1784
Anal. Chem. 1987, 5 9 , 1784-1789
Laser-Excited Atomic Fluorescence Spectrometry with a Laboratory-Constructed Tube Electrothermal Atomizer F r a n c i s R. Preli, Jr., Joseph
P.Dougherty, a n d Robert G . Michel*
Department of Chemistry, University of Connecticut, Storrs, Connecticut 06268
A shod-tube (8 mm long) electrothermal atomlzer (ETA) was constructed as an atom cell for laser-excited atomic fluorescence spectrometry (LEAFS). A variety of graphite tube designs were evaluated In order to achleve heatlng rates comparable to a commercial atomlc absorptlon ETA. An excimer pumped dye laser was used to excite nonresonance (dlrect Ilne) fluorescence from Ag, Co, Cu, In, W, and TI, and resonance fluorescence from Mn. For nonresonance fluorescence measurements, It was important to spatially discriminate the furnace blackbody radiation from the atomic fluorescence signal. For resonance fluorescence, careful alignment of the laser excitation beam was necessary to mlnhnlze stray light. Detection Umits obtained were comparable to the best reported values for LEAFS in a cup ETA and 2 to 500 tbnes better than for atomic absorption using an ETA. Calibration curves were linear, with a relative dope of 1, for 4.5 to 6 orders of magnitude.
Laser-excited atomic fluorescence spectrometry (LEAFS) has been shown to be a very sensitive technique for the determination of trace metals. The best flame LEAFS detection limits are comparable to atomic absorption spectrometry (AAS) with an electrothermal atomizer (ETA AAS) (1-3). LEAFS detection limits have been improved, by factors of 10 to 1O00, by substituting an electrothermal atomizer for the flame (ETA LEAFS) (4,5). In addition, the linear calibration range of ETA LEAFS is approximately 5 to 6 orders of magnitude. Most of the ETA LEAFS work has been done with a graphite cup as an atomization cell ( 4 , 5 ) . The atomic fluorescence is commonly viewed above the cup in an attempt to minimize the furnace blackbody radiation that reaches the detection system. Graphite cups have provided excellent detection limits but are not readily applicable to analyses of real samples because of the severe matrix interferences that occur in the cool environment above the cup (6). Bolshov et al. (7)have found that the cup furnace can be used for the analysis of real samples when operated under a vacuum, but the detection limit was degraded by 2 orders of magnitude relative to operation at atmospheric pressure. The matrix interference problems for LEAFS are probably the same as those found in ETA AAS. Cup ETAS are no longer used for ETA AAS and have been replaced by the tube ETA. Matrix interferences have been minimized, in ETA AAS, by using relatively fast furnace heating rates (approximately 1500 O C s-l) and by delaying the introduction of atoms into the furnace by use of the L'vov platform technique (8). The atoms are released from the platform into a nearly isothermal environment that minimizes condensation and compound formation of the analyte. A recent publication has described the use of a commercial tube ETA for the LEAFS determination of P b (9),but the detection limit was a factor of 45 worse than that obtained by Bolshov et al. ( 4 ) with a graphite cup. We have studied LEAFS in both a commercial atomizer and a laboratory-constructed atomizer and have achieved detection limits 0003-2700/87/0359-1784$0 1.5010
Table I. Dye Laser Parameters max
W ~ n , element Ag
nm
dye"
concentration
328/338 DCM 5 mM in MeOH/DMF
power, pJ/pulse 4
(21)
Co Cu
304/341 R610 5 mM in MeOH 325/510 DCM 5 mM in MeOH/DMF
In
410/451
13 6
(2:l)
Mn
Pb T1
DPS 279/279 R 560 283/405 R 575 377/535 BBQ
0.6 mM in dioxane 5.5 mM in MeOH 5.0 mM in MeOH 0.8 mM in cyclohexane
19 3 7
250
DCM, 4-Dicyanomethylene-2-methyl-6-p-(dimethylamino)styryl-4H-pyran; R 610, N,N,N',N'-tetraethylrhodamine; DPS, 4,4'-diphenylstilbene;R 560, Rhodamine 560; R 575, Rhodamine 575; BBQ, 4,4'"-bis[(butyloctyl)oxy]-p-quaterphenyl; MeOH, methanol.
that are comparable to those obtained with cup atomizers. Our laboratory-constructed furnace used a tube that is supported, in the center, by two graphite electrodes. This design was chosen primarily because it was easy to construct the electrode assembly to fit between the pole faces of a small electromagnet for Zeeman background correction (10). An alternative design would be to hold the tube a t the ends, as done in the Perkin-Elmer HGA series of atomizers. We report on ETA LEAFS in the Perkin-Elmer HGA-500 design in another publication (11). This communication describes the development and performance of the laboratory-constructed furnace system along with detection limits and calibration ranges obtained for ETA LEAFS. EXPERIMENTAL SECTION LEAFS Instrumentation. The details of the LEAFS instrumentation have been given elsewhere (10) but are briefly summarized here. The instrument used an excimer pumped dye laser, with the capability of frequency doubling. The laser was operated at a pulse repetition rate of 80 Hz. A small portion of the excimer laser beam was used to trigger a boxcar averager to process the fluorescence signals from a monochromator/photomultiplier tube (PMT)/preamplifier assembly placed at a right angle with respect to the laser excitation beam. A summary of laser operating parameters is given in Table I, including the dye concentrations and the maximum pulse energy available for each element. The laser pulse energy was measured at the output of either the dye laser or, where appropriate, the frequency doubler and was sufficient to saturate the Cu, In, Pb, and T1 transitions. The excitation and fluorescence wavelengths are given in Table I for each element. The dye laser radiation was frequency doubled for all elements, except In and T1. Electrothermal Atomizer. A detailed schematic of the atomizer is shown in Figure 1. The assembly consists of two brass electrodes attached to a stainless steel base. Each brass electrode is water cooled, and one of the brass electrodes is insulated from the base but permanently attached to the base with four screws. The other electrode is able to slide perpendicularly along the base, by means of an "inverted T" machined into the base and electrode. The pressure of the electrodes against the furnace tube maintains electrical contact and can be adjusted with a spring plunger. The 4 mm diameter laser beam was passed through ports drilled 0 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 14, JULY 15, 1987
4 D -
dimensions in
1785
mm
TOP V I E W iC15.I
ELECTRODE
GRAPHITE ELECTRODE
I
____ U
U
+77
STEEL BASE
------=
VI,
U
b
a
80
cl
7
END VIEW
SIDE VIEW
Figure 1. Top, side, and end views of the laboratory-constructed ETA.
See text for a detailed description. U
Table 11. Furnace Tube Design Data
c2
tube
vendor
o.d., mm
i.d., mm
wall, mm
length, mm
a b cl c2 d e
Varian Ringsdorff Perkin-Elmer Perkin-Elmer POCO POCO POCO POCO
5 10 8 8 9.6 9.6 9.6 9.6
3 7.5 6 6 6.8 7.2 7.6
1 1.25 1 1 1.4
9
1.5 4
8.0
0.8
8 8 8 8 8 8 8
f g
1.2
1.0
port, mm
2
4 4 4 4 4
through the sides of the tube, and fluorescence was viewed at a right angle, through the bore of the tube. A Perkin-Elmer HGA-2000 power supply was used to heat the furnace. The furnace assembly was enclosed inside an aluminum and Plexiglas enclosure that was purged with argon, except during the atomization step when the gas flow was stopped. A removable baffle/window assembly located at the front of the enclosure allowed a 10-pL sample aliquot to be placed upon the wall of the furnace tube. The baffle was used to minimize extraneous furnace emission, from the walls of the tube, from reaching the detector. Fluorescence Collection Optics. Two 50 mm focal length lenses were placed between the furnace and the monochromator to collect the fluorescence signal. For Zeeman background correction studies (IO), two lenses were required to physically separate the magnet from the PMT, in order to eliminate interference caused by the magnetic field. The same optical system was used here so that a comparison could be made of detection limits obtained with and without Zeeman background correction. A 1:l image of the furnace tube was focused onto the entrance slit of the monochromator. The monochromator was placed on its side, such that the entrance slit was parallel to the direction of propagation of the laser beam. Preparation of the Furnace Tubes. Furnace tubes were constructed from either Ringsdorff RW-0 or POCO AXF-5Q1 graphite. The performance of these tubes was compared by measuring the heating rate and maximum temperature attainable for each tube. In addition, the heating rates of several modified commercial ETA AAS tubes were measured for comparison. The laboratory-made tubes were pyrolytically coated in a laboratory-constructed apparatus at approximately 2000 "C with 5% methane added to an argon atmosphere. The various tube designs are depicted in Figure 2, and the pertinent dimensions are given in Table 11. Tube a was a standard Varian AAS design (Varian, Palo Alto,CA). This tube was chosen to provide a performance reference since the furnace design was initially based upon the Varian CRA-63. Designs c l and c2 were Perkin-Elmer HGA-600 pyrolytically coated ETA AAS tubes (Perkin-Elmer, Norwalk, CT) that were reduced in length from 28 to 8 mm. All remaining designs were made from Ringsdorff RW-0 (Sigri, Somerville, NJ) or POCO Graphite AXF-5Q1 (POCO Graphite, Decatur, TX) graphite rod stock. Tubes a and c l had only a single port (the ETA AAS sample introduction port), while the remaining designs had two ports to allow passaa-of the laser radiation through the tube at a right angle to the fluorescence
U
defg
Flgure 2. Furnace tube designs that were evaluated with respect to heatlng rate: (a) Varian ETA AAS tube: (b)tube made from Ringsdorff RW-0 graphite for LEAFS; (cl) Perkin-Elmer ETA AAS tube modified by reducing the length; (c2) Perkin-Elmer ETA AAS tube modified for LEAFS by reducing the length and adding laser ports. (d-g) ETA LEAFS tubes made from POCO AXFdQ1 graphite with wall thickness of 1.4, 1.2, 1.0, and 0.8 mm, respectively. See Table I1 for dimensions of tubes.
axis. For all studies, 4.6 mm, pyrolytically coated, Ringsdorff RW-0, curved-contact, laboratory-constructed electrodes were used. Temperature and Heating Rate Measurements. An Ircon Model 1100 optical pyrometer (Ircon, Skokie, E)was focused on the outside of the graphite tube wall. The pyrometer signal was amplified by a factor of 100 with a 741 op-amp and collected by an Epic Instruments Wavesaver I (Epic Instruments, Foster City, CA). The wavesaver had the capability of displaying the heating profile on an oscilloscope or strip chart recorder. Temperatures were determined by comparison of the pyrometer output with calibration curves supplied by the vendor and by use of an emittance value of 0.7. The heating rates were determined by measurement of the time required to heat from 100 "C (the temperature used to dry the sample aliquot prior to atomization) to 2200 "C. The power supply was set to the maximum output for the heating rate measurements. Temperature reproducibility was determined from the standard deviation of 16 consecutive measurements of the final temperature reached during the atomization step. The temperature was measured at the end of a 5-s atomization step. The power output of the furnace controller was set to give a final temperature of 2200 "C. Procedures. The detection limits were determined, after subtraction of the blank signal, by extrapolation of the calibration curve to a signal level equal to three times the standard deviation of 16 measurements of the blank noise. For elements where the contamination level in our instrument laboratory was relatively high (e.g., Pb), the detection limits were determined in two ways. First, the detection limit was obtained by exciting fluorescence from a blank solution at the analytical wavelength. This measurement gave the detection limit that was degraded due to the environmental contamination. Second, the laser was tuned off the analytical wavelength, by approximately 0.1 nm, and the blank measurements were repeated. This represented the ultimate detection limit that would be obtained if all sources of environmental contamination could be eliminated. Atomic fluorescence due to saturation broadening was not detected at the "off line" wavelength. This was checked by comparison of the "off line" signals obtained with and without atomization of the blank solution. The same signal was obtained whether or not the blank solution was present in the furnace, which indicates that fluorescence due to saturation broadening was not affecting the detection limit measurement. The linear dynamic range was determined by use of a combination of PMT voltages and neutral density filters as previously
1788
ANALYTICAL CHEMISTRY. VOL. 59, NO. 14. JULY 15. 1987
Table 111. Heating Rates and Maximum Temperatuns heating rate, ‘C 8-’ (100-2200 OC)
tube a b
mal temp, OC
860 700 720 1400
el e2
d e f
900
loo0 1400 2300
g
8td
de”, ‘C
2700 2600
10
2600 2940 2660 2700 2750
10
10 10 10 15
2900
Y
I
w
a r 3
4
cc
W
n.
6 r
0
2
6
4 TIME
8
IO
(51
Fbuv 4. Typlcai heating rate temperature profile.
3. SEM photomicrographs (1OOOX magnlflcatbn) of tube surfaces: (a1 Ringsdodl RW-0 graphke as recelved from vanda: (b) pou3 AXF-501 qaphne as recewed horn vendor. (c)Poco A X F W l graphne afler undergo ng surtace oxdation treatment -re
described 110). The solutions that were used to determine the detection limits and linear ranges were prepared daily, in a clagp 100 dean air environment. hy serial dilution of a 1 mg/mL standard. A medium of 0.04 M nitric acid in deionized water was used. RESULTS AND DISCUSSION Pyrolytic Coating. It was found that the furnace tubes fabricated from POCO graphite required a different procedure to apply a pyrolytic coating than the tubes made from the Ringsdorff graphite. The Ringsdorff graphite was relatively easy to coat and additional coating cycles led to thicker coatings that strengthened the tube. In Figure sa. a scanning electron microscope (SEM) photomicrograph (1000X magnification) of the uncoaled surfare of a Ringsdorff tube shows a high occurrence of large voids. The void fraction and average void size were determined from the photomicrograph. The void fraction was found to be 24% with an average void area of 110 wm2. When the pyrolytic coating was applied directly onto the POCO graphite, the coating easily flaked off of the tube. A SEM photomicrograph of an uncoawd POCO tube is shown in Figure 3b. The void fraction was determined to be 20% with an average void size of 5 pm2. It is thought that the small void size did not allow for the pyrolytic coating to adhere to the surface of the tube. To effectively coat the POCO graphite. it was necessary to first heat the tubes to
approximately 2000 OC in an argon environment that contained a small concentration of air, prior to the introduction of methane. The resultant pyrolytic coating adhered well to the graphite. The SEM photomicrograph (Figure 3c) of the pretreated surface shows that the surface contains many more voids of larger area than the untreated tube. The void fraction was determined to be 40% with an average void size 44 pm*. SEM photomicrographs of the pyrolytic coatings of both types of graphite appeared similar to those published by Ortner e t al. (12) (Figures 5a and 5b in ref 12). Heating Rates. Table 111 summarizes the heating rates, maximum temperature. and temperature reproducibility for each furnace tube. Figure 4 depicts a typical heating profile. The data for the Varian AAS tube (a) in Table 111show that the heating rate with the Perkin-Elmer HGA-2000 power supply was comparable to the heating rate with a Varian CRA-90 power supply (13). A maximum temperature of 2700 OC was reached. Temperature reproducibility was within 10 OC. The remaining heating rate studies compared furnace tube designs that were practical for use in LEAFS. The data for the modified Perkin-Elmer AAS tubes (cl and c2) show the effect of the laser ports. An increased heating rate was observed upon addition of the laser ports and was probably due to the loss of mass from the tube. Also, the laser ports were positioned directly between the two electrodes that supported the tube, and the current had to flow around the laser ports, which heated the ends of the tube more efficiently than the tube without laser ports. In addition to the heating rate, the maximum temperature increased with the addition of the laser ports for similar reasons. Tubes d through g were fabricated from POCO AXF-5Q1 graphite. I t was found that this material had superior me-
ANALYTICAL CHEMISTRY, VOL. 59, NO. 14, JULY 15, 1987
1787
Table IV. Graphite Properties
property density, g/cm3 resistance, pQ cm hardness, Shore
flexural strength, N/mm2 thermal conductivity, W/mK porosity, % > 1 pm maximum grain size, pm ash, ppm
Ringsdorff RW-0" 1.75 850 30 39 146 13 75 2
POCO AXF5Qlb
28002600 -
1.84 1270 70 82
V
0
a
121
3
19 25 5
,Q
I-
2200
W
a I
:2000
Ringsdorff Spectrographic Carbon Catalog; Ringsdorff-Werk GmbH: Bonn-Bad Godesberg. *POCO Graphite Catalog; POCO Grauhite: Decatur. TX. a
chanical properties in comparison to the Ringsdorff RW-0 graphite (tube b in Table 111)and also provided faster heating rates. Table IV lists the physical and electrical properties of each type. The increased mechanical durability of the POCO graphite was attributed to finer grain size and greater hardness. A greater electrode pressure could be applied, which further enhanced contact between the furnace tube and the electrodes. Furnace tubes were fabricated from POCO graphite with wall thicknesses of 1.4, 1.2, 1.0, and 0.8 mm. As shown in Table 111, the heating rate increased with decreasing wall thickness, from 900 "C s-l for a 1.4-mm wall to 2300 "C s d for a 0.8-mm wall. The temperature precision for the 1.4 mm, 1.2 mm, and 1.0 mm wall tubes was better then 10 "C, the minimum temperature difference that could be measured. A slightly worse precision was obtained for the 0.8 mm wall tube (15 "C). A comparison of the heating rates for tubes b and e (Table 111) shows that a better heating rate was obtained from the tube made from POCO graphite than from the tube made from Ringsdorff graphite when both tubes had approximately the same dimensions. This was attributed to the larger electrical resistance of the POCO graphite (Table IV) than the Ringsdorff graphite. The heating rate was degraded when the HGA-2000 power supply was set to any output value less than the maximum. To compensate, the power supply was set to the maximum temperature (and hence the fastest heating rate), but for a shorter amount of time than was required to reach that temperature. In this way, low atomization temperatures became accessible with high heating rates. Figure 5 shows the temperature calibration curves for POCO tubes (d-g), as a function of time, when the furnace power supply was set to maximum output. While this approach was used to improve the detection limit of some metals, it would not be practical for real sample matrices because a constant final temperature is required to minimize interferences in the furnace. To achieve the maximum sensitivity for analytes in real samples, a modern furnace controller capable of fast heating rates over a wide range of atomization temperatures (9, 11) would be required. The HGA-2000 was adequate to determine the detection limits and calibration ranges for ETA LEAFS. A 3.5-s atomization time at maximum power was used for cobalt. These conditions provided a heating rate of 690 "C s-l with a final temperature of 2600 "C, using tube b. For Cu and Mn, a 2-s atomization time at maximum power was used, with tube f, providing a final temperature of 2400 OC and a heating rate of 1300 "C s-l. For the remaining elements, the atomization time was 1.5 s, which provided a heating rate of 1400 "C s-l and a final temperature of 2200 "C with tube f. Tube Lifetimes. Tube lifetimes and coating thickness for some of the tubes are found in Table V. The coating thickness was determined from SEM photomicrographs. The tube
mm mm mm
1400 0
1
2
3
TIME
(5)
4
5
Figure 5. Temperature calibration curves for POCO graphite furnace tubes with controller set to maximum output. Atomization time was set to achieve the desired atomizatlon temperature.
Table V. Furnace Tube Coating Thickness and Lifetimes
wall thickness, o.d., mm
mm
graphite
10 10 10
1.25 1.25 1.25 1.25 1.2 1.0 0.8
Ringsdorff Ringsdorff Ringsdorff Ringsdorff POCO POCO POCO
10 9.6 9.6 9.6 a
coating cycles
coating thickness, mm
0
4 8
12 1 1 1
lifetime"
15 22 32 50 91
0.120 0.240 0.330 0.005 0.005
111
0.005
35
See text.
lifetimes were determined by using the normal heating program, with an atomization temperature of approximately 2000 "C for 5 s, followed by a cleanout step of approximately 2700 "C for 5 s. The tube lifetime is the number of heating programs completed before the mechanical failure of the furnace tube. The number of pyrolytic coating applications (2 min each at 2000 "C) had a significant effect on the durability of the Ringsdorff tubes. A maximum lifetime of 50 atomizations was obtained for a tube that had undergone 12 coating applications. The increase in Ringsdorff tube lifetimes as a function of coating thickness was presumably due to the added mechanical strength imparted by the pyrolytic coating. One coating application was used for the POCO tubes and the result was a very thin pyrolytic graphite layer. The POCO tubes did not require the application of a thick pyrolytic coating to obtain useful lifetimes. The lifetimes of several POCO tubes with wall thickness ranging from 0.8 to 1.2 mm are shown in Table V. The 1.0 and 1.2 mm wall tubes had similar lifetimes while the 0.8 mm tube lifetime was only 35 atomizations. This indicates that a compromise must be made between tube life and heating rate. The 1.0 mm wall POCO tube (f) was used to obtain most of the analytical data presented here because it offered both good lifetime (number of usable atomization cycles) and fast heating rate. A 1.25 mm wall Ringsdorff tube (b) was used to obtain the detection limit for Co. Imaging Considerations. The imaging of the fluorescence upon the monochromator entrance slit was found to be of critical importance. When properly focused, the furnace tube
1788
ANALYTICAL CHEMISTRY, VOL. 59, NO. 14, JULY 15, 1987
8 1300 1
0
Table VI. Comparison of Detection Limits (pg)
z w
l
this work
element Ag
“I
1 L’J
500
l o
300
1
0.5 1.0 1.5 LENS WSlTlON (rnrn)
0
2.0
Figure 8. Pb fluorescence slgnal(283/405 nm) as a function of lens position for the two-lens optical system. The lenses were placed between the furnace and monochromator as described in ref 10: (0) lens 1; (X) lens 2.
blackbody emission could be spatially resolved from the atomic fluorescence signal and was observed as a bright ring around the entrance slit. The entrance slit was adjusted to exclude as much of the furnace emission as possible. Figure 6 shows the effect of small deviations of the two imaging lenses from the optimum vertical position for the nonresonance fluorescence determination of lead. As shown, deviations as small as 0.5-1.0 mm dramatically reduced the amount of fluorescence collected, showing that care must be taken to focus the analytical region of the furnace (the region through which the laser beam passes) onto the entrance slit. In addition, the spatial resolution between the furnace blackbody emission and the fluorescence was degraded as the lenses were moved from their optimized positions. This led to a concomitant increase in the noise and degradation of the detection limit. Effect of Slit Width. The effect of slit width on the fluorescence signal, background signal, and blank noise was studied for 100 pg (10 ng/mL) of a lead standard. The signal-to-noise ratio decreased by a factor of 2 when the slits were increased from 0.5 to 2 mm. A detailed study of the noise sources for our instrument has not yet been undertaken, but for Poissons statistics, and a constant signal-to-background ratio, the signal-to-noise ratio should be independent of the slit width. The observed degradation of the signal-to-noise ratio may have been due to the nonuniform intensity distribution of the furnace blackbody radiation in the plane of the entrance slit. We cannot interpret the data further because it is debatable whether or not a factor of 2 change in the signal-to-noise ratio is significant. For Pb, as well as for the other elements reported here, the detection limits and calibration curves were determined with a 0.5-mm slit width. Detection Limits. A comparison of the detection limits obtained for this work (using a tube furnace) with the best reported ETA LEAFS values (all determined by using a cup furnace and viewing the fluorescence above the cup) is found in Table VI. In addition, ETA AAS values are included. The LEAFS detection limits obtained here are comparable to the best reported values. The detection limits for all elements studied, except T1 are within a factor of 5 better or worse than the literature values. For Tl, the transition used here (377/535 nm) is not the most sensitive one. Presumably, the more sensitive 277 f 353 nm transition was used to obtain the literature value (the transition is not mentioned in ref 5). We have obtained an ETA LEAFS detection limit of 0.006 pg in an HGA-500 furnace at the 277 f 353 nm transition. For Mn and Pb, the detection limits were determined by measuring the background first at the analytical wavelength and also at 0.1 nm away from the analytical wavelength, as explained in the Experimental Section. The degradation of the Mn detection limit, from environmental contamination,
co
cu In Mn Pb T1
ETA LEAFS on off line line 0.02 0.3 0.6 0.02 0.4 0.2 0.1
0.1 0.01
lit. ETA LEAFS
lit. ETA AAS”
O.lb 0.06b 0.2b 0.1c 0.2* 0.002b 0.003c
0.5 2 1 9 1
5 10
From ref 8. From ref 4. From ref 5. was only a factor of 4, but for P b it was a factor of 20. Very careful control of the environment around the LEAFS instrument would be necessary to eliminate contamination, in order to achieve the “off line” detection limits for real sample analyses. The literature detection limits for Ag, Co, Cu, Mn, and Pb, in Table VI, were also determined by using the “off line” method for measurement of the blank noise. It is not known how the blank noise was determined for the literature detection limit values, listed in Table VI, for In and T1. For all of the elements studied here, except Mn, the detection limits were determined by using a nonresonance (direct line) fluorescence transition. The background consisted primarily of furnace blackbody emission. The detection limits were optimized by careful alignment of the collection optics to exclude the emission from reaching the detector. For resonance transitions, care was also required to minimize stray laser radiation that reached the detector. In our instrument, the laser excitation radiation had to pass through the pole piece of an electromagnet (a distance of 84 mm) before passing through the furnace tube. (The electromagnet was not used for the work reported here but, for convenience, served as part of the enclosure for the ETA.) Misalignment of the laser beam through the pole piece, or the furnace tube, caused some of the radiation to be reflected into the collection optics, and a degradation of the detection limit resulted. The reported Mn detection limit was measured after careful alignment of the laser beam. Scatter of the laser radiation from particles within the furnace, during the atomization step, was not found to be significant for the aqueous standards used here. The detection limits reported here are all better than the detection limits for ETA AAS. The improvement in detection limit ranges from approximately a factor of 2 for Cu to a factor of 500 for Pb. Linear Dynamic Range. The linear dynamic ranges for Ag, In, Pb, and T1 were found to be 6 orders of magnitude. For Co and Cu, the linear ranges were 5.5 and 5 orders of magnitude, respectively. The linear dynamic range for Mn, using resonance fluorescence, was 4.5 orders of magnitude. Curvature of the LEAFS calibration curve due to self-absorption is generally worse for resonance fluorescence, in comparison to direct line nonresonance fluorescence, because the concentration of atoms that can reabsorb the fluorescence, in the resonance case, is usually greater than for the nonresonance case. In either case, however, the calibration curves are linear for much larger concentration ranges than for ETA AAS. For ETA AAS, the linear range is limited to 2-3 orders of magnitude.
CONCLUSIONS A comparison of the results reported here with those obtained with LEAFS in a commercial, 28 mm long, tube atomizer (11) shows that the tube length has relatively little effect on the detection limits and linear dynamic ranges for
1789
Anal. Chem. 1987, 5 9 , 1789-1794
the volatile elements or for transitions that are observed in the shorter wavelength regions of the spectrum where furnace blackbody emission intensity is low. The detection limits for Ag, Co, In, Mn, Pb, and T1 were found to be the same, within about a factor of 3, for either the short-tube or the long-tube atomizer. The nonresonance detection limit, for Cu, was 10 times worse in the long-tube atomizer relative to the short tube. This was because it is more difficult to spatially resolve the furnace emission from the fluorescence signal in the long-tube atomizer at higher atomization temperatures (11) and because the detection wavelength was 510 nm, where an intense furnace emission signal was observed. A comparison of the furnace electrode system used here with the commercial atomizer used in ref 11shows that the tube lifetime is much longer for the commercial atomizer. The electrodes in the commercial ETA contact the tube at the ends, along its strongest axis. The laboratory furnace electrodes contact the tube in the center, along a mechanically weak axis. A longer tube lifetime would be an important advantage for the routine implementation of ETA LEAFS. We have obtained detection limits in the commercial AAS atomizer with and without the L’vov platform (11).The use of the platform did not degrade the detection limits. Similar results would be expected for the laboratory furnace if it were modified to provide temperature feedback, which would allow the use of the L’vov platform in conjunction with a fast heating rate. We are currently modifying the commercial atomizer used in ref 11to accept a short furnace tube to improve the detection limits for elements that require both a high atomization temperature and a long fluorescence detection wavelength. In addition, we are modifying a commercial electromagnet to provide Zeeman background correction for ETA LEAFS in the commercial atomizer. This will allow the investigation of ETA LEAFS, using modern furnace technology, for the determination of trace metals in samples.
ACKNOWLEDGMENT We thank Fred Galli for providing the SEM photomicrographs.
Registry No. Ag, 7440-22-4; Co, 1440-48-4;Cu, 7440-50-8; In, 1440-74-6;Mn, 7439-96-5; Pb, 7439-92-1;T1,7440-28-0.
LITERATURE CITED Human, H. G. C.; Omenetto, N. N.; Cavalli, P.; Rossi, G. Spectrochim. Acta, Part B 1984, 398, 1345. Epstein, M. S.; Bayer, S.; Bradshaw, J.; Voightman, E.: Winefordner, J. 0. Spectrochim. Acta, Part B 1980, 358, 233. Horvath, J. J.; Bradshaw. J. D.; Bower, J. N.: Epstein, M. S.; Winefordner, J. D. Anal. Chem. 1981, 5 3 , 6 . Bolshov, M. A.; Zybin, A. V.; Smirenkina, I. I. Spectrochim. Acta, Part B 1981, 368, 1143. Omenetto, N. Analyticai Laser Spectroscopy: Martellucci, S., Chester, A. N., Eds.; Plenum: New York, 1985; p 139. Aggett, J.; West, T. S. Anal. Chim. Acta 1971, 5 5 , 349. Bolshov, M. A.; Zybln, A. V.; Koloshnokov, V. G.; Mayorov, I. A.; Srnlrenkina, I. I. Spectrochlm. Acta, Part E 1988, 478, 487. Slavin, W. Grephite Furnace AAS A Source Book; The Perkin-Elmer Corp.: Ridgefleld. CT, 1984; pp 13-16. Dlttrich, K.; Stark, H.J. J . Anal. Atom. Spectrom. 1988, 7 , 237. Dougherty, J. P.; Preil, F. R.; McCaffrey, J. T.; Seltzer, M. D.;Michei, R. G. Anal. Chem. 1987, 5 9 , 1112. Dougherty, J. P.; Preli, F. R.; Mlchel, R. G. J. Anal. Atom. Spectrom., in press. Ortner, H. G.; Schlemmer, G.; Welz, B. Spectrochim. Acta, Part B 1985, 408, 959. Culver, B. R.. Analytical Methods for Carbon Rod Atomizer; Varian Techtron: Springvale, Victoria, Australia, 1975.
RECEIVED for review January 5,1987. Accepted April 1,1987. This work was supported by the National Institutes of Health under Grant GM 32002. R.G.M. was supported by a Research Career Development Award from the National Institute of Environmental Health Sciences under Grant ES 00130. Some of the equipment used in this research was purchased under grants from the Research Corporation, the University of Connecticut Research Foundation, and the donors of the Petroleum Research Fund administered by the American Chemical Society. This work was presented in part at the XI11 Annual FACSS Conference, St. Louis, MO, as papers 475,476, and 477, at the 3rd Biennial National Atomic Spectroscopy Symposium in Bristol, England, July 23-25, 1986, and at the 25th Eastern Analytical Symposium, New York, NY, October 20, 1986, as paper 19. R.G.M. also presented parts of this material for the Society of Applied Spectroscopy Tour Speaker Program during April of 1986.
Determination of Picomolar Levels of Cobalt in Seawater by Flow Injection Analysis with Chemiluminescence Detection Carole M. Sakamoto-Arnold and Kenneth S. Johnson* Marine Science Institute, University of California, Santa Barbara, California 93106
Flow Injection analysls (FIA) was used to automate the detennlnatlon of cobalt In seawater by the Co-enhanced chemllumlnescent oxldatlon of galllc acid In alkaline hydrogen peroxlde. A preconcentratlonhparatlon step In the FIA manlfold with an In-line column of lmmoblllzed 8-hydroxyquinoline was lnduded to separate the Co from alkaline-earth Ions. One sample analysls takes 8 mln, including the 4-mln sample load period. The detection limit Is approxlmately 8 pM. The average standard devlatlon of replkate analyses at sea of 80 samples was f5 % The method was tested and Intercalibrated on samples collected OH the CalHornla coast.
.
The biogeochemistry of Co in the ocean is of interest due to its association with vitamin BI2(1,2)and its accumulation in marine manganese nodules (2).However, little is known 0003-2700/87/0359-1789$01.50/0
of its oceanic distribution or speciation because very low concentrations (
