Negative peaks with SAX column in HPLC

[stela13_77]

When injecting water I get 2 negative peaks in the HPLC analysis
They only show up when I inject pure water, when injecting the mobile phase they don't show up.
The mobile phase consists of 0.1 M KH2PO4 adjusted to pH 3 with H3PO4.
UV detection at 220 nm, flow 1,0 ml/min and isocratic elution.
Rt peak 1: 1.7 min and peak 2: 2,9 min
Any suggestions what causes the two negative peaks?
Especially the second peak puzzles me
I have added a chromatogram showing the negative peaks:

https://ibb.co/sp7KG4dT

Has you can see it is not two "blips" a one has replied.

[Multidimensional]

It sounds like you are using HPLC for the first time without any formal training (are you a student at school? If so, please ask your professor to provide a trained chromatographer to help you. No one can learn to operate an HPLC on their own). The "peaks" you are seeing would be normal when you inject a liquid that is not 100% identical to the mobile phase. Injecting water onto the column that is equilibrated in aqueous buffer should in fact show two small "blips" on the baseline at the column's void volume. These are normal. Please be sure to work with an experienced chromatographer as doing so without any formal training often leads to a great deal of confusion and misinterpretation of results. *It takes an average of five years of full-time industrial training just to learn the basics of liquid chromatography. Even more years to learn method development! This is not a technique that you can learn by taking class, reading books or watching web videos.

[vmu]

The two negative peaks in question are likely to be the vacancy peaks caused by disturbance of the equilibrium between the components of the eluent and the stationary phase when pure water is injected. If the column here is 250 mm x 4.6 mm, the first peak elutes close to the column interparticle volume (suggesting the presence of ion-exclusion effects for this peak) and the second peak elutes close to the column hold-up volume (the sum of the interparticle volume and the intraparticle pore volume).

[stela13_77]

Thank you for the excellent and insightful explanation vmu.
I wanted to know the reason for the negative peaks, they have no direct impact on the analysis. When I changed the composition of the eluent going from pH 6 to pH 3 I started to see the second peak. Both eluent are based on phosphate salts/acid. The first peak close to the voidtime was also present when using the phosphate buffer with pH 6.
The method will be used for quantification of nitrate and nitrite in a pharmaceutical product.
Never worked with SAX in HPLC, more used to regular RP-HPLC with C18 columns.

[vmu]

The first peak close to the voidtime...

For a 250 mm x 4.6 mm column filled with fully porous particles, the retention volume of your first peak is closer to the column interparticle volume (aka "exclusion limit") than to the column total void volume, which is the sum of the interparticle channels volume and the intraparticle pore volume. Ion-exclusion mechanism is involved in formation of the first peak. The second peak elutes closer to the total void volume (hold-up volume).