Borate carry-over problem as hard nut to crack. Help needed!

[Jam]

Dear All,

I turn to you after trying almost everything to resolve this tricky carry-over problem (at least we think its carry-over). We are detecting borax with HPLC system having a RI detector. The standard solution prepared from boric acid dissolved in water is 0.25 mg/mL. System parameters are as follows:

Mobile phase: 0.25 mmol/L perchloric acid
Injection volume: 20 µL
Column: Shim-pack SCR-102H (8.0 mmI.D. x 300mm) Shimadzu Co.
Column oven and RI detector T: 40°C
Polarity /sens.: +32

First we used simply mobile phase as a needle-wash solution. Mobile phase is injected after six standard injections. The carry-over peak area varied between 1.2 - 5.4 % compared to standard peak area. The same phenomenon was noticed in Waters Alliance 2695 and Agilent 1100 systems. The reason for % carry-over variance remains unknown, even though it seems that carry-over is smaller, when mobile phase is injected before any standard injections after having washed the system throughly. Then different tests were carried out with Waters system:
1) needle wash solution 50%MeOH; carry-over 3.2%
2) needle wash solution 100%MeOH; carry-over 4.5%
3) needle wash solution mobile phase, but vials without septums and every mobile phase injection from separate vials; carry-over 3.4%
4) needle wash solution mobile phase, but no boric acid standard was introduced to system, only borax sample; carry-over 3.2%
5) when injection volume was doubled the carry-over peak area was doubled as well as standard; carry-over 4.1%
6) Standard and mobile phase were analyzed for 60 minutes and no peaks eluted after borate peak.

In every test the needlewash time was extended.

The system was washed with water and methanol, and passivated with 6N nitric acid. The purge injector, primeneedlewash were performed with all wash solutions and nitric acid. Tests were carried out after this.
7) needle wash solution water; carry-over 0.8%
8.) the test 7) was repeated; carry-over 1.7%
9) needle wash solution 6N nitric acid; carry-over 2%

And that's it. My mind is empty. Has anyone played around with this kind of phenomenon before? Any advices?

Best regards,
JAM

[Uwe Neue]

A few more pieces of information are necessary before we can conclude that it is true carry-over. Don't forget, with an RI detector, you will see anything.

1. What is the retention time of the carry-over peak? Does it agree with the retention of the analyte peak?

2. How do you know that the peak that you get is the analyte peak, and not a peak from the sample solvent? (You will need to vary the analyte concentration and not just the injection volume.)

3. Does the peak area change from injection to injection? True carry-over peaks decline from injection to injection, assuming that you are injecting blank mobile phase. If you get the same peak with the same peak area from a mobile phase injection, it is related to a subtle difference in the composition of the injection and the mobile phase eluting from the column. The reason for this could be manifold, but it is not carry-over.

[LarryHouse]

perhaps you can try to narrow it down further by making sure it isn't a peak getting knocked out of the column? try removing the column, injecting a stock solution, watch the signal response that shows up. then try injecting mobile phase- and see if you see a signal.. ?

[Jam]

Dear All,

Problem solved!! Yesterday after having sent the post, I went home, threw myself on the sofa to take a nap, and then I realized the reason for this phenomenon.

This far we have used regular laboratory glassware to prepare standards and samples, also the vials have been glassvials. Of course we have to use plastic vials, flasks etc., because Boric acid and borates are used in glass manufacture, what is why we got a small peak also from mobile phase. Today we have tested this idea, and there are absolutely no peaks in the mobile phase chromatograms, when plastic vials and bottles are used. Jihaaa!

Thank you Uwe and Larry for your input anyway.

Best regards,
Jam

[Jam]

..... so it was not a real carry-over problem in the first place...

Happy new Year!!

Jam