Approaches to overcome matrix effects in gas chromatography

Matrix-induced response enhancement, first described by Erney et al. [99], is a matrix effect well-known mainly in the GC analysis of pesticide residues in food, negatively impacting quantitation accuracy of certain susceptible analytes [98]. When a real sample is injected, the matrix components tend to block active sites (mainly free silanol groups) in the GC inlet and column, thus reducing losses of susceptible analytes caused by adsorption or degradation on these active sites. This phenomenon results in higher analyte signals in matrix-containing vs. matrixfree solutions, thus precluding the convenient use of calibration standards in solvent only, which would lead to overestimations of the calculated concentrations in the analysed samples.

In theory, elimination of active sites or matrix components would overcome the matrix-induced enhancement effect; however, absolute and permanent GC system deactivation or thorough sample clean-up are virtually impossible in practice [100]. Careful optimization of injection and separation parameters (such as the injection technique, temperature, and volume; liner size and design; solvent expansion volume; column flow rate; and/or column dimensions) can lower the number of active sites (due to a decreased surface area) or shorten the ana-lyte interactions with them. This results in a reduction but rarely complete elimination of the effect. For instance, application of a pressure pulse or temperature programming during the injection (to reduce residence time or thermal degradation in the injection port) may serve as examples of this effort [101-104].

Since an effective elimination of the sources of the matrix-induced response enhancement is not likely in practice, the analysts often try (or are required) to compensate for the effect using alternative calibration methods. The current compensation approaches include the use of matrix-matched standards, standard addition method, and isotopically labelled internal standards (not feasible in multianalyte analysis due to their unavailability and/or prohibitive price). All of these techniques require extra labour and costs; moreover, they may still lead to quantitation inaccuracies because the extent of the effect depends on analyte concentration and matrix composition [105] (problems in the case of standard addition and matrix-matching, respectively).

Matrix-matched standardization is thus far the most widely used approach in spite of its imperfections including a rather time-consuming and laborious preparation of matrix-matched standards and need for an appropriate blank material (ideally the same as the analysed samples). The matrix-matching procedure becomes especially onerous when different commodity types are to be analysed in one batch of samples, which is often the case in routine pesticide residue analysis.

Recently, a concept of ''analyte protectants'' has been introduced in the GC analysis of pesticide residues [106,107], but can be applied to other analytes as well. Analyte protectants are compounds that strongly interact with active sites in the GC system, thus decreasing degradation and/or adsorption of co-injected analytes. The concept idea is to add suitable analyte protectants to sample extracts as well as matrix-free (solvent) standards to induce an even response enhancement in both instances, resulting in effective equalization of the matrix-induced response enhancement effect. A mixture of 3-ethoxy-1,2-propanediol, gulonic acid g-lactone, and sorbitol was found to be the most effective for the volatility range of GC-amenable pesticides [107] as demonstrated in Figure 6, which compares peak shapes and intensities of three selected pesticides obtained in solvent standards and matrix extracts with and without the addition of the above mixture of analyte protectants.

In addition to the compensation for matrix-induced response enhancement, the application of analyte protectants can also significantly reduce another matrix effect called matrix-induced response diminishment [26,108]. This effect is caused by gradual accumulation of non-volatile matrix components in the GC system, resulting in formation of new active sites and gradual decrease in analyte responses. The use of analyte protectants provides GC system deactivation in each injection, resulting in improved ruggedness and a less frequent need for the GC system maintenance [107].

Another way how to overcome gradual build-up of non-volatile matrix components is to remove them from the system after each analysis. This can be accomplished using a special injection technique called direct sample introduction (DSI) [109]. In DSI, a liquid (or solid) sample is placed in a disposable microvial. After this step, the microvial is introduced into the injection port using a manual probe or more recently using an autosampler. In the automated version, called DMI (difficult matrix injection) or recently introduced Linex (automated liner exchanger), the liquid sample (up to about 20-30 mL) is injected into the microvial placed in a liner, which is then inserted into the inlet [110-114]. After solvent evaporation, the inlet is rapidly heated and analytes transferred to the column for a GC separation. When the GC run is completed, the liner with the microvial is removed from the system together with non-volatile matrix components, which remain in the disposable microvial. In comparison with other injection techniques, DMI incorporates greater glass surface area in the inlet, which needs to be deactivated. Also, the activity of the inlet may vary greatly throughout the GC sequence because a new (different) liner and microvial are introduced into the system each time. In this respect, the use of analyte protectants was demonstrated to offer more convenient and effective solution than standard silanization of glass surfaces, thus another benefit in addition to the compensation for matrix-induced response enhancement [113].

A) without analyte protectants

B) with analyte protectants

A) without analyte protectants

B) with analyte protectants

matrix (fruit extract) and solvent (acetonitrile) solutions (A) without and (B) with the addition of analyte protectants (ethylglycerol, gulonolactone, and sorbitol). The numbers demonstrate signal (peak height) enhancement factors (signal in matrix vs. solvent) obtained without the use of analyte protectants and improvement in o-phenylphenol signal intensity in matrix with the use of analyte protectants. Reprinted with permission from Ref. [107].

matrix (fruit extract) and solvent (acetonitrile) solutions (A) without and (B) with the addition of analyte protectants (ethylglycerol, gulonolactone, and sorbitol). The numbers demonstrate signal (peak height) enhancement factors (signal in matrix vs. solvent) obtained without the use of analyte protectants and improvement in o-phenylphenol signal intensity in matrix with the use of analyte protectants. Reprinted with permission from Ref. [107].

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