Topic: GSR Nitrite Swab control issues

[Travis Melland]

Hello Everyone!


Hoping someone here can help troubleshoot an issue we are having with our nitrite swab controls for GSR distance testing. They will work when we make them fresh, but when we go to use them again (about a month or longer sometimes), they do not react with Modified Griess (on HP photopaper). We have gone through multiple batches of swabs and have came across the same issue each time - they seam to lose their 'potency'/reactiveness after a short period of time. Everything we have read says they should be stable 'indefinitely' if sealed and room temperature, but maybe not? Has anyone else had these issues? Confirmed our reagents are good because they react with known gunshot patterns.


Below is the formula we have for making our swabs - again, fairly standard as far as we can tell from what we can find published. Maybe we should up the concentration?


9 grains of Sodium Nitrite in 100mL of DI water. Soak cotton swabs in the solution (we let it saturate quite well), then allow to air dry. We store in an airtight brown glass bottle.


Any suggestions or troubleshooting ideas are welcome!


Thank you,


Travis Melland
MN BCA

[Doug Lancon]

Travis, it sounds like you're doing everything right.  Have you tried different batches of swabs?  I can't imagine that'd make much difference but might be worth a try. 

[Julie Knapp]

I recommend not making/storing the swabs, but making and storing the solution of Sodium Nitrite.  When you are doing the case, just pour some into a small beaker and dip your clean swabs into the solution, mark your corners, or whatever your procedure is...etc. 

[Travis Melland]

Doug, yeah we have been forced to make a new batch basically every time we need to perform the test. Which we don't get a frequent amount of requests, so they tend to sit. The new batches always work...for a while?


Julie, we may have to look into changing out method/SOPs and keep the solution. Has your lab found that keeping nitrite swabs over time tended to lessen the potency? Or have you always made fresh swabs prior to testing?


Thank you both for a response!

[Greg Laskowski]

I ran into that problem years ago.  My solution was to seal the swabs in Kapak and store them at laboratory temperature.

[Julie Knapp]

I have never really like the swabs because I have found them to be not as reactive over time, but never had a problem with the solution being kept and just using it on clean swabs as I need it.  I have never tested that though - just personal observation. 

[Justine Kreso]

We have not had that issue, but they are stored in a bag, in a box, in a drawer.


We do put a year expiration on them, however so they never sit longer than that.


It could also be your photo paper potentially?  Have you tried a new batch of photo paper?

[Evan Thompson]

Consider Quantofix from CTL scientific.  Certified, stable paper, for the detection of nitrites.


https://www.ctlscientific.com/cgi/search.cgi

[Stacey Phetteplace]

Nice, Evan, but he still needs nitrite test swabs for controls   We recently validated QF and I love it!


Our SOP states to follow the procedure you have for storing dried swabs, but all of us have all switched to making fresh the day of use for that very reason. One of our examiners published our method validation and he describes the limitations of prepared & stored versus prepared fresh.

One of the observations he noted was the dried swabs took longer to produce a positive result (30-45 seconds versus 10 second with a freshly made swab).

[Chris Gunsolley]

We've seen the same thing with regard to nitrite swabs not lasting forever and, since we rarely even perform this testing, prepare them fresh for each exam.  Another thing of note... we had a jar of alpha-napthol that was decades old that, while a preparation of sheets using it passed Q.C. tests (using nitrite swabs), it performed poorly in casework.  That is, its sensitivity was diminished.  Because the nitrite swabs were *loaded* with nitrite, it seemed fine but even processing test fired panels produced poor results.  Because of that experience, I think there's some room for improvement regarding the Q.C. process.  Maybe perform a serial dilution of the nitrite swab solution to see that which most closely mimics that seen in casework? Or just use known/test panels as part of the actual Q.C. process?

[Holli Worden]

Travis, I have also noticed the weakness of the pre-prepared nitrite swabs over time. Ours are stored in a plastic container, such as one for pencils/pens. I think others suggestions make sense as far are just wetting a swab with nitrite solution day of.