Desorption Efficiency Determination

Transfer the primary section of XAD-4 resin together with the associated glass wool plugs from 20 sample tubes to 20 2-ml vials. Spike five vials with 10-pl portions of alkaloid secondary standard solution, five vials with 20-pl portions, and five vials with 50-pl portions. The remaining five vials are blanks. Cap and store all vials in the same manner in which samples are handled. Uncap, add quinoline (internal standard) solution, desorb with solvent, and analyze all spiked samples and blanks as described above. Correct the results obtained on spiked samples by subtracting the blank average from the average for each spiked level. Calculate desorption efficiency (DE) for each alkaloid as follows  

If DE is different for the three levels prepared (ANOVA or Mest, P 0.05), construct a plot of DE vs. weight of alkaloid found. If DE is equivalent for the three levels, but different from 100%, (Mest using pooled results, P 0.05), use the pooled arithmetic mean for DE correction. DE should be determined two or more times until consistency is demonstrated. 

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Evans, P. R., Horstman, S. W. Desorption Efficiency Determination Methods for Styrene Using Charcoal Tubes and Passive Monitors, Am. Ind. Hyg. Assoc. J. 42, 471 (1981)... 

Determination of Desorption Efficiencies in the 3M 3500 Organic Vapor Monitor," presented at American Industrial Hygiene Conference, Houston, Texas, May 20, 1980. 

DCP in 15% (v/v) acetone in cyclohexane. In other tests, desorption efficiency was determined prior to capacity tests. 

A series of experiments was also performed to determine the sample blank and the optimum extraction time. When required, several solvents were evaluated to obtain optimal extraction efficiencies. A desorption efficiency of at least 0.8 was required. 

Determination of Accuracy and Precision of the Analytical Procedure. The desorption efficiency was determined for each method at widely separated analyte quantities to establish the average recovery to be expected. The spiking and analysis procedures for these tests were similar to those described earlier for the preliminary desorption efficiency tests. For HCCP and... 

Once the desorption efficiency is determined, the effects of other factors can be examined by laboratory experimentation and field validation. 

The most common technique in determining desorption efficiency is to inject the compound or a solution of the compound directly into the solid sorbent (13). The mixture is allowed to stand overnight and then desorbed and analyzed. Gases and highly volatile compounds are usually introduced as a mixture in air or nitrogen from a SARAN film bag or a cylinder. 

Once is determined, the desorption efficiency can be calculated for any solid and liquid ratio. Another equation... 

Another useful equation ( 15) has been derived which can be used to determine the ratio n necessary to obtain a desired desorption efficiency Z ... 

Determination of Desorption Efficiencies" and Some Exensions for Use in Methods Development ... 

Desorption efficiencies were determined for each sampling device in our laboratory. Analysis of passive dosimeter samples and calculation of concentrations were done as recommended by each device s manufacturer 07, 8). Charcoal tube samples were analyzed as recommended by NIOSH Sampling and Analysis Method... 

For accurate analysis, we must know the total volume of air passed through the adsorbent tube, the mass of the analyte trapped, and the desorption efficiency of the solvent. Before sampling, the pump must be calibrated using a bubblemeter, a rotameter, or a gasometer to determine the flow rate. Using the flow rate and the time sampled, the total volume of air sampled can be determined. 

Air analysis for some of the individual pesticides of this class has been published by NIOSH. These pesticides include mevinphos, TEPP, ronnel, malathion, parathion, EPN, and demeton (NIOSH Methods 2503, 2504, 1450). In general, pesticides in air may be trapped over various filters, such as Chro-mosorb 102, cellulose ester, XAD-2, PTFE membrane (1 pm), or a glass fiber filter. The analyte(s) are extracted from the filter or the sorbent tube with toluene or any other suitable organic solvent. The extract is analyzed by GC (using a NPD or FPD) or by GC/MS. The column conditions and the characteristic ions for compound identifications are presented in the preceding section. Desorption efficiency of the solvent should be determined before the analysis by spiking a known amount of the analyte into the sorbent tube or filter and then measuring the spike recovery. 

For the study of desorption efficiency, or recovery, of vinyl acetate from the charcoal, known amounts of vinyl acetate, either neat or in solution in cyclohexane, were metered onto 100-mg beds of charcoal. The samples were desorbed with 1 mL of carbon disulfide after 1, 5, or 15 days storage at room temperature. The resulting solutions were analyzed by gas chromatography to determine the amount of vinyl acetate that was desorbed. The desorption efficiencies were then calculated according to the following equation ... 

Other corrections that must be considered are the collection efficiency of the charcoal tube and the desorption efficiency of carbon disulfide for this specific solvent. TABLE 1 lists the recommended collection tube for each solvent, flow rate to be used in samplings, and desorption efficiency of many organic compounds. (6) The desorption efficiency of carbon disulfide with the charcoal tubes can be determined by injecting a known amount of solvent onto the charcoal. At least five charcoal tubes are sampled and the 100 mg portion removed and placed in a septum sealed vial. A concentration applicable to the threshold limit value of the organic solvent in question is injected onto the 100 mg of charcoal by piercing the septum cap with a microliter syringe. Several concentrations of solvent should be checked to determine the variation in desorption efficiency with solvent concentration. In like manner, standards are prepared by adding the same amount of solvent to the carbon disulfide solution in the vial. The standards are analyzed with the samples. The percent desorption efficiency (D.E.) is determined as ... 

The affinity that the solvent vapor has for the activated charcoal or the charcoal s adsorptivity is reflected in the "collection" efficiency. Early studies(i> 2) show that for many solvents, collection efficiencies are similar. No generalization is without exception and therefore test atmospheres should be generated where this information is important. The collection efficiency and desorption efficiency, together with the analytical precision and accuracy are incorporated into the total coefficient of variation for the method. Many solvent vapor sampling methods are not this thoroughly documented in the literature because of the difficulty of generating known test atmospheres. In this study both direct injection and flowing of vapor-air mixtures over the charcoal were used for efficiency determinations these values are reported in table 1 and required much time and effort to obtain. 

The unique properties of carbon nanotubes include adsorbent characteristics that offer the prospect of yet more efficient hydrogen storage. Dillon et al. [64] used temperature programmed desorption to determine the hydrogen storage capacity of single-wall nanotubes. The authors predict a storage capacity of —50 kg Hi/m at ambient temperature and pressures for nanotubes with 20 A diameters. 

Factors which cause changes in recovery are insidious, and unless these are understood and can be related to the chemical and physical properties of the compounds collected, errors can go undetected. The desorption efficiency is the most significant of the factors in defining the sorption-desorption system. Although desorption efficiency cannot always be isolated, it can be determined experimentally and is one of the first indicators of a potential problem in a suggested method. 

Once K is determined, the desorption efficiency can be calculated for any solid and liquid ratio. Posner further developed this concept and derived several other equations for calculating the desorption efficiency for any solid and liquid ratio and for determining the ratio necessary to obtain a desired desorption efficiency. Equation (5) is a modification of Eq. (5) and can be used to calculate the expected desorption efficiency when the volume is changed. 

It should be noted that the partition ratio at equlibrium predicts the optimum desorption effiency attainable, and other experiments may be necessary to detect nonequlibrium situations. Desorption efficiency should not be taken as the recovery since other factors may have a significant effect. After a solvent/sorbent system is selected and tested using the phase equilibrium technique, direct injections of the test compound are made into collection tubes with and without air being pulled through. If the desorption efficiencies as determined by direct injection are considerably lower than phase equilibrium values, interaction or reaction on the sorbent surface is indicated. If the total recovery from the simulated air collection is lower than the direct injection efficiency (even through no breakthrough has occurred) hydrolysis, oxidation, or another reaction may indicated. 

A number of studies on desorption efficiency and recovery have been reported in the literature. Krajewski compared three methods of determining desorption efficiencies to dynamically prepared standards and found the phase equilibrium data somewhat higher in most cases. Evans and Horstman also showed that the recovery of styrene from dynamically sampled tubes was 18% lower than the direct spike method. They later showed that this was not due to a storage problem since they found no significant change in the recovery of styrene after six days Other researchers reported the effects of co-adsorbed compounds on... 

The solid sorbents are desorbed with a suitable solvent in a similar manner as discussed for tube/pump sampling. The desorption efficiency (DE) must also be determined for each compound. Once the mass of the contaminant (M) is determined, its time-weighted-average concentration (C) in air can be calculated ... 

Other sorbents are not compound specific and, as a result, trap a wide range of compounds. Unless specifically desired (see later in this section the discussion of DNPHcompounds collected do not react with the sorbent. It is an unfortunate reality that the efficiency of recovery of most adsorbed compounds from the sorbents is less than 100%. For this reason, the efficiency of the sorbent for the desired compounds and the extraction or desorption efficiency of the compounds from the sorbent must be determined. 

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