Instrumental Analysis Exam 3

• Mass spectrometry can analyze almost everything. Species are detected based on their mass to charge ratios. This technique is highly sensitive, specific, and broadly acceptable.
• Atomic mass spectroscopy is based on a series of steps

  1. atomization - the act of splitting atoms into smaller parts; fragmentation
  2. conversion of the fractions into a stream of ions (usually contain slightly positive charge; 1)
  3. separating the ions based on their mass to charge ratios
  4. counting the number of ions for each type or measuring the amount of current produced when ions strike the transductor
• Most mass spectroscopy produces a spectra based on the relative abundance of ions. The relative abundance is the relative intensity of mass peaks with the highest peak being assigned to 100. The highest peak represents the base peak and the smaller peaks represent molecular ion peaks.
• Mass spectrometry employs precise definitions about mass based on the atomic mass scale. This included the unified atomic mass unit and the scales in terms of atomic mass units and isotopic.
• Unified atomic mass unit = 1/12 * mass of a carbon-12 atom

  ex:$12 (g/mol)/6.022E23 (atoms/mol) =$$1.992646E-26*$$1/12 =$$1.660538E-27 kg$

  1 u =1 dalton
• The mass of everything is then scaled with respect to the unified atomic mass unit.

  ex: 35,17 Cl is 2.91407 times stronger than C-12. This number is multiplied by 12 for 34.98853 Da
• Mass values reported in mass spectroscopy as also in isotopic standards. This is isotopic sensitive and specific.

  ex: CH4= (12.000000*1)+(1.008*4)+16.03200 Da [this is carbon-12]

  ex: C(H2)H3 = (13.0000*1)+(1.008*3)+(2.0161*1)=18.0400 Da [this is carbon-13]
• Resolution is a measure of the ability of a mass spectrometer to differentiate between masses. The closer the m's are, the higher the resolution.

  R = (mass of the first peak)/(mass difference between the two peaks)
• The instrumentation of mass spectroscopy is the most important aspect. As analysis occurs, the take off (ionization/desorption), in-air flight (ion sorting), and landing (mass detection) has to be managed.
• Mass spectroscopy analysis is based on completing all three steps: ionization, fragmentation and detection. The functions of various steps are in a different card

  
• For mass spectroscopy instrumentation, there are three different spectrometers

  • The purpose of the inlet system is to introduce a small amount of sample into the ion source, allowing the sample to be converted into gaseous ions through bombardment with electrons and photons.
  • Ionization works by applying thermal or electrical energy generating an output of ions commonly positively charged.
  • The mass analyzer works very similar to a monochromator but the dispersion depends on the mass to charge ratio rather than wavelength.
  • The ion transductor converts the beam into electrical signal.
• Mass spectroscopy requires a vacuum to keep low pressure. This prevents infrequent collisions within the free electrons. This is around 10^-5 to 10^-8 torr
• One of the ionization and desorption techniques for mass spectroscopy is electron ionization. A single electron is stripped from the stable molecule and accelerated to a high speed. There are stages of interaction because it is very rare to have interactions between the electrons. In the resulting spectra, the molecular ion peak may not be present because of high fragmentation.
• Another ionization/desorption technique for mass spectroscopy is chemical ionization. Fragmentation occurs through proton and hydride transfers. Similarly, chemical ionization can be coupled with field ionization. This is coupled with high electric field to form hydrides.
• Another ionization and desorption techniques is field ionization and desorption. Field ionization is when a beam of electrons is fired at a sample and the electrons are ionized. This is a high amount of energy. (ionization) Simultaneously, a high energy field is applied with heating to develop a hydride (desorption). Desorption is better at reading the molecular weight. Electron ionization reveals the structure and functional groups present in comparison.
• Another ionization/desorption technique is MALDI, matrix-assisted laser desorption/ionization. The sample is shot with lasers which allows desorbed matrix and analyte ions to be released from the sample to the extraction grid. This matrix is normally made of proteins and the laser must be at a different wavelength depending on the matrix being used. There is no fragmentation and leads to higher masses together?
• MALDI in mass spectroscopy stands for matrix assisted laser desorption/ ionization
• Another ionization/desorption technique is electrospray ionization in which liquid samples are sprayed into tiny droplets under high electric fields. This includes an ambient pump with a sample that is either vapor or liquid. The liquid/ vapor is sent through a metal needle into a capillary which acts to dissolve and remove the solvent. Once this is done, the second pumping occurs into a quadrupole mass spectrometer.
• With electrospray ionization, this is the difference between surface tension and coulombic fragmentation. This consists of multi-charged ions with little fragmentation
• Ambient ionization and ejection is what gets the molecule to take off. This is where the sample ionization or injection is done in an open field
• There are two main types of mass analyzers: quadrupole based and time of flight.
• Quadrupole based mass analyzers consist of 4 quadrupole magnets, two positively charged and two negatively charged, an ion source, ion transductor, and RC and DC voltages. As the ion source enters the space between the magnets, the ion will go on an unstable trajectory to reach the transductor.
• For a quadrupole, there are two types of ion flow. A shows that the ions are moving inward the same direction as the electric field. B shows that the ions are moving outward in the opposite direction as the electric field. A represents the alternating current and B represents the direct current
• For quadrupole mass analysis, a sin graph will be the result. This will be positive biased. A will be higher than B. There is also a relationship with weight. The heavier ions will have a larger momentum. The lighter will be more responsive to alternating current, A.
• Another mass spectroscopy analyzer is time of flight. This is the time required for positive ions to travel from the source to the detector to measure the mass. There are two different types of detectors: linear and orthogonal.
• Time of flight analyzers consist of a few distinct parts. The ions are released from the sample chamber with an electric field to drive them into the inlet with the same kinetic energy. They first enter the ionization and acceleration region, then the ion spreads because of different velocities, and then the ions arrive sequentially at the ion detector.
• For time of flight, the kinetic energy is constant. As the kinetic energy decreases, the weight increases and the velocity decreases. Since the kinetic energy is the same, the heavier ions will arrive last and the lighter will arrive first.
• In a time of flight linear drift tube, there is no potential added into the free drift region. Since there is one shot, there is limited resolution because of the length. This tube is only 1-2 meters long.
• For orthogonal time of flight, this is oriented in a V-like shape. The extraction region occurs before the ionization source. This acts like a gate which controls which ions will be released at what times. The ions travel down to an ion mirror where they are reflected up another tube. The heavier ions are detected last. This method has much more resolution.
• The orthogonal time of flight set-up has a greater resolution because of the extraction region, the reflection off the ion mirror, and the length which is longer leading to better resolution.
• There are two main types of transductors for mass spectroscopy: electron multipliers and microchannel plates.
• Electron multiplier transductors are used in mass spectrometers to amplify the signal from the ionization chamber. The ion signal enters the detector and then will bounce off a series of panels, each time amplifying. This is off of copper/ beryllium alloy plates continuously. This has a gain of electrons 10^5 fold.
• Microchannel plates are lead coated glass microtubes which can individually respond to incoming electrons/ stimulus. Sensitivity and ability to quantify are simultaneous. The ions enter the system through mass spec magnets. When the ions reach the microchannel plante, the ions are amplified to electrons and then photons at a phosphor screen.
• One of the mass spectrometry case studies is Inductively Coupled Plasma Mass Spectrometry (ICP MS). This method is very good for elemental detection. It is very fast and highly sensitive. This type of spectrometry cannot be used on proteins because it would break down the compound to just carbons and lose all characterization.
• The torch within ICP MS is consistent of 3 concentric tubes. This is the first step in instrumentation and serves as the atomizer. A sample of argon aerosol or vapor is sent through the tubes. The temperature of the vapor increases as it approaches the load coils. This center of this torch is hotter than the sun at 10000K.
• For ICP MS instrumentation, the torch is coupled with an interface that works at atmospheric pressure with a low pressure mass spectrometer. A very small amount of gas from the torch will enter the second step called the skimmer. This is where positively charged ions are removed into the quadrupole

  
• ICP MS generally has the highest detection limit for most elements. ICP optical emission spectroscopy generally has the worst
• Tandem Mass Spectroscopy allows the mass spectrum of preselected and fragmented ions to be obtained and used again for a second cycle. You want to use a second cycle because this generates different fragmentation patterns. This allows for the sample to be further resolved and better identification of ions with the same mass to charge ratios.
• The collision gas inlet within the tandem MS system is used in order to achieve separation for the fragmented ions.
• Mass spectroscopy can be coupled with different techniques. One of the common ones is chromatography and mass spectroscopy coupling. The sample beings by going through a HPLC mechanism. The sample must be gaseous to begin with. Once it has passed through HPLC, it enters the MS detector.