Chromatography Forum

Hi , i am working on one pesticide .I 've do my LOD AND LOQ AS mentioned by John Doland ( 3*SD/SLOPE) .

I USE RP-HPLC SYSTEM with c18 ( 250mm *4.6mm)

mobile phase ( 70% methanol in water ) isocratic system

flow rate =1 ml/min

i really want to go down in my LOQ to determine this pesticide in water to about 10 ppb. HOW can i lower this LOQ

any sugestion would be very appreciate it

Thinner column (e.g. 3 mm diameter) or/and smaller particle size. If you have excessive resolution, a shorter column could add to the other 2 suggestions. The latter may “costâ€

Newchromatographer,

You didn't mention what kind of detection you're using. LOQ will be determined by the detection technique also

Best regards

[quote="danko"]Thinner column (e.g. 3 mm diameter) or/and smaller particle size. If you have excessive resolution, a shorter column could add to the other 2 suggestions. The latter may “costâ€

LOQ and LOD are ultimately controlled by Signal / Noise ratio. There are only two ways to improve that:
- more signal
- less noise
zokitano and danko have both made good suggestions on increasing the signal.

Because you are defining LOQ in terms of concentration (rather than mass on column) and your sample (water) is much weaker than your mobile phase, the easiest way to increase the signal is to increase the injection volume. (in effect, doing on-column concentration of your pesticide). If it were my problem, I would do a series of tests going up by successive factors of 3 (1, 3, 10, 30, etc) with a mid-range standard until I saw peak shape problems beginning to occur. I've successfully done 10 mL injections of water in an ion chromatography system.

Note that this will not increase the linear range: just move the entire range downward.

...now the LOQ is 0.01 PPM .I will try your suggestion because i want to lower to 10 ppb !!

Job done?

zokitano
my detection is Uv

tom jupille
it is written in my instrument ( Shimadzu ) the maximum injection is (20micro m) , so this mean i will not be able to inject more .I would be gratefull if you explain more

JA
i don't understand your comment

thanks all

Dear Newchromatographer,

JA is totally right: 0.01 ppm = 10 ppb

You've already got the desired LOQ of 10 ppb or 0.01 ppm whatever you prefer more

Regards

tom jupille
it is written in my instrument ( Shimadzu ) the maximum injection is (20micro m) , so this mean i will not be able to inject more .I would be gratefull if you explain more

Many autosamplers will accommodate a range of syringe and loop volumes so that they can be configured for different injection volume ranges. However, as has been pointed out, you are already at your goal (I have to learn to read the posts more carefully! ), so there is no further need to modify the autosampler.

Dear Newchromatographer,

JA is totally right: 0.01 ppm = 10 ppb

You've already got the desired LOQ of 10 ppb or 0.01 ppm whatever you prefer more

Regards

oooooh my god sorry i mean ( 1ppb ) otherwise you are absloutely right hhhhhhh

tom jupille
it is written in my instrument ( Shimadzu ) the maximum injection is (20micro m) , so this mean i will not be able to inject more .I would be gratefull if you explain more

Many autosamplers will accommodate a range of syringe and loop volumes so that they can be configured for different injection volume ranges. However, as has been pointed out, you are already at your goal (I have to learn to read the posts more carefully! ), so there is no further need to modify the autosampler.

seriously tom , you and your friends help me a lot a lot , and it is my mistake that i wrote 10 instead of 1 ( i have to learn to write carefully )

I also work with pesticides. I have found that lowering the LOQ means further cleanup of sample prior to injection. This may mean SPE or Liquid/Liquid partition. Water might seem like a clean matrix but depending on the source there can be a suprising degree of interference. Simple syringe filtration through Nylon, PTFE or GHP can help. Also it may be necessary to concentrate your water sample if you are running into a detection problem at low concentration. Rotovap-ing water is difficult and time consuming there are better ways just search the internet.

It is likely that you can change the size of the sample loop in the autosampler to increase the injection volume. This will give you bigger signal. Changing parameters (column, mobile phase, etc) to give better peak shape may also help, but increasing the injection volume would be my first try. What detector are you using? If UV, have you optimized the wavelength for your compound?