Anal. Chem. 1982, 54, 125-128
(7) Can0 PavBn, J. M.; Pino, F. Anal. Lett. 1974, 7 (2), 159-165. (8) Savostina, W. M.; Astakhova, E. K.; Peshkova, V. M. Vestn. Mosk. Univ. Ser. 2: Khlm. 1963, 2. ( 9 ) Meites, L. “Handbook of Analytical Chemistry”; McGraw-Hill: New York, 1963. (10) Charlot, G. “Les reactions chimiques en solution”, 6th ed.; Masson et cie Oditeurs: Paris, 1969.
LITERATURE CITED Christopherson, H.; Sandell, E. B. Anal. Chim. Acta 1954, IO, 1. Babko, A. K.; Mikhel’cron, P. B. Tr. Kom. Anal. Khim., Akad. Nsuk SSSR Otdel. Khlm. h’auk 1954, 5 (E), 61-67. Wenaer. P. E.: Monnier, D.: Bachmann-Chacruis. W. Anal. Chim. Acta 1956 15, 473-483. Banks, C. V.; Barnurn, D. V. J . Am. Chen?. Soc. 1958, 80, 3579. Erdey, L. “Gravimetrio Analysls Part 11”; Pergamon Press; Oxford, 1985.
Mu%OzLeyva, J. A,; Can0 PavBn, J. M.;Pinos, F. An. Quim. 1973, 69,
251-252.
125
RECEIVED for review June 1, 1981. Accepted September 18, 1981.
Gas-Phase Chemilurninescence with Ozone Oxidation for the Determination of Arsenic, Antimony, Tin, and Selenium Kitao Fujiwara, * Yutaka Watanabe, and Keiichiro Fuwa Department of Chemistry, Faculty of Science, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan
J. D. Winefordner Department of Chemistry, University of Florida, Gainesville, Floriah 326 11
Gas-phase chemilumlntescence for the ldetermlnatlon of As, Sb, Sn, and Se is investigated. The procedure is based on the reaction between ozone and the hydrides of the analytes. The nondispersive detection system for chernlluminescence emlsslon Is applied to improve the sensitivity and detectlon power. The llmlting detectable amounts; are as follows: As, 0.15 ng; Sb, 10 ng; Sni, 35 ng; Se, 110 ng. These amounts are llmlted by the contamlnatlon of As In the reagents used. The wlde llnear dynamic range ( 104-105),the hlgh detection power for As, high stability of background slgnal, and simple handllng and low-cost c:onstruction of the instrumental system are advantages of the present method.
Applications of Chemiluminescence spectrometry for trace metal analysis are very rare (1) except for a condensed phase indirect method (2-7), !wherethe chemiluminescent oxidation of gallic acid, luminol, or luciferin by alkaline HzOzare utilized to detect metal ions, e.g., Co, Cr, Os, Ru, Fe, etc. However, indirect chemiluminescence is accompanied by several difficulties, including strong chemical interference of various concomitants as well as spectral interferences and poor selectivity for anwlyte. Such drawbacks of the indirect chemiluminescence method in the condensed phase are ascribed to the complexity of tlhe reaction mechanism. In the present paper, the gas-phase chemiluminescence detection of some metalloids with ozone oxidation is investigated for the first time. The chemiluminescence caused by ozone oxidation of nitrogen monoxide lhas been previously reported for monitoring air pollutants (8,9). The technique presenkd here is based upon the same approach. In a previous paper (IO),chemiluminescence resulting from oxidation of arsine (AsH3) and stibine (SbH3) in a diffusion flame was used to measure concentrations of As and Sb in the 10-100 ng range. However, use of a flame as the analytical media of chemiluminescence imposes several restrictions including flame background emission and turbulence which limit the minimum detectable amounts of these elements. The cherniluminescence produced by ozone oxidation method overcomes these difficulties, and detection of nanogram to subnanogram amounts of arsenic, antimony, tin, and selenium becomes
Table I. Standard Sdution for 1000 ppm Analyte As
Bi Ge Hg
Pb Sb
Se Sn Te NO,NO,-
Na,HAsO,, CH3AsO,Na,.6H,O, (CH,),AsO,Na. 3H,O-dissolved in 0.1 HCl Bi shot-dissolved in 5% HNO, K,GeO,-dissolved in 0.2 N KOH HgSO,-dissolved in 1 N H,SO, Pb( NO,),-dissolved HNO, K(SbO)C,H,O,~~/,H,O-dissolvedin aqueous solution H,SeO,-dissolved in aqueous solution metallic tin-dissolved in HC1 (28%)/H,SO, (4%) solution H,TeO,-dissolved in aqueous solution NaN0,-dissolved in aqueous solution NaNO,-dissolved in aqueous solution
possible. The detection power is limited, however, by the purity of reagents used in this experiment.
EXPER,IMENTAL SECTION Chemicals. The standard reagents used for testing chemiluminescence are listed in Table I. Sodium borohydride (4% aqueous solution with 0.1 N NaOH) and 0.1 N HC1 are used for the hydride generation. A mixture of ascorbic acid and 10% potassium iodide solution is used as the ozone trap. When the ascorbic acid is exhausted by ozone and the color of solution turns brown (formation of iodine), more ascorbic acid is added. Apparatus. The system for chemiluminescence detection is shown in Figure 1. The ozonizer is a silent discharge type, made by Nippon Ozone Co., Ltd. (type 0-1-2). The emission detection system consists of a R-446 photomultiplier (Hamamatsu) with a high-voltage power supply made by Hamamatsu (type HTVC665);the output signal is amplified by a dc amplifier made by Keithley (type 427) and is recorded with a strip chart recorder made by Hitachi Ltd. (type 056). The reaction chamber (volume 22 cm3) for chemiluminescence is made of Pyrex glass enveloped with a brass cylinder which is directly connected to a photomultiplier shield as shown in Figure 2; the chamber and shield are also covered with aluminum foil for noise reduction. Procedure. A sample of about 20 mL (acidity is adjusted with 0.1 N HCl) is placed in the hydride generation vessel (HGV). After the HGV was purged with He for 30 s to eliminate air, the hydride trap (HT) is immersed in liquid nitrogen (HT is a U-tube glass which is half-packed with quartz wool). After the HT is completely cooled, 3 mL of sodium bromohydride solution is slowly injected
0003-2700/82/0354-0125$01 . m O 0 1981 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 1, JANUARY 1982
jg HGV
WT
B
U
,. './
OT OT
Figure 1. Gas flow system for chemiluminescence with ozone oxidation: He, helium cylinder; FM, gas flow meter; WT, water trap; HGV, hydride generation vessel; HT, hydride trap; Op,oxygen cylinder; OZ, ozonizer; RC, reaction chamber; PM, photomultiplier; PS, power supply; AMP, amplifier; R, strip chart recorder; OT, ozone trap.
I
0.5 1.o He FLOW RATE, L/min. Figure 3. Dependence of chemiluminescence signal on flow rate of carrier gas (helium): (0)arsenic; (X) antimony; (0)selenium. Oxygen flow into the ozonizer was 0.2 L/min. 0
-- He+ANALYTE HYDRIDE
-
OZONE EXHAUST
Figure 2. Reaction chamber for chemiluminescence detection. into the HGV. The HT is then transferred to the water tub 1 min after the injection of sodium borohydride. The analyte hydride vaporized from the HT is then introduced into the reaction chamber with helium (carrier) and is mixed with ozone as shown in Figure 2.
RESULTS AND DISCUSSION Chemiluminescence of Hydrides with Ozone Oxidation. The mixing of ozone and arsine, stibine, tin hydride, or hydrogen selenide gives a blue-white emission. In the case of arsenic, emission extended from 350 nm upward (similar to that in the flame chemiluminescence shown in ref 10). However, contrary to the flame chemiluminescence results (IO),the characteristic emission bands of As0 around 250 nm are not observed. Therefore, the present chemiluminescence (produced by the ozone oxidation) seems to be based on the following process: MH, 03 = H,MO,*
+
H,MO,* = H,MO,
+ hv
where M is arsenic, antimony, tin or selenium, and H,MO, might be H3M04or HMO3, etc. Because of the broad emission bands, we used a nondispersive system as mentioned in the Experimental Section to increase the detection power. Optimization of Detection System. Most of the conditions in the hydride generation process, Le., amount and injection speed of reducing reagent, and times mentioned in the previous section are optimized according to ref 11. The flow rate of oxygen and applied voltage to the ozonizer, carrier gas (helium), and sample volume are investigated here because these conditions are directly of concern in the chemiluminescence reaction. Oxygen Flow Rate. The dependence of chemiluminescence intensity (for arsenic, antimony, and selenium) on ox-
-
1 ' :o-6
10-5
10-4 10-3 10-2 A M O U N T S OF A N A L Y T E S ,
10-1
1
10
mg
Figure 4. Analytical calibration curves for various elements. ygen flow rate in the ozonizer showed no significant difference among the analytes (the luminescence intensity maximized between 0.2 and 0.6 L/min and gradually decreased at higher flow rates). (In the present system, 40 g/m3, the maximum ozone concentration, is attained at 0.2 L/min of oxygen flow rate.) Applied Voltage to Ozonizer. The variation of chemiluminescence intensity (similar for arsenic, antimony, and selenium) with applied voltage for the silent discharge ozonizer showed a gradual increase with voltage and a plateau for voltages above about 60 V (corresponds to an ozone concentration of 13 g/m3). No difference among the analytes is found. Flow Rate of Carrier Gas. The relationship between signal level (for arsenic, antimony, and selenium) and carrier gas (helium) flow rate is shown in Figure 3. (Argon and nitrogen are inconvenient carrier gases for long-term use because of clogging the hydride trap by liquefaction.) Since a helium flow rate over 1.0 L/min sometimes caused damage to the connection of the gas tube, 0.8 L/min is used for routine analyses. Sample Volume. When the volume of the hydride generation vessel is 100 mL, the sample volume has little effect upon the signal level. Detection Limits and Analytical Calibration Curves. Analytical calibration curves for arsenic, antimony, tin, selenium, and nitrite ion (NaN02) are shown in Figure 4. The linear dynamic ranges of these analytes are 103-106. Chemiluminescence signal recordings for various amounts of each
127
ANALYTICAL CHIEMISTRY, VOL. 54, NO. 1, JANUARY 1982
F
A
B
I
1
-. Flgure 5. Chemiluminescence signals of various amounts of several analytes: (A) blank; (B) arsenic (50 ng); (C) ,antimony (250 ng); (D) selenium (5 pg); (E) tin (‘I pg); (F) NaNO, (600 pg).
Table 11. Detection Limit of Analyte 0.15 ng Sb 10 mg Sn 35 ng Se 110 11g Taking 50 mL of sample.
As
a
I
2 1 RETENTION TIME(min)
0
Flgure 6, Differential volatilization of hydrides from liquid nitrogen trap. Trap was removed from liquid nitrogen Dewar at time, t = 0 (trap was packed with quartz wool): (A) arsenic (10 ng); (B) tin (1 pg); (C) seleniuim (5 pg); (D) antimony (500 ng); (E) NaN02 (0.5 mg). A
(0.003 p p b a ) (0.2 PPba)
( 0 . 7 PPbU)
(2.2 PPbU)
analyte are shown in Figure 5. The detection limits of arsenic, antimony, tin, and selenium are noted in Table 11, the detection limits are based upon a signal-to-blank standard deviation of 3 (for five blank measurements). The major signal noise source in the present studies is the dark current shot noise of the photomultiplier (in the previous paper (IO),the major noise is the flame background emission noise); the signal level is high because of the high gain of photodetection without spectral dispersion. However, minimal detectable amounts of analytes are limited in this work by fluctuation of the emiesion signal in thle blank due to the arsenic contamination in the hydrochloiric acid used in this experiment. The slopes of‘ the log-log calibration curves for arsenic, antimony, tin, selenium. and NOz- shown in Figure 4 are unity except for the high and low extremities which are less than unity because of a decrease in generation efficiency of the hydrides. Bismuth and mercury also give chemiluminescence emission by the present method, but the analytical calibration curves of these elementi3 deviated considerably from linearity and so are not included in Table I1 and Figure 4. The cause of these deviations seemed to be in the generation process of the hydrides according: to the procedure noted in the Experimental Section. The quantitative detection of these elements by chemiluminescence might be possible if the hydridle generation process could be improved. Other elements such as lead, germanium, the tellurium whiclh also produce gasphase hydrides by the sodium borohydride reduction did not give chemiluminescence under the present conditions; also nitrate ion (NaNQ3) did not give chemiluminescence emission. Arsenic in organic forme, [CH3As03Nab(CH3)2As02Na]could not be detected indicating that CH3AsH2and (CH3)2AsHdo not react with ozone. Separation of Analyte Emission. Since the present system is based on nondispersive detection of emission spectra, the observed signal is not specific for each analyte. Therefore, except for arsenic each malyte suffered considerable spectral interference when arsenic, antimony, tin, selenium, and nitrite ions were simultaneously present. However, spectral determination of arsenic witih the present method provided higlh
15
10
5
I
0
RETENTION TIME(min1 Figure 7. Chromatographic separation of hydrides. A stainless tube
(3 mm 4 X 50 crni) packed with Porapak Q 801100 was set between the hydride trap and the reaction chamber in the gas line: (A) arsenic (3 pg); (B) tin (200 pg); (C) antimony (50 pg).
selectivity (12) against interference in various samples, such as, soil or biological materials, because arsenic is over 200 times more sensitive than the other hydride elements. The calculated selectivities (12) of the present method are as follows: As, 57; Sb, 1.2 X Se, 1.2 X Sn, 4.3 X However, in order to measure tin, selenium, and antimony with the present system, time resolution might possibly be useful. The hydrides can be partially separated by selective volatilization, i.e., after the hydrides are trapped at liquid nitrogen temperature, the trap is allowed to warm up to room temperature. In this manner, the antimony and selenium can be separated from arsenic as well as the interference of nitrite ion (see Figure 6). However, the peak heights of all the analytes are redluced about 20% by this method compared with the trap soaking in water and selenium and antimony are not separated from each other. If the trap is soaked in a dry ice-acetone mixture instead of leaving it at room temperature, all peaks overlapped. Quartz wool is a superior packing material to zeolite with respect to separation efficiency. In Figure 7, a chromatograpahic separation resulting with a 50 cm stainless column (3 mm 4) packed with Porapack Q 80/100 (purchased from Waters Co.) is shown. However, here
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Anal. Chem. 1982, 54, 128-132
arsenic, tin, and antimony are completely separated a t room temperature. Application. The present method is applied to the determination of arsenic concentration in orchard leaves (SRM 1571, NBS); 1g of orchard leaves is digested with a mixture of sulfuric acid and nitric acid using a 20-mL Kjeldahl flask. The results (9.2 f 1.2 and 10.8 f 1.2 pg) obtained agreed well with the recommended value (10 f 1pg/g), The contents of tin, antimony, and selenium are too low (13-15) for the present method, and should not interfere with the results for arsenic.
LITERATURE CITED Stedman, D. ti.; Tammaro, D. A. Anal. Lett. 1976, 9,81-89. Isacsson. U.: Wettermark. G. Anal. Chim. Acta 1974. 68. 339-362. Nau, V.; Nleman, T. A. Anal. Chem. 1979, 51, 424-426.' Montano, L. A.; Ingle, J. D.,Jr. Anal. Chern. 1970, 51, 919-926. Stieg, S.; Nleman, T. A. Anal. Chem. 1977, 49, 1322-1325.
16) MacDonald. A,: Chan. K. W.: Nleman, T. A. Anal. Chem. 1979.. 57.. 2077-2082. (7) Marlno, D. F.; Wolff, F.; Ingle, J. D., Jr. Anal. Chem. 1979, 57, -2051-2053 . -. ... (8) Fontljln, A.; Sabadell, A. J.; Ronco, R. J. Anal. Chem. 1970, 42, 575-579. (9) Stevens, R. K.; Hodgeson, J. A. Anal. Chem. 1973, 45, 443 A-449
A.
(IO) Fujlwara, K.; Bower, J. N.; Bradshaw, J. D.; Wlnefordner, J. D. Anal.
Chim. Acta 1979, 709,229-239. (11) Watanabe, Y. thesis for masters degree, University of Tokyo, 1981. (12) Fujlwara, K.; McHard, J. A.; Foulk, S. J.; Bayer, S.; Wlnefordner, J. D. Can. J. Spectrosc. 1980, 2 5 , 18-24. (13) Fuwa, K.; Notsu, K.; Tsunoda, K.; Kato, H.; Yamamoto, Y.; Okamoto, K.; Doklya, Y.; Toda, S. Bull. Chem. SOC. Jpn. 1978, 51, 1078-1082. (14) Byrne, A. R. J. Radioanal. Chem., 1974, 20, 627-837. (15) Byrne, A. R. J. Radioanal. Chem. 1977, 37, 591-597.
RECEIVED for review June 5,1981. Accepted October 16,1981.
Adsorption of I-Pentanol on Alkyl-Modified Silica and Its Effect on the Retention Mechanism in Reversed-Phase Liquid Chromatography Karl-Gustav Wahlund" and Ingegerd Beljersten Department of Analytical Pharmaceutical Chemistry, Biomedical Center, University of Uppsala, Box 574, $75 1 23 Uppsala, Sweden
The adsorption Isotherm of 1-pentanol between the chromatographic solid phase of LlChrosorb RP-8 and a mobile aqueous buffer has been determined. At low concentrations of 1-pentanol a monolayer is formed on the solld phase. Hlgher concentratlons result in a bulk layer of 1-pentanol. The change In separation selectivlty that occurs when the adsorbed amount of 1-pentanol varies indicates that at the higher coating degrees only a fractlon of the adsorbed 1pentanol is present as a bulk phase. The total retentlon of carboxyllc acids can then be explained as a comblnatlon of adsorption to the solid phase and partltlon to the bulk phase of 1-pentanol. Only at the maximal coatlng the partltlon mechanism domlnates the retention. At low coating degrees the retentlon Is governed by adsorptlon to the solld phase accordlng to the Langmulr adsorptlon Isotherm.
In a previous paper (I) we studied the coating of alkylmodified silicas with 1-pentanol in order to produce reversed-phase columns for liquid-liquid chromatography. The coating was done by equilibrating the column packing with buffer solutions containing various concentrations of l-pentanol up to saturated solutions which contain 2.30 vol % 1-pentanol. It was noted that the amount of 1-pentanol adsorbed to the column during such an equilibration increased with increasing concentration of 1-pentanol in the buffer (the mobile phase). The increase of the adsorbed amount of 1pentanol affected the retention of carboxylic acids differently depending on the hydrophobicity of the acid. This was explained by an interaction with the support. It has often been recommended that liquid-liquid chromatography should be performed with a large loading of
stationary liquid phase and with a support of moderately high surface area in order to obtain pure liquid-liquid partition (2). In classical liquid chromatography often diatomaceous earth, which has a rather low specific surface area, has been used as the support. When the modern silica supports, with a rather high specific surface area, are used in partition chromatography, the interaction of the solute with the support might be more pronounced. This need not be a disadvantage but can lead to a change of the expected separation selectivity especially if one wants to regulate retention by a change of the stationary phase loading. When the support interaction is pronounced the change of the liquid stationary phase loading can cause a change of the retention in the wrong direction, i.e., a decreased loading gives an increased retention (1, 3). For silica supports, coated with 3,3'-oxydipropionitrile ( 4 ) , water ( 5 ) , or formamid (5) and eluted with hexane or dichloromethane, pronounced influence from the support on the retention was observed for polar uncharged compounds and even at high loadings. The same observation was made with organic ammonium ions on silica columns coated with buffer and eluted with chloroform + l-pentanol(3), but the support interaction was small a t the maximal loading of the buffer. When alkyl-modified silica was coated with 1-pentanol (I) or tributyl phosphate (6)and eluted with aqueous mobile phases, the support effects were negligible for hydrophilic compounds at high loadings. In the present paper we have studied the relation between the adsorption of 1-pentanol and the retention of hydrophilic carboxylic acids covering a broad range of coating degrees. The intention has been to study the role of the adsorbed 1-pentanol for the retention. The adsorption isotherm of 1-pentanol has been determined and it is shown that by changing the concentration of 1-pentanol in the mobile phase
0003-2700/82/0354-0128$01.25/00 1981 American Chemical Society
