Experimental Considerations in Headspace Gas Chromatography - Pharmaceutical Technology

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Experimental Considerations in Headspace Gas Chromatography
In this case study of amines, the authors discuss several parameters to be considered in developing a headspace GC method.

Pharmaceutical Technology
Volume 34, Issue 5, pp. 76-79

Results and discussion

Figure 4
Sample volume. The sample volume for each of the amines was varied by running replicates of 100 l, 500 l, 1 mL, 2 mL, and 5 mL of solution in the headspace vials. Figures 2–4 show that sample volumes higher than 1 mL do not yield the expected increase in signal. The experiments with allylamine were originally run using DMF, which resulted in the opposite response to that shown by using DMSO as the diluent. The response of allylamine decreased with increasing amounts of sample in the headspace vial, thereby demonstrating that the choice of diluent is important.

Figure 5
Vial equilibration time. The sample equilibration time in the headspace oven was varied by running replicates at 5, 10, 15, 30, and 60 min. Figures 5–7 demonstrate that equilibration times longer than 5 or 10 min do not yield a significant increase in signal. For TEA, the 15-minute time point shows more scatter in the data than the other time points, which could be indicative of a secondary reaction or interexperimental differences because of different columns and analysts. It is of interest to note that USP recommends incubating the samples for 60 min. These data show 60 min to be excessive and indicate that the same results can be obtained in one-quarter of the time, with less risk of secondary reactions occurring.

Figure 6
Diluent composition. Often, analyses require detecting small amounts of an analyte in the matrix. This function can highlight the power of headspace because one can use large amounts of sample in the headspace vial to get at the sensitivity required for detection without having to inject large amounts of the matrix onto the column. There are cases, however, when increasing sample size is insufficient, and other techniques are required to increase the signal. One option studied was to vary the percentage of water in the diluent to determine if the concentration of volatile solvents in the headspace could be affected by mixing the diluent with a solvent that has a higher vapor pressure. The following set of experiments was conducted testing the amines, using diulent systems containing some aqueous (as water, 0.001M NaOH, 0.01M NaOH, or 0.1M NaOH) and an organic solvent. Base was added to ensure that the amine was a volatile-free base and not present as a salt. Figure 8 presents the response of TEA with increasing percentages of water in the diluent. At 50% water, the peak splits into two peaks, likely due to water interactions with the amine and the liquid phase of the column. For these samples, the two peak areas are combined (denoted by –S). Using 0.01M NaOH does not appear to be necessary, because the responses with and without added base were similar. For TEA, the ratio of 80:20 DMSO:water appears to give the best response, thus affording an increase of about 60% over straight DMSO. Figure 9 presents the response of n-butylamine with increasing percentages of water in the diluent. Addition of the base 0.1M NaOH does appear to have an effect, as using only water results in lower responses. For n-butylamine, a ratio of 90:10 DMSO:0.1M NaOH appeared to give the highest response; however, the increase was only about 6% more than pure DMSO. The solvent selection for allylamine was more difficult, because the original solvent system of DMF showed low-area responses for the standard 1 mL sample volume as observed in the vial volume experiment previously outlined. The mixture of 90:10 DMF:0.001M NaOH does appear to yield a higher signal than other DMF solvent mixtures, but it is still much lower than similar mixtures using DMSO and a slightly stronger base. Other combinations were tried (e.g., DMF:water; DMF:0.0001M NaOH); however, nothing appeared to work better than neat DMSO (data not shown).


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