Minimization of volatile nitrogen oxides interference in the determination of arsenic by hydride generation atomic absorption spectrometry

doi:10.1016/S0584-8547(01)00335-4

Spectrochimica Acta Part B: Atomic Spectroscopy

Volume 56, Issue 10, 31 October 2001, Pages 1883-1891

https://doi.org/10.1016/S0584-8547(01)00335-4 Get rights and content

Abstract

In this study emphasis was given to minimize the interference of volatile nitrogen oxides from digestion procedures with nitric acid on the determination of arsenic by hydride generation atomic absorption spectrometry (HG AAS). Sulfamic acid (SA) is proposed to minimize this interference by employing three procedures for the digestion of hair in closed systems: conventional and microwave (MW) heating in polytetrafluorethylene (PTFE) vessels and by MW heating in glass vials. Hair samples were digested with H₂SO₄+HNO₃ or HNO₃+H₂O₂ mixtures. Concentrated hydrochloric acid was added for the digestion for the procedure in glass vials. The accuracy of the procedures with PTFE vessels was verified by the spike recoveries of organic (p-aminobenzenearsonic acid and dimethyl arsinic acid, from 92 to 101%) and inorganic (sodium arsenate, from 98 to 102%) arsenic compounds. For the procedure in glass vials the recovery was from 86 to 97% for organic As and from 97 to 102% for inorganic As. The results obtained for a certified hair reference material using the three digestion procedures were well within the 95% confidence interval of the certificate when SA was added to the solutions. However, when SA was not added, recoveries were low and non-reproducible signals and high background levels were observed. Urea, benzoic acid and hydroxylamine hydrochloride were also studied (maximum As recovery of 90% using hydroxylamine hydrochloride) but the best results were obtained with use of SA.

Introduction

Hydride generation atomic absorption spectrometry (HG AAS) has become one of the most powerful and well established techniques for the determination of arsenic. Hydride generation accessories are relatively inexpensive and offer high relative sensitivity, low limits of detection and can be easily automated [1]. In spite of these advantages HG AAS is prone to some interference, mainly in the condensed phase and in the gas phase, e.g. during hydride generation and in the atomizer [2]. Most of the interference studies in HG AAS are dealing with the effect of concomitants in the sample, namely transition metals [3], [4], [5] and hydride-forming elements [6], [7], however, papers about the influence of volatile nitrogen oxides in procedures involving HG AAS are scarce. This aspect is important because most digestion procedures for biological samples in closed systems use nitric acid alone or in combination with other reagents, such as sulfuric acid, hydrogen peroxide, or hydrochloric acid [8], [9]. The problem is more pronounced with partial digestion procedures where organic compounds and nitrogen oxides remain in the solution. A similar interference was also found in the determination of Hg by flow injection cold vapor atomic absorption spectrometry [10], [11].

The main products of the reduction of concentrated nitric acid in the final digests are NO, N₂O₄ and NO₂ [12], and the resultant brown color is mainly due to the presence of nitrite in equilibrium with NO₂ molecules. When the final determination is made by flame or electrothermal atomic absorption spectrometry, the influence of nitrogen oxides on the signal can be disregarded. However, in determinations by HG AAS, when sodium tetrahydroborate is used as the reducing agent, the signals can be strongly affected by residual nitrogen oxides. Some reports can be found in the literature about the effect of these species [13], [14], [15], but often this kind of interference is referred to as being due the simple ‘presence of HNO₃’ or its sub-products without specific investigation of the interference. Some reagents are proposed in the literature to minimize this interference, such as formic acid [16] and sulfanilamide [17]. Volatile nitrogen oxides were proposed to be transported together with the analyte hydride upon the addition of sodium tetrahydroborate, and sulfamic acid was used to minimize this effect [18]. Purging of the final solution with argon, and evaporation of digests were also tried [19], however, excessive heating of acid digests to remove such interference is not only time-consuming but may also be undesirable in view of possible analyte losses.

In this work, a study was undertaken to investigate the interference effects of volatile nitrogen oxides on arsenic determination by HG AAS in digests of hair using nitric acid. Three wet digestion procedures for hair in closed systems were developed, using polytetrafluorethylene (PTFE) vessels and glass vials. The effect of nitrite in the liquid phase was also studied, and sulfamic acid is proposed to minimize these interference. The accuracy was verified by spike recoveries of organic and inorganic arsenic compounds and by the analysis of certified reference material.

Section snippets

Apparatus and instrumental parameters

A Perkin–Elmer Model 3030 atomic absorption spectrometer with an arsenic hollow cathode lamp, operated at 15 mA, and deuterium background (BG) corrector, equipped with a Perkin–Elmer MHS-10 batch hydride generation accessory, was used throughout this work. The quartz cell was periodically cleaned by immersion in a 1.44 mol l⁻¹ HNO₃+0.28 mol l⁻¹ HF solution for 30 min. Argon (99 996%, White Martins, Brazil) was used as the purge gas. Integrated absorbance was used instead of peak height for

Sample digestion without sulfamic acid addition — initial studies

Initially sample digestion procedures A1, A2 and A3 were evaluated without the addition of SA. The H₂SO₄+HNO₃ digestion mixture was chosen because of its efficiency, its rapid attack of the sample [21], [22] and because of its suitability for the relatively mild conditions used in procedure A3. The use of hydrogen peroxide was avoided to facilitate the investigation of the effect of nitrogen oxides only. The results for the determination of total arsenic in the certified hair reference material

Conclusion

Sulfamic acid has been demonstrated to be an effective masking agent for removing NO₂⁻ prior to sodium tetrahydroborate reduction of As. Good recoveries for organic and inorganic arsenic compounds were obtained using the three digestion procedures investigated. Good agreement was obtained with the certified value for a reference hair sample. Hence, SA can be proposed as an efficient masking agent for the analysis of hair digested with nitric acid and most likely also for most other biological

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After the reduction, sulfamic acid was added in order to avoid the interference of nitrite and the gaseous byproducts of nitrate reduction in the selenium volatile species generation. Sulfamic acid reduces rapidly nitrite to nitrogen; its addition while heating was found to be more effective. [16–21,40]. The experiments were carried out as mentioned in ˈ2.3.3.1 Simultaneous reduction of Se(VI) to Se(IV) and nitrate eliminationˈ.
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In order to develop as sensitive, accurate and dependable methods for determination of Arsenic (III) in blood is very important for human health protection. However many techniques are developed for the determination of Arsenic (III) including, flame atomic absorption spectroscopy (FASS), inductively coupled plasma/mass spectroscopy(ICP-MS), graphite furnace atomic absorption spectroscopy (GFAAS), inductively coupled plasma/atomic emission spectroscopy (ICP-AES), hybrid generation atomic absorption spectroscopy (HG-AAS), Plasmon resonance sensor (PRS) and atomic fluorescence spectrometry (AFS) [4–10]. However these techniques are required sophisticated and expensive equipment, high operating cost, professional and skilled staffs to carry out the study for the detection of Arsenic (III).
Strontiumtitaniumtrioxide (SrTiO₃) decorated β-cyclodextrin (β-CD) nanocomposite modified glassy carbon electrode was developed for the efficient and accurate determination of poisonous Arsenic (III) ions in water and blood serum samples. The synthesized SrTiO₃/β-CD nanocomposite was successfully characterized by spectral and electrochemical methods. The present SrTiO₃/β-CD nanocomposite modified GC electrode was provided an excellent electrochemical performance due to higher surface area, comparable to glassy carbon electrode with SrTiO₃ and β-CD separately. The fabricated electrochemical sensor (SrTiO₃/β-CD/GCE) was showed a sensitive electrochemical response for the oxidation of Arsenic (III) in 0.1 PBS. The oxidation peak current of Arsenic (III) was obtained by Amperometric, under the optimized conditions, which linearly increased with increase the concentration of the range between 10 and 140 μM and the detection limit was found to be 0.02 μM. The present SrTiO₃/β-CD/GC electrode was also showed better stability, good sensitivity and reproducibility and is applied successfully for the detection of Arsenic (III) in water and serum samples.
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In this work a new procedure for the determination of ultra-trace concentrations of Mo in plants is proposed. Microwave-assisted acid digestion of plants were performed without H₂O₂ in order to minimize contaminations and obtain lower procedure's blank. Solutions with different HNO₃ concentrations were evaluated to digest the samples and 7 or 1 mol L^− 1 HNO₃ solutions led to similar carbon residual contents. Since, analytical blank was a critical issue all samples were microwave-assisted digested using only 5 mL of 1 mol L^− 1 HNO₃. For Mo determinations in plant materials an ICP-MS with tandem configuration (ICP-MS/MS) with an octopole reaction system (ORS³) pressurized with oxygen gas and operating in MS/MS mode was employed to overcome polyatomic interferences caused by K, a major element in plants. Higher sensitivity was achieved for the determination of Mo using the mass-shift dioxide form (MoO₂⁺) obtained with O₂ gas flow rate at 0.2 mL min^− 1. The limit of detection of the analytical procedure was 260 pg g^− 1. Molybdenum was quantified in six certified reference materials: apple leaves (NIST 1515), peach leaves (NIST 1547), trace elements in spinach leaves (NIST 1570), tomato leaves (NIST 1573), wheat flour (NIST 1567a) and rice flour (NIST 1568b) for accuracy assessment. Recoveries for Mo were in the 100.0–109.6% range and analytical results were in agreement with certified values at a 95% confidence levels (t-test). The procedure was applied for determination of Mo in 7 different samples of sugar cane, corn leaves and roots, and rice flour and the concentrations were in the 0.39 to 3.81 μg g^− 1 range.
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Vietnam is an agricultural country with a population of about 88 million, with some 18 million inhabitants living in the Red River Delta in Northern Vietnam. The present study reports the chemical analyses of 68 water and 213 biological (human hair and urine) samples conducted to investigate arsenic contamination in tube well water and human arsenic exposure in four districts (Tu Liem, Dan Phuong, Ly Nhan, and Hoai Duc) in the Red River Delta. Arsenic concentrations in groundwater in these areas were in the range of < 1 to 632 μg/L, with severe contamination found in the communities Ly Nhan, Hoai Duc, and Dan Phuong. Arsenic concentrations were markedly lowered in water treated with sand filters, except for groundwater from Hoai Duc. Human hair samples had arsenic levels in the range of 0.07–7.51 μg/g, and among residents exposed to arsenic levels ≥ 50 μg/L, 64% of them had hair arsenic concentrations higher than 1 μg/g, which is a level that can cause skin lesions. Urinary arsenic concentrations were 4–435 μg/g creatinine. Concentrations of arsenic in hair and urine increased significantly with increasing arsenic content in drinking water, indicating that drinking water is a significant source of arsenic exposure for these residents. The percentage of inorganic arsenic (IA) in urine decreased with age, whereas the opposite trend was observed for monomethylarsonic acid (MMA) in urine. Significant co-interactions of age and arsenic exposure status were also detected for concentrations of arsenic in hair and the sum of IA, MMA, and dimethylarsinic acid (DMA) in urine and %MMA. In summary, this study demonstrates that a considerable proportion of the Vietnamese population is exposed to arsenic levels of chronic toxicity, even if sand filters reduce exposure in many households. Health problems caused by arsenic ingestion through drinking water are increasingly reported in Vietnam.
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The present work evaluates the influence of vessel cooling simultaneously to microwave-assisted digestion performed in a closed system with diluted HNO₃ under O₂ pressure. The effect of outside air flow-rates (60–190 m³ h⁻¹) used for cooling of digestion vessels was evaluated. An improvement in digestion efficiency caused by the reduction of HNO₃ partial pressure was observed when using higher air flow-rate (190 m³ h⁻¹), decreasing the residual carbon content for whole milk powder from 21.7 to 9.3% (lowest and highest air flow-rate, respectively). The use of high air flow-rate outside the digestion vessel resulted in a higher temperature gradient between liquid and gas phases inside the digestion vessel and improved the efficiency of sample digestion. Since a more pronounced temperature gradient was obtained, it contributed for increasing the condensation rate and thus allowed a reduction in the HNO₃ partial pressure of the digestion vessel, which improved the regeneration of HNO₃. An air flow-rate of 190 m³ h⁻¹ was selected for digestion of animal fat, bovine liver, ground soybean, non fat milk powder, oregano leaves, potato starch and whole milk powder samples, and a standard reference material of apple leaves (NIST 1515), bovine liver (NIST 1577) and whole milk powder (NIST 8435) for further metals determination by inductively coupled plasma atomic emission spectroscopy (ICP-OES). Results were in agreement with certified values and no interferences caused by matrix effects during the determination step were observed.

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^☆: This paper was presented at the 6th Rio Symposium on Atomic Spectrometry, held in Concepción and Pucón, Chile, December 2000, and is published in the Special Issue of Spectrochimica Acta Part B, dedicated to that conference.

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Minimization of volatile nitrogen oxides interference in the determination of arsenic by hydride generation atomic absorption spectrometry☆

Abstract

Introduction

Section snippets

Apparatus and instrumental parameters

Sample digestion without sulfamic acid addition — initial studies

Conclusion

Spectrochim. Acta Part B

Spectrochim. Acta Part B

Anal. Chim. Acta

Talanta

Spectrochim. Acta Part B

Anal. Chim. Acta

Anal. Chim. Acta

Mechanisms of transition metal interferences in hydride generation atomic-absorption spectrometry. Part 1. Influence of cobalt, copper, iron and nickel on selenium determination

Analyst

Determination of arsenic and tellurium by hydride generation atomic spectrometry: minimizing interferences from nickel, cobalt and copper by using an alkaline sample solution

Analyst

Mechanisms of transition metal interferences in hydride generation atomic-absorption spectrometry. Part 2. Influence of valency state of arsenic on the degree of signal depression caused by copper, iron and nickel

Analyst