Linear Temperature Programmed Retention Indices Research Articles

Modeling the effect of temperature on solid-phase microextraction of volatile organic compounds from air by polydimethylsiloxane coating using finite element analysis

A development of analytical methods based on solid-phase microextraction (SPME) is a very time- and labor-consuming task. The finite element methods have found a wide application in SPME modeling for faster and more accurate optimization of analytical methods. In this work, a computational model for predicting the effect of temperature on extraction of VOCs from air onto SPME coating based on polydimethylsiloxane (PDMS) has been developed using COMSOL Multiphysics® (CMP) software. Most suitable methods and models for estimating the diffusion coefficients of analytes in air and coating, and coating-air distribution constants of the analytes at different extraction temperatures were chosen. The Fuller method was chosen for calculating diffusion coefficients of analytes in air due to its simplicity and reliability. Coating-air distribution constants at different temperatures were estimated using van't Hoff equation. A combination of inverse gas chromatography on a capillary column with a similar stationary phase for estimating diffusion coefficients and linear temperature programmed retention indices (LTPRI) for estimating coating-air distribution constants at initial temperature were chosen for modeling purposes because in most cases it provided lowest values of root-mean-square difference from experimental extraction profiles from 125 mL bulb at 25 and 40 °C. The developed model can be recommended for faster and simpler optimization of the methods of air sampling using PDMS SPME fiber. It can also be used for obtaining extraction profiles at fluctuating temperatures.

Valorization of coffee silverskin industrial waste by pyrolysis: From optimization of bio-oil production to chemical characterization by GC × GC/qMS

Coffee bean tegument (silverskin) is a by-product derived from the coffee roasting process. This residue has been discarded, which constitutes a serious environmental problem. The depletion of oil reserves and environmental issues promoted by the combustion of fossil fuels has generated interest in the use of biomass to obtain fuel and chemicals products. The liquid product obtained by biomass pyrolysis is commonly called bio-oil. Bio-oil is a complex mixture of compounds, and its detailed chemical characterization is necessary to prospect its potential uses. Therefore, this study optimized the final temperature of pyrolysis and N2 flow rate parameters for the pyrolysis of silverskin in a fixed bed reactor, by using a central composite design and response surface. The bio-oil chemical composition was evaluated by comprehensive two-dimensional gas chromatography coupled to rapid-scanning quadrupole mass spectrometry (GC × GC/qMS), combined with the use of standards and linear temperature-programmed retention indices (LTPRI). The optimal values ​​calculated were 560 °C for the final pyrolysis temperature and 49 mL min−1 for the N2 flow rate. The organic phase yield was 15.2% under these conditions. At the optimal conditions, 228 compounds were identified (90.1% of the sample chromatographic volume) in the organic phase. The major chemical class, in terms of volume percentage, were the phenols (26.70%), followed by nitrogen compounds (18.51%). In addition, it is worth mentioning the high representability of the saturated hydrocarbons (8.28%), unsaturated aliphatic (6.69%), and aromatics (7.77%), which together account for 22.74% of the sample chromatographic volume. These results showed that the silverskin bio-oil may have the potential use as a source of chemical inputs.

Gas chromatographic-mass spectrometric determination of glycosides without prior hydrolysis

The article presents for the first time the linear temperature programmed retention indices on a column with stationary phases of 5% phenylpolydimethyl silicone and the mass spectra of trimethylsilyl (TMS) derivatives of 71 glycosides (both commercial preparations and compounds extracted from plant tissues) which were not characterized earlier by these parameters. Converted to their TMS derivatives, the glycosides were thermally stable: they exhibited single peaks on their chromatograms without products of thermal decomposition. Therefore this work demonstrates the suitability of high resolution–high temperature gas chromatography (HR-HT/GC) to analyse different groups of glycosides including compounds with disaccharide moieties without the necessity of their hydrolyses. Since a limited number of commercial and plant-isolated glycosides were available, an attempt was made to assess their retention indices using the known “structure–retention relationships” approach. It was demonstrated that the retention indices of silanised glycosides and their aglycones were characterized by a linear dependence.

Separation of C5-C7 hydrocarbon components on Al2O3 capillary column and its application

The separation and qualitative analysis of 54 common C5-C7 hydrocarbon components on three Al2O3 capillary columns, including S type, KCl type and M type, were investigated in detail. The results showed that the separation of the 54 C5-C7 hydrocarbon components were partly different on these three Al2O3 capillary columns, and the most of C5-C7 hydrocarbon components could be well separated, except that some of them couldn't achieve a baseline separation or co-eluted. Linear temperature programmed retention indices of the 54 C5-C7 hydrocarbon components, including 15 C5 hydrocarbons, 25 C6 hydrocarbons and 14 C7 hydrocarbons, were determined on these three Al2O3 capillary columns. The determination of linear temperature programmed retention indices for these C5-C7 hydrocarbon components on Al2O3 capillary columns provided a basis for the qualitative analysis of them. At the same time, a real pyrolysis gas sample from one of the petrochemical plants was qualitatively analyzed according to the linear temperature programmed retention indices, and the contents of C5-C7 hydrocarbon components were determined. The application field of Al2O3 capillary column was expanded and it is helpful for the analysis of light hydrocarbons in petrochemical enterprises.

Gas Chromatographic and Mass Spectrometric Characterization of Trimethylsilyl Derivatives of Some Terpene Alcohol Phenylpropenoids

This paper reports mass spectra and linear temperature programmed retention indices (I(T)) for 31 esters of mono- and sesquiterpene alcohols with hydroxycinnamic acids. In this study, 14 phenylpropenoids were synthesized by esterification of terpenols with p-coumaric, ferulic, caffeic and sinapinic acids. Other phenylpropenoids were semi-quantitatively isolated by column chromatography from exudates covering the buds of two birch species (Betula pubescens and B. litwinowii). Main diagnostic ions in phenylpropenoids mass spectra were determined by GC/MS analysis. The possibility of predicting I(T) values was demonstrated with standard error of prediction between 3 and 11 of retention index units.

Detection of extraction artifacts in the analysis of honey volatiles using comprehensive two-dimensional gas chromatography

Extraction using headspace solid phase microextraction (HS-SPME) coupled to comprehensive two-dimensional gas chromatography with flame ionisation detection (GC×GC–FID) was employed to evaluate the effect of SPME fractionation conditions (heating time and temperature) on the generation of artifacts. The occurrence of artifacts was more pronounced at higher fractionation temperatures and times which caused significant changes in the chromatographic profiles. The identification of the volatile fraction of the honey blend was performed through a two-dimensional gas chromatograph coupled to a mass spectrometer with time of flight analyser (GC×GC–ToFMS) by comparing the first dimension linear temperature programmed retention index (1D-LTPRI) with the peak’s identities provided by the mass spectral similarity search. Several artifacts were found and identified – such as hydroxymethylfurfural, methyl–furone and furfural – and some of them were not previously detected as such in honey samples. These compounds were either the result of hydrolysis or thermal decomposition of components already present in the honey samples. This occurrence was attributed to the increased detectability provided by GC×GC compared to conventional GC. The possible emergence of previously unknown extraction artifacts as a general tendency related use of GC×GC instead of conventional GC is discussed as a result of these observations.

Botanical and Geographical Origin Characterization of Polish Honeys by Headspace SPME-GC× GC-TOFMS

Volatile organic compounds (VOCs) composition of Polish honeys obtained from various geographical regions was studied. Headspace solid phase microextraction (HS-SPME) using Divinylbenzene/Carboxen/Polydimethylsiloxane-coated fibers was used in combination with comprehensive two-dimensional gas chromatography–time–of–flight mass spectrometry (GC× GC–TOFMS) because of the complexity of the samples. Acacia, linden, rapeseed, buckwheat and honeydew honeys were studied in this project.A total of 329 compounds were identified during the investigations. Positive identification of 82 compounds was achieved using analytical standards, while tentative identification of 247 compounds was based on comparison of deconvoluted mass spectra and linear temperature programmed retention indices (LTPRI) with NIST 2005 library and literature values, respectively. Many VOCs identified in Polish honey samples were commonly present in honeys from other geographical regions of Europe (e.g. Italy, Corsica). They included certain acyclic and cyclic alkanes, acyclic and cyclic alkenes, aromatic hydrocarbons, oxygenated aromatic compounds, alcohols, aldehydes, ketones, esters, ethers, nitriles, organic sulphides, phenolic compounds and terpenes. GC× GC–TOFMS profiles for the same types of honey samples were remarkably similar. Eight botanic discriminants were selected among all compounds. Their identity was confirmed by comparison with authentic standards. Furthermore, possible geographical discriminants for Polish honeys were identified, including 19 for acacia, 3 for linden, 3 for rapeseed and buckwheat and 6 for honeydew honeys. Keywords: Discriminants, honey, HS–SPME/GC× GC–TOFMS, origin traceability, volatile organic compounds.

Analysis of fractions and bio-oil of sugar cane straw by one-dimensional and two-dimensional gas chromatography with quadrupole mass spectrometry (GC × GC/qMS)

This paper describes the use of GC×GC/qMS for the characterization of bio-oil from the pyrolysis of sugar cane straw and its fractions (from pressurized liquid fractionation by solvent elution). This is the first use of this technique for a bio-oil characterization. The GC×GC/qMS allied to some computational tools, allowed the identification and classification of the compounds according to many factors such as chemical class, number of carbons, number of substituents in the carbonic chain, molecular weight and main peaks in fragmentation spectra. It used also the LTPRI (linear-temperature-programmed retention index) system for the confirmation of the identification of some of the compounds in the bio-oil and fractions. It found a total number of 331 compounds tentatively identified in the crude bio-oil and 166 were confirmed by LTPRI identification. The fractionation step promoted a better selectivity of the compounds in the fractions, allowing the identification of a greater number of compounds.

Simultaneous sampling and analysis of indoor air infested with Cimex lectularius L. (Hemiptera: Cimicidae) by solid phase microextraction, thin film microextraction and needle trap device

Air in a room infested by Cimex lectularius L. (Hemiptera: Cimicidae) was sampled simultaneously by three different sampling devices including solid phase microextraction (SPME) fiber coatings, thin film microextraction (TFME) devices, and needle trap devices (NTDs) and then analyzed by gas chromatography–mass spectrometry (GC–MS). The main focus of this study was to fully characterize indoor air by identifying compounds extracted by three different microextraction formats and, therefore, perform both the device comparison and more complete characterization of C. lectularius pheromone. The NTD technique was capable of extracting both (E)-2-hexenal and (E)-2-octenal, which were previously identified as alarm pheromones of bedbugs, and superior NTD recoveries for these two components allowed reliable identification based on mass spectral library searching and linear temperature programmed retention index (LTPRI) technique. While the use of DVB/CAR/PDMS SPME fiber coatings provided complementary sample fingerprinting and profiling results, TFME sampling devices provided discriminative extraction coverage toward highly volatile analytes. In addition to two alarm pheromones, relative abundances of all other analytes were recorded for all three devices and aligned across all examined samples, namely, highly infested area, less infested area, and control samples which were characterized by different bedbug populations. The results presented in the current study illustrate comprehensive characterization of infested indoor air samples through the use of three different non-invasive SPME formats and identification of novel components comprising C. lectularius pheromone, therefore, promising future alternatives for use of potential synthetic pheromones for detection of infestations.

HS-SPME/GC–MS analysis of volatile and semi-volatile organic compounds emitted from municipal sewage sludge

The aim of the research involved identification and semi-quantitative determination of unknown volatile and semi-volatile organic compounds emitted to air by sewage sludge formed in the process of municipal wastewater treatment in a sewage treatment plant. Samples taken directly after completion of the technological process as well as the sludge stored on the premise of the sewage treatment plant were analyzed. A simple method using off-line headspace solid-phase microextraction combined with gas chromatography-mass spectrometry has been proposed for extraction and detection of organic pollutants. For reliable identification of compounds, combination of two independent parameters: mass spectra and linear temperature programmed retention indices were employed. Over 170 compounds of different structure were identified including aliphatic and aromatic hydrocarbons, alcohols, esters, carbonyls, as well as sulfur, nitrogen, and chlorine containing compounds. The prevailing substances included: ethyl ether, n-hexane, p-xylene, o-xylene, mesitylene, m-ethylbenzene, limonene, n-decane, n-undecane, and n-dodecane. A few compounds such as methanetiol, dimethyl polisulfide, octaatomic sulfur, phthalic anhydride, and indoles were identified in the sludge for the first time.

Polydimethylsiloxane-based permeation passive air sampler. Part I: Calibration constants and their relation to retention indices of the analytes

A simple and cost effective permeation passive sampler equipped with a polydimethylsiloxane (PDMS) membrane was designed for the determination of time-weighted average (TWA) concentrations of volatile organic compounds (VOCs) in air. Permeation passive samplers have significant advantages over diffusive passive samplers, including insensitivity to moisture and high face velocities of air across the surface of the sampler. Calibration constants of the sampler towards 41 analytes belonging to alkane, aromatic hydrocarbon, chlorinated hydrocarbon, ester and alcohol groups were determined. The calibration constants allowed for the determination of the permeability of PDMS towards the selected analytes. They ranged from 0.026cm2min−1 for 1,1-dichloroethylene to 0.605cm2min−1 for n-octanol. Further, the mechanism of analyte transport across PDMS membranes allowed for the calibration constants of the sampler to be estimated from the linear temperature programmed retention indices (LTPRI) of the analytes, determined using GC columns coated with pure PDMS stationary phases. Statistical analysis using Student's t test indicated that there was no significant difference at the 95% probability level between the experimentally obtained calibration constants and those estimated using LTPRI for most analyte groups studied. This correlation allows the estimation of the calibration constants of compounds not known to be present at the time of sampler deployment, which makes it possible to determine parameters like total petroleum hydrocarbons in the vapor phase.

Identification and profiling of volatile metabolites of the biocontrol fungus Trichoderma atroviride by HS-SPME-GC-MS

In the present study we describe a method, which is based on solid phase microextraction (SPME) coupled to gas chromatography–mass spectrometry (GC-MS) and which can be used for the profiling of microbial volatile organic compounds (MVOCs) in the headspace (HS) of cultures of filamentous fungi. The method comprises the following successive steps: 1. growth of the fungus on a solid culture medium directly in headspace vials, 2. measurement of volatiles by HS-SPME-GC-MS, 3. deconvolution of mass spectra, 4. identification of volatiles by comparison of measured, deconvoluted mass spectra and linear temperature programmed retention indices (LTPRI) on two stationary GC phases with database entries and LTPRI published in the literature, and 5. profiling of the identified MVOCs. The developed method was successfully applied to cultures of the biocontrol fungus Trichoderma atroviride. An in-house library consisting of mass spectra and LTPRI values of fungal VOCs was established and used to study the profiles of MVOCs of this fungus. In total, 25 different MVOCs were identified by applying strict criteria (spectral match factor at least 90% and a maximum relative deviation of LTPRI of ± 2% from literature values). The MVOCs were assigned to the compound classes of alcohols, ketones, alkanes, furanes, pyrones (mainly the bioactive 6-pentyl-alpha-pyrone), mono- and sesquiterpenes, 13 of which have never been reported to be produced by Trichoderma spp. before. Eleven of these volatiles have been additionally confirmed using authentic standards. Finally, time course experiments and cultivation of T. atroviride in the presence of the mycotoxin fusaric acid demonstrated the potential of the method to study the dynamics of MVOC profiles as well as the effect of different environmental/biological conditions on the expression of MVOCs of filamentous fungi.

Gas chromatographic retention indices of biologically and environmentally important organic compounds on capillary columns with low-polar stationary phases

This work presents linear temperature programmed retention indices on the columns with stationary phases of 5% phenylpolydimethyl silicone of 389 organic compounds, including extractive substances of plant tissues and environmentally important compounds. Certain factors which influence the values and reproducibility of retention indices during gas chromatographic analysis of multicomponent mixtures are discussed.

Gas chromatographic investigation of phenylpropenoid glycerides from aspen ( Populus tremula L.) buds

20 phenylpropenoid glycerides were synthesized by esterification of glycerol by p-coumaric, ferulic and caffeic acids. Main diagnostic ions in mass spectra and linear temperature programmed retention indices (LTPRI) of trimethylsilyl derivatives of these compounds were determined by GC/MS analysis. On the basis of these analytical parameters 39 in various degree substituted phenylpropenoid glycerides were for the first time identified in diethyl ether and ethyl acetate extracts from aspen buds.

Application of Pentafluorobenzyl and Hexafluoroisopropyl Esters for Retention Indexes in GC-Negative Ion Chemical Ionization MS

In order to use the linear temperature programmed retention index (LTRI) for the negative ion chemical ionization (NCI) mode in which n-alkanes cannot be detected, LTRIs were newly defined using homologous series of pentafluorobenzyl and hexafluoroisopropyl esters. The method for converting LTRI based on n-alkanes to the new LTRIs was also developed and applied to PCB and chlorinated pesticide analysis. The results demonstrated that the LTRI databases, which were developed based on n-alkanes, could be utilized for NCI analysis.

Rapid headspace solid-phase microextraction–gas chromatographic–time-of-flight mass spectrometric method for qualitative profiling of ice wine volatile fraction: I. Method development and optimization

An analytical method for the determination of volatile and semi-volatile compounds representing various chemical groups in ice wines was developed and optimized in the presented study. A combination of the fully automated solid-phase microextraction (SPME) sample preparation technique and gas chromatographic–mass spectrometric (GC–MS) system to perform the final chromatographic separation and identification of the analytes of interest was utilized. A time-of-flight mass spectrometric (TOF-MS) analyzer provided very rapid analysis of this relatively complex matrix. Full spectral information in the range of m/ z 35–450 was collected across the short GC run (less than 5 min). Divinylbenzene/Carboxen/Polydimethylsiloxane (DVB/CAR/PDMS) 50/30 μm fiber performed best during the optimization experiments and it was used in the headspace SPME mode to isolate compounds from ice wine samples, consisting of 3 mL wine with 1 g salt addition. After the sample incubation and extraction (both 5 min at 45 °C), analytes were thermally desorbed in the GC injector for 2 min (injector maintained at 260 °C) and transferred into the column. The MS data acquisition rate of 50 spectra/s was selected as optimal. The optimized analytical method did not exceed 20 min per sample, including both the isolation and pre-concentration of the analytes of interest, the final GC–TOF-MS analysis and the fiber bake-out. Both a linear temperature-programmed retention index (LTPRI) method using C 8–C 20 alkanes loaded onto the fiber and a mass spectral library search were employed to identify the target compounds. The repeatability of the developed and optimized HS-SPME-GC–TOF-MS method for ice wine analysis, expressed as relative standard deviation (RSD, %, n = 7), ranged from 3.2 to 9.0%.

Rapid headspace solid-phase microextraction-gas chromatographic–time-of-flight mass spectrometric method for qualitative profiling of ice wine volatile fraction: II: Classification of Canadian and Czech ice wines using statistical evaluation of the data

The previously developed and optimized headspace solid-phase microextraction (HS-SPME)-GC–time-of-flight (TOF) MS analytical method for the determination of compounds with a wide range of polarities and volatilities was successfully used in this study to characterize and classify a large set of ice wines according to their origin, grape variety and oak or stainless steel fermentation/ageing conditions, based on a statistical evaluation (principal component analysis (PCA)) of the measured data. More than 130 ice wine samples collected directly from Canadian and Czech wine producers were analyzed in this study. The SPME step was beneficially carried out utilizing the new-generation super elastic divinylbenzene/Carboxen/polydimethylsiloxane (DVB/CAR/PDMS) 50 μm/30 μm fiber assembly. One fiber was used for the whole sequence of ice wine samples, control and blank experiments, which consisted of more than 600 individual extraction/injection cycles. Utilizing the high-speed TOF analyzer, full spectral information within the range of 35–450 u was collected for the entire GC run (as short as 4.5 min) without compromising in the detection sensitivity, as compared to other scanning mass analyzers operated in selected ion monitoring or MS n mode to achieve similar sensitivity. The identification of analytes was performed by a combination of the linear temperature-programmed retention index (LTPRI) approach with the comparison of the obtained spectra with three libraries included in the ChromaTOF software. A total of 201 peaks were tentatively assigned as ice wine aroma components and 58 of those compounds were evaluated in all of the examined samples.

Application of the chromatographic retention index system for the estimation of the calibration constants of permeation passive samplers with polydimethylsiloxane membranes

The paper presents the results of research on the calibration of permeation passive samplers equipped with polydimethylsiloxane (PDMS) membranes using the physico-chemical properties of the analytes. Strong correlations were found between the calibration constants of the samplers and the linear temperature-programmed retention indices of the analytes determined on columns coated with pure PDMS ( r 2 = 0.914). These correlations make it possible to estimate the calibration constants for unidentified analytes, which is impossible when using conventional procedures. This, in turn, enables the deployment of permeation passive samplers in the same way in which active samplers are deployed. The reproducibility of the calibration constants determined in different laboratories and retention indices determined using different chromatographic systems was very good, indicating that the calibration constants estimated using this approach should be reproducible as well. The approach proposed should lead to more widespread use of permeation passive samplers.

Distribution coefficients of aliphatic alcohols, carbonyl compounds and esters between air and Carboxen/polydimethylsiloxane fiber coating

Lower aliphatic carbonyl compounds, esters and alcohols were found in open atmosphere, as well as in volatiles of microorganisms, living plants and leaf litter. Quantitative determination of their emissions into the gas phase with the aid of solid phase microextraction (SPME) requires the knowledge of their distribution coefficients K fg between fiber coating and gas phase. This paper reports the results of experimental determination of K fg values for 17 volatiles belonging to the above-mentioned classes of organic compounds. Mixed Carboxen/polydimethylsiloxane stationary phase, which proved to be effective during SPME determination of more volatile pollutants, was chosen as the fiber coating. Values K fg were determined at temperature range 15–30 °C, which permitted the dependence between log K fg and T −1 to be determined. Distribution coefficients for 23 other compounds from the same homologue series were calculated using two-parameter empirical equation, where physico-chemical parameters of compounds (boiling temperature, linear temperature programmed retention index) served as descriptors.

New Developments and Applications of Solvent-Free Sampling and Sample Preparation Technologies for the Investigation of Living Systems

In recent years there has been considerable interest in developing techniques to monitor levels of biologically active compounds in living systems in natural environments. These efforts represent a significant departure from conventional ‘sampling’ techniques, where a portion of the system under study is removed from its natural environment, and the compounds of interest extracted and analyzed in a laboratory environment. An in vivo sampling approach can eliminate errors and reduce the time associated with sample transport and storage, and can therefore result in the collection of more accurate, precise, and faster analytical data. An ideal in vivo sampling technique should be portable, solvent-free, and offer integration of the sampling, sample preparation and sample analysis steps. These requirements are met by two techniques based on coated fibre and membrane technologies, presently under development in our laboratory.[1] Fibre solid-phase microextraction (fibreSPME) involves exposing a polymer-coated fused silica fibre to a sample. The analytes partition into the fibre coating until an equilibrium is reached. The fibre is then removed from the solution and the analytes are desorbed in the injector or injection loop of an analytical instrument such as a gas chromatograph (GC). The fibre is contained in a syringe-like device to facilitate handling.[2] Fibre-SPME can be used for both spot and time-averaged sampling. For spot sampling, the fibre is typically exposed to a sample matrix until the partitioning equilibrium between sample matrix and the coating material is reached. In the time-averaged technique the fibre remains in the needle during exposure of the SPME device to the sample. The fibre coating works as a trap for analytes that diffuse into the needle. In membrane extraction with a sorbent interface (MESI) a polymeric hollow fibre or a flat sheet membrane, in contact with a sample, is fitted directly into the carrier gas line of a GC equipped with a sorbent trap.[3]Analytes partition into the polymeric phase of the membrane and, after diffusion through the membrane, are carried by the gas to the sorbent trap. The concentrated analytes are periodically delivered onto the front of the column by a thermal pulse. MESI is a dynamic system, where the rate of analyte intake is dependent on both the diffusion coefficients of analytes in the membrane material and the membrane/sample matrix distribution constant. Similar to fibre-SPME, MESI can be used for both spot and time averaged monitoring. Both fibre-SPME and MESI techniques integrate sampling, sample preparation, and sample introduction to the analytical instrument, into a simple procedure. In fibreSPME mechanical movement of the fibre is necessary, as the sampling and sample introduction steps are separated in space, allowing one instrument to analyse large numbers of fibres. MESI, on the other hand, requires a dedicated, permanently attached instrument to one or several membrane/sorbent systems, eliminating the need for mechanical movement and therefore reducing the possibility of failure. MESI is thus suitable for continuous operation, allowing conversion of the analytical separation and detection instrument into a sensor-like device suitable for monitoring operations. Calibration procedures can be made very simple in both methods. For example, in air monitoring, the air/coating distribution constant can be estimated using the linear temperature-programmed retention index system (LTPRI). This allows quantification, even without identification, as long as the stationary phase used in the analytical column is identical to the fibre coating. The diffusion coefficient can be calculated by knowing the molecular weight of the target compound. To facilitate analysis of very polar analytes, a derivatization procedure can be used. For example, the validation field measurement of formaldehyde in ambient air, using both spot and time average sampling, was conducted using several techniques. Similar results were obtained for this challenging analyte using fibre-SPME and other more established procedures.[4]