Why? Is the older cat not eating/driinking/using the litter box?

How long will you separate them for? Hopefully not too long, as that
could be counterproductive for the "getting to know you" process that
the cats have to go through, however stressful it might be for the
humans involved to watch ;-)

9 weeks is *very young* for a kitten to leave her mother and siblings.
She should have stayed at home at least 3 more weeks to learn cat
language, but that can't be changed now :-(, so you will have to let her
learn cat language from your older cat.

Camilla

It took my older cat about 3-4 days to accept the new kitten, and a
full week before they were really comfortable and playing with each
other.  Ming was curious about the kitten, but if she turned around,
he'd run and hide.  Give it a little time- within a week or so they
should be fine.  We've had the kitten for not quite 3 weeks, and the
two chase each other up and down the hall now, with a "wrassle" at the
end.  I was concerned at first because Ming was so much bigger and
could seriously hurt the kitten.  This hasn't been a problem- he keeps
his claws sheathed and only pins and "bites" her when she gets too wild
(he puts his open mouth on her, but doesn't clamp down).  On the other
hand, at first the kitten hadn't figured out how to restrain herself...
she bit Ming hard, and he ended up with a huge abscess on his
shoulder.  It's responded well to treatment, and it hasn't stopped the
"kitty races" in the hallway.  It should be noted that the kitten was a
feral rescue-  I took her when she was only 4 weeks old, so she really
didn't fully learn cat social behavior from her mother- it's been up to
Ming to teach her.  She's gotten much better- Ming isn't pinning and
holding her down nearly so often, and she's not screaming when he does.
She just relaxes,  he lets her up and it's off to the races!

Katrina
--

*snip*

Putting them in different rooms is fine - generally what others recommend is
that you put the kitten in a room by itself and let the older one have run
of the house.  The next day, put the older cat in the same room the kitten
was in and let the kitten explore the house for an hour or so, then put
reverse the order.  This way you are letting each cat get used to the other
cat's scent without causing any confrontation problems.  Do this for about a
week, then let kitten out and about w/older cat - only when you are around
to supervise.  Until they become acclimated to one another I would keep
kitten seperate.

Give them a bit of time, it's an adjustment for the older cat.  Some of mine
tolerate each other, others are best friends, it depends on the cat -some
are fine with other cats, while others prefer not to be bothered.  Two of
mine tolerate each other - Elmo and Stumbles - every now in then one of them
decides they can't stand the other one and the chase begins.  Then again,
Elmo is generally miserable and doesn't want to be bothered.  The
interesting thing, when I started fostering he was the first to investigate
the kittens and would start playing w/them.  Then one bit him on the paw
(and it became infected) so no he avoids kittens.

Give it time, and be patient!

JoJo
https://home.comcast.net/~zookeepr/Fosters.htm