Charcoal sorbents

The charcoal sorbent, used to monitor worker exposures, is desorbed with 0.04 N NaOH and the sulfuryl fluoride undergoes partial hydrolysis ... 

Notes Useful for compounds that cannot be recovered from the charcoal sorbents the most serious problem with silica is the effect of water, which can cause desorption of the analytes of interest, and the heating effect involved can sometimes initiate reactions such as polymerization or hydrolysis of the analyte... 

Air (benzene, ethyl benzene) Collect sample on charcoal sorbent desorb with carbon disulfide GC/FID 10-100 ppb NR NIOSH 1984c (Methods 1500 and 1501)... 

The rationale for the development of such fibers is demonstrated by their appHcation in the medical field, notably hemoperfusion, where cartridges loaded with activated charcoal-filled hoUow fiber contact blood. Low molecular weight body wastes diffuse through the fiber walls and are absorbed in the fiber core. In such processes, the blood does not contact the active sorbent direcdy, but faces the nontoxic, blood compatible membrane (see Controlled RELEASE TECHNOLOGY, pharmaceutical). Other uses include waste industrial appHcations as general as chromates and phosphates and as specific as radioactive/nuclear materials. 

Sorbent tubes Small glass tubes that contain sampling media such as silica gel or activated charcoal. 

Prior to the development of modern SPE formats, liquid-solid partitioning with charcoal, silica, Florisil, and/or alumina was common to aid in the removal of lipids in the determination of nonpolar pesticides, but these sorbents are less useful in the cleanup of semi-polar and polar pesticides owing to the large elution volumes needed. Applications of modern SPE are discussed in Section 3.2. 

Various forms of carbon are used to sample those analytes whose breakthrough volume is too low on Tenax for sufficient preconcentration [8,395-399]. Charcoal, graphitized carbon blacks, and ceurbosieves with wface areas from 5 to 900 w /g are commercially availablJ Bhe high surface area sorbents are used... 

Filter through a 0.5-pm Zefluor absorb on a solid sorbent tube containing coconut shell charcoal desorb with ammonia hydroxide and hydrogen peroxide dilute. 

Air Adsorb to solid sorbent tnbe (e.g., charcoal) desorb in CS2 eqnilibrate inject aliqnot toGC GC/FID 0.1 mg/ 5-10 mL sample 96-106 NIOSH 1994... 

From another perspective chromatographic methods employing charcoal or other similsir carbonaceous sorbents can also be considered as pre-dav PIV I VdlinvInB the rmit sUitHr--Im Al driltald tt... 

Since charcoal is such a good sorbent and is readily available, the solution to some sampling problems is to find a way to increase the recovery of that compound from charcoal. One way is by increasing the solvent/sorbent ratio as discussed in the phase equilibrium section. Two other approaches are the use of mixed solvents and the two-phase solvent system. 

Other solid sorbents have been found more suitable than charcoal for a number of compounds. Silica gel and alumina have been used as a complement to charcoal when sampling polar compounds, but water vapor is strongly adsorbed on these sorbents which leads to deactivation of the sorbent and breakthrough of the compounds by frontal elution. Difficulties also arise with compounds that hydrolyze easily. Alternative sorbents for the collection of polar organic compounds which are sensitive to hydrolysis are porous polymers such as the Chromosorb porous polymer series, Porapak porous polymer series, Tenax-GC and Amberlite XAD sorbent series. 

Carbonized Resins. A special sorbent made by controlled thermal pyrolysis of polyvinylidene chloride (Dow developmental Adsorbent XF-4175L) (34) was shown to be three to five times more effective for the collection of highly volatile compounds, such as vinyl chloride (Figure 5) and methyl chloride, than the best available activated charcoal (31,36,37). Although this sorbent is not commercially available, Carbosive and Carbosive S show similar collection properties and they are available from gas chromatographic supply houses or may be obtained already packed in small collection tubes (SKC Inc., Eighty Four, PA). 

The sorbent tube is placed near the worker s breathing zone, and the outlet of the tube is attached to a calibrated personal sampling pump. A known volume of air is drawn through the tube. Alternatively, several passive charcoal badges are currently commercially available. No sampling pump is required for these devices,... 

Surface Area and Mesh Size. 20/40 mesh sorbent is generally used to minimize the pressure drop across a sorbent tube. Some sorbents, such as Tenax-GC, are not available in these mesh sizes, but can still be used by increasing the cross-sectional area of the sampling tube to lower the overall pressure drop for these smaller sorbent particles. Surface areas vary from very low in Tenax-GC to over 1000 m2/g in activated charcoal. In some cases, the greater surface area may increase the capacity of the sorbent. 

Table I shows some of the characteristics of the commonly used sorbents. The charcoals are by far the most frequently used solid sorbent for organic vapors. Over 130 methods have been validated in our five-year study using coconut, petroleum, and synthetic charcoal. The other sorbents include silica gel, used primarily for amines, and porous polymers, used for substances not amenable to collection on charcoal or silica gel. Other researchers have used other sorbents, including Florisil, alumina, and molecular sieves.

Chemical Properties. If a substance is highly polar, it may be readily collected by charcoal, but it may be difficult or even impossible to recover it. Also, some substances may be readily hydrolyzed and it may be best to collect these on hydrophobic sorbents like a porous polymer. 

The sorbents most commonly used in industrial hygiene sampling are charcoal, silica gel, and the porous polymers. In addition, a number of methods have been developed using coated sorbents. Each sorbent is discussed briefly in this section. 

Charcoal. Activated coconut charcoal has gained the status as the almost universal solid sorbent. Petroleum-based charcoal is less active, but is also widely used. Charcoal is a very effective sorbent and is generally used for collection of nonpolar organic solvent vapors. It also collects polar organics, but they frequently cannot be recovered. However, many organic substances that are reactive, polar, or oxygenated (e.g., chloroprene, acetic acid, and acetone) have been successfully collected and recovered from charcoal. Substances for which charcoal tube methods have been validated are listed in Table II. 

---