Anal. Chem. 2001, 73, 2012-2017
In-Column Pyrolysis: A New Approach to an Old Problem T. Go´recki*,† and J. Poerschmann‡
Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1, and Department of Remediation Research, UFZ-Center for Environmental Research, Leipzig-Halle, Germany
High-molecular-weight fragments produced during pyrolysis of both natural and synthetic materials often carry the most significant structural information. Their diagnostic value is usually limited when using commercial pyrolysis devices because of analyte discrimination on transfer from the pyrolysis unit to the GC column. A device enabling pyrolysis in line with GC column was developed to overcome this problem. Pyrolysis is carried out in a segment of deactivated stainless steel tubing. One end of the tubing is connected through a restrictor to a standard GC injector, and the other end is connected to a precolumn followed by a GC column. Pyrolysis is carried out by passing a pulse of electric current from a capacitive discharge power supply through the tubing. Nondiscriminated alkane pattern up to C-58 (limited by the temperature limit of the GC stationary phase) was observed for the pyrolysis of polyethylene. A comparison of conventional pyrolysis with in-column pyrolysis indicates that the range of semivolatile pyrolysis products that can be detected in the pyrograms extends much further toward higher-boiling compounds for the technique proposed. The new approach has also proved very useful in methodical variations of pyrolysis, including thermochemolysis using tetramethylammonium hydroxide.
that relative rates of chemical reactions control the pyrolysis process rather than transport limitations. Limitation (ii) is also very significant, as evidenced by both literature studies6-9 and our own experience. The highly desirable products that are often lost during transfer from the pyrolysis unit to the column include long-chain alkanes, alkylbenzenes, fatty acids, and dicarboxylic acids (as methyl esters after tetramethylammonium hydroxide (TMAH)-assisted thermochemolysis, cf. below) as well as steranes and hopanes. The high diagnostic value of these compounds stems from the fact that they cannot be formed during pyrolysis from other HOM subunits or functional groups, which is in contrast to many smaller compounds often seen in pyrograms. For example, phenol in pyrograms might originate from the HOM network itself (i.e., from phenol units in the HOM network), but it might also be a breakdown product of lignin, carbohydrates, and proteins, which are the starting material for HOM. We believe that limitation (ii) can be vastly reduced or even entirely eliminated by changing the way in which pyrolysis is carried out. This paper presents a new device, which allows pyrolysis to be performed in line with the GC column (patent has been applied for). No discrimination related to analyte transfer should occur in this approach, because the pyrolysis unit forms an integral part of the GC column train and is located inside the GC oven.
Analytical pyrolysis can be a very useful tool for characterization of complex materials, including man-made polymers (e.g., plastics), and natural organic polymers such as humic organic matter (HOM).1-3 Unfortunately, conventional pyrolysis of materials of the latter type has significant limitations.4 The limited value of pyrolysis in HOM research is mainly attributed to (i) formation of carbonaceous residue, which essentially has no diagnostic value, and (ii) losses of high-boiling pyrolysis products of significant diagnostic value on transfer from the pyrolysis unit to the GC column. Limitation (i) cannot be circumvented with these crosslinked polymers. The phenomenon occurs also when using insource pyrolysis under high-vacuum conditions,5 which indicates
EXPERIMENTAL SECTION A schematic diagram of the in-column pyrolysis apparatus is presented in Figure 1a. The design of the pyrolysis device is presented in Figure 1b. The device consists of an inert Silcosteel metal capillary (1) (0.53 mm i.d.; Restek, Bellefonte, PA), connected through butt connectors (2) (Supelco, Bellefonte, PA) to a fused-silica restrictor (3) (100 µm i.d.; Polymicro Technologies, Phoenix, AZ) and a precolumn-GC column assembly (4). The capillary is heated rapidly to pyrolysis temperature by passing electric current from a custom-built capacitive discharge power supply connected to the capillary through contacts (5) and leads (6). A similar power supply was successfully used for fast thermal desorption of volatile analytes from SPME fibers10,11 and microsor-
†
University of Waterloo. UFZ-Center for Environmental Research. (1) Warmler, T. P. J. Chromatogr. A 1999, 842, 207-220. (2) Schulten, H. R.; Schnitzer, M. Biol. Fertil. Soils 1997, 26, 1-15. (3) White, D. M.; Irvine, R. L. Environ. Monit. Assess. 1998, 50, 53-65. (4) Poerschmann, J.; Kopinke, F.-D.; Balcke, G.; Mothes, S. J. Microcolumn Sep. 1998, 10, 401-411. (5) Poerschmann, J.; Kopinke, F.-D.; Remmler, M.; Mackenzie, K.; Geyer, W.; Mothes, S. J. Chromatogr. A 1996, 750, 287-301. ‡
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(6) Almendros, G.; Martin, F.; Gonzalez-Vila, F. J.; Del Rio, J. C. J. Anal. Appl. Pyrol. 1993, 25, 137-147. (7) Saiz-Jimenez, C. Sci. Total Environ. 1992, 117/118, 13-25. (8) Ko ¨gel-Knabner, I.; Hatcher, P. G.; Tegelaar, E. W.; de Leeuw, J. W. Sci. Total Environ. 1992, 113, 89-106. (9) Saiz-Jimenez, C.; Hermosin, B.; Ortega-Calvo, J. J. Water Res. 1993, 27, 1693-1696. (10) Go´recki, T.; Pawliszyn, J. Anal. Chem. 1995, 67, 3265-3274. 10.1021/ac000913b CCC: $20.00
© 2001 American Chemical Society Published on Web 03/23/2001
Figure 1. (a) Schematic diagram of the in-column pyrolysis apparatus. (b) Design of the in-column pyrolysis device: 1, silcosteel capillary; 2, butt connectors; 3, restrictor; 4, (pre)column; 5, electrical contacts; 6, electrical leads.
bent traps.12 The distance between the contacts (5) is kept constant at 48 mm by mounting them in a special bracket (not shown). This ensures that the resistance of the segment of Silcosteel tubing between the contacts is reproducible. Pyrolysis temperature is controlled by adjusting the voltage to which the capacitors in the power supply are charged. The sample (powder or viscous liquid; typical size, ∼100 µg) is kept inside the steel tubing by means of two fused-silica wool plugs. The exact amount of sample is determined by weighing the tubing with one plug in place, loading the sample, and weighing the tubing again. The pyrolysis capillary is disposable and is used only once unless the products of pyrolysis carried out at a lower temperature are to be pyrolyzed at a higher temperature. When the desired amount of a sample is much larger than 100 µg, a simple split line consisting of a glass Y-connector and a piece of a small-diameter fused-silica capillary can be connected downstream from the pyrolysis unit to prevent column and detector overload. The entire split assembly is kept inside the GC oven to prevent cold spots in the system. Carrier gas flows from the GC injector through the restrictor (3) to the pyrolysis capillary (1). The role of the restrictor (3) is to prevent backflush of the reaction gases formed during the pyrolysis step. The downstream end of the pyrolysis capillary (1) is connected through a segment of fused-silica capillary serving as a precolumn to the GC column (4). The outlet of the column is connected to the detector (MS). The precolumn serves two purposes: it helps focus the injection band through the retention gap effect, and it protects the column itself against the buildup of very high boiling material, which could spoil chromatography. Because some buildup is inevitable, each chromatogram should (11) Go´recki, T.; Pawliszyn, J. Field Anal. Chem. Technol. 1997, 1 (5), 277284. (12) Segal, A.;Go´recki, T.;Mussche, P.;Pawliszyn, J. J. Chromatogr. A 2000, 873, 13-27. 2000.
be carefully examined. Once chromatography starts to deteriorate, a 20-30 cm piece of the precolumn should be cut off and discarded. The following is the description of the pyrolysis procedure. The GC oven is opened, and carrier gas pressure is reduced to 0.5 psi. The downstream butt connector is loosened and disconnected from the Silcosteel capillary. The open end is immediately plugged with a piece of wire of the same diameter to prevent large amounts of air from entering the column and the MS. The upstream connector is then disconnected, and the electrical contacts are removed from the capillary. A new pyrolysis capillary, packed with the sample as described above, is then mounted inside the oven between the electrical contacts. The upstream butt connector is mounted first, and the capillary is flushed with helium for a few seconds to remove air. Next, the plugging wire is removed from the downstream butt connector, which is subsequently quickly mounted onto the other end of the pyrolysis capillary. The GC oven door is closed, and carrier gas pressure is restored to its original setting. The system is flushed with carrier gas for at least five minutes, and the sample is pyrolyzed. The GC run is started simultaneously with the pyrolysis. The following GC/MS systems were used for the experiments: HP 6890/HP 5973 and HP 5890 series II/HP 5970 (both from Hewlett-Packard, Palo Alto, CA). Data acquisition was carried out in full-scan mode. Columns that were used included 30 m × 0.25 mm × 0.25 µm HP-5 and 30 m × 0.25 mm × 0.1 µm HP-5 TA (Hewlett-Packard). The columns were equipped with 2-m fused-silica precolumns, connected with glass press-fit connectors (Supelco). Different temperature programs were used, depending on the requirements of the particular analysis, but in general, the oven was kept at 40 °C for 1 min, and then the temperature was programmed to 300 °C or 335 °C, depending on the column, at rates varying from 7 to 10 °C/min. Thermochemolysis of peat-derived HOM was carried out by refluxing a degreased sample with TMAH, as described in ref 5, followed by pyrolysis. This approach yields results that are qualitatively very reproducible. Quantification is difficult in this method, because the exact fraction of HOM in the end product, viscous syrup, can only be estimated. Thermal and electrical characteristics of the pyrolysis capillary were determined using a digital oscilloscope (PMC 3365 A, Philips Electronics). Heating profiles of the capillary were measured using 0.002-in. (∼50 µm) thermocouple wires (type K, chromel-alumel, Omega Engineering, Stamford, CT) that were spot-welded to the capillary one on top of the other. Temperature distribution along the capillary was determined using a 0.003-in. (∼75 µm) type K bare wire thermocouple (Omega Engineering). The thermocouple wires were threaded through ∼5-cm segments of 0.25-mm-i.d. fused-silica capillary tubing, terminating about 0.5 cm from the thermocouple junction on each end. This served to prevent contact of the bare thermocouple wires with the internal walls of the Silcosteel capillary. The entire assembly was threaded inside the Silcosteel capillary. Peak temperature along the tube was measured at ∼1-cm intervals. Three measurements were carried out at each point. The electrical characteristic of the device was determined using a 0.001 Ω shunt (Keithley Instruments, Inc., Cleveland, OH) connected in series with the discharge circuitry. Peak current was Analytical Chemistry, Vol. 73, No. 9, May 1, 2001
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Table 1. Temperature Reproducibility at Different Capacitor Voltages for a Single Capillary and n ) 10 Measurementsa UCD (V)
mean E (mV)
RSD (%)
70 75 80 85 90 91.7
20.2 22.5 26.1 29.2 32.6 34.0
1.7 1.0 1.2 0.8 0.6 0.5
a U , capacitive discharge voltage; E, thermoelectric force generCD ated by the thermocouple; RSD, relative standard deviation.
Figure 2. Peak discharge current vs capacitor voltage. Table 2. Temperature Reproducibility at Different Capacitor Voltages for Two Different capillariesa tube 1, E (mV)
tube 2, E (mV)
UCD (V)
E1
E2
E3
mean
E1
E2
E3
mean
80 85 90
24 27 30
24 27 30
24 27 30
24 27 30
24.5 27.5 30.5
24.5 27.5 30.5
24.5 27.5 30.5
24.5 27.5 30.5
a U , capacitive discharge voltage; E , thermoelectric force generCD i ated by the thermocouple at the ith measurement. A difference of 0.5 mV corresponds to ∼12 °C at 750 °C.
Figure 3. Typical temperature profiles of the pyrolysis capillary in stagnant air.
determined by measuring the momentary voltage drop across the precisely calibrated shunt. Conventional pyrolysis experiments were carried out using Pyroprobe 1000 (CDS, Oxford, PA), as described earlier.4,5 RESULTS AND DISCUSSION Electrical and Thermal Characteristics of the Pyrolysis Device. Both electrical and temperature characteristics of the system were determined. Figure 2 presents the dependence of peak discharge current on capacitor voltage. At the maximum voltage examined (90 V), peak current reached 920 A. Current rise was very rapid, maximum value being reached in 0.875 µs. After that, current decayed exponentially, going down to 10% of its maximum value after 4.6 ms. The temperature characteristic of the pyrolysis capillary was determined using very fine thermocouple wires (∼50 µm), spotwelded to the center of the tubing. Because of the very small diameter of the wires and their negligible thermal mass, the thermocouple formed in such a way had extremely good dynamic properties (very short response time) and did not disturb the heating process through heat dissipation. Typical temperature profiles recorded in stagnant air are presented in Figure 3. Maximum temperature reached at 90 V was 750 °C, and the rise time (0-95%) was ∼13 ms. Thus, the heating rate at the highest voltage exceeded 50 000 °C/s. The cooling rate depended on the intensity of air convection. In stagnant air, the tubing returned to initial temperature after ∼16 s, but in the GC oven with the fan turned on, the cooling time was 16. On the other hand, alkylbenzene pattern reaching as far as C27 could be clearly seen in the in-column pyrogram, which is in accordance with geochemical considerations.15 This indicates that the in-column approach effectively eliminates analyte discrimination on transfer from the pyrolysis unit to the GC column. It should be emphasized that discrimination of high-molecular weight compounds is by no means unique to the particular device used in this study. Similar phenomena were observed for other types of commercial pyrolysis units, including filament16 and Curie point pyrolyzers.17,18 Figure 6 presents another example of a nondiscriminated pyrogram that was obtained for polyethylene using the system described. It is clear from this Figure that no discrimination occurred, even for very long chain aliphatic hydrocarbons. Broadening of the peaks past ∼C-45 was related to the isothermal part of the temperature program (last segment, constant temperature of 335 °C). The usual R,ω-alkadiene/n-alk-1-ene/n-alkane pattern19 was observed for lower-boiling products; at higher temperatures, the peaks coeluted as a result of insuffcient efficiency of the column or too fast a temperature program. (15) Schulten, H.-R.; Schnitzer, M. Org. Geochem. 1993, 20, 17-25. (16) Asperger, A.; Engewald, W.; Fabian, G. J. Anal. Appl. Pyrol. 1999, 50, 103115. (17) Goebbels, F. J.; Puettmann, W. Water Res. 1997, 31, 1609-1618. (18) Challinor, J. M. J. Anal. Appl. Pyrol. 1993, 25, 349-360. (19) Wampler, T. P.; Levy, E. J. Analyst 1986, 111, 1065-66.
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Figure 6. Nondiscriminated pyrolysis of polyethylene. Labels indicate the number of carbon atoms in the hydrocarbon chain. Inset: C-15 alkadiene/alkene/alkane range.
Figure 7. Fatty acid methyl esters (FAME) in thermochemolysis products of peat-derived HOM (500 °C): (a) conventional pyrolysis, (b) in-column pyrolysis. Numbers indicate the length of the carbon chain in the carboxylic acid.
Nevertheless, the alkadiene/olefine/alkane pattern could still be seen when monitoring individual masses (e.g., m/z ) 57 amu and m/z ) 55 amu; see Figure 6). The proposed approach can also be very beneficial for other methods based on pyrolysis. For example, thermochemolysis using tetramethylammonium hydroxide (TMAH) is an excellent method to recognize fatty acids and dicarboxylic acids in the polymeric backbone.20 These acids undergo decarboxylation in conventional pyrolysis, and thus, can hardly be detected with this method. Thermochemolysis using TMAH suffers from the same discrimination effects as those observed in conventional pyrolysis. Figure 7(a) shows a fatty acid methyl ester (FAME) pattern of peat-derived HOM subjected to TMAH thermochemolysis at 500 °C using a slightly modified commercial system. An attempt was made to eliminate the discrimination by applying additional heating (300 °C) to the transfer line between the pyrolysis unit (20) Challinor, J. M. J. Anal. Appl. Pyrol. 1989, 16, 323-333.
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Figure 8. FAME in thermochemolysis products of a root of a helophyte plant. Numbers indicate the length of the carbon chain in the carboxylic acid. Inset: “i” denotes iso- form, “a” denotes anteisoform; numbers after the colon indicate the number of double bonds in the alkyl chain.
and the GC injector. This additional heating proved beneficial, as evidenced by the fatty acid methyl ester pattern extending to C28. Figure 7b presents a pyrogram obtained for the same sample using in-column pyrolysis. Discrimination in the conventional system is still evident when comparing the abundances of C24, C26, and C28 peaks to that of C22 in both pyrograms. The comparison shows that the FAME pattern is characterized by the highest peak abundances in the C24 region (lignoceric acid) with the in-column approach, whereas in the conventional approach, the pattern is seemingly shifted toward lower molecular weight compounds, despite the additional efforts aimed at reducing discrimination. Figure 8 shows the FAME profile of a root of a helophyte plant, determined by in-column pyrolysis. In the framework of our phytoremediation studies, helophytes were used to study the rhizospheric “root effect”, the basics for which are described in ref 21. FAME pattern extending as far C28 is clearly visible, with the highest peak intensities around the C16-C18 range. As with peat, pronounced even-over-odd discrimination points to natural origin. This example illustrates the applicability of the method proposed for biological samples. CONCLUSIONS Preliminary results reported herein indicate that the proposed new technique of nondiscriminated in-column pyrolysis is a very promising approach. The method extends the range of pyrolysis products to include high-boiling-point compounds, which are typically lost during the transfer from commercial pyrolysis devices to the GC column. The experiments presented in this paper aimed mainly at proving the concept. We are currently working on optimizing the method so that it requires fewer manipulations. The method can be used for almost any kind of material, but the greatest benefits can be realized for samples whose pyrolysis products of the greatest diagnostic value are characterized by high boiling points. Small sample amount requirements, good reproducibility of the results (due to extremely high heating rate), and the quantitative character of those results should make this (21) Pakeman, R. J.; Hankard, P. K.; Osborn, D. Rev. Environ. Contam. Toxicol. 1998, 157, 1-23.
method a very useful tool in many areas, including forensic analysis, catalyst research, product control, etc. Other examples of methodical varieties of pyrolysis that can benefit from the suggested approach include the addition of hydrogen (reducing atmosphere)22,23 or oxygen24 to the inert carrier gas; the addition of catalysts, including porous silicates or zeolites, to study distinct reactions;25 reactive pyrolysis using oxalic acid;26 combustion/ isotope ratio MS,27 etc. In addition, thermochemolysis studies with TMAH can be run sequentially, with the first pyrolysis carried out at 500 °C, followed by pyrolysis of the same sample at 750 (22) Shakkottai, P.; Kwack, E. Y.; Lawson, D. Rev. Sci. Instrum. 1990, 61 (7), 1958-1965. (23) Rocha, J. D.; Brown, D. S.; Love, G. D.; Snape, C. E. J. Anal. Appl. Pyrol. 1997, 40/41, 91-103. (24) Bilbao, R.; Alzueta, M. U.; Millera, A. Ind. Eng. Chem. Res. 1995, 34 (12), 4531-09. (25) Williams, P. T.; Horne, P. A. J. Anal. Appl. Pyrol. 1995, 31, 15-37. (26) Sato, H.; Mizutani, S.; Tsuge, S.; Ohtani, H.; Aoi, K.; Takasu, A.; Okada, M.; Kobayashi, S.; Kiyosada, T.; Shoda, S. Anal. Chem. 1998, 70, 7-12. (27) Schulten, H.-R.; Gleixner, G. Water Res. 1999, 33, 2489-2498.
°C. This sequential approach yields information on the kinds of bonds in the sample-ester bonds that are cleaved at the lower temperature, whereas ether bonds are cleaved only at the higher temperature. Studies of this kind will be presented in the upcoming paper. ACKNOWLEDGMENT This paper was presented in parts at PITTCON 2000, March 12-17, 2000, New Orleans, and the 23rd International Symposium on Capillary Chromatography, June 6-9, 2000, Riva del Garda (Italy). The research was supported in part by the Natural Sciences and Engineering Research Council of Canada. Restek is gratefully acknowledged for supplying the Silcosteel tubing.
Received for review August 3, 2000. Accepted February 14, 2001. AC000913B
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