Effect of ionization suppression by trace impurities in mobile phase water on the accuracy of quantification by high‐performance liquid chromatography/mass spectrometry

Abstract

The high-performance liquid chromatography (HPLC) column is capable of enrichment/pre-concentration of trace impurities in the mobile phase during the column equilibration, prior to sample injection and elution. These impurities elute during gradient elution and result in significant chromatographic peaks. Three types of purified water were tested for their impurity levels, and hence their performances as mobile phase, in HPLC followed by total ion current (TIC) mode of MS. Two types of HPLC-grade water produced 3-4 significant peaks in solvent blanks while LC/MS-grade water produced no peaks (although peaks were produced by LC/MS-grade water also after a few days of standing). None of the three waters produced peaks in HPLC followed by UV-Vis detection. These peaks, if co-eluted with analyte, are capable of suppressing or enhancing the analyte signal in a MS detector. As it is not common practice to run solvent blanks in TIC mode, when quantification is commonly carried out using single ion monitoring (SIM) or single or multiple reaction monitoring (SRM or MRM), the effect of co-eluting impurities on the analyte signal and hence on the accuracy of the results is often unknown to the analyst. Running solvent blanks in TIC mode, regardless of the MS mode used for quantification, is essential in order to detect this problem and to take subsequent precautions.

Identification of two novel trace impurities in mobile phases prepared with commercial formic acid

“…Liquid chromatography coupled to mass spectrometry (LC/MS) is becoming ever more important in the identification and characterization of unknowns in complex systems . Soft ionization techniques such as electrospray ionization (ESI), as well as the wide variety of separation phases available, make LC/MS an appealing option for separations containing molecules that are able to accommodate a charge.…”

“…Impurities may include plasticizers, polymers, human‐derived proteins, and others, and sources may include solvents, glassware, storage vials, and reagents used to prepare samples . Depending on the nature of the impurity, it may be concentrated during gradient separations and elute as a peak . Some impurity peaks may go unnoticed in LC/MS separations, especially when targeted methods are used.…”

“…Some impurity peaks may go unnoticed in LC/MS separations, especially when targeted methods are used. However, impurities can cause a variety of problems, such as signal suppression of analyte peaks, when the impurity peak elutes near, or concurrently with, an analyte of interest, and this can lead to mass errors and inaccurate quantitation …”

“…However, impurities can cause a variety of problems, such as signal suppression of analyte peaks, when the impurity peak elutes near, or concurrently with, an analyte of interest, and this can lead to mass errors and inaccurate quantitation. 1,8 During routine work in our laboratory, several extraneous peaks were observed in both solvent blanks and samples. The peaks persisted even after extensive re-equilibration of the column (6-8 h) and after running several blanks.…”

Accounting for matrix effects of pesticide residue liquid chromatography/electrospray ionisation mass spectrometric determination by treatment of background mass spectra with chemometric tools

“…[21,22] Even for pure water gradient elution the presence of this type of ions has been shown. [23] Second type ions, in the case of sample analysis ionisable matrix compounds may be present. In addition to yielding ions, the matrix compounds may either cause ionisation suppression/ enhancement of the analyte signal or have no influence on analyte ionisation.…”