Electrothermal atomic absorption spectroscopy-Present understanding and future needs

S,mtmhmw Acta Vol. 38% No. il/l2. pp. 14334446. 1983. Printd in Creal Ballsin.

0%.8s4?/83 53.00 + xi0 Perpamon Prew Lid.

ELEJECTROTRERMAL ATOMIC ABSORPTION SPECTROSCOPY -

PRESENT UN~~STAN~~NG AND FUTURE REEDS

Abstract - A comparison is made between Massmann-type furnaces (with and without the L'vov platform) and constant temperature atomizers. It is shown that there is no major difference between these types of furnaces with regard to peak height sensitivities. On the other hand, the ~ass~nn-type furnaces showed to a greater extent susceptibilities towards matrix interference effects. The effect of the sample residence time on gas phase interference effects has been investigated at various constant temperatures for lead in large excesses of iron chloride and sodium sulphate, respectively. These experimental results are discussea and they are correlated to data obtained by high temperature equilibrium calculations. As s conclusion we found that there is a need for a better control of the gas phase inside graphite tubes. Advantages of separating the volatilization and atomization processes are discussed. The potentialities of constant temperature atomizers for atomic emission spectroscopy are lined out.

Since its inception, conventional GFAAS has been developed considerably with regard to methodology and instr~entation. The technique has been essentialb improved by the introduction of e.g., automatic sample devices, the L'vov platform technique, matrix modifications, pyrolytically coated graphite, automatic background correctors, adequate signal evaluation and rapid controlled heating of the atomizers. In spite of this progress there still remain problems in connection with the vaporization/atomization of samples. In conventional Massmann- type furnaces, the temperature at which an element is vaporized depends on its volatility and usually effective atomization temperatures are often too low for complete atomization. An additional disadvantage comprises difficulties in relating absorbance signals, which may originate from different stomiaation intervals, to true amounts of an element. Many of these problems inherent in Massmann~t~e furnaces can be eliminated by vaporizing samples into atomizers which are kept at a constant temperature. This concept was employed in the first graphite furnace ever built for analytical AAS [l], but due to the technical complexity of the isothermal approach, it has only been realized on a minor scale and therefore little is known about its limitations.

By vaporizing samples from a platform 12,s) inserted into Massm~n-tee furnaces, the problems arising from non-isothermal atomization can often be minimized in a relatively simple way. In particular for volatile elements it is possible to approach conditions of constant temperature atomizers by the combined use of the platform technique with an element stabili- zing modifier solution f4,51.

The aim of this paper is to characterize isothermal as well as ~assmann-tee atomizers (equipped with and without platforms) with respect to sensitivity and susceptibility to interference effects as veil as identifying future needs in order to develop the graphite furnace technique further.

Instrumentation

The basic design of the home-made two-step furnace used for most experiments has been described earlier l61. However, several modifications have been made to improve the performance. The furnace was constructed by combining two Varian CRA 90 carbon rod atomizer workheads. A graphite tube was attached to the upper workhead and a graphite cup to the lower. The cup was fitted tightly into an aperture in the bottom of the tube, see Fig. 1. The terminal blocks of the upper workhead were modified to allow four instead of two electrodes to support the tube. Faoh eLectrode was spring-loaded to achieve an even pressure against the tube, and the dis- tance between two electrodes in the 883116 terminal block was llmm. In order to minimize gas consumption and deterioration of graphite parts, a brass housing was placed around the gra-

1435

1436 W. FRECH er al.

Fig. 1. Two-step graphite furnace configuration

phite parts. The size of this housing was increased to allow graphite tubes as long as 25 mm to be used. The housing was covered with a quartz window equipped with an opening for sample injection. The furnace was installed into a Varian-Techtron AA-6 atomic absorption spectrometer, provided with background correction and fitted with two separate power supplies, one for each workhead. Both transformers were capable of delivering 7.5 kW and both the tube and the cup were operated at 15 V. Facilities for precise temperature control of the graphite tube and cup were installed. Fibre optic cables directed towards the outer surfaces of the tube and the cup were mounted on the terminal blocks of the furnace, and the light emitted from the tube and the cup was monitored with photodiodes, sensitive to infrared radiation. The principle of this optical feed-back control has been described earlier [7]. Temperature settings referring to the outer surfaces of the graphite tube and cup were calibrated with an optical pyrometer (Keller Spezialtechnik Pyro Werk GmbH, Model PBO 6AF3). Gas phase temperatures were determined spectroscopically by using the line pairs pb(283.3 nm)/Pb(280.2 nm/ pb(368.3 mu) [al.

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 origi- nal 260 ms to 10 ms, as described earlier [9]. A peak reader module was connected to the re- corder output of the spectrometer [lo] providing simultaneous monitoring of the peak height and the peak area. 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 [ll].

For some of the experiments a Perkin-Elmer AA 3030 atomic absorption spectrometer was used, equipped with an HGA 500 graphite furnace, an AS 40 autosampler and a PR 100 printer. Maxi- mum heating rates were used throughout.

For the emission measurements, the center of the graphite tube of the home-made two-step furnace was focused onto the entrance slit of a SpectraSpan III Echelle monochromator. A quartz refractor plate wavelength modulator, similar to that described by Harnly et al. [121, was installed to correct for background emission. The signal from the photomultiplier tube was via a current-to-voltage converter connected to an Alpha LSI minicomputer [ill, which also controlled the movement of the quartz plate.

Material

Graphite cups and tubes (i.d. 4 mm and 16-25 mm long) for the two-step furnace were nanufac- tured from RWOl high density graphite (Ringsdorff-Werke GmbH, Bonn-Bad Godesberg, FRG). Some of the 1.8-mm tubes were pyrolytically coated by Ringsdorff-Werke. The coating had to be removed where the electrodes were in contact with the tube to avoid sparking. End caps (circu- lar graphite disks; 1 mm thick, o.d. 4 mm, i.d. 2 mm) were pressed onto both ends of some 25-mm tubes, see Fig. 1.

Pyrolytically coated graphite tubes with pyrolytical platforms were used to the Perkin-Elmer HGA 500.

Electrothermal atomic absorption spectroscopy 1437

Procedure

5-40 ul of a standard solution was dispensed into the graphite cup through 8 small opening in the graphite tube. The sample was dried by heating the tube to 400 'C and the CUP to 100 OC for 20-b0 .s, After this drying step the tube was heated to preselected final temperatures. Hhen these temperatures were reached, heating of the cup started. The heating rates of the tube and the cup could be varied. The furnace was Slushed with argon (2 1 min-'1. Background correction was used throughout.

High temperature equilibrium calculations

The calculations were perSormed as described earlier [13-151.

RESULTS AND DISCUSSION

Sensitivities for isothermal and non-isothermal atomizers

The two-step constant temperature atomizer shown in Fig. 1 was developed in order to study its potential advantages compared to conventional Massmann-type furnaces (with and without platforms). The construction of the two-step furnace made it possible to use tubes of different designs (change of tube length, tubes with and without end caps) and still maintain constant temperature conditions. This equipment enabled us to study the effect of atom residence time on sensitivity and interference effects.

Figure 2 shows signals for 0.5 np. of lead for three tube confiaurations and two different heating rates of the cup. As &-be seen, there is only a small increase in the peak height but a large increase in the peak area for longer tubes with end caps. The rate at which atom vapour diffuses from a tube with end caps can be calculated from 11.63

SC is the transverse

M f = No - exp-(2DSc/tcV) * ‘c

cross-sectional area of the apertures in the caps, t,.is the thickness of the caps, V is the volume of the cuvette, II is the atomic vapour diffusion coefficient, MO is the mass of vapour at the initial instant of time, and 'c is the time.

For lead in water (left-hand figures) an increase of the heating rate of the cup by a factor of 2 has no marked effect on the signal. These results are consistent with theories presented elsewhere [16,17]. For example, L'vov [16] expresses the peak height as a function of atomization time and mean residence time of atoms ~z. In our experiments ~~ is favourably small compared to ?z and in this situation moderate changes Of the ratio fl/~z do only slightly affect the peak height.

The rate of atom formation is known to be critically dependent on the local environment of the graphite surface [la], a property which is dependent on the composition of the sample. On the right-hand side of Fig. 2 we show lead signals obtained in the presence of sodium nitrate. Even when atomization times are short and residence times are long (e.g. t,fr, s 0.13 in the left-hand bottom of Fig. 2) the presence of a matrix does still affect the peak height. It should be observed that the sodium nitrate matrix does not change the atomization efficiency as is evident from the peak area values given in Fig. 2. Table 1 summarizes the results given in the figure. The findings discussed so far indicate that, at least for lead, an optimum peak height sensitivity can be reached rather easily but that it might be difficult to design an atomization system which gives complete freedom from solute volatilization interference effects.

A summary of some peak height sensitivities obtained at optimum conditions with different graphite furnaces is given in Table 2a. In order to be able to compare these values, we have normalized the data with respect to the cross-section diameter and the results are given in Table 2b. For some elements the efficiency of atomization has been determined for the Perkin- Elmer HGA 72 and HGA 2200 [17,19], respectively. The normalized sensitivity values of the HGA 500 1201 have been recalculated for 100% efficiency by using the data of Sturgeon et al. 1191 and Hroek and de Galan f173. Although these data have been generated with slightly different instrumentation, except for aluminum, the recalculated sensitivity values presented in Table 2b agree surprisingly well. This indicates that for the elements investigated the obtained sensitivity is close to the maximum attainable. It should be mentioned that several elements (e.g. those which form very stable carbides), are in general difficult to atomize from a graphite surface and for these the peak height sensitivities are far from those corresponding to an ideal situation. For example, for rare earth elements Sycbra and co-workers [21] have reported a large increase in peak height sensitivity when using metal at.omizers in-

1438 W. FRECH et al,

stead of graphite furnaces.

Table 3 shows a comparison betueen peak area sensitivities (in pg/O.Ok4 A*s) obtained with a HOA 500 f22] and tbe two-step furnace. Except for arsenif the two-step furnace with a 25 mm Lang tube gives about half the amounts of the EGA 500, As illustrated for lead the values can be still lowered by simple means (caps and plug). However, the analytical advantages of such measures could be of Limited value with regard to the detection limit since the uncertainty caused by baseline shifts increases with longer measuring times. In this connection it should be stressed that peak are& evaluation should be used for the majority of ana&tical applications since greater freedom from solute volatilization interference effects is obtained. For that reason it would be natural to compare even peak area sensitivities. How- ever, such values are normally not provided.

t . I . , . 8 . ,

0 8 I . , , . ,

2 4 6 0 2 4 13 8

Fig. 2, Signals for 0.5 ng of lead obtained for different tube configurations. upper part: 16 mm open tube. Middle part: 25 mm tube with end caps. Bottom: 25 mm tube with end caps, injection port plugged. The left-hand side shows signals from aqueous solutions for two different heating rates of the cup. The right-hand side shows corresponding signals in 5 ug of sodium nitrate. The increase of cup and tube temperatures are shown.


Table 1. Recoveries ($) for 0.5 ng of lead in 5 ug of sodium nitrate. Values are related to aqueous solutions

Heating rate, 1000°C/s Heating rate, 500°C/s

Tube Peak height Peak area Peak height Peak area

16 mm 82 101 78 100 25 mm with cap 93 99 74 101 25 mm with cap,plug 92 100 83 104

Table 2a. Peak height sensitivities in different furnaces (pg/O.O044 A). If not otherwise stated the most sensitive line was used. The tube length of the two-step furnace was 25 mm

Element L'vov Varian furnace+ GTA 95

Perkin-Elmer Two-step HGA 500 furnace

1161 t231 [241 [201

Ag 0.1 1.0 3.0 0.56

Al ia ;*: 14 As 018

16 ;:;I Be 0.03 0.4 0.7 0.32 Bi 4s 7.0 12 3.7 Cd 0.08 0.3 0.2 0.5 0.20 0.14h Cr 2 2.5 3.5 2.5' Mn 0.2 ::;: 10 2.8 1.1 Pb* 2 13 4.0h Sb 5e 9.0 27 2.; Sn 2 22 17' lgb

t) cuvette diam. 2.5 mm a) 197.2 nm, b) 306.8 nm, c) tube length 18 mm, d) 283.3 l ) 231.1 1) 224.6 nm, O) no modifier used,

nm, nm, a) tube with end caps and plug in in-

jection port

Table 2b. Peak height sensitivities normalized to 1 mm2 tube area (pg/O.O044 A). The tube areas were calculated to be: Perkin-Elmer HGA 500 = 26.4 mm*, Varian GTA 95 = 19.6 mm*, two-step furnace = 15.2 mm* (the cup volume was included when the effective tube area was calculated)

Element L'vov Varian Perkin-Elmer Two-step furnace GTA 95 HGA 500 furnace

[161 t231 1241 [201

Ag 0.02 0.05 0.11 0.05' 0.04 Al 0.20 As 0.98.

0.36 0.53 0.05 0.24 0.41 0.61 0.63

Be 0.006 Bi 0.5ab

0.04 0.015 0.026 0.021 0.36 0.45 0.24

Cd 0.016 0.015 0.008 0.019 0.010 0.013 o.oo9c Cr 0.41 0.13 0.13 0.14 Mn 0.041 0.036 0.11 0.05 0.072 Pb 0.41 0.31 0.38 0.49 0.34 0.32 0.26' Sb 0.5= 0.46 1.0 Sn 0.41 1.12 0.64~

0.44 1.05

a) recalculated to 193.7 nm assuming = b) ”

sensitivity 193.7/197.2 1/1.6 223.1 nm W I, 223.21306.8 = 111.4

C) 8, 217.6 IUII I1 II

*) not corrected for different wavelengths used 217.6/231.2 = 112

e) tube with end caps and plug in injection port f) recalculated for 100% atomization efficiency

1440 W. FKECH etal.

Table 3. Peak area sensitivities in pS/O.O044 A's. The tube length of the two-step furnace was 25 mm

Element Perkin-Elmer HGA 500 1221

Two-step furnace

Tube temp. OC

As l7 18 1900 Al 10 5.9 2250 Cd 0.35 0.23 0.15b 1100 Mn 2.0 0.9 1730 Pb 11 5.2 4.1a

2.ab 1150

14=

atube with end caps; btube with end caps, '16 mm tube

injection port plugged;

Susceptibility to interference effects for isothermal and non-isothermal atomizers

It is difficult to make an unambiguous comparison between different atomization systems in regard to their susceptibility towards interference effects. One way to make such a comparison is to ensure that the analyte and the total amount of the interferent is to the greatest possible extent simultaneously vaporized during the atomization sequence. This can be accomplished by using rapid heating during the atomization in order to prevent selective volatilization and by excluding sample pretreatment steps during which uncontrollable amounts of the interferent might be removed.

We performed some experiments for lead and aluminum in the presence of copper chloride according to these propositions. Figures 3 and 4 show that the two-step constant temperature atomizer is least susceptible to interference effects. The changes in the recovery obtained with these three atomization systems can be explained by differences in gas phase temperatures during atomization.

Fig. 3. Recovery of 1 ng of lead in the presence of different amounts of copper chloride for i) HGA 500 with atomization from tube wall (A), atomization from the L'vov platform (0); ii) the two-step furnace (0). Recoveries are related to aqueous solutions. Peak areas were evaluated throughout. The atomization temperature was selected to be 1900 OC in all experiments. Samples, 10 nl, were only dried, background correction was used throughout.

Elcctrothermalatomicabsorption spectroscopy 1441

For atomization from the tube wall, samples are volatilized at relatively low temperatures and hence the temperatures experienced by the analyte are appreciable lower than the preset temperature. For the platform on the other hand, a higher tube temperature is reached when the sample is volatilized. However, as has been shown elsewhere [81 the gas phase temperatures in the L'vov platform-Massmann tube are still lower than the preset ones, and therefore the L'vov platform is less efficient in reducing interference effects than the constant temperature atomizers. However, for many analytical problems the advantage shown for the two- step furnace is of minor significance in view of the fact that efficient matrix modifiers are available for a great number of element-matrix combinations [251. We have, for example, been able to eliminate completely the effects from 80 pg of copper chloride (log CuCl, = 1.9) on aluminum by using the platform together with recommended matrix modifiers and optimum pretreatment temperatures t26l.

- r I-

0.3 & 1.5

logCuCl2 'pg

Fig. 4. Recovery for 1 ng of aluminum in the presence of different amounts of copper chloride for i) HGA 500 with atomization from tube wall (A), atomization from the L'vov platform (0); ii) two- step furnace (0). Recoveries are related to results obtained for aluminum in aqueous solutions. Peak areas were evaluated throughout. The selected atomization temperature was 2400 OC. Samples, 10 ~1, were only dried. With the two-step furnace argon containing 0.2% of methane was used. Background correction was used.

Gas phase interference effects in isothermal atomizers

As discussed above, solute volatilization interference effects can be controlled by means of peak area evaluation in combination with atomization at a constant temperature. For this reason the two-step furnace facilitates systematic studies of gas phase interference effects and this means that optimum atomization temperatures can be established easily for given analytical conditions. In order to find a gas phase composition which gives maximum atom formation we have proposed the use of high temperature equilibrium calculations.

Systems which are far from a state of equilibrium are known to be very susceptible to varia- tions in the' analytical conditions. For graphite furnace atomic absorption spectroscopy it should therefore be advantageous to work under conditions for which a state close to equilibrium is attained. The crucial factors for this are temperature and time. The agreement between the results obtained for 1 ng of lead in 40 up of iron chloride with long (0) and short (0) tubes as shown in Fig. 5 indicates that rapid kinetics are involved. The calculated recoveries for lead based on different assumptions with regard to the amount of chloride and water present in the graphite cup immediately before the onset of atomization are also shown in Fig. 5. It is evident from the complete overlap of the signal traces for the iron chloride background and lead in iron chloride (Fig. 6), that iron chloride and lead is simultaneously volatilized. This means the application of the calculations should be straightforward since no selective volatilization takes place and since no condensed iron chloride phases are present. The assumptions with regard to the amount of chloride left in the cup for the theoretical curves shown in Fig. 5 are based on experimental results of separate experiments [27] and the curves which enclose the shaded area represent the extreme values. The theoretical results were also found to be critically dependent on the amount of residual water assumed to be left in the cup before atomization. As we have found in earlier studies [2'71, this

1442 W. FRECH et al.

amount varies considerably for different types of graphite. is used in our experiments,

For non-pyrolytic graphite which amounts in the micromole range were found to be still left after

the drying step. In Fig. 5 two different amounts of water are compared, zero (lowest curve) and 3 umol, respectively. The good fit between theoretical and experimental results for the higher amount of water shows that the amounts of water as well as chloride have been esti- mated accurately. Further evidence for the correctness of our assumptions will be given in the next paragraph. in which we will discuss the atomization of lead in the presence of a sodium sulphate matrix.

100

50

0

0.6

b c 0.4

9" G

z 0.2

0

1300 1700 2100 2500

Temp I K

Temperature 1540 K

ri . I. I * I. 1. I

2 4 6 8 10

Time/s

Fig. 5. Comparison between equilibrium calculations and experimental results for 0.5 ng of lead in the presence of 0.25 urn01 of iron(II1) chloride for two different tubes, 25 mm (0) and 16 mm (0). The recoveries are related to aqueous solutions. Peak areas were evaluated throughout. For the upper curves we used input amounts corresponding to 3 umol of water while for the lowest curve no water was assumed to be present. The shaded area shows theoretical results for two input amounts of chloride, 0.50 and 0.25 umol, respectively. The lowest curve gives values assuming 0.50 urn01 of chloride as input amount.

Fig. 6. Absorbance signals for 0.5 ng of lead in iron chloride and in aqueous solutions. The background absorbance for iron chloride and the tube temperature are given as well.

In Fig. 7a we show, for different input amounts of water, calculated recoveries of lead in the presence of sodium sulphate. Experimental recoveries are given as well. The upper curves which were based on the assumption that 3 and 1 umol of water were present during atomization, correlate best with the experimental results, in particular at temperatures above 1500 K. Here we can notice that our assumption with regard to water fits two different systems. In separate calculations we have found that under the normal working conditions of a graphite furnace, oxygen should not play any significant role in the formation of atomic lead in the presence of iron chloride. On the other hand, oxygen is of importance for the recovery of lead in a sodium sulphate-matrix, and this has been discussed elsewhere (281.

If we use the input amounts for the upper curve of Fig. 7a and assume that gaseous reaction products do not further react with solid carbon, we obtain the lower curve shown in Fig. 7b. The upper curves in Figs. 7a and 7b are identical. Thus the shaded area of Fig. 7b gives the uncertainty for various degrees of completeness for the heterogeneous reactions. The filled symbols of Fig. 7b show the experimental recoveries obtained with a 16 mm pyrolytically coated tubes. The differences in the results, comparing the tubes, can be explained by incomplete reactions due to shorter reaction times, smaller amounts of water left and lower reactivity of the pyrolytical surface. It is interesting to note that no agreement between experimental and theoretical recoveries is obtained below 1600 K irrespective of tube length. Similar results have been reported elsewhere 1131.

The results discussed in the last two paragraphs indicate that rather high temperatures may be required in order to obtain a state close to a gas phase equilibrium. This means, it is important to consider kinetics in order to understand gas phase interference effects.


100

a@ . E

f

!50-

Qd I I I 1

lwo 1700 2100 2soo

TemplK

100

50

0 I 1 1 1 1300 1700 2100 2!500

Tamp I K

Fig. 7 a. Caaparison between equilibrium calculations (solid lines) and experimental results (0) for 0.5 ng of lead in the presence of 0.05 umol of sodium sulphate. The recoveries are related to aqueous solutions. Peak areas were evaluated throughout. The tube length was 25 mm. The solid lines represent recoveries for the following input amounts of water (umol, from lower to upper curve): 0, 0.1, 1, 3.

Fig. 7 b. Curve A the same as in Fig. 7 a. Curve B gives calculated recoveries assuming that gaseous reaction products are formed by a quantitative reaction between carbon and oxygen (from water and sulphate). In order to simulate an incomplete reaction between gas phase products and the graphite wall we considered only gas phase equili- bria. The filled symbols give experimental results for a 16 nrm tube.

It should be stressed that the aim of the studies illustrated in the last figures was not to establish optimum conditions for atomization but to show the applicability of theoretical calculations in order to get some insight into the complex chemistry involved in graphite furnace atomic absorption spectroscopy.

The accuracy of the results given in Figs. 5, 7 a and 7 b depends, to a great extent, on the reliability in the measurement of the graphite tube temperature. One way to reduce problems caused by the emissivity of graphite or by heterogeneous temperatures along the tube is to relate the equilibrium calculations to spectroscopic temperatures. Because of uncertainties in the relative gf-values and line shapes of the various spectroscopic lines the necessary constants for the calculation of temperatures were determined experimentally 181. The correlation between temperatures as measured with an optical pyrometer and spectroscopic temperatures is given in Fig. 8. The results indicate that temperature gradients along the tube (25 mm) are negligible. The spectroscopic temperatures [al, obtained with a 16 mm tube which was centrally heated by only one electrode pair were, depending on the final temperature, 50 to 150 K lower compared to the tube heated by two electrode pairs, indicating that temperature gradients appear in the 1.6 nun tube if heated by only one electrode pair.

W. FRECH et al.

2600 -

Y .

2 2200

t ;.

,o

g 1600-

I I I I I

1400 1600 2200 2600

Temp (sped I K

Fig. 8. Correlation between tube wall temperatures as measured with an optical pyrometer and spectroscopic temperatures. The pyrometer was focused on the outer tube wall between one of the electrode pairs.

CONCLUSIONS AND FlJl'tJHJI ASPECTS

As shown in this paper, promising results have been obtained with the two-step constant perature atomizer. However, the present design is not as convenient as the commercially available Massmann-type atomizers. For example, the contact electrodes do consume large

tem-

amounts of power. An additional drawback is the greater complexity of such a system in which six electrodes, one tube and one cup are involved. However, it is possible to realize the principles of a two-step furnace by simpler designs by sacrificing some flexibility. Recently one of the authors presented an atomizer based on an end-heated tube combined with an inde- pendently heated cup 1291. It should be pointed out that a two-step furnace can be easily combined with existing instruments. Thus it is possible to apply autosamplers directly and in addition it should be possible to utilize existing methodology.

I I

0 1.2 214 3.6 4:6 60

TIME Is

Fig. 9. Graphite furnace emission signals for lead and aluminum. The outer tube wall temperature was 2350 OC. A pyrolytically coated 16 mm long tube was used.

Electrothermalatomicaborptionspectroscopy 1445

More work is needed, however, in order to completely understand the latent potential of the two-step furnace concept. For example, the fact that tube and sample cup can be heated sepa- rately permit controlled volatilization of the analyte into a constant temperature environment which facilitates speciation studies. As has been shown elsewhere, with the separate control of volatilization and atomization, identical analytical conditions can be used for many elements which facilitate multielement determinations [121.

Another interesting field comprises graphite furnace atomic emission spectroscopy. Constant temperature atomizers should be suitable for volatile elements in particular, since high end controlled excitation temperatures can be provided. Some preliminary results for graphite furnace emission spectroscopy are given in Fig. 9. It can be seen that the detection limits for lead and aluminum differ by only 5 to 10 times. This should be compared with the corresponding data obtained with atomization from a platform 1301 for which the detection limit for aluminum is 113 times lower than that of lead.

As discussed in connection with high temperature equilibrium calculations, the fraction of free atoms which is formed in a graphite tube, is strongly dependent on the composition of the gas phase. Under the normal working conditions of a graphite furnace a pure inert gas, with low buffer capacity, is used. This means the composition of the gas phase can be strongly affected by sample constituents and by the surface properties of the graphite tube. Con- sequently, in order to better control the quantitative formation of atoms, it is desirable to use gas buffers instead of pure inert gas atmospheres. However, much more work is needed to establish and apply the specific optimum conditions for different element/matrix combinations.

In view of the results presented in this paper it seems obvious that future atomizer develop- ments should be directed towards constant temperature units which permit separate control of the vaporization step. It should be stressed, however, that matrix interference effects can- not be completely reduced even for constant temperature atomizers [311 which means that matrix modifiers will play a very important role in practical analytical work even in the future.

REFERENCES

[ll

[21

[31

I41

[51

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Documents

Electrothermal atomic absorption spectroscopy-Present understanding and future needs