A gas-sampling glow discharge coupled to hydride generation for the atomic spectrometric determination of arsenic

Spectrochrmica Acta, Vol. 488. Printed in Great Brltain

No

10. pp.

1207-1220,

1993 0

056‘M547/93 $5 00 + 03 1993 Pergamon Press Ltd

A gas-sampling glow discharge coupled to hydride generation for the atomic spectrometric determination of arsenic J. A. C. BROEKAERT,* R. PERErRo,t T. K. STARN and G. M. HIEFTJE$ Department

of Chemistry, Indiana University, Bloomington, (Received 29 January

IN 47405, U.S.A.

1993; accepted 8 April 1993)

Abstract-A

gas-sampling glow discharge (GSGD) has been coupled with a continuous-flow hydride generator and optimized for the determination of As. The discharge characteristics of the source, operated at constant voltage, and its sampling capabilities have been studied for helium, neon and argon used as discharge gases. The evolved arsine, the accompanying hydrogen and the noble support gas were drawn into the discharge with an efficiency as high as 78%. Spatially resolved maps were obtained of the As I 228.8 nm line, the spectral background and the fill-gas lines, all as a function of the discharge pressure, in order to identify optimal working conditions. Under these conditions, noise amplitude spectra of analytical signals are completely white. Detection limits for As with helium (operated at 6.5 torr (1 torr = 133.332 Pa) and 650 V), with neon (operated at 2.5 torr and 750 V) and with argon (operated at 1.5 torr and 550 V) are 54 ng/ml, 30 ng/ml and 20 @ml, respectively. The linear portions of calibration curves extended to 10 &ml in the argon discharge and to 2 @ml for neon and helium.

1. INTR~DUC~~N THE atmospheric-sampling glow discharge developed by MCLUCKEYet al. [l] has been proven to be a useful soft ionization source for mass spectrometry. The device is able to extract atmospheric gases through a small cathodic orifice behind which is located an anchored glow discharge in a reduced-pressure zone. Because the extracted atmospheric sample must pass through the glow region, its constituents are efficiently ionized for subsequent mass spectral analysis. Of course, such sources can also be used to determine the constituents in other atmospheric pressure gases such as helium [2, 31. Not surprisingly, the discharge characteristics of these devices are rather similar to those of other glow discharges, which have a long and successful history as sources for atomic spectrometry (for a survey, see, e.g. Refs [4-61). Recently, the atmospheric-sampling glow discharge has been modified for the emission-based determination of non-metals in gases and vapors [7]; the modified unit has been dubbed the gas sampling dc glow discharge (GSGD). The GSGD is equipped with a narrow-bore silica capillary, cemented to the orifice leading to the low-pressure glow region, for introducing gases and vapors. The result is a convenient apparatus for the sensitive near-site analysis of non-metal containing vapors. In the present study, the GSGD was applied to the determination of As in solution, following its conversion to the volatile hydride by means of a continuous-flow hydridegeneration system. The latter system is similar to those commonly used in plasma atomic spectrometry (for a review see Ref. [8]). Th e combination has been explored with helium, neon and argon as support gases. In coupling the GSGD with the hydride-generation system, the gas-sampling rate from the hydride-generation apparatus was found to be particularly critical. The influence of operating conditions (pressure, voltage, current) was evaluated and the spatial distribution of line and background intensities was measured for the As I 228.8 nm line and for atom and ion lines of the fill gases. The zones of high signal-to-noise levels were examined further by means of * On leave from: Department of Chemistry, University of Dortmund, P.O. Box 500500, D-4600 Dortmund, F.R.G. ‘rOn leave from: Department of Physical and Analytical Chemistry, University of Oviedo, 33006 ,Oviedo, Spain. *Author to whom correspondence should be addressed. 1207

J. A. C. BROEKAERTet al.

1208

PTFE to rotary Pump

window Fig. 1. GSGD source.

noise amplitude spectra were determined.

and analytical

figures-of-merit

under optimized

conditions

2. EXPERIMENTAL 2.1. Instrumentation The GSGD, as described earlier [7, 91, was interfaced to an all-glass hydride-generation cell of conventional design [lo, 111. Special attention was paid to optimizing the gas-sampling rate and to drying the analyte gases. 2.1.1. GSGD source. The all-metal lamp (Fig. 1) was made of Ni-Cr steel and was equipped with two quartz side-viewing windows of 25.4 mm diameter [7, 91. The circular anode and cathode (diameter of 37 mm) are oriented in horizontal planes and separated by 9 mm. For convenience of sample introduction, a silica capillary of 0.25 mm internal diameter (Polymicro Technologies) was attached by means of silicone cement to the cathode orifice. The whole source was placed on a computer-controlled translation stage, which moved the assembly horizontally in a direction perpendicular to the observation path. The source could be adjusted vertically by means of a manual translation stage that had a reproducibility of better than 0.7 mm. With a 1:l.l image of the source on the entrance slit of the spectrometer, spectral features could be mapped with a spatial resolution of 0.68 mm vertical and 0.45 mm horizontal. Data acquisition for spatially resolved measurements was controlled by a National Instruments NB-MIO-16XL multi-function A/D board and LabVIEW@ 2 software operated on a Macintosh II PC; 512 data points were collected across each image. The source was powered by a dc supply [7, 91 that was operated at constant voltage (400-750 V, depending on discharge gas and pressure) for most experiments. A 2.8 H inductor was inserted in the output of the supply to mitigate the effect of current breakthroughs that occasionally occur in the discharge. Discharge pressure was controlled by a rotary-vane mechanical pump (Balzers, displacement capacity of 60 mYh) throttled by a butterfly valve and monitored with a capacitance pressure transducer (Baratron). 2.1.2. Hydride-generation system. The hydride-generation apparatus (Fig. 2) is based on the flow-cell type of system used in ICP spectrometry [ll]. A column, filled with 3 mm diameter glass beads, was used to ensure complete reaction and to dissipate the reaction heat [12]. The beads are held in the column by a fritted glass plate. The reagents are brought together in a glass “T” with an internal diameter of 2 mm. The support-gas flow (argon, helium or neon) enters a second side arm of the same glass tube. A condenser placed downstream from the reaction tube serves to remove moisture from the warm reaction gas mixture of hydride, hydrogen and carrier gas. The temperature of the condenser was maintained near 0°C with a circulating water bath. A 6 mm i.d. gas-liquid separator, inserted between the reaction tube and the condenser, enabled the condensed liquid to be drained away continuously. Despite its relative complexity, the hydride generator has an internal volume of less than 50 ml, because of the space occupied by the glass beads; as will be shown later, analytical response is accordingly rapid. In order to obtain the highest possible sensitivity, it is desirable to match as closely as possible the flow rate of product gases from the reaction system (Fig. 2) to the natural uptake rate of the silica capillary. In order to gauge the closeness of these flows, a side arm was incorporated into the sampling line, and excess gas escaping from it directed into a 50 ml water-filled

As determination by gas-sampling

-

1209

2 M HCl / sample

w

5 %NaBH4

to GSGD

water 0 OC

Fig. 2. Continuous-flow hydride-generation system (dimensions in mm): (a) reaction column; (b) fritted glass disc; (c) gas-liquid separator; (d) condenser.

Erlenmeyer flask (see also Fig. 1). Any evolved gas not entering the GSGD therefore vents from the side arm and produces visible bubbles in the flask. By controlling the carrier gas flow, the flow of the hydride-generation reagent (5% NaBH4 solution) and by throttling the displacement volume of the vacuum pump with the aid of a butterfly valve, the sampling efficiency could be maintained at >75%. 2.1.3. Atomic emission spectrometric measurements. A 1:l.l image of the discharge was formed by means of a quartz lens on the entrance slit of a 0.35 m Czerny-Turner monochromator. A 3 mm aperture placed in front of the entrance slit produced full illumination of the grating during analytical measurements, whereas a 1 mm aperture was used to provide better vertical spatial resolution during mapping studies. The gas-sampling glow discharge (GSGD) was operated for at least an hour prior to taking measurements. This warm-up time gave the source body a constant temperature. No adverse effects due to source generated heat were noted. Further details of the instrumentation are given in Table 1. 2.1.4. Noise amplitude spectra. For recording the noise amplitude spectra, the signal from the R928 photomultiplier was fed into a current amplifier (Keithley, Model 427), with a 0.01 ms time constant. The signal was then low-pass filtered at 130 Hz with a Krohn-Hite model 3342

Table 1. Instrumentation

used for hydride-generation

GSGD atomic emission spectrometry

Power supply for GSGD

Kepco, model BHK 1000-0.2

Vacuum system roughing pump pressure measurement

Bakers, model DUO 06OA MKS Baratron capacitance pressure transducer

Peristaltic pumps Spectrometric equipment lens aperture stops monochromator entrance/exit slit width photomultiplier PMT power supply picoammeter

Gilson, Type II pumping rates: O-5 ml/min 13.5mm focal length, 50 mm dia. 1 mm for spatially resolved studies and 3 mm for analytical measurements 0.35 m GCA-McPherson model EU-700; grating: 60 X 60 mm; 1180 grooves/mm, blazed for 250 mm 40 km Hamamatsu RX?& -800 V Keithley, model 244 Keithley, model 414s

1210

J. A. C. BROEKAERT et al.

filter and then oversampled at 150 Hz with 0.1 Hz frequency resolution by a Macintosh data collection system [ 131. 2.1.5. Muss spectrometric measurements. The first stage of a Balzers quadrupole mass spectrometer, originally designed for inductively coupled plasma mass spectrometry [14], was altered to produce a glow discharge within the first stage between the sampling plate (cathode) and the skimmer cone inside the interface. A BTFE spacer separated the cathode from the rest of the mass spectrometer, held at ground potential. The cathode geometry was planar, rather than step-shaped as in the emission experiments, because the skimmer cone protrudes toward the cathode. The distance between the sampling plate (cathode) and the skimmer (anode) was 1 cm. The second and third stage ion optics and the pressure in the first stage of the mass spectrometer were optimized for maximum electrometer signal from the argon dimer ion at mass 80. This species was readily detectable, and was close to the mass range of interest (mass 75 through mass 79). A LabVIEW@ 2 program was written to collect and store the mass data. 2.2. Reagents An arsenic stock solution was prepared by dissolving AsZOs (Fisher Scientific) water with the addition of a minimum amount of NaOH. Standards were obtained by dilution of the stock solution with 2 M HCI, and 2 M HCl solutions were used as all measurements analar grade NaBHJ (Aldrich) and analar grade HCI (J. T. Baker)

3. RESULTS

AND

in distilled appropriate blanks. For were used.

DISCUSSION

Discharge characteristics and spectral features The GSGD can be operated stably with any of several the use of helium, neon and argon has been investigated. 3.1.

gases.

In the present

study,

The As I 228.8 nm line was found to be highly sensitive and relatively free from spectral interferences for all three gases. Use of the As I 193.7 nm line was also feasible, even without nitrogen purging of the optical path. However, compared to the As I 228.8 nm line, the 193.7 nm line produced a signal-to-background ratio that was 2 X poorer, an intensity that was lower by a factor of five, and a spectral background pattern that was less smooth. Consequently, the 193.7 nm line was not used for further work. It was found that the greatest net analyte signal was produced in argon at about 1.5 torr (1 torr = 133.332 Pa), in neon at 2.5 torr and in helium near 6.5 torr. In all three cases, the analyte response was found to increase with pressure as well as with voltage; both changes cause the current density in the discharge to go up (Fig. 3). With neon, however, the rise in signal with current density occurred only within a limited range. The spectra in Fig. 4 were recorded using the hydride-generation conditions and discharge characteristics compiled in Table 2. It was found that operation of the discharge at constant voltage and pressure and with the discharge current as the dependent parameter led to the best signal reproducibility. This mode of operation was consequently adopted for all analytical measurements reported in this paper. 3.2. Hydride-generation and sampling The best conditions for continuous-flow hydride-generation are well known from papers on hydride-generation combined with ICP spectrometry [lo-121. However, different conditions might prove to be optimal with the GSGD, since they also influence strongly the robustness and excitation properties of the discharge. Accordingly, several parameters were optimized: the concentrations and flow rates of the NaBH, solution and HCl, the length of the sampling capillary, the carrier gas flow rates, the discharge current and the discharge pressure. With a 5% NaBH, solution (stabilized with 5% NaOH) and 2 M HCl, which are known to deliver enough hydrogen for quantitative conversion of As into AsH3, the sampling efficiency into the discharge was optimized by varying the length of the fused-silica capillary. The percentage of gas extracted into the discharge can be determined from the difference between the volume of gas flowing from the hydride

As determination

by gas-sampling

1211

Helium:

5.6 5.8 6.0 6.2 6.4 6.6

Pressure, torr

Neon:

Argon:

...................__

Fig. 3. Intensity/voltage/pressure behavior of the GSGD for the As I 228.8 nm line. Hydridegeneration conditions as given in Table 2. Solution concentration: 2 pg/ml As.

generator reaction chamber (the noble carrier gas flow plus the hydrogen and hydride) and the overflow at the gas bubbler. Over the range of analytical interest, the As signal grows linearly with the sampling efficiency (Fig. 5(a)). This behavior is consistent with experiments that revealed the influence of the NaBH4 solution flow rate (Fig. 5(b)); here, the As I 228.8 nm line intensity declines in parallel with the sampling efficiency. This decline is due to an excess of hydrogen production with added NaBH,, causing the fraction of evolved hydride reaching the discharge to drop. Both results indicate that excitation conditions are not drastically altered by the addition of hydrogen to the discharge. Because the flow rate through the capillary will depend on the viscosity of the sampled gas, and thus also on its temperature, the temperature of the cathode plate was allowed to stabilize before further measurements were taken. For all three discharge gases (Ar, Ne, and He), signals increased when the carrier gas flow rate was lowered. However, a drop in carrier gas flow lengthened the required time for a particular analysis. Consequently, the flow rates were chosen as a compromise between greatest signal and shortest analysis time. An argon flow rate of 86 ml/min, together with a 0.3 ml/min flow rate of the 5% NaBH, solution, proved suitable.

1212

J. A. C. BROEKAERT et

al.

NKJn

L- -d

Argon

J

Fig. 4. Spectral scans across the As I 228.8 nm line emitted by the GSGD under optimized operating conditions (Table 2). The asterisk indicates the As I 228.8 nm line. Solution concentration: 2 &ml As.

These flow rates resulted in a sampling efficiency of 78%. The chosen flow rates for helium and neon, together with the discharge conditions for maximum signals are included in Table 2.

3.3. Spatially resolved measurements Spectral features were spatially mapped by computer-controlled lateral movement of the discharge chamber, with the signal at each location being integrated for

Table 2. Optimum analytical conditions and performance (detection limits were determinated to-background noise of 3)

at a signal-

Working conditions: 2 M HCI supplied to the hydide generator at 3.8 mUmin 5% NaBH, solution (in 5% NaOH) at 0.3 ml/min length of sampling capillary: 3.5 cm flow rate into GSGD (mUmin)* discharge pressure (torr) voltage (V) current (mA)

helium 234 6.5 650 -165

neon 103 2.5 750 -160

argon 86 1.5 550 -165

Figures-of-merit: detection limit (ng/ml) upper end of linear range (@ml)

helium 54 2500

*The GSGD sampling efficiency was always in the range 75-85%

neon 30 2500

argon 20 loo00

As determination

1213

by gas-sampling

a>

sr .t: .5

3

60.0 -

$ '4

40.0.-,

t $

20.0 0.0 0.0

27 17

50 I t IO.0

'. I I

20.0

10

I 1 30.0

5

I 40.0

, f 50.0

3.5 ,

(*)

60.0

Percentageofgas sampled

b) 100.0

-

@

2.j

*O.O -

:\u--.-, 20.0 , 0.2

0.3

0.4

0.5

0.6

FlowrateNa33H~

0.7

0.8

09

solution,mhin

Fig. 5. Influence of sampling efficiency on the net signal for the As I 228.8 nm iine in the argon GSGD operated under optimized conditions (Table 2). (a) Effect of sampling efficiency (%) on the signal from 10 &ml As. Sampling efficiency was kontroHed by adjusting the length of the sampling capillary (* numbers above the axis denote Fpillary length in cm). (b) Change of the signal from 5 &ml As (0) and of the sampling efficiency (0) with the flow rate of NaBH, solution. The principal effect of the NaBH, is to alter the production rate of HZ.

1.5 s. This procedure was repeated at different vertical positions across the 9 mm anode-cathode distance. Bidimensional, 900 pixel maps comprising 25 ho~zontal lines could be measured in 35 min. Unfortunately, the image of the upper region of the discharge was truncated by the circular viewing port and does not represent either a geometrical feature of the discharge nor the true spatial distribution of a spectral feature. 3.3.1. Measurements of&e argon discharge. Under the optimum analytical conditions (1.5 torr, 550 V and -165 mA) intensity maps were constructed for carrier gas lines, for As I 228.8 nm emission and for a blank (measured at the analyte line). The images for Ar I 420.1 nm and Ar II 450.7 nm (Fig. 6) indicate that the discharge is laterally quite uniform and that support gas atom and ion lines have a very similar intensity distribution across the discharge. As showd by the vertical intensity profiles derived from these maps (Fig. 7), the location of greatest emission from the support gas lines coincides roughly with that of the analyte and iis background; however, the latter persist farther from the cathode. This skew might be related to the kinetics of arsine dissociation, which no doubt progresses as the entering gas proceeds from the cathodic orifice and into the discharge. However, this explanation fails to account for the skew that occurs also for background emission at the arsenic line. To ascertain whether the distributions in Figs 6 and 7 are due to the location of the sample input port and the vacuum connection, the sample introduction capillary was moved from the center to the edge of the cathode, but still along the viewing axis. Because this sample input position is farther from the vacuum port, the arsine should

1214

J. A. C. BROEKAERT et

al.

Ar I 420.06 nm

4.0803-S A

imm

-8 mm

0

8mm

Ar II 450.70 nm

5.1723-8

A

8.5 mm

17 mm -6

Fig. 6. Intensity

mm

0

8mm

maps of argon lines in the GSGD under optimized (Table 2).

6.0.10-9

working conditions

6.0.10-*

c 3 5.0.10-g _m 2B 4.0.10-g

5.0.10-s

2 3.0.10-g $ 7 2.0.10-g EJ ‘Z z 1.0.10-g 2 o.o.loO

4.0.10-s

,s I: &

3.0.10-s

q

1

L!?. B 2.0.10-8 _?L > 1.0.10-s

o.o.loO

0

4 a 12 16 Vertical displacement, mm

20

Fig. 7. Vertical intensity profiles obtained for the argon GSGD under optimized conditions (Table 2): (O), net signal for As I 228.8 nm; (A), blank intensity; (0), intensity of Ar I 420.1 nm; (+), intensity of Ar II 450.7 nm. Solution concentration: 5 pg/ml As.

As determination

by gas-sampling

1215

0

9.9453+9

-amm

0

8mm

Fig. 8. Image representing the slope of the dependence of As line intensity on discharge pressure, obtained by regression fitting of emission maps obtained at 1, 1.5 and 2 torr. The smallest change in analyte signal with pressure occurs in the center of the discharge (the dark region between 6 and 8.5 mm), which is also the zone of greatest analyte emission. Solution concentration: 5 p&ml As; argon GSGD with other conditions as listed in Table 2.

enjoy a longer residence time in the discharge. Yet, both the maps and the electrical characteristics of the discharge remain the same. Clearly, the spatial features apparent in Figs 6 and 7 are attributable to the discharge itself and not to the vacuum or sample introduction arrangements. It is useful to identify the discharge zones that are most immune to pressure fluctuations since such changes can constitute the dominant noise in emission from a glow discharge [13]. These zones might be found by subtracting images of analyte emission taken at two different pressures. However, a better approach is to determine the image that corresponds to the slope of the dependence of arsenic emission on pressure. Maps were acquired at pressures of 1, 1.5 and 2 torr and the intensity at each pixel regressed against the pressure itself; the result is the display in Fig. 8. From this slope image it appears that there is a central zone where the influence of pressure is least. Conveniently, this zone is centered and in the vicinity of the highest analyte signal. 3.3.2. Measurements of the helium discharge. Spatially resolved measurements of the helium discharge were performed at 6.5 torr, 650 V and 165 mA as the optimized conditions (Table 2). As can be seen from Fig. 9, the support gas line (He I 492.2 nm) and the Ha line (at 486.1 nm) exhibit maximum intensity at a location centered between the electrodes. The analyte line and background are both of much lower

z $j 1 $ &

3510.9

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3.0.10-S

2 -1.5 10.7 B

2.5,10-' 2.0.10-s

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1.510-9

9 -5.0.10-9

-j 1.0.10.9 m % 5.0.10.'0 z o.o.lo" 0

4

s

12

Is

Vertical position, mm Fig. 9. Vertical intensity profiles obtained for the helium GSGD under optimized conditions (Table 2): (0) net signal for As I 228.8 nm; (A), blank intensity; (0), intensity of He I 492 .2 nm; (+), intensity of H, line (H 486.1 nm). Solution concentration: 2 ug/ml As.

J. A. C. BROEKAERTet al.

1216 a) I\ie 208 .5 nm

6.8863-g

-8mm

0

A

8mm

b) AS 228.8 nm 4.2453-g

A

4.2453-g

A

8.5 mm

mm c) Blank

-8 mm

0

8mm

0.0

I

-8 mm

b

mm

Fig. 10. Maps of various spectral features in the neon GSGD operated conditions (Table 2). Solution concentration: 2 kg/ml As.

under optimum

intensity (note the different vertical axes on Fig. 9), peak a bit closer to the cathode and tend, as was found for argon, to extend toward the anode. The He discharge was found also to be laterally very s~metrical. 3.3.3. ~e~ure~ents of the neon discharge. Under analytically optimum conditions for the Ne discharge (2.5 torr, 750 V, -160 mA), the Ne I 208.6 nm line, the As I 228.8 nm line and the spectral background all extend farther from the cathode (top of Fig. 10) than in the He or Ar discharges. Moreover, for all spectral features, a dip is found in the center of the profile. The feature nearly disappeared in a spatial map of Ne I at a discharge gas pressure of 6.5 torr, indicating that the notch might be related to gas flow dynamics. This hypothesis is in compliance with our observations

As determination

1217

by gas-sampling

a) Helium 0.20

b) Neon

Frequency, Hz

0.25

o.oo.O c) Argon

Frequency, Hz

0.25 7

o.ool 32.5 0

65 97.5 Frequency, Hz

130

Fig. 11. Noise amplitude spectra recorded from the GSGD with different discharge gases operated under optimized conditions (Table 2). Emission feature: As I 228.8 nm; solution concentration: 2 &ml As.

made on the helium plasma at 6.5 torr. At this pressure, no dip is found, whereas at a lower source pressure [7], a dip similar to that in Fig. 10 occurs. Not surprisingly, the signal-to-background ratio is low in the dip region, making it analytically unfavorable. 3.4. Noise amplitude spectra Noise amplitude spectra, calculated as described elsewhere [15], were recorded for the analyte line as well as for a blank (at the analyte line) under optimal operating conditions for each of the discharge gases. In every case, only white noise was found (Fig. 11). An investigation of the low-frequency regions showed also no discrete peaks near 1 Hz. This absence suggests that fluctuations that might occur as a result of the dropwise introduction of the NaBH, solution are smoothed, possibly by the reaction column. Moreover, the formation of droplets at the exit of the gas-liquid separator seemed not to contribute any discrete noise peaks. The absence of discrete noise peaks underscores the importance of a high-quality roughing pump with high displacement volume and the throttling valve used for pressure control. These findings are in agreement with those obtained from a Grimm-type glow discharge lamp used for the atomic spectrometric analysis of solids [13, 161.

1218

J. A. C. BROEKAERTet nl.

- 10 x .‘f 2 d .E $

‘3 m Ti ti

i 2,

vuidf t

analyte entering the cell

t leaving the cell 40 Time, minutes

Fig. 12. Long-term stability of the argon GSGD. Solution concentration: 100 pg/mI As; argon carrier gas Bow rate: 0.4 llmin; other conditions as in Table 2.

3.5. Hydride-generation

with the GSGD for As determination The scan in Fig. 12 shows the long-term stability of the GSGD (over 30 min) to be adequate for quality analytical work. To obtain this degree of stability, the vacuum in the source must be held within f 0.02 torr. Otherwise, pressure fluctuations influence the sampling rate through the capillary and also the excitation characteristics of the discharge. The carrier-gas flow rate used to produce the stability curve shown in Fig. 12 is roughly half the flow rate used for analytical measurements (0.4 l/min vs 0.860 I/ min). This change ensures a worst-case condition in which all the reaction products end up in the source (except the water vapor). Thus, if excess hydrogen or coating of the electrode surfaces by excess analyte were detrimental to long-term stability, this experiment would indicate the problem. It can be seen also from Fig. 12 that the rise and decay times of analytical signals are modest and depend on the analyte concentration. Because the scan in Fig. 12 was obtained with a 100 pg/rnl solution, a low gas-sampling efficiency was used (about lo-20%). In scans taken with concentrations below 1 pglml and a sampling efficiency above 75%, the maximum intensity level was reached more rapidly, within about 1 min. There is an abrupt change in signal level when changing from the analyte to a blank (on the right side of Fig. 12). This sudden drop is likely caused by the momentary cessation of hydrogen production that alters source pressure as the solutions are switched. After the initial rapid decline in signal, the return to baseline is more gradual; the signal drops to below 2% of its m~imum within 2 min. The detection limits measured with neon and argon as discharge gases were lower than with helium (Table 2). This behavior would be expected to arise if the breakdown of the arsine and not the excitation of the atomic species is the limiting step in signal generation. It also should be emphasized that the detection limit for the argon plasma might have suffered from the existence of other nearby spectral features (Fig. 4). This spectral interference is also probably responsible for a small offset noticeable in the analytical curve in argon (Fig. 13). In contrast, no blank contributions were detected for either neon or helium as discharge gases. This spectral interference would not be likely to occur if the 0.35 m monochromator were replaced by a spectrometer of higher resolving power. The importance of arsine breakdown in the GSGD is confirmed by the abundance of hydrogen-intoning As species in the mass spectrum of the discharge (Fig. 14). However, the mass spectrum of Fig. 14 was recorded at a discharge pressure of 0.2 tort- and at a low discharge current, so the polyatomic species present there might be in much lower abundance in the GSGD optimized for emission measurements. Also, the source geometry for mass spectrometry is different in that a conical skimmer is required for the anode, rather than a flat plate, in order to sample the expanding gases into the mass spectrometer. In both cases, however, the cathode-to-anode separation is similar (10 mm for the mass spectrometer as opposed to 9 mm for emission). The linear range of calibration curves for As I 228.8 nm with all three discharge

As determination

2 Fig. 13. Calibration

by gas-sampling

1219

4 6 8 As concentration, ug/ml

10

curve for As I 228.8 nm. Argon GSGD under optimized (Table 2).

70

12

74 16 Mass, Daltons

18

conditions

80

Fig. 14. Mass spectrum obtained from argon GSGD. Solution concentration: 10 t&ml As; argon pressure: 0.2 torr; voltage: 760 V; current: 20 mA. Other conditions as listed in Table 2. Ouadrupole mass spectrometer (Balzers); vacuum in 3rd stage: 2 x 10m5 tot-r. Signal detection: secondary electron multiplier.

gases is rather limited (Table 2). This limitation arises from the influence of added molecular species (e.g. arsine, hydrogen) on discharge characteristics. It could be that their dissociation leads to an increase in the cathode temperature, and therewith to deterioration of the sampling through the silica capillary, as this directly relates to the viscosity of the entering gases. However, this hypothesis is weakened by the observation, mentioned earlier, that high amounts of hydrogen accompanying the arsine do not seem to influence the discharge greatly.

4. CONCLUSIONS

AND

FUTURE PROSPECTS

The experiments above have shown that GSGD sources, which were originally developed as soft ionization sources for the mass spectrometry of organic molecules, are useful also as excitation sources for atomic emission spectrometry. Effective breakdown of weakly bound molecules as well as efficient excitation of the liberated atoms allow a sensitive determination of elements such as arsenic. However, atomic emission detection limits obtained with the gas-sampling glow discharge are still considerably higher than for many other atomic spectrometric techniques. In atomic absorption spectrometry (AAS) for example, detection limits

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J. A. C. BROEKAERTet al.

for As with a heated quartz-tube atomizer are in the sub-ng/ml region (see textbooks on AAS such as Ref. [l?]). With cold-trapping or hot-trapping in a graphite furnace, the last of which has been explored by STURGEONet al. [18], the detection limit for As was as low as 0.04 ng/ml. Further, with a different sampling technique and a microwave-induced plasma source, the detection limit for As was down to 0.4 @ml. Here, however, excess hydrogen had to be removed before the arsine was sent into the Beenakker cavity [19]. This drawback was overcome recently by means of a hollow-fiber membrane used for the on-line selective removal of excess hydrogen and moisture ahead of the low-power MIP; a limit of detection of 0.32 ng/ml was obtained [20]. In ICP-OES using hydride-generation in a flow-cell, detection limits are in the ng/ml range [lo, 111. Further developments involving the GSGD and hydride-generation are analytically interesting. Discrete sampling, perhaps by flow injection, could be useful. The source would then be minimally exposed to the sample, and the linear range could be increased. That approach was proven in our laboratory in preliminary measurements, which have shown that signals obtained with continuous sample aspiration could be matched by injecting volumes as low as 100 ~1. The source might be useful also for the detection of other elements that can be liberated as gaseous species. This group includes all elements that form hydrides and those such as nickel that form volatile carbonyls [21]. By freeze-trapping the analyte species, one might obtain far higher absolute powers of detection. Also, the source offers potential as an element-specific detector for gas chromatography, as already discussed elsewhere [7]. The application of alternative dry aerosol generation devices in conjunction with the GSGD should likewise be investigated. Perhaps the full analytical capabilities of the GSGD to trace and ultratrace element detection will only be realized when it is employed as an ion source for elemental mass spectrometry. Acknowledgements-This work was supported by the National Institutes of Health through grant ROl GM46853 and by the National Science Foundation through grant CHE 90-20631. Financial support by NATO through the CRG grant 900039 is gratefully acknowledged. R. PEREIROthanks the “Ministerio Espafiol de Educacidn y Ciencia” for providing a scholarship. T. STARNthanks Boehringer Mannheim for providing a fellowship.

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