What is an Electron MicroProbe Analyzer (EMPA)
An EMPA is an analytical instrument designed for non-destructive chemical analysis of minute solid samples. It is the same as an SEM, with the difference being the microprobe has the ability of chemical analysis. The EPMA has the ability to acquire precise, quantitative elemental analyses on spots as little as 1-2 microns by wavelength-dispersive spectroscopy (WDS) which makes it possible to analyze rocks and minerals to resolve complex chemical variation within single phases and by energy dispersive systems (EDS).
Analyses are performed by focusing a beam of electrons onto the sample surface. The electrons, are generated from a glowing filament and are accelerated toward the sample using a voltage potential of 15,000 volts or more. The high-energy electrons knock electrons from atoms in the sample, creating an “excited atom”. As each atom becomes less excited, it drops back down to its normal energy state thereby emitting energy in the form of X-rays. Most X-ray analytical methods are based on the fact that each element has its own characteristic wavelength and energy measured by WDS and EDS respectively. The number of X-rays emitted from a particular spot in the sample, and comparing that number with the number emitted from a standard, we can determine the concentration of the element of interest in our sample.
For example, we are measuring iron in an unknown sample, and we get half the number of counts that we found on our iron standard. We would estimate that our sample had half the amount of Fe as that standard. However, a ZAF matrix corrections are the used to account for the presence of other elements in the sample which interfere with the Fe X-rays. Some elements in the sample will, because of their high atomic numbers, essentially reflect electrons away from the sample before they’ve had a chance to excite Fe atoms. This decreases the number of X-ray counts that we might otherwise expect from Fe in our sample, and we must correct for this atomic number effect (Z). On the other hand, some elements absorb Fe X-rays before they leave the unknown, and these are corrected for the amount absorbed (A). In some cases, elements emit their own X-rays, which in turn can cause Fe to emit even more X-rays than it would if Fe were the only element present which is accounted for through the fluorescence correction.
These three matrix effects are known as:
Z – Atomic number effect
A – Absorption effect
F – Fluorescence effect
Our microprobe is equipped with two principal imaging modes both of which can be captured digitally for detailed petrographic reports. The two imaging modes are:
(1)Backscattered electron (BE) imaging shows compositional contrasts in your sample. These contrasts are greatest at higher operating current.
(2)Secondary Electron (SE) imaging shows topographic/surface contrast, with highest resolution at low operating current.
How does the EPMA work?
1. A tungsten filament is the electron source which is fitted to the cathode in the gun area of the EPMA.
2. A series of alignment coils, a condenser lens, and objective lens located in the column of the instrument are used to condense and focus the electron beam.
3. A sample chamber houses a brass stage which can be moved in X, Y, and Z space. The EPMA is also equipped with an optical microscope to aid in focusing and viewing the area to be analyzed.
4. The EPMA is under constant vacuum to prevent molecules in the atmosphere from interfering with the electron beam as it interacts with the sample.
5. Arranged around the sample chamber are spectrometers, and commonly an EDS detector that are used to collect x-rays and electrons emitted from the sample.
Quantitative analysis involves the measurement of the characteristic X-rays from an unknown sample relative to a set of standards analyzed under the same conditions. Correction factors (ZAF correction) for various effects are calculated by the computer. The precision of measurements on the electron microprobe is a function of x-ray counting statistics, which depends on the total number of x-ray counts collected on both the standard used for calibration, and also on the counts collected on the unknown sample. The minimum precision attainable on the instrument is in the vicinity of 0.5% relative. Spectrometer mechanical reproducibility is considered to be the limiting factor in precise measurements on the instrument. Therefore, at low total counts collected, counting statistic errors dominate, and at high total counts collected, instrument reproducibility dominates. Precision also depends on the chemical homogeneity of both the standard used for calibration and that of the unknown sample.
The accuracy of measurements on the electron microprobe depends on accurate knowledge of the composition of the primary calibration standard, and the “correctness” of the algorithm used to convert from x-ray intensity to concentration units (i.e. the ZAF correction). A global accuracy statement cannot be made. However, the accuracy is typically better than 5%, but may be worse for elements subject to peak interferences, or where there is a large compositional difference between the standard and sample and a large correction factor is observed (i.e. x-ray absorption, for example).
Our electron microprobe is also equipped with an energy-dispersive spectrometry (EDS) system, which we use primarily for qualitative identification of elemental abundances.
Unlike WDS, the EDS system does not recognize specific X-rays. Instead, the EDS detector receives and counts all X-rays at once, separates the energy spectrum into different channels. An EDS spectrum of X-ray energies from 0 to 10 kV can be collected and displayed on a computer screen. Peaks will show up on the spectrum, corresponding to energies of elements present in the sample. These spectra can be saved and imported as digital captures into reports.