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草稿:電噴霧游離

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電噴霧(奈米噴霧)游離

電噴霧游離(Electrospray ionization, ESI)是一種質譜分析常用的技術,原理是把高電壓施加在液體上,產生氣溶膠,進而產生離子。ESI 對於分析大分子有很大的幫助,因為它能避免大分子在游離過程中容易斷裂的問題。與其他基質輔助雷射脫附游離法(matrix-assisted laser desorption/ionization, MALDI)不同的是,ESI 能產生多重帶電的離子,如此便能將分析儀的質量範圍往上延伸,以檢測到蛋白質或多肽片段中 kDa–MDa 級別的質量。[1][2]


使用電噴霧游離的質譜法被稱為電噴霧游離質譜法(Electrospray Ionization Mass Spectrometry, ESI-MS),或較少見地被稱為電噴霧質譜法(electrospray mass spectrometry, ES-MS)。ESI 是一種軟性游離技術,因其產生的碎裂 (fragmentation) 非常少。優點為可以觀察到分子離子(或更精確地說,是偽分子離子)。然而,從這類簡單的質譜圖中能獲得的結構資訊有限。這項限制可以透過將 ESI 與串聯質譜法(tandem mass spectrometry, ESI-MS/MS)結合來克服。此外,ESI 另一個重要的優點是,它能將溶液相中的特性保留到氣相中。


電噴霧游離技術(electrospray ionization technique)最早由山下正道 (Masamichi Yamashita)和約翰·費恩(John Fenn)於 1984 年首次報導[3],同年,蘇聯的莉迪亞·蓋爾(Lidia Gall)及其同事也獨立發表了相關研究。不過,莉迪亞·蓋爾的研究工作在西方科學文獻中直到 2008 年翻譯版本發表後,才被廣泛認可。電噴霧游離技術在生物大分子分析上的發展[4],也讓約翰·班奈特·費恩(John Bennett Fenn)與田中耕一(Koichi Tanaka)於 2002 年獲得諾貝爾化學獎。[5]約翰·班奈特·費恩使用過的其中一台原始儀器,目前在美國賓夕法尼亞州費城的科學歷史研究所(Science History Institute)展出。


歷史

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電噴灑游離法正離子模式示意圖:電噴霧在高電壓作用下,泰勒錐體會噴射出液滴。液滴中的溶劑逐漸蒸發,使液滴上的電荷不斷增加。當電荷超過瑞利極限時,液滴會劇烈地分裂,產生帶電(正)離子的串流。

在 1882 年,雷利勳爵(Lord Rayleigh)在理論上估算了液滴能夠承載的最大電荷量,超過這個量時,液滴會噴射出細小的液體射流。這個現象現在被稱為雷利極限(Rayleigh limit)


在 1914 年,約翰·澤勒尼(John Zeleny)發表了關於液滴在玻璃毛細管末端行為的研究,並提出了電噴霧的不同模式證據。[6]威爾遜與泰勒(Wilson and Taylor) 以及諾蘭(Nolan)在 1920 年代探討了電噴霧現象, [7]而馬基(Macky)則在 1931 年進行了相關研究。[8][9]而電噴霧錐(現稱為泰勒錐)由傑弗里·英格拉姆·泰勒爵士(Sir Geoffrey Ingram Taylor)描述。


第一次將電噴霧游離技術與質譜分析結合使用,是在 1968 年由馬爾科姆·多爾(Malcolm Dole)報導的。[10][11] 約翰·班奈特·費恩(John Bennett Fenn)則因於 1980 年代末期發展電噴霧游離質譜法,而在 2002 年榮獲諾貝爾化學獎。[12]

游離機制

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Fenn的首台與單四極桿質譜儀結合的電噴霧游離源

含有目標分析物(濃度通常需要在 10⁻⁶ 至 10⁻⁴ M 之間[13])的液體,會透過電噴霧分散成細小的氣溶膠。[14]由於離子的形成涉及大量溶劑蒸發,也稱為脫溶,電噴霧游離通常使用的溶劑是水與揮發性有機溶劑(如甲醇、乙腈[15])的混合物。為了減少初始液滴的大小,溶液中會添加能提高導電度的化合物,例如醋酸。這些化合物同時也提供了質子來源,以促進離子化過程。對於大量的電噴霧,除了電噴霧熱源的高溫外,若搭配加熱的惰性氣體,如氮氣或二氧化碳的霧化,也能提升效果。[16]氣溶膠接著會透過一根帶有約 3000 伏特電位的毛細管,從而被引入質譜儀的第一級真空區。毛細管加熱可以幫助帶電液滴進一步脫溶。當溶劑從帶電液滴中蒸發後,液滴達到雷利極限後會變得不穩定[17]。此時,隨著液滴尺寸不斷縮小,液滴內部相同電荷產生的靜電斥力超過了維持液滴完整的表面張力,液滴因此發生形變並產生庫倫裂變,也就是液滴「爆裂」成許多更小、更穩定的液滴。這些新液滴會持續脫溶,並經歷進一步的庫倫裂變。裂變過程中,液滴會損失少部分的質量(1.0–2.3%),相對的,也會獲得較多比例的電荷(10–18%)。[18][19]


氣相離子最終生成有兩種主要理論支持:離子蒸發模型(IEM)和電荷殘留模型(CRM)。IEM 認為,當液滴的半徑達到某一臨界值時,液滴表面的電場強度會增強到足以促使溶劑化離子發生場脫附現象[20][21]。CRM 則認為,電噴霧產生的液滴經歷蒸發與裂變的循環,最終產生平均每個液滴僅包含一個、或更少分析物離子的子液滴。[10]當殘留的溶劑分子蒸發後,氣相離子就會形成,分析物會帶著液滴原本攜帶的電荷。


IEM、CRM 和 CEM 示意圖

小離子(如小分子)是透過離子蒸發機制直接或間接地釋放到氣相中[21][22];而較大的離子(例如摺疊蛋白質)[23]則是透過帶電殘留機制形成。[24][25][26]


第三種模型提出了結合帶電殘留與場發射的機制。[27]另一種名為鏈噴射模型(chain ejection model, CEM)則是針對無序高分子(如未摺疊蛋白質)提出的。[28]


質譜觀察到的離子可能是經由添加氫陽離子(H⁺)產生的準分子離子,表示為 [M + H]⁺,或是添加其他陽離子(例如鈉離子 [M + Na]⁺),或者是去除氫原子核 [M − H]⁻。常見的還有多重帶電離子,如 [M + nH]ⁿ⁺。對於大型高分子,可能會有多種電荷狀態,形成特徵性的電荷態包絡圖。這些都是偶電子離子類別:與其他游離源不同,它們並不涉及電子本身的添加或移除。分析物有時可能參與電化學過程,導致對應質譜峰位置的偏移。這種現象可在貴金屬(如銅、銀和金)的直接電噴霧游離中觀察到。[29]


在電噴霧游離(ESI)中,產生小分子氣相離子的效率會根據化合物結構、使用的溶劑以及儀器參數而有所差異。[30]離子化效率的差異可達到百萬倍以上。


衍生技術

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在低流速下運作的電噴霧會產生更小的初始液滴,這有助於提高游離效率。1993 年,Gale 和 Richard D. Smith 報告指出,使用更低流速(甚至到 200 nL/min)時,靈敏度顯著提升。[31]1994 年,兩個研究團隊分別提出了「微電噴霧(microspray)」名稱,用來形容低流速下的電噴霧。Emmett 和 Caprioli 在 300–800 nL/min 的流速條件下,證明了在 HPLC-MS 分析中的性能提升。[32]Wilm 和 Mann 進一步證明,將玻璃毛細管拉製成數微米尺寸,讓其尖端可在約 25 nL/min 的毛細管流速下持續產生電噴霧。[33]這項技術在 1996 年被重新命名為「奈米電噴霧(nanospray)」。[34][35]目前,奈米電噴霧這個名稱也用來指由泵提供低流速的電噴霧,而不僅限於自我供給的電噴霧。雖然對電噴霧、微電噴霧和奈米電噴霧的流速範圍並沒有嚴格定義,相關研究探討了液滴裂變前分析物分配的變化。[36][37]這篇論文比較了三個研究團隊的結果,並測量在不同流速下,訊號強度比值 [Ba²⁺ + Ba⁺]/[BaBr⁺] 的變化。[38][39][40]


冷噴霧游離(cold spray ionization)是一種電噴霧技術,將含有樣品的溶液經由一個小型冷卻毛細管(10–80 °C)進入電場中以產生帶電冷液滴的細霧。[41] 此方法應用於分析一般電噴霧游離無法研究的脆弱分子和客體-主體相互作用。


電噴霧游離(electrospray ionization)也已經能在低至 25 托(torr)的壓力下進行,稱為次環境壓力奈米電噴霧游離(subambient pressure ionization with nanoelectrospray, SPIN),其技術基於 Richard D. Smith 及其同事開發的雙階段離子漏斗介面。[42]SPIN 系統利用離子漏斗協助將離子有效集中並傳送至質譜儀的低壓區,從而提高了靈敏度。奈米電噴霧發射器由一根細小的毛細管構成,孔徑約 1–3 微米。為了確保足夠的導電性,該毛細管通常會鍍上導電材料,例如金。奈米電噴霧游離只需消耗幾微升樣品,並能形成更小的液滴。[43] 在低壓條件下,特別適合用於低流速,因為較小的電噴霧液滴尺寸可以更有效的脫溶並使離子形成。最後,研究人員展示了液相離子向氣相離子轉移、以及經由雙離子漏斗介面進入質譜儀的整體離子化利用效率超過 50% 的成果。[44]


環境電離

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DESI 环境电离源示意图

在環境電離中,離子的形成發生在質譜儀外部,無需樣品前處理。[45][46][47] 電噴霧被廣泛應用於多種環境電離源中,用於產生離子。


解吸電噴霧游離(DESI)是一種環境游離技術,其中溶劑電噴霧會被導向樣品。[48][49]透過對樣品施加電壓,電噴霧被吸引到樣品表面。樣品化合物被溶劑萃取,溶劑隨後再次霧化為帶高電荷的液滴,這些液滴蒸發後形成帶高電荷的離子。游離後,離子進入質譜儀的大氣壓介面。DESI 可在大氣壓下直接對樣品進行游離,幾乎無需樣品前處理。


SESI 环境电离源示意图

萃取電噴霧游離是一種噴霧型的環境遊離方法,使用兩股合併的噴霧,其中一股是由電噴霧產生的。[46]


基於雷射的電噴霧環境遊離是一個兩步驟的過程:脈衝雷射先從樣品上脫附或燒蝕材料,接著材料羽流與電噴霧相互作用以產生離子。[46] 在環境遊離中,樣品材料通常沉積在電噴霧附近的目標物上。雷射從樣品表面脫附或燒蝕材料,並將其噴射到電噴霧中,形成帶高電荷的離子。範例包括電噴霧雷射脫附游離(electrospray laser desorption ionization)、基質輔助雷射脫附電噴霧游離(matrix-assisted laser desorption electrospray ionization)以及雷射燒蝕電噴霧游離(laser ablation electrospray ionization)。

SESI-MS SUPER SESI 与 Thermo Fisher Scientific-Orbitrap 联用

靜電噴霧游離(ESTASI)用於分析位於平面或多孔表面上的樣品,或微通道內的樣品。樣品區域上會放置含有分析物的液滴,並施加脈衝高電壓。當靜電壓力大於表面張力時,液滴和離子就會被噴出。

二次電噴霧游離(SESI)是一種噴霧型的環境遊離方法,其中電噴霧產生帶電離子。當這些離子與氣相中的蒸氣分子碰撞時,會使蒸氣分子帶上電荷。[50][51]


在紙噴霧游離(paper spray ionization)中,樣品會被塗佈在一片紙上,並加上溶劑。當對紙片施加高電壓時,就會產生離子。

應用

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LTQ 质谱仪上的电喷雾接口的外部。

電噴灑技術被用來研究蛋白質的摺疊。[52][53][54]


液相層析-質譜儀(LC-MS)

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電噴霧電離(ESI)是液相層析質譜(LC-MS)分析時首選的離子化來源。這種分析可以在線上直接進行,將從LC管柱洗脫出的液體直接導入電噴霧,或者離線進行,先收集分段,再使用傳統的奈米電噴霧-質譜(nanoESI-MS)分析。

在ESI-MS的多種操作參數中,針對蛋白質分析,[55]電噴霧電壓已被確立為在ESI LC-MS梯度洗脫過程中應特別注意的重要參數[56]

此外,針對不同溶劑組成[57](例如TFA[58]或醋酸銨[59]、超增效試劑[[60][61][62]、衍生基團[63])或噴霧條件[64],對電噴霧-LCMS譜圖及/或奈米ESI-MS譜圖的影響,也已有相當多的研究。[65]

毛細管電泳-質譜儀(CE-MS)

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毛細管電泳-質譜法由太平洋西北國家實驗室的理察·史密斯 (Richard D. Smith)及其同事開發並獲得專利的 ESI 接口實現,並被證明可廣泛應用於分析非常小的生物和化學化合物混合物,甚至擴展到單個生物細胞。

非共價鍵氣相相互作用

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電噴霧游離(ESI)也被用於研究氣相中的非共價鍵相互作用。電噴霧過程被認為能將液相中的非共價鍵複合物轉移到氣相,且不會破壞這些非共價鍵相互作用。然而,在利用 ESI-MS 或奈米電噴霧質譜(nanoESI-MS)研究配體-受質複合物時,已發現存在一些問題[19][66],例如非特異性相互作用[67]。其中一個有趣的例子是研究酵素與作為酵素抑制劑的藥物之間的相互作用。[68][69][70] 例如,針對 STAT6 與抑制劑之間的競爭研究[70][71][72],就利用 ESI 作為篩選潛在新藥物候選物的方法。


電噴霧游離甚至可用於研究分子量超過 1 MDa 的蛋白質複合物。[73][13]


參見

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参考

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