#a levelbr

[a-level-study-bunny]

MASS SPECTROMETRY (PART 1)

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 positive ions are accelerated using an electric field. This results in them all having the same kinetic energy. (Super important for the next stage!)

3) ION DRIFT

The +1 ions travel through the middle of a negatively charged plate (through a hole) into a tube. This is the stage where the ions drift through a field with no charge. The thing that differentiates these ions is their mass/charge ratio. The higher the mass/charge ratio, the heavier the ion, the slower it goes (and vice versa). This means they hit the detector at different times (thus showing the abundance - but I'll explain that in a moment!)

4) DETECTION

The 1+ ions hit a negatively charged plate. This contact causes the ions to lose their charge as they collect an electron from the plate. This results in a movement of electrons, which produces an electric current that we can measure. The more electrons that hit the plate at once, the larger the current. This then translates on the mass spectrum as a larger spike.

If you've made it this far, well done!! You deserve a cookie (or something... I don't have one for you, but you deserve one!!)

Now that you've looked at all of that, it's time to look at the results! But since this post is already rather long, I will be making another post to cover that! (Sorry!)

Useful AQA resources:

https://filestore.aqa.org.uk/resources/chemistry/AQA-7404-7405-TN-MASS-SPECTROMETRY.PDF - This has pictures etc and is official AQA material :D

[a-level-study-bunny]

BIOCHEMICAL TESTS FOR SUGARS (PART 1)

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 colour change goes in this order: blue - green - yellow - orange - red - This colour change happens as the copper (I) oxide is created! (If no colour change happens, there is no sugar).

If there is only a small amount of reducing sugar present, the amount of precipitate created would be less and so the reaction may appear more green or yellow. This just means that not all of the Benedict's reagent has been used up in the reaction.

Any colour change indicates the presence of a reducing sugar. It's the concentration that determines how far along the colour scale the solution will turn. (Even if the solution looks green, there has to be copper (I) oxide present since it is being turned green by the red precipitate being formed!) - This means that the test can also be used to get an idea of how much reducing sugar is in the solution.

Non-Reducing Sugars

A negative test doesn't definitively mean that there's no reducing sugars present - It just means you might have to break down the sample into monosaccharides and repeat the test. This can be done like so:

1) Get a new sample of the test solution in a clean test tube and add dilute hydrochloric acid to the mixture.

2) Carefully heat the sample in the heated water bath (heated to 100°C)

3) Neutralise the sample with sodium hydrogen carbonate (baking powder!)

4) Carry out the test like you did the first time!

If the test forms a coloured precipitate, the test has come back as positive. If the test stays blue, the test is negative

REAGENT TEST STRIPS

There's another test that you can do called the Reagent test strip. This is literally a commercial test strip that can be used to identify the presence of reducing sugars.

The test is carried out by dipping the strip into the test sample and comparing the colour it turns to a chart (the chart helps to estimate the reducing sugar concentration.

For a real world application of the reagent test strips - These are often used to test for the presence of sugar in urine (useful if diabetes is suspected!)

[a-level-study-bunny]

REDUCING & NON REDUCING SUGARS

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 :)

I will be ending the post here as this isn't actually on the specification (but is interesting and hopefully gives a bit more clarification about what a reducing and non reducing sugar actually is!) - I will go into the actual biochemical tests we need to know in another post :)