JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, JANUARY 1988, VOL. 3 53

Direct Determination of Trace Amounts of Silicon in Iron Oxide by Graphite Furnace and Flame Atomic Absorption Spectrometry Using Halocarbon Vapour for Fluoride Evolution*

Tibor Kantor and Manuel Alvarez Prietot Technical Analytical Research Group of the Hungarian Academy of Sciences and Institute for General and Analytical Chemistry, Technical University of Budapest, Budapest 152 I , Hungary

The fluorination of different chemical forms of silicon has been studied using copper(l1) hydroxyfluoride solid reagent and C2CI3F3 vapour in a graphite tube furnace. The method of transportation of the evolved substances from this type of furnace into an acetylene - dinitrogen oxide flame and suitable techniques for handling solid samples were elaborated. The internal gas (Ar) of the furnace was used as the carrier on volatilisation of the halocarbon liquid, which ensured simple operation. The detection limit was 0.84 pg 1-1 of Si in iron oxide when taking 40 mg of solid sample and evaluating the peak absorbance measurements. The average RSD was 4.7% in the concentration range 8-36 pg 1-1 of Si. Keywords: Graphite furnace and flame combined source; silicon tetra fluoride evolution; fluorination with copper(l1) h ydroxyfluoride and trichlorotrifluoroethane; solid sample analysis of iron oxide; silicon determination

Silicon is one of the critical contaminants in iron(II1) oxide that is used for manufacturing ferrite based electronic devices. The usual demand is for the determination of silicon in the 5-50 p.p.m. range, which is a difficult analysis. Considering the atomic spectroscopic methods which involve the prepara- tion of 1% mlV sample solutions, the sensitivity of graphite furnace atomic absorption spectrometry (GFAAS) and that of inductively coupled plasma atomic emission spectrometry is adequate for the concentration levels in question. However, in the former method matrix modifiers should be used to inhibit the formation of silicon carbide and these reagents are potential sources of contamination. 1 In ICP-AES, the degree of sublimation of the heated parts of the quartz torch is the limiting factor in silicon determination at trace levels.* However, chlorinating iron samples with carbon tetrachloride in a sealed ampoule and introducing the silicon chloride vapour into the ICP source (off-line method of evolution and detection) resulted in a useful analytical method.3 This probably means that the main limitation in ICP atomic emission methods is the contamination from fusion - dissolu- tion reagents and procedures. which applies to dissolution methods in general.

An on-line version of fluoride evolution and detection was introduced 30 years ago for arc emission spectrography by Paterson and Grimes.4 They used CuF2.2H20 layered under an iron oxide sample in a graphite cup electrode for the evolution and excitation of silicon and boron. The same reagent and technique were used by Tymchuk et al. for the selective distillation of several trace elements from metallic copper samples.5

Chapman and Dale6 introduced a furnace and flame combined source for the evolution and detection of silicon and boron using CuOHF. The laboratory-constructed, resistance wire furnace contained a changeable graphite crucible for loading the reagent and the sample. The volatile reaction products were sucked into the mixing chamber of fuel and oxidant gases by a modified "water jet pump'' used as a gas injector. Solutions were appiied on to the top of the reagent layer and dried before heating up to 800 "C. Graphite powder based standards were mixed with the reagent (20%) before loading. The evolved SiFj vapour was atomised in an

* This paper is dedicated to the memory of Professor John M. Ottaway. Presented in part at the XXV CSI Post-Symposium on Graphite Furnace Atomic Absorption, Huntsville, Ontario, Canada, 28th June-2nd July, 1987.

-t Present address: Departamento de Quirnica Analitica, Instituto de Materiales y Reactivos para la Electronica, Universidad de La Habana. Cuba.

acetylene - dinitrogen oxide flame. Based on peak-absorbance measurements, a characteristic mass of 0.25 pg Si could be attained.

A basically similar graphite furnace and flame combined source has been developed independently in this laboratory with the adaptation of a Varian Techtron CRA-90 furnace.7.8 This system has been used predominantly for studying high-temperature processes including halogenation reactions with halocarbon vapours.9-10 Halogenation with CC14 and CC12F2 vapours has also been applied to d.c. arc excitation of different solid samples for exploring its potentiality in both selective distillation of certain constituents and the complete evaporation of carbide forming elements.9.11 This method of halogenation was also introduced independently by Kirk- bright and Snook in ICP-AES with the use of graphite furnace vaporisation as the sample introduction method. Halocarbon vapour introduction has also proved to be useful when utilising graphite rod direct sample insertion into the ICP source.13

The results obtained by Chapman and Dale seemed to be promising, and our work was initially directed towards adapting their method to a commercially available graphite tube atomiser as an evolution device. A different method of transporting the evolved substance from this type of furnace and suitable techniques for handling solid samples have been elaborated. Fluorination of Si metal, S i02 and S i c with CuOHF were studied through volatilisation curves and also with the use of an Ar - C2C13F3 vapour atmosphere in the furnace. The gaseous reagent proved to be more advan- tageous in several respects and the analytical method pre- sented here is based on its application.

Experimental Instrumentation

A Perkin-Elmer HGA-400 graphite furnace was used with slight modifications (Fig. 1). An alumina tube (Purox, Morgan Refractories, UK) was used for the collection of the evolved substance. A small graphite tube was fitted tightly into the end of the alumina tube as shown, for performing analytical measurements under fast heating conditions. For volatilisa- tion studies, an alumina tube with a shorter graphite tube terminal was used, which extended the alumina tube by 2 mm. A small part of the graphite ring behind the alumina tube (not shown) should be taken off to insert the alumina tube into the position seen in Fig. 1. The alumina tube is removable, and is held in place by a device (made from a sheet of aluminium), which is fixed by a screw to the original furnace. Thus, the alumina tube can easily be removed for the application of the

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54 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, JANUARY 1988, VOL. 3

Ar

Fig. 1. Cross-section of the Perkin-Elmer HGA-400 graphite furnace, supplemented with a suction tube for transportation of the evolved substance and with an inlet tube for introducing auxiliary outer gas. (a) Graphite tube with stoppers for analysis under fast heating; and ( b ) graphite probe for volatilisation studies under slow heating conditions

solution samples or for changing the internal graphite tube. Another small graphite tube is fixed into the right-hand side hole of the graphite cylinder (this hole was originally made to allow pyrometric temperature control), through which the auxiliary sheath gas (Ar) is introduced to compensate for the gas removed by suction (see below). A small part of the graphite ring should also be taken off behind the port of this graphite tube, which can be conveniently carried out using a rasp.

The original electrographite tube is supplemented with two graphite stoppers in the tube ends for handling powder samples [Fig. l(a)]. The diameter of the coaxial sampling hole is enlarged to 2.5 mm, through which the solid sample can be introduced with the use of a small funnel. The HGA-400 apparatus has a pneumatic clamping system for holding the sample tube, which allows it to be easily removed and replaced. Thus, batch processing of solid samples can be utilised. Standard solutions were also introduced in the same type of tubes (with stoppers), but on to the surface of a L'vov platform made of solid pyrolytic graphite (see below).

To ensure the well defined location of a small amount of solid sample inside the graphite tube, a graphite probe was fabricated [Fig. l (b ) ] and used in the volatilisation studies. The graphite probe and the other supplementary graphite parts were machined from RWI graphite rod (Ringsdorff Werke, FRG). The right-hand side holder of the quartz window of the HGA-400 furnace (with O-ring sealing) was removed for inserting and removing the graphite probe. For performing volatilisation studies, the temperature of the bottom of the graphite probe was measured with a Pt - Pt/Rh thermocouple and also with a vanishing wire optical pyrometer, under slow heating conditions. The temperature versus time characteristics found can be deducted from Figs. 3 and 4.

The evolved substance was monitored by a Varian Techtron Model AA 6 spectrometer operated with an acetylene - dinitrogen oxide flame (burner slot 5 cm). The flow-rate of the fuel was 3.84 dm3 min-1 (calibrated by the manufacturer). The flow-rate of the oxidant was selected to produce a reducing zone (red feather) of about 20 mm in height without soot formation (see also below). Positioning of the burner head was made during aspiration of K2Si03 standard solution (observation height of 7 mm) to attain the highest available sensitivity. A spectral band width of 0.2 nm at 251.6 nm and a Varian Techtron hollow-cathode lamp (1.5 mA) were used.

When changing to using the furnace and flame combined system, the pneumatic sprayer was replaced by a gas injector constructed from a plastic T-shaped junction.14 The metal capillary tube inside this device was covered by a sintered plastic foil to avoid the possibility of reaction with SiF4 vapour.5 The upper end of the alumina tube (Fig. 1) was loosely connected to the gas injector through a 40 cm long tube made of natural rubber (synthetic silicon-rubber tubing releases SiF4 when CuOHF is used in the furnace.) Dinitrogen oxide gas was supplied to the mixing chamber through the capillary of the gas injector and through the auxiliary branch supply line of the oxidant gas. The flow-rate of the former fraction could be regulated separately with an additional valve (modification). This valve was used for adjusting suction flow-rate through the alumina tube shown in Fig. 1 to 0.6 dm3 min-1. (The same flow-rate of argon was also applied as an auxiliary sheath gas as mentioned above.) The combined (total) flow-rate of the oxidant was 5.5 divisions on the arbitrary scale of the flow meter. To measure the suction flow-rate, the alumina tube was connected to a flow meter when maintaining the acetylene - dinitrogen oxide flame. Readjustment of the suction flow-rate was necessary only in the initial heating-up period of the burner.

Volatilisation studies using CuOHF were also made with the quartz furnace and flame system described in detail elsewhere. 15

Procedure for Halocarbon Vapour Introduction

For introducing halocarbon vapour into the sample tube, the internal gas supply of the HGA-400 was used for carrier vol- atilisation of 1,1,2-trichloro-1,2,2-trifluoroethane (CZC13F3). The carrier argon gas stream was passed above the surface of the halocarbon liquid (without bubbling) in a glass vessel of 4 cm in diameter.11 The volume of the liquid was 2-3 ml and it was neither thermostated nor stirred in contrast with previ- ously recommended conditions.11 The total internal flow of argon was sub-divided into two equal streams by the original gas supply unit. One gas flow stream was passed above the halocarbon liquid and the two streams were then combined and passed into the furnace unit where the stream was subdivided again towards the two ends of the graphite sample tube. Because of the high volatility of the C2C13F3 liquid (b.pt. 47.6 "C), a low flow-rate of the carrier gas (15 cm3 min-1; half of the total internal gas) was selected. Based on the vapour pressure data of the halocarbon concerned16 and the relevant method of calculation,ll the flow-rate of the halocarbon vapour is 7.8 cm3 min-l at 20 "C with the use of 1.5 cm-3 min-1 carrier gas flow-rate, if the equilibrium conditions of satura- tion are satisfied. However, from mass loss measurements of the liquid under the conditions described, an actual rate of 2.4 cm3 min-1 vapour was estimated (about one third of the equilibrium value). These conditions of halocarbon vapour introduction were used both in the volatilisation studies and in the quantitative analytical measurements.

The graphite sample tubes and the graphite probes were purified by heating to 2700 "C (manual heating mode) for 5 s in a halogenating atmosphere. The purification was monitored by the flame atomic absorption signal of the silicon. The remains of any solid sample left after the silicon atomic absorption measurements were poured from the tubes, and the empty tubes were purified again for multiple use.

No damage to the quartz windows and the metallic parts of the HGA-400 furnace were caused by the use of halocarbon vapour during the course of this work. However, for extended use it might be advisable to exchange the quartz windows for graphite discs, machined from a graphite rod. As the decomposition products of halocarbon vapours are poisonous, the use of an effective hood above the flame is mandatory. Although the decomposition products released from the furnace are sucked into the flame, it is also advisable to extend

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JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, JANUARY 1988, VOL. 3 55

the ventilation of the hood above the graphite furnace, to avoid accident a1 hazards.

Procedures for Volatilisation Studies

Volatilisation (evolution) studies of silicon with the quartz furnace and flame atomisation (QFAFA) system were perfor- med with mixtures of different chemical forms of silicon and copper hydroxyfluoride reagent (Spex Industries, Edison, NJ, USA). Four mixtures were prepared separately containing 10% Si02, 10% window glass, 5% silicon metal and 5% Sic, and 10 mg of each these mixtures were loaded into the graphite cups. A constant heating rate of 50°C min-1 was applied up to 950 "C; the flow-rate of the furnace gas was 0.5 dm3 min-1.

Using the graphite furnace and flame atomisation (GFAFA) system for volatilisation studies with the CuOHF, heating conditions similar to those applied with the QFAFA system were specifically used. These were approximated by selecting a 1500°C final tube temperature (nominal) and a ramp time of 998 s (maximum available value). From the calibration mentioned above, heating rates of 58 and 83 "C min-1 were effective at 400 and 800 "C. Mixtures contain- ing the same amounts of silicon as for QFAFA were introduced into the graphite probes and the internal flow-rate of argon was 300 cm3 min-1 in the graphite tube.

For volatilisation studies with halocarbon vapour, a final tube temperature of 2000°C and a ramp time of 998 s were selected. From the experimental temperature versus time characteristics (temperature of the bottom of the probe), the effective values of the heating rate were 125 and 150 "C min-1 at 750 and 1500 "C, respectively. The different chemical forms of silicon being investigated were diluted with graphite powder (Ringsdorff Werke, FRG) to give the same concentra- tions as above, and 10-mg amounts of these mixtures were applied to the graphite probes.

Procedures for Analytical Measurements

The iron oxide base of the silicon standards was prepared from slices of a spectral pure iron rod (Johnson Matthey, Royston, UK) dissolved with 1 + 1 nitric acid in a platinum crucible (with the lid on) under heat from a laboratory flame. After homogenisation of the iron oxide in an alumina mortar (Fritch Pulverisette, Type 4, FRG), portions were measured into plastic vessels and wetted completely with aliquots of K2Si03 standard solution of 10, 20 and 40 p.p.m. added silicon concentration (analyte addition). An IR lamp was used for drying, and homogenisation was carried out in an alumina mortar. Portions of the iron oxide base were diluted with high-purity graphite powder (Ringsdorff) in proportions of 1 + 1 and 1 + 3 to extend the calibration to the lowest detectable concentration of silicon.

Sample masses of 20-50 mg were weighed into the graphite tubes [Fig. l(a)], and the powdered fillings were compacted by tapping the tubes. A suitable plastic tool and a plastic container were fabricated to hold the sample tubes at a near horizontal position during sample loading and storage.

Matrix-free K2Si03 standard solutions were also used for calibration and these were pipetted (10 p1) on to the surface of the L'vov platform within the sample tube, as is the convention in the furnace AAS technique.

The heating and monitoring programme used for both solution and solid samples is shown in Table 1. It was observed in preliminary studies that the introduction of water vapour released during drying of the solutions into the transporting system (step 1) resulted in a decrease of the silicon AAS signal. This was probably due to the partial hydrolysis of SiFj vapour in the wetted tubing system. Therefore, in the later work, the suction tube (Fig. 1) was inserted above the sample tube after the drying step had proceeded. (The start of the

recorder in step 2 was a sign for inserting the suction tube.) The temperature settings in Table 1 are nominal values of the HGA-400 power supply.

Results and Discussion Characteristics of the Analyte Transport System

The graphite tube atomisers, such as Perkin-Elmer HGA systems, have been applied as vaporisation devices in combi- nations with ICP sources by several workers.17-19 In these previous investigations one of the water cooled ends of the graphite tube has normally been used to remove the vapour - aerosol mixture (see below) formed from the sample, and this gas stream was passed into the ICP through appropriate tubes. With this type of coupling, transportation is performed in a closed or semi-closed system during vaporisation, which allows the use of a low sample gas flow-rate, an important factor with the argon ICP. The use of a gas-injector increases the total flow-rate of the carrier (sample) gas, but this is not, however, a handicap with a flame observation source. When the suction produced by the gas injector is utilised for transportation, the need for a gas-tight enclosure around the heated parts of the furnace is circumvented, which makes the loading of the sample easier. In addition, the gas injector fixed in the mixing chamber prevents the streaming of the flamm- able gas mixture in the direction of the vaporisation device, which is a pre-condition for safe operation. These are sufficient reasons for an increase in the use of gas injectors with the furnace and flame combined systems described.G10,2" It is noted that in a spark discharge and a nitrogen ICP combination (4.5 dm3 min-1 sample gas), a gas injector was also used for the transportation of the dispersed material.21

The particular configuration of the suction tube with the graphite terminal adjacent to the sampling orifice of the tube (Fig. 1), resulted in a higher efficiency for transporting a gaseous analyte (SiF4) than using the shorter graphite terminal mentioned above. This was observed under fast heating conditions (Table 1). Similar comparative measurements, using the longer and shorter graphite tube terminals, were also made for the transportation of the evolution products of zinc and chromium, when the halocarbons were introduced. For these elements, the use of the longer graphite tube terminal resulted in a lower sensitivity. These contradictory observa- tions are probably related to the different forms of the substances being transported. From the evolution products of zinc and chromium, an aerosol should be produced for transportation and an aerosol can be obtained by mixing the hot vapour with a turbulent flow of a cold gas.22 This condition is not as effectively satisfied when using the the longer graphite terminal in the suction tube (Fig. 1), which actually rests on the wall of the hot graphite sample tube.

Table 1. Heating and monitoring programme for the determination of silicon using halocarbon vapour

Step

Parameter 1 2 3 4 Temperature/"C . . . . 320 20 320 1800

1 1 1 Ramptimek . . . . . . 50 9 Holdtimels . . . . . . 10 3 15 + Recorder . . . . . .

+ * +i Read (10-s integration) . . Baseline . . . . . . Mini-flow-rate$/cm3 min-1 30 30 30 30

* The integrated signal measured in step 3 was subtracted from that of step 4 to account €or the base-line drift between the subsequent heatings.

- + +

- - -

- - -

t The start of the integration in step 4 was delayed for 2 s. $ Half of the internal gas (15 cm3 min-i) was used for carrier

volatilisation of the halocarbon liquid (see text).

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The high susceptibility of SiF4 to hydrolysis was probably a factor that contributed to the decrease of the silicon sensitivity observed without use of a closely attached suction tube end. The argon gas used in these experiments probably contained a low concentration of water vapour. It is likely that the water reacted with the decomposition products of the halocarbon vapour on heating, and thus no hydrolysis of SiF4 occurred. By mixing the evolved gas with an external argon gas stream of low water content, partial hydrolysis of SiF4 will probably take place. Aggregates are formed from the silicon dioxide produced by this reaction, which have a lower efficiency of transportation than the gaseous SiF,.

Volatilisation Studies With the Use of Copper Hydroxyfluoride

In these studies the vapour was continuously swept from the heated part of the furnace using slow heating, and thus the temperature of the vapour did not increase significantly above its evolution temperature. Therefore, the absorbance versus time, or absorbance versus temperature curves recorded under these conditions may be considered as “volatilisation curves. ”

In Fig. 2, the volatilisation curves for silicon that were obtained when different chemical forms of silicon were reacted with CuOHF in a quartz furnace (QFAFA system) are shown.15 It was expected that the HF gas evolved from the reagent (see below) would react with the wall of the quartz furnace, and this is represented by curve A in Fig. 2. This curve was recorded with the use of 10 mg of CuOHF alone, and it can be considered as a “blank” recording in respect of the recordings with sample and reagent mixtures (curves B-E). In Fig. 3, the results of similar experiments are shown, with the exception that fluorination occurred in the HGA-400 graphite furnace (GFAFA system).

A 120 I II

Temperature/”C - Fig. 2. Volatilisation curves for silicon using the quartz furnace and flame combined system (QFAFA), introducing 10 mg of solid samples in graphite cups. A, CuOHF reagent (R); B, 10% silicon dioxide + R; C, 10% glass + R; D, 5% silicon metal + R; and E, 5% silicon carbide + R. Numbers on peaks are temperatures in “C

When using the quartz furnace the volatilisation peaks of silicon metal and silicon carbide (curves D and E in Fig. 2) at Ti = 280 and T, = 360°C are larger than the “blank” peak (curve A in Fig. 2 ) , which implies a contribution of the reaction products to the signals from the samples. [The terms and symbols “initial observation temperature of volatilisa- tion” (Ti) and “temperature related to the maximum rate of the volatilisation” (T,) have been discussed elsewhere, 101 However, from the curves recorded with the use of the graphite furnace (curves C and D in Fig. 3), it is clear that evolution of silicon from silicon metal and silicon carbide in this temperature range is insignificant. Apart from this inconsistency, the shape of the corresponding volatilisation peaks and their characteristic points are in acceptable agree- ment, when the results of QFAFA and GFAFA systems are compared.

Conventional thermoanalytical methods (TG, DTG, DTA, DSC) have been used in conjunction with X-ray diffraction, IR spectroscopy and standard analytical methods for identifi- cation of the decomposition products of copper( 11) fluoride dihydrate23.24 and copper(I1) hydroxyfluoride.23 Concerning the decomposition of CuF2.2H20, two major decomposition steps were found with the evolution of HF gas in the 10CL200 and 300460 “C ranges. However, from the identification of the solid decomposition products, different mechanisms were anticipated. It was also mentioned that the moisture and oxygen concentration have considerable influence on the decomposition pr0cesses.23,~~ In this context, the observations seen in Fig. 2 and Fig. 3 suggest that the commercial reagent

Temperature/”C -

T

0 2 4 6 8 10 12 Time/mi n

Fig. 3. Volatilisation curves for silicon using the graphite furnace and flame combined system (GFAFA), introducing I0 mg of solid samples on graphite probes. A, 10% silicon dioxide + CuOHF reagent (R), B, 10% glass + R; C, 5% silicon metal + R; and D, 5% silicon carbide + R. Numbers on peaks are temperatures in “C

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used in our work was actually a mixture of CuF2.2H20 and CuOHF compounds.

According to the reaction steps suggested by Wheeler and Haendler,23 the dihydrate fraction of the reagent mixture underwent a low-temperature decomposition with H F gas evolution T, = 132°C:

2(CuF2.2H20) + CuOHF.CuF2 + H F + 3H2O (1) The H F released by this process reacted with silicon dioxide powder (curve B in Fig. 2 and curve A in Fig. 3) and with metallic silicon powder (curve D in Fig. 2 and curve C in Fig. 3) with the formation of SiF4 gas. This reaction did not take place to a detectable degree with a smooth silica surface (curve A in Fig. 2), with powdered glass (curve C in Fig. 2 and curve B in Fig. 3) and with silicon carbide (curve E in Fig. 2 and curve D in Fig. 3).

The largest fraction of the reagent mixture used was probably as CuOHF, which decomposed at a higher tempera- ture23 Tm = 345°C:

4CuOHF- CuOHF.CuF2 + 2CuO + H F + H20 (2) The basic salt produced by reactions (1) and (2) was

decomposed in a third step [reaction (3)] in the absence of water T, = 420 "C:

The HF gas released by the processes (2) and (3) underwent reactions with silica and glass producing SiF4 gas, according to the curves A-C in Fig. 2 and curves A and B in Fig. 3. However, formation of a greater amount of SiF4 is not observed from silicon metal (curve C in Fig. 3) and from silicon carbide (curve D in Fig. 3) in the 30W50 "C range, for the graphite furnace.

For the interpretation of the vaporisation peaks observed above 450"C, as seen in Fig. 3, no relevant data could be found in the literature. It may be supposed that Cu2SiF6 was formed from the metallic silicon, with HF gas released by reactions (2) and (3), under the reducing effect of graphite. This compound is known to be decomposed with the liberation of SiF4, and the peak at T = 720°C may correspond to this process. According to reaction (3), CuF2 is formed above 500 "C, and this compound decomposes at 950 "C, if present alone.16 However, it probably reacts with silicon carbide, and the highly erratic peak observed at 790°C is caused by this exothermic reaction.

CUOHF.CUF~+ CUO + C U F ~ + H F (3)

Volatilisation Studies Using Halocarbon Vapour

In Fig. 4, volatilisation curves of different chemical forms of silicon, mixed with graphite powder, are shown when using an Ar - C2CI3F3 internal furnace atmosphere. From the estimated flow-rate of the halocarbon vapour as above, its concentration was 7.4% V/V in the furnace atmosphere.

It is known from conventional atomic absorption spec- trometry that the greater reducing character of the acetylene - dinitrogen oxide flame (fuel to oxidant ratio) is a dominant factor in enhancing the atomised fraction of silicon,2s and this also holds with the present combined system. To attain the highest available sensitivity, the flow-rate of the acetylene gas was increased to the limiting value indicated by the production of carbon soot in the flame. This highly reducing flame was maintained without introducing halocarbon vapour or heating the furnace. However, by heating the furnace a positive shift of the base line appeared as illustrated by curve A in Fig. 4. When introducing halocarbon vapour into the furnace, a further shift of the base line is apparent in curve B of Fig. 4. Inspection of curves C-F in the same figure indicates that base-line shifts occurred of a similar magnitude to that in curve B.

TemDeraturePC - 1000

I 710

1540 F I/

I I I I I I 1 1

7 8 9 10 11 12 13 14 15 Time/min

Fig. 4. Volatilisation curves for silicon introducing C2C13F3 vapour into the graphite furnace (GFAFA system), with the sample + graphite powder mixtures (10 mg) on graphite probes. A, Blank recording without introducing halocarbon vapour; B, as A but introducing halocarbon vapour; C, 10% silicon dioxide; D, 10% glass; E, 5% silicon; and F, 5% silicon carbide. Numbers on peaks are temperatures in "C

Curve C in Fig. 4 represents the halogenation process of Si02 powder, which takes place in a relatively narrow temperature range (670-1060 "C). According to curve D, the halogenation of glass powder is a much slower process. It was also observed visually that the flame emission of calcium (as a constituent of glass) disappeared at the same time as the signal for silicon ( T = 1540 "C). The major fraction of metallic silicon (curve E) is halogenated in the 670-1000°C range, but this process lasts up to 1200 "C. The decrease in the reaction rate at higher temperatures is probably due to a partial formation of silicon carbide in the metal - graphite powder mixture. A minor fraction of the silicon carbide sample (curve F) is halogenated at a similarly low temperature as the silicon dioxide (curve C), while the halogenation of the major fraction of the silicon carbide sample takes place at a much higher temperature (1000-1540°C). It is to be noted that a small fraction of the silicon carbide sample also reacted with the CuOHF reagent, as indicated by curve D in Fig. 3. The silicon carbide sample used in these experiments was of "industrial grade" and presumably it contained silicon dioxide as an impurity.

It appears that the initial temperature of halogenation of silicon metal and silicon dioxide is determined by the temperature at which reactive species are produced from the C2C13F3 vapour. It is expected that the fluorine gas and the fluorine containing radicals formed by decomposition of C2C13F3 are the major reactants in the present halogenation processes, but the role of chlorine containing species cannot be excluded. In fact, the CC14 vapour proved to be much less effective for liberating silicon in the temperature range covered by the present experiments.

Shape of Silicon Signals With Fast Heating Conditions Using Halocarbon Vapour Introduction

In order to achieve complete evolution of silicon in a reasonably short period of time, the heating conditions

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summarised in Table 1 were selected by preliminary experi- ments. In the first stage of the experiments, sample solutions (K2Si03) were pipetted on to the wall of an electrographite tube. Although the integrated signals showed reasonable reproducibility, the shape of the signals was rather flat and the precision of the peak-height signals was poor. In the view of the volatilisation studies described above, this problem could be attributed to the possible formation of silicon carbide on the surface of the graphite tube during the heating-up period of the halocarbon vapour. As is known, the application of a L’vov platform results in a lower temperature rise of the condensed phase relative to that of the gas phase in the first period of heating. Therefore, it may be expected that silicon carbide is not formed on the surface of a platform made of solid pyrolytic graphite until there is a sufficient concentration of the fluorinating species from the thermal decomposition of the halocarbon vapour.

Indeed, by pipetting the solution standards on to the surface of a L’vov platform, reproducible silicon signal shapes could be obtained. This is demonstrated in Fig. 5 , using a 40-s recording time. Although, the heating time was 0-10 s the base line did not return to zero before about 20 s. In Fig. 5(c), recordings are shown which were made after measurements with variable amounts of silicon. The shape and height of these post-heating signals are seemingly independent of the amount of silicon applied in the previous steps. These post- heating signals can be correlated with the base-line shift mentioned above (Fig. 4). It can be estimated that the broadening of the silicon peaks at the decaying edge is caused by the same type of base-line shift. This problem could be diminished by using a less reducing flame at the expense of a decrease in the atomic absorption sensitivity of silicon (see below). By applying a deuterium lamp for background correction, the base-line shift in question could not be eliminated.

In Fig. 6(a), recordings made with 20-mg solid standards are shown, while Fig. 6(b) shows the post-heating signals in line with the corresponding silicon signals. The same heating and monitoring programme was used for both the “first” and the “post” measurements. The broadening of the decaying edges of the silicon signals is again similar in magnitude to the post- heating signals and are independent of the silicon content of the standards. In comparison with the recordings discussed above (Fig. 5 ) it can be estimated that the base-line shifts in question are somewhat larger for solid samples than for solution samples. However, this difference can also be due to a change in the flame composition, because the recordings in

b)

1.5 pg Si

I

I (c’ 0.5 pg Si

1.5 pg Si c-- 0 20 40 0 20 40 0 20 40

Timels

Fig. 5. Recordings of (A) and (B) silicon si nals and (C) post- heating signals under fast heating conditions &able l), for 10-yl volumes of standard solutions (potassium silicate). Using L’vov platform and halocarbon vapour

Fig. 5 and Fig. 6 were made at different times, and also different cylinders of gases were used. Because of the relatively poor reproducibility of the base-line shift, the calculation of the net integrated signal was made by subtract- ing the post-heating signal from the relevant bulk (total) signal, both measured with the same sample. Integrations were made over an interval of 2-12 s during the heating period (step 4 in Table 1).

By these double heatings of iron oxide samples, metallic iron globules were formed, which had a highly inelastic structure. The mass of these globules was &12% higher than expected from stoicheiometric calculation (considering pure Fe203 and iron) in spite of the observation that a small fraction of the iron was evaporated. This can be understood by supposing the formation of steel globules of high carbon content, the carbon being produced from the decomposition of halocarbon vapour.

Effect on Flame Background of Changing the Fuel to Oxidant Ratio

The base-line shift observed with the combined source increased with the fuel to oxidant ratio, and also with the temperature of the furnace. Because of the heat expansion of the argon gas in the furnace, it may be expected that the air is more effectively excluded from the transporting system. If the later assumption holds, the base-line shift can be attributed to the same basic cause.

It was therefore thought that the base-line shift (or change in the flame background) should also be observable when using the conventional pneumatic nebulisation of solutions, and this method was selected for further studies. De-ionised water and matrix-free potassium silicate solutions were

t

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Fig. 6. Recordings of ( a ) silicon signals and ( b ) post-heating signals under fast heatin conditions (Table l), for 20-mg solid standards (iron oxide matrixf. Using graphite tubes with stoppers [Fig. l ( a ) ] and halocarbon vapour. Programme: 1800°C; ramp 1 s ; and hold 9 s

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JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, JANUARY 1988, VOL. 3 59

nebulised. The flow-rate of the dinitrogen oxide was main- tained at 7.3 dm3 min-], and the flow-rate of acetylene was varied from 3.44 to 4.25 dm3 min-1 (calibrated by the manufacturer). The observation height was 7 mm, as in the experiments described above.

In Fig. 7(b) , the change of the flame background with water aspiration is shown as a function of acetylene flow-rate, when the base line was adjusted to zero for the lowest flow-rate of the fuel. The flame background increased slightly when measured with a deuterium lamp at 251.6 nm (adjusted with the use of the silicon hollow-cathode lamp), and this also holds for the measurement at 250.7 nm with the use of the silicon hollow-cathode lamp. These small increments seen on curves 1B and 2B can be related to the soot formation above an acetylene flow-rate of 3.8 dm3 min-1, which could also be observed visually. However, when the measurement was made at 251.6 nm with the use of the hollow-cathode lamp (curve 3B), the increase in the flame background with the flow-rate of the fuel was significant. It must therefore be related to a sharp absorption band overlapping the Si 251.6-nm line. Consultation of the literature on flame band spectra has not revealed the identity of the queries involved.

According to the curves seen in Fig. 7(a) , the net signal of silicon increases considerably with the flow-rate of the fuel for both of the silicon lines used. At the highest acetylene flow-rate plotted, the fuel to oxidant ratio is 0.582, which is close to the value (0.602) used by Rasmuson et al.25 for attaining the highest available sensitivity for silicon. The atomised fraction of silicon calculated was 0.12 under the highly reducing flame conditions applied by these workers (observation height of 6 mm).

Solid versus Gaseous Reagents

From preliminary studies with CuOHF, using fast heating conditions, we obtained sharp evolution peaks for silicon,

0.3

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0.1

- k c. m -5 O.OE c a m

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Fig. 7.

I I I I I

( b )

3.4 3.6 3.8 4.0 4.2 Acetyleneidms min - 1

Variation of (a ) the net silicon signal and ( b ) the flame background with the acetylene flow-rate, when using pneumatic nebulisation of solutions. (1A) Deuterium lamp 251.6 nm, 500 ug ml-1 of Si; (2A), Si hollow-cathode lamp (HCL), 250.7 nm, 500 pg ml-1 of Si; (3A), Si-HCL 251.6 nm, 200 yg ml-1 of Si; (1B) deuterium lamp 251.6 nm; (2B) Si-HCL 250.7 nm; (3B) Si-HCL 251.6 nm. Using an N,O flow-rate of 7.3 dm3 min-1 and h = 7 mm

similar to those demonstrated by Chapman and Dale.6 However, the precision of the measurements was far from satisfactory for iron oxide samples of low silicon content (admixtures with 20% reagent). By applying solid reagent alone in an empty graphite tube, variable silicon signals were detected, probably due to the contamination of the graphite tubes during their preparation. The purification of the graphite tubes with the use of the solid reagent was, however, a rather time-consuming procedure. However, the purifica- tion of the graphite tubes by using the gaseous reagent could easily be performed and its completeness could be controlled fully. Therefore, even when the solid reagent is preferred for certain purposes (see below), the gaseous reagent is recom- mended for the purification of the graphite tubes and probes. Obviously, the mixing of the solid reagent with the sample or its layering below the sample are also tedious procedures and are potential sources of contamination. The need for these procedures are circumvented by the use of the gaseous reagent.

From the curves seen in Figs. 2 and 3, the use of the solid reagent makes it possible to differentiate between certain silicon compounds (speciation) and this might be important for future considerations.

Analytical Curves and Performance Data

A calibration graph for 40-mg solid standards, prepared by analyte additions, is shown is Fig. 8 (line A). The points plotted are the averages of triplicate measurements, the straight line was determined by linear regression analysis ( r = 0.985, n = 9). A silicon concentration of 0.159 p.p.m., was found for the iron oxide base by this calculation. Line B in Fig. 8 is for the measurements made with the diluted iron oxide base, the slope of which is 3.6% less than that of line A. This calibration was based on peak-absorbance measurements, which proved to be more reliable at low concentration levels.

A comparison of the signal versus mass function was made for solution and solid standards, evaluating both peak-absor- bance and integrated absorbance measurements. The slope of the calibration graph using peak-absorbance measurements was 1.3 times lower for solution than for solid standards. The net values of the integrated absorbances were determined by the method described above, and the average values of 3-5 replicates were plotted as a function of the silicon mass present in the loaded amount of solution and solid standards (20 mg). The straight calibration graph in Fig. 9, was obtained by linear

0 5 10 15 20 Concentration Si, p.p.m.

Fig. 8. Calibration graphs for silicon (based on peak absorbance measurements) with 40 mg of solid standards (Fe203 matrix) prepared by: A, analyte addition, and B, by dilution of the iron oxide base with graphite powder

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60 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, JANUARY 1988, VOL. 3

Mass of Si/pg

Fig. 9. Calibration gra h for silicon (based on integrated absorbance measurements) with (07 solution standards (K2Si03) and (0) 20 mg of solid standards (Fe203 matrix)

Table 2. Analytical performance data with peak- and integrated absorbance measurements. cslms, characteristic value, concentration to mass; cLImL, limit of detection (3s), concentration to mass*

Silicon in Silicon in solutiont (10 pl) solid sample$ (40 mg)

Parameter Peak Area Peak Area cs . . . . . . 2.8 pg ml-1 1 .O pg ml-1 0.55 pg g-1 0.24 pg g-1 cL . . . . . . 4.7pgml-1 7.2pgml-1 0.84pgg-’ 1.2pgg-’ mslpg . . . . 0.028 0.010 0.022 0.0097 mLlpg . . . . 0.047 0.072 0.034 0.050 RSD (average),$

Range . . . . 0.5-2 pg 8-36 pg g-

* cs, Characteristic concentration; ms, characteristic mass; cL, limit of detection in units of concentration; and mL limit of detection in units of mass.

% . . . . 7.9(21) 5.6(21) 4.7(17) 6.2(17)

t As potassium silicate. $ Powdered iron(II1) oxide matrix. § Average value of relative standard deviation (number of

measurements) in the given ranges of mass and concentration.

graphite tube used as sample holder [Fig. l(a)] 80 mg of powdered iron oxide takes up approximately 314 of the free volume, i.e., it can be accommodated if necessary.

The data characterising the analytical performance are summarised in Table 2. The integrated absorbance (peak area) was measured on a calibrated scale (A s). The characteristic mass values (absorbance of 0.0044) based on peak-absorbance measurements can be compared with those reported by Chapman and Dale,6 and the values found in the present work are better by about one order of magnitude. It is noted here, that with pneumatic nebulisation of matrix free standard solutions, the characteristic concentration was 3.2 pg ml-1 of Si under the flame conditions which result in the highest signal (see Fig. 7).

The detection limits were determined from the standard deviations (SD) of the post-heating signals. The values were much poorer with integrated rather than with peak-absorption measurements due to the late base-line shift. This is also reflected in the poorer detection limits with integrated absorbance measurements. Applying 40 mg of solid sample, 4 pg g-1 of silicon could be determined with an RSD of 12% for peak-absorbance measurements, which is adequate at these trace levels.

Conclusions The fluoride evolution, transportation and detection system described may be used for the determination of other fluorides of high volatility. However, the matrix can exert a strong influence on the fluorination and selective distillation of a particular constituent, which should be studied experimen- tally. l1 From a preliminary investigation, As and B in addition to Si can be distilled from an iron oxide sample utilising the gaseous reagent proposed in this work.

Most of the elements that have volatile fluorides require the use of a highly reducing acetylene - dinitrogen oxide flame to attain acceptable sensitivity for low concentration levels. As no water is supplied to the flame with the present sample introduction method, a higher flame temperature and more reducing character are likely to result relative to the situation when applying the pneumatic nebulisation of aqueous solu- tions. These are favourable conditions to improve the analytical sensitivity for several elements.

The authors express their gratitude to Laszl6 Bezur who fitted the electronics for communication between the Perkin-Elmer HGA-400 graphite furnace and the Varian Techtron AAS spectrometer. They acknowledge also the help of Ilona Cyranski in the experimental work.

~~

regression ( r = 0.983) for solid standards. The slope of the calibration graph calculated for the solution standards ( r = 0.978) was lower by 11% than that for the solid standards. Considering the problem associated with the determination of the net integrated signal’ (the late base-line shift discussed above) it can be stated that the deviation of the calibration graphs with solid and solution standards is within experimen- tal error. This suggests that the solid samples concerned can be analysed for silicon by performing the calibration with matrix- free solution standards, provided that integrated absorbance signals are evaluated. However, a further confirmation of this statement is needed with the use of certificated solid stan- dards, which have not been available in this laboratory.

The measurements associated with Fig. 9 were performed with synthetic solid standards of 20 mg. When applying 40 mg of these solid standards, the slope of the signal versus silicon mass function decreased by 1.21 and 1.16 times for peak- absorbance and integrated absorbance measurements, respec- tively. This means that the analytical sensitivity is not exactly proportional to the mass of the loaded sample, which may be due to a decrease in the halogenation efficiency and/or transportation efficiency with increasing sample mass. In the

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Paper J7/130 Received 6th July, 1987

Accepted 17th September, 1987

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