Dycor MA200M Mass Spectrometer
Updated 22nd April 2015
The Dycor mass spectrometer is a small instrument intended for gas and vapour identification and analysis. The instrument is linear and is sensitive to gaseous chemicals having concentrations from sub ppm to 100% levels. The instrument is portable and has travelled off site by car to solve industrial and environmental problems. A small generator or a battery-inverter combination can be employed for remote use.
MassportMASSPORT is a DOS based program. It is not out of date, as it can now be run on modern PCs by using the program DOSBOX. Pressing "Print Screen" with Picasa running will cause the plot window to be saved for immediate viewing and printing. The MASSPORT data is also saved in the normal way.
A mass spectrometer sorts out gas mixtures into its constituents as well as producing fragments of each constituent of the gas mixture. The gas mixture enters an ion source where charged molecular fragments are produced by collision with electrons. The ions produced are extracted and are sorted out in a mass filter according to their mass to charge ratio (m/z). The ions are detected as electrical signals with an electron multiplier or a Faraday plate, which is simply a hollow metal tube. The ions land on the surface and are neutralised by extracting electrons from the Faraday plate. The current produced is measured with an electrometer or high impedance amplifier. The electron multiplier improves the signal to noise ratio at least 100 times. The electron multiplier voltage is usually set to produce a gain of about 1000.
Low mass ions are displayed as a vertical line at the left end of a scale while heavy ions are displayed towards the right. The length of a line represents the quantity of that ion. It relates directly to the proportion of that ion forming component in the gas mixture.
Multiple Ion Plot
Another way of recording information is to select particular m/z ratios as being representative of a particular compound. Their variations with time are recorded and in this way several components of a mixture can be followed in real-time. These are called multiple ion plots. A logarithmic y axis shows related ions well.
It is useful to have a spectral library for reference. Recorded spectra can be compared to this library using computer based search methods. An example is shown above.
The term partial pressure is often used with this instrument. It is expressed in torr which is 1/760 of a standard atmosphere. It is simply a number approximately related to the real partial pressure of a component in the vacuum system of the mass spectrometer. Sensitivity factors can be derived for each species by calibration and applied to future work.
Dycor Quadrupole Mass Filter
The Dycor quadrupole, shown at right, behaves as a mass filter. The filter has four 6.5mm diameter stainless steel rods with the opposite rods electrically connected. The rods have an ion source mounted above and two ion detectors sited below, namely a Faraday plate and an electron multiplier. Ions created by the ion source are accelerated along the central axis of the rods by a -70 volt electric field.
An oscillating RF/DC electric field is applied to the rod pairs which causes a spiral ion motion around the axis The applied RF frequency is about 6.7MHz with an amplitude up to a few hundred volts. The RF peak voltage is several times that of the DC voltage so an electric field reversal takes place at twice the RF frequency i.e 13.4MHz.
The ion motion may or may not be stable in the electric field. If the ion motion is stable then the ion reaches the detector and it is measured as a small electric pulse. With many ions the pulses add to produce a continuous signal. If the ion motion is unstable, the ion is neutralised on a surface and it is not measured. The rods with the positive DC voltage applied transmit ions above a particular mass to charge ratio. The rods with the negative DC voltage applied transmit ions below a particular mass to charge ratio. The value of the DC or RF voltage determines which ion is selected. The ratio of the RF to DC voltage determines the resolution by slightly overlapping the low mass range with the high mass range.
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Base Peak Mass Spectrometry
How a Quadrupole works
Graphical Quadrupole Theory
Dycor MS Startup
Spectral Database SDBS
MASSPORT screen views ...
TabDVM ion plot.
Mass Spectrum Library.
Mass Spectrum Library.
Mass Spectrum Library.