2520
Anal. Chem. 1985, 57,2520-2526
Direct Current Plasma as a Radiation Source for Flame Atomic Fluorescence Spectrometry Philip A. Goliber, Martha S. Hendrick, and R. G. Michel*
Department of Chemistry, University of Connecticut, Storrs, Connecticut 06268
A two-electrode direct current plasma was used as a source for the excitation of flame atomic fluorescence by aspirating hlgh concentratlong of a metal into the plasma. The shapes of atomic emlssion callbratlon curves In the plasma and atomlc fluorescence curves In the flame showed that under certain experimental conditions the plasma could be considered to be either a narrow spectral line source or a pseudocontlnuum source. These condltlons depended upon whether the sample lntroductlon tube was positioned directly below the plasma or in posltlons displaced horizontally to the front or back of the plasma. Limits of detection were typically in the range 1-30 ng/mL, although two elements, arsenlc and lead, were worse than this at 25 and J pg/mL, respectively. These detectlon ilmlts are the same or only silghtiy worse'than literature detectlon limits wlth a varlety of other llght sources used for atomic fluorescence spectrometry (AFS).
Probably the most important factor which limits the realization of the potentially very high sensitivity of atomic fluorescence spectrometry (AFS) is the lack of suitably intense radiation sources. PuIsed dye lasers can be considered such sources, but they are expensive and even they are limited in their intensity at wavelengths in the UV region of the spectrum where most elements have their resonance lines. Conventional radiation sources are so much less expensive than lasers that a continued search for more intense conventional radiation sources is justified. Of the radiation sources that are currently available, the high-pressure xenon arc is useful for continuum source excited AFS which allows for rapid sequential multielement analysis (1,2). This technique gives good detection limits (low ng/mL) but not sufficiently good to be competitive with established multielement techniques. In addition, special attention (2-4) must be paid to background correction of spectral interferences in order to ensure high accuracy. The most successful line sources have been hollow cathode lamps (HCLs) (5,6) and microwave (7-9) and radio-frequency (10, 11)excited electrodeless discharge lamps (EDLs). Detection limits by boosted HCL excited AFS in an inductively coupled plasma (ICP) are well into the low ng/mL range (5, 6). Detection limits for some less-volatile elements are not good by HCL-ICP due to high background levels which prevent use of the most efficient atomization regions of the plasma. The use of an ICP as a radiation source for excitation of atomic fluorescence in a flame has been studied in some detail by Winefordner et al. (12,13). A high concentration of any particular metal (typically 20 000 pg/mL) was nebulized into the ICP, and the resulting radiation was used to excite atomic fluorescence of the same metal aspirated as the analyte into a flame. The ICP proved to be an excellent narrow line source because of its freedom from self-reversal and its high stability. Atomic fluorescence detection limits were in the low ng/mL range for some elements but were poor (bg/mL) for others. This work was extended by Winefordner et al. (14,15) by using
an ICP to excite atomic fluorescence in a second ICP. Detection limits were better or about the same as in a flame and similar to HCL-ICP detection limits. High background caused limitations in the choice of viewing region in the ICP just as it does for the HCL-ICP technique. Messman et al. used the DCP as a pseudo-continuum source for the measurement of atomic absorption in a flame (16). Very high concentrations (about 5000 pg/mL) were introduced into the DCP in order to broaden the line profile. The resultant pseudo-continuum output, although self-reversed, was used in the continuum source atomic absorption arrangement of Messman et al. (17). The analytical detection limits were poor, relative to xenon arc excited atomic absorption ( I T ) , because of instabilities in the DCP which were assigned to self-reversal flicker noise. The work reported here explored the possibility of using a direct current plasma (DCP) as a radiation source for excitation of atomic fluorescence in a flame. The broad selfreversed lines used by Messman et al. and commonly observed (18)are not desirable for flame AFS, because that would result in increased scatter signals which increase noise levels without concomitant increases in signal-to-noise ratio. However, the DCP has potentially many of the same advantages as the ICP when in a similar experimental arrangement, including the potential to function as a narrow line source which is free from self-reversal. We found that the spectral output can be a narrow line for the elements tested but only under carefully controlled experimental conditions concerned with the way that the required high concentration is nebulized into the DCP.
THEORETICAL CONSIDERATIONS One focus of this paper was to determine whether or not the DCP can act as a line source for excitation of atomic fluorescence. In order to obtain information about the line profile, the approach reviewed in detail by Omenetto et al. (12) was used. In brief, when the emission from the DCP is monitored via the excitation of fluorescence in a flame, three types of plots can be obtained: the usual emission curve of atomic emission intensity from the plasma vs. analyte concentration in the plasma; the excitation curve, which is a plot of the flame fluorescence signal from a constant analyte concentration vs. the analyte concentration nebulized into the plasma; and the fluorescence curve of growth, which is a plot of the flame fluorescence signal vs. the analyte concentration in the flame at a constant concentration of analyte in the plasma. The shapes of these three types of plots can be described in terms of several limiting cases. At low concentrations, in all three types of plots, the slope of the log/log curves will be unity. At high concentrations the slopes are different for each type of curve depending on the line profile. Emission curves reach a limiting slope of one-half as increasing atom density in the source causes self-absorption of the emitted radiation. Excitation curves have a limiting slope of zero as increasing concentration causes self-absorption, while a negative slope indicates that the high concentrations are sufficient to cause self-reversal. Fluorescence curves of growth have a negative
0003-2700/85/0357-2520$01.50/00 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985
2521
Table I. Apparatus and Operating Conditions apparatus
model no.
plasma current gas flow: anode and cathode gas flow: nebulizer solution flow spectrometer: monochromator entrance and exit slits middle slit bandpass photomultiplier tube (PMT) PMT housing photon counter microcomputer
nr P
x
CHOPPER
manufacturer Spectrametrics, Inc., Andover, MA
Spectrajet I1
6A
1.3 L/min
__
DH-BOA 0.2 m double monochromator
9893QB/350 PR1400RF 1112 with 1120 discriminator Pet 4016
Y
4k
conditions
SEPARATED FLAME n SPHERICAL MIRROR
Figure 1. Instrumental layout.
slope a t high concentrations if the source is a narrow line and a slope of zero if the source is either a continuum or broad enough to behave like a continuum. Inspection of the three types of curves then allows for qualitative estimates of the line profile being emitted from the plasma. EXPERIMENTAL SECTION Apparatus. Figure 1 shows the experimental arrangement used. The aerosol delivery tube (ADT) was removed from the Spectrajet I1 DCP (Spectrametrics, Inc., Andover, MA) and mounted separat,ely to allow for control over the position of the ADT. The plasma was positioned so that the YZ-plane (Figure 2) centered on the electrodes was facing the flame. Measurements were taken with the orifice of the ADT positioned approximately 5 mm beneath the opening of the ceramic electrode sleeves and horizontally positioned along the X-axis (Figure 1)either in front of the plasma with respect to the flame, centered under the plasma, or behind the plasma. The entire image of the DCP was focused onto a mechanical chopper operating at 80 Hz and equipped with an LED-phototransistor arrangement to provide a reference signal. Bright emissions occurring from the tips of the electrodes were excluded from the excitation image by use of light stops. The second lens was used to provide a magnified and defocused image on the flame. Care was taken to ensure that this image was at least as wide as the flame to minimize pre- and post-filter effects. A nitrogen-shielded flame was used to reduce the flame background. The burner head and nitrogen separator were of laboratory construction. The signal from the photomultiplier tube was set in phase with the reference signal from the chopper and processed through a photon counter operating in a chopped mode. The photon counter was interfaced to a Pet 4016 microcomputer for data processing. Operating conditions used for the duration of the experiment are summarized in Table I, with the exception that an Instruments SA, HR 320 monochromator equipped with an EM1 9789QB photomultiplier tube was substituted while measuring detection limits. This mono-
2.4 L/min 1.4 mL/min
0.25 mm 2.0 mm 0.5 nm
Instruments SA, Metuchen, NJ
Thorn EMI-Gencom, Inc., Plainview, NY Products for Research, Danvers, MA Princeton Appl. Res., Princeton, NJ Commodore, Wayne, PA chromator was equipped with variable entrance and exit slits, which were adjusted so that the bandpass of the monochromator encompassed groups of lines for those elements for which this was possible (Table IV). When two-source background correction was used, it was carried out by using a Varian Associates VIX300 UV EIMAC xenon arc operating at 15 A, according to our previously published procedures (19). The nebulizer arrangement was the standard system supplied with the plasma by Spectrametrics, Inc. Procedure. Solutions of cadmium, magnesium, and strontium in 0.04 M HC1 were made from 20000 pg/mL stock solutions of cadmium iodide and strontium chloride (Baker-Adamson reagents) and magnesium rod (Spex Industries, Inc.). Excitation curves were obtained by aspirating a low analyte concentration into the flame (1 pg/mL cadmium and strontium, 50 ng/mL magnesium) while varying the concentration of the solution aspirating into the plasma (250,500,1000,5000,10000, and 20000 pg/mL). Analytical curves of growth were obtained by aspirating each of the solutions listed above in turn into the plasma while varying the concentration of the solution aspirating into the flame (1, 10, 100, 1000, 5000, and 10000 pg/mL). For both types of curves a similar procedure was followed. Three 7-s integrations of a 0.04 M HC1 blank were followed by three integrations with the analyte being nebulized into the appropriate device (either the plasma or the flame),followed by three more blank trials. The analyte solution for a given trial was selected in a random fashion. The average of the blank values flanking the appropriate trial was subtracted from the average signal obtained while the analyte was being aspirated, and the resulting value was taken as the signal obtained for the trial. Solutions used for measuring detection limits were made from Spex purified metals or reagent-grade salts when the former were unavailable. RESULTS AND DISCUSSION Initial experiments with the DCP as an excitation source for flame AFS revealed that the position of the sample tube beneath the plasma and along the X axis (Figure 1)critically affected the shapes of the three types of plots that were described above. This is probably because of the way in which the nebulized analyte droplet cloud spreads around the plasma. Any significant atom population toward the front of the plasma (on the flame side of the plasma) is able to reabsorb radiation emitted further back. This causes selfabsorption and, where temperature and/or concentration gradients exist, self-reversal of the radiation from the plasma. Coleman and Allen (20) have characterized the flow patterns of the nebulized droplets around the plasma. Their work indicated that there exists a plasma skin produced by thermal, electrical, and magnetic barriers which deflects the droplets around the plasma rather than through it. Hence, a sheathing effect is obtained which is caused by atoms in the viewing direction that are in a position to cause self-absorption and self-reversal. Coleman and Allen managed to decrease the sheathing effect by making the orifice of the aerosol delivery
2522
ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985 AEROSOL DELIVERY TUBE
\
.ECTRODE
-3mm
BEHIND
I
-
-0-CENTERED
-
".& -/
-3mm
/
I N FRONT
CERAMIC S L E E V E
ry X
-
DIRECTION OF FLAME
Axes which describe the relative positioning of the aerosol delivery tube (ADT). Top view of plasma showing only the ceramic sleeves, electrode tips, and ADT tip. The scale-3 mm behind, 0 centered, and 3 mm in front-refers to the centerline of the ADT tip. The Z axis is perpendicular to the X Y plane. Figure 2.
Table 11. Summary of the Slopes of Emission Curves" ADT position (see text) in front 3 mm centered behind 3 mm
cadmium 0.6 0.6 0.6
strontium 0.8 0.7 0.6
b2
1
I
P L A S MA
I
I
io4
CONC E NT RAT I 0N ( u g /m I)
Excitation curves for cadmium. ADT posltions: (a) 3 mm behind the plasma, (b) centered under the plasma, (c) 3 mm In front of the plasma. Figure 3.
magnesium
104 i
0.8 0.8 0.7
Q
z
e .... cn w
"The slopes listed above are based on a least-squares analysis of the emission data for the respective metals over the same concentration ranges as the excitation curves (Figures 3, 4, 6). All the curves were linear with the above slopes.
V
z
-
W 0
Vl W
0
tube smaller. However, this caused increased alignment and flow-rate difficulties. The main direction of the work described here was to look a t the effect of the position of the ADT with respect to the plasma. Three to five locations were studied, ranging from a position where the center of the ADT was 3 mm in front of the center of the plasma to a point 3 mm behind the plasma center. These axes are defined in Figure 2. Six elements, cadmium, strontium, magnesium, zinc, manganese, and lead (for wavelengths see Table IV), were studied. The first three were studied in the most detail. These elements were chosen in order to provide measurements in both the ultraviolet and visible spectral regions and also to illustrate the use of the DCP as a light source, using elements that are known to be sensitively determined by fluorescence in a flame (Cd, Zn) and by atomic emission in the DCP (Sr, Mg). In addition, some intermediate elements (Pb, Mn) were studied. Effects of ADT Position. In Table I1 are given the slopes of the atomic emission calibration curves that were obtained over the same concentration range as in Figures 3, 4, and 6 and for three different positions of the ADT (3 mm in front of the center line of the plasma, centered under the plasma, and 3 mm behind the plasma; Figure 2). Some self-absorption was apparent in all the data because all slopes were less than unity. Cadmium showed the greatest self-absorption with slopes close to the theoretical limiting slope of one-half. Excitation curves for cadmium are shown in Figure 3. It can be seen that the slopes of the cadmium excitation curves at high concentrations were about zero when the ADT was placed toward the back of the plasma. This indicated selfabsorption without self-reversal. As the tube was moved forward, through the central position toward the front of the plasma, the slopes of the cadmium excitation curves at high concentrations became negative. This signifies that in the forward positions self-reversal was being caused by a sheath of atoms in the front of the plasma, i.e., in the observation direction.
3 1 !L
1 o\
PLASMA CONCENTRATION
(ug/mll
Excitation curves for strontium. ADT positions: (a) 3 mm behind the plasma, (b) centered under the plasma, (c) 3 mm in front of the plasma. Figure 4.
Similar results to those obtained for cadmium were also obtained for strontium. The excitation curves in Figure 4 show that the positions of the ADT toward the back, centered, and toward the front of the plasma all caused the curves to reach a limiting slope greater than zero at high concentrations. This signifies that the strontium line was not self-reversed but was self-absorbed to some degree. The least self-absorption was observed when the ADT was toward the back of the plasma because the most positive limiting slope was obtained for that curve. The effects of self-reversal and self-absorption on the width of the atomic line in the plasma can be seen by inspection of the limiting slopes of the atomic fluorescence curves of growth in the flame (12). The data obtained for cadmium (Table 111) show that when the ADT was centered under the plasma or in front of the plasma, the curves achieved a negative slope when nebulizing low plasma concentrations. This indicated that the plasma was acting as a line source when low concentrations were being introduced into the plasma. A t high plasma concentrations the DCP behaved like a pseudo-continuum source because the spectra line was broadened sufficiently by self-absorption and self-reversal to result in a limiting slope of zero. The limiting slope at high concentrations in the flame should be -0.5 if the DCP was acting as a pure line source. However, the negative slopes in Table I11 often are more negative than -0.5. This is probably a result of self-reversal effects in the flame at high flame concentrations. Despite the possible existence of self-reversal in the flame qualitative
ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985
2523
Table 111. Limiting Slopes of Cadmium Fluorescence Growth Curveso plasma concentration, rg/mL
slopes at three ADT positions (see text) in front centered behind
250 500 1000
5 000 10 000 20 000
-0.5 -0.5 -0.3
-0.5
0.0 0.1
0.0 0.1
-0.6 -0.6 -0.7 -0.7 -0.5
0.3
0.1
-0.4
-0.5 -0.4
"Growth curves were examined over a concentration range of 1-10000 pg/mL. The limiting slopes listed above are the result of a least-squares analysis of the data points in that region of the calibration curve after the plot reached a maximum at high concentrations of analyte in the flame. An example set of growth curves is given in Figure 5. All curves gave a slope of unity at low concentrations (from the 1-10 pg/mL region to lower concentrations). 104
I
/ 1oo $00
I
1101
I
I
1103
FLAME CONCENTRATION
I
t b 4
(ug /m I)
Figure 5. Fluorescence curves of growth for cadmium. ADT centered for curves a and b, and to the back of the plasma for curves c and d. Analyte concentratlons in the plasma were 10 000 pg/mL (curves a and c) and 5000 pg/mL (curves b and d). Actual fluorescence signals have been normalized to one point on the linear portion (slope of unity of 1 pg/mL) of the fluorescence curves in order to demonstrate the differences occurring in the limiting slopes at high analyte concentrations.
conclusions can be drawn here by looking a t the changes in the slopes of the curves as a function of plasma concentration. Trends from zero slope to steadily more-negative slopes with decreased plasma concentration or with change in ADT position indicate trends from continuum-source behavior to increasingly narrow line-source behavior. When the ADT was moved to a position toward the back of the plasma, the sheathing effect was decreased and the plasma acted as a line source for plasma concentrations as high as 20 000 pg/mL as evidenced by the negative slopes (Table 111) at high concentrations obtained in all cases. Examples of the effect of the sample tube position on the cadmium growth curves can be seen in Figure 5. In the centered ADT position (Figure 5a,b), high analyte concentrations in the plasma caused a broadening of the spectral line emitted from the plasma as evidenced by slopes close to zero. When the ADT was moved to the rear of the plasma (Figure 5c,d), a decrease in broadening occurred as demonstrated by the change from a limiting slope of approximately zero to a negative slope. The curves of growth for strontium were very similar to the cadmium curves discussed above. In the front and centered ADT positions a gradual transition was observed from linesource behavior (negative limiting slope) at low plasma con-
P L A S M A CONCENTRAT I ON ( u g / m l )
Figure 6. Excitation curves for magnesium. ADT positions: (a) 3 mm behind the plasma, (b) centered under the plasma, (c) 3 mm in front of the plasma, (d) 5 mm in front of the plasma.
centrations to continuum-source behavior (zero limiting slope) at high plasma concentrations. When the ADT was moved toward the rear of the plasma the DCP exhibited line-source behavior at all of the plasma concentrations that were examined. A slight upward lift was observed in the fluorescence curves of growth at high plasma concentrations (Figure 5a). This was the result of DCP radiation being scattered in the flame and was confirmed by using two-source background correction (19) to subtract out the scatter. These data were not obtained for all the curves, so none of it is reproduced here. It was not a severe enought problem to change the interpretation of the data. Magnesium showed similar trends to those obtained fqr the other two metals that were examined. Self-absorption was more apparent in the forward aerosol tube positions than toward the back of the plasma as demonstrated by the slopes of the emission curves (Table 11) and the excitation curves shown in Figure 6. The growth curves followed the trend in that they showed the plasma to be a line source at lower plasma concentrations and a continuum source at higher plasma concentration for the forward and centered ADT positions (growth curves not shown). As before, this was caused by the sheathing effect of magnesium atoms in the observation direction. When the ADT was moved toward the back of the plasma, the curves of growth indicated line-source behavior at lower plasma concentrations but only intermediate between line-source and continuum-source behavior at high plasma concentrations. This is different from the other two metals where high concentrations produced good line-source behavior. The reasons for this probably lie in a combination of many factors which cannot be separated, with any certainty, by inspection of the data presented here. However, the curves of growth for magnesium at different plasma concentrations peaked at different flame concentrations. This did not occur for the other metals and suggests that the magnesium a-parameter, which is a function of the ratio between collisional broadening and doppler broadening (21), changed with an increase in magnesium concentration in the plasma. The equivalent of this was not observed in the ICP for magnesium (12). It appears that collisional broadening caused by the argon plasma gas is not the only broadening process in the DCP and that a role may be played by Stark and Holzmark broadening caused by high concentrations of magnesium atoms. Other Metals. Zinc, manganese, and lead were studied in somewhat less detail in that comparisons were not made, at every concentration for each sample tube position. Nevertheless, similar trends were observed. When the ADT was
2524
ANALYTICAL CHEMISTRY, VOL. 57, NO. 13, NOVEMBER 1985
Table IV. Limits of Detection (ng/mL)
elementa As Ca Cd co
Cr
cu Fe Mg Mn
Pb Zn
(193.6) (234.9) (422.6) (228.8) (240.7) (241.1) (241.4) (242.5) (357.8) (324.7) (327.4) (248.3) (248.8) (249.0) (285.2) (279.5) (279.8) (280.1) (283.3) (213.9)
DCP-flameb
ICP-flamec
Xe-flamed
HCL-ICP'
DCP-AASg
20d"
22 000
268 000 8 4 31
ICP-ICPf
-
5000 4 0.8
0.08 0.8 5
11
-
-
11
8
2 2
10 1 -
7
6
10
1 2
0.09
534 6
-
0.2 0.3
2
-
800
25 0.5
0.5
"Analyte wavelength in nm given in parentheses. Where more than one line is given then the spectral bandpass of the monochromator included all the lines given. bThis work; dc plasma excited atomic fluorescence spectroscopy in a flame; average of 10 10-s (1s) integrations; S I N = 3. CTakenfrom ref 13; inductively coupled plasma excited atomic fluorescence spectroscopy in a flame; time constant = 3-s or 16 1-s integrations; SIN = 3. dTaken from ref 1;xenon arc excited atomic fluorescence spectroscopy in a flame; 10-s integration; S I N = 2. "Taken from ref 5; hollow cathode lamp excited atomic fluorescence spectroscopy in an inductively coupled plasma; 10-s integration; S I N = 2. f Taken from ref 15; inductively coupled plasma excited atomic fluorescence spectroscopy in an inductively coupled plasma; 10-9 integration; SIN = 2; ion lines studied gave comparable results. #Taken from ref 16; dc plasma as a radiation source for atomic absorption spectrometry in a flame; 20 5-s integrations; S I N = 3. Wide bandpass optical interference filter used, 189.0-193.0 nm; all other HCL-ICP results used a monochromator with 3.2-nm band-pass. placed toward the back of the plasma all the metals allowed the plasma to behave like a line source and, as the ADT was moved forward, continuum-source behavior was evident. Plasma Concentration. The fluorescence excitation curves discussed above (Figures 3 , 4 , 6 ) serve to demonstrate the effect on the fluorescence of increasing metal concentration in the plasma. For cadmium and strontium it is apparent that, with the ADT either centered or in front of the plasma, relatively low analyte concentrations were sufficient to produce self-absorption effects &e., loss of line-source character through line broadening). On the other hand, when the ADT was placed to the rear of the plasma, these effects were minimized or avoided. The excitation curve for magnesium, however, shows that for this metal some self-absorption effects at higher concentrations were unavoidable, even with the ADT located behind the plasma. This meant that it was impossible, under the conditions that were used, to avoid broadening of magnesium lines a t plasma concentrations exceeding approximately 1000 ng/mL. This is not a major impediment to the use of the DCP as an excitation source because it is unlikely that broadening of the magnitude seen here would result in significant spectral interferences. The increased DCP emissions at higher analyte concentrations did result in an increase in the fluorescence intensity and hence in improvements in detection limit. Ion Lines. Strong emissions from ionic transitions are characteristic of the DCP and are often employed for analytical applications in atomic emission spectrometry. Two ion lines were briefly examined using the DCP-APS technique: the magnesium ion line a t 280.2 nm and the strontium ion line a t 407.8 nm. An excitation curve for magnesium was obtained with the ADT centered under the plasma and for the same plasma concentration range as in Figure 6. A straight-line graph with a slope less than 1demonstrated that some self-absorption occurred. Fluorescence growth curves for strontium, shown in Figure 7 , demonstrated continuumsource behavior for the centered sample tube position. The upward lift a t the high concentrations in the flame was due
I
1
o5
FLAME C O N C E N T R A T I O N (ug/mli
Figure 7. Fluorescence curves of growth for strontium ion. Plasma concentration (pglmL): (a) 10000 and (b) 1000. ADT centered under the plasma.
to scatter of DCP radiation in the flame as explained earlier. These results are consistent with the data presented previously for atomic transitions for these metals, where similar self-absorption effects were demonstrated for the centered ADT position. The results presented here for ion lines, while similar to the results for atomic lines, are incomplete and in need of further examination. It is expected, however, that due to the poor qualities of a flame as an ionization source, fluorescence excited from ions in a flame would be rather weak. Better results would be forthcoming from an atom cell capable of promoting ionization to a greater extent, such as an ICP (15) or a second DCP serving as the atom (ion) cell. Detection Limits. The above data indicated that the best line-source behavior for all metals was when the ADT was about 3 mm behind the plasma away from the viewing direction. In this position any increase in the concentration in the plasma led to an increase in the fluorescence signal and hence to optimum detection limits. The detection limits listed
ANALYTICAL CHEMISTRY, VOL. 67, NO. 13, NOVEMBER 1985
in Table IV were obtained at the optimum ADT position and with 20 000 mg/mL of each particular metal nebulized into the plasma. The measurement procedures used were those advocated by Long and Winefordner (22) which are based on extrapolation of a calibration curve and a signal-to-noiseratio of three. This was done at two time constants (Table IV). It is true that the two-electrode plasma does not have a very stable output compared to the more modern three-electrode plasma. Use of the latter plasma for such work as this may well improve detection limits a little. It is unlikely that such improvements will be startling because the change in detection limits with time constant followed an approximately square root relationship (Table IV). Noise caused by instability, if it had been limiting, would have prevented the change in time constant from improving the detection limits because such noise increases linearly with signal size (proportional or l/f noise behavior). In fact, light source noise is not usually the limiting noise at the detection limit in atomic fluorescence measurements because it is a fixed proportion of the total fluorescence signal and hence decreases with signal size. Flame background noise was the limiting noise sowce at the detection limit (low analyte concentrations). Plasma drift was the limiting noise source at high concentrations in the flame. The increased plasma stability that can be obtained with the three-electrode DCP would therefore result in increased precision at the higher concentrations but not significantly improved detection limits. We took care in this work to normalize all results to blank and standards measurements in order to avoid spurious data caused by plasma instability. The detection limits shown in table IV are typically in the range 1-30 ng/mL, although two elements, arsenic and lead, were worse than this at 25 and 5 pg/mL, respectively. In general these detection limits are similar or only slightly worse than literature detection limits for several atomic fluorescence techniques which, for convenience, are also shown in Table IV. The HCL/ICP results have recently been improved significantly (6),but it is striking that, except for arsenic, all the detection limits in Table IV are very similar. It may be the case that the DCP/atomic absorption detection limits of Messman et al. (16) could be improved by reduction of self-reversal upon moving the ADT as suggested here. This is because noise associated with the self-reversal had an effect on their measurements. This would bring the atomic absorption detection limits a little closer to the atomic fluorescence data. The DCP/AFS detection limits could be improved slightly by better imaging of the DCP onto the flame by using mirrors or lenses with more light gathering power. This is difficult because of the small area of the plasma that emits and the resulting necessity to expand the image of the plasma onto the flame in order to avoid such things as post-filter effects. Most of the rest of the instrumentation was efficient in its potential to measure fluorescence. Size of the ADT. The movement of the ADT to the various positions toward the front or back of the plasma has a rather similar effect to the change in size of the ADT as described by Coleman and Allen (20) for atomic emission measurements. With large-diameter ADTs, the aerosol that skirts the plasma results in substantial ground-state atom concentrations along the optical axis between the DCP and the detector. Coleman and Allen demonstrated that this skirting caused various bending effects on the emission calibration curves, and they minimized the bending by using smaller diameter ADTs. We carried out some exploratory experiments with ADTs of reduced size which did indeed optimize the line source character of the DCP for excitation in AFS. The magnitude of the effect was similar to moving the normally sized ADT to an optimum position with respect
2525
to the plasma, but the two effects were not additive. Hence, a combination of the two approaches was not fruitful. In addition, the alignment and stability problems associated with changing the size of the ADT were severe, with the stability effects significantly degrading the detection limits.
CONCLUSIONS A possible application of the DCP excited flame AFS approach is as an adjunct to conventional DCP emission analyses. Where spectral interferences are problematic, the DCP could be used to excite fluorescence in a flame and so eliminate spectral interferences without further sample preparation. This is similar to the approach of Epstein et al. (13) for ICP-excited AFS in a flame, Kosinski et al. (14) for ICP-excited AFS in an ICP, and Cavalli et al. (23) for ICP-excited AFS in a flame. The potential sacrifice will be a slight sensitivity loss and the possibility of some matrix interferences in the flame. However, this sort of addition to a DCP plasma instrument would not be expensive to implement. A further consideration in the implementation of the DCP-AFS technique is the ability to utilize the closely associated technique of analysis of DCP emissions using a flame as a resonance monochromator (DCP-RM). The instrumental arrangement for this technique is the same as was employed here for DCP-AFS. In DCP-RM, however, a pure analyte solution is aspirated into the flame while the sample is aspirated into the plasma. A calibration curve is obtained by bracketing the sample solution with a series of standards aspirated into the DCP. The excitation curves discussed previously in this paper constitute one example of utilizing the flame as a resonance monochromator. In a real sample analysis situation, use of DCP-RM has the advantage of extending the upper range of the analyte concentration which can be determined without resorting to dilutions. When a sample concentration exceeds the concentration at which the fluorescence growth curve loses linearity, the DCP-RM technique can be used to extend direct analysis ranges into the percentage levels. Using an ICP-RM technique Omenetto et al. (24) obtained detection limits in the low pg/mL ranges for several metals. Although 250 pg/mL was the lowest concentration used in the DCP in this work, the magnitude of the fluorescence signals implies that detection limits at least comparable with those of ICP-RM could easily be obtained. The xenon arc and DCP excited AFS detection limits are almost the same. This indicates that the DCP may be a viable alternative to the xenon arc (1) in a sequential slew scan multielement instrument. This is an advantage because the xenon arc approach is hampered by spectral interferences which, although relatively minor in magnitude and extent (4, are nonetheless discouraging. The DCP, because it can be made to act as a line source, will not be affected by spectral int'erferences. A further advantage of the DCP over the xenon arc is its ability to measure metals such as arsenic with resonance lines below about 210 nm. Only a new (less than 10-20 h old) xenon arc has enough UV output to excite metals that far into the UV (25). Some analytical applications of the DCP/AFS instrument described here will be reported in a future paper, including details of the magnitude of scattering of DCP radiation that can be expected with this technique. Our continuing studies involve the use of an easily ionized element (EIE) to enhance the emission (26-29) from the DCP with a view to improving the detection limits in the flame. ACKNOWLEDGMENT We thank the US. Coast Guard, Research and Development Center, Groton, CT, for the loan of the DCP. Registry No. As, 7440-38-2; Ca, 7440-70-2; Cd, 7440-43-9; Co, 7440-48-4; Cr, 7440-47-3; Cu, 7440-50-8; Fe, 7439-89-6; Mg,
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7439-95-4; Mn, 7439-96-5; Pb, 7439-92-1; Zn, 7440-66-6.
LITERATURE CITED (1) Johnson, D. J.; Plankey, F. W.; Winefordner, J. D. Anal. Chem. 1975, 47, 1739-1743. (2) Ullman, A. H.; Pollard, B. D.; Boutilier, G. D.; Bateh, R. P.; Hanley, P.; Wlnefordner, J. D. Anal. Chem. 1979, 51,2382-2387. (3) McCaffrey, John T.; Mlchel, R. G.; Anal. Chem. 1983, 55,488-492. (4) McCaffrey, John T.; Wu, Man-Li Wang; Michel, R. G. Analyst (London) 1983, 106, 1195-1208. (5) Demers, D. R.; Allemand, C. D. Anal. Chem. 1981, 53, 1915-1921. (6) Lancione, R. L.; Drew, D. M. Paper No 8, presented at the 1l t h Annual (7) (8)
(9) (10)
Meeting of the Federation of Analytical Chemlstry and Spectroscopy Societies, Phlladelphia, PA, Sept 17, 1984. Haarsma, J. P.; De Jong, G. J.; Agterdenbos, J. Spectrochim. Acta, Part B 1975, 298, 1-18. Michel, R. G.; Coleman, Julla; Winefordner, J. D. Spectrochlm. Acta, Part B 1978, 338, 195-215. Seltzer, M. D.; Michel, R. G. Anal. Chem. 1983, 55, 1817-1819. Novak, John W., Jr.; Browner, Richard F. Anal. Chem. 1978, 50,
1453-1457.
(11) Walters, P. E. Spectrochlm. Acta, Part B 1983, 368, 889-898. (12) Omenetto, N.; Nikdel, S.;Bradshaw, J. b.;Epstein, M. S.;Reeves, R. D.; Winefordner, J. D. Anal. Chem. 1979, 51, 1521-1525. (13) Epstein, M. S.; Nlkdel, S.;Omenetto, N.; Reeves, R.; Bradshaw, J.; Winefordner, J. D. Anal. Chem. 1979, 51, 2071-2077. (14) Kosinski, M. A.; Uchlda, H.; Wlnefordner, J. D. Anal. Chem. 1983, 55, 688-692. (15) Long, Gary; Wlnefordner, J. D. Appl. Spectrosc. 1984, 38,583-567. (16) Messman, Jerry D.; O’Haver, Thomas C.; Epstein, Michael S . Anal. Chem. 1985, 57, 416-420. (17) Messman, J. D.; Epsteln, M. S.;Ralns, T. C.; O’Haver, T. C. Anal. Chem. 1983, 55, 1055-1058. (18) Skogerboe, R. K.; Urasa, I. T. Appl. Spectrosc. 1978, 32,527-532.
(19) Mlchel, R. G.; Hall, M. L.; Ottaway, J. M.; Fell, G. S. Analyst (London) 1979, 104,491-504. (20) Coleman, G. N.; Allen, A. M. Appl. Spectrosc. 1982, 36, 116-120. (21) Wlnefordner, J. D.: Schulman, S. 0.; O’Haver, T. C. “Luminescence Spectroscopy in Analytical chemistry”; Wlley: New York, 1973. (22) Long, G. L.;Winefordner, J. D. Anal. Chem. 1983, 55,713A-724A. (23) Cavalli, Paolo; Rossi, Guglielmo; Omenetto, Nicolo Analyst (London) 1983, 108,297-304. (24) Omenetto, N.; Cavalli, P.; Rossi, G. Rev. Anal. Chem. 1981, 5 (3/4), 185-205. (25) Wu, Man-Li Wang, Ph.D. Dissertation, Unlversity of Connecticut, 1984. (26) Johnson, G. W.; Taylor, H. E.; Skogerboe, R. K. Appl. Spectrosc. 1980, 34, 19-24. (27) Nygaard, D. D.; Gilbert, T. H. Appl. Spectrosc. 1981, 35, 52-56. (28) Decker, R. J. Specfrochim. Acta, Part 8 1980, 358, 19-31. (29) Eastwood, D.; Hendrick, M. S.;Mlller, M. H. Spectrochlm.Acta, Part 8 1982, 378 293-302.
RECEIVED for review April 4, 1985. Accepted July 11, 1985. This work was presented in preliminary form a t the 13th Northeast Regional ACS Meeting, June 26-29, 1983, at the University of Hartford as paper 23 and at the 9th Annual FACSS meeting, Philadelphia, PA, Sept 19-24,1982, as paper 196. R.G.M. was supported by a Research Career Development Award from the National Institute of Environmental Health Sciences under Grant ES 00130. The work was supported in part by the donors of the Petroleum Research Fund, administered by the American Chemical Society, and Research Corporation.
Red and Near-Infrared Photodiode Array Atomic Emission Spectrograph for the Simultaneous Determination of Carbon, Hydrogen, Nitrogen, and Oxygen J. M. Keane, D. C. Brown,l and R. C. Fry* Department of Chemistry, Willard Hall, Kansas State University, Manhattan, Kansas 66506
I n a single exposure, a small, crossed Crerny-Turner photodiode array spectrograph with a coarsely ruled gratlng covers an unusually large “wlndow” for atomlc spectra (650-950 nm). Slmultaneous monltorlng of C, H, N, and 0 atomlc emlsslons Is performed In mlcrowave or Inductively coupled plasmas. Detector coollng and Image lntenslflcatlon are unnecessary. Intense emlsslon spectra are obtalned In mllllsecond exposure times as the result of (1) enhanced red and near-Infrared (near-IR) sensltlvlty of photodlode arrays, (2) favorable fhumbers of short focal length spectrographs, and (3) large elemental concentrations occurrlng In purlfled Samples of Interest In synthetlc chemistry. Atomlc emlsslon spectra of organic compounds are relatlvely simple In the red and near-IR reglon. For compounds contalnlng C, H, N, and 0, Interference-free lines have been located In splte of the low resolutlon of this Instrument. Qualltatlve elemental analysls of organic compounds Is asslsted using Xerox transparent overlays of labeled master reference spectra.
Samples pertaining to applied disciplines such as geology or metallurgy often contain a large number of easily excited metals. The corresponding air-path ultraviolet and visible Present address: H a r r i s Corp., Rochester, NY.
atomic emission spectra in hot plasmas can be far too complex to sort out with photodiode arrays. A trade-off arises between (a) the need for extreme ultraviolet dispersion and resolving power to minimize spectral interference and (b) the conflicting desire to have a large spectral range or multielement UV “window” covered by the relatively small photodiode array chip. Unfortunately, these two needs cannot be simultaneously met with present day photodiode arrays which are only about 25 mm in length and typically have only 512 or 1024 channels for “dividing up” the entire spectrum. The use of self-scanned, linear photodiode arrays as detectors in ultraviolet atomic emission spectrometry has therefore been primarily limited to a few diagnostic studies where high resolving power was not a concern or where it was not necessary to cover a large spectral range simultaneously. Examples include (a) studies of one or two elements dissolved in distilled water (1-3) and (b) 90’ detector orientation for spatial source profiling involving a single UV wavelength (4-9). Another limitation of unity gain photodiode arrays in atomic spectrometry is poor sensitivity in the UV region where most metallic emissions occur. This often leads to the use of detector cooling and long exposure times or costly image intensifiers (accompanied by an additional 2- to 4-fold loss in resolution). Photodiode arrays are actually most sensitive to red and near-infrared (near-IR) radiation. A preliminary report from this lab included qualitative detection of inductively coupled
0003-2700/85/0357-2526$01.50/00 1985 American Chemical Society
