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Chapter 5: Q29P (page 117)

State when standard additions and internal standards, instead of a calibration curve, are desirable, and why.

Short Answer

Expert verified

The cause of desirability is described in the below steps

Step by step solution

01

The definition of the question

In this job, That will be discussed when standard additions and internal standards are preferable to a calibration curve.

02

Desirability of standard additions

They are advantageous when the sample matrix is unknown or difficult to duplicate, as well as when unexpected matrix effects are anticipated.

03

Desirability of internal standards

They are advantageous when instrument conditions vary from turn to turn and they are added to an unknown sample matrix at the start when uncontrolled and severe sample loss occurs.

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Most popular questions from this chapter

Blind Samples: Interpreting Statistics-- The U.S. Department of Agriculture provided homogenized beef baby food samples to three labs for analysis.4Results from the labs agreed well for protein, fat, zinc, riboflavin, and palmitic acid. Results for iron were questionable: Lab A, 1.59±0.14(13); Lab B, 1.65±0.56 (8); Lab C, 2.68±0.78(3) mg/100 g . Uncertainty is the standard deviation, with the number of replicate analyses in parentheses. Use two separate t tests to compare results from Lab Cwith those from Lab A and Lab B at the 95 % confidence level. Comment on the sensibility of the t test results and offer your own conclusions.

Standard addition graph. Tooth enamel consists mainly of the mineral calcium hydroxyapatite,C10PA46OH2. Trace elements in teeth of archeological specimens provide anthropologists with clues about diet and diseases of ancient people. Students at Hamline University used atomic absorption spectroscopy to measure strontium in enamel from extracted wisdom teeth. Solutions were prepared with a constant total volume of 10.0mLcontaining role="math" localid="1667792217398" 0.750mgof dissolved tooth enamel plus variable concentrations of added Sr.

(a) Find the concentration of Sr and its uncertainty in therole="math" localid="1667792593357" 10-mLsample solution in parts per billion =ng/mL.

(b) Find the concentration of Sr in tooth enamel in parts per million =μgg.

(c) If the standard addition intercept is the major source of uncertainty, find the uncertainty in the concentration of Sr in tooth enamel in parts per million.

(d) Find the 95%confidence interval for Sr in tooth enamel.

In a murder trial in the 1990 s, the defendant's blood was found at the crime scene. The prosecutor argued that blood was left by the defendant during the crime. The defense argued that police "planted" the defendant's blood from a sample collected later. Blood is normally collected in a vial containing the metal-binding compound EDTA (as an anticoagulant) at a concentration of ~4.5mMafter the vial is filled with blood. At the time of the trial, procedures to measure EDTA in blood were not well established. Even though the amount of EDTA found in the crime-scene blood was orders of magnitude below ~4.5mM

, the jury acquitted the defendant. This trial motivated the development of a new method to measure EDTA in blood.

(a) Precision and accuracy. To measure accuracy and precision of the method, blood was fortified with EDTA to known levels.

Accuracy=100×meanvaluefound-knownvalueknownvaluePrecision=100×standarddeviationmean=coeffcientofvariation

For each of the three spike levels in the table, find the precision and accuracy of the quality control samples.

(b) Detection and quantitation limits. Low concentrations of EDTA near the detection limit gave the following dimensionless instrument readings: 175,104,164,193,131,189,155,133,151, and 176. Ten blanks had a mean reading of 45 - 1 . The slope of the calibration curve is1.75×199M-1. Estimate the signal and concentration detection limits and the lower limit of quantitation for EDTA.

Consider a sample that contains analyte at the detection limit defined in Figure. Explain the following statements: There is approximately a 1%chance of falsely concluding that a sample containing no analyte contains analyte above the detection limit. There is a50%chance of concluding that a sample that really contains analyte at the detection limit does not contain analyte above the detection limit.

Internal standard calibration curve. Figure 5-10 is a graph of Ax/Asversus[X]/[S]=(mol%vinylacetateunits)/(mol%ethyleneunits)=q/p in Reaction 5-13.

(d) From the uncertaintyubof the intercept, find the95%confidence interval for the intercept. Does this interval include the theoretical value of zero?
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