Tenax sampling rate

A solid sorbent Tenax cartridge has a capacity of 100 fig of toluene. If samples were collected at a rate of 5 liters/min, calculate the maximum ambient concentration which can be determined by an hourly sample and a 15-min sample. 

A commonly used method of sampling and analysis for volatile organic compounds In ambient air Is by concentration of the compounds on a solid sorbent such as Tenax and subsequent thermal desorption and GC/MS analysis of the collected compounds. The analysis phase, although not trivial, can be done well If proper care Is taken. However, the sampling phase of this process apparently Introduces artifacts and unusual results due to, as yet, unknown factors. A method to detect some sampling problems has been proposed and tested (7 ). This distributed air volume method requires a set of samples of different air volumes to be collected at different flow rates over the same time period at the sampling location. Each pollutant concentration for the samples should be equal within experimental error since the same parcel of air Is sampled In each case. Differences In results for the same pollutant In the various samples Indicates sampling problems. 

Table I contains a list of some of the compounds that have been submitted to this type of analysis. The recovery data is intended to be illustrative only since recoveries depend strongly on several important method variables. Recoveries are expressed as a percentage of the amount added to organic free water. The purge time was 11-15 minutes with helium or nitrogen, the purge rate was 20 ml/minute at ambient temperature, and the trap was Tenax followed by Silica Gel. Data from the 5 ml sample was obtained with a custom made purging device and either flame ionization, microcoulo-metric, or electrolytic conductivity GC detectors. Data from the 25 ml sample was obtained with a Tekmar commercial liquid sample concentrator and a mass spectrometer GC detector using CRMS.

Active Sampling. These techniques use the dynamic passage of the sampled air at a specified rate through a substrate (e.g., a filter), an absorbant (e.g., Tenax (diphenylphenylene oxide) or activated charcoal), or a detector (e.g., a photometer) that measures a parameter that is proportional to detectable quantities of a contaminant. 

Heat the sample for 30 min at 30°C while purging helium at a rate of 25 ml/min. Collect volatile compounds on the trap (packed with Tenax or equivalent) and thermally desorb at 180°C onto a 30-m x 0.32-pm i.d. x 1-pm film thickness fused-silica capillary column. After desorption is complete, hold the initial temperature for 1 min at -20°C and then program the temperature to ramp to 220°C at 6°C/min. Set the injector and the detector temperatures at 260°C and 280°C, respectively. Use helium as carrier gas at a flow rate of 3.0 ml/min and a split ratio of 20 1. 

The analysis of particle bound PAH involves collection of PAH bound to dust particles on 0.8 pm glass fiber or silver membrane filters, desorption of PAH from the particles into a suitable organic solvent, and analysis of the extract by a capillary GC using an FID. Between 500 and 1000 L air at a flow rate of 120 L/h is recommended for sampling, which can give a detection limit of 0.15 to 0.50 pg/m3 for each compound (Riepe and Liphard, 1987). The method suggests the installation of an absorber resin, such as XAD-2 or Tenax, after the... 

Adsorbed over Tenax (1 to 2 g) in a cartridge analyte desorbed with an inert gas under heating and transferred onto a cold trap and then onto the front of a precooled GC column at 70°C column heated analyte eluted and detected by GC/MS, ECD, or FID recommended flow rate 0.5 L/min sample volume 100 L. 

Alternatively, air drawn through a cartridge packed with either Tenax (1 g) or carbon molecular sieve (0.5 g) cartridge heated at 350°C under helium purge analyte transported to the front of a precooled GC column, temperature programmed ethylbenzene determined on a PID, FID, or a mass spectrometer recommended air flow rate 0.5 L/min sample volume 50 L. 

Adsorbed over Tenax ( 2 g) in a cartridge desorbed by heating under He purge transferred into a cold trap and then to the front of a GC column at -70°C column heated analyte detected by ECD or MSD (U.S. EPA Method TO-1) recommended flow rate 100 mL/min sample volume 10 L. 

For purge-and-trap analysis, a portion of the extrudate was ground and 15 g were placed in a two-necked sample flask with 30 g of NaCl and 100 ml of distilled water. Headspace components were collected on activated Tenax TA by purging nitrogen gas through the sample suspension at a flow rate of 400 mL/min for 2 hours. 

Preliminary experiments, the results of which are summarized briefly under Results and Discussion, led to the following optimized sampling method. Volatile collection was carried out for 30 min at the cold storage temperature (4 c) and with a vacuum pump flow rate of 50 mL/min. Then, the sampled Tenax tubes were removed, capped and ready for GC analysis the same day. 

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