Ignitable Liquids Best Practice: Packaging for Analysis

So how would you like me to package this?

On our travels to fire scenes throughout the world this is the one question that we are asked on a fairly regular basis by crime scene investigators (CSI). The response to our answer is usually ‘Oh, we were never told about that!’.

As part of their normal duties CSIs are expected to attend and investigate fire scenes, however due to the infrequency of fires scene investigations, CSI’s may only attend one or two fire scenes a year. All CSIs ‘know’ that ‘arson’ bags are to be used for exhibits that are suspected of holding ignitable liquid residues. I have been involved in fire investigation training for CSI for over a decade and have found that individuals find it fairly difficult to differentiate between certain ignitable liquids from their odours alone and some find it much more difficult than others. For example, a highly evaporated petrol may smell similar to a paraffin or a diesel. So it is unreasonable, in my view, to expect a CSI to accurately determine whether the ignitable liquid residues they have found are from a specialist solvent, petrol, paraffin, diesel or another ignitable liquid. However samples with specialist solvent residues on them and samples with petrol/paraffin/diesel residues on require different packaging to retain the ignitable liquid residues. ‘Arson’ bags are made of a polymer called ‘nylon’ that retains hydrocarbon based residues and if only a nylon bag is used to package a sample then specialist solvent residues, for example, would not be retained.

Ignitable liquids may be detectable on items which have either held, or had such liquids poured or splashed on to them. To a greater or lesser extent all ignitable liquids are volatile and the residues of such liquids can be lost through evaporation. Whether such liquids are subsequently detected during laboratory analysis will depend upon a number of factors including the quantity originally present, the nature of the ignitable liquid, the time delay before packaging and the effectiveness of the recovery method.

In the UK ‘nylon’ bags are normally used to package fire scene debris for ignitable liquid analysis. A correctly sealed nylon bag (a swan neck, nylon to nylon seal secured with cable ties or adhesive tape) will retain hydrocarbon based liquids, namely those obtained from the fractional distillation of crude oil such as lighter fluid, petrol, white spirit, kerosene based liquids (paraffin) and diesel.

However not all ignitable liquids are hydrocarbon based, for example polar group liquids such as alcohols (e.g. methylated spirits) and ketones (e.g. nail varnish remover or MEK, an industrial solvent) are flammable and can be used to assist the development of a fire. These chemicals will not be retained by nylon and should be packaged in polythene bags or a glass jar. Therefore if a CSI only uses a nylon bag to package fire debris for ignitable liquid analysis then potential evidence could be lost.

If a mixture of ignitable liquids is suspected or the nature of the fuel is uncertain, then there are two good methods of packaging which can be used. One is to place the item into a glass jar with a screw cap and seal it tightly. The second is to double wrap the item, first in ‘nylon’ and then in polythene.

Given the environment CSIs work in and the amount of space available to them, it has long been deemed unviable to use glass jars at crime scenes. Therefore the preferred method for packaging fire debris to retain ignitable liquids for analysis should be to double wrap first in nylon and then in polythene making sure that both bags are correctly and individually sealed. Each layer of packaging should be separately sealed by twisting the opening end of the bag and folding it over to create a ‘swan neck’ seal, this should be secured with cable ties or adhesive tape to ensure a good ‘nylon’ to ‘nylon’ and polythene to polythene seal. The self-seal police evidence bags should not be used for the polythene bag as certain solvents can degrade the adhesive thus destroying the seal and potentially resulting in lost evidence.

If a combination of nylon and polythene bags is used for packaging items then any ignitable liquid residues will be retained for analysis as long as the integrity of the packaging is maintained. Therefore it is important if any sharp items are identified in a debris sample that they are packaged to protect the integrity of the bags i.e. a cardboard box or a plastic box or tube.

If a pool of liquid is identified on a surface then there are a number of methods available to the CSI to recover it. If the pool is large enough then pipetting the liquid into a metal tin, and sealing with a metal insert and screw cap may be a viable method. If not then some CSIs may opt to swab the liquid with a biology swab often used for blood, however the surface area of these swabs is relatively small and sometimes the CSI may just place the swab in its sheath and in a self-seal polythene bag rather than the packaging sealed as described above. This would mean that the sample cannot be meaningfully analysed and any potential evidence would be lost. A preferable method would be to use a quantity of paper towel or other large surface area, absorbent material to soak up the liquid and then package in nylon and polythene.

It is possible for nylon and polythene bags to become contaminated with low levels of ignitable liquid residues, primarily during their manufacture but also afterwards during storage and handling. The residues detected from contamination tend not to be the lighter, highly volatile compounds as these are easily and quickly lost through evaporation. To explore the contamination issue control packaging should be prepared in all cases where ignitable liquids are suspected of being used but especially where low levels of ignitable liquid residues are suspected of being present on casework samples. The control bags should come from the same batch of bags used to package the samples and both control and samples are analysed similarly. If any additional packaging (cardboard boxes) or sampling methods (tissue paper) are used then these must also be included in the control.

Throughout my career as a forensic scientist specialising in fire investigation I have been fortunate to work on thousands of laboratory based cases that have involved ignitable liquid analysis. Unfortunately I have seen too many exhibits submitted for analysis that have been rejected due to poor (holes/tears or badly sealed) or incorrect packaging (paper sacks or self-seal evidence bags). If this article can stop even one item being rejected by a forensic provider then it would have been well worth writing it.