Collector Selection:

When selecting a collector, there are intrinsic characteristics of the collector that will determine the success of the collection and analysis. A properly chosen collector will exhibit no color or texture that would be interpreted by the software as a droplet stain, and will bond with the spray reliably so that residue does not inadvertently flake or dust off in handling, confounding the data.


Considerations: All of the following should be evaluated in advance of going to the field.


Surface roughness: The surface should be as smooth as possible. While higher roughness will increase collection efficiency of very fine droplets, it can also create texture in a 4800 DPI scan. Evaluate this by scanning blank cards and observing the scans. A blank collector scan should be thresholded in imageJ and analysed to determine where “zero” is. This procedure is defined here


Absorptiveness: How will the collector interact with the spray? Hydrophobic surfaces, such as the styrene petri dishes are constructed of, polypropylene or other plastic films, and metal plates may cause spray to bead, bounce, or possibly even roll off, confounding results. The dried residue may also not bond to the substrate, making handling and processing difficult.  A good collector should make some degree of bond with the spray so that reasonable handling will not result in deposit loss or transfer. Too much absorptiveness, such as one would find with alpha-cellulose filter papers, is also not good, as the wicking will result in dilution of the stain and irregular spreading, again, confounding analysis.


Color: How pure is the background color of the collector? The more homogeneous, the better. Droplet analysis will involve setting the software to recognize a range of colors that are known to be the spray tracer (detailed more below). If the collector has color impurities, this will confound the data. The blank collectors should exhibit ZERO instances of color that would overlap with the color of the tracer. 


Sampler stability: Is your collector stable in the conditions that will be encountered? This is particularly applicable to water and oil sensing papers, and coated collectors such as magnesium oxide coated slides (Chaskopoulou et al. 2013). Soft substrates, high amounts of dust or non-spray material, wet or high humidity environments, can confound collectors that rely on an impact or reaction with water. Also, the method described here involves contact with the surface of the flatbed scanner, so the collector should be stable when physically contacted.


Sampler thickness:  Thinner is better for this method. Multiple collectors can be placed on a flatbed scanner for simultaneous processing; as thickness increases, shadowing at edges of the collector can affect the result.


1 side or 2?: If the front and backside coverage need to be differentiated, assure that either 1) both sides are in fact the same or 2) you have adequate collector area to fold so that back and front can be collected and analysed uniformly.


            A couple of recommended collectors for 4800 DPI scanning:


Kromekote/ Chromalux: Note: as of late 2012, Mohawk Paper Company ceased production of the venerable Kromekote C1S (1 sided) and C2S (2 sided). While there are still some supplies around on the internet, Chromalux C1S White is Mohawk’s future replacement for Kromekote.  Kromekote and Chromalux are both cast-finish high-gloss printing papers. They have a very smooth, high gloss surface that is designed to bond with applied inks and minimize wicking. This has historically made them the collector of choice when using colored sprays (Matthews et al. 2014). Chromalux is offered as C1S only in quite a few weights, so 2-sided samplers must be made by cutting longer strips and folding the collector. This makes scanning easier and less confusing (see the section on using scanners on the website for more information).


Water and Oil Sensing Paper (WSP): Produced by Syngenta and widely available through agricultural

retailers, this is a common collector. WSP is a dual-layer finish paper, where moisture deposited on the surface dissolves and reacts the two layers to yield a blue stain. While an excellent ad hoc tool for quick and easy results, WSP should be used with caution in studies for two reasons. First, any moisture, including high humidity, will develop the blue tint. On humid days or in deep canopy conditions, this can lead to a blue shift in the background color. Unintended blue can lead to difficulty in thresholding (see below) and a subsequent confounding of data. If using, care should be taken to quickly dry and store collectors in a low humidity environment. Also, due to process control limitations with the film thicknesses, droplets under 20-30 microns may develop unreliably or at varying rates in relation to their true size. (Syngenta, pers. comm.)


Collector processing: Collectors should be bagged individually. The computer will be able to detect the thresholded (chosen) colors at intensities much lower than the human eye can detect. Don’t risk transference between collectors to save a little money on plastic sample bags.