LUMAS-30 TIME-OF-FLIGHT MASS SPECTROMETER WITH GLOW-DISCHARGE IONIZATION

Description

Technique

Time-of-flight mass-spectrometry with pulsed discharge in hollow cathode.
By dint of combination of gas-discharge ionization system with time-of-flight ion detection technique there have been realized high efficiency of sputtering of sample surface, high speed of mass spectra registration over all range of detected masses and high sensitivity for most elements.

The principle of operation is based on the following processes:

• high-efficiency atomization of analyzed samples due to cathode sputtering in pulsed glow discharge for both current conducting and non-conducting solid-state materials;
• pulsed ionization of sample atoms in glow-discharge plasma during not only glow period, but also afterglow period that results in attainment of similar sensitivities for large number of elements;
• high-speed registration of time-of-flight spectra (up to 3000 spectra /s);
• fast entry air lock for rapid introduction of large, up to 30mm dia., specimens (typically 10-17mm dia.,);
• vacuum systems with UHV component and modular design ensures that the various analysis methods and existing systems can be upgraded at any time.

Advantages:

• possibility for registration of a large number of spectra during period of one sample sputtering that results in improving of signal-to-noise ratio due to statistical averaging of recorded spectra;
• direct analysis of solid-state samples including gases dissolved in the samples, with high-economical flow rate of discharge gas and consumption of sample material due to time matching of pulsed ionization and time-of-flight registration of mass spectrum that results in appreciable decreasing of detection limits;
• high efficiency of sputtering and ionization of sample elements in pulsed discharge and, as a result, low detection limits (50-200 ppb);
• large dynamic range of detected element concentrations (up to 7 orders of magnitude) that by 2-3 orders of magnitude greater than detection limits of other methods of solid sample direct analysis;
• high-efficient suppression of gas components due to time discrimination and using of gas mixtures with addition of hydrogen as the reaction gas;
• wide range of analyzed objects including dielectrics and semiconductors in addition to metals. It is ensured by using of narrow pulses (1-80 s) of discharge current providing sputtering of non-conducting and weakly-conducting materials;
• possibility for direct mass spectrometric analysis of layer inhomogeneity of chemical composition for various objects (with layer resolution about 3 nm);
• possibility for direct mass spectrometric analysis of multilayer thin-film coatings;
• no necessity for dissolution in the course of sample preparation.

Technical characteristics:

Analyzed objects:

• metals;
• semiconductors;
• dielectrics;
• objects with mixed layer structure dielectric-metal, metal-semiconductor and dielectric-semiconductor (e.g. corrosion films on metal surface);
• powdered samples.

Main special features of Lumas-30

Pulsed discharge

Pulsed glow discharge is generated by sequence of voltage narrow pulses and, similar to radio-frequency discharge, can be applied for direct analysis of both current conducting and non-conducting samples. Representative pulse duration for this type of discharge is within the range from several microseconds up to several milliseconds. As a rule, power consumption for DC glow discharge is about 1-4W, for radio-frequency discharge - about 20-50 W that provides a signal with strength approximately similar (by order of magnitude) to DC discharge signal strength generated at lower power consumption. As for pulsed discharge, instantaneous power in this case can attain several kilowatts that provides rate of sample sputtering during pulse time approximately by two orders of magnitude greater than for DC discharge. Such high power results in signal increasing by 1-4 orders of magnitude in comparison with DC discharge.

Hollow cathode

There are two main types of glow-discharge sources applied for analysis of solid-state samples: glow discharge with flat cathode (Grimm-type discharge) and glow discharge in hollow cathode. In comparison with the Grimm-type discharge, for discharge in hollow cathode there are realized higher rate of sample sputtering and ionization of sputtered atoms. As a result, discharge in hollow cathode has lower detection limits. Pulsed discharge in hollow cathode provides possibility for even more increasing of sputtering and ionization rate and, moreover, for suppressing due to time discrimination of gas components that interfere with detection of some elements.

Time-of-flight mass spectrometer

Among the mass-spectrometric systems the most oriented for operation with pulsed sources of ions is the time-of-flight mass spectrometer because in this case there is realized the highest efficiency of ion detection.

Operating procedure

The device switching-on and bringing to operating condition is performed automatically.
Analyzed sample can be placed into the device by two ways. For the first way the sample is made as a disc 10 mm in diameter and 3-6 mm in thickness. It can be either solid one or prepared as a compacted (pressed) powder tablet. The sample is fixed as the bottom of hollow cathode made of high-purity Al, Nb or other metal.
For the another way, in case of solid material, the sample is turned to the shape of a cylindrical hollow cathode.

Ballast gas Ar or Ar-He mixture is fed into discharge chamber where the sample is fixed. Due to pressure difference between discharge chamber and differential pumping-out zone the generated sample ions together with ballast gas via opening in the sampler are entered into differential pumping-out zone and then into drift tube (being orthogonal to ion beam) with repelling grids. As the detector there is used chevron assembly consisting of two microchannel plates.
Designed interface of the device with movable discharge chamber provides possibility for in-operation change of samples without interruption of device functioning. After sample installation there is performed pumping-out of discharge chamber during 2-3 minutes, and then the device begins to be ready for measurements. Operator selects exposure time depending on requirements to the device sensitivity and then turns to measurement mode.
Acquired information is logged and archived.
To change the sample there is required to close the gate valve of movable discharge chamber, then take the holder out and replace the sample.

For the device calibration there are used the appropriate State Reference Standards (GSO). The device control under condition of data processing and logging is performed by unified control program.

The control program interface is presented below.

Control and registration mode: - automatic registration and processing of spectra with speed up to 3000 spectra/s, - automatic peak indexing according to built-in database, - graphical display of condition of vacuum aggregates, - automatic maintaining of specified pressure in ion source, - monitoring of pressure level by three manometers simultaneously, - graphic monitoring of 8 peaks in real time, - system setting of nominal values for power supply and spectrum registration.

Processing and logging mode - graphic selection of set of monitored parameters, - automatic processing of measured concentration results according to established calibration plots, - automatic logging and recording of the experimental results, - possibility for enlargement of database.

Terms of delivery:

• the system is made by request during 6 months;
• consultations and completeness according to customer's demands;
• start-and-adjustment works and personnel training;
• servicing.

1. Analysis of impurities in electrode copper

Device calibration - by State Reference Standards (GSS) for copper:
1) GSS No 945
2) GSS No 9410

Parameters:
Pmixture= 2.5 torr (Mixture composition: Ar - 70%, He - 29%, H - 1%)
Total number of spectra - 1000000
Analysis time - 5 min

Spectra of copper reference standard No 9410 (masses from 61 to 65 are cut out)

Measurement of mean concentrations of different elements in sample of electrode copper

As the given Table indicates, Lumas-30 provides measurement of proper results even if concentrations of different elements in copper are about ppm level.

2. Analysis of impurities in lead

Lead with tin impurity

Total number of spectra 106 Analysis time, min 5

In the given spectrum there is clearly marked calcium that is hard to detect by mass spectrometry.

3. Analysis of lead - antimony alloy composition

Total number of spectra 106 Analysis time, min 9.5

The given alloy is characterized by high concentration of selenium whose isotopes are clearly marked in spectrum.

Element concentrations in alloy sample

4. Analysis of vitrified slag composition

5. Analysis of steel

Spectrum of steel 55Õ7ÂÑ. Analysis time - 3 min.

6. Analysis of silicon sample composition

Silicon spectrum

The given examples of elemental analysis performed with using of LUMAS-30 time-of-flight mass spectrometer demonstrate the device ability to analyze current-conducting materials, metals and metal alloys (illustrated by examples: Cu, Pb, Pb-Sb, Fe), semiconductors (Si) and insulators (vitrified slag). In the all cases in mass spectra there have been registered observance of isotope abundance being typical for chemical elements.