Category Archives: Blog

Sample Retention in Nylon Evidence Bags

Nylon bags are frequently used in fire investigations to preserve samples such as fire debris for later analysis. They are considered to have very good resistance to vapour loss [1]. However, this does not mean that they hold onto all vapours forever, and this could make the difference between detecting accelerants or not, or mis-identifying the type of accelerant.

We showed in an earlier blog the type of profile seen for petrol (gasoline) vapours. Petrol and volatile solvents, such as naphthas are the most vulnerable to vapour loss because the molecules are very small and can diffuse through the polymer into the air.

We used bags from 2 different suppliers, adding 1µl of a synthetic mix of hydrocarbons in the gasoline boiling range to dry cellulose fabric, sealing the bag with a ‘swan-neck’ cable tie closure and encasing the bag with a second bag, also sealed with a swan-neck closure. The bags were then stored at ambient temperatures and the atmosphere inside each inner and outer bag tested after 20 days.

We used a synthetic mix in order to see if particular types of hydrocarbons migrated through preferentially.
This is the headspace GC-MS chromatogram of the synthetic mix (red chromatogram), showing C0-C3 aromatics (benzene through to trimethylbenzene) in the green chromatogram, alkanes from C5 pentane to C12 dodecane (purple chromatogram) and cycloalkanes and alkenes (C5 to C8) in the black chromatogram. The total aromatics content of the mix was 34%, similar to petrol.


Hydrocarbon synthetic mix


Hydrocarbon GCMS

Here are the chromatograms of the vapour from the inner and outer bags after 20 days ambient storage.

Hydrocarbon sampled from bag1 after 20 days

Nylon Bag 1

Hydrocarbon sampled from bag2 after 20 days

Nylon Bag 2

The red chromatogram is vapour from the Inner bag and the green chromatogram is for vapour sampled from the Outer bag.

After 20 days the levels of vapour in the inner bags were significantly lower than the vapour concentrations immediately after sealing the bags. The concentration of vapour in the outer bags was lower than for the inner bags and there appears to be a preferential loss of aromatic hydrocarbons. This could affect the interpretation of the analysis as the presence of the C1 and C2-benzenes in particular helps to define whether petrol (gasoline) is present or not.

In both the types of nylon bags used, volatile hydrocarbons permeated or diffused  through to the outer bag, and onwards to the atmosphere from the outer bag. But is this because of the inefficiency of the swan-neck closure, allowing volatiles to escape from the bag, or are the hydrocarbons permeating through the polymer bag itself ? [2]. We will be examining this in our next blog on this subject.

To summarise, it is best if analysis of fire debris for accelerants is carried out as soon after sample collection as possible. Contact SMS Analytical to discuss our rapid accelerant testing service.


[1] E. Stauffer, JA Dolan, R Newman: Fire Debris Analysis, Academic Press, Elsevier, 2008

[2] Strijnik and Hong-You: Evaluation of the Effectiveness of Nylon Bags as Packaging for Fire Debris, 2004, Proceedings of 56th Annual Meeting of the American Academy of Forensic Sciences.


Kerosene and Diesel Fuel Analysis By GCMS

In this article we show how we examine Kerosene  and Diesel fuel using GCMS.


Here is a typical chromatogram of a kerosene using a Mass Spectrometer as the detector. The number and distribution of the peaks forms a fingerprint of the fuel.

The chromatogram shows all the major peaks showing that this kerosene has an alkane range from heptane (C7) to hexadecane (C16). Kerosene contains different types of hydrocarbons: alkanes (or paraffins), cycloalkanes (also called naphthenes) and aromatics.

We can use selective ion monitoring to separate out these different classes of compounds.

The red chromatogram shows the whole range of compounds in the kerosene. By using selected ions we can see the straight chain and branched chain alkanes (green chromatogram) and the profile of the cycloalkanes (purple chromatogram).

The green chromatogram shows the alkylbenzenes present in the kerosene, ranging from toluene to C6 alkyl benzenes.

Diesel Fuel

Diesel fuel shows a mixture of hydrocarbons with a wider boiling range.

The alkanes in this diesel fuel range from nonane (C9) to dotriacontane (C32). The chromatogram shows an unexpected peak (labelled A). The mass spectrum from this peak identified it as di-octyl phthalate, a common plasticiser often found in plastic containers.

The green chromatogram shows the alkane distribution in the diesel fuel.

Sometimes contaminants are not seen in the main chromatogram because the hydrocarbons overlap the contaminant peak.

Selective ion monitoring can reveal contaminants.


The green chromatogram shows the selected ion chromatogram for alkyl benzenes. The peak at 29.24 min is unusual and indicates a possible contaminant. By checking the mass spectrum of this peak against the NIST mass spectral library, we found a match with triphenylphosphine oxide, which is not a normal component of diesel fuel.

The purple chromatogram shows the same peak is present in the chromatogram produced for the two main fragment ions for triphenylphosphine oxide.


Detection Of Fire Accelerants using GCMS

Accelerants may be used to start fires and SMS Analytical get a lot cases from insurance investigators relating to suspected arson using accelerants such as petrol.

Debris from the scene is placed in nylon bags and tied using the recommended swan neck technique [1] before placing in an additional outer nylon bag.

SMS Analytical receive this sealed package and can determine if traces of accelerants are present. To maximise the detection of just volatile hydrocarbons we take a sample of the vapour from the inner bag using a special head-space syringe inserted through the bag wall and then immediately seal-up the puncture :

headspace sampling for the fire analysis of accelerants

The contents of the syringe are injected directly into a GCMS for detection of relevant molecular ions.

The figure below shows a vapour sample from fire debris (bottom) compared with a sample of vapour from an aged petrol (top). The identity of the compounds was confirmed using both mass spectral library [2]  matches and reference compounds injected separately. Toluene, xylenes (C2-benzenes) and C3 and C4-benzenes are prominent in the fire debris vapour. The alkanes are, as expected, mainly branched chain or cyclic. The presence of petrol, a known accelerant, at the site of the fire was established.

example of GCMS mass spectral chromatogram of an accelerant


Find out more about our consultancy and laboratory services and an obligation free quote.

Article References:

(1) Fire Debris Analysis By Eric Stauffer, Julia A. Dolan, Reta Newman

(2) NIST Mass Spectral Library from Perkin Elmer



Why use SMS Analytical ?

Here at SMS Analytical, we’ve been thinking about what makes our business attractive to our customers so we decided to present a list of reasons they return to us with more work:

Understanding The Problem.

Quick to grasp the details of problem that is being solved. With experience in a wide range of chemical ‘areas’ we are able to suggest analyses outside of the typical international and national standards or aternative techniques to overcome matrix interferences.


Attention to detail and methodology is key to being an analytical scientist. In this respect we have been refining our operations and have brought in changes as the result of obtaining our ISO9001 certification in 2016. All quotations and reporting is peer reviewed before sending to our customers and we seek to obtain feedback once the report has been read.


Proportionate use of analysis techniques which may be changed as the results from previous techniques are made. This means we would normally phone and email our client to discuss the preliminary results from the first analysis especially as it could be that we would advise not wasting time and money by analysing further using the techniques previously agreed in the quote. Instead, we may suggest alternative and more appropriate techniques, based on what we’ve discovered so far, or even recommend a stop at that point.

Turnaround Times

As a guideline, we normally would expect to deliver the customer’s report in about 10 working days, depending on the nature of the work and the techniques involved. For non routine analysis, this is a very competitive situation compared to many of the big laboratories out there.


Being a small company we are unhindered by layers of administration or red-tape and so can respond in a timely manner to customer queries such as pre quotation questions or post reporting questions.

Any additional thoughts about these would be welcome…


FTIR Acquisition

cary 630 FTIR

Our latest analytical acquisition, an Agilent Cary 630 FTIR with diamond ATR is proving remarkably sensitive – a real advantage when analysing small quantities of sample. The whole instrument only has the bench foot print of about 2 CDs and is light enough to be used in the field as well:

cary 630 FTIR

Cary 630 FTIR


We purchased the instrument with a KBr optics option rather than ZnSe as we wanted to scan over a larger range and don’t intend taking the instrument into high humidity environments. Although this purchase was for the ATR module, there are a variety of other modules for liquid samples and quantitative analysis which are easily interchanged.

Polystyrene Spectrum Picture

Polystyrene Calibration Spectrum


The equipment has already proved its worth in analysing thin deposits on paint surfaces and in analysing minute powder deposits on electrical cabling :

White Deposit on Electrical Cable Picture

White Deposit on Electrical Cable


MicroLab Picture


We have found that the quality of the spectra from particulates of about 500microns or even less has been excellent. The sample spectrum (in red, above) was obtained from just two barely visible particles each of about 200 microns diameter, taken from the electrical cabling shown above. From elemental analysis using SEM-EDX, the material was known to be a mixture and, unsurprisingly, the library match wasn’t particularly good. However the FTIR data, together with the elemental analysis was sufficient to conclude that the source of the white powder was from a powder fire extinguisher.

Our Conclusion

The short path length within this instrument gives quick and impressive results using only a small amount of bench space. However, having already used Agilent FTIRs with Resolutions Pro software in the past our biggest wish with this instrument is that we could directly control the data collection from Resolutions Pro software rather than the ‘simple’ MicroLab software which we don’t like – it is more limited in functionality and easy to get lost within the series of screens. Instead we have to pass the data via the Resolutions Pro launcher button every time we wish to anything useful with a spectrum and the two pieces of software store data in different Windows file paths as well as being very different in their design’s look and feel.

Overall this instrument does a very good job for a sensible price and is only let down, in our view, by the MicroLab software. For those on a small budget one can start with a single attachment and buy others later.

Contact SMS For Analysis Quote