Chromatography Forum

My 7693A ALS injector has recently started occasionally injecting about 50% larger volume that it should. We are injecting 5uL from the standard 10uL syringe. Several times now, we are seeing apparent injection of ~7.5uL. e.g. a 1000 ppb standard is suddenly 1500 ppb and the surrogate is correspondingly higher at 150%. We can see that the solvent peak is distintively fatter.

If this is normal wear and tear is there a kit for servicing the detent system that controls the syringe needle pull out?

Have you caught it in the act and seen it do this? It would be quite a large error to happen mechanically. Is it possible that there is something unreliable going on in the inlet and it's a different efficiency of vaporisation/split flow or something???

It happened out of sight. I suppose it could be a flow controller failure. I examined the injector mechanism. Its got detents for 1,2,3,4, and 5 uL. But if it misses that 5th position; the maximum volume is 7.5uL. Which is what I think got delivered. We reran the same vials the next day and it was all good. But this happened 3 times in one run.

There is a solenoid controlling the detent mechanism by pushing the detent arm against the matching steps in the syringe train. It seems to have free action (no stickiness). The surfaces all match, the notches seem clean and sharp. I've never seen this happen in 20 years of using it once or twice a week.

ah, okay, that makes sense. Good luck!

I experienced a similar problem years ago. Our injection was a 1.0 uL split, occasionally we would see 200% area. It turned out the solenoid was periodically sticking and would deliver 2.0 uL. We had sent it out for repair and have not seen this issue since.