We need an accurate mass calibration solution which covers the whole mass range without contaminationg our system too much. Actually we use a mix containing Ultramark 1621 and several other compounds of lower masses. We don't like to use sodium salt clusters, since it causes many sodium aducts even after cleaning. We were also not really satisfied with PEGs as well as CsI.Therfore we are looking for about 2-3 compounds in the mass range of 2000 to 4000 whitch are avialabel in a good qualty, give a good signal in positve and negative mode, are cheap, and do not contaminate our system too much.
Christian,
which type of MS are you using? Ultramark is a typical calibrant for Orbitrap (and other Thermo) mass spectrometers. If you are using an orbitrap you will only observe a marginal change in mass accuracy when you increase your calibrant mass range.
Since you are talking about NaI, CsI and PEG as calibrants you are maybe using a TOF-MS. Then you have differnent options in my opinion:
1) Try phosphoric acid. It forms cllusters up to ~3800 Da (Depending on the instrument settings. Simply dilute metha phosphoric acid to a final concetration of 1% using ACN/Water (50:50, v/v).
2) Use the charge envolope of a large intact (bio-)molecule like IgG. It should perfectly fit into your mass range. There are some references commercially available (Waters). The main issue is that depending on your instrument the resultion may not be sufficient to determine the top of the peak exactly. thus leading to a bigger mass error.
3) Use 'ESI-L Low Concentration Tuning Mix' from Agilent which highest mass is ~2750 m/z. You would have to accept a slightly bigger mass error at the upper end of your mass range.
4) Rethink about NaI as calibrant. For me it works quite well and I dont observe large ammounts of sodium clustering. Maybe you should dilute your calibrant further.
best wishes
PPG 2700 is what we are using - recommended by AB Sciex and Thermo for the higher mass range. It is cheap, gives good signal, but it is sticky. Not more though than Ultramark. So, just dilute it down.
Appropriate references can be found here:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2523269/?report=classic
http://www.sciencedirect.com/science/article/pii/S104403050900227X
the best way is to create your own by tryptic/chymotrypsin digestion of BSA ( in 50mM Ammonium bicarbonate) in large quantities in lab. This will ensure more stringent control over buying from a company.
Thanks for the awnser! We use a TOF instrument (Synapt G2 HDMS).
With the NaI we had just problems with sodium adducts. We use Phosphoric acid for other instruments, but we are not very happy with the low mass range since it produces also very high signals below m/z 200. I think Igg would be an option too but it's a bit expensive and not really available in a good qualty. PEGs are also not really the molecules we wanted to have in our system as well as CsI. What we would like most would be two or three singel molecules, maybe some peptides within that range. Which are cheep and available.
If you are in MS for a long time then, prepare your own peptides using mol bio grade BSA as a source.
Ultramark 1621 is good from about 1900 to 2200, but it is hard to get rid of in the plumbing. Sodium trufluoroacetate clusters can go to high m/z. See Moini et. al. JASMS 9:977-980, 1998. Also King and Fales JASMS 10:2730276, 1999.
I meant m/z 900 to 2200 for the Ultramark. Of course, if you want to avoid the salt clusters, then the NaTFA clusters are not an option. We used PEG with an amine terminating group with good results for ESI. It required less sample than underivatized PEG, and proton attachment dominates the spectrum with little or no alkali metal cation attachment. You can use PEG sulfates in negative-ion mode.
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