## 3 SPME Process
### Quantitation
#### 3.5.1 Traditional Calibration Methods  
Traditional calibration methods include external standards (calibration curve), internal standard or standard addition method.  
SPME calibration can be performed either by equilibrium or pre-equilibrium calibration, respectively. In equilibrium calibration a partitioning equilibrium between the sample matrix and extraction phase is reached. The convection conditions do not affect the amount of extracted analytes due to the present partitioning equilibrium [22]. In pre-equilibrium calibration the amount of extracted analytes is related to the extraction time [22]. The convection/agitation thereby is constant. Applying equilibrium calibration for quantitation purpose is more sensitive and does not depend on the time. However, if the equilibrium extraction is too long, the pre-equilibrium extraction is preferred [22].  
The internal standard method is widely applied for quantitation in SPME methods. It has been applied for the quantitative analysis of environmental [165, 166] and food [167, 168] samples. For heterogeneous samples the standard addition calibration method is preferred [169, 170]. Using an internal or a surrogate standard (preferably mass-labeled internal standards) compensates for matrix effects and instrumental or sample preparation variability [171, 172, 173, 174, 175]. In addition, external standards are widely used [22, 176, 177, 178, 179, 180].  
\begin{table}
\begin{tabular}{l l l l l} \hline \hline
**Calibration method** & **Advantages** & **Disadvantages** & **Applications** \\ \hline
**Traditional** & External standard & No extensive sample preparation & Need for availability of blank sample & [176, 177, 178] \\  & & & matrices & \\  & & & Need for stable sampling procedure & \\  & & & and chromatographic conditions & \\  & & & & \\  & Standard addition & & Extensive sample preparation and & [165, 166, 171, 172] \\  & & & analysis & \\  & & & & \\  & Internal standard & Correction of sample matrix effects & Limited availability of suitable internal & \\  & & & standards & \\  & & Compensation of matrix effects and & High cost and limited availability of & \\  & & losses of analytes during sample & isotope-labelled standards & \\  & & & preparation and irreproducibility in & \\  & & & parameters (injection in GC/LC) & \\
**Equilibrium** & & Possibility to calculate concentration of & Need for knowledge about distribution & [181, 182] \\  & & analytes by amount of extracted & coefficients & \\  & & analytes & & \\  & & Independence of amount of extracted & \\  & & analytes of sample volume & & \\  & & & \\  & & & \\  
\begin{table}
\begin{tabular}{p{108.4 pt} p{108.4 pt} p{108.4 pt} p{108.4 pt} p{108.4 pt}} \hline
**Calibration method** & **Advantages** & **Disadvantages** & **Applications** \\ \hline Diffusion-based & [PERSON]’s first law of diffusion & Suitability for TWA sampling & Sorbent should be zero sink for target analytes & [184, 185] \\  & & Independency of sampling rate of face velocity & Very low sample rate for water sampling & \\  & Interface model and cross-flow & Minimizing of competitive effects for solid coating through high sampling & Need for controlled or determined flow velocity of sampling matrix & [186, 187] \\  & mode & rate and short time & Application limited to linear sampling & \\  & Suitability for on-site sampling & regime & \\ Kinetic calibration with standard & Suitability for TWA sampling & Need for determination of standard loading & [188, 189] \\  & Standard-free & No need for standard loading & Need for stable sampling conditions & \\  & kinetic calibration & Possibility to calculate concentrations of all extracted analytes in sample & & [190] \\ \end{tabular}
\end{table}
Table 2: _Cont._