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Chromatografy Methods for Environmental - Ando D.J.

Ando D.J. Chromatografy Methods for Environmental - Wiley publishing , 2003. - 265 p.
Download (direct link): chromatography2003.pdf
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Purge gas in
Trap
Vent
GC column
Figure 9.3 Illustration of a typical layout for purge-and-trap extraction of volatile organic compounds from aqueous samples - in ‘purge mode’: —? indicates sample pathway.
Volatile Compounds 169
Purge gas in
Figure 9.4 Illustration of a typical layout for purge-and-trap extraction of volatile organic compounds from aqueous samples - in ‘desorb mode’: —? indicates sample pathway.
with nitrogen to send the sample to the GC column. Typical desorption times are
2-4 min, with nitrogen flow rates of 1-2 mlmin-1, for narrow-bore columns. This allows the desorption of volatile organic compounds in a narrow band. The desorbed compounds are transferred via a heated transfer line to the injector of a gas chromatograph for separation and detection. In order to maintain the integrity of the trap, it is periodically cleaned. This is achieved by heating the trap to remove contaminants and residual water. A higher temperature than the desorb temperature is used for ca. 8 min. A typical procedure used for the thermal desorption (purge-and-trap) of aqueous samples is described in Figure 9.5.
9.3.1 Example 9.1: Purge-and-Trap Extraction of BTEX from Water
9.3.1.1 Extraction Conditions These were as follows:
Sample volume: 2-10 ml
170
Methods for Environmental Trace Analysis
Figure 9.5 A typical procedure used for the purge-and-trap (thermal desorption) extraction of volatile organic compounds from aqueous samples.
• Purge-and-trap conditions: sample sparged for 2-5 min using N2; BTEX mixture trapped on ‘Tenax’ trap maintained at 20oC for 1-5 min; analytes desorbed by rapid heating to 260oC for 1 min
Comments GC column initially maintained at 50oC to concentrate analytes.
9.3.1.2 Analysis by GC
Separation and identification of the BTEX mixture was carried out on a Carlo Erba HRGC 5300 Mega Series gas chromatograph, with split/splitless injection and flame ionization detection. A 30 m x 0.25 mm id x 0.1 ^m film thickness DB-5 capillary column was used, with temperature programming from an initial temperature held at 50°C for 3 min before commencing a 16°Cmin-1 rise to 120°C, with a final hold time of 7 min. The detector temperature was set at 250°C.
9.3.1.3 Typical Results
These are shown in Figure 9.6 [1].
Volatile Compounds
171
(a)
(b)
Concentration (ng ml 1)
Sample volume (ml)
Figure 9.6 Results obtained for the purge-and-trap extraction of BTEX from water, showing (a) the calibration graphs, and (b) the influence of sample volume: ?, benzene; ?, toluene; ?, ethylbenzene; x, m-, p-xylene; ?, o-xylene [1] (cf. DQ 9.4).
DQ 9.4
Comment on the results obtained in this study (see Figure 9.6).
Answer
Linear calibration graphs (Figure 9.6(a)) are shown for all of the BTEX components in the 0-10 ng ml-1 range. In addition, it is observed (Figure 9.6(b)) that the larger the sample volume, then the larger the signal. This situation is obviously important if you are seeking to carry out trace analysis. For such analysis, more sample is required in order to achieve a lower detection limit.
172
Summary
Methods for Environmental Trace Analysis
The methods described in this chapter are rather specialist in nature and you may not come across them in the undergraduate laboratory. However, their importance cannot be underestimated in the ‘real’ world where contamination from volatile organic compounds can cause health problems and hence require regular monitoring.
References
1. Leconte, A., ‘Comparison of purge-and-trap-GC with headspace solid-phase microextraction-GC for the analysis of BTEX in water’, MSc Dissertation, Northumbria University, Newcastle, UK, 1997.
Methods for Environmental Trace Analysis. John R. Dean
Copyright © 2003 John Wiley & Sons, Ltd.
ISBNs: 0-470-84421-3 (HB); 0-470-84422-1 (PB)
Chapter 10
Pre-Concentration Using Solvent Evaporation
Learning Objectives
• To understand the need for pre-concentration for organic compounds in organic solvents.
• To be able to carry out rotary evaporation in a safe and controlled manner.
• To understand the requirements for Kuderna-Danish evaporative concentration.
• To understand the requirements for an automated evaporative concentration system.
• To be able to carry out gas ‘blow-down’ in a safe and controlled manner.
10.1 Introduction
Pre-concentration is concerned with the reduction of a larger sample into a smaller sample size. It is most commonly carried out by using solvent evaporation procedures after an extraction technique (see, for example, Chapters 7 and 8). The most common approaches for solvent evaporation are rotary evaporation, Kuderna-Danish evaporative concentration, the automated evaporative concentration system (EVACS) or gas ‘blow-down’. In all cases, the evaporation method is slow, with a high risk of contamination from the solvent, glassware and blow-down gas.
174 Methods for Environmental Trace Analysis
DQ 10.1
When would solvent evaporation be required?
Answer
Solvent evaporation would be used when you are trying to perform ultra-trace analysis. It allows a sample extract to be further preconcentrated. An example would be analysis of pesticides in natural water. Initially, solid-phase extraction may have been performed to concentrate the pesticides from the water. However, to detect pesticides in natural waters will require additional pre-concentration via solvent-evaporation procedures.
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