| |
| In the course of monitoring MeBr in workplace, field, and ambient atmospheres, sampling and analytical methods of different sensitivities and complexities have been developed. Depending on the sampling device that is used for collecting air samples, MeBr can either be in a contained atmosphere (such as canisters) or adsorbed on a solid adsorbent (such as activated carbon or a porous polymer) prior to analysis. For the past two decades, quantitation of MeBr has used gas chromatography exclusively, and electron-capture detectors (ECD) are usually selected over the other types of detectors due to its high sensitivity to halogenated compounds (Scudamore, 1988), although very high sensitivity is also found with photoionization detectors (PID) (Dumas and Bond, 1985). The main reported sampling and analytical methods for analyzing atmospheric MeBr are summarized in Table 2. |
| |
Container Methods |
| Among the container methods, steel canisters were used for sampling volatile toxic chemicals in air, such as MeBr, by Jayanty (1989) and Gholson et al. (1990), and good stability and sensitivity were achieved for all the selected analytes. Cryogenic preconcentration was required prior to the delivery of samples into the GC column. Yagi et al. (1993, 1995) used 500-ml canisters for sampling MeBr to obtain flux measurement under field conditions. Sampling with canisters is labor-intensive since the container has to be evacuated before sampling, and the contents must be cryogenically concentrated before injection, which limits the number of samples that can be collected and analyzed. Sampling with canisters is therefore not suitable for extensive sampling as needed in volatilization flux measurement under field conditions, though the sensitivity could be very high if a proper detector is used. Using canisters is also not compatible with active (flow-through) chambers that are used for continuous sampling of the atmosphere. |
| |
| Another container method involves collecting an air sample using a gas-tight syringe, and injecting the contents directly into a gas chromatograph. In a study of the transport of MeBr in soil after fumigation, Kolbezen et al. (1974) used glass syringes to take and temporarily store soil air samples. The plungers were coated with a film of Triton X-100 to eliminate rapid leakage, and the needle was embedded in a MeBr-impervious sponge. Loss of MeBr was determined to be insignificant within 6 h, but 5-7% was lost after 22 h. The analysis was made by direct injection of the air sample in the syringe into a GC. This method has also been employed in small-scale laboratory experiments (for example, Gan et al., 1998a). |
| |
Adsorbent Methods |
| The most commonly used method for sampling atmospheric MeBr is pumping a relatively large volume of air through one or a series of adsorbent tubes. Methyl bromide in the air stream is trapped in the sample tube containing the solid adsorbent due to its high affinity to the adsorbent. Two types of adsorbent material have been recorded for use with MeBr: activated carbon (charcoal) (Eller, 1984; Woodrow et al., 1988; Lefevre et al., 1989; Gan et al., 1995a,b; Majewski et al., 1995; Gan et al., 1995a,b; Yates et al., 1996abc; Yates et al., 1997; Wang et al., 1997a), and porous polymeric adsorbent such as Tenax GC (Brown and Purnell, 1979; Dumas, 1982, Dumas and Bond, 1985; Krost et al., 1982). |
| |
| Activated carbon or charcoal tubes are low in cost (about $1 each), can accommodate large sample volumes, and need minimum preparation before sampling. A typical charcoal tube consists of two adsorption beds: a primary bed (A) and a backup bed (B) in a sealed glass tube. The charcoal can be derived from either coconut or petroleum. Polyurethane spacers are used to separate the two adsorption beds, and a plug of glass wool is usually placed in front of the primary bed to hold the charcoal in the sample tube. Before use, a tube is broken at both ends, and then connected to a vacuum source to draw the air to be sampled into the tube. Depending on the sampled volume, air flow rate and MeBr concentration, multiple tubes connected in series may be required to eliminate loss through breakthrough (Gan et al., 1995a). The number of tubes should be increased when a high flow rate or a long sampling interval is used. Gan et al. (1995a) found that for a single 600 mg coconut charcoal tube at a flow rate of 100 ml min-1, a sampling interval of #2 h resulted in no breakthrough loss. |
| |
| Methyl bromide adsorbed in charcoal tubes may be analyzed by two different methods: solvent extraction followed by injection from the solvent phase, and the so-called headspace-GC method. In solvent extraction, charcoal is transferred into a vial, a known amount of extracting solvent such as carbon bisulfide (CS2) is added into the vial, and the vial is sealed (Eller, 1984; Lefevre et al., 1989). After the solvent-charcoal mixture is mechanically shaken, an aliquot of the solvent is injected into a GC. This method has the drawbacks of manual sample preparation, and presence of other compounds in the final sample solution that may elute with or interfere with MeBr during chromatography (Gan et al., 1995b). This method allows for multiple injections of each sample so that multiple analytes may be measured using different methods or detectors. |
| |
| An alternative method is the headspace-GC method. In headspace-GC analysis, the charcoal is equilibrated with an organic solvent in a closed headspace vial at an elevated temperature for a given period of time, and an aliquot of the headspace containing the analyte is then introduced into the GC column for detection. Benzyl alcohol is often used as the solvent due to its high boiling point (210EC) (Woodrow et al., 1988; Gan et al., 1995b). When the vial size, solvent volume, and equilibrating temperature and time are fixed, automated headspace injectors give high reproducibility and sample throughput. Gan et al. (1995b) found that the equilibration temperature and time in the headspace autosampler, the size of headspace vials, as well as the amount of solvent all had an effect on the signal output for a given sample. The sensitivity of analysis can thus be maximized by choosing an optimal combination of these parameters. For instance, to analyze a sample tube containing 600 mg coconut charcoal, the best conditions were determined to be: 9-ml headspace vials; 1.0 ml benzyl alcohol; 110EC equilibration temperature, and 15 min equilibration time (Gan et al., 1995b). Using this method, analysis of a MeBr-containing sample tube takes only 3-4 min, and as many as 300 samples can be analyzed within 24 h. This method is appropriate for analyzing samples from large-scale field studies measuring MeBr volatilization, when a large number of samples is required (Yates et al., 1996bc; Yates et al., 1997; Wang et al., 1997a). This method has the disadvantage of being destructive, where each charcoal sample can be analyzed with only a single injection. |
| |
| There are many kinds of porous polymer adsorbent material that have been used for collecting volatile compounds in the air, and these include the Chromosorb series, the Porapak series, Ambersorb XE-340, and others. The most popular adsorbent, however, is Tenax-GC, which is a polymer of 2,6-diphenyl-p-phenylene oxide. Brown and Purnell (1979) estimated the safe sampling volume for MeBr to be 0.14 L for sample tubes packed with 0.13 g Tenax-GC. When coupled with a cryofocusing technique, the whole sample can be introduced into the GC column following thermal desorption, which greatly enhances the sensitivity. Detection limits of 500 pg/L (Krost et al.,1982) and 35 ng (Dumas and Bond, 1985) were reported when this method was used. Compared with charcoal tubes, polymer samplers need to be conditioned before sampling, the safe sampling volume is smaller, the cost is higher, and each analysis takes a longer time. |
| |
Other Methods |
| Other than the container method and the adsorbent method, cryogenic concentration in a cold trap has also been used for collecting MeBr (Kallio and Shibamoto, 1988; Kerwin et al., 1996). The cold traps include mixtures of dry ice-acetone, liquid nitrogen and dry ice-2-propanol. The weakness of this technique is the long time and many steps involved in handling one sample, but it is useful when sample throughput is not a factor and very low detection limits are sought. |
| |
| When extremely high sensitivity is pursued, such as in the case of monitoring MeBr in ambient air, a technique called O2-doping could be useful (Grimsrud and Miller, 1978; Kerwin et al., 1996). Grimsrud and Miller (1978) first reported that addition of a fraction of O2 in the carrier gas drastically increased the sensitivity of ECD detection of halogenated methanes including MeBr. When 3-5% of O2 was added to the carrier gas, signal response was enhanced about 2 orders of magnitude for MeBr. Using cryogenic concentration and O2-doping, Kerwin et al. (1996) reported a detection limit as low as 0.23 pmol or 22 pg. |
| |
|
Printable Version
E-mail this page
