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Void/Dead time in LC/MS
Posted: Sat Oct 06, 2007 4:07 am
by BillW
What is an appropriate compound for void time determination for LC/MS for
1. reverse phase
2. HILIC
Are there standard compounds that are always used for these determinations ? Note that the detector is MS, not UV.
Thanks,
Bill
Posted: Fri Oct 12, 2007 7:00 pm
by tom jupille
The only requirements are that the "probe" be unretained and detectable. I'm no mass spectrometrist, but I would assume that the standard probes used with UV (e.g., uracil for reversed-phase) would also be detectable with MS.
Posted: Sat Oct 13, 2007 4:59 pm
by BillW
Yes, I agree with the criteria that you described. The question is what compounds meet those criteria. It is essential that the compounds are not retained at all. Many compounds, although not well retained, are actually slightly retained.
The problem with a compound such as uracil is that it will be retained in some conditions in reverse phase.
Posted: Sun Oct 14, 2007 12:21 am
by tom jupille
If you go back and search the current Forum and the archives, you'll find a number of threads on dead time determination. The problem is that there are varying degrees of "unretained", with either charge- or size-exclusion a possibility.
The "best" suggestions for reversed phase (in principle, anyway) were tritium oxide (from Hans Mueller, if memory serves) or deuterium oxide (from Ben Buglio, again, if memory serves). These are small enough to get into all the stationary phase pores, hence no size exclusion, and non-ionic, hence no charge exclusion. I have no idea about MS detectability.
In the case of HILIC, even these choices are problematic because of the water bound to the stationary phase (which means that water is, to a certain extent, "retained").
Of course, a slight degree of retention may not matter unless you are doing detailed calculations of things like distribution coefficients. For tracking a separation, I would think that consistency is more important.
Posted: Mon Oct 15, 2007 6:45 am
by HW Mueller
Yes, tritiated water is quite good in RP, but is retained in HILIC (extend depending on amt. of organic). I thought that 22-Na+ at a low pH and high ionic strenth mobile phase was quite good but am not sure anymore (possible that there is some exclusion). 36-Cl- at such conditions might be on the "dot". (Note that if you are too basic 22-Na+ will be extremely retained and 36-Cl- will be excluded).
For those who have no access to radioisotpes some have suggested toluene (which I yet want to compare to the radioisotpes).