Headspace GC and TCD give water absorption capacity

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  • Published: Jun 15, 2018
  • Author: Ryan De Vooght-Johnson
  • Copyright: Image: Sea Wave/Shutterstock
  • Channels: Gas Chromatography
thumbnail image: Headspace GC and TCD give water absorption capacity

Flour water absorption capacity measurement needs improvement


Credit: Sea Wave/Shutterstock.

A key parameter of flour is its water absorption capacity, which is typically measured by filtration techniques in which flour is filtered from water and then weighed. However, these methods can be awkward to run, since blockages and fine particles passing through the filter can be a problem. Such problems make filtration processes difficult to automate without regular checking. Carrying out filtrations at temperatures other than ambient is also rather challenging without specialised equipment.

The researchers from Guangzhou and Shenyang used headspace GC to determine the absorption capacity of wheat flour. Drawing a graph relating the added water to the area of the water peak in the GC gives a clear change in the graph when the capacity is reached. This is because adding further water gives only an increased amount of free water (as opposed to water bound to the flour) and hence little or no increase in the peak area value, since the latter depends on the amount of water in the vapour phase. A similar process had previously been applied to measuring the water content of paper material.

GC with thermal conductivity detector used to measure water levels

Weighed samples of wheat flour (0.30 g each) were placed in 21.6-mL GC vials and varying weighed quantities of water were added. The vials were placed in the GC autosampler and mechanically shaken (for 25 minutes for equilibration at 35 °C).

GC used a Thermo Scientific TriPlus 300 automated headspace sampler and an Agilent 7890A GC instrument, the latter being fitted with a J&W GS-Q capillary column. The headspace apparatus employed a sample loop (3 mL volume), which was used to transfer the headspace contents onto the column. Nitrogen was used as carrier gas, with a flow rate of 2.7 mL/min. The oven temperature was 105 °C. A thermal conductivity detector (TCD) was used, which gave a distinct peak for water, although this was not particularly sharp as it ‘tailed’ somewhat on the downward slope. A small oxygen peak was also noted.

For each flour sample, which was split between different vials with varying amounts of water, a graph was drawn showing the relationship between the percentage of water added and the resulting GC peak area. Initially these graphs gave a more or less straight line, with the GC peak area value increasing with increasing amounts of water. They then had a turning point, beyond which the GC peak area value became virtually constant (i.e. the graph became flat): this point was the water absorption capacity of the flour sample.

The method was shown to give good repeatability: three samples were each run four times, giving similar results. The %RSD values for the three samples ranged from 1.75 to 3.48%. A comparison was also carried out, using ten samples, between the new GC method and the conventional filtration technique. The results were similar, with the percentage difference between the two techniques varying between 0.92 and 5.56%, confirming that the GC results were in line with the current method.

New GC method for water absorption capacity suitable for automation

This novel method of measuring water absorption capacity avoids the need for difficult and messy filtrations. It also allows the measurement to be automated, which is a great advantage when laboratories are dealing with large numbers of samples. As well as flour, this technique could doubtless be applied to many other materials.

Related Links

Journal of Separation Science 2018, 10, 416-422. Xie et al. "10.1002/jssc.201800073"

Journal of Chromatography A 2016, 1443, 62-65. Xie et al. "Rapid determination of moisture content in paper materials by multiple headspace extraction gas chromatography."

Wikipedia. Thermal Conductivity Detector

Article by Ryan De Vooght-Johnson

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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