BS EN ISO 10927:2011 pdf free download – Plastics – Determination of the molecular mass and molecular mass distribution of polymer species by matrix- assisted laser desorption/ ionization time-of flight mass spectrometry (MALDI-TOF-MS) (ISO 10927:2011).
8 Data acquisition and processing
8.1 General
Data acquisition can depend on the type of data acquisition system and computer software used. The raw- data file generally consists of data pairs (signal intensity and the corresponding time of flight) which, through the use of a calibration curve, enables a mass spectrum (signal intensity versus mass) to be constructed.
8.2 Calibration
8.2.1 General
For the calibration of the mass axis, use one of the two methods described in 8.2.2 and 8.2.3.
8.2.2 Calibration of mass axis using synthetic-polymer standards
8.2.2.1 Selection of standards
In this method of calibration, a synthetic-polymer standard with known repeating units and end groups is used. Use a well-characterized synthetic-polymer standard with a molecular mass lying within the molecular mass range of the polymer being investigated. Previous calibration of the mass spectrometer using a biopolymer can be used to assist in assigning oligomers to the correct masses. If such a biopolymer is used, its molecular mass shall be in the mid-range of that of the synthetic-polymer calibrant. The main peak obtained from the biopolymer is assigned to its mass as given in Annex A.
8.2.2.2 Sample preparation
Prepare solutions of polymer standards used for calibration using one of the procedures given in 7.2. Synthetic polymers used for the final calibration shall be run under the same conditions (same matrix and same laser fluence) as used for the test samples.
Prepare biopolymer standards used for calibration using the procedure given in 7.2.4.
8.2.3 Calibration of mass axis using biopolymer standards
Biopolymers from Annex A shall be used for mass axis calibration. Prepare a fresh solution of each biopolymer used. Select polymers of at least four different molecular masses which bracket the anticipated molecular mass range of the polymer being investigated. The molecular masses of the salt and the matrix can also be used for calibration.
8.2.4 Self-calibration method
The principle of self-calibration using the polymer which is being investigated can be used if the oligomer structure and the structure of the end groups are known. However, it is essential to have previously carried out an exact calibration. Using this method, a single peak of the test sample can be attributed to its theoretical molecular mass.
The self-calibration method should only be used exceptionally and is to be regarded as a fine tuning of the calibration methods described in 8.2.2 and 8.2.3.
8.3 Generation of calibration curve
Generally, an instrument will have software to derive a calibration curve for that instrument. This curve shall
be calculated using at least four calibration points obtained by one of the methods described in 8.2.2 and 8.2.3.
8.4 Signal intensity axis calibration
Calibration of the signal intensity axis is not necessary for polymers with a polydispersity of less than 1,2. Calibration with an internal standard, often used in biochemistry. cannot be done with synthetic polymers.
9 Expression of results
9.1 Calculation of molecular mass distribution
Once the MALDI spectrum of the polymer has been recorded, the intensity of the signal corresponding to each species, i, in the distribution is determined. The limits of this calculation are defined by the species with the lowest and highest molecular mass. A signal-to-noise ratio of at least 3:1 shalt be used as the threshold for the integration of the areas under the peaks.
Integration shall be performed over all the isotopes related to a peak. The molecular mass assigned to the peak is taken as that corresponding to the apex. M. or the centroid, Mc, of the peak. The choice of M or Mc shall be consistent with the choice made when generating the calibration curve.
Poly(ethylene glycol) polymers with high molecular masses (>10 000 g.mo[1) are known to form fragments which impede the correct identification of peaks at low signal intensities which are necessary for the calculation. In such cases, alternative sample/rnatrixisaht mixtures (e.g. a DCTB1) matrix and potassium or caesium salts) should preferably be used.
9.2 Calculation of the average molecular masses
The number-average, mass-average and z-average molecular masses (Me, M and M) can be computed using Equations (1) to (3).