Chromatography Forum

I am analyzing a mixture of C8 olefinic ketones (from a petroleum stream). The MW and therefore m/z is supposed to be 126. I have a standard of a pure C8 olefinic ketone which is also supposed to be MW 126. Analysis is by GC-MS on a Varian Saturn 2000 Ion Trap.
Instead of 126, I keep getting 126.8 or 126.9 (searches as 127). Essentially M+1. I spoke to someone in tech service at Varian and he mentioned that this happens some time with ion trap that the M+1 is generated. This is causing me problems since I am not getting "hits" when I search my compounds against the NIST library which is all generated by quadrupole MS. I did a quick analysis of standard on Agilent Quadrupole benchtop MS and got 126 as expected. Can anyone shed add'l light on this?

What concentration are you using? If greater than 10ppm, I have sometimes noticed the M+1 effect with my saturn 2000 ion trap. However, usually if you click somewhere not at the top of the peak, you get the expected results. Try clicking at other points on the peak.

Also, if you have changed flow rate, make sure you have done an autotune with the flow rate being used by the method. The Varian ion trap is designed for optimal use with flow rates between 0.75 - 1.5 mL/min.

Additionally, you could also save your own generated MS to a user library to be searched against.

-Aaron