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Converging Annular Time-of-flight Mass Spectrometer

Schematic of CAT

The schematic shows Kore's novel spectrometer geometry (patented) which starts with a ring shaped ion source. The ions are extracted perpendicular to the ring and deflected inwards so that as they proceed through the spectrometer each annular bunch of ions converges to a relatively small size by the time it reaches the detector. This allows a relatively large source to be used with a small detector giving the best of both worlds in terms of sensitivity, mass resolution and cost.


A Short Guide to the Working Principles of MS-200

Introduction

The MS-200, (which is based upon Kore's Converging Annular Time-of- flight (CAT) mass spectrometer) transports the advantages of mass spectrometer analysis from the laboratory into the field.

It was designed for in-situ analysis of volatile organic compounds (VOCs) in air, in a concentration range from the very low p.p.b. up to percent levels. The immediate availability of the analysis results offers a flexible sampling strategy compared to the rigid strategy that has to be applied when using adsorption tubes or sample bags. In comparison to commonly used adsorption tubes the MS-200 offers enormous additional versatility, saving both time and money. The immediate availability of the results in the MS-200 also offers the possibility of emergency screening and fast response in chemical accidents or hazards.

The MS-200 is unique and as such opens up many questions for the potential user. To answer these questions this guide explains the working principles of a membrane inlet mass spectrometer (MIMS).

This section is not aimed at the specialist mass spectrometer user. It is intended to guide the non-mass spectrometer expert, with some scientific background, through the different parts of the MS-200 and explain its basic working principles.

In this report we also answer the most frequently asked questions about our system. We discuss the different parts of the system, and follow the route a sample takes, from being introduced into the system until the user can see the result of the analysis on the computer screen.

What are the different components of the MS-200?

The MS-200 consists of several components that perform different tasks involved in the handling and analysis of the sample.

The different physical main components of the MS-200 are named below and their interaction with the other components is shown. Each of these components and the part they play in the analysis will be described in more detail.

Block diagram

These four parts are the main elements of interest. Of course, there are additional supporting parts like power supplies and vacuum pumps. However these will not be mentioned explicitly as they are not essential for understanding the system.

The sample is collected by the inlet system and led into the mass spectrometer. Here the mass of the sample molecules is determined. The timing & control electronics controls the process of determining the mass and delivers the collected information to the lap top computer. The software calculates the mass spectrum and displays the result of the analysis. The computer is also used to define analysis parameters and communicate them to the timing & control electronics.

Each of these main components will be described below.

The Inlet System

The sample has to pass from the atmosphere into the vacuum of the mass spectrometer. To prevent interactions between the sample and residual molecules in the mass spectrometer, analysis has to be performed at a very low pressure. The inlet system has to control the flow of sample into the vacuum. This must happen at a relatively low flow rate to allow the vacuum pumps to maintain low pressure.

Inlet schematic

Because of the very high efficiency of the time of flight mass spectrometer method only very small amounts of sample are required to obtain good mass spectra. The interface that regulates the delivery of sample into the mass spectrometer is the inlet system. This inlet system could be a pinhole, a capillary column or a semi-permeable membrane. We have chosen the semi-permeable membrane as the most appropriate for use in the MS-200.

Permeation of Chemicals through a Semi-Permeable Membrane

Having a membrane as the interface between a high pressure area and the ultra high vacuum of the mass spectrometer is a common technique in a gas chromatograph mass spectrometer system. In this case the membrane is mainly used to limit the amount of sample entering the vacuum of the mass spectrometer.

For a direct inlet membrane introduction mass spectrometer (MIMS), another property of membranes is used. A membrane also has the capability to filter chemicals, depending on their physical and chemical properties. The polydimethylsiloxane (silicone) membrane used in the MS-200 lets volatile organic compounds, (VOCs) pass much easier than substances that are generally not of analytical interest, such as water and nitrogen.

As this membrane is preferably permeable to VOCs we can both minimise the total amount of sample that is introduced into the mass spectrometer and increase the concentration of the VOC analyte of interest.

The rate of permeation mainly depends on:

Concentration of the Sample in the Double Membrane Inlet

Double membrane schematic

As shown by this picture, the selectivity of the membrane causes more of the VOC analyte (red) to pass than the background (blue). The lower pressure on the inner side of the membrane means that there are less molecules per volume than on the outer side, but the concentration of analyte compared to the background is higher than on the high pressure side of the membrane.

With the double inlet system used by the MS-200 the sample is concentrated twice. Therefore we can achieve a very high concentration of the analyte in the analyser chamber. This together with the high efficiency of our patented time of flight mass spectrometer technology produces results with a very high sensitivity.

The Analyser

A mass spectrometer is capable of identifying a chemical analyte by measuring its molecular weight. It therefore is analogous to a very sensitive balance.

There are a number of different ways that mass spectrometers separate molecules of different mass. Some systems measure the deflection of an ionised and accelerated beam of analyte in a magnetic or electric field. This deflection is dependent on the mass and therefore ions of different mass are separated spatially. Another method is to filter the beam by passing it through a high frequency electrical field. So that depending upon the electrical field applied, only ions of a particular mass are allowed through an aperture to hit a detector. However the analyser used in the MS-200 is a time-of-flight mass analyser.

Measurement of ion masses by Time-of-Flight Mass Spectrometry (TOF MS)

Time-of-flight mass spectrometers measure the time a sample molecule requires to fly a known distance. Charged molecules of the sample are accelerated in an electric field with a known energy.

The speed a molecule gains is then:

Velocity=SQRT(2*Energy/Mass)

From this equation the mass can be calculated, by measuring the time such a molecule requires to fly a known distance. Heavy ions will take longer than light ones.

TOF MS schematic

This picture shows the working principle of a time of flight mass spectrometer. To allow the ions to fly through the flight path without hitting anything else, all the air molecules have been pumped out to create an ultra high vacuum. In the vacuum in the MS-200 chamber an ion can fly on average 600 metres (mean free path) before it will hit an air molecule.

At the end of the ion flight path is a detector which produces electrical pulses as the ions impact the detector. The time measurement is done by the timing electronics, which applies a pulse of voltage to accelerate the ions and measures the time between this pulse, and the electrical impulse from the detector.

The MS-200 mass spectrometer has an flight path length of around 0.6m. A molecule with a mass 26 amu (atomic mass units) requires 6 µsec (6 x 10-6 seconds) to fly through the flight path.

For a sufficient distinction between different masses the timing electronics is capable of measurement with a resolution of 2 nsec (2 x 10-9 seconds)

Every 20 µsec the analyte in the ionisation area is accelerated and the masses of the molecules are recorded. In an experiment of 1 second, 50 000 analysis cycles are performed. Therefore the gathered spectra is a good representation of the sample.

Ionisation of the Sample Molecules and Production of a Mass Spectra

As described before, the sample in the spectrometer is accelerated with a known energy and from the gained speed (distance per time) the mass is calculated. This acceleration is done in an electric field, which requires the molecules to be charged.

Ionisation schematic

Charging of the molecules is achieved by bombarding them with electrons emitted from a filament. When a molecule is hit, it is very likely to loose one or more of its electrons and therefore will be charged and becomes an ion.

If a chemical in the spectrometer was simply ionised and accelerated then the analysis would deliver one peak.

A one peak spectrum

This peak represents the molecular weight of the chemical. For example a peak at 78 amu for benzene.

Electron impact ionisation not only charges the molecules of the sample, it also breaks parts of the sample down into different fragments.

Fragmentation schematic

In our example of benzene (C6H6) this delivers 29 different peaks with different intensities. These 29 peaks represent the different fragments of benzene and the likelihood of their occurrence (with the different isotopes of carbon) during the process of ionisation.

Benzene spectrum

This fragmented spectrum is unique for each component, like a fingerprint. When using 70eV impact ionisation this pattern can be compared with the NIST database. This database includes more than 100 000 different chemicals. Comparison of these fingerprints enables the identification of unknown chemicals in the sample.

Software for Control and Analysis of the Mass Spectrometer

Control of the mass spectrometer and analysis of the results is done from a lap top computer. This computer features GRAMS® (Thermogalactic Corp.), spectral analysis software that operates under Windows®. We adapted this software to suit the requirements of our MS-200. After switching on the mass spectrometer, all subsequent control and analysis of the data is operated via this software.

Mass Spectra Analysis

The timing electronics delivers information about the ions detected in the form of an ion detection-time histogram. From this raw information the software calculates the raw mass spectrum using the relationship:

t = Cb*SQRT(m) + t0

Next the mass peaks in the spectrum are located and a number of counts computed at each mass number. The result is usually referred to as a stick-plot and looks rather like the spectrum for benzene shown above.

When analysing a gas mixture with the MS-200, the resulting spectrum is, to a good approximation, a linear superposition of the different spectra from each of the components in the sample.

Graphical superposition

Given a set of ideal spectra, each one for a different pure compound, Kore's mixture analysis software can calculate what proportions of the ideal spectra to add together to give the best fit to a measured spectrum. A background (sometimes called null gas) result is also used in the calculation. When the system is calibrated, the concentrations of the individual components can be calculated and displayed in a simple table.

If the collected spectrum contains unknown chemicals, then these can often be identified by comparing the peak pattern of the unknown chemical with the NIST database.


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Last updated: Wednesday, September 14, 2005, 16:49

© Kore Technology Limited 2005