MS-200: Frequently Asked Questions
- Mass Spectrometer
- What is at the analytical heart of the MS-200?
- What are the advantages of a Time-of-Flight mass analyser?
- What is the mass range of the mass spectrometer?
- What GC column is used on the MS-200?
- What is the inlet method for sampling on the MS-200?
- After admitting a gas sample, how long does it take to acquire a mass spectrum?
- What are the advantages of the membrane inlet system?
- What determines the acquisition time?
- Can I sample continuously?
- Operational Performance
- What is the detection sensitivity of the MS-200?
- What are the repeatability and reproducibility of the MS-200?
- What are the typical costs of running the MS-200?
- How often must the mass spec be calibrated?
- How do I calibrate the system?
- How often do I need to calibrate the system?
- How much time is required at the start of the day to be ready for analysis?
- How much time is required to shut down at the end of the day?
- What software is used to control the MS-200?
- With no GC in front of the mass spectrometer, how can the MS-200 analyse a mixture of different organic compounds?
- How many components can the software distinguish?
- What are the weight and size of the MS-200?
- What is the power consumption of the MS-200 running on the battery?
- The analyzer operates at high vacuum. Don't I need a separate pumping station to regenerate the vacuum periodically?
- What are the main applications of the MS-200?
- Can you measure CO and CO2?
- Can you measure VOCs in water?
- Can I connect a GC to the MS-200?
- Mass Spectral Library Option
- What are the current MS-200 specifications?
The MS-200 utilizes a mass spectrometer, but, unlike a GC/MS that typically has a quadrupole mass spectrometer, the MS-200 uses a patented "time-of-flight" mass spectrometer (ToF-MS). Widespread use of time-of-flight mass analysers is a relatively recent phenomenon, but because of the advantages they offer, they are now widely accepted and are used in a range of commercial analytical equipment.
Parallel detection: Time-of-flight-mass spectrometry offers parallel detection of all species. This means that all mass fragments from all components entering the mass spectrometer are detected, providing high sensitivity and rapid analysis on samples with multiple components. Quantification of components in complex mixtures is achieved using library fitting tools within the instrument control software.
This is in contrast to quadrupole analysers which are scanning devices. When a scanning device is analyzing a specific mass, all others that may be entering the spectrometer are not detected.
Rapid analysis: the sampling rate on the MS-200 is 50KHz, and a mass spectral analysis with suitable statistics is typically achieved in 1 second. A typical collection time for concentrations in the ppb range is 10 seconds.
The minimum mass resolution of the MS-200 system is 200 Full Width Half Maximum (FWHM) at 78 daltons (acceptance criterion).
The MS-200 has a mass range of 0-1000 Daltons, and in practice this covers the range of masses for most organic gases that are the principal application for this analyser.
In principle, a suitable ToF spectrometer is capable of measuring masses out to several hundred thousand Daltons. In practice, the practical mass range of a ToF analyser is limited by three major factors:
- The ability to create high mass ions without fragmentation,
- The ability to convert high mass ions to electrical signals at the detector, and
- The ability to transport heavier non-volatile species into the ion source
Energetic ionisation processes such as electron impact result in significant fragmentation of the target molecules, producing a characteristic spectrum from a compound containing several peaks. Such a fingerprint spectrum can however be used to good effect to help clarify molecular structure, and also for unambiguous identification of the compound.
The MS-200 does not have a gas chromatograph front end and is thus not a GC-MS. Instead it uses two semi-permeable membranes to achieve pre-concentration of organic compounds prior to injection into the mass spectrometer. The MS-200 is therefore an example of a MIMS system (see below).
The MS-200 uses a dual membrane inlet system. Thus, the appropriate acronym for the MS-200 is MIMS; Membrane Inlet Mass Spectrometry. The basis of this method is the selective transport of analyte molecules across a semi-permeable membrane. The permeation of the host gas (typically air) is smaller than the flow of organic analytes of interest and thus an enrichment factor of 10-100 is obtained with each membrane. The membranes serve a second function: to create two pressure reducing steps to the high vacuum environment of the mass spectrometer. Gas introduction is via a standard1/8" Swagelock fitting. Sample is drawn at ~100ml per minute (simply to guarantee a fresh and representative sample on the membrane). From there the sample permeates into the intermediated vacuum space at around 1/1000 atmospheric pressure, and then permeates through the second membrane into the mass spectrometer where it is mass analysed.
It takes ~3 minutes for the inlet system to equilibrate after introduction of a new gas sample. After equilibration, the mass spectral analysis is complete in ~10 seconds. Thus a complete mass spectral acquisition can be performed in less than 5 minutes. This is typical for most analyses undertaken with the MS-200.
A MIMS system provides:
- Simple and rugged performance (perfect for on-site applications)
- Long-term operation with the same membrane for many months
- Immunity to dust and other particles
- Detection limits of a few ppb without sample pre-treatment
- Measurements where 'matrix effects' are small
- A clean environment for the ion source (no fouling of the source)
- Short response times with low memory effects
- Linear dynamic range of several orders of magnitude with good quantitative accuracy.
This question depends on the problem. If an unknown component with a high concentration has to be identified, then an analysis time of 1 second might result in sufficient counts. If however, a component of a very low concentration has to be quantified with an acceptable precision, then an experiment will have to last maybe 20 seconds. This longer time is required to acquire enough counts of the analyte at the mass peaks of interest, to have a statistical significance for quantification.
The MS-200 can be programmed to acquire data automatically for a user-defined time (from 0.5 to 105 seconds), and these can be acquired at a user-defined interval from 1 second to 10 5 seconds.
The semi-permeable membranes serve to concentrate most VOCs compared to air molecules, but the concentration factors vary between 10-100 for each membrane used. Consequently it is difficult to quote a general detection limit. Below are some typical 1 sigma values that have been determined empirically on the MS-200 with a ten second acquisition:
Xylene 500 ppt, Toluene 850 ppt, Benzene 1ppb, Dichloromethane 5 ppb, Trichloroethane 3 ppb, Ethylacetate 25 ppb, n-Hexane 6 ppb, Methanol 120 ppb, 2 Propanol 2 ppm
Each MS-200 is required to pass a factory acceptance test specifying repeatability of better than 10% for 3 consecutive measurements of 1 ppm benzene in a nitrogen standard, with a background check between measurements. Typical achieved performance is better than 5%. Also, a linearity of response of better than 10% is required from mixtures of benzene in nitrogen over the range 200 to 1000ppb of benzene.
The MS-200 is designed to operate in ambient temperatures up to 35°C. To maintain constant permeation properties of the membrane, the inlet is operated at approx 45°C to minimize the effects of ambient temperature fluctuations over the range 15-30°C. The temperature is controlled to within 0.1 degrees around the set value. At ambient temperatures of ~35°C the membrane temperature has to be set higher. The maximum temperature set point of the membrane in the present system is 90°C.
The daily running costs of the MS-200 are very low compared to a GC/MS. The only true consumable (other than electricity) is the tube of the peristaltic pump that is used to maintain the intermediate pressure step of 1 mbar (between the outer and the inner membrane). This tube is 35cm long and lasts between 80 and 100 operating hours. The price of a 15meter length of this tubing (sufficient for about 40 pump replacements ) is ~£200.
In normal use the outer membrane (~1cm² in area) does not degrade with time. However, should it need to be replaced because of accrual of dirt in a dusty environment, the price of 20 membrane squares, pre-cut to fit outer membrane holder, is ~£150.
Kore recommends that the electron multiplier detector be replaced approximately every four years (depending upon frequency of use, naturally), and this requires sending the MS-200 back to the factory. The total cost is ~ £2800, including regeneration of the vacuum system.
Very infrequently. Once a 'mass calibration model' has been established empirically, this is stable for many weeks at a time. Also, it is common in the MS-200 to convert the mass spectrum to a 'stick plot', in which the height of a peak represents the peak area. In such 'stick plots' the peak is positioned at the 'nominal' mass rather than the absolute mass; thus a peak due to C2H 3Cl3 (absolute mass 131.930) will appear at 132. Minor shifts of the mass peak position are thus irrelevant in stick plots.
If you require quantitative analysis then the system has to be calibrated. The easiest way to perform a calibration is to obtain a gas bottle with the required analyte or mixture of analytes in nitrogen at a known concentration (a standard gas bottle). The concentration of this standard would be similar (ideally) to the expected concentration to be measured. The standard is attached to the inlet of the MS-200 and analysis is performed. The result screen of the mixture analysis offers the option to calibrate. After this "button" is hit, one simply chooses the component to be calibrated and tell the software the actual concentration of the standard. The linearity of the MS-200 over a wide concentration range does not require a multi-point calibration and therefore simplifies the process to a task of a few minutes.
By system calibration we mean running a standard with a known concentration to verify the system response. The frequency of this depends upon the required accuracy of your analytical result. For high accuracy, a daily calibration should be sufficient.
The software, once started, automatically checks the instrument status and begins warming the inlet line. It takes approximately 5 minutes for the inlet (including the membranes) to reach operating temperature. A gas sample introduced at that time will take a few minutes to equilibrate, after which a mass spectrum may be acquired; thus the time elapsed before a spectrum can be acquired at the start of the day is ~ 10 minutes.
There are no special procedures for closing down the MS-200. Switching off simply involves the time to shut down the computer.
The software is an augmented version of GRAMS/AI by Thermo Corporation,running on a laptop computer. The communication to the instrument is via a USB connection. More details of GRAMS/AI may be found here.
With no GC in front of the mass spectrometer, how can the MS-200 analyse a mixture of different organic compounds?
Kore has developed mixture decomposition software within the GRAMS 32 environment. It uses a least squares method to match fragment patterns with those in a reference database, and thus identify and quantify components in complex mixtures. The "fingerprint" spectra used in the reference database can be produced by the user's MS-200 on a 'pure' compound or can be imported from an electron impact (EI) database like the NIST mass spectral database.
Mathematically, the software can deal with over 100 library components (profiles) at the same time. Practically, it has been tested with 10 -15 different components in the MS-200 and found to work reliably. In a trial study for NASA, a mixture of 9 VOCs in the low to mid ppb concentrations were analysed in the MS-200 and were detected and distinguished. The majority of components were quantified within plus/minus 30ppb of their (later disclosed) values.
The weight of the instrument, including the carrying case and the battery to operate it, is 21kg. If a separate 12V d.c. power source is available (e.g. from a car/van) the battery can be removed reducing the weight to ~13 kg. The laptop to control the instrument is separate and weighs ~4kg. The encased instrument has the dimensions 531 x 328 x 213 mm.
The battery is sealed lead-acid 12V unit and delivers 10 Amp hours. The power consumption varies across the various modes from "pumping only" to "middle of acquisition". Here is a guide (though not all states are mentioned):
|Instrument switched off (ion pump only)||1.1 W|
|Instrument ready for analysis||17 W|
|Average (1 data point every 1 minute)||21 W|
|Average (1 data point every 5 minutes)||18 W|
The analyzer operates at high vacuum. Don't I need a separate pumping station to regenerate the vacuum periodically?
No. Unlike other portable mass spectrometry-based detectors, the MS-200 is a sealed unit with a novel, onboard pumping system. The amount of gas taken into the analyzer via the two membranes is low and is pumped adequately by the onboard ion pump, maintaining high vacuum in the analyzer. Under typical sampling conditions, the system vacuum is maintained for longer than the life of the detector. At the time of renewal of the detector, the vacuum system is re-generated.
The main applications are:
- The analysis of volatile and semi-volatile organic components (VOCs and SVOCs) on remediation sites as a fast and cost-effective screening process prior to site renewal
- Urban air quality monitoring research
- Industrial hygiene: monitoring of VOCs produced by factories
- Emergency response testing for industrial spills
Yes, however the MS-200 was designed for the analysis of VOCs. Therefore the membrane material was chosen to provide concentration of these compounds. The sensitivity for CO and CO2 is thus relatively poor. Additionally their mass spectra are almost identical. Owing to these two factors it is not recommended to use the MS-200 for measuring CO and CO2 levels.
At the moment this is not possible. This is because if the background matrix is water, there will be too much water permeating into the analyser vacuum and analysis would become impossible. However, we are currently developing an interface for the MS-200 which will allow us to perform this analysis in future on the MS-200.
There is nothing to stop a user from interfacing a GC to the MS-200. The MS-200 has been created to circumvent some of the time overheads associated with running samples through a GC column, but a user would be able to connect a GC to the MS-200 if desired. Kore does not supply such a column.
When the MS-200 is to be used frequently for the characterisation of unknown gas samples, we recommend purchase of the mass spectral library option. With this option the NIST/EPA/NIH electron impact mass spectral library is installed onto the MS-200 system. This library contains mass spectra from over 100,000 compounds compiled by NIST over many years. Data in the NIST library was produced by 70eV electron impact ionisation, the same as in the MS-200.
The NIST software package contains many powerful tools, such as mass spectral interpretation tools, library search facilities and so on. Spectra from the MS-200 can be exported to the NIST software for search/identification purposes. Furthermore, spectra from the NIST library can be brought in to the MS-200 software to allow the building of customised databases directly within the MS-200 software.
(The detailed procedures and data providing specs 1-5 are supplied on request)
- 1. Detection Sensitivity
- The sensitivity shall be more than 20 counts/ppb of Benzene as measured in a 10 second acquisition on the mass 78 peak (C6H6) for a 1ppm standard in nitrogen. Sensitivity is measured from the integral of mass 78: span counts - zero counts for a 'span' and a 'zero' gas respectively.
- The detection limit of 1sigma( shall be better than 5ppb for a ten second measurement, as measured from 30 measurements of 'zero' gas (nitrogen) using the mixture analysis software on Benzene.
- 2. Mass Resolution
- The mass resolution shall be >200 as measured from the full width half maximum value (FWHM) at mass 78 (C6H6) from Benzene.
- 3. Repeatability
- On three consecutive measurements of a 1ppm Benzene standard followed by a 'zero' gas measurement, the Benzene concentration shall be calculated using the sensitivity value determined in 1(a) above. None of the three measurements shall deviate by more than 10% from the stated concentration.
- 4. Linearity of Response
- Using a gas divider system to create 0, 20, 40, 60, 80 and 100% mixtures of 1ppm Benzene in Nitrogen, the maximum deviation from a straight line shall be no more than 10%.
- 5. Battery Voltage Variation
- If the battery voltage varies between 13.5 volts and 11.3 volts, the result on quantification of a 1ppm Benzene standard shall not vary by more than 10% from the stated concentration.
- 6. Battery Running Time
- Continuous Analysis: 2.5 hours; Pumping Only: 2.5 days
- 7. Mass Range
- The maximum mass range of the time-of-flight spectrometer is 1000 daltons
- 8. MS-200 Case and Contents
- Weight 21Kg (18Kg with no battery), Size 53.1cm x 32.8 cm 21.3 cm
Last updated: 16:34 21/09/2017
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