GC Application Note How to Develop a Method using In-tube Extraction (ITEX) for GC and GC/MS? www.palsystem.com How to Develop a Method using In-tube Extraction (ITEX) for GC and GC/MS? Joeri Vercammen, Ph.D. www.is-x.com Introduction Throughout our lives we are continuously exposed to substantial quantities of volatile organic components (VOCs). Residual solvents from carpets, paints and glues are part of the indoor air we daily inhale. Offices, workspaces and even our living rooms, it’s practically impossible to breathe clean air nowadays. Perhaps we are luckier with the food we eat and the beverages we drink? Unfortunately, I am afraid not. Solvents from packaging foils and printing inks may pose a severe risk for the consumer, while the water we drink or use to make soda, beers, etc is polluted with traces of desinfection agents and other environmental contaminants. No wonder ‘VOC analysis’ plays such a vital role in many environmental and QC laboratories around the globe. Gas chromatography (GC) really loves VOC analysis and VOC analysis loves GC. They form a steady couple and have a long and happy marriage with many offspring. It’s quite remarkable to see how some of these offspring have matured substantially throughout the years, whilst others seem to remain stuck in puberty. One of these techniques that has difficulties meeting expectations is the subject of this contribution. The technique is called in-tube extraction (ITEX) and was introduced about ten years ago as an option to the popular Combi-PAL platform from the Swiss manufacturer CTC Analytics. The solution was, primarily, developed to address the severe drawbacks associated with classic purge & trap instrumentation (see below) yet allowing equal sensitivity. The initial hardware was upgraded substantially 2 IngeniousNews 02/2013 and became much more user-friendly. Today, distributed as ITEX-2, the technique is truly modular and can be easily added to any Combi-PAL system. Before the introduction of ITEX, it required quite a lot of expertise and hocus pocus if you had the intention to report VOC concentrations below 0.25 ppb (= parts-per-billion level) with static headspace. Although this level might seem sufficient for the majority of applications, the enforcement of ever stronger regulations has made the classic headspace approach, i.e. with gastight syringe, more or less redundant. In order to compete with classic purge & trap instrumentation, one has to be able to detect VOC with a sufficient degree of certainty at low or even sub ppb levels. Typical reporting limits usually range between 0.01 and 0.02 ppb. Purge-and-trap analysis has been a popular approach for VOC analysis for several decades. Although very powerful, it suffers from quite some drawbacks as well. First of all, it is particularly well suited for the analysis of VOC in relatively clean samples, such as drinking water. However, when applied to the analysis of more contaminated samples, strict safety measures need to be taken. Due to the immense concentration effect of purge & trap, these systems are easily overloaded and contaminated, often by mistake. Particularly less volatile components, such as naphthalene tend to stick to valves, fittings and o-rings and elute very slowly from the dead volumes that are typical for these systems. In order to avoid elevated blank levels and/or regular system downtime, laboratories often include a sniffer step, which acts as a pre-screener to protect the system from unwanted exposure to high VOC amounts. Other points of attention have to be considered, though they are far less critical than system contamination and blank levels, e.g. blocking of the cold trap due to improper removal of residual water vapors and excessive sample foaming during purging. Many of these issues have been addressed by the introduction of ITEX-2 (details can be found here). ITEX-2 consists of a dedicated gastight syringe (1.3 mL), which is fitted with a needle with microtrap. The microtrap contains the packing material (or a combination of materials) that is used to enrich the volatile components from the headspace. A heating element is positioned around the microtrap to release trapped volatiles after sampling. The principle of enrichment is very simple and closely resembles classic static headspace analysis with syringe. First of all, it is recommended to equilibrate the sample at elevated temperature. Afterwards, the vial is punctured and its headspace sampled repeatedly. This is accomplished by slowly moving the plunger of the ITEX-2 syringe up and down. Finally, the device is moved to the GC injector, where trapped components are released by flash thermal desorption. Due to the relatively small amount of trapping material, particularly compared to purge & trap, it is possible to desorb the microtrap directly into a split/splitess injector without the need for cryofocusing. This avoids freezing of residual water vapor. How to develop an ITEX Method? Before starting with the actual development of an ITEX method, you have to decide which type of packing material you are going to use to trap analytes. This is mainly determined by the volatility of the components you would like to enrich. Please be aware that the material you finally pick can have a tremendous effect on the ITEX cycle time as well, but more on that later. Each ITEX-2 kit is delivered with two microtraps, each filled with approximately 40 mg Tenax TA. Since it does not retain water, Tenax is a very powerful trapping material, particularly for classic applications, such as BTEX in water. Unfortunately, more volatile target components, such as vinyl chloride for example, cannot be enriched by Tenax, even not at trap temperatures as low as 35°C. These components are simply much too volatile. Here, it is necessary to use a mixed bed microtrap. We usually suggest a trap that contains two separate beds, one Tenax TA and one Carboxen 1000 in a 1:1 ratio. The much stronger Carboxen bed is placed on top of the Tenax bed, such that components that are not retained on Tenax are trapped on the Carboxen. Why don’t we use a 100% Carboxen 1000 bed instead, you might ask yourself? The reason is simple. The Tenax plays an important role too, i.e. it serves as a protective layer to capture less volatile components (both target and/or matrix) that will destroy the Carboxen material permanently due to irreversible adsorption when directly exposed to it. The use of a mixed bed trap has another important consequence, as briefly highlighted in the beginning of this paragraph. Instead of using trap temperatures close to room temperature, mixed bed sampling can be carried out at temperatures as high as 50°C! Since it takes quite some time to cool down the microtrap after injection/conditioning, a lot of valuable (cycle) time can be recovered easily. Equilibration times and temperatures are determined similar to classic static headspace injections and will not be discussed here. Just one small suggestion; we usually do not wait for the sample to equilibrate. Five minutes of equilibration is usually enough for most applications, while ITEX sampling itself might take quite some time as well. Once again several tens of minutes can be recovered from the final ITEX cycle time. This is particularly important when throughput is an issue in your lab! ITEX variables that have an immediate impact on method performance include the number of sampling strokes, the plunger speed during sampling and the trap conditioning parameters. It’s quite clear that increasing the number of strokes will increase analyte response. The extent of this increase is, however, not really proportional to the number of strokes, since it depends not only on compound volatility but also on the complexity of the sample matrix (heavily loaded matrices induce displacement of analyte traces). At the same time, you should be careful increasing the number of sampling strokes, since it has a massive effect on total cycle time. Let’s assume, for example, an extraction speed of 25 µL/sec; extracting 1000 µL implies, in fact, 2000 µL per single stroke (one upward and one downward movement) or in total approximately 80 seconds in time per individual cycle. Five strokes will take 400 seconds, 20 strokes as much as 1600 seconds, which is almost half an hour! Bearing in mind that the cycle time of state-of-theart GC/MS methods for VOC is situated somewhere around 20 minutes, it’s quite clear that 20 strokes are unacceptable when you are aiming at a smooth and efficient process. Please note that when it’s necessary to actually sample 20 times to achieve sufficient sensitivity, it is probably much more convenient to consider a more powerful packing material and sample faster. Throughout the years we have implemented several ITEX projects. Today I will discuss the results of the analysis of sub ppb levels of VOC in heavily loaded waste waters. Other projects that we carried out include the analysis of trace levels of benzene in soft drinks, residual VOC in plastics, fire accelerants in suspected arson studies and others. Some typical extraction conditions are summarized in Table 1. These settings are based on the macro ‘ITEX_VolatileRev02’. We advise our customers to use this macro, which is part of the ITEX-2 kit, due to one of its particular characteristics. Namely the slow upwards plunger movement during trap heating, which reduces the loss of the most volatile target components of your sample. Parameter Setting Incubation temp, °C 75 Incubation time, min 5.00 Agitation speed, rpm 500 Syringe temp., °C 85 Extraction vol., µL 1000 # Extractions 20 Extraction speed, µL/sec 50 Desorption temp, °C 280 Desorption speed, µL/sec 750 Conditioning temp., °C 300 Conditioning time, min 3.00 Peak identification and results are summarized in Table 2. A typical chromatogram of a VOC standard at 10 ppb (GC/MS in SIM mode) is depicted in Figure 1, insert shows vinyl chloride at 0.1 ppb, 10 ppb and 100 ppb. Table 1: Typical settings for ITEX-2. IngeniousNews 02/2013 3 Tr, min Component Limit of Detection, ppb Linear range, ppb 1.15 2.01 Vinylchloride 1,1-Dichloroethylene 0.015 0.006 0 – 100 0 – 100 2.71 t-1,2-Dichloroethylene 0.012 0 – 100 3.44 1,1-Dichloroethane 0.019 0 – 100 4.21 c-1,2-Dichloroethylene 0.016 0 – 100 4.51 Bromochloromethane 0.018 0 – 100 4.72 Chloroform 0.012 0 – 100 4.88 CCl4 0.024 0 – 100 5.02 1,1,1-Trichloroethane 0.019 0 – 100 5.55 Benzene-D6 (IS) N/A N/A 5.59 Benzene 0.013 0 – 100 5.87 1,2-Dichloroethane 0.018 0 – 100 6.37 Trichloroethylene 0.022 0 – 100 7.01 Bromodichloromethane 0.012 0 – 100 7.33 Bromotrichloromethane 0.008 0 – 100 7.43 1-Bromo-2-chloroethane 0.013 0 – 100 7.78 Toluene-D8 (IS) N/A N/A 7.82 Toluene 0.012 0 – 50 8.15 Tetrachloroethylene 0.019 0 – 50 8.33 1,1,2-Trichloroethane 0.013 0 – 100 8.44 Dibromochloromethane 0.011 0 – 100 9.07 Chlorobenzene 0.013 0 – 100 9.05 Ethylbenzene-D10 (IS) N/A N/A 9.12 Ethylbenzene 0.008 0 – 100 9.13 1,1,1,2-Tetrachloroethane 0.014 0 – 100 9.23 p/m-Xylene 0.015 0 – 50 9.56 o-Xylene 0.019 0 – 100 9.59 Bromoform 0.01 0 – 100 10.13 n-Propylbenzene 0.013 0 – 100 10.17 1,1,2,2-Tetrachloroethane 0.017 0 – 100 10.24 o-Chlorotoluene 0.015 0 – 100 10.28 1,2,3-Trichloropropane 0.009 0 – 100 10.37 p-Chlorotoluene 0.016 0 – 100 10.58 1,2,4-Trimethylbenzene 0.011 0 – 100 10.81 1,3-Dichlorobenzene 0.013 0 – 100 10.88 1,4-Dichlorobenzene 0.016 0 – 100 10.92 1,2,3-Trimethylbenzene 0.016 0 – 100 11.2 1,2-Dichlorobenzene 0.007 0 – 100 11.81 2,4-Dichlorotoluene 0.015 0 – 50 11.83 2,6-Dichlorotoluene 0.006 0 – 50 11.85 1,2,4-Trichlorobenzene 0.014 0 – 50 12.24 3,4-Dichlorotoluene 0.008 0 – 50 12.34 Hexachlorobutadiene 0.012 0 – 50 12.36 1,3,5-Trichlorobenzene 0.014 0 – 50 12.6 Naphthalene 0.01 0 – 100 12.75 1,2,3-Trichlorobenzene 0.012 0 – 50 Table 2: Peak identification and linearity details. 4 IngeniousNews 02/2013 Figure 1: Typical chromatogram, 10 ppb VOC std. Figure 2: Robustness, no carryover. Conclusion About the Author References Today we focused on in-tube extraction (ITEX) as a powerful alternative to classic purge & trap analysis. The approach is particularly well suited for the analysis of trace organic compounds with a wide range of volatilities. Due do the simplicity of the system, it can be applied successfully to the analysis of heavily loaded samples without the risk of cross contamination and permanent system failure. Joeri received his Ph.D. in chromatography from Ghent University (Belgium) in 2002. He is currently employed as managing expert at IS-X, an independent team of true chromatographers that deliver chromatographic method development solutions (prep-to-rep), method validation, expert training and quality control. [1]In-Tube Extraction of Volatile Organic Compounds from Aqueous Samples: An Economical Alternative to Purge and Trap Enrichment Jens Laaks, Maik A. Jochmann, Beat Schilling, and Torsten C. Schmidt Anal. Chem. 2010, 82, 7641–7648 [2]In-tube extraction for enrichment of volatile organic hydrocarbons from aqueous samples. Jochmann MA, Yuan X, Schilling B, Schmidt TC J. Chromatogr. A, 2008; 1179; 96-105 Contact: [email protected] Website: www.is-x.com IngeniousNews 02/2013 5 Legal Statements Imprint CTC Analytics AG reserves the right to make improvements and/or changes to the product(s) described in this document at any time without prior notice. Date of print: 08.2014 CTC Analytics AG makes no warranty of any kind pertaining to this product, including but not limited to implied warranties of merchantability and suitability for a particular purpose. CTC Analytics AG Industriestrasse 20 CH-4222 Zwingen Switzerland T +41 61 765 81 00 F +41 61 765 81 99 Contact: [email protected] Under no circumstances shall CTC Analytics AG be held liable for any coincidental damage or damages arising as a consequence of or from the use of this document. © 2011 – 2014 CTC Analytics AG. 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