Hello everyone,
I am trying to dissolve azathioprine in a concentration of 1mg/mL for forced degradation studies. It is soluble in dilute alkali, but for forced degradation i need it to be dissolved in a neutral solvent, preferably not crashing out when acid is added. I have managed to dissolve it in 50:50 dimethylformamide : water, but now i am getting a m/z signal of 74 (corresponding to DMF+H) which is tailing far beyond the void volume (my injection volume is 10 uL). I have found divided opinions on DMF; some say not to use it in MS, others say it won't do any harm. I am interested if prolonged usage of this solvent could harm the column (C8) or MS. Any experiences regarding this topic? I am using ESI-ion trap in positive ion mode, with water and acetonitrile with 0.1% formic acid as mobile phases.
Also, any suggestions regarding neutral solvents for azathioprine or post-column addition for increase of sensitivity, seeing that DMF causes a bit of ion suppression for my analytes.
Thank you!
I'm assuming you are asking about the use of DMF to dissolve the sample prior to injection. This is commonly done. My experience is that it won't harm the column, and it (or DMSO) are a useful solvent for injections onto a column. However, depending on how the compound elutes (concentration of organic solvent), it may affect the purification. If the compound elutes with mainly organic solvent, the effects of DMF and DMSO are minimal. If it elutes in mostly aqueous, a larger injection can "drag" the sample down the column and do anything from make poor peak shape, to loss of resolution, to compound eluting near the void (no retention). You don't want to use DMF or DMSO as a mobile phase due to back pressure!
As for injecting directly into the MS, I found it does suppress ions, and dilution works fine.
I'm assuming you are asking about the use of DMF to dissolve the sample prior to injection. This is commonly done. My experience is that it won't harm the column, and it (or DMSO) are a useful solvent for injections onto a column. However, depending on how the compound elutes (concentration of organic solvent), it may affect the purification. If the compound elutes with mainly organic solvent, the effects of DMF and DMSO are minimal. If it elutes in mostly aqueous, a larger injection can "drag" the sample down the column and do anything from make poor peak shape, to loss of resolution, to compound eluting near the void (no retention). You don't want to use DMF or DMSO as a mobile phase due to back pressure!
As for injecting directly into the MS, I found it does suppress ions, and dilution works fine.
Hello,
thank you for your answer!
Regarding the elution, i have dissolved my standards in the mixture and the retention times haven't changed much, so no worries of eluting in the void volume or loss of resolution. Although I am getting some noise at the full scan in MS after it elutes, I will most probably use a SIM so no worries there too. So no harm can be done to the instrument, that's what i wanted to know, thank you very much!
DMF is not really good for ESI. It has a really high vapour pressure, so it can hang around inside your mass spectrometer for a very long time. Typically you would want to use a few percent of either DMF or DMSO, to minimize contamination of the MS.
DMF is also much worse than DMSO for use in a MS in general. DMF tends to degrade the pump oil you use in the backing pumps for the turbos. This can lead to reduced lifetimes, and something you will need to keep in mind if you are using DMF regularly and at high concentration.
Hi, thanks for your answer!
I have tried to reduce the percentage of DMF in the solvent and have come to 30%, really cannot go lower than that and none of the usual solvents can dissolve my samples. I am aware that the MS source will need a bit more cleaning than usual and am hoping to have to use DMF only for these two or three methods so as not to damage the instrument.
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