JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, APRIL 1990. VOL. 5 139

Furnace Atomic Non-thermal Excitation Spectrometry With the Furnace as a Hollow Anode

James M. Harnly US Department of Agriculture, Beltsville Human Nutrition Research Center, Building 161, BARC-EAST, Beltsville, MD 20705, USA David L. Styris and Nathan E. Ballou Pacific Northwest Laboratory, P.O. Box 999, Richland, WA, USA

Furnace atomic non-thermal excitation spectrometry with the furnace as the hollow anode (HA-FANES) was characterised for the determination of Cd and Cu. Operating in the abnormal glow discharge region, the current was limited by the current density at the cathode. Higher currents were attained with a larger diameter cathode and at higher pressures. The emission signals were dependent on the current density. More intense signals were observed when viewing the top and left of the discharge. This positional dependence of the signal strength was much more pronounced for Cu than for Cd. Peakareas and signal to noise ratios increased linearly with the discharge pressure to a maximum of 25.3 kPa. Emission intensities were constant as a function of the discharge current above a threshold of 20 mAfor Cd and 50 mAfor Cu. Both Cd and Cu gave the largest peak areas at the lowest atomisation temperature (1390 "C). The peak areas decreased roughly in proportion to the increased diffusion coefficients at higher temperatures. The detection limits for HA-FANES (0.8 and 1.5 pg for Cd and Cu, respectively) are comparable to those for conventional FANES with the furnace as a hollow cathode and graphite furnace atomic absorption spectrometry. Keywords: Glow discharge; hollow anode; furnace atomic non-thermal excitation spectrometry; cadmium and copper determination

Furnace atomic non-thermal excitation spectrometry (FANES) combines the efficient vaporisation/atomisation of the graphite furnace with the efficient excitation of the hollow-cathode discharge to provide a highly sensitive emission source. 1.2 FANES has detection limits comparable to or slightly better than those for graphite furnace atomic absorption spectrometry (GFAAS), and has the same ability to handle micro-volume samples. The simplified optics (as compared with GFAAS) make FANES ideal for multi-ele- ment furnace determinations, and therein lies its true appeal.

The excellent detection limits, but lengthy cycle time, of the graphite furnace atomiser have produced considerable interest in multi-element GFAAS. Interest in multi-element AAS, in general, has existed since shortly after its inception as a single-element technique . 3 4 A key component, however, has always been lacking, namely an intense source that could provide detection limits at all wavelengths comparable to those for conventional AAS using a hollow-cathode lamp. Graphite furnace atomic emission spectrometry (GFAES) circumvents the need for a source but does not provide sufficient thermal energy for efficient excitation of elements with emission wavelengths below 300 nm.7 Indeed, this same lack of intensity in the UV region has been the downfall of multi-element GFAAS using a continuum source.8~9

Hollow-cathode discharges have long been known to be highly intense emission sources for metals and non-metals. The high intensities achieved are a result of the high electron temperatures and the lack of local thermodynamic equilib- rium at the low operating pressures (typically less than 13 kPa). Falk et al. 1 were the first to establish a hollow-cathode discharge in a furnace, using the furnace as the cathode and an external anode. Their work has been followed with consider- able interest by the atomic spectroscopy community and has given rise to a commercial instrument. 10 Unfortunately, there has been little work with FANES, apart from in one or two laboratories.*

An alternative FANES design was developed with the furnace as the hollow anode (HA-FANES)." This work began as an attempt to develop a replicate FANES source. A prototype design in a bell jar, however, soon revealed that it

was difficult to maintain the discharge inside the furnacc cathode without external shielding to restrict the electrical field lines." This is not an uncommon problem for hollow- cathode discharges,Q hence the use of a glass sleeve around the cathode in hollow-cathode lamps. The lack of control of the position of the discharge led to experiments with the polarity reversed. Using the furnace as the anode, with a carbon rod down the centre of the furnace for the cathode, provided a discharge with excellent stability and no need for electrical shielding. Initial results suggested that, with further optimisation, results comparable to those given by FANES with a hollow cathode (HC-FANES) could be achieved.

This paper describes the optimisation of HA-FANES for the determination of Cd and Cu with respect to the discharge pressure and current, the size of the cathode and the atomisation temperature.

Experimental Hollow-anode FANES has been described previously. 11 The hollow-anode furnace consists of an integrated contact cuvette mounted between two copper clamps supported on hollow copper feed-throughs. The feed-throughs are mounted in a 150-mm flange which seals one end of a six-way 100 mm diameter vacuum cross. The discharge current is introduced into the cross through a high-voltage feed-through in the same flange just described. Inside the vacuum cross, a Macor block supports a pyrolytic graphite coated carbon rod which passes down the centre of the furnace and serves as the cathode. The components and support electronics are listed in Table 1.

Several changes have been made since the initial report." First, a 1.0 mm diameter carbon rod is now used as the cathode in place of the 0.5 mm diameter rod and a new Macor cathode support was machined to hold the larger cathode.

Second, a custom-made 150-mm flange was constructed of aluminium to accommodate the custom-made copper feed- throughs. These cylindrical feed-throughs are hollow with stainless-steel tubes inserted along the length of the cavity to give a tube-within-a-tube configuration. Circulating water enters through the stainless-steel tube and returns on the

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140 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, APRIL 1990, VOL. 5

Table 1. Components of HA-FANES

Component Model Company

(Bonn, FRG) Integrated contact cuvette - Ringsdorff- Werke

0.5- and 1.0-mmcathodes - Ringsdorff- Werke Furnace power supply . . HGA-500 Perkin-Elmer

(Bodenseewerke, Uberlingen, FRG)

BHK 1000-0.2M Kepco (Flushing, NY, USA)

Baratron 122AA MKS Instruments (Andover. MA, USA)

High-voltage power supply

Capacitance manometer

Thermocouple vacuum gauge . . . . . . Convectron275 GranvillePhillips

(Boulder, CO, USA)

(Skokie, IL, USA) Vacuumpump . . . . 1376 Sargent- Welch

Constant-temperature circulator . . . . . . FK Haake-Buchler

Instruments (Saddlebrook, NJ, USA)

USA) Opticalpyrometer . . . . 11 x 30 Ircon (Niles, IL,

Table 2. General analytical procedure

Step Procedure 1 Deposit sample 2 Drysample:

Ar atmosphere 15-s ramp

150 "C 15-s hold

3 4 5 6 Atomise:

Evacuate chamber to 0.013 kPa Back-fill with Ar to desired pressure (1.33-25.3 kPa) Start glow discharge (5-100 mA)

0-s ramp

(1390-2070 "C) 3-s hold

outside. The water temperature is maintained at 15 "C using a constant-temperature circulator.

Third, a new high-voltage supply (Table 1) was acquired to drive the glow discharge. This power supply permits operation in the constant-current mode. The internal jumpers have been reset to provide the fastest response time.

Finally, a new furnace power supply (Table 1) was obtained to heat the integrated contact cuvette. Although impedance matching is not optimum, the diode feedback system is used to allow the maximum heating rate. The diode has been mounted in a window viewing the back of the furnace (180" from the spectrometer). Although the diode does not view the furnace directly (the Macor cathode support is in the way), there is sufficient reflected furnace emission that the most sensitive diode scale (<lo00 "C) can still be used. As the diode views less direct light, and hence lower intensities, from the furnace, the temperature scales are no longer appropriate. The lowest temperature attainable is approximately 1300 "C. Lower temperatures could be achieved by positioning the diode to observe more direct furnace emission.

An optical pyrometer was used to calibrate the furnace temperature but was not used routinely. The pyrometer was focused inside the sample port and an emissivity of 1.0 was assumed. The setting on the front of the diode housing and the furnace temperature setting were varied independently to give the flattest shape possible to the top of the peak. A 3-s atomisation was used. The maximum heating rate (starting at ambient temperature) was approximately 1100 "C s-1.

A 1 : 1 image of the middle (aligned in depth with the sample port) of the furnace is focused on the entrance slit of the kchelle polychromator. 11 The multi-element operation of the kchelle and the computerised data acquisition and processing for emission measurements have been described previously.13 Wavelength modulation is used to compute the background- corrected emission intensities accurately at a rate of 56 Hz. In this study, data for Cd (228.8 nm) and Cu (324.7 nm) were acquired simultaneously. The analytical sequence used in this study is shown in Table 2. The atomisation temperature and the discharge pressure and current were varied as described in the text. Throughout this study, argon was used as the fill gas. Other variables which were investigated but are not shown in Table 2 are the cathode diameter and the region of the glow discharge viewed by the spectrometer.

Signal to noise ratios are computed for each atomisation. The peak height and the optimised peak area (integrated from the peak appearance until the decay to l/e2 of the maximum to include 9&95% of the peak area) are determined as described previously.14 The base-line standard deviation of the peak area is determined from the standard deviation of the individual intensities, si, measured during the pre-atomisation period:

sat-,, = si v t c t m

where tc is the time constant for the individual intensities (0.018 s for a 56-Hz modulation frequency) and t , is the measurement time for each element (0.5 s for Cd and 1.5 s for Cu). This assumes that the limiting noise is statistical in nature and does not change owing to the atomisation process ( i . e . , black-body , line and molecular emissions were negligible). This was shown to be a valid assumption for the wavelengths for Cd and Cu, for the atomisation temperatures used and for standards in dilute nitric acid. This algorithm allows a base-line peak-area noise to be computed for each atomisa- ticin.

The data processing program also measured selected peak event times: appearance, peak maximum, decay to half maximum, decay to l /e maximum, decay to l/e2 maximum and the peak, end.13

Results and Discussion Current - Voltage Characteristics

Fig. 1 shows the current versus voltage characteristics for a 1 .O-mm diameter cathode operated at nine different discharge pressures. These values were obtained at room temperature and were accurate until temperatures in excess of 1600 "C were reached. Above this temperature, the release of thermionic electrons from the carbon surface resulted in enhanced currents for a constant potential as reported previously.* As the high-voltage power supply was operated in the constant- current mode, it was observed that the current remained steady, but the potential dropped dramatically.

In general, the plots in Fig. 1 represent the abnormal glow discharge region. Complete coverage of the cathode by the plasma was achieved from approximately 250 V at the higher pressures to 300 V at the lower pressures. Potentials higher than those shown in Fig. 1 significantly decreased the stability of the discharge.

Qualitatively, an unstable discharge is characterised by random bright, localised flashes, arising from very rapid but apparently low current arcs, superimposed over the seemingly steady cathodic glow discharge. These flashes usually occurred near the junction of the cathode and the Macor support. In the abnormal glow discharge region (the voltages covered in Fig. 1), these rapid flashes did not lead to degeneration of the plasma into an arc. Although the flashes were accompanied by an unsteadiness of the potential (as shown on the voltage meter of the high-voltage supply), the current level remained constant. At higher potentials (greater

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JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY. APRIL 1990, VOL. 5 141

600 I 1 .- E 500- w

$2 5 400 - m r rn 6 300-

I I I I I 1 0 20 40 60 80 100 120

200J

Discharge current/mA

Fig. 1. Discharge voltage versus current for a 1.0-mm diameter cathode at various argon pressures: A. 1.33; B, 3.99; C, 6.65; D, 9.31; E. 12.0; F, 14.6; G, 17.3; H, 20.0: and 1. 22.6 kPa

5-

1, A

C

.................. ..................

Fig. 2. End-on view of the HA-FANES emission source: A, furnace wall; B, negative glow region or corona; and C, cathode. The image of the source on the spectrometer was shifted using the lens so that the entrance slit viewed five different regions of the corona: 1, above; 2, top; 3. centre; 4, bottom; 5 , left; and 6, right

than those shown in Fig. l), an arc discharge formed with termination of the cathodic glow. Repeated arcs occurred as the potential was periodically re-established.

A stable discharge, characterised by very steady voltage and current levels, was free of flashes. Stable behaviour was readily established within the limits shown in Fig. 1.

Instances of instability of the discharge within the limits shown in Fig. 1 appeared to be related to the morphology of the carbon surface. Operation of the glow discharge without firing the furnace for periods of greater than 5-10 min produced unstable behaviour and intensely bright spots on the anode (furnace) surface. Heating the furnace above 1600 "C eliminated the bright spots and re-established stable behav- iour.

The highest attainable discharge currents, with continued stable operation, were approximately twice as large for the 1.0-mm diameter ( D ) cathode than the 0.5-mm diameter (d ) cathode. This observation suggests that current density limits the maximum current. It is interesting that the cathode current density, J , is limited by space charge-induced distortion of the fields.15 This current density for an anode diameter (A) , is calculated from the Poisson equation in cylindrical coordi- nates to be inversely proportional to 6 In (A/6) where 6 is the cathode diameter ( D or d). This implies that, for any given voltage, the ratio (.JD/Jd) of current densities for the two cathodes (1.0 versus 0.5) should be 2.75. By using the present data for the 1.0-mm diameter cathode and the data from reference 11 for the 0.5-mm diameter cathode, the magnitude

Table 3. Signal comparison for 0.5- and 1.0-mm diameter cathodes. Results obtained viewing the left of the corona at 9.31 kPa and 1845 "C atomisation temperature

Cathode

Parameter 0.5 mm diameter 1 .O mm diameter Current/mA . . . . 25.0 50.0 Cathodearedmm2 . . 34.6 69.1 Current density/

Cd peak arealls Cupeakaredls

mAmm-2 . . . . 0.72 0.72 . . 137.0 f 8.0 ( n = 5) . . 1191.0 k 19.0 (n = 3) 1246.0 f 36.0 (n = 5)

155.0 k 9.0 ( n = 3)

Table 4. Peak area versus viewed region of the image. Results obtained at 9.31 kPa, 50 mA, atomisation temperature 1845 "C, with a 1.0-mm diameter cathode. Precisions determined as l a of the base-line noise

Region Peak aredl s of corona viewed" Cd c u

1 Above . . . . 199k0.4 686 k 0.8

3 Centre . . . . 152k0.4 78 f 0.6 4 Bottom . . . . 103 k0 .3 706 f 0.7 5 Left 155 k 0.3 1283 k 0.9

2 Top . . . . . . 191 k0 .5 1233 k 1.7

. . . . . . 6 Right . . . . 151 f 0 . 4 611 +- 1.8

* See Fig. 2 for diagram of viewed region.

of JD/Jd is calculated to be 5.2 at 3.99 kPa and 300 V. This discrepancy can only be due to the more intense thermionic emission of electrons from the smaller cathode. It has a smaller mass and a smaller radiation surface, which leads to higher temperatures and greater thermionic emission of electrons. This source of charge carriers is not accounted for in the above solution of the Poisson equation. The observed equality in current densities at maximum stable current occurs because the limit of the cathodic thermionic emission current (Richardson - Dushman equation) has been reached in both instances. At this point the current can be increased only by increasing the cathode temperature. Increasing the voltage will have no effect.

The current densities of HA-FANES, however, are much higher than those for HC-FANES. The conventional graphite furnace (28 mm long and 6 mm in diameter) used in the commercial HC-FANES system has a surface area of 528 mm2 compared with 34.6 and 69.1 mm2, respectively, for the 0.5- and 1 .O-mm diameter cathodes of HA-FANES (the integrated contact furnace is 19 mm long and the cathode extends 3 mm further before entering the Macor mount). For a 50-mA current, the current densities for the three cathodes are 0.095, 1.44 and 0.72 mA mm-2, respectively.

Cathode Diameter

All previously reported data were acquired using a 0.5-mm diameter cathode." As shown in the previous section, a 1.0- mm diameter cathode offers higher current levels, but comparable current densities. Table 3 presents the results obtained for repeated atomisations of 20 pl of a 100 ng ml-1 concentration of Cd and Cu using 0.5- and 1.0-mm diameter cathodes. The current for the larger cathode was twice that of the smaller cathode, giving the same current density in each instance. Each value was obtained on a separate day and is the average of at least three determinations. The data were collected over a period of several months, using only two integrated contact furnaces, although five or six cathodes were used. The life of the furnaces is fairly long (no statistics are available) as atomisation takes place in a very pure argon atmosphere at reduced pressures. The wear on the 0.5-mm

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300

v) 200-

? - -. Y (0

2 100-

JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, APRIL 1990, VOL. 5

- -

Discharge pressure/kPa

Fig. 3. Peak area for 2 ng of Cu as a function of argon discharge pressure at (A) 30, (B) 50 and (C) 70 mA. Atomisation at 1845 "C and viewing the left of thc corona

t I I I 0 10 20

Discharge pressure/kPa

Fig. 4. Peak area for 2 ng of Cd as a function of argon discharge pressure at (A) 30, (B) 50 and (C) 70 mA. Atomisation at 1845 "C and viewing the left of the corona

cathode was considerably greater than that on the furnace and arose from the glow discharge. Close examination of the glow discharge showed it to be slightly more intense at the point where the cathode enters the Macor support. After approxi- mately 100-150 atomisations the cathode was visibly thinner in this region. Continued operation caused the cathode to break at this point. The analytical results were not affected by the decrease in the cathode thickness. The wear on the 1.0-mm cathode has not been as noticeable and none have yet broken during operation.

The results in Table 3 show that comparable analytical signals are achieved with comparable cathodic current densi- ties of the discharge. The day-to-day reproducibility of the signals is excellent, despite some uncertainty in the viewing area. The relative standard deviations for the integrated signals averaged 5.9% and 2.4% for Cd and Cu, respectively. The relative standard deviations for the peak heights were approximately twice the values found for the integrated areas. Previous results had shown unusually large uncertainties between repeated atomisations. These poor precisions were attributed to the large time interval between atomisations (approximately 20 min as there was no water cooling of the furnace). With the improvements described under Experimental, an atomisation can be made every 5 min. Even more rapid atomisation rates could be achieved with automa- tion of the evacuation and back-filling of the system (Table 2, steps 3 and 4). The relative standard deviations in Table 3 are acceptable for Cu despite the fact that all the samples were

1200)

0 20 40 60 D C u rre nt/m A

Fig. 5. Peak areas for (A) 2 ng of Cd and (B) 2 ng of Cu, and signal to noise ratios for (C) Cd and (D) Cu as a function of the discharge current. Atomisation at 1845 "C, a pressure of 9.31 kPa and viewing the left of the corona

Table 5. Peak areas versus atomisation temperature. Data acquired at 9.31 kPa, 50 mA, viewing left of corona, using a 1.0-mm diameter cathode. Each value is mean of four determinations

Cd c u Temperature

"C Area Height Area Height 1390 153 924 1511 946 1604 145 800 1401 1287 1845 13 1 798 1220 135 1 2070 125 600 1037 1596

delivered to the furnace by manual pipetting. The relative standard deviations for Cd, however, are less acceptable. The very rapid nature of the Cd signal may be the major source of imprecision. The full width of the Cd signal at 6.65 kPa is less than 0.5 s (compared with 1.5 s for Cu). During this period, 28 background-corrected intensities are computed. Better rela- tive precisions may be achieved with a faster data acquisition rate.

For both cathode sizes, double peaks were observed at lower discharge currents (less than 20 mA for the smaller cathode and less than 40 mA for the larger cathode). The double peaks arose from the condensation and re-atomisation of the analyte on the cathode. At higher currents (25 and 70 mA, respectively), the cathodic current produced sufficient heating of the cathode that its temperature no longer lagged so far behind that of the wall temperature. Consequently, at the higher currents, only single peaks were observed.

Viewed Region

Viewed from the end of the furnace (Fig. 2), the glow discharge appears as a halo or corona with a dark inner region surrounded by a luminous ring with a dark outer region between the corona and the wall. Any portion of the corona or the dark regions can be imaged on the entrance slit (500 pm high and 100 pm wide for this work). In this study, six different regions of the corona were viewed, as shown in Fig. 2 and Table 4. The analytical signals were obtained using 20 pl of 100 ng ml-1 concentrations of Cd and Cu and each value is the average of at least three determinations. The standard deviations shown in Table 4 represent the base-line noise and thus reflect the background emission intensity of the plasma in each of the six regions.

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JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, APRIL 1990. VOL. 5 143

Table 6. Detection limits, assuming 30 of the base-line noise as the detection limit and a 28 mm furnace length

Detection limit/pg

HC-FANES This

Element work Reference 2 Reference 10 GFAAS" Cd . . . . 1.5 0.6 - 0.3 c u . . . . 0.8 1.5 1.5 2.0

The data show a surprising horizontal and vertical asym- metry for Cu and a vertical asymmetry for Cd, although the extremes of the intensity distribution are less pronounced for Cd. These results are reproducible and have been observed on a variety of days for both the 0.5- and 1.0 mm-diameter cathodes. Three factors may have influenced these results. Firstly, the optical system was aligned visually. The position- ing of the lens and the focusing of the image of the corona on the entrance slit was done at visible wavelengths. The transient nature of the signals made it impossible to use the Cd and Cu emissions for positioning the optics. Hence the image for Cd at 228.8 nm is expected to be more defocused than that for Cu at 324.7 nm. This probably accounts for the fact that the variation in the pattern of the Cd signals (approximately a factor of two difference between the highest and lowest values) is much less extreme than that of the Cu signals (a factor of 150).

Secondly, the cathode is centred vertically but is offset horizontally towards the right wall of the furnace so that samples may be deposited through the sample port without striking the cathode. This off-centre position might result in differences in field strengths on the two sides of the cathode and produce the vertical and horizontal asymmetries observed for the analytical signals. A horizontal asymmetry was observed for Cu (a factor of two difference between the left and right of the corona) but not for Cd. Both elements show a vertical asymmetry; in both instances the signal from the top of the corona is approximately a factor of two larger than the signal from the bottom. A vertical concentration gradient has been observed for Cu in the graphite furnace using atomic absorption.16 It is, however, the opposite of the pattern observed in Table 4. The volatile nature of Cd makes it unlikely that the observed asymmetry is due to a concentration gradient within the furnace.

Finally, a 19-mm deep source is not the ideal geometry for emission measurements. It is possible that the long axis of the cathode does not lie in line with the lens, the entrance slit and the collimating mirror of the polychromator. Misalignment of the cathode might result in a shortening of the image to either side and on the top or bottom, thus producing the asymmetric results. Further experiments are planned to investigate these results.

It was found that the background noise levels were determined by the glow discharge, even in the dark regions outside the corona. Emission from the furnace was not a factor at these temperatures (2070 "C maximum), these wavelengths (324.7 nm for Cu and 228.8 nm for Cd) and with standards in dilute nitric acid. In general, the background emission levels and, consequently, the standard deviation of the back- ground, were proportional to the analytical signal levels. The most notable exception was between the horizontal regions viewed for Cu. The highest signal, to the left of the image, was accompanied by a relatively low noise level, while the low signal to the right of the image had a higher noise level than observed on the left side.

Discharge Pressure The analytical signals and background noise were also examined as a function of the discharge pressure and current.

The Cu and Cd analytical signals, as a function of pressure, are shown in Figs. 3 and 4, respectively. The signals for both elements increased linearly with pressure at all three currents while the background noise levels decreased slightly as a function of pressure. These data were acquired viewing the left of the image and the optical alignment was not changed during the course of the experiment.

It would appear that the nature of the discharge does not change significantly with pressure. The background de- creased only slightly with pressure. As the size of the glow discharge appeared to shrink slightly with pressure while the optical alignment remained unchanged, the reduction in noise could be a result of viewing a slightly different region of the discharge. The increased signal for both elements is predict- able from the increased residence time of atoms within the furnace arising from the increased pressure and the accom- panying reduction of the diffusion coefficient. Any non-linear response would suggest a change in the excitation mechanism or population of the atomic and ionic states.

The increased residence times of Cu and Cd with pressure resulted in increased signal widths (from the appearance time until decay to l/e2 of the peak maximum). For Cu the signal widths were 0.868 and 1.304 s at 4.0 and 25.3 kPa, respec- tively, whereas for Cd the widths were 0.327 and 0.423 s. Base-line noise levels were determined for integration periods of 0.5 s for Cd and 1.5 s for Cu. If the exact widths were used as integration limits, the base-line noise would increase with pressure. As a consequence of using the larger integration intervals, the base-line noise levels remained constant and the signal to noise ratios for both elements increased linearly with pressure.

Discharge Current

The variations of the Cu and Cd signals as a function of the discharge current are shown in Fig. 5. Both elements were viewed simultaneously at the left of the image at a pressure of 9.31 kPa. Both elements appeared to give a constant signal as a function of current after a threshold current had been attained. The threshold was higher for Cu (50 mA) than for Cd (20 mA). The signal to noise ratio was optimum for both elements between 20 and SO mA. At present, no explanation can be offered for these observations. Further experiments are planned. It should be noted that there was no observed signal for either element in the absence of a discharge (Fig. 5 , 0 mA). At an atomisation temperature of 1845 "C, there was no thermally excited emission.

Atomisation Temperature

Table 5 shows the effect of the atomisation temperature on Cu and Cd. The results were obtained at 9.31 kPa and 50 mA while viewing the left of the image. For both elements, the peak area decreased as the temperature increased. The peak height of Cu increased with increase in temperature whereas the peak height of Cd decreased.

The results for Cu and Cd are close to those expected for a diffusion loss-limited system. At 1604, 1845 and 2070"C, decreases in peak area of 17, 30 and 40%, respectively, are predicted (assuming that the diffusion rate is proportional to the temperature to the power 3/2) compared with peak areas at 1390°C. The decreases for Cu and Cd were 8 , 1 9 and 31% and 6, 14 and 26%, respectively. The lowest atomisation temperature (1390°C) was not sufficient for Cu. The signal was unusually broad (1.72 s from the appearance to decay to half the maximum of the peak) and had not returned to the base line at the end of the 3-s atomisation cycle. The respective peak widths were not statistically different for any of the signals for Cd or for any of the Cu signals at the three highest temperatures. The Cu peak height increased, as previously observed for GFAAS, but the Cd peak height decreased with increasing temperature.

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144 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, APRIL 1990, VOL. 5

The signal to noise ratios varied directly with the peak area as the background levels were dependent only on the glow discharge and were unaffected by the furnace temperature. The integration limits were kept constant. The optimum compromise temperature for Cu and Cd is slightly above 1390 “C for peak-area determinations. A considerable com- promise would be required for peak-height determinations as the maximum values were obtained at the highest and lowest temperatures, respectively.

Detection Limits

The computed detection limits for this study (HA-FANES) are given in Table 6 with values reported for two HC-FANES systems and conventional GFAAS. The HA-FANES results were obtained with atomisation at 1845 “C, 25.3 kPa and 70 mA, and viewing the left of the image. To permit an accurate comparison, the HC-FANES results were corrected math- ematically to reflect a detection limit of three times the base-line standard deviation. In addition, the HA-FANES values were decreased by a factor of two to account for the shorter furnace (19 mm long) used in this study as compared with conventional furnaces (28 mm long). Frech et d.18 reported a factor of two poorer sensitivity with the smaller furnace. The correction was made because there appears to be no reason why a longer integrated contact cuvette cannot be manufactured.

The results reported in Table 6 for HA-FANES were obtained simultaneously for Cu and Cd. Improved detection limits could be obtained individually for Cd at lower tempera- tures and in a different viewed region. Consequently, it seems reasonable to conclude that the detection limits for HA- FANES are comparable to those for HC-FANES and conventional GFAAS.

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Paper 91041 25 F Received October 3rd, 1989

Accepted November I6th, 1989

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