- Email: [email protected]
Design and application of a gas chromatograph–gas chromatograph transfer line
Journal of Chromatography A, 1210 (2008) 234–238
Contents lists available at ScienceDirect
Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma
Short communication
Design and application of a gas chromatograph–gas chromatograph transfer line ∗ ˛ Łukasz Dabrowski University of Technology and Life Sciences, Faculty of Chemical Technology and Engineering, Department of Chemistry and Environmental Protection, ul. Seminaryjna 3, 85-326 Bydgoszcz, Poland
a r t i c l e
i n f o
Article history: Received 9 April 2008 Received in revised form 9 August 2008 Accepted 22 September 2008 Available online 27 September 2008 Keywords: GC–GC interface Transfer line GC–MS–AED Coupled techniques
a b s t r a c t A simple innovation is proposed in this work: to use interface (transfer line, TL), which allows quantitative mass transfer between two gas chromatographs. This approach assumes that one chromatograph serves as a conventional GC apparatus and the second one only as a thermostat with the appropriate detector(s) mounted. Two possible applications of the TL were tested: an “inlet–TL–detector” system and a system for dual-detection analysis, i.e. mass spectrometry and atomic emission detection. Results obtained for both of the systems show that the interface enables effective connection of two independent chromatographs. The transfer line is a manufacturer-independent innovation (with its own electronics), easy to set up and maintain. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In a modern gas chromatography laboratory, several apparatus are usually used for various purposes. Each of them is equipped with different, sometimes unique devices, according to the procedure needed. However, some problems may arise when gas chromatographs designated to a specific application are used for another one. It may happen that the instrument needs an upgrade with the addition of an injector or/and detector, which is not always possible or economically viable. Rapid development of a large variety of gas chromatographic techniques brings about many changes in the instrumentation and construction design of the chromatographic devices. In many cases the manufacturers of chromatographic equipment discontinue well-known products and introduce new ones, sometimes not compatible with the old devices. The only way of upgrading an old chromatographic system is often the purchase of a new one. In addition, this carries with it an upgrade of the software, personnel training, conversion of the laboratory and others. In effect, the total cost of such a transformation is much higher than the nominal cost of the apparatus. On the other hand, sometimes the only problem is that the required device (injector and detector) is mounted on a separate gas chromatograph (available in the laboratory) and cannot be connected to the considered one because of incompatibility. A similar
∗ Tel.: +48 52 3749108. E-mail address: [email protected]. 0021-9673/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.chroma.2008.09.072
situation occurs when the manufacturers do not foresee the possibility of coupling two detectors to one gas chromatograph (for example GC–MS–AED). A simple solution to this problem is proposed in this work. It involves using a special interface, which allows quantitative mass transfer between two individual gas chromatographs. A GC system created in this way is coupled by a heated interface. This allows the utilization of various devices (inlets and detectors) mounted on each of the apparatus without any further upgrades. Transfer lines are mounted in commercially available GC–MS or GC–atomic emission detection (AED) systems. They are also used in headspace analysis systems combined with GC [1–3], in the coupled techniques LC–GC, solid-phase extraction–GC [4] and also in GC–inductively coupled plasma MS [5–7]. Construction of the interface is dependent on the purpose. The simplest transfer lines are used to transfer liquid media, used for example in a LC–GC interface. In this case it is usually a stainless steel capillary, sometimes wrapped with thermal insulation [4]. A more complicated interface is used when constant temperature should be maintained. It must include a thermostatic controller. GC–MS or GC–AED systems are usually equipped with the heated prim pipe (inside is a GC capillary) wrapped with thermal insulation. This construction is surrounded by a wide pipe for tidiness and protection. The aim of this work was to design the interface between two independent GC systems before using various devices installed on each of the chromatographs. This approach assumed that one chromatograph serves as a conventional GC apparatus and the second one only as a thermostat with the appropriate detector(s) mounted.
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235
glass fibre tape and glasswool (25 mm thickness) resistant to heat up to 700 ◦ C (Rockwool Polska, Cigacice, Poland). The total length of the transfer line is 150 cm. On both ends it was bent at an angle of 40◦ (Fig. 2). Both ends of TL were bent and then were inserted into GC ovens through approximately 1.5 cm holes in the upper walls of the chromatographs. Flexibility is an important feature of the transfer line. With up to 40–50◦ of bending there is no problem inserting a capillary column into the TL. 2.3. GC–GC transfer line thermal characteristics
Fig. 1. Cross-section of the transfer line. (1) Deactivated fused silica capillary; (2) copper tube; (3) glass fibre tape; (4) heating cord; (5) glass wool.
2. Experimental 2.1. Chemicals and reagents Dichloromethane (DCM, GR grade for analysis), acetonitrile (gradient grade for liquid chromatography) and aluminium oxide (90 active neutral for column chromatography) were obtained from Merck (Warsaw, Poland). Sodium sulfate (anhydrous pure analytical-reagent grade) and copper were purchased from POCh (Gliwice, Poland). Pesticide standards (TCL Pesticides Mix, 2000 g/ml in toluene/hexane) were obtained from Supelco ´ Poland). (Poznan, 2.2. GC–GC transfer line design A cross-section view of the transfer line (TL) is shown in Fig. 1. To make the GC–GC TL flexible, copper tubing (150 cm × 1.5 mm I.D., with 12 mm O.D.) was used. In fact, there were three copper pipes: bundled together side-by-side in order to achieve at least the minimum bending radius (6 mm) of the heating cord required by the manufacturer. Laboratory heating cord HSS-series (5 m, 600 W, up to 450 ◦ C; Roth, Karlsruhe, Germany) was coiled on the tubing. The heating cord was connected to a HT 30 temperature control unit (equipped with the temperature probe Pt 1 0 0 inserted into the copper pipe wall; all from Roth). The transfer line was isolated with
From the user’s point of view, it is important to know, after switching on, when the TL is ready to use and how to adjust the heating power to obtain precise temperature control. In order to validate the thermal conditions in the constructed interface, two experiments were carried out. Time needed for the TL to reach the set temperature from 23 ◦ C (air-conditioned laboratory) was determined using maximum output power (i.e. 600 W). To characterise the precision of the temperature held in the TL, various output power were set and the temperature variation versus time was measured for several different temperatures, i.e. 170 ◦ C, 200 ◦ C, 220 ◦ C, and 250 ◦ C. 2.4. Instrument setup Two possible applications of the TL were tested. The first one: an “inlet–TL–detector” system, in which the specific inlet (cool oncolumn) was not available on the GC coupled with the MS detector and had to be “taken” from another GC. Another implementation of the TL was dual-detector analysis where the detectors were mounted on separate GC instruments (i.e. MS and AED systems). Although the GC–MS–AED solution had been already proposed [1,2] it had been constructed with an old, nowadays unused, AED model (HP 5921). Recently [3], a MS–AED connection approach was proposed by the current AED manufacturer (joint analytical systems, Moers, Germany) but it is designed only for specific MS and AED models and is quite expensive. 2.4.1. Instrument setup for cold-on-column–TL–mass spectrometry analysis A Hewlett-Packard 6890 gas chromatograph equipped with a cool on-column (COC) injector was connected by a transfer line
Fig. 2. Overall view of GC–MS–AED system. (1) MS detector; (2) deactivated fused silica capillary (0.5 m × 0.18 mm); (3) temperature probe PT 100; (4) transfer line; (5) supply cords; (6) temperature control unit; (7) capillary column; (8) Y-type splitter; (9) AED; (10) deactivated fused silica capillary (3 m × 0.18 mm); (11) PTV inlet.
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Table 1 Analytes tested with the dual AED and MS detection system, LODs (S/N = 3), RSDs (n = 4) Compound
Formula
Ion 1 (m/z)
Ion 2 (m/z)
RSD TICa (%)
RSD AEDb (%)
LOD MS SIM (Ion 2) (g/ml)
LOD MS scan (g/ml)
LOD AED (Cl 479) (g/ml)
␣-HCH ,␥-HCH ␦-HCH Heptachlor Aldrin Heptachlor epoxide Endosulfan 4,4 -DDE Dieldrin Endrin Endosulfan II 4,4 -DDD Endrin aldehyde 4,4 -DDT Endosulfan sulfate Endrin ketone Methoxychlor
C6 H6 Cl6 C6 H6 Cl6 C6 H6 Cl6 C10 H5 Cl7 C12 H8 Cl6 C10 H5 Cl7 O C9 H6 Cl6 O3 S C14 H8 Cl4 C12 H8 Cl6 O C12 H8 Cl6 O C9 H6 Cl6 O3 S C14 H10 Cl4 C12 H10 Cl6 O C14 H9 Cl5 C9 H6 Cl6 O4 S C12 H8 Cl6 O C16 H15 Cl3 O2
181 181 181 272 263 353 277 246 263 263 197 235 250 235 272 317 227
219 219 219 337 293 355 339 318 380 281 207 165 345 165 387 281 228
2.1 2.1 5.8 5.2 3.0 5.2 3.5 4.7 3.1 4.2 4.1 5.3 6.8 6.9c – 5.7 4.3
2.0 0.3 1.9 4.3 2.9 3.1 3.8 5.0 3.7 5.0 5.3 5.5 3.6 6.9c – 7.0 6.9
0.004 0.003 0.004 0.015 0.006 0.005 0.017 0.018 0.026 0.027 0.016 0.002 0.009 0.018 0.016 0.063 0.028
0.042 0.038 0.067 0.094 0.043 0.075 0.068 0.062 0.074 0.093 0.080 0.061 0.150 0.177c – 0.117 0.274
0.058 0.043 0.045 0.095 0.040 0.057 0.043 0.078 0.048 0.079 0.055 0.067 0.106 0.111c – 0.108 0.812
a b c
Calculations based on peak area—TIC. Calculations based on peak high. 4,4 -DDT and endosulfan sulfate: calculation made for the coeluted substances.
with a Hewlett-Packard Model 5890 Series II gas chromatograph, equipped with a 5972 mass-selective detector—all from HewlettPackard, CA, USA. The effluent from the separation column was transferred via a fused silica tube (in the transfer line) to 5890 GC and then to MS. Injector’s (COC) initial pressure was 34.5 kPa (5 psi) then operated in constant flow mode. Injection volume was 2 l. An Rtx-5 MS capillary column, 30 m × 0.25 mm, 0.25 m from Restek (Bellefonte, PA, USA), was used. To connect two chromatographs a fused silica capillary (5 m × 0.18 mm) was used. Initial oven temperature of the HP 6890 was set to 35 ◦ C. Then the oven was heated with a rate of 30 ◦ C/min to 130 ◦ C and heated with a rate of 10 ◦ C/min to 250 ◦ C (held 2 min). The temperatures of the TL and HP 5890’s oven were set to 250 ◦ C (isothermal). The MS detector was operated in the selected ion-monitoring mode. For each of the substances analysed two characteristic ions (quantitative and qualitative) were monitored during the analysis. 2.4.2. Instrument setup for dual-detector analysis A Hewlett-Packard 6890 gas chromatograph equipped with AED and programmable temperature vaporization (PTV) injector was connected by transfer line with a Hewlett-Packard Model 5890 Series II gas chromatograph equipped with a 5972 massselective detector—all from Hewlett-Packard (Fig. 2). The effluent from the separation column was split by a Y-type connector (Vitreous Silica Outlet Splitter; SGE, Ringwood, Australia) and transferred by a fused silica tube (in transfer line) to 5890 GC and MS and also by a short piece of fused silica tube to the AED system. The PTV inlet (Hewlett-Packard) was used in the solvent vent program with an initial temperature of 60 ◦ C (held 0.3 min) heated with a rate of 720 ◦ C/min to 270 ◦ C (held 5 min) and finally 250 ◦ C till the end of the analysis. To eliminate the solvent (dichloromethane), the injector vent time was set to 0.2 min (vent flow: 100 ml/min) and purge time was set to 1.5 min (purge flow: 50 ml/min). Initial pressure of carrier gas (helium) was set to 103.4 kPa (15 psi) resulting carrier gas flow rates (at 60 ◦ C) of 0.57 ml/min for MS and 0.65 ml/min for AED. PTV was operated in constant flow mode during the analysis. Injections of 10 l solution were performed manually. The temperatures of the AED system heated zones were 280 ◦ C for the transfer line and 280 ◦ C for cavity. The solvent vent was switched on 6.5 min after injection. Instrument default methods
were used for elemental detection of chlorine (479 nm), hydrogen (486 nm) and carbon (496 nm). The reagent gas used was oxygen at 275.8 kPa (40 psi). A solvent delay of 4 min was applied. The MS system was operated in the selected ion monitoring (SIM) or in the scan mode. Monitored ions (in SIM mode) for the analytes are given in Table 1. Scan range of the collected spectra was: 50–500 u. Electron multiplier voltage was increased (from the auto tune value) by 200 V to achieve higher sensitivity. Transfer line to MS was heated to 280 ◦ C. The column used in the system was HP-5MS. Other operating conditions of the system (i.e. column dimensions and temperature program) are the same as in COC–TL–MS setup. 2.5. Data acquisition and software used Data acquisition was performed on two PII computers by running the Hewlett-Packard GC ChemStation Rev. A.05.02 [273] and G1034C ver. C02.00. A remote-start cable between two apparatus was constructed and used to enable the simultaneous start of the chromatographs’ programs and data acquisition systems. The AED signal could be converted to MS data using the HP AED File Conversion Utility software. It is possible to generate ASCII files from MS spectra using Chemstation or the macro found on the internet (www.hp2ascii.mac) [7]. 2.6. Sample preparation To evaluate the cold-on-column–TL–MS system (described in Section 2.4), standard solutions of the mixture of organochlorine pesticides, PCBs and PAHs at a concentration of 1 g/ml were analysed. Several repetitions of the manual injections were performed to obtain standard deviations of the results. To evaluate the GC–AED–(TL)–MS system two types of the samples were analysed: standard solutions and real samples. Standard solutions of organochlorine pesticides (Table 1) were prepared at concentrations between 50 g/ml and 0.3 g/ml. Soil samples from a cultivated field were analysed using the procedure described previously [6]. Soil samples (15 g) were extracted with DCM (40 ml) in an ultrasonic bath for 30 min (2× 15 min). Standards were added (to the samples with standard addition) before the extraction. The extract was concentrated in a rotary evaporator and then placed under a nitrogen stream till dry. The dried residue was dissolved
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Fig. 3. Temperature variation as a function of the percent of nominal heating cord power—600 W.
in 1.25 ml of acetonitrile and cleaned up on alumina (1 g) SPE bed with a layer of sodium sulfate (0.5 g) and copper, using 10port SPE Vacuum Manifold (Agilent, Palo Alto, CA, USA). A fraction of 5 ml was eluted with acetonitrile. After the solvent exchange to dichloromethane the sample was analysed by GC–MS–AED system. 3. Results and discussion 3.1. GC–GC transfer line thermal characteristics The time needed to heat the TL from 23 ◦ C to various temperatures was relatively short and varied from 3 min to 5 min, respectively, for the temperatures 170 ◦ C and 250 ◦ C. After that period, the power of the supply device needed to decrease. Depending on the power of the heating cord supplied, temperature changes in the transfer line could be observed. To adjust the optimum effective power when the TL reaches the set temperature, the temperature variation versus power was determined (Fig. 3). Whatever the temperature, the heating impulse needed adjustment to 20–30% of nominal heating cord power (i.e. 600 W) to obtain a temperature sinusoidal variation below 0.1 ◦ C. This could be explained by good thermal insulation of the constructed interface. Thermal stabilisation of the TL was reached approximately 10 min after switching it on. 3.2. Standards analysis with cold-on-column–TL–MS system Calculations of the RSDs were based on the area of the analytes’ peaks. In case of all of the analysed compounds (polychlorinated biphenyls, polycyclic aromatic hydrocarbons, and organochlorine pesticides) the RSDs were less than 5%, which is typical for the manual injections with COC inlet and no negative influence of TL could be noticed. 3.3. Dual-detector instrument setup—system performance test Several analyses of standard solutions were performed to estimate the basic characteristics of the system. The results are shown in Table 1. Both the RSDs and the detection limits for both of the detectors are low enough to use this system in the trace analysis of pesticides. The limits of detection (LODs) for the pesticides determined with the atomic emission detector were on average 10-fold higher than those obtained with the MS detector in SIM mode. This proportion could be changed by using different splitter or different geometries of the capillaries (from splitter to detectors).
Fig. 4. Chromatograms obtained for the soil sample (A) and soil with standard (5 ng/g) addition (B) MS-SIM (m/z = 181 MS and m/z = 219) and traces of C (496 nm) and Cl (479 nm). (1) ␣-HCH; (2) ,␥-HCH; (3) ␦-HCH.
The extract injected into the GC system usually contains the analytes in concentrations above the LODs given in Table 1. The LODs of the C-trace for the analytes are approximately three times higher than the results achieved for the Cl-trace, but the C-trace serves only as a tool for determining the time shift between these two detectors. 3.3.1. Real sample analysis Typical chromatograms obtained for a soil sample are shown in Fig. 4A and for a spiked soil sample in Fig. 4B. When only GC–MS (SIM) chromatograms are available, misinterpretation of HCH could occur. HCH isomers usually have retention times between 10.5 min and 12.0 min. On both of the ion chromatograms (m/z = 181 and m/z = 219) there is a peak (marked with vertical dashed line), which could be misinterpreted as one of the HCH isomers. Retention times of this peak and standards are different, but there is always a possibility of a retention time shift, when analysing real samples. Interpretation of the Cl-trace obtained from AED shows that the suspected peak does not contain chlorine at all and therefore cannot be a HCH isomer. The “HCH-like” compounds often occur on MS (SIM) chromatograms of soil and sediment samples from various sources. Using an additional element specific detector connected via TL allowed the identification problem to be resolved. 4. Conclusions Results obtained either for the COC–TL–MS system or the PTV–AED–TL–MS system show that the interface enables effective connection of two independent chromatographs into one system. Connection of two chromatographs equipped with MS and AED system using the GC transfer line, enables simultaneous analysis of a single sample by two detectors on two differ-
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ent instruments. GC–MS–AED provides structural information of the analytes and interferences, which prevents misinterpretation of the obtained data. The detection limits obtained were low enough to measure organochlorine pesticides in the environmental and food samples. A dual-detector system can be a useful tool in the analysis of a real sample in a complex matrix. The constructed transfer line is a universal device, which could be used for many different purposes—connecting two chromatographs to use various equipments mounted on both of the apparatus (inlets, detectors and other). The transfer line is a manufacturer-independent innovation (with its own electronics), easy to setup and maintain. The GC–GC interface enables one to create universal and powerful systems, which could be used in many GC laboratories.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.chroma.2008.09.072. References [1] H. Mol, T. Hankemeler, U. Brinkman, LC–GC 17 (1998) 544. [2] L. van Stee, P. Leonards, R. Vreuls, U. Brinkman, Analyst 124 (1999) 1547. [3] K. Ziegenhals, in: Presented at Pittcon 2006, AED Technical Session, Orlando, FL, 2006. [4] M. Schweigkofler, R. Niessner, Environ. Sci. Technol. 33 (1999) 3680. [5] T. Zimmermann, Ph.D. Thesis, University of Marburg, Marburg/Lahn, 2005. ˛ ˙ [6] Ł. Dabrowski, H. Giergielewicz-Mozajska, M. Biziuk, J. Gaca, J. Namie´snik, J. Chromatogr. A 957 (2002) 59. [7] http://www.amdis.net/About/Dejanews/Convert HP ChemStation 3D MSD data to ASCII-1.htm.
Contents lists available at ScienceDirect
Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma
Short communication
Design and application of a gas chromatograph–gas chromatograph transfer line ∗ ˛ Łukasz Dabrowski University of Technology and Life Sciences, Faculty of Chemical Technology and Engineering, Department of Chemistry and Environmental Protection, ul. Seminaryjna 3, 85-326 Bydgoszcz, Poland
a r t i c l e
i n f o
Article history: Received 9 April 2008 Received in revised form 9 August 2008 Accepted 22 September 2008 Available online 27 September 2008 Keywords: GC–GC interface Transfer line GC–MS–AED Coupled techniques
a b s t r a c t A simple innovation is proposed in this work: to use interface (transfer line, TL), which allows quantitative mass transfer between two gas chromatographs. This approach assumes that one chromatograph serves as a conventional GC apparatus and the second one only as a thermostat with the appropriate detector(s) mounted. Two possible applications of the TL were tested: an “inlet–TL–detector” system and a system for dual-detection analysis, i.e. mass spectrometry and atomic emission detection. Results obtained for both of the systems show that the interface enables effective connection of two independent chromatographs. The transfer line is a manufacturer-independent innovation (with its own electronics), easy to set up and maintain. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In a modern gas chromatography laboratory, several apparatus are usually used for various purposes. Each of them is equipped with different, sometimes unique devices, according to the procedure needed. However, some problems may arise when gas chromatographs designated to a specific application are used for another one. It may happen that the instrument needs an upgrade with the addition of an injector or/and detector, which is not always possible or economically viable. Rapid development of a large variety of gas chromatographic techniques brings about many changes in the instrumentation and construction design of the chromatographic devices. In many cases the manufacturers of chromatographic equipment discontinue well-known products and introduce new ones, sometimes not compatible with the old devices. The only way of upgrading an old chromatographic system is often the purchase of a new one. In addition, this carries with it an upgrade of the software, personnel training, conversion of the laboratory and others. In effect, the total cost of such a transformation is much higher than the nominal cost of the apparatus. On the other hand, sometimes the only problem is that the required device (injector and detector) is mounted on a separate gas chromatograph (available in the laboratory) and cannot be connected to the considered one because of incompatibility. A similar
∗ Tel.: +48 52 3749108. E-mail address: [email protected]. 0021-9673/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.chroma.2008.09.072
situation occurs when the manufacturers do not foresee the possibility of coupling two detectors to one gas chromatograph (for example GC–MS–AED). A simple solution to this problem is proposed in this work. It involves using a special interface, which allows quantitative mass transfer between two individual gas chromatographs. A GC system created in this way is coupled by a heated interface. This allows the utilization of various devices (inlets and detectors) mounted on each of the apparatus without any further upgrades. Transfer lines are mounted in commercially available GC–MS or GC–atomic emission detection (AED) systems. They are also used in headspace analysis systems combined with GC [1–3], in the coupled techniques LC–GC, solid-phase extraction–GC [4] and also in GC–inductively coupled plasma MS [5–7]. Construction of the interface is dependent on the purpose. The simplest transfer lines are used to transfer liquid media, used for example in a LC–GC interface. In this case it is usually a stainless steel capillary, sometimes wrapped with thermal insulation [4]. A more complicated interface is used when constant temperature should be maintained. It must include a thermostatic controller. GC–MS or GC–AED systems are usually equipped with the heated prim pipe (inside is a GC capillary) wrapped with thermal insulation. This construction is surrounded by a wide pipe for tidiness and protection. The aim of this work was to design the interface between two independent GC systems before using various devices installed on each of the chromatographs. This approach assumed that one chromatograph serves as a conventional GC apparatus and the second one only as a thermostat with the appropriate detector(s) mounted.
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235
glass fibre tape and glasswool (25 mm thickness) resistant to heat up to 700 ◦ C (Rockwool Polska, Cigacice, Poland). The total length of the transfer line is 150 cm. On both ends it was bent at an angle of 40◦ (Fig. 2). Both ends of TL were bent and then were inserted into GC ovens through approximately 1.5 cm holes in the upper walls of the chromatographs. Flexibility is an important feature of the transfer line. With up to 40–50◦ of bending there is no problem inserting a capillary column into the TL. 2.3. GC–GC transfer line thermal characteristics
Fig. 1. Cross-section of the transfer line. (1) Deactivated fused silica capillary; (2) copper tube; (3) glass fibre tape; (4) heating cord; (5) glass wool.
2. Experimental 2.1. Chemicals and reagents Dichloromethane (DCM, GR grade for analysis), acetonitrile (gradient grade for liquid chromatography) and aluminium oxide (90 active neutral for column chromatography) were obtained from Merck (Warsaw, Poland). Sodium sulfate (anhydrous pure analytical-reagent grade) and copper were purchased from POCh (Gliwice, Poland). Pesticide standards (TCL Pesticides Mix, 2000 g/ml in toluene/hexane) were obtained from Supelco ´ Poland). (Poznan, 2.2. GC–GC transfer line design A cross-section view of the transfer line (TL) is shown in Fig. 1. To make the GC–GC TL flexible, copper tubing (150 cm × 1.5 mm I.D., with 12 mm O.D.) was used. In fact, there were three copper pipes: bundled together side-by-side in order to achieve at least the minimum bending radius (6 mm) of the heating cord required by the manufacturer. Laboratory heating cord HSS-series (5 m, 600 W, up to 450 ◦ C; Roth, Karlsruhe, Germany) was coiled on the tubing. The heating cord was connected to a HT 30 temperature control unit (equipped with the temperature probe Pt 1 0 0 inserted into the copper pipe wall; all from Roth). The transfer line was isolated with
From the user’s point of view, it is important to know, after switching on, when the TL is ready to use and how to adjust the heating power to obtain precise temperature control. In order to validate the thermal conditions in the constructed interface, two experiments were carried out. Time needed for the TL to reach the set temperature from 23 ◦ C (air-conditioned laboratory) was determined using maximum output power (i.e. 600 W). To characterise the precision of the temperature held in the TL, various output power were set and the temperature variation versus time was measured for several different temperatures, i.e. 170 ◦ C, 200 ◦ C, 220 ◦ C, and 250 ◦ C. 2.4. Instrument setup Two possible applications of the TL were tested. The first one: an “inlet–TL–detector” system, in which the specific inlet (cool oncolumn) was not available on the GC coupled with the MS detector and had to be “taken” from another GC. Another implementation of the TL was dual-detector analysis where the detectors were mounted on separate GC instruments (i.e. MS and AED systems). Although the GC–MS–AED solution had been already proposed [1,2] it had been constructed with an old, nowadays unused, AED model (HP 5921). Recently [3], a MS–AED connection approach was proposed by the current AED manufacturer (joint analytical systems, Moers, Germany) but it is designed only for specific MS and AED models and is quite expensive. 2.4.1. Instrument setup for cold-on-column–TL–mass spectrometry analysis A Hewlett-Packard 6890 gas chromatograph equipped with a cool on-column (COC) injector was connected by a transfer line
Fig. 2. Overall view of GC–MS–AED system. (1) MS detector; (2) deactivated fused silica capillary (0.5 m × 0.18 mm); (3) temperature probe PT 100; (4) transfer line; (5) supply cords; (6) temperature control unit; (7) capillary column; (8) Y-type splitter; (9) AED; (10) deactivated fused silica capillary (3 m × 0.18 mm); (11) PTV inlet.
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Table 1 Analytes tested with the dual AED and MS detection system, LODs (S/N = 3), RSDs (n = 4) Compound
Formula
Ion 1 (m/z)
Ion 2 (m/z)
RSD TICa (%)
RSD AEDb (%)
LOD MS SIM (Ion 2) (g/ml)
LOD MS scan (g/ml)
LOD AED (Cl 479) (g/ml)
␣-HCH ,␥-HCH ␦-HCH Heptachlor Aldrin Heptachlor epoxide Endosulfan 4,4 -DDE Dieldrin Endrin Endosulfan II 4,4 -DDD Endrin aldehyde 4,4 -DDT Endosulfan sulfate Endrin ketone Methoxychlor
C6 H6 Cl6 C6 H6 Cl6 C6 H6 Cl6 C10 H5 Cl7 C12 H8 Cl6 C10 H5 Cl7 O C9 H6 Cl6 O3 S C14 H8 Cl4 C12 H8 Cl6 O C12 H8 Cl6 O C9 H6 Cl6 O3 S C14 H10 Cl4 C12 H10 Cl6 O C14 H9 Cl5 C9 H6 Cl6 O4 S C12 H8 Cl6 O C16 H15 Cl3 O2
181 181 181 272 263 353 277 246 263 263 197 235 250 235 272 317 227
219 219 219 337 293 355 339 318 380 281 207 165 345 165 387 281 228
2.1 2.1 5.8 5.2 3.0 5.2 3.5 4.7 3.1 4.2 4.1 5.3 6.8 6.9c – 5.7 4.3
2.0 0.3 1.9 4.3 2.9 3.1 3.8 5.0 3.7 5.0 5.3 5.5 3.6 6.9c – 7.0 6.9
0.004 0.003 0.004 0.015 0.006 0.005 0.017 0.018 0.026 0.027 0.016 0.002 0.009 0.018 0.016 0.063 0.028
0.042 0.038 0.067 0.094 0.043 0.075 0.068 0.062 0.074 0.093 0.080 0.061 0.150 0.177c – 0.117 0.274
0.058 0.043 0.045 0.095 0.040 0.057 0.043 0.078 0.048 0.079 0.055 0.067 0.106 0.111c – 0.108 0.812
a b c
Calculations based on peak area—TIC. Calculations based on peak high. 4,4 -DDT and endosulfan sulfate: calculation made for the coeluted substances.
with a Hewlett-Packard Model 5890 Series II gas chromatograph, equipped with a 5972 mass-selective detector—all from HewlettPackard, CA, USA. The effluent from the separation column was transferred via a fused silica tube (in the transfer line) to 5890 GC and then to MS. Injector’s (COC) initial pressure was 34.5 kPa (5 psi) then operated in constant flow mode. Injection volume was 2 l. An Rtx-5 MS capillary column, 30 m × 0.25 mm, 0.25 m from Restek (Bellefonte, PA, USA), was used. To connect two chromatographs a fused silica capillary (5 m × 0.18 mm) was used. Initial oven temperature of the HP 6890 was set to 35 ◦ C. Then the oven was heated with a rate of 30 ◦ C/min to 130 ◦ C and heated with a rate of 10 ◦ C/min to 250 ◦ C (held 2 min). The temperatures of the TL and HP 5890’s oven were set to 250 ◦ C (isothermal). The MS detector was operated in the selected ion-monitoring mode. For each of the substances analysed two characteristic ions (quantitative and qualitative) were monitored during the analysis. 2.4.2. Instrument setup for dual-detector analysis A Hewlett-Packard 6890 gas chromatograph equipped with AED and programmable temperature vaporization (PTV) injector was connected by transfer line with a Hewlett-Packard Model 5890 Series II gas chromatograph equipped with a 5972 massselective detector—all from Hewlett-Packard (Fig. 2). The effluent from the separation column was split by a Y-type connector (Vitreous Silica Outlet Splitter; SGE, Ringwood, Australia) and transferred by a fused silica tube (in transfer line) to 5890 GC and MS and also by a short piece of fused silica tube to the AED system. The PTV inlet (Hewlett-Packard) was used in the solvent vent program with an initial temperature of 60 ◦ C (held 0.3 min) heated with a rate of 720 ◦ C/min to 270 ◦ C (held 5 min) and finally 250 ◦ C till the end of the analysis. To eliminate the solvent (dichloromethane), the injector vent time was set to 0.2 min (vent flow: 100 ml/min) and purge time was set to 1.5 min (purge flow: 50 ml/min). Initial pressure of carrier gas (helium) was set to 103.4 kPa (15 psi) resulting carrier gas flow rates (at 60 ◦ C) of 0.57 ml/min for MS and 0.65 ml/min for AED. PTV was operated in constant flow mode during the analysis. Injections of 10 l solution were performed manually. The temperatures of the AED system heated zones were 280 ◦ C for the transfer line and 280 ◦ C for cavity. The solvent vent was switched on 6.5 min after injection. Instrument default methods
were used for elemental detection of chlorine (479 nm), hydrogen (486 nm) and carbon (496 nm). The reagent gas used was oxygen at 275.8 kPa (40 psi). A solvent delay of 4 min was applied. The MS system was operated in the selected ion monitoring (SIM) or in the scan mode. Monitored ions (in SIM mode) for the analytes are given in Table 1. Scan range of the collected spectra was: 50–500 u. Electron multiplier voltage was increased (from the auto tune value) by 200 V to achieve higher sensitivity. Transfer line to MS was heated to 280 ◦ C. The column used in the system was HP-5MS. Other operating conditions of the system (i.e. column dimensions and temperature program) are the same as in COC–TL–MS setup. 2.5. Data acquisition and software used Data acquisition was performed on two PII computers by running the Hewlett-Packard GC ChemStation Rev. A.05.02 [273] and G1034C ver. C02.00. A remote-start cable between two apparatus was constructed and used to enable the simultaneous start of the chromatographs’ programs and data acquisition systems. The AED signal could be converted to MS data using the HP AED File Conversion Utility software. It is possible to generate ASCII files from MS spectra using Chemstation or the macro found on the internet (www.hp2ascii.mac) [7]. 2.6. Sample preparation To evaluate the cold-on-column–TL–MS system (described in Section 2.4), standard solutions of the mixture of organochlorine pesticides, PCBs and PAHs at a concentration of 1 g/ml were analysed. Several repetitions of the manual injections were performed to obtain standard deviations of the results. To evaluate the GC–AED–(TL)–MS system two types of the samples were analysed: standard solutions and real samples. Standard solutions of organochlorine pesticides (Table 1) were prepared at concentrations between 50 g/ml and 0.3 g/ml. Soil samples from a cultivated field were analysed using the procedure described previously [6]. Soil samples (15 g) were extracted with DCM (40 ml) in an ultrasonic bath for 30 min (2× 15 min). Standards were added (to the samples with standard addition) before the extraction. The extract was concentrated in a rotary evaporator and then placed under a nitrogen stream till dry. The dried residue was dissolved
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Fig. 3. Temperature variation as a function of the percent of nominal heating cord power—600 W.
in 1.25 ml of acetonitrile and cleaned up on alumina (1 g) SPE bed with a layer of sodium sulfate (0.5 g) and copper, using 10port SPE Vacuum Manifold (Agilent, Palo Alto, CA, USA). A fraction of 5 ml was eluted with acetonitrile. After the solvent exchange to dichloromethane the sample was analysed by GC–MS–AED system. 3. Results and discussion 3.1. GC–GC transfer line thermal characteristics The time needed to heat the TL from 23 ◦ C to various temperatures was relatively short and varied from 3 min to 5 min, respectively, for the temperatures 170 ◦ C and 250 ◦ C. After that period, the power of the supply device needed to decrease. Depending on the power of the heating cord supplied, temperature changes in the transfer line could be observed. To adjust the optimum effective power when the TL reaches the set temperature, the temperature variation versus power was determined (Fig. 3). Whatever the temperature, the heating impulse needed adjustment to 20–30% of nominal heating cord power (i.e. 600 W) to obtain a temperature sinusoidal variation below 0.1 ◦ C. This could be explained by good thermal insulation of the constructed interface. Thermal stabilisation of the TL was reached approximately 10 min after switching it on. 3.2. Standards analysis with cold-on-column–TL–MS system Calculations of the RSDs were based on the area of the analytes’ peaks. In case of all of the analysed compounds (polychlorinated biphenyls, polycyclic aromatic hydrocarbons, and organochlorine pesticides) the RSDs were less than 5%, which is typical for the manual injections with COC inlet and no negative influence of TL could be noticed. 3.3. Dual-detector instrument setup—system performance test Several analyses of standard solutions were performed to estimate the basic characteristics of the system. The results are shown in Table 1. Both the RSDs and the detection limits for both of the detectors are low enough to use this system in the trace analysis of pesticides. The limits of detection (LODs) for the pesticides determined with the atomic emission detector were on average 10-fold higher than those obtained with the MS detector in SIM mode. This proportion could be changed by using different splitter or different geometries of the capillaries (from splitter to detectors).
Fig. 4. Chromatograms obtained for the soil sample (A) and soil with standard (5 ng/g) addition (B) MS-SIM (m/z = 181 MS and m/z = 219) and traces of C (496 nm) and Cl (479 nm). (1) ␣-HCH; (2) ,␥-HCH; (3) ␦-HCH.
The extract injected into the GC system usually contains the analytes in concentrations above the LODs given in Table 1. The LODs of the C-trace for the analytes are approximately three times higher than the results achieved for the Cl-trace, but the C-trace serves only as a tool for determining the time shift between these two detectors. 3.3.1. Real sample analysis Typical chromatograms obtained for a soil sample are shown in Fig. 4A and for a spiked soil sample in Fig. 4B. When only GC–MS (SIM) chromatograms are available, misinterpretation of HCH could occur. HCH isomers usually have retention times between 10.5 min and 12.0 min. On both of the ion chromatograms (m/z = 181 and m/z = 219) there is a peak (marked with vertical dashed line), which could be misinterpreted as one of the HCH isomers. Retention times of this peak and standards are different, but there is always a possibility of a retention time shift, when analysing real samples. Interpretation of the Cl-trace obtained from AED shows that the suspected peak does not contain chlorine at all and therefore cannot be a HCH isomer. The “HCH-like” compounds often occur on MS (SIM) chromatograms of soil and sediment samples from various sources. Using an additional element specific detector connected via TL allowed the identification problem to be resolved. 4. Conclusions Results obtained either for the COC–TL–MS system or the PTV–AED–TL–MS system show that the interface enables effective connection of two independent chromatographs into one system. Connection of two chromatographs equipped with MS and AED system using the GC transfer line, enables simultaneous analysis of a single sample by two detectors on two differ-
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ent instruments. GC–MS–AED provides structural information of the analytes and interferences, which prevents misinterpretation of the obtained data. The detection limits obtained were low enough to measure organochlorine pesticides in the environmental and food samples. A dual-detector system can be a useful tool in the analysis of a real sample in a complex matrix. The constructed transfer line is a universal device, which could be used for many different purposes—connecting two chromatographs to use various equipments mounted on both of the apparatus (inlets, detectors and other). The transfer line is a manufacturer-independent innovation (with its own electronics), easy to setup and maintain. The GC–GC interface enables one to create universal and powerful systems, which could be used in many GC laboratories.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.chroma.2008.09.072. References [1] H. Mol, T. Hankemeler, U. Brinkman, LC–GC 17 (1998) 544. [2] L. van Stee, P. Leonards, R. Vreuls, U. Brinkman, Analyst 124 (1999) 1547. [3] K. Ziegenhals, in: Presented at Pittcon 2006, AED Technical Session, Orlando, FL, 2006. [4] M. Schweigkofler, R. Niessner, Environ. Sci. Technol. 33 (1999) 3680. [5] T. Zimmermann, Ph.D. Thesis, University of Marburg, Marburg/Lahn, 2005. ˛ ˙ [6] Ł. Dabrowski, H. Giergielewicz-Mozajska, M. Biziuk, J. Gaca, J. Namie´snik, J. Chromatogr. A 957 (2002) 59. [7] http://www.amdis.net/About/Dejanews/Convert HP ChemStation 3D MSD data to ASCII-1.htm.