Mass Spectrometry

Background Mass spectrometry (Mass Spec or MS) uses high energy electrons to break a molecule into fragments. -이온화, 분자의 분해된 조각 Separation and analysis of the fragments provides information about: Molecular weight Structure -질량대 전하비(mass-to-charge rato) -분자질량, 분자구조 예측

기기 구성

Molecular Ions give us the molecular mass Dislodges an electron Electron Impact M -e• 2e-• M+• Chemical Ionization M H+ [M+H]+ Weighs one more than MW

Ionization source

Fragmentation or B+ + A· EI [M·]+ A+ + B· (neutral) Better carbocation wins and predominates (“Stevenson’s Rule”) CI [M+H]+ PH+ + N (neutral) The “Even Electron Rule” dictates that even (non-radical) ions will not fragment to give two radicals (pos• + neutral•) (CI)

Fragmentation

Background The impact of a stream of high energy electrons causes the molecule to lose an electron forming a radical cation. A species with a positive charge and one unpaired electron Molecular ion (M+) m/z = 16

Background The impact of the stream of high energy electrons can also break the molecule or the radical cation into fragments. m/z = 15

Background Molecular ion (parent ion): The radical cation corresponding to the mass of the original molecule The molecular ion is usually the highest mass in the spectrum Some exceptions w/specific isotopes Some molecular ion peaks are absent.

Isotope Patterns in Ion Clusters Here are two molecular ions of nearly the same m/z. One of them is “carbon-rich”, and has a larger number of 13C’s The other, presumably has proportionately, more heteroatoms C24H50 C12H22O11

Background Mass spectrum of ethanol (MW = 46) M+ SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology, 11/1/09)

Why is this Important? A rule of thumb, made possible by knowing the isotopic abundance is that the number of C in a formula is given by: N= All 12C 1 13C C10 C100 2 13C From this, it is clear that for large or macromolecules, there will be practically no population having all 12C or even only 113C

The “Nitrogen Rule” Molecules containing atoms limited to C,H,O,N,S,X,P of even-numbered molecular weight contain either NO nitrogen or an even number of N This is true as well for radicals as well. Not true for pre-charged, e.g. quats, (rule inverts) or radical cations. In the case of Chemical Ionization, where [M+H]+ is observed, need to subtract 1, then apply nitrogen rule. Example, if we know a compound is free of nitrogen and gives an ion at m/z=201, then that peak cannot be the molecular ion.

Background The cations that are formed are separated by magnetic deflection.

자기장 부채꼴 분석기(magnetic sector analyzer) : 단일초점분석기   영구자석 또는 전자석을 이용하여 이온화 장치에서 나온 이온살이 180°, 90°또는 60°되는 원형 통로를 지나가도록 함 → 90°형 기기를 보여줌 → 전자충격에 의해 생긴 이온은 가속 → 슬릿 B를 통해 금속 분석기로 들어감 → 금속 분석관의 내부압력은 약 10-7torr를 유지 → ⓐ 자석의 자기장의 세기를 변화 ⓑ 슬릿 A와 B 간의 가속 전위를 변화시키면서 → 질량이 다른 이온들이 출구 슬릿 위의 초점에 모일 수 있도록 주사 → 출구 슬릿을 통과한 이온은 수집전극에 도달하여 이온전류를 냄 슬릿 B에서 나오는 질량 m, 전하 z인 이온의 병진 즉, 운동에너지 KE는 → ⓐ V는 A와 B사이의 전압, ⓑ v는 가속된 이온의 속도, ⓒ e는 전자의 전하량(1.60 x 10-19C) → 같은 전하, z를 갖는 모든 이온들은 가속된 후 질량에 관계없이 같은 운동에너지를 가짐 → 이 가정은 단지 대략적으로만 사실이다 → ∵ 가속되기 전 이온들은 통계적인 속도분포(속도와 방향)를 가지므로 가속된 후에도 이온들은 비슷한 분포를 하기 때문 → 이런 가정에 대한 문제점은 이중-초점 기기를 설명한 다음 절에서 설명 → 슬릿을 빠져 나온 모든 이온들은 거의 같은 운동에너지를 가지므로 무거운 이온은 자기장내에서 낮은 속도로 움직임

* 자기장에서 일정한 질량과 전하를 갖는 이온이 움직이는 통로는 이 이온에 작용하는 두 힘사이의 균형에 따라 결정   ㉠ 자기력 FM은 → B는 자기장의 세기 ㉡ 균형을 나타내는 원심력 FC는    → r은 자기장 부채꼴 곡면 반지름 → 한 이온이 원형 통로를 통과하여 수집관에 도달하기 위해서는 FM과 FC가 같아야함 → 다시 정리하여 → 식 20-8을 식 20-4에 대입하여 정리하면 → 식 20-9는 세 변수(B, V 또는 r)중 두 변수를 일정하게 유지하면서 한 변수를 변화시키면서 질량스펙트럼을 얻을 수 있음을 보여줌 ㉮ 대부분의 최신 부채꼴 질량분석계는 V와 r을 일정하게 유지하면서 자석에 통하는 전류, 즉 B를 변화시키는 전자석을 가지고 있음 ㉯ 광사진 검출법을 이용하는 부채꼴 분석계에서는 B와 V를 일정하게 유지하면서, r을 변화시킴

사중극자 질량 분석기(quadrupole mass spectrometer)   * 자기장 부채꼴 기기보다 ⓐ 부피가 작고 ⓑ 값도 싸며 ⓒ 튼튼하다. * 탁상용 질량분석기에 사용 : 현재 가장 많이 사용되는 질량 분석기 * 장점: 짧은 주사시간(<100ms). → 크로마토그래피의 봉우리를 실시간에 주사하는데 특히 유용 비행시간(time-of-flight, TOF) 질량분석기 전자, 이차이온 또는 레이저 광자의 짧은 펄스로 시료에 충격을 주면 주기적으로 양이온이 생성 (이온화 펄스) → 생성된 이온들은 103∼104V의 전기장 펄스에 의해 아무런 장이 걸리지 않는 관(field-free drift tube) 속으로 가속 → 질량차이로 인한 이온의 분리는 이온이 관의 끝에 놓여져 있는 검출기로 이동하는 동안 일어남 → 관에 들어온 모든 이온은 같은 운동에너지를 가짐 → ∴ 관속에서 이들의 속도는 이들의 질량의 제곱근에 역비례(식 20-4) → 보통 비행시간은 1∼50μs(정교하고 빠른 전자부품 필요) * 다른 종류의 질량분석관에 비교한 여러 장점들 ⓐ 기기가 단순하고, ⓑ 튼튼하며, ⓒ 이온화 장치를 가까이 설치하기 쉬우며, ⓓ 사실상 무제한의 넓은 질량범위 등 * 단점 ⓐ 제한된 분리능 ⓑ 제한된 감도, 재현성과 질량확인 용이성이 떨어짐 → 몇몇의 시간-비행 기기가 시판되고 있지만 자기장이나 사중극자 질량분석계보다 널리 사용되지 않음

Background Only cations are detected. Radicals are “invisible” in MS. The amount of deflection observed depends on the mass to charge ratio (m/z). Most cations formed have a charge of +1 so the amount of deflection observed is usually dependent on the mass of the ion.

Background The resulting mass spectrum is a graph of the mass of each cation vs. its relative abundance. The peaks are assigned an abundance as a percentage of the base peak. the most intense peak in the spectrum The base peak is not necessarily the same as the parent ion peak.

Background The mass spectrum of ethanol base peak M+ SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology, 11/1/09)

Background Most elements occur naturally as a mixture of isotopes.(동위원소) The presence of significant amounts of heavier isotopes leads to small peaks that have masses that are higher than the parent ion peak. M+1 = a peak that is one mass unit higher than M+ M+2 = a peak that is two mass units higher than M+

Easily Recognized Elements in MS Nitrogen:(홀수) Odd number of N = odd MW SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology, 11/2/09)

Easily Recognized Elements in MS Bromine: M+ ~ M+2 (50.5% 79Br/49.5% 81Br) 2-bromopropane M+ ~ M+2 SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology, 11/1/09)

Easily Recognized Elements in MS Chlorine: M+2 is ~ 1/3 as large as M+ M+ M+2 SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology, 11/2/09)

Easily Recognized Elements in MS Sulfur: M+2 larger than usual (4% of M+) M+ Unusually large M+2 SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology, 11/1/09)

Easily Recognized Elements in MS Iodine I+ at 127 Large gap M+ Large gap I+ SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology, 11/2/09)

Fragmentation Patterns The impact of the stream of high energy electrons often breaks the molecule into fragments, commonly a cation and a radical. Bonds break to give the most stable cation. Stability of the radical is less important.

Fragmentation Patterns Alkanes Fragmentation often splits off simple alkyl groups: Loss of methyl M+ - 15 Loss of ethyl M+ - 29 Loss of propyl M+ - 43 Loss of butyl M+ - 57 Branched alkanes tend to fragment forming the most stable carbocations.

Fragmentation Patterns Mass spectrum of 2-methylpentane

Fragmentation Patterns Alkenes: Fragmentation typically forms resonance stabilized allylic carbocations

Fragmentation Patterns Aromatics: Fragment at the benzylic carbon, forming a resonance stabilized benzylic carbocation (which rearranges to the tropylium ion) M+

Fragmentation Patterns Aromatics may also have a peak at m/z = 77 for the benzene ring. 77 M+ = 123

Fragmentation Patterns Alcohols Fragment easily resulting in very small or missing parent ion peak May lose hydroxyl radical or water M+ - 17 or M+ - 18 Commonly lose an alkyl group attached to the carbinol carbon forming an oxonium ion. 1o alcohol usually has prominent peak at m/z = 31 corresponding to H2C=OH+

Fragmentation Patterns MS for 1-propanol M+-18 M+ SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology, 11/28/09)

Fragmentation Patterns Amines Odd M+ (assuming an odd number of nitrogens are present) a-cleavage dominates forming an iminium ion

Fragmentation Patterns

Fragmentation Patterns Ethers a-cleavage forming oxonium ion Loss of alkyl group forming oxonium ion Loss of alkyl group forming a carbocation

Fragmentation Patterns MS of diethylether (CH3CH2OCH2CH3)

Fragmentation Patterns Aldehydes (RCHO) Fragmentation may form acylium ion Common fragments: M+ - 1 for M+ - 29 for

Fragmentation Patterns MS for hydrocinnamaldehyde 91 M+ = 134 105 SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology, 11/28/09)

Fragmentation Patterns Ketones Fragmentation leads to formation of acylium ion: Loss of R forming Loss of R’ forming

Fragmentation Patterns MS for 2-pentanone M+ SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology, 11/28/09)

Fragmentation Patterns Esters (RCO2R’) Common fragmentation patterns include: Loss of OR’ peak at M+ - OR’ Loss of R’ peak at M+ - R’

Frgamentation Patterns 105 77 M+ = 136 SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology, 11/28/09)

Rule of Thirteen The “Rule of Thirteen” can be used to identify possible molecular formulas for an unknown hydrocarbon, CnHm. Step 1: n = M+/13 (integer only, use remainder in step 2) Step 2: m = n + remainder from step 1

Rule of Thirteen Example: The formula for a hydrocarbon with M+ =106 can be found: Step 1: n = 106/13 = 8 (R = 2) Step 2: m = 8 + 2 = 10 Formula: C8H10

Rule of Thirteen If a heteroatom is present, Subtract the mass of each heteroatom from the MW Calculate the formula for the corresponding hydrocarbon Add the heteroatoms to the formula

Rule of Thirteen Example: A compound with a molecular ion peak at m/z = 102 has a strong peak at 1739 cm-1 in its IR spectrum. Determine its molecular formula.

GC-Mass Spec: Experiment 23 Mass Spec can be combined with gas chromatography to analyze mixtures of compounds. GC separates the components of the mixture. Each component is analyzed by the Mass Spectrometer.

GC-Mass Spec: Experiment 23 Assignment: Observe the GC-mass spec experiment Record experimental conditions Analyze the mass spectrum of each component of your mixture: Parent ion peak? Heteroatoms apparent from spectrum? A minimum of 1 or two significant fragments and their structures

GC-Mass Spec: Experiment 23 Assignment (cont.): Using the Mass Spec data, retention times, and boiling points, identify the components of your mixture. Write three paragraphs (one per compound) summarizing and interpreting all data. See your data sheet for more details.