1264
Anal. Chem. 1966, 58, 1264-1265
Electrothermal Vaporization for Sample Introduction into a Three-Electrode Direct Current Argon Plasma William G . Elliott, H e n r y k Matusiewicz,' and Ramon M. Barnes* Department of Chemistry, University of Massachusetts, GRC Towers, Amherst, Massachusetts 01003-0035 The direct current plasma (DCP) configuration developed by Elliott (1) has been shown to be an excellent source for analytical atomic emission spectrometry (2). The application of DCP excitation for atomic emission (AES) measurements of trace metals has, during the past 10 years, enhanced the versitility of AES as an analytical technique ( 3 , 4 ) . In normal use, sample is pumped continuously into a ceramic cross-flow nebulizer, and the resulting aerosol is transported to the excitation region of the plasma. This approach is satisfactory for routine analyses, but under special circumstances benefits may be obtained from the use of electrothermal vaporization (ETV) sample introduction (5). In this study we explored the adaptation of a commercial electrothermal vaporization system for sample introduction into the three-electrode DCP. The ETV technique reduces the sample volume required for analysis and is especially convenient for volumes in the 5-20 pL range. Recent studies of aerosol approaching the plasma indicate that elimination of the solvent by ETV may permit more efficient introduction of the sample directly into the excitation region of the DCP than with conventional nebulization (6). Three elements chosen for this feasibility investigation (aluminum, copper, and manganese) cover a range of vaporization and excitation conditions and illustrate problems expected in analytical applications. For each element the effects of carrier gas flow rate and observation region were investigated using 1pg/mL solutions. The conditions providing the best signal-to-background ratio were employed to measure the calibration function and estimate the detection limit. EXPERIMENTAL SECTION Instrumentation. The equipment consisted of a three-electrode DCP (Spectrajet,SmithKline Beckman, Inc.) with its power supplies,a 0.75-m Czerney-Turner monochromator (SPEX Model 1700) having a reciprocal dispersion of 1nm/mm, a programmable picoameter, and a PDP 11/03 computer for data acquisition and processing. The DCP was imaged at unity magnification onto an aperture 0.2 mm high which in turn was imaged onto the monochromator entrance slit (0.1 mm wide). The size of the observation region was therefore comparable to that normally used. A sample introduction adapter was constructed as shown in Figure 1 from 6-mm i.d. Pyrex tubing with a 4.4 mm 0.d. X 2.4 mm i.d. glass tube mounted coaxially to carry the sample vapor. Auxiliary argon fed into a side arm on the outer Pyrex tube provided a coaxial argon sheath. A modified controlled-temperature graphite furnace (Instrumentation Laboratory Model IL655) was used to generate the sample vapor (7,8). Samples were introduced onto a pyrolytically coated microboat in a round graphite tube (8). Drying and ashing cycles were employed as in atomic absorption spectrometry prior to vaporizing the sample. During the high-temperature vaporization cycle, the sample vapor was transported by argon carrier gas through a 6 mm i.d. glass tube from the ETV chamber to the DCP sample introduction adapter approximately 0.5 m away. The monochromator photomultiplier (RCA 1P28A) current corresponding to the emission intensity was sampled at 0.1-s intervals by an analog-to-digitalconverter (Analog Devices Model 1251) and the data were stored on the computer floppy disk for subsequent processing and plotting. Reagents. Analytical A.C.S. reagent grade (Fisher Scientific) chemicals and deionized distilled water were used. Stock solutions Present address: Department of Analytical Chemistry, Technical University of Poznad, 60-965 Poznad, Poland.
Table I. Furnace Temperature Program cycle
stage
temperature, " C
time, s
drying
1 2 3
ambient-80 80-115 115-140
20 10 20
ashing
4
5
140-400 400
10 0
6
400-2500
5
vaaorization ~~
~~
~
were prepared at 1000 pg/mL, and the measurement solutions were freshly prepared by dilution with deionized water. Measurement Procedure. Conditions for the ETV were set as determined previously (7) and are given in Table I. In addition, the outer argon flow rate was 5.0 L/min, operation mode was set for AUTO, and the temperature feedback was ON. The DCP was ignited using the electrode currents and gas flows normally employed (7.5 A, 1L/min). The argon sheath flow was set at 2 L/min and the ETV carrier gas flow set to its experimental value. The DCP was then positioned so that the experimental observation region was imaged onto the spectrometer aperture. For each experimental condition a 5-pL aliquot of solution was deposited manually with a 10-pL micropipet (Eppendorf) onto the pyrolytically coated graphite microboat as described previously (8) and the ETV sequence initiated. The desolvated sample vapor was carried into the DCP, and the resulting transient emission signal was measured. This measurement was repeated with 1pg/mL Al, Cu, and Mn solutions for carrier gas flow rates ranging from 0.5 L/min to 3.0 L/min and observation regions from 2 mm below to 2 mm above the normal observation region. The conditions that provided the best signal-to-background ratio were then used for measurements of blank solutions and concentrations of 0.1,0.3,1.0,3.0, and 10 rg/mL. The monochromator wavelength setting for each element was adjusted using an appropriate hollow cathode lamp located on the optical axis. Spectral lines selected were A1 I 396.15 nm, Cu 1324.75 nm, and Mn I1 257.61 nm. RESULTS AND DISCUSSION For the three elements tested, the best signal-to-background ratio was found to coincide with the observation region normally used with aerosol sample introduction, i.e., approximately 2 mm below the intersection of the anode plasma columns (Figure 2). Although the optimum carrier gas flow rate was dependent upon the element, compromise conditions can be used for multielement analysis. Both manganese and copper gave good results at 2.2 L/min, but A1 showed a temporal double peak (Figure 3). Increasing the flow rate to 2.8 L/min gave a more intense single peak for Al, but caused severe instability of the plasma. Further investigation of vaporization chamber design may reduce this effect. The Mn signal as a function of time for vaporization of 0.3, 1.0, and 3.0 pg/mL solutions is presented in Figure 4. Detection limits were estimated by calculating the standard deviation for the signal after the ETV transient settled for 0.1 and 0.3 pg/mL. Measurements of blanks gave similar results except for aluminum, which showed memory effects. The standard deviations for all three elements were approximately 10 ng/mL, corresponding to detection limits of approximately 30 ng/mL. These detection limits would be expected to improve substantially when a monochromator of
0003-2700/86/0358-1264$0 1.50/0 0 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986 f.O
4.4 mm 0.d. x 2.4 m m i.d.
0.9
.
-
0.7
.
0.6
-
0.5
-
% 0.4 ! 4
-
2
0.3
-
8
0.2
-
0. 1
-
?2
Coolant Tube 9 m m 0.d. x 6 m m i.d.
*
~
2l- 0.8
*
-I
ant
n
I
0
Ground Joint
1265
I
I
I
I
I
I
I
43 Argon Carrier 8. Sample Vapor Figure 1. Sample introduction adapter. The outer tube provides in coolant sheath and the inner tube directs the sample vapor into the excitation region.
0.8
m J
0.7
.
0.6
-
0.5
-
0.4
-
0. l m9 O
E
2
n
Ir
-r
?2
LI
2
0.3
-
8
0.2
-
0. f
{
-I
0
1.0
-,
0. 9
2-
0. 8
4
2
-
!-
0.7 0. 6
2
Lu )r
0.5 0.4
+-I
0.3 -J
Lu 0.2 &
0
1
2
3
4
5
T I M E (S) Flgure 2. Time-intensity profiles for 1 pg/mL Mn viewed at (A) 1 mm, (6)2 mm, and (C) 3 mm below the intersection of the plasma anode columns. The carrier flow rate is 2 L/min.
greater spectral resolution is employed, because the limiting noise was due to background fluctuation. The echelle spectrometer normally used for analytical purposes with this plasma provides approximately 10 times more resolution with the same size apertures; comparable improvement in the detection limits would be anticipated. They would then be similar to values reported for aerosol sample introduction (4). Analytical performance of the technique appears to be limited by the electrothermal vaporization chamber and sample deposition method. In general, a variety of matrix effects have been reported and the use of standard additions or matrix matching is recommended (5). Reproducibility for a given sample is approximately 6% (5). This feasibility study demonstrates the capability for electrothermal sample introduction with the three-electrode DCP. Although the exploratory detection limits are not as low as those obtained by furnace atomic absorption or ETVICP (5, 7), the approach does offer the opportunity for multielement analysis and provides a convenient method for microsample analysis with the DCP.
RECEIVED for review June 17, 1985. Accepted October 24, 1985. This investigation was supported in part by Department of Energy Contract DE-AC02-77EV-0432 and the ICP Information Newsletter.
