紅外光譜的樣品制備.doc

紅外光譜的樣品制備 第一部分每年各地紅外光譜的實(shí)驗(yàn)室制備和利用紅外光譜儀分析成千上萬個(gè)樣品。 這些樣品范圍從商業(yè)產(chǎn)品像高聚物顆粒和液體表面活性劑，一直到高純度有機(jī)化合物。為了從這些不同的材料中得到高質(zhì)量的紅外譜圖，我們必須采用多種多樣的制樣技術(shù)。這篇文章的旨在與您交流紅外制樣技術(shù)。在這篇文章中，將對(duì)基于樣品的物理特性進(jìn)行的技術(shù)選擇作討論。液體液樣的制備是將少量樣品涂于兩片紅外透明的窗片（KBr、NaCl等）之間。窗片的互相擠壓形成一個(gè)樣品薄層，樣品的成分決定了選擇哪種窗片。對(duì)于無水的樣品，窗片材料是KBr。對(duì)于含水樣品, KRS-5 較為適合。固體固體樣品對(duì)光譜學(xué)家提出挑戰(zhàn)。樣品的熔點(diǎn)為我們指出首先該考慮哪種技術(shù)。對(duì)于熔點(diǎn)低于72。C的樣品，用適當(dāng)?shù)娜軇悠啡芙?，成膜于KBr窗片上是最先考慮的。如果因?yàn)榛€不好或是溶解性差而不成功，可以考慮在兩片KBr窗片內(nèi)熔化成膜。如果這也不行，樣品可進(jìn)行KBr壓片。對(duì)于熔點(diǎn)高于72。C的樣品，首選的技術(shù)是KBr壓片。對(duì)于聚合物樣品，成膜法是首選，接著是熱熔法和壓片法。對(duì)于熔點(diǎn)未知的樣品，結(jié)晶度的檢測(cè)將會(huì)指明哪種技術(shù)將會(huì)成功。高結(jié)晶度的樣品用KBr壓片法較好，對(duì)于低結(jié)晶度的樣品，成膜和熱熔會(huì)得到更好的譜圖。紅外光譜的樣品制備 第二部分液體樣品液體樣品的分析有多種方法。在本文中，我們主要探討所使用的制樣方法及一些有關(guān)的潛在問題。純樣品技術(shù)分析液體樣品的最常用方法就是將一滴液體夾在兩片鹽片中間，過程如下：將一滴樣品滴于合適的鹽片上，幾秒鐘后，將另外一塊鹽片合上，這樣液體被夾在兩塊鹽片之間，變成薄膜狀。當(dāng)然，選用的鹽片要與分析的液體樣品兼容。不含水的樣品可采用KBr（325mm）鹽片，含水樣品則采用KRS-5鹽片，這幾種晶體材料的選用主要是根據(jù)它們?cè)诩t外段的透光范圍（優(yōu)于4000450cm-1）和穩(wěn)定性。每次一個(gè)樣品做好后，用帶合適的溶劑的棉花清洗，然后在倒有甲醇的鹿皮或雞皮上拋光。KBr鹽片需要經(jīng)常進(jìn)行拋光，以維持其表面的光潔。由于KRS-5晶體有毒，所有只有當(dāng)其表面被劃傷或污染時(shí)才需要拋光，而且要求專業(yè)人員來完成。ATR技術(shù)水平的單反射ATR主要是由一個(gè)ZnSe晶體的凹槽組成，盡管ZnSe晶體的截止頻率為650700CM-1，但它比其它寬頻帶的材料要更加耐用。 在樣品分析好后，要用適當(dāng)?shù)娜軇悠窙_掉，再用棉花球擦洗干凈，這種材料不需要經(jīng)常拋光。潛在問題：但它最大的問題就是樣品譜圖的非線性，主要指峰位的位置和強(qiáng)度不滿足BeerLambert法則：A = abc 這里, A =吸收值a =摩爾吸收系數(shù) b = 光程c =濃度The Beer-Lambert Law is typically discussed in regards to quantitative analysis. Spectral searching is a type of quantitative analysis, since most searching algorithms use band intensity as a factor. Linearity due to sampling typically occurs in transmission analysis and not in ATR, due to the nature of the ATR technique. Beer-Lambert定理主要是針對(duì)定量分析的，譜圖檢索是定量分析的一種類型，因?yàn)樽V圖檢索是以吸收強(qiáng)度為基礎(chǔ)的，非線性主要出現(xiàn)在透射中，在ATR中則沒有，這主要是由于ATR的技術(shù)特點(diǎn)導(dǎo)致的。The most common cause of non-linearity is inconsistent path length across the sampling area. For our instruments at 2 cm-1 resolution, the sampling area is 6 mm at focus. If there are any air bubbles or unevenness in the sampling area, the path length of the sample will vary across the sample area. 最常見的導(dǎo)致非線性的原因是透過樣品的光程的不連續(xù)性。分辨率為2時(shí)，樣品區(qū)紅外光的聚焦點(diǎn)僅有6MM，如果此時(shí)樣品區(qū)樣品厚度薄厚不均或碰巧有氣泡或，就會(huì)引起此處光程不同。These variations will cause inaccuracies in the band intensities in the spectrum. In other words, the bands will be weaker or stronger than they should be. This condition will lead to reduced spectral search quality. Figure 1 is a plot of a spectrum of 3,4-dichlorotoluene analyzed as a neat with a consistent path length. The peak intensity of the band at 808 cm-1 is 0.39, while the peak intensity of the adjacent band at 870 cm-1 is 0.24 (A808 cm-1/A870 cm-1 = 1.6).這些不同將導(dǎo)致譜圖在各個(gè)波段的吸收強(qiáng)度與實(shí)際值產(chǎn)生差異，從而降低譜圖檢索的質(zhì)量，圖是純3,4-dichlorotoluene的吸收?qǐng)D，在808 cm-1波段處的吸收度為0.39，鄰近870 cm-1處是0.24 (A808 cm-1/A870 cm-1 = 1.6)Figure 2 is the same sample analyzed with uneven coating of the sample on the crystals. As you can see, the band intensities are different. For this spectrum, the peak intensity of the band at 808 cm-1 is 0.76, while the peak intensity of the adjacent band at 870 cm-1 is 0.62 (A808 cm-1/A870 cm-1 = 1.2). This represents a change of 25% from the previous spectrum. This could lead to a poor spectral search match. We 870 cm-1check for unevenness by analyzing the sample using a 6 mm sampling area, followed by analysis with a 3 mm sampling area. The two spectra are subtracted from each other and the residual spectrum is examined. 圖二是同一個(gè)樣品但是通過在晶體上做成一層薄厚不均的膜而得到的譜圖，它的吸收率與上圖已經(jīng)有差別了，808 cm-1處的吸收率是0.76而870 cm-1處卻為0.62(A808 cm-1/A870 cm-1 = 1.2)，與圖一相比，已有25的差距，這必然會(huì)導(dǎo)致譜圖檢索結(jié)果正確率的下降，將樣品聚焦斑為6mm時(shí)得到的的圖與聚焦斑為3mm時(shí)得到的圖相減，用差譜的結(jié)果來進(jìn)行分析：Non-linearities due to unevenness would show up as a large residual. We also minimize unevenness by frequent polishing of the crystals. 由薄厚不均導(dǎo)致的非線性將會(huì)使差譜的結(jié)果非常明顯，我們可以通過定期對(duì)晶體進(jìn)行拋光來降低這種誤差。The second cause of non-linearity in the spectrum of a liquid sample is sample thickness. A large percentage of spectra of liquid samples submitted to us for spectral search analysis have been run too thick. Most FT-IR spectrometers/detectors have a linear response up to approximately 1.2 absorbance units. Any band above 1.2 absorbance units is of questionable linearity. Non-linearitycan sometimes be seen as “choppiness” on the top of the band. We only accept spectra from our lab that have a maximum absorbance value of 1.2 absorbance units. Figure 3 is a plot of the spectrum of the same sample used previously. However, the sample thickness was much larger than the previous spectrum. 經(jīng)常引起液體樣品譜圖的非線性的另一個(gè)原因是樣品的厚度。液樣太濃將會(huì)導(dǎo)致譜圖的吸收太強(qiáng)，而多數(shù)紅外儀器的檢測(cè)器的線性響應(yīng)范圍是0到1.2個(gè)吸收單元，大于1.2時(shí)就會(huì)引起線性問題。有時(shí)非線性會(huì)使譜圖中吸收峰的頭部成平頭狀，在我們的實(shí)驗(yàn)室中只接受吸收單元1.2的譜圖，圖三也是上面提到的樣品的譜圖，但樣品的厚度卻遠(yuǎn)遠(yuǎn)大于前者。The strongest band in the spectrum is over 30 absorbance units. For this spectrum, the peak intensity of the band at 808 cm-1 is 1.66, while the peak intensity of the adjacent band at 870 cm-1 is 0.94 (A808 cm-1/A870 cm-1 = 1.76). This represents a change in relative intensity of 10%, compared to the normal spectrum in Figure 1. This change in relative band intensities will have a negative effect on search accuracy. 譜圖中最強(qiáng)的吸收單元已經(jīng)超過了30個(gè)吸收單元，808 cm-1處的吸收度為1.66，870 cm-1處為0.94(A808 cm-1/A870 cm-1 = 1.76)，相比而言，產(chǎn)生了10的誤差，這種不同波段的吸收值的相對(duì)性的差異將會(huì)給譜圖檢索帶來負(fù)面影響Sample Preparation for IR Spectra - Part III紅外樣品的制備 第三部分III成膜技術(shù)Film Technique在 Sadtler的實(shí)驗(yàn)室，涂膜技術(shù)是用在熔點(diǎn)低于72。C的樣品和低結(jié)晶度的樣品，比如象高聚物，涂膜法也可在其他方法失敗后試用。涂膜的主要程序 先將樣品溶于適當(dāng)?shù)娜軇┲小Ｈ缓髮?shù)滴溶液滴于惰性的基質(zhì)上，溶液揮發(fā)后在基質(zhì)上留下一層薄膜。如果惰性基質(zhì)是紅外透明的，可直接檢測(cè)或?qū)⒈∧兿聶z測(cè)。選擇合適的溶液 選擇溶液最主要的標(biāo)準(zhǔn)是容易去除（除了最明顯的一點(diǎn)，可溶解樣品）。這意味著必須要有低的沸點(diǎn)。蒸發(fā)溶劑所需的熱量越少，樣品所受的影響就越小。另外，溶劑越容易去除，越少的溶劑將會(huì)留下。在我們的實(shí)驗(yàn)室，以下列出的溶劑將首先考慮：氯仿(BP. 61.2 C)，丙酮(BP. 56.2 C)，三氯乙醇(BP. 151 C)，鄰二氯苯(BP. 180.5 C)和水(BP. 100 C)。在選擇成膜技術(shù)時(shí)這五種溶劑適用于85的樣品。純?nèi)芤旱墓庾V也應(yīng)準(zhǔn)備著作為參照。將溶劑的譜圖與成膜樣品的譜圖作比較是判斷是否有溶劑殘留的一個(gè)好方法。每取用一次溶劑便將其參比譜圖更新一下也是一個(gè)好習(xí)慣選擇基質(zhì) 在 Sadtler的實(shí)驗(yàn)室，沒有試圖將薄膜從基質(zhì)上取下，基質(zhì)和薄膜是一起放入光譜儀的。所以需要對(duì)紅外透明的基質(zhì)。除了溶劑是水采用KRS-5晶體外，一般最常用的基質(zhì)是KBr晶體。如果決定將薄膜取下，玻璃將是不錯(cuò)的選擇。成膜 經(jīng)驗(yàn)告訴我們最好使用少量的稀溶液（35滴），在基質(zhì)上形成幾層薄膜，這將比用濃溶液形成的厚膜和大量的溶液一次成膜要好的多.這將使薄膜中的溶劑殘留最少。有時(shí)，當(dāng)你成膜的是晶體樣品，譜圖上會(huì)顯示非常嚴(yán)重的散射和基線傾斜。這在單層成膜時(shí)經(jīng)常發(fā)生，在多層成膜時(shí)也會(huì)出現(xiàn)。我們認(rèn)為這是因?yàn)樽钕瘸恋淼木w成為了形成大晶體的晶核，正是這些大晶體造成光的散射，使基線傾斜。在我們實(shí)驗(yàn)室為了防止這種問題的發(fā)生，我們經(jīng)常在晶體的兩面都涂上一層薄膜，有時(shí)在兩塊晶體的兩面都涂上一層薄膜，一共形成四層膜。這個(gè)能解決絕大部分的散射問題。在蒸發(fā)溶劑時(shí)，使晶體上的溶液保持流動(dòng)。這將幫助您得到厚度均勻的膜。我們經(jīng)常將晶體放在一小片可反復(fù)使用的紙卡上（大約2”3”），后不停的敲擊紙卡的背面，使溶液保持流動(dòng)，或者用移液管末端不停的攪拌攪拌如果去除溶劑需要加熱，而晶體又是水溶性物質(zhì)，比如KBr，那你應(yīng)該先加熱卡片，去除其中含有的水汽。如果你不這樣作，晶體的底部會(huì)吸水霧化，這將使你的譜圖的基線傾斜。在我們的實(shí)驗(yàn)室，我們使用加熱燈來慢慢清除水汽，如果是在一個(gè)較為的環(huán)境中，因該一直用燈加熱 以去除環(huán)境中水汽的影響。注意任何必須預(yù)防的情況，尤其是在使用易燃溶劑時(shí)。潛在問題 在成膜技術(shù)中最嚴(yán)重的兩個(gè)問題是薄膜厚度不均勻和溶劑殘留。薄膜的厚度不均將導(dǎo)致譜圖的非線性。而在薄膜技術(shù)中應(yīng)該時(shí)刻注意溶劑殘留的問題?？偸菍⒔Y(jié)果譜圖與溶劑譜圖的主峰作比較。如果結(jié)果顯示有溶劑殘留，有時(shí)可通過繼續(xù)加熱來去除溶劑。如果你不能確定某個(gè)特征峰是溶劑還是樣品產(chǎn)生，那樣品必須用另一種方法檢測(cè)或使用另一種不會(huì)產(chǎn)生該特征峰的溶劑。另一個(gè)可能產(chǎn)生的問題是，某些樣品在加熱和有氧氣的情況下易發(fā)生氧化。這將導(dǎo)致在1740 cm-1上有一個(gè)C=O 的小峰。有幾種方法可以防止或最小化這種氧化。在惰性氣氛中蒸發(fā)溶劑，比如在氮?dú)庵?，這樣可以減少氧氣的存在。或是減少加熱量來化小這個(gè)問題?？赡艿脑?，你可以使用更低沸點(diǎn)的溶劑，或用真空泵來抽取溶劑。紅外樣品制備IIIFilm Technique制膜技術(shù)In Sadtlers laboratories, the film technique is used for samples with melting points below 72C and for samples with low crystallinity, such as polymers. The film technique is also attempted when other methods fail to produce a good spectrum.在薩特勒實(shí)驗(yàn)室，制膜技術(shù)主要應(yīng)用于熔點(diǎn)小于72C而且是低結(jié)晶率的樣品，例如聚合物，有時(shí)當(dāng)一個(gè)樣品用其它方法都作不出好圖時(shí)，也會(huì)采用制膜法。General Procedure for Casting a Film 制膜流程The sample is first dissolved in a suitable solvent. Several drops of the solution are then placed on an inert substrate and the solvent is evaporated off leaving behind a thin film of the sample on the substrate. The film is then either peeled off and placed in the spectrometer; or, if the substrate is transparent in the infrared, the substrate with the sample can be placed into the spectrometer.首先將樣品溶到溶劑中 ，滴幾滴溶液放到惰性基底上，將溶劑蒸發(fā)，然后在基底上留下一層薄膜，再將薄膜揭下來放到儀器中作圖。如果惰性基底能透紅外光，可將它們一塊放到儀器中測(cè)量。Selecting a Suitable Solvent選擇合適的溶劑 The main criterion for a solvent (besides the obvious one, dissolving the sample) is that it should be easy to remove. This implies a low boiling solvent. The less heat applied to evaporate the solvent, the less the sample will be affected. In addition, the easier it is to remove the solvent, the less solvent will be left behind.對(duì)溶液而言，最關(guān)鍵的是要便于分離，即溶的沸點(diǎn)低，稍微加熱即可將溶劑揮發(fā)而溶質(zhì)幾乎不受影響，而且溶液越容易分離，溶劑揮發(fā)的就越干凈。In our laboratories, the following list of solvents are attempted first: chloroform (BP. 61.2 C), acetone (BP. 56.2 C), trichloroethanol (BP. 151 C), o-dichlorobenzene (BP. 180.5 C), and water (BP. 100 C). These five solvents are suitable for about 85% of the samples for which the film technique is attempted. The spectra of the pure solvents should also be kept on hand as a reference. 在我們實(shí)驗(yàn)室，常用下列溶劑：chloroform (BP. 61.2 C), acetone (BP. 56.2 C), trichloroethanol (BP. 151 C), o-dichlorobenzene (BP. 180.5 C), 和 water (BP. 100 C). 對(duì)準(zhǔn)備采用制膜測(cè)量的樣品，85的樣品可運(yùn)用上述的五種溶劑，純?nèi)軇┻€可用來測(cè)參考譜圖。Comparison of the solvents spectrum to the cast film is considered mandatory to determine if any solvent was left behind. It is also a good practice to generate a new reference spectrum each time a solvent is reordered.對(duì)照純?nèi)軇┑淖V圖，可檢驗(yàn)是否有溶劑遺留在樣品中，同時(shí)樣品的譜圖又可作為經(jīng)過重新處理的樣品的譜圖的參照?qǐng)D。Selecting the Substrate 選擇基底In Sadtlers laboratories, no attempt is made to peel the film off the substrate. The substrate is placed with the film into the spectrometer. Therefore a sub-strate which is transparent in the infrared is needed. The most common choice is a KBr crystal unless the solvent is water, in which case a KRS-5 crystal is used. If the decision is made to peel off the film, then glass is generally a good choice.在薩特勒實(shí)驗(yàn)室，從不直接從基底上撕膜，而是直接將基底帶膜一塊放到儀器中，這就需要一個(gè)對(duì)紅外 光完全不吸收的基底，只有溶劑不是水，大多數(shù)我們都采用KBr晶體，如果是水則用KRS-5，如要揭膜下來，完全可以采用玻璃作基底。Casting the Film鑄膜 It is our experience that it is best to use a small amount of a dilute solution (3-5 drops) and cast several thin films on top of each other, rather than to produce one thick film from a concentrated solution or from using a large amount of the solution. This minimizes the possibility of trapping any solvent within the film.我們?cè)趯?shí)驗(yàn)中，經(jīng)常只采用35滴經(jīng)過稀釋的溶液，重復(fù)的澆多次薄膜，而不是用濃溶液作一張厚膜或是用大量的溶液，這都是為了盡可能減少遺留在薄膜中的溶劑。Sometimes, when casting films of crystalline materials, a spectrum may show a significant amount of scattering and give a poor sloping baseline. This problem is more common when trying to cast a single thick film, but also occurs when casting several thin films on top of each other. It is our belief that this is a result of the first crystals precipitating out and acting as ”seed” crystals to produce larger crystals. It is these large crystals which cause the light to be scattered, hence a poor baseline. To avoid this problem in our laboratories, we will often cast a thin film on both surfaces of the crystal, and occasionally on both surfaces of two crystals, resulting in a total of four thin films. This technique corrects most scattering problems.有時(shí)，當(dāng)我們鑄成膜后，在譜圖中會(huì)由于散射而導(dǎo)致基線傾斜，多數(shù)情況是出現(xiàn)在一次性澆鑄成的厚膜上，有時(shí)經(jīng)多次澆鑄的薄膜也會(huì)出現(xiàn)這種情況。首先我們要相信這是晶體沉淀的結(jié)果，即就象晶體種子“逐漸生成大晶體一樣。大晶體容易引起散射從而出現(xiàn)基線傾斜，為了避免這種情況，我們實(shí)驗(yàn)室經(jīng)常在晶體的兩面同時(shí)鑄膜，偶爾也會(huì)在兩塊晶體的兩面進(jìn)行，總共生成四層膜，以解決散射問題。When evaporating off the solvent, keep the solution moving over the crystal. This will help you get a film of even thickness. We usually place the crystal on a small disposable cardboard card (about 2” x 3”) and either rock the card back and forth to keep the solution moving, or use the end of the pipet to smear the solution over the crystal. If heat is required to remove the solvent and the crystal is a hydroscopic material such as KBr, then you should first heat the card to drive off any residual moisture in the card. If you do not do this, the bottom surface of the crystal will absorb this moisture and cause it to become cloudy, which can cause a sloping baseline in your spectrum.在溶劑蒸發(fā)過程中，盡量使溶液在基底上不停流動(dòng)，這將有助于得到一張薄厚均勻的膜。具體方法是將澆有溶液的晶體放在一塊小板子（2” x 3”）上，然后前后晃動(dòng)板子，使溶液不停運(yùn)動(dòng)，或者可以用吸耳秋不停的對(duì)著溶液吹，如果需要加熱，而基底又類似KBr之類，那您需要先對(duì)基底加熱出濕，如果不這樣，基底將會(huì)吸濕潮解，從而引起譜圖的基線不準(zhǔn)。In our laboratories, we use a heat lamp to gently evaporate off the solvent. This should always be done in a fume hood to remove the vapors from the lab environment. Observe all necessary precautions, especially when working with flammable solvents.在我們實(shí)驗(yàn)室中，常常使用紅外照燈，除去環(huán)境中的濕汽，切記不可用于可燃性溶液。Potential Problems 潛在問題The two most serious problems that can occur with the film technique are an uneven film or residual solvent in the spectrum. The problem with an uneven film is spectra