrier gas flow rate (0.24 L min−1), the limit of detection (LOD, 3σ) for 111Cd was 1.6 ng g−1. The proposed method was applied to the determination of Cd in 81 soil, sediment, and rock standard reference materials (SRMs). The results for 68 of these geological SRMs were in good agreement with the reference values. The Cd levels in 10 limestone SRMs (GSR-22, GSR-23, GSR-24, GSR-27, GSR-28, GUI-1, GUI-2, DIAN-1, DIAN-2, and DIAN-3) were reported for the first time, and reference Cd values for other 3 geological SRMs (GSD-7, GSD-19, and GSM-1) were updated by this method. The results showed that this method has great potential for Cd determination in geological samples.

Determination of sub-ng g−1 Au in geological samples by ion molecule reaction ICP-MS and CH4 plasma modifier

Direct determination of sub-ng g−1 levels of gold by inductively coupled plasma mass spectrometry (ICP-MS) is complicated because of the presence of serious mass interferences, and the high first ionisation energy of Au (9.225 eV) also results in poor analytical sensitivity. A reliable method based on the combination of ion molecule reaction (IMR) ICP-MS and addition of a CH4 plasma modifier technique was evaluated for the direct determination of Au in geological samples. The interfering 181Ta16O+ and 180Hf16OH+ (the sample matrix source) on the mono-isotope 197Au were successfully eliminated by oxidation using O2 as the reaction gas. The deduced IMR mechanism in the reaction cell involved the oxidation of 197TaO+ and 197HfOH+ to the higher oxides 213TaO2+ and 213HfO2H+, 229TaO3+ and 229HfO3H+, and 245TaO4+ and 245HfO4H+, while the target Au does not react with O2. In addition, to further improve the methods signal to background ratio (SBR), a CH4 modifier was introduced to the ICP plasma at 2 mL min−1; the poor sensitivity of Au+ was increased by a factor of four and the background signal (at m/z 197) was decreased to 50%. The improvement of the SBR (eight fold) was due to both the carbon enhancement effect (for Au) and the carbon competition effect (with the interfering oxide ions) in the CH4 modified plasma. The proposed method was applied to the direct determination of Au in ten different types of geological standard reference materials (SRMs). The results of all SRMs were found to agree well with the certified values. This method has great potential for the direct determination of trace levels of Au in various geological samples.

Method Development for the Determination of Total Fluorine in Foods by Tandem Inductively Coupled Plasma Mass Spectrometry with a Mass-Shift Strategy

A method for total F determination in food and tea samples based on a mass-shift strategy using tandem inductively coupled plasma quadrupole mass spectrometry (ICP-MS-MS) was developed. This method consists of four steps: (1) conversion of the hardly ionized F atoms to BaF+ via ICP, (2) use of the first quadrupole (Q1, set at 157) to ensure only the m/z 157 ions (i.e., 138Ba19F+, 157Gd+, and 138Ba18OH+) enter the reaction cell (RC), (3) shifting 138Ba19F+ to a new mass 138Ba19F(14NH3)3+ by reacting with NH3 in RC to avoid the interfering ions (i.e., 157Gd+ and 138Ba18OH+), and (4) passing interference-free 138Ba19F(14NH3)3+ to the second quadrupole (Q2, set at 208) for detection by the MS detector. The mass-shift process of the target F (in ICP and RC) expected to follow the path: F + 138Ba+ → 157BaF+ + 3NH3 → 208BaF(NH3)3+, while the reaction pathway of dominant 157Gd+ in RC proposed to 157Gd+ + NH3 → 157Gd14N1H+ + nNH3 → 157Gd14N1H(14N1H3)n+ (n = 0-5). Under the optimized setting of tandem MS (Q1 → Q2 = 157 → 208) and RC reaction gas flow rate (NH3/He = 10:90, 8.0 mL min-1), the background equivalent concentrations (BECs) and limits of detection (LODs) were 0.021 and 0.022 μg mL-1, respectively. The calibration curve was linear in the range between 0.1 and 10 μg mL-1, with a correlation coefficient of R2 = 0.9999. The results obtained for 14 different food-related standard reference materials (SRMs) were in good agreement with the certified values on a 95% confidence level. The proposed method was then employed to evaluate the F contents of 13 branded tea samples. The total F concentrations ranged from 39.2 to 93.2 μg g-1. The tea infusions contained F between 23.5 and 85.4 μg g-1, with an extraction efficiency of 56.0-91.6%, and the water-soluble F contents of a Puer brick tea were 58.7, 21.4, 3.82, and 1.41 μg g-1 for filtrates 1, 2, 3, and 4, respectively.

Direct determination of cadmium in geological samples by slurry sampling electrothermal atomic absorption spectrometry

Cadmium (Cd) is an important element for the assessment of environmental pollution. An accurate and high throughput method involving slurry sampling electrothermal atomic absorption spectrometry (ETAAS) was developed to detect trace Cd content in geological samples. After sonication of the sample with 0.5% v/v HNO3 and 0.5% v/v Triton X-100 solution, the slurry was directly introduced into a graphite atomizer and detected by AAS. This simple method was shown to take 80% less time, and resulted in 90% less reagent waste than the conventional acid digestion method. Under optimized conditions (sample size ≤ 75 μm, sonication time: 20 min, pyrolysis temperature: 450 °C, and atomization temperature: 1600 °C), the characteristic mass and limit of detection for Cd were 0.8 pg and 0.002 μg g−1, respectively. The proposed method was applied to the determination of Cd in 86 soil, sediment, and rock standard reference materials (SRMs). The results for 78 of these materials were in good agreement with the reference values. The Cd levels in five limestone SRMs (GUI-1, GUI-2, DIAN-1, DIAN-2, and DIAN-3) and one clay SRM (GBW03103) were reported for the first time. The proposed method shows great potential for the direct determination of trace Cd in various geological samples.

Using citric acid stabilizing reagent to improve selective hydride generation-ICP-MS method for determination of Sb species in drinking water

An improved selective hydride generation-inductively coupled plasma-mass spectrometry (SHG-ICP-MS) method using citric acid as the stabilizing reagent was established and evaluated for the speciation of inorganic Sb in drinking water. The use of citric acid as the stabilizing reagent improved the sensitivity of the Sb species over 2-fold, when compared to the conventional pH-dependent SHG strategy. The SHG reaction in the presence of citric acid was studied using high-resolution quadrupole-Orbitrap mass spectrometry (HR-MS). Almost 100% of the Sb(V) and Sb(III) reacted with citric acid to produce [Sb(OH)3(C6O7H5)]− and [Sb(C6O7H6)2]−, respectively. The resultant Sb(V)-complex ([Sb(OH)3(C6O7H5)]−) cannot be reduced by NaBH4, while the resultant Sb(III)-complex ([Sb(C6O7H6)2]−) originating from Sb(III) can be reduced thoroughly by NaBH4. The ICP-MS signal intensity for Sb was initially low due to its high first ionization energy of 8.61 eV; however, an increase in sensitivity was observed with an addition of 2.0 mL min−1 CH4 to the ICP. Three elements (Ge, Sn, and Te) were evaluated as potential internal standards (ISs) to improve the stability of the HG signal. After careful evaluation, 118Sn was selected as the IS, and the relative standard deviation (RSD) for the of Sb(III) signal (1.0 μg L−1) was improved from 5 to 2%. Under optimized conditions, excellent detection limits (4.0 ng L−1 for Sb(III) and 6.0 ng L−1 for total Sb) and satisfactory recoveries for Sb(III) (from 93.1 to 108%) and total Sb (from 98.5 to 103%) were obtained. The proposed method was employed for analysis of three types of drinking water: bottled water, tap water, and well water. The average Sb(III) and total Sb contents were 0.032 and 0.320 μg L−1, respectively, in bottled water (N = 10), 0.197 and 1.05 μg L−1, respectively, in tap water (N = 10), and 0.211 and 1.93 μg L−1, respectively, in well water (N = 40). Thus, this high throughput method has great potential for the determination of inorganic Sb species at ultra-trace levels in drinking water.

Direct speciation of Cr in drinking water by in situ thermal separation ETAAS

In situ speciation of Cr(VI) and Cr(III) based on thermal separation and electrothermal atomic absorption spectrometry (ETAAS) detection has been developed. The complexant 2-thenoyltrifluoracetone (TTA) was used to selectively react with Cr(III); the product of the Cr(III)–TTA complex vaporized at 400 °C, while Cr(VI) quantitatively remains up to a temperature of 1200 °C and was subsequently detected by AAS. The complexation reaction products of the volatile Cr–TTA complex were verified using electro-spray ionization high resolution quadrupole-Orbitrap-mass spectrometry (ESI-HR-MS). The Cr(III) concentration was established by calculating the difference between the total Cr (Cr(T)) and Cr(VI) concentrations. Under optimum experimental conditions, the detection limits for Cr(VI) and Cr(T) are 0.046 μg L−1 and 0.039 μg L−1, with RSDs for 1.0 μg L−1 of Cr(VI) and 1.0 μg L−1 of Cr(T) of 3.1% and 2.8% (n = 5), respectively. The accuracy of this method was validated by testing two water standard reference materials (SRMs) for total chromium. The proposed method was employed for analysis of drinking water. The Cr(VI) concentrations for tap water, bottled water, and well water were found to be 1.41, 0.53 and 0.68 μg L−1, respectively, which is in good agreement with the reference values measured by ion chromatographic ICP-MS. This high throughput method has great potential for screening of Cr species at ultra-trace levels in drinking water.

Gas collision for improving the precision and accuracy of 11B/10B ratios determination in ICP-QMS and its application to determining wine provenance

Boron accumulates in plants with the same isotopic ratio as found in the source soil and water, producing isotope ratios (11B/10B) that reflect those of the sources, thus indicating the provenance of products derived from vegetative matter. We developed and validated a simple analytical method based on gas collision inductively coupled plasma quadrupole mass spectrometry (ICP-QMS) for the determination of B isotope ratios to distinguish the geographic origins of wines. Use of the gas collision technique (using Ne as the collision gas) in ICP-QMS can effectively improve the precision and accuracy of 11B/10B determination, which may be due to improvement of the ion transmission or sensitivity (via collisional focusing) and a reduction in plasma noise (via collisional energy damping). Compared with the conventional method (without Ne gas collision), the precision of the 11B/10B ratio was improved 3.2-fold (from 3.15‰ to 0.94‰) and the accuracy was improved from an error of −5.5% to −0.1%. The −0.1% error represents mass bias, resulting from in-cell gas collision, and can be accurately corrected using an external bracketing technique with NIST SRM-951 B isotope standard. Direct dilution of the wines by a factor of 100 with 1% HNO3 was found to substantially reduce matrix-induced mass discrimination. Other important parameters such as detector dead time, dwell time per data acquisition, and total integration time per isotope were also optimised. Twenty wines from nine countries were analysed, and δ11B values ranged from +1.73 to +46.6‰ with an average external precision (N = 5) of 0.82–1.63‰. The proposed method has sufficient precision to distinguish between 20 wine brands originating from four different geographic regions.

On-site separation of Cr(VI) and Cr(III) in natural waters by parallel cartridge ion-exchange columns

A simple, fast, portable, and solvent-free method is developed for field separation of Cr(VI) and Cr(III) in natural waters. The method involves passing a water sample through parallel cation- and anion exchange resin cartridge columns at the field site. In a matter of seconds, all the Cr(III) is retained on the cation-exchange column and Cr(VI) passes to the effluent, while the Cr(VI) is quantitatively retained on the anion-exchange resin and Cr(III) passes to the effluent. The two collected solutions are preserved and determined later in the laboratory using inductively coupled plasma mass spectrometry (ICP-MS) or any other elemental analysis technique sufficiently sensitive to measure the Cr concentrations of interest. Cr(VI) (or Cr(III)) can be separated from Cr(III) (or Cr(VI)) at the pH range 1–10 at Cr(III)/Cr(VI) (or Cr(VI)/Cr(III)) concentration ratios as high as 10u2006000 (or 500). The limits of detection for Cr(III) and Cr(VI) are 0.03 and 0.01 μg L−1, respectively. Repeatability expressed as relative standard deviation is determined to be 7.1% for Cr(VI) and 5.7% for Cr(III), using a drinking water sample containing 1.14 μg L−1 Cr(VI) and 0.72 μg L−1 Cr(III). The parallel cartridge ion-exchange resin columns can be used continuously for 40 drinking water samples without any regeneration or cleaning procedure. The accuracy is validated by analysing Cr(III) and Cr(VI) in three water standard reference materials (GBW080257, GBW080403, and GBW080404). Furthermore, the proposed method is applied to the on-site separation of Cr(III) and Cr(VI) from natural waters.