2376
Anal. Chem. 1980, 52, 2376-2383
EvaIuat ion of Plate Electrodes for Laser-Enhanced Ionization Spectrometry George J. Havrilla‘ and Robert B. Green* Department
of Chemistry, Chemistry Building, University of Arkansas, Fayetteville, Arkansas 7270 1
Flat sensing electrodes (plates) have been suggested as a better alternative to cylindrical electrodes (rods) for laserenhanced ionization (LEI) spectrometry because of thelr lncreased resistance to electrical effects produced by high ion concentrationsin the flame. When a sample contains an easily Ionized element, interferences due to electrical effects largety determine slgnal behavior. This paper examlnes the slgnal collection role of plate electrodes In detail and establishes optimum conditions for their use. By use of these conditions, the best signal recovery to date is reported for an anaiyte in a low lonlzatlon potential matrix. Additional advantages and disadvantages of plate electrodes are also demonstrated. Expansion of the laser beam and hlgh voltage interferent removal show promise for further Improving LEI signal collection.
Laser-enhanced ionization (LEI) in flames was first reported in 1976 (1). Independent observation of the same phenomenon followed within a year (2,3). Since then, several investigators have explored the potential of LEI as a new technique for analytical flame spectrometry (4-1 I ) . In LEI spectrometry, a dye laser tuned t o a discrete absorption transition of an analyte atomized in a flame enhances the thermal (collisional) ionization of the analyte atoms. The conductivity change in the flame is detected with electrodes and measured with conventional electronics. Detection limits which are superior to those obtained with existing methods of flame spectroscopic analysis have been reported for many metals (44).Since the analyte’s transition probability and ionization potential both play significant roles in determining LEI signal strength, many transitions which are unsuitable for conventional analysis produce low limits of detection by LEI spectrometry (4-6). Two photon (6) and stepwise excitation processes (7,8) have been used to increase the selectivity and sensitivity of LEI measurements. Excited states of molecules produced by seeding a hydrogenlair flame have also been observed (9). A four-level model of the atom has been used to describe the LEI signal production mechanism by a combination of optical and collisional processes (10). Previous studies of electrodes for LEI spectrometry have indicated a strong preference for flat electrodes (plates) rather than cylindrical ones (rods) because of their increased resistance to electrical effects produced by high ion concentrations in the flame (6, 11). This background interference is due to ions contributed by the analyte, its matrix, and flame combustion reactions. When a sample contains an element with a low ionization potential, these interferences may become significant in determining signal behavior (5,6,11). Up t o now, a complete characterization of plate electrodes has not been reported. The present work examines the signal collection role of plate electrodes in detail and establishes conditions for their optimum use. By use of these conditions, the best signal recovery to date with nonintrusive electrodes is reported for an analyte in a low ionization potential matrix. This work also demonPresent address: Center for Analytical Chemistry, Rm. 212, Bldg. 222, US. National Bureau of Standards, Washington, DC 20234. 0003-2700/80/0352-2376$01 .OO/O
strates additional advantages and disadvantages of plate electrodes. The introduction of laser beam expansion and high-voltage interferent removal show promise for further improving LEI signal collection.
EXPERIMENTAL SECTION Apparatus. The experimental apparatus is illustrated in Figure 1. It is similar to the apparatus described in ref 11with the exception of the electrodes. In this study the LEI signal was detected with a pair of plates. Either 0.5 mm thick molybdenum plates (The Rembar Co., Inc., Dobbs Ferry, NY) approximately 152 mm long and of varying width (5, 10, 20, and 30 mm) or 2 mm thick graphite plates (Atomergic Chemetals Corp., Plainview, NY) 100 mm long and 21 mm wide were used. Both electrodes were negatively charged, and the burner head was used as the anode in some experiments. In other experiments, one electrode was used as the anode and the other the cathode. A charge of -1400 V was applied to the cathode(s) unless otherwise stated. Dissolved samples were aspirated into a standard premix burner with a 5-cm single slot head. An acetylene/air flame was used except where noted. A Chromatix CMX-4 flashlamp-pumped tunable dye laser was the excitation source. The laser output consisted of 0.8-ps pulses with a 10-Hz repetition rate. An intracavity birefringent filter was used for wavelength tuning and narrowed the beam to 0.1 nm for the rhodamine 6G dye fundamental radiation. A frequency doubling option provided tunable radiation in the ultraviolet spectral region. The laser beam was maintained nominally parallel and perpendicular to the burner slot. The laser beam cross-sectional area was varied using 1 in. diameter UV grade fused silica lenses (Oriel Corp., Stamford,CT). Other details about beam pceition, electrode separation, and height above the burner head are noted in the text. The signal pulse was separated from the dc background current with a high pass filter on the input of a current preamplifier. The LEI signal pulse was displayed on an oscilloscope and processed with a boxcar signal averager. The signal was read out on a strip chart recorder. Reagents. Indium was chosen as the model analyte for these studies for reasons cited in ref 11. Atomic indium was excited at its 303.9-nm resonance line in these experiments. Aqueous standards were prepared from 99.97% indium metal (Matheson Coleman and Bell, Norwood, OH) according to procedures described in ref 12. Reagent-grade sodium chloride (Fisher Scientific Co., Fair Lawn, NJ) was used to prepare the matrix solutions. Sodium has a low ionization potential and produces severe electrical interferences with the LEI signal (4-6,II).Rhodamine 6G (rhodamine tetrafluoroborate) laser dye was obtained from Exciton Chemical Co., Inc., Dayton, OH. Procedure. The data are reported in terms of “percent signal recovery” (11) and “LEI signal”, i.e., the raw LEI signal for the aspirated sample in arbitrary units. The latter is used for direct comparison of signals for various electrode widths and excitation heights. In these cases, the LEI signal plots are much more instructive than relative percent signal recovery curves. Since the percent signal recovery and LEI signal curves were meant to indicate trends, error bars were not included to simplify figures. A &4% error was typical. A voltage was applied to the electrodes which was well into the region where further voltage increases produced no change in LEI signal, Le., saturation. RESULTS AND DISCUSSION It has been demonstrated that plates exhibit different signal collection characteristics from rods (6,10,11). The use of plate electrodes in preference to rods permits recovery of LEI signals in higher concentrations of low ionization potential matrices. 0 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 14, DECEMBER
1980
2377
CLRRENT TRIGGER
RECORDER
Figure
1.
t SIGNAL AVERAGER
loot
Experimental apparatus for LEI spectrometry.
The present study documents the characteristics of plate electrodes and reports optimum conditions for analysis. In order to fully understand the relationship of the electrodes and the resulting LEI signal, we will make comparisons with a previous rod study (11). An analytical calibration curve for indium using plate electrodes was linear for a 3 orders of magnitude concentration range. The upper end of the linear dynamic range was determined by preamplifier saturation and could be extended with a smaller feedback resistor (11). The limits of detection obtained with 10-mm plates were a t least an order of magnitude better than those achieved with rods. A calibration curve with 10 pg/mL of sodium added was also linear over the same range but produced another order of magnitude decrease in limits of detection due to signal enhancement, i.e., the calibration curves with and without sodium are parallel but the curve for the sodium matrix solution intersects the abscissa at a lower concentration. With extreme enhancement, e.g., 50 pg/mL sodium matrix, the calibration curve became nonlinear a t low indium concentrations. In cases where the matrix concentration suppresses the LEI signal, analyte detection limits are commensurately higher as a consequence of the reduced signal. T o an extent limited by noise, signal enhancement with low ionization potential matrices could be advantageous because it permits the detection of smaller analyte concentrations, but further investigations of the nonlinear behavior are necessary. Electrode Area. The area as well as the shape of the electrode plays an important role in determining the strength of the LEI signal. (Since the electrode length remains the same throughout, a change in the electrode width implies a change in the electrode area.) Figure 2 illustrates a comparison of the effect of different electrode widths on the LEI signal for 100 ng/mL of indium in an acetylene/air flame. The laser beam was positioned at 10 mm above the burner head and the rods and the 5 mm and 10 mm wide plates were centered a t that height. The lower edge of the 20 mm and 30 mm plates was 5 mm above the burner head. The burner head was used as the anode. As the width of the electrodes was increased the LEI signal increased until a plateau was reached at 10 mm. Rods 6.2 mm in diameter gave a higher LEI signal than 5 mm wide plates. The maximum LEI signal was obtained with a plate width of approximately 8 mm. This suggests that beyond a certain electrode width, the collection efficiency remains approximately constant. This is apparently determined by the laser beam diameter which, in turn, determines the “effective” electrode width necessary for complete ion collection. Up to 10 mm, noise increased as plate width increased but signal improvement was marginally greater. This resulted in slightly larger signal-to-noise ratios as the plate width was increased. Signal-to-noise ratios for 10-mm and 20-mm plates were nominally equal. As with rod electrodes ( 1 1 ) , plots of signal-to-noise ratios a t increasing sodium concentrations re-
I
I
5
lc
ELECTRODE W ’ O T H
+--i4\ 1 m ~
ic
Flgure 2. Effect of the electrode width on the LEI signal for 100 ng/mL indium: (0)rods; ( X ) plates.
400r
Na CONCENTRATION
lug/ml 1
Figure 3. Effect of sodium matrix concentration on the LEI signal for 100 ng/mL indium: (a) 1-mm rods; (b) 1.6-mm rods; (c) 2.4-mm rods; (d) 3.3-mm rods; (e) 6.2-mm rods; (f) 5-mm plates; (9) 10-mm plates. sembled signal vs. sodium concentration curves under the same conditions. A more informative presentation illustrating the difference between rods and plates is seen in Figure 3. The sodium matrix concentration ranges are indicated for the various electrode widths up to the 10-mm plates. The “matrix concentration range” is d e f i e d as the region between zero matrix added and the concentration which produces 0% signal recovery. I t was observed that only a 0.5-mm change in the cathode separation shifted the sodium matrix concentration range for both rods and plates so these data were taken at the cathode separation and position which gave the maximum
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 14, DECEMBER 1980
sodium matrix concentration range. The sodium matrix concentration range increased with increased electrode width. By increasing the electrode width the density of the dynamic sheath of positive ions which surrounds the cathodes is diminished (10, 11). The distribution of the sheath over the surface of the cathode effectively increases the potential of the applied voltage a t the excitation site. This means that the wider electrodes are able to tolerate larger sodium ion concentrations. A dramatic difference in collection behavior is evident if Figure 3e,f is examined. These plots are for the 6.2-mm rods and 5-mm plates, respectively. Even though the rods gave a higher LEI signal for indium than the plates, the sodium matrix concentration range attained with the rods was considerably lower. The field intensity at the surface of a cylindrical conductor depends inversely on its radius. Even with larger diameter rods the field strength will still be greater than that for plates and hence the cation sheath will be more dense. A later section will deal with signal threshold and saturation in these terms. S a m p l i n g Height. The LEI signal for 100 ng/mL of indium was measured as a function of both height above the burner head and fuel-to-oxidant ratio. In these experiments, the relationship of the laser beam and electrodes was fixed. This was done first in the horizontal anode-cathode (HAC) mode in order to eliminate the LEI signal dependence on anode-cathode separation. This effect is significant and might bias these measurements if the split cathodes/burner head anode configuration, i.e., the vertical anode-cathode (VAC) mode, were used. In the HAC mode, the laser beam is centered vertically and horizontally between the anode and cathode. The electrode separation is adjusted to maintain a nominally constant air gap with the flame. The LEI signal for 100 ng/mL indium was plotted vs. the acetylene flow rate for 1-mm rods for several sampling heights. The maximum signal was obtained at 1.92 L of acetylene/min. The maximum LEI signal at 30 mm above the burner head required 2.85 L of acetylene/min and was about 80% of the maximum signal measured at 10 mm. This indicates the upward movement of the interconal zone (i.e., the region of highest analyte concentration) within the acetylene/air flame as the fuel-tooxidant ratio is increased. The same experiment performed with 10-mm plates yielded different results as seen in Figure 4. The LEI signal for indium peaks and then drops off as the laser beam/electrodes are translated vertically in the flame and only about 55% of the maximum signal can be recovered by increasing the acetylene flow rate. When the height above the burner head at the plate center is 15 mm (Figure 4b), a maximum signal results a t an acetylene flow of approximately 2.45 L/min. Further increases or decreases in height result in decreases in LEI signal intensity. The reason for this difference in the sampling height dependence may be attributed to geometrical differences between rods and plates. This may be explained as follows. The rods can follow the contour of the flame. The proximity of the plates to the flame is determined by their width. A t positions in the flame where the cross-sectional width is increasing as the height increases, the electrode-flame separation a t the lower edge of the plates will be greater than a t the upper edge. In fact, it is possible to simulate Figure 4 with rod electrodes if a constant separation is maintained over the height of the flame. The electrodes were not allowed to come into contact with the flame in order to prevent corrosion, fouling, and conductive heating of the high-voltage leads. Sampling height profiles in the VAC mode remained essentially the same as for the HAC mode if the applied voltage was maintained to give a saturated LEI signal. As long as the
2001 t
t 1
’
e 1
b
180
I
I
1
1
2 2c
2 60 C2HZ
I
I
I
J
3 00
ml
Flgure 4. Effect of acetylene flow rate on the LEI signal for 100 ng/mL indium. 10 mm wide plate electrodes with beam centered in HAC mode at the following heights above the burner head: (a) 10 mm; (b) 15 mm; (c) 20 mm; (d) 25 mm; (e) 35 mm.
minimum voltage for saturation is maintained, changes in the anode-cathode separation should not affect the LEI signal. Although the profiles were essentially the same in each mode, the absolute LEI signals were much higher in the VAC configuration. Figure 4 gives no information about sampling heights between 10 and 15 mm above the burner head. Therefore, the region between 9 and 17 mm above the burner head was profiled in 2-mm increments in the VAC mode. Plots of these data indicated a sampling region from 11 to 15 mm above the burner head which gives the same signal but a t slightly different fuel-to-oxidant ratios corresponding to optimum positions of the interconal zone. Past the 15-mm sampling height the LEI signal begins to diminish. Thus the maximum signal can be attained by positioning the plates within this region and adjusting the fuel-to-oxidant ratio. The HAC and VAC modes of signal collection were compared for both 1-mm rods and 10-mm plates. In each case the VAC mode exhibited a greater signal and increased sensitivity. In fact, the signal for 10-mm plates in the VAC mode was 1.4 times greater than for the HAC mode. The most dramatic difference in the two modes of signal collection was the resistance to matrix interferences; the VAC mode was superior by a factor of 4. The increase in the LEI signal for the VAC mode may be attributed to the increased surface area of the burner head anode. The increased electron flow results in an increased LEI signal. Different migration pathways in each mode may also contribute to signal differences. Assuming that the natural flame and matrix ions are symmetrically distributed horizontally from the center of the flame at a specific height, the same ion concentration gradient will exist on either side of the excitation region. When the indium atom is excited and thermally produces an ion and electron, in the HAC mode, the electron must pass through a flux of 50% of the flame and matrix ions traveling in the opposite direction. This increases the likelihood of an electron collision resulting in signal suppression. On the other hand, in the VAC mode, the
ANALYTICAL CHEMISTRY, VOL. 52, NO. 14, DECEMBER 1980
2379
r
1501 200
1
e
e
I
80
2 20
2 60 C2H2
1 1
I
I
I
3 00
ml
Figure 5. Effect of acetylene flow rate on the LEI signal for 100 ng/mL indium. The cathodes are 20 rnrn wide plates, VAC mode, with the lower edge 8 rnm above the burner head. Excitation height above the burner head: (a) 9.5 rnm; (b) 13 mrn; (c) 18 mrn; (d) 23 mm; (e) 26.5 rnm.
electrons move toward the burner head away from the positive ion flux. Similar reasoning may be used to explain signal threshold data for the two modes and two different electrodes. A minimum voltage must be applied to the electrodes before a LEI signal is observed. When an increase in applied voltage no longer increases the signal, a saturation condition exists. In the VAC mode, the thresholds for rods (-300 V) and plates (-110 V) were lower than in the HAC mode and the applied voltages required for saturation were less. A higher applied voltage was needed to overcome the ion sheath for rods ( 4 0 0 V) and plates (-200 V) in the HAC mode. Relative Excitation Height. With plates, the interaction volume of the laser beam is small compared to the area of the electrodes. Therefore when the burner head is used as the anode, there should be an optimum excitation height where the electrostatic forces due to the applied voltage =e balanced by laser-related ions and electrons to give a maximum LEI signal. This behavior is illustrated in Figure 5 . In these experiments, the laser beam is translated with respect to stationary plates. These curves were obtained with 20 mm wide plates a t -1400 V, 12.5 mm apart, with the lower edge 8 mm above the burner head. The LEI signal reached a maximum at an excitation height of 13 mm above the burner head which corresponds to 5 mm from the lower edge of the 20-mm plate. The important parameter for this study is the excitation height relative to the plates. The excitation height above the burner head may be varied as “sampling height” studies have shown but only a t the expense of signal. As expected there is some dependence upon the acetylenelair ratio similar to that explained in an earlier section. This was evidenced by the attainment of signal maxima at higher excitation heights by increasing the acetylene flow. This is due to the upward movement of the interconal zone. What is most interesting about the plot is the decrease in LEI signal with increasing excitation height. In fact a t positions above the center of the plate, severe signal loss was observed. This is apparently a result of the electrostatic repulsion of the laser-related electron due to the negative applied voltage on the cathodes. Both the 10-mm and 30-mm plates exhibited behavior similar to the 20-mm plates. Further evidence supporting electrostatic effects is illustrated in Figure 6. This figure shows plots of the LEI signal
.O’
L
C2H2 I l / m l
Flgure 6. Effect of acetylene flow rate on the LEI signal for 100 ng/mL indium. The cathodes are 10 mm wide plates, VAC mode. centered at 13 rnm above the burner head. The solid lines are for -1400 V applied;the broken lines are for -800 V applied. Excitation height above t h e burner head: (a) 8 mm; (b) 13 mm; (c) 18 rnm; (d) 8 mm; (e) 13 rnm; (f) 18 mrn.
intensity for 100 ng/mL of indium at various acetylene/air ratios, for a 10 mm wide plate centered 13 mm above the burner head. The excitation height was varied and the results compared for two different voltages. The solid lines indicate -1400 V, and the broken lines, -800 V applied to the plates. At the 8-mm excitation height, -800 V applied gives a lower LEI signal than -1400 V. The greater signal for the -1400 V applied a t the low excitation position is a result of the increased signal collection due to the higher applied voltage. The 13-mm excitation height gives equal signals for the two applied voltages and constitutes an inversion point for the LEI signals a t -800 and -1400 V. For positions above the center of the plates, the -800 V applied produces a greater signal than -1400 V applied. The lower LEI signal at the -1400 V applied at the 18-mm excitation height is apparently due to electrostatic repulsion of the laser-related electrons. The higher applied voltage impedes the flow of electrons toward the burner head anode more a t positions above the center of the plate than the lower applied voltage. The maximum LEI signal for the three plate widths 10,20, and 30 mm occurs at an excitation position of 13 mm. In each case the acetylene/air ratio and the LEI signal for 100 ng/mL indium were the same. The effect of excitation height is also demonstrated in plots of the LEI signal for indium as a function of sodium matrix concentration. The sodium matrix concentration ranges illustrated in Figures 7 and 8 surpass previously reported results for plate cathodes (25 mm wide at -1500 V) by more than 150 gg/mL sodium (6). The present results have been obtained by optimization of both plate and excitation positions as discussed here. Lower applied voltages result in both decreased signal and truncation of the sodium matrix concen-
2380
ANALYTICAL CHEMISTRY, VOL. 52, NO. 14, DECEMBER 1980 250-
200-
150-
, \ 100 Na
CONCENTRATION
lug/ml I
Effect of sodium matrix concentration on the LEI signal for 100 nglmL indium. The cathodes are 10 mm wide plates, VAC mode, centered at 13 mm above the burner head. Excitation height above t h e burner head: (a) 9.5 mm; (b) 13 mm; (c) 16.5 mm. Figure 7.
200 N a CONCENTRATION
300 fug,ml~
J 400
Effect of sodium matrix concentration on the LEI signal for 100 ng/mL indium. The cathodes are 20 mm wide plates, VAC mode, centered at 18 rnm above the burner head. Excitation height above the burner head: (a) 9.5 mm; (b) 13 mm; (c) 18 mrn; (d) 23 mrn; Flgure 8.
(e) 26.5 mm. tration range; e.g., for the 10-mm plates, a t -800 V applied, total signal suppression at an excitation position of 13 mm occurred at 150 Fg/mL sodium, whereas total signal suppression at -1400 V applied did not occur until more than 450 pg/mL sodium had been added. In comparing Figures 7-9, several features are apparent. First, the indium signal is approximately the same for all three plate widths a t excitation heights of 9.5 and 13 mm. Other excitation heights produce significantly differing values due to the changes in the field symmetry. It is important to note that as the excitation height is increased above 13 mm, the signal decreases. However, the resistance to sodium interferences increases as the excitation height and plate width are increased up to plate widths of 30 mm. Considering only the 30 mm plates, the sodium matrix concentration range did increase up to an excitation height of 23 mm. The lower overall resistance to the sodium matrix interference in the 30-mm plate case was due to increased plate separation which will be discussed later. In all cases, the increased resistance to sodium interferences is due to the dispersal of the positive ion sheath. The shapes of the curves produced a t higher excitation heights are also instructive. An interesting pattern is observed when the curves for excitation heights 23 mm or greater in Figure 9 are examined. The LEI signal is first suppressed, then enhanced, and then suppressed again. This behavior was also observed a t lower voltages with correspondingly lower sodium matrix concentration ranges. The behavior of these curves can be explained in terms of the formation of the positive ion sheath about the cathodes and electrostatic forces. As illustrated in Figure 9, when the indium atoms are excited higher on the plate cathodes, a dip begins to appear in the otherwise characteristic signal recovery curves. With
2ool
N a CONCENTRATION
I p g ‘ml I
Effect of sodium matrix concentratlon on the LEI signal for 100 ng/mL indium. The cathodes are 30 mm plates, VAC mode, centered at 23 mm above the burner head. Excltation height above the burner head: (a) 9.5 mrn; (b) 13 mm; (c) 18 mm; (d) 23 mm; (e) 28 mm; (f) 33 mm. Figure 9.
excitation high on the plates, the indium LEI signal is suppressed at low sodium concentrations because the laser-related
ANALYTICAL CHEMISTRY, VOL. 52, NO. 14, DECEMBER 1980
Table I. Comparison of Threshold Voltages and Saturation Voltages as a Function of Electrode Width electrode width, mm
threshold voltage, V
saturation voltage, V
1 1.6 2.4 3.3 5 6.2 10
300 280 220 210 180 190 110 110 120
1100
20 30
1050
1025 1000 900 975
600 700 550
electrons must traverse the bulk of the negative field to reach the burner head. When the sodium matrix concentration is increased, the sodium ion sheath on the negative plates becomes more dense, partially neutralizing the field, and produces a second maximum in recovered LEI signal a t higher matrix concentrations. In some cases, the signal is not only totally recovered but enhanced as well (Figure 9d-f). After a secondary signal maximum has been attained the sodium ions again begin to suppress the signal. Similar behavior may also be recognized in Figures 7 and 8. The changes in signal behavior due to increasing sodium concentration are also accompanied by signal delay and broadening of the LEI signal peak. LEI signal delay may be another manifestation of electrostatic repulsion. This delay was dependent upon the excitation position and increased with increasing excitation height. The delay ranges from approximately 0.2 ps a t 18-mm excitation height to almost 2 ps at 26.5-mm excitation height for the 20-mm plate. (There was no delay observed below 18-mm excitation height.) This delay was observed to decrease corresponding to increases in the sodium matrix concentration as the sheath attenuated the electrostatic repulsion due to the applied voltage. Almost no signal delay was observed a t the sodium matrix concentration which gave the highest signal recovery for Figure 9d-f. This same effect was also observed in a similar broadening of the signal pulse. As mentioned earlier the 30-mm plates gave a smaller sodium matrix concentration range than the 20-mm plates although deviating from the observed trend for increasing plate widths. This was due to the physical limitations of the experimental system when the plates are positioned perpendicular to the burner head. As the plate width increases, the separation of the plates a t their lower edge increases because the flame cross section increases with increasing height over the sampling region. Therefore, the 30-mm plates, although positioned a t the same height above the burner head as the 10-mm and 20-mm plates, required a greater horizontal cathode separation in order for the additional width to be accommodated and remain out of contact with the flame. The increased gap between the lower portion of the plates and the flame decreased the sodium matrix concentration range that was attained even though the plates were wider. Similar experiments at lower cathode heights confiim that as the plate width is increased, the resistance to matrix interferences increases if a constant flame-plate separation is maintained. If the plates were allowed to remain in contact with the flame, severe fouling, corrosion, and heating of the high voltage leads resulted. The present data are therefore the best obtainable within the constraints of the physical dimensions of the plates and flame. Signal Threshold a n d Saturation. The results in Table I illustrate the effect of increasing electrode width on the threshold voltage. Plate electrodes exhibit thresholds which are substantially less than those observed for rods (11). The decreasing threshold voltages as the cathode width increases
2381
can be attributed to a decrease in the positive ion sheath. The increasing electrode width decreases the effective shielding due to the sheath resulting in a greater field intensity a t the excitation site. The anomalous variations in the larger plate thresholds and saturation voltages are due to cathode separation effects. The saturation voltages are also given in Table I. The saturation voltage is the voltage above which increases in applied voltage will result in no increases in LEI signal. For comparison of the saturation voltages of all the electrode widths, the saturation voltage was interpolated from the data a t a 95% maximum signal. Of particular interest is the rather sharp drop in saturation voltage for the lo-, 20-, and 30-mm plates. This indicates a much better signal collection efficiency than any of the other electrode widths. Even the 5-mm plates exhibit better threshold and saturation voltages than the 6.2-mm rods. Laser Beam Diameter. Since the UV laser beam has a diameter of approximately 2.5 mm (4.9 mm2 cross sectional area) only a very small volume in the flame is subject to laser excitation. Signals for focused, unfocused, and expanded laser beams were compared. The beam was focused using an 1 in. diameter fused silica lens with 250-mm focal length. The focused beam had a average diameter of less than 0.8 mm2 cross sectional area over the length of the flame. This lens focused the laser beam approximately in the center of the 25 mm long flame. Beam expansion was achieved by placing a second similar lens in front of the first lens. This resulted in a slightly divergent elliptical beam rather than a collimated one. The average cross sectional area of the expanded beam was 16.5 mm2 with the ellipse’s major axis perpendicular to the burner head. The most obvious effect of beam expansion was the increased signal due to the irradiation of a larger flame volume. No spatial filtering to reduce scattered excitation light was necessary to improve signal-to-noise ratios because of nonoptical LEI detection. The LEI signal for 100 ng/mL indium ranged from 10 to 42 to 77 for the focused, unfocused, and expanded beams, respectively. The sodium matrix concentration range using laser beams with different cross sectional areas remained about the same; however, the signal maxima occurred at slightly lower matrix concentrations as the cross sectional area increased. Charge recycling promoted by a net increase in charge carriers (matrix ions) has been suggested as a possible mechanism for signal enhancement (11). This may also partially account for the shifted maxima. In the expanded beam the signal maximum appeared at the lowest matrix concentration, approximately 260 pg/mL sodium. This is because during a single laser pulse, there are fewer photons per unit cross sectional area to recycle or reexcite ground-state indium atoms which have been produced by collisions with sodium atoms. Also with the expanded beam, the average migration distance to the cathodes is reduced thereby reducing collisions. Therefore suppression predominates at lower sodium concentrations than for the unfocused and focused beams. The peak signal enhancement for the unfocused and focused beams occurred at 300 and 325 pg/mL sodium, respectively. Similar results were obtained by using 1-mm rod electrodes although signal levels were lower. High-Voltage Interferent Removal. The results obtained with wider plates suggested that interfering ions might be removed prior to detection. Since severe electrical interferences are limited to low ionization potential matrices and most analytes are not significantly ionized in the acetylenelair flame, such a technique could be widely applicable. In this experiment, the dual cathodes/ burner head anode configuration (VAC) was used to remove sodium ions and a second anode and cathode were positioned above the “interferent
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 14, DECEMBER 1980
Figure 10. Arrangement of interferent removal and signal detection plates with respect to the burner head (end-on)and laser beam (*).
the acetylene/air flame was intermediate, 70 pg/mL sodium. These data show that although natural flame ions contribute to electrical interferences, the flame temperature is the single most important factor when solutions contain low ionization potential elements. The neat indium solution gave a greater LEI signal in the acetylene/& flame than in the hydrogen/nitrous oxide flame. A limitation of the experimental configuration may have contributed to the lower signal in addition to the lower indium atom fraction. The nitrous oxide based flame is considerably thicker than the acetylene/air flame. In fact, the acetylenelnitrous oxide flame is almost 20 mm wide a t the excitation site 10 mm above the burner head. This means that the indium ions produced by laser excitation and thermal ionization may be a full centimeter from the cathodes. Further work with nitrous oxide based flames is under way. Beam expansion or use of a burner head which produces a more compact flame will improve the signal collection. It also may be possible to detect LEI signals at higher positions in the flame in the horizontal anodelcathode configuration. Evidently the applied voltage threshold for this flame is considerably greater than any other flame studied. Vitreous carbon plates should also be more appropriate for the hotter nitrous oxide flames.
CONCLUSIONS removal" electrodes to collect the LEI signal (HAC). The experimental configuration is depicted in Figure 10. The detection and removal electrodes were held a t -loo0 V. The acetylene/& ratio was adjusted to obtain the maximum signal for 100 ng/mL indium. There was over a fourfold increase in the matrix concentration range using interferent removal but, as predicted by Figure 4, the signal was less than in the VAC mode. Although there was a great improvement with interferent removal, the resistance of the HAC mode to sodium interference was not as good as the VAC mode of signal collection. The feasibility of interferent removal prior to signal collection has been demonstrated in this preliminary experiment. This method of interferent removal coupled with the optimum signal collection techniques described in the present work should further increase the usable concentration range of low ionization potential matrices. Graphite Electrodes. The use of graphite plate cathodes was investigated. A 2 mm thick vitreous carbon sheet was cut into 21 mm wide plates. Although the performance of these cathodes for the criteria evaluated here was essentially the same as the molybdenum plates used for the bulk of the work, they do have several practical advantages over metal electrodes. The carbon electrodes are less susceptible to fouling and corrosion, can withstand higher temperatures, and conduct less heat to the high-voltage leads. These factors permit closer placement of the cathodes to the flame thereby increasing the LEI signal. Their prime disadvantage is greater expense, e.g., approximately 10 times greater per unit area than molybdenum. S u r v e y of Several Flames. The electrical interference produced by a sodium matrix was compared in several flames. The flames and their relative temperatures are as follows; hydrogenlair < acetylenelair < hydrogen/nitrous oxide < acetylene/nitrous oxide. The cathodes were 20 mm wide plates centered 15 mm above the burner head with a 20-mm excitation height. Hydrogen-based flames have essentially no natural flame ions and therefore a very low electrical background. The hydrogenlair flame gave a constant signal within experimental error for concentrations greater than 100 Kg/mL. At the other extreme, there was no indium signal observed in the acetylenelnitrous oxide flame although this may be a consequence of a low indium atom fraction as much as LEI signal collection. The matrix concentration range in
The superior characteristics of plate electrodes for LEI spectrometry have been documented and the optimum experimental conditions have been identified. The LEI signal depends on the position of the plates (sampling height, electrode separation, electrode-flame separation), relative excitation height, fuel/oxidant ratio, and applied voltage. By use of the optimum conditions, the best LEI signal recovery to date in a low ionization potential matrix has been reported. These results may be translated to other systems keeping in mind their relative nature, and, in some cases, the need for optimization of parameters for a specific apparatus. Perturbation of signal collection processes is clearly the largest source of the observed electrical interferences. For plates, electrostatic interactions should be included among the possible perturbations. The present examination of the characteristics of plate electrodes defines an experimental base for subsequent studies of signal collection and therefore further improvement of signal recoveries. Alternate electrode geometries and expanded beams should improve signals and resistance to electrical interferences. The development of a high temperature flame and a compatible LEI detection system for use with refractory elements is an immediate need. Vitreous carbon electrodes, possibly immersed in the flame, and an expanded beam may be useful with higher temperature flames. The present results emphasize the need for the control of the variables identified here for maximum sensitivity and reproducible results for valid comparisons. A versatile electrode positioner is a necessity for maximizing LEI signals (13). The LEI signal delay characteristics reported in this paper be useful for monitoring flame processes.
LITERATURE CITED Green, R . E . ; Keller, R . A,; Schenck, P. K.;Travis, J. C.; Luther, G. C . J . Am. Chem. SOC.1978, 9 8 , 8517-8518. Alkemade, C. Th. J. Proc. Colloq. Spectrosc. Int. 20th and Abstr. Pap.-lnt. Conf. At. Spectrosc. 7th, 1977, 1977. van Dijk, C. A. W.D. Dissertation, University of Utrecht, The Netherlands, 1978. Turk. G. C.:Travis. J. C.: DeVoe. J. R.: O'Haver, T. C. Anal. Chem. 1978, 50, 817-820. Travis, J. C.; Turk, G. C.; Green, R. 8.ACS Symp. Ser. 1978, No. 85, 91
Turk, G. C.; Travis, J. C.; DeVoe, J. R.; O'Haver, T. C. Anal. Chem. 1979, 57, 1890-1896. Turk, G. C.; Mallard, W. G.: Schenck, P. K.; Smyth, K. C. Anal. Chem. 1979. .- . ., 5 - 1 ,. 2408-2410 - .- .. Gonchakov, A. S.;Zorov, N. B.; Uuzyakov, Yu. Ya.; Maveev. 0.1. Anal. Lett. 1979, 12. 1037-1048.
Anal. Chem. 1980, 52, 2383-2387 (9) Schenck, P. K.; Mallard, W. G.; Travis, J. C.; Smyth, K. C. J . Chem. Phys. 1978, 69, 5147-5150. (10) Travis, J. C.; Schenck, P. K.; Turk, G. C.; Mallard, W. G. Anal. Chem. 1979, 57, 1516-1520. (11) Green, R. B.; Havrilla, G. J.; Trask, T. 0. A w l . Spectrosc. 1980, 3 4 , 561-569. (12) Smith, 8. W.; Parsons, M. L. J. Chem. Educ. 1973, 50, 679-681. (13) Havrilla, G. J.; Green, R. B., submitted for publlcation in Anal. Chem.
RECEIVED for review June 30, 1980. Accepted September 18,
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1980. This research was supported by the National Science Foundation under Grant No' CHE-79-18626*This work was presented in part a t the 179th National Meeting of the American Chemical Society, Houston, T X , March 1980. This paper was taken in part from the dissertation written by G. J. Havrilla in partial fulfillment of degree requirements for a Doctor of Philosophy in Chemistry from West Virginia University, Morgantown, WV.
Disposable Potentiometric Ammonia Gas Sensors for Estimation of Ammonia in Blood M. E. Meyerhoff" and R. H. Robins Department of Chemistry, The University of Michigan, Ann Arbor, Michigan 48 109
A simple, rapid, and Inexpensive method for estimating the ammonia content of various blood samples is presented. The method utilizes a newly developed ammonium selective membrane electrode-based potentiometric ammonia gas sensor in conjunction with a multiple standard addition procedure. The assay requires 250 pL or less of sample and is shown to yield good accuracy (relative errors, -7.0 to +14%) and precision (relative standard deviation, 2.5-12.0 YO) in the normal ammonia concentration range. Results obtained for pooled human plasma and serum samples correlate well with a current manual enzymatic procedure. Additional Information concerning the design and analytical function of the new disposable gas sensor is also presented.
The application of membrane electrodes and sensors for the direct determination of discrete ions and gases in physiological samples has grown rapidly in recent years (1-4). Potentiometric sensors offer several significant advantages for such measurements, including, simple instrumentation requirements, minimal additional reagents, low cost, and freedom from sample turbidity and color problems which often interfere with the more traditional photometric assays. We have recently introduced a new type of potentiometric ammonia gas sensor ( 5 ) . In this paper we present a method for using this disposable sensor for the rapid estimation of ammonia in serum, plasma, and whole blood samples. (Throughout this paper, in order to be consistent with previous clinical chemistry literature, the terms ammonia or ammonia in blood refer to the total concentration of ammonia gas plus ammonium ions present, i.e., total ammonia nitrogen concentration.) T h e measurement of ammonia in blood is an important diagnostic test for several disease states including hepatic coma and the fatal childhood disorder, Reye's Syndrome (6). Recent outbreaks of Reye's Syndrome have demonstrated the need for a rapid, simple, and reliable method to detect ammonia in blood; perhaps one which could readily be performed in a small laboratory or physician's office. Manual determinations of ammonia can be made by a number of methods, including ion exchange (7), isothermal diffusion (8),and enzymatic assay (9, 10). The ion-exchange and isothermal procedures are slow and require many sample manipulation 0003-2700/80/0352-2383$01 .OO/O
steps which can decrease the accuracy of these techniques. The enzymatic assay employs glutamate dehydrogenase (GLDH, EC no. 1.4.1.2) to catalyze the reaction
NH4+ + NADH
+ a-ketoglutarate
-
glutamate
+ NAD
(1)
The decrease in absorbance a t 340 nm, due to the disappearance of reduced nicotinamide adenine dinucleotide (NADH) from the assay mixture, is proportional to the ammonia present. The manual method suffers from poor precision and accuracy in the normal ammonia concentration range ( I I ) , and other drawbacks, including interferences from competing enzymatic reactions, generation of ammonia during the reaction, and interferences due to GLDH inhibitors which may be present in the sample (i.e., drugs) (12). Normal values of ammonia found in blood vary considerably depending on the method used. For most blood specimens, normal values ranging from 20 to 80 kmol/L have been reported (13) with whole blood levels usually slightly higher than serum or plasma values. Commercially available ammonia-selective gas sensors, utilizing p H glass membranes as internal sensing elements, have previously been used for blood ammonia determinations (12,14-16). Use of these sensors requires operation a t a pH >10.3 so that all ammonia in the blood sample is present as free dissolved gas. Under these alkaline conditions, the labile amide groups of the amino acids glutamine and asparagine may hydrolyze to give false elevated ammonia values and drifting electrode potentials (12-14). Prior perchloric acid precipitation of proteins has been employed (12) to reduce these problems, but increased sample handling and adjustment of the sample to pH 11.0 is still required. In addition, the size of the commerical ammonia sensors has necessitated the use of the rather large volumes of blood samples (2-3 mL). Here we describe a simple, multiple standard addition procedure to directly estimate the concentration of ammonia in serum, plasma, or whole blood. The method employs a newly developed polymer membrane electrode-based ammonia-selective gas sensor (5) which has improved detection limits over existing commercial sensors. Measurements take place under mild buffer conditions, pH 8.5, and, therefore, the possibility of hydrolysis reactions which liberate additional ammonia is greatly reduced. Furthermore, the amount of sample required for the assay is 250 FL or less. Potentiometric 0 1980 American Chemical Society
