Blank Subtraction

The solvents and measurement system may give rise to signals from elements present in a sample. For example, classroom buildings typically have significant blanks from Ca2+ due to chalk dust. Sulfuric acid is frequently slightly contaminated with Pb2+ from the lead pipes commonly used to move the acid through the manufacturing process (PbSO4 is insoluble in concentrated sulfuric acid, so lead is comparatively inert to attack by sulfuric acid). Si from quartz is a trace contaminant from ICP torches. Sodium and potassium are ubiquitous. Thus, the amount of sample in an analyte is not the total amount measured; it is the amount in excess of the ambient background. Working curves frequently include the blank signal as part of the background correction. In such case, a background is NOT subtracted from the signal; rather, the background/blank is included in the working curve. Alternatively, if one carries out a determination, substituting distilled water or some other suitable, clean material for the specimen, but otherwise carrying out a determination according to protocol, one can measure the concentration of the analyte in the reagents. Thereafter, the true concentration of analyte is that found through normal measurement, minus the blank.
Data Reduction in Atomic Spectroscopy
Sample Problems
 1 A sodium emission line at 588.9 nm, measured with 0.1 nm bandwidth for a solution containing 100 ppb Na+ as NaCl, gives rise to a signal of 43756 analog-to-digital converter counts for a 1 s integration. A measurement at 589.1 nm measures the background emission as 1210 counts for a 1 s integration. If a 10 ppb sample is aspirated, what signal is expected? If background were not measured and subtracted, what concentration would the 10 ppb specimen appear to have, and what percent error would be obtained? Answer 2 In reality, stray light in the instrument means that the background measured at 589.1 nm includes 1% stray light from the 588.9 nm emission line. What is the true background at 589.1 nm (i.e. background from the plasma, not stray light)? How does this change the formulation of the background subtraction problem? Answer 3 A reagent blank for determination of chromium in steel shows a signal corresponding to 0.02% chromium. If the raw signal from a steel sample appears to indicate 4.01% Cr, what is the true percentage of Cr in the steel? What information is missing to decide if the steel actually has at least 4.0% chromium? Answer
 Atomic Emission Spectroscopy