Practical Example Determination of Ethylene Oxide in a PVC Tube

From the foregoing discussion it is apparent that there are several possible ways to carry out a quantitative headspace analysis. A systematic approach to develop the most suitable calibration technique is therefore desirable. This is illustrated by the following example of the determination of ethylene oxide (EO) in a sterilized PVC tube. Ethylene oxide is widely used for sterilization, but due to its toxicity the residual concentration must be carefully controlled down to a safe limit (e.g. 1 ppm). This example covers several of the aspects discussed above. Since PVC tubing is a solid sample, the solution approach was naturally the first choice. The sample was dissolved in dimethylacetam-ide and calibration was by multilevel standard addition (see Figure 6), resulting in an EO concentration of 19.95 ppm with a precision as expressed by the correlation coefficient of 0.9961.

Figure 6 Determination of EO in a sterilized PVC tube by standard addition calibration (STA) and by multiple headspace extraction (MHE). Gas chromatographic conditions: Perkin-Elmer SIGMA 2000, HS100 Automatic Headspace Sampler; 2 m x 3.2 mm stainless steel column, packed with Chromosorb 101, 80/100 mesh; temperature programme: 100°C (7.5 min), 15°C min-1, 200°C; detector: FID; carrier gas: He, 20 mL min-1; calibration standard: aqueous solution of EO (10.3 |ig iiL-1).

(A) STA. Sample preparation: 1 g PVC tube, dissolved in 2 mL dimethyl acetamide, equilibrated 90 min at 90°C; calibration by adding 10, 20 and 30 |L of the calibration standard; result: 19.95 |ig g — EO, regression coefficient r= 0.9961.

(B) MHE. Sample preparation: 1 g PVC tube, cut in pieces of 3 mm x4 mm, 1 mm thick, equilibrated as above; calibration by external vapour standard, prepared by TVTof 8 |L; total area counts from sample analysis: 258 464, from calibration standard: 104 132; result: 19.53 ig g-1 EO; regression coefficients: sample (S), r=— 0.99914; calibration standard (C), r=— 0.99990.

Figure 6 Determination of EO in a sterilized PVC tube by standard addition calibration (STA) and by multiple headspace extraction (MHE). Gas chromatographic conditions: Perkin-Elmer SIGMA 2000, HS100 Automatic Headspace Sampler; 2 m x 3.2 mm stainless steel column, packed with Chromosorb 101, 80/100 mesh; temperature programme: 100°C (7.5 min), 15°C min-1, 200°C; detector: FID; carrier gas: He, 20 mL min-1; calibration standard: aqueous solution of EO (10.3 |ig iiL-1).

(A) STA. Sample preparation: 1 g PVC tube, dissolved in 2 mL dimethyl acetamide, equilibrated 90 min at 90°C; calibration by adding 10, 20 and 30 |L of the calibration standard; result: 19.95 |ig g — EO, regression coefficient r= 0.9961.

(B) MHE. Sample preparation: 1 g PVC tube, cut in pieces of 3 mm x4 mm, 1 mm thick, equilibrated as above; calibration by external vapour standard, prepared by TVTof 8 |L; total area counts from sample analysis: 258 464, from calibration standard: 104 132; result: 19.53 ig g-1 EO; regression coefficients: sample (S), r=— 0.99914; calibration standard (C), r=— 0.99990.

Due to problems with solvent impurities and also to achieve a better sensitivity, a solvent-free method was expected to be superior. A six-step MHE procedure with the sliced solid sample was calibrated with a five-step MHE of an external vapour standard (see Figure 6) and after the necessary volume correction a nearly identical concentration of 19.53 ppm was obtained, but now with a four-times higher sensitivity. Also the precision was better, as expressed by the linear regression coefficients of — 0.99914 for the sample and — 0.99990 for the calibration standard. This allowed reduction of the MHE procedure to three steps, but still with a linear regression calculation, and a two-step procedure is sufficient if the highest precision is not required.

From these good results some more conclusions may be drawn to simplify the analysis further. The good linearity over the whole working range indicated that the solid PVC matrix was behaving as a partition system; this conclusion allowed the application of standard addition calibration in the form of gas-phase addition. Even the use of an internal standard appears feasible, since from a successful gas-phase addition it can be concluded not only that EO partitions between both phases, but that any other compound will do the same, provided it is similar in its chemical properties (e.g. dimethyl ether). As discussed above, a calibration factor has to be determined first, which comprises not only the differences in the detector response but also the different solubilities (partition coefficients) in the PVC matrix. For this example it is no problem to obtain a PVC tube without any EO in it.

Taking into account all these possibilities, the final decision as to what is a suitable calibration technique may depend on other considerations such as the simplicity of sample handling or the sample throughput in an automated headspace sampler. For example, the standard addition calibration needs a series of vials to be subsequently analysed, thus occupying the corresponding number of places in the turntable of an autosampler, while for the MHE procedure the determinations are all carried out from the same vial and the sample throughput in an autosampler will therefore not be affected. On the other hand, the addition of an internal standard to every sample is tedious and prone to errors, particularly if pipettes are used to transfer solutions with highly volatile compounds.

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Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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