Spectroscopic measurement of carbon monoxide in a graphite furnace

.Specnochimica Acar. Vol. 436, Nos 4/S. pp. 421438. Printed in Great Britain.

1988.

0584-8547188 Pergamon

S3.00+.00 Press pk.

Spectroscopic measurement of carbon monoxide in a graphite furnace* R.E. STURGEON+ Division of Chemistry, National Research Council of Canada, Ottawa, Canada KIA OR9

and H. FALK Central Institute for Optics and Spectroscopy, Academy of Sciences GDR, Rudower Qauseel 5, 1199 Berlin, German Democratic Republic (Received 20 August 1987; in revised form 28 October 1987) Abstract-Time-resolved, direct spectroscopic measurements of the PC0 in an unperturbed high-temperature graphite furnace are reported, based on the attenuation of the CO(0,0) transition at 154.3 nm excited in a hollow cathode discharge. A model is presented to account for the kinetic and “diffusion” control regimes observed for oxidation of electrographite, pyrolytic graphite-coated and glassy carbon tubes. Heterogeneous equilibrium appears to be established only at T > 2200 K incoated tubes. Above 1500 K, glassy carbon tubesareas reactive toward 0, as coated tubes. The major source of O2 in the unloaded furnace tube is ingress of ambient atmosphere through the sample dosing hole, giving rise to a steady-state PC0 of 1.2 x 10-j atm at 2600 K. Release of oxidants from decomposition of sample matrices and their slow disappearance by diffusion/reaction processes contribute to an elevated PO, in the tube at temperatures < 2000 K.

1. INTRODUCTION determination of the partial pressure of oxygen in the high-temperature graphite furnace remains elusive despite significant effort from several research groups [l-16]. Interest in this parameter continues unabated because the central role played by gaseous and surface chemisorbed oxidants affects, to some degree, all aspects of analyte atom formation

ACCURATE

cm

Characterization of the major gaseous constituents within the furnace has been attempted by thermodynamically modelling the system [S, 8,9, 123 or through indirect measurements of the partial pressures of 02, CO, CO, and hydrocarbons in the efRuent purge gas [6,11,13,14]. The former approach assumes (i) the system reaches equilibrium, (ii) that the major source of 0, in the furnace is impurity in the (Ar) purge-gas and (iii) that the furnace can be treated as a closed system. In reality, none of these assumptions is fulfilled. Heterogeneous equilibria appear to be established only at temperatures in excess of 2200 K [13]. The presence of a sample dosing hole in the center of the tube dictates that the furnace is an open system and, consequently, the major amount of oxygen may not arise from impurities in the purge gas but from ingress of O2 from the ambient atmosphere as well as from sample decomposition reactions [ 151. Indirect measurements of the partial pressures of gaseous reactants suffers from the need to transport these species out of their high-temperature environment via a flow of purge gas to a (cold) detector. The possibility of both heterogeneous and homogeneous reactions taking place in the sample transfer line cannot be ignored and thus unambiguous interpretation of the results is not assured. As well, it is not possible to study the system in a purge gas stop mode, a condition normally used for analytical purposes. Direct, in situ spectroscopic measurement of the gaseous reactive species of interest is preferred as this approach is non-invasive and hence, non-perturbing to the analytical system. We report here on the direct vacuum ultraviolet (VUV) spectroscopic measurement of CO in a dry, unloaded graphite furnace undertaken in an attempt to obtain additional information on the high-temperature oxidation of the graphite tube and on the partial *An Invited Paper at the XXV CSI, Toronto, Canada. +Author to whom correspondence should be addressed. 421

422

R.E.

STURGEON

and H.

FALK

pressure of CO, since it is presumably the major reactive component in the furnace. Such measurements also permit verification of the accuracy of earlier, indirect gas chromatographic analysis of this gas [ 131 and time-resolved concentration information in the gas stop mode to be obtained which is otherwise unavailable. Spectroscopic measurements of CO conducted in the VUV region are experimentally superior to those in the IR because background emission from the incandescent furnace walls is considerably less and the sensitivity of determination in w 103-fold greater. 2. THEORY In contrast to other models [4,9, 151, the graphite furnace is not assumed to be in a state of chemical equilibrium. Further, the major source of free oxygen in the tube is not assumed to be impurity in the inert Ar purge gas; a more comprehensive consideration is given to the possible diffusion of ambient O2 into the tube through the sample dosing hole. A simplified model aimed at describing the partial pressure of CO (I’,,) as a function of experimental variables and furnace tube geometry at steady-state temperatures is given below for a pyrolytic graphite-coated tube. Reference is made to the geometry of the furnace tube as outlined in Fig. 1. With the furnace at steady-state temperature T, and in purge gas stop mode, diffusional processes account for the major movement of gaseous species. It is assumed that (i) the concentrations of all species depend only on x and T; (ii) CO forming reactions take place only within 1x ( < l&2; (iii) there is a linear concentration gradient for 1,/2 >, 1x1> 1,,/2. Considering that oxygen may diffuse into the tube both through the sample dosing hole and from the ends, leads to the following boundary conditions:

no(x,r) = n,,(x,

r) =

ng = constant for 1x13 1,/2, r B rl + 1, for (xl < 1,/2, r < r,+l, i no < ng 0

for 1x12 1,/2, r 2 r, + 1, constant for Ix I < 1,,/2, r < rr + 1,

co -I no

(1) (2)

where n denotes number density of the subscripted species (elemental oxygen and carbon monoxide), r, is the inner tube radius, 1,is the tube length, 1, is the effective length of the dosing hole (2 tube wall thickness) and I,, is the effective length of the tube over which significant reaction occurs. The production rate of CO is given by: dNC0 ~ dt =

kc0( TM& 0,,/2)

where N,, is the total number of molecules of CO, k,,(T) is the rate constant for the reaction, A, is the active surface area of the tube participating in the reaction [ 171,and no is the number density of elemental oxygen at 1x1< 1,,/2. The rate of loss of CO may be described by: dNco

-

dt

=

-2Dco(~Mc&(l, -L-d -Dco(~~4n~olld

Fig. 1. Geometric projection of the furnace tube.

(4)

423

CO in a graphitefurnace

where D,,(T) is the diffusion coeflicient for CO in the system, A, is the cross-sectional area of the tube and A, is the cross-sectional area of the sample dosing hole. The input of 0 is given by: dNo = 20, ( T)A, [nz - $,l/(11 - 2,,) + Do (0% dt

[6

- &l/l,

(3

where D,(T) is the temperature-dependent diffusion coeficient for 0 in the system. For steady-state conditions, the equality: dNco ---=dt

dNo dt

(6)

holds [where the left hand side of the equation is the summation of Eqns (3) and (4)], from which the concentration of CO in the center of the tube will be given by: (7) Two limiting cases arise: Case 1, at high temperature: [2&/(/, - 1,~)+

4,lk,1l(‘b (7’V,&) e 44, (T)

hence,

(8)

ngo = n@oVY&o(T). Case 2, at low temperature:

[2Ac/(l, - kr) + A,,/[, I/(J’Q,V’V&

%-A,Do (T)

hence, n:, =

4 4tT

: co 2&-o t 7’)C2AclG - Lr) + A,%! n ’ (”

(9)

According to a simple Arrhenius treatment: ko( T) = A exp ( - KW’)

(10)

where A is the frequency factor and E. the activation energy for the reaction. It is to be noted that the general behavior of the diffusion controlled input and loss to and from the tube is very similar for tubes with and without a sample dosing hole. At high temperature, Eqn (8) shows that the concentration of CO in the furnace is dependent upon the ambient concentration of oxygen and the ratio of diffusion coefficients for oxygen and CO and will therefore exhibit little temperature dependence. At lower temperatures, where the surface oxidation is reaction rate controlled, the CO concentration is primarily dependent on the reaction rate constant and will show pronounced temperature dependence [cf. Eqn (lo)]. 3. EXPERIMENTAL 3.1. Apparatus A schematic of the experimental system is shown in Fig. 2. Measurements were made of the gas phase in a Perkin-Elmer model HGA-2200 graphite furnace. The sole modification to the furnace workhead was the replacement of the quartz end windows with identically-sized LiF windows and insertion of thin brass washers of 0.4 cm id. over these windows to optically isolate the central axis of the tube volume. Rotameter-type gas flowmeters were used to control the internal and external purge gas flows of Ar to the workhead. Conventional pyrolytic graphite-coated tubes obtained from Perkin-Elmer Corp., as well as glassy carbon and uncoated electrographite tubes, obtained from Ringsdorff-Werke GmbH (FRG), were studied. The latter tubes were supplied without dosing holes. These were added before use. The area of

424

R. E. STURGEONand H.

FALK

Source

Fig. 2. Schematic of experimental system. the resulting dosing hole in these tubes was 25 “, larger than those in the glassy carbon and pyrocoated tubes. The furnace was coupled to the entrance slit assembly of a McPherson model 225 1m VW scanning monochromator and to the output of a demountable Cu hollow cathode lamp radiation source via Arpurged guides. The LiF biconvex lenses located in these guides provided a vacuum tight enclosure for the lamp and slit assembly while permitting the furnace to be operated at ambient pressure. The furnace and lamp were rigidly positioned on an optical rail with the LiF lenses providing a parallel beam of radiation at 150 nm which was directed through the furnace. The entrance slit of the monochromator was located in the focal plane of the right hand lens while the slit-shaped diaphragm of the lamp (cf. Fig. 3) lay in the focal plane of the left hand LiF lens. The entire system was initially aligned by illuminating O-ring

,

h\

LiF-lens

Diaphragm

umping tube

Fig. 3. Cross-section of hollow-cathode discharge lamp.

CO in a graphite furnace

425

the spectrometer from the exit slit using a He-Ne laser while the grating was in the zero order position. A schematic of the demountable hollow cathode lamp is shown in Fig. 3. The unique feature of this lamp, previously described by BECKER-ROSSand FALK [18], is the provision for admission of reactive gases directly into the hollow cathode discharge. For these experiments, Ar containing 10m3 atm CO was bled into the lamp (- 12 ml/min) in order to permit excitation ofa molecular spectrum of CO. The space between the hollow cathode diaphragm and the tirst LiF lens was continuously purged with Ar (- 20 ml/min) to prevent severe self-absorption of the resulting emission bands. The lamp was evacuated to _ 1.4 torr using a single stage rotary pump and a stable discharge established at typically 800 V d.c. and 250 mA using a Sorensen DCR 1500 watt high voltage power supply operating in the current regulated mode. A 250 R ceramic load resistor was placed in series with the lamp. The monochromator was fitted with a 600 line/mm MgF,-coated grating blazed at 200 nm giving a first order reciprocal linear dispersion of 1.66 nm/mm. Entrance and exit slits were set for 25 pm, resulting in a bandpass of _ 0.04 nm. The monochromator was typically operated at a pressure < 1 x 10m6 torr. A simple dc. detection system was assembled in which the output from an EMI 50 mm end-on type 62558 photomultiplier tube with sodium salycylate coating was fed to a model 417 high-speed Kiethly picoammeter. The emission signal was recorded on a Gould model OS4100 digital storage oscilloscope, the time base of which was triggered by a voltage pulse from the furnace power supply at the commencement of the atomization cycle. Signals were subsequently transferred to a strip chart recorder for evaluation. The response time of the detection system (10-90 Id) was estimated to be _ 50 ms. Temperature-time measurements of the interior tube surfaces were made by sighting a calibrated automatic optical pyrometer, series 1100 (Ircon Inc., Niles, IL), through the sample dosing hole. Blackbody conditions were assumed. Relative longitudinal temperature-time protiles of the exterior surface of the tube were also made by replacing the standard tube contact rings by a set of extended rings which increased the separation between the metal cooling jackets. These contact rings were drilled with four 2 mm diameter holes at distances of 1.0,3.5,6.0 and 10.0 mm from the center of the furnace tube. These holes provided viewing ports for making temperature measurements. 3.2. Procedure Emission lines from the source lamp lying in the range 14&170 nm were examined and the CO(0,0) and CO(l,2) bands at 154.3 nm and 163.1 nm, respectively, were selected for study. Under optimum conditions, a dark current of 0.2 nA and a signal current of 7.5 nA were recorded at the CO(0,0) band for a PMT voltage of 730 V. Calibration curves for CO and O3 were constructed at room temperature for the 154.3 and 163.1 nm bands using standards for each gas prepared by volumetric dilution in Ar, as described earlier [13]. In a similar manner, a number of other gases (i.e. N,, CH,, C,H,, CzH4, H,O and CO,) were studied as potential interferents. All gaseous mixtures were introduced into the furnace using a 200 ml syringe via a silicone septum port temporarily located in the internal purge gas line for this purpose. An absorption path length of 7.4 cm was obtained, this being the distance between the LiF end windows on the furnace. Absorption-time measurements at both CO bands were recorded during the ramp heating of the dry, unloaded furnace tubes. External Ar sheath gas was maintained at 1 I/min for these studies whereas the internal Ar purge gas flow rate was varied between 0 and 400 ml/mm Based on a knowledge of the release energetics for CO from the tube surface (computed from an Arrhenius plot), the effective path length for absorption (based on a convolution of the longitudinal temperature profile of the tube with the release energies), the Boltzmann factor for the lower level of the transition and the experimental absorption coefficient, the absorption-time data were converted to partial pressure of CO-time results. 3.3. Data manipulation Absolute values of PC0 were derived from the absorbance-time 154.3 nm band, according to the relationship given in Eqn (11):

(Pco),=

Ar

4L*l~~elT),

measurements made at the (0,O)

(11)

where A, is the measured absorbance at time t, k is the experimental absorptioncoetlicient (atm- ’ cm-‘) and BF,,is the effective Boltzmann factor defining the relative population of the lower level (v” = 0) of the absorption transition, i.e. [19]:

w (-E,n=o/W

~&!!LcJ=, N 101

I+

i i=O

exp(-E,.=JkT)

R. E. STURGEON and H. FALK

4-Izo -l=i

t T

At

(Pcolt=

Wfj-ej-y-J,

I/t --

Boltzmann Factor (BF):

BF=

N,QJ

exp(-E,,+

-

/RT)

z

N tot

l;+OeXp(-E,-i

/FIT)

BF

Relative CO Generation: exp(-E,

( pco)IQ

/FiTI

)l(BF,ff)l

exp(-E,/RT1=,)l(BFeff)I=o

t-o-

1 Fig. 4. Schematic outline of data treatment.

For convenience, only the first five vibrational levels were taken into consideration in calculating the vibrational partition function. E,. values were taken from a compilation by KRUPENIE [20]. As time is a surrogate for temperature in the graphite furnace, the absorption coefficient, k, is temperature dependent in that its experimental value, obtained through room temperature calibration, changes with the density of the CO and for these purposes ideal gas corrections (i.e. T,,/300) were applied to the measured absorbance data. Changes in the value of k due to changes in the line shape factor with temperature (e.g. Doppler broadening) were not taken into account. Although this will introduce a small systematic error into the PC0 data, it will not influence the conclusions drawn from the measurements. Data treatment is schematically illustrated with the aid of Fig. 4. Longitudinal temperature-time profiles of the tube were constructed for a given heating regime. The effective Boltzmann factor was then computed at each increment of time, ti, according to the longitudinal temperature distribution in the tube at that time. BF,, was defined as the ordinate values of BF which bisected the area under a plot of BF vs 1.

The effective length of the absorption volume of CO was calculated with the aid of Eqn (13):

(“‘)’

exp( - 4IRWW,d, Oc exp( - EJRT, _ o)/(BF,,),

_o

(13)

for which it was assumed that the rate of production of CO along the length of the tube was only dependent on the temperature at that position and could be characterized by a release energy, E,, which was experimentally determined (cf. Eqn 4.5). A plot of the relative P,, as a function of 1 at any point in time enabled leRto be calculated as the ordinate value at which the areas A,, and A,, shown in Fig. 4, were equal. The parameter (T,,), was similarly calculated as the ordinate value of Twhich bisected the area under a plot of T vs 1 in the interval of Ietr. 4. RESULTSAND DISCUSSION An emission spectrum from the lamp, obtained 140-170 nm, is shown in Fig. 5. A spectral bandpass

by scanning the wavelength interval of 0.025 nm was used. The CO fourth

CO in a graphite furnace

-

427

WAVELENGTH

Fig. 5. Emission spectrum from hollow-cathode source.

positive band system, ascribed to an A’II-X’C transition [21], is evident with the bands being degraded to the long wavelength side. This emission spectrum is, with the exception of band intensity ratios, similar to the absorption spectrum recorded for CO in this wavelength region [21]. In addition to CO transitions, an intense CI line is seen as are two NI impurity lines. The presence of the former line indicates that decomposition of CO is taking place in the source. Essentially the same spectrum could be realized daily with variations in the intensity ratios with discharge conditions. 4.1 Figures of merit Calibration curves prepared with CO at room temperature using the CO(0,0) 154.3 nm band were linear to 1.1 absorbance, corresponding to an upper limit of 1.5 x 10h4 atm CO. Detection limit, calculated from three times the standard deviation of the signal from 4 x 10-s atm CO, was estimated to be 1 x 10m6 atm for an absorption path length of 7.4 cm. This figure will naturally be degraded to 4-5 x 1O-6 atm as the absorption path length is decreased to reflect the dimensions of the hot zone of the furnace tube. This detection limit is comparable to that for non-dispersive i.r. at 4.7 pm using a 20 m path cell. The sensitivity of the measurement, defined from the (inverse) slope of the calibration curve, is 6 x lo-’ atm CO/O.0044 A.U. The experimental absorption coefficient is 2.2 x lo3 atm-‘cm-‘. This value is approximately 3-fold greater than that reported by MYER and SAMSON[21], who used similar instrument resolution, but an S-fold lower estimate of the integrated absorption coefficient of this (0,O) band [22]. Unless the instrumental bandpass is less than the width of the spectral features, calculated absorption coefficients represent lower limits to the true values. Judging from the molecular dissociation in the source and the normal hollow cathode operating temperatures, usually reported near lOOOK [23], the width of the emission line is not intinitely narrow compared to that of the room temperature adsorption line and decreased sensitivity results. Absorption by CO at the 163.1 nm (1,2) band was not detected either at room temperature nor when 2 x 10m3 atm CO was introduced into the furnace with the latter at 2000 K. 4.2. Interferences As with many spectroscopic techniques, interferences from other species which may be present in the furnace can be expected. Relative to CO, the measured absorbance by several gaseous species is summarized in Table 1. Absorption by HNO, and NO, vapors were calculated from literature values of their absorption coeficients [24] rather than being experimentally measured. Many of these gases have been identified as components in the

R. E. STURGEON and H. FALK

428

Table

1. Interferences

Gas

at CO (0,O)

Relative absorbance

co

1.000 0.000 0.000 0.002 0.007 0.01s 0.019 0.062 0.109 0.145

N, CH, C,H, C,H, Hz0 CO, *HN03 0, *NO, *Calculated.

high-temperature effluent gas from a graphite furnace [13] and others, such as HNO, and NO,, are expected to be present during the decomposition of samples acidified with HNO,. It is apparent that, even if these gases were present in the furnace at concentrations equal to those of CO, only 0, is likely to cause a significant interference in a dry, heated tube. 4.3. Atmospheric ingress The response to 0, was of interest and was measured at both the CO(0, 0) and (1,2) transitions where, in agreement with literature values of the absorption coefficients [24], 3fold greater signals were obtained at the (0,O) band which had an (inverse) sensitivity of 6 x 1o-6 atm/0.0044 A.U. Calibration was linear to 5 x 10e4 atm 0, for the (0,O) band. These figures are based on a 7.4 cm absorption path length. One of the more noteworthy controversies concerning 0, in the graphite furnace is its source. Most researchers assume that the major source of 0, is the (Ar) purge gas [3-51 which may typically contain l-5 x 10e6 atm O2 impurity. Other possible sources include permeation of 0, from the ambient atmosphere through the plastic hoses conducting the purge gas to the furnace workhead. Figure 6 presents room temperature results for the absorbance by air (primarily due to 0,), diffusing into the optical path of the furnace tube, as a function of the flow rate of internal Ar purge gas. The external Ar purge gas was maintained at 1 l/min. An internal flow rate of

FLOW RATE, ml/min

Fig. 6. Absorbance signal due to ingress of air through sample dosing hole vs internal purge gas flow.--1 I/min external Ar flow; - - - - no external Ar flow; absorbance scale reduced l&fold.

CO in a graphite furnace

429

400 ml/mm was arbitrarily selected as the reference zero absorption level from which the absorbance signals at other flow rates were calculated. With each change of flow rate, a 10 s ingress period was permitted prior to measurement. This time period was particularly important as the internal flow rate was reduced. At stop flow, the continuous ingress of air eventually resulted in complete occultation of the beam. Simple order-of-magnitude calculation suggests that atmospheric ingress is the major source of O2 within the unloaded furnace. Assuming the entire path length (7.4 cm) to be uniformly filled with 0, at the end of a 10s ingress period and that complete conversion to CO occurs at a temperature of 2200 K, the PC0 in the furnace would be approximately 2 .x 10-3 atm at this temperature. Experimentally, the Pc, is found to be 1 x 1O-3 atm at 2200 K (cf. Section 4.4). The latter value is based on a 7 s ingress period at stop flow. Changing, by a factor of 5, the length of tygon tubing supplying the internal and external sheath gas to the furnace workhead produced no measurable difference in the rate of ingress of 0, into the furnace, suggesting that the major point of entry of 0, is through the sample dosing hole. This conclusion is supported by the spatially resolved absorbance-time measurements made by RAYSON and HOLCOMBE during a study of the atomization of Sn [25]. A steep gradient in absorbance (i.e. Sn atom density) on passing from the bottom of the furnace tube (a CRA-90 device) where the sample is deposited to the top where atomic Sn disappears due to rapid recombination with ingressing 0, is ascribed to this effect. Based on measurements made in this study, the 1 I/min flow of Ar external purge gas is > 99.9 ‘;/,efticient in preventing the diffusion of 0, into the furnace tube during a 10 s interval at 0 ml/min internal purge gas flow. The dashed line in Fig. 6 shows data for the ingress of air into the tube in the absence of an external purge gas. It is to be noted that the scale is reduced by a factor of 10 for these data. It is evident from these observations that the furnace is not a closed system and should not be treated as such in thermodynamically modelling thechange in Po, with temperature [5,9]. 4.4. PC0 USt characteristics Figure 7 shows the change in PC0 with temperature over the course of a 5 s atomization cycle for the heating of dry unloaded tubes of electrographite, pyrolytic graphite-coated electrographite and glassy carbon. The temperature-time profiles at the centres of the tubes are given as the accompanying dashed lines. Similar ashing temperatures, rates of heating and hnal temperatures were used in all cases and were, in the interests ofcomparison, kept relatively low, as dictated by the low rates of heating which could be attained with the glassy carbon tubes [26]. A 25 s ashing stage was used, the last 7 s of which the flow rate was reduced from 300 ml/min internal purge gas to zero ml/min.

0.0

1.0

2.0

3.0

4.0

5.0

TIME, s

Fig. 7 Dynamic change in PC0 with time during 5 s atomization with internal gas stop. Temperature profile at tube center: - - - glassy carbon; -. pyrocoated tube, - - - - uncoated tube.

430

R.E. STURGEON and H. FALK

For each tube, a rapid rise in PC0 is noted as the temperature is increased and a steady-state P,, is achieved as the central tube temperature reaches a steady-state. Qualitatively, these observations are in agreement with the model which predicts that a steady-state will be achieved at sufficiently high temperature wherein the rates of diffusion of O2 in and CO out of the furnace are balanced. If the internal purge gas flow is maintained, the absorption by CO decreases, to a first approximation, inversely as the flow rate. However, the relative absorbance at steady-state temperature decreases more slowly with flow rate in an uncoated tube than in a coated or glassy carbon tube. Quantitation of the PC0 in these tubes in the presence of a gas flow was not attempted due to difficulties in estimating an absorption path length. Decrease in the absorption signal may be related to a number of factors, foremost being a decrease in the rate of ingress of 0, into the tube, a decrease in the residence time and hence extent of reaction as well as possible decreases in the gas temperature and absorption path length with increased gas how. Oxidation of graphite originates with the dissociative chemisorption of 0, onto the surface at specific active sites. Formation of adsorbed CO is generally accepted to be the rate limiting step [27]. The rate of oxidation of graphite may vary by lOOO-fold,depending on the state of the carbon surface [28]. It is well recognized that the high porosity surfaces and reactivity of electrographite can be considerably decreased when a layer of dense, pyrolytic graphite is deposited over this substrate. The (low temperature) chemical reactivity and porosity are reduced and resistance to oxidation is further improved with glassy carbon. This material exhibits freedom from discontinuities at crystallite boundaries that probably account for its inertness [29]. The density of active surface sites on these materials, and hence their reactivity toward oxygen, would be expected to increase on going from glassy carbon to pyrolytic graphite to electrographite [6,30], This trend is reflected in the data shown in Fig. 7. Detectable CO formation begins at 1000, 1200 and 1400 K for uncoated, coated and glassy carbon tubes, respectively. This is in accord with the characteristics of thermal desorption of surface oxides from graphite, which is known to be essentially complete only at temperature above 1200 K [31,32]. Quantitatively, the PC0 in the uncoated tube > coated > glassy tube. The absolute magnitude of the PC0 obtained for a coated tube in this study is in good agreement with values reported earlier [ 131 using gas chromatographic detection of CO. In this latter instance, a PC0 of 1.5 x 10m3 atm was obtained using a T-tube furnace at a steady-state temperature of 2300 K and an internal purge gas flow of 25 ml/min Ar. This compares favorably with the 1.I x 1o-3 atm reported here. This PC0 value is also in reasonable agreement with earlier estimates based on the calculated ingress of 0, into the tube. The greater quantity of CO evolved during heating of the uncoated tube may reflect not only an enhanced reaction rate for oxidation, but the possible permeation of 0, (as CO) directly through the hot, porous wall of this tube. This process is precluded with the coated and glassy tubes because they are essentially impermeable to gases. Figures 8-10 illustrate the change in P, with temperature for each tube with internal purge gas stop and with various initial rates of heating. A given line of data thus describes the Pco(T) during a typical atomization cycle where the temperature is that measured at the center of the tube. For the coated and glassy carbon tubes, the limited rate of heterogeneous oxidation is reflected in the decreased P,,(T) as the rate of heating is increased. Above 1500 K, the oxidation characteristics of glassy carbon are nearly identical to those of pyrolytic graphite coated tubes. Although this observation contrasts with the extremely high corrosion resistance often attributed to glassy carbon [33], this characteristic is based only on experience gathered at relatively low temperatures where this material is used for wet and melt decompositions. SCHLEMMER and WELZ [34,35] have earlier alluded to the increased high-temperature reactivity of glassy carbon while its resistance to oxidation at lower temperatures is clear from studies devoted to the effect of elevated PO, on the signals for volatile elements [lo]. Data for the uncoated tube (Fig. 10) reveal that, over a similar range of heating rates, the PC0 is apparently not influenced by the initial rate of heating but determined only by the

CO in a graphite

1000

1500

431

furnace

2000

2500

TEMPERATURE, K Fig. 8. Dynamic

change

in PC0 with central tube temperature (K SC’) of a pyrocoated tube. Internal

1000

Fig. 9. Dynamic

change

for various gas stop.

2000 1500 TEMF’ERATURE, K

initial rates of heating

2500

in PC0 with central tube temperature for various (K s- ‘) of a glassy carbon tube. Internal gas stop.

initial rates of heating

surface temperature. Apart from the immediate conclusion that the oxidation reaction must be significantly faster for this tube, several other factors merit consideration. The greater density of surface active sites associated with electrographite permits chemisorption of a significantly larger amount of oxygen onto the tube surface during the 7 s period prior to the atomization cycle when the tube temperature is 760 K and the internal purge gas flow is arrested. Additionally, the high permeability of, electrographite (0.1-10 cm2 s-r for He) compared to pyrolytic and glassy carbon (lo- ’ ’ cm2 s-r) [36] permits a significant amount of CO to enter the graphite tube at high temperatures. Rapid desorption of substantial CO from the tube surface at 1000 K, coupled with the influx of CO via 0, permeation through the wall, may mask the reaction rate (limited) processes which occur at lower temperatures (< 1400 K)and suggest that the oxidation reaction is sufficiently fast at T > 1000 K as to be

432

R. E. STURGEON and H. FALK 2.0-

1.5 m z : % a

IO-

8

0.5-

0.0’

cd=

1000

I

1500

I

2000

I

2500

TEMPERATURE, K Fig. 10. Dynamic

change

in PC0 with central tube temperature (K s-‘) of an uncoated tube. Internal

for various gas stop.

initial rates of heating

only under diffusion control and hence, independent of the initial rate of heating. Data presented in Section 4.5 will clearly show that this is not the case and that the oxidation reaction is under kinetic control at T < 1400 K. 4.5. Energetics of C-O, reaction Equations (8b(lO) predict that, provided a sufficient temperature range is studied, an Arrhenius treatment of the Pco(T) data will yield two distinct regions; a low temperature kinetic region where the rate of reaction is determined by the reactivity of the surface-a process with a significant temperature dependence, and a high temperature region in which the P,, is nearly independent of temperature because the reaction rate is sufficiently fast that the oxidation becomes diffusion controlled. Figure 11 presents an Arrhenius treatment of the P,, data obtained for each type of tube. Temperatures are those measured at the centre of the tube at the end of a 5 s heating cycle while the P,, data are the corresponding steady-state P,, values measured at the end of the

Fig. 11. Arrhenius

plot of 5 s steady-state PC0 data. Internal gas stop. 0 uncoated tube, A glassy carbon tube.

tube, 0 pyrocoated

CO in a graphite furnace

433

cycle. These data were derived from experiments with a least three tubes of each type obtained on at least three occasions over the course of a 6 month study. Error bars represent one standard deviation of the mean. The two temperature regions, corresponding to kinetic and “diffusion” regions, clearly manifest themselves in this plot. It is also readily apparent that, within experimental error, the reactivity of the glassy carbon tube is approximately equal to the coated tube at T > 1500 K. The uncoated tube is apparently much more reactive than the others, as evidenced both by the elevated levels of Pco( T) as well as the fact that the “diffusion” control regime in this tube is established at 1470 K; 400” lower than for glassy carbon and 500” lower than the coated tube. Apparent activation energies derived from the slopes of these lines are summarized in Table 2. These values are apparent because varying diffusion effects have not been taken into account. Essentially the same values are evident for all three tubes, indicating that the mechanism of oxidation is the same for each type of graphite. The heterogeneous reaction between O2 and graphite involves the transport of O2 to the surface, chemisorption onto active sites (with physisorption of a mobile layer on the a-b plane), desorption of CO and transport of this product away from the surface. This description does not account for any secondary homogeneous (i.e. CO + O2 --t COz) and heterogeneous (i.e. COz + C + CO) reactions which may also occur simultaneously. Thus, the factors likely to control the reaction rate are diffusion or chemical reactivity, depending on which is the slower process. The general rate expression for the overall reaction is given by Eqns (3) and (10). At low temperatures the Arrhenius energy can be considered as rate controlling. From a mechanistic viewpoint, the reaction surface may be almost completely saturated by the oxide complex (A, -+ 0). Thus, decomposition of this complex (i.e. desorption of CO) may be the rate limiting step in the overall reaction. Because desorption of surface oxides is usually considered complete at temperatures > 1200 K [31,32] and since data presented here are, with the exception of a single point, collected above 12OOK,then the carbon surface must be considered to be free of oxide complex as desorption occurs almost instantaneously after adsorption. The rate limiting step then becomes chemisorption of O2 onto active surface sites. KELEMENand FREUND[37] have studied the energy barrier for dissociative adsorption of O2 onto glassy carbon and reported a value of 13 kcal/mol at 600 K and with a surface coverage, 0, of 0.1 (i.e. 1 oxygen atom per 10 surface carbon atoms). This barrier increases with I3and temperature. The apparent activation energy of 19 kcal/mol obtained in this study may reflect this process. At sufficiently high temperatures, the supply of O2 becomes diffusion limited and the temperature dependence of the reaction rate diminishes. As both diffusion rates and gas viscosity increase with temperature, the net effect should make the reaction rate almost independent of temperature. Equation (8) describes this condition. Apparent activation energies of 4 kcal/mol measured at high temperature should not strictly be interpreted as activation energies for diffusion, but rather the net effect of an unbalanced temperature coefficient in the (Do/Dco) ratio, in keeping with the relationship: 5 s atomization

/ I- \n

D=D'+

.

(14)

()

Table

2. Apparent activation kcal/mol

energies,

Regime Tube

kinetic

“diffusion”

uncoated coated glassy

1s+2 20*2 19+2

4_+1 5*1 4*1

434

R.E. STURGEON

and H. FALK

4.4. Estimation of PO,

The establishment of the diffusion control region at high temperature does not necessarily imply that a state of equilibrium has been attained. Earlier experiments with simultaneous GC detection of CO and COz in the high temperature Ar effluent from a pyrocoated tube suggested that heterogeneous equilibrium may have been reached only at T > 2200 K for an internal purge gas flow of 25 ml/min [13]. It was clear from these results that this “equilibrium” temperature was dependent on internal purge gas flow rate and thus, at stop flow, the inflection point at 2040 K for coated tubes shown in Fig. 11 may signal the onset of equilibrium in this tube. Given the above, it follows that there is excess free oxygen (i.e. above equilibrium values) in the tube in the kinetic region, i.e. at T < 2000 K, that has not been consumed by reaction. Qualitatively, this conclusion is in agreement with earlier results of FRECH et al. [S], Lvov [16] and STURGEON et al. [6]. PINGXIN and TIEZHENG [15] argue that air ingressing through the sample dosing hole cannot influence the P o2 in the tube because it must traverse the incandescent wall, whereupon it will be reduced. This argument is based on the assumption that the system is at equilibrium. The reactivity of the exterior surface of the tube cannot be expected to be significantly different from that of the interior and thus, it is clear that for T < 2000 K the kinetically limited heterogeneous reaction will not result in complete conversion of ingressing O2 during its transit through the dosing hole. Reasonable agreement between PC0 data obtained in this study (Fig. 11) and that reported earlier using GC detection (Fig. 3, Ref. [ 111)suggests that the Pco, data cogenerated in that study may be used to quantitate the PO, in the furnace in the kinetic regime [16]. Homogeneous equilibrium is likely established in the gas phase at T > 1500 K and thus, following LVOV[16], reactions 15-17 may be used to calculate a (PO,),:

co+o*co~ 0+0+02 (PO,), = PO, + 0.5 PO.

(15)

(16) (17)

Calculated values of (PoJr range from 6 x lo- lo to 5 x 1O-8 atm. over the temperature interval 170~2000 K, respectively. These values are higher than those reported by FRECH et al. but lower than those of LYOV[16]. A probable reason for the discrepancy with Lvov’s [ 1l] values is that dry, unloaded furnace tubes are used here as opposed to sample (i.e. Sn) loaded tubes in which excess oxidants may be released during decomposition of the sample matrix (cf. Section 4.7 and Ref. [37]). (P o2) z values reported by FRECH et al. [8] are lower than those calculated above because these authors based their measurements on work with uncoated tubes [39]. Furthermore, it should be noted that the PO, scale calculated by FRECH et al. [S] was derived on the basis of reactions taking place at the tube surface and therefore reflect conditions in the vicinity of the surface rather than in the bulk gas phase. With purge gas stop, heterogeneous equilibrium appears to be established at T > 21OOK, thereby permitting calculation of (Po2)z from reactions 16-18: c,,, + 02 + 2co.

(18)

In the temperature interval 210&2600 K the calculated (PO,), varies from 7.2 x lo-i5 to 4.8 x lo- l3 atm, respectively. The (Po2)z may be higher than this when samples are atomized

WI.

Recently, L,VOV[ 161 utilized the (Pco,/Pco) data reported by STURGEON et al. [ 131 to calculate the (PoJz at temperatures in excess of23OOK in a T-shaped pyrocoated tube having an internal Ar flow of 25 ml/mm. Without accounting for the PO, measurement blank, (Po,)r values of lo-’ atm were derived. These, however are in error at 2620 K [16] because, at this temperature, the blank is comparable to the measured total Pcox. The presence of an internal purge gas in the above system resulted in an apparent system equilibrium at T > 2200 K- and thus (PO,), values, as calculated by L’vov [16], should reflect reality in that system at T < 2200K.

CO in a graphite

435

furnace

4.7. Atomization of samples The simplest sample that can be introduced into the furnace is deionized, distilled water (DDW). However, a 1% (v/v) solution of HN03 is most commonly used. Figure 12 shows absorbance signals at the CO(0, 0) 154.3 nm band obtained during the “atomization” of 20 ~1 volumes of DDW and 1% HN03 introduced into an uncoated electrographite tube. Absorbance is given in this figure rather than I’,-, because of the uncertainties in estimating an absorption path length. Internal purge gas stop was used. Various pretreatment temperatures were studied, ranging from 780-1410 K for periods of 20 s with a 300 ml/mm internal Ar purge flow. In all cases the tube was then briefly cooled and recycled through the single temperature ramp (78&2260 K) shown in Fig. 12. For this purpose a further 25 s, 780K “ashing” step was used, the last 7 s of which the internal purge gas was arrested. A sharp, early increase in the absorbance signal is produced in the presence of HNOJ, reflecting the extensive chemical oxidation of the graphite surface. In agreement with results of FRECHand CEDERGREN [40], it is seen that the graphite tenaciously retains water and decomposition products of HN03. Absorbance signals from the sample are elevated over that from the dry tube following pretreatment temperatures as high as 1210 K. It is probable that all traces of these samples are removed from the tube at higher temperatures since, above 1200 K it is generally accepted that all surface oxides are desorbed from graphite [31,32]. No traces of free oxidants in the gas phase could be detected at the CO( 1,2) 163.1 nm band. Similar increases in the absorbance signals at the CO(O,O) band were seen during atomization of 20 ~1 volumes of 1 ‘x HN03 in pyrolytic graphite coated tubes although the signal enhancements were not as dramatic as for the uncoated tube as a result of less extensive soaking of the aqueous phase into this surface. Figure 13 presents signals obtained during the atomization of 20 pg masses of Mg and Ni, introduced into a pyrolytic graphite coated tube with a 20 ~1 volume of 1% HN03. The 20 pg masses used arecommon for these two elements when they are employed as matrix modifiers or in those circumstances where they may be present as concomitants in complex samples, particularly Mg in dissolved sediments and biological tissues or samples of saline water. Signals were registered at both the CO(0, 0) and CO(l, 2) bands. Those obtained at the (1,2) band reflect absorption by oxidants in the system, which may include H20, HN03, 02, NOz and NO released as decomposition products from the samples [41]. Signals at the (0,O) band reflect absorption by both oxidants and CO released from the surface. At low temperatures (80&l 100 K) an initial rapid release of oxidants occurs, reflecting decomposition of the samples. Absorbance peaks in this region were found to be proportional to the mass of element atomized in the range l-20 pg. These signals appear to decay by simple diffusion but reduction to CO and CO1 at higher temperatures obviously compliments this removal process. The significant point of this observation is that oxidants

0.0

1.0

2.0

3.0

4.0

5.0

TIME, s

Fig. 12. Dynamic absorbance by CO at 154.3 nm in an uncoated tube. Internal purge gas stop. 20 ~1 injections of H,O or 1 TL (v/v) HNO, with various thermal pretreatment temperatures. - - - Temperature protile at tube center.

R.E. STURGEON and H. FALK

0.6 0.4 0.2 0.0 2400

Y

0.8

2000

g

0.6

1600

$ i-r

0.4 0.2

TIME, s Fig. 13. Dynamic absorbance signals generated by sample atomization in a pyrocoated tube. Internal gas stop. ---Temperature profile at tube center. 20 pg Mg, 0 720 K ashing temperature; 0 20 p’g Mg, 1080 K ash; A 20 pg Ni, 780 K ash; A 20 pg Ni, 1080 K ash; -20 11 1% HNO,.

generated reduction

during sample matrix decomposition evolve into the gas phase without complete by the graphite. This same conclusion was reached earlier by DROESSLER and HOLCOMBE [ 141 who spectroscopically detected release of free O2 during decomposition of 1 c(g amounts of Ni(NO,), in a CRA-90 atomizer. Additionally, FRECH and CEDERGREN [42] have suggested matrix decomposition as a possible source of excess 0, within the furnace. Since active sites on the tube surface are likely blocked by chemisorbed species at temperatures below lOOCL1200 K, little oxidant reduction can be expected in this temperature interval or earlier than 1 s. Significant amounts of oxidants appear to persist to temperatures as high as 2000 K. It is perhaps coincidental that this is the temperature at which the oxidation rate of a pyrolytic graphite-coated tube transforms from the kinetic to the “diffusion” regime. At high temperature, substantial amounts of oxidants probably exist only at the cooler extremities of the tube where they decay by condensation onto cool surfaces and by back diffusion into the hot reaction zone at the center of the tube. Most oxidants can be eliminated by ashing the sample at 1110 K, as can be seen by the decreased signal for Mg at the (1,2) band and the complete removal of the Ni signal following a 1080 K thermal pretreatment. However, there are several elements which cannot tolerate such high pretreatment temperatures and, when large amounts of concomitant Mg is present, may suffer gas phase interference effects due to the release of excess oxidants. These signals were generated using a relatively low heating rate of 500 K s- ‘. With maximum power heating at 2000 K s-l, oxidant release is expected to be more rapid and persist to high temperatures. Signals measured at the (0,O) band reflect the convolution of absorbance by both oxidants and CO. At c > 2.5 s the signals are produced primarily by CO only. It is noteworthy that the signal for Ni is partially resolved and reflects the early oxidant release process and decay as well as subsequent release of CO. This is more evident following an ashing of the sample at 1080 K. In this situation the oxidant release peak is not observed, the major matrix decomposition having been completed during thermal pretreatment. These observations are consistent with the sequence of reactions outlined below: 550 < T < 1000 K: Ni(NO,),

.6H,O(,) + NiO(,) + H,O + HNO,

+ NO, + NO + 0, (18)

T > 1080 K: NiO,,) + C,,, + Ni,,, + CO

(19)

T > 1590 K: NJ,, + Ni(,).

(20)

CO in a graphite furnace

431

5. CONCLUSIONS This study has demonstrated the utility of spectroscopically probing the furnace gas phase in the VUV region. The major source of CO in the unloaded furnace does not arise from reduction of 0, impurity in the Ar sheath gas but from the reduction of 0, diffusing into the furnace through the sample dosing hole. The furnace is thus an open system and should not be thermodynamically modelled otherwise. C-O, reactions are seen to become rapid only at T > 1200 K in a coated tube and are kinetically controlled by the reactivity of the surface at T < 2000 K. Kinetic control precludes establishment of heterogeneous equilibrium in the system at T < 2000 K during gas stop operation and leads to elevated levels of PO,. The nature of the graphite surface significantly affects reduction kinetics; uncoated tubes permit establishment of a “diffusion” control reaction regime (and presumably heterogeneous equilibrium) at T > 1470 K. Although glassy carbon is chemically resistant to oxidation at low temperatures, it becomes as reactive to 0, as pyrolytic graphite at T > 1500 K. This observation may account for the extensive low temperature pulse shifting of signals noted for volatile elements atomized in this tube in the presence of an elevated P o, [IO] as well as the elevated appearance temperatures measured for many elements atomized in pure Ar [lo]. The poor analytical performance (2-fold higher characteristic masses for many elements by both peak height and area measurement) of the glassy carbon tube [ 10,34,35] may be linked, at low temperatures, to a suppression of oxide dissociation due to elevated levels of PO, in the tube and, at high temperatures, to increased chemical reactivity coupled with a macroporous surface [36]. Excess oxidants are present in the furnace during atomization of samples and, in accord with conclusions drawn by PINGXINand TIEZHENG[ 151, will be the major source of (Po,k at T < 2000 K in the coated tube. Kinetically limited uptake of such pulse-released oxidants from the surface, particularly at lower temperatures, can affect appearance temperatures, shift absorption pulses, suppress oxide dissociation, etc. The amount of oxidant released will depend upon the nature and amount of sample (as well as concomitant matrix), the nature of the graphite surface (activated or blocked surface sites) and the thermal pretreatment temperature. Obviously, a thermal pretreatment temperature should be used which is as high as possible. Matrix modifiers, which are usually employed to achieve this, may exacerbate the problem of oxidant release. In particular, it should be obvious that Ni and Pd modifiers, whose oxyanion salts decompose at relatively low temperatures, are preferable to use of Mg salts (cf. Fig. 13). Release of oxidants during sample atomization may also account for the elevated PO, detected by Lvov and RYABCHUK [4] during the high temperature atomization of Sn. Acknowledgements-The authors thank K. HUBER (Herzberg Institute of Astrophysics) for use of the McPherson monochromator, B. HUTSCH (Ringsdorff-Werke FRG) for the glassy carbon tubes and G. GARDNER (NRC Chemistry) for preparation of standard gas samples. NRCC No. 28572.

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[l] [Z] [3] [4]

[S] [6] [7] [8] [9] [IO] [l l] [12] [I33

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