Potassium ferrocyanide (KFC) is an anti-caking compound added to table salt. Although KFC is a common non-toxic food additive, its spectroscopic response is representative of analogous cyanide compounds. Trace detection of other cyanides in food products is essential to the safety of consumers, as they can be toxic at oral consumption levels as low as 20 μg/g. This application demonstrates rapid trace analysis of potassium ferrocyanide in table salt with Misa (Metrohm Instant SERS Analyzer), in a simple assay format with minimal use of laboratory reagents.

Federal guidelines state that KFC levels may not exceed 13 μg/g as an additive to table salt. This application note describes detection of trace amounts of potassium ferrocyanate in table salt with Misa and Metrohm Raman’s silver nanoparticle (Ag NP) solution.

A standard SERS reference spectrum (Figure 1) for KFC (100 μg/g, Ag NPs) is dominated by a prominent peak at 2133 cm-1 corresponding to the C≡N stretching mode. The C≡N cyanide stretch is distinctive, as it is one of very few Raman peaks which appear above 1800 cm-1. In a y-corrected spectrum, this strong, distinct cyanide peak appears for any cyanide moiety, whether benign KFC or toxic hydrogen cyanide gas, making SERS an excellent technology for cyanide detection.

Figure 1. Representative SERS spectrum of cyanide.

To prepare spiked samples, regular table salt was doped with potassium hexacyanoferrate(II) trihydrate dissolved in water to generate a concentration range of test samples: 10,000, 1,000, 100, 10, 1 μg/g, and 100 ng/g. Following the addition of analyte, samples were air-dried.

For SERS analysis, 100 mg of each sample was dissolved in 0.5 mL water. A 100 μL aliquot of the sample was mixed thoroughly with 800 μL Ag NPs and 50 μL 50 mmol/L NaCl in a glass vial. The sample vial was inserted into the vial attachment on Misa for testing.

Table 1. Experimental Parameters
Instrument Acquisition
Firmware 0.9.33 Laser Power 5
Software Misa Cal V1.0.15 Int. Time 1 s
Misa Vial Attachment 6.07505.040 Averages 10
ID Kit - Ag NP 6.07506.450 Raster ON

Figure 2 displays overlaid Ag NP SERS spectra acquired for different concentrations of KFC, demonstrating detection down to 1 μg/g.

Figure 2. Overlaid, baselined, background-subtracted SERS spectra of KFC in table salt with Misa and Ag NPs.

Detection of potassium ferrocyanide in the field

Using the large end of the scoop, add 3–4 scoops of sample to a 2 mL vial. Add water to the vial until ~1/4 full. Cap and shake the vial gently to dissolve solids, then let the sample rest for 2 minutes. Fill a clean vial halfway full with Ag NPs. Using pipettes, add 2 drops of sample solution and 1 drop of NaCl solution to Ag NPs, then cap and shake the vial gently to mix. Insert into vial attachment on Misa for measurement.

Table 2. Requirements for Field Test Protocol
ID Kit - Ag NP 6.07506.450
includes: Silver nanoparticles (Ag NP)
Disposable pipettes
2 mL glass vials
NaCl solution 3 g NaCl in 100 mL water
Test settings Use ID Kit OP on Misa

Misa’s rapid, facile trace detection of potassium ferrocyanide in table salt down to 1 μg/g is an order of magnitude below permissible levels. The simple procedure described herein requires minimal sample preparation and is ideally suited for on-site testing.

Given the similarity of SERS cyanide spectra, the results of this study suggest that Misa may be used as a robust cyanide sensor.


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