Thermal degradation observed with different injection techniques: Quantitative estimation by the use of thermolabile carbamate pesticides

An experiment has been designed to study the thermal degradation of thermolabile compounds caused by various injection techniques. The four carbamate pesticides aminocarb, bendiocarb, carbaryl, and dioxacarb decompose thermally into methylisocyanate and the corresponding phenol. The carbamets and the phenols arising from them were separated on a 25 m SE-54 fused silica column; all compounds exhibited sharp peak shape indicating that the degradation observed took place completely within the injector. When cold on-column injection was employed no thermal degradation was observed whereas with hot splitless injection at 220°C decomposition of the carbamates was almost complete. PTV injection was found to produce intermediate results. When packed with glass wool and operated with glass wool and operated with starting temperatures lower than the boiling point of the solvent, decomposition was found to be almost complete. Applying isothermal conditions at 140°C (30°C above the boiling point of toluene) aminocarb and bendiocarb underwent only slight decomposition while carbaryl and dioxacarb were about half degraded. Results from PTV injection with an empty insert resembled those obtained using cold on-column injection and in this mode the application of temperatures up to 200°C resulted in no visible degradation. This can be explained by the short residence time of the sample in the injector.     

Mechanisms of direct and TiO2-photocatalysed UV degradation of phenylurea herbicides.

Phenylurea herbicides undergo low-yield (phi(PI) <15 %) monophotonic photoionisation upon 193-nm laser flash excitation. The so-formed radical cations (phenylurea.+) are highly acidic (-1.5 < pKa <0.5) and deprotonate readily to yield the corresponding neutral radical (phenylurea.). Pulse radiolysis experiments allowed limitation of the reduction potential of phenylurea.+ within 2.22 V versus the normal hydrogen electrode (NHE) < E degrees (phenylurea.+/phenylurea) < 2.43 V versus NHE. The main photoproducts of UVC (lambda=193 nm) photodegradation of phenylureas correspond to a photo-Fries rearrangement. One-electron reduction with e-(aq) yields the corresponding radical anions (phenylurea.-), for which 4.3< pKa < 5.33. The rate constants for reaction with e-(aq) show that in photocatalysis the generation of phenylurea.- and O2.- on the surface of the photocatalyst may be competitive. High reactivity toward e-(aq) is predicted from linear free-energy relationships (LFER) for phenylureas bearing electron-withdrawing groups. Reaction with HO. takes place mainly via addition to the aromatic ring and/or H. abstraction from a saturated carbon atom (98 %), rather than one-electron oxidation (2 %). High reactivity toward oxidation by HO. is predicted from LFER for phenylureas bearing electron-donating groups. Adsorption studies for TiO2 in its polymorphic forms of rutile and anatase, as well as with the commercial mixture Degussa P-25, show photocatalysis is independent of the specific area of the catalyst. A variety of compounds are generated during the photocatalytic degradation of Diuron, while only two hydroxychloro derivatives are observed upon prolonged direct 365 nm irradiation. The photocatalytic degradation proceeds mainly by oxidation of the Me group of the side chain, hydroxylation of the aromatic ring, and dechlorination. The photoproducts of photocatalytic degradation differ from one polymorphic form of TiO2 to another.     

Performance of programmed temperature vaporizer, pulsed splitless and on-column injection techniques in analysis of pesticide residues in plant matrices.

A programmed temperature vaporizer (PTV) injection technique has been recently implemented in our laboratory. In present paper its performance is compared with other GC injection techniques commonly used in trace analysis of organic contaminants. Twenty-six pesticides representing different chemical classes were selected for the study. This group comprised compounds typically subjected to discrimination in the injection port of the gas chromatograph, e.g., polar organophosphorus pesticides and thermolabile carbamates. In the first set of experiments standards in pure solvent were injected into GC systems employing different types of injection, i.e., (i) on-column, (ii) pulsed splitless, (iii) PTV solvent split, (iv) PTV splitless, and the responses of analytes were compared. Discrimination of troublesome compounds was significantly decreased with the application of PTV solvent split injection. In the second set of experiments repetitive injections of purified wheat samples were performed, with aims to evaluate the long-term stability of responses, as well as matrix effects in different stages of system contamination for each injection technique. The tolerance of the GC system to co-injected matrix components was increased in the order: on-column相似文献   

Liner as the Key of Injection Optimization in Pesticide Analysis

Regulations for pesticide residue analysis in food require very low detection limits; thus requiring maximum sensitivity in the gas chromatographic determination. This is accomplished by an overall method optimisation, which includes optimisation of injector parameters. Here we study the effect of the inlet liner design on the optimisation by comparing five liner designs in splitless and pulsed splitless injection modes, using a test mixture of fifteen pesticides analyzed by GC-ECD. Possible links between the injection parameters and liner types were evaluated, with the result that, accurate choice of inlet liner and injection parameters can reduce detection limits by up to 300%. Revised: 25 October 2005 and 9 January 2006     

Funktionell substituierte zinnorganische Verbindungen. III. (2-Aminoethyl)-triorganostannane

Functionally Substituted Organotin Compounds. III. (2-Aminoethyl)-triorganostannanes The preparation of (2-aminoethyl)-triorganostannanes 1 from 2-chloroethylamin and triorganotin halides is reported. Like primary amines the compounds 1 react with aldehydes, ketones, acyl chlorides or acetic anhydride, isocyanates, isothiocyanates, and hydrogen halides yielding the corresponding triorganostannylethyl substituted azomethines 2 , amides 3 , and 5 , ureas and thioureas 6 , and ammonium halides 7 . Compounds 7 undergo thermal induced fragmentation reaction studied by DTA/TG analysis.     

Possibilities and limitations of fast GC with narrow-bore columns

In this work, two narrow-bore capillary columns with different internal diameters (I.D.) 0.15 mm (15 m length, 0.15 microm film thickness) and 0.10 mm (10 m length, 0.10 microm film thickness) with the same stationary phase (5% diphenyl 95% dimethylsiloxane), phase ratio and separation power were compared with regard to their advantages, practical limitations and applicability in fast GC on commercially available instrumentation. The column comparison concerns fast GC method development, speed and separation efficiency, the sample transfer into the column utilizing split and splitless inlet, sample capacity, detection (analysing compounds of a wide range of polarities and volatilities--even n-alkanes C16-C28 and selected pesticides) and ruggedness (in the field of ultratrace analysis of pesticide residues in real matrix). Under conditions corresponding to speed/separation efficiency trade-off 0.10 mm I.D. versus 0.15 mm I.D. column provides a speed gain of 1.74, but all other parameters investigated were better for the 0.15 mm I.D. column concerning more efficient sample transfer from inlet to the column using splitless injection, no discrimination with split injection. Better sample capacity (three times higher for the 0.15 mm than for the 0.10 mm I.D. column) resulted in improved ruggedness and simpler fast GC-MS method development.     

Influence of the solvent on the gas chromatographic behaviour of urea herbicides

Summary Degradation products of chlorsulfuron, chlortoluron, diuron, fluometuron, isoproturon, linuron, metabenzthiazuron, metobromuron, and monuron formed in the gas chromatographic injector have been used for identification of the respective herbicides. Mass spectra of the derived compounds were obtained with a quadrupole mass spectrometric detector working in scan mode (20–450 amu). The compounds generated often depended on the solvent used for phenylurea herbicide injection (ethanol, methanol, dichloromethane, and acetonitrile). When methanol and ethanol were used as solvents the major products formed from phenylureas were carbamic acid esters. When acetonitrile or dichloromethane were used the main derivatives were phenylisocyanates. Chlorsulfuron and metabenzthiazuron, however, generated a triazine plus a phenylsulfonamide and a benzothiazolamine, respectively, irrespective of the solvent used. Linuron and diuron behaved similarly and gave degradation products with the same mass spectra. The thermal reactions occurred instantaneously in the injector block and were promoted by the high temperature selected (300°C). Detemination of the compounds derived from urea herbicides, by use of a 30 m BP10 column and a selected ion registering (SIR) program based on two or three ions, can be used for sensitive detection of the presence of urea herbicides in environmental extracts. With standards in methanol instrument detection limits ranged from 0.1 pg for chlorsulfuron (detected as 2-chlorobenzensulfonamide) to 1 pg for monuron and metobromuron (both detected as their carbamic acid methyl esters).RSD were below 9% at the 5 ng L⁻¹ level. The response was linearly dependent on quantily (r>0.9986) in the 5 ng L⁻¹ to 25 μg L⁻¹ range. Unequivocal identification of some phenylurea herbicides was not always possible because some herbicides with similar structures, for example diuron and linuron, gave the same derivative.     

Thermal degradation characteristics of rigid polyurethane foam and the volatile products analysis with TG-FTIR-MS

Thermal degradation characteristics of rigid polyurethane (PUR) foam in both air and nitrogen gaseous environments were studied using thermogravimetry and differential scanning calorimetry (TG-DSC) hyphenated techniques. And in situ Fourier Transform Infrared (FTIR) was employed to investigate the characteristic functional groups of the decomposition residues at different temperatures. It is found that the thermal degradation of PUR material in air and N₂ present a three-stage and a two-stage process, respectively. And the degradation reaction rate of PUR in air is accelerated significantly due to the presence of oxygen. The thermal degradation mechanism of PUR under non-oxidizing gaseous environment was evaluated using a TGA instrument coupled with Fourier Transform Infrared and mass spectrometer (TG-FTIR-MS). HCFC-141b served as blowing agent is detected at the initial stage. The urethane bond groups of PUR start to break up into isocyanates segments and polyols segments from about 200 °C. With an increase of temperature, the polyols decompose into some kinds of aliphatic ether alcohol. In the temperature range of 350–500 °C, the dominant volatile products are primary amines, secondary amines, vinyl ethers and CO₂.     

The effects of inlet liner configuration and septum purge flow rate on discrimination in splitless injection

An unmodified split/splitless inlet system using forward-pressure controlled pneumatics was operated in splitless injection mode with several inlet liners under a range of septum purge flow rates. The relative recovery (discrimination) of hydrocarbons ranging from n-C₈ to n-C₂₀ depended strongly upon the injected sample volume with open-ended liners at high septum purge flow rates of e.g. 50 mL/min. Little or no discrimination was observed at septum purge flows of 2–3 mL/min. The same inlet was also operated in a back-pressure regulated configuration that produced mass discrimination similar to that observed with the higher septum purge flows in the forward-pressure configuration. An inlet liner with a restricted inlet and outlet gave mass-discrimination levels independent of septum purge flow rate, but in the reverse sense of that observed with open-ended liners. Preferential volatile-component losses out of the inlet liner to the septum purge vent are principally responsible for the observed mass discrimination with openended liners, while mass-dependent losses with doubly-restricted liners seem due to slow sample evaporation.     

Analysis of headspace samples using off-line sorbent trapping coupled with automated thermal desorption and splitless injection onto a cryogenically cooled wide bore capillary column

Gas chromatographic equipment and procedures are described for automated splitless injection of pseudo-static headspace samples collected externally onto a sorbent trap. The GC microprocessor controls, in sequence, carrier gas backflushing of the sorbent trap for water removal, splitless thermal desorption into a cryogenically cooled wide bore (0.53 mm i. d.) capillary column and oven temperature programming. The method has been routinely applied for profiling the mid-to-high boiling compounds (bp 80–225°C) in the headspace of a variety of foods and beverages. Method criteria, advantages and limitations are discussed. FID and NPD chromatograms for brewed coffee and peanut butter volatiles are presented as typical examples.     

Using electronic pressure programming to reduce the decomposition of labile compounds during splitless injection

A split/splitless capillary injection port has been developed for electronic pressure programming (EPP) in gas chromatography. The inlet may be operated in several modes: constant pressure, constant flow, vacuum compensation (for gas chromatography–mass spectrometry (GC-MS)), pressure-programmed, or a combination mode enabling a pressure program to be followed by constant flow. A pressure-programming technique has been tried which uses high pressure (high column flow rate) at the time of injection followed by reduction in inlet pressure to a value required for normal chromatography. Sample is swept rapidly from the inlet and into the column, reducing contact with the hot, active inlet surfaces which cause sample decomposition. The decomposition of endrin and 4,4′-DDT, two labile pesticides, can be substantially reduced using this technique and modest improvements were also observed with the carbamate pesticide carbaryl.     

Mass spectrometric analysis of nitrogen- and phosphorus-containing pesticides by liquid chromatography-Mass Spectrometry

A series of nitrogen- and phosphorus-containing pesticides (amines, anilides, carbamates, phosphonates, phenylureas, sulfonylureas, and triazines) was examined by thermospray (TSP) ionization. A method is described that employs off-line and on-line solid-phase extraction and TSP liquid chromatography-mass spectrometry (LC-MS) with time-scheduled selected ion monitoring (SIM) for environmental monitoring of these pesticides in aqueous samples. SIM detection limits for the pesticides analyzed in conjunction with reversed-phase high-performance liquid chromatography range from 40 to 600 pg. In addition, methods for inducing fragmentation in thermospray LC-MS are presented. The structural information gained therefrom can be used to confirm a tentative identification. Therefore, fragmentation pathways under certain experimental conditions were investigated. Atmospheric pressure chemical ionization, electrospray, fast-atom bombardment, ²⁵²Cf plasma desorption, and collision-activated dissociation spectra are presented for several pesticides to confirm the proposed pathways and to gain additional and complementary information. Further confirmation may be achieved by postcolumn addition of different alkylated amines to the carrier stream in the TSP operation to induce postcolumn on-line derivatization (POD) reactions in the condensed phase of the vaporizer probe with selected pesticides. Additional clustering reactions in combination with solvent-mediated chemical ionization are observed by the POD technique. Both processes can be used to enhance the structural information from TSP spectra and thus the specificity of the method.     

Separation of phenylureas by capillary electrochromatography on porous graphitic carbon

Summary In this work the potential of porous graphitic carbon (PGC) as a new stationary phase in, capillary electrochromatography has been explored. Its behavior under the action of an applied potential is described for the separation of phenylureas. First, it was shown that porous graphitic carbon enables high efficiency in capillary electrochromatography over a wide range of mobile phase velocities. It was then demonstrated that this material might be responsible for degradation of the solutes at frit-PGC interfaces. Although electrolytic degradation reactions are suspected to occur on this type of conductive material, voltamperometric measurements furnished no clear evidence. A specific injection procedure is proposed for avoiding degradation of the solutes at the inlet interface before their chromatographic separation. Comparison of the retention behavior of phenylureas on PGC in liquid chromatography and in capillary electrochromatography show that the retention propertiets of PGC are altered by application of an electrical field, because this modifies the donor-acceptor interactions between the solutes and the stationary phase.     

High performance liquid chromatography with post-column photolysis of pesticides for generation of fluorophores

Summary Several classes of pesticides have been found to respond to a high performance liquid chromatography post-column reaction detector that employs UV photolysis with an optional reaction with o-phthalicdicarboxaldehyde-2-mercaptoethanol (OPA-MERC) reagent followed by fluorescence detection. Photolysis of most of the N-methylcarbamates, carbamoyl oximes, carbamothioic acids, dithiocarbamates, and phenylureas tested produced primary amines which reacted with OPA-MERC to form the respective derivatives. In some cases, substituted aromatic pesticides such as phenylcarbamates, phenylamides, and phenylureas photolyzed to chemical species which had native fluorescence. This technique was successfully applied to a method for pesticide analyses in groundwater and vegetation. For aldicarb sulfone, a pesticide that did not absorb UV light, use of acetone as a photosensitizer enhanced detection.     

Direct injection of large sample volumes into capillary columns with packed column injector

A direct injection method for large volume samples which avoids severe tailing of the solvent peak has been developed using a packed column injector (up to 100 μl) leading into an ordinary capillary column (0.3 mm i.d.). Modifications are made to the cooler zones of the inlet port and on the carrier gas flow control system. This injection technique is based on the effective use of phase soaking and cold trapping using a retention gap. The large volume of solvent vapor is rapidly purged out of the injector with a higher flow of carrier gas while the solutes trapped at the head of the column are subsequently analyzed with another optimum flow rate. The proposed carrier gas flow regulation system is also compared with conventional split/splitless injection methods.     

Optimization and evaluation of low-pressure gas chromatography-mass spectrometry for the fast analysis of multiple pesticide residues in a food commodity

A fast method of analysis for 20 representative pesticides was developed using low-pressure gas chromatography-mass spectrometry (LP-GC-MS). No special techniques for injection or detection with a common quadrupole GC-MS instrument were required to use this approach. The LP-GC-MS approach used an analytical column of 10 m x 0.53 mm I.D., 1 microm film thickness coupled with a 3 m x 0.15 mm I.D. restriction capillary at the inlet end. Thus, the conditions at the injector were similar to conventional GC methods, but sub-atmospheric pressure conditions occurred throughout the analytical column (MS provided the vacuum source). Optimal LP-GC-MS conditions were determined which achieved the fastest separation with the highest signal/noise ratio in MS detection (selected ion monitoring mode). Due to faster flow-rate, thicker film, and low pressure in the analytical column, this distinctive approach provided several benefits in the analysis of the representative pesticides versus a conventional GC-MS method, which included: (i) threefold gain in the speed of chromatographic analysis; (ii) substantially increased injection volume capacity in toluene; (iii) heightened peaks with 2 s peak widths for normal MS operation; (iv) reduced thermal degradation of thermally labile analytes, such as carbamates; and (v) due to larger sample loadability lower detection limits for compounds not limited by matrix interferences. The optimized LP-GC-MS conditions were evaluated in ruggedness testing experiments involving repetitive analyses of the 20 diverse pesticides fortified in a representative food extract (carrot), and the results were compared with the conventional GC-MS approach. The matrix interferences for the quantitation ions were worse for a few pesticides (acephate, methiocarb, dimethoate, and thiabendazole) in LP-GC-MS, but similar or better results were achieved for the 16 other analytes, and sample throughput was more than doubled with the approach.     

Rapid and sensitive determination of phenylurea herbicides in water in the presence of their anilines by extraction with a Carbopack cartridge followed by liquid chromatography

A rapid and sensitive method for determining phenylurea herbicides in environmental aqueous samples in the presence of their anilines is described. The water sample is preconcentrated by passage at a flow-rate of ca. 150 ml/min through a 250-mg graphitized carbon black (Carbopack B) cartridge. After washing with 0.6 ml of methanol, the Carbopack B trap is connected with a cartridge containing a strong cation exchanger. Organics trapped by the Carbopack cartridge are eluted by passage of 6 ml of methylene chloride-methanol (95:5, v/v). Anilines and other basic compounds are quantitatively subtracted from the solvent system while flowing through the cation-exchange cartridge. After evaporation and redissolution, the sample is subjected to reversed-phase gradient elution high-performance liquid chromatography with UV detection at 250 nm. Recoveries of phenylureas added to water at levels between 30 and 3000 ng/l were higher than 92%. The limit of detection was about 1 ng/l, for a 2-1 sample. With respect to an octadecyl (C18)-bonded silica cartridge, the Carbopack B cartridge had a far better extraction efficiency for polar phenylureas.     

Practical aspects of splitless injection of semivolatile compounds in fast gas chromatography

Possibilities and practical aspects of implementation of splitless injection of larger volumes for fast GC purposes utilizing narrow-bore column, hydrogen as carrier gas, fast temperature programming under programmed flow conditions and commercial instrumentation were searched. As a model sample semivolatile compounds of a broad range of volatility and polarity (7 n-alkanes and 19 pesticides) were chosen. Peak shapes, peak broadening and peak areas and its repeatability were evaluated under various experimental set-ups (liner/injection technique combinations). Various factors, such as liner design, injection technique, retention gap length, compound volatility and polarity, the solvent used, initial oven temperature influenced compound focusation and/or maximal injection volume. Combination of analytical column (CP-Sil 13 CB 25 m long, 0.15 mm i.d., film thickness 0.4 microm) with normal-bore retention gap (1 m long, 0.32 mm i.d.) allowed maximal injection volume 8 microl for 4 mm i.d. liner used without any peak distortion when solvent recondensation in the retention gap was employed.     

Critical aspects of the determination of pentafluorobenzyl derivatives of aldehydes by gas chromatography with electron-capture or mass spectrometric detection: Validation of an optimized strategy for the determination of oxygen-related odor-active aldehydes in wine

This work presents a thorough study of some aspects critical to the quantitative performance of methods for the determination of volatile aldehydes previously derivatized to pentafluorobenzyl hydroxylamine oximes. The conclusions of the study are further applied to the validation of an optimized procedure for the determination of oxidation-related aldehydes from wine. Aspects studied include the chromatographic injection, the analytical performance of electron-capture detection (ECD) or MS detection, and the way in which the derivatization is carried out. Different injection techniques have been optimized and compared (classical splitless-hot injection; cold splitless; and large volume solvent split injection). All of them were carried out in a programmed temperature vaporization (PTV) injector, with a 350 microL internal volume liner (3.4 mm internal diameter). Classical splitless injection of hexane extracts is troublesome and requires large carrier gas flows (>10 mL min(-1)). Cold splitless injection was clearly superior. Large volume solvent split injection has been also optimized. ECD has been found to lack the necessary selectivity for the determination of unsaturated aldehydes in wine, although the quantitation of several minor aldehydes is possible. MS detection has proven to be superior for the determination of these compounds in wine. The way in which the reagent is applied is also critical and for the case of wine is important to ensure that the reagent is applied after some of the major wine aldehydes have been eliminated. The finally proposed method is extremely sensitive. Method detection limits ranged from 0.002 microg L(-1) (for unsaturated aldehydes) to 0.73 microg L(-1) (for phenylacetaldehyde) and precision (measured as relative standard deviation) is < or =6% in all cases. The method makes it possible to determine quantitatively in a single run the wine aldehydes with sensory significance (isobutyraldehyde, 2-methylbutanal, isovaleraldehyde, (E)-2-hexenal, (E)-2-heptenal, (E)-2-octenal, (E)-2-nonenal, methional and phenylacetaldehyde).     

Analysis by desorption of volatile impurities from an ionic liquid solution in an unmodified gas chromatograph inlet

A procedure for using ionic liquids to determine volatile impurities in compounds or matrices that are soluble in an ionic liquid is described. Using a conventional autosampler a droplet of ionic liquid solution is suspended in the inlet of the gas chromatograph and analytes are desorbed onto a GC column using splitless injection conditions. Results are presented for 1,2-propanediol, nonane, N,N-dimethylacetamide, and mesitylene in two different compounds in the ionic liquids trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide and trihexyltetradecylphosphonium dicyanamide.