Dycor MA200M Mass Spectrometer
Updated 26 September 2018
Updates: Ion Pumped Mass Spectrometer
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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.
MASSPORT 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 It is not out of date, as it can now be run on modern PCs by using the program DOSBox.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 a suitable imaging program running will cause the plot window to be saved for immediate viewing and printing. The MASSPORT data is also saved in the normal way. There may be some problems running this software on modern 64 bit PCs.
The Dycor system 2000 software requires the older control units to be updated before communication can be established. Presumably this is a PROM upgrade.
Like with all recent technology you are slowly being forced to upgrade everything. Fortunately I am at an age where this matters little to me, but I do like reviving systems I used in the past, even for a short while. I have rewritten MASSPORT so it functions with some later PCs, but not the most recent. The file MASSPORT2 runs on some later computers and is mass spectrometer only.
I have just tried MASSPORT with DOSBox on a Raspberry Pi 3B. I used a USB to RS232 cable and set the configuration file to look for TTYUSB0. Everything seems to work. The Raspberry Pi could be mounted inside, and powered from, the Dycor console.
My Raspberry Pi configuration file is dosbox-0.74.conf. This is a file in a hidden folder: home/pi/.dosbox/dosbox-0.74.conf
My MacBook Air also runs MASSPORT without problems using DOSBox. The USB to serial cable driver needs to be installed and the configuration file needs the serial entry "serial1=directserial realport:/cu.usbserial-FTBUQ219" Many other systems should work since DOSBox runs on many computer platforms. I am confident that Linux should work. ChromeOS may work.The text files produced by MASSPORT are named either *.bar for SPECTRET bar spectra or *.dat for TABDVM multiple ion plots. A typical ion plot file might be named Run1.dat for example. DOS only allows up to 8 letter file names followed by up to 3 letters as a suffix. This is an 8.3 file name. These files can be renamed to *.txt and viewed with a simple text editor or spreadsheet.
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. The horizontal axis is the mass to charge ratio (m/z) in daltons.
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.
Ion Pumped Mass Spectrometer
Recently my MA200 mass spectrometer has been off-site, monitoring the output of a pilot plant. I have looked at the old parts I have from defunct equipment formerly used by others. With some component exchanges there was enough to build up an ion-pumped Mass Spectrometer. Previously I built several mass spectrometers in the years from 1988 to 1996. The old components I now have are therefore about 22 to 30 years old.
The ion pump is a Varian 8 litre per second diode ion pump with a small 3500 volt Varian controller. This is connected in place of the normal turbo pump. A backing pump is only needed for start-up. Otherwise the set-up is conventional, apart from the inlet system. A manual for the ion pump is here. Some further information about ion pumps is in the manuals available from Duniway Stockroom. A wide range of vacuum equipment is available from this source. Look under Documents - Data Sheets for Ion Pump operation, applications and troubleshooting guides.
Ideally a turbo pump is required for starting an ion pump. Alternatively a Sorption Pump could be used with liquid nitrogen. My method, using just a backing pump, is to heat the mass spectrometer while rough pumping. The high vacuum valve is closed and the pump is disconnected. As the mass spectrometer cools some additional vacuum is created which is sufficient to start the ion pump. The clean mass spectrum shows that no oil backstreaming occurred.
Any inlet system needs to be matched to the ion pump so that the gas admission rate matches the pumping speed. This is more critical for ion pumps with lower pumping speeds. In my case I have developed a simple membrane inlet which allows for standby operation when not in use.
This design is uses a KF16 vacuum centering ring with a built-in sintered metal filter. A disk of 0.5 mm thick silicone rubber, slightly larger than the filter, is wedged in place by a 9.8 mm OD washer. Excess silicone rubber is cut away and some RTV silicone sealant is applied around the edges of the washer to complete the seal. The active diameter of the inlet, represented by the hole in the washer, is 3.75 mm. The optimum diameter is probably a bit less than this or a second membrane layer can be used. The required exposed diameter will depend on the membrane thickness used. Additional 3.75 mm diameter silicone discs can be cut and added to the membrane in the centre, until the pressure settles to an optimum value. This silicone rubber adheres to itself and is sold in electronic shops as weather proofing electrical tape. Before use, I heated the membrane at 40°C for a few hours to remove any volatile organic compounds. In my case, the working pressure is about 4 x 10-6 torr for two 0.5 mm thick silicone rubber membrane layers with an exposed diameter of 3.75 mm. At this pressure the pumping speed is almost constant over a moderate pressure range so pump characteristics should not influence experiments.
On the other side of the KF16 centering ring a drilled aluminium plug was machined and pressed in place and sealed with 5-minute Epoxy adhesive. The short 1.5 mm OD stainless-steel line to the mass spectrometer was pressed in place and sealed with Loctite 243. My approach to the use of adhesives in vacuum systems is to use only tiny amounts. Choose materials which don't smell of any solvents.
Running with the membrane inlet continuously in an open state will shorten the life of the ion pump. I devised a simple plug with an O-ring seal which reduces the mass spectrometer residual pressure to well below 10-7 torr. The rest of the plug occupies the space above the membrane. A short 3.75 mm diameter spigot is machined to lightly press on the membrane surface. A little silicone grease helps with the initial sealing. Thereafter the increasing vacuum holds everything in place. The power consumed by the ion pump in this state is less than 100 milliwatts. The power supply produces much less waste heat than a neighboring LCD display. There is usually an initial pressure pulse when the plug is fitted, which momentarily increases the ion pump current. I may add a screw thread above so the plug behaves like a tap.
There is no need for perfection here. Any reading in the low 10-7 torr range would do. The aim is simply to move into a pressure range where the pump lifetime is reasonable. The 8 l/s ion pump specification gives an approximate lifetime of 40000 hours at 1 x 10-6 torr. A low standby pressure also means that moderate power cuts can be tolerated.
There are many ways to design a membrane inlet. At 0.5 mm thickness a self supporting membrane is possible which would simplify the design. The layers would consist of a membrane seat, the membrane, a washer and a threaded cap which would bear down on the washer. To allow for the absence of the backing filter, and with half the membrane thickness, an exposed membrane diameter of about 2 to 3 mm would be a good starting point. A 1/8 inch Swageloc fitting with nylon ferrules could be adapted. The membrane disc would be fitted between the ferrules with the outer ferrule inverted. The main requirement is no leaks and the abilty to close off into a low vacuum stand-by state. I simply did a little bit of lathe work and used components that were available here.
Residual Mass Spectrum
The residual mass spectrum of an ion pumped mass spectrometer can look a little unusual. The spectrum has masses for hydrogen (2), water (2, 16, 17, 18), Air (28, 32), argon (40) and carbon dioxide (44). There are traces of nitric oxide (30, 14), carbon monoxide (28, 12), residual hydrocarbons and secondary peaks for the above gases. Note that the residual pressure is very low at about 5 x 10-8 torr. The residual pressure can be reduced further after warming the mass spectrometer to about 100°C for a few hours. The following mass spectrum showing residual ions was emailed from my Raspberry Pi computer.
Residual Mass Spectrum
Some species such as carbon monoxide and nitric oxide are created by the Penning discharge in the ion pump. There are carbon inclusions in the titanium plates of the pump as well as residual hydrocarbons and water. Argon is elevated because there is no chemical reaction with titanium in the ion pump. Burial of argon by metal sputtering is the only pumping mechanism. There will be some argon signal instability as already buried argon is sometimes released due to sputtering of titanium metal. Triode pumps are more stable in this respect as the pump envelope can act as a surface to trap neutral gases with no sputtering. Carbon dioxide is elevated because the membrane inlet admits this gas at about 5 times the rate of many other gases, probably because of the linear molecule shape. The same applies to hydrogen, which also comes from residual water. Similar components are found in outer space where energetic charged particles modify the chemistry of planets, asteroids and comets. Note that this is my explanation for what is observed. There may be other interpretations. Using dry gases for experiments will help.
For evolved gas experiments from heated samples I would use an aquarium pump, a silica gel drier and a small temperature controlled tube furnace. The exit flow would be cooled and passed over the membrane inlet. The transparent cover shown can be machined and drilled to suit. When not required the ion pump can be put into a standby state by inserting the plug.
Air Mass Spectrum
It remains to be seen if a small 8 l/sec ion pump can perform well long term. Routine operation requires some care in use. Reading the ion pump manual is very helpful. My old 20 l/sec Varian triode ion pump was very useful in the past, but I do not have a suitable power supply.
JEPSPECTRO - Home Page
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Base Peak Mass Spectrometry
How a Quadrupole works
Quadrupole Mass Filter
Graphical Quadrupole Theory - pdf file
Spectral Database SDBS
Dycor Mass Spectrometers
Ion Pump History
Spectral Database SDBS
Pfieffer TCP015 controller
MASSPORT Software - zip file
Masport2 Software - zip file - for later PCs
MASSPORT Documents - pdf file
Dycor MS Startup - pdf file
Raspberry pi configuration file dosbox-0.74.conf
MASSPORT screen views ...
TabDVM ion plot.
Mass Spectrum Library.
Mass Spectrum Library.
Mass Spectrum Library.