Preliminary investigation of a medium power argon radiofrequency capacitively coupled plasma as atomization cell in atomic fluorescence spectrometry of cadmium

Talanta 76 (2008) 1170–1176

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Preliminary investigation of a medium power argon radiofrequency capacitively coupled plasma as atomization cell in atomic fluorescence spectrometry of cadmium Tiberiu Frentiu a,b,∗ , Eugen Darvasi a,b , Marin Senila a , Michaela Ponta b , Emil Cordos a,b a b

National Institute for Research and Development of Optoelectronics Bucharest – Research Institute of Analytical Instrumentation, Donath 67, 400293 Cluj-Napoca, Romania “Babes-Bolyai” University, Faculty of Chemistry and Chemical Engineering, Arany Janos 11, 400028 Cluj-Napoca, Romania

a r t i c l e

i n f o

Article history: Received 21 February 2008 Received in revised form 14 May 2008 Accepted 14 May 2008 Available online 21 May 2008 Keywords: Atomic fluorescence spectrometry Capacitively coupled plasma Cd determination

a b s t r a c t The single ring electrode radiofrequency capacitively coupled plasma torch (SRTr.f.CCP) operated at 275 W, 27.12 MHz and Ar flow rate below 0.7 l min−1 was investigated for the first time as atomization cell in atomic fluorescence spectrometry (AFS) using electrodeless discharge lamps (EDL) as primary radiation source and charged coupled devices as detector. The signal to background ratio (SBR) and limit of detection for Cd determination by EDL-SRTr.f.CCP-AFS were compared to those obtained in atomic emission spectrometry using the same plasma torch. The detection limit in fluorescence was 4.3 ng ml−1 Cd compared to 65 ng ml−1 and 40 ng ml−1 reported in r.f.CCP-atomic emission (AES) equipped with single or double ring electrode. The lower detection limit in EDL-SRTr.f.CCP-AFS is due to a much better SBR in fluorescence. The limit of detection was also compared to those in atomic fluorescence with inductively coupled plasma (0.4 ng ml−1 ), microwave plasma torch (0.25 ng ml−1 ) and air–acetylene flame (8 ng ml−1 ). The influence of light-scattering through the plasma and the secondary reflection of the primary radiation on the wall of the quartz tube on the analytical performance are discussed. The non-spectral matrix effects of Ca, Mg and easily ionized elements are much lower in EDL-SRTr.f.CCP-AFS compared to SRTr.f.CCP-AES. The new technique was applied in the determination of Cd in contaminated soils, industrial hazardous waste (0.4–370 mg kg−1 ) and water (113 ␮g l−1 ) with repeatability of 4–8% and reproducibility in the range of 5–12%, similar to those in ICP-AES. The results were checked by the analysis of a soil and water CRM with a recovery degree of 97 ± 9% and 98 ± 4%, for a confidence limit of 95%. The present EDL-SRTr.f.CCP-AFS is a promising technique for Cd determination in environmental samples. © 2008 Elsevier B.V. All rights reserved.

1. Introduction The driving forces determining the development of atomic fluorescence spectrometry (AFS) are the well-established sensitivity and selectivity, the simple and low-cost instrumentation compared to other spectrometric methods such as atomic emission and absorption spectrometry (AES and AAS) as well as the simplicity of spectra compared to those recorded in AES. Most of the reported results with applications of AFS have been obtained for volatile elements or elements converted in volatile hydrides, which can be atomized efficiently at moderate temperature [1–6]. The trends in the development of AFS regarding the atomization cells,

∗ Corresponding author at: “Babes-Bolyai” University, Faculty of Chemistry and Chemical Engineering, Arany Janos 11, 400028 Cluj-Napoca, Romania. Tel.: +40 264593833; fax: +40 264420667. E-mail address: [email protected] (T. Frentiu). 0039-9140/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2008.05.020

primary radiation sources, detection devices, background correction methods and applications have been published for years as reviews [7–13]. Almost all plasma sources have been used as atomization cells for AFS, such as inductively coupled plasma (ICP-AFS) [14–17], microwave-induced plasma (MIP-AFS) [18], microwave plasma torch (MPT-AFS) [19] and direct current plasma (DCP-AFS) [20,21] with different primary radiation sources and dispersive and non-dispersive optical detection systems. Very recently a new atomizer based on atmospheric pressure non-thermal dielectric barrier discharge microplasma was designed for AFS coupled with a hydride generator (HG-DBD-AFS) [22]. The development of the new AFS instrumentation has been focused on the application of laser excited atomic/ionic fluorescence spectrometry (LEAFS and LEIFS) and the use of charge coupled detectors (CCDs). Among the laser sources, those with diode are very attractive for LEAFS due to their potential for low-cost, narrow spectral line width, compactness and ease of wavelength tunability through temperature and electrical manipulation. The applications of laser diode in atomic

T. Frentiu et al. / Talanta 76 (2008) 1170–1176

spectrometry and their features were reviewed by Niemax and co-workers [23] and Galbacs [24]. Laser-induced fluorescence technique was used for the diagnosis of reactive capacitively coupled radiofrequency discharges operated in pure CF4 , by determining the fluorine atom number density using the loss rate constant of the CF2 radicals in the afterglow of the plasma [25]. To date, radiofrequency capacitively coupled plasmas (r.f.CCPs) have been investigated as atomization source in AAS [26,27], excitation source in AES [27–32], and ion source in mass spectrometry (MS) [33,34], but there is no report regarding their use in analytical applications for AFS. In our laboratory an Ar r.f.CCP with a Mo tubular electrode and a single ring electrode (SRTr.f.CCP) [35–38] or a double ring electrode (DRTr.f.CCP) [35,37–42] operated at higher power (275 W, 27.12 MHz) than previously mentioned plasmas were investigated in AES for the determination of elements in liquid samples introduced by pneumatic nebulization without aerosol desolvation. Also, a very low power Ar r.f.CCP (<75 W, 13.56 MHz or 10.9 MHz) was developed in our laboratory for the direct analysis of nonconductive solid samples by r.f. sputtering [43] and pneumatically nebulized liquid samples without aerosol desolvation [44] using AES detection. The aim of this paper is to present for the first time several figures of merit of SRTr.f.CCP as atomization cell in AFS in a particular

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configuration using the r.f. electrodeless discharge lamp (EDL) as primary radiation source and CCD as optical detector. The development of the plasma within a quartz tube diminishes the diffusion of N2 , O2 and CO2 from the air resulting in a lengthen of the plasma tail plume, most favourable for the AFS measurements. The use of a good quality quartz tube with the 160 nm cut-off limit enables the approach of the UV range useful for elements generating a high fluorescence signal in this region. Operating conditions and non-spectral matrix effects of Ca, Mg and easily ionized elements (EIE) (Li, Na and K) on Cd determination by EDL-SRTr.f.CCP-AFS were compared to SRTr.f.CCP-AES. Also a comparison of the detection limits with those obtained in SRTr.f.CCP-AES, DRTr.f.CCP-AES, EDL-ICP-AFS, HCL-MPT-AFS and flame atomic fluorescence spectrometry (FAFS) are given. The new technique was investigated for the determination of Cd in water, soil and industrial hazardous waste and checked using certified reference materials (CRMs). 2. Experimental 2.1. Instrumentation The experimental set-up includes the SRTr.f.CCP torch (INCDOINOE 2000 Bucharest, Research Institute for Analytical Instrumen-

Fig. 1. Experimental set-up for EDL-SRTr.f.CCP-AFS.

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Table 1 Operating parameters for the experimental EDL-SRTr.f.CCP-AFS set-up Power supply

Free-running r.f. generator, 275 W, 27.12 MHz (INCDO-INOE 2000, Research Institute for Analytical Instrumentation, Cluj-Napoca, Romania)

Plasma torch

Capacitively coupled; coaxial geometry with molybdenum tubular electrode (inner diameter 3.5 mm) and single outer ring electrode of copper (SRTr.f.CCP) mounted at a distance of 5 mm above the tubular electrode; working gas: 5.0 quality argon (Linde gas SRL, Cluj-Napoca, Romania); argon flow rate 0.4–0.7 l min−1 ; 16 mm i.d. quartz tube, 10 cm length, 160 nm cut-off (H. Baumbach & Co. Ltd, Ipswich Suffolk, UK).

Sample introduction system

Meinhardt pneumatic nebulizer (PerkinElmer, Norwalk, USA) equipped with a spray chamber with impactor without aerosol desolvation; aerosol intake into the plasma via the tubular electrode (0.4 ml min−1 , 5% nebulization efficiency)

Primary radiation source

Electrodeless discharge lamp supplied in continuum mode at 240–450 mA from a power supply model CT06859 (EDL, PerkinElmer, Norwalk, USA)

Optics

HR 4000 Microspectrometer Ocean Optics (200–420 nm, 50 ␮m entrance slit, 1200 groves mm−1 grating, Toshiba CCD with 3648 pixels, collimating system 74-UV, fibre optic QP 600 ␮m, 25 cm length) (Dunedin, USA)

Optical arrangement

Radial–radial and axial–radial geometry at 90◦

Data acquisition

Spectrasuite soft (Ocean Optics); 10 and 30 s integration time; manually background correction

tation, Cluj-Napoca, Romania) as atomization cell, a 200–420-nm multichannel high resolution HR 4000 microspectrometer (Ocean Optics, Dunedin, USA) equipped with a 600-␮m fibre optic to collect the fluorescence signal, a Meinhardt pneumatic nebulizer and a spray chamber with impactor without aerosol desolvation, an EDL as primary radiation source supplied at different currents from a power supply model CT06859 (PerkinElmer, Norwalk, USA). The schematic diagram of the new experimental EDL-SRTr.f.CCP-AFS set-up is presented in Fig. 1, while the operating parameters in Table 1. Two optical arrangements were used to excite and measure the fluorescence signal (Fig. 2). In the radial profiling experiment (Fig. 2a) the fluorescence signal was measured in the radial direction through the quartz tube of the torch, while the primary excitation beam from EDLs was also collimated in radial direction onto the plasma. In the axial profiling (Fig. 2b), the fluorescence signal was imaged on the entrance slit of the microspectrometer using the fibre optic mounted in the radial direction related to the plasma, whereas the EDL was positioned in the axial direction. In both arrangements, the EDL, optical collimating system and fibre optic were fixed on a precision translation stage and aligned at right angle. The focusing and collimating of the EDL primary radiation onto the plasma were investigated. A silica lens (50 mm diameter and 75 mm focal length) was used to illuminate the plasma.

A mortar grinder Retsch RM 100 and a sieve shaker Retsch AS200 (Haan, Germany), an overhead shaker REAX20/8 Heidolph (Kelheim, Germany) and a closed-vessel microwave system Berghof MWS-3+ with temperature control mode (Eningen, Germany), and a Sartorius vacuum filter equipment (Goettingen, Germany) were used during sample preparation. Results found by EDL-SRTr.f.CCPAFS were compared to those obtained by AES using the ICP multichannel spectrometer SPECTRO CIROSCCD (Spectro Analytical Instruments, Kleve, Germany). 2.2. Reagents, standard solutions and CRMs Stock solutions of 1000 ␮g ml−1 of Cd, Li, Na, K, Ca and Mg purchased from Merck (Darmstadt, Germany) were used in this study. A solution of 400 ng ml−1 Cd was used to determine the optimum operating conditions of EDL-SRTr.f.CCP-AFS. Solutions of 400 ng ml−1 Cd in the presence of 1–500 ␮g ml−1 Li, Na, K, Ca and Mg were used in the study of the non-spectral interferences on the Cd fluorescence and emission. All dilutions were made with 1% HNO3 (v/v). Hydrochloric acid 37% and nitric acid 69% (GR for analysis ACS) purchased from Merck were used for digestion of samples. The soil CRM 025-050 RTC-Laramie (New York, USA) and the water from LGC Proficiency Testing, Aquacheck Scheme (distribution 331, group 17C)—LGC Promochem GmbH (Wesel, Germany) were used in the internal quality control of Cd determination. Argon (5.0 qual-

Fig. 2. Optical arrangement for radial–radial (a) and axial–radial (b) geometry.

T. Frentiu et al. / Talanta 76 (2008) 1170–1176 Table 2 Operating conditions for the microwave digestion system Stage

Temperature (◦ C) Ramp time (min) Hold time (min) Power (%)a a

1

2

3

4

180 5 25 60

100 1 10 20

100 1 1 10

100 1 1 10

100% power corresponds to 1450 W.

ity) from Linde Gas SRL Cluj-Napoca, Romania was used as working gas. 2.3. Sample preparation Ten replicate amounts of 1.000 g of CRM 025-050 or contaminated soil were digested with 10 ml aqua regia in closed PTFE containers of the microwave system according to the digestion program presented in Table 2. The procedure and settings were those recommended by the equipment producer for this type of samples. Ten replicate amounts of 175 g industrial hazardous waste were leached with distilled water (<10 ␮S cm−1 ) at 2:1 liquid to solid ratio at 20 ± 2◦ C for 24 ± 0.5 h (SR EN 12457/1:2003) with the Heidolph shaker at 16 rpm. Cadmium was determined in the filtered solutions and in water CRM by EDL-SRTr.r.CCP-AFS and ICP-AES after calibration on the range of 20–400 ng ml−1 Cd at 228.812 nm. 3. Results and discussions 3.1. Optimization of operating parameters The operation of EDL-SRTr.f.CCP-AFS was optimized with respect to the Ar flow rate, observation height, lamp current and non-spectral interferences of Ca, Mg and EIE on Cd. 3.2. Influence of Ar flow rate and observation height A single Ar stream was used as plasma support gas and sample carrier. The effect of Ar flow rate on 400 ng ml−1 Cd fluorescence signal in EDL-SRTr.f.CCP-AFS with radial–radial arrangement is shown in Fig. 3.

Fig. 3. Effect of Ar flow rate on Cd fluorescence signal at 228.812 nm (400 ng ml−1 ) for different observation heights in EDL-SRTr.f.CCP-AFS with radial–radial arrangement. Ar flow rate: (A) 0.4 l min−1 ; (B) 0.5 l min−1 ; (C) 0.6 l min−1 ; (D) 0.7 l min−1 . Experimental conditions: 275 W power level; 350 mA lamp current; 10 s integration time; collimated primary radiation.

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The measurements were performed at different observation heights above the tubular electrode for gas flow rates in the range of 0.4–0.7 l min−1 . Although a single gas flow is used, two factors control the Cd fluorescence signal: efficiency of the pneumatic nebulizer and the cooling of the discharge occurring at higher gas flow rates, which is favourable in fluorescence measurement. It can be seen that the maximum of the fluorescence signal is reached at higher heights in the plasma tail, where the temperature and the background of the discharge are lower. Therefore for a flow rate of 0.7 l min−1 Ar considered as optimum for both nebulizer operation and plasma development, the maximum fluorescence signal occurs at 72 mm above the tubular electrode. Unlike fluorescence, the maximum of the Cd signal in SRTr.f.CCPAES is reached at lower observation height (16 mm). The emission signal decreases as the Ar flow rate increases and the optimum value is considered to be 0.4 l min−1 . The results are similar to those obtained in ICP-AES and ICP-AFS [45]. Whereas the maximum of emission in ICP-AES occurs at lower heights (15–20 mm), the fluorescence signal for most metals in ICP-AFS is maximum in the tail plasma zone. Also, in ICP-AFS an increase of the injector gas flow to 1.5–2 l min−1 is required compared to 1.0 l min−1 in ICP-AES. The dependence of the signal to background ratio (SBR) for 400 ng ml−1 Cd on the observation height in the axial–radial geometry of EDL-SRTr.f.CCP-AFS (0.7 l min−1 Ar) and SRTr.f.CCP-AES (0.4 l min−1 Ar) is presented in Figs. 4 and 5, respectively. The axial–radial arrangement of the AFS set-up was chosen for comparison with AES as in this geometry the interference caused by the secondary reflection of the EDL primary radiation at 228.812 nm could be avoided unlike to the radial–radial geometry where this undesirable phenomenon limited the fluorescence sensitivity. Therefore the integration time was increased to 30 s in the axial–radial arrangement, which improved the fluorescence signal of 1.7 times. As shown in Figs. 4 and 5 the maximum SBR of Cd in fluorescence and emission occurs at 72 mm and 16 mm height, respectively. The SBR decreases in fluorescence at lower heights as a result of the diminished primary radiation due to the prefilter phenomenon and increase of background. Although the background decreases over 72 mm, SBR decreases due mainly to air diffusion into the plasma and quenching of the Cd fluorescence as well as increase of the light-scattering interference through the cool reactive oxidant environment. Since the SBR in fluorescence is 15 times higher that in

Fig. 4. Dependence of signal to background ratio for Cd at 228.812 nm on the observation height in the axial–radial arrangement of EDL-SRTr.f.CCP-AFS. Experimental conditions: Cd concentration 400 ng ml−1 ; 275 W power level; 0.7 l min−1 Ar flow rate; 350 mA lamp current; collimated primary radiation; 30 s integration time.

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Fig. 5. Dependence of signal to background ratio for Cd at 228.812 nm in SRT r.f.CCP-AES on the observation height. Experimental conditions: Cd concentration 400 ng ml−1 ; 275 W power level; 0.4 l min−1 Ar flow rate; 30 s integration time.

Fig. 6. Dependence of the Cd signal at 228.812 nm on lamp current in EDL-SRTr.f.CCP-AFS with axial–radial arrangement. Experimental conditions: Cd concentration 400 ng ml−1 ; 275 W power level; 0.7 l min−1 Ar flow rate; collimated primary radiation; 30 s integration time.

emission the SRTr.f.CCP is more suitable for atomization than for excitation.

3.4. Non-spectral matrix effects of EIE, Ca and Mg in EDL-SRTr.f.CCP-AFS and SRTr.f.CCP-AES

3.3. Operating current for EDL

The non-spectral matrix effects of Li, Na, K, Ca and Mg in concentrations up to 500 ␮g ml−1 on the fluorescence signal of 400 ng ml−1 Cd in EDL-SRTr.f.CCP-AFS in the radial–radial and axial–radial arrangements for the optimum observation height (72 mm) are presented in Fig. 7a and b, while in SRTr.f.CCP-AES at an observation height of 16 mm in Fig. 8. The non-spectral matrix effect (%) was calculated according to the equation:

The optimum operating condition of Cd EDL was established by measuring the fluorescence signal for a concentration of 400 ng ml−1 Cd at different input currents. The dependence curve obtained for EDL-SRTr.f.CCP-AFS in the axial–radial arrangement for an observation height of 72 mm is illustrated in Fig. 6. The fluorescence signal is linearly proportional to the operating current up to 325 mA, which means that the de-excitation rate of the Cd atoms is faster than the excitation rate. The limitation of the fluorescence signal at the operation current of 350 mA is a consequence of the beginning self-reversal of the EDL. It should be noted that the fluctuation of the EDL primary radiation is not strongly reflected in the fluorescence signal at this current as the curve slope in this point is the lowest. Consequently the 350 mA operation current was considered as optimum.

ME =

S − S0 × 100 S0

where S and S0 are the Cd signal in the presence and absence of a particular concomitant, respectively. The non-spectral matrix effects of Ca, Mg, Li, Na and K are similar in both arrangements used in the study of fluorescence and much smaller compared to emission. Thus, the influence of the Ca, Mg and Na matrices are usually in the range ±10% up to a concentration of 500 ␮g ml−1 concomitant. The results show that

Fig. 7. Non-spectral matrix effects of Ca (A), Mg (B), Li (C), Na (D) and K (E) in the range 0–500 ␮g ml−1 on the fluorescence of Cd (400 ng ml−1 ) at 228.812 nm in EDLSRTr.f.CCP-AFS with radial–radial (a) and axial–radial (b) arrangement. Experimental conditions: 275 W power level; 0.7 l min−1 Ar flow rate; 72 mm observation height; 350 mA lamp current.

T. Frentiu et al. / Talanta 76 (2008) 1170–1176

Fig. 8. Non-spectral matrix effects of Ca (A), Mg (B), Li (C), Na (D) and K (E) in the range 0–500 ␮g ml−1 on the emission of Cd (400 ng ml−1 ) at 228.812 nm in SRTr.f.CCP-AES. Experimental conditions: 275 W power level; 0.4 l min−1 Ar flow rate; 16 mm observation height.

less the volatility of the matrix and higher the ionization energy of the concomitant, smaller the non-spectral matrix effects. The vaporization–atomization interferences of the refractory elements (Ca, Mg) in EDL-SRTr.f.CCP-AFS are fewer and smaller than in flames and furnaces, and are comparable to those reported in ICP-AFS [45]. Unlike fluorescence, in emission the non-spectral matrix effects on Cd are slightly depressive in the case of Ca and very depressive in the case of Mg, Li, Na and K. The decrease of Cd emission signal in the presence of these concomitants follows the order: Ca < Mg < Li < Na < K. The higher depressive interference in emission compared to that observed in fluorescence is the result of easily liberated electrons by EIE, which affect the equilibrium balance between atoms and ions with the consequent alteration of the energy-transfer pathway for excitation in the plasma. In short, SRTr.f.CCP is suitable as atomization cell in AFS in the presence of complex matrices. 3.5. Limits of detection Detection limit was calculated using the 3 criteria. In this method the SBR was determined for a concentration of 200 ng ml−1 Cd, while the relative standard deviation of the background (RSDB) was calculated from 10 successive measurements of the background signal in the proximity of the Cd 228.812 nm line.

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Fig. 9. Detection limit of Cd at 228.812 nm for EDL-SRTr.f.CCP-AFS in radial–radial (A) and axial–radial (B) arrangement as a function of observation height. Experimental conditions: 275 W power level; 0.7 l min−1 Ar flow rate; collimated primary radiation; 10 s integration time for radial–radial arrangement and 30 s for axial–radial arrangement.

The dependence of the detection limit for Cd on the observation height for EDL-SRTr.f.CCP-AFS in radial–radial (A) and axial–radial (B) arrangement is presented in Fig. 9. Table 3 gives a comparison of detection limits for Cd in EDL-SRTr.f.CCP-AFS, SRTr.f.CCP-AES, DRTr.f.CCP-AES, as well as in AFS using other atomization cells. According to Fig. 9, the lowest detection limit in EDL-SRTr.f.CCPAFS for both arrangements is obtained at 72 mm observation height, where SBR is maximum. As shown in Table 3, the Cd detection limits in EDLSRTr.f.CCP-AFS with radial–radial and axial–radial arrangement using pneumatic nebulization, without aerosol desolvation are 6.4 ng ml−1 and 4.3 ng ml−1 , respectively. The improvement of the detection limit by a factor of 1.5 in the axial–radial arrangement is the result of a diminished interference caused by the secondary reflection on the quartz tube of the primary radiation onto the lens of the fibre optic. The Cd detection limit in EDL-SRTr.f.CCP-AFS was about 10–15 folds lower compared to SRTr.f.CCP-AES and 6.3–9.3 folds compared to DRTr.f.CCP-AES. The comparison with the other AFS systems is difficult because of significant differences in their experimental set-up. Several comments could be made. The limit of detection in EDL-SRTr.f.CCP-AFS is lower than that obtained in LEAFS using an air–acetylene flame, where the background signal is higher. Although the detection limits obtained in EDL-SRTr.f.CCP-AFS are higher than in EDL-ICP-AFS, the advantages of our atomization cell are lower background, low operation power and total Ar consumption, resulting in low-cost

Table 3 Detection limits (3 criteria) for Cd determination by EDL-SRTr.f.CCP-AFS in radial–radial and axial–radial arrangement, SRTr.f.CCP-AES, DRTr.f.CCP-AES, EDL-ICP-AFS, HCLMPT-AFS and LEAFS Method

LOD (ng ml−1 )

Operating conditions −1

Reference

EDL-SRTr.f.CCP-AFS radial–radial arrangement

6.4

275 W; 0.7 l min

Ar; 72 mm observation height; 10 s integration time; CCD

This paper

EDL-SRTr.f.CCP-AFS axial–radial arrangement

4.3

275 W; 0.7 l min−1 Ar; 72 mm observation height; 30 s integration time; CCD

This paper

EDL-ICP-AFS

0.4

EDL and solar-blind photomultiplier tube

[14]

HCL-MPT-AFS

0.25

80 W; dry aerosol; boosted HCL; solar-blind photomultiplier tube

[19]

SRTr.f.CCP-AES

65

275 W; 0.4 l min−1 Ar; 16 mm observation height

[35]

DRTr.f.CCP-AES

40

275 W; 0.4 l min−1 Ar; 12 mm observation height

[35,39]

Air–acetylene flame; laser excitation

[46]

a

LEAFS a

8

LEAFS—laser flame atomic fluorescence spectrometry.

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Table 4 Analytical determination of Cd in reference materials Sample

Measure units

Certified content

LGC—aquacheck scheme water CRM 025-050 soil

␮g l−1 mg kg−1

113 ± 7 369 ± 41

Found content EDL-SRTr.f.CCP-AFS

ICP-AES

111 ± 4 358 ± 31

117 ± 4 365 ± 19

Values are reported with 95% confidence limit (n = 10).

Table 5 Cadmium content in soil leached in aqua regia and industrial hazardous waste leached in water obtained by EDL-SRTr.f.CCP-AFS compared to ICP-AES Sample

Contaminated soil Contaminated soil Industrial hazardous waste

Found content, mg kg−1 EDL-SRTr.f.CCP-AFS

ICP-AES

16 ± 1 51 ± 2 0.40 ± 0.02

19 ± 1 52 ± 1 0.38 ± 0.02

Values are reported with 95% confidence limit (n = 10).

operation. The poor limits of detection compared to HCL-MPT-AFS are due to the use of CCD detector in our configuration compared to the solar-blind photomultiplier tube with higher sensitivity used in the method of comparison. The difference in the detection limits is also the result of sample introduction as wet aerosol in our approach. 3.6. Analytical determination Statistical data of the calibration curve y = (51 ± 106) + (26 ± 1)C (n = 5, 95% confidence limit) over the concentration range of 20–400 ng ml−1 Cd (correlation coefficient 0.9999) obtained by EDL-SRTr.f.CCP-AFS in the axial–radial set-up show the lack of systematic errors. Standard deviation of reproducibility (sR ) and accuracy in the internal quality control were assessed on the basis of two CRMs with different Cd content. The found results compared to the reference values are given in Table 4. The standard deviation of reproducibility in EDL-SRTr.f.CCP-AFS was 5.0% and 12.0% for Cd determination in water and soil, at recovery degrees of 98 ± 4% and 97 ± 9% (95% confidence limit), comparable with ICP-AES. Repeatability in EDL-SRTr.f.CCP-AFS was checked by carrying out 10 replicates on separate soil and industrial hazardous waste subsamples and compared with the standardized ICP-AES method. The data in Table 5 show a precision range of 4–8% in both methods. 4. Conclusions The figures of merit of a new atomization cell in AFS for pneumatic nebulization of aqueous samples without desolvation using EDL as primary excitation source and CCD for detection were evaluated. The advantages of the r.f.CCP as atomization cell in AFS include low-cost as instrumentation and maintenance, excellent discharge stability and high tolerance to complex matrix as well as a good capability for sample dissociation and atomization. Non-spectral matrix effects were much lower than those observed in SRTr.f.CCPAES. The method exhibits a detection limit of Cd 10 times better than in DRTr.f.CCP-AES due to an improved signal-to-background ratio. Acknowledgment The present investigations are supported by the Romanian Ministry of Education and Research, PNCDI II Program (Project FLUOROSPEC no. 71019/2007).

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