CHEM 2302 Lecture Notes - Lecture 4: Sodium Chloride, Chromatography, Stoichiometry

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29 Sep 2016

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Standard addition: known quantities of analyte are added to the unknown. Consider a standard addition in which a sample with unknown initial concentration of analyte [x]i gives a signal intensity ix. Then a known concentration of standard, s, is added to an aliquot of the sample and a signal is+x is observed for the second solution. Addition of standard to the unknown changes the concentration of the original analyte because of dilution therefore the diluted concentration of analyte will be [x]f ( f = final). We designate the concentration of standard in the final solution as [s]f (remember that x and s are actually the same analyte species). Concentration of analyte plus standard in final solution. S signal from initial solution signal from final solution. For an initial volume v0 of unknown and added volume vs of standard with concentration [s]i, the total volume is v=v0+vs and the concentrations in the above equation are:

Related Questions

A known standard mixture of pseudoephedrine (A) and 4-aminoacetophenone (B;
internal standard, IS) and an unknown urine sample gave the following GC data (using
a 30-m HP5 capillary column held at 260Â° C).
Compound A Concentration 500 ug/mL - Peak Area 141.2
Compound B Concentration 500 ug/mL - Peak Area 119.9
A in urine sample Concentration unknown - Peak Area 147.7
B added to urine sample Concentration (calc from below) - Peak Area 218.9

A 5.00 mL urine sample was taken for GC analysis. To this sample was added 100 Î¼L
of the 5 mg/mL of the internal standard (B). The mixture was then treated with ammonia
and passed through a RP18 SPME cartridge. The analyte and internal standard were
both retained on the SPME cartridge and were then eluted with 2.0 mL methanol. The
eluate was evaporated down to about 0.5 mL and 1 Î¼L of this concentrate was
analysed by GC. The observed retention times and peak areas obtained for the signals
are included in the table above.
What was the concentration (in Î¼g/mL) of A in the urine?
(peak area of analyte signal)/(analyte concentration) = F (peak area of IS signal)/ (IS concentration)

This is a prior exam question example rather than a real world result. This is all the information the lecturer has provided for a 10 point exam question. I'm expecting the calculations are to be done from concentration and peak area per the formula rather than using retention time and creating a calibration curve. Needing help with the dilution calculation for the B+sample concentration please.

Consider a solution containing 3.78 mM of an analyte, X, and 1.39 mM of a standard, S. Upon chromatographic separation of the solution peak areas for X and S are 4001 and 10867, respectively. Determine the response factor for X relative to S.

a) F = ?

To determine the concentration of X in an unknown solution, 1.00 mL of 8.50 mM S was added to 3.00 mL of the unknown X solution and the mixture was diluted to 10.0 mL. After chromatographic separation, this solution gave peak areas of 6299 and 4703 for X and S, respectively.

b) Determine the concentration of S (in mM) in the 10.0 mL solution.

c) Determine the concentration of X (in mM) in the 10.0 mL solution.

d) Determine the concentration of X (in mM) in the unknown solution.

Internal Standards

Some analytical methods are subject to one or more common sources of variability in the instrument response. One method aimed at overcoming this variability is the internal standard method. In this method, a known amount of a compound similar to the analyte is added to each standard and sample. Since we know the concentration of the internal standard (IS), we can use the IS response as an indicator of variability. In this type of calibration curve, the quantity plotted on the y-axis is not simply the instrument signal for the analyte, but rather the ratio of the analyte signal to the IS signal.

The IS method is commonly used in gas chromatography, because of the variability inherent in the small volumes injected into the instrument. In the forensic lab, blood alcohol measurements are one of the most common analyses performed. This dry lab example will use blood alcohol data.

You will use ethanol standards that you have purchased. You have 5 of these, with concentrations of 0.050, 0.080, 0.100, 0.200, and 0.300 %w/v of ethanol. You also have a blank, which doesnât contain any ethanol.

You will use n-propanol as the internal standard. You have made up a 0.0300% (v/v) solution of propanol.

You have one sample to run.

You add 2 mL of each of the standards and the sample to GC vials. Then you add 0.450 mL of the internal standard n-propanol to each vial. You get the data below.

Ethanol concentration, % (w/v)	Peak area for ethanol (pA*sec)	Peak area for n-propanol pA*sec)	Ratio of ethanol area to propanol area

First, make a calibration curve using only the area of the ethanol peak and the concentrations of the standards. As in Part 1 a and b, prepare and print a calibration curve including the linear least squares equation of the line equation and the R² value, and print a copy for your report. Also, calculate the concentration of your sample and the uncertainty (95% confidence interval), as well as the uncertainty of the unknown determination on a percentage basis, and include these values in your report. .

Next, make a calibration curve based on the ratio of ethanol area to propanol area and the concentrations of the standards. As in the previous question, prepare and print a calibration curve including the linear least squares equation of the line equation and the R² value, and print a copy for your report. Also, calculate the concentration of your sample and the uncertainty (95% confidence interval), as well as the uncertainty of the unknown determination on a percentage basis, and include these values in your report. .

At a blood alcohol concentration of 0.08% (w/v) or more a person is considered impaired. Briefly discuss whether the calculated results for the sample in e and f above support criminal charges of impairment.

Report Requirements:

Spreadsheet with graphs and calculations. Prepare separate sheets in one excel file for each of the following:

Data Set 1

Data Set 2

Calibration with external standard

Standard addition with one standard (optional, could submit hand or type written calculation only)

Standard addition with multiple additions

Internal standard peak areas

Internal standard peak area ratios

Print-outs of graphs

Data Set 1

Data Set 2

Calibration with external standard

Standard addition with multiple additions

Internal standard peak areas

Internal standard peak area ratios

Results

Fill in the table below with results from the spreadsheet. Remember to include units.


	Concentration of unknown	Uncertainty of unknown	Uncertainty of unknown in terms of %

	Concentration of unknown	Uncertainty of unknown	Uncertainty of unknown in terms of %

2d	Concentration of unknown	Uncertainty of unknown	Uncertainty of unknown in terms of %

	Concentration of unknown	Uncertainty of unknown	Uncertainty of unknown in terms of %

	Concentration of unknown	Uncertainty of unknown	Uncertainty of unknown in terms of %

Short answers/additional tables

Section 1c:

Section 1d:

Section 1e2:

Section 2a Table:

Section 3b Table:

Section 3b4 calculation

Section 3b5

Section 4d Table with calculated ratios

Section 4g

CHEM 2302 Lecture Notes - Lecture 4: Sodium Chloride, Chromatography, Stoichiometry

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