Archive for the ‘Kromatografi’ Category

GC-MS and LC-MS typically use totally different mechanisms for ionisation. In GC-MS the sample is usually ionised directly (Electron Impact, EI), or indirectly (Chemical Ionization, CI) by an electron beam. The high-energy electrons cause the formation of free radical ions. These are ions because they’ve lost an electron, so they have the same mass as the parent, but an odd number of electrons.

In fact the electron beam is often energetic enough to cause substantial fragmentation. The fragments are also free radicals, and form the “fingerprint” that is used in confirmation of identity. This fingerprint is compared with library fingerprints; NIST is probably the most widely used library.

An example might be Aspirin, acetyl-salicylate. By EI in GC-MS, this has a tiny peak at 180amu, corresponding to the molecular ion (free radical; the MWt of acetyl salicylate is 180Da). But the major peaks are all fragments: 120, 138, 92, 43amu, and many others.

In contrast, the spectrum below is acetyl-salicylate measured in LC-MS, using electrospray ionisation, ESI (in negative mode):

Notice that the major ion is now 179amu. This is a pseudomolecular ion, formed by loss of H+. This is quite typical in electrospray: the major ion is formed without fragmentation (unlike EI), and it is formed by loss of H+ in negative mode, or by gain of H+, Na+, or some other ion (ammonium and potassium are quite common) in positive mode. Very often in positive mode, one sees a mixture. Peaks 22amu apart nearly always mean the H+ and Na+ ions of the same thing.

Acetyl salicylate is a very easily fragmented molecule, so two fragments are still visible even under the mild conditions of ESI. These are the ions at 137 and 93amu. Unfortunately ESI collision-induced fragmentations are often not very peak-rich, so they are often not such good “fingerprints” as EI spectra. They can also vary with instrument and conditions.

Notice also the peaks at 225 and 381amu. These are adducts. Unfortunately analytes often associate weakly in the spray-chamber in ESI, and then appear as adducts of two things together. 225amu is probably a formate acid adduct of acetyl salicylate (46+179), and 381 is a sodium dimer (179+179+23; notice there is still a single negative charge). Adducts are most problematic at high concentrations.

 

Download (gift from Dr. Lionel Hill, John Innes Centre, UK) :

1. Modul Training HPLC

2. Modul Training MS

 

UOP Methode 539 mengatur prosedur untuk menentukan komposisi berbagai campuran gas hydrocarbon yang diperoleh dari kilang (refinery gas,  LPG) maupun dari alam (natural gas).  Analisa dengan metode ini  menggunakan Gas Chromatography (GC) dengan multi column yang dioperasikan sacara otomatis dengan menggunakan  valve switching.

Proses pemisahan dan analisa dilakukan dalam 3 rentang, yaitu : 1) C3 – C5 boiling range; 2) Intermediate boiling range (carbon dioxide, ethylene & ethane); 3) Light gases (hydrogen, oxygen + argon, nitrogen, methane & carbon monoxide).

Konfigurasi peralatan:   1)Injection system : spesifikasi Injection System tidak ditentukan secara spesifik; 2) Oven  : spesifikasi Oven tidak dijelaskan secara spesifik. 3) Column, terdiri dari 3 columns yang dihubungkan secara seri. Column 1, untuk menentukan C3 – C5 boiling range. Column 2, untuk menentukan intermediate boiling range (carbon dioxide, ethylene & ethane). Column 3, untuk menentukan light gases (hydrogen, oxygen + argon, nitrogen, methane & carbon monoxide).  4) Control System: Automatic atau manual. 5) Detector: 1 unit thermal conductivity detector dengan spec 20 sqmm/0.1% mol min response (for carbon).  6) Data Aquisition : tidak dijelaskan secara spesifik.

Berikut adalah GC untuk aplikasi UOP 539 dari beberapa vendor.

gc-uop-539

 

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ASTM D-4815 mengatur prosedur untuk menentukan kandungan ethers dan alcohols dalam gasolines dengan menggunakan Gas Chromatography (GC). Secara spesifik, jenis senyawa yang ditentukan meliputi methyl tert-butylether (MTBE), ethyl tert-butylether (ETBE), tert-amylmethylether (TAME), diisopropylether (DIPE), methanol, ethanol, isopropanol, n-propanol, isobutanol, tert-butanol, sec-butanol, n-butanol dan tert-pentanol (tert-amylalcohol).

Rentang pengukuran yang ditangani oleh standard ini adalah Ethers: 0.1 s/d 20.0 mass % dan Alcohols: 0.1 s/d 12.0 mass %.

Konfigurasi GC. Konfigurasi GC yang disyaratkan dalam standard ini adalah: Injection System, bisa berupa Splitting-type injection system, jika menggunakan capillary column atau FID detector. Atau bisa menggunakan on-column injector & autosampler sejauh dapat memenuhi batasan sample size, efisiensi dan detector linearity. Atau bisa juga menggunakan microlitre syringes yang dilengkapi dengan automatic syringe injectors dan liquid sampling valve.

Oven, (Main) oven berisi nonpolar column dan polar column serta valve. Bisa juga dilengkapi auxiliary oven yang digunakan untuk menempatkan polar column dan valve (untuk kasus ini, main oven hanya berisi nonpolar column).

Column, terdiri dari polar column dan nonpolar column. Polar column, digunakan untuk pra-separasi oxygenates terhadap volatile hydrocarbons. Polar column berupa TCPE micro-package column 560 mm, 1.6 mm outside diameter, 0.76 mm inside diameter, SS tube package with 0.14-0.15 g of 20% TCEP on 80% mesh chromosorb P (AW). Nonpolar column atau analytical colum, terbuat dari WCOT Methyl silicone column (capilary), 30-m long, 0.53 mm inside diameter fused silica WCOT column with 2.6 micron film thicness of cross-linked methyl silixane.

Control system, terdiri dari flow control & indicator, pressure regulator dan automatic valve switching & backflushing valve.

Detector, berupa TCD atau FID, dengan spec sensitivity min 2 mm.

Data aquisition, bisa berupa computer, recorder, analog electronic atau secara manual.

Berikut adalah contoh GC untuk aplikasi ASTM D-4815 dari beberapa vendor.

 

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GC, dalam Pandangan Instrument Engineer