Using Energy Dispersive Spectroscopy to Analyze Gun Shot Residue

Scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) is an industry-approved technology for the identification and analysis of typical Gun Shot Residue (GSR) primer particles, thanks to its speed and ability to analyze individual particles.

Systems, which encompass a large amount of analysis area utilizing built-in SEM/EDS controls, are developed for automated analysis of GSR particles with less user intervention. Software and hardware components are employed to identify particles of choice and then further organize them into classes of interest that are characteristic of GSR.

When a GSR system is specifically developed to increase the data collection efficiency, more sampling area can be covered in a reduced amount of time, which results in higher productivity and a greater possibility for detecting even the smallest GSR particles present in a sample.

Analytical Requirements

The image detection of each GSR particle and the collection of spectral data from each particle from a whole set of stubs in the sample are two major aspects of a run. It is essential to precisely and efficiently acquire key parameters and details like particle morphology, stage location, classification matching, and chemistry.

Several GSR samples can contain hundreds or even thousands of particles of interest, thus even small reductions in collection time per particle, without affecting the classification accuracy, will be advantageous to the total throughput and productivity of a GSR analysis run. Moreover, advanced follow-up analyses of classified GSR particles validate the match and improve the overall confidence in results.

EDS Silicon Drift Detector (SDD) Collection Parameters and Optimization

A number of factors are vital in the total EDS data collection process, such as:

  1. Input count rate
  2. Output count rate
  3. Scan speed or collection time

Higher count rates and more rapid collection speeds will result in an increased speed of data collection. It is essential to take each aspect into consideration when expressing the detector’s collection speed for the analysis.

  1. Input count rate (ICR) refers to the number of X-rays that strike the detector and is given in counts per second (CPS). This factor is a function of the sample, SEM and detector geometry (which means some low atomic number samples do not generate a high number of CPS, while metallic particles generate a high number of CPS). The SEM operation, kV, and beam current have two key variables. The widely established practices for GSR analysis use a beam voltage of 20–25 kV, which is extremely suitable for the production of high CPS. The SEM’s beam current can be easily modified to attain high ICR, while still retaining the needed quality to detect and image even the tiniest required submicron GSR particles. Finally, the detector geometry considerations will have an effect on the input count rate; however, usually, any size detector will readily obtain the highest required count rates on the basis of the sample and SEM conditions. Yet another factor is the window material. Although windowless detectors are not appropriate for GSR due to contamination problems1, developments in thinner window materials enable improved input count rate, while still guaranteeing SDD protection and cleanliness.
  2. Output count rate is a very vital factor in the EDS data collection process. It is a function of how efficiently the electronics of detector turn the raw input count rates into the useable X-ray counts in a spectrum. For data characterization, high input count rates can be converted with low dead time into usable counts but only with a fast processing time, typically below 1 μs. Additionally, the detector needs to maintain a high-quality resolution at faster processing time. Instruments having a resolution of about 130 eV for the Mn Kα peak will more unambiguously resolve close peaks, for example, the Ca Kα and Sb Lα1 peaks.1
  3. When the SDD can be optimized for count rates, spectral collection time becomes a secondary function. Collection times as rapid as 1 second or better provide a quality GSR classification and analysis with a detector that delivers high efficiency, as well as quality resolution. For instance, an SDD, which obtains more than 100K CPS with less than 30% dead time, while still retaining a resolution of 130 eV or more may be employed. This will guarantee an analytical method where countless numbers of particles from several samples and stubs can be analyzed in fewer hours when compared to the conventional overnight runs. Shorter run times provide users the opportunity to manually verify the results several times per day, rather than just once as with an overnight run. 

Figure 1 shows a 500X magnification of a sample area of a GSR stub. At 2048 x 1600 pixel resolution spectra from particles as small as 0.25 µm can be collected.

Figure 1 shows a 500X magnification of a sample area of a GSR stub. At 2048 x 1600 pixel resolution spectra from particles as small as 0.25 μm can be collected.

Figure 2 shows the analyzed results of the 308 particles found in this single field of view. At one second collection time, this data was collected in about 5 minutes.

Figure 2 shows the analyzed results of the 308 particles found in this single field of view. At one second collection time, this data was collected in about 5 minutes.

Figure 3 shows a spectrum collected in 1 second. Octane Elite yielded high input count rate (117 KCPS) and low dead time (26%) with a high output count rate of 86 KCPS with a 130 eV quality resolution, so there is high confidence in the statistical accuracy of the classified 3-component GSR particle.

Figure 3 shows a spectrum collected in 1 second. Octane Elite yielded high input count rate (117 KCPS) and low dead time (26%) with a high output count rate of 86 KCPS with a 130 eV quality resolution, so there is high confidence in the statistical accuracy of the classified 3-component GSR particle.

Figure 4 shows a zoomed-in area of the 3 component particle that corresponds with the spectrum in Figure 3.

Figure 4 shows a zoomed-in area of the 3 component particle that corresponds with the spectrum in Figure 3.

EDAX Octane Elite Series

The EDAX Octane Elite EDS System has numerous prominent design characteristics which make it ideal for GSR applications. The most noteworthy benefit is the innovative Silicon Nitride window material, which is designed to be almost 10 times thinner than conventional polymer windows, while still maintaining the detector integrity. This will generate higher input count rates to the detector. Moreover, Octane Elite electronics are one of the fastest available in the market with processing times less than 0.2 μs, which guarantees the efficient and reliable conversion of input count rates into output count rates.

Additionally, the Octane Elite electronics provide resolution stability and will guarantee the detector resolution quality to separate close peaks and offer precise GSR classification solutions. In a nutshell, the most recent design technologies of the Octane Elite detectors take GSR analysis to a whole new level.

Reference

  1. Guide for Primer Gunshot Residue Analysis by Scanning Electron Microscopy/Energy Dispersive X-Ray, Spectrometry 11/29/11

This information has been sourced, reviewed and adapted from materials provided by EDAX Inc.

For more information on this source, please visit EDAX Inc.

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