Chromatography Forum

Hi folks ,

My next job is to analyse two toxic pesticide in water using ( RP-HPLC ) .Actually , i have done many analysis by preparing standard in clean water but what worry me is the limit of QUANTIFICATION .For example , my LOQ IS ( 0.01 PPm ) .I don't know what my mentor think when he said measure in waster water and we know that the MRL of pesticide in water is less than ( 10 ppb ) .

this is first

second because i will use filtration by filtering the waste water the directly measuring in hplc is this ok ? or do you think using other teachnique will be better like what ?


Please help me mates and thanks in advance

Since 0.01 ppm = 10 ppb, you will not be able to quantitate below that level with your present procedure.

Depending on the details of how you are doing the analysis, however, you may be able to improve on that substantially by increasing the injection volume. Because your sample is in a weak solvent (water), dissolved hydrophobic compounds are concentrated on the head of the column during the injection. This is the "other side of the coin" of the problem with strong solvent diluents which shows up frequently on the forum. You will, of course, have to experiment to find out just how large an injection volume you can use.

Re filtration, this is not necessarily a problem, but you *will* have to do a recovery study to establish that your analyte is not lost by sticking to the filter, and also confirm that you are not leaching interferences from the filter.

Since 0.01 ppm = 10 ppb, you will not be able to quantitate below that level with your present procedure.

Depending on the details of how you are doing the analysis, however, you may be able to improve on that substantially by increasing the injection volume. Because your sample is in a weak solvent (water), dissolved hydrophobic compounds are concentrated on the head of the column during the injection. This is the "other side of the coin" of the problem with strong solvent diluents which shows up frequently on the forum. You will, of course, have to experiment to find out just how large an injection volume you can use.

Re filtration, this is not necessarily a problem, but you *will* have to do a recovery study to establish that your analyte is not lost by sticking to the filter, and also confirm that you are not leaching interferences from the filter.

Thanks Tom.Actually , i am using a very low walvelength which my analyte has the highest uv absorption on it .Also, there is no way to change my three long columns ( 250mm ,phenomenex ) due to economic restrictions .So , as i undersatnad from you that increasing the injection volume will optimise the loq ( say less than 10 ppb ) .Therefore, i have to change the loop as i read in the LC-GC troubleshooting book .i don't know if this expensive or not ? because i am using 20 micron ( the loop )

Regarding the filter you are reading my mind because i am now studying the recovery of my analyte in the filter paper .but please Tom can you give me more illustration in which other things i have to investigate in analysis pesticide in waste water because i looked in google but i really lost .Do you thing SPE WOULD BETTER than filtration ?

thanks Tom for your help i really appreciated

One of the better ways to do trace analysis in water samples is to enrich the analytes on a SPE cartridge, then elute from the SPE cartridge in a small volume and inject this enriched sample into the HPLC system. My colleague Michael Young has done many of these extractions from real environmental waters. Enrichment factors of 1000 to 1 are possible.

Essentially you use a Oasis HLB cartridge (water wettable and water compatible), suck the desired amount of water through the cartridge, and elute with a small amount of methanol. Further, more specific tricks are available in the Oasis handbook on the Waters webside, or I can give you additional details if you contact me.

One of the better ways to do trace analysis in water samples is to enrich the analytes on a SPE cartridge, then elute from the SPE cartridge in a small volume and inject this enriched sample into the HPLC system. My colleague Michael Young has done many of these extractions from real environmental waters. Enrichment factors of 1000 to 1 are possible.

Essentially you use a Oasis HLB cartridge (water wettable and water compatible), suck the desired amount of water through the cartridge, and elute with a small amount of methanol. Further, more specific tricks are available in the Oasis handbook on the Waters webside, or I can give you additional details if you contact me.

Dear Uwe Neue, thanks very much for your help .Actually , i really want to give more details as i am completely new in this Area .Also, could you please show me where can i find your colleague "Michael Young " work to read and get i dea.

and finally , where can i find the Oasis handbook please ?

Sorry for this several questions

thanks you very much again i start to see the road

Go to the Waters website: http://www.waters.com/waters/home.htm

In the upper right corner is a search field. Put in "Oasis Applications Notebook", and you will get it as the first entry.

You may need to fill out your name at some point in order to get it.

Thanks Uwe Neue,

it look for me is like acommercial advertisament !

Well, the booklet contains a lot of proven methods, well thought out and demonstrated to work. An example close to the work that you want to do is the analysis of asulam in river water on page 12 with a 100 to 1 enrichment. There are many methods of the same type in this booklet. Of course, they were done with our packing material. If you think that proven and functional methods are only advertisement, so be it...

Uwe Neue

I really feel shy from your generous help.I am terribly sorry if i post any things that may annoy you

Thanks indeed for you and sorry a gain

Don't worry... I see that you have some difficulties with English...

If you have more questions about the details of some of the methods, you can contact me at the e-mail below.