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Analytical Chemistry

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Introduction to Mass Spectrometry



Mass spectrometry is an analytical technique that enables the identification and quantification of unknown compounds within a sample, and the determination of their structure.

In mass spectrometry, gas phase ions are generated from the atoms or molecules in a sample. The ions are then separated based on their mass-to-charge ratio, symbolized by m/z.

This separation enables the determination of quantitative and qualitative information about a sample, such as their mass and structure.

This video will introduce the basic concepts and instrumentation of mass spectrometry, and demonstrate its use in element quantification.

A mass spectrometer is composed of an ionization source, a mass analyzer, and a detector. At the ionization source, the compounds are ionized, usually to a single positive charge.

Ions can be generated using various techniques, such as impact with an electron beam, plasma, or lasers, each resulting in a range of fragmentations that aid in the determination of molecular structure. These methods are loosely grouped into "hard" and "soft" ionization.

Hard ionization techniques cause extensive fragmentation, resulting in more fragments of lower mass.

Soft ionization techniques result in less, or almost no, fragmentation with a high molecular mass range.

If the fragmentation is too great, valuable structure information can be lost. If it's too little, small molecules will not be efficiently ionized. Thus, the selection of an ionization method depends on the analyte of interest and the desired degree of fragmentation.

The ions are then accelerated in an electric field as they enter the mass analyzer, where they will be separated.

The most basic mass analyzer is a magnetic sector, which is composed of a curved magnet that produces a homogeneous magnetic field. The attractive force of the magnet, plus the centrifugal force of the accelerating ions causes them to travel in a circular path through the curve.

The radius of the ions circular path depends on the accelerating voltage, the applied magnetic field, and the mass-to-charge ratio.

The voltage and magnetic field can then be selected to only allow certain mass-to-charge ratio species through the curved path. Other ions crash into the sides of the magnetic pathway and are lost. By scanning the magnetic field strength, desired ions reach the detector at different times, thereby identifying each species precisely.

Another type of mass analyzer is the quadrupole mass filter. The quadrupole consists of two pairs of parallel metal rods, with each pair of opposing rods electrically connected.

A direct current voltage is applied to the rod pairs, and their potentials continuously alternated so the pairs are always out of phase with the other.

The ion beam is then directed through the center of the four rods. Ions travel in a corkscrew-like path, due to the constant attraction and repulsion from the rods. Depending on the ions mass-to-charge ratio, the ion will either travel the full path of the quadrupole and reach the detector, or will crash into the rods.

Now that the basics of the mass spectrometer have been described, lets take a look at its use in the laboratory.

The mass spectrometer used in this experiment is an inductively coupled plasma, or ICP, ionizer, with a quadrupole filter. The instrument will be used to detect and quantify a metal component in a sample.

To begin the experiment, fill all polypropylene tubes with 5 mL of 0.1 M hydrochloric acid in order to remove any contaminating trace of iron. Place the tubes in a water bath for 1 h at 50 °C.

After incubation, wash the tubes with 5 mL of deionized water, and dry the tubes in an oven or chemical hood.

In the clean tubes, add 1.8 mL of concentrated nitric acid and 200 μL of sample containing the isotope of interest.

Follow safety precautions when using concentrated acid.

Place the tubes in a water bath overnight. The temperature can be increased to shorten digestion time, if necessary.

After the sample has been digested, let the tubes cool to room temperature.

Next, add 8 mL of deionized water to dilute the samples, and to obtain a nitric acid concentration below 20%. The final dilution of the sample is 1/50. The ideal concentration for ICP is in the parts-per-billion range. Centrifuge the tubes to pellet any remaining macroscopic residues.

ICP is a method of hard ionization that uses coupled argon plasma at about 10,000 °C that is electrically conductive to ionize the sample molecules.

Begin the instrument set up by inspecting the ICP torch to ensure that it is clean.

Then, inspect the sampler and skimmer cones to ensure they are also clean. These cones enable the sampling of only the inner portion of the ion beam generated by the ICP torch and act as a barrier to the high vacuum of the mass spectrometer.

Check the argon pressure and start the chiller. Start the plasma and liquid flow into the system. Wait 20 min for the system to warm up fully.

Next, aspirate a standard test solution, which contains various known elemental standards. The test solution should be selected to cover the expected mass range of the analyte solution.

When the solution flow is established, initialize and test the instrument according to the manufacturer's guidelines.

To run the instrument, first select the elements and isotopes of interest. Then set the scan mode to peak hopping.

Select five replicates per measurement. Set each replicate to contain 40 measurement sweeps, each sweep with a dwell time of 50 ms. The total integration time is 2,000 ms per replicate.

Prepare a calibration curve for the elements of choice by measuring pre-prepared standard solutions.

Finally, run the sample, in this case, iron-oxide nanoparticles. Determine the concentration of iron using the iron calibration curve.

Mass spectrometry is used in a wide range of applications using various ionization and mass analysis techniques.

In this example, a type of soft ionization mass spectrometry, called matrix assisted laser desorption ionization time-of-flight, or MALDI-TOF, was used to analyze high molecular weight proteins. With MALDI, molecules are stabilized with a matrix, to decrease fractionation when the large molecules are ionized.

The protein solution and matrix were both spotted on the clean MALDI plate, and dried. The MALDI plate was inserted into the instrument, and the sample analyzed.

The analysis of volatile and oxidation sensitive compounds was measured using electron ionization mass spectrometry, a hard ionization technique.

First, a lockable tube system was designed in order to enable full evacuation of the tube, followed by loading of the sample under cooling by liquid nitrogen.

The sample tube was connected to the inlet port, and the sample loaded into the instrument. The mass spectrum of the sample in this case tris(trifluoromethyl) phosphate, was then analyzed.

A molecular beam mass spectrometer coupled with synchrotron radiation was used to explore the electronic structure of gas phase molecules and clusters.

The molecular beam, integrated with synchrotron radiation, provided a selective ionization method to probe molecules in the gas phase.

The sample was loaded into the nozzle, the nozzle reloaded into the instrument, and the photon beam allowed to enter the chamber.

The mass spectrum was then collected and compared to photoionization efficiency data in order to determine the electronic structure of molecules.

You've just watched JoVE's introduction to mass spectrometry. You should now understand the basic instrumentation of mass spectrometry, and how to run a basic mass-spectrometry-based analysis.

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