684
Anal. Chem. 1987, 59,684-687
as sensitive as the conventional FPD, the detection limit of which was about 0.04 pg/mL. In conclusion, the major advantages of the present technique are the improvement of the sensitivity by about a factor of 5 as well as the accurate correction for static backgrounds. Nevertheless, the improvement of the sensitivity by the present method is not as good as expected. This may be due to the incompleteness of the modulated magnetic field used, because the accuracy of the period and peak strength is estimated to be about *1%. The purpose of this work is to compare the performance of this new technique relative to that of conventional FPD. In order to get performance higher than that presented here, it will be necessary to use an improved modulated magnetic field (higher modulation frequency, a more completely modulated magnetic field), a burner, optical system, sample introduction system, combustion conditions, etc.
and K. Fukuda of the National Chemical Laboratory for Industry for useful discussions.
LITERATURE CITED (1) Nozoye, H.; Someno, K. Bunseki Kagaku 1985, 8 , 508-509. (2) Hashlmoto, S.: Fujiwara, K.; Fuwa, K. Anal. Chem. 1985, 5 7 ,
1305- 1309. (3) Murphy, J.; Riley, J. P. Anal. Chim. Acta 1962,27, 31-36. (4) Brody, S.S.;Chaney, J. E. J . Gas Chromatogr. 1988,4 , 42-46, (5) Wakayama, N. I.; Ogasawara, I.; Nishikawa, T.; Ohyagi, T.; Hayashi, H. Chem. Phys. Lett. 1984. 107, 207-211. (6) Wakayama, N, I.; Nozoye, H.; Ogasawara, I.; Fukuda, K. Japanese Patent 60-43516.
Nobuko I. Wakayama* Hisakazu Nozoye Ichiro Ogasawara National Chemical Laboratory for Industry Tsukuba Research Center Yatabe, Ibaraki 305, Japan
ACKNOWLEDGMENT The authors thank T. Katayama of the Electro Technical Laboratory for sharing his knowledge of magnets and S. Nishi
RECEIVED for review July 22,1986. Accepted November 12, 1986.
AIDS FOR ANALYTICAL CHEMISTS Purge Gas Enhancement of Peak Resolution in Differential Scanning Calorimetry Guang-Way Jang, Ranjana Segal, and Krishnan Rajeshwar* Department of Chemistry, The University of Texas a t Arlington, Arlington, Texas 76019-0065 Overlapping peaks are a nuisance in the analyses by differential scanning calorimetry (DSC) of mixtures and compounds that undergo more than one thermal event. The strategy for enhancing peak resolution in DSC usually is based on a reduction in heating rate or sample mass. This method, however, suffers from the handicap associated with prolonged analysis times and lowered sensitivity, respectively. This paper describes an alternative method based on the use of a thermally conductive purge gas (e.g., He) to enhance peak resolution in DSC. The theory is verified through use of computer simulations and experiments on model compounds and mixtures.
THEORY The equivalent-circuit model previously developed for DSC cells of the heat-flux type (1) is shown in Figure 1. The slope, d(dq/dt)/dTsH, of the leading edge of a DSC peak may be written as d(dq/dt) - 1 dTsH Rs
1 RD'
1 2 1 +-KRD' ++(1) KRG KRG
.
In eq 1 and as before ( I ) , dq/dt is the heat-flow rate, TSH is the sample-holder temperature, Rs is the thermal resistance between sensor and sample, RD' is the thermal resistance of the disk between the sample and reference platforms, RGis the thermal resistance of the gas phase separating the heater block from the sample and reference, RG' is the gas thermal resistance between sample and reference, and K is the thermal lag term containing sample and instrumental contributions. It is important to emphasize that Rs also contains contributions from the thermal resistance of the interfacial region between the sample pan and the sample platform. Narrowing of the peak width and thus better separation of overlapped DSC signals can be achieved by minimizing the magnitudes 0003-2700/87/0359-0684$0 1.50/0
of Rs and the other component resistances (cf. eq 1) through the use of a purge gas with facile heat conduction characteristics.
EXPERIMENTAL PROCEDURES All experiments were performed on a Du Pont Model 1090 thermal analysis system fitted with the Model 910 DSC accessory module. Fusion endotherms were recorded usually after one or two initial "conditioning" heat-cooled cycles through the transition. Commercial samples of In and adipic acid (99.99% purity or better) were used as received. The purge gas (either Ar or He) was flushed through the DSC cell at the rate of ca. 80 mL/min. Sealed A1 sample pans were used in all the cases. Computer simulations of DSC thermograms were carried out on an IBM PC-XT fitted with a Hewlett-Packard Model 9575A graphics plotter. These simulations were facilitated by decomposing the overall thermograms into pretransition, transition, and posttransition regimes. Input parameters comprised the scanning temperature limits, resolution element size, transition enthalpy, transition temperature, and the heat capacities of sample plus container. Multiple transitions were individually simulated prior to summation. Parameters such as sample mass, heating rate, and the component thermal resistances in eq 1 were then systematically varied to assess their influence on DSC peak shapes and resolution. Further details of the simulation protocol will be published elsewhere (2).
RESULTS AND DISCUSSION For initial investigations on purge gas enhancement of DSC peak resolution, we chose the melting transitions of indium (mp 156.6 "C; heat of fusion, AHf = 28.4 J/g) and adipic acid (mp 152.0 "C, A",= 253 J/g) as model systems. The simulated thermograms in Figure 2 parts a and b, for In and In/adipic acid mixtures, respectively, confirm our expectations (vide supra) on (a) increase of the slope of the leading edge of the DSC endotherm, (b) narrowing of the endotherm, and ( c ) enhancement of peak resolution when the purge gas is (C 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 4, FEBRUARY 15, 1987
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180
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3 E \
Flgure 2. Computer-simulated DSC thermograms illustrating (a) the enhanced slope of the leading-edge and peak-narrowing in He vs. Ar for the I n melting transition and (b) the enhanced peak resolution for an Wadipic acid mixture in He vs. Ar. The inset in Figure 2a contains a set of DSC thermograms for I n in He and Ar atmospheres. The sample mass was 10.16 mg and the heating rate was 10 OC/min.
switched from Ar to He. Figure 3 contains experimental data, attesting to the positive influence of He on peak resolution. Figure 4 contains experimental thermograms of In/adipic acid mixtures, wherein the purge gas peak resolution technique is compared with the usual method of reducing sample mass or heating rate. We have employed the peak resolution parameter, P, defined by previous authors for chromatogram analyses (3)for quantitative comparisons. With reference to Figure 5, this parameter is given by p =f/g (2) . Base line resolution (P= 1.00) is achieved at identical heating rate and comparable sample mass when the purge gas
140
158
188
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178 (OC)
Figure 4. Influence of varying sample mass, heating rate, and purge
gas atmosphere on peak resolution in DSC thermograms for In/adlpic acid mixtures. Parts a-c pertain to Ar atmosphere and part d shows data in He atmosphere. The heating rate was 10 OC/min for parts a, b, and d, whereas the thermogram in Figure 4c was run at 5 'C/min. The sample masses were (a) 8.00 mg, (b) 4.00 mg, (c) 7.93 mg, and (d) 8.02 mg.
atmosphere is switched from Ar to He (compare Figure 4 parts a and d). On the other hand, peak resolution is incomplete (cf. Figure 4b, P = 0.81) for a 50% reduction in sample mass. Complete resolution can be achieved, however, by halving the heating rate (cf. Figure 4c, P = 1.00), but at the cost of ap-
686
ANALYTICAL CHEMISTRY, VOL. 59, NO. 4, FEBRUARY 15, 1987
J
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'
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Figure 7. Comparison of DSC thermograms for l a in Ar (a) and He (b), respectively. The sample mass was 2.60 mg and the heating rate was
4
:
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:
:
:
:
:
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Figure 6. Comparison of DSC thermograms in Ar (a and b) and He (c) with In and adipic acid samples contained in separate AI pans. The heating rates were 10 OC/min in parts a and c, and 5 'C/min in part b. The sample masses were malntalned constant for In and adipic acid at 4.94 mg and 3.04 f 0.04 mg, respectlvely. proximately double the analysis time and ca. 50% reduction in analytical sensitivity. Different samples were utilized for the data in Figure 4 parts a-d, to preclude complications from slow volatilization of adipic acid. The relative weight fraction of adipic acid/In in the mixture was maintained constant at a ratio of 3862 in all the cases. A subtle feature of purge gas effects is that the enhancement of the endotherm slope does depend on the sample thermal conductivity (cf. eq 1). Thus, the influence of He on the endotherm slope may be expected to be more pronounced for In than for a sample with lower thermal conductivity such as adipic acid. This was verified experimentally. In fact, when this happens, the diminution in time to peak maximum that always results when He is used (cf. eq 14,ref l),may have an overriding influence on analytical sensitivity. This effect is actually seen in a careful examination of the thermograms in Figure 4a and Figure 4d, wherein the adipic acid peak amplitude is attenuated in He, contrasting the In case. It should also be noted that ordinate recalibration is necessary when the purge gas is switched from Ar to He. The dramatic effect of He on peak resolution is better revealed in an experimental format wherein the two samples (e.g., In and adipic acid) are stationed in different pans (i.e., reference and sample pans, respectively). Complications arising from chemical reactions between the mixture components are also thereby avoided. Figure 6 illustrates representative data utilizing this "twin-pan" strategy. After extensive evaluation of this purge gas resolution technique using model compounds and mixtures, we have
80
100 120 Temperature
140
160
(OC)
Figure 8. Comparison of DSC thermograms for l b in Ar (a) and He (b and c). The heating rates were 10 OC/min in parts a and b and 5 'C/min in part c. The sample masses were 1.15, 1.72, and 1.49 mg, respectively, in parts a, b, and c. begun to utilize it for the analyses of "real-world" samples. For example, this technique has proved to be useful for the study of deaquation and dealkylation reactions of cobalt complexes, 1, containing the bis(dimethy1glyoximato) equatorial ligand. ,H
0 '
' 0
la: R = CH,CHCH,; L = HzO b: R = C6H,0CH,CH2
As illustrated by the thermograms in Figure 7 for compound la, use of He (instead of Ar) alone suffices to completely separate the dealkylation exotherm from the deaquation en-
Anal. Chem. 1987, 59, 687-688
dotherm. An extremely intractable case is that involving compound l b (cf. Figure a), which requires a combination of He and reduced heating rate to bring about base line resolution. A 50% reduction in the heating rate or sample mass alone failed in this case. The idea that peak resolution in DSC may be enhanced with the use of a high thermal conductivity purge gas is not entirely new (cf. ref 4). However, this idea has not been verified previously by experiments and computer simulations. Furthermore, earlier claims (4)as to the negative influence of He on analytical sensitivity need not be universally true. It has been shown herein how the thermal characteristics of the sample itself play an important role in this regard.
ACKNOWLEDGMENT The authors thank the Du Pont Co.Jor instrumental sup-
687
port and Kenneth L. Brown for gifts of the cobalt samples.
Registry No. He, 7440-59-7. LITERATURE CITED (1) Jang, 0.-W.; Rajeshwar, K. Anal. Chem. 1988, 58, 416. (2) Jang, G.-W.; Rajeshwar, K., manuscript in preparation. (3) Morgan, S.L.; Deming S. N. J . Chrometogr. 1975, 112, 267. (4) Baxter, R. A. In ThermalAna&s/s; Schwenker, R. F., Jr., Garn, P. D., Eds.; Academic: New York, 1969; Voi. 1; p 65.
RECEIVED for review August 11, 1986. Accepted October 6, 1986. This research was supported by the Texas Advanced Technology and Research Program and the Strategic Defense Initiative Office, Innovative Science and Technology Branch, through the Defense Nuclear Agency, Contract No. DNA 001-85-6-0181.
Automated Ion-Exchange Column System for Biological Sample Fractionation Y. T. Kim and Christiaan Glerum* Ontario Tree Improvement and Forest Biomass Institute. Ontario Ministry of Natural Resources, Maple, Ontario, Canada LOJ 1EO Ion-exchange resins are widely used in chemical laboratories for separation of many organic compounds. In our laboratory we have been extracting amino acids and sugars from tree tissues and separating the various fractions by passing them through cation- and anion-exchange columns prior to separating them on thin-layer chromatographic plates (1). Ionexchange columns are needed not only for separation of compounds but also for increasing the concentration of samples as well as for purifying the samples (2). When tree tissues are extracted for various compounds, the ion-exchange columns are a neccessity prior to chromatographic analyses (1, 3-5). When ion-exchange resin columns are used, the flow rate has to be carefully controlled and the solution level on top of the resin should not be allowed to go below the top of the resin because air bubbles in the resin will decrease the resin's efficiency, which would be a serious source of error. Furthermore, only a few columns (about four) could be run manually a t one time. To avoid the time-consuming work of watching the meniscus in the columns from not getting past the top of the resin, a device was developed that automatically stops the solution flow at the desired level in the column. With this system one can operate many columns a t any one time. We now operate 10 columns concurrently in our laboratory. The construction of this device is described below.
DESCRIPTION OF APPARATUS The device consists of three components: (1)power supply, (2) solenoid, and (3) detector control. The electronic circuit diagram with the materials used is shown in Figure 1, while the assembled circuit is shown in Figure 2. Because we operate 10 columns with this system, the power supply has to provide the voltages for 10 integrated circuits (IC) and 10 solenoids. The 23 V dc is regulated by the IC LM317 voltage regulator for the IC LM1830 and the 25 V dc for the solenoids is directly connected to the bridge rectifier. Two platinum wires are inserted as probes through the glass wall of each chromatography column, 19 cm above the Teflon stopcock and about 1cm above the ion-exchange resin. The column is 25.0 cm high and 1.05 cm i.d. with a 200-mL reservoir on the top. The resin bed is 15 cm high. Silicon rubber tubing is attached to the delivery tip and passed through a 24-V solenoid, which controh the flow by opening and closing the silicon 0003-2700/87/0359-0667$01.50/0
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Flgure 1. Electronic circuit diagram and materials used: 1, LM1830 (10); 2, 2N5195 (10): 3, 2N3904 (10): 4, 1N645 (10): 5, 100-V 4-A bridge rectifier (1): 6, power transformer, 115 V, 60 Hz in, 24 V dc, 3 A (1); 7, LM317 voltage regulator (1); 8, capacitor, 4000 pF 75 V dc (1); 9, 50 pF 50 V dc (1); 10, 0.05 pF 100 V (10); 11, 0.001 pF (10); 12, vector board (1); 13, neon indicator (1): 14, fuse holder (1): 15, toggle switch (2); 16, resistor 2.7 K (20): 17, resistor 7.5 K (10): 18, capacitor, 100 p 25 V dc (10); 19, wnnector (1); 20, case (1). The numbers in the diagram are matched with the numbers of the materials. The numbers in parentheses are the number of parts needed for a
complete circuit. tubing. The control signal from the platinum probes is generated by the IC LM1830 and sent to the solenoid via a current-boosting transistor. This IC works as an oscillator and as a threshold detector, which generates about 7-kHz ac signal that goes through the probes and solution. It has an internal reference register (nominal 13 kR) of 10-kRthreshold resistance, 680-mV threshold 0 1987 American Chemical Society
