NJDEP-SRP Low Level TO-15 Series: Part 6 – Carryooooooover…

I recently received some feedback on the previous posts in this blog series and was asked the question “what kind of carryover am I seeing?” The truth of the matter is that I struggled to answer this question. I believe this question needs to be qualified a little bit in order for me to answer it. So I am going to break the aforementioned general question into the following two specific questions:

Am I seeing carryover at concentrations typically encountered (i.e., <100 ppbv)?
Am I seeing carryover at concentrations out of this world (i.e., >500 ppbv)?

Why the qualification? Well, the answer is different for both questions. Also, the first scenario (i.e., concentrations <100 ppbv) is going to account 99.9% of ambient air samples a laboratory will encounter. In addition, remember that NJ LL TO-15 states that the calibration range “must be from 0.20 to 40 ppbv.” This means that the >500 ppbv scenario represents a blown-out soil vapor sample, which needs to be diluted and/or injected at a smaller volume so that it falls within the calibration range. In any event, in an attempt to answer these questions I analyzed a series of samples followed by blanks as follows:

Experiment 1: Blank – 200 ppbv – Blank – 200 ppbv – Blank – 200 ppbv – Blank
Experiment 2: Blank – 1,000 ppbv – Blank – 1,000  ppbv – Blank – 1,000  ppbv – Blank

I observed an average carryover of 0.03 and 0.17% for experiments 1 and 2, respectively. Refer to figures 1 and 2 (below) for a visual representation of what experiment 2 looks like.

Figure 1: Black trace = 1 ppmv, blue trace = typical blank, red trace = carryover sample analyzed immediately after 1 ppmv.

Refer to Figure 2 for a zoom.

Figure 2: Zoom of Figure 1. Black trace = 1 ppmv, blue trace = typical blank, red trace = carryover sample analyzed immediately after 1 ppmv.

Is this good? IDK… The interesting thing is that I cannot genuinely tell you what % carryover is good or bad, as I have never seen anything published by any of the manufacturers. So if I answered the question like this, I am not sure where this would take us. Personally, I think these numbers look great! Think about it… less than 1% carryover on an instrument expected to meet method criteria of ±30%. BUT…

We should note that these same carryovers correspond to 0.03 and 1.14 ppbv in the carryover runs for experiments 1 and 2, respectively. Now what does this mean? It means that when loading “hot” soil vapor samples onto your system for analysis, it would be wise to run them at the end of the sequence and with blanks/conditioning runs in between. Certainly do not try to analyze sub slab samples from Long Island and then samples from the High Sierras.

So to answer the original question: I would say nothing to speak of at typical concentrations and about what I would expect for a “hot” sample analyzed via a headspace, purge and trap, air preconcentrator, or any other analytical instrument containing a solid sorbent bed.