P T technique

LODs obtained with P T techniques are typically in the ng/L to low pg/L range and thus often 100 to more than 1000 times lower than those achieved with static HS techniques. P T does have shortcomings, however. The main ones are95... 

The P T technique is frequently used for determination of volatiles in food and beverages [40, 41], plants [42], natural water and wastewater [43, 44], sediments [45], and cow slurries [46]. The technique is widely recommended by the EPA. 

Before analysis, the bacterial cultures should be transferred into standard 20 ml headspace vials and sealed with PTFE-lined Teflon caps to equilibrate the headspace. Sample handling is a critical step affecting the analysis by E-nose. The quality of the analysis can be improved by adopting an appropriate sampling technique. To introduce the volatile compounds present in the headspace (HS) of the sample into the E-nose s detection system, several headspace sampling techniques have been used in E-nose. Typically, the methods of headspace sampling (Ayoko, 2004) include static headspace (SHS) technique, purge and trap (P T) technique, stir bar sorptive extraction (SBSE) technique, inside-needle dynamic... 

If this author is asked to estimate the concentration level of VOCs and static HS or P T techniques are not available, he would employ Hxd-FFE EPA Method 3820 from the SW-846 series is a useful starting starting point for this LEE technique. Seven different approaches to preparing environmental samples to determine VOCs are presented in SW-846. We will discuss each of these with an emphasis on the operational aspects. 

The P T method to isolate, recover, and quantitate VOCs in various environmental sources of water has and continues to remain as the premier technique for this class of environmental contaminants. The technique was developed at the EPA in the early 1970s (36) and remains the method of choice, particularly for environmental testing labs that are regulatory driven. P T has had its most success with drinking water samples when combined with gas chromatography and element-specific detectors. GC detectors will be discussed in Chapter 4. In this section, we will discuss the EPA methods that use the P T technique to achieve the goals of TEQA. EPA Method 502.2 summarizes the method as follows (37) ... 

The P T technique is recommended as an extraction technique for VOCs in several standard methods (Table 23.3). The US EPA has proposed different standard protocols for the analysis of volatiles in water using P T. These methods can be used for most of VOCs that have boiling points below 200°C and are insoluble or slightly soluble in water. The type of sample matrix being analyzed determines the implemented configuration of the extraction technique. The 500 series EPA methods are addressed to potable waters, whereas the 600 series refer to analysis of wastewaters. The analytical methods for determining hazardous waste are known as the 8000 series methods (US EPA SW-846). 

Water is evaporated and trapped from aqueous or moist samples as well. Most of which is disposed of by the dry purge step, particularly when using Tenax adsorption traps with low water retention. Residual moisture can still be transferred to the GC column during the desorption step (Madden and Lehan, 1991). As the resolution of highly volatile substances on capillary columns would be impaired and the detection by the mass spectrometer would be affected, additional devices are used to remove water. In particular, where the P T technique is used with ECD or MS as detectors, reliable water removal is necessary. Different technical solutions working automated during the desorption phase are in use with the P8dT instruments of different manufacturers. 

How then do the techniques differ For this, the terms recovery and sensitivity must be defined. For both methods, the recovery depends on the vapour pressure, the solubility and the temperature. The effects of temperature can be dealt with because it is easy to increase the vapour pressure of a compound by raising the temperature during the vaporization step. With the P T technique, the term percentage recovery is used. This is the amount of a compound which reaches the gas chromatograph for analysis relative to the amount which was originally present in the sample. If a sample contains 100 pg benzene and 90 pg reach the GC column, the percentage recovery is 90%. In the static headspace technique, a simple expression like this cannot be used because it is possible to use a large... 

A comparison with actual concentration values makes the differences between the static headspace and P T techniques very clear. The percentage recovery for... 

Comparable ratios are obtained in the analysis of a solid sample, for example, the analysis of residual solvents in a technical product. A run using the P T technique and 10 mL of sample at 150 °C gave a recovery of 63% for toluene. The sample contained 1.6 ppm, which corresponds to a quantity of 101 ng. The partition coefficient in the static headspace technique at 150 °C (for a sample of 1 g) is 95. The quantity of residual solvent in 19 mL of headspace is therefore 17 ng. For an injection of 0.5 mL, 0.4 ng are injected. The quantity injected is therefore smaller by a factor of 250 than that in the P T analysis. Furthermore, the reproducibility of this analysis was 7% for the P T technique and 32% for the static headspace analysis (RSD). 

The analysis of seepage water (Figure 4.10) and the condensate residue from the incineration of gas from landfill sites (Figure 4.11) are two examples of the effective simultaneous control of the volatile halogenated hydrocarbon and BTEX concentrations. The P T technique was chosen here so that as wide a spectrum... 

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