1766
Anal. Chem. 1981, 53, 1766-1771
Determination of Antimony(III), Antimony(V), and Methylantimony Species in Natural Waters by Atomic Absorption Spectrometry with Hydride Generation Meinrat 0. Andreae, * Jean-Frangois Asmodi,' Panayotis Foster,* and Luc Van7 dack3 Department of Oceanography, Florida State University, Tallahassee, Florida 32306
The antlmony specles antimony(111), antimony(V), methylstlbonic acld, and dimethyistlbinic acid are reduced in aqueous matrlx by sodlum borohydrlde to stlbine, methylstlbine, and dimethyistibine. Sb(II1) and Sb(V) are reduced together to stlbine under hlghly acldlc condltlons in a solutlon containing Iodide. Sb(II1) Is selectlveiy reduced at near-neutral pH, where no reductlon of Sb(V) takes place. Sb(V) Is then obtained by subtraction. The methylantimony acids are reduced at acld pH without addltlon of iodlde. The stlblnes are collected on a Iiquld-nitrogen-cooled trap and separated chromatographically after heatlng the trap. They elute Into a quartz cuvette burner or graphite furnace mounted In the beam of an atomic absorption spectrophotometer; the resulting absorbance signals are recorded as chromatographic peaks. The absolute detectlon limit is 30-60 pg of Sb, depending on species and operating conditions; the concentration limit of detection is 0.3-0.6 ng L-' for a lOOmL sample. Analytical results from river and estuarlne waters are presented. Sb(III), Sb(V), methyistibonlc acld, and dlmethylstlblnic acid were found In natural waters.
Antimony enters the aquatic environment as a result of the weathering of rocks (which contain an average of 0.16 ppm Sb), from soil runoff, through effluents from mining and manufacturing, and from municipal discharges. Typical concentrations in unpolluted waters are in the sub-partsper-billion range ( I , 2), but in leachates from secondary lead smelters levels of several parts per million have been observed (S. Johnston, Department of Environmental Regulation, Tallahassee, FL, personal communication, 1981). Antimony and its compounds have been listed as priority pollutants by the U.S.Environmental Protection Agency. The determination of antimony in aqueous solutions by reduction to stibine with sodium borohydride was first suggested by Braman et al. (3). Detection of the stibine was by atomic emission from a dc discharge. The combination of hydride generation and atomic absorption detection (without cold-trapping of the stibine) was described by Fernandez (4). As will be discussed below, these procedures and similar ones in the literature are applicable only to solutions containing Sb(III), inasmuch as Sb(V) is not completely reduced under the conditions described. The reduction of this species requires the presence of iodide ion in an acid reaction medium (5). This effect has been used for the selective determination of Sb(II1) and Sb(V) (6);at low pH and in the presence of zirconium(IV),the reduction of Sb(II1) is suppressed less than that of Sb(V), so that under these conditions only Sb(II1) is determined. Total inorganic antimony is then measured by DBpartement d'Environnement, Universit6 Paris VII, 2 Place Jussieu, 75221 Paris Cedex 05, France. * Institut Universitaire de Technologie I, Rue Franpois Raoult, 38000 Grenoble, France.
Departement Scheikunde, Universitaire Instelling Antwerpen, Universiteitsplein 1, B-2610 Wilrijk, Belgium.
0003-2700/81/0353-1786$01.25/0
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reduction in acid solution in the presence of iodide, and Sb(V) is calculated as the difference. This method is not highly selective; interference of Sb(V) in the determination of Sb(II1) starta to become significant at a ratio of 41. As will be shown below, most natural waters have Sb(V)/Sb(III) ratios on the order of 100 or more. None of the methods published to date combines the sensitivity and selectivity required to determine Sb(II1) and Sb(V) at the concentrations found in natural waters. No organoantimony compounds have been previously observed in nature. In preliminary work, we observed peaks eluting after the stibine peak, which we tentatively attributed to methylantimony acids, in analogy to the methylarsenic acids, methylarsonic acid, and dimethylarsinic acid, the biogeochemistry of which we have discussed previously (7). The compounds methylstibonic acid [CH3SbO(OH)2,dihydroxo(oxo)methylantimony] and dimethylstibinic acid [(CH&'3bO(OH), hydroxo(oxo)dimethylantimony], were prepared by Meinema (B), who made samples of these compounds available to us. As discussed below, we have established the identity of the synthetic compounds with those found in natural waters. This paper presents details on the analytical procedure for the determination of inorganic and methylantimony species by selective hydride generation and atomic absorption detection. The performance of the procedure and its application to selected samples of natural waters will be discussed.
EXPERIMENTAL SECTION Apparatus. The apparatusfor the reduction of the antimony species to the corresponding hydrides, their collection on a cold trap, and their determinationby atomic absorption is shown in Figure 1. It consists of a reaction vessel which contains the sample and any added reagents. Reaction vessels of different sizes (25-250 mL) can be attached to the apparatus. Helium enters the solution through a fritted bubbler and serves to carry the stibines from the reaction vessel into the cold trap. Sodium borohydride solution is injected with a plastic syringe into the reaction vessel through a side arm, which is closed off by a Teflon-coated silicone septum held in a Teflon Swagelok union (6.4 mm 0.d.). All connections in the apparatus are made glass-to-glassby short Teflon sleeves, using 6.4 mm 0.d. Teflon tubing which creates a gastight fit around 6 mm 0.d. glass tubing. All glass parts are constructed from Pyrex glass. The stibines are collected in a cold trap immersed in liquid nitrogen. It consists of a 6 mm 0.d. Pyrex U-tube; the length of the U is about 30 cm. About two-thirds of the U is filled with a chromatographic packing (15% OV-3 on Chromosorb WAWDMCS 60/80 mesh). About 1m of Chrome1wire (-3 Q) is wound around the outside of this trap/column and connected to a variable transformer. This permits heating the trap at a controlled rate after the removal of the liquid nitrogen bath. The outlet of the cold trap/column is connected to the atomic absorption detection system. The internal surfaces of the system, with the exception of the reaction vessel, are deactivated by treatment with silylation reagents. Treating the reaction vessel with the silylating reagent does not improve the performance of the system and results in excessive foaming. The surfaces are initially treated with Sylon 0 1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981 He in
'-1
to Vorioc
Quartz Cuvette Burner Pocking
Flgure 1.
Apparatus for the determination of antimony species in
water. CT, a commercial preparation of 5 % dimethyldichlorosilane in toluene (Supelco, Bellefonte, PA). After packing the trap, it is conditioned for a few hours at 150 "C, and then two times 25 pL of Silyl-8 (Pierce Chemical Co., Rockford, IL) are injected onto the trap and it is then left to condition at the same temperature overnight. This treatment is repeated if a deterioration of the column performance is observed after prolonged use. This passivation minimizes peak tailing and prevents the irreversible sorption of the stibines to the walls of the apparatus which may otherwise cause a negative interference. We have investigated the use of both the quartz cuvette burner described by Andreae (9) for the determination of arsenic and the graphite furnace atomizer modified for the determination of gaseous compounds (IO)which we had developed for the determination crf germanium. The former consists of a quartz glass tube, 9 mm i.d. and 7 cm long, which is mounted in the beam path of the atomic absorption spectrophotometer. The carrier gas is mixed with air, and the mixture then enters the burner cuvette from the front. Hydrogen is introduced from the opposite side. A hydrogen-rich flame burns inside the burner cuvette and combusts the stibines, generating a transient antimony atom population which is recorded as a peak. To determine the stibines using the Perkin-Elmer HGA 400 graphite furnace as a detector, we have disconnected the internal purge inlets at the detector and connected a glass connection tube to one of them. The carrier gas from the trap is premixed with argon and fed into this connection tube. The gas stream containing the stibines is thus forced through the graphite furnace and escapes through the other purge gas port. The burn cycle of the graphite furnace is initiated about 5 s before the elution time of the unsubstitued stibine and the furnace held at temperature until the last peak has eluted (ca. 50 s). Either detector is mounted in a Perkin-Elmer 5000 atomic absorption spectrophotometer. We use the 217.6-nm antimony line from an electrodeless discharge lamp operating at 8.5 W. The slit setting is 0.2 nm, and the deuterium background corrector is used. The absorbance signal is recorded on a strip-chart recorder. Standards and Reagents. A solution of antimony potassium tartrate containing lo00 ng Sb L-* is used to prepare the standards for the determination of antimony(II1). The antimony(V) standard is prepared by dissolving potassium antimonate(V) in dilute hydrochloric acid and dilutingwith water to make a solution containing ca. 1000 ng Sb L-l. This solution is calibrated (after further dilution) against the Sb(II1) standard to correct for the somewhat variable stoichiometry of the antimonate(V). The standards for methylstibonic acid and dimethylstibinic acid have been obtained from H. A. Meinema (Institute for Organic Chemistry, TNO, Utrecht, The Netherlands). Methylstibonic acid (MSA) was dissolved in distilled water and diluted to make a stock solution containing 300 mg of MSA L-I* The sample of dimethylstibinic acid (DMSA) used did not dissolve completely in water. The addition of 1mL of HC1 per 100 mL of solution was necessary to bring the fine, floccularresidue into solution. As discussed below, this material is probably antimony
1767
trioxide formed from partial decomposition of DMSA during storage. These standard solutions were diluted to form intermediate standards with concentrations of 0.1 mg L-I, from which microliter amounts were pipetted into 100 mL of deionized water or natural water samples to form the final standard solution. The intermediate standards were made on the same day that they were to be used. We have not observed any changes in the composition of the standards at high concentrations over several months. The methylantimony acids appear to be quite stable against hydrolysis of the carbon-antimony bonds in neutral and slightly acid solution. The stability of the dilute antimony(II1) solutions is highly variable. We have found oxidation to antimony(V) to be almost complete after only 31h in some instances. On the other hand, some solutions showed little oxidation in over several montlis. We suspect that trace amounts of oxidant compounds may be present in variable amounts in our water (reverse osmosis, four-step ion exchange) and cause the oxidation of antimony(II1). As a precaution, we prepare the standards immediately before analysis and add ca. 1g of ascorbic acid L-* of solution. Sodium borohydride (Fisher Scientific, Pittsburgh, PA) is dissolved to make a 4% solution. After addition of 1 mL of 2 N NaOH per 100 mL of reagent, this solution is stable for several days. All other chemicals are reagent grade. The Tris-HC1buffer soltuion for the determination of Sb(II1)is prepared by dissolving Tris [tris(hydroxymethyl)aminomethane,Sigma Chemical Co., St. Louis, MO] in deionized water to make a concentration of 1.9 M and adding concentrated HC1 to this solution until a pH of about 6 is indicated by a glass electrode. No reagent blanks are found for Sb(II1) and the methylantimony acids. For Sb(V),a reagent blank of 0.6 ng is present, due largely to the sodium borohydride. This blank is, however, quite constant and does not interfere with the analysis. Methods. Antimony(III), total inorganic antimony (Sbi),and the methylantimoniah are determined in separate aliquots. The selective reduction of Sb(1II)takes place at near-neutral pH. The methylstibonic acid requires a lower pH for its complete reduction (54). At a pH near 1and with the addition of iodide, both Sb(II1) and Sb(V) are reduced, and Sb(V) is calculated by difference. The sample size to be used for analysis depends on the comcentration of the antimony species sought, as the method determines the absolute ,mount of antimony present in the sample volume. The sample ishould contain between 0.05 and 20 ng of Sb in the species to be determined. Different reaction vessels are used to accommodate samples of 25, 50, 100, or 250 mL. Smaller samples are pipetted into the 25-mL reaction vessel and diluted to 25 mL. The followingreagents are added per 100 mL of sample: 1mL of Tris-HC1 (1.9 M) for Sb(III), 0.5 mL of HCl (6 N)for the methylantimonials, or 2 mL of HCl(9 N)and 3 mL of KI (1M) for the determination of Sbi. The reaction vessel is then attached to the apparatus and purged with helium for 3 min to remove enclosed air. The trap/column is then immersed in liquid nitrogen and the borohydride solution injected into thle reactor (3 mL for Sb,, 1 mL for the other species). A plastnc hypodermic syringe with a stainless steel needle is used for the injection. In the case of Sbi, the borohydride is injected slowly (45-60 s) into the solution to avoid excessively rapid evolution of hydrogen from the strongly acidic solution. After 6 min of reaction and stripping time, the liquid nitrogen is removed and the variac is switched an to heat the trap/column (ca. 13 V, 4 A). If the graphite furnace detector is used, its program cycle is initiated. The chart recorder is turned on and the peaks are recorded. After the peaks have eluted, the trap is heated for another 2 min to remove condensed water from the trap. The helium carrier gas flow rate is 100 mL/min; the air and hydrogen flow rates to the quartz cuvette burner are 240 and 300 mL/min, respectively. The analytical sensitivity is not very strongly dependent on these gas flow rates; variations of &30% in the gas flow rates change the sensitivityby less than 10%. For the graphite furnace detector a helium flow rate of 100 mL/min is used; to this an argon flow of 70 mL/min is added.
RESULTS AND DISCUSSION Hydride Generation. The criteria for the optimization of the hydride generation process are species selectivity,
1788
ANALYTICAL CHEMISTRY, VOL. 53,NO. 12, OCTOBER 1981 0.15
r
r
t
0.05-
P-
1c
0.10
I
H
L
LiL_ 1 -2 6 7 8 a
OooO
.
2
0
PH
Figure 2. Relationship between the pH of the reaction solution and the reduction yield (expressed as peak absorbance) for SbQIT) and Sb(V).
maximal yield, minimal blank, speed, and convenience of manipulation. As the methylated species can be separated from inorganic antimony on the basis of their chromatographic behavior, the hydride generation step has to differentiate only between Sb(II1) and Sb(V). The efficiency of the hydride generation process depends strongly on the reaction p H the reduction yield normally decreases sharply above the pH corresponding to the pK,, of the species concerned. Thus As(V) (arsenic acid, pKa, = 2.3) can be separated from As(II1) (arsenous acid, pKa = 9.2) by reducing the former at pH 1.5, the latter at pH 7 (10). We applied this approach to the speciation of antimony, which shows a similar difference in pKa, for the +3 and +5 species: 2.7 and 11.0, respectively. We found Sb(1II) to be reduced at constant yield in the pH range from 1 to 7 (Figure 2). Due to the alkalinity of the NaBH4, the pH of the reaction solution is higher after the addition of the borohydride than its initial value. This is represented in Figure 2 as horizontal bars, the left ends of which represent the pH of the sample solution before the addition of the borohydride reagent and the right ends the pH at the end of the reaction. Hydrochloric acid, acetic acid, potassium biphthalate, and Tris-HC1 were used to control the solution pH. A citric acid/citrate buffer at pH 4 was tested but gave unacceptably high Sb(II1) blanks and poor yield for the methylantimonials. The yield for Sb(V) increased as expected at low pH but remained less than half of the yield observed for Sb(II1) even at pH 0.8. About 2% of the Sb(V) was reduced in a solution buffered at pH 4 with potassium biphthalate; no Sb(V) reduction took place at pH 6-7 (Tris-HC1). This permits the selective reduction of Sb(II1) at near-neutral pH. Figure 3 shows the effect of the amounts of 6 M HC1 and 1 M KI added to 35 mL of the reaction medium containing 10 ng of Sb(V). In the absence of KI, the yield increases with the amount of HC1 added, but levels off at about 0.05 abs. Holding the amount of HC1 added constant at 1 mL, the addition of KI brings about a sharp increase in Sb(V) response, leveling off at 5 mL of KI added, corresponding to an iodide concentration of 0.12 M in the final solution. The response obtained under these conditions for Sb(V) (0.130 abs) is identical with that obtained for Sb(II1) (0.132abs) within the experimentalerror. The best yields for the reduction of MSA and DMSA were obtained in mildly acidic colution (0.5 mL of 6 M HC1 per 100 mL of solution). Under these conditions, the peak absorbances for the methylantimony compounds (Figure 4) are comparable to those of inorganic antimony (response per nanogram of Sb Sb(II1) 0.021 abs, MSA 0.022 abs, DMSA 0.024 abs). When the methylantimony acids are reduced under the conditions given for Sb(II1) at near-neutral pH, the yield for DMSA does
0.5 mL HCI
1.0 0
6
3 rnL K I
Figure 3. Reduction efficiency for Sb(V)as a function of the amount of 6 M HCi added to 35 mL of reaction solution. The left side of the figure represents the effect of the addition of HCI with no addition of KI; on the right side, the HCi addition is kept constant at 1 mL, and increasing amounts of KI are added. n?? " V0 0
E 0
0
I mtn
2
0
I
min
2
Figure 4. Chromatograms showing the peaks of stibine and methyiand dimethyistibineresulting from (a)a standard solution containing ca. 2 ng each of antimony in the form of Sb(III),methyistibonic acid, and dimethylstibinic acid; (b) 100 mL of surface seawater from a station in the eastern Gulf of Mexico (12 March 1981, latitude 27°15.16fN, longitude 96O29.88'W). not change significantly, while the MSA peak decreases by about 30%. Under many circumstances, the labor savings from combining the determination of Sb(II1) and the methylantimonials will outweigh this minor loss in sensitivity. The acid dissociation constants of MSA and DMSA are not known. The arsenic analogues, methylarsonic acid and dimethylarsinic acid, have a pKa, of 3.6 and 6.2, respectively. Due to the larger ionic radius of antimony, slightly higher values of pK,, are to be expected for the antimony analogues. This is in good agreement with our observations about the pH dependency of their reduction yields. Our observations on the reduction of the antimony species disagree with the findings of Foreback (11),who claims complete reduction of Sb(II1) in unbuffered solutions and of Sb(V) at pH 1.5-2.0 without addition of KI. We have observed diminished yields of about 20-30% for both species under these conditions. Later, Braman and Tompkins (12)analyzed seawater using the same method. They report a concentration of 27 ng L-l, which is very close to what we find if we do not add KI. In the presence of iodide, however, we find a concentration of ca. 120-140 ng Lml(depending on the origin of the sample). (Also in contrast to our results, Braman and Tompkins did not observe the presence of organoantimony
ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981
0'25r
Table I. Recovery of Standards of Methylstibonic Acid and Dimethylstibinic Acid Added to Seawater amt of Sb, ng L-I MSA DMSA seawater 5.3 3.2 standards added 5.3 4.2 predicted 10.6 7.4 found
9.6
1769
L
8.4
compounds.) Fleming and Ide (5) also observe that the presence of iodide is a requirement for the reduction of Sb(V) at high yield. Borohydride Concentration a n d Amount. The concentration of the borohydride reagent was chosen at 4% to provide convenient volumes (1-3 mL) for injection into the reaction solution. We have not observed changes in yield as a result of different borohydride reagent concentrations. The amounts of borohydride solution injected have been chosen to lie about 50% above the amount at which a plateau in the yields as a function of the borohydride addition is reached. As the borohydride ion hydrolyzes rapidly in acid solution, it is necessary to inject the reagent slowly (45-60 s) into the reaction solution for the Sb(V) determination. Injection rate is not critical a t the pH of the Sb(II1) reduction. Trapping, Volatilization, and Separation of t h e Stibines. The hydrides stibine and methyl- and dimethylstibine generated in the borohydride reduction are swept by the helium carrier into the cold trap/column. This trap consists of a U-tube filled to about two-thirds with a chromatographic packing. The first third of the tube is left open to allow for the condensation of water vapor from the gas stream. This eliminates the need for the water trap used in earlier systems (10)which consisted of a U-tube immersed in dry ice, inserted into the gas stream ahead of the cold trap/column. The trap/column is immersed in liquid nitrogen before the borohydride in injected into the reaction solution. The stibines are frozen out in the trap and collect at the head of the chromatographic packing. After the reaction and stripping time, the liquid nitrogen is removed and power is supplied to the heating coil around the trap. It then functions as a very simple "temperature-programmed" gas chromatograph. The separation and peak shape available with this simple system are surprisingly good (Figure 4) when a packing of 15% OV-3 on Chromosorb W 60/80 mesh is used. Other packing materials, e.&, glass wool, OV-17, etc., typically give more tailing and poor separation of the methylstibinepeaks. The retention time can be controlled by varying the voltage supplied to the heating coil. With increasing rate of heating, the retention time decreases and the peaks become narrower and steeper, thus increasing the signal-to-noise ratio and decreasing the detection limit. If the heating rate is too high, however, the separation of the peaks decreases, thereby limiting potential gains in sensitivity. If the determination of the methylantimonials is not required, higher signal-to-noise ratios can be achieved for inorganic antimony by using a trap filled with glass wool instead of the chromatographic packing and by going to higher heating rates. Stibine is still resolved from the methylstibines under these conditions, but the separation of methyl- and dimethylstibine is inadequate. The identity of the methylantimony compounds in natural water samples has been established on the basis of the element-specific detector (which proves that they are antimony compouncls) and by cochromatography of the natural compounds with the authentic samples of MSA and DMSA. For this purpose, amounta of the synthetic compounds comparable to those found in seawater samples were added to a sample of seawater (Table I). The natural and the authentic com-
0.10
1
.A
b - iL
U
2000
.
L
-
-
2500
OC
Influence of graphite furnace temperature on the peak absorbance resulting from 10 ng of Sb(II1)by borohydride reduction. Flgure 5.
pounds cochromatographed perfectly, and the recovery was quantitative within the precision of the method. The peaks attributed to MSA and DMSA cochromatographed with the authentic compounds on both columns packed with OV-3/ Chromosorb W and with OV-l7/Chromosorb W, which supports further the identity of the natural and the synthetic compounds. Detection of t h e Stibines. Two different detectors were investigated for their performance with respect to the determination of the eitibines: the quartz cuvette burner (1'0) and the graphite furnace detector modified for the intiroduction of gaseous samples (9). The sensitivity of the graphite furnace detector is about half of that of the quartz cuvette system. In many cases, especially for the determination of total inorganic antimony,this disadvantage may not be critical, and either system could be used. T h e Graphite F'urnace Detector. The most critical variables for the sensitivity of the graphite furnace detector are the flow rates of helium and argon into the furnace and the atomization temperature. The influence of the latter is shown in Figure 5. In contrast to the behavior of germanium, which has a broad plateau of the response as a function of atomizationtemperature (9),antimony has a relatively narrow atomization maximum at 2200 "C. It is interesting to note that for both elements the optimum atomization temperature is considerably lower for the gas flow-through system than for norpal operation using injection of liquid samples. This is a significant advantage for the operation of the furnace as a chromatographic detector, as the long burn times would destroy the graphite tubes relatively rapidly if high furnace temperatures were required. We run the furnace at 2200 "C and bring it up to this temperature as soon as the cold trap is removed from the liquid nitrogen. This allows potential accumulations of interferents, which may have happened during the stripping and reaction cycles, to be burnt off before the stibines elute. The furnace is maintained at this ternperature for 75 s. Under these conditions, the graphite tubles show no significant deterioration after a day's use. Helium is used for stripping the stibines from solution and as a chromatographic carrier gas because (in contrast to nitrogen and argon) it does not condense in the cold trap cooled with liquid nitrogen. For rapid and efficient stripping of the stibines, the helium flow rate should be kept between ca. 50 and 100 mL/min. Higher flow rates do not increase the performance and lead to .excessive back-pressures in the system. If pure helium is supplied to the graphite furnace, however, the graphite tubes deteriorate very rapidly and sensitivities are low. Therefore, argon is added to the carrier gas stream before it enters the furnace. The argon flow is regulated by the flow controller ("Miniflow") in the furnace
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981
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Table 11. Replicate Analyses of Solutions Containing 10 ng and 2 ng Sb(II1) 10 ng of Sb(II1) 2 ng of Sb(II1) peak abs integral peak abs integral 0.302 0.303 0.318 0.330 0.329 0.306
i-_,
200
L - _ _ i _ - _ I L - . . - U
400
300
500
rnL/min
Figure 6. Peak absorbance (10 ng of Sb) with the quartz cuvette burner detector as a functlon of the flow rate of air and hydrogen. For
the experiments with varying flow rate of air, the hydrogen flow rate was kept at 300 mL/min. When varying hydrogen flow, the air flow rate remalned at 240 mL/min.
programmer unit. For He flow rates between 50 and 100 mL/min, the peak response increases from 0 to 50 mL/min of argon added. Above 50 mL/min Ar, no significant difference was found with flow rates up to 80 mL/min Ar. For routine operations, an argon flow of 70 mL/min was selected. The peak response also increases with the He flow rate from 0.15 to 0.26 abs for a 10-ng sample when going from a flow rate of 50 mL/min to one of 100 mL/min. The latter flow rate was used for routine analysis. The Quartz Cuvette Detector. The quartz cuvette detector (IO)is about twice as sensitive as the graphite furnace system. The detector is operated continuously and need not be switched on before each analysis. It requires three supply gases: helium ~ 1 8carrier gas and hydrogen and air to produce the flame in which atomization takes place. Three flames actually burn in this detector: one where the hydrogen supplied from the back of the burner meets the air/helium mixture from the front inlet and two at the ends of the cuvette tube, where the excess hydrogen is combusted. Atomization takes place largely in the central flame; whether the outer flames are lit or not does not strongly influence the peak absorbance. The sensitivity shows a slight decrease with increasing hydrogen flow rate (Figure 6). If the hydrogen flow is adjusted below about 200-250 mL/min, however, the flame tends to "pop" during the stripping cycle and one of the external flames is extinguished. We select a hydrogen flow just high enough to avoid this problem. The exact flow rate at which this occurs depends on the particular burner geometry. The sensitivity is quite independent of the air flow rate above a threshold level of about 190 mL/min and declines very rapidly below this level (Figure 6). For routine analysis, we are using an air flow rate of 240 mL/min. Analytical Figures of Merit. Sensitivity and Detection Limits. The sensitivity depends on a number of parameters, which were discussed above. Depending on which species is being determined and which trap and detector are used, the sensitivity is between 110 and 220 pg/0.0044 abs. When antimony standard solutions were injected into the graphite furnace under the conditions suggested by the manufacturer (drying 10 s at 110 O C , ashing 10 s at 700 "C, atomization 5 8 at 2700 "C, internal purge flow stopped during atomization), a sensitivity of 57 pg/0.0044 abs was achieved. This difference can be attributed to the differences in the peak width between the peaks introduced by atomization off the graphite surface and those introduced as chromatographic peaks, differences in purge gas composition and flow, and the different geometry of the atom reservoir in the case of the quartz cuvette burner. As the total volume of sample represented by the peak is more than 3 orders of magnitude higher in the case of the
mean std dev
%RSD n
0.300 0.313 0.013 4.1 7
501 499 496 496
0.0553
100
0.0603
87 104
0.0583
505 507
501
0.0580
4.6 0.9 6
0.0025 4.
97 8.9 9.
3
3
borohydride procedure (up to 250 mL vs. 50 pL), the concentration detection limits of this method are by a similar factor above those of normal graphite furnace AA. The standard deviation of the base line noise is 0.0006 abs (using the electrodeless discharge lamp), resulting in a noise-limited absolute detection limit of 30-60 pg of Sb at the 95% confidence level. Sb(V) has a measurable reagent blank; the standard deviation of this blank is about twice that of the base line, resulting in detection limits about a factor of 2 poorer for this species than for Sb(II1) and the methylantimonials. With sample volumes of 100 mL, these absolute detection limits translate into solution detection limita of about 0.3-1.2 ng L-I, depending on the operating conditions and the antimony species determined. Precision, Accuracy, and Recouery. The precision of the determination of antimony was evaluted by replicate analysis of solutions containing 10 ng of Sb(II1). An estimate of the precision at lower concentrations was obtained from three replicate analyses of solutions containing 2 ng of Sb(II1). The results are summarized in Table 11. The peak integrals were determined by use of the electronic integration feature of the AA 5000. At low concentrationsof antimony, peak integration did not improve the precision of the determination, while at the 10 ng level a RSD of 0.9% was obtained with integration cqmpared to 4.1% for peak height. The integration results for small peaks suffer from the poor base line handling capabilities of the integrator and the fact that the integration interval is controlled by a time sequence rather than by electronic peak detection. With an electronic integrator of the type used in gas chromatography, the precision could be improved for small amounts of antimony also. There are no certified standards for antimony in aqueous solutions or natural waters currently available, making the direct evaiuation of accuracy impossible at this time. We have tested for the possibility of systematic errors by checking the standards for the different species against each other and by standard additions to samples of natural waters (seawater and river waters). As a reference standard we use antimony potassium tartrate, which is available at high purity and with stoichiometric composition. At the 1000 ppm level, this standard is highly stable in aqueous solution. At low concentrations, oxidation of Sb(II1) to Sb(V) may take place at unpredictable rates. Some solutions have been observed to oxidize within half an hour, while others were stable for several months. The rate of oxidation appears to depend on the quality of the water used to make the dilutions. We suspect that small amounts of oxidants, e.g., active chlorine, present in the water may be responsible for the oxidation. In fact, a somewhat unorthodox, but highly efficient, solution to this problem was to make the 10 and 0.1 ppm intermediate standards for Sb(II1) in water from a local river, which typically contains less than 5 ppt of Sb(II1). These solutions were
ANALYTICAL CHEMISTRY, VOL. 53, NO. 12, OCTOBER 1981 ~~
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Table 111. Antimony Species Concentrations (Expressed as ng Sb L-*) in Fresh, Estuarine, and Marine Waters Sb(II1) Sb(V) MSA DMSA Ochlockonee River 3.2 22.9 0.5 ND ND 1.9 30.3 ND Trinity River 0.9 145.0 0.8 ND Mississippi River
