Anal. Chem. 1987, 59, 539-540
with the calculated ratios of 100:9937:6. It should be noted that subsequent cooling of the Bz03 sample to ambient temperature is accompanied by the reformation of protonated boron oxides. This process is observed while the sample surface is continuously bombarded by the primary Xe beam during cooling. If, after the sample is cooled, the primary beam is turned off for a few minutes and then turned on, a sharp increase (about 200%) in the relative abundances of protonated boron oxides is observed. The enhanced abundances will decrease within a few seconds until equilibrium abundance ratios (Figure 2A) are reestablished. The experiment demonstrates that adsorbed HzO and B,03-Hz0 reaction products are continuously and rapidly renewed on the surface of the sample. The high-temperature FAB sample holder is useful in identifying ions of hygroscopic samples by their isotopic abundance ratios by eliminating interfering protonated or hydrated species that would be detected at the same nominal mass-to-charge ratios. It is of particular utility in identifying high-mass, low-abundance ions that cannot be separated from
539
interfering species by high-resolution techniques. Equally important, the high-temperature capability enables the use of tandem mass spectrometry (MS/MS) techniques such as collision-induced dissociation to obtain structural information about ions, such as [B&]'+ ( m / z 140), which cannot be separated from interfering ions ( [H11B310B06]+,m / z 140) by the low-resolution, parent-ion selection inherent in MS/MS. While the example presented here involved the use of a vitreous material, the technique should be useful with any sample that does not undergo thermal decomposition and maintains its adhesive properties above its melting point.
LITERATURE CITED (1) Barber, M.; Bordoli, R. S.; Elliot. G. J.; Sedgewlck, R. D.; Tyler, A. N. Anal. Chem. 1982, 5 4 , 645A-657A. (2) Golubkov, V. V.; Tltol, A. P.; Vasilevskaya, T. N.; Poral-Koshlts, E. A. Fiz. Khlm. Stekla 1977, 3 , 312-315. (3) Mackenzie, J. D. J. Phys. Chem. 1959, 6 3 , 1875-1070.
RECEIVED for review August 20, 1986. Accepted October 10, 1986.
Electrothermal Atomic Absorptlon Spectrometry wlth Improved Tungsten Tube Atomizer Kiyohisa Ohta* Department of Chemistry, Mie University, Mie, Tsu, 514,Japan Syang Yang Su Department of Chemistry, Virginia Commonwealth University, 1001 West Main Street, Richmond, Virginia 23284 In recent years, a metal microtube atomizer has been demonstrated to be an excellent atomization device for atomic absorption spectrometry (1-5). As compared with flame or graphite tube atomization systems, the significant advantages for atomic absorption spectrometry with a metal tube atomizer are higher sensitivity, no carbide formation, lower background emission from atomizer surface, no absorption arising from carbon particles, long lifetime, better reproducibility, and lower costs for instrumentation and maintenance. Chakrabarti et al. (7,8)reported that peak absorbances and integrated absorbances of relatively involatile elements increased exponentially with increasing heating rates and that relatively volatile elements increased in the peak absorbances with heating rates. Therefore, to improve sensitivity and detection limits further, it is necessary to heat an atomizer to its maximum temperature a t an increased rate and/or develop a new adequate tube atomizer having a higher atomization efficiency and a longer residence time for neutral atoms. In this study, we report a novel metal tube atomizer made from small inner diameter (1.5 mm) tungsten tubing. The atomizer is evaluated for various elements.
EXPERIMENTAL SECTION Apparatus. The apparatus, including a microcomputer used
in the present work, has been described in a previous publication (2). The absorption signal from an amplifier and the output signal from a photodiode measuring atomizer temperature were monitored on a memoriscope (Iwatsu MS-5021) and were simultaneously fed into and processed by a microcomputer (Sord M223). The temperature of the atomizer was calibrated against the photodiode voltage by using an optical pyrometer (Chino Works). A tungsten tube atomizer (40 mm long, 1.5 mm i.d., and 0.05 mm wall) made from a high-purity tungsten foil (99.95% Goodfellow
Metals, Ltd.) is shown in Figure 1. Two legs made from molybdenum sheets (0.3 mm thickness) support the tube at both ends. A 0.3 mm diameter hole was drilled at the midpoint of the tube to enable a sample to be injected into the atomizer. When an absorption signal was measured, the hole was closed with a movable cover to confine the analyte vapor. There were two pinhole apertures in front of and behind the atomizer to provide a narrow beam of light about 1.0 mm in diameter and to remove background emission from the atomizer surface. Hollow-cathode lamps and electrodeless discharge lamps (Hamamatsu photonics KK.) were used as light sources. The atomic resonance lines for measuring absorption signal are listed in Table I. The atmosphere in the absorption chamber was purged by a mixture of argon and hydrogen gases, the ratio of the flow rates of which varied from one element to another as shown in Table I. The addition of hydrogen to the argon purge gas serves to protect the atomizer from oxidation by traces of oxygen in argon and also gives a favorable effect on the electrothermal atomization of some elements (9). The best flow ratio of purge gas for each element was chosen. Reagents. Stock solutions (1mg/mL) were prepared as nitrate salts, except for antimony (tartarate), arsenic (trioxide), and selenium (oxide), in 0.1-6 N acid. The solutions for measuring atomic absorption were diluted from the stock solutions with distilled-deionized water just before use. All chemicals used were of analytical grade purity. Procedure. A total of 50 NLof sample solution was pipetted, 10 p L at a time, into the tungsten tube atomizer. Each of the five 10-rL subportions was individually dried at 350 K for 10 s. After the last subportion was dried, the sample introduction port was covered, and then the temperature of the atomizer rapidly raised to 2570 K under the optimized conditions.
RESULTS AND DISCUSSION Compared to a molybdenum microtube atomizer previously described (4),the improved tungsten tube atomizer described here was assembled from a large piece of tubing (40 mm as
0003-2700/S7/0359-0539$01.50/00 1987 American Chemical Society
540
ANALYTICAL CHEMISTRY, VOL. 59, NO. 3, FEBRUARY 1, 1987
Table I. Sensitivities and Detection Limits with a Improved Tungsten Tube Atomizer for Various Elements
Purge gas, mL/min Ar + element
line, nm
Ag A1 As Bi Cd Cr
328.1 309.3 193.7 233.1 228.8 357.9 324.8 248.6 285.2 279.5 232.0 217.0 217.6 196.1 224.6 460.7 214.3
cu
Fe Mg Mn Ni Pb Sb Se
Sn Sr Te Reference 6.
Reference 9.
HZ
480
400 480
400 480
absolute sensitivity (pg)/l% absorption metal carbon microtube present work atomizeP atomizerb
+ 20
+ 100 + 20
+ 100 + 20
0.067 0.11 2.9 0.40 0.0048 0.96 0.010 0.14
0.010 0.021 0.13 0.10 3.7 3.8 1.1 0.0050 0.84
0.1 1 4 0.08 2 0.6 1 0.04 0.2 9 2 5 9 2 1 1
0.14 0.74 2.8
1.3 0.014 3.4 0.075 2.5 1.1 0.68 2.1 2.8 0.15 0.5
detection limit, pg present work 0.051 0.099 2.4 0.72 0.0044 0.57 0.0060 0.086 0.0087 0.024 0.12 0.36 12 2.1
1.7 0.0035 2.3
graphite atomizerC 0.08 0.8
6 8 0.02 0.3 0.5 0.8 0.02 0.05 5 0.6 6 5 3 0.8 3
metal microtube atomizerb 1 1 5 0.6 0.05 2
0.01 2 0.08 2
0.8 2 2 4 0.3 1
Reference 10.
Flguro 1. Improved tungsten tube atomlzer
opposed to 17-25 mm) to lengthen the absorption path and, with taller supports (15 mm as opposed to 3-7 mm), to provide better, more uniform thermal insulation for the absorption chamber. In addition, the atomizer was heated rapidly (8 K/ms as opposed to 0.5-4 K/ms) and has a relatively small inner diameter, as well as a movable cover for the sample injection port. Therefore, it is predicted that a longer residence time for analyte atoms will result in a denser cloud of atoms. The absolute sensitivities and detection limits investigated with the tube under optimum experimental conditions are listed in Table I and compared with those of carbon atomizers (6, IO) and a metal microtube atomizer (9). The absolute sensitivity was defined as the mass of element giving an absorption of 1% (0.004absorbance unit). The detection l i i i t was the weight of analyte which gave an atomic absorption signal equal to three times the standard deviation of an background. The tabulated detection limits with a graphite atomizer, summarized by Parson et al. (IO), were recalculated for a 5O-pL injection of sample solution. The sensitivities and detection limits obtained for the tungsten tube atomizer are better than those reported for the conventional atomizer (9, IO),except for elements in groups Va and VIa (groups 5 and 6 in 1985 notation), such as selenium, antimony, and tellurium. This presumably proves that the density of atoms formed in
the improved atomizer appears to be several times larger than in the conventional atomizers in most analyte elements. The tungsten atomizer is not permeable by sample solution, such that good reproducibility (
