Anal. Chem. 1986, 58, 1973-1977
1973
Spatially Isothermal Graphite Furnace for Atomic Absorption Spectrometry Using Side-Heated Cuvettes with Integrated Contacts Wolfgang Frech* a n d Douglas C. Baxter
Department of Analytical Chemistry, University of UmeA, Sweden, S-901 87 UmeA, Sweden B r u n o Hutsch
Ringsdorff Werke GmbH, Bad Godesberg, 5300 Bonn 2, Federal Republic of Germany
A side-heated graphite cuvette with integrated contacts (ICcuvette), which provldes spatial isothermaiity, has been constructed (patents pendlng) and Its analytkai performance Investigated. The entlre IC-cuvette Is manufactured from a single graphite plece, thus avoiding contact problems. For invdatHe elements, lower atomizationtemperatures could be used than In a Massmann-type furnace to provide similar Sensnhrlties. At the same the, less problems were observed with signal taiilng, condensation, and memory effects. I n combinatlon with a L'vov platform, the IC-cuvette provlded hlgher vapor phase temperatures and consequently reduced spectral and nonspectral Interference effects In comparison to platform equlpped Massmann-type furnaces.
The majority of atomizers used for graphite furnace atomic absorption spectrometry (GFAAS) are of the Massmann type (I). In such, the sample is placed directly onto the tube wall, and atomization is accomplished by heating the tube by a high current pulse. This atomizer concept is very simple to use but suffers from two main inherent disadvantages. First, analyte compounds are already volatilized a t relatively low tube temperatures under conditions of temporal nonisothermality, i.e., before the tube has reached its final temperature. As a result, the degree of atomization is often low and, in addition, matrix dependent (2). Second, since the tube extremeties are in contact with water-cooled cones, a temperature gradient exists, the magnitude of which increases as a function of both time and temperature (3). This spatial nonisothermality is responsible for vapor condensation and recombination of atoms at the cooler areas of the tube, thereby causing memory effects ( 4 ) and spectral interferences (5). Problems due to spatial nonisothermalityhave been addressed by utilizing tubes heated from the side through one pair of electrodes (6). However, with this configuration, temperature gradients can be avoided only by using relatively short tubes. This results in poor sample containment and, hence, inferior efficiency (7).Assuming a constant heating rate, short residence times also result in relatively low mean gas phase temperatures (81, thus amplifying interference effects. Longer tubes, side heated by two pairs of contact electrodes pcwitioned such that the tube ends are heated before the center which contains the sample, have been used by Lawson et al. (9). In this way, the sample vaporizes when the mean gas phase temperature is higher, thus depressing interference effects. Unfortunately, this type of side heated tube is difficult to install and has a limited lifetime, particularly at high atomization temperatures, since even small misalignmentsbetween tube and contact electrodes lead to overheating of the tube/electrode interface. An additional drawback arises from the fact that the area upon which the sample is deposited heats 0003-2700/88/0358-1973$01.50/0
by conduction, Le., slower than the tube ends. This makes it difficult to determine refractory elements with such furnaces. Problems encountered with temporal nonisothermality can be alleviated by introducing samples into preheated Massmann-type atomizers, e.g., by the probe technique (IO),or by using a separately heated sample compartment (11). It should be mentioned that although the background resulting from sample constituents which have condensed on cooler parts during pretreatment steps can be removed before analyte atomization in such systems, there is still a potential risk for recombination and condensation of atomization products at cooler parts of the tube. In the design described by Siemer (II) such problems were avoided by adjusting the wall thickness along the length of the tube and by providing exit holes for the vapor before it could contact the support electrodes. Additionally, the complexity of such systems has hindered their use on a routine basis. A very simple way of reducing interference effects arising from temporal nonisothermality involves the use of the L'vov platform (4,12),a small graphite plate placed within the tube upon which the sample is deposited. Since the platform is heated primarily by tube radiation, sample volatilization is delayed until higher tube and, hence, gas-phase temperatures have been reached. However, when the platform in Massmann type furnaces is used, the problems of condensation and recombination are accentuated compared to constant temperature atomizers (IO,11),since background components liberated during pretreatment cannot always be completely removed from the analytical volume and may therefore cause interference effects during atomization. In principle, such effects could be overcome by applying the L'vov platform in spatially isothermal tubes. In this paper, we characterize the analytical performance of a side-heated spatially isothermal graphite tube with integrated contacts (IC-cuvette). The effects of platform atomization, vapor phase temperatures, and completeness of sample removal during atomization have been investigated, and the results obtained are compared with conventional Massmann-type furnaces.
EXPERIMENTAL SECTION Instrumentation. The basic design of the IC-cuvette is shown in Figure 1. The L'vov platform had a shape similar to that used in the Perkin-Elmer HGA-600 but rested on the tube bottom on four feet and had cuts along ita lateral edges in order to minimize contact with the tube walls. The entire IC-cuvettewas manufactured from one single piece of graphite of RWO quality (Ringsdorff Werke GmbH) and incorporated into a previously described home-made furnace (13). The cuvette contact areas were clamped in new terminal blocks designed to accommodate the IC-cuvette and to exclude air from entering the furnace. The furnace was installed in a Varian Techtron AA-6 atomic absorption spectrometer, provided with an H2lamp for background cor@ 1986 American Chemical Society
1974
ANALYTICAL CHEMISTRY, VOL. 58, NO. 9, AUGUST 1986
-
2800 25002200
le00 1600-
.
1300-
a
Figure 1. Schematic drawing of the ICcuvette: (1) cuvette contact area,clamped in terminal blocks, (2) injectbn port, (3)apettwe for fiber
8 1Ooo-
c
optics.
700
-
Table I. Conditions for the HGA-74 and IC-Cuvette for the Comparison of Lead and Silver Measurements in the Two Systems
2
atomization"
wavelength, element
nm
time, s
Ag
328.1 283.3
3 3
wall temp, "C
platform time, s temp, "C
d
I
Pb
1500 1500
3 5
"Neither ashing nor matrix modification was used. For the HGA. eas stoD was used during atomization. rection. A transformer capable of delivering 7.5 kW and operated at 15 V or 18 V was connected to the furnace. Provisions for temperature-controlled heating were incorporated with a fiber optic cable mounted in the terminal block and directed toward the tube through a 2-mm hole in the cuvette contact (see Figure 1). The principle of this optical feed-back control has been described earlier (14). Temperature settings referring to the inner surface of the graphite tube were calibrated with an optical pyrometer (Keller Spezialtechnik Pyro Werk GmbH, Model PBO 6AF3). An additional photodiode could be installed in two positions to monitor the temperature at the upper wall of the tube at the center and end. For some measurements the furnace incorporating the IC-cuvette was replaced by a Perkin-Elmer HGA-74 which was also provided with temperature-controlled heating (14). The signal damping of the AA-6 readout module was modified to obtain a faster response time from the electronics. The value of the DAMP A time constant was thus altered from the original 260 ms to 10 ms, as described earlier (15). In order to study in detail the time dependence of the analytical signals as well as the temperature, a fast on-line data acquisition system was connected to the spectrometer (16). This system was also used to calculate the peak height and peak area values. For comparative experiments a Perkin-Elmer Zeeman-3030 atomic absorption spectrometer equipped with an HGA-600 graphite furnace, an AS-60 autosampler, and a PR-100 printer was used. The instrumental parameters are summarized in Tables I and 11. Reagents and Materials. The NBS 1566 oyster tissue and the anorthosite rock sample were prepared according to ref 17 and 18,respectively. All standard solutions and matrix modifiers
I
0
2000 2000
1
2
3
4
5
Time I s
Figure 2. Temperature curves measured at the center (-) (- - -) of an IC-cuvette.
and end
were prepared from SR grade or higher purity. Only pyrolytically coated tubes were used (Ringsdorff Werke GmbH). Platforms were manufactured from pyrolytic graphite (Ringsdorff Werke GmbH). Argon of SR grade was used throughout. So far a limited number of tubes have been used but the typical useful lifetime appears to be 600 determinations of aqueous solutions using atomization temperatures between 2000 "C and 2600 "C.
RESULTS AND DISCUSSION Heating Characteristics. As mentioned in the introduction, tubes that provide spatial isothermality confer superior analytical conditions. In order to investigate the extent to which the IC-cuvette conforms to the ideal of spatial isothennality, temperature profiles were measured at the center and at the end of the upper wall. As can be seen from Figure 2, the temperatures are identical during heating, but when temporal isothermality is established, a slight (always less than 100 "C) temperature gradient develops. It was observed that the side wall temperature, as measured with an optical pyrometer focused between the central contact pair, closely followed the upper wall end temperature. This means the cuvette is devoid of any cool parts where condensation can take place. However, since the entire cuvette surface is heated, higher currents must be used to achieve heating rates comparable to Massmann-type furnaces. It should be mentioned that IC-cuvettes can be used with relatively thin tube walls since no mechanical pressure has to be exerted across the cuvette to ascertain electrical contact. This is achieved by maintaining a vertical pressure at the terminal blocks over the cuvette contact areas. Since the entire cuvette is manu-
Table 11. Conditions for the IC-Cuvette and HGA-BOO Atomizers, Gas Stop during Atomization
element
wavelength, nm
ash" temp, "C
Bad Crd Mod Pb' Snf TJ
553.6 357.9 313.3 283.3 286.6 214.3 318.4
1000 1000 1000 600 800 600 lo00
Vd
IC-cuvette atomization time, s temp, "C 10 10 10 5 9 5 10
2500 2300 2450 2000 2400 2000 2500
ashbtemp, "C 1200 1600 900 1000 1000 1400
HGA-600 atomizationc time, s temp, "C 10 10 5 5
5 10
2300 2650 2000 2400 2000 2650
a Ramp 8 s, hold 12 8. Ashing followed by 40-5 'cool down step" at 200 O C when platform used. Ramp 10 s, hold 20 8. Ashing followed by 20-5 "cool down step" at 200 "C when platform used. cHGA-600maximum heating rate. Atomization followed by 6 s clean stage at 2650 "C. dWall atomization. Background correction not used. 'Platform atomization. Matrix modifier, 10 Mg of NH4H2POI + 3.5 pg of Mg(NO&. fPlatform atomization. Matrix modifier, 200 pg of NH40H. #Platform atomization. Matrix modifier, 20 pg of Ni as nitrate.
2k
ANALYTICAL CHEMISTRY, VOL. 58,NO. 9,AUGUST 1986
I
1.2
Peak area
1
a.... ". .....,
.... 0.8
H f
-1
-
-2
-1
-2 0
b,......,.
...'...
'.a ............a
...".O. ......_...__
- ----- ------
1
2
3
4
5
0
1
2
3
4
5
Number of empty firings
0.4
-
Figure 4. Carry-over contamination following 20 ng (0)and 200 ng (0) of Cr standards as a function of the number of empty firings. The dashed lines are the detection limits. Data points on dotted lines are from Harnly et al. (79),with wall atomization at 2700 "C for 7 s and gas flow of 50 mL min-', followed by a 6-sclean stage using a pyrocoated tube in an HGA-500. The other points on solid lines were obtained with an IC-cuvette, conditions as in Table 11.
0I
I
I
0
1
8
4
Time /s
Figure 3. Signal transients for 4.0 ng of V (-) (- - -) using wall atomization in an IC-cuvette.
and 250 pg of Cr
Table 111. Effects of Final Atomization Temperature and Heating Parameters on Vanadium Signals max
15 15 15 15 18
0
kheight
I
a
transformer: V
...... ..__, "0
0.
*..,:.
1975
current: A
620 700
final With 4 ng of vanadium temp, "C peak height peak area 2400 2500 2580 2630 2500
0.446 0.512 0.508 0.511 0.615
0.664c 0.835 0.849 0.832 0.840
Without load. bNottrue root mean square value. 'Sample not completely volatilized. factured from a single piece of graphite, contact problems were not observed, even when maximum temperatures were used. The temperature overshoot evident in Figure 2 is avoided when the upper wall emission is used for optical feedback temperature control. The bending of the temperature curve is partly due to the nonlinear response of the photodiode. By modification of the contact configuration, the heating characteristics of the IC-cuvette can be changed easily. Thus, it is possible to selectively heat the cuvette ends before the center to increase vapor phase temperatures when determining volatile elements. Determination of Involatile Elements. Signals for chromium and vanadium obtained with the IC-cuvette are shown in Figure 3. In spite of the relatively low tube temperature (with the Perkin-Elmer HGA-600 a furnace temperature of 2650 "C has to be used), the vanadium signal returns fairly rapidly to the base line and the peak maximum is positioned just before the final temperature is reached. Indeed, from Table I11 it can be seen that atomization temperatures above 2500 OC do not improve the peak height sensitivity. The constancy of the peak areas indicates complete volatilization of vanadium at temperatures equal to or above 2500 OC. The lower atomization temperatures, which can be used with the IC-cuvette, will also be beneficial for tube lifetime. In Massmann-type furnaces the determination of involatile elementa is problematic due to condensation and reevaporation processes prevailing at the cooler parts of the tube which ultimately result in peak tailing and memory effects. A comparison of the memory effects obtained following the atomization of large amounts of chromium in the IC-cuvette
and a Perkin-Elmer HGA-500 (taken from Harnly et al. (19)) is given in Figure 4. Although higher atomization temperatures and a convective gas flow were used in the HGA-500, the memory effects are significantly worse. According to Harnly et al. (19),the carry-over contamination still persisted in all cases even after seven empty firings. A comparison of characteristic masses for barium, vanadium, chromium, and molybdenum between the IC-cuvette and an HGA-BOO operated under recommended atomization conditions showed both atomizers give similar results. Since the cross sectional areas of the tubes are comparable (IC-cuvette 25 mm2 and HGA-600 26.4 mm2)and the HGABOO tube is longer (28 mm; IC-cuvette 19.4 mm), this indicates a much shorter effective tube length in the HGA-600 in which the atomic population can be maintained, since, in the cooler regions of the tube, the number of free atoms will be reduced by condensation. It should be observed that at these high temperatures, transport of atoms is dominated by diffusion processes (7). Another aspect of involatile element determination is that wall atomization must be used. Hence, sample solutions are free to spread along the tube wall during drying. In Massmann-type furnaces, analyte atoms located away from the center will consequently experience lower atomization temperatures, which might reduce the contribution to the signal from these atoms. The degree of spreading increases with sample volume and with reduced surface tension. In Figure 5 the effect of increasing the molybdenum sample volume is shown for the two atomizer systems. Since the signals obtained with the IC-cuvette are independent of the sample volume, calibration curves can be constructed simply by using different volumes, and signals should not be influenced by surface tension. Platform Atomization. With the platform technique, it is desirable to heat the platform only by tube radiation, in order to achieve a maximum temperature difference and rapid platform heating rates (4). This permits the highest possible vapor phase temperature to be established during atomization. With the platform used in the IC-cuvette, the area in contact with the tube walI is relatively small, thus limiting conductive and resistive heating. In order to compare platform heating characteristics in the IC-cuvette with the HGA furnace, platforms of the same surface area and mass were used in both systems under conditions giving similar tube heating rates. Figure 6 shows silver signals and tube temperature profiles obtained in the two atomizers. The substantial signal shift toward higher temperatures observed in the IC-cuvette shows that the delay in platform heating is more pronounced compared to the HGA, resulting in a higher gas phase or spectroscopic temperature (8) in the former; see Table IV. Table V shows characteristic mass data for the IC-cuvette and the
1976
ANALYTICAL CHEMISTRY, VOL. 58, NO. 9, AUGUST 1986 ~
1 I
u)
E
! 9
~~
Table V. Characteristic Mass Data for Silver and Lead in T w o Atomizer Systems Using the Same Heating Rate elemeat
HGA-74 wall
peak height
Ag
peak area
Pb Ag Pb
1.0 7.8 3.8 33.9
0.5
0
characteristic mass; pg IC-cuHGA-74 IC-cuvette wall plat vette plat
0.5 4.4 1.4 13.3
0.9 7.7 1.7 23.2
0.7 5.0 1.7 14.3
" Mass required to give 0.0044 A or As. Table VI. Recovery of Lead and Tellurium from Sodium Chloride Matrices Using Two Atomizer Systems peak area analyte analyte in in NaCl H20
E
t
SI
0.5
0
4
analyte, ng
NaC1, Pg
atomizer
0.4 P b
20
1.0 T e
100
HGA-600 plat IC-cuvette plat HGA-600 plat IC-cuvette plat
0 70
140
280 cow. /pg
210
I-'
Flgure 5. Peak area (upper trace) and peak height (lower) absor0) bances for Mo using 5 pL samples of various concentrations (0, and using 5, 10, 15 and 20 pL aliquots of a 70 pg L-' solution ( 0 ,m):
0.122 0.083 0.202 0.161
0.105 0.086 0.146 0.165
rel" H20, Ti 86.1 103.6 72.4 102.5
a Matrix modifiers were used throughout and all values were corrected for blanks.
solid lines, HGA-BOO; dashed lines, IC-cuvette.
1
HGA
- 800
0.5
1
ICC
-
0.8
8 rp
e
B n
0
1
2
TW/.
4
0.4
-
3
4
5
,
1
.
.
0
1
2
3
1
4
1
5
TLm /.
F&e 7. S i a i transients for 1.O ng of Te in H20 (lower traces) aM 100 pg of NaCl (upper .traces)matrlces uslng two atomizer systems. The dotted lines show the background signals. A matrix modtfier was
used.
0I
1
0
1
1
2
3
Time I s
Fi@re 6. Signal transients for 100 pg of Ag and temperature curves using platform atomization In an HGA-74 (-) and an IC-cuvette (-
- -).
Table IV. Spectroscopic Temperatures in Different Atomizer Systems Using Lead as the Thermometric Species (Reference 8) instrum temp: "C
atomizer
spec temp, "C
atomizer
spec temp, OC
1500 2000 2000
HGA-74 wall plat HGA-BOO platb
1340 1770 1710
ICC wall plat ICC platb
1350 1900 1960
Instrumental temperature (HGA-BOO) or surface temperature as measured through injection port (HGA-74, ICC). *Matrix modifier added.
HGA when wall as well as platform atomization is used. Compared to platform atomization, the sensitivities are even more noticeably improved in the IC-cuvette. Nevertheless, the spectroscopictemperatures in both systems are about the
same as shown in Table IV, which means that there is no difference in the rate of atom removal due to diffusion (7). Consequently, other processes like convection must increase the rate of atom removal from the HGA, processes that are grobably caused by spatial nonisothermality and by differences in the construction of the atomieers. As an example, in contrast to the futnace incorporating the IC-cuvette, the HGA has provision to maintain a convective inner gas flow directed from the tube ends to the center. When this inner gas flow is stopped during atomization,the tube gases can only expand freely through the injection port thus creating a convective flow. Table VI shows resulb obtained with the HGA-600and the IC-cuvette for lead and tehrium in sodium chloride matrices when using the so-called "stabilized temperature platform furnace" concept (STPF) (12). This incorporates the use of a modifier, a L'vov platform and rapid furnace heating, as is illustrated in Figure 7 for tellurium. As can be seen from the figure, a t the time of the analyte peak maximum the background components have beeh completely removed from the IC-cuvette, while a considerable background signal persists throughout the atomization in the HGA which, however, is accurately compensated for by the Zeeman background corredor. The i n c r e a s background ~ in the HGA can be ascribed to condensation and revolatilization, as well as recombination processes. It should be noted that the small background
ANALYTICAL CHEMISTRY, VOL. 58, NO. 9, AUGUST 1986
Figure 8 that the IC-cuvette provides a greater specific signal relative to the background, as well as lesser nonspectral interference effects; see also results for lead and tellurium in Table VI. These reductions in interference effects can be attributed to more effective dissociation of sample constituents brought about by the higher gas phase temperatures in the platform-equipped IC-cuvette (Table IV). The advantage of obtaining higher gas-phase temperatures is further illustrated in Figure 9 where lead was determined in NBS oyster tissue 1566. In this particular application, the IC-cuvette gives a result that is close to the certified value, when an aqueous ealibration is used. The IC-cuvette described here realizes the advantages of using a spatially isothermal atomizer and shows how the GFAAS technique can be readily improved.
2400 "C
I
I
1977
ICC
HGA -600
. ..
ACKNOWLEDGMENT I
i
I
0
1
2
3
4
5
Time /s
Figure 8. Signal transients for Sn in a dissolved anorthosite rock sample, using two atomizer systems. The dotted lines show the background signals. A matrix modifier was used. The upper traces have been digitally smoothed.
The authors are indebted to Bernhard Welz, Perkin-Elmer, FRG, for providing the Zeeman-3030 AA spectrophotometer and to Erik Lundberg and Gnders Cedergren for valuable suggestions. Registry No. V, 7440-62-2;Cr, 7440-47-3;Mo, 7439-98-7;Ag, 7440-22-4;Pb, 7439-92-1;Te, 13494-80-9;NaC1,7647-14-5;HzO, 7732-18-5; Sn,7440-31-5; Ba, 7440-39-3; graphite, 7782-42-5.
LITERATURE CITED (1) Massman, H. Spectmhlm. Acta, Part B 1888, 238, 215-226. (2) Frech, W.; Cedergren, A.; Lundberg, E.; Siemer, D. Specfrochlm. Acta, Part B 1983, 36B, 1435-1446. (3) Falk, H. Spactrochim. Acta, Part 8 1985, 408, 533-542. (4) L'vov, B. V. Specfrochh. Acta, Part B 1978, 338, 153-193. (5) L'vov, B. V. Spectrochim. Acta, Part B 1989, 24b, 53-70. (6) Matousek, J. P.;Brcdie. K. 0.Anal. Chem. 1973, 45, 1606-1609. (7) van den Broek, W. M. G. T.; de Galan, L. Anal. Chem. 1977, 49, 2176-2186. (6) Sierner, D. D.; Frech, W. Spectrochm. Acta, Part B 1984, 398, 26 1-268. (9) Lawson, S. R.; Dewalt. F. G.; Woodriff, R. Prog. Anal. Atom. Spec~ S C 1983, . 6 . 1-48. (10) Littlejohn, D.; Cook, S.; Durie, D.; Ottaway, J. M. Spectrochim. Acta, Part 8 1984, 398, 295-304. (11) Siemer, D. D. Anal. Chem. 1983, 5 5 , 692-697. (12) Slavln, W.; Carnrick, G. R.; Manning, D. C.; Pruszkowska, E. At. Spectrosc. 1983, 4 , 69-66. (13) Frech, W.; Lundberg, E.; Cedergren, A. Can. J . Specfrosc. 1985. 3 0 , 123- 129. (14) Lundgren. G.; Lundmark, L.; Johansson, G. Anal. Chem. 1974, 46, 1028-1031. (15) Lundberg. E. Chem. instrum. 1978, 6 , 197-204. (16) Lundberg, E.; Frech. W. Anal. Chem. 1981, 53, 1437-1442. (17) Lundberg, E.;Frecb, W.;Llndberg, I. Anal. Chim. Acta 1984, 760, 205-215. (18) Lundberg, E.; Bergmark, B. Anal. Chim. Acta, in press. (19) Harnly, J. M.; Miller-Ihli, N. J.; O'Haver, T. C. Spectrochim. Acta, Part B lg84, 398, 305-320.
LA..--
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Signal transients for 0.4 ng of Pb in H20(lower traces) and Pb in oyster tissue (upnsr traces) using two atomizer systems. The dotted ilnes show thft background signati. A matrix modifier was used. the observed lead conCentrations using aqueous calibration were 0.22 and 0.42 bg g-' for thq HGA-600 and IC-cuvette, respectively. Certified value was 0.48 A 0.04 pg g-' Pb.
observed for tellurium in water iq the HGA is a consequence of the Zeeman-effect background correction. Figure 8 shows sign+ for tin iri aorthosite rock obtained with the two atomizerg, In this case neither the hydrogen lamp nor the Zeemqn effect background correctors can adequately compensate for the nonspecific abssrbance. It can be seen from
RECEIVED for review January 6,1986. Accepted April 2,1986. This work was supported by the Swedish Natural Science Research Council.
