Persistent ghost peak

[Jonno02]

Hi,

Looking for a little help with an annoying persistent ghost peak on my LC system.

I'm using:
Hypersil GOLD C18 column
0.1% orthophosphoric acid in ACN (45%)
0.1% orthophosphoric acid in HPLC water (55%)
DAD detector set at 272nm
0.3ml/min flowrate
sample run time of 5 minutes
pre-injection derivitisation using Pentafluorobenzaldehyde in ACN:H20 (9:1) - also used as my blank


This method is used to quantify aqueous formaldehyde in water samples. It has worked well for the past year with no issues, all we've had to do is change our column twice and the usual routine maintenance.

Recently, we've noticed our calibration works perfectly (standards of 0.5, 1, 2, 4, 6, 8, 10 and 12 ppm). After this, we run 3 blank samples as per normal procedure. Normally all of these blanks read no formaldehyde.

However, we're now getting around 1.2ppm of formaldehyde detected on every single blank; no matter how many we run. Our normal sample retention time is 2.881 minutes. With the 'ghost peak' we're seeing retention times of 2.850 minutes.

So far, we have:

- Completely renewed all mobile phase; and pre-washed the containers in the relevant solvents.
- Changed the column for a brand new one that was conditioned with mobile phase for 24 hours; still detecting the same peak.
- Checked the system for leaks.
- Tried a 100% ACN wash for 2 hours.
- Tried a h20 1 hour wash, followed by 50/50 ACN/IPA wash for 1 hour.
- Raising the sample run time to 20 minutes.

Nothing seems to have worked, so I'm turning to the experts in the hope somebody could give me some pointers.

Greatly appreciate any help.

[Zoraku]

Have you looked at fresh Pentafluorobenzaldehyde? If you inject just Acetonitrile or some other solvent do you still see the peak? Or is it only when you inject the actual blank?

[Jonno02]

Hi zoraku,

Yes I've tried fresh pentaB and yet our ghost peak remains.

I've just done a few no volume injection runs and our ghost peak wasn't present, does this perhaps mean it's something within our sampler?

[Zoraku]

Hi zoraku,

Yes I've tried fresh pentaB and yet our ghost peak remains.

I've just done a few no volume injection runs and our ghost peak wasn't present, does this perhaps mean it's something within our sampler?

I would start leaning toward that as the possibility. If you didn't see the peak initially, then after you saw it replaced all consumable components (mobile phase, all fresh solvents in sample preparation, new column, etc...) then I would start to look at possible system contamination somewhere.

Do you use a needle wash after or prior to each injection? What is the solvent? I'd look at this as a possible source. Flush the syringe a few times and do your best to try to clean the system.