Samsung RT34M Manuale Utente Scaricare il pdf (Pagina 36)

Dushyant Patel LC-MS/MS Method

S.K.P.C.P.E.R., Kherva 24 M .Pharm. Thesis

• The droplets are vaporized in a high temperature tube (the vaporizer).

• A high voltage is applied to a needle located near the exit end of the tube. The

high voltage creates a corona discharge that forms reagent ions through a series

of chemical reactions with solvent molecules and nitrogen sheath gas.

• The reagent ions react with sample molecules to form sample ions.

• The sample ions enter the mass spectrometer and are analyzed.

¾ APCI is a gas phase ionization technique. Therefore, the gas phase acidities and

basicities of the analyte and solvent vapor play an important role in the APCI process.

¾ In the positive-ion mode, sample ionization occurs in a series of reactions that start with

the electron-initiated cation formation. Typical examples of primary, secondary, and

adduct ion formation are shown below:

Primary ion formation

−

+ N

→N

. + 2e−

Secondary ion formation

+ H

O →N

+ H

O →H

+ HO.

Proton transfer

+ M → (M+H)

+ H

¾ In negative-ion mode, (M-H)

is typically formed by the abstraction of a proton by OH−.

¾ APCI is typically used to analyze small molecules with molecular weights up to about

1500 U. APCI is a very robust ionization technique. It is not affected by minor changes

in most variables, such as changes in buffers or changes in buffer strength.

¾ APCI use in positive or negative ion polarity mode. For most molecules, the positive-ion

mode produces a stronger ion current.

¾ This is especially true for molecules with one or more basic nitrogen (or other basic)

atoms. An exception to the general rule is that molecules with acidic sites, such as

carboxylic acids and acid alcohols, produce more negative ions than positive ions.

¾ Although, in general, fewer negative ions are produced than positive ions, negative ion

polarity is sometimes the mode of choice.

¾ This is because the negative ion polarity mode sometimes generates less chemical noise

than does the positive mode. Thus, selectivity might be better in the negative ion mode

than in the positive ion mode.

1 2 ... 31 32 33 34 35 36 37 38 39 40 41 ... 92 93

Nessun commento

Samsung RT34M Manuale Utente Pagina 36