JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, APRIL 1991, VOL. 6

Fluorination and Volatilization of Refractory Elements from a Furnace for Sample Introduction into an Inductively Coupled Using a Polytetrafluoroethylene Slurry

Min Huang,* Zucheng Jiangt and Yun'e Zeng Department of Chemistry, Wuhan University, Wuhan 430072, China

22 1

Graphite Plasma by

A method was developed to determine refractory elements by inductively coupled plasma atomic emission spectrometry (ICP-AES) with sample introduction by electrothermal vaporization. A slurry of polytetrafluoroethylene was used to form fluorides rather than carbides of the elements. In this way, the refractory elements were efficiently vaporized and subsequently introduced into the plasma. The detection limits for Zr, V, Cr, W, Mo, B and Ti were improved by a factor of 7-1 19, compared with those of electrothermal vaporization ICP-AES without the fluorinating agent. No memory effects were observed and adequate precision was obtained. The fluorination process is also discussed. Keywords: Electrothermal vaporization; refractory elements; polytetrafluoroethylene slurry; inductively coupled plasma atomic emission spectrometry; fluorinating agent

The most popular sample introduction technique used in in- ductively coupled plasma atomic emission spectrometry (ICP- AES) is pneumatic nebulization. It is simple and convenient to operate. However, the technique suffers from low efficiency of sample transport and presents some difficulties with viscous, high salt content solutions and micro-volumes of samples.

The use of electrothermal vaporization (ETV) as a sample introduction technique for ICP-AES has been reported.I-l0 The detection limits were improved by one or two orders of magni- tude for most of the elements tested, compared with those of pneumatic nebulization. However, a problem occurred when refractory elements which combine strongly with carbon to form refractory carbides were determined.6 Very low sensitivi- ty and severe memory effects were encountered. Tantalum filament vaporizers have been used as a means of avoiding carbide formation,' but the tantalum metal becomes brittle after repeated heating cycles.

The addition of a halogenating agent to improve the vola- tilization process was used originally in arc spectroscopy.' In previous work with ETV-ICP-AES, Kirkbright and Snook6 and Satumba et al.' added trifluoromethane or chlo- rine to the injector gas and improved the detection limits of refractory elements. Ng and Caruso8 added ammonium chlo- ride (7% m/v) as a means of vaporizing the analytes as their chlorides.

In the present study a polytetrafluoroethylene (PTFE) slurry was used as a fluorinating agent to promote the vaporization of refractory elements from a graphite furnace for determination by ICP-AES. The analytical characteristics of the technique and the advantages of PTFE as a fluorinating agent were ex- plored. The fluorination process and the optimization of the conditions were also investigated.

Experimental Instrumentation and Operating Conditions In this study, a graphite furnace WF-4, which is similar to an HGA 500, was employed as the vaporization device. The original silica windows at the two ends of the furnace were removed and replaced with two PTFE cylinders, one of which was connected to the injector tube of the plasma torch viu a plastic tube (4 mm i.d. x 0.5 m). The other was blocked to

* Present address: Centre of Material Research and Testing, Wuhan

t To whom correspondence should be addressed. University of Technology, Wuhan 430070, China.

prohibit air from entering the graphite furnace. The analyte was vaporized and carried to the excitation source by a stream of argon gas.

The instruments used in this work are listed in Table 1 and the optimized operating conditions are given in Table 2.

Preparation of Slurry Sample With ultrasonic wave vibration and in the presence of OP sur- factant [RC2H40(C2H,),,H 1, very fine PTFE powder was dis- persed into the propanol medium which can be mixed with water in any ratio. The slurry containing 60% m/v PTFE is also commercially available and remains stable for a long time. The slurry sample was prepared by adding PTFE slurry to a liquid sample. Unless otherwise stated, the PTFE content of the slurry sample was 1.8% m/v.

Table 1 Instrumentation

Plasma power Plasma generator, 2 kW (Beijing Broadcast Instru- ment Factory, China); frequency 27.12 MHz; power output, G 2 . 0 kW; load coil, 2.5 turns

Monochrometer, WDG 500- 1 A Type (Beijing Second Optics, China); Czerny-Turner mounting with 1200 lines mm-' grating blazed at 250.0 nm; focal length, 0.5 m with reciprocal linear disper- sion, 1.6 nm mm-'

Plasma imaged in a 1 : 1 ratio onto the entrance slit. Slit-width, 25 pm

Conventional type silica torch. Injector tube, 1.5 mm i.d.

Signal from photomultiplier (R456, Hamamatsu, Japan) was measured using potentiornetric recor- der LZ3- 104 (Shichuang Fourth Instruments Works, Shichuang Province, China)

Model WF-4 electrothermal device (Beijing Second Optics, China), which is similar to an HGA 500

Plastic tube, 4 mm i.d. x 0.5 m

Spectrometer

Optics

Plasma torch

Read-out

Graphite furnace

Interface

Table 2 Operating conditions

Incident power Observation height Carrier argon gas Coolant argon gas Auxiliary argon gas Heating cycle

of graphite furnace

Sample introduction

1.0 kW 15 mm (above working coil) 0.8 1 min-' 13 1 min-I 0.8 1 min-' Drying time, I0 s at 100 "C; ashing time,

10 s at 250 "C; atomization time, 6 s at 2500 "C

20 p1 micropipette with disposable poly- ethylene tip

Publ

ishe

d on

01

Janu

ary

1991

. Dow

nloa

ded

by U

nive

rsity

of

Prin

ce E

dwar

d Is

land

on

27/1

0/20

14 2

0:28

:54.

View Article Online / Journal Homepage / Table of Contents for this issue

222

B C Ik w u

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, APRIL 1991, VOL. 6

t \

A

lr- 10 s H

B

.$ f -

Fig. 1 Signal profiles for Ti, 20 pI sample: A, 1.0 pg ml-' solution without PTFE; B, residual signal of the first firing; and C, 1.0 pg ml-I Ti solution with 1.8% m/v PTFE, intensity I x 20

10 s w

t \

f-

Fig. 2 Signal profiles of V, 20 pl sample: A, 1.0 pg ml-I V solution without PTFE; B, residual signal of the first firing; and C, 1.0 pg ml-' so- lution with l .8% m/v PTFE, intensity I x 10

Table 3 (from reference 12)

Boiling points of carbides and fluorides for various elements

Boiling point/"C

Element

Ti V Cr Mo W B Zr C

Element

3400 3000 2672 4804 5927 2500 4377

3652 (sublimation)

Carbide Fluoride

4820 284 3900 1000 3800 1 100

35 6000 17.5 3500 - 99 5100 800

-

Procedure A 20 pl volume of the test sample was pipetted into the graph- ite furnace. The sample inlet hole was blocked using a graphite rod. After being dried and ashed, the analyte was vaporized and carried into the plasma under optimized conditions. The peak height was measured for calibration.

Results and Discussion Principles of Fluorination-Vaporization In the absence of PTFE, the signal intensities of the refractory elements in ETV-ICP-AES were very weak, and broad signal profiles with long tails were recorded as shown in Figs. 1 and 2. Furthermore, severe memory effects were observed. No signal was detected for 1 .O pg ml-I of zirconium. The residual signals for titanium and vanadium were almost the same as the original ones.

I I A 3s I I H I

f-

Fig. 3 Recorder tracings for 20 pl samples: A, 0.1 pg ml-I Zr in 1.8% PTFE; B, residual signal of the first firing after vaporizing 20 pl of 10 pg ml-I Zr in 1.8% PTFE; and C, residual signal for the second firing

However, the addition of PTFE changed the situation as fluorine can combine very strongly with the refractory ele- ments. In this situation, the fluorinating reaction is the main re- action taking place in the graphite tube at high temperatures, and the fluorides produced can be vaporized in the graphite furnace without any problem because the boiling points of fluorides are much lower than those of carbides, as shown in Table 3.

A sharp and intense signal profile was recorded when fluorination-vaporization was applied. The signal profiles of Ti and V with and without the presence of PTFE in the samples are shown in Figs. 1 and 2, respectively. Similar results were observed for Zr, Cr, Mo, W and B. It was found that there were no memory effects on the performance after high concentrations of these elements were vaporized. As shown in Fig. 3, the residual signal intensity is less than 0.1% for 10 pg ml-i of Zr under the experimental conditions used.

The slurry was prepared prior to injection of a 20 p1 aliquot into the furnace. Another experiment in which the sample and PTFE sluny were pipetted separately into the furnace was also carried out. However, the signal intensity enhancement was not sufficient owing to separation of the PTFE and the refrac- tory elements, which reduced the probability of forming the fluoride. Furthermore, this method is not convenient.

Ashing Curves for Fluorination-Vaporization The ashing curves of the elements are shown in Figs. 4 and 5 , from which the following conclusions can be drawn: (i) no losses were found for Zr, Ti and V in the ashing step until the furnace temperature was much higher than the de- composition temperature of PTFE (415 "C) and near the va- porization temperatures of their fluorides; (ii) losses of Ti, Mo, W and B began to occur at a temperature near the de- composition point of PTFE; (iii) the ashing temperature (Ta\,,) for the element whose fluoride has a vaporization tem- perature higher than 415 "C is controlled by the vaporization temperature of the fluoride, and for the element whose fluoride has a vaporization temperature lower than 415 "C, the ashing temperature is controlled by the decomposition temperature of the PTFE, i.e., Ta,h >415 "C should be chosen. One of the most important advantages of PTFE as a fluorinating agent is its high decomposition temperature. For example, the vaporization temperature of BF, is as low as -99.7 "C, while the ashing temperature used can be as high as 415 "C. As a result many matrices, particularly organic matrices, can be removed in the ashing step.

Optimization of PTFE Content of the Slurry Sample The PTFE content was adjusted to 0.3, 0.6, 1.2, 2.4 and 6% m/v in a series of samples. Fig. 6 shows that the signal in- tensity increases with increasing content of PTFE and reaches a plateau at 1.8%. However, with 1.8% PTFE and using the heating cycle given in Table 2, the plasma dis- charge became unstable due to the vigorous decomposition

Publ

ishe

d on

01

Janu

ary

1991

. Dow

nloa

ded

by U

nive

rsity

of

Prin

ce E

dwar

d Is

land

on

27/1

0/20

14 2

0:28

:54.

View Article Online

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, APRIL 1991, VOL. 6 223

L I 1 1 I 400 800 1200

Trh PC

Fig. 4 pg ml-I; and V. 1.0 pg ml-'

Ashing curves for 20 pl samples of Cr, 0.5 pg rn1-I; Zr, 0.5

' 150 - m

C 4- .-

t $ 9 0 g Q - z

30

1 I 200 600 1200

Trh PC

Fig. 5 and Mo, 0.5 pg ml-'

Ashing curves for 20 pl samples of Ti, B and W, 1.0 pg ml-I:

0.6 1.8 3.0 PTFE concentration (% m/v)

Fig. 6 sample of a 0.5 pg ml-' solution of Zr

Effect of PTFE concentration on fluorination-vaporization; 20 pl

Table 4 Detection limits

Detection limit/ng

This work

Wavelength/ Without With Element nm PTFE PTFE Reference 6* Reference 8t

Ti 334.9 2.0 0.018 - - V 309.3 2.0 0.05 - 0.3 Cr 267.7 0.6 0.05 0.05 0.2 Mo 202.0 0.5 0.008 0.1 - W 207.9 1.2 0.16 0.16 - B 249.7 1.7 0.05 0.0s - Zr 257. I 0.4 0.004 0.01 0.02

* Cr 357.9 nm; Mo 3 13.3 nm; and W 276.4 nm. t V 437.9 nm; and Cr 357.9 nm.

5 s I .

t -

Fig. 7 Typical signal reproducibility with 20 pl samples containing 1.8% m/v PTFE and 0.5 pg ml-I Cr

of the PTFE. In order to solve this problem, a higher ashing temperature, but one which was still below the decomposi- tion temperature of the fluorides, was used and made the de- composition of PTFE less vigorous. The elements combine with fluorine as soon as they are released from the PTFE in the ashing step and are subsequently vaporized when the furnace temperature is raised in the analytical cycle. In this way, a PTFE concentration as high as 30% m/v in the sample has no effect on plasma stability. For the vaporiza- tion of samples containing a low content of the elements, 1.8% of PTFE was used.

Detection Limits and Precision The limit of detection is defined as the analyte concentration yielding a signal equal to three times the standard deviation of the background noise. Results obtained for ETV-ICP-AES with PTFE are similar to the work reported in references 6 and 8 in which other types of halogenating agents were used (Table 4). The limits of detection ranged from 4 to 160 pg. The proposed technique therefore has potential for the deter- mination of trace elements.

With fluorination-vaporization, no memory effect existed and good reproducibility was obtained. Fig. 7 represents a 3% relative standard deviation (RSD) for five replicate measure- ments of 0.5 pg ml-' of Cr. For 0.5 pg ml-' of Zr and 0.5 pg ml-1 of V, the precisions were 4 and 2%, respectively. Con- sidering that a sample size of only 20 p1 was employed, the precision obtained is reasonable.

Cali brat ion The calibration graph for Zr by using this technique is linear over a dynamic range of three orders of magnitude, from 0.01 to 10 pg ml-I.

Advantages of PTFE as a Fluorinating Agent Compared with other halogenating agents, PTFE has several advantages. (i) Fluorine is sufficiently chemically active to combine with refractory materials. (ii) The PTFE has a high fluorine content. (iii) The PTFE contains few inorganic im- purities and does not introduce any cations as do inorganic ha- logenating agents, therefore, there are no consequent vaporization or spectral interferences. (;I?) The PTFE does not act as a fluorinating agent below 415OC, enabling higher ashing temperatures to be used. (I?) The use of a finely dis- persed PTFE SIUKY ensures that the fluorination reaction is efficient. ( 1 1 i ) The PTFE slurry is easy to prepare in propanol and can be diluted with water, in any ratio, to form a stable emulsion.

Publ

ishe

d on

01

Janu

ary

1991

. Dow

nloa

ded

by U

nive

rsity

of

Prin

ce E

dwar

d Is

land

on

27/1

0/20

14 2

0:28

:54.

View Article Online

224 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, APRIL 199 I , VOL. 6

Conclusion 4 Schmenmann, S. M., Long, S. E., and Browner, R. F., J. A n d . At.

A method using PTFE as a fluorinating agent for the deterrni- nation of the refractory elements Zr, Cr, Mo, W, B, Ti and V by ETV-ICP-AES has been studied. Some advantages of using PTFE as a fluorinating agent have been demonstrated. The de- tection limits for the elements mentioned were from picogram to sub-nanogram levels and were improved by one or two orders of magnitude in comparison with the technique in the absence of fluorination. Further work on application/ interference studies and determination of rare earth elements using the fluorinating technique is being undertaken.

5

6 7

8 9

10

I 1 12

Specwom., 1987, 2, 687. Aziz, A., Broekaert, J. A. C., and Leis, F., Specwochim. Acta. Part B , 1982,37,369. Kirkbright. G. F., and Snook, R. D., Anal. Chem., 1979,51, 1938. Satumba, R. T., Boots, R. A., and Matousek. J. P., /CP /nf. Nend., 1987, 13,22. Ng, K. C., and Caruso, J. A., Analyst , 1983, 108,476. Jiang, Z.-c., and Fassel, V. A., Fen.\-; Shivanshi, 1987,6,6. Huang, M., Xu, L.-f., Jiang, Z.-c., and Zeng, Y., Chem. J. Chin. Univ.. 1989,7,709. Kantor, T., Specrrochim. Acra. Part B . 1983,38, 1483. Handbook of Chemistry and Physics, ed. West, R.C.. Chemical

1 2 3

Rubber Company, Cleveland, OH, 59th edn., 1978.

References Ng, K. C.. and Caruso, J. A., Appl. Spectrosc., 1985, 39, 7 19. Matusiewicz, H.,J . Anal. At. Sperwom., 1986, 1, 171. Nixon, D. E., Fassel. V. A., and Knisely, R. N.. Anal. Chem.. 1974, 46,2 10

Paper OlO2.548H Received June 7th, I990

Acrepted September I I th, I990

Publ

ishe

d on

01

Janu

ary

1991

. Dow

nloa

ded

by U

nive

rsity

of

Prin

ce E

dwar

d Is

land

on

27/1

0/20

14 2

0:28

:54.

View Article Online