What is the mineral and chemical composition of your soil samples? What is the purpose of the research? Why do you use HNO3 2M and how? What were the test portion weights of the soil samples?

What AAS technique do you use, because there are at least two main ones - FAAS and GFAAS, in different technical variants, and whether the absorption background corrector in your spectrometer is a deuterium corrector or some other. What is the spectrometer (manufacturer, model)? What are the supposed concentrations of these elements and associated elements in your sample solutions? What are the reagent blank concentration values? Do you use any soil reference materials and do they have certified values for the content of these elements after digestion with HNO3 2M?

ZJ

You should make a difference between the blank and the standard solutions, the blank should not contain any mineral and the standards should contain the mineral but with different concentrations.

If you are talking about standard solutions it is normal that some have a higher concentration than your sample. It is also possible that your blank has been contaminated during handling.

G'day Naziru,

It is possible that a blank can be higher than the sample, even with no contamination etc. If the difference is low, within say 3 sd of the blank. This is because every measurement has an associated method uncertainty value.

Cheers,

Adam

Sure. The blank soil sample can have a spectral interference that occurs at the same wavelength as your elements of interest. This spectral interference is not found in blanks prepared from the reagents alone. You can use 'standard additions', a 'spike' sample in every run, a NIST traceable standard with a known concentration, Zeeman effect AAS maybe.

How, exactly, did you get a blank soil? It's hard to find soil without any lead, nickel or cadmium. Since you ended up in a liquid matrix you really should use the digestion blank (DI water, acid) as your instrumental zero.

Now, if you're running water samples, especially DI water samples, you may find that the sample is cleaner than your DI water. That is pretty common.

However, it sounds like in your case the "blank" is a soil and it's really hard to get a soil without at least some concentration of those analytes. Perhaps you mean background soil rather than blank soil?

How did you prepare your blank sample? For AAS analysis matrix matching analysis is very important, and base on my past experiences of environmental chemistry testing you did not properly digest your sample. Did you dry your soil samples at 60C in an oven, until you achieved a constant weight? Did you use a Phillips beaker or microwave digestion vessel? Typical soil digestion testing method require using concentrated Nitric, hydrogen peroxide, and Hydrochloric acid not a 2M Nitric acid solution. This is an important step to oxidize your metal analytes to the correct oxidation state and have them captured in the sample matrix for reproducible AAS analysis.

Are you using analytical grade II lab water? You could be experiencing interference from your poor digestion method (need your analytes unbounded and readily soluble in the sample matrix), contaminated glassware, or spectral interference. For example the AAS line for Cd is 228.8nm, and As also absorbs at 228.812. Have you checked your instrument software for interfering elemental correction (IEC)? Typically this would cause higher Cd concentration determination if your sample contained high concentration of As, and the only way to correct this would be matrix matching (matching sample As level to your calibration Std solutions, determining As levels in your sample and subtracting out As contributions based on concentration, and/or improper IEC settings in the software.

When preparing your blank sample it must undergo the same digestion procedure as your samples, and your glassware should be acid and base washed, properly rinsed, and stored opening faced down. Since you did not provide levels of positive concentrations in your blank, we can only propose theoretical sources for false positives. With out knowing your software and analytical lines for data collection, you need to investigate your sample preparation and optimize your sample prep techniques.

Zbigniew Jońca Thank you for your valuable questions. I used FAAS and the weight portion of my soil sample is 5g. I used 2M HNO3 according to the methodology agreed upon by our supervisor, possibly to have our analytes in the same form (for both vegetation and the soil samples), because aqua-regia or stronger digestion acids may be too strong for the vegetation. Other questions you raised are very helpful.

Nice Kindombe sure, our blank doesn't contain any trace of the soil sample. It is only the reagent (s) used without the sample. It's possible that contamination may come into play. Thank you for suggesting that.

Adam Crawford thank you for at least informing me that it's possible. But does sd here means Standard Deviation?

Mark Krause thank you, but you got me wrong. My blank is not a soil sample. It is just the reagent (s) and DI water without the sample.

Michael Sanford I found your answer very detailed and thanks for the explanation. All the questions raised are very helpful.

Thank you all for finding time to respond to my questions. Although it is very unusual to have a blank higher than the sample result, your explanations provided possible justifications for it.

This is due to the matrix error indeed.

You should carefully handle during the matrix preparation or if you could purchase it then you must be purified it before use.

Hope this will helpful to you.

Yes, it is possible. There could be one or two reasons.

1. Blank has contamination. Matrix effect creates a lower absorbance in the sample in comparison with the blank.

2. There are huge amounts of Ca, Mg, Na, K in the soil solutions and their emission increases the total light that falls on PMT or solid-state detector and creates the negative absorbance.

No, it is not possible for blank results to be higher than sample results in AAS (Atomic Absorption Spectroscopy) analysis.

In AAS analysis, the blank sample is used to measure the background level of absorbance or interference from the solvent and other materials used in the analysis. The blank value is subtracted from the sample value to obtain the net absorbance, which is used to calculate the concentration of the analyte in the sample.

If the blank value is higher than the sample value, it would indicate that there is a significant interference or contamination present in the blank sample. In this case, the analysis would need to be repeated, and steps would be taken to identify and remove the source of interference or contamination.