CH141_lab2_F09 - Determination of the Elemental Molar...

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CH141 -1- GC-MS Determination of the Elemental Molar Masses of Chlorine and Bromine Pre-lab Assignment: Reading: 1. Sections 1.5, 1-8, and 2.4 in Brown, LeMay, Bursten, and Burdge. 2. This lab handout! Questions (show all work): 1. What molecules are you using in this lab to determine the elemental masses of chlorine and bromine? 2. What specifically will you inject into the GC-MS? 3. How many peaks do you expect to see on the chromatograph? INTRODUCTION: The purpose of this lab is to give you insight into the concept of elemental molar mass. Many elements have more than one stable isotope. As a result, individual atoms of the same element can have different masses. In the laboratory we are almost always working with large numbers of atoms (moles). We are, therefore, interested in the average mass of a mixture of stable isotopes of the element, rather than the individual isotope masses. In this lab you will use a mass spectrometer to measure the masses and abundances of the isotopes of Cl ( 35 Cl and 37 Cl) and Br ( 79 Br and 81 Br). From the mass and abundance data you will be able to calculate the elemental molar mass of Cl and Br and compare your answer to the published result. This experiment is a “discovery”-type experiment. The procedure will be carefully described, but the analysis of the data is left purposely vague. You will work in small groups to decide how best to work up the data.
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CH141 -2- GC-MS What is a mass spectrometer? A mass spectrometer is an instrument that accurately measures the masses and relative abundances of atoms, molecules, or pieces of molecules (fragments). Figure 1 is a diagram of one of our instruments. A mass spectrometer consists of three general parts: the ion source, the mass separator, and the ion detector. In our instrument a sample is introduced into the ion source as a gas. An electron beam fragments the gas molecules into positively charged ions. The positive ions are accelerated away from the positively charged accelerator plate, through a set of charged focusing lenses and into the mass separator. A mass separator consists of a magnetic field that deflects ions toward or away from the detector. Note : Figure 1 shows a slightly more complicated mass separator used in one of our instruments at Colby. {In our instrument, the magnet is actually a set of four electromagnets called a quadrupole. The quadrupole magnetic field oscillates very quickly causing the positively charged fragments to travel in an oscillating path. Only fragments with the correct
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CH141_lab2_F09 - Determination of the Elemental Molar...

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