Interview Questions.

Question 1. What Is Hplc?

Answer :

HPLC stands for High-Performance Liquid Chromatography (previously known as High-Pressure Liquid Chromatography).

It is a chromatographic method used to split the components in a combination, to discover each issue, and to quantify each aspect.

In wellknown, the approach involves a liquid pattern being exceeded over a strong adsorbent fabric packed into a column the use of a drift of liquid solvent. Each analyte inside the pattern interacts slightly in another way with the adsorbent cloth, for this reason retarding the waft of the analytes. If the interaction is weak, the analytes glide off the column in a brief amount of time, and if the interaction is powerful, then the elution time is lengthy.

Question 2. What To Do When Back Pressure Increases?

Answer :

An boom in back-strain normally indicates either a shield or analytical column trouble. To locate precisely in which the trouble lies we advocate you take away the protect column (if you are the usage of one) and replace the vintage cartridge with a new one.
If the original stress is restored, you solved the trouble.
If the pressure remains high, disconnect the analytical column from the device, backflush it (do NOT connect the column to the detector while doing so) and run a few column volumes of your cellular section through the column.
If the trouble nevertheless persists you may have a few strongly retained contaminants on your column coming from your preceding injections.
Run the ideal recovery methods, as recommended by the column producer, and retest the column.
If the preliminary stress isn't always restored you may must alternate the inlet frit or replace the column.
Always run your device (2 to 5 ml/min) with out the protect column and the analytical column to affirm that your strain isn’t coming from any other source, like a blocked in-line column prefilter, blocked/kinked tubing, particulates blocking your injector and so on.
Always work your manner from the detector returned to the pump to isolate the problem.
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Question 3. How Do I Determine The Void Volume In Hplc?

Answer :

The void extent of a system is normally decided by means of injecting an unretained general (Uracil in RP-HPLC) that has no or very little retention on a selected segment. Slight variations in this value are defined by way of the extra column dead extent of your unique device configuration and set-up.

Multiply the elution time of the unretained compound by way of the float fee to get the real void quantity of the gadget and column. To determine the column void volume alone you'll need to subtract the system void quantity decided with out the column connected.

Question 4. Why Should I Use A Guard Column With My Analytical Or Preparative Column?

Answer :

A guard column is usually recommended to guard the analytical/preparative column from infection from particulates from the injection, debris from worn pump seals/injector rotor seals or unfiltered mobile stages.
Filtration through a zero.22um to zero.Forty five um ought to be achieved a good way to dispose of particles and help degas the cell section at the equal time.
Solid Phase Extraction or Liquid Liquid Extraction also help produce a purifier pattern for direct injection.
Failure to use a defend column immediately exposes the analytical or preparative column to contamination and consequently reduces its sensible lifetime.
Care have to be taken to use, on every occasion feasible, the equal fabric within the protect column as in the analytical/preparative column mainly while doing approach improvement.
Typical analytical protect columns are 1 or 2 cm long with either 2.0 (2.1) or four.Zero (4.6) mm depending at the column dimensions and 1cm lengthy for 10mm, 21.2mm & 30mm column identification.
Whether you choose 1 or 2 cm lengthy protect columns is tied to how harsh your cell segment is and the way messy your pattern is. Ideally, that allows you to preserve your chromatography and to avoid growing the machine stress, you have to use the shortest shield column to be had and use the equal identity as the column on every occasion available. Otherwise you have to select the nearest smaller id shield column to be had.
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Question five. What Happens If My Sample Solvent Is Stronger Than My Mobile Phase?

Answer :

We do no longer advise injecting in a stronger solvent as it typically consequences in top distortion, broadening, terrible sensitivity, and shortening of retention instances.

This occurs due to the fact a few analytes will tend to journey too fast through the column, in place of eluting in a symmetrical band.

If you certainly should do that, keep the quantity as small as feasible and ensure the solvents are miscible.

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Question 6. Can I Get A Sharper Peak By Injecting My Sample In A Weaker Injection Solvent?

Answer :

In this state of affairs, the sample is initially concentrated onto the pinnacle of the column and movements via the column in a decent band.
This technique is every now and then used to decrease band broadening for a larger quantity pattern injection.
Keep in mind that your sample additives should be soluble inside the mobile phase as well, so as for this to work.
Question 7. How Much Sample Can I Inject On My Lc Column?

Answer :

Two different sorts are feasible:

Mass overload (too much analyte injected at the column)
Volume overload (an excessive amount of liquid injected on the column)
The chromatograms are really one of a kind in those 2 conditions.

In mass overload, the analyte molecules saturate the silica on the inlet give up of the column which causes the excess molecules to transport forward down the column with out a great deal interplay, lowering in turn the analyte retention time and displaying a "shark fin" top shape (fronting).

Volume overload occurs while the injected pattern volume is massive sufficient to carry analyte molecules through a considerable share of the interstitial extent inside the column and leads to shark fin kind peaks as nicely and later elution times.

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Question 8. What Size Threads Are On The End Fittings Of My Hplc Column?

Answer :

Most fittings in your HPLC and UHPLC systems and columns have 10-32 threads. However, you may locate that fittings and columns from Waters, Rheodyne and SSI (Lab Alliance) have distinctive seating depths.

Question 9. What Is The Internal Diameter Of My Lc Tubing?

Answer :

The inner diameter of HPLC grade stainless-steel tubing is identified by using the color coded band at the pre reduce tubing while HPLC PEEK tubing is likewise colored consistent with its inner diameter.

Typical encountered shades used for 1.16” od HPLC tubing (shade coded band for SS or strong colour for PEEK) are:

Black = 0.004” ID
Red = 0.Half” ID
Yellow = zero.007” ID
Blue/Tan = zero.010” ID
Orange = 0.020” ID
Green = zero.030” ID
Please notice that those colorings may additionally fluctuate depending at the producer mainly with regards to HPLC stainless-steel tubing. Please take a look at together with your tubing provider/manufacturer to affirm tubing ID coloration coding. Please observe that the HPLC chrome steel tubing is available in precut lengths as it is without a doubt impossible to supply clean, clean, bur-free cuts with out the producer’s particular machinery equipment. As for SS in HPLC, it additionally will become extraordinarily tough to supply, in-house, bur-unfastened, ideal cuts in particular when the use of the narrower IDs (less than zero.1/2” ID).

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Question 10. How Much Should I Change My Injection Volume If I Change The Size Of My Column?

Answer :

Optimal injection volumes are without delay associated with the cylinder volume of your column and are, consequently, dependent on the move sectional vicinity (A=π r2) and period (L) of your column. Since that is the case, you may estimate any adjustment from an present method for injection extent.

If you are changing to a one of a kind size ID (with packing cloth and duration remaining the equal), just multiply your modern volume by using the ratio of the radii squared to determine the best volume for your new technique. For example, if you are presently injecting 20 µL on a one hundred fifty x 4.6 mm column after which transfer to a one hundred fifty x three.Zero mm column, you could estimate the adjusted volume through multiplying 20 x (1.52)/(2.32). Your new volume ought to be about 8.5 µL.

Question 11. Can I Get A Sharper Peak By Injecting My Sample In A Weaker Injection Solvent (such As one hundred% Water For Reverse Phase)?

Answer :

In this scenario, the pattern is to begin with concentrated onto the top of the column and actions through the column in a good band. This method, known as “on-column compression” or “point-injected”, is every now and then used to reduce band broadening for a larger quantity sample injection. Keep in thoughts that your pattern additives need to be soluble in mobile section as properly, in order for this to work.

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Question 12. How Do I Address An Increase In Backpressure And Other Signs Of Blockage Or Contamination?

Answer :

Please comply with the guidelines in our weblog “Building up Pressure on HPLC?”. This will assist you discover or affirm the source of pressure. If the column itself is located to be the supply of issue, please observe the column regeneration steps given on our LC Column Cleaning Recommendations web page. To help keep away from destiny occurrences, Restek incredibly recommends the use of guard columns to guard your reversed segment and regular section analytical LC columns from both frit blockage and column contamination.

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Question 13. How Do I Determine Total Column Volume Or Void Volume For Lc?

Answer :

The term “column volume” commonly refers back to the void extent, which represents the extent of cell segment that is among the silica debris. This place is referred to as the interstitial area. You can estimate void volume by using multiplying the whole column extent (pi x radius2 x length) by means of a thing that estimates the everyday packing performance for a selected column type. For absolutely porous columns, the equation for void extent (in mL) is V = (zero.68) pi r2 L, wherein V = column volume in mL, r = column radius in cm, and L = column length in cm. For superficially porous columns, along with our Raptor columns, the element is special and the equation is V = (0.50) pi r2 L.

Void quantity is more commonly predicted experimentally by using injecting a standard containing an analyte that is regarded to have no, or negligible, retention on that specific column segment. A proper example of this for reversed-section HPLC is uracil. One ought to be conscious that this estimation is likewise situation to more column dead extent for the specific device that is getting used, so it may vary slightly.

Question 14. Why Am I Seeing Bleed From My Biphenyl Column On My Uv But Not On My Mass Spec?

Answer :

A small amount of segment bleed is inherent for all phases, which include phenyl phases, and is extremely depending on the size and dimensions of the column. This bleed is typically negligible and does no longer have an effect on retention times, however can be seen, specifically by way of UV detection. It can frequently be reduced after conditioning. Bleed can also be minimized with the aid of using an isocratic elution, a shallower gradient, and/or incorporating a gradient flush between runs.

Question 15. What Should I Use To Analyze Explosives (as Per Epa Method 8330b) By Hplc?

Answer :

While no person LC column can provide baseline separation for all of those analytes blended, the Raptor Biphenyl and Raptor ARC-18 columns from Restek are an incredible preference for primary and confirmation analysis. Fully porous HPLC particles, particularly the Ultra C8 and Ultra Aromax columns, are also an option. Keep in thoughts that an expansion of column stages may additionally provide partial solutions for this method, however Restek has observed these pairs to provide most efficient effects.

Question 16. Is Special Conditioning Needed For The Raptor Biphenyl Column Prior To Its First Use, Or If It Has Been Sitting Idle?

Answer :

For the maximum element, the Raptor Biphenyl column behaves just like another reversed-phase column. However, in certain instances, longer equilibration instances may be wanted. Switching among organic solvents, inclusive of acetonitrile and methanol, may additionally require a 15-20 minute flush in high natural cell phase.

Question 17. How Much Equilibration Time Is Required In Between Gradient Runs On A Raptor Biphenyl Column?

Answer :

Whether you are the use of fully porous silica or SPP silica, a few equilibration time is wanted among runs if you are the usage of a gradient and the amount of time is similar for each sorts of columns. Usually, the equal of 7 column (void) volumes is sufficient unless you are using an ion-pairing approach.

Question 18. What Mobile Phase Solvents Are Compatible With Spp Or Raptor Columns?

Answer :

Any solvent this is typically used for reversed-segment LC will work excellent, consisting of but now not restrained to water, methanol, and acetonitrile.

Chemical Engineering Interview Questions
Question 19. Can I Pump Solvent Through The Raptor Biphenyl Column Backwards To Clean It?

Answer :

Similar to UHPLC columns, it is not recommended to opposite the float for those columns. However, you may still pump via a sequence of solvents, so long as they're miscible.

Question 20. How Much Can I Inject Onto A Raptor Column?

Answer :

Injection volume depends on various of factors such as column dimensions, sample solvents, and evaluation necessities. As is continually an excellent practice with chromatography, try to inject as little as possible and in the same or weaker solvent than your mobile segment.

Question 21. How Do Raptor Arc-18 Columns Differ From Ordinary C18s?

Answer :

The tremendous difference is the ruggedness of the bonded segment. With the ARC-18, any residual silanol companies are shielded and made inert thru steric protection. The end result is a much broader working pH variety of one.Zero–8.Zero. The ARC-18 is specially useful between a pH of one.Zero and three.0, wherein different C18 column phases might also start to degrade below these harsh situations. Like the Raptor Biphenyl column, the desk bound section is bonded to superficially porous silica particles (SPP).

Question 22. How Well Does The Raptor Arc-18 Column Work For Acids And Bases?

Answer :

The ARC-18 presents added retention for charged bases and, in many cases, is favored over a traditional stop-capped C18. For impartial acids, it really works properly and is favored over cease-capped C18 stages, mainly at pH < 3. The ARC-18 additionally works for neutral bases and charged acids, however offers extra advantages and performs nice on the decrease pH ranges.

Question 23. Can The Raptor Arc-18 Column Be Used With one hundred% Aqueous Mobile Phases?

Answer :

No. We advise using the Raptor ARC-18 column with at least five% natural within the cell section. For programs requiring better aqueous content, we endorse the Ultra Aqueous C18 or Pinnacle DB Aqueous C18 columns.

Question 24. How Do We Know That The Raptor Columns Are Rugged?

Answer :

We use frits which might be much less at risk of clogging from sample matrices, and the column packing is less probable to be broken through better pressures that could develop. Added protection of a protect column is likewise available and recommended to extend the lifestyles of the column similarly.

Quality Center (QC) Interview Questions
Question 25. Which Fittings Can Be Used For Uhplc?

Answer :

You can either use the stainless-steel fittings that are just like the ones that come with your UHPLC system or you may use EXP fittings. The EXP fittings may be used up to twenty,000 psi while tightened with a wrench. In any case, constantly make certain you're the usage of a fitting with 0 useless extent so that the high efficiency supplied with the aid of UHPLC isn't always compromised by greater dead quantity.

Question 26. How Do I Tighten My Fittings?

Answer :

Our polymer-based regularly occurring connectors and PEEK union connectors only want to be hand-tightened, whereas any of the stainless steel fittings want to be wrench-tighten.

The EXP fittings can be used either way: They are hand-tightened for deplete to eight,seven-hundred psi or wrench-tightened for burn up to twenty,000 psi. Note that over-tightening reasons galling and will damage the threads. Fittings that want to be wrench-tightened normally require ¼-flip beyond hand tight to attain a leak-tight seal. Unfortunately, there's no everyday torque placing.

Question 27. What Injection Solvent Should I Use For Hilic Separations?

Answer :

The injection solvent need to be as close of a suit as possible to the preliminary cellular section situations, that are high in natural content for HILIC separations. By matching the injection solvent to the preliminary cell phase situations, you get better height shape, extended retention, and better sensitivity.

Chromatography Interview Questions
Question 28. What Kind Of Ph Effects Do I Have To Be Aware Of With Hilic Separations?

Answer :

The effect of pH on analyte price country varies based on each compound’s pKa, to pH results ought to be evaluated carefully in the course of approach improvement. With HILIC strategies, the excessive attention of natural solvent inside the mobile phase raises the pH, and the actual eluent pH can be 1–1.5 gadgets higher than within the aqueous element on my own. The price state of the column itself also can be affected.

For example: in a Raptor HILIC-Si column, the bare silica has a pKa among three.8 and 4.Five, so the cellular phase pH modifications the rate of the silica floor, making it neutral in very acidic situations and ionized (negatively charged) as the pH starts to approach three.Eight and above. For this motive, if your analyte has one or greater protonated amine or quaternary amine agencies, it’s a terrific candidate for evaluation on a Raptor HILIC-Si column.

Question 29. Can I Use Buffers For Hilic Separations? What Kind And What Concentration?

Answer :

Many HILIC separations use a mass spectrometer for the detector, so volatile buffers like ammonium formate and ammonium acetate are very common. However, the excessive organic content of the cell phase can cause buffer salts to precipitate, that could lead to downtime for device preservation. In addition, excessive buffer concentrations can impact chromatography by using lowering analyte retention.

To keep away from these effects, method optimization is required and 10 mM is a good start line for buffer awareness. Both the A and B cell phases should be buffered similarly with a purpose to preserve the ionic power consistent in the course of a gradient for the maximum regular MS detector reaction. Check along with your MS supplier for the maximum buffer attention they propose on your ESI source.