Journal of Chromatography A

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1 Journal of Chromatography A, 1190 (2008) Contents lists available at ScienceDirect Journal of Chromatography A journal homepage: Optimisation of programmable temperature vaporizer-based large volume injection for determination of pesticide residues in fruits and vegetables using gas chromatography mass spectrometry Darinka Štajnbaher a,, Lucija Zupančič-Kralj b a Public Health Institute, Environmental Protection Institute, Prvomajska 1, 2000 Maribor, Slovenia b Faculty of Chemistry and Chemical Technology, University of Ljubljana, Aškerčeva 5, 1000 Ljubljana, Slovenia article info abstract Article history: Received 21 October 2007 Received in revised form 29 February 2008 Accepted 4 March 2008 Available online 7 March 2008 Keywords: Programmed-temperature vaporizer (PTV) Large volume injection (LVI) Pesticide residue analysis Multiresidue method GC MS The applicability of programmable temperature vaporizer (PTV) solvent vent injection to the gas chromatographic (GC) determination of pesticide residues in fruits and vegetables was evaluated with the aim of miniaturizing the current multiresidue method. For that purpose 24 pesticides representing different chemical classes were initially chosen for optimisation of the large volume injection (LVI) parameters. Various parameters related to the optimum injector performance were tested for several types of packed and empty liners using both fast (at-once) and speed-controlled PTV solvent vent injection of standard solutions in ethyl acetate. In the next step, several packed and empty liners were evaluated for their suitability for pesticide multiresidue analysis. Parameters identified as optimal were then applied for PTV solvent vent injection of sample extracts prepared using the miniaturized multiresidue method to assess the long-term stability of the system. The combined use of large volume injection of 10 l ethyl acetate extract into an empty multi-baffled or a CarboFrit packed liner using PTV injectors and GC MS analysis enabled the detection and quantification of 124 pesticides in fruit and vegetable samples at the 0.01 mg/kg level using miniaturized reversed-phase solid-phase extraction (RP-SPE) of diluted acetone extract and clean-up on a small anion-exchange SPE column Elsevier B.V. All rights reserved. 1. Introduction Increased public concern over potential health hazards associated with exposure to pesticides has led to the development of highly sensitive and selective analytical procedures to determine residues of these compounds in a variety of food matrices [1 8]. Gas chromatographic methods with mass spectrometric detection (GC MS) have been used successfully for the analysis of many volatile pesticides and they offer simultaneous quantitation and confirmation of a large number of pesticides. The introduction of the sample into the gas chromatograph is a very important step, influencing sensitivity, accuracy, precision and resolution. The most important injection techniques are split/splitless, on-column and programmed temperature vaporisation (PTV) techniques, the last being capable of offering the greatest analytical flexibility, e.g. temperature programming and the possibility of large volume injections (LVI). Combining a cool injection step with controlled vaporisation eliminates or improves a number of important disadvantages Corresponding author. Tel.: ; fax: address: darinka.stajnbaher@zzv-mb.si (D. Štajnbaher). associated with the use of conventional hot splitless injection such as discrimination and thermal degradation as well as the adverse effects of non-volatiles present in the sample [9]. The quantitative performance of the PTV injection system appears to be comparable to that of on-column injection and much better than that of hot splitless injection [10 14]. In the GC separation high-molecular weight compounds present in food extracts can cause retention time shifts, poor peak shapes resulting in loss of resolution, reduced analyte response due to adsorption or degradation of labile analytes and/or decreased reliability of measurements as a result of matrix-induced response enhancement or diminishment. Therefore, sample cleanup is a very important step in maintaining long-term instrument performance. The trends towards miniaturization in sample preparation are driven by the need to increase the sample throughput and decrease the consumption of chemicals. The use of large volume injection in gas chromatography enables injection of a larger portion of the extract (up to several hundred microliters), reduction of sample size processed without affecting detection limits or omission of the offline extract concentration [2,5]. For the successful use of PTV-based LVI in pesticide residue analysis the system must be robust enough to enable stable performance [14,15 17] /$ see front matter 2008 Elsevier B.V. All rights reserved. doi: /j.chroma

2 D. Štajnbaher, L. Zupančič-Kralj / J. Chromatogr. A 1190 (2008) In our previous paper [1], we described development of a multiresidue method for determination of 90 pesticides in fruits and vegetables using a GC MS. This paper describes the application of LVI using a PTV injector as a tool to enable miniaturization of the multiresidue method [1] routinely used in our laboratory. 2. Experimental 2.1. Chemicals and reagents All solvents used were pesticide residue grade and were obtained from J.T. Baker (Deventer, The Netherlands). Certified pesticide standards were obtained from Dr. Ehrenstorfer (Augsburg, Germany). Individual stock standard and composite working standard solutions at four concentration levels (0.25, 0.50, 5.0 and 12.5 g/ml) were prepared in ethyl acetate. Calibration standards were prepared in both ethyl acetate solutions and blank extracts of fruits and vegetables at the concentration levels corresponding to 0.01, 0.02, 0.20 and 0.50 mg/kg. Deuterium-labelled [ 2 H 6 ]malathion was used as a procedural internal standard. Ethyl acetate solution of malathion-d 6 (16.0 ng/ l) was added to all homogenized samples prior to extraction. Its recovery served to assure that the performance of the method for each sample was within the acceptable limits for routine analysis and it was not used to correct for recovery. Pentachlorobenzene (PChB) solution (20.9 ng/ l) in ethyl acetate was used as an internal standard for calibration. It was added to the final extracts prior to the GC MS analysis to correct for volumetric errors. Matrix-matched calibration solutions were prepared in the following way: the extracts were evaporated to approximately 720 l under a gentle stream of nitrogen and 40 l of working standard solution in ethyl acetate containing the studied pesticides was added. In addition, 20 l of each of the internal standards were added to give a final volumeof800 l. The standard solutions for the optimisation of LVI parameters (std LVI 0.20) were prepared the same way, replacing the sample extract with ethyl acetate. Standard solutions (std CS 0.20) used for 1 l cold splitless injection were prepared by mixing the same working solutions in ethyl acetate at 10 times higher concentration. LiChrolut EN sorbent was purchased from Merck (Darmstadt, Germany) as a bulk material and was weighed into the 15-ml SPE columns with reservoirs (Macherey-Nagel, Düren, Germany) in the laboratory. Cartridges filled with DEA (100 mg/1 ml) were purchased from Varian (Middelburg, The Netherlands). Deionised water was prepared by a Nanopur reagent-grade water system from Barnstead (Dubuque, IA, USA) Sample extraction A 10 g portion of sample previously homogenized using a Blixer 3 Plus mixer (Robot Coupe, Jackson, MS, USA) was weighed into a 50-ml polypropylene centrifuge tube (Sarstedt, Nümbrecht, Germany) and fortified with 400 l of appropriate standard solution to give the concentration level corresponding to 0.02 or. In addition, 200 l of internal standard solution (malathion-d 6 )was added, followed by 18 ml acetone and the sample was mixed for 2 min using a Genie 2 vortex mixer (Scientific Industries, Bohemia, NY, USA). The extract was centrifuged for 10 min at 4000 rpm using a Sigma 3 18 K centrifuge (Sigma Laborzentrifugen, Osterode am Harz, Germany). The supernatant was transferred to a 25-ml volumetric tube and acetone water mixture (2:1) was added to the 25-ml mark. After thorough shaking a 2.5-ml aliquot (1/10) corresponding to 1 g of sample was transferred to a 15-ml reservoir of a funnel-shaped SPE column that had previously been filled with 150 mg of LiChrolut EN sorbent, washed with 4 ml of ethyl acetate and pre-conditioned with 4 ml of methanol followed by 5 ml of deionised water. After the addition of the aqueous acetone extract, the reservoir was filled to the 10-ml mark with deionised water. The diluted extract ( 18% acetone) was passed through the sorbent with a flow rate of approximately 5 10 ml/min. The sorbent was dried by passing air through the sorbent bed until the colour of the sorbent changed from brown to orange (ca. 20 min). A 1-ml SPE column containing 100 mg of DEA sorbent topped with approximately 1 cm of anhydrous sodium sulphate and washed with 3 ml of ethyl acetate was placed beneath the LiChrolut EN column. The pesticides retained on the LiChrolut EN phase were then eluted with 4 ml of ethyl acetate. The eluate was concentrated under a gentle stream of nitrogen to approximately 780 l and 20 l of PChB solution was added prior to GC MS analysis GC MS analysis An AT 6890 gas chromatograph (Agilent Technologies, Wilmington, DE, USA), configured with a CIS-4 Plus PTV inlet (Gerstel, Mühlheim, Germany) and an Agilent 5973 mass-selective detector with inert ion source was used. Samples were injected with an AT 7683B autosampler, which is capable of speed-controlled injections. The PTV injector was operated in the cold splitless mode (as a reference method) and the solvent vent mode. Peltier cooling was used to maintain the injection port at 20 C during injection and solvent elimination. The design and properties of tested liners are listed in Table 1. For the GC separations a HP-5MSi fused silica capillary column (30 m 0.25 mm, 0.25 m) coated with 5% diphenylmethylpolysiloxane stationary phase (Agilent Technologies) was used with helium as the carrier gas in the constant pressure mode at kpa. The MS was operated in electron ionization (EI) mode at 70 ev. Temperatures of the transfer line, ion source, and quadrupole were set at 280, 230 and 150 C, respectively. The column temperature was held at 70 C for 2 min, programmedat25 C/min to 150 C, then at 3 C/min to 200 C and finally at 8 C/min to 280 C, which was held for 14 min. Quantitative analysis of pesticides was performed using selected ion monitoring (SIM). The pesticide peaks were confirmed with retention times and abundance ratios of target ions and two qualifier ions (see Table 3). The retention time locking (RTL) feature of the Chemstation software was used to maintain stable retention times after trimming the column by locking a method to chlorpyriphos-methyl at a retention time min. The temperature programme and other conditions were chosen according to the method proposed by Agilent for use with deconvolution reporting software (DRS). The method uses constant pressure mode together with a relatively high initial pressure. Consequently, the flow rate at the lowest column temperature was 2.4 ml/min causing faster/more efficient sample transfer from the inlet to the column Long-term stability study The tolerance of the PTV injector and GC system for successive injection of matrix loaded sample extracts was tested using the optimised conditions in a long-term stability study. For this purpose the GC sequence consisted of 10 sets of 10 injections of matrix-matched standard solutions, prepared as described above, bracketed with a standard solution in ethyl acetate at the same concentration level. Two concentration levels corresponding to 0.20 and were tested. As the number of ions per group influences the sensitivity of GC MS in SIM mode, the longterm stability of response was tested for all pesticides included in the method (124 pesticides) to make the comparison under

3 318 D. Štajnbaher, L. Zupančič-Kralj / J. Chromatogr. A 1190 (2008) Table 1 The desing and other properties of the tested PTV liners Liner Empty CIS 4 liner, multibaffled, deactivated (Gerstel) PTV liner, multibaffled, deactivated (Agilent Technologies) Siltek deactivated multibaffled glass inlet liner (Restek) PTV liner, porous sintered glass on the inner surface, deactivated (Agilent Technologies) Packed liners Glass liner packed with PDMS foam with 10 mm packing length for intermediate volatiles (Gerstel) Liner filled with pesticide grade glass wool, deactivated (Gerstel) Siltek deactivated liner filled with Siltek glass wool (Restek) Siltek deactivated liner with CarboFrit plug (Restek) Properties, dimensions For all GERSTEL liners the deactivation is only stable at temperatures up to 275 C. Higher temperatures can be used, but this will create more active sites inside the liner, 2 mm I.D. (3 mm O.D.), length 71 mm and 200 l volume 1.5 mm I.D., length 71 mm and 150 l volume The maximum temperature of Siltek deactivation is 350 C, which enables higher temperatures during splitless transfer and/or baking out period, 1.5 mm I.D. (3 mm O.D.), length 71 mm and 200 l volume 1.5 mm I.D., length 71 mm and 150 l volume Polydimethylsiloxane (PDMS) (maximum temperature: 300 C, minimum temperature 10 C). It is an ideal packing material if trapping on glass wool is too weak and on Tenax TA too strong. It allows trapping at moderate temperatures, reducing the need for cryogenic cooling compared to glass wool. This nonpolar, highly inert material is suited for LVI of analytes with a wide range of volatility. It offers strong retention of nonpolar analytes and can be used for analytes with boiling points as high as n-c 40, 2 mm I.D. (3 mm O.D.) and length 71 mm Pesticide grade glass wool packed inlet liners are specially deactivated after the glass wool is inserted into the liner, 2 mm I.D. (3 mm O.D.) and length 71 mm The borosilicate glass wool was the only type of wool available because of increased brittleness of fused silica wool after treatment with Siltek deactivation, 2 mm I.D. (3 mm O.D.) and length 71 mm Carbofrit is a filigree network of carbon-type material treated at high temperature. It offers higher thermal stability (450 C) and no bleed from deactivation, 2 mm I.D. (3 mm O.D.) and length 71 mm conditions as close as possible to the conditions during routine analysis. 3. Results and discussion 3.1. Optimisation of solvent vent PTV parameters The applicability of PTV-based LVI and the evaluation of the PTV liners were initially assessed using a test mixture of 24 representative pesticides that varied widely in polarity, thermolability and volatility and included compounds prone to degradation and susceptible to matrix effects/tailing (in bold in Table 3). The use of PTV-based LVI with solvent elimination requires careful method optimisation for maximum accuracy and reproducibility. The following parameters were taken into consideration: design of the liner, the presence and type of packing in the liner, injection speed, solvent evaporation temperature (initial injector temperature), solvent venting time, vent flow-rate, inlet pressure, and splitless time/transfer time. Since the flow of carrier gas through the liner causes the solvent and any volatile fraction of the sample to evaporate and be swept with the carrier gas out through the split vent, special attention was paid to the optimisation of conditions for volatile compounds. Losses of such compounds may be reduced by careful optimisation of the injection parameters when empty liners are used or by the use of an adsorbent as a trapping material [12,18]. Another group of analytes that requires special attention includes the compounds prone to thermodegradation and/or adsorption. Therefore, the degradation products of carbofuran (7-phenol carbofuran), carbaryl (1-naphtol), captan (1,2,3,6-tetrahydrophthalimide), folpet (phthalimide), dicofol (dichlorobenzophenone), dichlofluanid (N,N-dimethyl-N-phenylsulphamide, DMSA), tolylfluanid (4-dimethylaminosulphotoluidide, DMST), and iprodione (3,5- dichlorophenyl hydantoin) were also included in the method (not added to standard solutions) to assess the level of degradation in the injector/column/matrix. To enable rapid heating, the internal diameter of PTV liners is generally smaller than those of conventional split/splitless injectors. The disadvantage of the low internal volume is the limited amount of the liquid sample that can be injected at once. The volume of liquid that can be retained by the liner determines the maximum volume. When the sample volume exceeds the maximum volume of the PTV liner, the sample should be introduced either by repetitive (multiple) injection, or by speed-controlled injection. The PTV-based LVI was optimised by comparing the peak areas obtained for cold splitless injection of 1 l of a standard solution (std CS 0.20) and solvent vent injection of 10 l of 10-fold diluted standard solution (std LVI 0.20). In addition to analyte recoveries, measured as the ratio of the response for solvent vent to that for cold splitless injection expressed as a percentage, the degradation rate and peak shape were also taken into account when assessing the optimal conditions and/or evaluating the liners Optimisation of parameters for speed-controlled injection The speed-controlled injection was necessary for empty liners, because tests showed that the maximum injection volume under the conditions used was below 10 l. The speed-controlled injection was preferred to repeated injection because of better performance as a result of increased efficiency of solvent trapping of analytes in the liner during solvent elimination [9,17]. The parameters for solvent vent injection into empty liners using solvent vent were optimised for a Siltek deactivated multibaffled liner Optimisation of parameters for solvent venting. The successful separation of the solvent from the most volatile analytes in the injector depends on the solvent evaporation temperature, solvent evaporation time, inlet pressure and the vent flow-rate [11,12]. Also, the design of the liner as well as the physico-chemical properties of the solvent need to be taken into account. For separation of analytes from the solvent without significant losses, it has been found that the initial PTV temperature should be about 250 C below the boiling point of the most volatile analyte [19 21]. In this work, ethyl acetate was chosen for its good properties as a SPE elution solvent. It is less favourable as a solvent for PTV-based LVI injection because of its relatively high boiling point (77.1 C). The low boiling point of dichlorvos (234.1 Cat Pa [22]) would require sub-zero cooling of the inlet to achieve the 250 C rule. One way to extend the application range towards more volatile analytes is to stop solvent venting shortly before completion of solvent evaporation since it has been found that losses of volatile analytes occur mainly with evaporation of the last few microliters of solvent [18 20]. Prior to this, most compounds are efficiently

4 D. Štajnbaher, L. Zupančič-Kralj / J. Chromatogr. A 1190 (2008) retained in the liner in a film of solvent (solvent trapping) aided by the fact that the temperature at the evaporation site can be much lower than the initial PTV temperature due to the cooling caused by solvent evaporation [20]. When the solvent is completely removed, the temperature returns to the initial PTV temperature and evaporation of the most volatile analytes increases. In this situation, the proper optimisation of vent time is very important. Our results showed that this approach is beneficial also for labile pesticides (mainly carbofuran and carbaryl) which showed less degradation for empty liners when the solvent was not completely removed. Probably the simultaneous evaporation of analytes and the solvent is beneficial for labile compounds, because thermal stress as well as degradation due to adsorption is reduced [12,13]. The interrelated vent parameters such as vent flow rate, vent pressure and vent time had to be mutually optimised. Each of these interactive parameters must also be optimised with respect to the other parameters, especially the evaporation temperature and vent flow-rate. The head pressure during solvent elimination was set to 0 kpa ( flow stop mode), which in the case of the vacuum outlet means the actual pressure at the chosen vent flow rate was approximately 5 kpa. A pressure close to ambient pressure speeds up the evaporation of solvent, and at the same time lowers the amount of solvent entering the column during solvent elimination. Longer vent times were necessary if a lower vent flow rate and higher vent pressure were used. Since higher vent flow rates lead to losses of more volatile pesticides, a longer vent time together with low pressure was used instead. When using higher flow rates, good recoveries were only possible for the conditions that lead to the severe peak fronting of the more volatile analytes. With the longer vent times needed to avoid the flooding of the column entrance that caused the peak distortion, the recoveries of volatiles dropped to unacceptable values. Although good peak shapes were obtained with the use of a retention gap that accommodates the recondensed solvent [12], it was not used in this study (see below). Only with lower vent flow rates were good recoveries of volatile pesticides and good peak shapes possible. Lower flow rates were also beneficial in reducing the actual pressure in the stop-flow mode resulting in higher evaporation rates. The degradation rates of thermolabile pesticides increased with increasing vent flow rates, which was probably due to the smaller amount of remaining solvent in the liner. On the other hand, the quantity of remaining solvent needed be low enough to avoid causing excessive flooding of the column, since no retention gap was used. The recoveries of volatile analytes were better at lower initial PTV temperatures, while the influence was not significant for less volatile compounds. At constant vent conditions, raising the initial PTV temperature led to losses of more volatile analytes and also to increased degradation of thermolabile pesticides (carbaryl, carbofuran) as a consequence of complete solvent removal. The vent time under optimised conditions was determined experimentally by increasing the time stepwise until the peak distortion due to band broadening in space caused by excessive recondensation of solvent vapour in the column was no longer present. The final condition chosen for solvent elimination were mild (see Table 2). Using a low initial PTV temperature of 20 C, low vent flow rates and slow injection speed combined with longer vent times, the losses of dichlorvos were not significant Optimisation of sample transfer from the PTV injector to the GC column. Minimal diffusion and mixing of sample vapours with the carrier gas is required for a rapid and efficient transfer of the sample from the liner onto the column. This is effectively achieved through the use of narrow liners with higher linear carrier gas velocity enabling faster transfer of the compounds to the column at lower temperatures [19]. In theory, the sample transfer time for PTV injectors using narrower liners ( 2 mm I.D.) should be substantially shorter in comparison with conventional splitless injection [21,23]. Our experimental data showed that times longer than expected were needed for efficient transfer, which was also noticed by Grob et al. [23]. With reduced splitless time, the responses of high boiling compounds quickly decreased, whereas this effect was much less pronounced for more volatile pesticides. The heating rate of the liner is more important, when there is more solvent remaining in the liner, since the small internal volume of the liner can easily be overloaded by developing solvent vapours. An optimised inlet heating rate (5 C/s) enabled efficient transfer of analytes from the inlet without liner overflow. The influence of the heating rate is more pronounced for less volatile pesticides (deltamethrin) with better responses at higher heating rates. For thermolabile compounds the opposite was observed and the responses of degradation products of dichlofluanid and tolylfluanid increased significantly at higher heating rates. The PTV temperature at the time of sample transfer had a similar influence on responses as the heating rate. For the efficient transfer of less volatile pesticides the necessary PTV temperature was between C. At higher temperatures the response of the last eluting analyte (deltamethrin) increased only slightly. However, higher transfer temperatures caused an increased degradation rate of labile pesticides (carbaryl, carbofuran, tolylfluanid, etc.). The temperature chosen (240 C) was a compromise between Table 2 The optimised conditions for LVI using fast and speed-controlled injection and the conditions for refence cold splitless method (the values in italics are the values used in the optimisation process) Fast injection Speed-controlled injection Cold splitless (reference method) Injection volume (syringe volume) 10 l (25- l syringe) 10 l (25- l syringe) 1 l (5- l syringe) GC initial temperature (initial time) 70 C (2.0 min) (50, 60, 70, 80 C) 70 C (2.0 min) (50, 60, 70, 80 C) 70 C (2.0 min) Injection speed (time needed) fast 35 l/min (0.286 min) (20, 25, 30, 35 l/min) fast Vent time 0.77 min (many different values) 0.45 min (many different values) Purge time (vent time + splitless time) 2 min (0.8, 1.0, 1.5, 2.0, 2.5 min) 2.5 min (0.5, 1.0, 1.5, 2.0, 2.5 min) Vent pressure 0 kpa 0 kpa Vent flow 20 ml/min (20, 30, 40, 70, 100, 200 ml/min) 40 ml/min (20, 30, 40, 70, 100, 200 ml/min) PTV initial temperature (initial time) 20 C (0.82 min) (10, 20, 25, 30, 35, 40 C) 20 C (0.50 min) (10, 20, 25, 30, 35, 40 C) 20 C (0.50 min) PTV heating rate 1 5 C/min (2, 3, 4, 5, 7, 9, 12 C/min) 5 C/min (2, 3, 4, 5, 7, 9, 12 C/min) 5 C/min PTV transfer temperature (hold time) 240 C (3 min) (180, 200, 220, 240, 260, 280, 300, 320 C) 240 C (3 min) (180, 200, 220, 240, 260, 280, 240 C (3 min) 300, 320 C) PTV heating rate 2 10 C/s 10 C/s 10 C/s PTV final temperature (hold time) 280 C (10 min) 350 C 280 C (10 min) 350 C 280 C (10 min) 350 C Pressure 21.5 psi 21.5 psi 21.5 psi Splitless time = purge time vent time = purge time vent time 2.0 min

5 320 D. Štajnbaher, L. Zupančič-Kralj / J. Chromatogr. A 1190 (2008) these effects. Also, a lower PTV temperature during sample transfer was used to prevent the non-volatile or high boiling matrix components from entering the analytical column. The co-injected matrix components and the presence of retaining dirt material in the liner may impair the transfer of analytes. Therefore, optimisation of heating rate and PTV temperature at sample transfer was repeated in the presence of matrix (green pepper extract). No significant differences were noticed. During the first stage, the injector was programmed to reach the lowest temperature required for transfer of the heaviest pesticides and during the second stage the higher boiling sample matrix compounds were diverted out of the split vent during the injector baking-out phase. The final PTV temperature chosen was 280 C because the maximum temperature for the deactivation of Gerstel liners is 275 C. Using mild conditions for solvent elimination, especially low vent flow rates, caused the recoveries of less volatile pesticides (deltamethrin) to fall below acceptable values. The use of pressure pulse injection was tested to achieve a better response of late eluting compounds. Increasing the inlet pressure during splitless sample transfer helps to reduce the residence time of the analytes in the liner at elevated temperatures as well as increase the amount of especially late-eluting analytes transferred onto the column. Pressure pulse injection gave better recoveries of high boiling pesticides (10 20%). The improvement was not substantial enough to justify the use of this technique, given that the transfer of high boiling matrix components was also higher. The column temperature during sample introduction into the column influences the transfer of the solvent vapours as well as the initial band widths of the analytes and can therefore have a great impact on the peak shapes [9,19,23]. Column temperatures some C below the pressure-corrected solvent boiling points leads to solvent recondensation at the beginning of the column, effectively stopping the migration of the starting band (solvent effect). However, recondensation of large solvent volumes causes peak distortion of analytes of high and moderate volatility [12,19].Toavoid this peak distortion, the initial GC column temperatures should be only a few degrees below the pressure-corrected boiling point of the solvent when only a small volume of solvent recondenses [23]. In our experiments, the influence of the column temperature on pesticide response during sample transfer of the analytes was not significant, but it was important for the peak shapes. Higher initial head pressure enabled faster sample transfer and increased solvent boiling point, which made solvent recondensation possible even at the initial column temperature of 70 C, which was close to the normal solvent boiling point. At the chosen pressure (148.2 kpa), the boiling point of ethyl acetate was calculated from the Antoine equation and was equal to C [24]. Under the conditions chosen (Table 2), the best results were achieved for initial GC temperatures between C. A higher initial GC oven temperature (90, 100 C) led to an increasing peak splitting of early eluting compounds (dichlorvos, mevinphos, degradation product of carbofuran). A lower initial GC oven temperature (40, 50 C) led to peak fronting of early eluting compounds caused by column flooding as a result of increased solvent recondensation at the column entrance. Even though the use of a retention gap is beneficial in connection with solvent vent injection because more solvent can be transferred to the column without deterioration of peak shape, a retention gap was not used in our study because of the additional active sites that may cause peak tailing of certain susceptible analytes. Also, with careful optimisation of the PTV and GC conditions, good peak shapes were obtained without it. Optimised and tested parameters for speed-controlled PTV solvent vent injection using empty liners are summarized in Table The optimisation of parameters for fast (at-once) injection For large volume injection the use of packed liners is often recommended in order to prevent the liquid sample from reaching the base of the injector, resulting in losses via the split exit, or flooding the column inlet. Packed liners are capable of retaining a large volume of liquid sample allowing rapid introduction (fast or at-once injection). Compared to empty liners the packing in the liner significantly increases the solvent elimination rate, as a result of the increased gas-solvent contact area [10,11]. The use of packed liners improves sample vaporization, leading to increased responses of less volatile analytes, better reproducibility, improved peak shapes and more efficient trapping of non-volatile sample residue [9]. The use of liners packed with a suitable adsorption material is another efficient method of minimising losses of volatile analytes [10,18,25]. The use of packed liners can also have disadvantages such as adsorption of active compounds, variable performance due to inconsistent packing densities and contamination by decomposition products of the packing material itself and/or deactivation agents. In particular, the most commonly used glass wool is difficult to deactivate [9,13,19,25]. The selection of packing material depends on the volatility and the polarity of the analytes. Various packing materials were tested since higher capacity packed liners enable fast (at-once) injection, leading to a simpler optimisation process due to a lower number of interrelated parameters that require optimisation. For the optimisation of fast injection a Siltek deactivated liner packed with a CarboFrit plug was used. With this, the maximum volume that could be retained by the liner was at least 10 l, thus fast injection was possible. Due to the more efficient solvent removal for packed liners, milder conditions for solvent venting were chosen, namely lower vent flow rate and consequently longer vent times. Most of the conclusions of the optimisation process for empty liners also apply to packed liners. The heating rate had little influence on the response of pesticides and in contrast to empty liners the degradation was not significantly higher at higher heating rates. For the liner packed with a Carbofrit plug there was no significant increase in degradation of labile pesticides at higher PTV transfer temperatures ( C), especially when pressure pulse was used. In the absence of the pressure pulse the degradation was slightly more pronounced, but still lower than for empty liners. The responses were not significantly dependent on PTV temperature during sample transfer and an almost complete transfer of deltamethrin was achieved even at 220 C demonstrating more efficient sample transfer for packed liners. The influence of matrix (apples) on the transfer was again not significant under the conditions used. The desorption PTV temperature chosen was 240 C, since a lower temperature helps to reduce the amount of high boiling matrix components reaching the column. The recoveries of late eluting compounds were good (significantly better than for empty liners) and a pressure pulse was not necessary Evaluation of liners for solvent vent PTV-based injection The choice of a suitable liner is especially important in pesticide residue analysis using PTV-based LVI. The volatility of the analytes as well as the sample volume determines the preferred injection mode and type of liner. Packed liners can lead to low recoveries of thermolabile and adsorptive compounds, even for well-deactivated packing materials. In this case the recoveries can be improved by using empty liners with small internal diameters. The residence time of pesticides in this type of liners is shorter and analytes leave the PTV injector at lower temperatures [9,25]. Three of the liners selected in this study were empty multibaffled liners, one of which was empty with porous sintered glass on the inner surface and the rest were packed liners. Since pesticides

6 D. Štajnbaher, L. Zupančič-Kralj / J. Chromatogr. A 1190 (2008) Table 3 Tested pesticides (in bold are representative pesticides chosen for initial optimisation of PTV injection parameters), target and qualifier ions, and relative standard deviations (RSD, %) of responses and calculated matrix effects together with their relative standard deviations (RSD, %) for both types of liners and concentration levels (n =100) Pesticide Target/qualifier ions Relative standard deviation (RSD, %) Average matrix effects (RSD, %) CarboFrit a, Metamidophos 141, 94, (10.5) 113 (6.4) 108 (2.5) 107 (2.7) Dichlorvos 109, 185, (2.8) 101 (2.9) 101 (4.2) 106 (1.7) Dichlobenil 171, 173, (1.8) 100 (2.0) 94 (2.9) 103 (1.2) Biphenyl 154, 152, (1.6) 101 (2.3) 103 (3.8) 106 (1.1) Mevinphos 127, 192, (3,2) 106 (3.4) 116 (7.3) 108 (2.0) Propham 93, 137, (3.0) 100 (1.8) 96 (3.3) 105 (1.5) o-phenylphenol 170, 141, (2,5) 103 (1.6) 105 (3.7) 108 (1.6) Heptenophos 124, 250, (3,2) 104 (2.6) 103 (4.5) 107 (1.8) Tecnazene 261, 203, (2.5) 100 (1.7) 96 (3.4) 104 (1.8) Diphenylamine 169, 167, (2.5) 101 (1.6) 99 (3.4) 105 (1.5) Chlorpropham 127, 213, (2.9) 102 (1.9) 98 (3.3) 108 (1.7) Trifluralin 306, 264, (3.1) 101 (1.6) 96 (3.5) 107 (1.7) Phorate 75, 231, (3.9) 103 (1.7) 110 (3.5) 111 (1.8) Dichloran 176, 206, (2.8) 102 (2.0) 96 (3.6) 107 (2.7) Dimethoate 87, 93, (6.8) 114 (6.3) 129 (9.6) 114 (2.5) Carbofuran 164, 149, (3.2) 104 (2.7) 112 (8.2) 108 (2.2) Atrazine 215, 200, (2.6) 101 (2.0) 101 (3.1) 107 (1.7) Lindane 181, 183, (2.6) 100 (2.1) 97 (3.0) 104 (1.6) Quintozene 237, 295, (3.0) 101 (1.8) 97 (3.6) 104 (2.0) Fonofos 246, 109, (2.8) 101 (1.7) 98 (3.4) 106 (1.5) Propetamphos 138, 194, (2.9) 101 (1.8) 97 (3.2) 107 (1.7) Propizamide 173, 255, (2.7) 101 (1.8) 98 (3.2) 106 (1.9) Pyrimethanil 198, 200, (2.6) 101 (1.8) 98 (3.1) 106 (1.7) Diazinon 179, 304, (2.7) 101 (1.8) 98 (3.5) 107 (1.6) Phosphamidon I 264, 127, (9.2) 113 (6.6) 162 (18.1) 114 (2.8) Paraoxon-methyl 247, 109, (13.4) 118 (10.3) 148 (17.0) 112 (4.5) Chlorothalonil 266, 264, (8.5) 88 (14.6) 50 (27.8) 101 (9.4) Pirimicarb 166, 72, (2.6) 103 (1.7) 98 (3.2) 109 (2.0) Phosphamidon II 264, 127, (4.9) 112 (5.2) 147 (11.9) 115 (2.6) Parathion-methyl 263, 109, (3.7) 111 (5.0) 109 (4.9) 112 (3.0) Chlorpyriphos-methyl 286, 125, (3.3) 108 (3.8) 106 (4.1) 110 (2.1) Vinclozolin 198, 285, (2.9) 102 (1.7) 102 (3.5) 107 (1.7) Carbaryl 144, 115, (4.9) 108 (6.0) 139 (18.5) 109 (4.4) Tolclofos-methyl 265, 125, (2.4) 102 (2.2) 99 (3.1) 107 (1.6) Malaoxon 127, 268, (8.1) 118 (8.7) 186 (19.2) 107 (6.7) Fenchlorphos 285, 287, (3.7) 108 (4.2) 104 (3.7) 110 (2.2) Prometryn 241, 226, (2.3) 102 (1.7) 100 (3.3) 107 (1.9) Metalaxyl 206, 249, (4.5) 101 (1.9) 99 (3.5) 107 (2.0) Paraoxon-ethyl 275, 109, (3.6) 107 (3.6) 119 (7.5) 110 (2.7) Fenitrothion 277, 125, (6.4) 110 (4.7) 107 (4.2) 112 (2.7) Pirimiphos-methyl 276, 290, (14.6) 103 (19.8) 103 (3.4) 102 (1.7) Dichlofluanid 123, 167, (2.2) 112 (9.9) 121 (8.9) 119 (10.2) Malathion 173, 127, (4.7) 112 (5.2) 109 (5.7) 111 (2.1) Metolachlor 238, 162, (2.7) 101 (1.8) 98 (3.1) 106 (1.7) Fenthion 278, 125, (2.7) 104 (2.6) 102 (3.4) 109 (1.7) Chlorpyriphos-ethyl 197, 314, (2.6) 101 (1.8) 100 (3.1) 106 (1.7) Parathion-ethyl 291, 109, 125, (3.1) 102 (2.1) 101 (3.6) 108 (2.3) Triadimefon 208, 57, (5.7) 101 (1.9) 100 (3.8) 107 (2.0) Tetraconazole 336, 338, (6.3) 103 (1.9) 101 (4.9) 110 (2.6) Bromophos-methyl 331, 125, (4.5) 113 (5.8) 107 (4.3) 112 (2.7) Cyprodinil 224, 210, (2.8) 103 (1.8) 101 (3.1) 109 (1.8) Pirimiphos-ethyl 318, 333, (2.8) 102 (1.7) 100 (3.0) 109 (1.9) Penconazole 248, 159, (6.3) 101 (2.1) 102 (4.9) 108 (2.3) Chlorfenvinphos I 267, 323, (3.0) 103 (2.3) 105 (4.1) 110 (2.2) Thiabendazole 201, 174, (5.6) 107 (5.9) 117 (5.3) 110 (8.0) Captan 79, 264, (4.0) 103 (4.9) 107 (12.7) 106 (6.2) Tolylfluanid 137, 238, (2.9) 112 (8.5) 107 (8.2) 116 (8.4) Chlozolinate 331, 259, (2.8) 100 (1.9) 98 (3.6) 106 (1.7) Chlorfenvinphos II 267, 323, (3.0) 103 (2.3) 105 (4.1) 110 (2.2) Folpet 262, 297, (4.8) 104 (5.2) 122 (15.9) 108 (6.0) Quinalphos 146, 298, (2.7) 102 (2.1) 101 (3.7) 109 (1.8) Triadimenol I 168, 112, (7.2) 109 (6.3) 132 (6.4) 114 (5.4) Mecarbam 329, 159, (2.7) 101 (1.8) 103 (4.3) 107 (1.9) Fipronil 367, 369, (5.8) 104 (1.7) 104 (3.4) 109 (2.7) Procymidone 283, 285, (2.1) 101 (2.2) 102 (2.7) 107 (1.7) Triadimenol II 168, 112, (7.2) 107 (2.5) 107 (5.2) 115 (3.6) Hexythiazox 227, 156, (40.3) 104 (33.7) 101 (22.6) 129 (24.4) Methidathion 145, 85, (4.5) 118 (6.6) 119 (8.1) 115 (3.1) Endosulfan I 195, 339, (4.1) 100 (1.8) 98 (5.1) 105 (1.9) Tetrachlorvinphos 329, 331, (5.4) 118 (7.5) 137 (11.0) 115 (3.4) Mepanipyrim 222, 207, (3.3) 103 (2.1) 104 (3.0) 109 (2.3) Hexaconazole 214, 231, (5.6) 105 (2.1) 109 (4.9) 112 (2.8)

7 322 D. Štajnbaher, L. Zupančič-Kralj / J. Chromatogr. A 1190 (2008) Table 3 (Continued ) Pesticide Target/qualifier ions Relative standard deviation (RSD, %) Average matrix effects (RSD, %) CarboFrit a, Imazalil 215, 217, (6.7) 112 (3.8) 106 (3.6) 115 (4.4) Profenofos 339, 374, (3.4) 105 (3.4) 117 (6.5) 111 (2.2) Fludioxonil 248, 154, (5.1) 106 (2.6) 110 (3.6) 109 (2.8) Myclobutanil 179, 288, (5.6) 103 (2.5) 98 (5.5) 108 (2.8) Iprovalicarb I 119, 134, (7.0) 105 (2.1) 105 (5.8) 108 (3.3) Buprofezin 105, 305, (3.5) 108 (5.4) 107 (12.1) 107 (4.2) Bupirimate 273, 316, (3.1) 101 (1.8) 101 (3.2) 108 (2.4) Cyproconazole 222, 139, (5.3) 102 (2.4) 103 (4.7) 108 (4.9) Nitrofen 283, 285, (3.6) 103 (2.4) 107 (3.5) 109 (3.8) Krezoxim-methyl 116, 131, (3.5) 100 (5.4) 101 (3.4) 107 (2.3) Iprovalicarb II 119, 134, (5.2) 103 (2.8) 102 (4.1) 107 (3.5) Endosulfan II 195, 339, (3.6) 101 (2.0) 126 (3.3) 106 (2.2) Chlorfenapyr 408, 247, (4.8) 103 (2.6) 103 (5.0) 109 (2.2) Chlorobenzilate 251, 139, (3.3) 101 (2.0) 102 (3.3) 108 (2.2) Diniconazole 268, 270, (6.5) 107 (2.7) 102 (5.4) 106 (3.8) Oxadixyl 163, 233, (6.4) 101 (2.3) 104 (4.2) 107 (2.7) Ethion 231, 384, (3.4) 105 (2.6) 103 (3.5) 110 (2.3) Triazophos 161, 285, (3.5) 102 (4.5) 118 (6.9) 104 (3.3) Endosulfan sulfate 272, 237, (3.1) 102 (2.6) 102 (3.3) 108 (2.3) Benalaxyl 148, 325, (3.3) 101 (1.9) 101 (3.4) 107 (2.7) Quinoxyfen 237, 307, (3.2) 101 (2.1) 101 (3.2) 107 (2.3) Propiconazole 259, 173, (9.6) 103 (5.6) 102 (5.6) 107 (5.8) Trifloxystrobin 116, 222, (4.1) 103 (2.9) 106 (3.3) 116 (3.8) Tebuconazole 250, 125, (5.5) 103 (2.5) 106 (4.6) 108 (3.1) Propargit 135, 350, (4.7) 102 (6.2) 113 (4.6) 110 (3.5) Iprodione 314, 316, (5.2) 103 (3.0) 112 (6.8) 108 (3.1) Phosmet 160, 317, (8.1) 139 (11.7) 144 (14.0) 119 (5.5) Pyridaphenthion 340, 199, (4.9) 107 (4.5) 116 (7.3) 109 (3.7) Bromopropilate 341, 183, (4.1) 101 (2.2) 102 (3.3) 107 (2.7) Dicofol 251, 139, (19.2) 161 (46.1) 108 (40.1) 91 (17.0) Bifenthrin 181, 165, (3.4) 103 (1.9) 102 (3.4) 109 (2.8) Fenpropathrin 181, 265, (4.6) 101 (2.4) 101 (4.4) 108 (2.6) Tetradifon 356, 159, (4.4) 101 (2.3) 100 (3.7) 106 (2.5) Azinphos-methyl 132, 160, (12.5) 129 (14.7) 133 (18.3) 126 (6.9) Phosalone 182, 121, (5.2) 114 (6.1) 118 (8.0) 111 (4.3) Pyriproxyfen 136, 226, (4.6) 103 (2.3) 105 (3.5) 111 (3.2) Lambda-Cyhalothrin 181, 197, 199, (5.5) 108 (6.2) 102 (4.1) 109 (3.3) Fenarimol 139, 251, (5.3) 102 (2.7) 102 (4.7) 108 (2.9) Azinphos-ethyl 132, 160, (5.9) 109 (4.7) 118 (9.2) 110 (3.9) Pyrazophos 221, 232, (4.8) 107 (3.7) 107 (5.2) 108 (3.3) Acrinathrin 289, 181, (3.6) 107 (6.4) 102 (6.0) 109 (3.2) Bitertanol 170, 168, (9.0) 109 (4.1) 112 (5.7) 116 (6.3) Permethrin 183, 163, (5.2) 103 (2.5) 100 (7.0) 106 (7.4) Coumaphos 362, 364, (7.5) 115 (5.5) 115 (7.3) 110 (4.4) Cyfluthrin 163, 206, (5.3) 109 (3.2) 102 (5.8) 107 (3.7) Cypermethrin 181, 163, (5.9) 109 (13.2) 111 (8.1) 108 (3.4) Difenoconazole 323, 265, (13.2) 112 (7.6) 112 (12.0) 112 (6.1) Indoxacarb 527, 468, (7.2) 109 (4.0) 105 (12.9) 109 (4.2) Deltamethrin 181, 255, 209, (7.3) 102 (5.1) 99 (8.5) 107 (4.7) Famoxadone 330, 315, (12.9) 113 (9.5) 108 (7.1) 111 (5.7) Azoxystrobin 344, 388, (11.9) 109 (6.8) 109 (6.4) 110 (4.8) a Responses of first 20 injections (out of 120) were not taken into account because the liner was not previously conditioned. represent many different chemical classes, losses due to degradation and/or problems with desorption can readily occur. Therefore, stronger adsorbents were not tested as packing materials. Despite the well-known disadvantages of glass wool in pesticide residue analysis, two types of wool were also tested as their properties and type of deactivation were claimed to be improved. The liners tested and their dimensions and properties are listed in Table 1. For the comparison of liners a standard solution in ethyl acetate containing all pesticides listed in Table 3 was used. The criteria for selection of a liner were based on the recoveries (relative responses vs. responses obtained using cold splitless injection with the same liner) of the most volatile pesticide (dichlorvos), degradation rate of labile pesticides, recoveries of late eluting compounds (deltamethrin) and the overall performance of the liner. Finally, the responses obtained with the tested liner were compared to responses for the Siltek deactivated multibaffled liner, which was chosen as a reference. The parameters previously optimised for fast and speed-controlled injection were used as a starting point and were re-optimised if necessary. Comparison of the performance of different liners is not easy, because of the difficulty in achieving constant and repeatable conditions of the GC system over a long period of time. Therefore, the initial tests were performed in the absence of matrix. For every new type of the liner, a piece (about 10 cm) of the column entrance was trimmed off. No special conditioning of the liners was performed. Instead, several 10 l injections of standard solutions in solvent vent mode were made initially to assess the need for conditioning of the tested liner The applicability of empty multibaffled liners for pesticide residue analysis For compounds sensitive to degradation and/or adsorption on active sites the preferred liner type is an empty liner, so those were tested first. The optimised parameters for speed-controlled injection were used for comparison of three commercially available

8 D. Štajnbaher, L. Zupančič-Kralj / J. Chromatogr. A 1190 (2008) multibaffled PTV liners suitable for CIS 4 Plus injector (Table 1). The only significant difference between these multibaffled liners was that initially the background for the Agilent liner was much higher (a high peak for diethyl phthalate was present in the first run). The comparison of liners for solvent vent injection showed significantly lower responses for the Agilent liner, especially for high boiling compounds (deltamethrin). The Gerstel multibaffled liner gave similar results to the Siltek deactivated multibaffled liner, the latter giving the least degradation of the three liners (for carbaryl and carbofuran) Empty liners with porous sintered glass on the inner surface The liner with porous sintered glass on the inner surface is an empty liner, but with its specially prepared surface provides a higher capacity for liquid samples. For the assessment of this liner the conditions for fast injection were used, since its maximum sample volume under the chosen conditions was higher than 10 l. In comparison with the liners packed with adsorbents, the use of high desorption temperatures does not lead to thermal decomposition of the packing material. Also, the risk of decomposition of analytes is reduced compared to packed liners, although the higher surface compared to empty multibaffled liners makes it more active. Using the conditions previously optimised for fast injection in solvent vent mode, the recoveries of high boiling compounds were significantly lower. Further optimisation revealed that higher desorption temperatures were needed. Unfortunately, the use of higher desorption temperatures led to higher degradation rates for labile pesticides, especially carbofuran, carbaryl and dicofol. Recoveries for low boiling pesticides were good indicating no losses of volatiles. Comparison of responses for solvent vent injection with the Siltek multibaffled liner showed better responses for the multibaffled liner, especially for thermolabile pesticides (carbaryl and carbofuran) and polar pesticides (methamidophos, mevinphos) Liner packed with pesticide grade glass wool Pesticide grade glass wool packed inlet liners are specially deactivated after the glass wool is inserted into the liner. Comparison of responses for solvent vent injection between a straight liner packed with pesticide grade wool and a Siltek deactivated multibaffled liner showed good responses using the former for stable compounds, but as expected also higher degradation of labile pesticides (captan, folpet, carbofuran, carbaryl and dicofol). The responses were also worse for polar pesticides (methamidophos, paraoxonmethyl, azinphos-methyl). Peak shapes for all pesticides were good, which was more commonly the case for packed than for empty liners Siltek deactivated single notch liner packed with Siltek deactivated glass wool A liner packed with Siltek-treated wool was also tested because Siltek treatment proved to be efficient in preventing degradation of pesticides in multibaffled liners. The initial injections to assess the activity of a new liner showed two types of changes in responses. With continuing injections the responses of some pesticides decreased (carbaryl, carbofuran, etc.), while for others it increased (azinphos-methyl, paraoxon-methyl). Deltamethrin showed a distinct isomerisation with both peaks being almost equal in height. The degradation rate was quite high for most of the labile pesticides. Dicofol gave no response throughout the conditioning sequence (10 runs). The responses of polar pesticides (methamidophos, paraoxon-methyl) were very low and did not significantly improve with further injections. The comparison of responses of Siltek deactivated wool and pesticide grade wool showed much better results for the latter, with the difference being greatest for polar, labile and for late eluting pesticides. The performance of the liner was also much worse than the Siltek deactivated multibaffled liner Liner packed with PDMS foam Polydimethylsiloxane (PDMS) foam has an open porous structure with a maximum temperature of 300 C. A glass liner packed with 10 mm packing length PDMS foam intended for intermediate volatiles was used. As all previous liners, this liner was also initially conditioned through several solvent vent injections of standard solution. The responses of pesticides stabilized after 10 injections, and only azinphos-methyl gave variable responses. A comparison of the results for solvent vent injection with those for the Siltek deactivated multibaffled liner showed much lower responses, especially for high boiling (deltamethrin, azinphos-methyl) and polar compounds (methamidophos). The use of a higher desorption temperature (300 C) did not significantly improve the responses. The degradation rate was high, especially for carbofuran, carbaryl and dicofol Siltek deactivated single notch liner with Carbofrit plug CarboFrit packing offers the same advantages of glass wool, but with superior inertness for highly active compounds and higher thermal stability. The improved trapping of high molecular weight contaminants helps increasing column lifetime and decrease GC system maintenance requirements. Liners packed with a CarboFrit plug were used for conventional splitless injection, especially for injection of larger sample volumes (5 10 l) of unpurified food extracts [7,8]. Conditioning of CarboFrit plugs prior to use was found to be very important [7,8]. A larger internal diameter (3.2 or 3.4 mm) liner packed with a CarboFrit plugs but with has also been successfully used for solvent vent injection of food samples in pesticide residue analysis [3,26]. Because liners packed with a CarboFrit plug need proper conditioning before use, twenty-five 10 l injections of standard solution in ethyl acetate (std LVI 0.20) were initially made in solvent vent mode to determine the number of injection necessary to achieve a stable responses for the pesticides. The results showed that the responses of most pesticides were stable after only 2 3 injections of standard solution in the absence of matrix. Initially, the responses of high boiling compounds were low. The response of deltamethrin stabilized after 6 7 injections, while for azinphos-methyl (and phosmet) injections were needed after which the responses continued to increase insignificantly. The recoveries of solvent vent injection (compared to cold splitless) were good, especially for high boiling compounds. A comparison of responses for solvent vent injection with those obtained for Siltek deactivated multibaffled liner showed significantly (10 30%) higher responses for Siltek deactivated liner with a CarboFrit plug. The baking out phase (350 C for 10 min) during solvent vent injection did not affect the responses of pesticides or degradation products. Higher temperatures were also tested for baking-out in the presence of matrix which resulted in diminishing response of labile pesticides. This effect was also noticed by Fankhauser-Noti et al. [27] and was ascribed to carbonization of matrix components causing adsorption and/or degradation of analytes. Liners packed with a CarboFrit plug overcome many of the limitations of other packing material such as degradation on active sites in case of glass wool and difficulties with desorption with other types of packing (Tenax) and at the same time provide all the advantages of packed liners The choice of the best liners for further testing The best performance was obtained for the Siltek deactivated multibaffled liner and the Siltek deactivated liner with a CarboFrit