Reagents and Test Media
The following reagents and test media are required for the Modified Griess Test:
- Nitrite sensitive emulsion-coated test paper
- Nitrite test swabs (nitrite compound positive control samples)
- Solution of glacial acetic acid (15%) and distilled or deionized water
- Nitrite compound-free cheesecloth
Nitrite Sensitive Emulsion-Coated Test Paper
Desensitized photographic paper or photographic printer paper can be used as a medium for capturing the azo dye pigment patterns generated during the Modified Griess Test.
The test paper preparation procedure is as follows:
- Prepare a solution of 7.7 grains (.5 grams) of sulfanilic acid in 100 mL of distilled or deionized water.
- Prepare a solution of 4.3 grains (.28 grams) of alpha-naphthol in 100 mL of methanol.
- Combine equal volumes of the above solutions.
- Pour the combined solutions into a nonreactive photo processing tray. Briefly submerge and remove precut sheets of the test paper into the tray.
- Set the sheets aside to dry on an uncontaminated surface or hang to dry.
- Store any unused solution in a sealed container labeled according to laboratory protocol.
- Store dried test paper in a dry place.
Nitrite Test Swabs
- Prepare a solution of 9.3 grains (0.6 grams) of sodium nitrite in 100 mL of distilled or deionized water.
- Soak cotton tipped swabs in the nitrite solution.
- Set the swabs aside to dry.
- Store swabs in a sealed container labeled according to laboratory protocol.
Glacial Acetic Acid
- Combine 150 mL of glacial acetic acid with 850 mL of distilled or deionized water.
- Store the solution in a sealed container labeled according to laboratory protocol.
Cut nitrite-free cheesecloth to the size of the processed test paper. (If the cheesecloth contains nitrites, it may produce an orange background color on the processed test paper.)
The Modified Griess Test is carried out as follows:
- Confirm nitrite sensitivity of test paper.
Using a nitrite test swab saturated with 15 percent acetic acid solution, dab the corners of the test paper. Each corner should turn orange, confirming the sensitivity of the paper to nitrite compounds. A negative control should also be tested for each sheet of photographic paper to ensure that it is not contaminated.
- Mark reference points of evidence on test paper.
Place the evidence face down on the emulsion-coated side of the test paper. Using a pencil, index any seams, button holes, buttons, rips, pockets, suspected bullet holes, tears, cuts, etc., for reference. Do not use ink because it may transfer back onto the evidence item.
- Create layers of reaction items.
Soak an unused piece of cheesecloth in the acetic acid solution. Wring excess solution from the cheesecloth and spread it on top of the evidence (already placed on the photographic paper), completing the three-layer sandwich.
- Heat reaction layers.
Press the layers with a hot uncontaminated iron. Acetic acid steam is forced through the layers, causing the color-producing reaction.
- Examine and interpret the results.
Discard the cheesecloth and separate the evidence item from the test paper. Any orange indications on the paper are the result of a chromophoric reaction chemically specific for the presence of nitrite residues.
Note: It is possible that a spurious source of nitrite residues (not firearms-related) could be present. These are typically visualized as an orange haze; it is unlikely that such residues would alter the interpretation of the point reactions around a suspected bullet hole. Occasionally, background reactions can be observed on clothing items made from blue denim and from garments washed using certain detergents. Some disinfectants, as well as marijuana, can yield positive results.
- Label and retain test paper.
Mark the previous pencil marks placed on the test paper in ink when the paper is dry. Label with additional data as called for by laboratory protocol.
Reverse Modified Griess Test
Nonporous or thick surfaces will not permit the passage of the acetic acid-bearing steam through the reaction layers. These materials (e.g., vinyl, wood, sheet metal, thick leather, carpeting, floor tiles, drywall) are not suitable for the Modified Griess Test since passage of acetic acid steam is critical to the color-producing reaction.
The Modified Griess Test has been altered to accommodate these materials as follows:
- Confirm nitrite sensitivity of test paper .
Tape a piece of filter paper to the back of a piece of nitrite-sensitive test paper. Using a nitrite test swab saturated with 15 percent acetic acid solution, dab the corners of the test paper. Each corner should turn orange, confirming the sensitivity of the paper to nitrite compounds.
- Mark reference points of evidence on test paper .
Arrange the evidence item face up and the treated paper emulsion side face down on the questioned surface. Using a pencil, index any points of reference.
- Apply acetic acid solution to test paper .
Wipe the emulsion-coated side of the test paper with cheesecloth saturated with the acetic acid solution. Lightly apply the solution to the entire surface. (Heavy application may cause indistinct or hazy results.)
- Create layers of reaction items .
Place the test paper emulsion side down on the evidence item. Apply heat using an iron to the back of the treated paper. (The back of the paper is covered by filter paper to prevent the iron from sticking to the treated emulsion-coated paper.)
- Examine and interpret results .
Separate the treated paper from the evidence item. Any orange indications on the paper are the result of a chromophoric reaction chemically specific for the presence of nitrite residues.
- Label and retain test paper .
Mark the previous pencil marks placed on the test paper in ink when the paper is dry. Label with additional data according to laboratory protocol.
Additional Online Courses
- What Every First Responding Officer Should Know About DNA Evidence
- Collecting DNA Evidence at Property Crime Scenes
- DNA – A Prosecutor’s Practice Notebook
- Crime Scene and DNA Basics
- Laboratory Safety Programs
- DNA Amplification
- Population Genetics and Statistics
- Non-STR DNA Markers: SNPs, Y-STRs, LCN and mtDNA
- Firearms Examiner Training
- Forensic DNA Education for Law Enforcement Decisionmakers
- What Every Investigator and Evidence Technician Should Know About DNA Evidence
- Principles of Forensic DNA for Officers of the Court
- Law 101: Legal Guide for the Forensic Expert
- Laboratory Orientation and Testing of Body Fluids and Tissues
- DNA Extraction and Quantitation
- STR Data Analysis and Interpretation
- Communication Skills, Report Writing, and Courtroom Testimony
- Español for Law Enforcement
- Amplified DNA Product Separation for Forensic Analysts