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Solutions for impurity profiling and quality control of OLED materials

Analysis and management of impurities in raw materials, intermediates and final products are an important aspect in managing the characteristics of products, in the chemical industry, specifically the presence of trace amounts of impurities, e.g., organic molecules, can greatly affect product properties, safety and performance. Analytical technologies that can identify unknown impurities can provide a lot of benefits to industrial chemical processes such as improved yields, the reduction of defective products, and improved safety. Hence, rapid product development and quality control can be improved by adopting analytical technology that can reliably isolate, detect and quantify the main chemical components and potential low abundance impurities with good reproducibility.

Impurities analysis in manufacturing process

Various impurities (both organic and inorganic chemical components) may be introduced during the production of chemical products, and these impurity components may differ depending on the lot. For this reason, a method of product analytical testing that measures the purity of the main component and permissible concentration of impurities in each lot is widely used. Two suitable analytical testing techniques, liquid chromatography (LC) and mass spectrometry (MS), are routinely used for detecting and quantifying organic impurities during manufacturing in many industries. Organic impurity components that may be analyzed with LC and MS include :

  • Impurities in input raw materials and intermediates
  • By-products, decomposition products in reaction and manufacturing processes
  • Residual monomers
  • Residual substances from the previous process batch
  • Contamination from outside

Impurity profiling of OLED materials

Success in the market of high performance devices such as OLEDs can fundamentally depend on the quality of the materials that make up the individual components of OLED products. For example, organic chemical impurities contained in the material may lead to product degradation and shorten the lifetime of the device even when found at extremely low concentrations. In addition, since each material may differ in the type of impurity as a result of variability in the raw materials, synthesis, purification routes, and batch lots, it is necessary to utilize analytical technology that can detect and identify unknown organic impurities with high sensitivity over a broad chemical space.

In the analysis of OLED materials, HPLC combined with UV detector and a mass spectrometer is widely used for detecting organic impurity chemical components, in order to perform quality control.  Because these impurities are often found at low concentrations, it is important to keep in mind the following experimental aspects as methods are developed.

-Acquisition mode :

In order to detect unknown impurities, it is necessary to use a full scan data acquisition mode (i.e., covering a broad mass range or wavelength) rather than monitoring only at a specific wavelength or m/z. And, since the chemical components are likely unknowns, the acquisition mode of mass spectrometer will benefit from providing a means of identifying chemical components. One such methodology that high resolution mass spectrometry can provide elemental compositions that can in turn facilitate identification of chemical compounds by use of a database.

-Sensitivity :

In the case of trying to identify 0.01% concentration of impurities relative to the material, if the material is dissolved in a solvent at a concentration of 100 ppm(0.01%), the impurity concentration will be 10 ppb. For identifying unknown impurities, it is necessary to have a sufficiently sensitive detection limit that enables detecting and identifying peaks with concentrations in the range of several tens ppb in full scan mode.

-Data Processing :

Due to the searching nature of impurity profiling, it will also mean that an efficient workflows will be facilitated by a solution that can extract impurity peaks from enormous amounts of full scan data, provide elemental compositions for assisting with structural elucidation studies.

An example of impurity profiling of commercialized OLED material, E709

– Data acquisition

When measuring a trace amount of impurities relative to the main component, a large excess of the main component may interfere with impurity separation. By using the low dispersion UPLC system and UPLC column, it is possible to separate impurities that can-not be achieved with traditional lower resolution HPLC systems.

Three kinds of sample solutions below have been analyzed with UPLC/SYNAPT G2-Si. The SYNAPY G2-Si Q Tof system includes a Traveling Wave ion mobility (TWIM) separation functionality and dual collision cell. In order to conduct MS/MS, the SYNAPT was operated in MSE mode whereby the MS/MS acquisition mode alternates between a low energy scan to detect the precursor ions and then switches to a high energy scan to produce the product ions – all accomplished within a single analysis.

  1. Solvent Blank that is used as reference for comparing with sample solution.
  2. Low concentration sample that is used as reference for calculating relative concentration of impurity.
  3. High concentration sample that is used for identification of impurity and calculating its concentration (saturated main component is diverted to the waste)

As the data set was acquired with MSE mode which can provide precursor and fragment ion spectrum, information regarding the main component can provide clues to aid in the discovery and identification of impurities.

– Extraction of impurity peaks from total ion chromatogram

UNIFI software was used to process the data set. There are several approaches below to extract impurity peaks. UNIFI software can not only extract impurity peaks by these approaches, but also perform a database search using the elemental composition and generate structural elucidation using fragment match.

  1. Extract peaks with same fragment / neutral loss in common with the main component.
    Since the experiment was conducted in MSE mode, all precursor and product ion fragments were collected in one analysis. The information content then enabled the UNIFI software to search impurity peaks with fragment / neutral loss in common with the main component. The figure below shows the impurity with fragment in common with main component.
  2. Binary compare
    Often when comparing a total ion chromatogram of sample with another sample, it is difficult to extract peaks that are unique to only either one of samples. A graphic comparison of one chromatogram against a second called “binary comparer” can often highlight unique peaks in one sample over a second. To illustrate the principle, the figure below shows unique peaks detected from the sample by comparing sample and solvent blank.
  3. Search by synthetic pathwayIf the synthetic pathway is known (or can be proposed) based on prior knowledge, UNIFI can automatically search the related impurities by looking for transformations of main component structure.

– Elucidation of formula and structure

In high resolution mass spectrometry, elemental compositions are determined by a calculation based on the accurate mass and also incorporating the isotope pattern. The derived elemental composition can subsequently be used to perform a database search (both of online databases and locally developed databases). Since the extracted impurity peak also has appropriately aligned precursor ions and fragment ion information, fragment matches arising from fragment ion spectrum, can be compared with calculated fragments based on knowledge the chemistry and chemical structure in order to improve confidence of structural elucidation. These are sequentially processed in UNIFI software.

– Abundance of impurity

Several kinds of impurities were detected and structures elucidated (some examples are below). The lowest concentration of impurity was 0.005% (Imp05) and the highest of that was 0.127% (Imp06).

Once the impurities are found and identifies proposed in the research and development stage, a more focused study using either a photo diode array (PDA) and quadrupole MS can be applied to the QC monitoring and routine quantification analysis following manufacturing process.

By detecting and identifying unknown impurities like this, we are able to more clearly manage the manufacturing process and product quality and contribute to maximizing profit at each stage from development to production.

Proposal system solution for impurity profiling of OLED

UPLC with PDA detector and SYNAPT G2-Si

– UPLC column with high physical and chemical strength for expanding chromatographic selectivity

– Ultra high speed and resolution for method development and effective impurity separation.

– Photo Diode Array detector that realizes highly accurate profile by peak-purity test

– Accurate mass stability

– Dual collision cell that can effectively provide fragment ion from even rigid component

– Compatible with universal ion source, DESI, APGC, ASAP etc

– Ion Mobility Separation that is efficient for isomer separation and spectrum clean up

– TAP fragmentation (MS/MS/MS. Alignment between fragments)

– Appropriately aligned precursor and fragment ion spectrum

– Sequential elucidation workflow with Elemental composition, database search and fragment ion matching

– Automatically picking peaks with same fragment or neutral loss in common with main component

– Automatically extraction peaks based on predicted reaction and transformation.

Proposal system solution for quantification of impurity in QC

Alliance HPLC / ACQUITY Arc UHPLC with PDA and ACQUITY QDa

– Prefect method transfer to HPLC / UHLC that can produce same chromatographic picture as it obtained by UPLC

– Continuous blending of four solvents

– Photo Diode Array detector that realizes highly accurate profile by peak-purity test

– Easy-to-use Single Quadropole detector compatible with strong organic solvent

– Mass detection that can obtain mass information by same operation as optical detector.

– Intuitive operation

– Embedded functionality of purity test and spectrum match with PDA spectrum

– Customized calculation and reporting function

– Effective workflow from acquisition, processing to reporting for every operator.

– Regulatory compliance

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