Pesticide residue analysis in food with CGC — study of long‐term stability by the use of different injection techniques

Abstract

SummaryAn experiment was designed to study the long-term stability of analyses of pesticide residues in a complex food matrix using three different injection techniques. A spinach sample was spiked with five sulfur-containing pesticides of varying volatility and thermolability: arinphos-ethyl, dimethoate, methiocarb, ethiofencarb, and tri-allate. Tri-allate was selected as internal standard because of its thermal stability and its good chromatographic properties. Flame photometric detection resulted in chromatograms not subject to interference by peaks of compounds of the matrix. Starting with a clean and freshly, silanized injector an additional deactivation was found to occur on the first injection of a food sample resulting in an increased peak area for all pesticides in relation to tri-allate. Highest long-term stability was found with PTV injection. Best results for thermolabile carbamates were obtained using on-column injection. However, they were prone to faster deterioration. I IntroductionDue to its higher separation efficiency [I, 21 gas chromatography on capillary columns is far better suited for the detection of pesticide residues than techniques using packed columns. Ouantitation by capillary gas chromatography is affected by many factors [3,4]. When used for the analysis of real-life samples, cold on-column injection exhibits a number of disadvantages. Automated sampling is more complex. Even more important is the poor tolerance of deposits from matrix compounds. Therefore, multiresidue analysis is widely performed by hot splitless injection because of the ease of exchanging the injector inlet which retains the nonvolatile deposits from the food matrix [5, 61. This paper presents a systematic investigation of the long-term stability of gas chromatographic responses to pesticides in a food sample using three sampling techniques. Experimental InstrumentationA Sichromat 1 gas chromatograph (Siemens AG, Karlsruhe, FRG) was equipped with splitless injector and a flame photometric detector in the sulfur-mode. Additionally a programmed temperature vaporizer (Gerstel, Muhlheim, FRG), an on-column injector (Hewlett Packard, Palo Alto, CA, USA), and an autosampler (HP 7673) were also fitted. Gas ChromatographyA fused silica column 25 m x 0.32 mm i.d. SE 54 "bonded phase", df = 0.25 km (Nordion Analytica Espoo, Finland) was used with hydrogen as carrier gas adjusted to a flow rate of 60 cm/s. Temperature program: 1 min at 100 "C -10 "/min to 200 "C -2 min -stop. Injection TechniquesAll injections were performed by means of an autosampler using 10 p1 fixed needle syringes (Hamilton 80377) for hot-splitless and PTV injection and for on-column injection (Hamilton 80378). The injection volume in all experiments was 1 pl. The common hot-splitless injector was equipped with a glass insert of 260 p1 volume and filled with 10 mg of silanized glass wool. The filled glass insert was deactivated in a solution of 10 % dichloromethylsilane in toluene overnight. Immediately before mounting, the insert was rinsed with a s...

Optimization and application of the PTV injector for the analysis of pesticide residues

“…Nevertheless, repeated injections of "dirty" samples result in quick deterioration of injector performance [3] due to contamination of the front part of column with non-volatile deposits.…”

“…With on-column injection, more accurate results for troublesome compounds may be expected compared to splitless injection as long as a clean GC system is used. Nevertheless, repeated injections of "dirty" samples result in quick deterioration of injector performance [3] due to contamination of the front part of column with non-volatile deposits.…”

Trace level analysis of micropollutants in aqueous samples using gas chromatography with on-line sample enrichment and large volume injection

“…Moreover, thermal degradation is minimized, because the residence time of the components at elevated temperatures is reduced compared with conventional split and sp!itless injection. The quantitative performance of FIN injection systems appears to be comparable to that of on-column injection while at the same time column contamination due to the presence of residue components in the sample, which is frequently observed in on-column sample introduction, is absent [35]. Despite the good results obtained by Vogt using the PTV injector in early large volume sampling experiments, there was hardly any interest in the PTV injector for large volume sampling in the decade following that publication.…”

Pulsed Splitless Injection and the Extent of Matrix Effects in the Analysis of Pesticides

“…On-column and PTV injection represent other sample introduction alternatives which may be assumed to reduce and/or eliminate matrix effects. Direct injection of sample onto the analytical column in the former and slow evaporation of sample in the latter technique are the inherent features offering potential for elimination of discrimination, degradation, and other negative effects [21][22][23]. In spite of these assumptions, Mol et al have reported matrix effects occurring in some extent during analyses of nitrogen and phosphorus containing pesticides by PTV-GC [24].…”