26
February / March 2012
Trace Level Analysis of Aqueous Samples by Dynamic Headspace
by Dan Carrier, Applications Chemist, Anatune Ltd
Dan.carrier@
anatune.co.uk
Static Headspace analysis [1] is the most common sample introduction technique for volatile analysis by Gas Chromatography (GC). It works by analysing the vapour above the sample at equilibrium. However, for detection of trace level analytes, this may not be the most suitable method and some form of enrichment may be necessary. For certain analytes, over a 100 fold increase in sensitivity can be achieved with Dynamic Headspace (DHS) analysis when compared to Static Headspace analysis. This article compares DHS with other sample enrichment techniques such as Solid Phase Microextraction (SPME), and Stir Bar Sorptive Extraction (SBSE) using Twister stir bars.
Introduction
In this article, a herbal based liquor and three different orange juices were chosen for enrichment purposes. Both sets of drinks are aqueous based and require some form of enrichment and removal of water for trace level analysis by GC.
Slight differences in the ingredients can drastically change the taste of a drink. The unique smell and taste of different brands of drinks often are due to minor differences in their volatile components. Therefore, it is important to maintain a consistent composition of the drinks to ensure customer satisfaction. Many of the analytes in these drinks are volatile and most are present at trace level. Some of the trace level analytes cannot be detected with Static
Headspace analysis. DHS offers an approach to continuously enrich the analytes making it possible to detect and quantify them by Mass Spectrometry (MS).
This article compares DHS with other sample enrichment techniques such as Solid Phase Microextraction (SPME), and Stir Bar Sorptive Extraction (SBSE) using Twister stir bars.
In DHS, a sample is continuously purged with an inert gas, usually the GC carrier gas, and the volatile compounds are continuously retained onto an adsorptive trap. The trap can then be dried to remove any residual water which may have been collected. Reducing the amount of water is necessary to obtain good chromatography of the analytes which have been enriched. Figure 1 shows a schematic view of the automated
DHS Process.
Firstly, the sample is heated to a required temperature. This allows the sample to form an equilibrium between the liquid and the gas phase in the same manner as Static Headspace analysis. A dual needle then pierces the septa and an inert gas, usually helium, is passed through enabling analytes to be trapped onto the adsorptive trap. Some residual water may have been retained on the trap. Therefore, there is an option to dry purge the trapped sample. If this is required, the dual needle pierces a clean vial and the adsorptive trap is then dried with a set volume of helium at a specified temperature. The trap, containing sample analytes, is transferred into the thermal desorption unit (TDU). A fast temperature ramp is used to desorb the extracted analytes from the TDU onto the Cold Inlet System (CIS) which is set at a cold temperature to focus the analytes of interest. Once the analytes have been focussed into a tight band on the CIS, another fast temperature ramp is used to desorb the extracts onto the GC column.
Figure 1: Schematic view of the automated DHS process
Full evaporation technique (FET) is used to enhance the detection of volatile analytes by DHS. A low volume aliquot of each drink is placed into an empty headspace vial. The vial is heated to 80°C allowing the analytes in the sample to vaporise whilst leaving the low volatile matrix behind. The FET technique is performed by using a small volume of sample and vaporising the analytes in the headspace vial completely, without having to rely on establishing equilibrium between two
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60
