We are having a problem analyzing methanol and formaldehyde in an aqueous sample. We have a HP-Carbowax and Rtx-1 column. What is the best column for separation of methanol and formaldehyde? What is the best condition for separation these compounds? What should the oven temperature be and the flow rate of the gas carrier? I tried to use 80oC for the column oven (isocratic), but the separation was not very good. Can you give me suggestions for analysis of methanol and formaldehyde using GC-FID?
I wonder if the Rtx-1 column can handle formaldehyde and methanol, since they are relatively polar compounds. You should be able to check that in the product specs of the Rtx-1 column.
Moreover, if you check the boiling point of methanol, which is 67.4 C, and formaldehyde which is -18, your column temperature at 80 C is already above both. Which explains why your components didn't separate, they were both in gas phase from the start and would not have had any retention.
Take a gradient and start at 50 C go up to 80 C in a few minutes time, and your separation should improve. The formaldehyde will still come in the void volume, but your methanol should come a bit later.
Gas flow is typically something between 1.5 and 3 mL/min, it depends a bit on your setup.
Use the wax column - first, optimize flow rate (u-opt) for the carrier gas you are using. The column manufacturer will have specifics which are usually supplied with column, with FID use butane headspace as the non retained compound. I have attached some conditions for you. Be careful on how much aqueous you inject, keep in mind expansion volume of H2O is large, don't overload your injection sleeve/liner. I think we inject 2 uL. SEE ATTACHED --->
Formaldehyde by GC-FID is quite difficult. You will definitely need to use the polar (wax) column. You have to be careful in your preparation of standards. Since you're using a FID as your detector you are mass limited as to the amount of water you can inject; typically this is around 1-2 uL. With the wax column you'll want to start down around 40 C and then ramp up to around 150. Methanol chromatographs well; but the formaldehyde will tail, even on the wax.
You might try an old filling "Molecular sieve /Linde 5 a/", or similar inorganic sieve. This possibly will sepaate methanol from formaldehyde. Use He as a carrier gas at slow flow rate. Start analysis at moderately elevated temperature /40oC/ and and finish at about 100 oC /temp. gradient/. Injetor/detector temp. 120 oC. Column - 2.5 m, diameter 3.5 mm.
It is possible however, that sample can be analysed also in an isothermic mode. Use small wolume of the water sample of your analyte /1 microliter/ R.S.
Notice: after a series of analyses water from the columnis to be removed by longer time flushing with carrier gas at temperatures over 100 oC!
Maybe the following analysis should be better: use Chromosorb 102 /80 - 100 mesh/, fill 120 cm glas column with the sample direct injection, diameter of the column 0.35 cm. The injector temperature 170 oC, detector temp. 176 oC, and column temperature 160 or slightly higher. Carrier gas He, flow rate approximately 30 ml.min-1. If you use N2 as a carrier gas, increase the flow rate to some 48 ml.min-1 . Helium should give you better resoution. I am affraid, formaldehyde does not give sufficient signal on FID detector, I would prefer a TCD one.
R.S.
Note: for your analyt maybe lower column temperature /100 to 140 oC would do.
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