Lab 2: Mass spectroscopy

Using mass spectrometry information about for instance pollutants in the surrounding atmosphere can be obtained.

A Quadrupole Mass Spectrometer (QMS) consists of gas inlets, vacuum pumps, and a detector unit, as shown above.

Task 1

Investigate how a QMS works by studying what parameters have to be used to let ions pass through the QMS, and hit the end detector. Start with fluorine (F), m=19 amu. Via the link below you can do the simulation by setting the angle to 90 degrees, the DC voltage to 40 V. Try different AC voltages to make the ions pass the instrument. Determine the AC voltage interval which will give you a mass resolution of 1 amu. Use Virtuella laborationer, Massspektroskopi, to perform a QMS simulation. 

Task 2

Determine the constituents of a gaseous sample below. The intensities give the relative amounts of the atoms or molecules. The figure below shows the counter screen of a Mass spectrometer. One can choose different amplifying factors when recording the spectra, in this case 1X, 5X, and 50X (click to enlarge).

Try to determine which atoms or molecules can be seen in the simple spectra and their intensity relations. Click the boxes and check how the signals change. Often one has to use several amplifications in order to get an intensity relation between the peaks.

Task 3

Determine the molecule by looking at its Mass Spectrum. A lab instruction on Mass Spectrometry can be studied before doing the experiment. It shows in detail how the QMS works and describes the simulations above.

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Hints

How to select molecules: connecting the instrument to a gas inlet can result in various mass spectra. If you are born on e.g. the 3rd, 13th, or 23rd etc, then use spectrum #3 to work with. Try to analyze the molecule under investigation, such as CH2D2O. The only atoms in the gases are C, H, D (heavy hydrogen), N and O. The largest mass gives normally information about the total weight of the molecule, if the peak is not too small.

A program that gives suggestions on what molecules can present and their masses is available from the AIST Database: SDBS. Start with the largest mass and let the program suggest some five to ten possible molecules with about the same masses. Then select the first molecule and look at column with MS (Mass spectrometry). Click Y (yes) and a spectrum will be generated. Compare with the recorded spectrum.

Report

The report should be 2-3 A4 pages long. It should contain:

• Title. Name of lab exercise, author, and e-mail address.
• Aim. The purpose of the investigation and which parts it consists of.
• Experiment. Short description of the measurement. Table of measured values.
• Results. The result for the different parts of the experiment and analysis, calculations of the relevant values, preferably with uncertainties stated.
• Conclusions