Ion suppression is one form of matrix effect that liquid chromatography–mass spectrometry (LC–MS) techniques suffer from, regardless of the sensitivity or selectivity of the mass analyzer used. Ion suppression negatively affects several analytical figures of merit, such as detection capability, precision, and accuracy. The limited knowledge of the origin and mechanism of ion suppression makes this problem difficult to solve in many cases. Over the past decade and a half since the response-reducing phenomenon was exposed, however, protocols have been developed not only to test for its presence but also to account for its effects and eliminate the risk of ion suppression altogether. Because there is no universal solution for the matrix effect, some of the viable options are discussed briefly in this tutorial, which alone or in combination can help regain the quality of LC–MS analysis for the particular matrix–analyte combination. Two commonly used techniques to detect the presence of the matrix effect are illustrated. Modifying instrumental components and parameters, chromatographic separation, and sample preparation are all considered as means of reducing or possibly eliminating ion suppression. A variety of calibration techniques for compensating the effects of the phenomenon also are discussed.
Liquid chromatography–mass spectrometry (LC–MS) and tandem mass spectrometry (LC–MS-MS) have been established as the most sensitive and selective analytical techniques for biological samples. Starting in the early 1990s, however, many studies also have reported difficulties with reproducibility and accuracy when analyzing small quantities of analytes in complex samples such as biological fluids (1–3). Kebarle and Tang (2) originally described the matrix effect phenomenon as the result of coeluted matrix components, affecting the detection capability, precision, or accuracy for the analytes of interest. Ion suppression appears as one particular manifestation of matrix effects, which is associated with influencing the extent of analyte ionization (Figure 1). This change often is observed as a loss in response, thus, the term ionization suppression. However, depending upon the type of sample, it also can be observed as an increase in the response of the desired analyte (4). In this tutorial, the mechanism and origin of ion suppression will be investigated, as well as ways to validate the presence and circumvent or compensate for the effects in LC–MS.
Figure 1: Multiple reaction monitoring chromatograms for a constant postcolumn infusion of analyte (phenacetin). For the lower trace, protein precipitation blank plasma was injected, while the upper trace exhibits the chromatogram for an injection of pure mobile phase. The difference between the two traces directly shows the effect of endogenous plasma components on the analyte´s response. (Chromatographic traces reproduced from reference 25 by courtesy of John Wiley and Sons.)