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Organic solvents as interferents in arsenic determination by hydride generation atomic absorption spectrometry with flame atomization
Spectrochimica Acta Part B 61 (2006) 525 – 531 www.elsevier.com/locate/sab
Organic solvents as interferents in arsenic determination by hydride generation atomic absorption spectrometry with flame atomization Irina B. Karadjova a , Leonardo Lampugnani b,⁎, Jiri Dědina c , Alessandro D'Ulivo b , Massimo Onor b , Dimiter L. Tsalev a a Faculty of Chemistry, University of Sofia, 1 James Bourchier Blvd., Sofia 1164, Bulgaria C.N.R. Istituto per i processi chimico-fisici, Area della Ricerca di Pisa, Via Moruzzi 1, 56124 Pisa, Italy Academy of Sciences of the Czech Republic, Institute of Analytical Chemistry, Laboratory of Trace Element Analysis, Videňská 1083, 14220 Prague 4, Czech Republic b
c
Received 12 January 2006; accepted 22 March 2006 Available online 2 May 2006
Abstract Interference effects of various organic solvents miscible with water on arsenic determination by hydride generation atomic absorption spectrometry have been studied. Arsine was chemically generated in continuous flow hydride generation system and atomized by using a flame atomizer able to operate in two modes: miniature diffusion flame and flame-in-flame. The effects of experimental variables and atomization mode were investigated: tetrahydroborate and hydrochloric acid concentrations, argon, hydrogen and oxygen supply rates for the microflame, and the distance from the atomization region to the observation zone. The nature of the species formed in the flame due to the pyrolysis of organic solvent vapors entering the flame volume together with arsine is discussed. The observed signal depression in the presence of organic solvents has been mainly attributed to the atomization interference due to heterogeneous gas–solid reaction between the free arsenic atoms and finely dispersed carbon particles formed by carbon radicals recombination. The best tolerance to interferences was obtained by using flame-in-flame atomization (5–10 ml min− 1 of oxygen flow rate), together with higher argon and hydrogen supply rates and elevated observation heights. © 2006 Elsevier B.V. All rights reserved. Keywords: Hydride generation atomic absorption spectrometry; Organic solvent interference; Atomization interference; Miniature diffusion flame; Flame-in-flame
1. Introduction Hydride generation with atomic spectrometric quantification techniques is a major analytical approach for the separation, enrichment and determination of arsenic in the 10− 11–10− 5 g or 0.01–100 μg l− 1 range [1,2]. Hydride generation atomic absorption spectrometry (HGAAS) has been widely applied in analytical practice [1]. Nevertheless, it is well documented that HGAAS is prone to liquid-phase and gas-phase interferences [1,3]. Organic solvents are well recognized as interferents in determination of hydride forming elements and particularly of As by HGAAS in the analysis of alcoholic beverages like beer or wine [4–7]. There are scarce data on the degree and mechanisms of this interference. Organic solvents can depress the efficiency of arsine generation, transport and atomization. ⁎ Corresponding author. Tel.: +39 050 3152294; fax: +39 050 3152555. E-mail address: [email protected] (L. Lampugnani). 0584-8547/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2006.03.009
The aim of this work was to investigate and explain these interferences on arsenic HGAAS due to the presence of different organic solvents miscible with water and characterized with different physical parameters, such as vapor pressure and vapor density, summarized in Table 1 [8], by employing two types of flame atomizers with expected better tolerance to atomization interferences than the common quartz-tube atomizer (QTA) with external heating, namely a miniature diffusion flame (MDF) [9] and flame-in-flame (FIF) atomizer [10,11]. 2. Experimental 2.1. Reagents All reagents were of analytical or chromatographic reagent grade. Stock standard solution for arsenic was 1000 mg l− 1 As (III) AAS standard solution (Fluka, Buchs).Working arsenic standards of 0.3 mg l− 1 and 0.6 mg l− 1 were prepared daily by
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Table 1 Some characteristics of organic solvents used [8] Solvent
Boiling Vapor point, pressure, °C mm Hg (at 20 °C)
Methanol 65 Ethanol 78 i-Propanol 83 Acetonitrile 82 Ethanol 170 amine Ethylene 198 glycol Glycerol 290 Arsine a
Relative vapor density (vs. air = 1)
Heat of Relative vaporization, evaporation kJ mol− 1 rate (vs. butylacetate = 1)
97 43 33 73 0.4
1.1 1.6 2.1 1.4 2.1
39.2 40.5 42.1 34.2 –
5.9 3.3 3.0 5.8 –
0.06
2.14
58.7
–
0.0025a N760
– 2.7
76.1 –
– –
At 50 °C.
appropriate dilution in the required HCl medium and organic solvents. Aqueous solutions of sodium tetrahydroborate(III), NaBH4, were prepared fresh daily by dissolving the solid reagent (pellets reagent for AAS, BDH, Poole, UK) in 0.01 mol l− 1 NaOH. Deionized water by a Milli-Q system (Millipore, Bedford, MA) was used in all experiments. 2.2. Apparatus A Perkin Elmer Model 503 atomic absorption spectrometer and a Perkin Elmer arsenic electrodeless discharge lamp (EDL) powered at 8 W, wavelength of 193.7 nm and bandpass of 0.7 nm were used for all measurements. 2.3. Hydride generation Arsine was chemically generated from aqueous solution using NaBH4 reduction in a continuous flow system [9] (Fig. 1a). Sample and reductant flow rates were 4 and 2 ml min− 1, respectively. The total gas flow leaving the HG apparatus and entering the atomizer consists of two Ar flows: argon(I) (stripping gas) varied in the range 20–150 ml min− 1 added to the mixing cross, and argon(II) varied in the range 200–700 ml min− 1 and hydrogen varied within 50–300 ml min− 1 added to the gas–liquid separator. 2.4. Atomizer The atomizer, described in detail previously [12] (Fig. 1b), consisted of a vertical tube made of quartz (i.d. 6 mm). A nondeactivated gas chromatographic fused silica capillary (i.d. 0.53 mm) was axially positioned in the vertical quartz tube serving to introduce the oxygen (up to 20 ml min− 1) supporting a microflame. The capillary can be moved along the tube axis and could be fixed at desired distance from the atomizer top; this is denoted below as the “capillary distance”: e.g. capillary distance − 5 mm and 5 mm, respectively, stands for capillary tip position 5 mm below and above the atomizer rim. If not explicitly stated otherwise, the capillary distance 0 mm was employed. The
atomizer was operated in two different modes: (i) MDF mode, an Ar–H2 diffusion flame burned at the end of vertical quartz tube, no oxygen introduced to the capillary; and (ii) FIF mode, an oxygen microflame (oxygen flow rate to the capillary 1 to 20 ml min− 1) and Ar–H2 diffusion flame burned simultaneously at the end of vertical quartz tube. It should be noted that MDF is actually a limiting case (no oxygen introduced to the capillary) of FIF. The atom detection was performed in the region above the atomizer top. The optical beam axis (horizontal) and the atomizer axis (vertical) were in the same (vertical) plane. The optical beam cross section in the atomizer axis position was 3 mm (width) × 3 mm (height). The observation height, i.e. the distance of the atomizer top from the centre of the optical beam cross section, was 2.5 mm if not explicitly stated otherwise. The control of all gas flows was performed by ball rotameters except the microflame oxygen flow rate which was controlled by a mass flow controller (Cole Parmer, flow rate range of 0−20 ml min− 1). 3. Results and discussion Argon and hydrogen flow rates were optimized in order to obtain optimal signal-to-noise ratio, while also considering precision of measurements and avoiding extreme conditions of gas flows and high reductant levels. Flow rates of 30−50 ml min− 1 argon(I), 300 ml min− 1 of argon(II) and 80 ml min− 1 of H2 were found optimal. If not explicitly stated otherwise, these flow rates were employed in experiments described below. Organic solvents chosen for this study were various alcohols miscible with water but with different volatility, i.e. with different vapor pressure. Their depressive effect was studied within the reasonable concentration ranges of experimental variables: 1–10 % v/v of the organic solvent, 0.02 and 2 mol l− 1 HCl (at 0.6% m/v NaBH4) and 0.2–1.4% m/v NaBH4 (at 2 mol l− 1 HCl). It was found that the signal observed was not affected in the presence of non-volatile organic solvents like polyalcohols (e.g. ethylene glycol and glycerol). This is a proof that these organic solvents do not influence arsine generation in the liquid phase either due to different hydrolysis rates of NaBH4 or due to different mixing rates. In contrast, a strong interference was observed in the presence of volatile organic solvents (Fig. 2). The degree of this interference is presented as a ratio of the peak height absorbance in the presence and absence of alcohols (Ao/ Aaq). It appears that the volatility of the organic solvent is the most important parameter affecting the magnitude of the observed interference indicating that the interference takes place in the gaseous phase. This is supported by the observation that stronger signal depression is observed for all alcohols at higher HCl concentrations when the more vigorous reaction introduces larger amount of solvent vapors into the atomizer. The effect of the NaBH4 concentration is analogous, as shown in Fig. 3 for ethanol as an example: higher concentration of NaBH4 leads to more pronounced interference. The lowest tested NaBH4 concentration, 0.4 % m/v, performs best in terms of accuracy but the sensitivity is 25% lower. A compromise NaBH4 concentration
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(a) Ar II+H2 Ar I
p sample
NaBH4
Reaction coil
atomizer
u m p s
gas/liquid separator
waste
O2
(b) Optical beam cross section observation height Oxygen capillary
capillary distance
Arsine+Ar+H2
O2 Fig. 1. (a) The continuous flow hydride generator; (b) scheme of the atomizer with definition of the observation height and the capillary distance.
appears 0.6 % m/v since it produces only a minor sensitivity loss: − 10% vs. 0.9% m/v NaBH4. The tolerance to interference is 10% to 20% better with shorter reaction coil, obviously because of lower yield of solvent vapors. Consequently, a minimal length of reaction coil (1 cm) was adopted. In principle, the gaseous phase interference might take place either during arsine transport, on its way from reaction mixture to the atomizer and/or during arsine atomization, i.e. in the atomizer. The extent of the transport interference cannot be affected by atomizer parameters. However, the extent of ethanol interference in MDF (Fig. 2) is two orders of magnitude lower compared with that reported recently in externally heated QTA
[6]. Additionally, as discussed below, atomizer parameters can reduce the extent of interference in FIF atomizer. It can be concluded that, at the relatively low alcohol concentrations studied, the transport interference is not significant. Regarding atomization interferences, they could be either due to thermal expansion of the organic solvent vapor, resulting in expulsion losses of the analyte atoms or due to chemical processes in the atomizer. Among the three alcohols studied, the observed interference extent was lowest for methanol (Fig. 2) characterized by the highest vapor pressure (cf. the highest volatility) and the lowest vapor density (Table 1). The highest interference extent has been observed for i-propanol, the solvent with the lowest vapor pressure and the highest vapor density. It
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1.0
1.0 0.9
0.9
Ao/Aaq
Ao/Aaq
0.8 0.7
0.8 0.7
0.6 0.5
0.6
-1 2 mol l HCl
0.4
-1 0.02 mol l HCl
0.5 2
4
6
8
10
2
Methanol, % v/v
4
6
8
10
8
10
Methanol, % v/v
1.0
1.0 0.9
0.8
Ao/Aaq
Ao/Aaq
0.8
0.6
0.7 0.6
0.4
-1 2 mol l HCl 2
4
0.5
6
8
10
-1 0.02 mol l HCl 2
4
6 Ethanol, % v/v
1.0
1.0
0.8
0.8
Ao/Aaq
Ao/Aaq
Ethanol, % v/v
0.6
0.6
0.4 0.4 0.2
-1 0.02 mol l HCl
-1 2 mol l HCl 0.2 2
4
6
8
10
i-Propanol, % v/v
2
4
6
8
10
i-Propanol, % v/v
▪
Fig. 2. The interference magnitude (Ao/Aaq) versus different concentrations of the first three aliphatic alcohols in the absence of oxygen (MDF) (□) and at different O2 flow rates (FIF) 2 (▴), 3 (▾), 4 (●), 5 (♦), 6 (⋄), 10 ( ) and 15 ml min− 1 (○).
indicates that the molecular mass (namely the higher number of carbon atoms in the molecule) controls the magnitude of interference. In the case of the interference due to thermal expansion, the absorbance signal should decrease proportionally with the amount of solvent vapor entering the atomizer; the effect would be stronger for alcohols with higher vapor pressure. However, this is not observed experimentally: methanol, the solvent with the highest vapor pressure, is in fact less depressive. Therefore, the “chemical” interference remains to account for the observed depression of As signal in the presence of alcohols.
To assess the mechanism of the “chemical” interference, atomization mechanism in MDF has to be considered. As shown previously [10], hydride is atomized in the diffusion flame by interaction with hydrogen radicals formed in outer zone of the flame by reactions between hydrogen and ambient oxygen. Hydrogen radicals can thus diffuse to the cooler inner parts of the flame. Consequently, the whole flame volume is loaded by a high concentration of hydrogen radicals so that hydride is fully atomized already when passing the atomizer top. In a “pure” diffusion flame, in the absence of interfering
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1.0
Table 2 Effect of observation height (OH) for the flame of 700 ml min− 1 Ar and 300 ml min− 1 H2
0.8
Ao /Aaq
529
0.6
0.4
0.2 2
4
6 Ethanol, % v/v
8
10
Solvent, 10% v/v
Ao/Aaq
MeOH EtOH i-PrOH MeOH EtOH i-PrOH MeOH EtOH i-PrOH
no O2 10 ml min− 1 O2 15 ml min− 1 O2
OH = 2.5 mm 0.82 ± 0.03 0.67 ± 0.03 0.42 ± 0.04 0.97 ± 0.02 0.91 ± 0.02 0.75 ± 0.03 0.99 ± 0.02 0.97 ± 0.02 0.89 ± 0.03
OH = 8.5 mm 0.95 ± 0.02 0.81 ± 0.03 0.56 ± 0.03 0.99 ± 0.02 0.98 ± 0.02 0.85 ± 0.03 0.99 ± 0.02 0.99 ± 0.02 0.94 ± 0.02
Fig. 3. The effect of NaBH4 concentration in MDF. 2 mol l− 1 HCl, 0.4% (♦), 0.6% ( ), 0.9% (▴), 1.5% (▾) and 1.8% (●) m/v NaBH4, n = 3.
The effect of several other parameters on the interference magnitude should be highlighted:
species, analyte is present exclusively in the free atom form within the whole flame volume [10]. Considering the mechanism of hydride atomization [3,9], two mechanisms of the “chemical” atomization interference could be expected in the presence of interfering species: (i) the “radical population” one due to the depletion of hydrogen radical population in the atomizer and (ii) “analyte decay” one due to the acceleration of the decay of analyte free atom population caused by reactions of the free atoms. Volatile organic compounds are pyrolized in the inner flame zone of the diffusion hydrogen flame, close to the top of the atomizer, forming a variety of carbon radicals (e.g. C, CH, CH2, CH3) [13]. Since the carbon radicals are very reactive species, the formation of elemental carbon particles in the inner, oxygenfree, flame zone is expected. The particles are transported to the flame boundary, which is an oxygen-rich zone, to be oxidized, at least partially, there. Both kinds of carbon species, i.e. carbon radicals and particles, are potential interferents. In principle, carbon radicals as well as particles can interfere by both, radical population and analyte decay, mechanisms. The carbon particles are suspected to be a very efficient analyte decay interferent [12]. In summary, in the outer flame zone is lower concentration of carbon particles and much higher population of hydrogen radicals compared to the inner flame zone. Consequently, much more pronounced interferences should be expected in the inner flame zone. In the FIF atomizer, there is an additional oxygen-rich zone, just above the tip of the oxygen delivery capillary, supported by the oxygen flow introduced through the capillary. The extent of this oxygen-rich zone is controlled by the oxygen flow rate. The additional oxygen-rich zone actually reduces the size of the inner, oxygen-free, flame zone. Consequently, interference magnitude should decrease with increasing the oxygen flow rate. This scenario is in accord with the decrease of interference extent with the observation height (Table 2) and with increasing O2 flow rate (Fig. 2). Further optimization of O2 flow rate for each type and concentration of solvent results in almost complete removal of the organic solvent interference (Fig. 4).
Hydrogen flow rate Higher H2 flow rates are generally preferred in combination with high O2 flow rates thus slightly improving tolerance to organic solvents. However, the effect of high hydrogen flow rates is not very pronounced. Argon flow rate The tolerance to organic solvents interference improves with the increase of Ar flow rate (Fig. 5). This is compatible with a suppression of carbon particle formation at higher Ar flow rates. Capillary distance Three different capillary distances were tested: − 5 mm, 0 mm and 5 mm. Results with capillary distance 0 mm are presented at Fig. 2. A strong memory effect appeared with capillary distance − 5 mm: the signal could not reach the steady state. A gradual decline of the signal was observed followed by a second small peak upon switching standard to blank. The explanation is that carbon particles formed by 1.0 0.8 Ao/Aaq
▪
0.6 0.4 0.2 0.0 0
2
4
6 8 10 Oxygen, ml min-1
12
14
Fig. 4. Influence of O2 flow rate in MDF (for O2 flow rate of 0 ml min− 1) and FIF. 0.9% m/v NaBH4, 2 mol l− 1 HCl, 10% v/v methanol (▴), ethanol ( ), ipropanol (●), ethanol amine (♦), acetonitrile (▾) and 2% v/v acetonitrile (□), n = 3.
▪
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Fig. 5. Influence of argon II flow rate in MDF. H2 flow rate 300 ml min− 1, 2 mol l− 1 HCl, 0.9% m/v NaBH4, n = 3.
In order to prove validity of the above scenario, the interference magnitude due to two additional organic solvents, ethanol amine and acetonitrile, was studied. It is worth mentioning that acetonitrile is a common component of mobile phases employed in liquid chromatography. The dependence of interference magnitude in MDF for all investigated solvents is shown in Fig. 6. The observation that ethanol amine and acetonitrile, respectively, is the least and the most serious interferent is the further indication that the way of molecule pyrolysis, not the vapor pressure (compare Table 1) is responsible for the degree of interference. The influence of O2 flow rate is given in Fig. 4: even the highest feasible O2 flow rates cannot completely remove acetonitrile interference. It is noteworthy, however, that
applying higher O2 flow rates (15 ml min− 1) has brought improvement in absorbance–time profiles of signals even in the presence of acetonitrile—yet with substantial loss of sensitivity (2.5-fold) vs. aqueous As(III) standards. 4. Conclusions Organic solvents depressively interfere with arsine atomization in hydride generation flame AAS in the following order: ethanol amine ∼ methanol b ethanol b i-propanol b acetonitrile, and, for atomizers: FIF b MDF≪(externally heated) QTA. Important parameters of organic solvents are their vapor pressure, carbon contents and their pyrolysis pattern, presumably yielding carbon-containing radicals and carbon particles, which are the actual interfering species. Stronger interference is observed at higher HCl and NaBH4 concentrations, longer reaction coils, lower H2, Ar and O2 flow rates, and lower observation heights. Acetonitrile, a common component of many mobile phases in liquid chromatography is a very strong 1.0
0.8
0.6
Ao / Aaq
pyrolysis of solvent vapors are deposited on inner atomizer surface forming active centers to trap arsenic atoms. After stopping the sample introduction, the trapped analyte is volatilized and atomized in the diffusion flame, which is observed as the second peak. This explanation is supported by the formation of a visible black deposit on inner atomizer wall observed upon extinguishing of the diffusion flame when only the microflame on the capillary tip was burning. This proves that carbon particles can be deposited on inner atomizer wall; the process is faster when the atomizer wall is not heated by the diffusion flame. Compared with capillary distance 0 mm, the observed interference extent was more pronounced at capillary distance 5 mm. This corresponds to actual shifting of the additional oxygen-rich zone (formed by the microflame) upwards out of the region “observed” by the optical beam. Therefore capillary distance 0 mm has been adopted as optimal.
0.4
0.2
0.0
2
4
6
8
10
Solvent, % v/v
▪
Fig. 6. Interference magnitude in MDF due to different solvents. 0.9% m/v NaBH4, 2 mol l− 1 HCl, 10% v/v methanol (▴), ethanol ( ), i-propanol (●), ethanol amine (♦) and acetonitrile (▾), n = 3.
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interferent and calls for FIF atomization mode and for higher O2 flow rates. Acknowledgements The authors gratefully acknowledge funding by NATO Outreach Fellowship 219.34 to IBK, partial financial support from Bulgarian Ministry of Education, FNSF Project VYX-12/ 05 and the support by the Grant Agency of the AS CR (Project No. A400310507). References [1] J. Dědina, D.L. Tsalev, Hydride Generation Atomic Absorption Spectrometry, Wiley, Chichester, 1995. [2] D.L. Tsalev, Vapour generation or electrothermal atomic absorption spectrometry—both! Spectrochim. Acta Part B 55 (2000) 917–933. [3] J. Dědina, Interference of volatile hydride forming elements in selenium determination by AAS with hydride generation, Anal. Chem. 54 (1982) 2097–2102. [4] M. Segura, Y. Madrid, C. Cámara, Evaluation of atomic fluorescence and atomic absorption spectrometric techniques for the determination of arsenic in wine and beer by direct hydride generation sample introduction, J. Anal. At. Spectrom. 14 (1999) 131–137. [5] A. Martinez, A. Morales-Rubio, M.L. Cervera, M. de la Guardia, Atomic fluorescence determination of total and inorganic arsenic species in beer, J. Anal. At. Spectrom. 16 (2001) 762–766.
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[6] K. Tašev, I. Karadjova, T. Stafilov, Determination of inorganic and total arsenic in wines by hydride generation atomic absorption spectrometry, Microchim. Acta 149 (2005) 55–60. [7] I.B. Karadjova, L. Lampugnani, M. Onor, A. D'Ulivo, D.L. Tsalev, Continuous flow hydride generation–atomic fluorescence spectrometric determination and speciation of arsenic in wine, Spectrochim. Acta Part B 60 (2005) 816–823. [8] CRC Handbook of Chemistry and Physics, 67th edn., R.C. Weast (Ed.), CRC Press, Boca Raton, Florida. [9] J. Dědina, A. D'Ulivo, Argon shielded highly fuel-rich, hydrogen–oxygen diffusion microflame—a new hydride atomizer, Spectrochim. Acta Part B 52 (1997) 1737–1746. [10] J. Dědina, A. D'Ulivo, L. Lampugnani, T. Matoušek, R. Zamboni, Selenium hydride atomization, fate of free atoms and spectroscopic temperature in miniature diffusion flame atomizer studied by atomic absorption spectrometry, Spectrochim. Acta Part B 53 (1998) 1777–1790. [11] A. D'Ulivo, J. Dedina, L. Lampugnani, A. Selecka, Mechanism of atomization interference by oxygen at trace level in miniature flame hydride atomizers, Spectrochim. Acta Part B 60 (2005) 1270–1279. [12] A. D'Ulivo, J. Dedina, Interferences in hydride atomization studied by atomic absorption and atomic fluorescence spectrometry, Spectrochim. Acta Part B 51 (1996) 481–498. [13] C.S. McEnally, A.M. Schaffer, M.B. Long, L.D. Pfefferle, M.D. Smooke, M.B. Colket, R.J. Hall, Computational and experimental study of soot formation in a coflow, laminar, ethylene diffusion flame, Twenty-Seventh Symposium (International) on Combustion/The Combustion Institute, 1998, pp. 1497–1505.
Organic solvents as interferents in arsenic determination by hydride generation atomic absorption spectrometry with flame atomization Irina B. Karadjova a , Leonardo Lampugnani b,⁎, Jiri Dědina c , Alessandro D'Ulivo b , Massimo Onor b , Dimiter L. Tsalev a a Faculty of Chemistry, University of Sofia, 1 James Bourchier Blvd., Sofia 1164, Bulgaria C.N.R. Istituto per i processi chimico-fisici, Area della Ricerca di Pisa, Via Moruzzi 1, 56124 Pisa, Italy Academy of Sciences of the Czech Republic, Institute of Analytical Chemistry, Laboratory of Trace Element Analysis, Videňská 1083, 14220 Prague 4, Czech Republic b
c
Received 12 January 2006; accepted 22 March 2006 Available online 2 May 2006
Abstract Interference effects of various organic solvents miscible with water on arsenic determination by hydride generation atomic absorption spectrometry have been studied. Arsine was chemically generated in continuous flow hydride generation system and atomized by using a flame atomizer able to operate in two modes: miniature diffusion flame and flame-in-flame. The effects of experimental variables and atomization mode were investigated: tetrahydroborate and hydrochloric acid concentrations, argon, hydrogen and oxygen supply rates for the microflame, and the distance from the atomization region to the observation zone. The nature of the species formed in the flame due to the pyrolysis of organic solvent vapors entering the flame volume together with arsine is discussed. The observed signal depression in the presence of organic solvents has been mainly attributed to the atomization interference due to heterogeneous gas–solid reaction between the free arsenic atoms and finely dispersed carbon particles formed by carbon radicals recombination. The best tolerance to interferences was obtained by using flame-in-flame atomization (5–10 ml min− 1 of oxygen flow rate), together with higher argon and hydrogen supply rates and elevated observation heights. © 2006 Elsevier B.V. All rights reserved. Keywords: Hydride generation atomic absorption spectrometry; Organic solvent interference; Atomization interference; Miniature diffusion flame; Flame-in-flame
1. Introduction Hydride generation with atomic spectrometric quantification techniques is a major analytical approach for the separation, enrichment and determination of arsenic in the 10− 11–10− 5 g or 0.01–100 μg l− 1 range [1,2]. Hydride generation atomic absorption spectrometry (HGAAS) has been widely applied in analytical practice [1]. Nevertheless, it is well documented that HGAAS is prone to liquid-phase and gas-phase interferences [1,3]. Organic solvents are well recognized as interferents in determination of hydride forming elements and particularly of As by HGAAS in the analysis of alcoholic beverages like beer or wine [4–7]. There are scarce data on the degree and mechanisms of this interference. Organic solvents can depress the efficiency of arsine generation, transport and atomization. ⁎ Corresponding author. Tel.: +39 050 3152294; fax: +39 050 3152555. E-mail address: [email protected] (L. Lampugnani). 0584-8547/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2006.03.009
The aim of this work was to investigate and explain these interferences on arsenic HGAAS due to the presence of different organic solvents miscible with water and characterized with different physical parameters, such as vapor pressure and vapor density, summarized in Table 1 [8], by employing two types of flame atomizers with expected better tolerance to atomization interferences than the common quartz-tube atomizer (QTA) with external heating, namely a miniature diffusion flame (MDF) [9] and flame-in-flame (FIF) atomizer [10,11]. 2. Experimental 2.1. Reagents All reagents were of analytical or chromatographic reagent grade. Stock standard solution for arsenic was 1000 mg l− 1 As (III) AAS standard solution (Fluka, Buchs).Working arsenic standards of 0.3 mg l− 1 and 0.6 mg l− 1 were prepared daily by
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Table 1 Some characteristics of organic solvents used [8] Solvent
Boiling Vapor point, pressure, °C mm Hg (at 20 °C)
Methanol 65 Ethanol 78 i-Propanol 83 Acetonitrile 82 Ethanol 170 amine Ethylene 198 glycol Glycerol 290 Arsine a
Relative vapor density (vs. air = 1)
Heat of Relative vaporization, evaporation kJ mol− 1 rate (vs. butylacetate = 1)
97 43 33 73 0.4
1.1 1.6 2.1 1.4 2.1
39.2 40.5 42.1 34.2 –
5.9 3.3 3.0 5.8 –
0.06
2.14
58.7
–
0.0025a N760
– 2.7
76.1 –
– –
At 50 °C.
appropriate dilution in the required HCl medium and organic solvents. Aqueous solutions of sodium tetrahydroborate(III), NaBH4, were prepared fresh daily by dissolving the solid reagent (pellets reagent for AAS, BDH, Poole, UK) in 0.01 mol l− 1 NaOH. Deionized water by a Milli-Q system (Millipore, Bedford, MA) was used in all experiments. 2.2. Apparatus A Perkin Elmer Model 503 atomic absorption spectrometer and a Perkin Elmer arsenic electrodeless discharge lamp (EDL) powered at 8 W, wavelength of 193.7 nm and bandpass of 0.7 nm were used for all measurements. 2.3. Hydride generation Arsine was chemically generated from aqueous solution using NaBH4 reduction in a continuous flow system [9] (Fig. 1a). Sample and reductant flow rates were 4 and 2 ml min− 1, respectively. The total gas flow leaving the HG apparatus and entering the atomizer consists of two Ar flows: argon(I) (stripping gas) varied in the range 20–150 ml min− 1 added to the mixing cross, and argon(II) varied in the range 200–700 ml min− 1 and hydrogen varied within 50–300 ml min− 1 added to the gas–liquid separator. 2.4. Atomizer The atomizer, described in detail previously [12] (Fig. 1b), consisted of a vertical tube made of quartz (i.d. 6 mm). A nondeactivated gas chromatographic fused silica capillary (i.d. 0.53 mm) was axially positioned in the vertical quartz tube serving to introduce the oxygen (up to 20 ml min− 1) supporting a microflame. The capillary can be moved along the tube axis and could be fixed at desired distance from the atomizer top; this is denoted below as the “capillary distance”: e.g. capillary distance − 5 mm and 5 mm, respectively, stands for capillary tip position 5 mm below and above the atomizer rim. If not explicitly stated otherwise, the capillary distance 0 mm was employed. The
atomizer was operated in two different modes: (i) MDF mode, an Ar–H2 diffusion flame burned at the end of vertical quartz tube, no oxygen introduced to the capillary; and (ii) FIF mode, an oxygen microflame (oxygen flow rate to the capillary 1 to 20 ml min− 1) and Ar–H2 diffusion flame burned simultaneously at the end of vertical quartz tube. It should be noted that MDF is actually a limiting case (no oxygen introduced to the capillary) of FIF. The atom detection was performed in the region above the atomizer top. The optical beam axis (horizontal) and the atomizer axis (vertical) were in the same (vertical) plane. The optical beam cross section in the atomizer axis position was 3 mm (width) × 3 mm (height). The observation height, i.e. the distance of the atomizer top from the centre of the optical beam cross section, was 2.5 mm if not explicitly stated otherwise. The control of all gas flows was performed by ball rotameters except the microflame oxygen flow rate which was controlled by a mass flow controller (Cole Parmer, flow rate range of 0−20 ml min− 1). 3. Results and discussion Argon and hydrogen flow rates were optimized in order to obtain optimal signal-to-noise ratio, while also considering precision of measurements and avoiding extreme conditions of gas flows and high reductant levels. Flow rates of 30−50 ml min− 1 argon(I), 300 ml min− 1 of argon(II) and 80 ml min− 1 of H2 were found optimal. If not explicitly stated otherwise, these flow rates were employed in experiments described below. Organic solvents chosen for this study were various alcohols miscible with water but with different volatility, i.e. with different vapor pressure. Their depressive effect was studied within the reasonable concentration ranges of experimental variables: 1–10 % v/v of the organic solvent, 0.02 and 2 mol l− 1 HCl (at 0.6% m/v NaBH4) and 0.2–1.4% m/v NaBH4 (at 2 mol l− 1 HCl). It was found that the signal observed was not affected in the presence of non-volatile organic solvents like polyalcohols (e.g. ethylene glycol and glycerol). This is a proof that these organic solvents do not influence arsine generation in the liquid phase either due to different hydrolysis rates of NaBH4 or due to different mixing rates. In contrast, a strong interference was observed in the presence of volatile organic solvents (Fig. 2). The degree of this interference is presented as a ratio of the peak height absorbance in the presence and absence of alcohols (Ao/ Aaq). It appears that the volatility of the organic solvent is the most important parameter affecting the magnitude of the observed interference indicating that the interference takes place in the gaseous phase. This is supported by the observation that stronger signal depression is observed for all alcohols at higher HCl concentrations when the more vigorous reaction introduces larger amount of solvent vapors into the atomizer. The effect of the NaBH4 concentration is analogous, as shown in Fig. 3 for ethanol as an example: higher concentration of NaBH4 leads to more pronounced interference. The lowest tested NaBH4 concentration, 0.4 % m/v, performs best in terms of accuracy but the sensitivity is 25% lower. A compromise NaBH4 concentration
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(a) Ar II+H2 Ar I
p sample
NaBH4
Reaction coil
atomizer
u m p s
gas/liquid separator
waste
O2
(b) Optical beam cross section observation height Oxygen capillary
capillary distance
Arsine+Ar+H2
O2 Fig. 1. (a) The continuous flow hydride generator; (b) scheme of the atomizer with definition of the observation height and the capillary distance.
appears 0.6 % m/v since it produces only a minor sensitivity loss: − 10% vs. 0.9% m/v NaBH4. The tolerance to interference is 10% to 20% better with shorter reaction coil, obviously because of lower yield of solvent vapors. Consequently, a minimal length of reaction coil (1 cm) was adopted. In principle, the gaseous phase interference might take place either during arsine transport, on its way from reaction mixture to the atomizer and/or during arsine atomization, i.e. in the atomizer. The extent of the transport interference cannot be affected by atomizer parameters. However, the extent of ethanol interference in MDF (Fig. 2) is two orders of magnitude lower compared with that reported recently in externally heated QTA
[6]. Additionally, as discussed below, atomizer parameters can reduce the extent of interference in FIF atomizer. It can be concluded that, at the relatively low alcohol concentrations studied, the transport interference is not significant. Regarding atomization interferences, they could be either due to thermal expansion of the organic solvent vapor, resulting in expulsion losses of the analyte atoms or due to chemical processes in the atomizer. Among the three alcohols studied, the observed interference extent was lowest for methanol (Fig. 2) characterized by the highest vapor pressure (cf. the highest volatility) and the lowest vapor density (Table 1). The highest interference extent has been observed for i-propanol, the solvent with the lowest vapor pressure and the highest vapor density. It
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1.0
1.0 0.9
0.9
Ao/Aaq
Ao/Aaq
0.8 0.7
0.8 0.7
0.6 0.5
0.6
-1 2 mol l HCl
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-1 0.02 mol l HCl
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Methanol, % v/v
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6
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Methanol, % v/v
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0.6
0.7 0.6
0.4
-1 2 mol l HCl 2
4
0.5
6
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-1 0.02 mol l HCl 2
4
6 Ethanol, % v/v
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1.0
0.8
0.8
Ao/Aaq
Ao/Aaq
Ethanol, % v/v
0.6
0.6
0.4 0.4 0.2
-1 0.02 mol l HCl
-1 2 mol l HCl 0.2 2
4
6
8
10
i-Propanol, % v/v
2
4
6
8
10
i-Propanol, % v/v
▪
Fig. 2. The interference magnitude (Ao/Aaq) versus different concentrations of the first three aliphatic alcohols in the absence of oxygen (MDF) (□) and at different O2 flow rates (FIF) 2 (▴), 3 (▾), 4 (●), 5 (♦), 6 (⋄), 10 ( ) and 15 ml min− 1 (○).
indicates that the molecular mass (namely the higher number of carbon atoms in the molecule) controls the magnitude of interference. In the case of the interference due to thermal expansion, the absorbance signal should decrease proportionally with the amount of solvent vapor entering the atomizer; the effect would be stronger for alcohols with higher vapor pressure. However, this is not observed experimentally: methanol, the solvent with the highest vapor pressure, is in fact less depressive. Therefore, the “chemical” interference remains to account for the observed depression of As signal in the presence of alcohols.
To assess the mechanism of the “chemical” interference, atomization mechanism in MDF has to be considered. As shown previously [10], hydride is atomized in the diffusion flame by interaction with hydrogen radicals formed in outer zone of the flame by reactions between hydrogen and ambient oxygen. Hydrogen radicals can thus diffuse to the cooler inner parts of the flame. Consequently, the whole flame volume is loaded by a high concentration of hydrogen radicals so that hydride is fully atomized already when passing the atomizer top. In a “pure” diffusion flame, in the absence of interfering
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1.0
Table 2 Effect of observation height (OH) for the flame of 700 ml min− 1 Ar and 300 ml min− 1 H2
0.8
Ao /Aaq
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0.6
0.4
0.2 2
4
6 Ethanol, % v/v
8
10
Solvent, 10% v/v
Ao/Aaq
MeOH EtOH i-PrOH MeOH EtOH i-PrOH MeOH EtOH i-PrOH
no O2 10 ml min− 1 O2 15 ml min− 1 O2
OH = 2.5 mm 0.82 ± 0.03 0.67 ± 0.03 0.42 ± 0.04 0.97 ± 0.02 0.91 ± 0.02 0.75 ± 0.03 0.99 ± 0.02 0.97 ± 0.02 0.89 ± 0.03
OH = 8.5 mm 0.95 ± 0.02 0.81 ± 0.03 0.56 ± 0.03 0.99 ± 0.02 0.98 ± 0.02 0.85 ± 0.03 0.99 ± 0.02 0.99 ± 0.02 0.94 ± 0.02
Fig. 3. The effect of NaBH4 concentration in MDF. 2 mol l− 1 HCl, 0.4% (♦), 0.6% ( ), 0.9% (▴), 1.5% (▾) and 1.8% (●) m/v NaBH4, n = 3.
The effect of several other parameters on the interference magnitude should be highlighted:
species, analyte is present exclusively in the free atom form within the whole flame volume [10]. Considering the mechanism of hydride atomization [3,9], two mechanisms of the “chemical” atomization interference could be expected in the presence of interfering species: (i) the “radical population” one due to the depletion of hydrogen radical population in the atomizer and (ii) “analyte decay” one due to the acceleration of the decay of analyte free atom population caused by reactions of the free atoms. Volatile organic compounds are pyrolized in the inner flame zone of the diffusion hydrogen flame, close to the top of the atomizer, forming a variety of carbon radicals (e.g. C, CH, CH2, CH3) [13]. Since the carbon radicals are very reactive species, the formation of elemental carbon particles in the inner, oxygenfree, flame zone is expected. The particles are transported to the flame boundary, which is an oxygen-rich zone, to be oxidized, at least partially, there. Both kinds of carbon species, i.e. carbon radicals and particles, are potential interferents. In principle, carbon radicals as well as particles can interfere by both, radical population and analyte decay, mechanisms. The carbon particles are suspected to be a very efficient analyte decay interferent [12]. In summary, in the outer flame zone is lower concentration of carbon particles and much higher population of hydrogen radicals compared to the inner flame zone. Consequently, much more pronounced interferences should be expected in the inner flame zone. In the FIF atomizer, there is an additional oxygen-rich zone, just above the tip of the oxygen delivery capillary, supported by the oxygen flow introduced through the capillary. The extent of this oxygen-rich zone is controlled by the oxygen flow rate. The additional oxygen-rich zone actually reduces the size of the inner, oxygen-free, flame zone. Consequently, interference magnitude should decrease with increasing the oxygen flow rate. This scenario is in accord with the decrease of interference extent with the observation height (Table 2) and with increasing O2 flow rate (Fig. 2). Further optimization of O2 flow rate for each type and concentration of solvent results in almost complete removal of the organic solvent interference (Fig. 4).
Hydrogen flow rate Higher H2 flow rates are generally preferred in combination with high O2 flow rates thus slightly improving tolerance to organic solvents. However, the effect of high hydrogen flow rates is not very pronounced. Argon flow rate The tolerance to organic solvents interference improves with the increase of Ar flow rate (Fig. 5). This is compatible with a suppression of carbon particle formation at higher Ar flow rates. Capillary distance Three different capillary distances were tested: − 5 mm, 0 mm and 5 mm. Results with capillary distance 0 mm are presented at Fig. 2. A strong memory effect appeared with capillary distance − 5 mm: the signal could not reach the steady state. A gradual decline of the signal was observed followed by a second small peak upon switching standard to blank. The explanation is that carbon particles formed by 1.0 0.8 Ao/Aaq
▪
0.6 0.4 0.2 0.0 0
2
4
6 8 10 Oxygen, ml min-1
12
14
Fig. 4. Influence of O2 flow rate in MDF (for O2 flow rate of 0 ml min− 1) and FIF. 0.9% m/v NaBH4, 2 mol l− 1 HCl, 10% v/v methanol (▴), ethanol ( ), ipropanol (●), ethanol amine (♦), acetonitrile (▾) and 2% v/v acetonitrile (□), n = 3.
▪
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Fig. 5. Influence of argon II flow rate in MDF. H2 flow rate 300 ml min− 1, 2 mol l− 1 HCl, 0.9% m/v NaBH4, n = 3.
In order to prove validity of the above scenario, the interference magnitude due to two additional organic solvents, ethanol amine and acetonitrile, was studied. It is worth mentioning that acetonitrile is a common component of mobile phases employed in liquid chromatography. The dependence of interference magnitude in MDF for all investigated solvents is shown in Fig. 6. The observation that ethanol amine and acetonitrile, respectively, is the least and the most serious interferent is the further indication that the way of molecule pyrolysis, not the vapor pressure (compare Table 1) is responsible for the degree of interference. The influence of O2 flow rate is given in Fig. 4: even the highest feasible O2 flow rates cannot completely remove acetonitrile interference. It is noteworthy, however, that
applying higher O2 flow rates (15 ml min− 1) has brought improvement in absorbance–time profiles of signals even in the presence of acetonitrile—yet with substantial loss of sensitivity (2.5-fold) vs. aqueous As(III) standards. 4. Conclusions Organic solvents depressively interfere with arsine atomization in hydride generation flame AAS in the following order: ethanol amine ∼ methanol b ethanol b i-propanol b acetonitrile, and, for atomizers: FIF b MDF≪(externally heated) QTA. Important parameters of organic solvents are their vapor pressure, carbon contents and their pyrolysis pattern, presumably yielding carbon-containing radicals and carbon particles, which are the actual interfering species. Stronger interference is observed at higher HCl and NaBH4 concentrations, longer reaction coils, lower H2, Ar and O2 flow rates, and lower observation heights. Acetonitrile, a common component of many mobile phases in liquid chromatography is a very strong 1.0
0.8
0.6
Ao / Aaq
pyrolysis of solvent vapors are deposited on inner atomizer surface forming active centers to trap arsenic atoms. After stopping the sample introduction, the trapped analyte is volatilized and atomized in the diffusion flame, which is observed as the second peak. This explanation is supported by the formation of a visible black deposit on inner atomizer wall observed upon extinguishing of the diffusion flame when only the microflame on the capillary tip was burning. This proves that carbon particles can be deposited on inner atomizer wall; the process is faster when the atomizer wall is not heated by the diffusion flame. Compared with capillary distance 0 mm, the observed interference extent was more pronounced at capillary distance 5 mm. This corresponds to actual shifting of the additional oxygen-rich zone (formed by the microflame) upwards out of the region “observed” by the optical beam. Therefore capillary distance 0 mm has been adopted as optimal.
0.4
0.2
0.0
2
4
6
8
10
Solvent, % v/v
▪
Fig. 6. Interference magnitude in MDF due to different solvents. 0.9% m/v NaBH4, 2 mol l− 1 HCl, 10% v/v methanol (▴), ethanol ( ), i-propanol (●), ethanol amine (♦) and acetonitrile (▾), n = 3.
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interferent and calls for FIF atomization mode and for higher O2 flow rates. Acknowledgements The authors gratefully acknowledge funding by NATO Outreach Fellowship 219.34 to IBK, partial financial support from Bulgarian Ministry of Education, FNSF Project VYX-12/ 05 and the support by the Grant Agency of the AS CR (Project No. A400310507). References [1] J. Dědina, D.L. Tsalev, Hydride Generation Atomic Absorption Spectrometry, Wiley, Chichester, 1995. [2] D.L. Tsalev, Vapour generation or electrothermal atomic absorption spectrometry—both! Spectrochim. Acta Part B 55 (2000) 917–933. [3] J. Dědina, Interference of volatile hydride forming elements in selenium determination by AAS with hydride generation, Anal. Chem. 54 (1982) 2097–2102. [4] M. Segura, Y. Madrid, C. Cámara, Evaluation of atomic fluorescence and atomic absorption spectrometric techniques for the determination of arsenic in wine and beer by direct hydride generation sample introduction, J. Anal. At. Spectrom. 14 (1999) 131–137. [5] A. Martinez, A. Morales-Rubio, M.L. Cervera, M. de la Guardia, Atomic fluorescence determination of total and inorganic arsenic species in beer, J. Anal. At. Spectrom. 16 (2001) 762–766.
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[6] K. Tašev, I. Karadjova, T. Stafilov, Determination of inorganic and total arsenic in wines by hydride generation atomic absorption spectrometry, Microchim. Acta 149 (2005) 55–60. [7] I.B. Karadjova, L. Lampugnani, M. Onor, A. D'Ulivo, D.L. Tsalev, Continuous flow hydride generation–atomic fluorescence spectrometric determination and speciation of arsenic in wine, Spectrochim. Acta Part B 60 (2005) 816–823. [8] CRC Handbook of Chemistry and Physics, 67th edn., R.C. Weast (Ed.), CRC Press, Boca Raton, Florida. [9] J. Dědina, A. D'Ulivo, Argon shielded highly fuel-rich, hydrogen–oxygen diffusion microflame—a new hydride atomizer, Spectrochim. Acta Part B 52 (1997) 1737–1746. [10] J. Dědina, A. D'Ulivo, L. Lampugnani, T. Matoušek, R. Zamboni, Selenium hydride atomization, fate of free atoms and spectroscopic temperature in miniature diffusion flame atomizer studied by atomic absorption spectrometry, Spectrochim. Acta Part B 53 (1998) 1777–1790. [11] A. D'Ulivo, J. Dedina, L. Lampugnani, A. Selecka, Mechanism of atomization interference by oxygen at trace level in miniature flame hydride atomizers, Spectrochim. Acta Part B 60 (2005) 1270–1279. [12] A. D'Ulivo, J. Dedina, Interferences in hydride atomization studied by atomic absorption and atomic fluorescence spectrometry, Spectrochim. Acta Part B 51 (1996) 481–498. [13] C.S. McEnally, A.M. Schaffer, M.B. Long, L.D. Pfefferle, M.D. Smooke, M.B. Colket, R.J. Hall, Computational and experimental study of soot formation in a coflow, laminar, ethylene diffusion flame, Twenty-Seventh Symposium (International) on Combustion/The Combustion Institute, 1998, pp. 1497–1505.