Palladium metal is commonly used in the pharmaceutical, energy, electronics, and other industries. It may be toxic, and therefore needs to be removed from pharmaceutical formulations or anything else intended for ingestion.
Additionally, palladium is a rare metal. Recycling it will help sustain industries that depend on palladium.
Current methods to detect palladium, important as first and final steps in checking for the removal of palladium from a sample, are expensive and time consuming. A fast detection protocol would be advantageous.
Kazunori Koide (University of Pittsburgh, Pennsylvania) and coworkers have developed such a palladium protocol. The detection method works for a wide range of samples, and is not hindered by potentially competing molecules in the sample.
The detection method.
The scientists' detection method is based on a detection molecule that is originally nonfluorescent. In the presence of palladium metal and a phosphorous-based reactant molecule, the detection molecule undergoes a chemical reaction (a "deallylation" reaction) that turns on fluorescence.
The scientists' goal was to screen different phosphorous-based reactant molecules, to optimize the detection protocol. They found that tri-2-furylphosphine, in combination with sodium borohydride, accelerated the fluorescent chemical reaction faster than many of the other tested molecules.
Compatibility of the detection method.
In order for the scientists' protocol to be useful on a practical level, it must withstand common experimental conditions. Such compatibility issues include interference from dimethyl sulfoxide and the pH of the sample.
Dimethyl sulfoxide is commonly used in biological applications, such as the polymerase chain reaction (PCR), and to stabilize cells prior to freezing. The scientists' palladium detection method must therefore be insensitive to the presence of dimethyl sulfoxide.
The scientists found that dimethyl sulfoxide does not interfere with the detection protocol, up to a concentration of 10%. This is generally the maximum concentration of dimethyl sulfoxide used in biological applications.
Additionally, different biological applications will require different pH values. The scientists' palladium detection method must therefore be insensitive to acidic or basic pH conditions.
The scientists found that the detection protocol works over the pH 5-10 range, out of a possible 0 (highly acidic) to 14 (highly basic). This is a reasonable, commonly used range of pH conditions.
Palladium detection limits.
Fluorescence-based detection protocols are most useful if they give a linear response. In other words, each additional molecule to be detected gives a fixed increase in fluorescence.
This was true of the scientists' palladium detection method. The response is linear, over the 100 parts per trillion to 5.3 parts per billion palladium concentration range, after 1 to 4 hours at 24°C.
There was only a low amount of background interference. This means that fluorescence intensity (and thus palladium concentration) can be reliably measured.
Additionally, a relatively fast chemical reaction that works for all variants of palladium are essential for an effective detection protocol. Fulfilling the speed requirement, the detection reaction yields 14.1 fluorescent molecules per hour.
Fulfilling the reactivity requirement, the reaction is general for different palladium species (those of different chemical charges, e.g, +1, +2, etc), because all palladium ions chemically react to yield neutral palladium species. Thus, the scientists' palladium detection protocol reliably detects low concentrations of palladium, quickly and effectively.
Application to industry.
The pharmaceutical industry, among others, commonly uses palladium as catalysts in their chemical reactions. In order to ensure compatibility with such applications, the scientists tested their palladium detection method in the presence of ampicillin.
Ampicillin is an antibiotic. It was selected because it contains a wide variety of chemical subunits, posing a challenge to the scientists' detection protocol.
The scientists experimentally determined conditions (acidic pretreatment, followed by reaction at basic pH) under which 76% of the palladium was detected. Other molecules exhibited low interference as well.
Lowering the pH from basic to neutral generally eliminated more of the background interference. Although certain chemical units did interfere in the reaction under these conditions, the strongest interferants, α,β-unsaturated ketones, are uncommon in the pharmaceutical industry.
Molecules that strongly bind to palladium (and commonly used in the pharmaceutical industry), such as those containing sulfur, phosphine, or urea subunits, did not interfere in the detection protocol. This is due to the presence of the sodium borohydride in the protocol, a strong electron-donating molecule.
The scientists further successfully checked the performance of their detection protocol for other applications. These include polymer bead recycling (expensive materials used to remove metal ions from a sample), detecting insoluble "palladium black," and used catalytic converters (devices that reduce emissions from automobile engines).
Comparison to other detection protocols.
The scientists have demonstrated that their detection protocol works, under a range of conditions, for various applications. However, they also needed to compare it to other detection protocols, to prove that it is advantageous over other commonly used methods.
The scientists' palladium detection protocol was compared to the standard inductively coupled plasma mass spectrometry technique. The latter is a common technique to produce, separate, and detect metal ions, thus facilitating their identification.
A palladium sample was prepared according to the scientists' standard method, and then analyzed with both protocols. They yielded similar measured palladium concentrations, which means that both protocols are effective.
Overall evaluation.
These scientists' palladium detection protocol may not entirely supplant other palladium detection protocols, under all conditions. Its main advantage is that the technique is rapid and user-friendly, enabling samples to be prioritized for palladium extraction.
for more information:
Garner, A. L.; Song, F.; Koide, K.
Enhancement of a catalysis-based fluorometric detection method
for palladium through rational fine-tuning of the palladium species.
J. Am. Chem. Soc. 2009, 131, 5163-5171.