100
UPPER TRACE SELECTED ION SUMMATION PROFILE LOWER TRACE MASS CHROMATOGRAM
'51
SPECTRUM
"O
mle 358 360 362 364
mle 364
NUMBER
Figure 3. Comparison of GC/MS data single ion intensity and SIS plots for hexachlorobiphenyl isomers in Lake Superior lake trout
However, the spectrum does have four characteristic chlorine ion clusters at mle 263,277,345, and 380 that are generally used to identify dieldrin. Figure 2a is the conventional "mass-range chromatogram" where a sum of ion intensities for mass ranges 261-265, 275-279, 343-347, and 378-384 are plotted vs. spectrum number. The plot is complex, and dieldrin cannot readily be determined to be present in the sample. A less complex plot may be generated if single ion intensity profiles are used (Figure 2b); however, the sensitivity gained by a mass-range summation is sacrificed by using this technique. A search for dieldrin by using SIS Analysis is presented in the top trace of Figure 2b. The plot shows only one strong peak for which GC retention time may be readily calculated and/or a library search of the spectra at the peak maxima may be made. The confirmation of dieldrin has been accomplished rapidly and efficiently.
Table I. Average Increase in Sensitivity of PCB by Chlorination Number for SIS Peaks Compared to M 6 Peaks
+
Chlorination No. of PCB
Average increase in sensitivity
No. of peaks
3
6.1
4 5 6
3.4
3 5
7.5
6
7.1
6
The utility of SIS Analysis was further demonstrated when SIS Analysis was compared to single ion mass chromatograms for PCB's in Lake Huron walleye. Mass chromatograms for the M 6 ions of tri-, tetra-, penta-, and hexachlorobiphenyls were plotted on separate graphs along with SIS traces of the corresponding PCB. Figure 3 is a plot for the hexachlorobiphenyl isomers. No loss of peak resolution was observed from the single ion trace to the SIS trace. In addition, an increase in compound sensitivity was observed for PCB's that ranged from a factor of 3.4 to 7.5 (Table I).
+
(1) R.
LITERATURE C I T E D A. Hites and K. Biernann, Anal. Chem., 40, 1217 (1968).
(2) R. A. Hites and K. Biernann, Anal. Chem., 42, 855 (1970). (3) R. A. Hites and K. Biemann, in "Advances in Mass Spectrometry", Vol. 4, E. Kendrick, Ed., The Institute of Petroleum, London, 1968,p 37. (4) H. Nau and K. Biernann, Anal. Chem., 46,426 (1974). (5) J. E. Biller and K. Biemann, Anal. Left., 7 , 515 (1974). (6)J. W. Eichelberger. L. E. Harris, and W. L. Budde, Anal. Chem., 46, 227 (1974). (7) G. D. Veith, D. W. Kuehl, and J. Rosenthai, J. Assoc. Off. Anal. Chem., 58,
l(1975). ( 8 ) D. W. Kuehl, G. E. Glass, and F. A. Puglisi, Anal.
Chem., 46, 588 (1974).
RECEIVEDfor review November 4,1976. Accepted December 17, 1976.
Sealed Stainless Cell for Differential Scanning Calorimetry on Volatile Systems C. J. H. Schouteten, S. Bakker, B. Klarema, and A. J. Pennings" Department of Polymer Chemistry, State University of Groningen, The Netherlands
Differential scanning calorimetry ( 1) (DSC) is nowadays widely employed in studying thermal properties and transitions of many systems. Its range of applications has been extended to volatile materials by introducing sealed cells. Aluminum volatile sample pans, sealed by cold-welding ( 2 ) ,and stainless steel large volume capsules ( 3 ) ,sealed by rubber O-rings, are commercially available (Perkin-Elmer). Freeberg and Alleman ( 4 ) as well as Knappe, Nachtrab, and Weber ( 5 ) designed cells that can withstand considerably higher pressures in the order of 20 atm. These designs apply a Teflon disk or ring-shaped washer and a screw-thread to provide a tight seal between the cover and the pan of the cell. The purpose of this paper is to describe a simple cell of stainless steel that was sealed by resistance-welding by means of transformed capacitor discharge (6).This type of sealing appears to be quite suitable for this application, since the actual welding time is of the order of s so that the parts are heated only slightly. The welded capsule could withstand pressures over 80 atm and turned out to be particularly useful when applied to differential scanning calorimetry on volatile systems. EXPERIMENTAL The dimensions of the stainless steel (type 316) cover and pan are 522
ANALYTICAL CHEMISTRY, VOL. 49, NO. 3, MARCH 1977
indicated in Figure 1. Both parts, and in particular the rims, were ultrasonically cleaned in dichloromethane prior to sealing by resistance-welding.The welding was performed with a PECO Impuls schweiss Maschine Typ FP3K equipped with a ElektrodenkraftSteuergerat Typ LA40LE4 (PECO,Munchen-Passing,W. Germany). In this study cover and pan were welded by means of copper elec075
cover
Pa"
L
06 5
1
Figure 1. Dimensions'(in mm) of the stainless steel sealed cell
260
280
300
Tindim ted I O C I Figure 2. Melting thermogram of adamantane (25 mg) in a sealed cell at a scan speed o f 8 OC/min
I
1
LITERATURE CITED
-0
aJ c
E. M. Barrall and J. F. Johnson in "Techniques and Methods of Polymer Evaluation", Volume 2, Thermal Characterization Techniques, Marcel Dekker, New York, N.Y., 1970, Chapter 1, p 1. Perkin-Elmer, Norwalk, Conn., Instructions Volatile Sample, Sealer Accessory 219-0061, documentation No. 990-9338, December 1966. , , Perkln-Elmer, Norwalk, Conn , Instructions Large Volume Capsules (LVC) 319-0218, documentation No. 993-9115, July 1975. (4) F. E. Freeberg and T . G. Alleman, Anal. Chern., 38, 1806 (1966). (5) W. Knappe, G. Nachtrab, and G. Weber, Aflgew. Makromol. Chem., 18, 169 (1971). (6) H. Alberti and F. Frungel, Blech, 9, 678 (1961).
[3
.-u
-0
C .I-
with a sealed aluminum volatile sample pan (2) by measuring the melting peak temperature of 3.81 mg of tin as a function of the scan speed. The results, shown in Figure 3, indicate that the thermal lag of the welded cell is in between that of a sealed aluminum volatile sample pan and the Freeberg and Alleman design ( 4 ) . An interesting feature of our sealed cell is that it can be used in a greater temperature range since heat effects due to glass transition or fusion of the sealing material are absent. Furthermore, the broadening of the melting peaks was considerably less than in the case of the Teflon sealed cell. The welded cells could withstand higher pressures and were found to be particularly suitable for thermal studies in which the system is to be subjected to repeated heating and cooling cycles. The Teflon gasket turned out to leak under similar conditions. Preliminary measurements using nickel transistor caps instead of the cell of Figure 1also yielded favorable results. The resistance-welding technique is very simple, requires seconds, and can be applied to several metals and alloys so that the method of encapsulation described above can be employed in the study of the thermal behavior of many volatile systems.
520
RECEIVEDfor review October 7, 1976. Accepted November 1, 1976.
500 I 20
LO
Scan s p e e d (OC/,,,
I
Figure 3. Melting peak temperature of 3.81 mg tin in various cells as a function of the scan speed (0) Freeberg and Alleman design (brass; 1115.83 mg). ( 0 )Aluminum volatile sample pan sealed by cold-welding (28.88 mg). (0)Stainless steel cell sealed by resistance-welding (320.98 mg)
trodes. The welding pressure was lo00 kg/cm2 and the welding energy was 0.2 kJ. Calorimetric measurements were carried out using a Perkin-Elmer DSC model 1B.
RESULTS AND DISCUSSION At first it was established that resistance-welding of a cell filled with water did not cause any detectable change in mass. Furthermore, heating of this cell up to a temperature of 300 O C where the vapor pressure of water is 80 atm also did not lead to any weight loss. A melting thermogram of adamantane, that sublimes very readily, displays only a melting peak, and changes in the baseline due to sublimation or deformation of the sample pan could not be discerned (Figure 2). The thermal lag of the new sealed cell was compared with that of the cell designed by Freeberg and Alleman as well as
CORRECTIONS Theoretical and Experimental Characterization of Field-Flow Fractionation
In Figure 7 of this article by J. Calvin Giddings, Frank J. Yang, and Marcus N. Myers, Anal. Chem., 48, 1126 (1976), the exponents should be -8, -7, and -6 on each axis, replacing -9, -8, and -7, respectively. Calculator Program Yielding Confidence Limits for Least Squares Straight Line Regression Analysis
In Table I of this article by W. J. Blaedel and D. G. Iverson, Anal. Chem., 48,2027 (1976),the term in brackets in the expression of the confidence limit for parameter A should be raised to the power 1/2. In the legend to Table I, the definition of C should be B 2 - t2(sy2/Sx,),and (X2/df)o.osshould be a power series in ( N - 2) instead of in 1/(N - 2). The data point (700,4374) in the example (Table 11) should read (700,4347).
ANALYTICAL CHEMISTRY, VOL. 49, NO. 3, MARCH 1977
523
