Mass+Spectrometer

This is a device used to separate particles according to their masses and to measure their relative proportions. Chlorine for example consists of 35 Cl and 37 Cl. These two isotopes are present in different amounts in a substance. To be able to measure their percentage in a substance, we can use the mass spectrometer. The mass spectrometer separates 35 Cl and the 37 Cl according to mass and thus the the relative number of 35 Cl and 37 Cl in the substance Can be calculated.  A mass spectrometer works as follows:

**PART A: ** The substance to be tested (eg. chlorine) is turned into vapour at this stage. Once the substance has been vapourised, it flows through the tiny tube into region B. The tube is small in order to allow just a few particles to pass through every time, since the spectrometer needs to be maintained at a high vacuum.

**PART B: ** The atoms/molecules are converted into positive ions at this stage. This is done by bombarding the atoms/molecules with fast-moving electrons which then knock out and electron from the atom's outershell causing it to gain a single positive charge. The following equation shows what actually takes place:  X (g) + e - > X + (g) + 2e - - The substance X has been bombarded with electrons causing it to lose one electron and form a positive ion.

**PART C: ** The positively charged ions are passed through the two parallel electrodes, causing them to accelerate or in other words move faster. The reason why the ions move faster as they pass through region C is that there is a high potential difference between the two electrodes.

**PART D: ** There is a magnetic field in this region which causes the ions to be deflected. As any element has different isotopes (which have different masses), the isotopes are deflected to different extents. The more heavier ones are deflected less than the less heavier ones. In the case of chlorine, 37 Cl is heavier than 35 Cl so it is deflected less. <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">Usually the magnetic field is adjusted so that the particles of a certain mass are deflected and go round the bend to hit the plate at E. <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">For example, if the field was adjusted to detect a mass of 37, then, all 37 Cl particles would be deflected and hit the plate at E.

**<span style="font-family: Tahoma,sans-serif;">PART E: ** <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">At this part all the particles that hit the plate are of the same mass. The plate detects the amount of particles that hit it. Thus, you can find out how many particles of each mass are present in a sample of a substance. <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">This can be used to calculate something known as percentage abundances.

//<span style="font-family: Tahoma,sans-serif;">(PART F: it is connected to a vacuum pump which creates a high vacuum in the spectrometer. A high vacuum is necessary as this ensures that the ions don't hit into each other and thus the spectrometer functions properly.) //

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> //**Percentage abundances**// is a value that shows the percentage of each isotope that is present in a sample. <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">In the case of chlorine, we said that we have 35 Cl and 37 Cl. In any given sample of chlorine we have some amount of 35 Cl and some of 37 Cl. The percetage abundancy tells you that in any given sample of Chlorine what percentage is 35 Cl and what percentage is 37 Cl.

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">The presence of isotopes presents a problem, that is if we have both 35 Cl and 37 Cl in a sample, then how can we know the molar mass of the sample?? Is it 35 g mol -1 or 37 g mol -1 ??

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">To solve this problem we calculate an average mass using the percentage abundances of each isotope.

<span style="font-family: Tahoma,sans-serif; font-size: 14pt;">So we know that chlorine consists of 35 Cl and 37 Cl. We want to calculate the relative atomic mass of chlorine (or in other words the average molar mass of any sample of chlorine). To do this, we need to first calculate the percentage abundances of chlorine. We use the mass spectrometer to do this. <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">The mass spectrometer is set to detect the number of chlorine-35 particles present and then to detect the number of chlorine-37 particles. The results are displayed on something known as a mass spectrum, shown below: <span style="font-family: Tahoma,sans-serif; font-size: 14pt;"> <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">The above mass spectrum shows that in any sample of chlorine 75% is 35 Cl and 25% is 37 Cl. <span style="font-family: Tahoma,sans-serif; font-size: 14pt;">You must be wondering what the 'm/z' means. 'm/z' is the mass to charge ratio; that is the mass divided by the charge of the ion. As said earlier the atoms are converted to ions with a single positive charge so the 'm/z' is that same the mass number of the atom.

<span style="font-family: Tahoma,sans-serif; font-size: 19px; line-height: 27px;">From these values we know that in a sample of hundred chlorine atoms 75 will be <span style="font-family: Tahoma,sans-serif; font-size: 14pt; vertical-align: super;">35 <span style="font-family: Tahoma,sans-serif; font-size: 19px;">Cl and 25 will be <span style="font-family: Tahoma,sans-serif; font-size: 14pt; vertical-align: super;">37 <span style="font-family: Tahoma,sans-serif; font-size: 19px;">Cl. So we calculate the average mass as follows: So, the relative atomic mass of chlorine is 35.5. However, the reverse of this is also possible. That means you could be given the relative atomic mass of an element and be told to calculate the percentage abundances of the isotopes of that element. Example:  Iridium is composed of 191 Ir and 193 Ir. The molar mass is 192.2 g mol-1.Calculate the percentage abundances of the two isotopes. let the % of 191 Ir = x% thus % of 193 Ir = 100-x %. Calculate the molar mass as usual: so, the percentage abundances of 191 Ir and 193 Ir are 40% and 60% respectively.

** By the end of this lesson you should be able to: **
 * ** Describe how a mass spectrometer operates **
 * ** Using the value of percentage abundances calculate the relative atomic mass of the element **
 * ** Calculate percentage abundances when given the molar mass of an element **

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