New studies of the plasma emission detector - Springer Link

Mikrochim.Acta [Wien] 1989,III, 215 222

Mikrochimica Acta 9 by Springer-Verlag 1990

New Studies of the Plasma Emission Detector* Markus W. Bradter, Wolfgang H. Buscher, Karl Cammann**, Michael J. Faust, and Frank G. Winter Lehrstuhl fiir AnalytischeChemie,Westf/ilischeWilhelms-Universit~it,Wilhelm-Klemm-Strasse8, D-4400 Miinster, Federal Republic of Germany

Abstract. Being an element-selective detector, the plasma emission detector was used to determine carbon in organic compounds. Experiments were carried out with the aid of oscillating interference filters and lock-in amplifiers. The obtained signals were processed by a fast-Fourier-transform analyzer to study their frequency spectra. Results are given for the detection of carbon. Furthermore studies on the influence of angle-adjustment, microwave power', photomultiplier voltage and oscillation frequency on the determination of fluorine in organic compounds were carried out and results for detection limits and dynamic range of this method are presented.

Key words: plasma emission detector, microwave-induced plasma, elementselective gas chromatographic detector, fluorine determination, fast-Fouriertransform-analysis.

For quantitative trace analysis of organic compounds in environmental samples high resolution capillary gas chromatography (HRGC) has been well established as a standard method in analytical chemistry. This method provides a great variety of injection techniques and several very sensitive detectors are utilized in routine analysis. Besides the flame ionization detector (FID), the electron capture detector (ECD) and the mass selective detector (MSD) are widely used in organic trace analysis. Although they have undeniable advantages, these detectors have several drawbacks, which may cause problems in everyday analysis. In order to get quantitative results, a separate calibration for each compound to be delLected is required. Quantitative statements are often problematic if unknown compounds are to be detected or accurate standards for the surveyed compound are not available. The idea to develop an element-selective detector for HRGC, that avoids these problems, is based on the work of McCormack et al. [1] and of Bache and Lisk [2], published in 1965. Here a microwave-induced helium or argon plasma (MIP) has been used as an element-selective detector in a gas chromatographic system for * Presented in part at the 1989 European Winter Conferenceon Plasma Spectrochemistry,Reutte, Austria ** To whom correspondenceshould be addressed

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M.W. Bradter et al.

the first time. Since then, the G C - M I P coupling is widely spread in analytical research, which is shown in various excellent reviews [-3-9]. The plasma emission detector (PED) used in this work shows several differences to those normally used in atomic emission spectrometry. Instead of expensive polyor m o n o c h r o m a t o r systems interference filters with narrow band width are used for optical selection of the appropriate wavelengths. These filters perform a controlled oscillation in order to modulate the light passing through. The resulting modulated output signal can be easily detected and processed by frequency- and phase-selective lock-in amplifiers. More details have already been published in refs. [,-10-12, 14]. The output signal of the photomultiplier contains not only the reference signal with a modulated frequency of 20 Hz, but also frequencies of higher order. Therefore the frequency spectrum of the output signal has been measured by a fast-Fouriertransform analyzer [13]. In case of the determination of carbon results are given in this paper. In the second part, the detection of fluorine in organic compounds will be treated. The use of fluorinated organic compounds as coolant or propellant causes certain environmental problems and therefore the determination of fluorine is a very important part of modern analysis. Because the PED is well suited for the detection of halogenated compounds, research has been expanded to analyze fluorine [13]. This paper shows the optimization of measurement parameters as there are: oscillation frequency of the interference filter, photomultiplier voltage and microwave power, as well as the transmission characteristics of the optical filter is described. Furthermore the detection limit and dynamic range of this method to analyze fluorine are determined. E x p e r i m e n t a l

The principal experimental set-up is shown in Fig. 1. The GC-MIP-system including the signal-processing unit consists of the following parts: GI: gas inlet; GC: gas chromatograph; CAV: cavity; O: optics; FO: fibre optics; MWG: microwave generator; TFU: three filter unit; MC: motor control; HV: high voltage supply; PM: photomultiplier; FG: function generator; LI: lock-in amplifier; REC: recorder; FFT: fast-Fourier-transform analyzer. Detailed descriptions are given in [11, 14]. Materials

Trichloromethane(pro analysis), Merck-Schuchardt,Darmstadt, FRG. -- 4-Trifluoromethylbenzonitrile(TFB) (> 98~o),Merck-Schuchardt,Darmstadt, FRG. 1, 1, 1, 3, 3, 3-Hexafluoro-2-propanol(HFP) (>99.8~), Aldrich Chemic GmbH & Co. KG, Steinheim,FRG. Methanol (pro analysis), Merck-Schuchardt,Darmstadt, FRG. -- Helium 99.996~o, Westfalen AG, Miinster, FRG, cleared by Oxisorb, Messer-Griesheim, Diisseldorf, FRG. - -

- -

- -

Gas Chromatographic System

-- Gas chromatograph: HP 5890 A. Capillary:HP 5, 50 m length, 0.32 mm i.d., 0.52 #m filmthicknesswith a carrier gas flowof 1.5 ml helium per minute. - -

New Studies of the Plasma Emission Detector

217

,,]

1

I

FO

MC

O CAV GI TFU

PMpM . . . . .

REC

FFT

PLOTTER]

Fig. 1. Principle experimental set-up

GC-MIP-interface: special construction and adapted from "Ing. Bfiro fiir Analysentechnik A. H. F.", Feuerbacher, Ti.ibingen, FRG.

Microwave System - - Cavity: A. H. F., Tfibingen, FRG, MIP-resonator model H M W 25-471 N-W. - - Generator: A. H. F., T/ibingen, FRG, model G M W 24-301 DR, 2.45 GHz, 30-300 W. Discharge tube: Haldenwanger, Berlin, Alsint-ceramic, 1.5 mm i.d., 3 mm o.d., 35 mm length. Plasma gas flow rate: 300 ml min -1 (helium). -

-

-

-

Optical System Radiation source: 1 : 1 transmission of end-on-viewed helium plasma, focussed on the end ofa UV fibre optics. Fibre optics: (a) Schott-Gen., Mainz, FRG, one-string 1.8 m length, 3.0 mm i.d., fused silica; (b) ORIEL GmbH, Darmstadt, FRG, 0.9 m length, trifurcated, 5.5 and 3.2 mm i.d., fused silica. - - Optical interference filters: Omega Optical, Brattleboro, VT, USA. Carbon filter: central wavelength (CW): 247.5 nm, full width at half maximum (FWHM): 8 nm. Fluorine filter: CW: 685.43 nm, F W H M : 0.15 nm. - - Detector: photomultiplier R 1635, Hamamatsu, Herrsching, FRG, operated at 860 V. -

-

-

-

Signal Processing System -

-

Lock-in amplifier: Ithaco, Ithaca, NY, USA, H. M, S. Strassner, Leverkusen, FRG, Ithaco models 391A, 393 and 3921 with pre-amplifier 164. Fast-Fourier-transform analyzer: O N O SOKKI, model C F 940.

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M . W . Bradter et aL

Results and Discussion

Studies on the Oscillation of Interference Filters in Case of Carbon Determination The three filter unit is the central part of the plasma emission detector. In this unit three interference filters are placed on a single mechanical axis, which allows a controlled oscillation of all three filters regulated by a motor control unit. A function generator provides the oscillation frequency as well as the reference frequency for the lock-in amplifier. The resulting signal contains the oscillation frequency but also shows signals at higher frequencies. For the carbon channel the maximum signal-to-noise ratio was obtained with an oscillation frequency of 20 Hz of the interference filter. This has been achieved by feeding a constant stream of trichloromethane to the plasma gas and producing a constant signal in the PED. Next to the optimization of the oscillation frequency, the frequency spectra have been measured. The photomultiplier output has been connected to a fast-Fourier-transform analyzer via a resistor of 12 kf~ as current/ voltage-converter. Fig. 2 shows the frequency spectra of the carbon channel in a range of 0-100 Hz. The constant signal has been produced as described above. Besides the reference frequency of 20 Hz, additional frequency components can be seen at 40, 60 and 80 Hz. The dominating signal appears at 20 Hz and this was verified by scanning a frequency range of 0-1000 Hz. Remarkably, the signal produced by the general power supply (50 Hz) exceeds the examined signal by a factor of 10. The good discrimination against this disturbing frequency is proof for the efficiency of the lock-in amplifier to detect small signals with an underlying intense noisy background. The PED has been designed as a detector for a gas chromatographic system, therefore the dependency of frequency distribution on the concentration of the interesting compounds has been studied. In order to achieve this a set of variously concentrated samples of trichloromethane was determined. The samples covered a range from 50 to 1000 ng carbon absolute.

NAG ~V

x10O

X.-

PWR SPA 20. O O H z

h~n

LIN

IOOHz Y."

Fig. 2. Frequency spectrum of the carbon channel

. 172mV


219

I-

L rel.lntens.

-

5

Conc.

i 0

lOOHz

Fig. 3. Three-dimensionalplot of the frequencyspectra of the carbon channel Fig. 3 shows the results of the FFT-analysis of this set in a three-dimensional plot. In all cases the reference frequency of 20 Hz clearly dominates the frequency spectra, although some signals with higher frequency appear, but their intensity is very low compared to the base signal at 20 Hz. As a result it is shown clearly that the reference frequency proves to be best fitting to process signals produced by the photomultiplier. Studies on the Determination of Fluorine The PED permits the selective determination of fluorine by spectrometric examination of the fluorine emission line at 685.6nm. The compound 4trifluoromethylbenzonitrile (TFB) was used to determine the transmission characteristics of the interference filter. Fig. 4 shows a maximum of transmission in an angle position of + 4.5 ~ relative to the optical path. In addition to this, the oscillation frequency, microwave power and voltage at the photomultiplier were optimized. Oscillation frequency. The interference filter oscillates with ace, rtain frequency between the position of maximum and minimum transmittance. The oscillation frequency is controlled by a function generator, that also produces the reference frequency for the lock-in amplifier. Due to limitations of the mechanical set-up and of the stepper motor, a maximum of 26 Hz oscillation frequency is achievable. After optimization of other parameters like the time constant and the phase relation of the lock-in amplifier, a systematic variation of this parameter shows the optimal signal-to-noise ratio (S/N-R) to be at a frequency of 20 Hz (Table 1). Microwave power. As shown in Table 2 the best S/N-R will be achieved by a microwave power of 70 W. However, running an analysis with this low power the helium plasma can be easily overloaded by the solvent and therefore collapse. A setting to large power will cause the destruction of the ceramic tube. Therefore the smallest possible microwave power is chosen to secure a stable ]plasma and the

M. W. Bradter et al.

220

Relative Intensity 14

~2

10

6~

j 0 [

i

-20

-15

|

I

i

-10

__:

-5

0

L

i

i

5

I0

15

Angleposition

20

Fig. 4. Transmissioncharacteristics of the fluorine filter

Table 1. Frequency versus peak height Frequency Peak height

[Hz]

[cm]

8 10 12 14 16 18 20 22 24

-4.7 -6.2 -6.3 7 9.5 11.1 11.7 11 9.3

optimal S/N-R for all measurements. Using methanol as solvent, a power setting of 80 W is required. The current settings for the following studies are summarized in Table 3.

Temporary Detection Limits and Dynamic Range To determine the detection limit and the dynamic range of this method to detect fluorine, 0.5 #1 samples of T F B and H F P in methanolic solution were used. The


221

Table 2. Microwave power (MW) versus S/N-R MW [W]

S/N-R

65 70 75 80 85 95 105 115

18.1 20.51 17.09 13.06 12.47 8.43 7.04 4.2

Table 3. Optimized parameters for the determination of fluorine Parameter

Optimum setting

Angle Oscillation amplitude Microwave power High voltage Time constant Phase

+ 4.5 ~ declination from the vertical path of the beam _+4 ~ around the optimal angle 70 W (80 W using methanol) 860 V 0.03 s 90~ ~

observed concentration ranged from 5 to 2000 ng fluorine. The following temperature program proved to be the best to separate the compounds. 220~

4-Trifluoromethylbenzonitrile: (4-TFB)

30~

10 min

40~ 2 min

1, 1, 1, 3, 3, 3-Hexafluoro-2-propanol: ~ (HFP) 15~ 50~ 1 min

100~ 10 min

TFB shows a detection limit of 100 ng fluorine (5 ng s -1) absolute. Because of the high retention time (8.21 min) the concentration profile flattens very much, so that a peak height evaluation gives detection limits smaller than 100 ng absolute. Though the detection limit of 100 ng absolute seems to be poor, the dynamic range reaches up to 2000 ng, which allows an accurate and quick detection of fluorine in routine analyses. The retention time of HFB is considerably smaller (4.87 min) and therefore, caused by sharp peaks obtained, the detection limit is improved to 5 ng (2 ng s -1) fluorine absolute. However, the dynamic range only reaches up to 500 ng.

222


Compared to results published earlier with detection limits of 60 pg s -1 [15] and 1.8 pg s -1 fluorine [16] this described method cannot satisfy yet and needs improvement. Furthermore investigations have to be made concerning the selectivity of fluorine over carbon and other elements. However, simplicity, accuracy, speed and its low price justifies further developments of this technique, for instance by using sufficiently pure chemicals and gases as well as by raising the oscillation frequency. References [1] [-2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]

J. McCormack, S. C. Tong, W. O. Cooke, Anal. Chem. 1965, 37, 1470. C.A. Bache, D. J. Lisk, Anal. Chem. 1965, 37, 1477. L. Ebdon, S. Hill, R. W. Ward, Analyst 1986, 111, 1113. J. P. Matousek, B. J. Orr, M. Selby, Prog. Anal. At. Spectrosc. 1984, 7, 275. D. Deruaz, J. M. Mermet, Analusis 1986, 14/3, 107. P. C. Uden, Trends Anal. Chem. 1987, 6/9, 238. W. J. Boyko, P. N. Keliher, J. M. Patterson, Anal. Chem. 1984, 56, No. 5, 133R. P.N. Keliher, W. J. Boyko, R. H. Clifford, J. C. Snyder, S. F. Zhu, Anal. Chem. 1986, 58, No. 5, 335R. P.N. Keliher, D. J. Gerth, J. L. Snyder, H. Wang, S. F. Zhu, Anal. Chem. 1988, 60, No. 12, 342R. K. Cammann, H. Miiller, Fresenius' Z. Anal. Chem. 1988, 337, 336. H. Miiller, K. Cammann, J. Anal. Atom. Spectrom. 1988, 3, 907. M. Faust, K. Cammann, P. Stilkenb6hmer, F. Winter, M. Bradter, presented at the 17th Int. Symp. on Chromat., Vienna, 1988. M. Faust, W. Buscher, K. Cammann, M. Bradter, F. Winter, presented at the European Winter Conference on Plasma Spectroehemistry, Reutte, Austria, 1989. M. Bradter, W. Buscher, M. Faust, F. Winter, K. Cammann, GIT Faehz. Lab. 1989, 33, 166. W. R. McLean, D. L. Stanton, G. E. Penketh, Analyst 1973, 98, 432. K. Tanabe, H. Haraguchi, K. Fuwa, Spectrochim. Acta B 1981, 36, 633.

Received January 9, 1989. Revision September 25, 1989.