To Mik**, the reason why failed in the assay of Ractopamine lies on the sample pre-treatment. Here are some information from my note.
(1) Transfer 3.0 g of feedstuff sample into 25 mL solution of 2% Ammonia in MeOH and place onto Vortex 1 min, then sonicate and swirl 5 min. Pipet 10 mL of upper clear solution and transfer to a 1 ml flask, make the dryness @ 50 C by Rotatory Evaporator.
(2) Using 2 mL of MeOH and 2 mL of n-hexane to dissolve the residue, discard the upper solution, and repeat the above procedure twice. Collect the 10 mL of MeOH portion and blow with N2 to dryness @ 45℃. Dissolve the residue again with 5 mL of Ethyl Acetate, add some anhydrous sodium sulfate, and centrifuge the solution@5000rpm for 3 min.
(3) Apply Polystyrene SPE column purify the analyte: (a) Column solvation: apply 5 mL of Ethyl Acetate and 3 mL of Acetonitrile/Ethyl Acetate (1:1). (b) Interference elution: Elute Analyte by using 10 mL of MeOH/Ethyl Acetate (1:1), collect the elute. (c) Concentrate: Under 40 degree C, evaporate on a stream bath to dryness (or with Nitrogen blowing Instrument). Mixing with 0.1 mL BSTFA + 1%TMCS by using Vortex for 1 min, Heat in oven 80 C for 1 hr. Blow with N2 to dryness by Nitrogen blowing Instrument. Bring to the volume with 0.2 mL of toluene for GC-MS analysis.
GC Column: 5% Phenyl-Methylpolysiloxane 30 m×0.25 mm I.D.×0.25μm.
Inlet Temp: 300℃.
Oven: Initial 150℃ (3 min), 10℃/min to 230℃ (10min)， 20℃/min to 280℃ (10min).
Carrier Gas: Helium（99.999%）1.0mL/min
Injection: 1.0µL, pulsed splitless, 50 ng on-cloumn per Inj.
EI Temp: 230℃.
MSD Interface：280℃.
EM voltage: 1506V.
Scan Rage: 30～550U.
SIM (m/z): 267、250、502，Quantitative ion 250.
Solvent Delay：5 min. ( I can not forget the accident we made @ Tech Dec-2005. How about you?).
Please let me know if this sample pre-treatment method works.
BTW, How do you valuate the Beijing 2008 Olympic Game Opening ceremony? for me, feel humble in the front of 5000-years China History.
For Varia** AAS, I am NOT a Technical Expert, but according my 3-years experiences of Varia*: Coded Hollow Cathode Lamps is only good for the automatic lamp recognition SpectrAA software. So I prefer the Lower priced uncoded Hollow Cathode Lamps.
GC Column: 5% Phenyl-Methylpolysiloxane 30 m×0.25 mm I.D.×0.25μm.
Quantitation Database Globals:Type the Calibration Title, Default Multiplier, Default Sample Concentration, Reference Window/Non-Reference Window, Reference window for Internal STD,Correlation Window(For Scan: 0.05, For SIM:0.025)
Double click the Right button, type the chemical name. If it's a internal STD, Select the Internal icon. Move the “+” to select the targeted ions,click the Left/Right button at the same time. DO NOt forget "Save".
All the internal STD Need to present before compound quantitative
Type the Compound's Cas#. At 2nd Page, there is EnviroQuant for Envi for MDL. If the analyte is water/soil, then don't forget the MDL.
Calibration Plot
Calibrate/auto quant setup, -> AutoQuant Setup: Choose Compound Name,Update Calibration,Edit Compounds
Quantitate /Calculate, or data analysis ->Method /Edit Method -> Select Reports-> Quant Report saved with method.
Manual Qedit/Graphics
Changing Data States
Also under Positive ESI Mode, there are peaks of 336[M+H]+， 358[M+Na]+，371[M+K]+. So the conclusion is 339 could not be [M-H]-.
In practical of NegESI, it is difficult to explain the molecualr ion is the peak of 339: How could it disassociate a Fragments ion of 5 to obtain a Fragments ion of 334. Also, please double check the Mass Spectrum of Positive ESI, there is not Fragments ion of 341. Also confirm that the irrational of molecular ion of 339.
According my Tech lab Notebook(#2006-09), my answer to your question is: 100% Negative ESI Mode should be Good for Flavonoids.