Maximization of Sensitivities in Tantalum Ribbon Flameless Atomic Absorption Spectrometry Jae Y. Hwang, Charles J. Mokeler, and Paul A. Ullucci Instrumentation Laboratory Inc., 113 Hartwell Avenue, Lexington, Mass. 021 73 A flameless atomic absorption technique based on an enclosed atomization chamber with a tantalum ribbon as a heating element is critically evaluated with respect to maximizing sensitivity. Parameters involved in the study are temperature and height of measurements, chemical-thermal properties, and flow rates of the purge gases. Thirty-seven elements are presented with sensitivities and detection limits that range from 10-8 to lO-l8 gram. THE PROBLEM OF TRACE ANALYSIS has become critical during the last decade as the fields of material research, environmental science, biomedical science, and petrochemical science have developed and matured. Conventional flame atomic absorption spectroscopy (AAS) has been most useful in meeting the demands of these fields. There are, however, two main drawbacks to the wider application of conventional AAS in the field of trace analysis. They are the lack of sensitivity at the nanogram or picogram level and the volume of solution required for analysis; usually a minimum of 1 ml for each element analyzed. These two disadvantages are most serious when the amount of sample is limited and multielement analysis is required as in the case of biological and air particulate samples. To circumvent the problems associated with conventional AAS, a variety of flameless AAS techniques have been developed. Among those reported are the graphite crucible technique of L’vov ( I ) , the graphite cell technique of Massman (2), and the graphite furnace technique of Woodriff (3). West ( 4 ) has also used a similar graphite cup technique. Donega and Burgess (5) employed an atomization chamber which has a tantalum ribbon as a heating element for absorption measurements. Their technique was later modified and further pursued by Hwang et al. (6). The purpose of the present paper, however, is not to review or compare the details of the aforementioned techniques but to thoroughly investigate several parameters that affect sensitivity in a flameless AAS employed in our laboratory. GENERAL CONCEPT The peak atomic concentration, n, in atoms per cm3 of analyte in an atomization chamber is related to the analyte concentration, C, in moles per liter injected into the chamber by means of a pipet, by the following equation (7) if the analyte vaporizes and then diffuses out of the chamber slowly: n
=
6 X 1020 VCe@/V,
(1)
(1) B. V. L’vov, Spectrochim. Acta, 17,761 (1961). (2) H. Massman, ibid., 23B, 215 (1968). (3) R. Woodriff and G. Ramelow, ibid., p 665. (4) T. S. West and X. K. Williams, Aiial. Chim. Acta, 45,27 (1969). ( 5 ) H. M. Donega and T. E. Burgess, ANAL. CHEM.,42, 1521 ( 1970). (6) J. Y. Hwang, P. A. Ullucci, S . B. Smith, Jr., and A. L. Malenfant, ibid., 43, 1319 (1971). (7) J. D. Winefordner, “Atomic Absorption Spectroscopy,” R. M. Dagnall and G. F. Kirkbright, Ed., Butterworth, London, 1970, p 37. 2018
volume of analyte solution introduced, cm3 vaporization efficiency (8) = atomization efficiency (9) V , = inner volume of the atomization chamber, cm3
where V
=
e =
The vaporization efficiency, E , is the ratio of the number of all gaseous analyte species produced in the chamber to the number of analyte species injected into the chamber. This factor accounts for volatilization of the solid particles in the chamber. The atomization efficiency, p, is the ratio of the free analyte atoms in the chamber to the total analyte atoms in all gaseous forms. This factor accounts for incomplete dissociation of the analyte compound injected into the chamber, and for compound formation resulting from reactions of the heating element, analyte, and purge gas molecules. From Equation 1, it is clear that the sensitivity in the present technique is highly dependent on all the factors that affect E and p, provided that C and V, are given. For this reason, we have chosen four parameters that have significant effects on sensitivity in the present study. Among the parameters that we studied are temperature, composition of the purge gas, flow rate of the purge gas, and height of measurement. It should be stated that although temperature measurements were made on tantalum ribbons, there is non-uniform temperature, and also the sensitivity depends on the shape as well as the volume of the atom cloud. The latter two effects on the sensitivity were not elaborated in this study. EXPERIMENTAL Apparatus. For all absorption measurements, the Instrumentation Laboratory Inc. (IL) Model 253 double beam atomic absorption spectrophotometer was used. An IL Model 355 Flameless Sampler was used for atomization. Since the chamber used differs from the previous one employed for lead analysis in blood samples (6), it deserves a brief description as shown in Figure 1. Dimensions of the tantalum ribbons used in this investigation are also given. The upper ribbon with maximum volume of 100 p1 is designed specifically for elements that can be easily atomized at or below 2400 “C in argon atmosphere. The lower one with a maximum volume of 25 pl is so designed that intense localization of heat in the sample area allows the analysis of highly refractory elements such as Ti, Si, and V in hydrogen atmosphere. Reagents. STANDARDSOLUTIONS.All standards and reagents were prepared from reagent grade salts and metals using a minimum amount of acid when necessary. Distilled and deionized water was used throughout. PURGE GASES. All gases employed were the technical grade supplied by Linde Gas Company. Each purge gas system consists of a standard gas tank and a two-stage regulator which is fitted with a shutoff valve and a flow meter (8) J. D. Winefordner, C. T. Mansfield, and T. J. Vickers, ANAL. CHEM.,35, 1607 (1967). (9) L. deGalan and J. D. Winefordner, J . Quant. Spectrosc. Radiat. Transfer, 7, 251 (1967).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
GASOUTLET nose
Figure 1. Schematic diagram of the atomization chamber (No. RMA-6-SSB, F. W. Dwyer Mfg. Co., Michigan City, Ind.). A calibration curve supplied by the manufacturer was used to make corrections for the flow rates when different purge gases were required. Procedure. The atomization chamber is mounted on the premix burner system in place of the usual burner head. A narrow cardboard strip is placed and allowed to rest on the tantalum ribbon. The chamber is then slowly raised so that the tantalum ribbon starts obstructing the light beam from the hollow cathode tube. Thus, measurement is restricted to a very narrow zone immediately above the tantalum ribbon. Details of other manipulation procedures are given elsewhere (10, 11).
m i
RESULTS AND DISCUSSION
Height of Measurements. The height of measurements is defined as the distance between the top of the tantalum ribbon and the central portion of the light beam from the hollow cathode tube. Height at 0 mm is the point at which the light beam starts being obstructed by the tantalum ribbon. This can be easily observed by the deflection of the readout. Lead was chosen as an analyte, since it can be determined in a variety of purge gases. Argon, helium, nitrogen, and hydrogen were chosen as purge gases. As shown in Figure 2, the lead absorbance tends to decrease as the height of measurement increases in both hydrogen and argon gas atmospheres. This accounts for the recombination of atomic lead vapor due to the apparent temperature gradients above the glowing tantalum ribbon which was heated around 1500 "C in both cases. The profile seems to resemble the one obtained in a lean air-acetylene flame (12). It should, however, be pointed out that the present atomization chamber differs from the conventional flame atom reservoirs in a sense that the analyte atoms produced diffuse rapidly into a much colder atmosphere. This will result in recombination of analyte atoms and, in turn, loss of sensitivity. A similar finding was made (10) Operators' Manual for IL 355 Flameless Sampler, Instrumentation Laboratory Inc., Lexington, Mass., 1971. (11) J. Y . Hwang and P. A. Ullucci, A n ~ e r Lab., . 3, 41 (1971). (12) J. Y . Hwang, Can. Spectrosc., 16,43 (1971).
50
-4
I
-ARGON
\
GAS
--- H Y D R O G E N 2
GAS
6
4
H E I G H T OF M E A S U R E M E N T m m
Figure 2. Effect of height of measurementson lead sensitivity in different purge gases
by Alger et al. (13). It appears that the drastic change of sensitivity with the height in argon gas may be attributed to a steeper temperature gradient. A slower flow rate of argon, 7 l./min, compared to a flow rate of hydrogen, 20 l./min, may account for this steep temperature gradient due to a slow thermal convection process. Flow Rate of Purge Gases. Figure 3 indicates that optimal flow rate is important in maximizing sensitivity either in argon or hydrogen as a purge gas. If the flow rate is slow, the absorbance signal as a peak on the recorder tends to be broadened because of slow diffusion process. It is also possible that the low sensitivity at low flow rates is due to the incomplete removal of oxygen. If the flow rate exceeds the optimal rate, the plume of analyte atoms is flushed through the atomization chamber too rapidly for proper response, resulting in a loss of sensitivity. It is to be noted (13) D. Alger, R. G. Anderson, I. S. Maines, and T. S. West, Anal. Chim. Acta, 57, 277 (1971).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
2019
A r F L O W R A T E - l i t e r per m i n
I X W9 gm Pb
DYNAMIC
0STATIC
I 2 3 4 5 6 7 8 9 1 0
5 140"2
120-Ar
100-
?a
Figure 5. Effect of purge gases and modes of measurements on lead sensitivity
E 80:
8
16
24
32
Table I. Specific Heat and Thermal Conductivity of the Purge Gases Thermal conductivity [CalKsec) (cm2)1 Specific heat C, at 25 "C, ("C/cm) X 10-8 at 120 "C Gas Calk 0.12 45.46 Argon Helium 1.24 376.07 Hydrogen 3.41 471.11 0.25 65.71 Nitrogen
40
H 2 F L O W R A T E - l i t e r per m i n Figure 3. Effect of flow rate of purge gases on lead sensitivity
- Ar
130110-
-
so mfr
1050
30
-
IO. I
I I
800
A
I
1200
I o 0
moo
2400
aeoo
TEMP. .C
Figure 4. Effect of temperature of measurements on lead sensitivity in different purge gases
that hydrogen requires a faster flow rate than argon. This appears to be due to the differences in molecular or atomic weight of the two gases. This hypothesis is reinforced by studies of nitrogen and helium which exhibit a similar behavior in proportion to weight. Temperature. To calibrate the temperature control of the power supply used, a series of metals with known melting points was employed. As seen in Figure 4, the lead absorbance increases with temperature due to an increase in atomization. As the temperature exceeds the optimum, the sensitivity tends to decrease. This seems to be caused by mixed effects: a beneficial effect of raising temperature to assist atomization of lead; an adverse effect on the absorbance by a possible loss of sample through an abrupt spurt at high temperature prior to a proper atomization; the decrease in number of atoms per cubic centimeter because of 2020
thermal expansion at constant pressure; and a possible line broadening which decreases absorbance. A similar finding on manganese was made by Ebdon et ai. (14) in a flameless AAS and Kirkbright (15) believes that the adverse effect is due to expansion and added turbulence of the inert gas at higher temperature in their open system. Purge Gases and Modes of Measurements. Four different purge gases-argon, hydrogen, helium, and nitrogen-were tried in this study in two different modes of measurement. In the dynamic mode, measurements were made with a gas continuously purged through the chamber. In the static mode, the flow of purge gas was interrupted during absorption measurement. The latter mode was chosen to eliminate the cooling and turbulence effects of the purge gas during measurement. The results are summarized in Figure 5. Under optimum conditions, the maximum sensitivity for lead is achieved in an argon atmosphere using the dynamic mode of measurement. It is noteworthy that argon gas has the lowest specific heat and thermal conductivity among the purge gases studied (Table I). It seems that the thermal properties of the purge gas play a significant role in achieving maximum sensitivity. With the exception of nitrogen, each purge gas gives a better sensitivity in the dynamic mode of measurement compared to the static. Although the cooling effect by the purge gas of the tantalum ribbons is eliminated in the static mode of measurement, reduction of sensitivity was observed, possibly due to the overwhelming adverse effect of slow diffusion process as mentioned in the flow rate study. Choice of a Purge Gas. For an inert gas to be effective, it must have low thermal conductivity, low specific heat, and high atomic weight. The combination of these factors will lessen the cooling of the tantalum ribbon. However, some elements are better atomized in a hydrogen atmosphere. The basic mechanism that provides better results with hydrogen is its reducing nature. (14) L. Ebdon, G. F. Kirkbright, and T. S . West, ANAL.CHEM., Acta, 58, 39 (1972): (1 5) G. F. Kirkbright, Chemistry Department, Imperial College,
London S.W.7.U.K., private communication, Feb. 1972.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 12, OCTOBER 1972
Table LI. Sensitivities and Detection Limits slit Analyze width, P.M. Purge Temp, OC setting m voltageb gas 5 160 620 Ar 1200 10 2000 160 620 HZ 10 Ar 2400 160 7 ~ 1 ) 10 2400 160 620 Ar 10 2000 160 700 HP 2400 Ar 10 160 620 2200 9 160 700 Ar 2200 Ar 9 160 620 3 700 320 700 Ar 10 160 700 2000 HP 2400 10 Ar 160 520 1800 8 320 1000(2) Ar 160 530 1800 Ar 8 2000 10 320 530 Hz 2400 Ar IO 80 700 1800 8 160 700 Ar 4 900 320 530 Ar 320 620 Ar 8 1800 8 1800 Ar 320 1000(2) 7 Ar 2000 160 620 6 1400 80 530 Ar 8 1800 160 620 Ar 7 1600 Ar 160 530 10 2400 Ar 40 700 6 1400 Ar 160 700 2400 10 160 700 Ar 9 2200 Ar 320 1000(2) 2400 10 160 700(1) Ar 2000 10 320 700(1) HZ 10 2300 320 700 HP 2400 10 160 700 Ar 9 Ar 2200 160 700 10 2000 320 700 HP 10 2300 320 700 Hz 6 1400 160 620 Ar 2300 10 160 700 Ha 5 1200 160 620 Ar
Current, nm mA 328.1 9 Ag 10 309.3 AI 7 193.7 As 242.8 AU 9 553.6 Ba 10 Be 234.9 9 223.1 Bi 8 422.7 Ca 10 4 228.8 Cd 6 240.7 co 10 357.9 Cr 852.1 cs 10 324.8 9 cu 8 Eu 459.4 Fe 248.3 8 287.4 Ga 8 253. I 5 Hg In 303.9 8 766.5 K 8 10 670.8 Li 7 285.2 Mg 7 279.5 Mn 589.0 10 Na Ni 232.0 8 217.0 7 Pb 247.6 12 Pd 780.0 10 Rb 217.6 10 Sb 196.0 Se 8 Sia 251.6 9 Sn 286.3 8 460.7 Sr 7 214.3 Te 8 364.3 Tia 8 276.8 17 T1 318.4 10 Va 213.9 6 Zn a Based on modified Ta ribbon. I, An R372 tube was used unless stated otherwise. (1) R106. (2) R406.
Element
Absolute Sensitivities. Since the technique dictates a sequential process of evaporation-pyrolysis-atomization for absorption measurement, absolute sensitivies are used. Absolute sensitivity is defined as the minimum weight of analyte to give 1 absorption or 4.4 milliabsorbance. The instrumental and analytical conditions employed in attaining the sensitivities are summarized in Table 11. Flow rates of argon and hydrogen gases were 7 l./min and 20 l./min, respectively. Height of the measurements was 0 mm. Detection limits reported here are achieved by 2 standard deviations of the noise levels based on 10 consecutive readings. In conclusion, it is important to optimize the temperature and height of measurement and the chemical composition, thermal characteristics, and flow rate of the purge gas to achieve maximum sensitivity in the flameless technique reported. The technique is easy to use, sensitive, and has good precision (2-4W re1 std dev) at the nanogram level. It will be
Sensitiv., g 2 x 10-11 9 x 10-10 6 x 10-10 1 x 10-10 4 x 10-11 7 x 10-12 1 x 10-10 3 x 10-12 7 x 10-12 9 x 10-10 4 x 10-11 7 x 10-11 2 x 10-11 7 x lo-" 2 x 10-10 3 x 10-10 8x 3 x 10-10 3 x 10-12 7 x 10-12 2 x 10-18 2 x 10-11 3 x 10-12 4 x 10-10 3 x lo-" 2 x 10-9 2 x lo-" 8x 7 x 10-10 4 x 10-8 4 x 10-10 4 x 10-11 6 x 10-10 4 x 10-9 2 x 10-10 1 x 10-0 4 x 10-12
D.L., g x 10-12 x 10-10 3 x 10-10 2 x 10-11 1 x 10-11 2 x 10-12 4 x 10-11 1 x 10-12 2 x 10-12 3 x 10-10 2 x 10-11 7 x 10-11 1 x 10-11 3 x 10-11 1 x 10-10 1 x 10-10 2 x 10-10 1 x 10-10 1 x 10-11 2 x 10-12 1 x 10-13 7 x 10-12 1 x 10-12 2 x 10-10 1 x 10-11 7 x 10-10 1 x 10-11 2 x 10-10 7 x 10-10 1 x 10-8 1 x 10-10 1 x 10-11 3 x 10-10 1 x 10-9 7 x 10-11 4 x 10-10 1 x 10-12 4 3
very useful in the analysis of the trace and ultratrace elements in biological samples and air particulate samples since the technique provides excellent sensitivities and consumes only a few pl of sample solutions. Applications of the technique to directly determine Cr, Ni, Mn, Be, and Pb in human sera and blood samples are available. ACKNOWLEDGMENT
The authors wish to express their thanks to S. B. Smith, Jr., and G . P. Thomas for their time and effort in reading the manuscript and making suggestions for it. RECEIVED for review May 11, 1972. Accepted June 30, 1972. Part of this paper was presented at the 23rd Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1972, Cleveland, Ohio.
ANALYTICAL CHEMISTRY, VOL. 44,
NO. 12, OCTOBER 1972
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