Chromatography Forum

Hi everybody,

I would like to know how do you normally obtain a correct flow for the LCMS. Our LCMS (ESI) can only accept maximum 0.2mL/min of flow from the column so we normally use column with 2.1mm I.D. It could be interesting for us to acquire a flow splitter that could allow us to use also bigger column (4.6 mm I.D.).
I would like to hear from your experience which is the best way to split the flow for a LCMS (in relation to reproducibility, ease of use, costs etc.).

Thanks in advance for the response

Have a good day,

Davide

The easiest/cheapest way to split the flow is to use a simple "tee" right before the MS. My MS creates enough backpressure so that about 1/3 of the flow goes to the MS and 2/3 to the waste line.
If you need tighter control over the split ratio, there are adjustable flow splitters you can buy.

Hi Davide,

in my experience a simple T is not very reliable. The backpressure of the ESI-pipe changes. So you get changing split ratios and thus varying peak areas.

At least you should put some piece of small ID cappillary before the MS to get some 10s of bars backpressure. Try to determine the backpressure of your ESI alone; the capillary should have more.

On the other output of the T you can put another piece of capillary. Cut it until you reach the desired split ratio. Split ratio is the inverse of the pressure ratio.

Regards,

Hartmut

Dear All, thanks for the answers..

Dear Hartmut, could you please explain me a bit more the concept of: "Split ratio is the inverse of the pressure ratio". Am I understanding correctly that if I have a backpressure of 20bar on my MS (ESI probe+tube) and I have a pressure of 4bar on my other tube from the tee, I'm going to get a split ratio of 5? With a original flow of 1mL/min I will get 0.2mL/min into the MS?
Which internal diameter of the tube do you normally use on the other side of the tee? The tee is going to have some kind of effects on the peak shape?

Sorry for the long post and the question..

Have a good day,

Davide

Hello Davide,

your calculation is correct. What I tried to explain in my last post is, that the 20 bar backpressure on the ESI might change during your analysis series, so it is better to put a cappillary of e.g. another 50 bar backpressure before the ESI. Then you can put a cappillary that gives ca. 10 bar on the other side to get a 1:5 split.

Choose small ID tubing and a low dead volume Tee, so you will have no significant effect on peak shape. I used a PEEK-Tee from Upchurch and PEEK-tubing - in our lab we have a range of 1/16" tubing with different IDs available. I used very small IDs (0.004", 0.0025" and 0.001") to split 0.25 ml/min down to ca. 0.05 ml/min and keep the length as short as possible. I connected the pieces that were availble to a punp and recorded the backpressure. Then I chose two pieces that fit my needs.

To calculate the split ratio with different ID tubing, you can use the formula:

Split = (ID1^4 * L2) / (ID2^4 * L2),

with L1, L2 length of tubing
ID1, 2 int. diameter of tubing

Regards

Hartmut

sorry, formula should read :

Split = (ID1^4 * L2) / (ID2^4 * L1)