铵根-化学电离质谱法(NH4+ CI-MS)：高选择性，无碎片‘软’电离VOC

TOFWERK’s Vocus chemical ionization TOF mass spectrometers can be operated with multiple reagent ion chemistries to target different classes of compounds. One especially powerful technique uses ammonium-adduction-molecule chemistry for selective, soft ionization of volatile organic compounds (VOCs).

为更精确检测不同种类挥发性有机物（VOCs）, TOFWERK Vocus化学电离质谱仪可在多种母离子模式下运行。本文着重介绍其中一项高选择性，更‘软’的电离模式：铵根加成模式。

Basic Principles

基本原理

A DC plasma ion source generates high concentrations of NH4+reagent ions, which are mixed with target analyte molecules in the Vocus ion-molecule reaction chamber (IMR). Ligand switching reactions result in analyte-NH4+adducts without fragmentation (Figure 1). The bright ion source and efficient Vocus reactor are coupled to a TOF mass analyzer, yielding extremely low detection limits for even sub-second averaging times.

源自等离子体离子源的高浓高纯铵根离子在Vocus分子离子反应区（IMR）内与待测物分子发生有效碰撞。如图1所示，铵根母离子与待测物分子进行配体交换，从而达到无碎片的超‘软’电离效果。高能效的离子源，高效率的Vocus IMR和高分辨率的飞行时间检测器协同实现了在亚秒时间范围内的超低检测限。

Figure 1 NH4+ reagent ions are generated in a plasma source and ionize analyte molecules in the Vocus reactor.

图1 等离子源产生的铵根母离子与目标物分子在Vocus 反应区内发生碰撞继而电离。

Figure 2 Efficient ammonium-adduct formation creates a clean mass spectrum. Here spurious proton-transfer reactions involving the target analytes acetone, acetonitrile, and methyl ethyl ketone account for less than 1% of observed signals.

图2 高效铵根加成反应生成更容易解读的质谱谱图。丙酮，乙腈和丁酮因质子转移反应而形成的‘副’信号[M]H+强度不到铵根加成的产物离子[M]NH4+的1%。

Ensuring that only one type of ion-molecular reaction occurs in the IMR is important for making the spectrum easily interpretable. Adducts form efficiently in the Vocus IMR under low-energy collision conditions, and spurious ion-molecule reactions (e.g., proton transfer) that could complicate the mass spectrum are minimized (Figure 2).

确保Vocus IMR中电离反应过程中母离子的单一性是让仪器信号易于解谱的关键之一，这意味着只有同一个类型的电离反应发生。将Vocus IMR调控在一个低能级碰撞的运行模式，既有利于铵根加成产物的形成，也能最大程度的降低其他电离反应（比如质子转移反应）带来的影响。

Operating the IMR at low pressures improves the ability to control the energy of reagent ions and subsequent ionization processes impacting water vapor dependence, absolute sensitivity, and other characteristics. Electrostatic fields across the length of the reactor help control cluster distribution, and water vapor is added from the ion source to buffer against ambient water vapor changes. As when the Vocus CI mass spectrometer is used for proton transfer reaction ionization (PTR, H3O+ reagent ions), instrument response has negligible dependence on ambient humidity, greatly reducing instrument sensitivity to changes in environmental conditions (Figure 3).

与传统CIMS模式相比，让Vocus IMR内部压强保持在较低条件下可以提高对试剂离子能量的调控能力、降低电离过程样品对湿度的依赖性、提升仪器的灵敏度等。加载在IMR上的纵向静电场有助于控制母离子离子簇分布；离子源中加入水蒸气以缓冲实际样品湿度对测量结果的影响。类似于Vocus CI质谱仪在质子转移反应电离(PTR H3O+离子)模式下，使用铵根母离子的仪器响应对环境湿度的依赖性几乎可以忽略不计，大大降低了Vocus仪器对环境条件变化的敏感性(图3)。

The same Vocus ion source and IMR are used for NH4+adduct and PTR mass spectrometry, allowing fast, real-time switching between NH4+,H3O+ and other reagent ions. As shown in Figure 4, the switching is automated, reproducible and stable.

一套硬件，两种模式：无需任何硬件变动，Vocus CI质谱仪可在铵根离子NH4+、PTR H3O+和其他母离子间进行快速自动切换。如图4所示，Vocus质谱仪在每分钟一次的频率进行试剂离子切换，同时稳定的输出数据。

Benefits of NH4+ Adduct Chemical Ionization Mass Spectrometry (CI-MS)

铵根NH4+化学电离质谱（CI-MS）的若干优点

Proton-transfer-reaction (PTR) mass spectrometry is a powerful and widely used technique, yet it has drawbacks for some analyses. Chief among these is a high degree of fragmentation of certain functional groups, especially alcohols, peroxides, esters, and other highly oxidized molecules. NH4+-adduct ionization is much lower energy, or “softer,” than proton-transfer-reaction, producing mass spectra with less fragmentation, especially for key functional groups. Figure 5 shows Vocus CI mass spectra of the terpene aldehyde 2,6,10-trimethylundec-9-enal (farenal, MW210 Da, fragrance compound), acquired with H3O+ and NH4+reagent ions. When ionized by H3O+, farenal fragments by pathways common to PTR-MS, including dehydration of the aldehyde group. In contrast, thefarenal-NH4+ adduct is the dominant peak in the NH4+mode spectrum, with negligible fragmentation.

Figure 3 Vocus NH4+ CIMSs ensitivities remain constant across a wide range of ambient humidity. The datapoints for (H2O)NH4+ and (Acetonitrile)NH4+overlap.

图3 Vocus NH4+ CIMS的仪器灵敏度受样品湿度影响极小。注意：(H2O)NH4+和乙腈铵根加成产物的信号数据点有重叠。

Figure 4 Automated switching between NH4+and O2+ reagent ions. A fixed calibration gas wasmeasured for several hours. The reagent ion was switched every minute between NH4+ and O2+ reagent ions, allowing the real-time measurement of oxygenated species that typically fragment inconventional PTR-MS, as well as standard species such as BTX and acetone. The full time-series shows the average of one-minute data, and the insert shows 1-second data for a select period.

图4在NH4+和O2+试剂离子自动切换。该仪器以一秒的数据采集频率测量某固定标气几个小时。仪器每分钟在NH4+和O2+试剂离子模式间来回切换，可实时测量通常在传统PTR-MS中碎片率较高的含氧物种，同时涵盖BTX和丙酮等常见VOC。完整的时间序列显示的是一分钟的平均数据，而插图显示的是选定期间内的1秒数据。

质子转移反应(PTR)质谱法是一种功能强大、适用性及应用广泛的技术，但它在某些应用场景中也有不足。最主要的是在伴随某些官能团的物质电离过程会产生较高的碎片率，如醇类、过氧化物、酯类和其他氧化态高的分子。与质子转移反应相比，NH4+加成电离过程涉及到的能量要低得多，或者说 "软 "得多，电离上述的特殊官能团过程中产生的分子碎片较少。图5展示了用H3O+和NH4+试剂离子分别获得的萜醛类物质：2,6,10-三甲基-9-烯-十一醛（farenal，分子量210，芳香物质）的质谱谱图。当被H3O+电离时，farenal会遇到PTR模式常见的碎片途径，包括醛基脱水等。相反，farenal- NH4+加成产物分子是NH4+模式谱图中的主峰，碎片基本可忽略不计。

Figure 5 Comparison of Vocus CI mass spectra of farenal measured with H3O+ (top) and NH4+ionization (bottom). The structure of farenal is shown in the top panel.

图5 farenal分别在H3O+（上）和NH4+（下）模式下的Vocus CI质谱谱图。上图附有Farenal的分子结构式。

Additionally, while conventional PTR is a “broadband” technique that ionizes a wide variety of volatile organic compounds (VOCs), NH4+ is a selective technique, meaning that it is far more sensitive to certain groups of VOCs than others. This is especially helpful in situations where NH4+chemical ionization has a high sensitivity for a target analyte that is found in low abundance in a complex matrix with high concentrations of non-target VOCs. For example, crude oil contains high concentrations of aromatics (BTX), which rapidly deplete the H3O+ reagent ions used for PTR chemical ionization. The binding energy of the NH4+-aromatic adduct is weaker, allowing other VOCs in petroleum to be measured while small aromatics are ignored.

此外，传统PTR是一种 "宽频 "电离技术，可电离分析多种类的VOC，而NH4+的电离物种相对而言更有选择性，这意味着它对某些类别的VOCs更敏感。这在分析复杂样品中某些特定痕量VOCs目标物的应用场景中尤其重要：利用NH4+化学电离的选择性来实现对复杂样品中高丰度非目标分子的‘歧视’效应，同时保持对目标分析物具有较高灵敏度。例如，原油样品中含有高浓度的芳烃(BTX)，会大量消耗PTR化学电离的H3O+母离子，从而影响到分析的准确性。而铵根跟芳烃加合物的结合能较弱，相应灵敏度低，从而更好测量石油中的其他种类VOCs，芳烃在谱图中基本上可被忽略。

Collision-Induced Dissociation: Direct, Fast Quantification of Complex Substances

碰撞诱导解离：对复杂样品的直接快速定量分析

A major challenge of online mass spectrometry is the quantification of measured compounds (the determination of sensitivity). Often, dozens to hundreds of compounds are measured, and calibration of each with a cylinder or other calibration system is far too time-intensive and expensive to do practically. Many species of interest are difficult or impossible to purchase commercially or synthesize in significant amounts; others react or decompose in conventional calibration systems. Collision-induced dissociation is a method that overcomes these challenges by determining the sensitivity to all species in a sample of interest directly, simultaneously, and in near real-time. Collision-induced dissociation methods are often used in MS-MS to identify compounds based on their fragmentation. In contrast, when used with NH4+chemical ionization, low-energy CID is used to dissociate the NH4+ion adduct from the VOC molecule, rather than fragmenting the VOC.

在线质谱分析过程中的一个主要挑战是对检测到化合物信号进行量化（也就是要确定其灵敏度）。通常，一般的应用需要测量几十到几百种化合物，而用标气钢瓶或其他校准系统对每一种化合物进行一一校准既费时又费力，成本也昂贵。此外，许多物种很难或完全不能通过商业途径来购买或合成；其他物种则可能在校准系统中发生副反应或分解。碰撞诱导解离(CID)通过直接、近乎实时地同时测定样品中所有物种的灵敏度来克服上述的这些挑战。碰撞诱导解离方法通常用于串联式质谱（MS-MS）中，根据化合物的碎片和比值进行物种鉴定。当与NH4+化学电离一起使用时，低能CID是将VOC分子从其与铵根加成的物种解离出来，而不是让VOC碎片化。

In adduct-based CI-MS, the sensitivity is related to the collision frequency and the transmission of the adduct to the detector. The collision frequency is determined by the instrument temperature and pressure settings, given fixed sampling flow. The transmission of the adduct depends partly on the binding energy of the reagent ion to the VOC analyte molecule:weakly bound clusters dissociate easily, are transmitted less efficiently to the detector, and are therefore detected less sensitively. The binding energy has a unique value for each VOC. Binding energies can be calculated, but these calculations are computationally expensive and must be done by an expert. It is easier to empirically measure the binding energies. This is done by scanning through a range of voltages applied to ion-optic electrodes near the exit of the Vocus IMR. When the voltage is higher, ion energies are higher, and adducts dissociate. The stability of a particular detected VOC-adduct ion can be parameterized by the fraction of adducts that dissociate at each voltage(Figure 6).

在加成电离反应原理的CI-MS质谱法中，灵敏度与碰撞频率和加成产物在仪器内部传输效率有关。在采样流量固定的前提下，碰撞频率由IMR内部的温度和压力设置决定。而加成产物的传递效率则某种程度上取决于母离子与目标VOC分子的结合能：结合力较弱的分子簇更容易解离，也更有可能在传输过程中‘丢失’信号，也就是降低了下游检测器的传输效率，因此检测的灵敏度也较低。每个VOC的结合能都有一个特定值，可模型计算出结合能的具体数值。但这些计算过程耗时，成本相对昂贵，且对计算者的经验依赖较高。通过简单实验测量出结合能则比较实际。这是通过扫描施加在 Vocus IMR 出口附近的电子聚焦棱镜电极上的一系列电压来实现的。当电压差较高时，给予离子的能量较高，加成物就会解离。特定VOC-加成产物离子的稳定性可以通过在每套电压设定下解离掉的加成产物的百分比例来确定（图 6）。

Figure 6 Basic principle of sensitivity determined by collision-induced dissociation. High-sensitivity compounds form stable adducts with large binding energy, which can survive high-ion-energy conditions in the instrument. Low-sensitivity compounds dissociate at lower energies. The sensitivity can therefore be parameterized by the voltage.

图6 由碰撞诱导解离确定灵敏度的基本原理。高灵敏度化合物会形成更为稳定的加成产物，结合能大，能在仪器中的高离子能量设定下存活。相对低灵敏度的加成产物在较低的能量下就已经发生解离。因此，灵敏度可以通过扫描电压的数值来进行量化。

Once the binding energy of all detected VOC-adducts is determined, it can be converted to sensitivity by interpolating from the sensitivities and binding energies of a small number of compounds from a separate, single-time measurement of a standard cylinder.

一旦确定了所有检测到的待测物-铵根加成产物的结合能，就可以基于从标准样品气瓶的少量标气物种的灵敏度和结合能进行内插，将结合能相应的转化为灵敏度。

Example Applications of NH4+-Adduct CI-MS

铵根加成化学电离质谱的应用案例

NH4+-adduct CI-MS is ideal for a wide range of applications. Here we show examples from several disparate fields that demonstrate the advantages of NH4+ CI-MS for soft, sensitive and selective measurement of a target group of species, and how synergies with PTR-MS expand the measurement capabilities of a Vocus chemical ionization mass spectrometer.

在这里，我们展示了NH4+铵根加成CI-MS在多个不同应用领域的具体案例，这些数据都证明NH4+加成CI-MS的‘软’电离、高灵敏度和选择性在测量特定目标分析物上与生俱来的独特优势。同时与PTR-MS的协同效应也大幅扩展了Vocus化学电离质谱仪的测量能力。

Security

公共安全

Peroxide-based explosives are of particular interest to the law enforcement community. These highly oxidized compounds fragment easily when detected with conventional PTR-MS but are detected sensitively and selectively with NH4+-adduct CI-MS. Triacetone triperoxide (TATP) is one such compound. Trace TATP emitted from a canine-training aid carried by an individual can be detected in real time at parts-per-trillion level (Figure 7).

在安全领域里，因某些过氧化物爆炸物不含氮元素，其精确检测一直是执法部门的一个难点。这些氧化态高的化合物在传统PTR-MS检测模式下很容易碎裂，而采用铵根加成化学电离模式则能被高灵敏度和高选择性地检测到。三过氧化三丙酮（TATP）就是这样一种化合物。图7展示了铵根加成CIMS可以对犬类训练辅助工具中释放出的微量TATP在ppt的痕量级别上进行秒级检测（图7）。

Explosives are difficult to quantify because of the difficulty of introducing them into conventional calibration systems. In Figure 8, trace amounts of hexamethylene triperoxide diamine (HMTD), a low-vapor-pressureexplosive, are precisely quantified using the collision-induced-dissociationmethod.

爆炸物很难量化，因为很难将其引入校准系统中。在图8中，利用上述CID技术精确量化了检测到微量六亚甲基三过氧化物二胺(HMTD)信号。值得注意的是，HMTD是一种低蒸气压爆炸物。

Figure 7 An individual carrying a small sample of TATP walked twice past a window with the NH4+ CIMS instrument inlet. Trace levels of TATP were detected sensitively.

图7 一名携带极少量TATP样品的志愿者两次走过铵根CI-MS仪器进样口所在窗口。痕量浓度的TATP信号被近乎实时的准确检测到，铵根加成电离法的高选择性大大降低了检测的假阳性。

Figure 8 A sample of HMTD was opened after approximately 70 seconds and measured using NH4+-adduct CI-MS. Use of collision-induced-dissociation to quantify the instrument sensitivity to this compound determined a 1-second limit-of-detection of less than 1 pptv.

图8在打开HMTD样品大约70秒后，铵根加成CI-MS质谱仪开始检测到少量的HMTD信号。通过碰撞诱导解离来量化Vocus 化学电离仪器对HMTD的灵敏度，从而确定1秒的检测限优于1pptv。

Breath

实时呼出气体检测

Using NH4+ reagent ions, the Vocus CI mass spectrometer can directly measure low-concentration metabolites and other compounds in breath with reduced fragmentation relative to PTR. Figure 9 shows Vocus CI-MS measurements of individual breaths with H3O+ (PTR) and NH4+ reagent ions. The test subject ingested a eucalyptol capsule (molecular formula C10H18O), and the exhaled metabolites were monitored in real time. The reduced fragmentation of the NH4+spectra makes the technique better suited for untargeted analysis.

使用NH4+试剂离子，Vocus CI质谱仪可以直接测量呼吸中低浓度的代谢物和其他化合物，且相对于PTR模式而言，电离过程中碎片率大大降低。图9中展示了Vocus CI-MS对某志愿者呼出气体分别在H3O+ (PTR)和NH4+试剂离子模式下的测量结果。志愿者在口服桉叶胶囊之后（分子式C10H18O），实时监测到的呼出代谢物。由于NH4+谱图中分子碎片大幅减少，该技术也适合于非靶向分析。

Figure 9 Exhaled breath containing eucalyptol and metabolites. Left: two breaths measured with H3O+ reagent ions. Right: additional breaths measured with NH4+reagent ions, showing reduced fragmentation of monoterpenes and more selective measurement of oxygenated compounds.

图9 含有桉树醇和代谢物的呼气样品检测结果。左：用H3O+试剂离子测量的两次呼吸。右边：用NH4+试剂离子测量的呼气结果，单萜物质的碎片大幅减少，也使得含氧化合物的测量更具选择性。

Environment

环境监测

PTR-MS is an essential tool in atmospheric chemistry. The use of NH4+-adduct CI-MS enhances field and laboratory measurements by capturing highly-oxidized molecules important for the formation of fine particulates. An example of measurements of products resulting from ozone-initiated oxidation of α-pinene in a flow tube is shown in Figure 10. The figure shows a binding-energy scan of oxidation products containing up to seven oxygen atoms.

PTR-MS是大气化学领域的一类重要工具。而NH4+铵根加成CI-MS可通过捕获对新粒子形成过程很重要的高氧化态大分子来增强外场和实验室测量数据的维度和广度。图10所示为流动管实验中α-蒎烯氧化过程产生的光化学产物的测量实例。该图显示了含有多达7个氧原子的光化学产物的结合能扫描结果。

Most oxygenated adducts are more strongly bound than simple ketones, because more oxygen and polar functional groups leads to stronger binding. In general, oxidized organic molecules generally form strong, stable adducts. The adduct formation reactions are fast, so compounds are ionized at near the collision limit.

大多数含氧加合物与铵根的结合力比普通的酮类更强，因为更多的氧原子和极性官能团可产生更强的结合力。一般来说，高氧化态的有机分子一般都能形成强而稳定的加成产物。加成反应的速率很快，基本上可以认为化合物在接近碰撞极限的效率被电离成产物离子。

Figure 10 Example CID scan of highly oxidized molecules resulting from α-pinene oxidation. Highly oxidized compounds dissociate at much higher energies than simple ketones (such as MEK, shown in orange for reference).

图10α-蒎烯氧化产生的高氧化态分子的CID扫描实例。高氧化态化合物在比简单的酮类（例如MEK，橙色显示为参考）高得多的扫描电压设定下才会解离。

Petrochemistry

石油化工

Volatile organic compounds found in petroleum are dominated by alkanes, cyclolkanes, and aromatics. However, these compounds are not relevant to economically important analyses that determine the acidic and sulfur content. Use of NH4+ CI-MS to measure volatiles from crude oil highlights oxygen, sulfur, and nitrogen-containing functional groups, without being overwhelmed by the hydrocarbons present in the sample. In the NH4+adduct CI-MS data shown in Figure 11, hundreds of individual species present in crude oil headspace are visible, with no sample preparation and measurement time of less than a minute.

原油样品内含VOCs主要是烷烃、环烷烃和芳烃。然而，上述这些化合物与确定酸和硫含量的经济分析指数基本无关。使用NH4+ 铵根加成CI-MS来测量原油中的挥发物，可以更突出含氧、含硫和含氮的官能团，而不会被样品中存在的大量碳氢化合物所‘淹没’。在图11所示的铵根加合物CI-MS数据中，可见在无需样品制备的前提下，原油顶空气体中存在的上百个独立物种。每个样品的单独测量时间均不超过一分钟。

Figure 11 Headspace sample of three different crude oils (SJVH, XTO, and Tufflo 1200). A mass spectrum of each is shown at the top. The Kendrick mass defect plot on the bottom separates compounds by functional group (y-axis) and molecular weight (x-axis). The size of the dot indicates compound abundance, and color indicates the elemental composition. Many sulfur-, nitrogen-, and oxygen-containing compounds are visible.

图11三种不同原油（SJVH、XTO和Tufflo 1200）的顶空气体样品测量结果。顶部显示了每种油品的质谱。底部的Kendrick质量缺陷图按官能团（y轴）和分子量（x轴）进行化合物分离和鉴别。点的大小表示化合物信号相对丰度，颜色表示其元素组成。可以看到较多含硫、含氮、含氧的化合物。

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