At this point, by virtue of acquiring a Master's degree in the field, you've confirmed at least two things. First, you can dedicate yourself to working very hard for a reach goal. Second, you obviously have interest in working in this occupation since you chose this area of specialization. Emphasize these two accomplishments in the supplemental experience area of the application. Now, this is where you must learn to sell yourself.
Creativity is key, and writing style is always an indicator of abilities.
Site examples of writings, reports, classes, lab work, etc. to back this up. Read the job description you're applying for and look for an angle, or a niche, where they need a unique talent that you possess. Know the company, do some research and use this information. Does it require exceptional problem solving skills, attention to detail, creativity, and people skills. Compare this information to your own personal skills inventory.