Chromatography Forum

Hi, please can anybody help me as I have no clue how to solve this?

Determination of benzo[a]pyrene - sample was analyzed by LC with UV detection (ʎ = 254 nm). Benzochrysene was used as an internal standard - 200 µl (c = 50µg/ml) of standard solution was added to 2 ml of sample. Injection volume of prepared solution sample + standard was 20 µl.
Peak areas:
benzo[a]pyrene A = 3,21 cm2
benzochrysene A = 1,83 cm2
Molar extinction coefficients (ʎ = 254 nm):
benzo[a]pyrene ɛB(a)P = 39800 mol-1. l . cm-1
benzochrysene ɛB(b)CH = 24500 mol-1. l . cm-1
Molecular mass:
benzo[a]pyrene Mr(B(a)P) = 252,3
benzochryseneMr(B(b)CH) = 278,4.
What is the concentration of benzo[a]pyrene in the sample (µg/ml)?

It would help me a lot. Thanks!!

I may be missing something but shouldn't the ratio of the responses (areas) multiplied by the ratio of the molar extinction coefficients converted to mass equal the ratio of the concentrations? And you know what the concentration of internal standard is.

Hello

Have you run any calibration? Have you got calibration curve?

Regards

Tomasz Kubowicz

Steve,

Not necessarily (if only it can be so easy). Each molecule (even similar molecules) CAN have a different response to the detector. So the ratio of the peak response is meaningless. Thus, the calibration curve and even it is subject to matrix effects (sometimes).

I guess I assumed that the extinction coefficients were determined on that detector (or are close enough). Otherwise there isn't enough information to solve the problem.
That is why a multi-point calibration is normal. Matrix effects are a whole different can of worms.

The absorption spectra of polycyclic aromatic hydrocarbons shifts significantly depending on the solvent they're in; up to 40 nm or more. The molar extinction coefficients of the peak maxima will change at the same time. Your calibration curve must be obtained using the same solvent that you intend to use for the analysis.