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Chapter 5: Q5-6 P (page 114)

What is a blank and what is its purpose? Distinguish method blank, reagent blank, and field blank.

Short Answer

Expert verified

A blank is a sample supposed to have no analyte. Blank is commonly used to calibrate instruments such as calorimeters.

  • A method blank is taken through all the steps in the chemical analysis and used to evaluate the contamination from the preparation and analytical process.

  • A reagent blank and a method blank can be considered similar, but the reagent blank is not subjected to all sample preparation procedures.

  • A field blank is similar to a method blank, but it is taken into the field and exposed to the field environment.

Step by step solution

01

Definition of Blank

A blank is a sample supposed to have no analyte.

02

Purpose of blank and its types

Blanks are introduced into the experiment to take into account the interference made by the other species present in the system. Also, these blanks help to keep a record of the traces of analyte. These analytes are often used for sample preservation, preparation, and analysis.

Method blank -– It is a type of blank which contains all the present components in the system except the analyte. This type of blank is subjected to all sample preparation procedures. Before we can solve for the quantity of the analyte present in the sample, the response of the method blank must be deducted first from the response of a real sample.

Reagent blank - Unlike the method blank, this type of blank is NOT subjected to all sample preparation procedures. Moreover, it depends upon complete approximation regarding the amount of response introduced by the blank.

Field blank -– It is used in the same way as a method blank; however, this type of blank is exposed to the environment where the collection of samples is performed

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Most popular questions from this chapter

5-28. Ititandard addition. Lead in dry river sediment was extracted with 25wt%HNO3at35°Cfor1h.then1.00mLof filtered extract was mixed with other reagents to bring the total volume to V0=4.60mL.pb(ll)was measured electrochemically with a series of standard additions of 2.50pb(ll).


(a) Volume is not constant, so follow the procedure of Figures 5-5 and 5-6 to find ppmpb(ll)inthe1.00-mL

extract.

(b) Find the standard uncertainty and 95%confidence interval for the x-intercept of the graph. Assuming that uncertainty in intercept is larger than other uncertainties, estimate the uncertainty in ppm pb(ll)inthe1.00-mLextract.

Internal standard graph -- Data are shown below for chromatographic analysis of naphthalene (C10H8), using deuterated naphthalene (C10D8, in which D is the isotope 2H) as an internal standard. The two compounds emerge from the column at almost identical times and are measured by a mass spectrometer.

(a) Using a spreadsheet such asFigure 4-15, prepare a graph of Equation 5-12 showing peak area ratio(C10H8/C10D8)versus concentration ratio role="math" localid="1663559632352" ([C10H8]/C10D8) . Find the least-squares slope and intercept and their standard uncertainties. What is the theoretical value of the intercept? Is the observed value of the intercept within experimental uncertainty of the theoretical value?

(b) Find the quotientrole="math" localid="1663559638520" [C10H8]/[C10D8]for an unknown whose peak area ratio (C10H8/C10D8) is 0.652. Find the standard uncertainty for the peak area ratio.

(c) Here is why we try not to use 3-point calibration curves. For n = 3 data points, there is n - 2 = 1 degree of freedom, because 2 degrees of freedom are lost in computing the slope and intercept. Find the value of Student's for confidence and 1 degree of freedom. From the standard uncertainty in (b), compute the 95 % confidence interval for the quotient[C10H8]/[C10D8] . What is the percent relative uncertainty in the quotient[C10H8]/[C10D8]? Why do we avoid 3-point calibration curves?

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.

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.

Internal standard. A solution was prepared by mixing 5.00mLof unknown elementXwith2.00mLof solution containingrole="math" localid="1654777035083" 4.13μgof standard elementSper millilitre, and diluting to10.0mL. The signal ratio in atomic absorption spectrometry was (signal fromX)/ (signal fromS)=0.808. In a separate experiment, with equal concentrations ofXandS, (signal fromX)/signal fromS)=1.31. Find the concentration ofXin the unknown.
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