Need advice: Coworker negotiated higher salary, HR won't budge for me

Sorry for the long post: I’ve been a part-time medical interpreter at a prestigious hospital for almost a year. My qualifications:

* BA in Translation and Interpreting, MA in Spanish

* 10+ years teaching languages in higher ed. institutions in Europe, Canada and the USA

* 1 year medical translation experience in a hospital in Spain

* Recently acquired more on-job credentials

I understand some of my experience is not *directly* related with my current job. When hired, I accepted the initial offer without negotiation, wanting to be humble since I lacked FORMAL interpreting experience (although my BA is actually on that). Recently, a new interpreter with a similar background joined as PRN (as-needed basis). He just went full-time, negotiating a salary $10/hour higher than mine per hour (this is what I gathered from what he shared). I truly believe I’m more qualified.

What happened is that two full-time positions opened simultaneously. I had to take the later one due to family circumstances, although the first one was offered to me first. When I applied, HR asked for my salary expectations. They called me back this week and offered the same salary I currently earn, claiming “no room for negotiation” (why would they ask on the first place, then??!). This contradicts their request for expectations and ignores our previous conversation.

I tried negotiating, citing my experience, excellent reviews, new credentials and my ongoing training for Portuguese in order to be able to serve this population of patients. They insisted it’s the “same role, just longer hours.”, so they can’t do anything about that. I’m baffled. Why could my coworker negotiate while I can’t?

Important notes:

* I’m the only weekend interpreter, managing the entire patient load

* My boss trusts me and is very happy with my performance

* I’ve received ovations for my work

* Despite my qualifications, I’m living paycheck to paycheck

I approached my boss without mentioning my coworker situation, but she claims no control over HR decisions and can’t advocate for a higher salary.

I’m confused and hurt. How do I address this with HR? All I want is fair treatment because this raises concerns for me in terms of equity… Don’t you think ? I have another call with HR next week and I feel I need to bring this up, without affecting others, but I also feel I need to advocate for myself. If I’m missing something, I’d appreciate you let me know. Thanks for your help in advance.