Hello,
I recently started working with LC/MS.
The LC/Ms was bought without a new column so to start working we put in a Supelcosil LC-18 column
Now we have some money available to buy a new column adapted for LC/Ms.
Can anyone give me some suggestions wich column i should buy? (the new short columns are a older long column)
The only criteria are:
the column must separate as much different compounds as possible (esters,ethers,acids,alcohols,...)
many thanks in advance,
Kathy

Short columns, 3 cm or 2 cm, 3 or 3.5 micron, 2 mm i.d. to get into the best flowrate range of your MS. Atlantis dC18 from Waters can be used in 100% water and has excellent retention. Do not buy packings with embedded polar groups.

Hi Anon#1,

why should not buy packings with embedded polar group column? Any specific reason?

Thanks

Xu

Thank you for the recommandation.
For the moment i only got the prices of three different columns:
1)RP18; length 75 mm internal diameter 4 mm ; 3 µm
2)RP18; length 250 mm internal diameter 4.6 mm ; 5 µm
3)Prevail ; length 150 mm internal diameter 2.1 mm ;3 µm
Do you think i should go for column number 2 which is the longest ?
Kathy

All your columns are too long, 1 and 2 are too fat.

Dear Kathy,

the selection of your column dimension depends among other things on the interface you're using with your lc/ms.
- Thermospray is not suitable for flow rates less than 0.8 ml/min.
- APCI is ideal for higher flow rates (I guess around 1.2 ml/min) and columns with wider ID.
- API-ES is ideal for low flow rates and columns with lesser ID.

- Columns with polar embedded phases gives a higher background noise with lc/ms. When seperation doesn't require a polar embedded phase, don't use them with ms-detection.
- Columns with small particle size gives smaller and higher peaks, but in scan mode with wide mass range it's sometimes better to have broader peaks for more spectra information.

You can find on the websites of most lc/ms-manufacturers some basic advices about flow rates and column dimension.

Thanks for all the reactions!!

Because there was a special offer we bought a
Reversed phase Omnispher C18 column
ID 3 mm ; Length 15 cm ; Size 5 µm.
I work with APCI and ESI and normally at a flow rate of 0.5 ml/min.
I hope I 'll get better results !!!

Kathy

Reasonable compromise between all the different advice that you got, and a godd start. Sooner or later, you will go to a shorter column with a smaller i.d. and maybe with a smaller aprticle size.

Xu,

You don't want to use a polar embedded column chemistry for LC/MS detection methods as they tend to "bleed" i.e. produce spurious peaks in the LC/MS trace. The peaks are due to alkyl chain fragments and/or polar embedded groups being removed from the bonded material as solvent passes through it. These fragments only cause a problem is the bleed is severe or if the peaks produced interfer with analyte peaks in your sample. Under UV detection conditions, there would be no "extra" peaks detected.

This is not the case with polar endcapped columns though. They are more robust and tend not to bleed when used with LC/MS detection.

Rachel

The reason that "polar endcapped" columns do not bleed is because they do not have any endcapping - at least the ones that we have looked at.