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Determination of arsenic in sediments by chloride formation and d.c. plasma arc emission spectrometry
Analytica Chimica Acta, 107 (1979) 395-398 0 Elsevier Scientific Publishing Company, Amsterdam
-
Printed
in The
Netherlands
Short Communication
DETERMINATION OF ARSENIC IN SEDIMENTS BY CHLORIDE FORMATION AND D.C. PLASMA ARC EMISSION SPECTROMETRY
AKIRA
MIYAZAKI*,
AKIRA
KIMURA
and YOSHIMI
National Research Institute for Pollution and Resources, (Received
18th
October,
UMEZAKI
Ukima. Kita-ku. Tokyo
(Japan)
1978)
Summary. The sediment sample is heated in an induction furnace in hydrogen chlorideargon; arsenic trichloride evolved is trapped and then swcFt into a d-c. plasma arc. Calcium sulfate limit
addition
of detection
enhances
the signals
and suppresses
is 15 ng As; the relative
standard
inte;f*;rence deviatron
from
is 3.4%
organic
matter.
The
(n = 10) for 1.5 rg
AZ&O,.
Arsenic in sediments is usually determined after dissolution of the samples. This procedure is time-consuming and arsenic may be lost in the pretreatment_ A more direct method of analysis would therefore be advantageous_ Conventional arc or spark emission spectrometry is not satisfactory for the determination of arsenic in sediments because of its poor sensitivity and reproducibility. Morrison and Talmi [ 1, 21 reported that heating solids in an induction furnace is useful for microanalysis of solids, and several authors have since used this technique [3-g]. However, arsenic has not been determined with this device. This communication describes a method in which sediments are heated in an induction furnace in an atmosphere of hydrogen chloride [lo] _ Arsenic is converted to the volatile trichloride which is collected in a dry iceethanol trap. The trap is then heated, the chloride is swept into a d-c. plasma arc, and the arsenic is determined by emission spectrometry. Experimental
Apparatus. A Spectraspan Model 101 d.c. plasma arc emission spectrometer was used with a cross-type plasma jet (Spectrajet II), a Nippon-Denshi Kagaku Model U-125M recorder and a Hamamatsu R166 photomultiplier. The induction (r-f.) furnace (3 kW) was made by the Abe Trading Company. An RFS Model 2002 freeze drier and a Spex Model 8000 mixer/mill were used for the preparation of samples. The furnace and the associated gas flow system are shown in Fig. 1. The operating conditions for the d-c_ plasma arc emission spectrometer were as follows: plasma current, 7.5 A; electrode gas and carrier gas flow rates, 3.5 and 7.0 units, respectively (1unit = 0.47 1 min-‘); entrance and exit slit areas, 400 X 600 m and 200 X 400 pm, respectively; maximum voltage, 800 V; amplification, X 10. Reagents_ National spectrometric graphite powder L4100 (Union Carbide,
396
to Draft
HCl+Ar
Fig. 1. Apparatus for production, trapping and determination of arsenic trichloride. The U-tube is connected to taps A and B with Teflon tubing. The parts indicated by thick lines are heated with tapes to about 150°C to prevent condensation of A&I,_ Silica tubing is used where the tubes are heated inductively; other parts are of borosilicate glass.
U-S-A-) was used- All other chemicals were of analytical-reagent grade. Standards (10-200 pg As,03 g-‘) were prepared by mixing homogeneously As203, 20% graphite powder and 15% CaSO 4-$H20, made up with silica [ll] ; the As203 was first mixed with silica powder to give standards containing 100 or 1000 fig g-l, which were then diluted further as described. Procedure for determination of arsenic in sediments_ Set up the plasma spectrometer for the conditions given above. Freeze-dry the sample or standard and pulverize to <200 mesh. Add 20% of graphite powder and 15% of CaSO 4 -$H,O and mix well (15 min) with a Spex mixer. Transfer 30 mg of the mixture to a graphite electrode and place the electrode in the induction furnace_ With the stopcocks in the positions shown in Fig_ 1, heat the graphite electrode to 600°C in a hydrogen chloride--argon atmosphere and collect the arsenic trichloride in the cold trap for 90 s. Turn stopcocks A and I3 clockwise through 909 Remove the Dewar from around the U-tube and heat with an electric furnace at 200°C for 60 s to volatilize the arsenic trichloride. Open tap C fully to sweep the evolved arsenic trichloride into the arc. Measure the 235-O-nm As emission line intensity. Open the by-pass tap D slowly and return taps A, B and C to the positions shown in Fig. 1. Record the maximum emission intensity. Results and discussion Changes in the power of the furnace from 1.4 to 2.6 kW (300-1100°C) had little effect on the emission intensity of arsenic; a power of 1.8 kW (600°C) was used routinely- The effect of heating time on the signal from 1.5 pg of arsenic trioxide (l-l pg As) was examined_ The emission intensity increased linearly with time up to 70 s and became constant thereafter_ Consequently, a heating time of 90 s was chosen for later work. The trap was heated by a furnace at 200°C after removal from the cold bath, to vaporize the collected arsenic trichloride. The maximum emission intensity became constant after a heating period of 45 s. A heating time of
397
60 s was used routinely. Under these conditions the emission-time response from the plasma was a sharp peakInterferences. Addition of 5% (w/w) CaSO, - iHzO to standards containing Asz03 (O-005%), graphite (20%) and silica (74.995%) enhanced the intensity of the arsenic emission by about 20%. A constant enhancement by 40% was obtained when the mixtures contained lO-30% CaS04-iH,O, so that 15% was selected for routine use. In the presence of CaS04-iH*O, no interference was caused by 1.5 mg of Na*CO,, KICOB, CaO, MgO, A1203, NaCl, KMnO., or FeZ(S04)J(NH4)2S04- 24H20, in the determination of 1.5 pg of As203; these compounds also did not interfere in the absence of calcium sulfate. The effect of organic compounds which may be present in sediments was examined. Results are shown in Fig. 2. In the presence of calcium sulfate, no interference was observed for humic acid and tannic acid up to 3%; in its absence, these compounds seriously depressed the emission. Addition of sodium sulfate, in place of the calcium salt, was also successful in avoiding interference from these organic compounds. Recovery of other forms of arsenic. Recovery of arsenic(V) added as KH2As04 was 97%, calculated on the basis of calibration against As,O,. Thus almost all the arsenic in sediments is likely to be converted to arsenic trichloride (b-p. 130°C) in the proposed method. A preliminary result showed that recovery of cacodylic acid, (CH&As02H, which was not detected after conventional wet digestion, was 70%. Analytical results. The calibration graphs obtained under the recommended operating conditions were linear for O-3-4.5 pg As,O~. The relative standard deviation was 3.4% for ten determinations of 1.5 r.lgAs203_ The limit of detection, defined as the concentration giving a signal-to-noise ratio of 2, was 15 ng As or 5 pg As g-l. Table 1 compares the results obtained for the determination of arsenic in various sediments and sludges by the proposed method and by a conventional
0
1
2
Humic Tannic
3
Acid Acid
4 (“/O)
Fig. 2. Effects of humic acid (open symbols) and tannic acid (tilled symbols) on the emission from 1.1 pg of arsenic without calcium sulfate (GO) and with 15% calcium sulfate (a=).
398 TABLE
1
Results for arsenic (rg g-‘) in
sediments
Sample
Proposed method
Wet method
792 2a 194* 5 7605 31
80 189 790
River sediments arsenic mines
[ 121
near
A B C Sea sediments D
15?1
13
Sludges from industrial waste water E F
4723 7223
46 77
=Average deviation from the mean of 3 determinations.
wet method [12] in which samples were decomposed with nitric and sulfuric acids in the presence of permanganate- For the plasma method, the samples which contained large concentrations of arsenic, such as B and C, were diluted by mixing with silica powder before analysis_ Satisfactory results were obtained for all the samples_ The proposed method is rapid, because the time-consuming wet decomposition of samples is unnecessary, and is precise and accurate_ It is readily applied to sediments but should also be applicable to the determination of submicrogram amount-s of arsenic in soils and related samples_ REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12
G. H. Morrison and Y_ Talmi, Anal. Chem., 42 (1970) 809. Y. Talmi and G. H. Morrison, _4nal_ Chem., 44 (1972) 1455. F. J. Langmyhr and Y. Thomassen, Fresenius Z. Anal. Chem., 264 (19’73) 122. F_ J. Langmyhr and S. Rasmussen, Anal. Chim. Acta, 72 (1974) 79. F. J. Langmyhr, Y. Thomassen and A_ Massoumi, Anal. Chim. Acta, 67 (1973) 460. F. J_ Langmyhr, J. R. Stubergh, Y. Thomassen, J. E. Hanssen and J. Dole&l, Anal. Chim. Acta, 71 (1974) 35. D. G. Andrews and J. B. Headridge, Analyst, 102 (1977) 436. R. Nakashima and S. Sasaki, Bunseki Kagaku, 102 (1977) 145. R. Nakashima, Bunseki Kagaku, 27 (1978) 199. R. K. Skogerboe, D. L. Dick, D. A. Pavloca and F. E. Lichte. Anal. Chem., 47 (1975) 568_ A_ Yauchi, M. Tominaga and Y. Umezaki, Report of the Annual Meeting of the Mining and Metallurgical Institute of Japan, 1976. p_ 283. S. Terashima, Bunseki Kagaku, 23 (1974) 1331.
-
Printed
in The
Netherlands
Short Communication
DETERMINATION OF ARSENIC IN SEDIMENTS BY CHLORIDE FORMATION AND D.C. PLASMA ARC EMISSION SPECTROMETRY
AKIRA
MIYAZAKI*,
AKIRA
KIMURA
and YOSHIMI
National Research Institute for Pollution and Resources, (Received
18th
October,
UMEZAKI
Ukima. Kita-ku. Tokyo
(Japan)
1978)
Summary. The sediment sample is heated in an induction furnace in hydrogen chlorideargon; arsenic trichloride evolved is trapped and then swcFt into a d-c. plasma arc. Calcium sulfate limit
addition
of detection
enhances
the signals
and suppresses
is 15 ng As; the relative
standard
inte;f*;rence deviatron
from
is 3.4%
organic
matter.
The
(n = 10) for 1.5 rg
AZ&O,.
Arsenic in sediments is usually determined after dissolution of the samples. This procedure is time-consuming and arsenic may be lost in the pretreatment_ A more direct method of analysis would therefore be advantageous_ Conventional arc or spark emission spectrometry is not satisfactory for the determination of arsenic in sediments because of its poor sensitivity and reproducibility. Morrison and Talmi [ 1, 21 reported that heating solids in an induction furnace is useful for microanalysis of solids, and several authors have since used this technique [3-g]. However, arsenic has not been determined with this device. This communication describes a method in which sediments are heated in an induction furnace in an atmosphere of hydrogen chloride [lo] _ Arsenic is converted to the volatile trichloride which is collected in a dry iceethanol trap. The trap is then heated, the chloride is swept into a d-c. plasma arc, and the arsenic is determined by emission spectrometry. Experimental
Apparatus. A Spectraspan Model 101 d.c. plasma arc emission spectrometer was used with a cross-type plasma jet (Spectrajet II), a Nippon-Denshi Kagaku Model U-125M recorder and a Hamamatsu R166 photomultiplier. The induction (r-f.) furnace (3 kW) was made by the Abe Trading Company. An RFS Model 2002 freeze drier and a Spex Model 8000 mixer/mill were used for the preparation of samples. The furnace and the associated gas flow system are shown in Fig. 1. The operating conditions for the d-c_ plasma arc emission spectrometer were as follows: plasma current, 7.5 A; electrode gas and carrier gas flow rates, 3.5 and 7.0 units, respectively (1unit = 0.47 1 min-‘); entrance and exit slit areas, 400 X 600 m and 200 X 400 pm, respectively; maximum voltage, 800 V; amplification, X 10. Reagents_ National spectrometric graphite powder L4100 (Union Carbide,
396
to Draft
HCl+Ar
Fig. 1. Apparatus for production, trapping and determination of arsenic trichloride. The U-tube is connected to taps A and B with Teflon tubing. The parts indicated by thick lines are heated with tapes to about 150°C to prevent condensation of A&I,_ Silica tubing is used where the tubes are heated inductively; other parts are of borosilicate glass.
U-S-A-) was used- All other chemicals were of analytical-reagent grade. Standards (10-200 pg As,03 g-‘) were prepared by mixing homogeneously As203, 20% graphite powder and 15% CaSO 4-$H20, made up with silica [ll] ; the As203 was first mixed with silica powder to give standards containing 100 or 1000 fig g-l, which were then diluted further as described. Procedure for determination of arsenic in sediments_ Set up the plasma spectrometer for the conditions given above. Freeze-dry the sample or standard and pulverize to <200 mesh. Add 20% of graphite powder and 15% of CaSO 4 -$H,O and mix well (15 min) with a Spex mixer. Transfer 30 mg of the mixture to a graphite electrode and place the electrode in the induction furnace_ With the stopcocks in the positions shown in Fig_ 1, heat the graphite electrode to 600°C in a hydrogen chloride--argon atmosphere and collect the arsenic trichloride in the cold trap for 90 s. Turn stopcocks A and I3 clockwise through 909 Remove the Dewar from around the U-tube and heat with an electric furnace at 200°C for 60 s to volatilize the arsenic trichloride. Open tap C fully to sweep the evolved arsenic trichloride into the arc. Measure the 235-O-nm As emission line intensity. Open the by-pass tap D slowly and return taps A, B and C to the positions shown in Fig. 1. Record the maximum emission intensity. Results and discussion Changes in the power of the furnace from 1.4 to 2.6 kW (300-1100°C) had little effect on the emission intensity of arsenic; a power of 1.8 kW (600°C) was used routinely- The effect of heating time on the signal from 1.5 pg of arsenic trioxide (l-l pg As) was examined_ The emission intensity increased linearly with time up to 70 s and became constant thereafter_ Consequently, a heating time of 90 s was chosen for later work. The trap was heated by a furnace at 200°C after removal from the cold bath, to vaporize the collected arsenic trichloride. The maximum emission intensity became constant after a heating period of 45 s. A heating time of
397
60 s was used routinely. Under these conditions the emission-time response from the plasma was a sharp peakInterferences. Addition of 5% (w/w) CaSO, - iHzO to standards containing Asz03 (O-005%), graphite (20%) and silica (74.995%) enhanced the intensity of the arsenic emission by about 20%. A constant enhancement by 40% was obtained when the mixtures contained lO-30% CaS04-iH,O, so that 15% was selected for routine use. In the presence of CaS04-iH*O, no interference was caused by 1.5 mg of Na*CO,, KICOB, CaO, MgO, A1203, NaCl, KMnO., or FeZ(S04)J(NH4)2S04- 24H20, in the determination of 1.5 pg of As203; these compounds also did not interfere in the absence of calcium sulfate. The effect of organic compounds which may be present in sediments was examined. Results are shown in Fig. 2. In the presence of calcium sulfate, no interference was observed for humic acid and tannic acid up to 3%; in its absence, these compounds seriously depressed the emission. Addition of sodium sulfate, in place of the calcium salt, was also successful in avoiding interference from these organic compounds. Recovery of other forms of arsenic. Recovery of arsenic(V) added as KH2As04 was 97%, calculated on the basis of calibration against As,O,. Thus almost all the arsenic in sediments is likely to be converted to arsenic trichloride (b-p. 130°C) in the proposed method. A preliminary result showed that recovery of cacodylic acid, (CH&As02H, which was not detected after conventional wet digestion, was 70%. Analytical results. The calibration graphs obtained under the recommended operating conditions were linear for O-3-4.5 pg As,O~. The relative standard deviation was 3.4% for ten determinations of 1.5 r.lgAs203_ The limit of detection, defined as the concentration giving a signal-to-noise ratio of 2, was 15 ng As or 5 pg As g-l. Table 1 compares the results obtained for the determination of arsenic in various sediments and sludges by the proposed method and by a conventional
0
1
2
Humic Tannic
3
Acid Acid
4 (“/O)
Fig. 2. Effects of humic acid (open symbols) and tannic acid (tilled symbols) on the emission from 1.1 pg of arsenic without calcium sulfate (GO) and with 15% calcium sulfate (a=).
398 TABLE
1
Results for arsenic (rg g-‘) in
sediments
Sample
Proposed method
Wet method
792 2a 194* 5 7605 31
80 189 790
River sediments arsenic mines
[ 121
near
A B C Sea sediments D
15?1
13
Sludges from industrial waste water E F
4723 7223
46 77
=Average deviation from the mean of 3 determinations.
wet method [12] in which samples were decomposed with nitric and sulfuric acids in the presence of permanganate- For the plasma method, the samples which contained large concentrations of arsenic, such as B and C, were diluted by mixing with silica powder before analysis_ Satisfactory results were obtained for all the samples_ The proposed method is rapid, because the time-consuming wet decomposition of samples is unnecessary, and is precise and accurate_ It is readily applied to sediments but should also be applicable to the determination of submicrogram amount-s of arsenic in soils and related samples_ REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12
G. H. Morrison and Y_ Talmi, Anal. Chem., 42 (1970) 809. Y. Talmi and G. H. Morrison, _4nal_ Chem., 44 (1972) 1455. F. J. Langmyhr and Y. Thomassen, Fresenius Z. Anal. Chem., 264 (19’73) 122. F_ J. Langmyhr and S. Rasmussen, Anal. Chim. Acta, 72 (1974) 79. F. J. Langmyhr, Y. Thomassen and A_ Massoumi, Anal. Chim. Acta, 67 (1973) 460. F. J_ Langmyhr, J. R. Stubergh, Y. Thomassen, J. E. Hanssen and J. Dole&l, Anal. Chim. Acta, 71 (1974) 35. D. G. Andrews and J. B. Headridge, Analyst, 102 (1977) 436. R. Nakashima and S. Sasaki, Bunseki Kagaku, 102 (1977) 145. R. Nakashima, Bunseki Kagaku, 27 (1978) 199. R. K. Skogerboe, D. L. Dick, D. A. Pavloca and F. E. Lichte. Anal. Chem., 47 (1975) 568_ A_ Yauchi, M. Tominaga and Y. Umezaki, Report of the Annual Meeting of the Mining and Metallurgical Institute of Japan, 1976. p_ 283. S. Terashima, Bunseki Kagaku, 23 (1974) 1331.