Table 4 Overview of advantages and disadvantages of the semi-quantifying approaches

Method 1B. (*) External calibration curves obtained by adding standards to the sampling system

High accuracy due to identical sampling conditions between external calibrators and samples

Calibration curves are valid only for the specific sampling conditions (i.e. time or headspace volume)

Method 2. (*) External calibration curves obtained by adding standards to stir bars

Calibration curves are valid independent of sampling conditions

Lower accuracy due to different sampling conditions between external calibrators and samples

*NearestRT

High accuracy due to the usage of chemically similar compounds for semi-quantification

Several calibrators along the chromatogram need to be used

*Single calibrator

Statistical significance of the data is consistent, indicating that any calibrator is valid

A certain level of inaccuracy may result from a lack of chemical similarity between calibrator and sample VOCs

Method 3. Peak area/g fresh weight

It indicates the relative abundance among VOCs

There is no magnitude

Method 4. Normalized peak area/g fresh weight

Generally accepted as a precise mean to analyze relative abundance among VOCs

Bias due to differential stir bar adsorption between the internal standard and certain kinds of VOCs

There is no magnitude

Method 5. Single internal standard peak

Generally accepted as a precise mean for semi-quantification

Bias due to differential stir bar adsorption between the internal standard and certain kinds of VOCs

Method 6. NearestRT n-alkane

High accuracy due to the usage of chemically similar compounds for semi-quantification

A certain level of inaccuracy may result from a lack of chemical similarity between calibrator and sample VOCs

Method 7. Percentage analysis

Generally accepted as a precise mean to analyze relative abundance among VOCs

There is no magnitude