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Difficulties in the determination of arsenic by atomic absorption spectrometry
Amdyricu
Chhictr
69 ( 1974) 203-206 Publishing Company.
Scientific
SHORT
COMMUNICATION
Difficulties
in the determination
J. W. ROBINSON. Depurttmwt
(Reccivcd
203
Actu,
ci> Elsevicr
R. GARCIA.
of arsenic
G. HINDMAN
oJ’ Clret~ristry. Louisiotw
28th Scptcmbcr
Amsterdam
- Printed
by atomic
in The Netherlands
absorption
spectrometry
and P. SLEVIN
Stcrle Utliuersif_v,
Uufon Ro~qe.
Louisiutrn
7OW3 ( U.S.A.)
1973)
Previous communications from this laboratory have described a system for the continuous determination of lead1,,cadmium2*3 and mercury2*4*5 in the atmosphere. The present study was undertaken in an effort to extend this procedure to include arsenic. Although the resonance lines of arsenic lie in the vacuum ultraviolet at 189.0, 193.7 and 197.2 nm, this presented no problem as the system employed permitted the use of resonance lines as low as 184.9 nm’. The various techniques used in the calibration of the instrument for different metals have been compared6. An initial investigation showed that on occasion ambient air gave an arsenic atomic absorption signal even after correction for the molecular absorption in this spectral region. The results indicated *that arsenic in the atmosphere could be directly detected by atomic absorption. It was confirmed that the 189.0-nm line is the most sensitive for arsenic, followed by the 193.7-nm line7*8. Equipment
The equipment used has been previously described4. The r.f. carbon atomization cell was of the design described for cadmium determinations’. A Glomax demountable hollow-cathode lamp (Barnes Engineering Company) operated in a helium atmosphere was used as the radiation source. Calibration rvethotls The use of awine. The
initial approach adopted to calibrate the instrument for arsenic was to inject arsine gas diluted with clean air into the system. Arsine is readily decomposed at temperatures above 230” into arsenic and hydrogen. However, it was found that arsine decomposed even at room temperature, so that quantitative data based on the concentration of arsine introduced were unreliable. Moreover, some arsenic metal plated out in cool zones of the equipment between the carbon bed at 1600” and the water-cooled brass adaptor of the inlet head; and arsenic was held up in the carbon bed for varying unpredictable periods of time. Calibration by direct droplet hjection-H2 absorption. Any organic solvent introduced into the system was decomposed to carbon monoxide and hydrogen. The latter absorbed at wavelengths below 200 nm. In an effort to decrease this molecular background, formic acid was used as a solvent because of its low
204
SHORT
COMMUNICATION
4
5040.z 30P 3 %
20-
z a, k! IOa! rl 6 Time (mid
Fig. I. Absorption sigmls.
h-xc
l’or arsenic
10
injection
14
in rmnic
acid. A. points
ol’ injection:
B, dclaycd
xscnic
hydrogen content. Solutions of arsenic in formic acid were prepared by dilution of ktl-quantities of an aqueous 1000 116 ml - ’ arsenic stock solution with formic acid. A diagram of a typical absorption trace is shown in Fig. 1. These delayed absorption peaks were established to be due to arsenic atomic absorption rather than molecular absorption (measured by the deuterium lamp). To investigate this further. arsenic was introduced into the carbon bed by the the direct drop technique, and by injecting into a carbon disc and dropping disc onto the bed. Results indicated loss of arsenic on the carbon disc and again suggested interaction between arsenic and carbon. In further studies, the carbon bed depth was varied as was the air flow rate. The absorption signal increased with decreased contact with the carbon bed and with a decreased time between atomization and absorption measurement. Each of these observations indicated a carbon-arsenic interaction, although no C-As compounds have been reported in the literature. Vapor pressure data indicate that vaporized arsenic exists mostly in the form c$ As, molecules up to temperatures of 800”. The following equilibria exist at all temperatures. . As, # 2 Asz # 4 As
(1)
At a total pressure of 750 mm Hg, the partial pressures of the three species at temperatures of 800, 1000 and 1200” are given in Table I. These data indicate that the formation of As, molecules is favored at the temperatures present in the atomizer cell under equilibrium conditions and in the absence of other competing equilibria. If arsenic compounds are reduced to arsenic atoms in the atomizer, the latter will readily form As2 and As, molecules. The rate of formation depends on the kinetics of equilibrium (1). It would be expected that at low arsenic concendifferent trations, the rate of formation of As, and As, would be significantly from the rate for high concentrations of arsenic.
SHORT
COMMUNICATION
TABLE
I
PARTIAL
PRESSURES
----__
Sprcics
Porritrl 800
a
~~-.----._--._-696.2 Asa AS2 AS
’
47.6 5.21
OF
205
As, AND
As,,
p~‘cs.s~~w ( ~III of -~..--_.-.------.-------_-
As MOLECULES __-_ --._.
Hy)
i(MJ
1200
529.4 198.4 22.3
452.6 216.4 XI.0
-_--_--..-. ----
...--.-.
--_-. --.-
-.-----
.-.-- -.------.
----_-
------ - --.- ---.--
Since arsenic vapor is distributed in an equilibria involving arsenic molecules, it was decided to measure the molecular absorption of an arsenic system. A cell was evacuated and doped with. a small quantity of elemental arsenic; the cell was heated to various temperatures and the arsenic allowed to equilibrate. The molecular absorption was measured with a deuterium lamp as source: the spectral slit width was 0.125 nm. The results are shown in Fig. 2. Wide-band molecular absorption was observed for the arsenic vapor in the wavelength region 185-280 nm. The degree of absorption was temperature-dependent, again indicating the importance of equilibria conditions. The results showed that the As, species absorbed weakly but that As, absorbed more strongly across the spectral range examined. Moreover, there were variable degrees of molecular absorption across the three resonance lines of arsenic .It seemed clear that equilibria conditions would greatly affect the absorption measured for ;I given concentration of arsenic, which would be particularly important for flume atomizers: ilny small change in the part of the flame monitored would introduce a large change in the absorption signal. Many workers have found that the determination of arsenic by atomic absorp’tion has been difficult and unreliable. In flame atomizers atomic arsenic can be quickly changed to molecular arsenic with a severe loss in sensitivity. In non-flame atomizers the problem is further complicated by a possible interaction with carbon. Only under rigidly controlled conditions will reproducible data be obtained. However, with variation in sample composition such control may not always be possible.
Fig. 2. Molecular
absorption
spcctr;l l-or arsenic at
I IOU (--).
925”
(. * * *), and 775” (--
-).
206 Control
SHORT
This investigation was supported by research Ollice, Environmental Protection Agency.
grant
COMMUNICATION
R 800771.
Air Pollution
REFERENCES H. P. Hofton. C. M. Christian and J. W. Robinson. Specwosc. Lctf.. 3 (1970) lG1. C. M. Christian and J. W. Robinson, Aw/. Chim Acru, 56 (1971) 466. J. W. Robinson. D. K. Wolcott. P. J. Skin and G. D. Hindman. Awl. Clritn. Acm. 4 (1973) 13. J. W. Robinson, P. J. Shin. G. D. Hindman and D. K. Wolcott, /Ir~tr/. Chirn. Acrcc. 61 (1972) 431. J. W. Robinson and P. J. Slcvin. Artwr. Lob.. 48 (1972) 16. J. W. Robinson and D. K. Wolcott. AM/. Clzirn. kfo. 66 (1973) 333. H. Massmann. Z. Irwl. Clwn~. 752 (1970) I I I. 8 B. V. L’vov, Aronlic Ahsorpriort Spccrroscop~. lsracl Program for Scientific Trtmslntions. U. S. Dept. of Commcrcc, Springfield. Vn.. 1969. p. 33.
1 2 3 4 5 6 7
Chhictr
69 ( 1974) 203-206 Publishing Company.
Scientific
SHORT
COMMUNICATION
Difficulties
in the determination
J. W. ROBINSON. Depurttmwt
(Reccivcd
203
Actu,
ci> Elsevicr
R. GARCIA.
of arsenic
G. HINDMAN
oJ’ Clret~ristry. Louisiotw
28th Scptcmbcr
Amsterdam
- Printed
by atomic
in The Netherlands
absorption
spectrometry
and P. SLEVIN
Stcrle Utliuersif_v,
Uufon Ro~qe.
Louisiutrn
7OW3 ( U.S.A.)
1973)
Previous communications from this laboratory have described a system for the continuous determination of lead1,,cadmium2*3 and mercury2*4*5 in the atmosphere. The present study was undertaken in an effort to extend this procedure to include arsenic. Although the resonance lines of arsenic lie in the vacuum ultraviolet at 189.0, 193.7 and 197.2 nm, this presented no problem as the system employed permitted the use of resonance lines as low as 184.9 nm’. The various techniques used in the calibration of the instrument for different metals have been compared6. An initial investigation showed that on occasion ambient air gave an arsenic atomic absorption signal even after correction for the molecular absorption in this spectral region. The results indicated *that arsenic in the atmosphere could be directly detected by atomic absorption. It was confirmed that the 189.0-nm line is the most sensitive for arsenic, followed by the 193.7-nm line7*8. Equipment
The equipment used has been previously described4. The r.f. carbon atomization cell was of the design described for cadmium determinations’. A Glomax demountable hollow-cathode lamp (Barnes Engineering Company) operated in a helium atmosphere was used as the radiation source. Calibration rvethotls The use of awine. The
initial approach adopted to calibrate the instrument for arsenic was to inject arsine gas diluted with clean air into the system. Arsine is readily decomposed at temperatures above 230” into arsenic and hydrogen. However, it was found that arsine decomposed even at room temperature, so that quantitative data based on the concentration of arsine introduced were unreliable. Moreover, some arsenic metal plated out in cool zones of the equipment between the carbon bed at 1600” and the water-cooled brass adaptor of the inlet head; and arsenic was held up in the carbon bed for varying unpredictable periods of time. Calibration by direct droplet hjection-H2 absorption. Any organic solvent introduced into the system was decomposed to carbon monoxide and hydrogen. The latter absorbed at wavelengths below 200 nm. In an effort to decrease this molecular background, formic acid was used as a solvent because of its low
204
SHORT
COMMUNICATION
4
5040.z 30P 3 %
20-
z a, k! IOa! rl 6 Time (mid
Fig. I. Absorption sigmls.
h-xc
l’or arsenic
10
injection
14
in rmnic
acid. A. points
ol’ injection:
B, dclaycd
xscnic
hydrogen content. Solutions of arsenic in formic acid were prepared by dilution of ktl-quantities of an aqueous 1000 116 ml - ’ arsenic stock solution with formic acid. A diagram of a typical absorption trace is shown in Fig. 1. These delayed absorption peaks were established to be due to arsenic atomic absorption rather than molecular absorption (measured by the deuterium lamp). To investigate this further. arsenic was introduced into the carbon bed by the the direct drop technique, and by injecting into a carbon disc and dropping disc onto the bed. Results indicated loss of arsenic on the carbon disc and again suggested interaction between arsenic and carbon. In further studies, the carbon bed depth was varied as was the air flow rate. The absorption signal increased with decreased contact with the carbon bed and with a decreased time between atomization and absorption measurement. Each of these observations indicated a carbon-arsenic interaction, although no C-As compounds have been reported in the literature. Vapor pressure data indicate that vaporized arsenic exists mostly in the form c$ As, molecules up to temperatures of 800”. The following equilibria exist at all temperatures. . As, # 2 Asz # 4 As
(1)
At a total pressure of 750 mm Hg, the partial pressures of the three species at temperatures of 800, 1000 and 1200” are given in Table I. These data indicate that the formation of As, molecules is favored at the temperatures present in the atomizer cell under equilibrium conditions and in the absence of other competing equilibria. If arsenic compounds are reduced to arsenic atoms in the atomizer, the latter will readily form As2 and As, molecules. The rate of formation depends on the kinetics of equilibrium (1). It would be expected that at low arsenic concendifferent trations, the rate of formation of As, and As, would be significantly from the rate for high concentrations of arsenic.
SHORT
COMMUNICATION
TABLE
I
PARTIAL
PRESSURES
----__
Sprcics
Porritrl 800
a
~~-.----._--._-696.2 Asa AS2 AS
’
47.6 5.21
OF
205
As, AND
As,,
p~‘cs.s~~w ( ~III of -~..--_.-.------.-------_-
As MOLECULES __-_ --._.
Hy)
i(MJ
1200
529.4 198.4 22.3
452.6 216.4 XI.0
-_--_--..-. ----
...--.-.
--_-. --.-
-.-----
.-.-- -.------.
----_-
------ - --.- ---.--
Since arsenic vapor is distributed in an equilibria involving arsenic molecules, it was decided to measure the molecular absorption of an arsenic system. A cell was evacuated and doped with. a small quantity of elemental arsenic; the cell was heated to various temperatures and the arsenic allowed to equilibrate. The molecular absorption was measured with a deuterium lamp as source: the spectral slit width was 0.125 nm. The results are shown in Fig. 2. Wide-band molecular absorption was observed for the arsenic vapor in the wavelength region 185-280 nm. The degree of absorption was temperature-dependent, again indicating the importance of equilibria conditions. The results showed that the As, species absorbed weakly but that As, absorbed more strongly across the spectral range examined. Moreover, there were variable degrees of molecular absorption across the three resonance lines of arsenic .It seemed clear that equilibria conditions would greatly affect the absorption measured for ;I given concentration of arsenic, which would be particularly important for flume atomizers: ilny small change in the part of the flame monitored would introduce a large change in the absorption signal. Many workers have found that the determination of arsenic by atomic absorp’tion has been difficult and unreliable. In flame atomizers atomic arsenic can be quickly changed to molecular arsenic with a severe loss in sensitivity. In non-flame atomizers the problem is further complicated by a possible interaction with carbon. Only under rigidly controlled conditions will reproducible data be obtained. However, with variation in sample composition such control may not always be possible.
Fig. 2. Molecular
absorption
spcctr;l l-or arsenic at
I IOU (--).
925”
(. * * *), and 775” (--
-).
206 Control
SHORT
This investigation was supported by research Ollice, Environmental Protection Agency.
grant
COMMUNICATION
R 800771.
Air Pollution
REFERENCES H. P. Hofton. C. M. Christian and J. W. Robinson. Specwosc. Lctf.. 3 (1970) lG1. C. M. Christian and J. W. Robinson, Aw/. Chim Acru, 56 (1971) 466. J. W. Robinson. D. K. Wolcott. P. J. Skin and G. D. Hindman. Awl. Clritn. Acm. 4 (1973) 13. J. W. Robinson, P. J. Shin. G. D. Hindman and D. K. Wolcott, /Ir~tr/. Chirn. Acrcc. 61 (1972) 431. J. W. Robinson and P. J. Slcvin. Artwr. Lob.. 48 (1972) 16. J. W. Robinson and D. K. Wolcott. AM/. Clzirn. kfo. 66 (1973) 333. H. Massmann. Z. Irwl. Clwn~. 752 (1970) I I I. 8 B. V. L’vov, Aronlic Ahsorpriort Spccrroscop~. lsracl Program for Scientific Trtmslntions. U. S. Dept. of Commcrcc, Springfield. Vn.. 1969. p. 33.
1 2 3 4 5 6 7