Influence of the epitaxial composition on N-face GaN KOH etch kinetics determined by ICP-OES.

Roughening by anisotropic etching of N-face gallium nitride is the key aspect in today's production of blue and white light emitting diodes (LEDs). Both surface area and number of surface angles are increased, facilitating light outcoupling from the LED chip. The structure of a GaN layer stack grown by metal organic chemical vapour deposition (MOCVD) was varied in the unintentionally doped u-GaN bulk region. Different sequences of 2D and 3D grown layers led to a variation in dislocation density, which was monitored by photoluminescence microscopy (PLM) and X-ray diffraction (XRD). Thin-film processing including laser lift off (LLO) was applied. The influence of epitaxial changes on the N-face etch kinetics was determined in aqueous KOH solution at elevated temperature. Inductively-coupled plasma optical emission spectroscopy (ICP-OES) was used to measure the etch progress in small time increments with high precision. Thereby, the disadvantages of other techniques such as determination of weight loss or height difference were overcome, achieving high accuracy and reproducibility.

Quantitative capillary zone electrophoresis method for the precise determination of charge differences arising from the manufacture of heparan-N-sulfatase.

A rapid and reproducible high-resolution capillary zone electrophoresis (CZE) method capable of resolving the charge isoforms of intact heparan-N-sulfatase (HNS) has been developed to monitor the charge consistency across different batches of HNS. Separation was carried out using a bare fused silica capillary with a buffer system composed of 25 mM Tris, pH 8.0. This CZE method allowed the separation and integration of 14 peaks, each arising from differences in the amount of sialic-acid and mannose-6-phosphate bearing glycoforms, which were confirmed using enzymatically modified samples. Standard conditioning and rinsing conditions of the capillary were used to achieve optimal repeatability. Excellent day-to-day precision was obtained for migration times of each peak relative to the electroosmotic flow marker with relative standard deviation (RSD)≤ 0.5%. The precision of the relative peak areas (peak area percentages) ranged from 0.6% to 2.8% RSD for the major isoforms (peaks 3-12), from 4.0% to 5.0% RSD for peaks 1 and 2, and from 7.4% to 23.2% RSD for peaks 13 and 14. The method was able to discriminate charge variation across different batches of HNS, including those with both significant and minor process changes.

Determination of impurities in heparin by capillary electrophoresis using high molarity phosphate buffers.

Oversulfated chondroitin sulfate (OSCS), an impurity found in some porcine intestinal heparin samples was separated from intact heparin by capillary electrophoresis (CE) using a 600mM phosphate buffer, pH 3.5 as the background electrolyte in a 56cm x 25microm i.d. capillary. This method was confirmed in two separate labs, was shown to be linear, reproducible, robust, easy to use and provided the highest resolution and superior limits of detection compared to other available CE methods. Glycosoaminoglycans such as dermatan sulfate and heparan sulfate were separated and quantified as well during a single run. The heparin peak area response correlated well to values obtained using the official assay for biological activity. A high speed, high resolution version of the method was developed using 600mM lithium phosphate, pH 2.8 in a 21.5cm x 25microm i.d. capillary which provided limits of detection for OSCS that were below 0.1%.

Determination of impurities in the drug 5-aminosalicylic acid by micellar electrokinetic capillary chromatography using an electrolyte pH that approaches the isoelectric point of the parent compound.

Process impurities found in the drug substance 5-aminosalicylic acid were determined by micellar electrokinetic capillary chromatography (MECC). In order to enhance the detection of trace impurities, the parent drug was dissolved to an unusually high concentration of 5 mg/mL. To reduce the dispersive processes of electromigration dispersion and anti-stacking, both of which occur at high solute concentration, the electrolyte pH was adjusted to be close to the apparent isoelectric point (pI) of the zwitterionic drug. In this fashion, the net charge on the solute should approach zero thereby minimizing the aforementioned sources of band-broadening. Two additional developments are reported. Short sodium hydroxide washes were used to optimize the MECC reproducibility. The elimination of anti-stacking permitted the use of peak heights to quantitate low level impurities with improved precision. The method was compared to the high-performance liquid chromatographic (HPLC) method found in the United States Pharmacopoeia (USP). Both the MECC and HPLC methods were found to be similar with regard to migration-retention time reproducibility, peak area-height reproducibility, linearity and limit of quantitation. The MECC separation is 2x faster than HPLC. Both methods meet the system suitability requirements described in the USP.

Separation of phenols as neutral compounds by micellar electrokinetic capillary chromatography.

Separation of phenols as neutral solutes by micellar electrokinetic capillary chromatography provides a quantitative linear dynamic range of 6000-13,000. Since the compounds are injected and separated as neutral solutes, the dispersive processes of anti-stacking and electrodispersion are eliminated. Optimized conditions allow for sub-ppm quantitation of trace impurities in the presence of the major components at various stages of the production of high purity phenols. The background electrolyte consists of 100 mM sodium dodecyl sulfate in 50 mM phosphate buffer pH 7. The method is precise, reliable, and the limits of detection are superior compared to HPLC by a factor of 20.