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Anal. Chem. 1982, 5 4 , 2067-2072
(IO) Ekins, R. P.; Newman, B. Acta Endocrinol. 1970, 64, Suppi., 11-36. (111 C. I.Anal. Chem. 1979.. 51., 2306-2311. \ . . , . .Haifman. -
(3) Geiseler, D.; Bohner, J.; Ritter, M. Bull. S:chweiz. Ges. klin. Chem. 1080. 96-101. .--., 21 /3. .,..
~
~
~~
~~
1
(4) Ekins, R. P. Coll. int. Radioimmunologie, Lyon, 1981. (5) Robbins, I.; Johnson, M. L. I n "International Symposium of Free Thyroid Hormone"; Excerpta Medica: Amsterdam, Oxford, Princetown, 1979; pp 1-14. (6) Westphai, U. "Steroid-Protein-Interactions, Monographs on Endocrinology"; Springer: Berlin, Heidelberg, New York, 1971; Voi. 4. (7) Ramsden, D. B.; et ai. I n "Internationai (jymposium of Free Thyroid Hormone"; Excerpta Medica: Amsterdam,,Oxford, Princetown, 1979; pp 121-127. (8) Schiller, H. S.;Petra, P. H. J. SteroidBiochem. 1976, 7, 55-59. (9) Fletcher, I.E. J. fb,ys. Chem. 1977, 81 (No. 25), 2374-2378.
RECEIVED for review December 29, 1981. Accepted June 8, 1982, This work was supported by the ~ 1 v. H ~ ~~ boldtstiftung, granting a Fellowship to D.G., and was carried out in the Institute of Child Health, London, U.K., and the Hauptlaboratorium der Medizinischen Universitatsklinik, Tubingen, F.R.G.
Piezoelectric Crystail Thermogravimetric Analyzer for Temperature-Programmed Analysis of Deposited Films David E. Henderson,* Marle 6. DiTaranto,' and Wllllam G. Tonkin2 Department of Chemistry, Trinity College, hfartford, Connecticut 06 106
David J. Ahlgren, David A. G a t e n b ~ and , ~ Tuck Woh Shum4 Department of Engineerring, Trinity College, Hartford, Connecticut 06 106
A plezoelectric cryslal thermogravlmetric analyzer Is described whlch allows ,analysis of microgram samples at rates up to 100 OCImln. Siamples are deposited as fllms from solution In a volatile solvent. The varlatiion in frequency as a function of crystal temperature Is measured and subtracted from the TG curve by i3n LSI-11 minicomputer to yleld a curve of temperature vs. miass change. TG curves of samples as small as 200 ng were obtalned wlth the instrument described; however, the theoretlcal minimum sample Is at least 2 orders of magnltude less. Tho technique Is applicable at temeratures up to 570 OC uslng quartz crystals, but appllcatlons to temperatures In excess of 1000 OC should be possible wlth llthium nlobate piezoelectrlc crystals.
Thermogravimetry (TG) has developed into an important analytical technique since its introduction by Honda in 1915 (1). Recent applications have been extensively reviewed in this journal (2, 3) and have included studies of the decomposition of polymers ( 4 , 5 ) ,flame retardancy (6),oil stability (7, 8 ) , and the volatility of compounds in addition to more classical studies of minerals and precipitates ( 2 , 3 ) . A large portion of the use of TG has been in the routine screening of materials by industry, and much of this has been documented primarily in the technical applications literature. However, the references above provide 13ome examples of the diversity of applications of the technique. Commercial thermobalances employ beam, spring, cantilever, and torsion ballances. Measurements may be made under either isothermal or temperature programmed (nonisothermal) conditions. The latter provides considerably more information about a reaction since the exact temperature range in which a reaction occurs may be determined. Piezoelectric crystals have been used extensively both as sorption detectors Department of Clinical Chemistry, University of Connecticut, Storrs. CT 06268. * E.'I;du Pontde Nemors, Wilmington, DE 19898. 3Bell Laboratories, Holmodel, NJ 07733. Combustion Engineering, Windsor, CT 06095.
and to conduct isothermal TG. The original work by King (9) demonstrated the application of piezoelectric crystals for the detection of gas chromatographic effluents by sorption on crystals coated with films of liquids and isothermal T G of polymer oxidation (10). A wide range of analytical applications of this principle have been reported and are comprephensively reviewed in the report by Hlavay and Guilbault in this journal (11). The crystals (usually quartz) vibrate when electrically excited. The vibrational frequency is directly related to the mass of the crystal including any material on its surface. The resulting mass sensor (sometimes referred to as a quartz crystal microbalance) responds to very small changes in mass. Any reaction or process which can produce a change in mass on the crystal can be monitored with a high degree of sensitivity. Most piezoelectric microbalance studies have employed ATcut quartz crystals. The AT-cut crystal is a thin plate of quartz cut at a particular orientation to the axes of the quartz crystal. The AT-cut crystal oscillates in the thickness shear mode and is specifically preferred for this type of study due to its relatively low temperature coefficient of frequency near room temperature (1.2). The maximum temperature at which quartz exhibits piezoelectric behavior is referred to as the Curie point and is 573 OC. Other piezoelectric materials, specifically lithium niobate, exhibit much higher Curie points and may be employed at temperatures in excess of 1000 "C. Quartz crystals were used for this study and should be adequate for much routine T G however, the use of lithium niobate crystals should make possible T G studies throughout the entire temperature range normally employed for TG. The sensitivity of AT-cut quartz piezoelectric devices to changes in mass (eq 1) was originally derived by Sauerbrey (12). Often as little as lo-'* g of material can be detected. d F = (-2.3 X 106)F(dM,/A) d F = change in frequency due to coating
F = frequency in MHz
dM, = mass of coating A = area of coating
0003-2700/82/0354-2067$01.25/00 1982 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982
Applications of piezoelectric crystals as mass sensors have been limited, until now, to isothermal TG due to the large temperature coefficient of frequency. The theoretical frequency-temperature curve is defined by eq 2 (13). Isothermal T G studies a t elevated temperature were made possible only with careful temperature control.
dF = A , ( T - T,) + A J T -
+ A3(T-
f-ilr
2
(2)
dF = change in frequency in p p m from value at
I1
reference temperature
AI = -0.08538(&) A2 = (-0.134743 X
- (0.07833 X
A3 = (98.969
lo4) - (0.33 X 104)(Q)
X
Q = minutes of arc from zero angle for AT-cut quartz This study represents the first reported application of piezoelectric crystal for nonisothermal TG. This was accomplished through the use of a minicomputer to characterize the temperature frequency relationship for the crystal and to correct numerically the frequency-temperature-mass relationship to obtain the T G curve of the sample. A number of advantages are realized by using the piezoelectric crystal TG (PzTG) for temperature programmed TG studies. The high sensitivity allows very high heating rates to be employed. The limitation on heating rate in T G is the need t o maintain equilibrium conditions a t each temperature. Time is required for diffusion of gaseous products out of the sample. Thin films of material composed of one or two molecular layers should exhibit very rapid diffusion and thus permit thermal equilibrium to be maintained a t heating rates of 100 OC/min or greater. Thus, the smaller the sample size, the more rapidly it may be heated while maintaining thermal equilibrium. The sensitivity of piezoelectric crystals should allow the PzTG to approach this limit. Likewise, the effect of heating and cooling of the sample due to the reaction itself is less important for small samples. A typical T G curve using the PzTG employs a heating rate of looo C/min or more. The resultant high sample throughput makes this technique attractive for a variety of analytical problems. We foresee applications in pyrolysis, studies of flammability and flame retardancy, polymer characterization, aging studies of polymers, and evaluation of thin films and coatings.
EXPERIMENTAL SECTION Piezoelectric Crystals. Three types of quartz crystals were used in this study. The first were standard color TV 3.58-MHz AT-cut crystals. The electrodes were aluminum and the aluminum case was removed prior to use. High-temperature mounted crystals were 3/s in. diameter 5.0-MHz AT-cut (Bliley Electric Co., Erie, PA) and had 1/4 in. diameter gold electrodes. The electrodes were connected to the mounting leads using high temperature conductive epoxy. When the epoxy degraded with use, it was replaced with conductive silicone rubber (Tecknit, Cranford, NJ). Unmounted crystals were 3/8 in. diameter 5.0-MHz AT-cut quartz (Valtec, Hopkinton, MA). They had ' / 4 in. diameter gold electrodes and were fine ground. Construction of TG. The first prototype TG was constructed in a 2 X 2 X 6 in. aluminum chassis. A brass compression fitting (Parker-Hannifin) was modified and used to hold a four-hole Omegatite insulator (Omega Engineering Inc., Stamford, CT). The two crystal leads and a 0.005 in. diameter chromel-alumel (Omega Engineering) thermocouple were connected through the insulator to the oscillator and a 26-gauge ice point thermocouple. The oscillator used in the initial prototype was a Colpitts design (14); however, it did not produce sufficient drive to maintain oscillation at high sample loadings. It was used only for initial stability studies and replaced with a commercial oscillator kit (OX-1, International Crystal Manufacturing Co., Oklahoma City, OK)
IO
Figure 1. Prototype piezoelectric TG: (1) N, puge tube; (2) Omegatite insulator; (3) quartz crystal; (4) Pyrex tube heater; (5)Teflon ferrule; (6) compression nut; (7) compression plug (drilled and tapped for 9); (8)chassis; (9)0-seal male connector; (10) OX-1 printed circuit; (1 1) lead wires.
8-
7 High-temperature piezoelectric TG: (1) TG crystal; (2) mounting and feed through holes; (3) gold lead wire; (4) thermocouple crystal; (5) heater strip; (6) nickel lead wire; (7) MACOR ceramic base; (8) gold lead wire. Figure 2.
in all subsequent designs. This oscillator, while not truely optimized for the application,proved satisfactory to drive the crystal and was stable within the limits of the present digital frequency acquisition hardware. The second design used a 6 X 6 X 10 in. steel chassis and was similar to the first except that the crystal was mounted horizontally rather than vertically for easier sample placement. Compression fittings were used to provide a sealed system as shown in Figure 1. The thermocouple was mounted in the tube furnance such that it was positioned close to the crystal. The tube furnance was constructed from 24 mm Pyrex tubing. One end was sealed and a 8-mm glass tube attached as an outlet for the purge gas and for the thermocouple wire. The tube was wrapped with Scotch No. 69 Electrical Tape (3M, St. Paul, MN) and then with asbestas-insulatednichrome wire (Fischer Scientific Co., Pittsburgh, PA). The wire was wrapped to provide a noninductive heater and was connected to a variable voltage transformer (Staco, Inc.). The heater wire was held in place by a second layer of no. 69 electrical tape. The tube was then insulated with fiberglass. The third prototype TG was constructed to use unmounted crystals. A 1 in. diameter cylinder of Macor machinable ceramic (Duramic Products, Inc., Palisades Park, NJ) was machined as shown in Figure 2 and mounted on a transite board. Heat was provided by a flat, polished copper strip 3/8 X 1.5 in. which was silver soldered to 10-gaugecopper wire and connected as the tip of a 125-W Weller soldering gun. The strip was electroplated with nickel to prevent oxidation at high temperatures. One of the copper wires was grounded, allowing the strip to serve as one electrical contact for the crystal. Two spring clips were used to hold the crystal in place, one of which was also the electrical contact to the top of the crystal. The OX-1 oscillator was used
ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982
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2762-DI-PG, Data Translations Inc, MA). The latter included programmable gains of 1to 8. The output of the thermocouple was amplified with an instrumentation amplifier 1425-02, Teledyne-Philbrick, Dedham, MA) as shown in Figure 3. A digital frequency counter and real time clock circuit was constructed to acquire the frequency data. It was controlled by the digital output port and its data was read through the digital input port. The counter was constructed on a standard S-100 computer card (Vector Electronics Co., Sylmar, CA). A functional diagram of the counter is shown in Figure 4. Only one of the two counter inputs is shown, and the details of the counter interconnections are omitted for clarity. The counter board consisted of two 32-bit up counters using high-speed logic (74LS93) for the first eight bits in each to allow accurate counting of frequencies as high as 60 MHz. The counting was initiated by a timing signal from the digital output (Data Sent Lo) when the Start bit was also set. This changed the state of the D flip flop (IC17) and produced the Start signal which gated on the frequency input to the counter if enabled and also gated the output of a very stable 1.00-MHz temperature compensated crystal oscillator (TCCO-26MA, Bliley Electric Co.) into a series of divide by 10 circuits. The output of one of four counters was selected through a multiplexer and used to terminate the counting by resetting the flip flop. This signal also produced a Data Taken signal to the digital port to indicate the end of the counting period. The digital interface responded to Data Taken by setting the ready bit of the status register or generating an interrupt if enabled, thus signaling the end of the counting period to the program. The duration of the count period was programmable as 0.001, 0.01, 0.1, and 1.0 s. The data from the counters could then be selected through a multiplexer and read by the digital input port. The outputs of the divide by 10 circuits were separately multiplexed to the real time clock and external interrupt inputs of the A/D converter and were used to initiate A/D conversions of the thermocouple input during the counting period. Additional logic allowed the real time clock to operate continuously for general purpose applications by setting the Free Run bit in the output. Data Taken was returned immediately during free run, reset, or multiplex change operations by the logic in IC22.
470 K
vt
I-
Flgure 3. Thermocouple amplifler circuit diagram.
and the output was buffiered with a PAX-1 RF amplifier (International Crystal Manufacturing Co.). The temperature was measured with an identical crystal plated over most of ita surface with nickel as described b y King, Camilli, and Findeis (15). This crystal was placed beside the oscillator crystal on the strip heater. A gold-plated spring clip was used to contact the gold portion of the electrode on the surface while a niclkel clip was used to contact the nickel plating. The gold clip wai3 attached to copper wire as the copper-gold t_l
,
..
*._ ---*.-*+-
e - 2
-
80 120 4g TEM?E?ATURE (
160
C. >
20E
Flgure 8. TG curves of 35 pg of Cr(tfa), at approximately 100 "C/min. TG curves normalized to the total frequency change of the sample with
the greatest change to show variation between samples; nitrogen purge, 130 mL/min; sampling rate, 10 points/s. data for Cr(tfa), were very reproducible with an average temp(l/2) of 136 "C and a standard deviation of only 2.6 "C for the 25 determinations. Data for the hydrocarbon samples were not as reproducible having an average of 167 "C with a standard deviation of 10.3 "C. This was attributed to the tendency of these volatile liquids to begin evaporation even a t room temperature. This was a serious problem for tetradecane which could not be studied for this reason. Hexadecane was sufficiently less volatile that some data were obtained. Octadecane was not studied due to the need to keep operating temperatures low to prolong the life of the adhesive in the crystal mount. The metal complexes studied were all solids and showed no tendency to evaporate at room temperature. Thus the majority of the studies were conducted using these samples. A typical group of TG curves for Cr(tfa), is shown in Figure 8. The weight loss axis in Figure 8 is normalized to the greatest frequency change observed for the three samples and presented as percent weight loss. This is done to indicate the presence of considerable variability in the change in frequency observed for multiple samples and its significance in terms of weight loss. When multiple TG curves of a sample were individually normalized as in Figure 9, the resultant data were quite reproducible in terms of fractional weight loss. The effects of the variation in sample size on the observed temperature at the half point in the TG curve were also minimal.
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982
100,
100,
801
804
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f3
c
40.
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42
80
- =-V P E R 4 T U R E
,
120 (
160
200
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Flgure 9. TG curves of 35 p g of Cr(tfa),: conditions as in Figure 8; each TO curve normalized 0 to 100% weight loss individually and plotted on the same graph.
The major cause for the variation in frequency change observed for repetitive samples was believed to be the difficulty in obtaining identical placement and size of the sample films. Equation 1 indicates the inverse relationship between sample area and change in frequency. If the sample were applied to the entire active surface of the crystal, as has normally been the case in isothermal studies, the area term would become a constant in the equation. Coating of the entire surface, however, makes it difficult to determine the total sample weight applied independently of the measurement of the crystal frequency. In this study we wished to obtain a direct measurement of the sample size and did this by applying measured volumes of solutions of known concentration. The samples were applied in such a way that the entire sample remained on the active area of the crystal. As the amount of sample applied varied, the area of the sample also varied resulting in a poor correlation between total frequency change observed and sample size. The greatest sensitivities were observed for the smallest samples with a rapid decrease in sensitivity as the sample size increased. The sensitivity also differed for the hydrocarbon samples when compared to the metal complex samples. This may be due to the different area spots observed for these samples or it may be due to a different degree of coupling between the liquid sample and the crystal. The limitations implicit in these results were the difficulty in predicting the sensitivity for a given sample and the variation in sensitivity with the method of application to the crystal. However, the sensitivity predicted by the Saureby equation remained a reasonable estimate. Figure 10 shows T G curves of n-hexadecane at 1.4 pg of total sample. The upward drift of the base line prior to the sharp break in the curve may have been indicative of rapid evaporation of the sample even at low temperatures or possibly to spreading of the hydrocarbon similar to that observed in the work of King (9). A measurable break in the TG curves was obtained for samples as small as 0.20 kg.
CONCLUSIONS The data presented clearly demonstrate the ability to conduct T G studies at sample levels less than 1 pg using piezoelectric crystal mass sensors. The prototype instrument used to generate the results presented here is limited by the
0
62 120 180 TEMPE2ATURE ( O
240
r. )
300
Figure 10. TG curve of 1.4 p g of n-ClBH,,: conditions as in Figure 8; each TG curve normalized individually as to percent weight loss.
method of crystal mounting to a maximum temperature of approximately 200 "C. The third prototype instrument eliminates the adhesive from the crystal mount and has been used at temperatures approaching 500 "C. The use of lithium niobate piezoelectric crystals and a larger wattage heater will extend this to at least 800 "C. Several hardware and software changes are necessary to take full advantage of the improvements. Finally, the use of higher frequency crystals will lead to a considerable increase in sensitivity. When all of these changes are fully implemented, we are confident that it will be possible to obtain a usable T G curve on as little as 1 ng of sample. The potential of such a device when applied to the study of the volatilization of thin films and coatings is tremendous. The PzTG should also find application in the study of kinetics of a wide range of surface phenomena by the introduction of reactive agents into the purge gas. Studies of flammability of polymers as thin films will be possible rather than the studies of bulk properties required by the inadequate sensitivity of current TG instrumentation. The high speed of analysis with the PzTG will further enhance the usefulness of T G for routine analysis and screening of materials.
LITERATURE CITED (1) Honda, K. Scl. Rep. Tohoka Imp. Univ., Ser. 1 1915, 4 , 97-103. (2) Murphy, C. B. Anal. Chem. 1978, 5 0 , 143R-153R. (3) Murphy, C. E . Anal. Chem. 1980, 5 2 , 106R-151R. (4) Mitchell, J., Jr.; Chin, J. Anal. Chem. 1973, 4 5 , 273R-332R. (5) Cobler, J. G.; Chow, C. D. Anal. Chem. 1979, 5 1 , 287R-303R. (6) Hassel, R. L. Am. Lab. (Fairfield, Conn.) 1979, 9 , 35-43. (7) Hassel, R. L. J . Am. 0llChem. SOC. 1976, 5 3 , 179-181. (8) Nieschlag, H. J.; Hagemann, J. W.; Rothfus, J. A,; Smith, D. L. Anal. Chem. 1974, 46, 2215-2217. (9) Klng, W. H., Jr. Anal. Chem. 1964, 3 6 , 1735-1739. (IO) Flscher, W. F.; King, W. H., Jr. Anal. Chem. 1967, 3 9 , 1265-1273. (11) Hlavay, J.; Guilbault, G. G. Anal. Chem. 1977, 4 9 , 1890-1696. (12) Sauerbrey, G. Z . Phys. 1959, 155, 206-222.
(13) "Piezo Crystal Design Guide"; Piezo Crystal Co.: Carlisle, PA. (14) Manasse, F. K. "Semiconductor Electronics Design"; Prentice Hall: Englewood Cliffs, NJ, 1977; p 333. (15) Klng, W. H., Jr.; Camilli, C. T.; Findeis, A. F. Anal. Chem. 1968, 4 0 , 1330- 1335.
RECEIVED for review March 16,1982. Accepted June 28,1982. Paper presented at the 1981 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1981. This work was carried out under the support of a William and Flora Hewlett Foundation Grant of Research Corporation.
