Electrothermal vaporization and pyrolysis of materials for environmental analysis

37

Journal of Analytical and Applied Pyrolysis, 25 (1993) 31-48 Elsevier Science Publishers B.V., Amsterdam

Electrothermal vaporization for environmental analysis

and pyrolysis of materials

A. Golloch * and M. Haveresch-Kock Universitiit-GH-Duisburg, (Germany)

FB 6-instrumentelle Analytik, Lotharstr. 1, W-4100 Duisburg

W.G. Fischer Fischer Labor- und Verf~renstechn~k, Bonn (Germany)

Industriepark Kottenforst,

W-5309 ~eckenheim,

(Received October 15, 1992; accepted in final form November 23, 1992)

ABSTRACT An electrothermal vaporization unit (ETV) for solid and low-volume liquid sample introduction into inductively coupled plasma (ICP) and other excitation sources has been developed. The basis of the system is a graphite bridge with a ringshaped holder for exchangeable graphite cups of volume 500 or 700 ~1. PC controlled heating cycles can be applied to heat up the sample cups by resistance heating to desolvate, ash or pyrolyze and finally vaporize analyte material from the cup. The system is continuously purged by argon to protect it from oxidation and to carry the analyte vapor into the ICP. Solids can be weighed directly into sample cups without sample pretreatment. Starting the analysis, the cups are put into the graphite bridge via the entry port of the vaporization chamber. The greatest advantage of the system is the ability to exchange the cups quickly. No cleaning of the cups is necessary and therefore samples can be analyzed with high frequency. The ETV system was developed for coupling with ICP-atomic emission spectroscopy and used for several applications, e.g. determination of trace elements in sewage sludge, soils and biological samples. Electrothermal

vaporization;

environmental

analysis; inductively coupled plasma; pyrolysis.

The importance of trace elements in environmental analysis and other fields has been well recognized. Many different materials like soil, sediments, water, leaves or animal tissues must be analyzed. In particular, the concentration of heavy metals such as Cd and Pb must be controlled. Considering the analytical methods for trace element determination, atomic emission spectroscopy with an inductively coupled plasma (ICP) is one of the most

* Corresponding

author.

0165-2370/931$06.00 0 1993 - Elsevier Science Publishers B.V. All rights reserved

38

A. Golloch et al. / J. Anal. Appl. Pyrolysis 25 (1993) 37-48

commonly used methods, because of its good sensitivity and accuracy. However, one of the shortcomings of this technique is the necessity of a sample preparation step, because only liquid samples can be introduced into the plasma by nebulization with pneumatic nebulizers. Therefore, an additional step is necessary to dissolve or digest solid samples. Since the ICP technique was introduced, researchers have investigated the direct analysis of solid samples [l-6], because a number of analytical and practical advantages can be realized if solid samples are introduced directly into the ICP: contamination from reagents is minimized, dilution errors are eliminated, sample transfer losses arising from extra sample handling steps are avoided and time delay between sample collection and analysis is diminished. Ideally a method for direct trace element solid sample analysis should have the following attributes: (i) it should b e applicable to a wide range of sample compositions; (ii) it should be relatively fast; (iii) calibration should be simple; (iv) to avoid inhomogeneity problems, the method should be capable of handling fairly large samples; (v) reproducibility and accuracy should be suitable for the particular application. ELECTROTHERMAL

VAPORIZATION

Compared with established solid sampling techniques like arc, spark and laser ablation, electrothermal vaporization in combination with ICP is of such a quality that it is potentially attractive as a device for the direct trace analysis of solids [4,5]. The main advantage of ICP is the high temperature of the plasma attained in the zone of sample introduction, which may result in excellent breakdown of the sample, atomization and excitation. Also, the ICP is very stable and if the introduction of solids does not considerably disturb the plasma, then good precision of measurement (simultaneous or sequential multielement determination) will be possible. Using electrothermal vaporization (ETV) for sample introduction into the ICP, small liquid or solid samples can be vaporized by resistance heating and carried into the plasma by an argon stream. In contrast to the use of electrothermal devices for atomic absorption spectroscopy (AAS), it is not necessary to produce atoms of the analyte during the heating cycle. The analyte may be in any form (e.g. molecular, particulate), because the ETV simply dries and vaporizes the sample. Only a discrete pulse of analyte is transported into the plasma where the atomization and excitation take place. Since samples are introduced as dry particles, aggregates, molecules and partially atomized vapor clouds into the plasma, the energy is used more effectively in the atomization, ionization or excitation since there is no

A. GolIoch et al. / J. Anal. Appl. Pyrolysis

25 (1993) 37-48

39

need for desolvation. Of course, as in the application of AAS, premature loss of analyte during drying or ashing must be avoided. In comparison with nebulization techniques for sample introduction into ICP, ETV transfers analyte with substantially higher transport efficiency (60-80% instead of I- 10%) [5]. The ETV also introduces a transient cloud of analyte particles into the atom source with a population density greater than that attainable from pneumatic nebulization. Therefore, the concentration of analyte in the plasma is an order of magnitude higher than that from pneumatic nebulization, and detection limits are one order of magnitude lower than those from pneumatic nebulization. The separation of ETV and atom reservoir also facilitates independent optimization. Therefore, a single set of ETV operating conditions is usually sufficient for many elements.

EXPERIMENTAL

ETV sample holders Materials for ETV sample holders are similar to those which have been investigated for graphite furnace AAS (GFAAS) [1,3-61. These are either metals (W, Ta, Pt) or graphite. Available forms of the sample holders are a graphite rod, tube, cup or yarn, and metal filaments or loops. Graphite is prefered because many metals react with sample components at high temperatures, and some substrates may be vaporized and cause interference with many useful analytical lines. Design of the new ETV The basis of the system is a graphite bridge with a ringshaped holder for exchangeable graphite cups of volume 500 or 700 ~1 (Fig. 1). The durability of the cups is up to 300 heating cycles, depending on the temperature program. To achieve high maximum temperatures of the sample cups, graphite bridges of special design have been developed (Ringsdorff-Werke GmbH, Bonn) (Fig. 2). ETV 100 technical data ET V power surely The power supply is 230 V t_ loo/o, 50-60 Hz; power consumption is 50-3.600 VA; output voltage is O-10 VAC; output current is O-360A; data transfer is V.24/RS 232; capacity program is programmable via V.24, 1 to 100 current stages; resolution is 0.01 V resp. 0.1 s (Fig. 3).

A. Golloch et al. 1 J. Anal. Appl. Pyrolysis 25 (1993) 37-48

s

6 7

Fig. 1. Schematic diagram of the electrothermal vaporization unit. Graphite cup (1); graphite bridge (2); copper electrodes (3); glass dome (4); argon inlet (5); water supply (6); power supply (7).

Fig. 2. Schematic diagram of the graphite (quality RW 0) bridges and sample cups (Ringsdorff-Werke GmbH, Bonn, Germany). Certified impurities (pg gg’): B < 0.01; Ca < 0.2; Cu < 0.08; Fe = 0.2; Mg = 0.01; Si = 0.05; Ti < 0.5; V < 0.2.

Instrumental set up ICP A 27.5 MHz Spectroflame (Spectra A.1) spectrometer with a free running generator of power 800-2500 W was used. Gas flow rates were: coolant, 12-14 1 min-‘; auxiliary 0.5 to 1 1min-‘; nebulizer, 1 1min-‘.

A. Golloch et al. /J.

Anal. Appl. Pyrolysis

41

25 (1993) 37-48

3000

400

300

4 .

200

5

100

5 a ‘j 0

500

0

0 0

2

8

10

Fig. 3. Output voltage of the ETV power supply vs. output current and temperature of the graphite bridge.

Optical system

The monochromator (Spectra A.I.) had a Paschen-Runge mounting of 700 mm focal length, a holographic grating of 2400 grooves mm-‘, a wavelength range of 200-480 nm, and a dispersion of 0.5 nm mm-’ (1st order). The polychromator (Spectra AI) had a Paschen-Runge mounting of 700 mm focal length, a holographic grating of 2400 grooves mm-‘, a wavelength range of 200-490 nm, and dispersions of 0.5 nm mm-’ (1st order) and 0.25 nm mm-’ (2nd order). Analyzed samples

For studying the application of the ETV-ICP for environmental samples, various standard reference materials (SRM) from the Community Bureau of Reference (BCR) have been used (Table 1). Used anafyticai emission lines

Cd I, 228.8 nm; Pb II, 220.3 nm; Zn II, 213.8 nm; Al I, 220.4 nm. Evaluation of emission signals

Because the shape of the signals depends on various parameters, areas were used for quantitative analysis.

peak

Analysis of solid samples

Using ETV for solid sample introduction into ICP, PC controlled heating cycles can be applied for heating up the sample cups by resistance heating

42

A. Golloch et al. 1 J. Anal. Appl. Pyrolysis 25 (1993) 37-48

TABLE

I

BCR-SRMs

analyzed

using the ETV-ICP

Reference

technique

material

Certified Cd

BCR BCR BCR BCR BCR

Calcareous loam soil Light sandy soil Sewage sludge amended soil Sewage sludge of domestic origin Sewage sludge of mainly industrial origin City waste incineration ash Fly ash from the combustion of pulverized coal

141 142 143 144 146

BCR 176 BCR 038

0.36 0.25 31.1 3.41 77.7 470 4.6

value (pg g-‘) Zn 81.3 92.3 1272 3143 4059 25770 581

Pb 29.4 37.8 1333 495 1270 10870 262

in order to desolvate, ash or pyrolyze and finally vaporize analyte material from the cup. Solids can be weighed into sample cups directly without sample pretreatment. Starting the analysis, the cups are put into the graphite bridge via the entry port of the vaporization chamber. The system is continuously purged with argon to protect it from oxidation and to carry the analyte vapor into the ICP, where atomization and excitation take place. Measurements can be carried out with high frequency (lo-30 samples/h, depending on the kind of matrix), because no cleaning of the cups is necessary. Pyrolytically coated graphite cups provide a barrier between vapor-phase atoms and the cup material to avoid the formation of low-volatility carbides of some elements. Analysis

of heavy metals in soils, sewage sludges and ashes

As an example, the advantages of the electrothermal vaporization unit will be demonstrated on the analysis of different BCR-SRMs of soils, sewage sludges and ashes. The elements studied were Cd, Pb and Zn, because they are important in the environment and are of low or medium volatility. In most cases strong acid digestion, e.g. aqua regia, is applied for the determination of concentrations of heavy or toxic metals, if environmental samples have to be analyzed [ 71. However, using the ETV, sample weight was varied from 10 to 40 mg depending on the concentration of investigated elements. The volatile components of the solid sample were vaporized into ICP without any preparation. Optimization

of operating parameters

In the ETV-ICP technique, operating conditions such as rf power, observation height (depending on the excitation conditions), gas flow rates of the ICP, carrier gas flow rate and temperature programs of the ETV, have

43

A. Golloch et al. /J. Anal. Appl. Pyrolysis 25 (1993) 37-48

to be optimized. For simultaneous determination of several elements it is necessary to work under a compromise of conditions. As mentioned, the main difference between ETV and analyses of liquid samples is the introduction of a dry aerosol into the plasma. Since dry particles enter the plasma, excitation of the atoms or ions take place in a lower zone of the plasma. Therefore, the best observation height is about 5 mm above the load coil in contrast to a height of lo-20 mm when using pneumatic nebulizers. The operating conditions of the ICP were: power, 1200 W; coolant gas flow rate, 14 1min-‘; auxiliary gas flow rate, 0.75 1min-‘; ETV gas flow rate, 0.33 1min’; observation height, 5 mm above the load coil. Uptim~~ution of the temperature program

In order to obtain the optimum emission signal using ETV it is necessary to have a rapid rate of vaporization of analyte from the graphite cup and then rapid transport of the analyte vapor into the plasma. Deposition of the analyte on the vaporization chamber walls and connection tubes must be minimized, as must turbulent mixing of the aerosol with the carrier gas stream. Starting the analysis a low current is applied to dry the sample, followed by an optional step to “ash” (pyrolyze organic components or decompose the sample), a defined current is programmed to vaporize analyte from sample and finally a high current is applied to vaporize the matrix if necessary. The temperature program (Table 2) has been optimized for the simultaneous vaporization of the medium volatile elements Cd, Pb, and Zn from the SRMs using a monochromator to evaluate peakshape and reproducibility of emission intensity. On the one hand, organic solvents from the matrices should be removed before the vaporization of the analytes, but on the other hand, matrix elements should remain in the sample cup to avoid interference. TABLE

2

Temperature program sludges and ashes Stage

for the direct analysis

Time (s)

Voltage

10 10 10 10 5 5 5 30

1.5 2.0 2.5 3.0 4.0 4.5 5.0 6.0

of the heavy metals Cd, Pb and Zn from soils,

(V)

Temperature 560 780 1000 1200 1470 1600 1720 1900

(“C)

A. Golloch et al. /J.

44

RESULTS

AND

Anal. Appl. Pyrolysis 25 (1993) 37-48

DISCUSSION

Peak shape It has been found that under optimized operating conditions, the complete amount of Pb and Zn from the SRMs will be vaporized at a temperature of approximately 1000°C. There are no differences in times of appearance and peak shapes of the emission signals from different samples. Also, the peak shape is independent of the amount of analyte, and tailing of the signals has not been observed (Fig. 4). Quantitative analysis of the SRMs As for other direct solid sampling techniques, great attention must be paid to the method of calibration. Since real samples are very complex, preparation of artificial standards is difficult. However, complete matching of the standard material to the sample may be required for accurate measurements. Therefore, we investigated the applicability of SRMs for calibration. First, different quantities of the SRMs were analyzed to get information about the linear dynamic range of the method for each of the investigated elements, 15 samples of each SRM with increasing sample quantity were analyzed with repetition of two samples of 20 mg weight to take into account drift of the working parameters. It has been found that sample masses of 5-30 mg may be vaporized without extinguishing the plasma, and emission intensities lay within the linear working range.

0

I

I

BCR

141

ng Pb

1407

I

142 1617

Fig. 4. Signal profile obtained

143 I 5594

I

144 I 10207

after vaporization

I

146 I 25819

I

176 I 118374

I

038 I 26846

of lead from different

SRMs.

A. GoNoch et al. 1 J. Anal. Appl. Pyrolysis

45

25 (1993) 37-48

10

40

Mass of B&&M .

Fig. 5. Emission

intensity

Repetition

143 /r~?~~ samples

of zinc from BCR 143 vs. sample mass.

Since a linear correlation of emission intensity with sample mass (see Fig. 5) has been found the application of SRMs was tested to extend the calibration range. 5, 10, 15, 20, 25 and 30 mg of each available SRM were weighed without pretreatment into sample cups and vaporized into ICP. As shown in Fig. 6, calibration curves are obtained using SRMs similar to the samples. However, it has been observed that the slopes of calibration curves of the used SRMs differ a little. One reason may be the influence of

-2

1000000

a d

.2 100000 h

4

.Z

10000

cI(

g

1000

.I

i .C(

100 100

1000

100000

10000

Amount .

BCR

141

.

BCR

142

A

BCR

143

*

BCR

146

X

BCR

176

+

BCR

038

Fig. 6. Emission

intensity

1000000

of Lead /ng

of lead from different

0

BCR

SRMs vs. amount

144

of analyte.

46

A. Golloch et al. / J. Anal. Appl. Pyrolysis 25 (1993) 37-48

concomitant elements on vaporization characteristics of the analyte or changes in the excitation characteristics of the plasma caused by various matrix compositions of the SRMs. However, it may be noticed that the semiquantitative calibration of samples is possible using this method. Method of standard additions If suitable reference materials for the analysis of complex sample matrices are not available, the method of standard additions is often applied. Reasons for standard addition instead of calibration curves: (i) easy ionizable matrix elements influence the emission intensity; (ii) variation in emission intensity is caused by the effect of concomitants on transport of aerosol from the electrothermal vaporizer to the ICP; (iii) depression of analyte emission intensity is caused by matrix components decreasing the rate of analyte release; (iv) enhancement of analyte emission intensity is caused by the influence of a concomitant element giving matrix stabilization. In some cases calibration can be performed using liquid multielement standards. Vaporized analyte from the matrix may have the same vaporization characteristics as analyte from aqueous standard solution. However, in many cases interference is observed. Sample pretreatment For quantitative analysis of the BCR-SRM 143 and 146, 20 and 30 mg, respectively, of the sample were weighed into sample cups. Then 200 ~1 of aqueous standard solution was added to the solid. It was necessary for the solution to be completely mixed with the solid to obtain comparable vaporization behavior of each sample. The sample may be dried outwards the system or by means of an extra drying step before vaporization. Two different masses of samples have been used to control whether the slopes of the calibration curves are the same. Otherwise, multiplicative interferences may probably influence emission intensities. The third calibration curve was used to study the applicability of aqueous standard solutions for calibration. It must be considered that, in contrast to pneumatic nebulization of sample solutions into ICP, concentration of analyte and standard solution must be converted into absolute masses for calculation when ETV is used. Most of the results correspond well with the certified values of SRMs (see Table 3). The method of standard additions can be used for accurate determination in complex samples. However, this method is time consuming and not always the best choice for rapid multielement analysis. In most cases it is more important to get general information about the variety of analytes present and their semiquantitative concentrations; then calibration with SRMs may be used.

47

A. Golloch et u1.I J. Anal. Appl. Pyrolysis 25 (1993) 37-48

TABLE Results

3 of the analysis

of soils (method

of standard

addition)

value (pg gg

‘)

Value found

SRM

Element

Certified

BCR 143

Cd Pb Zn

31.1 f 1.2 1333 f 39 1272f30

30.6 + 6 1168 k 100 1231 F 102

BCR 146

Cd Pb Zn

77.7 f 2.6 1270+28 4059; k 96

75.5 + 11 1215 & 50 3468 + 143

(pg g-‘)

Selective vaporization

If complex matrices are analyzed, spectral interferences are often observed. Some of the interferences are caused by the electrothermal vaporization process while others are caused by changes in the plasma conditions. For example, the heating program does not only affect the degree but also the sign of the interference. Since elements of different volatilities reach the plasma at different times, this effect might be very useful in the elimination of some interference effects. If the interferent reaches the plasma either before or after the analyte, its effect on the analyte signal can simply be time gated out. See Fig. 7 for the temperature dependent vaporization of Pb and Al from BCR 146 (sewage sludge of mainly industrial origin).

D

8

6

Voltage Pb II: 220.35

nm,

iv

Al k220.46

nm

Fig. 7. Emission intensity of lead and aluminium vaporization from BCR 146.

vs. voltage

for temperature

dependent

48

A. Golloch et al. 1 J. Anal. Appl. Pyrolysis

25 (1993)

37-48

CONCLUSION

ETV is a useful method for solid sample introduction into different excitation units, e.g. ICP without any sample preparation. Available exchangeable sample cups offer the possibility of rapid analysis of different kinds of samples without cleaning the system. ETV-ICP can supply a lot of important information about environmental sample composition. The main advantage of this technique is the possibility of rapid multielement determination (sequentially or simultaneously) of unknown samples. A relative standard deviation of 3-10% was obtained. ACKNOWLEDGMENTS

This work was supported by the Institute for Environmenta and Environmental Analysis (IUTA), Duisburg, Germany.

Techniques

REFERENCES t M.W. Routh and M.W. Tikkanen, in A. Montaser and D.W. Golightly (Eds.) Inductively Coupled Plasmas in Analytical Atomic Spectrometry, VCH, New York, Chapter 12, 1987. 2 G.L. Moore, Introduction to Inductively Coupled Plasma Atomic Emission Spectrometry, in Analytical Spectroscopy Library, Vol. 3, Elsevier, Amsterdam, 1989. 3 P.W.J.M. Boumans, Inductively Coupled Plasma Emission Spectrometry, Part 2, Applications and Fundamentals, Wiley, New York, 1987. 4 K.C. Ng and J.A. Caruso, in J. Sneddon (Ed.), Sample Introduction in Atomic Spectroscopy, Analytical Spectroscopy Library, Vol. 4, Elsevier, Amsterdam, 1991. 5 H. Matusiewicz, J. Anal. Atom. Spectrom., 1 (1986) 171. 6 T. Kantor, Spectrochim. Acta, Part B, 38 (1983) 1483. 7 A.M. Ure, Fresenius’ Z. Anal. Chem, 337 (1990) 577.