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. Author manuscript; available in PMC: 2019 Jan 1.
Published in final edited form as: J Forensic Identif. 2018 Jan-Feb;68(1):11–27.

Silver Nitrate Grade and Its Effect on Physical Developer Performance–A Validation Study

Allison Coppes 1, Robert S Ramotowski 2, Brian A Jones 3, Michael E Manna 4, Esther V Chervinsky 4, Kim D Smith 4
PMCID: PMC5873944  NIHMSID: NIHMS945877  PMID: 29651198

Abstract

Silver nitrate is a critical component of the physical developer (PD) reagent. Significant increases in the cost of silver nitrate in recent years have caused the United States Secret Service’s Forensic Laboratory to look for a more economical way to produce physical developer. One possible solution to this dilemma is to use a lower grade of silver nitrate, which typically has a lower cost. This study compared the quality of fingerprints produced on various paper substrates by three physical developer working solutions, each prepared using either the American Chemical Society (ACS), U.S. Pharmacopeia (USP), or technical grade of silver nitrate. It was determined that the less pure grades of silver nitrate produced approximately the same quality of fingerprints as the ACS grade, which is currently used in the authors’ laboratory for making PD working solutions. In the experiments comparing PD prepared using the ACS and USP grades of silver nitrate, the ACS grade was superior only 5% of the time, whereas the USP grade was chosen 27% of the time. In the experiments comparing PD prepared using the ACS and technical grades of silver nitrate, the ACS grade was superior only 10% of the time, whereas the technical grade was selected 17% of the time. For the majority of the sample comparisons, no difference in fingerprint quality was observed. The overall conclusion was that either USP or technical grade silver nitrate can be used in place of the current, and more expensive, ACS grade of silver nitrate.

Introduction

The first stabilized physical developer (PD) solutions were prepared by Jonker and coworkers at Philips Research Laboratories (Eindhoven, the Netherlands) in the late 1960s [1–3]. The addition of specialized surfactants significantly extended the useful lifetimes of these developers. The first application of a stabilized physical developer for latent print development was by Morris and Goode in the early 1970s [4, 5]. The new reagent was formulated to develop the water-insoluble portion of latent fingerprints on porous surfaces that had been exposed to water. Prior to the introduction of this reagent, there was no reliable method for developing latent prints on papers that had been wetted. A formal user’s guide was introduced by the United Kingdom Home Office Police Scientific Development Branch (PSDB) in 1981 [6]. It was once thought that PD was the last process that could be used sequentially on porous substrates. However, Cole et al. noted that the use of a diluted bleach solution could improve the contrast of the PD-developed prints by bleaching stains from the background and by darkening the PD-developed prints [7]. This latter phenomenon was later attributed to the formation of silver oxide [8]. Additional toning treatments for post-PD use have been recommended, including the use of potassium iodide [9].

The reaction mechanism of the PD is quite complex, but it has been comprehensively studied and described by Cantu [10]. In summary, the PD reaction begins when silver ions (from silver nitrate) are reduced by ferrous ions (from ferrous ammonium sulfate). In the next step, the metallic silver particles that are formed are surrounded by negatively charged citrate ions (from citric acid monohydrate). This negatively charged colloid particle is then surrounded by two surfactants, n-dodecylamine acetate (positively charged) and Tween 20 (neutral charge). At this point, the silver-surfactant colloid complex is ready to interact with latent print residue. One possible explanation for this behavior is that the acidic environment of the PD working solution can induce a partial-positive charge on some of the constituents of the water-insoluble portion of the latent print (e.g., proteins) [11]. Thus, the first interaction would be an electrostatic one–the negatively charged silver colloid interacts with the positively charged protein. The subsequent reaction would then be autocatalytic. The initial silver particle would then act as a nucleation site for subsequent silver particles to aggregate. Colloidal gold or silver staining of proteins by a similar mechanism has been reported in the literature [11]. However, because PD appears to react with a variety of different pure chemicals, other mechanisms may also be possible.

Given the complexity of the physical developer reaction, it would follow that any attempts to alter the chemicals used in its preparation and usage could lead to changes that make the working solution less effective. However, several successful modifications have been reported. Several efforts have focused on the use of alternate acids for neutralizing basic papers in the prewash step [12, 13]. Several acids (e.g., malic, nitric, acetic) have been successfully used in place of maleic acid; however, there have been some concerns about higher background development caused by some of these alternatives [14, 15]. Houlgrave et al. reported on the improvement in the lifetime of the PD working solution after Tween 20 was successfully substituted for Synperonic N [16]. This improvement occurred without any significant loss of latent print development quality. These results were confirmed in a study reported by Sauzier et al. [14] They also observed that the order in which the chemicals in the redox solution are mixed makes no difference in the efficacy of the final working solution. They also noted that changing the concentrations of chemicals or removing them entirely could have negative consequences on print quality and background interference. Changes in water quality can impact the quantities of surfactants needed in the PD working solution. In the late 1990s, the PSDB adjusted downward the quantities of n-dodecylamine acetate and Synperonic N recommended for the detergent stock solution because of a switch from distilled to reverse osmosis-deionized water [17].

Silver nitrate is a critical ingredient in the PD working solution–without it, no development of latent print residue would occur. Schnetz and Margot explored the use of different silver salts (e.g., silver acetate, silver bromide, silver lactate) as the source of silver ions for their modified multimetal deposition reagent (MMD II) [18]. Although silver nitrate was successfully replaced by silver acetate in the nonstabilized (i.e., no surfactants added) PD working solution used in MMD II, no such modification has been attempted or proposed for the standard PD working solution. It is likely that silver nitrate was chosen for the PD reagent because of its high solubility in water (at 25 °C, the Ksp for silver nitrate in water is 51.6 [19]) and the fact that it was used in the first stabilized developer reported by Jonker et al. [2]

Chemicals come in a variety of grades, which are typically indicative of their purity. There is often a considerable difference in the cost for different grades of chemicals. Chemicals that meet the standards of the American Chemical Society are labeled as ACS grade. This is the current grade of silver nitrate that is used in the physical developer reagent in the authors’ laboratory. A chemical that meets the requirements of the United States Pharmacopeia and is acceptable for drug, medicinal, food, and laboratory use is labeled as USP grade. This grade is less pure than the ACS grade [20]. Chemicals labeled as technical grade are typically less pure than the other two grades. Technical grade chemicals are acceptable for industrial or commercial use, but they are not pure enough for drug, medicinal, or food use [20]. The price of silver nitrate in recent years has increased significantly, making the cost of the overall PD reagent more expensive. The use of a lower grade of silver nitrate could be more cost-effective; however, the potentially adverse effects of using the lower grade chemical and its potential impurities are not known. A survey of chemical supply websites in October of 2017 noted that the price of silver nitrate could vary by grade, from $2195.60 per 500 g for technical grade to $2750.50 per 500 g for USP grade silver nitrate (Spectrum Chemical, New Brunswick, NJ) [21]. On another manufacturer’s website, the price of ACS grade silver nitrate was listed as high as $5933.81 per 500 g (Fisher Scientific) [22]. It is important to note that chemical prices routinely fluctuate and discounts are offered for various groups (e.g., government, educational institutions), so the final price of these different grades of silver nitrate could be quite different than the prices quoted here.

The international standard ISO/IEC 17025:2005(E), section 5.4.5.2, specifies, “The laboratory shall validate non-standard methods, laboratory-designed/developed methods, standardized methods used outside their intended range and amplifications of standardized methods to confirm that the methods are fit for the intended use.” When substituting for a chemical in any reagent, including changes in reagent grade or purity or manufacturer, a validation study must be conducted to ensure no loss of process efficiency will occur. The performance of a new chemical must be compared directly against the effectiveness of the current, validated method (e.g., the use of split, depleted latent prints) [23]. Because it is such a critical component of the PD reagent, a limited-scope validation study was conducted to evaluate the efficacy of PD working solutions prepared using the aforementioned three different grades of silver nitrate.

Materials and Methods

Chemicals

The following chemicals were used in this study: ferric nitrate nonahydrate (Fisher Scientific, ACS grade), ferrous ammonium sulfate hexahydrate (Spectrum Chemical, ACS grade), citric acid monohydrate (EMD Chemicals, ACS grade), n-dodecylamine acetate (Pfaltz & Bauer, Inc.), Tween 20 (Sigma Aldrich), silver nitrate (Spectrum Chemical, ACS grade), silver nitrate (Fisher Scientific, USP grade), silver nitrate (Spectrum Chemical, technical grade), malic acid (Spectrum Chemical), and reverse osmosis-deionized (RO-DI) water (Elga Purelab Option-S 7/15). All chemicals were used without any further modifications.

Physical Developer Preparation

The PD working solution consists of a mixture of three stock solutions. The redox solution was prepared by mixing these compounds in the following order: 30 g of ferric nitrate nonahydrate, 80 g of ferrous ammonium sulfate hexahydrate, and 20 g of citric acid monohydrate in 900 mL of RO-DI water. The detergent solution was prepared by mixing 3 g of n-dodecyl-amine acetate and 3 mL of Tween 20 in 1 L of RO-DI water. Three versions of silver nitrate stock solutions were prepared to create the three different versions of physical developer working solutions. One was made with the ACS grade silver nitrate (99.0–99.97% purity), one with the USP grade of silver nitrate (99.8% purity), and one with the technical grade of silver nitrate (purity not specified). Each silver nitrate solution was prepared by mixing 20 g of the appropriate grade of silver nitrate in 100 mL of RO-DI water. Physical developer working solutions were prepared by mixing 900 mL of the redox solution, 40 mL of the detergent solution, and 50 mL of the appropriate silver nitrate solution. Once mixed, the PD working solutions were allowed to rest for at least 72 hours before the samples were processed [16]. The malic acid solution was prepared by mixing 25 g malic acid into 1 L of RO-DI water.

Sample Preparation

Seven substrates were chosen for the comparison: substrate A was white, premium business photocopy paper (Hammermill, 28#, 100 brightness); substrate B was white photocopy paper (Xerox, 20#, 92 brightness); substrate C was steno notebook white lined paper (Quill Brand, 6″ × 9″, Gregg ruled); substrate D was newsprint (Washington Post Express); substrate E was newsprint (Washington Post); substrate F was manila envelope paper (Quill, 28#); and substrate G was brown Kraft paper (ULINE). Split-depletion series latent prints were used for this study in accordance with guidelines published by the International Fingerprint Research Group (IFRG) [23]. One hundred and one depletion series strips were used in each experiment and each strip contained six sebaceous-rich latent prints. For experiment 1, four females and five males were selected. For experiment 2, four females and six males were selected. For substrates A, B, C, and E, 14 depletion series strips were used in each experiment. For substrates D, F, and G, 15 depletion series strips were used in each experiment. The age of deposited latent prints varied between as little as two months and up to one year prior to processing.

After the depletion series strips were prepared, the strips were distributed to the donors with instructions to deposit six sebaceous-rich latent prints on each strip, without recharging, starting from the top and moving towards the bottom of the strip. Donors were instructed to deposit sebaceous prints by very lightly wiping their forehead or nose and then by rubbing their fingers together vigorously to evenly distribute the residue. Samples were stored in manila envelopes under ambient laboratory conditions until they were processed between 2 to 12 months later.

Evaluation of Comparisons

The grading system that was used for this study was based on the scale first reported by McLaren et al. [24] The system used a scale of +2 to −2 to comparatively grade processed prints. Overall, if the left side had better fingerprint ridge detail, a positive score was given (a score of +2 was given for significant changes and +1 for minor ones). If the right side appeared to be better, then a negative score was given (similarly, a score of −2 was given for significant changes and −1 for minor ones). It should be noted that left and right depletion samples were processed randomly with PD reagents containing either the ACS grade or USP or technical grade silver nitrate. This procedure is done to minimize pressure bias during the deposition process. Samples that showed no differences in quality between the left or right sides received a score of zero (0). Although each sample consisted of six depletions, evaluators graded the overall or average quality of the ridge detail of each of these prints and provided a single grade (the range of values spanned from +2 to −2 rather than +12 to −12 if all six prints were evaluated). The scoring system is summarized in Table 1. The experience levels of the evaluators varied from nonexpert, a second-year trainee, and three IAI-certified latent print examiners with 13, 23, and 30 years of experience, respectively. Processed samples were imaged using the Foster and Freeman DCS 3 system (Evesham, Worcestershire, U.K.). Images were taken with a Fujifilm Finepix S2 Pro with a Nikon MicroNikkor lens using white light. For the final evaluation, photographic prints of these images were made using a Noritsu Model QSS-3302SM Pro Digital Printer.

Table 1.

A summary of the grading system used in this study.

Grade Description
+2 Left side shows significantly better development
+1 Left side shows slightly better development
0 No difference in development
−1 Right side shows slightly better development
−2 Right side shows significantly better development
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Experiment 1: USP versus ACS Grade Silver Nitrate

Two 4 liter bottles of physical developer solution were created at the same time using the same components except for the varying grades of silver nitrate. In this experiment, PD working solutions prepared using the ACS grade silver nitrate and the USP grade silver nitrate were compared. The depletion series fingerprint strips were cut in half prior to processing. Samples were first soaked and agitated in the malic acid prewash solution for 20 minutes. After draining the malic acid from the processing tray, one half of the strips were processed using the ACS grade PD and the other half using the USP grade PD. The samples were agitated during the 20-minute development process using an orbital shaker (model SHKA2000, Barnstead International). After draining the PD solutions, the samples were rinsed a minimum of three times with tap water and then dried using a photographic drum style dryer (model ST-22, Regal Arkay). All chemical reagents and rinses were disposed of according to federal and local environmental regulations. The total number of fingerprints that were processed and evaluated in this experiment was 606 (101 depletion series strips, 6 depletions per strip).

Experiment 2: Technical versus ACS Grade Silver Nitrate

The same procedure (as described in Experiment 1) was followed in this experiment, except that PD working reagents prepared using the ACS and technical grades of silver nitrate were compared. The total number of fingerprints that were processed and evaluated in this experiment was also 606 (101 depletion series strips, 6 depletions per strip).

Results

Experiment 1: USP versus ACS Grade Silver Nitrate

Overall, the five evaluators chose the current silver nitrate grade PD working solution containing the ACS grade as superior only 5% of the time. Prints developed using the PD working solution containing the USP grade of silver nitrate were chosen as superior 27% of the time, and no difference was selected 68% of the time. These results are broken down by individual evaluator in Chart 1.

Chart 1.

Chart 1

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A bar chart displaying the responses from the five evaluators for the ACS versus USP grade silver nitrate comparison. The consensus value is not a direct average; rather, it represents the majority opinion. The consensus total is only 100 because there could be no agreement with one of the samples. Evaluator data is listed in order (l to r) of years of experience, with evaluator 1 having the least (none) and evaluator 5 having the most (30 years).

Figure 1 shows two sets of depletion prints that were evaluated in this study. Figure 1a is an image of a set of depletion prints in which the majority of evaluators voted for the right-hand side (these prints were developed by a PD working solution containing the current ACS grade of silver nitrate). Figure 1b is an image of a set of depletion prints in which the majority of evaluators again voted for the right-hand side (these prints were developed by a PD working solution containing the USP grade of silver nitrate). Figure 2 is an example of an image of depletion prints in which no consensus could be reached by the evaluators. Two evaluators chose the right-hand side and two chose the left-hand side as being superior. The remaining evaluator decided that there was no difference between the two sides.

Figure 1.

Figure 1

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(a) An image of the first three depletions in the series of a sample from Experiment 1 in which the majority of evaluators selected the right side as being superior (the PD containing the ACS grade silver nitrate); (b) An image of the first three depletions in the series of a sample in which the majority of evaluators selected the right side as being superior (the PD containing the USP grade silver nitrate).

Figure 2.

Figure 2

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An image of a sample from Experiment 1 that resulted in mixed voting. In this sample, the PD containing the ACS grade silver nitrate processed sample is on the left and the PD containing the USP grade silver nitrate processed sample is on the right. There was one vote for 0 and two votes each for +1 and −1.

Experiment 2: Technical versus ACS Grade Silver Nitrate

Overall, the five evaluators chose the current silver nitrate grade PD working solution containing the ACS grade as superior only 10% of the time. Prints developed using the PD working solution containing the technical grade of silver nitrate were chosen as superior 17% of the time, and no difference was selected 71% of the time. These results are broken down by individual evaluator in Chart 2.

Chart 2.

Chart 2

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A bar chart displaying the responses from the five evaluators for the ACS versus technical grade silver nitrate comparison. The consensus value is not a direct average; rather, it represents the majority opinion. The consensus total is only 98 because there could be no agreement with three of the samples. Evaluator data is listed in order (l to r) of years of experience, with evaluator 1 having the least (none) and evaluator 5 having the most (30 years).

Figure 3 shows two sets of depletion prints that were evaluated in this study. Figure 3a is an image of a set of depletion prints in which the majority of evaluators voted for the right-hand side (these prints were developed by a PD working solution containing the current ACS grade of silver nitrate). Figure 3b is an image of a set of depletion prints in which the majority of evaluators again voted for the right-hand side (these prints were developed by a PD working solution containing the technical grade of silver nitrate). Figure 4 is an example of an image of depletion prints in which no consensus could be reached by the evaluators. Once again, two evaluators chose the right-hand side and two chose the left-hand side as being superior. The other evaluator decided that there was no difference between the two sides.

Figure 3.

Figure 3

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(a) An image of the first three depletions in the series of a sample from Experiment 2 in which the majority selected the right side as being superior (the PD containing the ACS grade silver nitrate); (b) An image of the first three depletions in the series of a sample in which the majority selected the right side as being superior (the PD containing the technical grade silver nitrate). (Depending on the paper used, the color of the print and background may vary slightly.)

Figure 4.

Figure 4

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An image of a sample from Experiment 2 that resulted in mixed voting. In this sample, the PD containing the ACS grade silver nitrate processed sample is on the left and the PD containing the technical grade silver nitrate processed sample is on the right. There was one vote for 0, two for +1, and one each for −1 and −2.

Discussion

Overall, the results of these validation tests were not totally unexpected. Investigations into the robustness of physical developer solutions by Barford et al. indicated that there was a considerable excess amount of silver nitrate in the reagent [25]. The authors noted that “… a PD solution in which the ferrous ion concentration has not fallen below 60% of that in the original fresh solution will develop marks of good quality as long as the silver ion concentration is at least 50% of that in the unused solution”. When reviewing the range of purities of silver nitrate evaluated in this study, none fell below the 95% purity mark. It is also unlikely that the silver nitrate concentration in the physical developer working solution would change significantly over time while in storage. Barford et al. noted that testing of such solutions using a silver ion selective electrode “… showed that the silver ion concentration remained almost constant over a period of 50 weeks”.1

This study found that, overall, the results indicate few or no differences were detected between prints developed with the three different silver nitrate-containing PD working solutions. However, even though the ACS grade was chosen over the USP and technical grades 5% and 10% of the time, respectively, nearly all of the latent prints were deemed of value for identification purposes. Interestingly, a noticeable trend was found in the data when years of experience were taken into account. Those evaluators at the lower end of the experience spectrum were more likely to grade samples as being equal in quality (i.e., a grade of 0). For the less experienced evaluators, this occurred 76% and 86% of the time during the USP and technical grade evaluations, respectively. The three IAI-certified examiners were more likely to make a decision as to whether one half was of superior quality compared to the other. For these experienced evaluators, a choice of “no difference” occurred only 54% and 58% of the time during the USP and technical grade evaluations, respectively. It is possible that in our specific validation study, experienced examiners would make more decisions on difficult latent prints (i.e., ±1 or ±2), whereas those with less experience would tend to declare those prints of equal value or grade “0”. An interesting and similar trend was observed by Langenburg, who noted that highly experienced examiners tended to be more conclusive with ambiguous fingerprint comparisons compared to examiners with significantly less experience, but he attributed these results to different human factors [26].

In some cases, differences in contrast may have played a role in choosing which half of the print was superior in overall ridge detail quality. Some of the technical grade samples exhibited an increase in background development of the substrate. Potential impurities from the lower grade of silver nitrate could have contributed to this phenomenon. The fingerprint ridge detail in several of the images of samples treated with PD containing the technical grade silver nitrate also appeared noticeably darker in appearance. This was more prominent with certain substrates, such as the manila envelope paper. This phenomenon was not seen in the USP samples, which could be a result of the technical grade containing more impurities than the USP grade.

The results from this study indicate that any of the three grades of silver nitrate could be successfully used to prepare the PD working solution. It should be noted that these results apply only to the specific chemicals tested during this validation study. Changing the manufacturer or the grade of silver nitrate beyond these three examples would necessitate a limited-scope validation. In the authors’ laboratory, making relatively minor changes to a well-established latent print development technique would require only a limited-scope validation study. These studies typically involve processing and comparing 50 to 100 samples, although more can be used. Typically, three donors and three evaluators can be used. The results of these limited-scope studies can more than adequately address whether modifications to any existing, well-established techniques or methods are scientifically and operationally justified. It is not realistic for forensic laboratories to perform full-scale validation studies for every modification (major or minor) to a method or instrument. Performing a full validation on a new method or instrument could take more than six months to complete and involve a considerable amount of laboratory resources as well as staff time. Such testing is typically reserved for novel, untested methods or instrumentation. Such novel methods or instruments would not be incorporated into the laboratory’s standard operating procedures until as many of the method’s or instrument’s variables (within practical limits) have been fully explored.

Conclusion

When comparing PD working solutions containing the current standard ACS grade silver nitrate to the lower purity USP and technical grades of silver nitrate, little difference in ridge detail quality was observed. The PD working solution that was prepared using the USP grade was found to produce better results in 27% of the samples compared to 5% of the time for the ACS grade. The PD working solution that was prepared using the technical grade was found to produce better results in only 17% of the samples compared to 10% of the time for the ACS grade. A significant trend was found in the data when years of experience was taken into consideration. However, there was a tendency at the lower end of the experience scale to determine that the two halves of a sample were equal (i.e., a grade of 0). The three IAI-certified examiners were more likely to choose one side as being of better overall quality. Although some issues of higher background development were observed when the technical grade silver nitrate was used, it did not significantly affect the value of those prints for identification. However, these differences in contrast could have played a minor role in influencing each evaluator’s final comparison decision. On the basis of these results, the authors have concluded that a lower grade of silver nitrate (e.g., USP, technical) could be used in the physical developer working solution and that it would be more cost effective to forensic laboratories without sacrificing the overall quality of fingerprint development.

Acknowledgments

The authors would like to acknowledge Forensic Photographer Scot Muntz, Forensic Services Division, United States Secret Service, for his assistance in preparing copies of the photographs used by evaluators for their comparisons.

Footnotes

1

Barford et al. noted that these PD working solutions were prepared using an impure batch of n-dodecylamine acetate that appeared to extend the shelf life of working PD solutions up to six months. The deterioration in performance was attributed to the gradual oxidation of ferrous ions to ferric ions.

Disclaimer: The views expressed in this article are those of the authors and do not necessarily represent the views of the United States Secret Service or the United States government. References to a specific manufacturer or product are for information purposes only and do not imply endorsement by the authors, their employers, or the United States government.

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