Accuracy - A measure of how close the analytical value provided by the equipment (wether percent concentration or some other value) is to the true value. This is usually expressed as the error which is the amount of difference between the number by instrument and the true value, and may be presented ein either of two forms: The absolute error is difference bwtween true and instrument (or calculated) values, - for instance, if the true concentration is 20.4% and the calculated value is 21.7%, the absolute error is +1.3% (concentration), the relative erroris the same difference between the two values expressed as a fraction or percentage of the true value. In the example above, this would be (21.7-20.4)/20.4=+6.37 (relative). In both cases, the + indicates the error is positive - the calculated value is geater than the true value.

Bremstrahlung - As desribed above, this is the flux of X-rays at all energies produced by the decleration of electrons in the sample. For microprobe, SEM and TEM analysis, this is the background in the spectrum above which rise the characteristic peaks from the elements present.

Concentration - The fraction of the element in the sample, usually by weight. In many cases, this is expressed as percent (%) or parts per million (10 000 ppm=1.0%). For some types of samples other units are appropriate; for instance, concentration of elements on filter paper may be given as micrograms per square centimeter. Concentration may be expressed either as elemnetal (e.g. 13.1% Al) or as compund (e.g. 24.8% Al2O3). In the later case, the X-rays from one element (Al) are mesured and the conversion from elemental concentration to comound concentration is simply based on the atomic weights and assumed composition of the compund.

Dead Time - The portion of the total analyzing time that is actually spent processing (mesuring) X-rays because during the time that each X-ray pulse is being measured, the system cannot mesure another X-ray that may enter the detector and is, therefore, said to be "dead". In an actual analysis, the total analyzing time is extended by the equipment so that the analyzing time (e.g. 100 seconds) set on the controls is the live time - the time spent waiting for the x-ray photons.

Detector Efficiency - The relative faction of the X-rays that leave the sample that are subsequently measured and counted. This includes the solid angle of X-rays from the sample that reach the detector as well as any losses (e.g. due to absorption in the window in front of the detector) that take place.

Escape Peaks - In Si(Li) detectors ecape peaks occurring 1.74 keV below each 'true' peak. Escape peaks are small (up to 1% of parent peak height) and its desirable that they should be removed from the spectrum for quantitative analysis.

Intensity - The number photons counted either for a specific element or the total number of photons of all energies being processed by the detector (total count rate). In some cases, (particulary with electron beam excitation as in the electron microprobe) the instensity is adjusted (normalized) for variations in the instrument by experssing it as counts per unit of beam current (e.g. counts per nanoampere). The intensity may be expressed either as the total number of counts during the analysis or as the number of counts per second.

Matrix - The total composition of the sample, expecially with regard to its effect on the measured intensity for a given element due to the primary absorption, secondary absorption and possibly secondary fluorescence, effects from the other elements in the sample.

Minimum Detectable Limit - The lowest concentration of the element in the sample that can be definitely be identified in as given analyzing time. Since this requires the peak to rise above background by a statistically significant amount, it is customary to speak of detection at the 95% confidence limit if the peak rises above the background by two standard deviation, or 99% for three "sigma". etc. The minimum detectable limit in a given matrix is proportional to (B/P)^(1/2) where B is background intensity an P is net peak intensity; it can be lowered (improved) by increasing total count rate, by reducing background relative to peak height, or bt extending the total counting time.

Overvoltage Ratio - The ratio of the incident electron voltage to the energy required to excite the atom of interest (its absorption edge energy). Generally, the higher this ratio the geater the excitation efficiency for the element - that is more X-rays per incdent electron that are produced.

Qualitative Analysis - Determination of which elements are present in the sample with little or no regard to the exact amouny of each.

Peak Separability - The ability of the system to separate peaks from different elements that lie close together in the spectrum or to separate an elemental peak from the background. This is dependant on many system parameters including detector resolution at various energies, amplifier parameters, and often computational capability to identify and strip peaks.

Precision - A measure of how reproducible the measurement is. This can be either the raw intensity data or final calculated value. In either case, is is normally expressed as percentage - for example, the mean intensity value is 51417 and, on repetitive runs, the variation is from 51720 to 51114. This variation is 51417 +/- 313 or 313/51417=0.61%. The statistical precision is the amount of variation that is expected simply from the probabilities inherent in counting random events such as X-ray photons and is simply Sigma=(N)^1/2. In the example above, the variation expected in 68% of the measurement would be 51417+/-227. In 95% of the measurement, the maxiumum variation expected would be two Sigma or 51417+/-454, in 99% of the measurements, the maximum variation expected would be three Sigma or 51417+/-680. Whether given for the counts or the final calculated value (e.g.concentration) the precision describes only the consistency of the data and not its accuracy.

Resolution - The "sharpness" of a single isolated peak in the spectrum, usually expressed as the "full width at half maximum" (FWHM) or, in other words, the width of the peak (in energy) at half its maximum height. Resolution varies with energy of the X-rays. Also, resolution is a measure of the performance of the detector, but does not adeqately define the total system ability to separate peaks from different elements.

Reproducibility - The ability of the instrument to repetedly give the same conditions and measurements. Normally, this is separated from the statistical contribution to precision which would be the only variation in mesurements if reproducibility were perfect. The reproducibility is also given as a percentage variation - for example, 0.1% reproducibility would mean that a mean intensity of 50 000 counts could vary by +/-50 counts.

Standards - Samples containing known concentration of various elements, preferable in somewhat similar proportions as the unknown specimen that are to be analyzed. Standards may be obtained by using actual samples that have been independently analyzed by another method, such as wet chemistry or they may be "made up" by mixing together accurately weighed pure chemicals. A combination of these techniques is to start with a few standards and mix them in various proportions to produce other standards. Another method is to "spike" a standard sample with known amounts of one or more pure compunds. In all cases involving mixing, it is of paramount importance to assure uniform mixing on as fine scale as possible. For electron excitation, it is usually not possible to obtain complex standards that are uniform on a sub-micron scale and, therefore, it is customary to use pure elements as standards.

Sum Peaks - Pulse pile-up causes 'sum peaks' to appear at energies corresponding to be sum of the energies of major peaks.

Semi-Quantitative Analysis - Determination of the approximate concentration of the elements in the sample, often as ratios to each other rather than directly as percent.

Absorption Coefficient - A parameter describing the distance of penetration of X-rays of a particular energy in a given material. The most common form, properly called the mass absorption coefficient, is "mi" in the absorption equation. Tables of values "mi" for X-rays of different energies, in specimens of different elements, cas be used to determine the "mi" value for any radiation in any matrix.

Stray Peaks - Spectrum contain small peaks which orginate from regions of the specimen other then analysed points, from the specimen holder, or from parts of the instruments. These are excited mainly by stray electrons (for example, those backscattered from the sample) and are more likely to be seen by EDS detector than by EDS detector. They can be minimized by good colllimation, but are difficult to eliminate completely.

Spectrum - A plot of X-ray intensity as a function of X-ray energy that shows peaks at energies corresponding to the characteristic shell binding energies of the elements in the sample, rising above a background composed primarily of scattered X-rays or Bremstrahlung.

Simultaneity - The ability to measure and store spectrum X-rays of different energies from the different elements all during the same analyzing time.

Sensitivity - A much abused word that has, at various times, been used to describe minimum detectable limit, detection efficiency, intensity for a given element, and other diverse and often undefined propertis of the analytical method, instrumnent or sample. The most often common use is to describe the intensity per unit concentration, ie. CPS/%.

ZAF Correction Method - Conversion the X-ray intensities to concentration based on measurement of the pure element intensities and compostion of corrected factors for the "atomic number effect" (which describes the depth of electron penetration and fraction of electrons that backscatter from the sample), the "absorption correction" (which describe the absorption of X-rays in the matrix - a function of composition and depth of electron penetration) and the "fluorescence correction" (which describe the secondary fluorescence of one element by the others present).