Mass spectrometry is a powerful analytical technique which has gained considerab
ID: 923132 • Letter: M
Question
Mass spectrometry is a powerful analytical technique which has gained considerable popularity in virtually all fields of science. The technique is based primarily on measurements of mass to charge ratio of analyte ions. The production, processing, and analysis of the ions is the operational backbone of the technique. Discuss the following parameters as they relate to mass spectrometry, showing their significance: Molecular ion Base peak Isotopic peak The nitrogen rule Daughter ion Three methods are used to ionize sample in mass spectometry (a) electrospray, (b) electron impact, and (c) chemical ioniztion. Discuss these methods, giving their weaknesses and strengths, if any.Explanation / Answer
1)
a)Molecular ion: In the mass spectrum the heaviest ion with highest m/z value is called as molecular ion.
b)Base peak:The tallest lie in the mass spectrum with certain m/z value is called as base peak. Ex: in the mass spectrum of n-pentane m/z 43 is the base peak.
c)Isotopic peaks:Ionic fragments of the same chemical formua are represented with M&M+1 peaks in the mass spectrum, these are called as isotopic peaks. Ex:CH3+ consists of normal mass 15 for 12C1H3+ & 16 for 13C1H3+
d)The nitrogen rule: The nitrogen rule states that organic compoundscontaining H,C,N,O,Si,P,S&the halogens either have 1) an odd nominal mass that indicates an odd no. of N atoms are present 2) even nominal mass that indicates even no. of N atoms or no N atoms are present in the molecular ion.
e)Daughter ion: An electrically charged product of a particular parent ion. In general such ions have a direct relationship with a particular precursor ion and may relate to a unique state of the precursor ion.The reaction need not involve fragmentation in the number of charges carried. Thus a fragment ion is a daughter ion but not all daughter ions are fragment ions.
2)
Electron spray: It is a tachnique used in mass spectrometry to prduce ions uing an electron spray in which a high voltage is applied to a liquid to create an aerosol. It is useful in producing ions from macromolecules.
Electron impact: It is an ionization method in which energitic electrons interact with gas phase atoms or molecules to produce ions. This is widely used in spectrometry for gaseous samples and volatile organic molecules.
Chemical ionization:It is a lower energy process than electron ionization. The lower energy yields less or sometimes no fragmentation and usually gives a simpler spectrum.