application of a simple photoionization detector for non-dispersive atomic spectrometry in the...

Talanra. Vol. 23. pp 187-192 Pergamon Press. 1976. Printed in Great Britain

APPLICATION OF A SIMPLE PHOTOIONIZATION DETECTOR FOR NON-DISPERSIVE ATOMIC

SPECTROMETRY IN THE VACUUM ULTRAVIOLET REGION

M. J. ADAMS, G. F. KIRKBRIGHT and R. M. TAYLOR

Chemistry Department, Imperial College, London, S.W.7, England

(Received 29 May 1975. Accepted 8 July 1975)

Summary-The construction and operation of a simple, solar-blind photoionization detector which responds to incident radiation between 125 and 140 nm is described. The detector, the response characteristics of which are controlled by the ionization potential of the ethylamine filler gas and the calcium fluoride window employed, is shown to provide for efficient detection of atomic line emission from carbon, oxygen and chlorine in this wavelength region. The spectral sources employed for non-dispersive work with this detector arc a microwave-excited argon plasma and a demountable hollow-cathode lamp.

Because of the numerous problems associated with the production and detection of radiation at wavelengths shorter than 200 nm this region has been somewhat neglected for the purposes of routine analytical atomic spectrometry. The atoms of many elements, including the halogens, carbon, sulphur, oxygen, nitrogen, phosphorus and mercury, emit intense resonance-line radiation in this region. A simple, non-dispersive system for detecting the atomic-line emission of these elements would be attractive for their detection and determination by the techniques of atomic emission, absorption and fluorescence spectrometry. Conventional spectrometry in the “vacuum ultraviolet” region is usually undertaken with large vacuum spectrometers and photomultiplier detectors. By analogy with the advantages of non-dispersive detection systems rather than grating or prism/ monochromator systems in analytical atomic spec- troscopy in the near ultraviolet and visible regions of the spectrum, use of a non-dispersive detection system in the vacuum ultraviolet might be expected to yield advantages of simplicity, large aperture and suit- ability for operation in simultaneous multi-element analysis. The photoionization detector (PID) appears to offer such possibilities. A typical PID consists of a chamber containing a gas at low pressure and two electrodes across which a constant d.c. voltage is applied. When the chamber is irradiated by photons having sufficient energy to cause ionization of the gas, an ionization current is produced and may be registered in the external circuit. The magnitude of the current is proportional to the incident ionizing photon flux. The spectral response of a PID is controlled on the short wavelength (high energy) side by the transmission characteristics of the material from which the window is made and on the long wavelength side by- the photoionization threshold of the filler gas. The construction and use of PID systems for physical research has been documented; detectors based on

this principle have found application in the examina- tion of the solar spectrum.‘,’ By careful selection of the window material and the filler gas, detectors with a spectral bandpass of only a few nm may be constructed, which will detect radiation in the range 10% 200 mn. In addition to high spectral selectivity, detectors of this type have high quantum efficiencies, low noise levels and the capability of serving as gas-gain multipliers. They may also be made to have large optical aperture, are “solar-blind” and may be rugged and compact. This paper describes the construction and evaluation of a simple PID system designed for non-dispersive work in the spectral range 125 140 nm, and its use for the detection of such radiation from a microwave-excited argon plasma at atmospheric pressure or from a flow-through demountable hollow-cathode lamp source.

EXPERIMENTAL

Apparatus

Photoionization detector. The PID assembly used in this work is shown in Fig. 1. A Pyrex cylinder (90 mm in length, 29 mm diameter, 2 mm wall-thickness) fitted with a B14/23 ground-glass cone and socket at one end and a side-arm fitted with a three-way 2-mm bore tap was employed as the detector envelope. The electrode assembly, mounted onto glass-to-metal sealing rods through the ground-glass cone. consisted of a cylindrical sheet copper cathode (80 mm in length, 25 mm internal diameter, 0.19 mm thickness) along the central axis of which a l-mm tungsten wire anode was located. The electrodes were insulated from each other by means of the glass and PTFE anode supports shown in Fig. 1. A polished calcium fluoride window (3 mm thickness, 25 mm diameter) was mounted in a cylindrical aluminium holder sealed with epoxy resin cement onto the end of the PID envelope. The aluminium window holder had an external annular depression in which an O-ring was positioned to permit vacuum sealing of the detector at the exit slit of the vacuum monochromator employed in some of the work. All components of the PID were carefully cleaned and degreased before assembly. The ground-glass joints were sealed with high-melting wax.

187

188 M. J. ADAMS, G. F. KIRKBRIGHT and R. M. TAYLOR

p.t.f.e. Supports

electrode Central tungsten wire electrode

Icm n

Fig. 1. Photoionization detector assembly.

A d.c. voltage (5-400 V) was provided across the detector from a dry battery supply. The photoioni~tion current was registered on a picoammeter (P24. Knick, Berlin, Ger- many) and the voltage output from this device was displayed on a potentiomctric chart recorder (Servoscribe Model RE 511).

Microwave plasma source. A microwave-excited argon plasma supported at atmospheric pressure in a fused silica tube (150mm length, 2mm i.d.) with a l/4-wave resonant cavity, was employed. Power (ca. 50 W) was supplied at 2450 MHz from a 200-W microwave power supply (Micro- tron Mk III, EMS Ltd.). The argon flow-rate was variable between 0.5 and l.OI./min and the plasma discharge was viewed “end-on” along the central axis of the silica tube.

Demountahle hollow-cathode lamp source. A- commercial demountable hollow-cathode lamp source (Mini~ow, Spectra Products Inc., North Haven, Conn., U.S.A.) was employed. The source was capable of operation at d.c. currents of between 5 and 50 mA and argon pressures between 2 and 10mmHg. The lamp assembly was capable of accepting a variety of cathodes. The cathode shells used in this work were aluminium (22 mm length, 4 mm internat diameter. 1 mm wall thickness) into which the material to be examined was pressed. The cathode assembly was water-cooled. The silica window of the demountabie hollow-cathode source was replaced by a calcium fluoride window (3 mm thickness) for the present studies.

Auxiliary equipment. The spectral emission characteristics of the source employed, and the spectral response characteristics of the PID, were established by using a I-m normal-incidence vacuum grating monochromator (Model E760, Rank Hilger Ltd.). This was equipped with an end- window photomultiplier tube (EM1 6258) the silica window of which had been coated with sodium salicylate (I- I .5 mg/cm2).

RESULTS AND DISCUSSION

For preliminary evaluation a PID was constructed to operate in the spectral range 120_14Onm, choice of this wavelength region being governed by the avail- ability of suitable window materials and of filler materials having suitable ionization potential and vapour pressure to enable the PID to operate effi- ciently at room temperature. In addition, a number of useful atomic lines from carbon, oxygen, chlorine, bromine and sulphur lie in this wavelength range and were thus available for assessment of the response characteristics of the PID. The vapour of ethylamine, which has an ionization potential of 8% eV (141.0 nm) was employed as the f&r material. The photoioniza-

tion efficiency curve for ethylamine is shown in Fig. 2.3 The short-wavelength response was controlled by the window material. In most of the work reported here a 3-mm calcium fluoride window was used. The transmission characteristics of this window are also shown in Fig. 2. The spectral response of the PID can be predicted to be between 120 and 140 run.

A simple vacuum and purging system was constructed to permit the filling of the PID (Fig. 3.). An- hydrous ethylamine (10 ml), b.p. 16.6, was placed in the pear-shaped flask and frozen by immersion of the flask in liquid nitrogen. The system was then eva- cuated with a two-stage rotary pump and gushed out several times with argon. After evacuation of the com- plete system tap A was closed to isolate the flask containing the ethylamine. The flask was then removed from the liquid nitrogen and allowed to warm to produce a steady increase in ethylamine vapour pressure. By first closing tap B and then open- ing tap A the system was flushed out several times with ethylamine vapour, followed each time by re- evacuation to a pressure of cu. 3 mmHg. Tap B was then finally closed and a preselected pressure of ethylamine vapour admitted to the system oia tap A. With taps A and B both closed, the detector was sealed by closing tap C and removed from the system for use. Even with the simple PID system constructed for this study, the PID envelope would maintain a pressure as low as 1 mmHg for periods of over 8 hr and was simply and rapidly refilled when necessary.

z I I 120 130 140 150

Wavelength, nm

Fig. 2. Photoionization efficiency curve for ethylamine and transmission characteristics of calcium fluoride

window.

Non-dispersive atomic spectrometry 189

taining CCI, provided useful line emissions from carbon, oxygen and chlorine suitable for evaluation of the spectral response characteristics of the PID.

The PID was filled with ethylamine vapour at 2 mmHg and positioned at the exit slit of the vacuum monochromator; the window of the ionization chamber formed the vacuum seal with the monochromator. The argon plasma, operated at 65 W and an argon how of 1 l./min, was viewed by the monochromator using a spectral half-band pass of 1.6 rmr. The emission spectrum of the argon plasma was recorded with the PID operated at 85V. The spectrum obtained is shown in Fig. 6, which also shows

Fig. 3. Evacuation and purging system employed for filling PID detector.

Spectral response characteristics of the PID

The spectral response characteristics of the ethylamine PID were studied with both sources available, i.e., a microwave-excited argon plasma and the demountable hollow-cathode lamp system. The back- ground emission spectrum from the argon plasma in the wavelength range of interest was first examined with the vacuum monochromator and photomultiplier assembly. Figure 4 shows a typical spectrum obtained. The principal features of the emission spectrum may be identified as lines originating from im- purities in the argon or from the walls of the silica plasma tubing, i.e., lines of carbon and oxygen from carbon dioxide, hydrocarbons and water vapour. Emission spectra were then obtained from the plasma when small quantities of other vapours were introduced into the argon supply. Figure 5 shows the spectra obtained in the presence of carbon tetrachloride, sulphur dioxide and iodine. Though SO2 and IZ gave no emission in the wavelength range of interest, the emission spectra of the argon plasma alone or con- i. hh

I I I I I I I

IjO li0 150 160 li0 lb0 lb0

Wavelength,nm

Fig. 5(a).

9 >

L

1 130 IL0 150 160 170 180 190 200

Wavelength,nm

Fig. 4. Emission spectrum from microwave-excited atmospheric argon plasma between 120 and 200 nm. Microwave power, 65 W; E.M.I. 6256B photomultiplier at 1600 V,

coated with sodium salicylate.

130 IL0 150 160 170 180 190

Wavelength,nm

Fig. S(b).


E N

--4

I I I I I I I I 130 IL0 150 160 170 180 190 200

Wavelength,nm

Fig. 5(c).

Fig. 5. Spectra from microwave-excited atmospheric argon plasma pressure of (n) Ccl,, (h) SOz, (c) 12, introduced into argon supply. Microwave power, 65 W, E.M.I. 6256B photomultiplier at 1600 V, coated with sodium salicylate.

the spectrum of the argon plasma obtained by using the vacuum monochromator at the same spectral band-pass (1.6 nm) but with the photomultiplier detector. It is clear that the PID responds only to

(bl

I I I I I I I I 120 130 140 150 160 170 180 190 200

Fig. 6. Emission spectra from microwave-excited argon plasma, obtained by using vacuum monochromator (1.6 nm-bandpass) and (a) PID detector operated at 85 V, 2 mm Hg pressure of ethylamine. (b) E.M.I. 6256B photo-

multiplier at 700V; coated with sodium salicylate.

radiation in the wavelength range predicted. Whereas the carbon and oxygen line emissions in the region 128-132 nm are observed, the more intense longer- wavelength carbon lines between 140 and 193 nm are not detected.

As a further test of the response characteristics of the ethylamine PID the demountable hollow-cathode lamp source with argon filler gas and an aluminium cathode containing a mixture of tungsten and mercuric chloride was employed. The spectrum from this source in the range 13s200nm (obtained with the vacuum monochromator and photomultiplier detector) consists principally of atomic line emission from chlorine, oxygen, carbon and mercury, as shown in Fig. 7~7. This spectrum was obtained with an argon- purged optical path (3 cm) between the source and the monochromator entrance slit. The experiment was then repeated with this 3-cm path-length purged with argon containing 1% methane. As methane absorbs strongly at wavelengths shorter than 140nm the emission intensity from the source at short wavelengths is attenuated. The spectrum obtained under these conditions is shown in Fig. 7h and clearly demonstrates the absorption of the oxygen and chlor-

ine atomic line emission between 130 and 140 nm and the unaffected intensities at longer wavelengths. These experiments were repeated with the PID in the non- dispersive mode, i.e., with the radiation from the demountable hollow-cathode lamp source falling directly on to the detector via the 3-cm purged path without use of the vacuum monochromator. The re-

(a)

(bl

I I I I I I I I 130 IL0 150 160 170 180 190 200

Wavelength, nm

Fig. 7. Emission spectra from demountable hollow-cathode lamp source. Argon filler gas, HgCl# cathode. (u) Optical path (3 cm) purged with argon. (b) Optical path

(3 cm) purged with l?;, CHI in argon.

Non-dispersive atomic spectrometry 191

sponse in the presence and absence of 1% methane in the argon purge-gas is shown in Fig. 8. The de- crease in the presence of methane correlates well with that between 130 and 140 nm shown in Fig. 7. It is thus apparent that it is the incident radiation below 140 nm to which the PID responds.

Effect of ethylamine vapour pressure and applied voltage on response of PZD

The radiation from the demountable hollow-cathode source with an aluminium cathode containing a mixture of tungsten powder and solid mercuric chloride was allowed to fall directly onto the PID in the non-dispersive mode. The effect of ethylamine pressure and the applied voltage on the response of the detector was examined. The current-voltage curves obtained are shown in Fig. 9.

For the detector geometry employed, in which a thin wire central electrode is located along the central axis of the large cylindrical electrode, the electric field E in the coaxial system at a distance r from the centre of the central wire electrode is given by

V

E = r In [a/b]

where V is the applied voltage and a and b are the radii of the cylindrical and central wire electrodes, respectively. In the applied voltage range between A and B (Fig. 9) for a given photon flux entering the detector the ions formed by the reaction.

C2H5NH, + hv+ C2H,NH: + e-

are collected with increasing efficiency as the field strength in the chamber increases with increasing voltage. Further increase in applied voltage, in the range from B to C, can then not result in further collection efficiency for the given geometry and cell dimensions, and a “plateau region” of relatively constant output current is obtained. At higher applied

I o-7

I I@

3cm path of argon

Fig. 8. PID currents observed in presence and absence of methane in optical path between hollow-cathode lamp

(HgCl&’ cathode) and detector.

I 2mmHgEt NH,

10-g I I I I 0 IO0 200 300 400

Voltage. v

lo” .

Hollow cathode /

15mA /’

F-•- .A*

a

21ji_ ~ _ yTz;z{ 0 100 200 300 400

Voltoge. V

Fig. 9. (a) Effect of ethylamine vapour pressure and applied d.c. voltage on response of PID. (b) Dependence of PID response on applied d.c. voltage, with different

operating currents for hollow-cathode lamp source.

voltages, as in the region of C and above in Fig. 9, the electrons produced by the initial photoionization of the ethylamine may achieve sufficient energy in the stronger field to cause secondary ionization on collision with ethylamine molecules, i.e., the energy of a fraction of the photoelectrons becomes greater than the ionization potential of the filler material. As the applied voltage is increased the increasing field strength thus leads to increasing electron multiplica- tion by secondary ionization and a consequent rapid increase in the current produced in the external circuit. This region is often referred to as the “gas-gain” region and can be employed to increase the sensitivity of the detector system. The effect shown in Fig. 9, where the PID current is inversely proportional to


Table 1. A selection of narrow bandpass PID systems in the wavelength range 105-l 84 nm

Window material Approx. thickness, mm Filler gas I.P., ev (Ref.) Spectral range of PID. nm _

Lithium fluoride

Calcium fluoride

Sapphire

Fused quartz

1

3

0.5

1

ethyl bromide 10.29 (2) 105-l 29 nitric oxide 9.25 (2) 105-l 34

benzene 9.25 (2,4) 122-134 toluene 8.82 (2,4) 122-141 p-xylene 8.44 (2) 142.. 147

trimethylamine 7.82 (5) 142-156 dimethylaniline I.14 (5) 16@ 172 triphenylamine 6.89 (5) 16&180

ferrocene 6.74 (5) 16&184

the pressure of ethylamine present over the range studied, can be explained on the basis of the mean free path of the photoelectrons. Thus, as the ethylamine pressure is increased, the mean free path of the electrons decreases so that there is insufficient time to permit their acceleration to energies sufficient to cause secondary ionization on collision with ethylamine molecules. This effectively delays the onset of the “gas-gain” region to higher applied voltages. The limiting case shown in Fig. 9 corresponds to the addition of helium as an inert filler gas at atmospheric pressure in the presence of 5 mmHg of ethylamine. Under these conditions the plateau region extends to above 300 V without significant secondary ionization.

Figure 9b demonstrates the dependence of the PID response on the incident radiation intensity from the source employed. An increase in operating current from 5 to 15 mA d.c. resulted in an increase in current of cu. one order of magnitude owing to the equivalent increase in observed intensity of the oxygen and chlorine atomic line emission between 125 and 140 nm. The proportionality between incident intensity and PID response is observed in both the plateau and gas-gain regions.

CONCLUSIONS

Our preliminary work reported here indicates that simple, high-efficiency, narrow spectral band-pass, solar-blind photoionization detectors may offer advantages for detection of radiation at short wave-

lengths in analytical atomic emission spectrometry They should prove most valuable when used for non- dispersive, on-line detection systems. The most useful wavelength range for such detectors lies between 120 and 200nm; Table 1 shows a selection of narrow spectral band-pass detectors which may be constructed with different window and filler materials. Detectors of this type may prove useful in continuous gas-analysis monitoring systems using plasma or glow-discharge sources, particularly for detection of oxygen, oxides of nitrogen and sulphur dioxide.

The detection and determination of low con- centrations of organic compounds in the vapour phase, particularly those containing halogen, sulphur or phosphorus atoms, may also be facilitated by use of this type of detection system. Work is at present in progress on the development of such systems and the application of PID detectors in atomic-absorption and fluorescence spectrometry; this will be reported in a later communication.

1.

2.

3.

4. 5.

REFERENCES

T. A. Chubb and H. Friedman, Rev. Sci. 1nstr.. 1955, 26. 493. J. H. Carver and P. Mitchell, J. Sci. Instr.. 1964, 41. 555. H. Hurzeler, M. G. Ingham and J. D. Morrison. J. Chem. Phys., 1958, 28. 76. F. I. Vilesov, Soviet Phys. Usp., 1964, 6. 888. L. S. Sorokin, V. K. Adamchuk and A. B. Dmitriev, Pribory i Tekh. Eksperim., 1970, 3. 216.

application of a simple photoionization detector for non-dispersive atomic spectrometry in the...

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