Furnace atomization plasma excitation spectroscopy: spectral, spatial, and temporal characteristics

Specnochrmrca A+ Vol 47B.No 4. pp.493-503. 1992

0584-8547/92 $5.00 t .oo @ 1992 Pergamon Press plc.

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Furnace atomization plasma excitation spectroscopy: spectral, spatial, and temporal characteristics T. D. HEITIPATHIRANA and M. W. BLADES* Department of Chemistry. The University of British Columbia, 2036 Main Mall, Vancouver, B.C., Canada V6T lY6 (Received 13 July 1991; accepted 3 September 1991) Abstract-Some fundamental characteristics of a furnance atomization plasma excitation (FAPES) source are presented and discussed. For this source, the electron ionization rate is 120 times higher in He compared with Ar resulting in easier ignition and operation with He. Background spectra are dominated by emission features from CO+, N;, N,, OH, NH, and He I. Emission is most intense near the central electrode and furnace walls. Excitation temperatures for Fe1 are in the range of 3000-4500 K.

FURNACE atomization plasma excitation spectrometry (FAPES) is a relatively new spectrochemical source for analytical atomic spectroscopy. The construction and operation of this source was first described by LIANG and BLADES [l]. The FAPES source enables the formation of plasma at atmospheric pressure inside an otherwise normal graphite furnace atomizer. The plasma is formed and sustained at atmospheric pressure by placing high voltage, radio frequency excitation (r.f.) on a conductive electrode, located inside the furnace with a co-axial geometry, while maintaining the furnace at virtual ground. During operation for analytical determinations, a sample (normally 5-50 ~1) is deposited inside the graphite furnace and atomized into the plasma using a “conventional” graphite furnace heating cycle. Atoms in the vaporized sample are excited by the plasma and emit at their characteristic wavelengths such that the FAPES source can be used for simultaneous multi-element analysis. Since the initial descriptions of the FAPES source [1,2] some analytical [3-53 and fundamental [6,7] characteristics have been reported. Even at this very early stage of development, FAPES promises to provide a method of carrying out simultaneous multi-element analysis on small liquid and solid samples with detection limits that are the same order of magnitude as those provided by graphite furnace atomic absorption spectrometry (GFAAS). Before optimum utility can be achieved with this new analytical source, a complete study of its fundamental characteristics must be undertaken. While FAPES could potentially combine the desirable characteristics of GFAAS and plasma emission spectroscopy, it could also present some of the non-desirable characteristics of each of these techniques. The challenge is to operate in a manner such that the former is enhanced and the latter is discriminated against. This paper describes some of the basic characteristics of the FAPES source including spectral, temporal, and spatial emission behavior for background species and excitation temperatures measured using iron as the thermometric species.

EXPERIMENTAL The source used for the collection of background spectra (Fig. 2) was an IL-455 (formerly Instrumentation Laboratory; now Thermo-Jarrell Ash, Waltham, MA) graphite furnace work head fitted with a central electrode as described in a previous paper [3]. This source operated *Author to whom correspondence

should be addressed. 493

494

T. D. HEITIPATHIRANA and M.W. BLADES

Fig. 1. Schematic diagram of the furnace atomization plasma excitation spectrometry source.

(FAPES)

at 27 MHz and could be used with both argon and helium as plasma gases. This source could be quite effectively purged and sealed allowing spectra to be collected without ingress of atmospheric gases. For the remainder of the experiments (Figs 3-6) a new FAPES source, shown in the cut-away schematic diagram in Fig. 1, was used. The design was based on that used for a low pressure, hollow-anode plasma source published by BALLOU et al. [8]. The main components of the FAPES source were a Ringsdorff-Werke (Bad Godesberg, Germany) pyrolytic-graphite coated, integrated contact graphite furnace (6 mm inside diameter), a Ringsdorff-Werke pyrolytic-graphite coated graphite rod 0.9 mm in diameter, and a highcurrent furnace support structure, machined from copper and Macor@, contained in a five-way hollow cube machined from aluminum. The hollow five-way cube was 6 x 6 x 6 in. with 5-in. diameter ports in five of the six sides. Each of the ports was fitted with a 5.5in. diameter “O”ring sealed aluminum flange. The furnace assembly was supported on one of the flanges similar to the method described by BALLOU et al. [8]. The FAPES source was viewed through a l-in. diameter quartz window on the front-side flange. A similar quartz window on the opposing backside flange allowed a light source to be directed through the graphite furnace such that absorbance or fluorescence measurements could be carried out. The central electrode was powered through an r.f. connector in the manner depicted in the diagram. Sample was introduced into the source through a small screw-top port mounted on the top flange. The five-way cube could be evacuated to 1 mtorr or pressurized to 2000 torr. The cube was mounted on a post which in turn was mounted on a crank-driven linear translation stage which allowed the source to be moved laterally relative to the detection system. For spatially resolved emission intensity measurements, the detection system was set to the appropriate wavelength and the entire cube and source was translated in increments of 0.1 mm. The movement of the platform was monitored using a Mitutoyo (Japan) Model 2047-11 precision displacement indicator gauge. For this latter experiment emission intensities were recorded on a chart recorder. The plasma was generated and sustained using an Advanced Energy (Fort Collins, Co) Model RFX-600, 13.56 MHz r.f. generator, an Advanced Energy Model ATX-600 automatic tuner, and an Advanced Energy Model 5017-000-G impedance matching network. The output of the impedance matcher was coupled to the central electrode through a variable l-10 PH inductor. This arrangement could be used to ignite and sustain a He plasma over the power range 5-150 W. When operating at incident powers below 50 W, during an atomization cycle, the reflected power was stable at below 2 W. At incident powers higher than 50 W, the reflected power could be maintained at a low value of l-2 W, except during an atomization cycle. During the atomization cycle, the reflected power increased to values as high as lo-20 W. In addition, at times, the reflected power would stay at these high values, even after the completion of the atomization cycle. An Ar plasma could be ignited at powers exceeding about 30-40 W, but would not operate in a stable manner at these high powers. If, after ignition, the power was rapidly reduced to

FAPES: spectral, spatial, andtemporal characteristics

495

8-10 W, a diffuse Ar plasma could be maintained at 13.56 MHz However, we have found that it is much easier to ignite and operate a stable Ar plasma at 27 MHz using a free-running oscillator r.f. supply. The furnace was heated using an IL-555 graphite furnace power supply. The temperature of the furnace was monitored using an Ircon Series 1100 (Ircon, 11) optical pyrometer which viewed the graphite furnace through a l-in. diameter quartz window on the top flange. A l-m Czemy-Turner monochromator (Schoeffel-McPherson, MA) model 2061 was used for the measurement of background spectra and iron spectra for excitation temperature measurements. This monochromator was equipped with a holo~aphic grating with 1200 lines/mm (Sch~ffel-McPhe~on, MA model AH-3264). An image of the plasma was formed at the entrance slit of the monochrornator using a 150-mm focal length, 50-mm diameter piano-convex fused silica lens. The imaging distance was adjusted to provide an image with a magnification of one from the center of the furnace. The entrance slit was 70 km wide and 2 mm high. The detector used was a Reticon (Sunnyvale, CA) Model RL-2048s linear photodiode array (LPDA). The array integration period and readout was controlled by using a Reticon Model RC1021 satellite controller board. Using an entrance slit of 70 p.m, the LPDA spectrometer provided a resolution of 0.06 nm while allowing the simult~eous measurement of a spectral window about 40 nm wide. A Melcor (Trenton, NJ) Model CP14-71-1OL thermoelect~c cooler mounted on the backside of the LPDA, allowed it to be cooled to -20°C. Data acquisition was carried out by interfacing the LPDA to an RC Electronics Model ISC-16 (Santa Barbara, CA) 1Zbit analogto-digital converter which was interfaced to a PC/AT compatible computer which was also used to set the integration time for the LPDA. A 0.35-m Mode1 270 Czerny-Turner monochromator (Schoeffel-MacPherson, MA) with a holographic grating with 1200 lines/mm was used for spatial and temporal emission measurements. The 1:l image of the plasma was formed at the entrance slit of the mon~hromator using a lOOmm focal length, 35-mm diameter fused silica lens. The entrance slit was 50 urn wide and 1 mm high. The detector used was a Hamamatsu (Middlesex, NJ) Model R955 photomultiplier amplified using a Keithley (Middlesex, NJ) Model 427 p&ammeter. Ouput from the current amplifier was converted to digital form using an RC electronics A/D converter and stored in a PC/AT microcomputer for further processing. For temporal measurements 2000 points were taken at a rate of 250 Hz.

REWLTS AND DISCUSSION The FAPES source has been operated using either He or Ar. However, it has been reported that the power required to ignite and sustain an Ar plasma is higher than that required to sustain a He plasma [7,9] and the Ar plasma is much more prone to the formation of striations or filaments. This at first seems counter-intuitive since the ionization energy of Ar is 15.75 eV and that of He is 24.59 eV; thus one would expect that Ar would break down and maintain a stable plasma at lower applied voltages. This observation can be rationalized by considering electron mobility and ion~ation efficiency in the two gases. In a constant electric field of magnitude E; an electron will acquire a constant velocity, called the drift velocity (P.), at which the energy lost per elastic collision with the surrounding gas (He or Ar) is equal to the energy gained from the field between collisions. The drift velocity is proportional to E and inversely proportional to the pressure (p). Drift velocities for a number of gases have been measured and compiled [lO,ll] as a function of the ratio E/p where E is the voltage per cm of electrode separation. Using the integrated contact furnace and 0.9 mm central rod, the electrode separation is 0.23 cm. For the FAPES source at an operating voltage of 1000 V, electron drift velocities in He and Ar are 5.5 x lo6 and 2.3 x lo6 cm s-l respectively. Neglecting the fact that the electric field strength varies with time, the fractional amount of the electrode separation that an electron moves during one half cycle of the r.f. field can be estimated. The purpose of this exercise is to get an intuitive feel for the probability of electron loss to the electrodes using different gases and different frequencies. At 13.56 MHz, this number is 0.88 for He and 0.37 for Ar; at 40 MHz, the number is 0.30 for He and 0.125 for Ar. It is possible to deduce some general conclusions from this simple calculation. Firstly, less electron loss to the walls in an Ar discharge relative to a He discharge leading to ignition and

T. D. HETTIPATHIRANA and M.W. BLADES

496

operation at lower applied voltages would be expected. This was not observed. Secondly, the rate of ion production in the plasma should increase with frequency since at higher frequencies, electron loss to the walls is lower, and hence electrons can ionize during more than one-half cycle of the r.f. field. Consequently, easier operation is expected at higher frequencies. This has been observed [7]. The reason for ease of operation of the FAPES source with He vs Ar is related to the ionization rate (ni) which is given by the product of the electron drift velocity and the first Townsend coefficient (a!). This gives the number of ionizing collisions per cm per torr for an electron. The Townsend ionization coefficient has been tabulated for a variety of gases for a variety of E/p values [lO,ll]. For the FAPES source at a voltage of 1000 V, the value for OLis 0.152 and 7.6 cm-i for Ar and He respectively. Therefore, the electron ionization rate in the two gases (the product of (Y and p) is 3.49 x 105 s-l in Ar and 4.18 x 10’ s-i in He. The ionization rate is 120 times higher in He compared with Ar! It should be pointed out that this result is highly dependent on the value of E/p. At E/p values lower than about 50 V cm-i torr-i, the electron ionization coefficient for He is higher than that for Ar. However, at higher values of E/p the reverse is the case. An additional comment is that at pressures higher than atmospheric, the E/p value will decrease and it should be even more difficult to ignite and operate an Ar- or He-FAPES source than at atmospheric pressure. Background

spectra

Most of the background spectral features arise from trace components in the plasma gas or combinations of these with species desorbed or vaporized from the furnace walls. The approximate concentration of impurities in the bottled helium (99.996%) used for our experiments were: H2 < 1 ppm; O2 < 3 ppm; N2 5-25 ppm; CO2 < l-2‘ ppm; HZ0 < 5 ppm; and total hydrocarbons < 5 ppm with similar values for the argon (99.996%). Typical background emission spectra for a He-FAPES source are provided in Fig. 2 for the spectral regions 215-300 nm (Fig. 2a), 300-400 nm (Fig 2b) and 400-520 nm (Fig. 2~). These spectra were collected using a photodiode array spectrometer and composite spectra were constructed by integrating 12 different spectral windows, each 40 nm wide, into a single background spectrum. Integration times ranging from 2 to 10 s were used. The intensity axis on the left of each spectrum allows a comparison of the relative intensities of the spectral features though the spectra have not been corrected for the spectral response of the measurement system (which is relatively poor in the ultraviolet). The spectral features observed are quite similar to those which have been observed previously [7]. The most prominent features in the background emission spectrum are CO+, OH, NH, N2, and N2+. In the 220-300 nm region, the spectra are dominated by emission bands of CO+ (Fig. 2a). The CO+ bands are not observed when Ar is used as the plasma gas. It is well known that CO+ is readily excited in He discharges as a result of selective excitation of the B2Z+ state of CO+ according to the following reactions [12,13]: He; + CO + CO+(B*C+) + 2 He

(1)

and He(21S,23S) + CO + CO+(B*Z+) + He(llS)

+ e-.

(2)

At these relatively high pressures and from vibrational distribution of the CO+ bands, the most probable reaction is the charge transfer reaction in Eqn (l).The presence of these CO+ bands suggests that the He-FAPES source could be quite effective for the selective excitation of analyte atoms and molecules in cases where the energetics are favorable. The other dominant feature is emission from the different vibrational bands of the first positive system of N2+. This N2+ system is also readily excited by chargetransfer from He; [12] which selectively populates the B*Z: electronic level leading to intense (B-X) band emission. The occurrence of these molecular bands presents a relatively intense background upon which analytical signals must be measured and it

FAPES: spectral, spatial, and temporal characteristics (a)

=

1200 1

CO+

(B?E+

--a X?E+)

ti

230

240

250

eao Wavelength

(b)

497

2;o

2iJO

2io

300

(nm)

20x10~

310

320

330

340

350 W.=-kngth

360

370

380

390

400

Cm)

Fig. 2. (a), (b).

is anticipated that the best performance will be obtained from the FAPES source using ultra-high purity plasma gases. In contrast to a previous report [7], emission from CI at 247.86 nm can be observed in He-FAPES background spectra, although in agreement with previous observations [7], the C I emission line is much more intense in an Ar-FAPES source. Figure 3 is a plot of the spatial distribution of emission from CO+ (229.96 nm) in the cube system observed by translating an image of the FAPES source laterally in increments of 0.1 mm. This distribution should be considered to be only approximate since the emission is spatially integrated from the entire length of the discharge and no attempt was made to optimize the imaging for this measurement. There are some features of the distribution which are interesting. The spatial profile peaks at the center of the source adjacent to the central electrode and also near the wall of the graphite tube. Similar behavior is exhibited for the He1 and N2+. However, the profiles for OH and NH show significant emission in the intermediate region between the central

T. D. HETI~PATHIRANA and M.W. BLADES

450

450

470

Wavelength @III)

Fig. 2. ~~ckg~~~~dspectra of He plasma. (a) 215-300 nm, fbf 3O&400 nm, and fc) 400-520 nm.

Fig. 3. Emission spatial profile for CO’.

electrode

and the tube walls. At this time,

it is not known whether

CO is formed

from

carbon vaporized from the electrode and the tube walls combining with oxygen in the plasma or from the dissociation of COZ. We have not measured the spatial distribution of analyte emission since the transient nature of the signal makes this a time consuming measurement prone to error. The use of a slittess spectrograph combined with an imaging detector similar to the system used by G~~M~~INOV [14] for the collection of absorption shadow-grams, is one possible method for acquiring this info~ation.

Tempard tmissioplprofiles The variation of emission intensity for He I, CO+, N,+, and OH was continuously measured during a dry (no aqueous sample was injected) atomization cycle in order to examine the effect of tube wall temperature on the emission intensity for these

FAPES: spectral, spatial, and temporal characteristics

499

species. The results are reproduced in Fig. 4a for HeI, Fig. 4b for CO+, Fig. 4c for N; and Fig. 4d for OH at r.f. powers of 14, 18, 22, 26 and 30 W as marked on each. The atomization temperature profile corresponding to these figures is reproduced in Fig. 4e. It can be seen that for He I (Fig. 4a), the emission is relatively stable throughout the atomization cycle although there is a small increase at the beginning of the cycle. For, CO+ (Fig. 4b), there is a dramatic increase in the emission intensity at the beginning of the atomization cycle, a depression during the peak of the atomization cycle, and a plateau as the furnace cools. The increase at the beginning of the atomization cycle is most likely due to an increase in the amount of CO+ in the furnace as a result of desorption of CO from the furnace walls and the electrode. The depression at the furnace wall temperature peak of the atomization cycle could be due to recombination of CO+ with thermionic electrons liberated from the walls and central electrode to form CO. If this were the case, both CO+ and N; could act as a buffer, minimizing the effect of thermionic electrons on the total electron density in the discharge. The data for N: (Fig. 4c) tend to support this suggestion since the temporal behavior at the peak of the atomization cycle for N; is similar to that observed for CO+ and the depression appears near the temperature maximum. However, the temporal behavior for CO+ and N: is complex and dependent on both the incident r.f. power and the furnace temperature. In general, as the r.f. power is increased, at a given point in the atomization cycle, the emission intensity is increased as a result of the increase in excitation. For OH (Fig. 4d), there is an initial increase, then a decrease in emission during the atomization cycle. Most probably, the temporal profile is the result of initial desorption of Hz0 and OH and subsequent dissociation of OH to form 0 and H. Excitation temperatures

Measured temperatures can be used to compare different plasma sources with respect to their ability to atomize, ionize, and excite analyte species. Spectroscopic methods are very popular for measuring temperatures since they are non-invasive and readily yield accurate and precise results. For plasma discharges, a variety of temperatures can be measured depending on which statistical distribution is being sampled. These are excitation temperatures (Boltzmann distribution), ionization temperatures (Saha distribution), and kinetic temperatures (Maxwell-Boltzmann distribution). Often the measurement of different temperatures is used as a test for the existence of thermodynamic equilibrium (TE) or local thermodynamic equilibrium (LTE) [15]. The existence of LTE is not necessarily advantageous for analytical purposes but can be useful for establishing models for an analytical discharge. STURGEON et al. [6] have reported several different temperatures for a FAPES discharge including excitation temperatures measured using HeI, a gas temperature measured using Doppler broadening of a Be I emission line, a He ionization temperature based on the measurement of electron density, and ionization temperatures for Mg, Fe,Cd and Zn. The iron excitation temperatures were measured using Fe(CO)S which was continuously introduced into the furnace chamber. The furnace was maintained at 970 K in order to dissociate the iron pentacarbonyl. Using this method Fe1 and Fe11 excitation temperatures were measured at an r.f. power of 100 W. Measurement at lower powers was not possible as a result of poor signal-to-noise ratio although He1 excitation temperatures were measured at r.f. powers between 25 and 100 W. Fe I temperatures The method used to measure Fe1 excitation temperatures for this paper differs from that used by Sturgeon et al. The source of Fe for the temperature measurement was from a standard micro-pipette injection of 5 ~1 of a 40 ppm acidified aqueous solution (from FeSO,). Five replicate injections were made for each power setting. The sample was dried at about 120°C for 1 min, with a 1 min thermal pre-treatment step at about 500°C (during which the plasma was ignited) and then ramp atomized to 1800 K.

T. D.

5ocl

HETRPATHIRANA

and

M.W.

BLADES

_I

0.0

0.5

1.0

I.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

3.5

4.0

4.5

5.0

5.5

6.0

Time(s)

-O.O 0.5

1.0

1.5

20

2.5

3.0

Time(s)

o’.“‘.““‘.“l”.l”“” 0.0

0.5

IO

1.5

o”‘.‘.““l”.“‘.‘.l”” 0.0

(e)

2.0

2.5

3.0

3.5

4.0

4.5

50

5.5

6.0

40

4.5

5.0

5.5

60

4.0

4.5

5.0

5.5

6.0

Ttme ( s )

0.5

1.0

15

2.0

2.5 3.0 3.5 Time(s)

0.5

1.0

1.5

2.0

2.5

1700

J 0.0

3.0

3.5

Time(s)

Fig. 4. Temporal emission behavior for (a) HeI, (b) CO+, (c) NC, and (d) OH at r.f. powers of 14, 18, 22, 26 and 30 W; (e) furnace temperature corresponding to these diagrams.

FAPES: spectral, spatial, and temporal characteristics

501

Table 1. Fe I transitions used for temperature measurement Wavelength (nm) 371.994 373.487 373.713 375.823 376.379 376.719

E,(cm-I)

~“&L! 1.38550 2.55167 1 93916 5:30828 8.25802 1.16964

x x x x x x

lo-” lo-” lo-l7 lo-l4 10m’J 1O-‘7

26875 33 695 27167 34 329 34547 34 692

Emission spectra were recorded by integrating the signal for 7.5 s. This is longer than actually required since the signal only lasts for a period of about l-2 s. However, difficulties in synchronizing the timing of the graphite furnace atomization cycle and the beginning of an array integration period precluded the use of a shorter integration time. The image of the plasma was taken from a radial distance of 1 mm from the electrode using a slit height of 2 mm and a slit width of 0.070 mm so that the area of the discharge sampled had these same dimensions (1:l imaging). The excitation temperature was measured using six Fe1 emission lines in the 370-377 nm spectral region which were recorded using the diode array spectrometer described in the Experimental Section. The lines that were used along with their spectral characteristics are listed in Table 1. The oscillator strengths were taken from BRIDGES and KORNBLITH [16]. The background subtracted emission intensities (I& were substituted into the Boltzmann equation from which the slope of log (2.303K7’-’ was evaluated from the regression of (I,,A3/g$,,) vs E,. The excitation temperature was evaluated from the slope. There are several advantages to the use of a photodiode array spectrometer. One is that the blank background can be recorded and spectrally stripped from the analyte spectrum, ensuring the removal of interfering background lines. A second advantage is that emission from all the lines is recorded simultaneously for a single sample injection which significantly improves the precision, particularly for a transient source like the graphite furnace. A “typical” emission spectrum for distilled water blank (a) and 200 ng of Fe (b) is provided in Fig. 5. This spectrum was recorded at an r.f. power of 30 W. The Fe emission spectrum has been displaced for clarity. Emission bands from N; can be seen in the background spectrum. For the measurement of net spectral line intensities, background correction can be accomplished either by measuring on either side of the lines in Fig. 5b or by subtracting the spectrum in Fig. 5b from that in Fig. 5a to yield a background corrected spectrum. The latter procedure was used for the temperatures reported in this paper. It should be pointed out that the Fe levels, used for the temperature determination, are lower levels. It has been amply demonstrated in the ICP that excitation temperatures for Fe1 yield no unique value, but are dependent on the excitation energy [15,17]. This non-linearity has been interpreted as being due to the relatively high radiative de-excitation rate for lower levels compared with upper levels [15,17,18]. For FAPES, STURGEONet al. [6] reported that Fe1 excitation temperatures are higher for upper levels compared with lower levels so there is some evidence that a similar situation exists, perhaps for a similar reason since both the ICP and FAPES are low frequency, atmospheric pressure plasma sources. The variation of the Fe I excitation temperature with r.f. power is presented in Fig. 6 which covers the power range lo-50 W. The temperatures measured over this power range are from 3100 to 4200 K. At a power of 100 W, STURGEONet al. [6] reported a temperature of 2920 K. For the FAPES source of this study, this would correspond to an r.f. power of just 10 W. It is unclear at this time whether the difference in Fe1

502

T. D.

~E~~PATHIRANA

and M.W.

BLADES

,

1298

1048

Diode

798

548

Number

Fig. 5. Emission spectrum for 200 ng of Fe.

temperature is due to variability in sources, spatial differences, or differences in the power coupling efficiency. The data in Fig. 6 demonstrate that the temperature increases monotonically with an increase in r.f. power over the range studied. For comparison, the Fe excitation temperature in a 27 MHz Ar-FAPES plasma at an r.f. input power of 18 W yielded a value of 4550 K. This Fe I excitation temperature would be the same as that observed for a low-flow Ar-ICP operating at a power of about 600 W 1191. It is clear that either the radiative and convective heat losses from a FAPES source is much lower than that from an ICP, or the electromagnetic field strength is much higher in FAPES, or both. Regardless, based on the Fe I excitation temperatures, FAPES clearly has the potential to be a potent excitation source for atomic spectroscopy.

CONCLUSIONS

The special characteristic of FAPES is that it couples an atmospheric pressure plasma as an excitation source with an electrothermal atomizer as a vaporization method. This characteristic is similar to FANES (furnace atomic non-thermal excitation spectrometry) [20] except that FANES normally operates at pressures between 1 and 10 torr. For both FAPES and FANES, the vaporization and excitation mechanisms operate

10

I.5

20

25

30

35

40

4.5

50

55

60

Power ( w ) Fig. 6. Iron excitation temperature

as a function of r.f. power.

FAPES: spectral, spatial, and temporal characteristics

503

essentially independently of one another such that, in theory, it is possible to optimize the operation of each. Unfortunately, in practice, the plasma properties are affected during the atomization cycle-perhaps the result of impedance matching problems or the presence of thermionic electrons or both. A FAPES plasma will operate using either He or Ar though ignition and operation is more facile with He and an analysis of the physical properties of He and Ar indicates that this is primarily due to differences in the electron ionization rate. This study found that the excitation temperature for the FAPES is in the range 3000-5500 K depending on the r.f. input power and the probe used for the measurement. The spectra for CO+ indicate that this plasma is capable of exciting energy levels as high as 19-20 eV. However, the energy transfer mechanism in this case is near-resonant. In most cases, excitation through electron and heavy body collisions is more probable. The electron energy distribution should be close to Maxwellian since FAPES operates at atmospheric pressure and the excitation frequency is relatively low compared with microwave plasmas. As is the case with most laboratory plasma sources, the FAPES discharge is spatially non-homogeneous and care must be exercised in the measurement and reporting of fundamental and analytical characteristics since these may be spatially dependent. There is also complex temporal behavior for both background and analyte species. Some aspects of the latter will be reported on in a future publication. Acknowledgements-The

authors would like to sincerely thank MARK VAGG of the U.B.C. Mechanical Services Shop for his expertise and advice in constructing the new FAPES source. Acknowledgement is made to the Natural Sciences and Engineering Research Council of Canada and the University of British Columbia for financial support.

REFERENCES [l] D.C. Liang and M. W. Blades, Spectrochim. Acta 44B, 1059 (1989). [2] R. E. Sturgeon, S. N. Willie, V. T. Luong, S. S. Berman and J. G. Dunn, J. Analyt. Atom. Spectrosc. 4, 669 [3] D. L. [4] R. E. (1990). [5] R. E. [6] R. E.

(1989).

Smith, D. C. Liang, D. Steel and M. W. Blades, Spectrochim. Acta 45B, 493 (1990). Sturgeon, S. N. Willie, V. T. Luong and S. S. Berman, J. Annlyt. Atom. Spectrosc. 5, 635

Sturgeon, S. N. Willie, V. T. Luong and S. S. Berman, Analyt. Chem. 62, 2370 (1990). Sturgeon, S. N. Willie and V. T. Luong, Spectrochim. Acta 46B, 1021 (1991). [7] R. E. Sturgeon, S. N. Willie, V. T. Luong and J. G. Dunn, Appl. Spectrosc. 45, 1413 (1991). [S] N. Ballou, D. L. Styris and J. M. Harnly, J. Analyt. Atom. Spectrom. 3, 1141 (1988). [9] M. W. Blades, T. Hettipathirana and D. C. Liang, 73rd Canadian Chemical Conference and Exhibition, Paper No. 48, Halifax (1990). [lo] S. C. Brown, Basic Data of Plasma Physics. Wiley, New York (1959). [ll] S. C. Brown, Basic Data of Plasma Physics-J966. Wiley, New York (1966). [12] C. B. Collins and W. W. Robertson, J. Chem. Phys. 40, 701 (1964). [13] M. Endoh, M. Tsuji and Y. Nishimura, J. Chem. Phys. 79, 5368 (1983). [ 141 A. Gilmutdinov, The dynamics of formation of atomic and molecular layers in GFAAS. Paper presented at the XXVII-CSI Pre-symposium on Graphite Atomizer Techniques in Analytical Spectroscopy, Lofthus, Norway (June 1991). [15] M. W. Blades, B. L. Caughlin, Z. H. Walker and L. L. Burton, Prog. Analyt. Spectrosc. lo,57 (1987). [16] J. M. Bridges and R. L. Kornblith, Astrophys. J. 192, 793 (1974). [17] Z. Walker and M. W. Blades, Spectrochim. Acta 41B, 761 (1986). [18] L. L. Burton and M. W. Blades, Spectrochim. Acta 45B, 139 (1990). [19] L. L. Burton and M. W. Blades, Appl. Spectrosc. 40, 265 (1986). [20] H. Falk. E. Hoffmann and Ch. Ltidke, Prog. Analyt. Spectrosc. 11, 417 (1988).