Selective removal of water in purge and cold-trap capillary gas chromatographic analysis of volatile organic traces in aqueous samples

The design and features of an on-line purge and cold-trap pre-concentration device for rapid analysis of volatile organic compounds in aqueous samples are discussed. Excessive water is removed from the purge gas by a condenser or a water permeable membrane in order to avoid blocking of the capillary cold-trap. Synthetic mixtures covering concentrations ranging from tenths to tens of ppb's and different chemical classes are used to study the effect of various process factors on the efficiency and selectivity of water removal as well as on the purging recovery. The importance of the concentration of the solutes, the flow rate in conjunction with the volume of the purge gas, and the temperature of the condenser, the cold-trap and the sample is emphasized. Theoretical models describing the purge process and the blocking of the cold-trap agree fairly well with the highly reproducible experimental results ( = 2-4%). Both the condenser and the Nafion membrane successfully remove water, although some compounds, dependent on volatility and polarity, are partly or completely lost. It is shown that non-polar volatile organic compounds are efficiently enriched so that recoveries between 80-100% and a detection limit of 1 ppt can be obtained. The applicability of the system is illustrated on some examples

A prototype model for evaluating SKA-LOW station calibration

The Square Kilometre Array telescope at low-frequency (SKA-Low) will be a phased array telescope supporting a wide range of science cases covering the frequency band 50 - 350 MHz, while at the same time asking for high sensitivity and excellent characteristics. These extremely challenging requirements resulted in a design using 512 groups of 256 log periodic dual polarized antennas each (where each group is called "station"), for a total of 131072 antennas. The 512 stations are randomly distributed mostly within a dense area around the centre of the SKA-Low, and then in 3 arms having 16 station clusters each. In preparation for the SKA Phase 1 (SKA1) System Critical Design Review (CDR), prototype stations were deployed at the Murchison Radio-astronomy Observatory (MRO) site (Western Australia) near the Murchison Widefield Array (MWA) radio telescope. The project involved multiple parties in an International collaboration building and testing different prototypes of the SKA1-Low station near the actual site. This resulted in both organisational and logistic challenges typical of a deployment of the actual telescope. The test set-up involved a phased build-up of the complex station of log-periodic antennas, by starting from the deployment of 48 antennas and related station signal processing (called AAVS1.5, where AAVS stands for Aperture Array Verification System), followed by expansion to a full station (AAVS2.0). As reference a station with dipole antennas EDA2 (EDA: Engineering Development Array) was deployed. This test set-up was used for an extensive test and evaluation programme. All test antenna configurations were simulated in detail by electromagnetic (EM) models, and the prediction of the models was further verified by appropriate tests with a drone-based radio frequency source. Astronomical observations on Sun and galaxy transit were performed with calibrated stations of both EDA2, AAVS1.5 and AAVS2.0. All 3 configurations were calibrated. EM modelling and calibration results for the full station AAVS2.0 and EM verification for the AAVS1.5 station are presented. The comparisons between the behaviour of the log-periodic antennas and the dipoles have advanced our understanding the calibration quality and the technological maturity of the future SKA1-Low array

Sample enrichment in high speed narrow bore capillary gas chromatography

Reduction of the column diameter has proved to be a highly efficient tool to increase the speed of analysis. Unfortunately, the requirements for instrumental design with respect to sample input band width, low dead volume interfacing, and time constants of detection and registration systems are the more critical the smaller the inside diameter. Recently we reported input band widths as low as 1 ms [1] for gaseous samples at ppm concentration levels, without any preconcentration, in a study with narrow bore columns and thermal conductivity detection. In this study a simple versatile micro on-column cold trap/thermodesorption enrichment system for narrow bore columns is introduced and evaluated. The combination of considerable sample enrichment and preservation of the compatibility of the required input band width with column dimensions is critically examined. The process of thermodesorption (reinjection) which is the most critical step, is particularly emphasized. The system consists of a short aluminum coated fused silica or metal capillary with a low mass and a low cost electrical heating. Input band widths down to 1 ms are obtained without extreme demands on electrical power (300 watt). The potential of the system is illustrated with some extremely fast separations

Quantitative aspects of cold temperature programmed sample introduction in capillary GC

In this study some aspects of quantitation in capillary gas chromatography are discussed for on-column and cold temperature proqrammed sample inlet systems. Discrimination due to differences in volatility, polarity and concentration of the solutes will be evaluated and compared for both systems. A mixture of n-alkanes with hydrocarbon numbers between 8 and 32 and a test mixture normally used for column quality control are used for this purpose. It will be shown that at optimal operating conditions highly accurate results can be achieved for samples with a wide range of volatility, polarity and concentration

Detectability and the resulting requirements for column-detector systems in capillary gas chromatography

Expressions for the minimum detectable amount Qo and the minimum analyte concentration Co as functions of the chromatographic parameters are derived for both mass and concentration sensitive detectors. The effects of pressure drop, column inner diameter and film thickness are given. The minimum analyte concentration for mass flow sensitive detectors. Com, can be reduced considerably by selecting the carrier gas velocity well above its optimum value (related to Hmin) , however, at the cost of long columns and long analysis times. For Qo the improvements can be neglected, and so the analysis can best be performed at Uopt. When the flow rate in the detector, Fd, is equal to the column flow rate Fc, the maximum permissible detector volume of concentration sensitive detectors is proportional to dc2 up to dc3, and so narrow bore columns require detectors of extremely small volume. Make-up gas has to be added when the actual volume is too large, thus deteriorating the detectability. Another approach, vacuum operation of the detector cel appears to be very attractive. On the other hand, when wide bore colums are used in combination with small volume concentration sensitive detectors, very small values of Qoc and Coc are obtainable when the abundant carrier gas can be removed before entering the detector cell. Digital noise filtering can further reduce the obtainable Qo and Co values, especially for broad peaks and thus for wide bore columns

Detectability and the resulting requirements for column-detector systems in capillary gas chromatography

Expressions for the minimum detectable amount Qo and the minimum analyte concentration Co as functions of the chromatographic parameters are derived for both mass and concentration sensitive detectors. The effects of pressure drop, column inner diameter, and film thickness are given. The minimum analyte concentration for mass flow sensitive detectors, Com, can be reduced considerably by selecting the carrier gas velocity well above its optimum value (related to Hmin), however, at the cost of long columns and long analysis times. For Qo the improvements can be neglected, and so the analysis can best be performed at uopt. When the flow rate in the detector, Fd, is equal to the column flow rate Fc, the maximum permissible detector volume of concentration sensitive detectors is proportional to dc2 up to dc3, and so narrow bore columns require detectors of extremely small volume. Make-up gas has to be added when the actual volume is too large, thus worsening the detectability. Another approach, vacuum operation of the detector cell, appears to be very attractive. On the other hand, when wide bore columns are used in combination with small volume concentration sensitive detectors, very small values of Qoc and Coc are obtainable when the abundant carrier gas can be removed before entering the detector cell. Digital noise filtering can further reduce the obtainable Qo and Co values, especially for broad peaks and thus for wide bore columns