Forensics Lab 5.4: Examine the Spectroscopic Characteristics of Soil


This article incorporates, in modified form, material from the not-yet-published Illustrated Guide to Forensics Investigations: Uncover Evidence in Your Home, Lab, or Basement.


Although wet chemistry tests are still commonly used in professional forensics labs, various types of spectrometry are used extensively to supplement or replace these older wet chemistry tests. Spectrometry has several advantages, notably that it is extremely sensitive, very accurate and precise, and can be used when only very small specimens are available for testing. The only real drawback to spectrometry is that professional-grade spectrometers are extremely expensive, costing from tens of thousands to literally millions of dollars.

Fortunately, the $25 Project Star Spectrometer is sufficiently accurate and sensitive to provide useful data about our soil specimens. (Just out of curiosity, we tested the Project Star Spectrometer by dissolving a few grains of table salt in a liter of distilled water and examining that specimen with the Project Star Spectrometer; the yellow sodium lines in the spectrum were visible even at that tiny sodium concentration.)

In this lab session, we’ll extract some of the ion species present in our soil specimens by treating them with 3 M hydrochloric acid, which reacts with many insoluble compounds to form soluble chlorides. Chloride salts are ideal for spectrometry, because most of them vaporize at low temperatures relative to other inorganic compounds. We’ll test those specimens with the Project Star Spectrometer and determine which ion species we can detect in our soil specimens. The limitations of our inexpensive spectrometer mean that our results will be qualitative (present/not present) rather than quantitative, but even qualitative results may provide data useful for discriminating among our specimens.

Required Equipment and Supplies

  • goggles, gloves, and protective clothing
  • Project Star Spectrometer
  • test tubes (6)
  • test tube rack
  • Eyedropper or Beral pipette
  • beaker, 150 mL (or similar container)
  • alcohol lamp or gas burner
  • stirring rod
  • inoculating loop (platinum or Nichrome)
  • digital voice recorder or tape recorder (optional)
  • hydrochloric acid, 3 M (~ 25 mL to 50 mL; see Substitutions and Modifications)
  • dry soil specimens, known and questioned

All of the specialty lab equipment and chemicals needed for this and other lab sessions are available individually from the Maker Shed or other laboratory supplies vendors. Maker Shed also offers customized laboratory kits at special prices, and a wide selection of microscopes and microscope accessories.


Hydrochloric acid is corrosive and emits strong fumes. Always wear splash goggles, gloves, and protective clothing when working in the lab.

Substitutions and Modifications

  • You may substitute hardware store muriatic acid for the hydrochloric acid. If you are using concentrated (37%) reagent grade hydrochloric acid, you can make up 25 mL of 3 M acid by carefully adding about 6.3 mL of concentrated acid to about 18.7 mL of distilled water. If you are using muriatic acid (31.45%, about 10.2 M), you can make up 25 mL of 3 M acid by carefully adding about 7.4 mL of the acid to about 17.6 mL of distilled water. The purity of hardware store muriatic acid varies. Some specimens are very pure (about lab grade); others are contaminated with various substances, often iron. If you use muriatic acid for spectrometric analysis, be aware that some of the emission lines you see in the spectrum may be from contaminants present in the acid. Actually, even reagent grade hydrochloric acid may contain impurities that show up in spectroscopic tests, which are extremely sensitive. That’s why professional forensics labs use spectroscopic-grade hydrochloric acid, which is extremely expensive.


This lab has two parts. In Part I, we’ll extract ion species from our soil specimens by soaking them in dilute hydrochloric acid. In Part II, we’ll test those specimens with the spectrometer and record the emission lines from elements present in our soil specimens.

Part I – Extract ion species from the soil specimens

  1. If you have not already done so, put on your splash goggles, gloves, and protective clothing.
  2. Transfer 25 mL to 50 mL of 3 M hydrochloric acid to the small beaker.
  3. Label six test tubes Q1 and K1 through K5.
  4. Transfer a sufficient quantity of each of your specimens to the corresponding test tube to have about one centimeter of soil in each tube.
  5. Use the eyedropper or Beral pipette to transfer sufficient 3 M hydrochloric acid to each test tube to bring the liquid level up to 4 or 5 centimeters, as shown in Figure 5-13.
  6. Swirl each tube intermittently for a minute or so to keep the soil suspended, and then replace the test tube in the rack.
  7. Allow the soil specimens to settle completely, which may require anything from a minute or two to 24 hours, depending on the soil specimen.


Figure 5-13. Extracting ion species from soil specimens for spectrometer tests

Figure 5-13 has two interesting features. First, the test tube second from the right is chipped, which we didn’t notice until we shot and processed this image. We discarded that tube immediately, as is proper procedure for any damaged glassware. Second, although we added the hydrochloric acid solution to the test tubes within seconds of each other, this image, shot about five minutes after we added the acid, shows the different settling characteristics of these specimens based on the clarity and color of the supernatant liquid in each tube. Specimen K4 (second from the right) shows the fastest settling, and specimen K5 (far right) shows the slowest.

Part II – Test the soil specimen extracts with the spectrometer

  1. If you have not already done so, put on your splash goggles, gloves, and protective clothing.
  2. Turn off the room lights, close the curtains, and take whatever other steps are necessary to darken the room. The room needn’t be completely dark, but the darker the room, the easier it is to observe the spectra.
  3. Light your alcohol lamp or gas burner.
  4. Dip the inoculating loop into pure hydrochloric acid and then hold it in the hottest part of the flame (the blue area near the tip) to burn off any contaminants. If necessary, repeat this step until the inoculating loop adds no color to the flame.
  5. Again dip the inoculating loop in the pure acid, and then hold the tip of the inoculating loop in the hottest part of the flame and examine the flame through the spectrometer, as shown in Figure 5-14. (It may be helpful to clamp or otherwise secure the spectrometer in position, pointing at the flame.) Any emission lines that appear are caused by the acid specimen itself or the inoculating loop. You can disregard these lines when you test your soil specimens.
  6. Carefully dip the tip of the inoculating loop into the liquid extract from specimen Q1, being careful not to disturb the soil at the bottom of the test tube.
  7. Put the tip of the inoculating loop in the hottest part of the burner flame and observe the emitted light through the spectrometer.
  8. Note the wavelength and intensity of as many lines as possible. (We used a digital voice recorder so that we could simply talk about the lines we observed and later transfer the data to written form.) The Project Star Spectrometer is graduated at 5 nanometer intervals, from 350 nm to 750 nm. You should be able to interpolate wavelength values to about one nanometer. As to intensity, we described the spectral lines as “extremely intense,” “very intense,” and so on, down to “extremely faint.” Although not strictly quantitative, these “semi-quantitative” intensity estimates will be useful later when you compare the observed spectra against known emissions lines for various elements. You may have to make several passes with each specimen to give yourself time to record all of the visible emission lines for that specimen.

Shooting Spectra

Instead of manually recording the wavelengths and intensities of the spectral lines, you can shoot images of them. To do so, you’ll need to make a clamp or bracket that holds the camera and spectrometer in fixed positions relative to the burner. If you use a digital camera, the images you capture will probably not match what you see visually. Digital cameras reproduce continuous analog color spectra in discrete digital chunks, so emission lines that are prominent visually may be much dimmer or even entirely absent photographically.

  1. Repeat step 4 to clean the inoculating loop, and then repeat steps 6 through 8 for each of your known specimens.


Figure 5-14. Barbara using the spectrometer to view emission spectra of soil specimens

It’s here that the difference between a $25 spectrometer and a $25,000 spectrometer becomes obvious. The professional instrument analyzes the elements present based on the observed spectrum, and displays or prints that information quantitatively. We have to do the comparison manually, which in practical terms limits us to comparing only a limited number of relatively bright spectral lines from a limited number of elements.

Table 5-5 lists the prominent emission line wavelengths and relative intensities for the six elements we chose to test for. The relative intensities listed in parentheses beside each characteristic emission wavelength provide a semi-quantitative metric. For example, the sodium lines at 589.0 nm and 589.6 nm are extraordinarily bright, while the lines at 371.1 nm and 568.8 nm are much fainter, but bright enough to be easily visible. We established an arbitrary cutoff at a relative intensity of 450, which should be readily visible even at the extreme ends of the visible spectrum where your eyes are less sensitive.

Nearly every (we are tempted to say “every”) soil contains sodium in amounts large enough to provide very prominent emission lines. Aluminum, although its emissions lines are fainter, is present in many soil specimens. Iron is present in some soils, particularly those with a reddish color, in amounts large enough to provide prominent emission lines. Barium and mercury are toxic heavy metals that (we hope) will not be present in your soil, but nonetheless some soil specimens will contain sufficient quantities of either or both metals to provide spectra bright enough to be visible with our inexpensive spectrometer. (You can look at a mercury spectrum simply by pointing your spectrometer at a fluorescent lamp and a barium spectrum by heating one crystal of any soluble barium compound with your inoculating loop.) Strontium is not widely distributed in soils, but may be present in some specimens, particularly because it is commonly used in fireworks (including simple sparklers) and road flares, where it produces an intense red light.

With a professional spectrometer, the question is, “what elements are present in this specimen?” With our simple spectrometer, we have to do a bit more work by asking, “is element X present in this specimen?” For example, we might decide to determine if sodium is present in one of our specimens. To do so, we look at Table 5-5 to determine which very prominent lines are present in the sodium spectrum. Knowing that the 589.0 nm are 589.6 nm “sodium pair” are extremely prominent, we look for those wavelengths in our specimen. If we see bright lines at those wavelengths, we strongly suspect that sodium is present. We confirm that by looking for the fainter sodium lines at 371.1 nm and 568.8 nm. If those lines are also present in the spectrum, it’s certain that sodium is present in the specimen. Repeating that procedure allows us to determine if aluminum, barium, iron, mercury, and/or strontium are present.

Table 5-5. Prominent emission line wavelengths and relative intensities for selected elements

Element Prominent emission line wavelengths (relative intensities)
390.1 (450); 394.4 (4500); 396.2 (9000); 466.7 (550); 559.3 (450); 600.6 (450); 607.3 (450); 618.3 (450); 620.2 (450); 624.3 (450); 633.6 (450)
413.1 (910); 428.3 (530); 455.4 (9300); 458.0 (580); 493.4 (6900); 553.5 (1830); 577.8 (740); 582.6 (610); 597.2 (700); 599.7 (620); 601.9 (610); 606.3 (840); 611.1 (880); 614.2 (1510); 634.2 (900); 645.1 (580); 648.3 (1770); 649.7 (900); 649.9 (1060); 652.7 (890); 659.5 (740); 667.5 (462); 669.4 (454)
404.6 (4000); 406.4 (1500); 407.2 (1200); 414.4 (800); 426.0 (800); 427.2 (1200); 428.2 (1200); 430.8 (1200); 432.6 (1500); 437.6 (800); 438.4 (3000); 440.5 (1200); 442.7 (600); 495.8 (1500); 516.7 (2500); 517.2 (500); 522.7 (1000); 527.0 (1200); 527.0+ (800); 532.8 (800); 534.1 (500)
404.7 (1800); 435.8 (2000); 546.1 (1100); 577.0 (1240); 579.0 (1100); 615.0 (1000)
371.1 (850); 568.8 (560); 589.0 (80000); 589.6 (40000)
403.0 (1300); 407.8 (46000); 421.6 (32000); 460.7 (65000); 472.2 (3200); 474.2 (2200); 478.4 (1400); 481.2 (4800); 483.2 (3600); 485.5 (500); 486.9 (600); 487.2 (3000); 487.6 (600); 487.6 (2000); 489.2 (1000); 496.2 (8000); 496.8 (1300); 515.6 (800); 522.2 (1400); 522.5 (2000); 522.9 (2000); 523.9 (2800); 525.7 (4800); 545.1 (1500); 548.1 (7000); 548.6 (1100); 550.4 (3500); 552.2 (2600); 553.5 (2000); 554.0 (2000); 638.1 (1000); 638.7 (900); 638.8 (600); 640.8 (9000); 650.4 (5500); 654.7 (1000); 655.0 (1700); 661.7 (3000); 664.4 (800); 679.1 (1800); 687.8 (4800); 689.3 (1200)


Dispose of the acidified soil specimens by diluting them with a large amount of water and flushing them down the drain.

Review Questions

Q1: Based on the spectra you observed for your soil specimens, how well can you discriminate between the specimens?