Question bank — Chromatography — TLC, column, GC, HPLC (principles)

False — bada $K$ matlab wo stationary phase prefer karta hai, isliye $k$ bada hai, $v=v_m/(1+k)$ chhota hai, aur wo sabse baad mein elute hota hai.

False — polar silica polar molecules ko pakad leti hai, isliye wo baseline ke paas ruk jaate hain aur $R_f$ chhota hota hai.

False — $R_f$ solvent polarity, plate, aur temperature par bhi depend karta hai; ye sirf fixed conditions ke liye reproducible hai, isliye knowns ko saath mein run kiya jaata hai.

False — separation ke liye alag $k$ chahiye; identical $k$ matlab identical average speed, isliye wo kitna bhi lamba column ho, alag nahi honge.

True — stationary phase non-polar (C₁₈) hai, isliye polar molecules barely stick karte hain, polar mobile phase ke saath chale jaate hain, aur jaldi elute ho jaate hain.

True — $k=0$ matlab stuck rehne mein koi time nahi, isliye $v=v_m/(1+0)=v_m$; ye ek unretained marker ka "dead time" mark karta hai.

False — order relative $k$ values se set hota hai; faster flow saare retention times ko chhotaa karta hai lekin sequence (sabse-kam-sticky-pehle) nahi badlata.

False — sugars vaporise hone se pehle char ho jaate hain; GC ko volatile, thermally stable samples chahiye, isliye aap UV-detected HPLC use karoge.

True — $k=K{V}_s/V_m$ $V_s$ ke saath badhta hai, isliye saare $k$ badhte hain aur sab kuch slow ho jaata hai (haalaanki separation improve ho sakti hai).

Pressure separate nahi karta; separation alag $K$/$k$ se aati hai. Pressure sirf solvent ko tiny (3–5 µm) particles ke through force karta hai, jinka large surface area resolution improve karta hai.

Ulta hai — high boiling point matlab wo film par zyada der liquid rehta hai, vapour ke roop mein kam time bitaata hai, bada $k$ hota hai, aur sabse baad mein elute hota hai; kuch "gir" nahi jaata.

Impossible — $R_f$ = spot distance ÷ solvent-front distance, aur ek spot front ko cross nahi kar sakta, isliye $R_f$ hamesha 0 aur 1 ke beech hota hai. 1 se upar ki value measurement error indicate karti hai.

Nahi — ye opposite hain. Normal phase (polar silica) polar analytes ko retain karta hai; reverse phase (non-polar C₁₈) non-polar analytes ko retain karta hai.

Galat — sticky matlab bada $k$, slow velocity, isliye wo sabse baad mein elute hota hai; sabse kam retained (sabse kam polar) component pehle niklta hai.

Zaroori nahi — $K$ concentrations compare karta hai, total amounts nahi; actually kitna fraction stuck hai ye phase volumes $V_s$ aur $V_m$ par bhi depend karta hai (isliye $k$, $K$ nahi, speed set karta hai).

Gravity rise ko oppose karti hai; solvent Capillary Action se chadhta hai — surface tension liquid ko gravity ke against fine silica pores mein kheenchti hai.

Kyunki ratio ye cancel kar deta hai ki aapne plate kitni der run ki; same compound same $R_f$ deta hai chahe front 5 cm chadhha ho ya 8 cm, isliye ye ek reproducible fingerprint ban jaata hai.

Mobile phase hi akela cheez hai jo physically flow kar rahi hai; stationary phase par stuck rehne ke dauran molecule anchored hota hai aur zero forward displacement contribute karta hai, isliye uski average speed uske stuck-time fraction se dilute ho jaati hai.

Gases bahut fast diffuse aur phases ke beech re-equilibrate karti hain, isliye bands narrow rehti hain; kam spreading matlab better-resolved peaks — GC ka core advantage.

Kyunki solvent polarity, plate material, aur temperature sabhi change karte hain ki compound kaise partition karta hai (Distribution / Partition Equilibrium), isliye $R_f$ sirf apne run conditions ke saath meaningful hoti hai.

Fine particles tightly pack hote hain aur flow resist karte hain (higher back-pressure), isliye ek high-pressure pump chahiye taaki solvent usable speed par move karta rahe, saath mein fine particles jo resolution offer karte hain uska fayda bhi mile.

Area poori band par signal accumulate karta hai, isliye ye kitna analyte pass hua uske saath scale karta hai; retention time reflect karta hai compound kaise partition karta hai, jo fixed conditions mein uski identity ka characteristic hai.

Kyunki affinity specific intermolecular forces (H-bonding, shape, functional groups) par depend karti hai, na ki kisi ek "polarity" number par, isliye unke $K$ values — aur hence speeds — phir bhi alag ho sakte hain.

$R_f=1$ — wo solvent front ke saath travel karta hai kyunki stationary phase par koi time nahi bita ($k=0$).

$R_f=0$ — wo itna strongly held hai ($K\to \infty$, $k\to \infty$) ki $v_{\text{analyte}}=v_m/(1+k)\to 0$ aur wo move nahi karta.

$k=K{V}_s/V_m\to 0$ sabke liye, isliye har compound $v_m$ approach karta hai aur kuch bhi separate nahi hota — differential lag create karne ke liye kuch stationary phase zaroori hai.

Mobile-phase polarity change karo (alag solvent) taaki unke $K$ values zyada diverge karein, spots ko alag kar dein; separation quality sirf molecules se fixed nahi hoti.

Nahi — protein vaporisation par decompose ho jaayega, isliye GC use galat padhega ya use destroy kar dega; protein ko solution mein rakhne ke liye HPLC (ya split methods) chahiye.

Ye $v_{\text{analyte}}=v_m$ ban jaata hai; ek unretained "dead-time" marker (jaise solvent front) full mobile speed par travel karta hai aur zero-retention reference define karta hai.

Do-phase tug-of-war collapse ho jaata hai — stable stationary phase ke bina koi differential retention nahi, isliye koi separation nahi hoti (ye incompatible solvents choose karne ka ek real hazard hai).