Volatility samples

For the charcoal, XAD, and PUF adsorbents discussed above, solvent extraction techniques have been developed for the removal and concentration of trapped analytes. Although thermal desorption has been used with Tenax-GC in some specialized air sampling situations [primarily with sampling volatile organic compounds (EPA, Method TO-17 )], this approach is not a viable alternative to solvent extraction for the charcoal, XAD, and PUF adsorbents. The polystyrene and PUF adsorbents are thermally unstable and the charcoal chemisorption bonding is more easily broken by... 

Headspace methods provide an indirect method of sample analysis suitable for the determination of organic volatiles [11,318-323]. The gas diase in contact with the sample and not the sample matrix itself is taken for analysis. If the sample is in thermodynamic equilibrium with the gas phase in a closed thermostated vessel, then this method of analysis is referred to as static headspace. If a carrier gas is passed over the sample and the sample volatiles accumulated in a cryogenic or sorbent trap, then the method is generally referred to as dynamic headspace. If the carrier gas is introduced below the surface of... 

Unfortunately, the preset split flow ratio is only an approximate indication of the sample split ratio. The latter depends in a complex way on many parameters, including the range of sample volatilities, sample solvent, volume of sample injected. 

There are several means of introducing samples into the mass spectrometer. The inlet system to be used depends mainly on the nature of the sample (volatility, molecular weight, polarity and thermal stability), and the ionisation method (in particular, the gas pressure in the ion source). The most common inlet systems are ... 

E. E. Stashenko, J. R. Martinez, Sampling volatile compounds from natural products with headspace/solid phase micro extraction, J. Biochem. Biophys. Methods, 70, 235 242 (2007). 

For the majority of applications, the sample is taken into solution and introduced into the plasma as an aerosol in the argon stream. The sample solution is pumped by a peristaltic pump at a fixed rate and converted into an aerosol by a nebulizer (see atomic absorption spectrometry). Various designs of nebulizer are in use, each having strengths and weaknesses. The reader is directed to the more specialist texts for a detailed consideration of nebulizers. There is an obvious attraction in being able to handle a solid directly, and sample volatilization methods using electric spark ablation, laser ablation and electrothermal volatilization have also been developed. 

HPLC is limited by sample volatility or thermal stability. It is also ideally suited for the separation of macromolecules and ionic species of biomedical interest, labile natural products, and a wide variety of other high molecular weight and/or less stable compounds (see Section 15.12). 

This technique is complementary to the thermospray technique. Relative advances of the particles beam technique over thermospray include library searchable electron impact spectra, improved reproducibility, easier use and increased predictability over a broad range of compounds. But since a particle beam requires same sample volatility, very large and polar compounds such as proteins may not provide satisfactory results using particle beam liquid chromatography-mass spectrometry. Additionally, certain classes of compounds such as preformed ions, azo dyes and complex sugars may not yield satisfactory electron impact spectra, but can be run on thermospray. In other words, both liquid chromatography-mass spectrometry techniques complement each other s limitations and the analyst may want to add both to address a broader range of samples. 

Several sampling procedures are applicable to volatile compounds but method application often depends on the compound(s) to be sampled (Dean, 2003). Part of the issue of sampling volatile compounds arises because some volatile substances sublime rather than boil, whereas other volatile substances emit significant quantities of vapor well below their boiling point. For sampling volatile hydrocarbons in the field, two procedures are generally recommended zero headspace and solvent extraction. However, these two procedures do not necessarily give equivalent results. 

Testing residual fuel oil does not suffer from the issues that are associated with sample volatility but the test methods are often sensitive to the presence of gas bubbles in the fuel oil. An air release test is available for application to lubricating oil (ASTM D3427 IP 313) and may be applied, with modification, to residual fuel oil. However, with dark-colored samples, it may be difficult to determine... 

At the Department for Chemical Ecology we have developed adsorption techniques for sampling and analyzing of volatile components in air. These gas chromatographic (GO methods have been used in allelochemical research, i.a. for analyses of volatiles emitted from plant leaves (1,2). We believed that such an adsorption method could be adapted for sampling volatiles in the soil by allowing for the high humidity in the soil and its effect on the adsorbent. 

De Raat, W. K., F. L. Schulting, E. Burghardt, and F. A. de Meijere, Application of Polyurethane Foam for Sampling Volatile Mutagens from Ambient Air, Sci. Total Environ., 63, 175-189 (1987). 

Faldt, J., Eriksson, M Valterova, I. and Borg-Karlson, A.-K. (2000). Comparison of headspace techniques for sampling volatile natural products in a dynamic system. Verlag der Zeitschrift flir Naturforschung 55c 180-188. 

If sample tube stoppers become loose during a run (a possible problem caused by the pressure of sample volatility), they can be secured using electrical tape placed around the neck of the sample tube and over the stopper top, between the tubes inserted in the stopper. 

In the case of foaming materials, the distillation must be carefully watched. Additional antifoaming agents can be used, but they will be coextracted and may cover some sample volatiles on the GC profile. Their use is not recommended. 

Basic Protocol 3 is a rapid GC method to evaluate limonene purity. The needle is coated with a residual limonene layer in the syringe. The sample volatilizes into the column in which the sensitivity of GC detectors (FID) provides a reliable indication of purity. This is known as a wet needle injection. 

The study described here demonstrates that ESCA provides information regarding the chemical nature of the surface of an unperturbed sample which would be difficult to acquire by other methods. A major weakness of ESCA, the necessity of exposing the sample to vacuum, together with its attendant problem of sample volatilization, can also be one of its strengths. The volatility of some nitrogenous species in atmospheric aerosol particles can be used to provide strong evidence for chemical identity of ionic compounds (e.g., ammonium nitrate) rather than simply ionic identities as provided by wet chemical methods. This volatility is accelerated by x-ray irradiation, so that similar results could be achieved only by extended vacuum exposure alone if another analytical technique were used. Also, with ESCA, volatile losses can be conveniently monitored since the sample remains in the spectrometer throughout the process. 

Sampling volatile analytes from samples having complex matrices usually takes place in the HS-SPME mode. This variant yields decidedly better results in the determination of aroma compounds59 and other volatile components.60 Moreover, HS-SPME prolongs the life of the fiber because it is not in direct contact with the sample. On the other hand, the direct extraction of less volatile compounds from solution is possible using DI-SPME. But in this case, the fiber deteriorates more quickly, increasing the cost of analysis. Headspace sampling is therefore employed whenever possible. 

The factor that makes FAB-MS so different from EI-MS is that, in its usual form, the sample coating the probe tip consists of a solution or suspension in a relatively nonvolatile matrix liquid such as glycerol. This provides for a continually renewed surface exposed to the atom beam and thus spectra that are stable over a period of many minutes. No heating of the sample is required other than the localized energy implanted in the sample by the atom beam. Although complications may result from interactions with the matrix liquid, they are often less than, or certainly no worse than, such complications as thermal decomposition or ion molecule reactions, involved in other techniques for sample volatilization. In addition, FAB-MS is looking at condensed-phase systems similar to those investigated by NMR or IR. Thus perhaps the data are easier to correlate. Several reviews or introductions to the method have appeared (4, 7-9,13, 15-22). 

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