Irma Thomas, Barbican, London

As is the way with R&B royalty, the Soul Queen Of New Orleans is heralded by her band's loosener, a muscular "Superstition" that gets the crowd primed for some fatback funk. But when she is announced, Irma Thomas is nowhere to be seen, her voice appearing to float down from heaven. She finally appears, led gingerly up the steps stage left, microphone in hand, her voice astonishingly clear for one now into her eighth decade.

Her longevity may have something to do with a relative lack of mainstream success: unlike many of her soul peers from the Sixties, Thomas has performed constantly since, and that workload has helped her voice retain its power and emotional subtlety. As she works through hits such as "Time Is On My Side", "Ruler Of My Heart", "It's Raining", the bustling "Break-a-Way" and the majestic "Take A Look", her ability to sustain high notes is quite extraordinary. It seems a truly effortless delivery, Thomas barely opening her mouth as she sings and mining each phrase for maximum impact: her focus on the emotional heart of a line is like a dart through the soul.

Punctuating her show with genial patter about the effects of ageing – which in her case seem minimal – Thomas draws material from all corners of her career, the early hits balanced by later successes such as "In The Middle Of It All", from the Grammy-winning After The Rain (2007), which was recorded after Hurricane Katrina destroyed her Lion's Den club in New Orleans.

Statuesque in a sparkly, floor-length white gown, Thomas sways gently as her band peels off groove after groove, deftly handling audience requests for even the most outré items in her back catalogue. "We'll get that one out of the way, then get to the easy ones," she instructs the musicians, before launching into "Yours Until Tomorrow". The song's pleading is handled with remarkable poise.

Occasionally, she consults a huge tome on a lectern, to remind herself of a lyric. The thickness of the book is testament to an extraordinary career. "I haven't done that one for over 20 years!" she marvels, while searching for "In Between Tears", which she sings acappella, since her band hasn't rehearsed the song.

When you've been performing for over half a century, you get the job done any way it takes.