Development and evaluation of needle trap device geometry and packing methods for automated and manual analysis

For air/headspace analysis, needle trap devices (NTDs) are applicable for sampling a wide range of volatiles such as benzene, alkanes, and semi-volatile particulate bound compounds such as pyrene. This paper describes a new NTD that is simpler to produce and improves performance relative to previous NTD designs. A NTD utilizing a side-hole needle used a modified tip, which removed the need to use epoxy glue to hold sorbent particles inside the NTD. This design also improved the seal between the NTD and narrow neck liner of the GC injector; therefore, improving the desorption efficiency. A new packing method has been developed and evaluated using solvent to pack the device, and is compared to NTDs prepared using the previous vacuum aspiration method. The slurry packing method reduced preparation time and improved reproducibility between NTDs. To evaluate the NTDs, automated headspace extraction was completed using benzene, toluene, ethylbenzene, p-xylene (BTEX), anthracene, and pyrene (PAH). NTD geometries evaluated include: blunt tip with side-hole needle, tapered tip with side-hole needle, slider tip with side-hole, dome tapered tip with side-hole and blunt with no side-hole needle (expanded desorptive flow). Results demonstrate that the tapered and slider tip NTDs performed with improved desorption efficiency.     

Needle microextraction trap for on-site analysis of airborne volatile compounds at ultra-trace levels in gaseous samples

Different capillary needle trap (NT) configurations are studied and compared to evaluate the suitability of this methodology for screening in the analysis of volatile organic compounds (VOCs) in air samples at ultra-trace levels. Totally, 22 gauge needles with side holes give the best performance and results, resulting in good sampling flow reproducibility as well as fast and complete NT conditioning and cleaning. Two different types of sorbent are evaluated: a graphitized carbon (Carbopack X) and a polymeric sorbent (Tenax TA). Optimized experimental conditions were desorption in the GC injector at 300°C, no make-up gas to help the transport of the desorbed compounds to the GC column, 1 min splitless time for injection/desorption, and leaving the NT in the hot injector for about 20 min. Cross-contamination is avoided when samples containing high VOC levels (above likely breakthrough values) are evaluated. Neither carryover nor contamination is detected for storage times up to 48 h at 4°C. The method developed is applied for the analysis of indoor air, outdoor air and breath samples. The results obtained are equivalent to those obtained with other thermal desorption devices but have the advantage of using small sample volumes, being simpler, more economical and more robust than conventional methodologies used for VOC analysis in air samples.     

Novel sample preparation technique with needle-type micro-extraction device for volatile organic compounds in indoor air samples

A novel needle-type sample preparation device was developed for the effective preconcentration of volatile organic compounds (VOCs) in indoor air before gas chromatography–mass spectrometry (GC–MS) analysis. To develop a device for extracting a wide range of VOCs typically found in indoor air, several types of particulate sorbents were tested as the extraction medium in the needle-type extraction device. To determine the content of these VOCs, air samples were collected for 30 min with the packed sorbent(s) in the extraction needle, and the extracted VOCs were thermally desorbed in a GC injection port by the direct insertion of the needle. A double-bed sorbent consisting of a needle packed with divinylbenzene and activated carbon particles exhibited excellent extraction and desorption performance and adequate extraction capacity for all the investigated VOCs. The results also clearly demonstrated that the proposed sample preparation method is a more rapid, simpler extraction/desorption technique than traditional sample preparation methods.     

Devising an adjustable splitter for dual-column gas chromatography

A flow controlled adjustable splitter was configured from a Deans switch and employed in an automated dual column gas chromatographic (GC) system for analyzing mono-aromatic compounds. Volatile organic compounds (VOCs), thermally desorbed from the sorbent trap, were split by the adjustable splitter onto two columns of different phases for separation and then detection by flame ionization detection (FID). Unlike regular splitters in which the split ratio is passively determined by the diameter and/or length of the connecting columns or tubing, the split ratio in our adjustable splitter is controlled by the auxiliary flow in the Deans switch. The auxiliary flow serves as a gas plug on either side of the column for decreasing the sample flow in one transfer line, but increasing the flow in the other. By adjusting the auxiliary flow and therefore the size of the gas plug, the split ratio can be easily varied and favorable to the side of no auxiliary gas. As an illustration, two columns, DB-1 and Cyclodex-B, were employed in this study for separating benzene, toluene, ethylbenzene, xylenes, denoted as BTEX, in particular the structural isomers of o-, m-, p-xylenes. This configuration demonstrates that BTEX cannot be fully separated with either column, but can be deconvoluted by simple algebra if dual columns are used with a splitter. The applicability of the proposed concept was tested by analyzing a gas standard containing BTEX at different split ratios and with various sample sizes, all leading to a constant ratio of m-xylene versus p-xylene.     

Construction of an automated gas chromatography/mass spectrometry system for the analysis of ambient volatile organic compounds with on-line internal standard calibration

An automated sampling and enrichment apparatus coupled with a gas chromatography/mass spectrometry (GC/MS) technique was constructed for the analysis of ambient volatile organic compounds (VOCs). A sorbent trap was built within the system to perform on-line enrichment and thermal desorption of VOCs onto GC/MS. In order to improve analytical precision, calibration accuracy, and to safe-guard the long-term stability of this system, a mechanism to allow on-line internal standard (I.S.) addition to the air sample stream was configured within the sampling and enrichment apparatus. A sub-ppm (v/v) level standard gas mixture containing 1,4-fluorobenzene, chloropentafluorobenzene, 1-bromo-4-fluorobenzene was prepared from their pure forms. A minute amount of this I.S. gas was volumetrically mixed into the sample stream at the time of on-line enrichment of the air sample to compensate for measurement uncertainties. To assess the performance of this VOC GC/MS system, a gas mixture containing numerous VOCs at sub-ppb (v/v) level served as the ambient air sample. Various internal standard methods based on total ion count (TIC) and selective ion monitoring (SIM) modes were attempted to assess the improvement in analytical precision and accuracy. Precision was improved from 7-8% RSD without I.S. to 2-3% with I.S. for the 14 target VOCs. Uncertainties in the calibration curves were also improved with the adoption of I.S. by reducing the relative standard deviation of the slope (Sm%) by an average a factor of 4, and intercept (Sb%) by a factor of 2 for the 14 target VOCs.     

Large Volume Headspace Analysis Using Solid-Phase Microcolumn Extraction

A rapid and simple large volume headspace (HS) sampling technique termed headspace solid-phase microcolumn extraction (HS-SPMCE) is described. HS gas above a liquid or solid sample is aspirated by attaching a gas-tight syringe onto a glass thermal desorption tube filled with Tenax sorbent. The trapped analytes are recovered by thermal desorption for gas chromatography–mass spectrometry (GC–MS) analysis. Benzene, toluene, ethylbenzene and the xylene isomers (BTEX) are used as model compounds to demonstrate the application of the extraction procedure for water samples. The results of the tests of the effect of agitation time and aspiration rate on recovery of the analytes show a good robustness of the method. BTEX are determined in the linear range from 0.5 to 50.0 μg L^?1 with limits of detection (3 σ) ranging within 0.09–0.14 μg L^?1 (MS was in scan mode). The method provides a good repeatability (RSD < 9%) and only a negligible carryover effect was observed ( ≤0.05%) when analysing BTEX at concentration 50.0 μg L^?1.     

Electrochemical immunosensor for prostate-specific antigen using a glassy carbon electrode modified with a nanocomposite containing gold nanoparticles supported with starch-functionalized multi-walled carbon nanotubes

We report on a simple, rapid, and efficient method for the extraction of volatile organic compounds (VOCs; including methanol, tetrahydrofuran, 2-hexanone and benzene) from air and solid samples. The system is based on the use of a laboratory-made syringe as the extractor. The needle of the syringe is placed in a chamber cooled by liquid nitrogen. The tip of the needle is placed in the headspace of a vial containing the sample. The headspace components then are circulated with a pump to pass the needle, and this results in freeze-trapping of the VOCs on the inner surface of the needle. The circulation of the headspace components is continued for 15 min, and the syringe is then removed and placed in a GC injector. The effects of volume of the sample vial, headspace flow rate, temperature and time of extraction and desorption were optimized. The overall time for sampling and analysis is <30 min. The method displays an extraction efficiency of >80%) and a good sample transfer efficiency into the GC column due to the absence of a sorbent inside the needle. No carry-over was observed after 30?s desorption at 260?°C. An external standard method was used for quantitative analysis. The relative standard deviation values are below 10% and the limits of detection range from 1.3 to 4.6?ng?g^?1.

Sampling and determination of volatile organic compounds with needle trap devices

In this study, a sorbent was immobilized inside a needle resulting in the development of a needle trap (NT) device. This device was applied to extract organic components from gaseous samples and to introduce an enriched mixture into a conventional gas chromatography (GC) injector. Construction of this simple and integrated sampling/extraction/sample introduction device was optimized by considering different ways to immobilize a sorbent in the needle, packing single and multiple-layer sorbent beds, and applying different desorption strategies into the GC injector. A carrier gas system was modified to minimize the carryover for the needle trap with a sealed tip (NT-1), and a narrow-neckliner was used for the blunt-tip needle trap (NT-2). Breakthrough in the device was investigated by connecting two NT-2 devices in series. The needle trap performed very well as an exhaustive spot sampler, as well as in a time-weighted average (TWA) operation. The linear velocity of the mobile phase has no influence on the sampling rate of the needle trap. Validation results against the standard NIOSH 1501 method using charcoal tubes for indoor air surveys demonstrated good accuracy for the NT approach. The reproducibility of the NT-2 was about 1% for benzene. The detection limits for FID detection and for 25 ml gas sample were 0.23 ng/l, 2.10 ng/l and 1.12 ng/l for benzene, ethylbenzene and o-xylene, respectively.     

Analysis of benzene, toluene, ethylbenzene, xylenes and n-aldehydes in melted snow water via solid-phase dynamic extraction combined with gas chromatography/mass spectrometry

The present study describes a method based on headspace-solid-phase dynamic extraction (HS-SPDE) followed by GC/MS for the qualitative and quantitative analysis of benzene, toluene, ethylbenzene, o-, m- and p-xylene (BTEX), and n-aldehydes (C(6)-C(10)) in water. To enhance the extraction capability of the HS-SPDE a new cooling device was tested that controls the temperature of the SPDE needle during extraction. Extraction and desorption parameters such as the number of extraction cycles, extraction temperature, desorption volume and desorption flow rate have been optimized. Detection limits for BTEX ranged from 19 ng/L (benzene) to 30 ng/L (m/p-xylene), while those for n-aldehydes ranged from 21 ng/L (n-heptanal) to 63 ng/L (n-hexanal). At a concentration level of 2 microg/L, the relative standard deviations (RSDs) for BTEX ranged from 3.9% (benzene) to 15.3% (ethylbenzene), while RSDs for n-aldehydes were between 6.1% (n-octanal) and 16.5% (n-hexanal) (n=7). Best results were obtained when the analyzed water samples were heated to 50 degrees C. At a water temperature of 70 degrees C GC responses decreased for all analyzed compounds. At a defined water temperature, a significant improvement of the GC response was achieved by cooling of the SPDE fiber during water extraction in comparison to an extraction keeping the fiber at room temperature. Evaluating the extraction cycles, for BTEX, the sensitivity was almost similar using 20, 40 and 60 extraction cycles. In contrast, the highest GC responses for n-aldehydes were achieved by the use of 60 extraction cycles. Optimizing the desorption parameters, best results were achieved using the smallest technical available desorption volume of 500 microL and the highest technical desorption flow rate of 50 microL/s. The method was applied to the analysis of melted snow samples taken from the Jungfraujoch, Switzerland (3580 m asl), revealing the presence of BTEX and aldehydes in snow.     

Combining membrane extraction with mobile gas chromatography for the field analysis of volatile organic compounds in contaminated waters

A mobile gas chromatographic device (Airmobtx HC 1000 monitor manufactured by Airmotec, Germany), originally designed for the analysis of benzene, toluene, ethylbenzene and xylenes (BTEX) in air, was connected to a flow cell for dynamic membrane extraction. Volatile organic compounds (VOCs) diffuse out of a water stream through a hollow fibre, are enriched onto sorption tubes integrated in the mobile device, and are then thermally desorbed and analysed by gas chromatography-flame ionisation detection. Battery operation of the device enables continuous on-site analysis of VOCs. Influences of the water flow-rate on system response and memory effects were investigated. The linear range of the method depends on the flow-rate of the water sample and did not exceed two orders of magnitude. The detection limits for trichloroethene, chlorobenzene and the BTEX compounds were found to be between 0.1 and 1.0 microg/l using a water flow-rate of 30 ml/min. Dynamic membrane extraction combined with the mobile gas chromatographic device was used for the on-site analysis of contaminated waters in the area of Leipzig.     

Perdeuterated n-alkanes for improved data processing in thermal desorption gas chromatography/mass spectrometry I. Retention indices for identification

The identification of organic compounds by GC/MS is useful in various areas such as fuel, indoor and outdoor air and flavour and fragrance applications. Multi-compound mixtures often contain isomeric compounds which have similar mass spectra and sometimes cannot be unambiguously identified by library search alone. Retention indices can help with confirmation of identification if they are reproducible. Using perdeuterated n-alkanes as a reference series for calculation of retention indices in GC/MS has a clear benefit because of the distinctive ion trace of m/z 34. Thermal desorption is useful for analysis of volatile organic compounds (VOCs) in air after sampling on appropriate sorbent cartridges. Comparison of indices between three systems, consisting of a thermal desorption unit, a gas chromatograph and a mass spectrometer, showed good agreement for compounds with well-defined peaks, whereas retention times varied.     

热脱附-气相色谱-三重四极杆串联质谱法测定环境空气中的挥发性有机物

采用热脱附(TD)结合气相色谱-三重四极杆串联质谱(GC-MS/MS)建立了环境空气中23种挥发性有机物(VOCs)同时检测的分析方法。空气样品通过主动采样的方式富集到装有Tenax-TA填料的热脱附管中,热解吸后在选择反应监测(SRM)模式下用GC-MS/MS进行检测,内标法定量。结果表明,23种VOCs在0.01~1 ng和1~100 ng低、高两个范围内线性关系良好,相关系数(r²)均大于0.99,方法定量限为0.00008~1 μ g/m³。加标水平为2、10和50 ng时,23种VOCs的平均回收率为77%~124%。除了最低加标水平的氯苯,相对标准偏差(RSD, n=6)均小于20%。对市内3个采样点的环境空气进行测定,其中苯、甲苯、乙苯、二甲苯、苯乙烯、1,2,4-三甲基苯和六氯丁二烯均有检出。实验证明,该TD和GC-MS/MS相结合的检测方法具有准确、可靠、灵敏度高等优点,适用于环境空气中VOCs的同时测定。     

Needle trap micro-extraction for VOC analysis: Effects of packing materials and desorption parameters

Combining advantages of SPE and SPME needle trap devices (NTD) represent promising new tools for a robust and reproducible sample preparation. This study was intended to investigate the effect of different packing materials on efficacy and reproducibility of VOC analysis by means of needle trap micro extraction (NTME). NTDs with a side hole design and containing different combinations of PDMS, DVB and Carbopack X and Carboxen 1000 and NTDs containing a single layer organic polymer of methacrylic acid and ethylene glycol dimethacrylate were investigated with respect to reproducibility, LODs and LOQs, carry over and storage. NTDs were loaded with VOC standard gas mixtures containing saturated and unsaturated hydrocarbons, oxygenated and aromatic compounds. Volatile substances were thermally desorbed from the NTDs using fast expansive flow technique and separated, identified and quantified by means of GC–MS. Optimal desorption temperatures between 200 and 290 °C could be identified for the different types of NTDs with respect to desorption efficiency and variation. Carry over was below 6% for polymer packed needles and up to 67% in PDMS/Carboxen 1000 NTDs. Intra and inter needle variation was best for polymer NTDs and consistently below 9% for this type of NTD. LODs and LOQs were in the range of some ng/L. Sensitivity of the method could be improved by increasing sample volume. NTDs packed with a copolymer of methacrylic acid and ethylene glycol dimethacrylate were universally applicable for sample preparation in VOC analysis. If aromatic compounds were to be determined DVB/Carboxen 1000 and DVB/Carbopack X/Carboxen 1000 devices could be considered as an alternative. PDMS/Carbopack X/Carboxen 1000 NTDs may represent a good alternative for the analysis of hydrocarbons and aldehydes. NTME represents a powerful tool for different application areas, from environmental monitoring to breath analysis.     

Atmospheric monitoring of volatile organic compounds using programmed temperature vaporization injection

A method has been developed for the automated determination of C₅ C₁₀ and C₂ C₆ volatile organic compounds in urban and rural air, using programmed temperature vaporization injection from a sorbent tube trap. A single activated charcoal sorbent tube was repeatedly used to collect samples of air with trapped VOCs being subsequently desorbed onto either a wide bore dimethyl polysiloxane (C₅ C₁₀) or porous layer open tubular (C₂ C₆) gas chromatography column without use of intermediate cryogenic refocussing. The high flow rates of helium used during the analysis resulted in the sample tube being cleaned and ready to reuse following the analytical separation. Examples of analysis of aromatic VOCs in urban air, and biogenic emissions in rural air, collected in a Sitka Spruce forest are presented. Using this method it is possible to quasi-continuously monitor concentrations of VOCs in locations where high sensitivity in situ analysis is required, but where cryogenic coolants may not be readily available or desirable.     

The application of two total transfer valve modulators for comprehensive two-dimensional gas chromatography of volatile organic compounds

Rotary and diaphragm 6-port 2-position valves have been evaluated as modulators for comprehensive GC. A total transfer methodology was used such that all analyte materials from the primary column were passed to the second separating column. The transfer methodology used repeated short periods of stop flow, but retained good primary column performance since primary carrier flow was partially maintained as pressure equilibrated along the length of the first column. Highly resolved separations of complex samples such as petrol and essential oils were achieved with equivalent performance (in terms of second column resolution) to thermal modulation with flame ionisation detection (FID). The valve modulators tested could outperform thermal modulators for very highly volatile organic species since the modulation process was insensitive to analyte vapour pressure. The diaphragm valve tested showed the best performance and was incorporated with a thermal desorption system to provide a high sensitivity separation of volatile organic compounds (VOCs) in air with LODs for individual VOCs of 2-4 pptV for a 1-L air sample.     

Thermal desorption and vapor transport characteristics in an explosive trace detector

Swipe-based explosive trace detectors rely on thermal desorption to vaporize explosive particles collected on a swipe. The vaporized material is carried by air flows from the desorption unit to the inlet of the chemical analyzer, typically an ion mobility spectrometer. We have observed that the amount of explosives detected from a swipe varies with the physical location of explosives collected on the swipe. There are two issues that may contribute to this effect: inhomogeneous or insufficient heating of the swipe during desorption and low velocity air flows that inefficiently transport desorbed vapor during the instruments analysis time. To better characterize this effect, we have simulated the air movements within a generic desorption unit using commercially available computational fluid dynamics software. Simulations are three dimensional, symmetric and solved under steady, laminar flow conditions. The calculated velocity field correlates directly with experimental detector response to the high explosive RDX. Results suggest that the limiting factor in this model thermal desorption unit is the flow-field around the swipe and flow rate into the detector, rather than heat transfer to the swipe itself. Buoyancy effects due to heating dominate the flow-field and produce a vertical bulk fluid motion within the domain that opposes much of the flow drawn into the analyzer.     

Construction and validation of an automated spray-and-trap gas chromatograph for the determination of volatile organic compounds in aqueous samples

An automated spray-and-trap (ST) chromatographic system was constructed for fast and efficient extraction of volatile organic compounds (VOCs) in aqueous samples with the capability to be deployed in the field for unattended continuous monitoring of surface or ground water. This system was built upon a commercial gas chromatograph with full automation capability using self-developed hardware and software. For sample analysis, fine droplets of the aqueous solution were generated in the extraction chamber by pressure expansion of a clean air stream through a spray nozzle. A portion of the VOCs distributed into the gas phase was retained by a multi-sorbent micro-trap kept at ambient temperature. Flash heating of the sorbent trap desorbed the enriched VOCs onto the gas chromatography (GC) with flame ionization detection (FID) for hydrocarbons or electron-capture detection (ECD) for halocarbons. In order to validate the performance of the ST method. it was compared with a more conventional method, i.e., a purge-and-trap (PT), by analyzing a serious of standard solutions containing benzene, toluene, ethylene. and o-, m-xylenes. Using a purge-and-trap method as a reference for complete extraction, the ST method showed less sensitivity. Extraction recoveries are in consistent with Henry's law constants. To test response time the ST-GC-ECD was periodically switched between tap and underground waters. Negligible carry-over of halogenated species and reproducibility better than 2% relative standard deviation (R.S.D.) can be achieved regardless of large concentration difference between the two sources, thus demonstrating applicability of the ST system for on-site monitoring.     

Determination of volatile organic compounds in different microenvironments by multibed adsorption and short-path thermal desorption followed by gas chromatographic-mass spectrometric analysis

A multiphase assurance approach was developed for the accurate and precise determination of volatile organic compounds (VOCs) in different microenvironments. This approach includes (i) development of a method including adsorption of VOCs onto a multisorbent media followed by short-path thermal desorption (SPTD) pre-concentration and gas chromatography (GC) coupled to a mass spectrometry (MS) quantification, (ii) validation of the sampling and analytical method and (iii) validation of the data using a multidimensional procedure. Tenax TA and Carbopack B sorbent combinations were used to collect 102 individual VOCs ranging from C5 to C12. Method parameters including thermal desorption temperature, desorption time and cryofocusing temperature were optimized. The average recoveries and method detection limits (MDL) for the target analytes were in the range 80-100% and 0.01-0.14 ppbv, respectively. The method also showed good linearity (R2 > 0.99) and precision (<8%) values. Validation of the method was performed under real environmental conditions at a gas station, in an office and a residential household to examine the influence of variation in meteorological conditions such as temperature and relative humidity and a wide range of VOC concentrations. The sampling and analytical method resulted in successful determination of VOC in different microenvironments. Finally, validation of the data was performed by assessing fingerprint and time series plots and correlation matrices together with meteorological parameters such as mixing height, wind speed and temperature. The data validation procedure provided detection of both faulty data and air pollution episodes.     

Mesoporous silicate MCM-48 as an enrichment medium for ambient volatile organic compound analysis

This study investigated the sorption/desorption properties of MCM-48 and its applicability as a sorbent for on-line gas chromatographic analysis of ambient volatile organic compounds (VOCs). To establish a valid comparison, commercially available carbon sorbents were evaluated under similar analytical conditions. Two trapping temperatures of 30 °C and −20 °C, representing ambient and sub-ambient temperatures, were tested by trapping a full range of VOCs from C₂–C₁₂. At ambient temperatures, due to the mesoporosity, the MCM-48 showed considerably limited trapping efficiency compared to microporous carbon sorbents on the highly volatile section of VOCs and only began to show effective trapping for compounds larger than C₇. Cooling to sub-ambient temperatures (e.g., −20 °C) extended the effective trapping down to C₄ VOCs, drastically increasing the applicability of MCM-48 as an in-line enrichment medium for gas chromatographic analysis of VOCs. The mesoporosity of MCM-48 also aided desorption. Much lower desorption temperatures (100–180 °C) were required for full desorption as compared to the temperatures (greater than 200 °C) required for carbon sorbents. Moreover, the easy desorption was accompanied by a low memory effect, as the large pores of MCM-48 can clean up more efficiently after desorption, with little residue left behind.     

Determination of gas phase nicotine and 3-ethenylpyridine,and particulate phase nicotine in environmental tobacco smoke with a collection bed – capillary gas chromatography system

A combined sampling and analysis technique for the determination of gas phase nicotine and 3-ethenylpyridine, and of particulate phase nicotine in environmental tobacco smoke with capillary gas chromatography is reported. The major advantage of the technique is that all of the collected particulate phase material is analyzed by thermal desorption of the collected material rather than by analysis of only a fraction of the sample extracted from the collection medium. A Teflon filter microtube is used to collect particulate phase nicotine. This microtube is follwed by a small Tenax sorbent bed to collect gas phase nicotine and 3-ethenylpyridine. After sampling, the Teflon filter is transferred to a clean glass tube and the tube becomes an insert for a modified packed column injector port where the material collected on the filter is heat desorbed to a cold capillary tubing trap. Gas phase nicotine and 3-ethenylpyridine are also transferred from the Tenax to the GC column by thermal desorption from the Tenax sorbent bed. Gas phase nicotine and 3-ethenylpyridine, and particulate phase nicotine are each determined by GC analysis of the desorbed material. Nicotine and 3-ethenylpyridine are quantitated by the use of external standards. This technique is straightforward and can be used for semi-real time determination of both gas and particulate phase compounds in environmental tobacco smoke. The results obtained by this technique compare well with those obtained by sampling with annular diffusion denuders.