Anal. Chem. 1989, 6 1 , 1017-1023
1017
Thermal Desorption Mass Analysis of Carbon Fiber Surfaces: Surface Oxygen Complexes D a n T. Fagan’ a n d Theodore Kuwana*
Center for Bioanalytical Research, University of Kansas, Lawrence, Kansas 66046
A thermal desorption mass spectrometry (TDMS) system was constructed and applied to study the removal of adsorbed material and the decomposition of surface oxygen complexes from carbon fiber surfaces. The system consisted of a Balzers QMG 51l quadrupole mass spectrometer contained within a vacuum system employing separate chambers for analysis and adsorption. Carbon fiber samples were heated by direct resistive heating in a h e a r increase up to 1000 OC under high vacuum. An inltlai desorptlon of “as-recelved” pitch-based fibers produced a variety of carbon-, oxygen-, hydrogen-, and nitrogen-containing fragments, as well as traces of species apparently contalnlng chlorine and sulfur. The inltial degassing step served to clean the carbon fibers for subsequent oxidation of the carbon with oxygen to form surface oxygen complexes. Such fibers then subjected to TDMS yielded carbon monoxlde, CO, and carbon dioxide, CO, as the major “desorptlon” products resulting from the decomposition of the surface carbon-oxygen complexes. The observation of multiple peaks in the desorption rate versus temperature profiles lndlcates that CO originates from possibly more than four different sites, and CO, from at least two sltes. The number of sites and the quantity of CO or CO, produced at each site depended on the time and temperature of the prior oxidative treatment with oxygen. The activation energy for CO, generation from the oxidized carbon surface was determined to be 187 f 13 kJ/moi.
INTRODUCTION Thermal desorption methods have been used extensively to study the adsorption properties and surface composition of carbonaceous materials. In a typical thermal desorption experiment, a sample is heated under vacuum or in a flowing stream of inert gas. Substances that are chemically bonded or weakly adsorbed to the surface are thermally removed and isolated for subsequent analysis (1-3) or detected directly by mass spectrometry (4-7), gas chromatography (8),gravimetry ( 9 ) ,or IR spectroscopy (10). Nearly all of the thermal desorption work reported previously for carbon has been limited to studies of finely ground or powdered samples of graphite and carbon black. In such samples, the grinding process may itself introduce contaminants or possibly alter the number and type of adsorption sites (11,121. To date, we are unaware of any reports concerning the direct thermal desorption analysis of surface oxygen complexes on intact carbon samples. Some studies have been reported on the residual gas analysis (using mass spectrometry) during vacuum-degassing of carbon fibers (13-15). Also, one report has described the use of chromatographic detection to study the thermal desorption of water from carbon fibers (16). We became interested in TDMS as an analytical technique because of its potential to provide confirmatory information concerning the chemical nature of carbon fiber surfaces. ‘Present address: The Upjohn Co., Kalamazoo, MI 49001. 0003-2700/89/0361-1017$01.50/0
Carbon fibers are industrially important in the preparation of high-strength composites (17). They are also important to electrochemists as microelectrodes ( 1 4 1 9 ) . Regardless of their application area, fibers are almost always subjected to some type of pretreatment prior to use. For example, they may be oxidized in air to improve fiberlresin bonding as a composite (20) or electrochemically oxidized to “activate” the surface for electron transfer in a sensor application (19). These oxidative treatments are known to introduce oxygen complexes onto the carbon surface (21,22). These surface complexes have been suggested to affect electron-transfer rates, adsorptive behavior, wettability, catalysis, and electrical properties (23-25). Various analytical methods have been used previously in our laboratory to study the surface of carbonaceous surfaces. These include X-ray photoelectron spectroscopy (26, 27), scanning electron microscopy (28),and electrochemistry (27, 29, 30). TDMS appeared attractive from the viewpoint of providing confirmatory structural and chemical information about carbon-oxygen complexes from mass analysis of the thermally “desorbed” species. Mass spectroscopy also has the requisite sensitivity to detect desorbed products from the relatively low surface area of carbon fibers. In addition, the high-vacuum environment required in the mass spectrometer sample chamber helps to preserve the integrity of “clean” carbon surfaces that have been subjected to pretreatment by high temperatures under vacuum prior to the subsequent oxidative treatments. The analytical capabilities of TDMS were applied to two problems that are discussed herein. A third problem concerns the adsorption of organic molecules, which will be discussed in a subsequent paper (31). Our initial studies concerned the cleanliness and chemical composition of the fiber surfaces as received from the manufacturer. A series of mass spectra were acquired a t a fast scan rate during the f i t vacuum-degassing to determine the initial desorption products. In this way the integrity of a freshly degassed fiber sample could be determined and repeatedly verified when fibers were subjected to gas-phase oxidation and TDMS analysis. Secondly, the type and nature of carbon-oxygen complexes formed by the oxidative reaction of fibers with molecular oxygen were studied by monitoring the “desorption” of CO and C02. These two species are the major components evolved from the thermal decomposition of surface complexes. What are the chemical identities of these complexes? It has been previously suggested (32-35)that carbonyls, quinones, phenols, carboxylic acids, and lactones could be formed oxidatively. Of these, the latter two functionalities could decompose thermally to yield C02, while the former ones seem to have CO as a principal decomposition product. The currently employed gas-phase oxidation will serve as a control method for future oxidative studies by TDMS. Several reports have suggested that the temperature a t which the oxygen is reacted with the carbon surface governs the type of functionality introduced (25,36,37). For this study the two temperature ranges of 300-330 OC and 600-650 O C were selected so that a comparison could be made among fibers 0 1989 American Chemical Society
1018
ANALYTICAL CHEMISTRY, VOL. 61, NO. 9, MAY 1, 1989 FEChT V I E A
.. . __ Figure 1. Schematic diagram of the TDMS system: 1, quadrupole mass spectrometer; 2, ionization chamber of quadrupole; 3, gate valves; 4, cry0 pump; 5, sorption pump; 6, glass viewpoint (main analysis chamber); 7, glass viewpoint (introduction/reactionchamber); 8, sample holder; 9, gas introduction port; 10, electrical feedthroughs; 11, thermocouple feedthrough; 12, rotary-linear motion magnetically coupled feedthrough; 13, thermocouple vacuum gauge. Not visible in diagram: turbomolecular pump (opposite viewport 7) and ionization vacuum gauge (opposite viewport 6). The arrow separates the two chambers.
5 SIDE V I E W
.1
reacted under relatively mild to rather severe conditions.
EXPERIMENTAL SECTION TDMS System. The TDMS system was constructed of stainless steel and ultrahigh-vacuum, metal seal components and is shown schematically in Figure 1. The system is divided into two sections. The main analytical chamber is that part shown to the left of the gate valve as indicated by the vertical arrow. A Balzers (Hudson, NH) QMG 511 quadrupole mass spectrometer (90' off-axis scanning electron microscopy detector) was mounted vertically in direct line of sight with the sample surface. High to ultrahigh vacuum was maintained with a Leybold Heraeus (Export, PA) RPK 1500 cryopump. The base pressure of the system was maintained at 5 x IO* Torr or less. A typical pressure during a desorption experiment was 2 X Torr. The pumping speed was sufficiently high, relative to the heating rate, that the mass spectrometer signal was directly proportional to the desorption rate as indicated by the peak shape (38). The use of a dynamic (high pumping speed) rather than a static (closed) system minimized the incidence of desorbed products back-reacting with the surface ( 5 ) . The sample introduction/reaction chamber is located in the section to the right of the indicated gate valve. This chamber can be isolated from the main chamber during adsorption of and reaction with oxygen to reduce evacuation time and prevent contamination of the quadrupole detector. High vacuum was maintained in this section with a Balzers TPU 50 turbomolecular pump. Rough pumping was done with a liquid-nitrogen-cooled molecular sieve sorption pump. The sample holder (described below) was mounted on a Huntington (Mountain View, CA) rotary-linear motion magnetically coupled feedthrough. This arrangement allowed the transfer of a sample between the two chambers without exposing the sample to ambient air. The introduction/reaction chamber was also equipped with a sample dosing unit for admitting organic adsorbant molecules in the gas phase. The dosing system is similar in design to one described by Madix (39). The sample holder is diagrammed in Figure 2. A bundle of 100-200 carbon fibers was held together at the ends with 0.025 mm thick Ta foil (Alfa Products, Denvers, MA). The fiber bundle (ca. 1 cm in length) was held between two stainless steel bolts that were attached to an insulating alumina support. This support was attached to the linear motion feedthrough. The samples were heated resistively by supplying electrical current from a Kepco (Flushing, NY) bipolar operational power supply/amplifier. Electrical contact to the fibers was made by the bottom of the sample support bolts being connected to copper wires extending into the vacuum system. The linear heating ramp was provided by programming the power supply with a linear ramp voltage. A linear-programmed heating rate has been shown to offer better
Figure 2. Top and side view of the sample holder: 1, carbon fiber bundle; 2, tantalum foil; 3, thermocouple wire; 4, alumina base; 5, stainless steel bolts for sample support and electrical contact; 6, stainless steel bolt for attachment to linear motion feedthrough. resolution than the frequently encountered temperature-step method (3). A temperature ramp of approximately 30 OC/s was found experimentally to produce satisfactory signal intensity and resolution. A chromel-alumel thermocouple constructed from 0.005 in. diameter wire (Omega Engineering, Stamford, CT) was placed in direct contact with the fiber bundle for temperature measurements (see Figure 2). By coiling the wire around the linear motion feedthrough, the wire moved between the two chambers with the sample. A Cerama-Seal (New Lebanon Center, NY) vacuum feedthrough provided contact between the thermocouple and an Omega Engineering TAC-386 thermocouple-to-analog converter, whose output was recorded on the x axis of a Houston (Austin, TX) Model 2000 X-Y recorder. The control unit of the Balzers quadrupole was interfaced to an Apple IIe computer with a 12-bit analog to digital-digital to analog interface (Interactive Microwave, State College, PA). Software developed in-house allowed up to 20 spectra to be recorded consecutively at a scan rate of up to 1 amu/ms. The time delay between each spectrum could be adjusted from 1 ms to several seconds. Three-dimensional plots of m / z vs time vs signal intensity were recorded with a Hewlett-Packard digital plotter. Thus, an entire range of m / z values could be examined during a single desorption experiment. Pressure measurements were made with either a GranvillePhillips (Boulder, CO) ionization gauge or a Varian (Lexington, MA) thermocouple vacuum gauge. Carbon Fibers/Oxygen. Two types of fibers, pyrolyzed pitch (PP) and heat-treated pyrolyzed pitch (HTPP), were received from Avco Specialty Products Division (Lowell, MA). Both were made by pyrolysis from purified coal tar pitch a t 1100 OC under N,. The HTPP samples had undergone a subsequent 3000 OC heat treatment under argon. Accordingto the manufacturer, these fibers do not have any type of coating or resin applied to their surfaces. The fibers with an average diameter of 33 pm were manufactured to be noncrystalline, similar in structure to glassy carbon. Young's modulus was (5-6) X lo6 psi. Linde (Somerset, NJ) hydrocarbon-free ultrahigh purity grade oxygen (CO, COP, and H20a t 3 ppm each) was used for the oxidative reactions.
ANALYTICAL CHEMISTRY, VOL. 61, NO. 9, MAY 1, 1989
1010
jB
A
28
20
30
44
50
40 M/Z
60
28
I
70 I 2 0 I
30
44
50
40
60
70
M/Z
1 ;
Flgure 3. Initial desorption products for PP (A) and HTPP (B) fibers from m / z = 20 to 70. The first of the three consecutive spectra in each case was recorded prior to desorption. At the time the third spectra was recorded, the sample temperature was approximately 900 "C. The scan rate was 1 ms/amu wlth a 2500-ms delay between scans.
Experimental Procedure. Two separate methods were used to record the data, depending on the type of experiment performed. To identify the mass of the fragments desorbed during thermal heating of samples, several mass spectra were recorded over a given m / z range. The resulting three-dimensional plot of m / z vs time vs signal intensity gave the initial "survey" spectra, indicating the chemical composition of desorbed material. It did not, however, indicate the temperature at which the various species were desorbed or the number of desorption sites. The second method involved monitoring a single m l z value as a function of temperature, resulting in a desorption rate (directly proportional to the mass spectrometer ion current) vs temperature curve. In this case, the temperature of desorption, number of sites, and relative amount desorbed for a particular species were determined. The procedure and method of data recording for each of the two experiments are outlined below. Initial Desorption Products. To determine the initial desorption products for "as received" fibers, a new fiber bundle was mounted in the vacuum system and evacuated to lo-* Torr for 12-24 h without heating. A computer-controlleddesorption survey scan was performed by recording 10 consecutive spectra while the sample was heated from 50 "C to approximately 1000 "C. Experiments were performed over m / z ranges of 1-51,20-70, and 51-151. A new fiber bundle was installed prior to each trial. The scan rate was 1 amu/ms with a delay of 2.2-2.5 s between each spectrum. Desorption of Surface Oxygen Complexes. Surface oxygen complexes were formed on vacuum-degassed fibers by reaction with oxygen and their decomposition monitored by recording the desorption rate vs temperature for CO and COP( m / z = 28 or 44, respectively). Several experiments were conducted with the same fiber bundle. The experimental steps are as follows: The sample was vacuum-degassed at 1000-1100 "C for at least 30 min at a pressure less than 5 X Torr and was allowed to cool to 70 "C or less. The reaction chamber was flushed three times with oxygen and filled to 200 mTorr. The sample was then heated to either 300-330 "C or 600-650 "C for a specified amount of time. Following oxidation, the chamber was evacuated for 5 min to lo4 Torr or less. The sample was then moved into the direct line of sight with the quadrupole. The desorption was recorded while the sample was heated from a temperature of 50 "C to 1000 "C at 30 "C/s and an m l z value of 28 or 44 was monitored. The background was obtained by recording the desorption a second time. R E S U L T S AND DISCUSSION Vacuum-Degassing-Initial Desorption P r o d u c t s . Three of the 10 spectra (to simplify the diagram) recorded
during the initial desorption for both PP and H T P P fibers are shown in Figure 3A and 3B, respectively, for m / z = 20 and 70. The first spectrum was recorded prior to heating; the third was recorded when the sample temperature reached approximately 900"C. A wide variety of low molecular weight fragments is observed as initial desorption products in these spectra. These fragments may arise from the desorption and decomposition of trapped substances formed during the carbonization process and from the decomposition of surface oxygen complexes present initially on the surface (14,15,40). The predominant peaks observed as the temperature increased were a t m l z values of 1, 2, 18, 28, and 44, corresponding to H, HZ,HzO, CO, and COz, respectively. The PP fibers were found to release larger amounts of desorption products, both in number and quantity, compared to H T P P fibers. This difference is expected since the H T P P fibers had been subjected to an additional heat treatment under inert atmosphere by the manufacturer. Some higher molecular weight hydrocarbon fragments, indicative of aromatic precursors, were observed in trace quantities, including m / z values of 78 and 91, which could be due to benzene- and toluene-like moieties. The reason for their presence is not known, although they could be physically entrapped species remaining from the manufacturing process. The existence of nitrogen-containing species was suggested by the presence of m / z values of 17 (NH, and/or OH) and 41,43, and 45 (fragments containing C, H, N). It is likely that some nitrogen-containing substances are present in the coal tar precursor from which the fibers are made. Besides fragments containing carbon, oxygen, nitrogen, and hydrogen, there were very minor peaks in the spectra with m / z values suggestive of fragments containing the elements chlorine and sulfur. For example, the peaks with m / z values at 35, 36,37, and 38 could be from carbon-chlorine moieties that decomposed to yield C1 and HC1 on heating. That these peaks were assignable to C1 was also supported by the peak intensities being in accordance with those expected from the isotopic ratios. That is, the 37C1/35C1and H37C1/H35C1peak height ratios had an average value of 0.38, which is close to the natural chlorine isotopic abundance ratio of 0.33. The PP fibers exhibited small peaks at m / z 64, 33, and 34, which could be assignable to SOz,HS, and HzS, respectively. HTPP showed only a peak at m l z 64. The presence of trace quantities of species containing chlorine and sulfur is not surprising
1020
ANALYTICAL CHEMISTRY, VOL. 61, NO. 9, MAY 1, 1989
Table I. Average Peak Maximum Temperatures for CO and COz Desorption from PP and HTPP Fiberso B
O2 reacn temp, "C
Tp,CO CY
P
Y
P
6
CY
897 (26) 1053 (24)
583 (14)
783 (15) 791 (12)
993 (24) 984 (26)
535 (12)
775 (15) 758 (15)
HTPP 344 (1Ub 637 (11)
624 (26) 689 (19)
748 (8) 838 (33)
325
603 (8) 690 (22)
728 (27) 752 (11)
820 (16) 956 (32)
PP (10)
640 (13) 0
200
400 600 800
1000
TEMPERATURE, ' C
Flgure 4. Desorption rate vs temperature for CO, desorption from HTPP fibers treated by vacuumdegassing (. . .), 325 "C oxidation for 5 rnin (- --), 325 "C oxidation for 45 min (- --), and 640 "C oxidation for 1 rnin (-). The linear heating rate was ca. 30 "C/s.
in view of the precursor material for the fibers being coal tar pitch. This type of pitch is known to contain sulfur, although the exact form is not known (41). It should be noted that peaks being assigned to the chlorine-, sulfur-, and nitrogencontaining fragments were much smaller than those for CO and C02. A second vacuum-degassing removed most of the fragments that appeared in the initial spectrum for both types of fibers. Surprisingly, the peak a t m / z 36 required a more extensive degassing for complete removal. Evidently some chlorine is incorporated in the bulk of the fiber and is not easily removed. It has been reported that polyacrylonitrile (PAN) fibers are sometimes carbonized in an atmosphere of HCl vapors to improve the yield (42). However, it is not known if these fibers received such a treatment. Further (30-60 min) vacuumdegassing at temperatures at or above 1000 "C was sufficient to reduce all of the peaks in the initial desorption spectrum to the background base line. The surface of the carbon fibers heated to 1000 "C, as described above, was assumed to be contaminant- and oxygen-free and was subsequently used for the oxidative studies. Desorption of C 0 2 . The desorption rate vs temperature curves for C 0 2 ( m / z = 44) are shown in Figure 4 for fibers that were pretreated by vacuum-degassing and then oxidized at 325 "C or at 640 "C. The results shown are for PP fibers. H T P P samples produce similar results with small shifts in Tp,the temperature of the peak maximum. The average peak temperatures are summarized in Table I for both types of fibers. As expected, the vacuum-degassed sample showed no C 0 2 desorption peak, indicating the absence of surface oxygen complexes with COz as their thermal decomposition product. For fibers oxidized at the low temperature, however, two peaks were observed. Thus, C 0 2 was produced from the thermal decomposition of a t least two different surface complexes. These two peaks are labeled a(C02) and p(C0,) for convenience of identification and correspond to the desorption maxima observed a t 535 "C and 775 "C, respectively. The ratio of the peak areas, a(C02)//3(C02),varied from about 1.0 for the 5-min oxidation to 2.5 for the 45-min oxidation. Thus, the effect of the extended oxidation with O2 at 325 "C was to increase the a peak intensity relative to that of the p peak. The desorption of COP from fibers oxidized a t 640 "C for 1 rnin was markedly different, as indicated by the solid curve in Figure 4. In this case, only the @(C02)peak was observed. The surface carbon-oxygen complex that decomposed to give
842 (19) 856 (21)
a HTPP = heat-treated pyrolyzed pitch carbon fibers; PP = pyrolyzed pitch carbon fibers. The heating rate was between 29.3 and 31.9 "C/s. *The values in parentheses are standard deviations.
the a(C02)peak was therefore unstable at the higher oxidative temperature. Notice also that the total amount of C02evolved (as indicated by the peak area) from a 1-min oxidation at 640 "C was comparable to that from a 45-min oxidation at 325 "C. This reflects the difference in the rate of surface oxygen complex formation at the two different temperatures. Clearly, the relative amounts of the two surface complexes can be controlled by changing the oxidative temperature and time. A brief statement can be made concerning the possible identity of the a and p desorbing groups. As discussed earlier, the two possible surface functionalities capable of producing C02 may be lactones and carboxylic acids, since these groups contain two oxygen atoms bonded to one carbon atom. Lactones would also evolve CO simultaneously with C02. Perhaps it is reasonable to suggest that the j3(C02) desorbing group is some type of surface carboxylic acid group since it is formed principally from the O2 reaction at the higher temperature. The a(C02) peak would then be derived from a lactone-type surface group. Notice that this peak is somewhat broader than the p(C0,) peak. This broadening could result from the breakage of multiple surface bonds (one C-C and one C-0) from a lactone-type group, whereas the narrower p(C02)peak results from the breakage of only one C-C bond of a carboxylic acid. It is important to note that these speculations are unproven and conclusive identification of surface functionalities is not possible based on TDMS data alone. Preliminary experimental results with adsorbed organics on oxidized fibers suggest that the site responsible for the p(C0,) interacts with basic molecules, giving further support for its acidic character (31). Desorption of CO. The desorption rate vs temperature curves for CO ( m / z = 28) were quite different from those of C02, as indicated in Figure 5 for both H T P P (A) and PP (B) fibers. The upper curves in the figure, labeled a, are for the low temperature oxidized fibers (325-344 "C, 5 rnin), and the lower curves, labeled b, are for the higher temperature oxidation (637-640 "C, 1 min). In both cases, four desorption peaks, labeled a(CO),p(CO), r(CO), and 6(CO), were observed. The T pvalues and the relative intensities of the four peaks were found to vary considerably from run to run and among different samples, and a "steady-state" desorption pattern was not attained for CO as it was for C02. There is some experimental evidence, based on scanning electron microscopic studies (31),that the outer layer of the fiber structure is removed after prolonged and repeated oxidation and thermal desorption. As the outermost carbon is removed, the newer exposed inner core carbon may have a slightly different structure than the initial surface. As is the case for C02,the vacuum-degassed surface (dashed line) showed no desorption
ANALYTICAL CHEMISTRY, VOL. 61, NO. 9, MAY 1, 1989
1021
Table 11. Amounts of CO and COz Desorbed from Carbon Fibers following Oxidation under Different Conditions
molecules desorbed" (X10E*2) COZ total
oxidn conditions
co
300-320 "C, 5 min 300-330 "C, 30 min 600-630 "C, 1 min
15.1 1.3 27.0 f 3.4 48.6 f 1.5
*
2.80 h 0.26 6.08 f 1.5 2.59 f 0.37
17.9 f 1.5 31.1 f 4.9 51.2 f 0.19
co, %
co*,%
co/co2
84.5 f 0.81 81.8 f 1.8 94.8 f 0.6
15.7 f 0.9 18.2 f 1.6 5.00 f 0.6
5.41 f 0.35 4.52 f 0.49 19.0 f 2.3
Calculated from measured heating rate, pumping speed, and MS gain. I
A 600
A
SO0
I
_-
b
0
__ _. ..__
ZOO
400
600 800
1000
0
TEMPERATURE.
ZOO
400 600
800
IO00
.C
Flgure 5. Desorption rate vs temperature for CO desorption at HTPP (A) and PP (8) fibers. The curves labeled a are for low-temperature oxidation (ca. 330 OC), and the curves labeled b are for high-temperature oxidation (640 OC). The dashed lines show the desorption from vacuumdegassed fibers. The linear heating rate was ca. 30 OCIS.
peak, indicating the absence of any surface carbon-oxygen complexes. In the case of CO desorption, as was previously observed for COz, the a peak predominates for the low-temperature oxidations. This observation is consistent with the existence of lactone or lactone-like moieties, since lactones can simultaneously evolve both CO and COz. Perhaps the two a peaks are from the same type of surface lactone. The relative amounts of CO and COPdesorbed at the two oxidative temperatures were determined by the integration of the peaks and calculation of the number of molecules produced using the measured heating rate, pumping speed, and mass spectrometer gain. These results are summarized in Table 11. Due to difficulties in determining the exact pumping speed, the quantitites given in absolute number of molecules are less reliable than the ratios (and percentage values) listed in the table. Notice that in all cases, CO was the principal desorption product (82-95 % ). The relative amount of the surface precursor responsible for CO increased a t the higher oxidative temperature, whereas the 300 "C oxidation for 30 min resulted in the most COz. Similar results were observed for oxidized graphite by Barton and co-workers (43) and for diamond powder by Matsumoto (6). That is, as the oxidative temperature increased, the CO/C02 ratio also increased. Nayak and Jenkins (IO),for example, reported CO/C02 ratios of 7.6 and 25.6 for graphitized furnace black oxidized at 200 and 500 "C, respectively. The similarity between literature values and those reported here for vitreous versus graphitic carbons suggests that different types of carbon react with oxygen to form a similar distribution of oxygen complexes. Activation Energy of Desorption. The activation energy of desorption, Ed, which represents the energy barrier to removal of surface-bound oxygen complexes as CO or COz, was determined for COPby using the heating rate variation method
Figure 6. Desorption of COPfrom carbon fibers after oxygen chemisorption at 325 OC (A) and 640 OC (B). The heating rates for the labeled curves are (in deg/s) as follows: a, 3.09; b, 6.90; c, 13.8; d, 29.0; e, 55.0; a', 3.40; b', 6.70; c', 13.9; d', 27.4; d', 52.0.
Table 111. Activation Energy of Desorption Values Obtained for C 0 2 Desorption" O2 reacn temp, " C
650 645 640 325 340
In (B/T,2) (slope)
Ed, kJ/mol
-22205 -22918 -25904 av -21591 -23909 av
185 191 215 197 f 16 180 199 190 13
*
In N (slope)
-21822 -19789 -21924 -21922 -23458
Ed, kJ/mol
181 165 182 176 f 16 182 195 189 f 9
" B = heating rate in "c/s;Ed = activation energy of desorption; N = desorption rate (proportional to mass spectrometer signal).
described previously by several authors (38, 44, 45). This technique involves recording the desorption rate vs temperature curves for a given species at several different heating rates and at the same initial coverage. The results for the decomposition of C 0 2 a t a variety of heating rates are shown in Figure 6 for oxygen complexes formed at 325 "C (A) and 640 O C (B). For the p(C02)peaks a t both temperatures, a plot of In (B/T,2)vs l / T p (where B is the heating rate in degrees per second and Tp is the peak maximum temperature in Kelvin) gave a straight line with slope equal to -Ed/R as shown in Figure 7A. The values of Ed,calculated from the results, are given in Table 111. Similarly, Ed was calculated from a plot of h (N) vs l/Tp as shown in Figure 7B, where N is the desorption rate (proportional to the mass spectrometer signal). Good agreement between the
1022
ANALYTICAL CHEMISTRY, VOL. 61, NO. 9, MAY 1, 1989
A
-12. 0
-12.6'
,
'b 10.2
' 9.3
9.4
9.5
9.6
9.7
9.8
9.9
10.0
10.1
1/1 ( I / W )
-a#.-20.6 -20.8
h
E-4
B
-21.0-21.2 -
ACKNOWLEDGMENT We thank R. J. Suplinskas of AVCO for the donation of carbon fibers and information concerning their fabrication and structure. Helpful discussions with Glen W. Hance, Yu-peng Gui, and Greg M. Swain are gratefully acknowledged.
-21.4-
-21.8
-22. 0
Vacuum-degassing a t or above 1000 "C removed essentially all of the adsorbed and surface-bonded substances to produce a clean, highly reactive surface. The thermal decomposition of surface carbon-oxygen complexes formed on clean carbon by reaction with molecular oxygen resulted mainly in CO and COz as desorption products. Carbon monoxide was evolved at temperatures between 600 "C and 1000 "C and gave four distinct peaks. The peak temperatures and relative intensity of these peaks varied considerably,depending on the number of oxidation/degassing cycles that the sample had experienced. Such variability suggests reaction at sites that are easily changed by hightemperature treatment, such as basal defects. The decomposition of surface oxygen complexes to give COz, however, was very reproducible, with desorption peaks being observed at 535 OC and 775 "C. It is suggested that these two peaks resulted from the decomposition of acidic surface groups such as lactones and carboxylic acids, respectively. The activation energy for the thermal "desorption" of COz averaged 187 f 13 kJ/mol as determined by the heating rate variation method.
i
\
LITERATURE CITED \
Bansal, R. C.; Vastola, F. J.; Walker, P. L., Jr. Carbon 1970, 8 , 443. S.S.; Phys. Chem. 19788 82,
-22.6 / / ) I / Barton, Harrison, 8. H.; Dolllmore, J. J. 9.3 9.4 9.5 9.6 9.7 9.8 9.9 10.8 10.1 10.2 290. I/I (l/K) E-4
Figure 7. Plot of in (BIT;) vs in, (A) and In ( N )vs i/rp ( B ) to determine the activation energy of desorption with the heating rate variation method.
values obtained with the two methods, using the same data, gives confidence in the reliability of the measurements (45). The values were nearly independent of the oxidative temperatures. The average Ed value for all of the trials is 187 f 13 kJ/mol. Literature values for Ed have been reported in the range of 100-300 kJ/mol (2, 10, 46). There is considerable variability, depending on the type of carbon and the analysis method used. Our value falls within those in the literature for various types of carbon and is reasonable considering that the C 0 2 results from the cleavage of a carbon-carbon bond. The energy of the C-C bond is 336 kJ/mol in an electrically neutral environment at 25 OC (22). The higher temperatures and the bonding of one carbon to an electronegative oxygen would effectively weaken the C-C bond and lower the energy required for its cleavage. The Ed values for the cr(COz) peak were not determined since the peak maximum temperature shifted only slightly with increasing heating rate as indicated in Figure 6. This is likely due to a relatively small activation energy coupled with a fairly broad peak which hides small shifts in T,,. I t was not possible to determine the value of Ed for co desorption due to the variability in the intensity and position of the four peaks observed in the desorption rate vs temperature plot. Furthermore, if the entire curve was smoothed to a single peak, small shifts in T pcould not be measured due to the broadness of the peak. Literature values of Edfor CO desorption from various carbons generally fall around 300 kJ/mol, indicating stronger bonds are involved in the precursorb) that produce COz.
CONCLUSIONS TDMS analysis of "as-received" carbon fibers identified the presence of surface oxygen complexes along with smaller amounts of chlorine-, sulfur-, and nitrogen-containing species.
Tremblay, G.; Vastola, F. J.; Walker, P. L., Jr. Carbon 1978, 16, 35. Sen, A.; Bercaw, J. E. J. Phys. Chem. 1980, 84, 465. Phillips, R.; Vastola, F. J.; Walker, P. L., Jr. Carbon 1970, 8 , 197. Matsumoto. S.;Kanda. H.; Sato, Y.; Setaka, N. Carbon 1977, 15, 299. Matsumoto, S . ; Setaka, N. Carbon 1979, 77, 303. Brooks, C. S.;Scola, D. A. J. Colloid Interface Sci. 1970, 32, 561. Tucker, B. G.; Mulcahey, M. R. F. Trans Faraday Soc. 1969, 6 7 , 274. Nayak, R. V.; Jenkins, R. G. Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 1984, 29, 172. Vastola, F. J.; Walker, P. J., Jr. J . Chim. Phys. 1961, 20, 58. Harker, H.; Horsley, J. B.; Robson, Carbon 1971, 9 , 1. Proctor, A.; Sherwood, P. M. A. J. Electron Spectrosc. Relat. Phenom. 1982, 39, 27. Drzal. L. T. Carbon 1977, 15, 129. Rand, B.; Robinson, R. Carbon 1977, 15, 257. Brei, V. V.; Pavlov, V. V. Theor. Exp. Chem. (Engl. Trans/.) 1979, 15, 253. Donnet, J. R.; Bansal, R. C. Carbon Fibers; Internatlonal Fiber Science and Technology -. Series; Marcel Dekker: New York. 1984: Vol. 3, pp 223-267. Edmonds, T. E. Anal. Chim. Acta 1985, 1 , 175. Feng, J.-X.; Brazell, M.; Renner, K.; Kasser, R.; Adams, R. N. Anal. Chem. l987, 59, 1863. Donnet, J. R.; Bansal, R. C. Carbon Fibers; International Fiber Science and Technology Series; Marcel Dekker: New York, 1984; Vol. 3, pp 115-136. Puri, B. R. I n The ChemistryandPhysicsof Carbon; Walker, P. L., Jr., Ed.; Marcel Dekker: New York, 1970; Vol. 6, pp 191-282. Engstrom, R. C.; Strasser, V. Anal. Chem. 1984, 53, 1366. Evans, J. F.; Kuwana, T.; Henne, M. T.; Royer, G. P. J. Nectroanal. Chem. Interfacial Nectrochem , 1977, 80, 409. Gunasingham, H.; Fleet, B. Anawst 1982, 107, 896. Golden, T. C.; Jenkins, R. G.; Otake, Y.; Scaroni, A. W. Proc. Nectrochem. SOC. 1984, 8 4 - 5 , pp 61-78. Mlller. C. W.; Karweik, D. H.;Kuwana, T. Anal. Ch8m. 1981, 5 3 , 2319. Fagan, D. T.; Hu, I.-F.; Kuwana, T. Anal. Chem. 1985, 5 7 , 2759. Kazee, B.; Weisshaar. D. E.; Kuwana, T. Anal. Chem. 1985, 5 7 , 2736. Hu, I.-F.; Karweik, D. H.; Kuwana, T. J. Electroanal. Chem. Interfacial Nectrochem. 1985, 188, 59. Hance, G. W.; Kuwana, T. Anal. Chem. 1987, 59, 131. Fagan. D. T. Ph.D. Thesls, The Ohio State University, 1987. Garten, V. A.; Weiss. D. E.; Willis, J. B. Aust. J. Chem. 1957, 10, 295. Boehm, H. P.; Diehl, E.: Heck, W.; Sappok, R. Angew. Chem.. Int. Ed. Engl. 1984. 3 , 669. Barton, S . S.; Harrison, B. H. Carbon 1975, 13, 283. Keleman, S . R.; Freund. H. Carbon 1985, 23, 723. Weller, S . W.; Young, T. F. J. Am. Chem. SOC. 1948, 7 0 , 4155. Smith, R. N. 0.Rev., Chem. SOC. 1959, 13, 287. Redhead, P. A. Vacuum 1962, 12, 203. McCarty, J.; Falconer, J.; Madix, R. J. J. Catal. 1973, 30, 235.
Anal. Chem. 1989, 6 1 , 1023-1025 (40) Mimeauk, V. J.; McKee, D. W. Nature 1969, 224, 793. (41) Donnet, J. R.; Bansal, R. C. Carbon Fibers; International Fiber Sclence and Technology Series; Marcel Dekker: New York, 1984; Vol. 3, 0 39. (42) bonnet, J. R.; Bansal, R. C. C8fbOn Fibers; International Fiber scien- and Technology Series; Marcel Dekker: New York, 1984; Vol. 3, D 24. (43) brton, S. S.; Harrison, B. H.; Dollimore, J. J . Chem. SOC., F8rad8y Tf8tlS. 1 , 1973, 69, 1039. (44) Falconer, J. L.;Madix, R. J. Surf. Sci. 1975, 4 8 , 393. (45) Falconer,J. L.; Schwarz, J. A. Catal. Rev.-Sci. Eng. 1983, 25, 141.
1023
(46) Dollimore, J.; Freedman, C. M.; Harrison, B. H.; Quinn, D. F. Carbon 1970, 8, 587.
RECEmD for review February 9,1988. Accepted January 23, 1989. We appreciate the generous support of this work bv a grant from-& National Science Foundation. The Suppori of the University and the Kansas University Endowment Association to the center is also hereby acknowledged.
Comparison of Fast Atom Bombardment Mass Spectrometry and Thermal Ionization Quadrupole Mass Spectrometry for the Measurement of Zinc Absorption in Human Nutrition Studies John Eagles,* Susan J. Fairweather-Tait, David E. Portwood, and Ron Self Institute of Food Research, Norwich Laboratory, Colney Lane, Norwich, U.K.
Adolf Gotz and Klaus G. Heumann Institute of Inorganic Chemistry, University of Regensburg, Universitatsstrasse 31, 0-8400 Regensburg, Federal Republic of Germany
A method Is descrlbed for the measurement of apparent zinc absorptlon In human nutrltlon studies. An enriched source of the stable Isotope “Zn was given to adult subjects together wlth a wheat cereal and the unabsorbed ”Zn measured In the feces. After drying, subsamples of the homogenized fecal material were ashed at 480 O C , purlfled for analysis by Ion exchange chromatography, and the “Zn/”Zn ratios determlned by both fast atom bombardment mass spectrometry and thermal lonizatlon quadrupole mass spectrometry. Good agreement was found between the two sets of results with mean preclslons of approximately 0.5 % for both techniques.
The quantitative measurement of stable isotopes of zinc in biological samples is essential for the advancement of research in human zinc nutrition. There is growing concern regarding the adequacy of zinc intake in Western diets (1, 2), especially among certain nutritionally vulnerable groups of the population, such as pregnant women (3). Zinc is not particularly well absorbed from the diet and absorption is influenced by many dietary and physiological factors (4). The most satisfactory means of studying zinc absorption and metabolism involves the use of zinc isotopes as tracers, but with the increasing concern about the safety of radioisotopes, stable isotopes are fast becoming the only viable option. This is particularly true for nutritionally vulnerable individuals to whom it would be unethical to administer a radiolabel. There are, however, certain disadvantages associated with the use of stable as compared to radioisotopes in zinc absorption studies. Apart from the relatively high cost, their natural occurrence in the environment is an added complication that necessitates very precise measurements for the accurate computation of enrichment in biological samples. In order that progress can be made in human metabolic studies, it is vital that these problems are overcome. With
this in mind, the present paper describes two different mass spectrometric techniques for measuring isotope ratios and compares the results obtained from the same set of biological samples. Earlier bioavailability studies on zinc employed neutron activation analysis of enriched 70Zn(5) or thermal ionization mass spectrometry (TIMS) using enriched 67Znand stabled isotope dilution (6). Inductively coupled plasma mass spectrometry has also been used (7). Isotope ratio measurements based on thermal ionization and single or double sector magnetic mass spectrometers have been shown to display high accuracy and precision. Recent work with the thermal quadrupole (THQ) instrument has confirmed that the results obtained are sufficiently accurate (8) and precise for many biological experiments (9). Similarly, the development of methods that use fast atom bombardment (FAB) ionization on a double sector “organic” mass spectrometer for quantitative inorganic analyses has shown that, based on the analysis of urine, feces, and saliva (10-22) it too can produce useful data for metabolic studies. However unlike TIMS which is internationally accepted as a definitive method of proven accuracy and precision, FAB mass spectrometry is relatively new and therefore in need of validation by accepted mass spectrometric techniques. Initial experiments utilized chelation with dithizone in the cleanup (12), but the ion exchange method (8) gives much higher concentrations of zinc and was adopted as the preparation procedure for all the work reported in this paper. This paper describes the measurement of 67Znenrichment in human feces collected for several days following an oral dose of enriched isotope given with a high-fiber cereal breakfast (13). Under these conditions all of the unabsorbed dose of 67Znwould have been excreted in the feces, together with some absorbed 67Znreexcreted via endogenous intestinal secretions. The difference between the oral dose of 67Znand the quantity of 67Zn,over and above natural abundance, excreted in the
0003-2700/89/0361-1023$01.50/00 1989 American Chemical Society
