Sorbent sampling methods

As a result of developing and validating filter/sorbent sampling methods, some additional criteria for testing were formulated and added to the original method testing criteria for these sampling trains. These are summarized below ... 

Two types of sorbent sample technique are commonly used active or pumped sampling, and passive or diffusive sampling. Table 14.8 shows the experimental conditions in which BTEX were determined from air using active sorbent sample methods. 

As well as typical sample preparation methods such as filtration and liquid-liquid extraction, newer developments are now extensively used. The first of these is solid-phase extraction (SPE). This is a rapid, economical, and sensitive technique that uses several different types of cartridges and disks, with a variety of sorbents. Sample preparation and concentration can be achieved in a single step. Interfering sugars can be eluted with aqueous methanol on reversed-phase columns prior to elution of flavonoids with methanol. 

A universal sampler applicable to the majority of pesticides would be an ideal sampling device. In this study, personal sampling and analytical methods were developed and validated in the laboratory for determining workplace exposure to several pesticides. The major objectives of the study were to standardize on specific sampling media and to develop and validate methods using filter/ sorbent sampling trains. 

Filter/Sorbent Methods. The methods developed using filter/ sorbent sampling trains are listed in Table III. The sampling train consists of a 37-mm diameter filter contained in a cassette filter holder followed by a sorbent tube as described above. Samples are collected at 1 liter/min to obtain the prescribed sample volume. After sampling is completed, the filter is removed from the filter holder and placed in a glass vial with the front sorbent section and capped. The combined sample is extracted with toluene and the resulting solution is analyzed by gas chromatography. 

Cryo-trapping is often used in combination with solid sorbents and therefore is less important as a stand-alone sampling method. 

The choice of sorbent, sampling and preparation methods includes the laboratory apparatus facilities and the purpose of the investigation. 

Real-time monitoring like PTR-MS is a good choice when the component of the sample gas will be changed using a traditional sorbent or canister sampling method. 

Adsorbed over coconut charcoal (100 mg/50 mg) desorbed with CS2 and analyzed by GC-FID flow rate 10 to 200 mL/min sample volume 0.5 to 2.5 L (NIOSH 1987) sample volume 10 L for 350 mg sorbent (OSHA Method 59). 

Hurley GF, Ketchem NH. 1978. A solid sorbent personal sampling method for the determination of acrolein in air. Am Ind Hyg Assoc J 39 615-619. 

Thermal Desorption Thermal desorption is an alternative GC inlet system particularly used for VOC analysis. However, the analytes subjected to thermal desorption must be thermally stable to achieve successful analysis. Otherwise, decomposition occurs. This technique is mainly used for determination of volatiles in the air. Such a methodology requires sample collection onto sohd sorbents, then desorption of analytes and GC analysis. Traditionally, activated charcoal was used as a sorbent followed by extraction with carbon disulfide. However, solvent desorption involves re-dilution of the VOCs, thus partially negating the enrichment effect. Therefore, the sampling method is to pump a sample of gas (air) through the sorbent tube containing certain sorbents in order to concentrate the VOC. Afterwards, the sample tube is placed in thermal desorber oven and the analytes are released from the sorbent by application of high temperature and a flow of carrier gas. Additionally, desorbed compounds are refocused in a cold trap and then released into the GC column. Such a two-step thermal desorption process provides a narrow chromatographic band at the head of the column. 

It is important in active sampling to choose appropriate sorbents and methods. Lindane (y-HCH), for example, is only present in the vapor phase, not in the particle phase [82]. Singh et al. [45] also reported lindane to be chiefly found in the vapor phase. DDT in contrast, when analyzed in rooms treated with DDT according to the National Malaria Eradication Program of India, was 34-78% particle-bound. 

The extraction solvent should be chosen such that the analyte is extracted with as little co-extraction of interfering substances as possible. Furthermore, choose a solvent that is compatible with the solid-phase extraction method that is being used. For example, samples that are high in fat are best homogenized with nonpolar solvents, such as hexane. The use of hexane as an extractant dictates normal-phase sorbents with silica, alumina, Florisil, or a CN sorbent. Samples with high water content are best homogenized with acids, bases, or polar solvents, such as methanol, acetonitrile, or acetone. The use of methanol or one of the water-miscible solvents suggests the use of reversed phase (C-18) and dilution of the extract with water or buffer. Thus, the choice of the extraction solvent will dictate the type of sorbent that will be used in SPE. 

Available data based on limited exposure tests in chambers and comparisons with pumped sampling methods in the laboratory and field show the techniques can be used to determine mean concentrations of VCX3s over periods of a day to several weeks for fixed site and personal monitoring. Problems of poor recovery at the desorption stage and possible losses by back diffusion means the investigator needs to consider carefully the choice of sampler and, in the case of thermally desorbable tubes, the optimum sorbent for the investigation. 

Matrix solid phase dispersion (MSPD) is an effective sample preparation technique that combines extraction and purification in one step. Barker et al. defined MSPD procedures as those that use dispersing sorbents with chemical modification of the silica surface (e.g.. Cl8, C8). Samples are blended and dispersed on particles (diameters of 40-100 p.m) using a glass or agate mortar and pestle (Pig. 4.3). The use of ceramic or clay mortars and pestles can result in loss of analytes. A disadvantage of the method is the traditionally high sorbent sample ratios... 

Other analytical methods used for the analysis of solid sorbent samples include ion chromatography, ion-selective electrodes, and polarography measurements. 

The SKC type sampler consisted of the SKC Passive Sampler for Inorganic Mercury (520 Series). The sampling method employed was OSHA ID-140 Mercury Vapor in Work Place Atmospheres. The sampling media consisted of Anasorb C300. The analytical method consisted of FGS-SOP-136 Analysis of Hg in Air via Capture on Sorbent Trap (NELAP and ISO-17025 Accredited) based on the principles of US EPA 1631 Revision E. 

Active personal exposure samples were at an approximate rate of 30% of the total. Active personal exposure samples were eolleeted when wind speeds were below 25 ft/min (feet per minute) or above 750 ft/min. The sampling method was eonsistent with OSHA ID-140 and employed low flow air sampling pumps with sorbent traps consisting of two beds of FSTM media which were analyzed by the FGS using the same method as that used for the analysis of the passive sample media. 

Fig. 14 shows adsorption isotherm of ethylene on the sol-gel derived CuCl/ y-AhOij sorbent (Sample 2 in Table 8). The isotherm is of type I. The equilibrium adsorption data of C2H4 at 1 atm pressure and 14-18°C on CuCl/ Y-AI2O3 (prepared by the solid state mixing method) was previously reported in the literature [68]. The amount of ethylene adsorbed on CuCl/ y-AUOs was only about 0.29 mmol/g. Yang and Kikkindes [72] reported very good data of adsorption of 0.73 mmol/g of ethylene on CuCl/y-AhOa adsorbent with support surface area of 340 m /g. The result of our sorbent is similar to the best result reported. 

To measure binary coadsorption equilibria by tbe volumetric-gravimetric method one proceeds as follows A sorbent sample of 1 g - 3 g and appropriate counterweights, typically lead or silver balls, are placed to the buckets of the microbalance. Then the sorbent is activated by exposing it to helium gas at higher temperatures, i. e. 433 K for activated carbons, 673 K for zeolites and inorganic molecular sieves. After cooling down and evacuation (< 10 Pa) the adsorption chamber is prepared for an adsorption experiment. 

To measure binary coadsorption equilibria by the densimetric-gravimet-ric method one should proceed as follows After preparing a sorbent sample, i. e. activating it, cp. Sect. 2.1 and weighing it in vacuum to determine its mass (m ), the adsorption chamber (AC) is filled with a gas mixture of known concentrations ( y, ) and the circulation pump is turned on. 

In practice a method proposed originally by 0. Jdntti [2.4, 4.21] some years ago and recently more elaborated by E. Robens et al. [4.22, 4.23] may prove useful in gravimetric measurements of adsorption kinetics. It allows one to curtail the micro weighing time i. e. to avoid measurements of the approach to equilibrium by calculating from 3 data points taken at 3 consecutive times the asymptotic uptake of mass in the sorbent sample with fair accuracy. 

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