1814 Anal. Chem. 1993, 65, 1814-1818

Ozone Gas-Phase Chemiluminescence for Silane and Its Application to the Determination of Silicate in Natural Waters

Kitao Fujiwara,' Masayuki Uchida,? Min-min Chen, Yu-ichiro Kumamoto, and Takahiro Kumamarut

Faculty of Integrated Arts and Sciences, Hiroshima University, Hiroshima 730, Japan

Chemiluminescence emission generated by mixing silane with ozone was investigated for the deter- mination of silicate in natural water. This chemi- luminescence spectra extends 400-850 nm provid- ing a maximum at 600 nm. Chemiluminescence spectra obtained by mixing diborane and ozone were also shown. For the purpose of generating silane from the aqueous solution of silicate, the following procedure was adopted: The sample solution was dried and mixed homogeneously with powdered lithium aluminum hydride in a Teflon tube or a molybdenum boat. When this mixture was heated at 200 OC, 97 f 3% of silicate in the sample was reduced to silane. After the generated silane was collected in the trap, the silane was mixed with ozone, and chemiluminescence was detected directly by a photomultiplier. The cal- ibration curve was linear from l to 500 pg Si, and the detection limit was 0.5 pg of Si for this method. Phosphorus and arsenic give the positive inter- ference, which is, however, negligible for the practical analysis of natural waters. The ana- lytical results for river waters and seawaters (the Tokyo Bay and the North Pacific Ocean) are in good agreement with those confirmed by molyb- denum yellow colorimetry.

Gas-phase chemiluminescence using ozone as the oxidant gas is one of the most popular methods in environmental analysis.'-3 This method has been first proposed for the measurement of NO, in the a tm~sphere ,~ where the reaction between ozone and nitrogen monoxide was used. Hills and Zimmerman proposed the application of ozone chemilumi- nescence technique to determine atmospheric isoprene gen- erated from leaves of the white oak as the byproduct of photosynthesis.5 Simplicity of the method, inexpensive equipment, and capacity of real-time measurement are the merits of using the ozone gas-phase chemiluminescence. Previously, the ozone gas-phase chemiluminescence technique was proposed to measure metalloid elements (As, Sb, Sn, and Se).6,7 In this method, these elements in the sample are converted to gas hydrides and mixed with ozone. Generation of gaseous hydride is a well-known technique in atomic

t Faculty of Science, Hiroshima University, Higashi-Hiroshima 724,

(1) Chisaka, F.; Yanagihara, S. Anal. Chem. 1982,54, 1015-1017. (2) Takeuchi, K.; Ibusuki, T. Anal. Chem. 1989, 61, 619-623. (3) Kanda, Y.; Taira, Y. Anal. Chem. 1990, 62, 2084-2087. (4) Fontijn, A.; Sabadell, A. J.; Ronco, R. J. Anal. Chem. 1970, 42,

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Japan.

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430.

0003-2700/93/0365-1814$04.00/0

spectrometry for the elements such as As, Ge, Pb, Sb, Se, Sn, and Te where tetrahydroborate is used as the reducing agent. Hydride generation for phosphorus is also possible, but in this case, drying a mixture of the sample and tetrahydroborate is required because the redox potential in the reduction of phosphate to phosphine is lower than that of water to h y d r ~ g e n . ~ , ~

Silane (SiH4) itself is poisonous to human health, Le., the American Conference of Governmental Industrial Hygienists has prescribed the safety guide for the concentration of silane in the atmosphere as 5 ppmv. With an increase in the use of silane, accidents have also happened: two students recently died in a silane explosion during an experiment to produce silicon wafers. These facts denote the necessity of an urgent development of a simple and sensitive spectrometric method to detect silane. One possibility is the spectrometric detection of silane by ozone gas-phase chemiluminescence. It has been pointed out that silane gives chemiluminescence on mixing with oz0ne1O-l3 and active nitrogen.14

Besides the importance of the silane detection in the atmosphere, the measurement of silicate ion through the method to generate silane is also important, especially for aqueous samples. The silicate ion is an important chemical species to limit the biomass in natural waters such as seawater and is enumerated as a nutrient along with phosphate and nitrate (+ nitrite). In spite of the importance of measurement, there are very few methods available to measure the silicate ion quantitatively in water samples. As shown in Table I, most of the methods for silicate measurement including liquid- phase chemiluminescence are based on the formation of molybdenum heteropolysilicic acid, which was facilitated by reacting silicate with ammonium molybdate under highly acidic conditions. However, this technique involves several drawbacks such as a large number of interfering species including phosphate and arsenate, a complex chemical procedure, and specific sensitivity, i.e., only sensitive to the orthosilicate ion. Although ICP atomic emission spectrometry is effective for measuring all the silicate species in the sample, atomic absorption spectrometry requires the chemical mod- ification of silicate. Otherwise, it cannot be employed because of low sensitivity to silicon.

In this paper, a method to determine silicate in natural water by ozone gas-phase chemiluminescence in which

(8) Hashimoto, S.; Fujiwara, K.; Fuwa, K. Anal. Chem. 1985,57,1305- 1309.

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107. 5891-5894. - - . , - - - - (11) Inoue, K.; Suzuki, M.; Kawabayashi, 0. Ger. Offen.

(Cl. GOlN21176) 1986; Chem. Abstr. 1986,104, 155074s. (12) Shirata, K.; Mukai, S.; Miki, S. Jpn. Kokai Tokkyo A

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0 1993 American Chemical Society

ANALYTICAL CHEMISTRY, VOL. 65, NO. 14, JULY 15, 1993 1815

Table I. Methods of Silicate Measurement method reaction

oxidation of thiamine by MHAa fluorometry colorimetry formation of MHA blue

formation of MHA yellow complexation of MHA and crystal violet complexation of reduced MHA and chromopyrazole I1 complexation of vanado-MHA and rhodamine B complexation of MHA and malachite green complexation of MHA and ethylrhodamine B oxidation of luminol by MHA

sorption of MHA on galssy carbon electrode

chemiluminescence polarography formation of MHA voltammetry AAS graphite furnace ICP-AES

detection range literature ref

30-600 pg/L 05-10 mg/L 3-100 pg/L 0.1-10 mg/L DL,b 120 pg/L

DL, 28 rg/L 0.3 pg/L+ DL, 52 pg/L 2.5 pg/L<c 0.2 pg/L<C 3-300 pg/L 50 pg-2 8 pg/L<C

0.4-10 pg/L

a MHA = molybdenum heteropolysilicic acid. DL = detection limit. The value shows the detection limit.

15 16 17 16 18 19 20 21 22 23 24 25 26 27

conversion from silicate ion to silane is examined. Spectro- scopic observation is also shown for the silane-ozone gas- phase chemiluminescence emission. As a spectral reference, the diboraneozone chemilumi-

nescence system was recorded, and the emission spectra are presented. Chemiluminescence spectra as to the boron were previously observed in the cases of colliding atomic B and organic oxides,2S*a reacting diborane derivatives with oxygen atom?O and reacting boron cluster molecular beam and NO2 (or N20),31 but not diborane and ozone.

EXPERIMENTAL SECTION Apparatus. The system adopted in the silicate measurement

is shown in Figure 1. The sample cuvette was inserted into a silane generator. The silane generator is a quartz tube surrounded by a nichrome heater. Helium was used as the carrier, and generated silane was concentrated in a silane trap, which is a U-tube made of Teflon. The silane trap was packed with quartz wool and was cooled by liquid nitrogen. The chemiluminescence was detected in a chemiluminescence chamber in which silane and ozone were mixed (ozone generator: Nippon Ozone Co., type OT-31-R-2). The chemiluminescence chamber is laboratory made, ita design is similar to the one previously menti~ned.~ The emission was detected by an electrically cooled photomultiplier (photon counter: Hamamatau C2130, photomultiplier: Hamamat-

(15) Limes, P.; Luque De Castro, M. D.; Valcarcel, M. Talanta 1986, 33.889-893. , - - - -~~ ~~

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(22) He, X.; Xu, S.; Wang, B. Fenxi Huaxue 1988,16,1086-1091; Chem.

(23) Pilipenko, A. T.; Terietakaya, A. V.; Bogoslovskaya, T. A. Zh.

(24) Chen,L.;H’k&,C. HaiyangXuebao 1989,11;5&62;Chem.Abstr.

(25) Er-Kang, W.; Meng-Xia, W. Anal. Chim. Acta 1982, 144, 147- 1989,111,2392322.

1.52. (26) Nater, E. A.; Burau, R. G. Anal. Chim. Acta 1989, 220, 83-92. (27) Boumans, P. W. J. M. Line Coincidence Table for Znductiuely

Coupled Plasma Atomic Emission Spectrometry; Pergamon Press: Oxford, 1980.

(28) Hosseini, S. M.; DeHaven, J.; Davidovits, P. Chem. Phys. Lett. 1982,86,496498.

(29) Bullitt, M. K.; Paladugu, R. R.; DeHaven, J.; Davidovits, P. J. Phys. Chem. 1984,88,4542-4547.

(30) Jeffers, P. M.; Bauer, S. H. J. Phys. Chem. 1984,88,5039-5042. (31) Devore, T. C.; Woodward, J. R.; Gole, J. L. J. Phys. Chem. 1988.

92,6919-6913.

COOLINO WATER EXHAUST

Flgure 1. Measurement system. 02, oxygen cylinder: He, hell n cyllnder; FM, flowmeter: HGV, hydrlde generatbn tube: TM, thermom eter; EH, electrlc heater: T, current controller: CL-RC, chemllumlnes- cence reactor; PM, photomuttlpller; CW-PS, power supply: R, recorder.

su R649, cooling unit: Hamamatau C659-A) without any spectral isolation: For quantitative measurement, the signal acquired to the photon counter was integrated for 10 s after the silane trap was warmed.

The spectral distribution of the chemiluminescence emission was observed by means of continuous mixing of the monosilane standard gas (380 and 5100 ppmv diluted by helium, purchased from Nippon Sanso Co. Ltd.) with ozone in the chemilumines- cence chamber (monochrometer: Jasco type CT-10, photo- multiplier: Hamamatau R456, power supply: Hamamatau C488R). In addition to silane, chemiluminescence emission spectra of diborane was also measured in the same manner. The standard gas of diborane (198 ppmv diluted by helium, purchased from Nippon Sanso Co. Ltd.) was used in this case. The spectral band width of the monochromator was 4 nm during the wavelength scanning measurement. Transparency of the window of the chemiluminescence chamber was carefully maintained during the measurement because the deposition of silicate caused fogging of the window after long-time use.

Reagents. Astandardsolution of silicate is produced by fusion of silica powder in sodium carbonate.

Several reagenta were tested as a potential reducing agent for reducing silicate to silane including NaH, KH, MgH2, and CaH2. Sodium tetrahydroborate (NaBH,), which is a typical reducing agent used for the generation of various hydrides and is capable of reducing phosphate to phosphine, is not effective in obtaining silane from silicate. Lithium aluminum hydride ( L u ) powder can be used to produce silane from silicate.

Procedure. The recommended procedure for conversion of silicate in the aqueous sample to silane and ita chemiluminescence detection is as follows: The sample solution is taken in a test tube made of Teflon (i.d. = 4 mm, height = 50 mm) and was dried overnight at 100 OC. About 50 mg of powdered lithium aluminum hydride was added to the dried sample and mixed promptlywith

1816 ANALYTICAL CHEMISTRY, VOL. 05, NO. 14, JULY 15, 1993

4 LIAlHq t ample

HerSIHq, - top vlow of molybdenum boa1

HORIZONTAL lY&

Flgure 2. Two types of hydrlde generation tube.

a spatula. The mixture of the lithium aluminum hydride and the silicate was inserted into the silane generation tube which was heated at 200 O C . In Figure 2, two types of silane generation tube are shown. A horizontal type generation tube is usable when the sample amount is small. In this case, a molybdenum or tungsten plate is convenient as a container for the sample, which is commercially available as a vessel for vacuum evaporation of metals. The time for drying the sample can be made short (less than 2 h) by using the metal container. The container made of quartz, which we used in the case of phosphine generation: is not appropriate due to the generation of silane from the container. Contact of lithium aluminum hydride with the generation tube wall (quartz) is the cause of the unexpected silane generation.

Generated silane was transferred to the trap by the carrier gas (flow rate = 65 mL/min). After 10-min collection, the trap was warmed to room temperature by soaking in a water tub, and the silane generated was transferred into the chemiluminescence chamber. Oxygen continuously flowed through the ozone gen- erator at a flow rate of 50 mL/min, under which 115 mg/L of ozone was produced in the output gas from the ozone generator.

The molybdenum yellow method was used to measure ortho- silicate in the natural waters to ascertain the analytical results obtained by the present chemiluminescence method: 50 mL of the sample was taken and mixed with 1.2 mL of 3 moVL of sulfuric acid and 2.0 mL of 10% w/v ammonium molybdate. Ten minutes after the mixing, the absorption of the solution at 380 nm was measured. All the samples analyzed were stored in polyethylene bottles.

Cautions. Highly concentrated silane is explosive, and more than a few mg of it should not be stored. Also, there is a possibility that ozone flows backward to the silane trap cooled at liquid nitrogen. Therefore, the use of a backward-flow preventor is preferable for the ozone flow line.

RESULTS AND DISCUSSION

Chemiluminescence Emission Spectra. Figure 3 shows the emission spectra obtained by mixing monosilane with ozone. The emission maximum appears at around 600 nm. Although some band structures are contained in the longer wavelength side of the emission peak, the attribution of these peaks is impossible, i.e., S i0 and Si02 do not provide emission bands in this regi0n.3~ Glinski et al.l0 inspected the chemi- luminescence of silane and ozone and concluded that the emission observed in the visible region was thought to be from HzSiO. Although some differences were found between Figure 3 and their observation in terms of the wavelength of the maximum emission and fine structure, the major species giving the broad emission band seems to also be HzSiO in

(32) Pearse, R. W. B.; Gaydon, A. G. The Identification of Molecular Spectra, 4th ed.; Chapman and Hall: London, 1976.

I . . . , 400 500 600 nrn

0 3 i

I . . , . . . , . . . . . 300 400 500 600 700 800 900 nm

rrcrn

Flgure 3. Chemllumlnescence spectra obtained by mlxing silane wlth ozone and diborane wlth ozone. CL = chernlluminescence. The spectra arrowed by ozone are the blank ozone signals.

Figure 3. In addition, the chemiluminescence emission spectra obtained on mixing diborane and ozone are also shown in Figure 3. In contrast with silane, diborane gives a distinct band structure in the ozone chemiluminescence spectra, which can be ascibed to BO and BOz. The difference between the chemiluminescences of silane and diborane is originated by the stability of the emission products.

Besides the difference in the band structure, silane gives emission in the longer wavelength region compared to diborane, Le., emission from silane continues up to 850 nm but those from diborane diminish at 600 nm. The charac- teristics observed in the chemiluminescence emission of silane appears to resemble those of phosphineg or arsine,33 and it is impossible to separate their emission spectroscopically from silane chemiluminescence by using a filter or monochromator.

Optimization of Conditions for Chemiluminescence Detection of Silane. Chemiluminescence emission intensity is dependent on the flow rate of ozone/oxygen and helium- (carrier). Both lower flow rates of helium and ozone/oxygen gave stronger emission intensity (higher peak height), i.e., decreases of helium flow rate from 800 to 65 mL/min or decreases of ozone/oxygen flow rate from 200 to 20 mL/min allow about a 50% increase in the signal intensity. Dilution of ozone and silane in the increased flow rates of oxygen and helium can be held responsible for this decrease (concentration of ozone decreases from 140 to 44 mg/L when oxygen flow rate increases from 20 to 200 mL/min). Merging silane with ozone at a high concentration is preferable for acquiring strong luminescence in our system. However, lower flow rates for both ozone/oxygen and helium reduced the reproducibility in the signal. Therefore, the gas flow rates were fixed at 50 and 65 mL/min for ozone/oxygen and helium, respectively.

A patent report with regard to the chemiluminescence detection of diborane and silane, however, was registered: 100 and 500 ppbv were reported as the detection limits for diborane and silane, respectively.ll The same detection limits can be obtained in our system. When collecting air in the trap cooled at liquid nitrogen temperature, 100 and 300 ng of diborane and silane are the detection limits, respectively. Figure 4 shows the dependence of the collection efficiency of silane in the trap on the flow rate of the sampling gas. The trap is a Teflon U-tube, in which various materials can be packed. The packing material chosen was 100 mg of quartz wool. Silica gel is also effective in the case of separating silane or diborane from polar hydrides such as phosphine, arsine, and stibine.%

(33) Fraser, M. E.; Stedman, D. H.; Henderson, M. J. Anal. Chem.

(34) Fujiwara, K.; Tsubota, H.; Tsumura, S.; Iwata, S.; Kumamaru, T. 1982,54, 12OC-1201.

Anal. Chim. Acta 1991, 246, 413-419.

ANALYTICAL CHEMISTRY, VOL. 65, NO. 14, JULY 15, 1993 1817

m-

e .\” .P

i” 0- 0 500 1000 1500

Carrier gas (He) flow rate (mL mn”) Flgure 4. Trapplng efficiency of silane at the trap packed with Teflon and quartz wools.

4 e 0

e

. - 0 I

O’ = 40 1;o 160 $0 200 i o 240 ’ Temperature(°C)

Figure 5. Dependence of sllane generatlon on temperature of the hydride generation tube.

Table 11. Chemiluminescence Signals Given by Various Smciea.

Si SiOIC 100 lob SiOa” 106 * 12 SiFS” 110 * 8

N NOS- P P o p As Asosa 5 * 3 Sb Sb8+ Sn Sna+ Pb Pb*+ B Boas

0 The vertical type silane generator was used. The amount of each element is 10 pg. The value given was the average and the standard deviation of five time measurements. *This value was referred to as 100.

r L E

iij m 101

0 z

Generation of Silane from Silicate. Figure 5 shows the dependence of signal intensity on the temperature of the silane generation tube, where the mixture containing 50 pg of Si (drying aqueous solution of sodium silicate) and powder of lithium aluminum hydride was inserted in the generation tube. Silane starts to be generated at 165 O C and becomes maximum at around 200 OC. However, the addition of heat to more than 200 O C to the generation tube causes a decrease of signal peak intensity. It was confirmed that monosilane decomposes when the temperature of the generation tube reaches over 200 OC. When injecting standard silane gas into a stream of the carrier gas just before the generation tube, 100% of the silane was preserved to pass through the generation tube at the temperature up to 200 “C, but about 70% of silane was decomposed by passing through the generation tube heated at 300 OC.

There is a possibility that the generated silane is not only the monosilane but also di-, tri-, and tetrasilanes. However, only a single peak appeared in the temporal emission of the chemiluminescence even under the condition of slow warming of the silane trap. (Ambient conditions were allowed to warm the trap, keeping the rate of warming low.) The appearance time of the chemiluminescence after warming the trap was almost the same as the case of trapping the monosilane standard gas. These facta suggest that more than 90 % of the gaseous silicon compounds produced in the present procedure is monosilane.

The silane generation was terminated within 6 min under conditions when 50 pg of silicate was mixed with lithium aluminum hydride and inserted into the generation tube heated at 200 OC. Therefore, it has been concluded that 10 min is sufficient time for trapping the generated silane.

Table I1 shows a comparison of silane generation efficiency from the different silicon compounds. Orthosilicate, meta- silicate, and hexafluorosilicic acid give the same conversion

5 10 COLORIMETRY (pssi mL-l)

Figure 6. Comparison of analytlcai results for natural waters. The bar In the flgure shows the relathre standard deviation of 10 measurements of the present chemliumlnescence method. The value obtained by the molybdenum yellow colorimetry was the averaged value of two measurements. 0, Northwest Pacific Ocean (22’ N, 175’ E); 0 , Tokyo Bay; 0, Nenotanl Rhrer in Etajlma Island, Japan.

rate to silane. According to the comparison of chemilumi- nescence for the standard monosilane gas, conversion rate from orthosilicate to silane is 97 f 3 % , which means almost all the silicone contained in the sample can be transformed to silane. This is one of the characteristics of the present method and is different from the case of phosphine and diborane generations from phosphate and borate, respectively. Both the conversion rates of phosphine and diborane in the previous reports9pa do not reach 50%.

The calibration curve was a linear l i e from 1 to 50 pg for silicon in silicate. The detection limit of the present method is calculated to be about 0.5 pg of silicon in silicate ion (three times the standard deviation of a 10-pg silicon sample, n = 10).

Interferences. In Table 11, the chemiluminescence in- tensity was obtained by processing several elements in the same manner as for silane generation from silicate. The data in Table I1 are obtained by use of the vertical type silane generator, where a Teflon mini test tube was used as the sample container. When the horizontal type silane generator was employed and a molybdenum boat (plate) was used as a sample container, phosphorus, arsenic, and antimony gave chemiluminescence as 20 f 4,185 f 27, and 27 f 1% of that of silicon in the signal peak height, respectively. The differences found in interference due to different generation tubes can be considered as the difference in the heat propagation in the sample container.

1818 ANALYTICAL CHEMISTRY, VOL. 65, NO. 14, JULY 15, 1993

Applications. The present method has been applied to the measurement of silicate in natural waters, where the silane horizontal type generation tube with the molybdenum boat was used. The Tokyo Bay and the Northwest Pacific Ocean where chosen as the representative sampling points in terms of the most polluted (coastal) and unpolluted (off-sea) seawaters, respectively. As can be seen in Figure 6, the analytical results obtained by the present method show good agreement with those obtained by the molybdenum yellow colorimetry: r (linear regression coefficient) = 0.997. Al- though some elements interfere with the present method as mentioned previously, these effects are negligible for seawater and river water. This result is approved by the fact that the natural waters such as seawater contain phosphorus (<0.1 pg/mL), arsenic (<5ng/mL), and antimony (<2 ng/mL) much lower than s i l i ~ a . ~ ~ , ~ ~

As to the hydride generation method under solid phase, we have previously proposed the method using sodium tetrahy- droborate as the reducing agent,8z9 which is effective to determine phosphate in natural water. Also, solid-phase reduction of borate to diborane has been proposed with the

use of powdered lithium aluminum hydride. However, this method is more effective for silicate than borate.

The precision is exclusively dependent on homegeneity of the mixture of the dried sample and lithium aluminum hydride powder. Lithium aluminum hydride suspended in organic solvent such as diglyme and tetrahydrofurane is commercially available, of which use may improve the homogeneity of the mixture (or reproducibility of the method). However, it cannot be used, Le., the vapor of organic solvent also gives the chemiluminescence, which cannot be separated from silane at the present conditions. Irrespective of this situation, the standard deviation shown in Figure 6 is small enough to apply the present method for the practical measurement of silicate in natural waters.

ACKNOWLEDGMENT

This study was supported by Granta-in-Aid 03202236 and 03248101 from the Ministry of Education, Science and Culture of Japan and a 1992 grant from Shimadzu Scientific Foun- dation.

(35) Broecker, W. S.; Peng, T.-H. Tracers in the Sea; Lamont-Doherty Geological Observatorv. Columbia University: Palisades. NY, 1982.

(367 Nakayama, E.:Suzzuki, Y.; Fujiwar; K.; Kitano, Y. Anal. Sci. 1989,5, 129-139.

RECEIVED for review December 28, 1992. Accepted April 2, 1993.