QuEChERS - basic questions

[lusi]

What mill would be best to use with the technique?

Also when you get your results from the MS, what calculations are needed to get the amounts per gram?

Does anyone know of any courses or labs (to do an intern) that does pesticide residue analysis using the technique?

[H.Thomas]

What mill would be best to use with the technique?

Any mill that you can operate with dry ice or liquid nitrogen. We have this one for small quantities
http://www.ika.net/owa/ika/catalog.prod ... ct=2900000
and this for a little larger samples
http://www.retsch.com/products/milling/ ... -features/
For really big samples, you can use a mill from the food processing industry.

Also when you get your results from the MS, what calculations are needed to get the amounts per gram?

Calculate which preparation and dilution steps you have performed with your sample. If you weigh e.g. 1 g of sample and get to a final volume of 10 ml, your calculation would be simply multiplying by 10 (to calculate µg/g from µg/ml) .

Does anyone know of any courses or labs (to do an intern) that does pesticide residue analysis using the technique?

If you know the basic of analytical chemistry, it should be sufficient to read some of the many articles floating around the internet. Or you could refer to the official methods EN 15662 or AOAC 2007.01.

[lusi]

Thanks H Thomas.

Calculate which preparation and dilution steps you have performed with your sample. If you weigh e.g. 1 g of sample and get to a final volume of 10 ml, your calculation would be simply multiplying by 10 (to calculate µg/g from µg/ml) .

Just to clarify. Lets say you use 1g of sample and the final volume is 10ml and your MS indicates 0.3ug/ml; then your actual concentration is 3ug/g?

The method uses 10g sample and adds 10ml ACN, but transfers only 1ml to be used for further clean-up. So is my final volume 1ml and sample 10g? If so would if the MS reads 0.3ug/ml; then your actual concentration is 0.03ug/g? Or should I account for the 10ml with 10g sample, then if the MS indicates 0.3ug/ml; then your actual concentration is 0.3ug/g?

I really need some help with the calculations.

Also do you know how to do the calculations using the Luke Method in the Pesticide Analytical Manual of the USFDA? We are being pressured to use this method but I have never done it before or have any calculations for it.

[H.Thomas]

Just to clarify. Lets say you use 1g of sample and the final volume is 10ml and your MS indicates 0.3ug/ml; then your actual concentration is 3ug/g?

Correct. 0.3 ug/ml = 3 ug/10 ml = 3 ug/g

The method uses 10g sample and adds 10ml ACN, but transfers only 1ml to be used for further clean-up. So is my final volume 1ml and sample 10g? If so would if the MS reads 0.3ug/ml; then your actual concentration is 0.03ug/g? Or should I account for the 10ml with 10g sample, then if the MS indicates 0.3ug/ml; then your actual concentration is 0.3ug/g?

If you take aliquots, that doesn't count. 10g sample in 10 ml ACN means a 0.3ug/ml result equals 0.3 ug/g, regardless how much you take for further processing. Except you have another dilution or concentration step in your procedure.

Also do you know how to do the calculations using the Luke Method in the Pesticide Analytical Manual of the USFDA? We are being pressured to use this method but I have never done it before or have any calculations for it.

We don't use Luke, and I'm really too lazy to look it up. If you could provide a brief sumary of all dilution and concentration steps...?

[lusi]

Thanks H. Thomas. Below is the method I am referring to, but we are going to be a rotavap instead:
- Weigh 100 g sample into blender jar and add 200 mL acetonitrile
- Filter
• Transfer filtrate to 250 mL graduate and record volume (F).
• Transfer measured volume of filtrate to 1 L separator.
• Carefully measure then add 100 mL petroleum ether to separator
Add 10 mL saturated sodium chloride solution and 600 mL water.
• Let layers separate, discard aqueous layer, and gently wash solvent layer with two 100 mL portions water. Discard washings, transfer solvent layer to 100 mL g-s graduate, and record volume (P).
• Add about 15 g sodium sulfate to graduate, stopper, and shake vigorously.
• Transfer solution directly to Florisil column, C1 or C2, or concentrate to 5-10 mL in K-D for transfer.

We are considering concentrating before clean-up.

• Calculate weight sample placed on Florisil column as:
g = S x (F/T) x (P/100)

where:
S = g sample extracted
F = volume of filtered acetonitrile extract
T = total volume (mL water in sample + mL acetonitrile added –
correction in mL for volume contraction). 5 mL contraction
volume is used for 80-95 mL water + 200 mL acetonitrile.
P = mL petroleum ether extract recovered
100 = mL petroleum ether into which residues were partitioned.

I do not know what they mean by contraction volume.

Place activated Florisil (4" or weight determined by lauric acid value)
in 22 mm id column; add about 0.5" sodium sulfate. Prewet column
with 40-50 mL petroleum ether.
Add extract.
Rinse tube with two 5 mL portions petroleum ether
• Elute column at about 5 mL/min with 200 mL 6% ethyl ether in
petroleum ether.
• Then elute at about 5 mL/min with 200 mL 15% ethyl ether/petroleum ether.
. Elute at about 5 mL/min with 200 mL 50% ethyl
ether/petroleum ether eluant.
Concentrate to <5 mL.

[H.Thomas]

[quote="lusi"]I do not know what they mean by contraction volume.

100 ml water + 200 ml acetonitrile do not give 300 ml, but less. This is the "contraction volume" you have to substract.

If you have calculated the amount of sample you have placed on the florisil-column, the rest of the calculation is easy - you have this amount of sample in your final volume after concentration.

[lusi]

Thanks so very much H. Thomas. I was not very sure given that there were so many dilutions within this method.

[pauloviedo]

Greetings,


We sold QuEChERS kit for PAH's extraction and analysis to the National Fisheries Institute. These kit were purchased based on the application note P/N 5990-5441EN Analysis of Aromatic Hidrocarbons in Fish using Agilent SampliQ QuEChERS AOAC Kit and HPLC-FLD (see attachment), several tests were performed in standards and the recoveries are good, the problem occurs when testing in real samples.

They are working with packaged precooked fish, but I think the application note is not suitable for these samples because being precooked the PAH´s are fixed in the fat of the sample and this may be the reason why the PAH's are not removed completely from the sample giving very low recoveries. We encouraged our costumer to work by prolonging the time of centrifugation and take care of cleaning material and solvents quality in addition to better homogeneize the sample but the low recoveries problem persists.

A collateral test was performed by adding a standard mix of PAH's in a raw fish fillet sample and applying the QuEChERS kit , but still the recoveries were not satisfactory.

We need your help to solve this problem.

[bdhedhy19]

One lab in San Francisco called Anersco lab does the pesticide residue on wool fat or any samples.