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Anal. Chem. 1985, 57,2002-2005
(3) Saito, M.; Schwan, H. P. "Biological Effects of Microwave Radiation"; Plenum Press: New York, 1960; Vol. 1. Valentlne, R. C. Blochim. Siophys. Acta 1982, 56, 33-42. .. (5) Hu, C. J.; Barnes, F. S.Radiat. Environ. Biophys. 1975, 12, 71-76. (6) Zlmmerrnan, U.; Vienken, J.; Pilwat, G. Z.Naturforsch., C: Biosci. 1881. 36C. 173-177. (7) Roger, T. J:; Bulish, E. Microbiol. Rev. 1980, 4 4 , 660-682. (8) Kiehn, T. E. Science 1979, 206, 577-580. (9) Persi, M. A.; Burnham, J. C. Appi. Environ. Microbial. 1981, 42, 364-369. (4) Furedl, A. A,;
' Present address: Faculty of Technology, Tokyo University of Agriculture
and Technology, Nakarnachi 2-24-16, Koganei, Tokyo 184, Japan.
Hideaki Matsuokal Eiichi Tamiya Isao Karube* Research Laboratory of Resources Utilization Tokyo Institute of Technology Nagatsuta-cho, Midori-ku, Yokohama 227, Japan
RECEIVED for review December 19, 1984. Accepted March 12, 1985. The work was supported by a grant from the Science and Technology Agency of Japan.
Simultaneous Molecular and Atomic Spectrometry with Electrothermal Atomization and Diode Array Detection Sir: Some papers dealing with the ultraviolet and visible study of molecular vapors produced by electrothermal atomizers were recently published (1-5). Various instruments were proposed depending on the aim of the work, from conventional flameless atomic absorption spectrometers (1-4) to dedicated equipment (5). Recently also the infrared range has been investigated, coupling an FT-IR spectrometer with a heated graphite atomizer, thus introducing further analytical applications of this device (6). At the same time new techniques have been developed for simultaneous multielement determination by graphite furnace atomic absorption (GFAA) or emission spectrometry (7-9). Considering these different applications, it seems advisable to explore the capability and behavior of electrothermal atomizers in acquiring, with the same equipment, molecular and/or atomic spectra for qualitative and quantitative analysis. This work reports some results obtained by a single instrument, made up of a graphite furnace and a diode array detector, in the ultraviolet investigation of vaporization phenomena of organic and organometallic compounds previously studied (2, 4 , 6). EXPERIMENTAL SECTION Apparatus. The equipment consists of a graphite furnace atomizer, Perkin-Elmer HGA-76 B, and Jasco KS-100 M multispectrometer modified to meet the requirements of this study. The atomizer is equipped with an AS-1 autosampler, while solids are sampled as previously described (6). The spectrometer includes a deuterium source, polychromator, data processor, and recorder. Deuterium lamp, atomizer, and polychromator are mounted on an optical rail. The diode array of the polychromator (512 elements 25 pm wide) is a Reticon 512 S. The wavelength range (190-355 nm) is focused on the detector diodes by a concave holographic grating (500 lines/mm) blazed at 230 nm. A baffle inserted in front of the entrance slit of the polychromator reduces the detection of light emitted by the graphite furnace tube walls. The calculated resolution of the polychromator is about 0.3 nm/diode and the stray light is evaluated below 0.3%. The data processor can acquire up to 16 spectra with collection times varying from 0.05 s to 1 s and interval times from 0.05 s to 25 a. The spectra are transferred by RS-232 C interface to a Perkin-Elmer 7500 professional computer for absorbance calculation, data storage, and representation. Reagents. Crude oils, pigments, and polymers are examined in this work. The oils are diluted 1to 10 with pure grade toluene. Pigment Blue 15 (copper phthalocyanine), Pigment Yellow 1 (monoazo derivative of 2-nitro-p-toluidine and acetoacetanilide), and Pigment Red 48:4 (Mn salt of monoazo derivative of 6amino-4-chloro-3-methylbenzenesulfonic acid and 3-hydroxy-2-
naphthoic acid) are commercial products previously examined ( 4 , 6). The polymer investigated is poly(viny1 chloride) (PVC) used for extruded industrial products. Organic and inorganic standards for atomic absorption spectrometry are employed to identify atomic lines in the high-temperature spectra. Procedure. The samples are vaporized by appropriate heating cycles; the vapor evolution is continuously monitored by collection of the ultraviolet spectra. The experimental conditions are described in Table I. The oils are examined after dilution (50 p L solution injection)following the thermal cycle 1 (2). The pigments, pressed in pellets ( 4 ) , are vaporized following cycle 2 (1-2 mg introduction). Finally extruded PVC (0.3-0.5 mg) is examined following cycle 3. A typical run takes 5-7 min comprehensive of spectra acquisition and absorbance calculation.
RESULTS AND DISCUSSION The equipment is a single beam spectrometer, therefore the deuterium spectrum (background) is required before the measurement. Spectra collected below 1000 "C can be processed using a single background recorded at room temperature, while high-temperature spectra require collection of different backgrounds recorded during a blank thermal cycle, high-temperature backgrounds being strongly affected by light emission from the walls of the glowing graphite furnace. The diode array, that works a t room temperature, has a dark current which represents 10% of the saturation current a t integration time of 0.5 s (10). With the same integration time and the maximum slit width permitted without reaching saturation over the whole spectral range, the dark current is 85% of the background signal at 190 nm. This unfavorable condition, due to concomitant low sensitivity of diodes and poor emission of deuterium at short wavelength, limits the useful spectral range between 200 and 355 nm. At temperatures exceeding 1000 "C atomic and molecular emission can take place in conjunction with atomic and molecular absorption; the equipment described does not employ any discriminating device; thus high temperature spectra result from algebraic combination of absorbed and emitted light. The occurrence of negative absorbance peaks is determined by prevailing of emission upon absorption. The apparatus described cannot match the sensitivity, as well as the resolution, of conventional GFAA, which uses monochromatic sources and more dispersive optics. For most elements the sensitivity achieved by this technique is close to or slightly worse than that obtained by flame atomic absorption spectrometry (using 50-kL injections). The repeatability of the molecular measurements below 1000 "C compares well with that obtained previously at fixed wavelength (4). The relative standard deviation of the atomic line in-
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Table I. Experimental Conditions for the Collection of Vapor-Phase Spectra thermal cycle 1 thermal cycle 2 1
2
3
150 20 60 300
1000 105 10 50
1500 6 25 300
4
1
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thermal cycle 3
2
3
4
1
2
3
1000
105 15 25
1500 6 22 300
2650 1 5 25
150 10 25 300
500 18 25 300
2650 3 4 25
*
*
*
Atomizer Parameters temperature, OC ramp time, s hold time, s argon flow, mL mi&
2750 1 7 5
150 10 25 300
Detector Parameters
*
spectra collection integration time, s interval time, s spectra
250
*
*
0.5 10
0.5 10 16
300
355
Flgure I. Vaporlzation cycle of Belaym crude oil (Egypt).
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IL
w u 4 z
ma
51 m 4
0200
2 50 300 WAVELENGTH, nm
355
Figure 2. Vaporization cycle of Es Sider crude oil (Libya).
tensities is about 10% and is effected by the graphite aging, which alters the emission of the tube walls. Vaporization cycles of crude oils are shown in Figures 1 and 2. The thermal cycle, consisting of a gradual heating and a final atomization, allows the evaluation of two distinct phenomena: evolution of molecular species below 1000 "C and atomization of nickel and vanadium above 2600 "C. The molecular evolution is related to the vaporization of oil compounds below 500 "C and of pyrolysis products originated from the decomposition of heavy molecules above 500 "C (2).In comparison with light crude oils like Es Sider (0.841 specific gravity), heavier crude oils like Belaym (0.874 specific gravity) show molecular absorption spectra shifted toward longer wavelengths due to the higher concentration of polyaromatic hydrocarbons. The features of the molecular spectra do not allow the identification of single compounds because of the complexity of the vapor-phase composition; nevertheless the vapor-phase behavior leads to tridimensional representations which are typical of the vaporized oil. These representations are used as first rapid screening in the "fingerprint"
8
16
I
200
WAVELENGTH,nm
$
0.5 0.5
I
250 300 WAVELENGTH, nm
I
355
Flgure 3. Atomlzation step of Belaym crude oil.
identification of spilled oils in seawaters. During the atomization step at 2750 "C some emission lines, not resolved by the polychromator, are observed: these lines are attributed to excited vanadium atoms (305.6,306.0,306.6 and 318.3,318.4,318.5nm) and to excited nickel atoms (lines from 335 to 350 nm) (11,12); as reported above, the system does not distinguish excited from ground state atoms, but simply detecta the sum of emission and absorption. Therefore it can be deduced that the excited atoms contribute significantly to the overall behavior of the vapor phase, in agreement with previous observations (13,14). Belaym crude oil (Figure l),having vanadium and nickel concentrations of 69 and 52 mg/kg, respectively, shows atomic emission lines more intense than those of Es Sider crude oil (Figure 2) having 3 mg/kg vanadium and 6 mg/kg nickel. A semiquantitative relationship between the intensity of these lines and nickel and vanadium content can be suggested and employed, together with molecular pattern, for the identification procedure. The behavior of Belaym crude oil at high temperature is shown in detail in Figure 3, reporting seven spectra (0.5 s interval time and integration time) acquired between 2200 "C and 2750 "C during step 4 of thermal cycle 1. Peaks and bands reported during the ramp time cannot be identified as atomic lines, hence originate from various molecular species. The intensity of these molecular absorption bands is generally proportional to the sulfur content in the crude oil. The closely spaced bands in the region 275-320 nm of the first spectrum are attributable to SO2(15).During the hold time the decrease of molecular absorption is followed by the growth of emission atomic lines. The persistence of atomic lines derives from the memory effect due to the formation of vanadium and nickel carbides, as nonpyrolytic graphite tubes were used in this study. Thermal cycles similar to those employed for oil identification are applied to solid samples of pigments. In this case
2004 0.21
ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUGUST 1985
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250 300 WAVELENGTH, nm
4
355
Flgure 4. Vaporization cycle of Pigment Blue 15 1 mixture.
+ Pigment Yellow 200
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Flgure 6. Vaporization cycle of Pigment Yellow 1
mixture.
+ Pigment Red 48:4
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Flgure 5. Vaporization cycle of Pigment Red 48:4.
atomic spectra differentiate organic from organometallic or inorganic compounds. Figure 4 shows the thermal behavior of a mixture made up of Pigment Blue 15 (PB 15) and Pigment Yellow 1 (PY 1). The spectra acquired below 500 "C originate from the vaporization of PY 1, while those between 500 "C and 1000 "C derive from the vaporization of PB 15. The 2650 "C atomic spectrum is determined by the organometallic nature of PB 15, two copper atomic lines (324.8 and 327.4 nm) being observed. The weak intensity of these lines confirms the conclusions taken by vapor-phase infrared spectrometry (5): copper phthalocyanine vaporizes without appreciable decomposition. The spectra of Pigment Red 4 8 4 reported in Figure 5 show a vaporization-pyrolysis step below 500 "C and the presence of several peaks at 2650 "C. The intense peak at 280 nm, attributed to Mn atomic absorption lines at 279.5,279.8, and 280.1, indicates that PR 4 8 4 is almost completely charred in the lower temperature steps. The remaining bands, which originate from molecular pyrolysis products, are found in crude oil spectra (Figure 3) and also during the heating of inorganic sulfates (e.g., CaS04). Anyway the reactivity of the graphite surface should be considered in explaining the vapor-phase molecular composition at high temperature. Figure 6 shows the behavior of a mixture PY 1 + PR 48:4 made up of an organic and an organometallic compound. The spectra acquired below 500 "C are molecular ones, originated by both components, while the 2650 OC spectrum is characteristic of the organometallic component only. The thermal resolution of the graphite furnace atomizer can be assessed in Figure 7, which shows atomic spectra of PVC acquired during an atomization ramp slackened in comparison to conventional GFAAS (-500 OC/s). In the first
250 300 WAVELENGTH, nm
355
Flgure 7. Vapor-phase behavior of poly(vlny1 chloride) during the atomization step.
spectrum a broad band is identified as molecular absorption of pyrolysis products and the overlapped peak as the atomic absorption line of cadmium at 229 nm (cadmium is added to PVC as a stabilizer). The following spectra indicate the consecutive atomization of zinc, lead, and copper also contained in the PVC. Therefore the thermal resolution at high temperature offered by the electrochemical atomizer (in this case -250 "C/spectrum) can be used, in conjunction with the spectroscopic resolution of the polychromator, for the identification of atomic species.
CONCLUSIONS The limitations in sensitivity and resolution concerning the atomic measurements have their counterpart in the wide spectral range examined, which allows the simultaneous detection of most elements commonly determined by GFAAS. At this stage the technique can be advanced for microanalysis as no chemical treatment and little samples are required. Moreover it could be applied to the study of chemical interferences in GFAAS and molecular evolution at temperatures not attainable by devices used in conventional ultraviolet spectrometry. Registry No. PVC, 9002-86-2; V, 7440-62-2; Ni, 7440-02-0; Cu, 7440-50-8;Cd, 7440-43-9;Zn, 7440-66-6;Pb, 7439-92-1;Mn, 7439-96-5;Pigment Blue 15,147-14-8;Pigment Yellow 1,2512-29-0; Pigment Red 48:4, 5280-66-0. LITERATURE CITED (1) Thompson, K. C.; Wagstaff, K. Analyst(London) 1979, 704,668-679. (2) Tittarelll, P.; Turrio Baldassarri, L.; Zerlla, T. Anal. Chem. 1981, 53, 1706-1708. (3) Tittarelli, P.; Ferrari, G.; Zerlia, T. At. Spectrosc. 1982, 3 , 157-160. (4) Tittarelli, P.; Zerlla, T.; Colli, A.; Ferrari, G. Anal. Chem. 1983, 55, .. 220-224. (5) Hutton, R. C.; Ottaway, J. M.; Epstein, M. S.: Rains, T. C. Analyst (London) 1977, 702,658-663.
Anal. Chem. 1985, 57, 2005-2007 (6) Tlttarelll, P.; Zerlla, T.; Ferrarl, G. Appl. Spectfosc. 1964, 3 8 , 715-720. (7) Lundberg, E.; Johansson, 0.Anal. Chem. 1976, 48, 1922-1926. (8) Harnly, J. M.; O’Haver, T. C.; Golden, 8.; Wolf, W. R. Anal. Chem. 1979, 57, 2007-2014. (9) Marshall, J.; LittleJohn,D.; Ottaway. J. M.; Harnly, J. M.; Mlller-Ihli, N. J.; O’Haver, T. C. Analyst (London) 1963, 108, 178-188. (IO) Horllck, G. Appl. Spectrosc. 1976,30, 113-123. (11) Parsons, M. L.; McElfresh, P. M. Flame Spectroscopy: Atlas of Spectral Llnes”. 1st ed.; IfVPlenum: New York, 1971. (12) Harrlson, G. R. “Wavelength Tables”; M.I.T. Press: Cambridge, MA, 1969. (13) Alder, J. F.; Samuel, A. J.; Snook, R. D. Spectfochlm. Acta, Pari S 1976, 316, 509-514. (14) Ottaway, J. M.; Shaw, F. Appl. Spectrosc. 1977, 3 1 , 12-17. (15) Pearse, R. W. B.; Gaydon, A. G. “The Identiflcatlon of Molecular
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Spectra”, 3rd ed.;Chapman 8. Hall: London, 1963.
Paolo Tittarelli* Rosa Lancia Tiziana Zerlia Stazione Sperimentale per i Combustibili Viale A. De Gasperi 3 20097 San Donato Milanese, Italy
RECEIVED for review December 26,1984. Accepted April 11, 1.985. This work was presented in part at the 11th FACSS Meeting, Philadelphia, PA, 16-21 Sept 1984 (p 100).
Differential Scanning Calorimetry for Rapid Exothermic Transitions Sir: In a recent paper by Eckhoff and Bagley (I), the freezing of distilled water was examined by differential scanning calorimetry (DSC), using the Perkin-Elmer Models DSC-1B and DSC-2. It was reported that the instruments gave incorrect energy values for the freezing exotherm, and the authors suggested that these instruments are unable to measure rapid thermal phenomena in a sample. In fact, DSC can be used for accurate measurement of any kind of thermal behavior, whether endothermic or exothermic, rapid or slow, provided that the instantaneous heat flow rate into or out of the sample does not exceed the maximum rating of the differential power measuring system. From the data presented by Eckhoff and Bagley, it is clear that this rating was exceeded in every experiment reported; the differential power measuring circuit was saturated for part of the exothermic transition, and therefore the transition energy was not measured accurately. That is, the experiments were carried out in such a way that peaks went off scale on the least sensitive range of the instrument. The purpose of this paper is to describe a procedure for measurements on highly supercooled samples and to report the results obtained with a 20-mg sample of distilled water, exhibiting 18.5 degrees of supercooling. The measurement was made on a Model DSC-2, and the measured exothermic transition energy agreed with the endothermic transition energy, as predicted. The paper will also include a discussion of the thermal equivalent circuit of the DSC sample holder. There are two mechanisms for providing the energy required by a sample material: through the thermal resistance from the sample holder to the surrounding enclosure at ambient temperature, and through a virtual thermal resistance created by the temperature control system. With this virtual resistance very small with respect to the other thermal resistances affecting heat transfer, heat exchange between the sample and the enclosure is made negligible. THERMAL EQUIVALENT CIRCUIT Figure 1shows the equivalent thermal circuit for the sample holder, using the nomenclature of the 1964 paper in which the theory of DSC was first presented (2).In this model, RT is the thermal resistance from the sample holders, at temperatures THSand THR, to the ambient temperature TA; a typical value for RT at ambient temperature is 600 deg s cal-l. Tp and Rp represent the temperature source and the thermal source resistance created by the closed-loop temperature control system. A typical value for Rp is 2 deg s cal-’. Ro is the resistance between the sample and the sample holder; a typical value for Ro is 200 deg s cal-’.
In the absence of thermal behavior in the sample, there is a steady heat flow rate from Tp to TA in each sample holder, through RP and R p This heat flow rate is a function of the total temperature gradient and the value of RT. As shown in the equivalent circuit, this heat flow rate bypasses the sample. The equivalent circuit shows the incremental heat flow rates associated with an assumed sample heat flow rate of W s cal s-l. Linear network theory predicts that this heat flow rate will be divided between Rp and RT in accordance with their resistance ratio. Thus the fraction of W , that flows through Rp, and is recorded by the calorimeter, is given by WM = (1 - p)wS where
Since RP is very much smaller than RT, the calorimetric calibration is essentially invariant with respect to RT; the ordinate calibration of a DSC varies by less than 2% over its operating temperature range, although RT varies by almost an order of magnitude. This is the fundamental difference between true power-compensated DSC and “heat flux” DSC. It should be understood that the measured heat flow rate WM is algebraically added to the existing heat flow rate through Rp. In the case of an exothermic transition, the heat flow rate out of the sample is accommodated by reducing the power supplied to the sample heater by the same amount. The heat flow rate through RT to the enclosure at ambient temperature is essentially unchanged. The thermal equivalent circuit also shows that the measurement of sample energy exchange will be unaffected by variations in Rotthe coupling resistance. It is impossible to avoid small variations in this parameter as different samples are introduced, but these variations influence only the rate of heat flow, not the integrated heat flow rate associated with a transition. In general, it is preferable to minimize Rot for improved temperature resolution and S I N ratio, but it is occasionally necessary to increase Ro, as will be discussed below. Finally, the equivalent circuit shows that phenomena on the reference side of a DSC sample holder are analyzed independently of those on the sample side. In a Model DSC-lB, the thermal coupling resistance between the two sample holders is of the same order of magnitude as RT, and the response is thus dominated by the much smaller resistance Rp; in the Model DSC-2, the sample holders are in separate cavities, and the thermal coupling vanishes. The orthogonality of the two temperature-control and power-measurement systems in DSC was analyzed in a study
0003-2700/85/0357-2005$01.50/00 1985 American Chemical Soclety
