(M+17)- in NEGATIVE ionization?

[Katrina]

Hello,
I am working in a Shimadzu LC-MS 2010EV, recently I' ve observed, in the negative mode an adduct M+18,
The probe is APCI, my column is new (Purospher RP8), my mobile phase is ACN/H2O (9:1), I use buffers (CH3COONH4) very often, I always wash the system and our samples are free from contaminants. My exact mass is 1483, i see the 1484 (M+1) in the possitive mode and a major peeak (100%) of 1500 (M+17) in the negative mode, {I also see 1518 (M+35), which is the Chloride adduct} The particular molecule has a Bromine and an Boronic ester, its generally non-polar (that's why i am using APCI, with the ESI probe the ionization could't take place), the sample is dissolved in DCM and then to ACN.
If someone has any suggestions....
Thank you in advance

[Katrina]

Hello again,
Complementary information always help:
I make a run in which I replaced the H20 of my mobile phase with buffer CH3COONH4 10mM PH=4.5, everything else remained the same.
This time I obtain in the possitive mode a major peak 100% of m/z 1501 (M+NH4)+ and in the negative mode a major 1500 (M+17)- 100% , a 1518 (M+Cl)- 40% , and a 1542 (M+Acetate)- 26%
I' ve googled the negative adduct of M+17, but I couldn't find nothing mentioned before,
I am looking forward for an answear
Thanks

[bhuvfe]

Do you have any reason to suspect an electron capture mechanism (since you have a Br)?

Is there a chance that you have an extra ionizable group that can form a ammonium salt, a COOH and SO3H, etc?

[sav]

1500: adduct [M+H2O-H]- or not adduct [M+OH]- or [M+X-Y]-* and ...
1518: + Cl? Why?

Can you post some mass spectrum and "picture"(or CxHyOz...) of molecule?

[Katrina]

1499.45: adduct [M+NH3-H]- or not adduct [M+OH-H]- or [M+X-Y]-* and ...
1518: + Cl? Why?

Can you post some mass spectrum and "picture"(or CxHyOz...) of molecule?

Can you be more spesific in your answear?
C71H124BBrO26, this is the molecular formula, I can not post you the structure (industrial regulations),
but the molecule has a basic core of fluorene, and two extra long etheric chains

[southp]

Maybe these are water adducts of the [M-H]-? Sav seemed to be thinking along that line too. If so, the Br isotope should give also 1502 and 1520.

1500 m/z = [M-H+H2O]-
1518 m/z = [M-H+2H2O]-

You didn't mention adding a source of chloride or seeing extra 1520 m/z, which you'd expect if 1518 were a chloride adduct.

[sav]

I think...
1500: [(RO)3B(-)OH]-
1518: [(RO)3B(-)OH+H2O]- (may be, cheсk isotope model)

+APCI: Isotope peak with 100% rel. abundance (rel. int.) [М]+* (radical ion) for C71H124BBrO26 is 1484.77 amu, [M+H]+ is 1485.77 amu

Without structure of molecule, MS parameters and mass spectra is very difficultly to predict [M+X-Y]- or...

Bororganic compounds often give some unpredictable results with ESI, APCI.