Atomization in Graphite-Furnace Atomic Absorption Spectrometry-Peak-Height Method vs. Integration Method of Measuring Absorbance: Heated Graphite Atomizer 2 100 R. E. Sturgeon, C. L. Chakrabarti,’ and P. C. Bertels Department of Chemistry, Carleton University, Ottawa, Ontario, K 1s 566,Canada
The signal integration technique developed and reported earlier has been used for measuring atomic absorption signals generated by the Heated Graphite Atomizer 2100. Cd, Zn, AI, Sn, Cu, Mol and V have been selected for this study. In theory, the lntegratlon method of measuring absorbance is superior to the conventlonal peak-helght as the measure of absorbance. In practice, integration does offer some advantages over the peak-helght method of measurement; absolute sensitivity is increased by a factor of 2- to 8-fold and the linear range of the working curves is Increased by a factor of up to 2. This study shows the effect of the better cell geometry of the HGA 2100 (as opposed to the Carbon Rod Atomizer 63) on the Integrated absorbance signals. Modifications to the Heated Graphite Atomizer 2100 which would improve the atomization conditions beneficial to the integration method of measuring are suggested.
In a previous paper ( I ) , a preliminary investigation into the performance and potential of an electronic integration method of measuring absorbance signals in graphite-furnace atomic absorption spectrometry was described. The measurements were carried out with a modified Carbon Rod Atomizer 63 (CRA). In that study, it was observed that the full potential of the integration method of measuring signals could not be realized because the cell geometry of the CRA 63 was not favorablr. for measuring integrated absorbance. L’vov has shown ( 2 )that the integrated signal Q N , is proportional to the product of the number of atoms of an element in the sample, No, and the average residence time of the atoms, 72, Le., Q N = N072. If the sample is pulse-vaporized, the peak signal N p e & = No. The ratio of the integrated to peak sensitivity is therefore governed by the magnitude of the residence time, 72. The open geometry of the CRA 63 cell produced large diffusional losses from the cell and residence times much less than one second for Zn, Cd, Al, Sn, Cu and approximately one second for Mo and V. This resulted in the sensitivities obtained by integrating over the respective absorption pulses being generally less than those obtained by the corresponding peak method of measurement. Despite the limitations imposed by the geometry of the CRA 63 cell, similar detection limits were obtained for each element by either method of measurement, the precision of both methods was comparable and the integrated absorbance signals from Cu, Cd, and Zn were independent of the atomization voltage, Le., independent of the kinetics of atomization. This paper critically examines the integrated absorbance signals produced by a Perkin-Elmer Heated Graphite Atomizer (HGA) model 2100. For comparison of the results obtained with the CRA 63 ( I ) , the same elements, Cu, Cd, Zn, Al, Sn, Mo and V have been chosen for this study. 1
Author to whom all correspondence should be addressed.
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ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
EXPERIMENTAL Apparatus. The integration control unit, source, and detectorrecorder system have been described in a previous paper ( 1 ) . For the present study, the working head and power supply of the CRA 63 were replaced with a Perkin-Elmer HGA 2100 and its atomization control unit. The base of the HGA 2100 furnace was modified so that it could be fitted to the optical rail of the Varian Techtron atomic absorption spectrophotometer, model AA-5. The PerkinElmer atomization control unit was used as supplied, except for slight changes (described below) t o the purge gas control. In the gas interrupt mode, the internal sheath gas (N2 or Ar) is cut off prior to the atomization stage. The electronic valves are designed such that the internal purge gas flow resumes after a period of about 1 2 seconds, irrespective of whether or not the atomization cycle is complete. As it was necessary to atomize for periods greater than this, the internal purge gas flow was regulated manually by a 3-way stopcock. In addition, a trigger pulse was extracted from the atomization control unit a t the onset of the atomize stage t o initiate the delay and integration timing cycles of the integration control unit. All temperatures were measured with an automatic optical pyrometer ( I ) (Ircon Inc., Niles, IL, series 1100). The HGA 2100 furnace itself is a semi-enclosed atomizer consisting of a graphite tube (coated with pyrolytic graphite), 28 mm long with an internal diameter of 6 mm, housed between two large graphite cones which provide the electrical contact between the tube and the housing. Removable quartz lenses fitted to the housing, in the form of windows, with O-ring seals, cap both ends of the atomizer tube. Ports in the housing allow for an independent system of external sheath and internal purge gas circulation. Reagents. All chemicals used were of certified ACS grade or of the highest purity commercially available. The stock solutions of metal standards were prepared as follows for each metal separately and contained 1000 pg/ml of that metal only. Solutions of Cu, Sn, Al, and Zn were prepared from the metals, Cd from cadmium carbonate, and Mo and V from molybdenum trioxide and vanadium pentoxide, respectively. The above metals or their compounds were dissolved in pure acids or bases where required and diluted with ultrapure water, obtained direct from a Milli-Q water system (Millipore Corporation). All test solutions were prepared immediately prior to their use by diluting with ultrapure water. The special laboratory where this study was done was clean enough to ensure very small and reproducible blanks. Gases. The argon and nitrogen gases used to sheath the atomizer were of 99.95% purity (Roncar Oxygen Co.). Procedure. Sample volumes of 5 p1 were introduced into the HGA 2100 furnace with an Eppendorf syringe fitted with disposable plastic tips. It was possible to employ larger sample volumes (100 pl) to reduce potential delivery errors, but this was not attempted in order to reduce dilution errors in the sample preparation, and to avoid long drying periods of the furnace and the possibility of fogging the end windows because of condensation. Nitrogen was used as the externaliinternal sheath/purge gas for Cd, Cu, Sn, Zn, Mo, and V, whereas Ar was used throughout the study of A1 as nitrogen gas formed aluminum nitride, resulting in a loss of sensitivity. N2 flow was maintained at 100 ml/min and Ar at 80 ml/min. Since the internal gas interrupt mode was used a t all times, the gas flow rate was not critical, the only requirement being that it must be sufficient to prevent oxidation of the graphite tube. The lamp currents, wavelengths, and spectral bandwidths used in this study, as well as the operation of the integration control unit, have been fully described in a previous paper ( 1 ) . Blanks were run throughout this study and their values were subtracted from the gross values to obtain the net values, which were reported.
_Table I . Effect of Atomization Temperature on P e a k and Integrated Absorbance XIrio,rbance
0.165 0.105 2700 0.188 0.115 2500 0.250 0.140 2100 0.198 0.225 1800 900 0.116 0.375 2700 0.257 0.145 z11 2 500 0.298 0.2 14 0.273 0.263 1900 0.141 0.332 1300 0.135 0.273 1100 cu 2700 0.044 0.086 2600 0.043 0,090 2100 0.028 0,084 AI 2700 0.059 0.109 2500 0.049 0.095 2200 0.010 0.046 si1 2700 0.156 0.24 1 2500 0.107 0.221 2200 0.062 0.202 f2 The temperature setting has been shown in the centigrade scale in accordance with that on the meter scale of the atomization control unit.
Cd
RESULTS AND DISCUSSION Optimization of Atomization Conditions. All peak and integration measurements were obtained using the internal gas in the interrupt mode. The cessation of internal purge gas in the HGA 2100 prior to and during the atomization cycle greatly increased the height and breadth of the absorbance signals. This is illustrated in Figure 1 for the atomization of 2.0 ng of Cu. The above increase can be explained on the basis that the interruption of the internal purge gas during the atomization cycle increases the residence time by preventing the sweeping out of the analyte atoms from the analysis volume. The atomization temperature (as indicated on the meter on the front panel of the HGA 2100 atomization control unit) and time were individually adjusted to give maximum values for both the peak and the integrated absorbance. The effect of the atomization temperature on the peak and the integrated absorbance is presented in Table I. The atomization temperature shown in Table I has not been measured and is merely the reading of the meter on the atomization control unit. Higher temperature, in reality, means applying higher atomization voltages to the furnace. Since Mo and V required nearly the maximum setting available on the meter for an observable absorption pulse, their absorption pulse characteristics could not be studied as a function of temperature. Therefore, Mo and V have been excluded from Table I. For each element, the atomization was carried out over a time period which extended beyond the duration of the absorbance signal. Increases in the atomization temperature setting result in higher voltages being applied to the atomizer, producing faster rates of rise of temperature of the furnace. This observation was confirmed by studying the temperature-time characteristics of the furnace with the automatic optical pyrometer. Of the elements studied, Cu was the only one for which the integrated absorbance was constant with respect to variation of atomization temperature, until a limit was
Figure 1. Oscilloscopic trace showing absorbance by 2.0 X
lo-'
g
copper Vertical scale: absorbance, O.l/scale unit. Horizmtal scale: sweep speed. 500 mseclscale unit. (A) Internal purge gas interrupt. (5)Internal purge gas on
reached, a t low voltage settings, where incomplete atomization occurred. For Zn and Cd, the integrated absorbance signal showed a sharp increase as the atomization temperature was progressively lowered, Because of the high temperature required for complete atomization of Mo and V, no lowering in the atomization temperature was possible for these eleqents. A1 and Sn gave a constant integrated absorbance signal over the temperature range 2670-3070 K and 2770-3070 K, respectively, but at lower temperatures a rapid decrease was observed, presumably because of compound formation ( I ) . Optimum atomization conditions and pulse characterization times corresponding to the optimum atomization conditions are presented in Table 11. Pulse characterization times have been explained in a previous paper (11. Of the elements studied, the absorbance values by the peak method varied with temperature in a complex fashion. It is interesting to note that, unlike the CRA 63 for Cd and Zn, the HGA 2100 does not give the highest absorbance by the peak method when the fastest rate of rise of temperature of the furnace is used. This difference between the CRA 63 and the HGA 2100 on the one hand, and between the elemefits of relatively high volatility (Cd and Zn) and the elements of relatively low volatility (Cu, Al, Sn, Mo, V) on the other hand, can be explained on the basis of the kinetics of formation and introduction of the analyte atoms into the analysis (observation) volume, and their dissipation-the term atomization process will be used here to include both these processes. The atomization process may be considered to consist of consecutive chemical reactions (3): kl
A-B-C
k2
where state A represents the number of atoms nl, not, yet in the optical path; state B, the number of observable atoms )22 in the analysis volume; and state C, the number of atoms n3 dissipated. If no represents the total number of the analyte atoms present in the sample, then n , = nl + np t 723. The constants k l and kp represent rate constants for first-order kinetics for the transformation to B and C, respectively. However, they are not rigorous constants as isothermal conditions do not exist in the furnace over the duration of the absorption pulse. The k l term relates to the rate of introduction of analyte atoms into the analysis volume, and the k2 term to loss of the analyte atoms (by diffusion and convection). A necessary condition for the production of an absorption pulse is that k i > kp. Second, for the absorption pulse to be sharp, the condition k l >> hp must ANALYTICALCHEMISTRY, VOL. 47, NO. 8, JULY 1975
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Flgure 2.
Oscilloscopic trace showing absorbance by 2.5 X lo-” g
Vertical scale: absorbance, O.l/scale unit. Horizontal scale: sweep speed,
200 msec/scale unit
Figure 3.
Sna
pb
cadmium
Oscilloscopic trace showing absorbance by 2.5 X IO-” g
zinc
2970 2970 2970
10
825
1600
8500
7670
1300 2600 118000 116700 1500 2800 >18000 >16500 For both the peak and the integration methods. T e n d and i d i s u i a v times could not be adequately measured for Mo and V since atomization was automatically and prematurely cut off when the external temperature of the furnace exceeded 60 “C. 20 20
absorption maximum or peak. Because of the open geometry of the CRA 63, losses occur by diffusion and convection. In such a case, the fastest rate of rise of temperature is therefore necessary for attaining large nzrnaX. Losses of analyte atoms from the analysis volume of the HGA 2100 are essentially due to diffusion. Since the rate of diffusion increases with increasing temperature, elements of relatively high volatility such as Zn and Cd produce the maximum number of analyte atoms in state B (nzrnax)a t lower atomization temperatures. Moelwyn Hughes ( 3 ) has shown that when k1 > k z ,
Vertical scale: absorbance, O.l/scale unit. Horizontal scale: sweep speed, 500 msec/scale unit
Figure 4.
Oscilloscopic trace showing absorbance by 1.0 X lo-’ g
copper Vertical scale: absorbance, O.l/scale unit. Horizontal scale: sweep speed, 500 msec/scale unit
exist in which case nzrnax= no in the limit of k l l k p a, where nzrnaxrepresents the maximum number of the analyte atoms. On this basis, the rate of supply of observable atoms in state B and their rate of loss may be discussed. An absorption maximum or peak occurs when dnzldt = 0 and the system is said to have reached a stationary state. Under this condition, an instantaneous maximum number of analyte atoms (nzmax)exists in the analysis volume. The maximum number of analyte atoms in state B (observable atoms) is given by ( 3 ) , +
n2max= where 1252
Tpeak
17,
exp(-k,T,,,)
is the titne required for the attainment of the
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
which illustrates the general principle that in a sequence of reactions the net rate is governed by that step which has the lowest rate constant. In the case of Zn and Cd, 121 is so much larger than k z (during the time represented by the rising segments of the absorption pulses) even a t low rates of rise of temperature, that k z governs the net rate of the entire atomization process. Elements having relatively high melting points and boiling points (e.g., Cu, Al, Sn) require a higher rate of rise of temperature to produce a sufficiently large value of k l even though the resulting higher temperature produces simultaneously an increase in the value of k z (because the rate of diffusion increases with increasing temperature). Therefore nzrnaxfor these elements occurs a t the highest temperature settings as shown in Table I. Mo and V (which have been excluded from Table I for reasons mentioned earlier) can be predicted to show an even more pronounced trend than that Of cu, A1, and Sn, and to give largest nzmaxvalues at even higher temperatures (higher than the maximum 2800 “C available on the control unit). Shapes of Absorption Pulses. The shapes of the absorption signals produced by the HGA 2100 are shown in Figures 2-8 and are seen to be significantly different from those of the CRA 63 pulses ( I ) . For comparative purposes, Figure presents a reproduction of the absorbance-time behaviour of identical masses of Cd, atomized under optimum conditions using the HGA 2100 and CRA 63. The increased delay time, as shown in Table 11, associated with the HGA 2100 is a direct reflection of the lower rate of rise of temperature of this furnace compared with that of the CRA 63. This delav time is associated with an “induction” time during which the furnace heats to a temperature sufficient to bring about an accelerated rate of atomization and the appearance of the absorption signal ( I ) . L’vov has esti-
Figure 5. Oscilloscopic trace showing absorbance by 5.0 X aluminum
lo-''
g
Vertical scale: absorbance, 0.1/scale unit. Horizontal scale: sweep speed, 1000 rnsec/scale unit
Figure 6. Oscilloscopic trace showing absorbance by 2.0 X
Figure 8. Oscilloscopic trace showing absorbance by 1.25X lo-'
g
vanadium
lo-'
Vertical scale: absorbance, 0.05/scale unit. Horizontal scale: sweep speed, 2000 rnsec/scale unit
g
tin Vertical scale: absorbance, O.l/scale unit. Horizontal scale: sweep speed, 1000 rnsec/scale unit
Figure 9. Absorbance time curves drawn from oscilloscopic traces showing absorbance by 1.0 X lo-" g cadmium with both atomizers ( A ) CRA 63; ( B ) HGA 2100
Figure 7. Oscilloscopic trace showing absorbance by 6.0 X molybdenum
lo-'
g
Vertical scale: absorbance, O.l/scale unit. Horizontal scale: sweep speed, 2000 rnsec/scale unit
mated (2, p 202) that in order that a sample be atomized rapidly (
