Sample Transfer Efficiency Research Articles

Two-dimensional gas chromatography coupled to isotope ratio mass spectrometry for determining high molecular weight polycyclic aromatic hydrocarbons in sediments

The accuracy of compound-specific isotope analysis (CSIA) of trace-level pollutants in complex environmental samples has always been limited by two main challenges: poor chromatographic separation and insufficient amounts of analytes. In this study, a two-dimensional gas chromatography-isotope ratio mass spectrometry (2DGC-IRMS) system was constructed for compound-specific δ13C analysis of high molecular weight polycyclic aromatic hydrocarbons (HMW-PAHs) in estuarine/marine sediments. This construction occurred through hyphenating an extra gas chromatography system (GC) to a conventional GC-IRMS using a commercially available multi-column switching-cryogenic trapping system (MCS-CTS). Compared with the previous 2DGC-IRMS strategy, which utilizes a Deans Switch device, the newly implemented 2DGC-IRMS scheme resulted in online purification of target analytes as well as enriched them online via duplicate injection and cryogenic trapping in CTS; this resultingly lowered the limits of detection (LOD) of CSIA. To improve the sample transfer efficiency to the IRMS, a broader-bore and longer fused-silica capillary was utilized to replace the original sample capillary running from the sample open split to the IRMS. A ẟ13C analysis of PAH standards showed accurate ẟ13C values, and high precisions (standard deviations 0.13–0.37%) were achieved, with the LOD of HMW-PAHs reduced to at least 1.0 mg/L (i.e., 0.07 to 0.09 nmol carbon per compound on-column). The successful application of this newly developed 2DGC-IRMS scheme provides a practical solution for the reliable CSIA of trace-level pollutants in complex environmental samples that cannot be measured using the conventional GC-IRMS system.

Picomolar-scale compound-specific isotope analyses.

We report modifications to compound-specific isotope analyses (CSIA) to enable high-precision isotopic analyses of picomoles of carbon for intact organic molecules. This sample size is two orders of magnitude below the amounts required for commercial systems. The greatly enhanced sensitivity of this system expands molecular isotope studies and applications previously prohibited by low concentrations and small samples. We utilize the resolving power and low volumetric flow rates of narrow-bore capillary gas chromatography to improve sample transfer efficiency while maintaining narrow peak widths. Post-column peak broadening is minimized using a micro-fluidic valve for solvent diversion, capillary combustion reactor, narrow-bore capillary transfer lines, and cryogenic water trap. The mass spectrometer was fitted with collector amplifiers configured to 25ms response times and a data logger board with firmware capable of rapid data acquisition. Carbon dioxide gas was introduced directly into the ion source to evaluate the dynamic range of the system and accuracy and precision of carbon isotope ratio (δ13 C value) measurements. The accuracy and precision for combusted compounds were evaluated using a suite of n-alkanes. For ≥30 pmol carbon introduced directly into the ion source, the mean difference between the measured and expected δ13 C values is 0.03‰ (1σ, n = 57) and the standard deviation of replicate measurements is 0.11‰ (1σ). The CO2 peak widths generated by the exponential dilution flask were 250ms and the peak widths produced by combusting n-alkanes were ca 500ms, less than 25% the width of conventional gas chromatography peaks. For a mixture of 15 n-alkanes (n-C16 to n-C30 ), the accuracy is 0.3‰ (1σ) and precision is 0.9‰ (1σ) for replicate δ13 C measurements with 100pmol carbon per compound on column. The pico-CSIA method described here offers improved chromatographic resolution and reduces sample size requirements by two orders of magnitude. These advances significantly broaden the available analytical window for CSIA in research areas frequently hindered by sample size limitations, such as forensics, paleoclimate, astrobiology, and biochemistry.

Development of an automated high-temperature valveless injection system for online gas chromatography

Abstract. A reliable method of sample introduction is presented for online gas chromatography with a special application to in situ field portable atmospheric sampling instruments. A traditional multi-port valve is replaced with a valveless sample introduction interface that offers the advantage of long-term reliability and stable sample transfer efficiency. An engineering design model is presented and tested that allows customizing this pressure-switching-based device for other applications. Flow model accuracy is within measurement accuracy (1%) when parameters are tuned for an ambient-pressure detector and 15% accurate when applied to a vacuum-based detector. Laboratory comparisons made between the two methods of sample introduction using a thermal desorption aerosol gas chromatograph (TAG) show that the new interface has approximately 3 times greater reproducibility maintained over the equivalent of a week of continuous sampling. Field performance results for two versions of the valveless interface used in the in situ instrument demonstrate typically less than 2% week−1 response trending and a zero failure rate during field deployments ranging up to 4 weeks of continuous sampling. Extension of the valveless interface to dual collection cells is presented with less than 3% cell-to-cell carryover.

Nanoliter Segmented-Flow Sampling Mass Spectrometry with Online Compartmentalization

We report a microfluidic device, using segmented flow in a two-phase system of immiscible liquids, which delivers aqueous droplets into a modified commercial mass spectrometer. The interface coupling the microfluidics to the mass spectrometer achieves up to 96% sample transfer efficiency to the vacuum chamber. Sample ionization is assisted by multipass infrared laser beam in the interface. The system achieves low femtomole detection limits of several analytes ranging from drugs to proteins. Sample ionization in this segmented-flow sampling was found to be remarkably insensitive to the presence of buffer salts and other matrices.

Sorbentless cryogenic needle trap device for the extraction of organic volatile compounds

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 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.

Estimation of Partition Coefficients of Benzene, Toluene, Ethylbenzene, and p-Xylene by Consecutive Extraction with Solid Phase Microextraction

It is very important to use partition coefficients of organic pollutants to predict their fate in the environment. A liquidliquid extraction technique was used to determine the partition coefficients of organic compounds between water and organic solvent. The concentration of the target compounds must be determined after equilibrium is established between the two phases. The partition coefficients can be estimated using the capacity factors of HPLC and GC. The solid phase microextraction (SPME) technique has been widely used for the analysis of volatile organic compounds (VOCs) in water and air; fragrances in perfume, food, and cosmetics; and many other emerging pollutants. In addition, the fact that SPME extraction is based on equilibrium drove scientists to develop a new approach to determine partition coefficient (Kd) values. Vaes et al. used the kinetics of partitioning to SPME fibers to estimate the Kd values for 19 organic compounds. Nardi used the “depletion technique” and a capillary tubing coated with siloxane to estimate the Kow values of BTEX. 6 The Kd values of hydrophobic compounds are generally known to have a close relationship with Kow. 7 Dean and colleagues showed a close correlation between water-fiber coefficients (Kfs) and octanol-water partition coefficients (Kow). 8 All these approaches use an absolute amount of analyte extracted from the SPME fiber. The liquid injection of a known amount of standard solution is a common technique to calibrate GC for quantitative analysis with the SPME fiber. This technique assumes that the sample transfer efficiency by the SPME fiber is the same as that of liquid injection. However, this study shows that several factors need to be considered in order to achieve higher sample transfer efficiency in the case of liquid injection, i.e., the liner, glass wool, and the temperature of the injector. The difference in sample transfer efficiency between liquid injection and the SPME fiber may result in errors in the estimation of Kfs values, i.e., Kow values. In this study, water-fiber partition coefficients (Kfs) were determined from consecutive extraction data by a SPME fiber. Instead of using the absolute mass of the analyte, the relative GC signal ratio was used to estimate the partition coefficients. An SPME fiber extracts target compounds multiple times from the same sample vial and inserts the fiber into a hot GC injector. The extracted amount on the fiber coating decreases exponentially after each extraction; Kfs of an analyte can be derived from the resulting GC responses. Theory. The following is the equation showing the relationship among the extracted amount (n, the number of moles of the analyte with an initial concentration of Co), the extraction phase volume, (Vf, typically 0.6 μL ), and the sample volume (Vs) when an SPME fiber has reached equilibrium with the analyte. Eq. (1) shows that the extracted amount is governed by the partition coefficient (Kfs) of the target analyte:

Calibration of solid-phase microextraction for quantitative analysis by gas chromatography

Calibration of SPME by the injection of liquid standard solution is a common method for SPME quantitative analysis by gas chromatography. The feasibility of this calibration method relies on the assumption that sample transfer efficiencies are the same for both the liquid injection and the SPME fiber injection. Sample transfer efficiencies for liquid injection and SPME fiber injection were studied in this paper. The results showed that the sample transfer efficiency for liquid injection was affected by several factors, such as the dimensions of the liner, the presence of the wool, and the temperature of the injector. The sample transfer efficiency for SPME fiber injection was affected by the cross-sectional area of the space between the column and the liner, the carrier gas flow rate, and the length of the column inside the liner. An equation was proposed to estimate the sample loss rate for SPME injection. It was found that the use of a direct injection (DI) liner and program temperature vaporizing provides high sample transfer efficiencies, for both liquid injection and SPME fiber injection. When a common SPME straight liner is used, large outer diameter (o.d.) pre-column will help to achieve high sample transfer efficiency.

Comprehensive two-dimensional capillary LC and CE for resolution of neutral components in traditional Chinese medicines

Comprehensive two-dimensional separation by capillary liquid chromatography coupled with capillary micellar electrokinetic chromatography was investigated. Fast micellar electrokinetic chromatography was achieved by speeding up electroosmotic flow and improving solute mass transfer in micellar pseudo-stationary phase. High efficiency separation was achieved by selecting operating conditions based on theoretical relationships and experimental data. A new gating interface from the first dimension to the second dimension based on effluent stacking-injection was developed. The reproducibility of the interface for sample transfer efficiency, migration time, and peak height was studied. The overall system performance was verified in the separation of complex neutral components. Resolution of hundreds of compounds present in traditional Chinese medicines was demonstrated.

Optimization of short-column gas chromatography/electron ionization mass spectrometry conditions for the determination of underivatized anabolic steroids.

A gas chromatographic/mass spectrometric method based on the use of short capillary gas chromatograph columns (3-5 m) and electron ionization mass spectrometry has been optimized and evaluated for the determination of underivatized anabolic steroids. The short-column gas chromatographic/mass spectrometric method was shown to result in short analysis times and to require minimal sample preparation, but suffered from some loss in sensitivity and chromatographic resolution compared with conventional gas chromatographic/mass spectrometric techniques for derivatized steroids. Therefore, short-column gas chromatographic conditions were optimized to maximize the sample transfer efficiency (sensitivity) from the gas chromatograph into the ion source of the mass spectrometer, while maintaining chromatographic integrity and minimizing thermal decomposition. Mass spectrometric conditions were optimized to maximize ionization efficiency with respect to the intensity of the molecular ion and degree of fragmentation such that positive identification of each steroid could be made based on the resulting mass spectra. Under optimized conditions, we have shown that underivatized anabolic steroids spiked into urine samples can be determined at low-nanogram levels using short-column chromatography/full-scan electron ionization mass spectrometry.