Addition of an internal standard to check solvent volume is recommended at this stage.
External calibration standards were analyzed with every set of samples, and recoveries of internal standards were checked against external calibration standards.
First, the chemical compatibility between sample and internal standard has been checked by acquiring a proton spectrum and, where required, a P NMR spectrum of the mixture right after preparation and again after 24 h.
The check of the internal standard in SPECT was, in general, extremely accurate.
As relaxation times largely depend on the mixture and the solvent, the evaluation of the relaxation times was carried out for entire mixtures (internal standard, analyte, and solvent) simultaneously with the compatibility check.
A check of the accuracy of the internal standard in the study with identifier 3 reveals an error of −2.2%%, and for identifier 10, the error in the internal standard was +1.7%%.
In biological/medical work it is common to use the natural 40K present in all tissues/body fluids as a check of the equipment and as an internal standard.
5-8 mg of lyophilized cells were resuspended in 0.6 mL of chloroform and 0.6 mL of methanol containing 15%%(v/v) H2SO4; 1 μL Trinonadecanoin (Nu-Check Prep Inc., Elysian, MN) was used as internal standard for quantification [ 44].
The method was validated for qualitative analyses only but to each sample, the internal standards were added and the recovery of the internal standards (criteria S/N of internal standard has to be >3) was used as a sensitivity and recovery check.
For tandem mass spectrometry 1 ml urine samples were neutralised and checked for pH, and purified after addition of an internal standard (1 – methylhistamine D3, Dr, Ehrensdorfer GmbH, Augsburg, Germany) by ion exchange (cation exchange Amberlite CG50, Sigma, Munich, Germany).
