#a levelbr

Mass spectrometry is a highly useful form of analysis, with the most commonly used type of mass spectrometry being time of flight (ToF) spectrometry. ToF spectrometry has multiple uses such as:

Finding out the relative atomic mass of an element by finding the mass and abundance of isotopes within the element

Finding out the relative molecular mass of a molecule

THE STAGES

There are multiple steps involved in the process and we need to know all of them (I know - I wish we didn't have to either but the AQA spec says so...)

Ionisation

Acceleration

Ion drift

Detection

1) IONISATION

There are actually two types of ionisation that we need to know that can happen in a ToF spectrometer. Electron impact ionisation (a.k.a. electron ionisation) and electrospray ionisation.

Electron impact ionisation (a.k.a. electron ionisation)

This process is done by first being vaporised and then bombarded with electrons that are fired from an 'electron gun' (i.e. a hot wire filament with a current that runs through it and emits electrons). This typically results in an electron being knocked off of each sample particle to form a 1+ ion. These 1+ ions are then attracted to a plate with a negative charge (this is where they're accelerated!) - However, this is a rather harsh form of ionisation and frequently results in fragmentation of the molecular ion (this results in fragments being picked up on the mass spectrum) - This is good to know, but also not actually in the specification! :)

Electrospray ionisation (this is a soft ionisation technique)

To start, the sample is mixed with a volatile solvent. This is then pushed through a hypodermic needle at high velocity. The needle has a high voltage held through it to shock the sample particles as they pass through. This results in a cloud of positively charged 1+ ions (they lose an electron because of the high voltage shock!) The ions are then accelerated by a negative plate.

2) ACCELERATION

The exams are heavily focused on making sure people understand the practical's, so I wanted to cover them properly in their own post :)

BENEDICT'S TEST - For detecting reducing sugars

For this test you will need:

A clean test tube

A sample (for testing) - needs to be a solution

Benedict's reagent (copper (II) sulphate)

A heated water bath

Method

1) Take 2ml of the solution being used for testing and add it to the clean test tube

2) Add 2ml of Benedict's reagent (copper (II) sulphate) to the same test tube (you must use equal amounts of the reagent and testing solution)

- by this stage, the mixture will be blue in colour as that is the colour of the reagent.

3) Take the test tube and partially submerge it in the heated water bath (the water should be at about 100°C) for about 3-5 minutes - the goal is to heat it through.

If a reducing sugar is present, then the solution will gradually go from the original blue colour to a brick red precipitation.

The reducing sugar would donate 1 electron to the Cu(2+) ions in the copper (II) sulphate. The result of this would be Cu+ ions, which would form the orange-red precipitate copper (I) oxide. - (i.e. the reducing sugar would reduce the Copper (II) sulphate to form copper (I) oxide).

This information is super important for understanding the practical's. In another post, I will go into the step by steps of how to carry out tests to detect the presence of reducing or non reducing sugars (as well as starch) - but for now, I will leave you with some information on what a reducing or non reducing sugar really is :)

REDUCING & NON REDUCING SUGARS

Whilst all sugars are soluble, they aren't all reducing. As a result, they can be divided into two groups: reducing sugars and non reducing sugars.

REDUCING SUGARS:

Reducing sugars are sugars that can give electrons to other molecules (which reduces the other molecule)

A reducing sugar has free aldehyde or ketone groups. These carbohydrates can act as reducing agents as a result of this.

It is worth noting that ALL monosaccharides are reducing sugars (so glucose, fructose and galactose are all good reducing agents), and some disaccharides are also (though not all!) - maltose and lactose are both reducing disaccharides.

These reducing sugars can be oxidised easily using a weak oxidising agent. They produce one or more compounds that have aldehyde groups when in an aqueous medium (this literally means that when it is dissolved in water, it makes various numbers of compounds containing aldehyde groups!)

NON REDUCING SUGARS:

A non reducing sugar does not have any free aldehyde or ketone groups. They also do not make good reducing agents because of this.

An example of a non reducing carbohydrate is sucrose (made out of one molecule of glucose and one molecule of fructose).

When in an aqueous medium (fancy talk again: it's been dissolved in water!) no compounds are produced containing aldehyde groups.

This is what an aldehyde and a ketone group looks like (the R groups are just abbreviations where other atoms can attach). These are easier to recognise when looking at open chain structures instead of the cyclic structure that we have been!

^ Here we have the open chain structure of glucose with the aldehyde group circled :)

^ And here we have the open chain structure of fructose with the ketone group circled :)