To develop a method for quantitatively determining ion suppression from LC/MS vials
The experiment was conducted according to a previous publication [Miller-Stein C, Bonfiglio R, Olah TV, King RC, Am. Pharm. Rev., 2000, 54]. Several mobile phase compositions [5%, 50% & 95% methanol] and additives [formic acid & ammonium hydroxide, both at 1 %] were evaluated and measured with the following protocol. Each vial was filled with 1.5 mL of a solvent mixture and equilibrated for 4 hours. The measurements were made by recording the effluent of both pumps with the injection valve in the load position [reference], followed by the injection valve in inject position [sample] and back to load position for confirmation. Each sequence was measured for 5 minutes for a total of 15 minutes. A hundred scans were combined for the reference and the sample.
A previous paper reported the effects of sample preparation techniques and mobile phase composition on the signal of ESI in positive mode using a simple setup for the quantification of ion suppression/enhancement [Mallet, CR, Lu, Z, Mazzeo, JR, Rapid Commun. Mass Spectrometry, 2004, 18: 49]. This work continues that study and focuses on the potential sources of suppression or enhancement of glass vials. The concern is related to the potential leaching of material from various compositions of glass and/or the presence of residues left behind from the manufacturing process. A high organic composition [95% methanol] showed a large amount of ion intensities at low mass range [100 – 400] and also multiple charge distributions in the 400 – 1000 mass range, which is indicative of polymer, detergents or oil contaminants. When compared to a standard and clean reference, the increase in signal intensity is clearly coming from the vial. Also, since those contaminants were detected with the high methanol composition, it suggests that they are not water soluble and more could potentially be seen if intermediate and non-polar solvents were used for testing. The use of acid or base showed mild signal increase in comparison to a common reference.
A method for quantitation of ion suppression from LC/MS vials has been developed and can clearly help to certify vials for use in the analytical laboratory.