Hello,
I try to quantify some diols in aqueous sample by GC-MS (EI source and quadrupole analyzer). I tried by direct aqueous injection but I obtained a very poor repeatability.
Some articles quote that water "decreases the vaccum in the ion source of mass selective detector" (Yu et al. (2014), Analyticals Methods, 6, 6931-6938).
Why water is more likely to decrease the vaccum than organic solvents?
Do you think this phenomenon caused the low repeatability?
I also noticed that most articles which practice the direct aqueous injection employ polar column to retain water for a longer time in the column. I used an apolar column: do you think I should try a polar column? Why?
Thank you for your answers,
Have a nice day!
Hello,
do not inject aqueous solution to polar columns (like wax-columns). The column will be broken down very soon.
D.
Thank you for your answers and your advices.
Some authors also used the polar column Stabilwax which is supposed to be resistant to water (http://www.restek.com/catalog/view/1414/43831). They also use a deactivated guard-column.
I also got an explanation about the vaccum problem : "water produces a vapor volume, which is more than seven times larger than of the same amount of organic solvent such as hexane. This leads to a significant decrease of the vaccum in the MS, which reduces the ionization efficiency and detector sensitivity" (Aeppli et al. (2008), Journal of Chromatography A, 1181, 116-124). These authors also monitored the pressure just after the water introduction into the MS source and they concluded that if you inject less than 1µL, the initial vaccum conditions are re-established within 5 minutes after the water introduction.
To answer to Zacharias: I used a liner and so I was limited by the injection volume at 0.4 µL. However, I used a 2 µL-syringe so the injection should be reproducible (as the injected volume is more than 10% the volume of the syringe).
I may test evaporation of my samples and reffiling with methanol.
Poor repeatability with water samples is due mainly to problems with pressure and flow pulses in the inlet caused by the very large vapour volume. You are right to inject small volumes, and using as low an inlet temperature as possible will also help. Lower inlet temperatures can lead to adsorptive peak tailing, especially with diols so you need to ensure that your inlet liner is clean and well deactivated.
Water can also damage the stationary phase of the column. Siloxane phases are more vulnerable, stabilized wax phases are robust to water as long as it is present only as vapour. If water condenses on the column as liquid it will permanently disrupt the phase, so do not use too low a starting temperature for the column programme; certainly not below 80C and preferably above 100C. A retention gap with polar deactivation will help. You will also see shifts in retention of polar analytes like diols due to the water temporarily increasing the polarity of the stationary phase. The less water you put onto the column the better, so use split injections rather than splitless if you can.
If the amount of water you inject is small enough not to compromise the chomatography, and it passes through the column as vapour, it is not likely to have a serious impact on the MS - with very polar analytes like diols the limiting factor on detectability is adsorption in the inlet and column, not detector sensitivity.
Your plan to test methanol as a solvent is a good one - it is much more tractable than water.
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