Samsung RT34M Uživatelský manuál stáhnout pdf (Strana 35)

Dushyant Patel LC-MS/MS Method

S.K.P.C.P.E.R., Kherva 23 M .Pharm. Thesis

molecules form negative ions in solution, and basic molecules form positive ions. The

ejection of sample ions from droplets is facilitated if the ionic charge and surface charge

of the droplet are of the same polarity. Thus, a positively charged needle is used to

analyze positive ions and a negatively charged needle is used to analyze negative ions.

¾ Sample ions can carry a single charge or multiple charges. The number of charges

carried by the sample ion depends on the structure of the analyte of interest and the

carrier solvent. The ESI process is affected by droplet size, surface charge, liquid surface

tension, solvent volatility, and ion solvation strength. Large droplets with high surface

tension, low volatility, strong ion solvation, low surface charge, and high conductivity

prevent good electrospray.

Figure 4.2 ESI process in the positive ion polarity mode

¾ Organic solvents such as methanol, acetonitrile, and isopropyl alcohol are superior to

water for ESI. Volatile acids and bases are good, but salts above 10 mM concentration

and strong acids and bases are extremely detrimental.

Atmospheric pressure chemical ionization:

¾ Atmospheric pressure chemical ionization (APCI) is a soft ionization technique, but not

as soft as ESI. APCI is used to analyze compounds of medium polarity that have some

volatility.

¾ In APCI, ions are produced and analyzed as follows:

• The APCI nozzle sprays the sample solution into a fine mist of droplets.

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