PbBr6*- and 14910 cm-I IvP1 for PbCls4- have been reported (19).
Cuvettes that were not made with lead-containing solder glass did not cause the formation of these complexes and their spectra did not show selective absorptions. Thus, the cuvettes referred to earlier as “active” were those that did contain, in their construction, leachable lead glass. The “inactive” ones were apparently fabricated either without the use of lead glass or, if lead glass was used, the cuvettes were designed to avoid contact of the solder glass joints with the sample. Indeed, substance has been added to this surmise when absorptions at 270, 302, and 357 nm were observed in spectra taken of halide solutions that had been in contact with fragments of lead-containing solder glass obtained from American Optical Company, Southbridge, Mass. A commercial product, Pyroceram Brand Cement (Corning Glass), in the form of finely divided glass designed for student use in the fabrication of glass apparatus, was treated in a similar way and again produced the characteristic absorptions at 270 and 301 nm. Of course, the possibility that even the “pure” salts used in the earlier work may have been contaminated with lead cannot be ignored. In our laboratory, LiCl from a previously opened
bottle produced a small absorption at 270 nm without contact with an “active” cell. Even after several recrystallizations, the peak (and, presumably, the lead contamination) was still present. Due to the high extinction coefficients of the halo-lead complexes at their absorbing regions, very small concentrations of lead are detectable. It is possible to detect total PbZ+ concentrations of 10-6M or less in concentrated halide solutions. This sensitivity may have analytical value in certain qualitative applications. We have been able to detect spectroscopically the presence of lead in the glaze of a certain brand of imported dinnerware which had been proclaimed by city health authorities as hazardous due to its lead content. ACKNOWLEDGMENT
The authors wish to thank W. B. Bridgman for productive discussions and encouragement. RECEIVED for review December 20, 1971. Accepted July 10, 1972. A portion of this investigation was performed by one of the authors, A. N., while supported by a National Defense Education Act fellowship.
Direct Volatilization-Spectral Emission Type Detection System for Nanogram Amounts of Arsenic and Antimony Robert S. Braman, Lewis L. Justen, and Craig C. Foreback Department of Chemistry, Uniaersity of South Florida, Tampa, Fla. 33620 An emission type detection system has been adapted for the detection and determination of arsenic and antimony. Arsenic and antimony compounds are converted to AsH3 and SbH3 in 1% aqueous NaBH4. Arsine and stibine are swept out of the solution using He carrier gas, then passed through a CaS04 drying tube and through a dc discharge detector. The 228.8 nm As and 252.5 nm Sb emission line intensities are monitored in a conventional type photometric system. Limits of detection are near 0.5 and 1 nanogram for Sb and As, respectively. Operating conditions of the technique have been studied in detail. The method has been applied to the quantitative analysis of natural waters and some marine sludges. This method is highly specific, and is much more sensitive and rapid than most current methods for arsenic and antimony.
INTERE~T IN THE DETECTION and determination of small amounts of arsenic and antimony compounds stems from their toxicity. Arsenic and antimony compounds have been widely used in insecticide products. A number of classical analytical methods have been developed for use in the field of forensic chemistry. The most widely used current methods for arsenic in low concentrations or small amounts are based upon colorimetric analysis after distillation of arsenic out of samples as arsine. The silver diethyldithiocarbamate method ( I ) for water analysis is an example. The lower limit of detection for this method is 0.2 pg As. Neutron activation methods (1) G. Stratton and H. C . Whitehead, J . Amer. Water Works Ass., 54, 861 (1962).
have a limit of detection of 1 nanogram ( 2 ) but are comparatively time consuming and complex. Atomic absorption methods for arsenic have limits of detection in the range 0.5 to 1 ppm in the aspirated samples (3, 4 ) . Using an electrodeless discharge lamp and an argon (entrained air) hydrogen flame, Menis and Rains (5) obtained detection limits near 0.1 ppm in the aspirated sample. Ando et al. (6) used a 91-cm path length and an argon (entrained air)-hydrogen flame to obtain a sensitivity (1%deflection) of 6 ppb. Neutron activation ( 2 ) methods for antimony have limits of detection similar to those of arsenic. Atomic absorption methods for antimony ( 7 , 8 ) have sensitivities (1 % absorption) in the 0.3- to 1.5-ppm range. Rhodamine-B, a sensitive colorimetric reagent for antimony, is useful in the 2- to 20-pg range after extraction into benzene (9) or isopropyl ether. (2) V. P. Guinn and H. R. Lukens, Jr., data in “Trace Analysis: Physical Methods,” G. H. Morrison, Ed., Interscience Publishers, New York, N.Y., 1965, p 345. (3) W. Holak, ANAL.CHEM.,41, 1712-13 (1969). (4) G. F. Kirkbright and L. Ranson, ibid., 43, 1238 (1971). (5) 0. Menis and T. C. Rains, ibid., 40, 95 (1968). (6) A. Ando, M. Suzuki, K. Fuwa, and B. L. Vallee, ibid.. 41, 1974 (1969). (7) S. Slavin and T. W. Sattur, At. Absorption Newslett., 7 (5), 99 (1968). (8) C. R. Walker, 0. A. Vita, and R. W. Sparks, Atzal. Chim. Acta, 47, 1 (1969). (9) E. B. Sandell, “Colorimetric Determination of Traces of Metals,” 3rd ed., Interscience Publishers, New York, N.Y., 1959, p 254.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972
2195
--To Monochromator System
Figure 1. Detail of volatilatizion system design
Electrical discharges in argon or helium are known to provide high sensitivity and selectivity in detection (10). During recent work by Braman (11) on the determination of small amounts of mercury, it was observed that arsine and stibine could also be detected with high sensitivity in discharges. Arsenic and antimony compounds were reduced to arsine and stibine in aqueous sodium borohydride solutions, carried out of the reaction chamber through a drying tube to remove water, and then passed through a dc discharge. A conventional monochromator and photometric readout apparatus was used to observe the 228.8 nm As and 242.5 nm Sb elemental emission lines. Responses were proportional to amounts of arsenic or antimony present. Limits of detection were near 1 nanogram. The method has subsequently been studied in detail and applied to the analysis of natural water samples and marine sludges. It is rapid, has few interferences, and has been found more sensitive and selective than any of the current colorimetric or conventional atomic absorption methods for arsenic or antimony. EXPERIMENTAL
Apparatus. The apparatus arrangement used consisted of a volatilization-reaction chamber, a drying tube, a dc discharge detector cell, a monochromator, and conventional electronic readout system. The dc discharge detector and optical readout system have been described in earlier work (11). Detail of the volatilization system design is shown in Figure 1. The volatilization chamber was a 100-ml graduate cut off near the 80-ml mark and had a 1-inch long, 6-mm side arm tube. A 0.25-inch Swagelok, Teflon (Du Pont) nut cut off an old tube fitting was sealed to the side arm tube with epoxy cement. A rubber septum completed the injection port design. Helium carrier gas was passed through a gas dispersion tube to pick up arsine or stibine generated in the chamber and was then passed through a drying tube and through the detector cell. Teflon tubing was used throughout the system. The drying tube was a 16-mm i.d. by 15-cm long polyethylene tube filled with 8-mesh color indicating anhydrous, calcium sulfate (Drierite) retained by small filter (10) R. S. Braman and A. Dynako, ANAL. CHEM., 40,95 (1968). (11) R . S. Braman, ihid., 43,1462 (1971). 2196
paper circles. Metal connectors and tubing and glass wool must be avoided in constructing the train to avoid absorption losses of stibine and arsine. Instrument Settings. The photomultiplier tube (RCA 1P28) voltage, discharge power, monochromator slit width, amplifier gain, and range of the strip chart recorder, all control the signal gain of the system. Several of these factors were varied systematically in this study. The following set of conditions was considered optimum for arsenic and antimony analysis: PM voltage, 800-900 V, dc discharge, 37 watts per lineal inch of discharge; 50-100 micron slit; and IOd6 or lo-’ ampere amplifier range. The amplifier output was 0.025 V for full scale range; the recorder had a range of 0-10 mV full scale. Reagents. Sodium borohydride from Alfa Inorganics was used as the reducing agent. An optimum concentration of freshly prepared 1 NaBH4 in water was used. There was no detected trace of arsenic or antimony in the NaBHI, any present would be evolved as arsine or stibine upon dissolution. American Cryogenics U.S.P. grade helium is of sufficient purity for use as a carrier gas without treatment. No residual arsenic or antimony was found in this gas. A 0.125inch by 1-meter long, flow restricting column was used just prior to the volatilization chamber. Procedure. The analysis procedure was studied systematically; the following procedure is optimum for arsenic and antimony analysis. Antimony and arsenic in samples must be in aqueous solution in the +3 or +5 oxidation state. Solution pH must be adjusted to pH 7-11 range. Large amounts of oxidizing agents and other materials which can seriously deplete the concentration of sodium borohydride reducing agent must be absent. Copper and silver interfere in arsine and stibine evolution and must be removed. This is done by passing sample solutions having more than 1 ppm copper or silver through a 16-cm by 0.8-cm i d . cation exchange column of Amberlite IR-124 (Rohm and Haas) 20-50 mesh, in the ammonium ion form. The volatilization chamber is filled to 25 ml with 1 NaBH4. Helium carrier gas at 200 ml per minute is allowed to sweep out the system for several minutes to displace air prior to initiating the discharge. The volatilization chamber is maintained at room temperature. Antimony samples are degassed with He prior to analysis to avoid the small positive response which dissolved nitrogen gas gives at 252.5 nm. Solutions of samples containing from 0.005 to 2 Mg of arsenic or antimony in up to 10 ml of solution are injected by
ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972
14
12
-
10
I
I.,-
N
I
8
0
d
B
4
2
h
ng Sb or As
Figure 3. Calibration curves (10-7 A amplifier range)
Figure 2. Typical response curves Sb, time scan at 252.8 nm (20 ng, lo-' A scale) b. As, time scm at 228.8 nm (500 ng, A scale) c. As, repetitive scans at 228.8 nm (500 ng, 10-6 A scale) a.
syringe into the volatilization chamber. After addition of approximately 30 ml of sample volume, the reducing solution is replaced. Responses are observed over a period of 3 to 6 minutes after sample injection and can be recorded in two ways. Repeated wavelength scans through the emission line region give results as shown in Figure 2c. If the monochromator is set to continuously observe line intensities as a function of time, the response as shown in Figure 2a or 2b is obtained. The continuous recording technique is preferred for ease of area integration. The repeated scan technique is superior for assuring specific detection of arsenic or antimony. Areas under response curves are integrated and are a linear function of the amount of arsenic or antimony present in samples. Calibration curves are prepared by syringe injection of arsenic or antimony solutions having appropriate concentrations of arsenic or antimony.
RESULTS AND DISCUSSION Response Curves, Precision, and Limits of Detection. Response curves for arsenic and antimony were obtained at or near optimum operating conditions for the sample size range from 3 ng to 1.5 pg. The smaller sample size calibration plots are shown in Figure 3. Area response calibration curves were linear for Sb throughout the 3-ng to 1.5-pg range. Arsenic response above approximately 0.2 pg becomes slightly curvilinear. Antimony is approximately two to four times more sensitively detected than arsenic under the conditions. From the response curve data and data on area noise as deter-
Table I. Arsenic and Antimony Responses at Selected Emission Lines Antimony Arsenic WaveResponse, WaveResponse, length, nm CmVPg" length, nm cm2,'pgb 287.8 259.8 252.8 217.5 206.8
a
b
1.7 8.8 15.2 5.6 0.3
286.0 278.0 235.0 228.8 197.2 193.6 189.0
3.2 8.6 20.9 33.3 6.3 12.2 2.9
PM, 800 V, 40-micron slit, IOe6A range. PM, 900 V, 40-micron slit, 10-6 A range.
mined by the method of Johnson and Stross (12) the limits of detection are 0.5 nanogram for antimony and 1 nanogram for arsenic. Assuming 1-ml sample volumes, the lower limits of detection are 0.5 and 1 part per billion for Sb and As. The upper limit of the method was not established but samples containing over 5 pg would give signals outside the usual range of the recording equipment. The lower limits of detection are a factor of 50 to 500 below those of spectrophotometric methods. Precision of the method was 1 5 % relative for samples giving area responses larger than 2 cm2. The precision of area responses less than 2 cm2was 10.15 cm?. Wavelength Selection. Several of the reported atomic emission lines for each element were studied for use in detection. Response in area per weight of sample was determined at each wavelength studied. Results are given in Table I. The maximum sensitivity found was obtained with the 252.8 nm Sb and 228.8 nm As lines. Use of a photomultiplier tube more suited to the 200-nm and below range could change the selection of emission line with the greatest response per (12) H. W. Johnson, Jr., and F. H. Stross, ANAL.CHEM., 31, 1206 (1959).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972
2197
24
t
.'
/
I
16
c
/
18 N
,
A
0
4 /
/ / /
c
./
/
/
/
12
I
I
16
20
I 24
I 28
I 32
I 36
Discharge Power. Watts (0.75 inch discharg)
Figure 4. Effect of discharge power level on responses
weight. The optimum emission lines were comparatively free of band systems from traces of nitrogen in the carrier gas. The 252.8 nm Sb line is near a violet degraded 244.80 nm N2 4th positive system band head and a 242.78 nm NO band. These are not an interference from the small traces of N2 impurity in the carrier gas. The 228.81 nm As line is near red degraded 228.15 nm N1 band and a 229.05 nm red degraded N2 band. Neither is a significant interference from carrier gas impurities. Slit Width Effect on Response. The limit of detection was determined as a function of slit width on the Heath Company monochromator. From an optical viewpoint and assuming a very narrow emission line width, limits of detection should not improve as slit widths increase. Increasing slit widths increases the total background radiation observed and thus the background noise also increases. This was not the case in this instance. An optimum response was obtained near 100 micrometers slit width. Electronic noise not associated with the discharge could have accounted for a large fraction of the total noise for slit widths below 100 micrometers. After 100 micrometers, further increases in slit width increased the noise signal from an increase in the discharge background noise. Power Level Effect. Figure 4 shows the response of the system for a fixed sample size as a function of power dissipated in the detector cell. Arsenic and antimony have slightly different power level response maxima. The optimum value for both elements is near 37 watts per lineal inch of discharge. Response is not a sensitive function of power level near the optimum. Effect of Carrier Gas Flow Rate. The effect of carrier gas flow rate on response was studied. Higher carrier gas flow rates reduce the time required to sweep out both arsine and stibine. Arsenic response per fixed sample size does not vary greatly over the 40 to 400 ml per minute flow rate range standard. Antimony response increases rapidly up to a maximum at approximately 200 ml per minute and remains comparatively constant to 400 ml per minute. The cause of the lost stibine signal at low flow rates was not found. The volatilization efficiency of arsine and stibine as a function of carrier gas flow rate was studied using spectrophotometric methods and 10- and 14-microgram sample sizes for antimony and arsenic, respectively. The Rhodamine-B 2198
method (4) was used for antimony and the silver diethyldithiocarbamate method ( I ) for arsenic. Arsenic exhibited a 100% recovery at all flow rates studied (40 to 150 ml He per minute). Antimony is apparently not quantitatively swept out of the system over the 40- to 400-ml per minute range studied but exhibits a 90% recovery maximum at 200 ml per minute. Losses may be surface absorption or oxidation of stibine after leaving the reducing solution. A carrier gas flow rate of 200 to 250 ml He per minute appears suitable for both elements. Effect of Volatilization Solution Volume. As samples are processed, the volume of the solution in the chamber increases. An increase in solution volume could decrease scrubbing efficiency. The response of 0.87- and 0.95-microgram samples of arsenic and antimony samples, respectively, was determined at several solution volumes. Responses decrease less than 10% for both elements as the solution volume rises from 10 to 40 ml. Removal of solution by syringe or complete replacement of the volatilization chamber solution is necessary after addition of 20 to 30 ml of total sample volume. Sample volumes of 1 ml are most convenient as 20 to 30 samples can then be analyzed using the same original reducing solution. Effect of NaBH4 Concentration. Quantitative volatilization of arsine and stibine requires freshly prepared sodium borohydride in water solutions at concentrations or higher by weight. Solutions of 1 to 2% NaBH4 by volume are optimum as the concentration decreases while sample is added. The evolution of large amounts of hydrogen gas can quench the discharge. Temperature Effects. Room temperature (23 "C) was the most suitable for operation of the method. Higher temperatures (above 40 'C) speeded both the decomposition of sodium borohydride by hydrolysis and decreased the operating time of the drying tube. Drying Agents. A drying agent is needed to remove water vapor from the carrier gas prior to introduction into the discharge. Water would otherwise quench the discharge. Several drying agents were used during the course of the experimental work. Anhydrous calcium sulfate (Drierite) and anhydrous calcium chloride quantitatively passed arsine and stibine while removing water. Silica gel absorbed all stibine and interfered in the passage of arsine. Anhydrous magnesium perchlorate absorbed approximately 30 to 40% of arsine and stibine, probably by oxidation. The amount of indicating Drierite used in the drying tube lasted for 4 to 6 hours of continuous operation at the 200 ml per minute flow rate before needing replacement. Used Drierite did not appear to have any appreciable effect on response or quantitative passage of arsine or stibine. Limitations and Interferences. Arsenic and antimony must be in solution and in the +3 or + 5 oxidation states prior to conversion to arsine or stibine. Stibine and arsine are detectable if present as such in sample solutions. Cacodylic acid, (CH&As02H, was not reduced to any detectable volatile arsenic compound by sodium borohydride. Consequently, many aryl or alkyl arsenic or antimony compounds may require oxidation prior to analysis. A similar sample treatment would also appear to be necessary for analysis of alloys and other binary compounds of arsenic and antimony. Samples must also be adjusted to a neutral to alkaline pH to avoid decomposition of the sodium borohydride. Large amounts of certain metal ions, if present in samples, will react with the sodium borohydride to produce metal borides or free metal in suspension.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972
Several cations and anions were tested for interference. None tested acted as a positive interference but some inhibited the evolution of arsine and stibine. Results are shown in Table 11. In addition to the data given, no interference was noted from seawater, nitrate ion, carbonate ion, and borate ion. The interference from copper and silver ions is in accord with other methods in which arsine and stibine are evolved prior to colorimetric analysis. Silver and copper ion interferences were removable by passing sample solutions through a cation exchange column in the ammonium ion form prior to analysis. No arsenic or antimony loss on the column was noted from solutions which were 0.2 to 0.8 ppm in these metals and 20 ppm in silver and copper ion. The removal of the very small amounts of copper and silver in seawater was not necessary. Arsenic and antimony do not interfere in the analysis of each other up to at least a 100 :1 ratio if the detector cell has no arsenic or antimony background. Large amounts of arsine or stibine (1-10 pg) will slowly build up a background (perhaps the elements or oxides) in the discharge chamber. This leads to a positive interference effect when mixtures of the two metals are to be analyzed. If, for example, a background of arsenic is built up, the injection of antimony alone will result in a signal at the arsenic wavelength. In this case, arsenic is being removed by the stibine gas. The removal of background arsenic or antimony in the detector cell can be easily done by injection of air, oxygen, or concentrated ammonium hydroxide into the system. Dissolved nitrogen will act as a positive interference in antimony analyses unless samples are degassed with helium. Dissolved nitrogen from air saturated water gives a signal equivalent to 20 ng Sb/ml or 10 ng As/ml. Since the evolution of stibine is comparatively rapid, the sweep out of nitrogen from samples and the sweep out of generated stibine occur at the same time. Dissolved nitrogen is not nearly the problem in arsenic analyses as it is in the case of antimony. Dissolved nitrogen is swept out almost completely prior to the evolution of arsine. Thus nitrogen is detected prior to the arsine. Applications. A large number of seawater, fresh water, and oxidized marine sludge samples have been analyzed for arsenic and antimony. Analysis of spiked samples of all the above types resulted in recovery of all antimony and arsenic added. Seawater samples (from the Tampa Bay area) all contained from 10 to 30 parts per billion of arsenic and antimony. Oxidized sludge samples had to be neutralized to pH 9-10 prior to analysis. Metal hydroxides, if any, were allowed to settle prior to sampling for analysis.
Table 11. Interferences in Arsine and Stibine Evolution from NaBHa Solutions Arsenic Antimony Ion ppm (0.81 ppm)" (0.24 ppm) Ag+
Ala+ Cd2+ cu2+
20 2 20 20
20
2 Cr2072-
Fe3+
Hg2+
Mn2+ Ni2+ Pb2+
20
20 5 2 20 20 20 1600 20
i
(-1 (-1 (7)
i
(-1 (-1 (-1
1
1
(-1 (-1 (-1 (-1 (-1 (-1 (-1 (-1 (-1 (-1
(-1 (-1
i, small amt (-) (-) (-)
(-1 (-1 (-1 (-1
Na2HPOd Zn2+ (-) = no interference at the indicated metal ion concentration. i = inhibits response by 10% or more at 20 ppm metal ion concentration.
Analysis of the water samples was possible without any pretreatment of any kind. This method for arsenic and antimony, particularly in natural waters, is rapid; analyses required from 3 to 5 minutes. The accuracy of the method was not studied in detail as standard water samples in the very low arsenic and antimony concentration ranges were not available. Prepared standards gave results in agreement with the calibration curve data. Several arsenous oxide and tartar emetic samples analyzed by iodometric titration also gave results in agreement with the calibration curves after dissolution to appropriate concentration andLsubsequent analysis. ACKNOWLEDGMENT
The authors wish to acknowledge the assistance rendered by Dominic Maggio in data reduction. RECEIVED for review February 25, 1972. Accepted July 7, 1972. Presented at the PittsbuFgh Conference on Analytical Chemistry and Applied Spectroscopy, March 8, 1972. This work was supported in part by the Undergraduate Research Participation program of the National Science Foundation under Grant Number GY 8814 and in part by the National Science Foundation under Grant Number NSF G P 31256.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 13, NOVEMBER 1972
2199
