What is matrix effect?
In my last post, What is Microflow, I talked about two main parameters, better sampling efficiency and reduced matrix effect, for microflow LC/MS offering higher sensitivity than its conventional counterpart.
Let’s take a close look at the matrix effect. It is a result of co-eluting components, usually the material in the sample other than the analytes or the mobile phase additives (can also be the contaminants from your LC mobile phase, yak!). For example, when performing LC/MS analysis with compounds in complex biological matrices, such as plasma or tissue, the major source of matrix effect is phospholipids. These background substances impact the ionization efficiency of the analytes through (1) competing for limited charges in the solution, (2) changing the surface tension of the solution, and (3) altering the physical or chemical properties of the analytes.
The matrix effect is usually defined as the change, positively or negatively, in MS response of an analyte in the sample matrix relative to the signal acquired with the standard solution. In order to evaluate the magnitude of the difference, the common approach is to establish the calibration curves of analytes in the matrix and in the standard solution and to compare the difference of these two curves (Figure 1)
Figure 1. The calibration curves of analyte A in standard solution and sample matrix
If you are extremely lucky, from time to time, matrix effect will boost the MS signal. However, it is rarely the case…. If you work with complex samples, you have probably already encountered a frustrating matrix effect. It’s pretty safe to say that anyone routinely dealing with complex sample matrices does not enjoy matrix effect, because it usually means time-consuming sample preparation or ion suppression.
There are a few commonly applied approaches to amend matrix effect. For example, conducting sample preparation to remove the matrix, dilution of the samples, or even modifying the instrument parameters. Each method has its benefits and negative aspects. Sample preparation usually means extra time and material investment. Sample dilution seems simple and straightforward, but it won’t work if the analyte concentration is low. Modifying chromatographic condition or ion source parameters may help with some analytes within the sample, but can also hurt others.
How does microflow LC provide improvements? The key is that the finer droplets formed by the microflow LC provide more surface area, which is the key parameter of the ionization efficiency. The higher the surface area, the better the ionization efficiency is. This compensates for the competition of ionization between the analytes and the matrix. Due to the formation of finer droplets at the ion source while the LC flow is significantly reduced, the ion suppression caused by matrix is alleviated (Figure 2).
Figure 2. The different degree of the signal reduction resulted from matrix effect in (a) microflow LC/MS and (b) conventional flow LC/MS
We have gone over the main benefits of the microflow LC/MS and the factors behind it. If microflow is so fantastic, why don’t we see it in every laboratory and being used in every field? In my next post, we will discuss the history and recent advancement of this novel-but-not-completely-new technology and how it can be implemented in the lab with minimal impact or changes to the current workflow and lab setup.