Why is there a negative dip in my HPLC chromatogram?

Because your solvent absorbs UV to some extent, too.

If you constantly see a negative dip, it may come from your sample, diluent, mobile phase, bottle, cap, glassware, column problems etc, that some compound is eluting out but does not absorb UV to the extent that your solvent does.

If there is a difference in composition between the injection solvent and mobile phase, your diluent (eg. water) could have a lower optical density than your mobile phase (eg. methanol) does. The negative dip will appear right before your sample is eluted.


*LC grade means that UV light absorbing impurities have been removed (not that the absolute purity is higher) and absorbance of specified wavelengths is kept within a specified range.

Lost prime

When your instrument prompts an error of a lost prime, it means pressure has dropped to below 25 psi for the last 50 cycles of plunger. Very often, gas bubbles interrupted the solvent path.


First, check that:

1. There is enough solvents in the bottles.
2. All solvent lines are properly connected and not drawing air into the system.
3. All solvents degassed and lines properly primed. It is more effective to prime each line at 100% for 5 min rather than 25/25/25/25 for A/B/C/D.


Other possible causes:

1. Solvent reservoir filters clogged.
2. Bacteria growth in the lines or inline filter and becomes clogged. May be able to clean out with 100% MeOH.
3. Primary inlet check valve failed. Ruby ball can get sticky over time esp if applications involve high ACN content. Ball gets stuck at the close position, so solvents cannot be delivered.


Passivating with 30% Phosphoric acid or 6N of Nitric acid not recommended.
This involves extreme cleaning, takes a lot of time, and unnecessary wearing of parts.

Another mild sexual harassment

My previous post on sexual jokes reminded me of my other encounter that happened 4 years ago.

I was working as an executive when my role was to manage the UAE market, United Arabs Emirates. I had to liaise with Arabic translators on frequent occasions. They were generally nice people, always ready to run off translating a new case.

There was once I was heading for a dinner appointment. It was off work and and I met Mr Y by the workplace exit. He offered to drop me at Lavender MRT, and I couldn't be more happy to gain a free ride.

Because I commented I was early for my dinner, he asked if I wanted to visit his Arabic school. Sure thing why not. Then I imagined something nice and welcoming, with a scale like an international school.

During the journey we talked about our families. And... he kept leading the conversation to becoming sexual. I can't remember the exact words it was something like this.

Mr Y, "My wife likes to do in the dark, I don't understand. How about you?"
Me, " Ah... (laughs), not telling you."

There was a lot more but I forgot the content already. After all, that was 4 years ago.

I almost wanted to get out of the car, and my alertness level shot up. I regretted agreeing to see the school, it was dark and a little shabby, with a long narrow staircase that leads to the 2nd floor. Like any other old shop house you would see.

In the classroom that was it. He showed me some facebook pictures, of his trips and his family on his computer.

Mr Y, "Do you want to sit?" Signaled for me to sit on him, pointing to his cock lap.
"No, I prefer the chair. (nearest one about 10 metres away, I just wanted to go already...)" 

Tried to appear amused and acted calm, but I was horrified. Grossed and freaked out. Thought of running away but then again it would be over-reacting. After all, we would still have to work with each other ---- everyday. I need to maintain my professionalism!!!
That happened during my very first job after graduation. Somehow such of my encounters were only with people of this skin color. Well fortunately he was only verbally challenging, and nothing happened.

I remembered I could rant 1-2 hours to my friends but now I could only remember the gist of it. Thinking back, guessed these people just enjoy harassing teasing females and I was too green and fresh out of school.

Sexual jokes, is it a form of sexual harassment?

28 January 2015 - Shanghai

There was a traffic jam and we were in a taxi.

Mr R showed me a whatsapp message, it's a joke sent by his friend.

Mr R, “Can you read it?” (All texts were in Malay)

Me, “Only very simple words like “dan means and” and “untuk means for”... So? What does this say?"

Mr R, “So this is saying, if you have sex once a week, you will have good physical health.” (Laughs)
Me, “Haha ok…” (Laughs)

Mr R, “If you have sex twice a week, you’ll have good mental health. 3 times a weeks, a good heart. 4 times… 5 times… 6 times… 7 times a week, it will boost your work performance. (Laughs) Malaysians like to share this kind of messages ah”

Like any mild sexual jokes you would see on fb, I expressed amusement, "Ha… you should tell your wife about this”.

Mr R, “Yeah I did. Here's another joke.”

Me, “Ok”

Now this was getting disturbing. Avideo about someone who drew a nude man and woman on his left hand, and by moving his pinky it looked like the man and woman are having sex. Too much details, too explicit. The man drew another 3-4 times more, showing the various positions. Oral sex, doggy style, whatever, you name it.

I was feeling disturbed. You don’t show these to someone you just got to know a week. I acknowledged with a grimace in response. 

And another ‘joke’, if you have sex in the dark will boost something, your heart or your brains or kidneys, than with lights on.

Yet another video of some guy playing with fireworks. Fireworks flew in the wrong direction and hit his naked butt. Only his backside was seen.

How would you have responded? 

I had a lot of thoughts. 

I was stunned.
It was not funny.

I stopped responding.

He stopped after 4 such jokes.

I can take sexual jokes and laugh them off, as any mild sexual jokes on fb.

But at what point does it become offensive?

Maybe I should have been firm and denounce his behavior.

Maybe I was too serious.

I did nothing.

On the next day, Mr R made an approach again, this time in class. A video about a China man who just got married, having sex needing to be supervised by his parents, it was non-censored. He laughed it off again.

All these while these goddamned jokes were only directed towards me. Not to the Malaysian guy he came with, or the other Chinese guys. I smiled lightly and shrugged, and continued with my work. 

But I was totally turned off.

  

Unfortunately there was no proof where I could document it. It wasn't intimidating, and I wasn't forced into any kind of submission.

It was just... very disturbing.