Tuesday, July 29, 2014

Part V - One Insane (But True) Thing About Mass Spectrometry



So....Where’s the high voltage?

In the last installment of our series on mass spectrometers, high voltage occurs in at least two areas of a mass spectrometer.  If you recall, there are four basic sections - the ionization & acceleration sections, deflection, and detection.

Ionization/Acceleration Chamber


The first place high voltage appears is in the ionization/acceleration chamber.  Often a high voltage power supply is used to power the electron gun that generates ions, or the cathode and anode grids used to accelerate the ions and create an electron beam.  VMI supplies high voltage diodes to many instrumentation power supply manufacturers.  Which diode is used depends on several system parameters, but some things to keep in mind are Vrwm, Trr, and operating frequency.  High voltage, lower current diodes are commonly used.
 

Deflection Chamber

A second location is in the deflection chamber where an electromagnet is used.  Electromagnets are basically large inductors that oppose changes in current.  When the power is switched off, a large voltage spike can occur, potentially damaging the windings of the electromagnet.  To remedy this, designers connect high voltage diodes across the windings so that when the power to the electromagnet inductor is switched off, the diode conducts in the forward direction.  This allows the voltage spike to dissipate to ground instead of appearing across the winding terminals. 


Diode Selection

Selecting the best high voltage diode for the application will depend on the anticipated voltage spike and inductive current.  Ultimately, the size and power of the electromagnet will influence the designer’s choice.  High voltage diodes with higher current capacity are often used.

Big Electromagnet - Photo Credit
  
If you’re not sure which diode is best for your application, contact us.  We can guide you through the selection process.

Friday, July 25, 2014

Part IV - One Insane (But True) Thing About Mass Spectrometry



Data Collection

 
Data collected by the detector is analyzed using a computer.  A typical mass spectrometer output is a graph with one or several vertical lines.   

Mass Spectrum

The mass spectrum graph typically shows the x-axis axis in atomic mass units (amu), the y-axis is relative intensity, or a similar term, that identifies how many ions of a given amu were detected.

Below is a molecular diagram of caffeine. 
Caffeine Molecule
Caffeine is composed of 10 hydrogen atoms, 8 carbon atoms, 4 nitrogen atoms, and 2 oxygen atoms.   
It is  represented by C8H10N402, and it’s molecular weight is 194.0805.

Below is a typical mass spectrum of caffeine


Mass Spectrum - Caffeine

The relative abundance of the vertical bar appearing at 194 on the x-axis reads “100”.  That means that the mass/charge ratio of a compound equal to that of caffeine was present in relative abundance of 100 compared to the other compounds.  Maybe the extra vertical lines has to do with whether the coffee came from Starbucks or Peet’s, or from Africa or South America, or....maybe the sample was in pill form.  You get the idea. 
   
At any rate, scientists can compare the results of a test sample to that of standardized mass spectrum samples to help identify the substance.  If you’re not a chemistry major, signature mass spectrums are available on-line, in subscription data bases, or at NIST

Next post we'll look at where high voltage applications might come into play inside a Mass Spectrometer.
 

Thursday, July 24, 2014

Part III - One Insane (But True) Thing About Mass Spectrometry



Okay, so far we’ve covered ionization, acceleration, and deflection.

Ionization, Acceleration, and Deflection Review

To review, once a particle has had one or more electrons stripped off, it is now a positively charged ion.  Next, positive and negative electric fields in the form of high voltage grids and cathodes are used to group and accelerate the ions into an ion beam.  At that point, they pass into the deflection chamber where magnetic fields are used to deflect the ion beam.  Depending on the strength of the magnetic field, the mass of the ion, and the charge of the ion, they will either be deflected towards the chamber walls where they’re neutralized, or continue on to the detector. 

The mass-to-charge ratio, m/z, is used to detect and identify the different materials.

By varying the strength of the magnetic field, scientists can tune the deflection chamber to allow a range of particles to make it to the detector, one type of particle at a time.  The lighter the ion, or the less charge it carries, the easier it is to deflect.  Likewise, to detect heavier ions, or ions with more charge, the stronger the magnetic field needs to be.

If you recall from the first and second posts, it’s kind of like dropping a bowling ball and a baseball off the building at the same time.  If the wind is blowing really, really, hard, the baseball will hit the side of the building where it’s smashed to smithereens, while the bowling ball continues to fall along its original trajectory.  In this case, the baseball simulates a light-weight particle, or one that is not highly charged.  The bowling ball represents a larger particle, or a highly charged one.  The ‘detector’ is the ground below the building, and the wind represents the applied magnetic force. 

Detection

When a particle hits the detector, it sets in motion a chain of events.  First, because the quantity of ions being detected is very low, often the quantity is amplified.  One method of amplification is the use of an  electron-multiplier.  A second method uses photo-multiplier-tubes (PMT) .

After amplification, the ions are recombined with a negative charge.  With electron movement comes current flow.  It is this current flow that is detected, recorded and analyzed.   

Part IV will discuss data collection and analysis. 

Friday, July 18, 2014

Part II - One Insane (But True) Thing About Mass Spectrometry



You may recall in Part 1 that the very first step that happens in analyzing material using a mass spectrometer is to strip the material of electrons, thus creating positively charged particles.  Once the particles have been ionized, they are accelerated, deflected, and detected.  In this post the discussion centers on the acceleration and deflection of ion particles.

What Happens in a Spectrometer
Parts of a Spectrometer and What Happens
 

Ionization and Acceleration

Using the laws of attraction and repulsion, positive and negative charged plates are used to accelerate the particles.   

Positive ions are propelled forward by the positively charged plate (like charges repel).  At the same time, the positive ion is attracted to negative plate (opposite charges attract).  The mixed ion stream beam is finely focused and contains ions of different charges and masses.  The acceleration through the use of strongly positive, high voltage plates on the order of 10kV, forces the stream into a deflection chamber  a magnetic field is used to deflect the ions.

Ionization takes place in a vacuum because the presence of air molecules would interfere with the charged particles. 

Deflection

The finely focused particle beam has its own electromagnetic field.  It is this field that enables a magnetic field to act upon the particles and deflect them.  Adjusting the strength of the magnetic field will deflect particles of different m/z because, for any given field strength, the lighter particles or ones with less charge, will be deflected the easiest.  Varying the magnetic field means particles can be selected.    

The combination of m/z ratio of the particles and the strength of the applied magnetic field determine which particles make it all the way to the detector.  Some interesting things happen in the detector, which we’ll cover in the next blog post.    

In the next blog post we’ll cover detection and analysis and where high voltage optocouplers and diodes are used.  Stay tuned for more.




References
The mass spectrometer - how it works. (n.d.). the mass spectrometer - how it works. Retrieved July 18, 2014, from http://www.chemguide.co.uk/analysis/masspec/howitworks.html

Harris, W. (2009, March 31). How Mass Spectrometry Works. HowStuffWorks. Retrieved July 8, 2014, from http://science.howstuffworks.com/mass-spectrometry3.htm

How the Mass Spectrometer Works. (n.d.). - Chemwiki. Retrieved July 18, 2014, from http://chemwiki.ucdavis.edu/Analytical_Chemistry/Instrumental_Analysis/Mass_Spectrometry/How_the_Mass_Spectrometer_Works