Mechanism of atom loss in graphite furnace atomic absorption

Loss through the sample injection port aperture accounts for approximately 20 % of thetotal losses whereas loss through the graphite walls contributes...
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Mechanism of Atom Loss in Graphite Furnace Atomic Absorption Spectrometry R. E. Sturgeon and C. L. Chakrabarti” Metal Ions Group, Department of Chemistry, Carleton University, Ottawa, Ontario, Canada K1S 5B6

The mechanism of loss of atomlc vapor from a Perkin-Elmer HGA-2100 graphite furnace varles with the physlcochemlcal properties of the analyte species, the heating rate and steady-state temperature of the atomizer, and the physicochemical properties of the surface of the furnace. The major factor contributing to loss of atomic vapor is diffusion to the cooler extremities of the furnace where condensation occurs. Loss through the sample injection port aperture accounts for approxlmately 20 % of the total losses whereas loss through the graphite walls contributes about 20 YO.

The sensitivity obtained with either the peak or the integration method of signal measurement in graphite furnace atomic absorption spectrometry is highly dependent on the rate of loss of atomic vapor from the analysis volume (1-3). Because the peak of these transient signals is attained when the rate of production of atomic vapor is equal to its rate of loss, a decrease in the rate of loss results in an increase in the peak amplitude (2,3). Similarly, the integrated absorbance is inversely dependent on the rate of loss of atomic vapor from the analysis volume (1, pp 119, 220). By determining the various modes of loss of atomic vapor from the graphite furnace and the factors which influence them, measures based on theoretical grounds may be taken to improve vapor confinement, thereby enhancing the sensitivity of determination. This study was undertaken to determine the relative contribution of the individual modes of atomic vapor loss to the total rate of loss of atomic vapor from a Perkin-Elmer Heated Graphite Atomizer model 2100 (HGA-2100). Five major factors may be cited as contributors to the loss of atomic vapor from commercial graphite furnace atomizers: (a) physical expulsion of excess atomic vapor if the volume occupied by the atomic vapor at the operating pressure and temperature of the furnace is greater than the analysis volume of the furnace; (b) diffusion and convection of atomic vapor through the apertures of the atomizer; (c) diffusion of atomic vapor through the walls of the graphite tube; (d) diffusion of atomic vapor to the cooler extremities of the atomizer where condensation may occur, and (e) “chemical” loss of the atomic vapor by reaction with the hot graphite surface and/or by recombination with other gas phase species (e.g., entrained oxygen, sheath gases, etc.) to form involatile or undissociated species. The rate of loss of atoms from the analysis volume may be conveniently studied by examining the decay portion of the absorbance-time signal given by the atomic vapor. Assuming that loss of vapor occurs solely by diffusion, the rate of diffusion may be expressed as follows:

W / d t = -D(dp /dx)S

(1)

where dM/dt is the rate of diffusion of mass, dpldx is the density gradient in the direction of diffusion, S is the area of a (imaginary) plane across which diffusion occurs, and D is the diffusion coefficient. The amount of diffusion taking place through the end apertures of a tubular cuvette (I) of length 1100

ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977

1 and cross-sectional area S is given by:

Mt

= Moexp(-8D,t/12)

(2)

where M, is the mass of the vapor at an initial instant of time arbitrarily selected from the decay portion of the absorbance-time signal, Mt is the mass of the vapor a t time = t , and D, is the gas phase diffusion coefficient. Similarly, the amount of diffusion which occurs through the porous walls of the cuvette (1) is given by: where D, is the diffusion coefficient for mass transfer through the wall, S , is the internal area of the walls of the cuvette, t , the wall thickness, V the volume of the cuvette, and M, and Mt are as defined above. Since absorbance is proportional to M, a plot of log(A,/A,) vs. t should yield a straight line from the slope of which D, or D, may be obtained. The general application of Equation 1 to commercial graphite furnaces is complicated by the fact that the temperature of both the furnace and the atomic vapor is time-dependent ( 4 ) ,and that the decay portion (tail) of the absorption pulse may contain a contribution from the continuing atomization of the analyte, especially when refractory species are atomized (2, 3 , 5 ) . To circumvent both of these difficulties, it is necessary to select, as probes for the measurement of the rate of vapor loss, elements which are sufficiently involatile that the decay regions of their absorbance-time profiles occur over the time period in which the graphite furnace has attained a steady-state temperature. Mo and V are two such elements which serve this purpose. Figure 1 shows the absorbance-time profiles for these elements superimposed on the temperature-time profile of a pyrolytic-graphite-coated tube. Although the temperature of the atomic vapor is somewhat lower than that of the surface of the furnace, the atomic vapor has attained a steady-state temperature over the decay region of the absorbance pulse ( 4 ) . The difficulty of working with Mo and V is that the absorbance peak for these elements does not mark the point at which atomization has been completed (followed thereafter by simple diffusional decay), but, rather, that the absorbance peak marks only the point of balance between the rate of formation and loss of atoms (2,3,5). Consequently, although the rate of atom formation beyond the absorbance peak becomes less than the rate of atom loss, the tail of the absorbance pulse represents a condition during which the analysis volume may be supplied with a significant number of additional atoms which are produced by the continuing atomization of the element. This problem is minimized by using graphite tubes which are coated with pyrolytic graphite (5). Penetration of analyte into the surface of such coated tubes a t high temperature is reduced because of the low porosity of the pyrolytic graphite surface. Such coated tubes yield high sensitivity and sharp absorbance pulses with well-defined decay regions (5). Equation 1 may therefore be used to investigate the mechanism of loss of atomic vapor in commercial graphite furnaces provided elements such as Mo and V are used as

TIME,s

Figure 1. Absorbance-time characteristics of Mo and V superimposed on the temperature-time'profile of the HGA-2 100 furnace. Furnace tube is coated with pyrolytic graphite; heating rate is 0.91 K ms-'

Figure 2. Furnace mounted on its stainless steel baseplate analytical probes. Since the gas phase diffusion coefficient is inversely related to the ambient pressure of foreign gas, the influence of an applied pressure of an inert gas on the rate of loss of atomic vapor may be used to assess the importance of gas phase diffusional loss.

EXPERIMENTAL Instrumentation. To investigate the effects of an external pressure on the rate of loss of atomic vapor, a special graphite furnace fitted with a pressurizable housing was constructed in this laboratory. The furnace was nickel-plated, fabricated from brass, and was of a size and design similar to that of the Perkin-Elmer HGA-2100. The commercially available graphite cones, tubes, and quartz end windows were used in this furnace. Provision was also made for a system of external and internal sheath gases. The furnace has been designed to provide larger water cooling ports and channels, solid unit construction for improved electrical contact and is capable of handling 1000 A at 15 V. Figure 2 shows the furnace fitted on a stainless steel circular baseplate. Through this baseplate, which also serves as a mount on the optical rail, pass all of the service connections to and from the furnace. These include the electricalconnections,the internal and external sheath gas lines, cooling water inlet and outlet lines, a gas pressure line input, and a gas pressure release valve. Each of the gas lines is equipped with an external regulator valve. The baseplate has a flange which is approximately 2 cm wide and is grooved to accept an "0"-ring and 12 equally spaced hex-head screws, as shown in Figure 2. These fittings allow the furnace to be enclosed in a pressurizable chamber which can be attached to the baseplate by the screw-type mount. The stainless-steel pressure chamber, shown in Figure 3, is fitted with two demountable (in threaded receptacles)quartz windows (1cm thick), a detachable quartz observation window mounted at 90" to the

Figure 3. Furnace enclosed within its pressure chamber optical axis, and an injection port containing a quartz window through which the furnace may be sighted by a pyrometer during pressurized runs. The chamber is also fitted with a pressure gauge (calibrated in psi) and a safety release valve (arbitrarily set at 200 psi). The furnace was powered by the commercially available Perkin-Elmer HGA-2100 power supply. Unless stated otherwise, investigations carried out at atmospheric pressure were made with the commercial Perkin-Elmer HGA-2100 furnace. Both regular and pyrolytic-graphite-coatedtubes (the coated tubes were obtained from Ultra Carbon Corporation, Bay City, Mich.) were used. The optical pyrometer, integration control unit, sources, and detector-recorder system have been described in a previous paper (2). Reagents. Stock solutions of metal standards (1000 pg/mL) were prepared for Mo, V, Cd, Cu, Al, and Pb. Solutions of Cd and Pb were prepared from their carbonates, Cu and A1 from the metals and Mo and V from the trioxide and the pentoxide, respectively. All test solutions were made immediately prior to their use by dilution with ultrapure water obtained direct from a Milli-Q water system (Millipore Corporation, Mississauga, Ontario, Canada). Gases. High purity (99.95%)argon, helium, and nitrogen gases were used both to sheath the atomizer and to pressurize the atomization chamber. Procedure. The sequence of operations followed for sample atomization under pressure is described below. Sample volumes of 5 WLwere introduced through the injection port of the pressure chamber with an Eppendorf pipet fitted with disposable plastic tips. The sample was subjected t o the usual drying and ashing with both internal and external sheath gases flowing. The heating sequence was then interrupted before the ashing stage was completed and the flow of both sheath gases was stopped. The injection port was then closed and the furnace was flushed once with the pressurizing gas (directly from a cylinder of the compressed gas) and then pressurized to the desired pressure (reproducibly set to 1 2 psig). The heating cycle was reinitiated and absorbance measurements were taken. The gas pressure was then released through a separate valve, the chamber repressurized,and the blank signal obtained. This sequence of operations requires approximately 2 min for completion (including the blank). The furnace was operated in the internal purge gas interrupt mode during runs at atmospheric pressure. An external sheath gas flow rate of 1.0 L min-' was maintained for all gases. The internal purge gas flow rate was 0.08 L min-I. Absorbancetime profiies for each of the elements were recorded on a storage oscilloscope and photographed with a Polaroid camera. Plots of log(A,/A,) vs. t were constructed for Mo and V using the data obtained from the tails of their absorbance pulses.

RESULTS AND DISCUSSION Effect of Internal P u r g e Gas. The effect of internal purge gas on the signal given by 4 X 10-l' g of Cu is shown in Figure 4. The initial few hundred ms following the appearance of the Cu signal are unaffected by the internal purge gas (curve B) because the atoms produced during this period ANALYTICAL CHEMISTRY, VOL. 49, NO.

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Table I. Relative Values of the Diffusion Coefficient in Various Sheath Gases and Relative Values of the Peak and the Integrated Absorbance by Cu Presented in Figure 5 Gas u-, nm‘ Dl,2b A, JAtdt Helium 0.200 3.63 0.53 0.48 Nitrogen 0.316 1.01 0.88 0.86 Argon 0.286 1.00 1.00 1.00 ’Diameters of the sheath gas taken from Ref. 7. Relative diffusion coefficients for Cu calculated from Equation 4, assuming the temperature of each diffusion medium is equal at all points in time. Atomic diameter of Cu (0.254nm) taken from Ref. 8.

Time

Figure 4. Oscilloscopic trace showing the effect of an internal purge gas on the signal from 4 X lo-’’ g Cu. (A) Internal purge gas interrupt mode; (6)Internal purge on. Absorbance: O.P/scale division. Time: 500 ms/scale division

Figure 6. Plot of log (AJAJ vs. time for Cd and Cu In an Ar atmosphere (1 atm pressure)obtained with an uncoated graphite tube at the heating rate of 1.23 K ms-’. 0 Cd, 0 Cu (atomic mass units), T is the absolute temperature, p is the pressure and gl,z is the collision diameter for the two species which is given by: UlJ

Time Figure 5. Oscilloscopic trace showing the effect of various sheath gases on the signal from 4 X 10”’ Cu. Absorbance: 0.2lscale division. Time 500 ms/scale division. (A) Ar, (6)NP, (C) He

of time do not escape from the analysis volume. The increased rate of convectional loss decreases both the peak height and the width of curve B. Since the peak of the absorbance pulse marks the point of balance between the rates of formation and loss of atoms, an increase in the rate of loss both decreases the peak height and shifts the maximum to an earlier point in time. Effect of Various Sheath Gases. The effects of various sheath gases on the signal given by 4 x lo-’’ g Cu are shown in Figure 5. Ar, N2, and He sheath gases were each maintained at a flow rate of 1.0 L min-’ with an internal purge gas flow rate of 0.08 L min-l. The furnace was operated in the internal purge gas interrupt mode to eliminate induced convectional loss of atomic vapor from the analysis volume. Substitution of N2 or He for Ar reduces the peak and the integrated absorbances. The effect of each gas on the relative peak and integrated signals is shown in Table I, which also shows the relative values of the diffusion coefficient for Cu; the latter is calculated from Equation 4 (6):

D I ,=~2.66 X 10-22T3’2[(M1+ M2)/2M1M2]1’2/po~,, (4) where Dlr2(m2s-’)is the diffusion coefficient for species 1 in medium 2, MI and M 2 are the masses of the two species 1102

ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977

= l/z(ul

+ 02)

(5)

where u1 and u2 are the individual diameters of the colliding particles (m). Although the analytical results are highly dependent upon the diffusion medium, the relative diffusion coefficients given in Table I do not quantitatively reflect the trends in the peak and the integrated signals. A major factor responsible for this discrepancy is the large temperature coefficient which has not been accounted for. Variation of the flow rate of external sheath gas from 0.5 to 2.0 L min-l produced no change in the absorbance-time traces. There is therefore no tendency for the flow of external sheath gas past the sample injection port to draw atomic vapor out of the furnace. Measurement of Loss Rates. The relative rates of loss of atomic vapor from the analysis volume were investigated by plotting log (AJA,) vs. time. The point A, was arbitrarily selected from the decay portion of the absorbance-time profile of the element, with the single condition that it be placed beyond the absorbance maximum. The difficulty in using elements such as Cd and Cu as working probes with this technique is illustrated in Figure 6. Straight lines are not obtained for these elements because the temperature of both the atomic vapor and the diffusion medium is rapidly increasing over the decay regions of these signals (3);hence, D1,2 increases with time (Equation 4) causing an accelerated diffusional loss of atomic vapor. A linear relationship is obtained with Mo and V, however. Figure 7 is a plot of log (A,/A,) VI. time for these elements in a pyrolytic-graphitecoated tube using an Ar sheath gas at atmospheric pressure. Because the decay portion of their absorbance-time profiles lies in a region of steady-state temperature of the furnace

I

d

5.0

Figure 7. Plot of log (A,lAt) vs. time for Mo and V in an Ar atmosphere ( 1 atm pressure) obtained with a pyrolytic-graphite-coatedtube at the heating rate of 0.91 K ms-’. 0 V, 0 Mo

Figure 8. Plot of log ( A , l A t ) vs. time for Mo at various pressures of Ar obtained with a pyrolytic-graphite-coatedtube at the heating rate of 0.91 K ms-’. 0 1.0 atm, 0 3.0 atm, A 5.1 atm, 0 7.1 atm, A 9.2 atm

-0.8

( ~ 2 8 0 K, 0 Figure l),straight lines were obtained. Although this does not imply that the temperature of the atomic vapor is equal to that of the furnace wall, it does indicate that the temperature of the atomic vapor is approximately constant over the tail of the absorbance pulse. The slope of the plot is a measure of the rate of loss of atomic vapor by all processes. Because the volume occupied by the atomic vapor under these conditions is less than 1% of the volume of the furnace, the possibility of its expulsion from the furnace may be ignored. The remaining loss processes, diffusional and “chemical”, may each be considered as first-order-rate processes. Correspondingly, a linear relationship between log (A,/AJ and time is expected in either case, with the slope of the curve being governed by the predominant mechanism of loss. Figure 7 shows that in an Ar atmosphere, the rate of loss of V > Mo by 17% (slopes are approximately 0.13 and 0.11 s-l, respectively), which strongly suggests that the mechanism of loss is a gas phase diffusional process. The ratio of D1,2for V and Mo, calculated from Equation 4, is 1.16:l.OO (atomic diameters of V and Mo used in Equation 4 are 0.134 nm and 0.139 nm (9),respectively). The relative rates of loss of Mo atomic vapor in an Ar and N2 sheath gas were found to be approximately the same. The rate of loss in a He atmosphere could not be investigated as the signals obtained were too small; larger analyte mass, necessary to obtain a measurable signal, could not be employed because of incomplete atomization (and hence, memory effects). The slope of the log ( A , / A , ) vs. t plot (i.e., the rate of loss) was found to be independent of the mass of analyte taken for measurement provided that complete atomization occurs. This latter observation indicates that the loss follows first-order kinetics and is consistent with Equations 1-3. An estimate of the diffusion coefficient may be obtained by using Equation 2. The slope of the log ( A , / A , ) vs. t plot is related to D , by:

D, = (2.303 12)slope/8

(6)

from which DM,= 1 X m2 s-’ in Ar at atmospheric pressure. This calculation assumes that gas phase diffusion to the ends of the furnace is the only atomic vapor loss mechanism and that the effective length of the analysis volume is 5.5 cm (i.e., the distance between the quartz end windows). Although the exact temperature of the Mo vapor is unknown, 2500 K is not an unreasonable estimate (4, cf. Figure 1). The DM,calculated above is within a factor of 5 (smaller) of similar data reported by L’vov (1, p 288) for Zn in Ar (taking into account the mass difference between Mo and Zn).

I

I

0

01

I

02

I

03

I

04

I

05

I

06

I

07

08

I

09

,

1.0

LOG P

Figure 9. Effect of an applied pressure of Ar on the rate of loss of Mo atomic vapor from a pyrolytic-graphite-coatedtube at the heating rate of 0.91 K ms-’

Effect of Injection Port Aperture. T o calculate the relative importance of the aperture (0.17-cm diameter) of the injection port in determining the total rate of loss of atomic vapor, measurements were made with the port sealed with a small plug of spectrographic graphite, machined to provide a gas-tight seal. Measurements of loss were also made in a second graphite tube whose injection port diameter had been increased to 0.34 cm. A series of measurements showed that the rate of loss of atomic vapor was decreased by 20% when the injection port was plugged, and increased by 16% when the area of the port was increased by a factor of 4. These data indicate that diffusional (and convectional) losses of atomic vapor through the injection port are small in comparison to other loss mechanisms. Although the presence of a graphite plug reduces the initial rate of heating of the furnace, the steady-state temperature attained by the furnace after 5 s of heating does not change. Similarly, the tube with the larger injection port has a slightly increased initial rate of heating (because of its decreased mass) but the steady-state temperature does not change. Consequently, temperature corrections do not have to be made for the above data. Effect of an External Pressure. The effect of an externally applied pressure of Ar gas on the rate of loss of atomic vapor from a pyrolytic-graphite-coatedtube is shown in Figure 8. The effects of an applied pressure on the analytical characteristics of the absorbance signals have been presented elsewhere (10). As the ambient pressure increases, the rate of loss decreases. The slope of the log ( A , / A J vs. t graph is directly proportional to the effective diffusion coefficient of the atomic vapor. Since the diffusion coefficient is inversely proportional to the pressure of the foreign gas (Equation 4), a logarithmic plot of the slope vs. the pressure should yield ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977

1103

“’cI 0.2

COOL

HOT

COOL

Figure 10. Distribution of atomic vapor within the graphite furnace and

the tendency of pressure both to contain the vapor against diffusional flow and to mechanically expel the vapor. Arrows indicate the direction of movement of the atomic vapor as a result of: (-) diffusion and mechanical expulsion; (- - -) pressure a straight line of unit slope. Such a plot is shown in Figure 9. Although a linear relationship has been drawn, the spread in the data points is too large to justify this relationship with certainty. The slope of the curve is 0.36. The effect of the applied pressure in lowering the rate of loss of the atomic vapor from the analysis volume is much less than what would be expected from a purely diffusional loss mechanism. An increase in the ambient pressure from 1to 9 atm reduces the loss rate by a factor of only 2. The low value of the slope of the curve in Figure 9 may be the result of other factors tending to increase the rate of loss of the atomic vapor at high pressure. Higher pressure reduces the mean free path of the atoms and decreases the rate of diffusional loss. Two other factors, however, tend to increase the rate of vapor loss. The decreased rate of diffusional loss allows a greater amount of the Mo atomic vapor to react with the incandescent graphite walls forming an involatile carbide (11-13). At high pressure, the atomic vapor may be mechanically expelled as the expanding gases are forced out of the analysis volume during the heating of the furnace. Although the analyte atomic vapor occupies less than 1%of the volume of the furnace, mechanical loss of the vapor entrained within the expelled Ar may occur. This process of vapor loss a t higher pressures has been observed by L’vov ( I , p 211). Thus, “chemical” and mechanical loss of the Mo atomic vapor may occur to a greater extent as the ambient pressure is increased, thereby reducing the effect of the applied pressure in lowering the overall rate of loss of vapor from the analysis volume. Figure 10 illustrates the various forces acting on the atomic vapor at high pressure-diffusion and mechanical expulsion tending to drive the vapor from the analysis volume and high pressure tending to contain it. Loss through t h e Graphite Wall. To determine the contribution of diffusion through the walls of the graphite tube to the total loss of atomic vapor, the rates of loss of Mo and V in a pyrolytic-graphite-coated tube were compared with those in an uncoated tube. Although uncoated tubes are subject to the problems discussed earlier, they may be used provided unused graphite tubes are employed and the number of atomization cycles is kept extremely low ( ~ 1 0 )Figure . 11 shows a log(A,/At) vs. t plot for Mo and V in an uncoated tube a t atmospheric pressure (Ar sheath gas). The slopes are 0.13 s-l and 0.14 s-l for Mo and V, respectively. These values indicate that the rate of loss of Mo and V atomic vapor from uncoated tubes is increased by 21% and 127’0, respectively, over that obtained with coated tubes. These increases reflect the combined contribution to the rate of loss arising from both diffusion through the wall of the tube and possibly increased “chemical” loss as a result of reaction of the atomic vapor with the more reactive incandescent graphite of the uncoated tube. These results indicate that the rate of loss of atomic vapor through the wall of the graphite tube is small in comparison to other types of loss (assuming that no loss occurs through the walls of the graphite tube which has been coated with pyrolytic graphite). L’vov ( I ) has shown that cuvettes made of either pyrolytic graphite or standard graphite which is coated with a layer of pyrolytic graphite prevent atomic vapor from diffusing through the graphite walls. Whereas L’vov ( I ) 1104

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TIME, s

Flgure 11. Plot of kg ( A J A J vs. time for Mo and V in an Ar atmosphere (1 atm pressure)obtained with an uncoated graphke tube at the heating rate of 1.23 K ms-‘. 0 V, 0 Mo

Table 11. Summary of the Rate of Loss of Atomic Vapor Slope of log (A,/At) vs. t plot, s-’ Coated Plugged Enlarged Uncoated Element tube port port tube Mo V

0.107 0.125

0.090

0.125

0.130 0.140

has shown that the rate of loss of atomic vapor through the porous walls of his uncoated cuvette is greater than the rate of loss of vapor from a similar but pyrolytic-graphite-coated cuvette, the results presented here indicate that diffusional loss through the graphite walls of an uncoated tube is rather small. The reason for this difference lies in the difference in geometry between the HGA-2100 furnace and L’vov’s ( I ) cuvette. Assuming that only gas-phase and wall-diffusional-losses occur (Equations 2 and 31, and considering the dimensions of the HGA-2100 furnace and L’vov’s cuvette ( I , p 205), it can be shown that gas-phase diffusional losses predominate over losses through the wall of the HGA-2100. The results of all experiments, summarized in Table 11, indicate that gas phase diffusion plays a major role in the loss of atomic vapor from the analysis volume of the HGA-2100. Loss of atomic vapor through the graphite walls is probably negligible in a pyrolytic-graphite-coated tube and accounts for approximately 20% of the losses in uncoated tubes. Gas-phase diffusional and convectional losses of atomic vapor through the aperture of the injection port are also small. At atmospheric pressure, the major loss mechanism is diffusion of atomic vapor to the cool extremities of the furnace where condensation of analyte occurs. This can be easily demonstrated by atomizing a few microliters of mercury in the furnace-a deposit of mercury is formed on the quartz end windows, their brass receptacles, and the graphite cones. Effect of E n d Windows. The role of the quartz end windows in determining the rate of loss of atomic vapor from the analysis volume in complex. Atomization of relatively easy-to-volatilize elements such as Cd and P b and medium volatility elements such as Cu in a furnace without end windows produces no change in either the absorbance-time profiles of these elements or their peak-height sensitivities. This effect was observed in both Ar and He sheath gases. Atomization of relatively involatile elements, such as Mo and V and elements whose appearance temperatures (2, 3) approach 2000 K (e.g., Al), in a furnace containing no end windows results in large decreases in both the peak absorbance and the integrated absorbance. Figure 12 shows the absorbance-time profiles for 1.5 X lo-’ g of A1 in an Ar sheath gas. Curve A was obtained with both end windows present, curve B with one window present, and curve C without end windows.

Flgure 12. Oscilloscopic trace showing absorbance by 1.5 X g AI in an uncoated tube at 1 atm of Ar and the heating rate of 1.23 K ms-’. Absorbance: O.l/scale division. Time: 500 ms/scale division. (A) End windows present, (€3) One window removed, (C) Both windows

removed ~~

The sample placed at the center of the furnace tube. (€3) The sample placed at either end on the grooved portion of the furnace tube

~~

Table 111. Effect of Rate of Heating of the Furnace on the Absorbance Signal by Al with and without the End Windows Presenta Peak absorbance Heating rate, With Without K ms-’ windows windows Ratiob 1.23‘ 0.9oc 0.7gd

9

Figure 13. Oscilloscopic trace showing absorbance by 1.2 X IO-’ AI in an uncoated tube at 1 atm Ar and the heating rate of 1.23 K ms- . Absorbance: 0.1/scale division. Time: 500 ms/scale division. (A)

0.360 0.145 0.175

0.155 0.045 0.035

2.3 3.2 5.0

a Ar sheath gas at 1 atm pressure, uncoated tube. The ratio of the peak absorbance given by A1 with the end windows present to that obtained without the end windows. ‘ Sample size = 1.5 x g. Sample size = 4.0 X g.

The presence of the gas-tight quartz end windows prevents convective loss of atomic vapor at the high temperature required for the atomization of this element. Although the HGA-2100 furnace is not symmetrical about the injection port (the left-hand is longer than the right-hand side), removal of either end window affects the signal equally. The increase in the geometric path length which results when the left-hand end window is removed does not increase the effective path length of the absorbing vapor. The reason for this is that the effective path length is not defined by the physical geometry of the graphite tube but by the distance from the center of the tube to a point whose temperature is sufficiently low that condensation of the analyte occurs (i.e., the effective path length is the hot zone of the graphite tube). Although the path lengths are unsymmetrical with respect to the injection port, the temperature gradient is symmetrical; consequently, the atomic vapor is lost at its point of contact with the cold walls. Table I11 shows the variation of the peak absorbance by A1 with the rate of heating of the furnace. The ratio of the signals obtained with and without the end windows being present indicates that the total amount of atomic vapor lost from the furnace increases as the rate of heating increases. The appearance temperature of the A1 signal is the same in each case (2080 K) and independent of the rate of heating of the furnace (14). The greater loss of atomic vapor at low rates of heating is the result of the increased time available for diffusion before the signal attains its maximum. The effect of varying the path length over which diffusion occurs, and hence, the time taken for losses to occur, is shown in Figure 13. Atomization of A1 was carried out at 1atm Ar with a furnace fitted with end windows. Curve A was obtained

by placing the sample in the normal central position within the furnace. Curve B (two traces are shown) was obtained by placing the sample at either extreme end of the graphite tube on the grooved portion of its surface. The sample placed close to either end window requires a longer period of time before the signal appears. This occurs because the rate of heating of the tube at either end is slower than that in the middle. Additionally, the signal from the sample placed a t either end is 60% smaller than that obtained with the centrally placed sample. This large decrease in sensitivity is the result of a shorter path length for diffusion between the sites of vapor production and loss (the cool extremities of the furnace). Whether or not the vaporized analyte reaches the window area of the graphite tube depends on the analyte element determined. This is a consequence of the time-dependent thermal gradient along the axial length of the tube. Depending on the analyte element (its atomic vapor pressure-temperature characteristics), the atomic vapor may reach the end of the tube and the end windows (as in the case of Hg) or it may condense within the tube somewhere between the injection port and the end of the tube. In the case of relatively involatile elements, removal of an end window increases the rate of transport of the atomic vapor to its site of condensation. Thus, a “wind” must be created by a pressure gradient within the graphite tube at high temperature when the thermal gradient is largest, thereby increasing the rate of loss. This effect is additional to the increased diffusion coefficients for these elements at higher temperatures. In addition, the sites of atomic vapor condensation for involatile elements are closer to the center of the graphite tube than those for easy-tovolatilize elements. It may be concluded that the end windows on the HGA2100 furnace are useful for reducing the mechanical expulsion of the analyte vapor at high temperatures. However, the windows have little or no effect on absorbance produced at low temperatures (eg., those by Cd, Pb). “Chemical” Loss of Analyte Vapor. Loss of the analyte vapor may occur as a result of formation of involatile and/or undissociated compounds between the analyte and the incandescent graphite and/or the matrix of the sample or gases in the atomizer. “Chemical” loss of Mo atomic vapor by formation of an involatile carbide through reaction with the incandescent graphite has been discussed earlier. “Chemical” loss of A1 by reaction with nitrogen in the presence of incandescent graphite can occur as a result of the formation of ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977

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undissociated AlCN (15),the sensitivity being reduced by over 60% when Nz is substituted for Ar as the sheath gas (11,14). “Chemical” loss of atomic vapor through the formation of involatile compounds (11) is the cause of incomplete atomization, and hence, “memory effects” of some elements (i.e., Mo and V). CONCLUSIONS At atmospheric pressure, condensation at the cooler extremities of the graphite tube and/or “chemical” loss are responsible for the greatest fraction of the total amount of atomic vapor lost. A t pressures above 1 atm, the major loss mechanism may change. Additional work is required to substantiate some of the conclusions drawn. In particular, a study of the distribution after atomization of radioactive tracer elements in the individual components of the furnace (end windows, tube extremities, interior surfaces of cones, etc.) for both pyrolytic-graphite-coated and uncoated tubes should allow the contribution of each loss mechanism to be assessed for easy-to-volatilize elements such as Cd. Additionally, such a technique may be used to determine whether mechanical expulsion of the atomic vapor occurs at high pressure. The loss mechanisms outlined for Mo and V should be compared with those obtained with a graphite tube lined with a metal foil (unreactive toward Mo and V) in order to determine the contribution of “chemical” loss to the total loss. ACKNOWLEDGMENT The authors thank E. A. Flood for valuable discussion.

LITERATURE CITED B. V . L’vov, “Atomic Absorption Spectrochemical Analysis”, translated by J. H. Dixon, Adam Hilger Ltd., London, 1970. R . E. Sturgeon, C. L. Chakrabarti, I. S.Maines, and P. C. Bertels, Anal. Chem., 47, 1240 (1975). R. E. Sturgeon, C. L. Chakrabarti, and P. C. Bertels, Anal. Chem., 47, 1250 (1975). R. E. Sturgeon and C. L. Chakrabarti, Spectrochim. Acta, Part 8 , (in press). R. E. Sturgeon and C. L. Chakrabarti, Anal. Chem., 49, 90 (1977). J. 0. Hirschfelder, C. F., Curtiss, and R. B. Bird, “Molecular Theory of Gases and Liquids”, John Wiley and Sons, New York, N.Y., 1954, p 14. W.J. Moore, ‘ Physical Chemistry”, 4th ed., Prenticakll, Inc., Englewood Cliffs, N.J., 1972, p 157. C. S. G. Phillips and R. J. P. Williams, “Inorganic Chemistry”, Voi. 11, Oxford University Press, Oxford, 1966, p 22. L. Pauling, “The Nature of the Chemical Bond”, 3rd ed., Cornell University Press, Ithaca, N.Y., 1960, p 403. R . E. Sturgeon and C. L. Chakrabarti, Spectrochim. Acta, Part 8 , (in press). R. E. Sturgeon, ph. D. Thesis, Carleton University, Ottawa, Ontario, Canada, 1977. J. H. Runnels, R. Menyfiekl, and H. B. Fisher, Anal. Chem., 47, 1258 (1975). Y. Taimi and G. H. Morrison, Anal. Chem., 44, 1455 (1972). R. E. Sturgeon, C. L. Chakrabarti, and C. H. Langford, Anal. Chem., 48, 1792 (1976). B. V. L’vov, “Electrothermal Atomization-The Way Towards Absolute Methods of Atomic Absorption Analysis”, presented as an invited paper at the 3rd FACSS meeting and the 6th International Conference on Atomic Spectroscopy, Philadelphia, Pa., Nov. 15-19, 1976.

RECEIVED for review January 17,1977. Accepted April 5,1977. The authors are grateful to the National Research Council of Canada for financial support of this project. One of the authors (RES) is grateful to the National Research Council of Canada for a post-graduate scholarship.

Atomic Absorption Spectrophotometry Based on the Polarization Characteristics of the Zeeman Effect Hideaki Koizumi“ Naka Works, Hitachi Ltd., Katsufa Ibaraki, 3 72, Japan

Kazuo Yasuda Instruments Division, Hitachi Ltd., Nishikubosakuraga wa, Minato- ku Tokyo, Japan

Mikio Katayama Department of Pure and Applied Sciences, College of General Education, The University of Tokyo, Meguro-ku, Tokyo, Japan

A new type of atomlc absorption spectrophotometer was developed by using the Zeeman effect and its polarlzatlon properties. A steady magnetic field was applied to a sample vapor in the direction perpendicular to the propagation of Incident light. The direction of polarlzatlonand the Intensity of light from a hollow cathode lamp were modulated wlth 100 Hz and 1.5 kHz, respectively. The polarlzatlon modulation makes both the background correction and the double beam measurement, and the Intensity modulation eliminates the signal caused by the emlsslon from a graphite atomizer. The dlfference of absorption was observed between light polarlzed parallel and perpendlcular to the field. The signal proportlonal to the atomic density can be obtained readily from the dlfference of absorption. The system is much more efficient than conventlonal Instruments In correctlng for high background absorbances. The present spectrophotometer could correct the background absorption up to 1.7, and Its baseline always had constant level. The present instrument can analyze, with high sensitlvlty, almost all of the elements that can be measured by conventional atomlc absorption spectrometry. 1106

ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977

A r.ew technique of atomic absorption spectrometry (AAS), such as the analysis of mercury by using the isotope shifted Zeeman effect, was proposed by Hadeishi et al. (1, 2). In previous papers (343, we also reported an improved technique of AAS using the Zeeman effect for the background correction and the same research was made by a number of investigators including Hadeishi and Stephens (6-9). A magnetic field was applied to a light source and the components of Zeeman emission lines were used for an absorbing and a reference light, respectively. Recently, the authors reported another type of the Zeeman atomic absorption spectrometry (10) in which a steady magnetic field was applied to the sample vapor perpendicular to the direction of light beam emitted from a conventional spectral source, and absorptions of radiation, perpendicular (Pl)and parallel (Pll)to the field, were observed alternatively. Figure 1shows the emission line of the hollow cathode lamp and the spectral pattern of the normal Zeeman components of the sample element. When the incident light is polarized perpendicular to the